Source to Sink Sedimentology and Petrology of a Dryland Fluvial System, and Implications for Reservoir Quality, Lake Eyre Basin, Central Australia.

Transcription

1 of a Dryland Fluvial System, and Implications for Reservoir Quality, Lake Eyre Basin, Central Australia. Bachelor of Science (Geology), University of Calicut, India. Master of Science (Geology), University of Kerala, India. Thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy Australian School of Petroleum Faculty of Science The University of Adelaide Australia March 2008

2 CHAPTER 7 INTERPRETATIONS OF MODERN SEDIMENTS 7.1 SYNTHESIS OF RESULTS IN RELATION TO POSITION IN SYSTEM This chapter brings together all of the observations discussed in the previous chapters. Sediment generation in a source region is a function of all the controlling variables such as structural style, subsidence, sediment feed supply and climate, but most particularly, the interaction of sediment supply and accommodation space generation (DeCelles and Giles, 1996; Critelli et al., 1997; Arribas et al., 2000). Also suggest that factors which control sediment composition and grain size are; tectonic setting, bedrock lithology, sediment contribution from the interfluves, river gradient and climate by the previous workers like Blair and McPherson, (1994), Critelli et al., (1997) and Arribas et al., (2000). The petrographic composition of clastic detritus carried by the Umbum Creek and its tributaries faithfully mirrors the complex geology of the Davenport Ranges. High-resolution modal analysis of the Umbum Creek network sands allowed the study to distinguish between the main metasedimentary provenances from the Davenport Ranges and the sedimentary provenances locations within the Eromanga and Lake Eyre Basins. In Umbum Creek modern sediments, grain size and compositions are mostly influenced by bedrock lithology, climatic conditions, drainage network and medium of transportation. The grain size and sediment compositional trends change as the main Umbum Creek intersects the minor tributaries along it s downstream course, especially in the distal, low relief area. The grain size and compositional variations in the tributaries can be traced to the sediment source area of sediments as well as to the bedrock lithology. The provenance of quartz populations (monocrystalline and polycrystalline) in the modern sands varies according to the mixing process which have effected differentially, with grains of different provenance lithotype. The results that support the interpretations outlined above and in the following chapter are summarised in Table

3 Table 7.1: Summary of the methods and key results of the analysis of Umbum Creek modern sands analysis

4 7.1.1 PROXIMAL The headwaters of the Umbum Creek tributaries are located in the Davenport Ranges, to the west of the study area. In this region, Proterozoic meta-sedimentary and Mesozoic sedimentary outcrops generate recycled sediments with considerable amounts of multicyclic quartz grains and metamorphic and sedimentary lithic fragments which agrees with the observations by Callen et al., (1995), Alley, (1998), Croke et al., (1998) and Krieg, (2000) (Appendix 10). Before the confluence of the first two tributaries (Hope and George Creek) in the proximal part of the system, the modal composition of the modern sediments varies with respect to metamorphic and sedimentary provenance regions with low monocrystalline quartz content and a prevalence of metamorphic grains noted (sample 14-1, 14-2, 13-1, 13-2 and 12-1; Figs 6.11 A and B). The QmFLt diagram (Fig. 6.9B) indicates that the composition of sands from the analysed proximal sector of Umbum Creek are less variable than in the other regions as both metamorphic and sedimentary sources form similar plot styled distributions on a QmFLt diagram. Both metamorphic and sedimentary provenance region lithologies are feldspar-poor and may produce variable quantities of quartz and lithic fragments. In the proximal part of the system, Sunny Creek generates more quartz grains than the other tributaries. In contrast, the three other tributaries (Davenport, Hope and George Creeks) provide more lithic fragments as they erode through the Cenozoic alluvial fans. The alluvial fans themselves also contribute a small amount of feldspar, indicates that these alluvial fans developed during wet climatic conditions which according to Dickinson, (1985), Cavazza et al., (1993) and Waclawik, (2006). Differences in sand composition are more evident when lithic grain contents are compared. The meta-sedimentary grains dominate river sand derived from the proximal section of the Umbum Creek catchment. Whilst monocrystalline quartz in the northern end of the proximal study area increases downstream of Hope and George Creeks confluence (sample 12-2). This increase cannot be related to a mixing process within the river network, because both the upstream sand samples 14 (2) and 12 (1) have lower monocrystalline quartz than sample 12 (2) (Fig. 6.9B). In this area, George Creek provides additional input of sediments into the system, most likely derived from reworking by aeolian processes. The supply of sedimentary lithic fragments remains homogeneous before and after the confluence of both Hope and George Creeks; however the metamorphic lithic fragments decrease due to downstream fluvio-aeolian mixing process

5 The entrance of Davenport Creek (sample 9-1) into George Creek produces a decrease in monocrystalline quartz (sample 9-2; Fig. 6.9B) and an increase in metamorphic and sedimentary lithic fragments. Thus, the mixing of sand from George and Davenport creeks appears to explain the increase in metamorphic lithic grain composition of the sample 9 (2) downstream of this point. The composition of sands analysed from the proximal sector of Sunny Creek seems to be homogeneous, with no drastic changes in composition related to supply from the provenance lithotypes. The sediment input from Sunny Creek retains the highest in monocrystalline quartz (74.0%; in sample 6-1 and 64.0%; in sample 5-1) throughout the entire Umbum Creek system. Carbonate lithic fragments are high in the proximal part of the river network compared to other parts. This is most likely due to the supply of carbonate grains from the meta-carbonate deposits in the Davenport Ranges. Finally proximal sand grain sizes generally coarsen downstream (medium to coarse) whilst moderate sorting is consistently observed. In the proximal part of the Umbum catchment, the quartz (monocrystalline and polycrystalline) provenance regions are strongly influenced by uplift and the rate of supply of sediments shedding off the Davenport Ranges. The modern sands in the source area of Sunny Creek (sample 6-1 and 6-2), Davenport Creek (sample 11-1 and 11-2) and Hope Creek (sample 13-1 and 13-2) are an abundant in quartz typical of plutonic/basement origin. Whereas George Creek samples (14-1 and 14-2) shows a elevated amounts of quartz of metamorphic origin (Appendix 9; Fig. 7.1). Along the course of the proximal network, the river system captures progressively more quartz from outcropping Mesozoic deposits of plutonic/basement origin. In contrast, quartz of volcanic origin is very low in abundance in samples throughout the proximal part of the Umbum Creek network. The minerals identified in the <2µm fractions XRD analysis of the proximal region samples (quartz, microcline, albite) are the weathering products of Mesozoic Sandstone rocks (Fig. 6.24). Hematite is present in samples as ferric oxide. Hematite (ferric oxide) typically precipitates in near-surface conditions in semi-arid fluvial deposits. The fine hematite particles present in the clay fraction suggest that they were generated from the abrasion of hematite-coated grains in the modern sedimentation setting during fluvio-aeolian transportation. Similarly SEM results also show grain coatings and overgrowths of alumino-silicate and quartz in sample grains from the proximal section samples (9-2 and 11-2) (Figs 6.25 and 6.26). Hematite, alumino-silicate and quartz coating along with quartz overgrowths in the modern sand grains indicate oxygenated conditions in a modern semi-arid climatic environment, agrees according to the observations of Bullard and White, (2002)

6 Figure 7.1 Quartz grain provenances in the Umbum Creek modern sediments from source to sink. This 3D map and histogram shows the various quartz provenances. Note proximal area represents the quartz derived mainly from plutonic and metamorphic source; the quartz sourced from the plutonic origin dominates the medial area. Distal area suggests quartz generated most likely from the reworking of both plutonic and metamorphic quartz

7 The stereo-zoom binocular microscopic results from the proximal samples show low aeolian grain interaction with fluvial grains is typical. However aeolian interactions in George Creek sands are only represented in one petrographic sample (12-2), were sub-rounded to rounded quartz grains are more common MEDIAL The medial section of the Umbum Creek network captures the greatest amount of sediment generated from the Mesozoic sedimentary rocks. This part of the Umbum Creek receives it s sediment load from the proximal part (Lang et al., 2004; Waclawik, 2006). The medial section includes the major section of Umbum Creek and the downstream section of Sunny Creek. In this region, Mesozoic sandstone, siltstone and mudstone outcrops generate sediments (recycled) with considerable amounts of quartz grains and subordinate metamorphic lithics. The modal composition of the medial section sands is consistent with a clastic sedimentary provenance, showing high monocrystalline quartz contents and a predominance of sedimentary lithic fragments (Figs 6.11 A & B). The medial part is influenced by small seasonal, intermittent tributaries with limited drainage basins flowing into the main river system (Fig. 5.6). The input from local tributaries causes an increase in monocrystalline and polycrystalline quartz proportion into the network. The sediment yield from the Mesozoic sedimentary rocks has the effect of decrease in the metamorphic lithic component and an increasing the amount of sedimentary lithic fragments. Volcanic lithic fragments are absent in the medial part of the system (Appendix 10). One of the most marked downstream variations along the Umbum Creek course is the increase in monocrystalline quartz in each downstream sample point after confluence (8-2, 15-2, 7-2). This can be attributed to sediment generated from Mesozoic sedimentary source rocks outcropping in the medial part, highlighting the relevance of these rocks as a major quartz grain producer. The sediment contributions from the proximal part of the network cause minor variations in the lithic and feldspar population of the downstream medial section samples (Appendix 10). Finally feldspar grains observed in the medial part represent grains generated from sedimentary rocks are dominant than transported from the proximal end of the drainage system. Although meta-sedimentary grains are transported downstream through the proximal part of the network, their relative abundance is noticeably less in the medial part of the drainage network. In the QmFLt diagram, (Fig. 6.17B) the samples show little variation in the sand composition because

8 the sediments are generated from a sediment source where abundant quartz grains and little feldspar and lithic fragments of a quartzose recycled sedimentary deposits. In contrast, quartz content in sand composition is varied proportionally and is evident in downstream samples after each minor confluence. This is most likely due to the Mesozoic sedimentary rocks input from the over bank outcrops controlled by the local drainage. These observations highlight the influence of local drainage patterns and the gradient for the modern sand generation. The supply of sedimentary lithic fragments remains almost homogeneous in the Umbum Creek and Sunny Creek samples, although there appear to be mixing of sediments from local drainage patterns. The Sunny Creek samples (4-1 and 4-2) show high polycrystalline quartz (18.8% and 13.6%) and low metamorphic lithic fragments (4.8% and 5.6%) as they capture more sediment from the Mesozoic sedimentary outcrops. Compare to distal region, more carbonate lithic fragments are identified in Umbum Creek in the upstream part of medial section, however they decrease dramatically in samples farther downstream (samples 15-1, 15-2, 7-1, 7-2, 4-1 and 4-2) (Fig. 6.11B). This is due to chemical weathering in this region as well as lack of production of carbonate grains from the Mesozoic sedimentary outcrops. The grain sizes of the medial samples generally show a trend of coarsening downstream (upper medium to lower coarse), moderately sorted with sub-angular to rounded grains. The monocrystalline and polycrystalline quartz from the medial section samples are predominantly sourced from the Mesozoic sedimentary rocks, which have their provenance in the Gawler Craton which consistent with Alley, (1988), Drexel and Preiss, (1995) and Krieg et al., (1995). The provenance of the quartz in the medial section is a mix of plutonic basement (60-65%) from Gawler Craton and metamorphics (30-35%) from the Davenport Ranges (Figs 6.23 and 7.1). This suggests that sediment supply is from a local drainage network where the sedimentary source rocks provenance from Gawler Craton. This localised drainage network controls the mixing effect on the sediment composition in the modern Umbum Creek medial region sediments. The absence of volcanic grains in the medial part indicates either complete weathering or a lack of supply from sedimentary outcrops. This supports the suggestion that the Eromanga Basin sediments are highly influenced by Gawler Craton basement rocks with low sediment input from the volcanic province of the Gawler Craton region. These observation agree with earlier workers like Drexel and Preiss, (1995) and Krieg, (2000)

9 The clay fraction minerals identified (Fig. 6.24) in the <2µm fraction XRD analysis of the medial sample confirm the relationship to their parent rocks. For example, the quartz fraction in this XRD samples represents the weathering product of the sandstone rocks whereby microcline is the weathering remnant of feldspar grains. Similarly albite in the modern clay mineral fraction was either derived from Mesozoic sedimentary rocks or from the albitisation of K-feldspar in the modern sands. If the albite in the clay fraction originated from the Mesozoic sedimentary rocks, the albite was probably developed in the deposit during the feldspar albitisation mesodiagenetic process similar to that described by Burley et al., (1985) and Bjorlykke, (1998). The kaolinite and illite in the clay fraction most likely originated from the decomposition of surrounding kaolinite-illite bearing Mesozoic sedimentary rocks. The other suggestion about the presence of kaolinite in the sediments is from the leaching of feldspar by meteoric water. Hematite (ferric oxide) and ilmenite (iron titanium oxide) fractions identified in the clay fraction of the medial section samples reflects the oxidising conditions typical of a semi-arid fluvial environment as illustrated by Burley et al., (1985) and Bullard and White, (2002). The fine particles of hematite and ilmenite present in the clay fraction were probably generated from the abrasion of hematite- and ilmenite-coated grains during transportation modern day sediment. The SEM results for the medial samples show magnesium alumino-silicate, hematite (iron-oxide), ilmenite (iron titanium oxide), and kaolin clay coatings (sample points 15-1 and 7-2). The overgrowth of quartz and feldspar are also noted on the grains (Figs 6.25 and 6.26). Hematite and ilmenite coating in the modern sand grains is indicative of the oxidising conditions within a modern semi-arid climatic environment. In such conditions K-feldspar precipitation occurs; a processes that requires high silica activity and high K + /H + ratios (Morad et al., 2000). Such conditions exist in the terminal splay complex area ( sink ) of current western Lake Eyre Basin. Refluxed residual brines associated with the halite evaporites are possible sources for K +. The aluminium and silica necessary for K-feldspar precipitation were internally sourced by the alteration of detrital silicates, particularly feldspars. The stereo-zoom binocular microscopic results from the medial part of the network show more aeolian-fluvial grain interaction. The stereo-zoom binocular microscopic results shows the presence of aeolian sand grains in the Umbum and Sunny Creek sands at sample points 8-2, 15-1, 7-2, 4-1 and 4-2 are evidence of additional input of sub-rounded to rounded quartz grains from the local aeolian sources

10 7.1.3 DISTAL The distal portion of the Umbum Creek drainage system starts after the confluence of Umbum and Sunny Creeks. This section of the network is influenced by small, seasonal, intermittent tributaries with aerially limited drainage basins carrying Palaeo-Neales River sediments. In addition to the input from upstream (proximal and medial section) sediments, Cenozoic sedimentary rock outcrops play an important role, providing sedimentary and metamorphic lithic fragments to the modern sediments, that agree with observations of Lang et al., (2004) and Krapf and Lang, (2005). The confluence of Umbum Creek with Sunny Creek shows a farther decrease in monocrystalline quartz and an increase in sedimentary lithic fragments due to the mixing of sediment and the additional supply of lithic fragments from Cenozoic deposits. The other significant downstream variation is the increase downstream of metamorphic lithic fragments proportions was noted in samples 3-1, 2-1 and 1-1 (Appendix 10). This increase in metamorphic sediments was most likely supplied from Cenozoic sediments derived from Palaeogene meta-sedimentary deposits of the uplifted Davenport Ranges. This agrees with observations like Callen et al., (1986), Alley, (1998) and Krieg, (2000). The feldspar content in sample point 3-2 is high (12.4%) due to the supply of feldspar grains from Mesozoic and Cenozoic sedimentary rocks, in which Cenozoic sediments are sourced from uplifted Davenport Ranges. However, Bulldog Shale and Etadunna Formations of Eromanga Basin deposits have approximately 5-8% feldspar content. In contrast, the feldspar content is generally reduced dramatically in these samples by the weathering of downstream feldspar in the distal part of the drainage system. The modal composition of the distal section sands is divided equally amongst sedimentary and metamorphic provenances, sources whereby low monocrystalline quartz proportions were typical. The QmFLt diagram (Fig B) shows that the composition of the sands from the analysed distal sector is variable due to the change of provenance from transitional recycled to quartzose recycled sedimentary deposits. The provenance of quartz in sample points 3-1, 3-2 and 2-1 were dominated by a plutonic basement provenance, but sample points 2-2, 1-1 and 1-2 were interpreted to have been dominated by a metamorphic quartz provenance domain (Table 7.1; Fig. 7.1). This highlights the influence of meta-sedimentary quartz grains supplied from the Peake and Denison Inliers (Davenport Ranges). And the quartz of plutonic basement provenance was supplied initially from greater Gawler Craton, and which was later recycled from Eromanga Basin

11 sedimentary deposits and Cenozoic sedimentary deposits, which agree with the authors like Callen et al., (1995) and Alley, (1998). The increase in polycrystalline quartz in the downstream distal section is related to the mixing effect of meta-sedimentary grains generated from Mesozoic deposits which transported to the distal end, with sediment yield from Cenozoic deposits. The supply of sedimentary lithic fragments remains homogeneous during the entire course of Umbum Creek within this distal part of the drainage basin. The absence of volcanic grains in the distal part is most likely due to either complete weathering or lack of supply from sedimentary outcrops. Sediments (Mesozoic and Cenozoic) in the medial part of the network contribute large volume of quartz content and some sedimentary lithic fragments in the upstream part of the distal section (sample point 3-1). It is noted that downstream sample points are highly variable due to local sediment generation (Appendix 10). The low carbonate lithic fragment content (< 3%) is notable in the Umbum Creek distal section (Fig. 6.11B), and is due to chemical weathering effects in this region and insignificant supply from local outcrops. The Umbum Creek terminal splay complex sediments contain materials that are high in organic matter compared to the proximal and medial parts (< 9.7%). Elevated organics in the terminal splay complex than in the other parts of the drainage network. The grain sizes of the distal samples generally show a downstream coarsening trend of upper medium to upper coarse grains, and are moderately sorted with sub-angular to well-rounded grains. The downstream section of the distal part (sample point 1-2) Umbum Creek system (delta/terminal splay) most contains coarse-grained sand that has been reworked by both aeolian and fluvial processes. This region is subjected to intense aeolian reworking, and much of the finegrained sands and silts on the terminal splay complex front are either blown away or moulded into low-amplitude shadow bars protected behind salt-adapted vegetation within channels (Fig. 7.2) (Lang et al., 2004). Several factors contribute to the downstream coarsening trend of the modern Umbum Creek sediments including the deflation of clay, silt and fine-grained fractions, the medium to coarse sedimentary lithic grain supply from Neogene deposits (mainly sourced from uplifted Davenport Ranges), and the coarser grain transportation events that occur in peak discharge during maximum flooding events. The mineralogy of the clay fraction (<2µm) XRD analysis indicates that composition of distal sands depends largely on the generation of sediments from parent rocks and from fluvial-aeolian

12 interaction. The quartz fraction suggests evolution as the weathering product of the sandstone breakdown and aeolian abrasion. It is interpreted that the albite content in the modern clay mineral fraction was either derived from the albitisation of K-feldspar in the modern sands, or formed during the feldspar albitisation mesodiagenetic processes in Mesozoic sedimentary rocks, as per the observations of Burley et al., (1985) and Bjorlykke, (1998). The occurrence of kaolinite in the clay fraction is attributed to the transportation of kaolinite-bearing Mesozoic sedimentary rocks into the distal part by flooding (Figs 6.25, 6.26). This is the process of surface water evaporating due to arid conditions, promoting higher salinity and the precipitation of evaporite minerals such as halite and gypsum. The aqueous geochemistry of a playa lake environment, including dissolved Al and Si and low ph, meets the conditions required for direct kaolinite precipitation from lake water (Drever, 1988; Morad et al., 2000). The fine hematite particles present in the clay fraction suggest generation from the abrasion of hematite-coated grains on the modern sand. Hematite fractions reflect oxidising conditions typical of semi-arid fluvial environments and emphasise the geological importance of ferruginous soils in warm climates with temperature averaging about 20 o C and seasonal rainfall of <1mm in the Umbum Creek catchment area, which agrees with the observations of Bullard and White, (2002). The sanidine clay fraction suggests evolution through the weathering of volcanic grains supplied from Cenozoic meta-sedimentary deposits of the Davenport Ranges. In general, clay fraction minerals suggest that there is considerable potential for aeolian abrasion to produce, or release, fine particles from natural dune sands. The SEM results for the distal grains show (sample point 1-2) coatings of magnesium aluminosilicate, hematite (iron-oxide), anatase (titanium oxide), halite and gypsum, and kaolin clay. The overgrowth of quartz and feldspar are also noted in the grains (Figs 6.23, 6.24). One of the most marked variations along the terminal splay is the presence of halite and gypsum coatings and an abundance of hematite and magnesium alumino-silicate (palygorskite) coatings, features related to oxidising conditions and the precipitation of saline lake water percolating into the modern sand grains in semi-arid to arid climatic conditions. Quartz filling in the cracks on quartz grain surfaces and quartz overgrowths were also observed and suggesting that the precipitation of quartz occurs from lake water during dry climatic conditions. The stereo-zoom binocular microscopic results show more aeolian grain interactions with the fluvial grains than any other part of the Umbum Creek drainage system. These aeolian interactions in the distal part of Umbum Creek sands are reported from every sample point, and are evidenced

13 by the presence of sub-rounded to rounded quartz grains. Because of the very low gradient of the distal network, aeolian transportation is more prominent than the fluvial transportation. However, flash floods enable surface runoff as a means of sediment transportation during occasional rain episodes. 7.2 PROVENANCE LITHOTYPE CONTROL This study have been identified five sediment provenance lithotype zones corresponding to five distinct framework grain compositions and textures, as well as on the spatial distribution of diverse grain lithotypes within different parts of the Umbum Creek network in the study area. These zones are listed below: 1. Archaean plutonic/ basement provenance of the Gawler Craton 2. Proterozoic volcanic provenance from the Gawler Volcanics and the Peake and Denison Inliers volcanics 3. Proterozoic metamorphic provenance of the Peake and Denison Inliers 4. Mesozoic sedimentary provenance 5. Cenozoic sedimentary provenance. The first two of these zones are not directly involved in present day sediment generation, but are indirectly involved as recycled sediments resident in the deposits of the three latter named provenance regions (Fig. 7.3). The five provenance lithotype zones and their impact on the composition of modern day Umbum Creek sediments are discussed below (Figs 7.1, 7.3). The quartz grain provenances are detailed in the ternary diagram (Fig. 7.4), which indicates, for example, that quartz grains are derived from of the Mesozoic sedimentary successions that were originally sourced from a mixed source including the plutonic/ basement setting; the Proterozoic metamorphic provenance of the Peake and Denison Inliers and the Cenozoic sedimentary provenance. The provenance lithotype controls over Umbum Creek modern sand in each of the sample points and the multiple provenances for the Mesozoic and Cenozoic sedimentary deposits are detailed in Table 7.2. A modal composition and texture analysis of provenance lithotype source rocks is detailed in Table 7.3; Appendix

14 NOTE: This figure is included on page 187 of the print copy of the thesis held in the University of Adelaide Library. Figure 7.2. Pictures obtained from the NASA website to show the dust storm causing the deflation in Umbum Creek Terminal splay area. Source: (A). Streams of dust blowing eastward October 28, (B). Dust storm blowing northerly - February 2, These processes are characteristic of dryland environments, especially around playa lakes

15 Figure 7.3 Schematic illustrating the relationship between the five provenance lithotypes with respect to origin of sediment in the Umbum Creek modern sands

16 Figure 7.4 Ternary diagram showing the relative percentage of quartz grain composition from various provenance lithotypes in the Umbum Creek catchment. The majority of the samples are dominated by plutonic/basement quartz grains

17 Table 7.2: Framework grain lithologies with respect to likely provenance regions

18 Table 7.3: Average grain size and sorting analysis of provenance lithotypes (source rocks) in the Umbum Creek catchment

19 7.2.1 ARCHAEAN PLUTONIC BASEMENT PROVENANCE Umbum Creek plutonic/basement framework grains were derived from the Archaean Gawler Craton and where they substantially deposited during the Proterozoic into the various units of the Peake and Denison Inliers. In the Mesozoic, these sediments were then further reworked into the Eromanga Basin sediments. In the Palaeogene, the presence and influence of the Gawler Craton plutonic provenance have been continued in Eyre Formation sediments, before the uplift of the Peake and Denison Inliers. Thus, Gawler Craton sediments have a strong control on all the modern Umbum Creek sediments through recycling mostly from Mesozoic and Early Cenozoic sedimentary deposits. The general dominance of recycled Gawler Craton plutonic grains is clearly observed in the proximal region of the Umbum Creek drainage basin, where localised Mesozoic sediments outcrop. The abundance of recycled Gawler Craton plutonic grains is well documented from the mixing of Mesozoic and Early Cenozoic sedimentary deposits. The distal part of the river network shows less abundance recycled Gawler Craton plutonic grains (Fig. 7.1) PROTEROZOIC VOLCANIC PROVENANCE The volcanic grains derived from the Proterozoic Peake and Denison Inliers (Davenport Ranges), were eroded and incorporated into primarily in Mesozoic Eromanga Basin sediments. The presence of volcanic grains in the modern sands has been demonstrated (this study) to be the result of the incorporation of recycled sediments from Mesozoic successions and directly sourcing of material from volcanic sediments of the Peake and Denison Inliers (uplifted Davenport Ranges). Most volcanic grains in the modern Umbum Creek sediments are shown to be from the proximal section of the river network. The lack of volcanic grains in the medial or distal parts may be due to chemical and mechanical weathering and/or the failing of recycled volcanic grains in Cenozoic sediments PROTEROZOIC METAMORPHIC PROVENANCE The presence of metamorphic grains in Neogene deposits in Lake Eyre Basin suggested that these grains were derived from the uplifted Proterozoic Peake and Denison Inliers (Davenport Ranges). It is not recorded in Table 4.1, because Eyre and Etadunna formations are not highly influenced by uplift. However, the later Neogene sediments of Lake Eyre Basin are obtained from the uplifted Davenport Ranges. These metamorphosed plutonic grains were originally sourced from Gawler Craton plutonic basement rocks, but then consequently metamorphosed in the

20 Proterozoic Peake and Denison Inliers deposits. The Davenport Ranges were uplifted in the Cenozoic and the subsequent erosion of metamorphic deposits occurred, then they were redeposited in the Late Palaeogene and Neogene Lake Eyre Basin. These metamorphic sediments exert a control influence on the composition of modern sediments of each tributary source area of Umbum Creek, where there is a predominance of recycled Cenozoic sediments. In the modern sediment, metamorphic grain dominance was observed from the beginning of the proximal section, particularly in sample points 14-1, 14-2, 13-2, 12-1, 12-2 and 9-2, where there are alluvial sediments from the Davenport Ranges. Dominance of metamorphic grains was also evident in some parts of the distal section (sample points 2-1 and 1-1) where there is sediment mixing of recycled Cenozoic sedimentary grains (Figs 7.1, 7.5, 7.6) MESOZOIC SEDIMENTARY PROVENANCE The sediments generated from the Mesozoic sedimentary provenance were deposited in the Eromanga Basin and represent recycled sediments of the plutonic/basement (Gawler Craton) and volcanics (Gawler Volcanics). Results show a consistent volume of recycled sedimentary grains in the modern Umbum Creek sand system are derived from Mesozoic deposits (Appendix 10). Sediment analysis suggests that Eromanga Basin sediments were reworked after the uplift of the Peake and Denison Inliers. Thus Eromanga Basin provenance sediments also exert a strong influence on the modern sediments through the recycling of mostly Mesozoic and Cenozoic sedimentary deposits. In the modern sediment, these recycled Mesozoic sediments are particularly apparent in the downstream part of the proximal section. These Mesozoic fragments are found throughout the medial section and distal portions of the Umbum Creek drainage network (Figs 7.1, 7.7, 7.8A) CENOZOIC SEDIMENTARY PROVENANCE A mixture of metamorphic and sedimentary source rocks contributed to the formation of the Cenozoic Lake Eyre sedimentary provenance. This provenance region is strongly influenced by recycled Mesozoic sediments, as well as by eroded metamorphic sediments from the uplift of the Davenport Ranges, particularly in the later Cenozoic sediments. Results show that these sediments record a strong distribution in the modern Umbum Creek sediments through localised recycling in the entire distal section of Umbum Creek drainage basin (Figs 7.1, 7.8 B C & D)

21 Figure 7.5 Sediment provenance comparisons of the modern Umbum Creek grains, recognizing the grains from its provenance lithotype in the source areas. Photomicrographs of typical Davenport Ranges provenance lithotypes and its rock fragments. A. Baltucoodna Quartzite under crossed polarized light. A. Modern sand grain derived from the Baltucoodna Quartzite. B. Duff Creek Beds quartzite B. Modern sand grain generated from the Duff Creek Beds quartzite. C. Mt Margaret Quartzite C. Modern sand grain derived from the Mt Margaret Quartzite. D. Skillogalee Dolomite, quartzite beds. D. Modern quartzite grain generated from Skillogalee Dolomite quartzite beds

22 Figure 7.6 Sediment provenance comparisons of the modern Umbum Creek grains, recognizing the grains from its provenance lithotype in the source areas. Photomicrographs of typical Davenport Ranges provenance lithotypes and its rock fragments. A. Wirriecurrie Granite under crossed polarized light. A. Modern sand grain derived from the Wirriecurrie Granite. B. Tidnamurkuna Volcanics B. Modern sand grain generated from the Tidnamurkuna Volcanics. C. River Wakefield Siltstone C. Modern sand grain originated from the River Wakefield Siltstone. D. Kalachalpa Siltstone D. Modern sand grain generated from the Kalachalpa Siltstone

23 Figure 7.7. Sediment provenance comparisons of the modern Umbum Creek grains, recognizing the grains from its provenance lithotype. Photomicrographs of typical Mesozoic provenance lithotypes and its rock fragments. A. Algebuckina Sandstone under crossed polarized light. A. Modern sand grain generated from the Algebuckina Sandstone. B. Cadna-owie Sandstone. B. Modern sand grain derived from the Cadna-owie Sandstone. C. Bulldog Shale, sandstone unit. C. Modern sand grain originated from the Bulldog Shale, sandstone unit. D. Coorikiana Sandstone. D. Modern sand grain generated from the Coorikiana Sandstone

24 Figure 7.8. Sediment provenance comparisons of the modern Umbum Creek grains, recognizing the grains from its provenance lithotype. Photomicrographs of typical Mesozoic and Cenozoic provenance lithotypes and its rock fragments. A. Oodnadatta sandstone under crossed polarized light. A. Modern sand grain generated from the Oodnadatta sandstone. B. Eyre Formation. B. Modern sand grain generated from the Eyre Formation. C. Etadunna Formation. C. Modern sand grain generated from the Etadunna Formation. D. Calcrete of Etadunna Formation. D. Modern sand grain generated from calcrete of the Etadunna Formation

25 7.2.6 MODAL COMPOSITIONAL TREND The modal composition of sands from Umbum Creek and tributaries is consistent with derivation from a mixture of metamorphic and sedimentary source rocks. This is shown by the generally low feldspar content of most samples with variable amounts of quartz and lithic fragments common. In the ternary plot (QmFLt), modern Umbum Creek sands (Figs 6.9, 6.17, 6.18) show variations in the composition and texture associated with the sediment supply from detritus source rocks along the fluvial course. Evolution of sand composition through the river system is mainly related to the relative proportion of bedrock lithologies in the corresponding drainage sub-basins. Thus, the greatest content of lithic components appears upstream (proximal), where meta-sedimentary detritus sources dominate over sedimentary and volcanic sources. However, the relative proportion of these source rocks does correspond to the lithic grain proportions in the sand, which contains more metamorphic than sedimentary lithics. A significant increase in sedimentary lithics and a decrease in meta-sedimentary lithics are observed in downstream samples in the proximal section of the network, probably due to the influx of sedimentary lithics from localised outcrop areas. Sunny Creek (proximal and medial section sample points 6-1, 6-2, 5-1, 5-2, 4-1 and 4-2) are located in a sediment source area where a large volume of monocrystalline quartz grains have been librated. Although this area is tectonically less active than other parts of the Umbum Creek, a significant number of grains have come from Proterozoic sediments as well as from Jurassic and Cretaceous (Mesozoic) fluvial to shallow marine sandstone formations in this region. In the more tectonically active parts of the Umbum Creek network alluvial fans of the Davenport Ranges also contribute a large portion of reworked sediments. The monocrystalline quartz proportions are elevated in the sediment provenance area but gradually decreases during transportation to the terminal splay. The compositional trend changes as the main Umbum Creek intersects the minor tributaries downstream, especially at the distal end in the low relief area. Polycrystalline quartz generally maintains the same proportions along the entire Sunny Creek system, but also experiences minor changes through influx from localised sources. Feldspar constitutes only a minor percentage, farther decreasing in the downstream direction, indicating chemical weathering during transportation. Lithic fragments at the Sunny Creek sediment source are low in percentage, but increase as Sunny Creek passes downstream interfluves sediment sources. A similar circumstances where exists for the silcretes (sedimentary grains), which are low in the source area, but increase in proportion downstream as the Umbum Creek takes in more Palaeogene and Neogene sediments from the low relief plains

26 Davenport Creek (proximal section sample points 11-1, 11-2, 10-1, 10-2 and 9-1) sediments originate from the Neo-Proterozoic sequences of the Davenport Ranges and contain comparatively low percentages of monocrystalline quartz and moderate proportions of lithic fragments. However, the percentage of monocrystalline and polycrystalline quartz tends to increase the quartz proportions after Davenport Creek meets George Creek in the proximal part of the network. In addition, feldspar contents are higher in the source area and rapidly decreases along with a short downstream transport distance. Silcretes are in greater supply at the Davenport Creek source than in other tributaries, and reduce downstream until Davenport Creek meets George Creek. Hope Creek (proximal section sample points 13-1, 13-2 and 12-1) is the second highest source for the mono and polycrystalline quartz into the system. However, their relative percentages decrease along the Hope Creek network due to the influence of local sediment influx from interfluve areas (Palaeogene and Neogene sediments). Occurrence of silcrete and feldspar is low in this sediment source area. Lithic fragment proportions are low initially but increase downstream and are comparatively high where Hope Creek intersects George Creek. Subsequently however, the percentages of lithics decrease farther downstream at the confluence of the main Umbum Creek. George Creek (proximal section sample points 14-1, 14-2,12-2 and 9-2) is the source of a high percentage of lithic fragments which supply the entire fluvial network due to the tectonically active fault zones. The lithic fragments observed are most likely derived from Neo-Proterozoic carbonates, evaporites, sandstone and siltstone. The percentage of lithic fragments reduces along the downstream course towards Davenport Creek. The amount of quartz (mono and poly crystalline) is also low compared to other areas, but increases at the confluence of Davenport Creek. Silcrete and feldspar are also observed in low proportions throughout the George Creek samples. Umbum Creek (medial and distal section 8-1, 8-2, 15-1, 15-2, 7-1, 7-2, 3-1, 3-2, 2-1, 2-2, 1-1, 1-2) forms after the merging of George and Davenport creeks. Therefore, Umbum Creek sediments are the mixture of the total sediments from Davenport, Hope and George Creeks. The Hope Creek and George Creek sediment source areas supply more lithic fragments and less quartz than the source area of Davenport Creek. Mixing of sediments from George and Davenport creeks reduces the amount of quartz in the stream sediments, although quartz grain proportions increases again downstream. However, polycrystalline quartz is generally maintained the proportions throughout

27 the Umbum Creek network. Feldspar contributes less to the sediments and varies according to the sediment supply from interfluves. Silcretes also show a lesser percentage in the Umbum Creek proximal end, but increases with the sediments incorporated from the adjacent low-relief area (Palaeogene and Neogene deposits). Sediment evolution and sediment fluxes are constrained in the Davenport Range source area of Sunny Creek because of the limited creek incision due to the tectonically inactive zone and the slow uplifting bedrock (Waclawik, 2006). More lithic fragments and other sediments are supplied from other tributary creeks as they lie in a tectonically more active zone. Present observation suggests that the alluvial fans on the foothills of the Davenport Ranges feed the sediments of the river system with a high amount of lithic fragments. However, the bulk of Umbum Creek s sediment load is derived from the interfluves rather than from the hinterland source areas. This is most likely due to the exposure of bedrocks on the drainage network and subjected to physical and chemical weathering processes, driven by climate and modulated by vegetation. The Umbum Creek fluvial system changes from a braidplain environment to the terminal splay (playa) fluvial system as it passes through an aeolian dune field. Large amounts of the hinterland are comprised of Mesozoic to Cenozoic deposits; however, the dry seasonal climatic condition provides more fluvio-aeolian sediment reworked from Palaeogene and Neogene deposits, rather than from deposits at the proximal section of the network. Contribution from the incision of the river network is limited because of the low rainfall in the area. The volcanic and feldspar grain content in Umbum Creek modern sand proportionally varies throughout the river network according to the supply. This supports the evidence for mixing processes caused by the introduction of sand supplied by the tributaries and the change in the provenance lithotype sources. 7.3 TRANSPORT PROCESS CONTROL Climate and topography in the source area are the main controlling factors of processes such as weathering and erosion, which determine the detrital spectrum supplied to first-order tributaries at the beginning of the dispersal system connecting source and basin (Weltje and von Eynatten, 2004). Compositional and textural characteristics of the initial detritus are modified by abrasion and

28 sorting during transportation. Mixing of detritus from multiple provenance sources may farther modify the initial sediment characteristics, especially when dispersal pathways are complex and involve the recycling of previously deposited sediments. While sediment is in transit, mechanical and chemical alteration act as important sediment textural modifiers in alluvial systems (Johnsson, 1993). Thus, the transport process largely reconstitutes the composition and textural maturity of sediments before they are deposited in the basin ( sink ). The grain-size distribution function approach provides a tool not only for numerical partitioning of the components, but also for understanding the mathematical laws of their grain-size composition and their controlling transport and depositional processes (Sun et al., 2002). Farther more, It is common for grain-size distributions to show a discontinuity of fluvial sediments, which implies that multiple components exist in these sediments (Robinson and Slingerland, 1998a; Sun et al., 2002). The discontinuities were commonly demonstrated using cumulative grain-size distribution curves (Figs 6.3A, 6.3B, 6.3C), but this study found that discontinuities are more obvious in frequency curves (Figs 6.2A, 6.2B, 6.2C). The wet season has a dominant role in sediment supply during flood periods. The sediment yield is at its maximum when dry climatic conditions are followed by a flood season and flash flooding. The rate of sediment supply and transportation in low-gradient areas is high during flash flooding events (Fig. 7.9). Umbum Creek modern sand is controlled by three types of transportation: fluvial, aeolian and combined fluvio-aeolian. Spatial changes in erosion rate and composition observed at Umbum Creek are due to variation in lithology and drainage patterns controlled by fault zones (Callen et al., 1995; Alley, 1998). The rate and nature of sediment supply to aeolian systems from fluvial systems is not only a function of sediment production and sorting, but also strongly depends upon the nature of the channel through which sediment is transported. The Umbum Creek system discharges mainly onto the exposed playa lake bed in the form of a subaerial terminal splay complex. After drying out, the terminal splay is subjected to intense aeolian reworking, and much of the fine-grained sands and silts on the delta front are either blown away or moulded into shadow bars on the delta front and on the delta plain (Lang et al., 2004). The wind also reworks the upper surface of the in-channel bars, resulting in the well-sorted and coarse-grained sediments in the system. Figure 7.10 illustrates the factors controlling sediment transfer processes in three main sediment storage areas for sand-sized material in dryland environments

29 Figure 7.9 Flash flood events recorded in Umbum Creek network. A. Flooding is active on the floodplain before it reaches the main channel, arrow pointing the flow of water to the channel. B. Rainwater filling in the channel and downstream flow. C. Evidence of the strength of fluvial flow and peak discharge during the major floods. D. Upside down tree transported during the flow of major flood event

30 Figure 7.10 Key links between the three main sediment storage areas for sand-sized material in dryland environments with respect to the western Lake Eyre Basin margin (modified from Bullard and Livingstone, 2002)

31 During periods of reduced runoff, wind erosion of fluvial deposits occurs, with fluvially derived sand being reworked into expanding aeolian dune fields. In the wet phase, there is increased runoff and sediments are supplied from the adjacent dune fields into the fluvial system. During dry phases, deflation of the sediments occurs due to changes in the water table levels. Deflation is prominent in the terminal splay area of Umbum Creek today. One of the reasons for the observed grain size coarsening downstream trend is wind erosion at the Umbum terminal splay, which removes fine grains and the clay fraction from these sediments (Fig. 7.2). This observations are consistent with the previous works like Krieg, (2000), Lang et al., (2004) and Waclawik, (2006) PROXIMAL Fluvial transportation is the dominant control on the movement of modern sands in the proximal part of the river network, due to the high gradient of the depositional plain and associated drainage pattern (Figs 5.5, 5.6). Sand collected from this part of the system has the greatest dilution of metamorphic lithic grains, providing evidence of the influence of the slope drainage pattern and fluvial transport. Abrasion during transport also influences sand composition and texture, particularly in higher gradient areas. The influence of abrasion is apparent in the disintegration of metamorphic lithic grains and the increase in monocrystalline quartz. Upstream and downstream sand samples in the proximal section show no evidence of any decrease in grain size due to transportation; however, this could be a mask effect caused by the mixing of coarser sediments from outcrops along the downstream course. Despite this sand composition shows no drastic variations in the QmFLt diagram along the tributaries of the proximal part of the system (Fig. 6.9B). Mechanical disintegration and slope gradient are considered the main mechanisms responsible for grain breakdown and compositional variation during fluvial transport in the proximal Umbum Creek network. Results indicate that chemical weathering plays only a minor role during transportation, as there is an abundance of carbonate grains in the sediment population MEDIAL Fluvial transportation is also the dominant control in the medial part of the network, although aeolian interactions also have intermittent influence. Variations in the content of lithic grain types correspond to supplies from the tributaries. There is an increase in sedimentary lithic grains and a decrease in metamorphic lithics as the system progresses from proximal to medial. Grain composition in medial sample points remains almost the same, but grain size tends to decrease

32 (Figs 6.4, 6.5, 6.8). There is no evidence that abrasion causes any significant changes in sand composition along this part of the river system. Sand composition in the medial section shows little variation because inputs from local drainage produce a consistent composition and because of the decrease in the proportion of metamorphic grains (Appendix 10). Grain sizes of the modern sediments vary from sample point to sample point, due to the disintegration of grains as well as the additional storage time for chemical weathering. The decrease in feldspar content can be attributed to chemical weathering of the grains in this area. Aeolian interaction is evident in sample points 4-1, 4-2, 7-2, 9-2 and 15-1 and thus maintains the grain size variation along the downstream course. Weathering is considered to have less impact on grain disintegration for sand composition variation in the medial section, but is responsible for decreases in grain size during fluvio-aeolian transport. Feldspar depletion becomes progressively more pronounced as chemical weathering proceeds in this area. This latter process appears to play a major role in the disintegration of sediments during the low gradient downstream transport and during temporary sediment storage time DISTAL The low gradient (<0.1 0 ) in the distal end of the Umbum Creek network supports aeolian rather than the fluvial processes with respect to the transport of sediment. Repeated cycles of weathering and abrasion during fluvio-aeolian transport eventually result in the destruction of composite and unstable grains and texture in the distal end sediments. The high percentages of quartz-rich sand and sub-rounded to rounded grains are the main outcomes of these processes. Results from this study illustrates that grain size tends to increase in the distal part of the network (Figs 6.4, 6.5). The sedimentary lithic grains composition continue to increase and metamorphic lithics composition continue to decrease as the system progresses from medial to distal (Fig. 6.8). The compositional variations in the distal section are highly dependent on the mixing of the sediments from nearby provenances regions (Appendix 10). The composition of the parent sediments is important in determining the quantity of resident fines that may be released during transportation and in the production of new particles. However, fine and clay material produced by fluvial transportation in the distal end of the system are removed from the terminal splay complex by the extensive winnowing of aeolian transport. The grain size variation in the distal end is due then to the addition of coarse grains from runoff and the removal of fine grains by wind erosion

33 It has long been recognized that the grain-size distributions of most hydraulic and aeolian sediments are poly-modal and represent different transport or depositional processes (Middleton, 1976; Ashley, 1978). In the Umbum Creek distal end, mechanical disintegration and chemical weathering are considered to be the mechanisms responsible for sand composition variation during fluvio-aeolian transport SUMMARY In summary, fluvial transportation processes dominate in the proximal part of the network, fluvial and aeolian processes dominate in the medial and fluvio-aeolian interactions are prominent in the distal end. The breaking of metamorphic lithic grains is a feature of the proximal section and results in the decrease of these grains. The increase in quartz grains in this part of the network is consistent with fluvial transportation effects. In addition, the storage of colluvium in the Davenport Ranges allows the removal of feldspar due to the impact of chemical weathering. Periods of fluvial activity interspersed with aeolian activity are characteristic of the medial part of the network. In this section, aeolian transportation facilitates the removal and addition of silt and clay sediments both out of and into the network, while fluvial transportation contributes to the dilution of feldspar and polycrystalline quartz. Dissolution of feldspar is also noticed in the medial section due to temporary storage during transport. Aeolian processes in the medial section, and more particularly in the terminal splay complex of the distal part of the network, cause sandy fluvial deposits to completely deflate to a stable residue of medium to coarser grains. The addition and subtraction of sediments by fluvio-aeolian processes in the terminal splay complex contributes to the characteristic medium to coarse grains with subangular to sub-rounded texture noted in most samples from this part of the Umbum Creek system. Sand composition features a high percentage of quartz, few lithic fragments and little feldspar (Appendix 9; Table 6.2; Fig. 6.8). Aeolian processes significantly rework channel deposits and barforms, the surrounding floodplain and the terminal splay complex during the long intermission between flood events. These processes rework grain sizes up to lower coarse-grained sand, which can then be redeposited

34 anywhere in the system, usually as a drape of well-sorted sands trapped behind vegetation (copous dunes). Catabatic winds blow with greatest intensity towards the lake in an easterly direction, creating sandy shadow barforms and ripples with a similar flow direction to those produced by fluvial processes. In ancient sediments, these may never be preserved as in-situ aeolian deposits, but nevertheless aeolian deflation is an important process for redistributing and sorting sand and has the potential to improve initial reservoir quality. 7.4 DEPOSITIONAL PROCESS CONTROL As described above, the sedimentary characteristics of modern Umbum Creek sand deposition are controlled by fluvial, aeolian and fluvio-aeolian processes. Sedimentary signatures formed by aeolian and fluvial processes have been widely recognised in stratigraphic sequences (Bullard and McTainsh, 2003). Different sedimentary characteristics indicate a change from an environment where aeolian deposition dominates to one where fluvial deposition dominates, or vice versa. However, temporal and spatial differentiation of dominant processes is not always clear, and mixed fluvio-aeolian deposits are also identified, as are sequences where the interplay of fluvial and aeolian depositional processes is very subtle (Mountney et al., 1998; Bullard and Livingstone, 2002). In the Umbum Creek catchment, there are locations where fluvial activity has an impact on aeolian processes and landforms and those where aeolian activity triggers a response in the fluvial regime, as well as areas where the two systems are co-dependent (Fig. 7.10). The compositional differences among depositional environments suggest mechanical disaggregation and hydrodynamic sorting in fluvial systems and aerodynamic sorting in aeolian systems (Johnsson, 1993; Heins, 1993). Fluvial, aeolian and fluvio-aeolian transportation directly control depositional processes in the western Lake Eyre Basin, and also modify composition, grain texture, sorting, grain size and early diagenesis (grain coating, cementation and overgrowth). Most of the modern sands are deposited in areas where fluvio-aeolian processes are dominant. The key area of sediment accumulation in the distal part of the system is in the terminal splay complex, which includes the floodplain, channels and the fan terminal splay area (sheet of sand deposition). In addition to these depositional areas, Umbum Creek has temporal storage areas in the medial part of the system

35 Sediment is ultimately deposited in a setting where it is buried and isolated from the weathering environment. The rapidity with which sediment is isolated from weathering may be important in determining its composition, especially where exposure to the weathering environment has been brief during pedogenesis and transport (Johnsson, 1993). When sediments are restored in alluvial sequences in an intense weathering environment, profound alteration of sediment composition may result (Heins, 1993; Johnsson, 1993). These processes give rise to a decrease in feldspar and carbonate grains, and ultimately produce the high percentage of quartz observed in the modern Umbum Creek sands. The medial part of the system is characterised by the temporary storage of sediments within the channels. Fluvial processes drive the dilution of feldspar and carbonate grains while in temporary storage areas, depending on the availability of saline water. Sample locations 4-2, 5-1, 7-2 and 15-1 from the medial part of the drainage network shows evidence of the interaction of aeolian grains with fluvial sediments (Appendix 9). Grains are modified through both transportation and depositional processes. Figure 7.11 details the comparison between fluvial, aeolian and fluvio-aeolian processes with respect to depositional environments, and their influence on the composition and texture of modern sediments in dryland environments. Sample locations 1-1, 1-2, 2-1 and 2-2 from the distal part of the network show the maturity of grain composition and texture characteristic of sediments deposited in a fluvio-aeolian environment. Most of the samples from the distal part show the medium to coarse grain size, moderate sorting, poly-modal and sub-angular to rounded grains, features typical of fluvio-aeolian depositional environments. Seasonal river activity delivers an influx of sediment, which is subsequently deflated (Fig. 7.2), a process which particularly affects modern sediments in the terminal splay complex. It is strongly evident from grain size analysis that fine and clay materials are removed by the wind from this area, leaving medium to coarse grains in the deposition (Figs 6.4, 6.5). The effect of wind on siliciclastic sediments (mostly quartz), however, is constrained to sorting by grain size, redeposition on the dry lake bed, and transportation to adjacent sub-environments, such as sand flats and dunes

36 Figure Outline of how fluvial, aeolian and fluvio-aeolian processes modifies the composition and texture of modern Umbum Creek sediments

37 Modern sediments in the distal part samples comprise a high percentage of quartz (70-90%), minor feldspar (<5%), minor carbonate (<4%) and subordinate lithic rock fragments (<15%). When this is compared with modern sediments from the proximal part of the network, which feature more abundant rock fragments (20-50%) and more significantly feldspar (2-8%), it is clear how transportation and depositional processes modify sediments, particularly through the influence of temporal fluvial depositional environments (Appendix 9). Grain angularity, grain size, and sorting are characteristically different in distal sediments (Table 6.2). The alteration and dissolution of feldspar and carbonates due to temporal storage during transportation through the medial and distal sections of the network continues through to the terminal splay complex area, resulting in a farther decrease in feldspar and carbonate grains (Table 6.2; Appendix 9) EARLY DIAGENETIC DEPOSITIONAL PROCESSES The spatial distribution of modern sediments in the terminal splay complex causes farther modification to composition and texture through various early diagenetic depositional processes, such as coating, cementation, precipitation and overgrowth. These processes are not only dominant in the fluvio-aeolian modified sediments, but are also seen in dune deposits as well as the fluvial deposits. Titanium oxide, haematite and alumino-silicate coatings mainly coat in the grains of dune deposits, which were recently re-deposited. The development of micro-quartz and coatings of clay and feldspar are typical of grains deposited in a fluvio-aeolian environment where there is saline water availability. Such conditions occur in the terminal splay complex. Halite, gypsum and quartz cementation indicates a fluvial depositional environment with the presence of saline water. Again, such circumstances are also found commonly in the terminal splay complex. The quartz overgrowth, grain coats and precipitation of quartz and feldspar are also common in this area in particular with depositional processes. However, these depositional processes (coating, cementation, precipitation and overgrowth) are related to the association of feldspar dissolution and precipitation of quartz and clay materials in the presence of salinity (Figs 6.24A, B & C; 6.25, 6.26). The presence of kaolinite clay material in the distal end of Umbum Creek drainage network suggests that the migration of meteoric water into surficial sediments is particularly favourable for kaolinite formation in fluvial environments, which consistent with the observation of Morad et al., (2000). The original detrital clay assemblages observed in Umbum Creek network were mostly illite, associated kaolinite, and subordinate smectite. However, during the present day Umbum

38 Creek semi-arid environment, original detrital clay undergoes repeated flooding and desiccation, smectite and illite were eventually eliminated and magnesium alumino-silicate (palygorskite) was formed. The presence of palygorskite in the terminal splay area is the result of transformation of smectite and illite in an Mg-rich environment CONTROLLING FACTORS FOR THE COARSENING DOWNSTREAM There is more than one controlling factor for the coarsening of sediment grain sizes downstream. According to Basu, (1976), Critelli et al., (1997) and Bullard and Livingstone, (2002) the cyclical changes in climate are reflected in sandy body thickness as well as the grain size and sorting. The effect of sediment mixing and variation of grain size in the Umbum Creek network depends on fluvio-aeolian interactions, surface run-off transportation and input from local sources lead the grain size coarsening downstream. For example, the distal part of Umbum Creek is an area where a considerable amount of the coarser fraction material from tributary streams mixes with surface runoff material during the wet periods. During these periods of increased rainfall and storm intensity, channels on terminal fans become entrenched and sediments entrained during this process are transported to the extremities of alluvial fans, adjacent to surrounding dune field (Fig. 7.9). Once the channel entrenchment ceases and aggradation starts, the quantity of sediment carried to the edge of the fans is reduced, resulting in a change in dunefield activity from sediment accumulation to dune modification and degradation. Thus, one of the main causes of the coarsening grain size trend observed of sand in the terminal splay of Umbum Creek is the net effect of aeolian interaction (Fig. 7.2). Coarser sands are buried below advancing bedforms on the lee sides of sand dunes, whereas finer sands are transported farther downstream, giving rise to a concentration of sediment of a narrow size range that is actively transported. Thus, a blanket of coarser sand lies below the successively deposited finer sand resulting in the development of a fining-upward sequence and a general progressive decrease in grain size downstream (Fig. 7.12A). This hypothesis matches the sample analysis of sample points 8(1) & 8(2) from the medial part of the Umbum Creek catchment. Aeolian deflation leaves the coarser grains and blows the finer grains, as seen in the terminal splay sample analysis (Figs 7.12B, C & D). On the terminal splay complex, low amplitude, slightly elongate triangular mouthbars occur either side of the shallow distributary channels. These bars contain an armoured top surface lag of gravelly coarse-grained sand, typically overlying medium to coarse-grained sand. Silty fine-to very fine-grained sand occurs throughout the distal part of these

39 bars and at the terminus of the distributary channels, but much of this is reworked by both aeolian and fluvial processes, becoming trapped by salt-adapted vegetation. In addition, a thin black-tobrown clay plug commonly mud-cracked and with a halite crust, fills the abandoned channel fill. Close-coupling of the fluvial and aeolian systems is evident in local sediment cycles in which sand deflated from the channel bed to form a dune is eroded by runoff and returned to the channel (sample locations 1-1, 1-2, 2-1 and 2-2). Bullard and White, (2002) demonstrated that in some sands, grain colour is related to the amount of iron oxide present in clay coatings on the grain surface. Modern Umbum Creek sediments from the distal samples show a high redness rating due to formation of the iron oxide coating during dune deposition. The presence of clay coatings may also provide an additional source of fine material. When angularity, particle size and sorting are held constant, particles with a clay coating yield more fine material on abrasion than those from which the clay coating has been removed. Clay coatings may be a significant source of fine material in areas where sands are dominated by sub-rounded and rounded particles rather than by more angular grains, observations are agree with Bullard et al., (2004). However, this is not likely the case in the Umbum Creek network, where the clay coatings and fine material produced via infiltrating meteoritic waters and abrasion in the aeolian and fluvio-aeolian environments are removed by aeolian deflation particularly in the Umbum Creek terminal splay complex area

40 Figure 7.12 Fluvio-aeolian stratification exposed in a shallow pit excavated into a sandflat. Scale bar 5cm for all figures. A. Fluvial depositional bedform (fining upwards cross-bed set). B. Pit from the proximal end of terminal splay complex showing the medium to coarse-grained deposits with fine grain lamination, within a braided-channel environment. Note the lag deposit produced by deflation processes. C. Pit from the medial part of terminal splay complex shows very coarse-grained deposits in the base and combined medium to coarse grained sand towards the top, where the significance of wind ablation is noticed. D. Pit from the distal end of terminal splay complex shows medium to coarse-grained deposit, with rip-up clasts and moderately sorted. The aeolian depositional processes are predominant in the Umbum Creek terminal splay

41 7.5 QUANTITATIVE ANALYSIS OF SAND COMPOSITION AND TEXTURE The compositional and textural properties of a siliciclastic basin fill are linked with the evolution of drainage basins through the principles of climatic-physiographic control of sediment production and supply. Application of these principles leads to a method of compositional analysis in which sequences controlled by high-frequency changes in the rate of accommodation can be distinguished from sequences controlled by high-frequency variations in the rate of sediment supply (order of 10kyr). Changes in rate and type of sediment supplied to depositional systems in response to environmental perturbations in drainage basins are explored in greater detail by studying sediment production under various scenarios of climatic and tectonic forcing (Weltje et al., 1998). The influence of abrasion will have most effect on gravel and cobble-sized material rather than the sand and silt sized grains. Sediment discharge from interfluves to basin is a function of two rates: soil production rate and soil transport rate. Both are controlled by the time-distribution of precipitation and temperature, as well as through the growth of vegetation which plays a role in the balance of apportioning incoming water between evapo-transpiration and runoff (Johnsson, 1993; Robinson and Slingerland, 1998b). Compositional and textural characteristics of the initial detritus in Umbum Creek network are modified by abrasion and sorting during transport, when sediments are carried away from their source area. While sediment is in transit, chemical alteration acts as an important sediment modifier during temporary storage of the sediment in alluvial systems. Chemical alteration and mechanical breakdown of source rock, followed by sorting of particles during transport and deposition, leads to preferential enrichment of specific materials in certain grain-size fractions, and hence, sediment composition tends to be a function of grain size. These observations are also consistent with Weltje and von Eynatten, (2004) COMPOSITIONAL VARIATION The Umbum Creek modern detrital sediments are complex mixtures of monocrystalline and polycrystalline grains, derived from a number of different lithological and tectonic units. As the compositional signatures of various sources of detritus are known (Figs 6.23 and 7.1; Appendix 4), the quantitative contribution of each lithostratigraphic unit to the modern Umbum Creek sand can be estimated directly from point-count data (Appendix 9). These estimates indicate that meta

42 sedimentary rocks of the Davenport Ranges represent a major source of sand-sized bed-load sediment in the proximal part of the Umbum Creek catchment. However, the modern sand contribution from Mesozoic and Cenozoic sedimentary rocks on the composition of medial part is significant due to the mixing effect of local inputs from the interfluves between Umbum Creek tributaries. The contribution of Mesozoic sedimentary rocks from the medial and distal part of the catchment is relatively high in the Umbum Creek terminal complex (Figs 6.8, 7.3), but overall is significant for the whole drainage basin. Cenozoic sedimentary rocks provide more than half of the sand-sized bed-load sediment in the distal portion of the Umbum Creek drainage system. In the proximal part of the Umbum Creek catchment, sediment composition comprises detritus derived from the Davenport Ranges and Mesozoic sedimentary rocks. These detritus input result in a higher concentration of metamorphic fragments than sedimentary fragments in the modern sediments (Fig. 7.13). The variation in quartz grain composition is directly proportional to the proportion of metamorphic fragments. In addition, the Sunny Creek sediment source area supplies an elevated percentage of quartz, but is low in metamorphic fragments (Fig. 6.8). The major compositional variation along the course of the proximal system is the increasing quartz content and the decreasing proportions of metamorphic fragments (Fig. 7.13). In the medial part, there is no dramatic change in compositional variation, although metamorphic rock fragments decrease, sedimentary rock fragments increase, and polycrystalline quartz increases while monocrystalline quartz decreases (Fig. 7.13). In the distal area, compositional trends change according the input of sedimentary rock fragments. However, exceptions are noted at sample points 2(1) and 1 (1) where fraction of monocrystalline quartz decreases in accordance with increased proportions of polycrystalline quartz, sedimentary and metamorphic fragments (Fig. 7.13). The modal analysis of the modern Umbum Creek sediment (sand fraction) was tabulated for all framework grain types and respective grain sizes (Appendix 11). Figure 7.14A illustrates the composition of the total grain types from all samples except sample location 1 samples ( source ) in the modern sand and Figure 7.14B classifies changes in composition at sample location 1 samples ( sink ). The comparison of relative amounts of modern sand composition considered from the average for the whole region (source) versus location 1 (sink) is to illustrate the concept of source to sink compositional variation. Where the sample location 1 is considered as the sink and all other sample locations with the generated sediment are the source for the sink sediments

43 Figure 7.13 Compositional variations of the modern sand samples from proximal, medial and distal. This frequency diagram shows the decline of metamorphic and carbonates lithic grain proportions and increase in sedimentary lithic grains and the gradual decrease in monocrystalline quartz from source to sink samples

44 Figure 7.14 Pie charts showing the relative amounts of Framework grain proportions. A. The average of the total grain counts in different grain types from the total samples ( source ) of the Umbum Creek catchment. B. The average of the total grain counts in different grain types from the sample points 1(1) and 1(2) ( sink ). Both showing the dominance of quartz with lesser sedimentary lithics and metamorphic grains, and low percentage in feldspar

45 Variations in grain size and roundness are quantitatively tabulated through modal analysis (Appendix 9; Figs 6.4, 6.5). Grain sizes of the entire modern sand samples are plotted against the respective framework grain types in Figure 7.15A. The grain sizes from location 1 (both sample points 1-1 and 1-2) are also plotted against the proportion of framework grain types to compare variations in grain size and the relationship of a particular grain type to a specific grain size (Fig. 7.15B). These plots suggest that monocrystalline quartz is the main constituent in the modern sand and maintains an average grain size of medium to coarse-grained (Figs 7.15A, 7.15B). The other framework grain types such as polycrystalline quartz and sedimentary and metamorphic lithics are preferentially coarser grained, where the whole rock proportions of the rock fragments equates to < 5 % (Fig. 7.15A). Grain sizes from location 1 ( sink ) show a minor variational trend in the overall grain size proportion as a function of and framework grain size with grain type (Fig. 7.15B). There is a relative decrease in monocrystalline quartz, whereas polycrystalline and sedimentary grains increase grain size dramatically in the terminal splay complex sample points (Fig. 7.15B). In figures 7.16A and 7.16B, the defined grain sizes of framework grains are plotted against the percentage of the respective grain types of all samples with respect to samples of location 1 (Appendix 11). These graphs show that monocrystalline quartz is the major grain type supplying more than 50% of coarse, medium, fine and very fine grain sizes (Figs 7.16A, 7.16B). Lithic fragments such as metamorphic, sedimentary and polycrystalline quartz contributes high percentages (15-35%) of very coarse to gravel sized framework grains to the system (Fig. 7.16A). The location 1 samples plot shows the abundance of larger sedimentary grains of gravel sizes and the very low content in other grain sizes. However, the monocrystalline quartz is the major grain type and comprises medium, fine and very fine grain sizes. In addition, feldspar grain size range from fine to very fine in location 1 samples (Fig. 7.16B). The total grains of modal analysed sands were tabulated and plotted according to size and grain type (Appendix 11) to achieve a better understanding of the distribution grain size which enriches particular grain type and vice versa. Following this, the categories of each grain type were plotted against respective grain size to determine how each category of grain type relates to grain size (Figs 7.17 A & B, 7.18 A & B, 7.19 A & B, 7.20 A & B). The monocrystalline quartz categories correspond to a wide range of very fine to very coarse-grained sediments in the Umbum Creek modern sands (Fig A). However, the majority grains of monocrystalline quartz are

46 represented in medium to coarse sands. Similarly medium to coarse grain size distribution is a general trend shown by the grain categories such as polycrystalline quartz, sedimentary lithics, metamorphic fragments and volcanic lithics (Figs 7.17 A & B, 7.18 A & B, 7.19 A & B, 7.20 A & B). In contrast however, framework grain lithologies such as feldspar and carbonate fragments represent fine to medium grain sizes. Quantitative analysis of mixed relationships between two different source rocks depends on whether both rocks generate clear diagnostic detrital grains (Molinaroli and Basu, 1993). As discussed previously, sands generated from Umbum Creek and its tributaries are of a mixed nature from a variety of sediment source rocks. The quantification of the abundance of the different source rock types in Umbum Creek permits comparison with the petrographic data from the sands (Figs 5.2, 5.3, 5.4). Furthermore, as lithic grains are unequivocally related to their provenance region, the contrast between their abundance in the sand and the areal extent of the provenance lithology in the drainage basin, allows us to evaluate the amount to which each grain type is representative of sediment source region. This agree the observation by Arribas and Tortosa, (2003). Such comparisons with respect to Umbum Creek sediments are illustrated in Figures 6.12 to The compositional variation graph of Umbum Creek modern sand indicates the changes associated with tributary supplies in relation to changes of source lithology in the drainage basin (Figs 6.12, 6.13, 6.14, 6.15, 6.16). It is clear from these graphs that sediment compositional variations show a direct relationship with the areal extent of their sediment sources. Quartz grain representation in modern sediments throughout the Umbum Creek network depends on the total amount of lithic grains generated from their respective sediment sources. The results of this study (shown in Appendix 9 and 11) indicate that the percentage of metamorphic lithic grains exceeds the relative proportion of these lithologies in the source area. This implies that metamorphic detritus concentrates in the lithic grain population, and is over-represented when compared to the provenance region (sample points 14-2, 13-2 and 12-1). In contrast, sedimentary lithic grains are concentrated in the medial and distal modern sands (as opposed to the proximal part of the system), a variation which correlates with the tectonic setting of the sediment source terranes. In addition, compositional trends show that modern sands are substantially affected by chemical weathering in the medial and distal parts, which reduces the content of feldspar and lithic grains of carbonate and volcanic origin (Figs 6.1, 6.8, 7.13)

47 Figure 7.15 Distribution of the different grain types with respect to their relative grain sizes. A. The total grain types in the Umbum Creek catchment ( source ) plotted against the relative grain sizes. Showing monocrystalline quartz dominance in the sand sized sediments. B. The total grain types in the location 1 ( sink ) (sample points 1(1) and 1(2)) plotted against the relative grain sizes. Showing the bulk of sand represented by monocrystalline quartz, but sedimentary lithics and polycrystalline quartz are important in coarse sands

48 Figure 7.16 Distribution of the relative percentage of particular grain types occurring in defined grain size categories. A. The total grain types and their percentages in the Umbum Creek catchment ( source ) plotted against the defined grain sizes. Showing the dominance of monocrystalline quartz in sand range throughout. B. The total grain types and their average percentages in the location 1 ( sink ) (sample points 1(1) and 1(2)) plotted against the defined grain sizes. Showing the dominance of sedimentary lithics in the gravel and coarse-grained sand and monocrystalline quartz dominance in coarse to coarse-silt grain size

49 Figure 7.17 Distribution of the relative percentage of monocrystalline and polycrystalline grain types with respect to defined grain sizes. Note various categories of monocrystalline quartz with total number of grains present, which dominates in very coarse to medium grained sand size. A. The percentage of monocrystalline grains in Umbum Creek catchment plotted against relative grain size. B. The percentage of polycrystalline grains in Umbum Creek catchment plotted against relative grain size. The graph shows the dominance of polycrystalline quartz varieties within coarse to medium grained size range

50 Figure 7.18 Distribution of the relative percentage of feldspar grains and sedimentary lithic fragments types with respect to defined grain sizes with total number of grains. A. The percentage of feldspar grains in Umbum Creek sediment plotted against relative grain sizes. Showing the various feldspar grains dominance in medium to fine grained sand size. B. The percentage of sedimentary lithic fragments in Umbum Creek sediment plotted against relative grain sizes. Showing the various sedimentary grains distribution in gravel to very coarse-grained size

51 Figure 7.19 Distribution of the relative percentage of metamorphic and carbonate lithic fragments types with respect to defined grain sizes with total number of grains. A. The percentage of metamorphic lithic fragments in Umbum Creek sediments plotted against relative grain sizes. Showing the various metamorphic grains proportions in coarse to medium grained sand size. B. The percentage of carbonate lithic fragments in Umbum Creek catchment plotted against relative grain sizes. Showing the various carbonate grains dominance in medium grained sand size

52 Figure 7.20 Distribution of the relative percentage of volcanic and other lithic fragments types with respect to defined grain sizes with total number of grains. A. The percentage of volcanic lithic fragments in Umbum Creek catchment plotted against relative grain sizes. Showing the various volcanic fragments is dominated in medium grained size. B. The percentage of other lithic fragments in Umbum Creek catchment plotted against relative grain sizes. This graph showing the various other grains are distributed in very coarse to medium grained sand size

53 7.5.2 MECHANICAL DISINTEGRATION The quantitative analysis of grain texture variation in the proximal and medial parts of Umbum Creek indicates that grain size is related more to source lithology than mechanical disintegration during the downstream course. However, at the terminal splay complex, sample grain size analysis supports the suggestion that grain size variation is due to the source rock lithology as well mechanical disintegration. In the latter situation this variation represents the weathering of rock fragments and the sorting of detritus during fluvial and aeolian transport as according to Arribas et al., (2000) and Bullard and McTainsh, (2003). The defined grain types versus grain sizes graphs suggest that mechanical disintegration enhances the fine to very fine-grained mono-crystalline quartz in the system (Figs 7.16A & B). The grain types observed in location 1 (terminal splay) sample sizes indicate the influence of sediment source lithology, and, mechanical and chemical weathering effects. Monocrystalline quartz gains from location 1 ( sink ) ranges from upper medium to very fine in grain size. In distinction, all others samples have the coarse to very fine-grained sands. This suggests that mechanical disintegration is the key factor which leads to the decrease in monocrystalline quartz grain size during the Umbum Creek course. However, the average grain size of location 1 sands maintained upper coarse to upper medium due to the additional mixing of coarser sedimentary lithic grains from the distal Umbum Creek network. The fluvio-aeolian transportation improve mechanical disintegration especially in distal part (Critelli et al., 1997; Arribas and Tortosa, 2003). Feldspar grain sizes also show a decrease in the location 1 samples, suggesting probable chemical weathering processes as opposed mechanical weathering, which agrees with the previous observations by Johnsson and Stallard, (1989). In general, there is a coarsening trend downstream, but the monocrystalline quartz grains decrease in size through mechanical disintegration. The highest proportion of the fine fraction comprises monocrystalline quartz especially in the medial and distal part of the Umbum Creek network. The relative grain size decrease was noted in polycrystalline quartz and metamorphic and sedimentary lithics are most likely due to the mechanical disintegration (Figs 7.16A & B). However, the elevated percentages of fine-grained quartz are also observed in the modern Umbum Creek source lithologies. Sediment suites derived from metamorphic sources show the greatest variation in grain size, with decreasing grain size downstream particularly notable in the proximal part of the network. This reflects the diverse

54 assemblage of sediment source rocks, which include metamorphic, sedimentary and quartz generated from plutonic rocks (Sunny Creek source area, sample points 6-1 and 6-2). Sediment suites derived from the medial area samples of the Umbum Creek catchment show considerable variation of grain size due to depositional mixing. In addition, the medial Umbum Creek drainage system captures sediments from various provenances. Mechanical and chemical weathering exerts less influence on grain size variation in this area in comparison to the influence of sediment source rock lithology and the mixing effect. Umbum Creek modern sediments show compositional variability according to their provenance, but grain size trends are generally maintained as predominately medium to coarse- grained along the system, largely because of wind ablation at the medial and distal sections. Although grain size variations are observed in downstream sample points, the subtraction of the fines from the modern sand by wind transport keeps the grain size similar throughout (Figs 6.4, 6.5, 6.6). The majority of lithic grains are initially coarse-grained to gravel in size; however, they decrease in grain size along the network, indicating that mechanical weathering is prominent in medial and distal areas. Proximal modern sands are typically angular to rounded, but distal samples generally comprise sub-angular to well-rounded grains, demonstrating the detritus effects of transportation downstream via mechanical fluvial and aeolian processes. In summary, the Umbum Creek modern sediments consist largely of detrital grains. Increasing quartz/feldspar ratios, the decreasing abundance of unstable fragments and increasing stable monocrystalline quartz are apparent trends in the modern sands as they progress downstream. Sediment input from tributaries is identified as a significant influence on grain size and composition. Variations in grain size and composition are also attributed to mixing of sediments from multiple sources. Tectonics has not been identified as a major factor influencing grain size as the rate of hinterland uplift and basin subsidence are slow in the present tectonic setting of the Umbum Creek catchment as according to Callen et al., (1995), Alley, (1998) and Croke et al., (1998). To re-emphasise, sediment generation from meta-sedimentary and sedimentary rocks is a major factor in the formation of the modern Umbum Creek sediments. However, compositional variation along the course of the network is a function of sediment input and sediment mixing from the entire source and drainage pattern (Figs 6.12, 6.13, 6.14, 6.15, 6.16). The high quartz content in modern Umbum Creek network system is the product of weathering of quartz-rich bedrock

55 sources. Subsequently grain-size variation is influenced by mechanical weathering due to fluvial and aeolian transport. 7.6 FORWARD MODELLING OF SAND COMPOSITION AND TEXTURE Forward modelling of sand composition and texture involves finding a statistically derived equation which best describes the observed compositional evolution and probabilistic prediction of textural development, considering the sedimentary cycle of a sample suite. Compositional trends are usually interpreted as resulting from specific geological processes such as changes of sediment source rock through time and location, sediment recycling, sediment mixing, and mechanical or chemical weathering (Eynatten, 2004). However, if these controlling processes are correctly interpreted as a function of compositional variation, then forward modelling trends allow for a quantitative description and analysis of the observed trend. The forward modelling approach for grain texture, considers the known parameters of source rock composition and texture along with sediment modifying factors such as climate, tectonic setting, gradient, transportation distance and medium, mixing processes, recycling processes, and early diagenetic processes. The relative contribution of each end member to the Umbum Creek network sediment flux was assessed with forward modelling of end-member mixing. The results from this study (calculated from the entire petrographic and mineralogical data set), indicate that the main sources of the sand generated in the Umbum Creek network are the meta-sedimentary to sedimentary (both first cycle and reworked) rocks exposed in the Davenport Ranges and with the Umbum Creek catchment area. The sedimentary rocks of the Eromanga and Lake Eyre basins have the highest capacity of sand-sized bed-load production, owing to their clastic lithology. The aim of forward modelling prediction is to characterise composition and texture during sediment evolution at the main stage of sediment generation from the source rocks. The extent to which generated sediment differs from the provenance lithotypes depends on the intensity and duration of weathering processes, which in turn depend primarily on climate (for intensity) and topography (for duration) (Heins, 1993; Heins and Kairo, 2006). This study simplifies the processes that are responsible for converting provenance lithotypes into generated sediment as a function of weathering due to climate and regional topographic gradient (Grantham and Velbel, 1988). The

56 generated sediments differ from their parent rocks most significantly when the chemical power of the environment is high (due to higher temperature and/or precipitation), and when the residence time in the environment is long (due to lower topographic gradient) (Heins and Kairo, 2006; Heins and Kairo, 2007). In order to make allowance for the contribution of a particular bedrock constituent of a compound source area to the derivative sand, the study used the quantitative dataset from the provenance lithotype analysis (Appendix 4). The sand generated from a particular bedrock type is expressed as the ratio between the proportional contributions of the bedrock to the sediment budget, and the outcrop area of that particular bedrock METHODS For the purpose of forward modelling prediction, 21 exhaustive and mutually exclusive categories, which classify every rock type into bedrock components capable of producing significant quantities of sand, were established (Table 7.4). This study used information about the provenance lithotype assemblage according to the classification to calculate the conditional probabilities that govern generated sand composition (Appendix 4, 14 and 15). The Umbum Creek catchment was divided into 33 grids which represented the bedrock classifications that contributed sediment to a particular sample suite (Fig. 7.21). The each grid comprises a total area of km 2. The resulting forward modelling calculations were based on the compositional distribution of provenance lithotypes and their bedrock classification. Constant climatic and topographic conditions, such as dry seasonal climate and low topographic gradients, were assumed for each specific provenance lithotypes (proximal ~5-6 0, medial ~1-2 0 and distal ~0.1 0 ) (Fig. 5.5). Other regional characteristics of relevance included the low weathering rates and flushing power, the short sediment transport distances, as well as recognising that the Umbum Creek drainage system represents small and slowly subsiding basin with confined alluvial depositional facies. The relative ability of each provenance lithotype to generate grain categories was based on the abundance of those specific grains, weighted by the relative abundance of each provenance lithotype

57 Table 7.4: Major rock components of provenance lithotypes

58 Figure 7.21 Geological map of the Umbum Creek catchment revealing provenance-oriented groups of sediment source rocks used for the sediment forward modeling investigation. The forward modeling study area divides the Umbum Creek catchment into 33 equal grids incorporating the drainage network

59 The drainage pattern and topographic relief within each grid area controls the sediment input into a particular sample point (Fig. 7.21; Appendix 12). The term full grid refers to a sample point with respect to the total number of grids (all upstream grids) which are supplying sediments related to their drainage pattern and topographic relief in the hinterland (Fig. 7.21). In contrast, limited grids are referred to the total grids (limited upstream grids) which are extremely influential on sediment supply to the related adjacent sample point. Which means term limited grid denotes the grids that are highly influenced in the drainage pattern and transport for any of the sediment yield to the related sample point. For example, the sediment input for sample point 5-1 is from grids 11, 12 and part of 16 and 17 in a limited grid system where as grids 4, 5, 10, 11, 12 and part of 16 and 17 for a full grid system (Fig. 7.21). The total number of grids pertaining to a particular sample point depends on the influence of drainage patterns, topographic gradient, along with mixing and transport processes. This is also subjected to the sediment yield from the source rocks, regardless of any sediment modifying factors. The number of grids included in the full grid and limited grids are varies from sample point to sample point. The limited grid also represents the grids control on generated sediment input into modern sediments. However, in limited grids the generated sediments are not taken into consideration in any sediment modifying processes (Appendix 13). This means the full grid might have laterally more control (influence of drainage patterns, topographic gradient, along with mixing and transport processes) on the sediment yield and input into the system than the limited grid. Other terminologies relevant to the forward modelling interpretation are defined below: Translator: represents the major bedrock components (outcrops) in the detritus source rock region within each grid (Table 7.4). Translator is formatted with respect to the proportion of original bedrock (provenance) lithology from which the sediment has generated (percentage of grains) that have incorporated into related latter provenance lithotype (meta-sedimentary or/and sedimentary deposits) (Appendix 12 and 13). This is calculated using the modal analysis data from the successions of Proterozoic to Cenozoic provenance lithotypes (five provenances) (Appendix 4; Table 4.1). In Translator a grid represents the proportion of provenance lithotype exposed in that particular grid. For example, grid 25 represent the entire Winton Formation and grid 16 represents various proportions of the Coorikiana Sandstone, Bulldog Shale, Oodnadatta Formation and Palaeogene alluvial sediments

60 Observed area-grids: are the total number of grids included in the forward modelling calculations according to the influence of sediment yield with respect to each sample point and area of related bedrock lithology classification (Appendix 12 and 13). The total area of a specified provenance lithology covers with respect to a particular grid varies according to the outcropping pattern of provenance lithology in that particular grid. This is calculated according to the sediment yield which is subject to drainage, topographic gradient and transport processes upstream of a particular sample point. For the limited grid calculation, the grids are considered limited to a particular drainage pattern, gradient and transportation control, which generate sediment that is then subsequently transported to a particular sample point. Calculated PL (provenance lithotype) mix: is the calculated percentage of provenance lithotypes contributed from the entire provenance lithotypes present with respect to total number of grids, which are considered as the source for the sample suite (Appendix 12 and 13). It is calculated using the translator and the number of grids, which is the sum of the multiplied particular bedrock lithology ratio present in the total observed area-grids with the percentage of that particular bedrock lithology proportions from the provenance lithotype (translator), divided by the sum of the total grid area of the entire bedrock lithology classification (observed area-grids). Predicted bedrock types (full grid): is equal to the result of the calculated PL mix with respect to the full grid. The full grid refers to the total number of grids that pertain to a sample point related to the sediment source (most the grids from upstream network) (Table 7.5). Predicted bedrock types (limited grid): is equal to the calculated PL mix with respect to the limited grid. The limited grid refers to the limited number of grids that supply the majority of sediments to particular sample point. (Table 7.6). Interpreted PL (provenance lithotype) mix: indicates the framework grain lithology categories in the modern Umbum Creek sediments (sand fraction) (Appendix 12 and 13). This is scheming as the sum of the calculated PL mix with the percentage of grain categories present in the sediment provenance lithotype of each source rock with respect to the sample suite divided the total number of grids. This infers that the generated sand grain lithology categories (% of grain lithology) ratio correlates to the abundance of the particular lithology categories (% of provenance) present in the provenance lithotype (source) within a grid area

61 Predicted grain categories (full grid): is equal to the result of the interpreted PL mix calculations with respect to the full grid from predicted bedrock types (Table 7.7). Predicted grain categories (limited grid): is equal to the calculations for interpreted PL mix with respect to the limited grid from predicted bedrock types (Table 7.8). Observed PL mix: is the modal analysis result from the modern Umbum Creek sand fraction (Table 7.9; Appendix 14) RESULTS The effectiveness of the forward modelling approach was tested against the modern sand modal analysis data set. Appendix 15 presents the comparison of data between the predicted grain categories from both full and limited grid datasets ( predicted ), and the observed ( actual ) grain categories of the modern Umbum Creek sand. Figure 7.22 shows the data from the predicted bedrock lithologies (full and limited grid) in comparison to the observed framework grain lithology categories of modern sand, presented in trilinear plots according to the provenance lithotype. Similar trilinear plots shown in Figure 7.23 compare the predicted and observed grain lithology categories

62 Table 7.5: Predicted provenance region bedrock lithology types in percentage using the combination of translator 1 and the full grid distribution. This table illustrates various rock lithologies used to simulate sediments at each sample point location with all upstream samples (Appendix 12 and 13)

63 Table7.6: Predicted provenance region bedrock lithology types in percentage using the combination of translator 1 and the limited grid distribution. This table illustrates various source rock lithologies used to simulate sediments at each sample point with limited upstream samples (Appendix 12 and 13)

64 Table 7.7: Predicted grain lithology categories in percentage using the combination of translator 1 and the full grid distribution. This table illustrates various grain lithology categories used to simulate sediments at each sample point with only upstream samples (Appendix 12 and 13)

65 Table 7.8: Predicted grain lithology categories in percentage using the combination of translator 1 and the limited grid distribution. This table illustrates grain lithology categories used to simulate sediments at each sample point with limited upstream samples (Appendix 12 and 13)

66 Table 7.9: Observed grain categories in percentage from the modal analysis of modern sands. This table illustrates the difference between observed and the predicted grain categories, difference in proportion indicates that the observed grain categories are subjected to sediment modifying factors (Appendix 14)