Part II. Drill Core Investigations

Transcription

1 Part II Drill Core Investigations 87

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3 Chapter 5 Sedimentology and stratigraphy of the DVDP-10, -11 and CIROS-2 Cores 5.1 Introduction The following section describes the DVDP-10, -11 and CIROS-2 drill cores. Because these cores were drilled during the 1970 s and 1980 s they have experienced varying degrees of degradation from long term storage, repeated logging and handling and in some cases extensive sampling. Logs of the DVDP cores have been constructed from the initial core logs of McKelvey (1979) and from the revisions of Wilson (1993). Detailed sedimentological analyses of the DVDP cores were conducted by Powell (1981) who defined eight sediment types from particle size analysis and related these to the depositional and environmental setting. The CIROS-2 log was constructed from relogging of the GNS Science archive half as part of this study and from the logs of Pyne et al. (1985) and Wilson (1993). Recently researchers of glacial sediments have abandoned the traditional interpretative facies terms of waterlain diamict and lodgement till (e.g. Hambrey et al. 1991) in favour of simplified glacial proximity curves with fours steps comprising immediate, proximal, distal and marine (e.g. Naish et al. 2009). In this study, the terms waterlain (W) and lodgement (L) are retained because the initial logs and subsequent work used these terms. The terms marine (M), distal (D), and proximal (P) are also used. Figure 5.1 presents modified sedimentation models that are adapted to the Taylor Valley after Hambrey et al. (1991) and the work of McKay et al. (2009) for the Pliocene and modern glacial maxima and minima. 89

4 Conceptual sedimentation models for the Pliocene through to modern glacial maxima and minima 2000 m Modern Glacial Maxima 0 m Lake DVDP-11 DVDP-10 Ross Ice Sheet (Polar) 5000 m Lithology Stratified Diamict Well sorted sand, silts and drop stones Stratified Diamict Massive Diamict Modern Glacial Minima 2000 m 0 m DVDP-11 DVDP-10 sea-ice 5000 m Lithology Fluvial/Aeolian sands, gravel Fluvial/Aeolian sands, gravel Aeolian sands, minor biogenic Pliocene Glacial Maxima 2000 m Matterhorn Taylor Glacier (Sub-Polar) 0 m Canada Glacier Commonwealth Glacier Mt Falconer Mt Coleman DVDP-11 Hjorth Hill DVDP-10 sea-ice Modern sea level melt water plumes 5000 m Lithology Massive Diamict Massive Diamict Stratified Diamict Sand, mud, silt, biogenic with IBRD Pliocene Glacial Minima 2000 m 0 m melt water plumes DVDP-11 DVDP-10 Modern sea level 5000 m Lithology Fluvial sands, gravel, clasts Fluvial/marine mud/sand, clasts Marine mud, rare clasts Diatomite, minor mud, rare clasts 90

5 Figure 5.1 (previous page): Modified sedimentation models that are adapted to the Taylor Valley after Hambrey et al. (1991) and the work of McKay et al. (2009). Modern glacial maxima result in damming the Valley mouths and the formation of freshwater lakes and deposition of lacustrine sediments. Glacial minima result in expansion of alpines glaciers and the deposition of aeolian sediments with minor deltaic sediments at the mouths of the Valleys. During Pliocene glacial maxima the Taylor Glacier advanced resulting in the deposition of massive and stratified diamicts in the fiords. Pliocene glacial minima are recognised by muddy, silty deposits in fiords and minima of ice berg rafted debris. 5.2 Drilling, core storage and background information DVDP The Dry Valleys Drilling Project (DVDP) was a collaboration between scientists from Japan, New Zealand and the United States of America. Drilling took place in the Austral summers between 1973 and 1975 using a wireline diamond drill rig. The aim was to recover continuous Cenozoic sedimentary records to determine the history of the Dry Valleys region (McGinnis, 1981). Three holes were drilled at McMurdo station and 11 more were drilled in the Dry Valleys. DVDP cores 10 and 11 are from the Taylor Valley and are the main focus of this study because they contain the most significant late Miocene - Pleistocene successions recovered during the drilling campaign (Ishman and Rieck, 1992). DVDP cores have been stored at -20 C at the Antarctic Marine Geology Research Facility at Florida State University, Tallahassee since they were drilled CIROS-2 The Cenozoic Investigation of the western Ross Sea (CIROS, see chapter 2) project comprises two cores which were drilled between 1984 and The principle aim of CIROS was to recover a record of the Cenozoic Geological history of the Ross Sea and Victoria land Region (Barrett et al., 1992; Barrett and Hambrey, 1992). The CIROS-2 core was drilled in New Harbor at the mouth of the Ferrar Fiord in 1984 and was specifically targeted to investigate the history of the Ferrar Glacier with an 91

6 aim of determining a minimum age of down cutting and the start of glaciation in the Transantarctic mountains. 5.3 CIROS-2 Lithostratigraphy Core recovery during CIROS-2 drilling was 67% with most core missing in the top 100 metres. The lithologic log is presented in figure 5.2 and the depths of erosion surfaces and lithostratigraphic units are presented in table 5.1. Logging was hampered by the poor condition of the core. Poorly lithified units from the upper portion of the core had almost completely disintegrated and finer grained units throughout the remainder of the core were also heavily fragmented. Well cemented diamicts and coarse, massive sandy units were better preserved. Because of the poor condition of the core, limited information was gained from the logging efforts therefore the original lithostratigraphic subdivisions of Pyne et al. (1985) and Barrett and Hambrey (1992) are retained for the upper 100 m and the subdivisions of Wilson (1993) are used for the lower portion. Figure 5.2 is the lithologic log which was developed for this study. Barrett and Hambrey (1992) subdivided the core into 13 Lithostratigraphic Units (LSUs) where the upper 100 m are units 1 through 7. Wilson (1993) subdivided the lower portion of the core into 10 LSUs (C1 at the base through C10) which form two major groups. The subdivisions of Wilson (1993) are retained in this study. For simplicity the two major groups of Wilson (1993) are summarised below and the subunits contained within each major group are stated in each LSU heading. LSU-1 ( m) The upper portion of LSU-1 is dominated by unstratified medium to fine sands with thin (mm-cm) mud interbeds. The lower 20 cm are well stratified with alternating mm-cm thick mud and sand interbeds (Pyne et al., 1985). LSU-1 is interpreted as a distal glaciomarine-glaciolacustrine deposit (Barrett and Hambrey, 1992). LSU-2 ( m) LSU-2 is comprises a sandy, muddy, mostly massive diamict (minor stratification is noted at 7.25 m). Clasts are subangular to subrounded and are dominated by granitic lithologies (Pyne et al., 1985). The unit is interpreted to be a lodgement till with minor 92

7 waterlain till. Ice was probably grounded over the drill site for most of the duration of LSU-1 (Barrett and Hambrey, 1992). LSU-3 ( m) LSU-3 is dominated by moderately well sorted, coarse to fine sandstone beds interbedded with millimetre bedded fine sands and siltstones. At the base of the unit is a 0.7 m thick, slightly stratified sandy, muddy diamict (Pyne et al., 1985). The upper portion of LSU-3 is interpreted as a distal to proximal glaciomarine-glaciolacustrine deposit with the basal diamict representing waterlain diamict (Barrett and Hambrey, 1992). LSU-4 ( m) This unit is dominated by coarse to medium sandstones, muddy gravely diamicts and sandy mudstones. The base of the unit contains a 1.3 m thick, unstratified sandy, muddy diamictite which is overlain by a 0.7 metre thick, sequence of alternately bedded thin (mm) mudstone and coarse to medium, sandstone beds. The upper portion of LSU-4 comprises stratified, sandy, muddy diamicts (Pyne et al., 1985). The unit is interpreted as mix of alternating lodgement and waterlain diamict and probably represents alternating grounded and floating ice (Barrett and Hambrey, 1992). LSU-5 ( m) LSU-5 is composed of coarse to medium, sandstone beds with minor, thin (mm-cm) mudstone beds. Sandstone beds are mostly unstratified at the top becoming more stratified near the base (Pyne et al., 1985). The unit is interpreted as a distal glaciomarineglaciolacustrine deposit (Barrett and Hambrey, 1992). LSU-6 ( m) The top 11 metres of LSU-6 comprises sandy, muddy, unstratified diamicts with a 1 m thick stratified interval at metres. Between m and m is a sequence of fine, muddy mm-cm bedded sands with slump deformation and rip up clasts. The base of the unit comprises a muddy diamict (Pyne et al., 1985). The upper 11 metres of this unit are interpreted as lodgement and waterlain till with ice being directly over the drill site and a brief episode of subaerial exposure and weathering at metres depth. The lower portions (below 79 metres) of the unit was deposited in a glaciolacustrine setting (Barrett and Hambrey, 1992). 93

8 LSU-7 ( m) LSU-7 is dominated by fine to medium, decimetre bedded basaltic sandstones. Between m and m is a weakly stratified, sandy, muddy gravelly diamict. The unit was deposited in an ice covered lacustrine setting (Barrett and Hambrey, 1992). LSU-8 ( m) C5 through C9 The base of LSU-8 contains a 4 metre thick diatomaceous mudstone with interbeds of stratified, muddy, sandy, gravelly diamict between m and m and between m and m (C5). C5 is disconformably overlain by a 1.2 m thick, sandy stratified diatomaceous mudstone (C6) which grades into a 23.5 m thick, muddy, sandy, gravelly diamict (C7) which is massive at the base becoming more stratified towards the top. A thin interval between m and m contains well stratified, white tuffaceous siltstone laminae. C7 is conformably overlain by a 1.2 metre thick muddy, medium sandstone (C8) and the upper 11 metres of LSU-8 (C9) are composed of a moderately stratified, muddy, sandy, gravelly diamict which rests disconformably on C8 (Wilson, 1993). The sediments from the base of LSU-8 (C5 and C6) are interpreted to have been deposited in a proximal to distal glaciomarine setting. The base of C7 is lodgement till with an upcore transition to waterlain till and finally proximal to distal glaciomarine conditions (C8) (Wilson, 1993). The upper portion of LSU-8 is interpreted as having been deposited under a floating ice tongue with episodes of ice grounding (Wilson, 1993). LSU-9 ( m) C1 through C4 The base of CIROS-2 penetrated granite gneiss basement which is overlain by a 1.1 metre thick, stratified, sandy, diatomaceous mudstone (C1). C1 is disconformably overlain by a 13 m thick massive, muddy, sandy, gravelly diamict (C2) which grades into a 1.2 m thick sandy, stratified diatomaceous mudstone (C3) and back to an 8 metre thick variable, massive to stratified, muddy, sandy, gravelly diamict (Wilson, 1993). LSU-9 records two episodes of ice advance and retreat. 94

9 Depth (m) 0 Lithology Glacial LSU LSU Description Depositional Proximity Environment M D P W L LSU-1 Wilson (1993) Unstratified medium to fine sands with thin (mm-cm) mud interbeds Distal glaciomarinelacustrine deposit LSU-2 Sandy, muddy, mostly massive diamict Grounded ice occasional ice liftoff 20 Diamict Mudstone Sandstone LSU-3 LSU-4 LSU-5 C10 Moderately well sorted, coarse to fine sandstone Siltstones Moderately well sorted, coarse to fine sandstone Stratified sandy, muddy diamict Massive diamict Coarse to medium sandstone Proximal glaciomarineglaciolacustrine Ice-proximal Ice-proximal - grounded ice Distal glaciomarineglaciolacustrine LSU-6 Massive gravelly, sandy, muddy diamictite Grounded ice - brief episode of subaerial exposure Basement 80 Stratified diamict Ice-proximal LSU-7 Medium bedded basaltic sandstone Ice covered lacustrine 100 C9 Stratified - massive diamict Grounded ice - floating ice C8 Muddy, medium sandstone Ice-proximal glaciomarine 120 Stratified diamict Ice-proximal glaciomarine LSU-8 C7 Massive diamictite Grounded ice 140 C6 C5 Stratified diatomaceous mudstone Diatomaceous mudstone intervals of Stratified, muddy, sandy, gravelly diamict Distal - proximal glaciomarine Alternating distal - proximal glaciomarine C4 Stratified to massive diamicts Floating ice tongue - episodes of ice grounding 160 LSU-9 C3 C2 Stratified diatomaceous mudstone Massive gravelly, sandy, muddy diamictite Distal glaciomarine Grounded ice C1 Stratified, sandy, diatomaceous mudstone Granite gneiss basement Distal glaciomarine 95

10 Figure 5.2 (previous page): Lithologic log and descriptions, glacial proximity curve (M = marine, D = distal, P = proximal, W = waterlain, L = lodgement), and environmental interpretations of CIROS-2 compiled from Pyne et al. (1985), Barrett and Hambrey (1992), Wilson (1993) and from observations made during this study. Table 5.1: CIROS-2 Lithostratigraphic units and erosion surface depths. Event Top depth (m) Bottom depth (m) LSU LSU LSU LSU LSU LSU C2 ES C2 ES LSU LSU C2 ES C2 ES C2 ES C2 ES C2 ES LSU C2 ES C2 ES C2 ES C2 ES C2 ES LSU = Lithostratigraphic unit, ES = erosion surface 96

11 5.4 DVDP-10 Lithostratigraphy DVDP-10 was drilled at the mouth of the Taylor Valley at the same site as DVDP-8 and -9. Difficulties with maintaining a stable hole and core recovery during the drilling of DVDP-8 and -9 resulted in the drilling of DVDP-10 during which 84% of a metres succession was recovered before drilling was abandoned (McGinnis, 1981). The lithologic log is presented in figure 5.3 and the depths of erosion surfaces and lithostratigraphic units are presented in table 5.2. The original log and lithostratigraphic subdivisions of McKelvey (1979) form the basis of the log which is presented in this thesis. The core was divided into 5 main LSUs by McKelvey (1979) and these subdivisions have been retained in this study. Notes made during the study of Wilson (1993) were not included in his work however, these have been obtained and are incorporated into the log presented here. LSU-1 ( m) LSU-1 comprises interbedded medium to very coarse sandstones with minor granule conglomerate and pebble conglomerate beds with rare shell fragments noted near the top and base. Pebbles range from 0.4 to 3.0 cm in size. Sandstone beds become more stratified towards the base of this unit. The unit is interpreted as an interlayered mix of fluvial, deltaic and beach deposits. LSU-2 ( m) LSU-2 comprises a 14.2 metre thick sequence of horizontally stratified granule and conglomerate beds with minor sandstone beds. Individual beds have gradational bases and some contain reworked tillite clasts. Average clast sizes are between 10 mm and 30 mm and the maximum clast size is 46 cm. Wilson (1993) interpreted the base of LSU-2 to represent an ice retreat succession from a glacial to a fluvial sedimentary system. 97

12 Depth (m) 0 Lithology Glacial Proximity M D P W L Ice source LSU Description Depositional Environment Interbedded medium to coarse sandstone Sub-tidal delta/beach LSU-1 Pebble-granule conglomerate 20 Ross Ice LSU-2 Stratified coarse to medium sandstone Pebble-granule conglomerate Fluvial 40 Massive diamict Conglomerate fines to sandstone Glacio-fluvial Massive diamict Conglomerate Diamict Breccia Mudstone Sandstone Taylor Glacier Ross Ice Taylor Glacier + Ross ice LSU-3 LSU-4 LSU-5 Conglomerate fines to sandstone Variable diamict Stratified, medium sandstone Massive diamict Poorly sorted, slightly stratified, fine to medium pebble conglomerate Massive diamictite Stratified sandy mudstone Stratified diamict Massive medium sandstone Poorly sorted, sandy breccia Muddy sandstone Shearing, Ice contact Massive gravely, sandy, muddy diamict Stratified sandy mudstones, sandstones and diamicts Massive gravelly, sandy, muddy diamictite Massive gravelly, sandy, muddy diamictite Stratified, interbedded, mudstone and diamict Stratified muddy, sandy and gravelly diamict Ice-proximal Grounded ice Ice-proximal Grounded ice Ice-proximal glaciomarine Ice-proximal-ice distal Ice-proximal glaciomarine Distal glaciomarine Grounded ice Grounded ice Ice-proximal glaciomarine Grounded ice Grounded ice Ice-proximal glaciomarine Distal glaciomarine Ice-proximal glaciomarine Grounded ice Distal glaciomarine Ice-proximal glaciomarine Ice-proximal glaciomarine Distal glaciomarine 98

13 Figure 5.3 (previous page): Lithologic log, Lithostratigraphic Units (LSU s), lithologic descriptions and environmental interpretations of DVDP-10 compiled from McKelvey (1979) and Wilson (1993). The glacial proximity curve (M = marine, D = distal, P = proximal, W = waterlain, L = lodgement) is from Wilson (1993) and sediment source is from Porter and Beget (1981). Table 5.2: DVDP-10 Lithostratigraphic units and erosion surface depths. Event Top depth (m) Bottom depth (m) LSU LSU LSU DV10 ES DV10 ES DV10 ES DV10 ES DV10 ES LSU DV10 ES DV10 ES LSU DV10 ES DV10 ES DV10 ES DV10 ES DV10 ES DV10 ES DV10 ES DV10 ES LSU = Lithostratigraphic unit, ES = erosion surface 99

14 LSU-3 ( m) LSU-3 comprises an interbedded mix of massive diamictites with minor interbedded fine to coarse, sometimes pebbly laminated sandstone beds and rare, thin interbedded mudstone and siltstone beds. Overall the diamictites from the upper portion of this unit are massive with large clasts. The high clast content (>20%) and glacial loading structures indicate that they were deposited immediately beneath a glacier. Diamictites from the base of the unit become stratified and current washed. Occasional thin sand beds and minor stratification within diamictites may represent short periods of glaciofluvial deposition during ice-liftoff. Thick sandstone beds are generally massive at the top becoming more stratified near the base. Most have granule-pebble conglomerate horizons or pebbles scattered throughout. Significant, erosive boundaries and sharp contacts are at m, m and m. LSU-4 ( m) LSU-4 is variable and comprises fine to coarse, massive and laminated sandstone, slitstone and mudstone beds, stratified and massive diamictites, breccia and thin conglomerate beds. The upper 12 metres are dominated by a texturally variable pebble diamictite with two thin sandy mudstone intervals and minor thin sandy mudstones throughout. The lower portion of LSU 4 contains massive and bedded sandstones, mudstones, conglomerates and breccia. The diamicts were probably deposited under an ice shelf or in grounded ice conditions as is evidenced by shear surfaces. The finer, bedded sediments were probably deposited during the retreat of the grounding line in iceberg conditions. LSU-5 ( m) LSU-5 is texturally variable and is dominated by massive and interbedded sandy mudstones and diamicts, medium to fine sandstones and pebble and breccia conglomerate beds. Mudstone units typically have minor (<1%) dispersed clasts comprising granules and pebbles. Pebble conglomerates are thin (<0.8 m) and clasts typically range in size from granules to a maximum size of 5 cm. All conglomerate beds have upper gradational contacts to sandstone or mudstone beds and two beds ( m and m) have sharp, erosive basal contacts. Provenance studies indicated that below 150 metres sediments are sourced from western Taylor Valley (Porter and Beget, 1981). 100

15 Wilson (1993) interpreted mudstones as being deposited in a distal glaciomarine setting and breccia and massive diamicts to be deposited in ice proximal or lodgement till setting. He identified significant erosive surfaces are at m, m, m, m, and m. However, Fielding et al. (2011) revisited DVDP-10 and suggested a more varied depositional environment. They agreed that deposition occurred in an ice distal to iceberg fiord setting, however they suggested that a thin interval (167.4 m m) is a paleosol with root structures indicating subaerial exposure. Ishman and Rieck (1992) designated the portion below m as the Ammoelphidiella uniformina foraminifera zone which indicates deposition in deep water ( m) and the interval between 163 m and 156 m as the Epistominella exigua foraminifera zone which they attribute to unstable and fluctuating environmental conditions. Between m and m Ishman and Rieck (1992) designated a thin Ammoelphidiella antarctica zone which they attributed to unstable, high energy environment. However, the higher and more consistent diatom diversity also indicates prolonged open water conditions and high surface productivity. 5.5 DVDP-11 Lithostratigraphy Drilling of DVDP-11 resulted in 94.1% recovery of a metre succession before drilling was abandoned (McGinnis, 1981). The original log and lithostratigraphic subdivisions of McKelvey (1979) and Powell (1981) are used to describe the upper 150 m of this core. The lower portion was relogged by Wilson (1993) who created detailed logs and reinterpreted its depositional history. The core was divided into 8 main LSUs by McKelvey (1979) and the upper 3 LSUs are retained for this study. Wilson (1993) subdivided the lower 175 m into 20 units (Units D1-through D20 where, D20 comprises the remainder of the core above m) based on lithology, thickness and sedimentary structures which form four major groups. For simplicity the four major groups of Wilson (1993) are summarised below (The subunits of Wilson (1993) are stated in each LSU heading). Environmental interpretations are based on the work of Powell (1981) and Wilson (1993). The lithologic log is presented in figure 5.4 and the depths of erosion surfaces and LSUs are presented in table

16 LSU-1 ( m) The top unit of DVDP-11 comprises a granule conglomerate and drilling breccia which is underlain by a laminated fine sandstone and granule conglomerate beds. The base of LSU-1 contains a 60 cm thick calcareous fine to medium sandstone with interbedded mudstones (McKelvey, 1979). The unit is interpreted to have been deposited in a braided river system with the fine grained beds being the remains of abandoned channels (Powell, 1981). LSU-2 ( m) LSU-2 is a 14 metre thick succession of prograding, moderately well sorted medium to medium coarse sandstone beds with three thin (>30 cm) coarse sandstone to granule conglomerate beds. Individual beds are initially horizontal with increasing up core dip of 7 (McKelvey, 1979). The unit is interpreted as a sequence of traction current beds with occasional soft sediment loading features indicating periods of rapid disposition (Powell, 1981). LSU-3 ( m) LSU-3 is dominated by homogeneous sometimes pebbly fine to very coarse sandstones and minor pebble beds (McKelvey, 1979). The unit coarsens downwards and is interpreted to have been deposited in a sub-tidal delta system which was proximal to grounded ice Powell (1981). LSU-4 ( m) LSU-4 is a thick variable unit with conglomerates (cobble to granule), very coarse to medium sandstones and thin mudstone beds. The base of the unit ( m) contains a 7 metre thick winnowed cobble to pebble conglomerate, which is overlain by a 5 metre thick pebbly, texturally immature (some sands are feldspathic), very coarse sandstone bed. Between m and m, a stratified pebbly coarse to medium sandstone is interbedded with thinner (up to 1.5 m thick) conglomerate beds (McKelvey, 1979). Wilson (1993) logged these sandstone beds as well stratified waterlain till with washed sand and pebble layers. Between m and metres, a fining upward succession of conglomerates and sand beds are capped with a 1 metre thick, bioturbated silty mudstone. The remainder of LSU-4 ( m) 102

17 comprises gradationally deposited, washed, pebbly medium and coarse sands and 2 metre thick pebble conglomerate centred at 80.5 metre depth (McKelvey, 1979). Winter and Harwood (1997) indicated that the upper 91 m of the succession is dominated by a diverse freshwater/lacustrine diatom assemblage. Wilson (1993) logged this upper portion as a waterlain till with interbedded washed pebbles and breccias. All contacts within LSU-4 are gradational with no significant unconformities interpreted from the lithostratigraphy, however, the basal contact is sharp and from biostratigraphic studies (Harwood, 1986; Ishman and Rieck, 1992; Winter and Harwood, 1997) it represents a significant unconformity. Powell (1981) suggested that this unit was deposited in an ice proximal setting with tidal influences and occasional subaerial exposure. LSU-5 ( m) The contact between LSU-5 and LSU-6 is disconformable and sharp with evidence of shearing. The upper 12 metres of LSU-5 are highly variable with interbedded pebble diamicts, laminated sandy mudstones and pebble conglomerates. The remainder of LSU-5 is dominated by a thick, weakly stratified to massive diamict with a 2 metre thick, washed pebble conglomerate above metres and a basal 0.7 metre thick interbedded sand and mudstone bed. A sharp disconformity is observed at m and gradational contacts occur at m and m. The lower portion of the basal diamict was probably deposited under an ice-shelf with an up-core change to grounded ice (Wilson, 1993). The upper, fine grained beds document the retreat of a grounding line and a change to more open water conditions with tractions currents and ice-bergs (Powell, 1981). LSU-6 ( m) Units D19 through D17 The lower portion of LSU-6 (D17) contains a 5.93 m thick stratified, muddy, sandy pebble diamictite with indications of current reworking and winnowing. The basal diamict is disconformably overlain by a 7.97 m thick stratified gravelly, sandy muddy diamict which has a 1.4 m thick basal fossiliferous mudstone (D18). The upper metres (D19) of LSU-6 comprises a poorly sorted, slightly stratified, fine to medium pebble conglomerate with sub-rounded to sub angular clasts typically between 1-5 cm in size (maximum clast size was 60 cm) (Wilson, 1993). Wilson (1993) interpreted the basal diamict to have been deposited in a very proximal glaiomarine setting with a change to shallow proximal glaciomarine to waterlain environment. The upper conglomerate is interpreted to have formed in a very shal- 103

18 low, very proximal marine to sub-ice current environment. Between 202 m and 193 m Ishman and Rieck (1992) designate the Trifarina spp. foraminiferal zone which they suggested indicates deposition in shallower water (>300m) with high current activity. LSU-7 ( m) Units D12 through D16 The base of LSU-7 contains a m thick poorly sorted, gravelly, sandy muddy massive diamict (D12) with average clast size of 5 cm (maximum 85cm) and ice loading and shear structures (McKelvey, 1981). A 4.4 m thick sequence of bioturbated, laminated sandy mudstone and cross bedded gravelly sandstone beds (D13) gradationally overlies the basal diamict. Unit D13 is disconformably overlain by a 4 metre thick moderately stratified muddy, sandy diamict bed with subangular to rounded clasts (D14) which is disconformably overlain by a metre thick sequence of stratified diamict, mudstone beds and well sorted sandstone beds (D15). In the upper beds of D15, between m and m, is a massive sandy mudstone with basaltic tuff at m, and basaltic lapilli and fine ash between and m (McKelvey, 1981) from which Prentice et al. (1999) obtained an 40 Ar/ 39 Ar age (see chapter 8). The upper 3.46 metres (D16) disconformably overly D15 and comprise a thick poorly stratified pebble conglomerate-breccia which grades into sandy, gravely mudstone. The unit fines upwards into sandy and bioturbated muddy horizons (Wilson, 1993). The basal diamict (D12) is interpreted as lodgement till and was probably deposited beneath overriding glacial ice. However the overlying mudstone and sandstone beds (D13) document an ice retreat phase with fluctuations between ice proximal and ice distal glaciomarine conditions (Wilson, 1993). Both D14 and D15 were probably deposited in an ice proximal glaciomarine environment with the cross-bedded sandstones indicating strong proglacial and subglacial currents (Wilson, 1993). The upper unit (D16) documents and ice retreat phase from proximal to distal glaciomarine conditions (Wilson, 1993). Ishman and Rieck (1992) designated this interval as the Epistominella exigua foraminifera zone. They recognised high species diversity with barren intervals indicating unstable and fluctuating environmental conditions. However, diatom flora in this interval (199 m m) indicate open ocean conditions and minimal ice cover (Winter and Harwood, 1997). 104

19 LSU-8 ( m) Units D11 through D8 The base of LSU-8 comprises a metre thick moderately well stratified muddy, sandy gravelly diamict (D8, Wilson, 1993) that contains several scour surfaces near the base. The central portion of the unit is less stratified and more massive with the upper few metres becoming more stratified with mudstone horizons which grade into 2.35 metre thick sequence of laminated sandy mudstone beds. The mudstone beds are uncomformably overlain by a 7.92 metre thick poorly sorted (bioturbated between and m) gravely, sandy, muddy diamict (D10) which is massive and internally sheared at the base (McKelvey, 1981). D10 grades into a metre thick stratified, interbedded, mudstone breccia/conglomerate (D11). Diamict beds are moderately to well stratified and mudstone beds are bioturbated and laminated with sand lenses (Wilson, 1993). The lower portion of LSU-8 (D8) records a water lain till setting with an upcore ice advance phase followed by proximal glaciomarine and iceberg conditions (D9). The massive diamict above the disconformity at metres (D10) was deposited beneath grounded ice. The remainder of LSU-8 records a retreat of the grounding line and a history of advancing and retreating glacier over the drill site (D11, Wilson, 1993). Ishman and Rieck (1992) designated the portion below 242 m as the Ammoelphidiella uniformina foraminifera zone which indicates deposition in deep water ( m). Winter and Harwood (1997) found that the entire interval between 239 m and 247 m contains a rich diatom assemblage which they attribute to minimal ice cover. LSU-9 ( m) Units D7 through D1 The base of DVDP-11 contains a 2.88 metre thick, moderately to well stratified muddy, sandy and gravelly diamict with thin mudstone interbeds (D1) and rounded to subangular clasts with average sizes between 2.5 cm and 7 cm (maximum 28.5 cm, McKelvey, 1981). D1 is overlain by a 0.27 metre thick very fine, diatomaceous mudstone (D2, Wilson, 1993). The basal units are disconformably overlain by a metre thick, muddy, sandy, gravelly diamict (D3) which becomes stratified and fines up core to a 5.18 metre thick diatomaceous sandy mudstone which is interbedded with gravelly mudstone (D4). A metre thick, mostly massive gravelly, sandy, muddy diamictite (D5) rests disconformably on unit D4 with the upper 5 metres becoming more stratified and grading into a 0.76 metre thick stratified sandy mudstone (D6) which grades back to a 4.6 metre thick sandy, muddy, gravelly diamict (Wilson, 1993). 105

20 The base of DVDP-11 records a proximal glacial environment with and abrupt shift to deep marine to distal glaciomarine environment. Above the basal units (D1 and D2) is a record of ice advance over the drill site with the deposition of lodgement till followed by ice retreat and shift to a dynamic glacial regime with multiple cycles of ice advance and retreat. The upper portion of LSU-8 (D5-D7) records a history of grounded ice over the drill site with ice retreat (D5) and a brief interval of distal glaciomarine conditions (D6) followed by a re-advance of floating ice over the drill site (D7) (Wilson, 1993). Winter and Harwood (1997) identified a diverse in situ diatom flora in a thin mud interval ( m) indicating open ocean, ice - distal conditions and Webb and Wrenn (1982) identified rich foraminiferal assemblages at m which indicate water depths of between 600 m and 900 m and productive, open-ocean conditions. 106

21 DVDP-11 Lithostratigraphy Depth (m) Lithology Glacial Proximity M D P W L Ice LSU source Wilson (1993) LSU LSU-1 LSU-2 Description Granule conglomerate and drilling breccia. Laminated fine sandstone and granule conglomerate beds. Prograding, moderately well sorted medium to medium coarse sandstone beds. Thin coarse sandstone to granule conglomerate beds. Depositional Environment Braided river LSU-3 Homogeneous sometimes pebbly fine to very coarse sandstones and minor pebble beds Ice-proximal sub-tidal delta Ross Ice DV11 ES-1 Fining upward succession of conglomerates and sand beds Subaerial exposure Conglomerate Diamict Breccia Mudstone Sandstone DV11 ES-2 DV11 ES-3 DV11 ES-4 Sharp contacts DV11 ES-7 H2/H3 DV11 ES-8 DV11 ES-9 H1 DV11 ES-11 DV11 ES-14 Lodgement till Waterlain till DV11 ES-15 Taylor Glacier Ross Ice Taylor Glacier + Ross ice D20 D19 D18 D17 D16 D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 LSU-4 LSU-5 LSU-6 LSU-7 LSU-8 Bioturbated silty mudstone Stratified pebbly coarse to medium sandstone beds with thinner conglomerate beds Texturally immature, very coarse sandstone Winnowed cobble to pebble conglomerate Interbedded pebble diamict, laminated sandy mudstones and pebble conglomerate Washed pebble conglomerate Weakly stratified to massive diamict Poorly sorted, slightly stratified, fine to medium pebble conglomerate Fossiliferous mudstone Stratified, muddy, sandy pebble diamictite Poorly stratified pebble conglomerate-breccia grading into sandy, gravely mudstone Stratified diamict and mudstone/sandstone beds Moderately stratified muddy, sandy diamict Bioturbated, laminated sandy mudstone and cross bedded gravelly sandstone Gravelly, sandy muddy massive diamict Stratified, interbedded, mudstone breccia/conglomerate Massive gravely, sandy, muddy diamict. Internally sheared at the base. Laminated sandy mudstone beds Moderately well stratified muddy, sandy gravelly diamict becoing massive at the top Massive gravelly, sandy, muddy diamictite Stratified sandy mudstone Ice-proximal glaciomarine Ice-proximal glaciomarine Very ice-proximal glaciomarine Very ice-proximal glaciomarine Ice-proximal-ice distal oscillations Ice-proximal-ice distal oscillations Ice-proximal glaciomarine Ice-proximal-ice distal oscillations Grounded ice Grounding line oscillation Grounded ice Ice-proximal glaciomarine Grounded ice Ice-proximal glaciomarine Grounded ice Distal glaciomarine Ice-proximal glaciomarine DV11 ES-16 DV11 ES-17 D5 D4 D3 D1/2 LSU-9 Massive gravelly, sandy, muddy diamictite Diatomaceous sandy mudstone which is interbedded with gravelly mudstone Muddy, sandy, gravelly diamict becoming stratified at top Moderately to well stratified muddy, sandy and gravelly diamict overlain by diatomaceous mudstone Distal glaciomarine Grounded ice Distal glaciomarine Ice-proximal glaciomarine Grounded ice Distal glaciomarine Ice-proximal glaciomarine 107

22 Figure 5.4 (previous page): Lithologic log, Lithostratigraphic Units (LSU s), lithologic descriptions and environmental interpretations of DVDP-11 compiled from McKelvey (1979) and Wilson (1993). The glacial proximity curve (M = marine, D = distal, P = proximal, W = waterlain, L = lodgement) is from Wilson (1993) and sediment source is from Porter and Beget (1981). Table 5.3: DVDP-11 Lithostratigraphic units and erosion surface depths. Event Top depth (m) Bottom depth (m) LSU LSU LSU LSU DV11 ES LSU DV11 ES DV11 ES DV11 ES LSU DV11 ES DV11 ES DV11 ES LSU DV11 ES DV11 ES DV11 ES LSU DV11 ES DV11 ES DV11 ES DV11 ES DV11 ES LSU DV11 ES Continued on next page

23 Event Top depth (m) Bottom depth (m) DV11 ES DV11 ES LSU = Lithostratigraphic unit, ES = erosion surface 5.6 Erosion surfaces in successions Glacial surfaces of erosion (GSE, Fielding et al. (2000)) and current eroded/winnowed surfaces are recognised in all three drill cores and their depths are listed in tables 5.1, 5.2, and 5.3, and displayed figures 5.2, 5.3, and 5.4. GSEs indicate ice has advanced and grounded over the drill site. They are typically recognised as sharp contacts between overlying massive diamicts which represent glacial advance and underlying units which may be ice distal facies. Other evidence of ice advance and grounding may be in the form of shearing or soft sediment deformation in sediments below the contact surface. The lower 50 metres of DVDP-10 contains several GSEs. Two well defined GSEs with scoured surfaces and soft sediment deformation associated with ice loading occur at and metres. Evidence of shearing and ice loading is recognised in DVDP-11 in a massive diamict between and metres where internal shear structures indicate ice loading. However, Fielding et al. (2000) indicated that when multiple stacked GSEs occur within a single diamict, where no interglacial facies are preserved, the erosion surfaces may be difficult to recognise. The massive, sheared diamict in DVDP-11 may be an amalgam of several phases of glacial advance. Erosion surfaces are also recognised as sharp, current eroded contacts with rip up clasts in current winnowed conglomerate between m and metres in DVDP- 11 and at m in DVDP-10 a thin current eroded disconformity is recognised between two muddy sandstone beds. These scoured erosion surfaces may formed by subglacial streams (Fielding et al., 2000) or subglacial/submarine current and outwash activity (McKay et al., 2009). 109

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25 Chapter 6 Paleomagnetic behaviour in drill core sediments 6.1 Introduction Paleomagnetic studies were conducted on specimens from the DVDP-10, -11, and CIROS-2 cores. A principal aim of these studies was to develop new magnetostratigraphies and chronologies for these drill cores using modern instruments and techniques. Where possible paired oriented specimens (5-10 cc) were demagnetised using different techniques (AF and thermal) to ensure the characteristic remanence was isolated and to help identify the remanence carrying mineralogy. Magnetic moment measurement were made using a 2G Enterprises DC , pass-through superconducting rock magnetometer (sensitivity of A/m), which is housed in a magnetically shielded room at the OPRF. Orthogonal component vector plots and PCA analyses were conducted using the multi-platform PuffinPlot software package (Lurcock, 2011). Sample preparation, demagnetisation methods, and data analysis techniques are discussed in detail in chapter 3. The following section describes the different demagnetisation behaviours seen in specimens. Behaviour categories are universally applied to sediments from the three drill cores. Ten types of paleomagnetic behaviour are recognised. Divisions are based on the stability of magnetisation, demagnetisation behaviour, and on the overall noise level of demagnetisation vectors. 111

26 6.1.1 Paleomagnetic behaviours Group A Group A specimens (Figures 6.1 and 6.2) typically had NRM intensities of between and A/m. Demagnetisation vectors were linear and directed to the origin although in some samples soft, viscous overprints which were easily demagnetised at low unblocking temperatures or weak AFs were recognised. The signal to noise ratio is low and PCA of all category A samples resulted in MAD values of between 0 and 3 for samples that were demagnetised in alternating fields and between 0 and 5 for thermally demagnetised samples. The reason for allowing for higher MAD values in thermally demagnetised samples is to allow for sample alignment errors when placing samples on the magnetometer tray between heating steps which differs from AF demagnetisation where samples are fixed to the tray for the entire demagnetisation process. An additional exception for the MAD limits is for samples which exhibit A behaviour where MAD values of up to 10 were allowed as long as the demagnetisation vector was straight and had a low noise level. Additional behaviour information is provided by the and suffixes. Group A samples terminated at the origin. A samples were directed to the origin but because of high coercivity minerals or because of thermochemical alteration during heating the demagnetisation vectors did not terminate at the origin. A samples were not directed to the origin and did not terminate there. Many of these samples reached a stable end point somewhere other than the origin which indicates a high coercivity or high temperature mineral carries a remanence which offsets the end point from the origin of the vector plot. Polarity determinations of group A samples are straight forward with good PCA fits with data and a low signal to noise ratio. The primary component that is directed to the origin of the orthogonal component vector plot is selected as the DRM. Behaviour DVDP-11 DVDP-10 CIROS-2 All specimens A 5% 5% 1% 4% A 6% 4% 3% 5% A 1% 0% 3% 1% Total A 12% 9% 7% 10% Table 6.1: Proportions of A type behaviour for New Harbour drill cores. 112

27 a) DVDP m, fine sandstone b) DVDP m, muddy claystone NOT ORIENTED vertical horizontal Dec MAD Inc MAD N,U 1 N,U 15 A A vertical horizontal 1 1 E,E Intensity (A/m) x 10-1 Intensity (A/m) x 10-2 AF (mt) 100 AF (mt) 100 W,H 5 5 E,H 5 5 c) DVDP m, silty claystone d) DVDP m, pebbly mudstone Dec Inc vertical A Dec Inc vertical A MAD MAD horizontal MAD MAD horizontal 2 N,U N,U E,S 5 20 E,H Intensity (A/m) x 10-1 AF (mt) Intensity (A/m) x 10-2 AF (mt)

28 Figure 6.1 (previous page): (a. DVDP m) Unoriented specimen with smooth, linear demagnetisation path that is directed towards the origin but does not terminate there because of a high coercivity mineral which is not sensitive to AFs. (b. DVDP m) Specimen with smooth and linear demagnetisation path that is directed to and terminates at the origin. An inclination of -72 is close to the expected inclination of the time averaged geomagnetic field (83 ) at latitude of the drill site (77 S). (c. DVDP m ) A reversed polarity specimen with smooth and linear demagnetisation path that is directed to and terminates at the origin. Note the viscous overprint which is demagnetised at 5 mt to reveal the ChRM. (d. DVDP m) A steeply magnetised specimen with smooth and linear demagnetisation path that is offset from the origin. This specimen is from a pebbly mudstone suggesting that a pebble within the sediment contains a high coercivity component that is not demagnetised by 60 mt. 114

29 a) DVDP m, muddy claystone Dec MAD Inc MAD vertical horizontal b) CIROS m, diamictite A Dec MAD Inc MAD vertical horizontal A 6 5Intensity (A/m) x N,U Intensity (A/m) x N,U Temperature ( C) Intensity (A/m) x Temperature ( C) 600 Magnetic susceptibility (S.I.) E,H 3 4 E,H 1 5 c) DVDP m, mudstone Dec MAD Inc MAD vertical horizontal A d) CIROS m, diamictite Dec MAD Inc MAD vertical horizontal A N,U 10 N,U 2 E,H Intensity (A/m) x 10-3 AF (mt) 100 AF (mt) E,H 115

30 Figure 6.2 (previous page): (a. DVDP m) Muddy claystone specimen with relatively smooth, linear demagnetisation path and a slight offset from the origin. This specimen was thermally demagnetised therefore higher MAD values are expected from sample handling and placement errors. (b. CIROS m) Specimen with relatively smooth and linear demagnetisation path that is directed to the origin. However, because of thermochemical alteration the demagnetisation path does not terminate at the origin. (c. DVDP m ) A reversed polarity specimen with viscous overprint followed by smooth and linear demagnetisation path that is directed to the origin. (d. CIROS m) A steeply magnetised specimen with relatively smooth demagnetisation at low AFs. Data are not included for demagnetisation data above 50 mt because of high noise level. The high noise level is attributed to high coercivity phases being agitated by high intensity AFs. Group B Group B specimens differ from group A mainly because demagnetisation data have more scatter resulting in a higher PCA MAD values. Specimens typically had NRM intensities of between and A/m. Demagnetisation vectors were usually linear with MAD values for AF demagnetised samples between 3 and 10 and between 5 and 10 for thermally demagnetised samples to allow for higher angular noise from sample placement between demagnetisation steps. In some specimens (e.g. figure 6.4, b. DVDP m ), a soft, viscous, overprint was present. However, this was easily demagnetised by the first few heating or AF steps to reveal a relatively stable ChRM. B sample demagnetisation paths are directed to and terminate at or near the origin, B samples are directed towards the origin but because of high coercivity minerals during AF demagnetisation or because of thermochemical alteration during heating demagnetisation vectors do not terminate at the origin (e.g. a. DVDP m, d. DVDP m, figure 6.3). B samples are not directed, and do not terminate at the origin (e.g. b. DVDP m, c. CIROS m, figure 6.4). In some cases, (e.g. d. DVDP m, figure 6.4), a stable end point is reached which is offset from the origin and it is clear from the shallow inclination of the ChRM that this component is controlled by clasts within the sediment rather then by the magnetisation of the sediment. Specimens with unusually shallow inclinations were not used to construct magnetostratigraphies. In some cases (e.g. figure 6.3, a. DVDP m ) demagnetisation was smooth 116

31 and relatively linear at low fields and more noisy at higher fields. This may be caused by physical disaggregation of a portion of the sample from vibrations during AF demagnetisation which was sometimes observed. Polarity determinations of group B samples are reasonably straight forward with a low to moderate signal to noise ratio. The primary stable component is taken as the ChRM. Behaviour DVDP-11 DVDP-10 CIROS-2 All specimens B 12% 13% 4% 22% B 24% 18% 24% 12% B 13% 8% 15% 11% Total B 49% 38% 42% 45% Table 6.2: Proportions of B type behaviour for drill cores. B type behaviour was by far the most common accounting for 45% of observed behaviours. 117

32 a) DVDP m, mudstone with disp clasts Dec 2.15 MAD vertical horizontal B Inc MAD N,U 30 b) DVDP m, silty mudstone Dec Inc MAD MAD N,U 1 B 1 2 E,E vertical horizontal Intensity (A/m) x 10-3 Intensity (A/m) x 10-2 AF (mt) 100 AF (mt) E,S 5 5 c) CIROS-2 40 m Sandstone Dec MAD N,U 5 Inc MAD B 5 vertical horizontal d) DVDP m, mudstone with granules Dec MAD Inc MAD N,U 5 vertical horizontal B Intensity (A/m) x E,H AF (mt) 60 Intensity (A/m) x 10-3 Magnetic susceptibility (S.I.) Temperature ( C) 600 W,H 2 E,H 20 2 S,D 118

33 Figure 6.3 (previous page): (a. DVDP m) Mudstone with dispersed clasts. Demagnetisation is relatively smooth at low fields. However, at higher fields data are more noisy. This could be caused by high coercivity minerals or by disaggregation of the sample from vibrations during demagnetisation. (b. DVDP m) Silty mudstone specimen with a single, relatively smooth, reversed polarity component that is directed to and terminates at the origin. (c. CIROS-2 40 m ) A normal polarity sample which has a relatively smooth demagnetisation path that terminates at the origin. Even though this a sandstone specimen, a significant silt-mud fraction must be present which holds the remanence. (d. DVDP m) Areversed polarity specimen with a noisy demagnetisation path that is directed to the origin. The specimen is of a mudstone with granules. The granules, which probably have varying mineralogy, may be responsible for higher noise level of this specimen, however, the component still exhibits a relatively linear trajectory to the origin and therefore represents an original, detrital magnetisation of the specimen. 119

34 a) DVDP m, clayey siltstone b) Sample: DVDP m, mudstone with disp clasts Dec Inc vertical Dec Inc vertical MAD MAD horizontal MAD MAD horizontal B B N,U N,U 20 Intensity (A/m) x 10-2 AF (mt) E,H 10 E,H Intensity (A/m) x 10-2 AF (mt) c) CIROS m, diamict Dec Inc MAD MAD vertical horizontal B d) DVDP m, claystone with granules Dec Inc vertical MAD MAD horizontal 20 N,U N,U 6 10 E,H 10 E,N Intensity (A/m) x 10-1 Intensity (A/m) x AF (mt) AF (mt)

35 Figure 6.4 (previous page): (a. DVDP m) A relatively smooth demagnetisation path that is directed to the origin dominates at low fields (data from higher fields are removed because of high noise level). (b. DVDP m) is of mudstone with dispersed clasts. A soft overprint is removed by the 20 mt step to reveal a relatively linear component that is slightly offset from the origin. (c. CIROS m) Linear demagnetisation to an offset origin from a sample of diamict. (d. DVDP m) Specimen has a relatively smooth demagnetisation profile that is not directed to the origin. PCA results were not included in the final magnetostratigraphy because of the shallow nature of the component, the offset from the origin and the nature of the sediment indicate that the remanence is probably dominated by a pebble rather than the enclosing sediment. Group C Group C specimens typically have NRM intensities of between and A/m. PCA angles are only broadly representative of the orientation of the demagnetisation vectors and MAD values are usually between 10 and 20. Samples often have more than one component of magnetisation (a. DVDP m and c. DVDP m, figure 6.5) and in some cases this component persisted to higher alternating fields or temperatures (350 C for sample a. DVDP m, figure 6.5). No divisions (e.g. C, C, C ) are made within group C specimens because accurate determination of principal components was not possible and thus accurately predicting the orientation of vectors was difficult. A subgroup within Group C are C2 specimens which are found only in thermally demagnetised specimens from CIROS-2 (Figure 6.5, d. CIROS m). Specimens with C2 behaviour have two components with very little overlap in the demagnetisation spectra. A low temperature component exists between 20 C and 250 C and a high temperature component with a different orientation (usually directed to the origin) persists above 300 C. PCA results of C2 samples are not included in the final magnetostratigraphy because of uncertainties about which of the two components represents the detrital remanence. Diagenetic alteration of sediment in CIROS-2 below 90 m has significantly altered the magnetic mineralogy. A more detailed discussion can be found at the end of this chapter in section

36 Behaviour DVDP-11 DVDP-10 CIROS-2 All specimens C 8% 11% 17% 11% C2 0% 0% 5% 1% Total C 8% 11% 22% 12% Table 6.3: Proportions of C type behaviour for drill cores. C2 behaviour was only seen in CIROS-2. Group D, E, and F Group D specimen demagnetisation data have high scatter and PCA is not used because it results in prohibitively high MAD values. Specimens typically demagnetise in a noisy but linear fashion and have NRM intensities of between and A/m. Group D specimens were not used for constructing magnetostratigraphies (see chapter 7). Samples from sandstone units from the upper portions of drill cores with very low unblocking temperatures or soft mineralogies were included in this group. Group E and F specimens have unstable magnetisations that do not produce meaningful demagnetisation data. Group E specimens have magnetisations that are dominated by low coercivity or low temperature components (CIROS m, figure 6.6) and Group F specimens do not demagnetise at all (DVDP m, figure 6.6). Specimens have NRM intensities of between and A/m. Individual magnetic components are rarely observed and PCA is not possible, therefore, Group E and F specimens were not used to construct magnetostratigraphies. Behaviour DVDP-11 DVDP-10 CIROS-2 All specimens D 8% 9% 6% 8% E 2% 18% 24% 11% F 21% 16% 0% 15% Total 31% 43% 30% 34% Table 6.4: Proportions of D, E, and F behaviours for drill cores. The high proportion of poorly behaved samples from DVDP is attributed samples from diamictites where clasts sometimes the remanence of the specimen. The high proportion of E type behaviour in CIROS-2 is attributed to the presences of diagenetic siderite cement. See section for a detailed discussion. 122

37 a) DVDP m, silty claystone Dec MAD Inc MAD N,U 30 8Intensity (A/m) x Intensity (A/m) x 10-2 vertical horizontal C b) DVDP m, mudstone Dec MAD Inc MAD N,U vertical horizontal C 4 8 E,H E,H Intensity (A/m) x 10-2 Magnetic susceptibility (S.I.) Intensity (A/m) x Temperature ( C) 600 AF (mt) 35 c) DVDP m, silty mudstone Dec Inc vertical C C 2 MAD MAD horizontal N,U 1 N,U 20 6 E,H d) CIROS m, sandstone vertical horizontal 80 Magnetic susceptibility (S.I.) Temperature ( C) E,H 30 5 Magnetic susceptibility (S.I.) 15 Temperature ( C)

38 Figure 6.5 (previous page): (a. DVDP m) Silty claystone with a reversed polarity magnetisation which is carried at high temperatures. A slight offset from the origin prevents PCA from being anchored. A viscous overprint is removed during the first heating step and more persistent noise is removed by the 300 C heating step. (b. VDP m) Mudstone specimen with high noise level. PCA is possible but results are questionable and were rarely included in the final magnetostratigraphies. Where data from these specimens were included they are clearly identified. (c. DVDP m ) Silty mudstone specimen where 80 precent of the magnetisation is dominated by a low temperature component that is not directed to the origin. The low temperature component is removed by the 150 C heating step to reveal a relatively stable, steep reversed polarity component that is directed to the origin. This component is taken as the ChRM of the specimen. (d. CIROS m) One of only 6 examples of C2 behaviour. This behaviour was seen in thermally demagnetised CIROS-2 samples only and comprises two discrete components that have little overlap of their demagnetisation spectra. PCA results of C2 data were not included in the final magnetostratigraphies. 124

39 a) DVDP m, mudstone D vertical horizontal N,U 20 b) DVDP m, medium sand vertical horizontal 5 D N,U 1 1 E,S 5 20 E,E 3 Intensity (A/m) x 10-3 Intensity (A/m) x AF (mt) AF (mt) 100 c) CIROS m, siltstone vertical horizontal E N,U 25 d) DVDP m, mudstone vertical horizontal F N,U E,S 5 25 E,H Intensity (A/m) x 10-4 Magnetic susceptibility (S.I.) Intensity (A/m) x 10-3 Temperature ( C) 600 AF (mt)

40 Figure 6.6 (previous page): (a. DVDP m and b. DVDP m) Two specimen from different lithologies that have D type behaviour. Components are noisy which prohibits the use of PCA. Because most of demagnetisation data are confined to the lower hemisphere it is assumed that samples acquired their detrital remanence in a reversed polarity field. (c. CIROS m and d. DVDP m) Examples of E and F type behaviour respectively. Noise levels are too high to allow PCA Sources of Viscous Remanent Magnetisations (VRM) Drill cores samples were obtained by continuous wireline diamond-bit coring. Drill bits and drill pipe typically have strong magnetisations which can result in a steep vertical, drilling overprints of the sediment (e.g. Florindo et al., 2005; Wilson et al., 2007b). Additional overprints may have been acquired at the core storage facility where cores are kept in a -20 C freezer on metal shelves. Other possible sources for the viscous overprints are from sample transportation (samples were shipped to the OPRF by Fedex) or during subsampling which was done using a dry, diamond saw blade (Chapter 3). As a result of exposure to varying magnetising fields many samples (e.g. figure 6.1, a, c, d) have viscous overprints with varying orientations that were acquired since the sediment was deposited, drilled, and archived. In all cases these viscous overprints were easily demagnetised in the first few heating or AF demagnetisation steps. 6.2 Magnetic mineralogy The following section discusses the magnetic mineralogy of drill core samples with the aim of demonstrating the stability of remanence. A detailed discussion of rock magnetic and environmental magnetic properties for each drill core is presented in chapter Comparison of demagnetisation techniques Most specimens were demagnetised using AFs to preserve their mineralogy for later analyses. However, when enough material was available sister specimens were subjected to comparative thermal and AF demagnetisation. This was done to determine the remanence carrying capability of specimens under different demagnetisation conditions 126

41 and rock magnetic properties of the sediment. In some cases, where diamictite samples were measured the direction of demagnetisation vector differed between the two techniques indicating that a clast was present in one of the specimens which was more strongly magnetised than the enclosing sediment. In these cases the shallow inclination specimen was ignored. In many cases AF demagnetisation did not completely remove the remanence by the 100 mt step (Figures 6.7 b, d, and 6.8 e, f) indicating that magnetic mineralogy is not purely magnetite. In these specimens thermal demagnetisation was more effective at demagnetising the specimen and temperatures of up to 650 C were needed to remove the remanence completely (Figures 6.7) indicating that maghemite or hematite are present in the sediment. Comparisons of demagnetisation spectra (Figure 6.8) reveal that fine grained samples from DVDP cores demagnetise easily in AFs and lose between 80% and 60% of the remanence by the 40 mt step with an average MDF of NRM of 15.3 mt. Thermal demagnetisation indicates that specimens hold a remanence to between 500 Cand 600 C. Rare samples from sandstone units (Figure 6.8, sample g/h) unblocked at very low temperatures and had very low coercivities which was attributed to the presence of only large, multidomain grains which are not suited to holding a remanence. Demagnetisation spectra from CIROS-2 (Figure 6.8, samples o, p, and q, r) show a clear change from below 90 metres. Above 90 metres in CIROS-2 samples retain a remanence to 550 C and 70% of the remanence is lost at 40 mt which indicates that magnetite is probably the dominant magnetic mineral. AF demagnetisation of CIROS-2 samples below 90 metres results in a complete loss of remanence by the 10 mt step and thermal demagnetisation indicates a low unblocking temperature with all remanence lost at 350 C. Sediments below 90 metres are dominated by mudstones and muddy diamicts which are suitable for paleomagnetic studies therefore a grain size control of demagnetisation spectra is unlikely. It is more likely that diagenesis of magnetic mineralogy has occurred. However, demagnetisation behaviour and spectra do not provide enough information to determine the magnetic mineralogy. Overall, thermal and AF demagnetisation techniques produced comparable demagnetisation vectors, and where AF demagnetisation successfully isolated the detrital remanence, thermal demagnetisation did too. All DVDP-10 and DVDP-11 samples from fine grained, muddy intervals in the lower portions of the drill cores have high unblocking temperatures indicative of magnetite, and minor quantities of maghemite, and hematite. Some sample from sandy lithologies in the upper beds of the drill core 127

42 a. DVDP m (Thermal) Dec Inc MAD NRM 2.04 x 10-2 A/m W,H N,U vertical horizontal E,H b. DVDP m (AF) Dec Inc MAD NRM 2.83 x 10-2 A/m W,H N,U vertical horizontal E,H Intensity (A/m) x Temperature ( C) Susceptibility (S.I.) Intensity (A/m) x AF (mt) 100 S,D S,D c. DVDP m (Thermal) Dec Inc MAD NRM 1.44 x 10-2 A/m vertical horizontal N,U d. DVDP m (AF) Dec Inc MAD NRM 1.33 x 10-2 A/m vertical horizontal N,U Intensity (A/m) x Susceptibility (S.I.) Temperature ( C) Intensity (A/m) x AF (mt) 80 W,H E,H S,D W,H S,D E,H e. DVDP m (Thermal) Dec Inc MAD NRM 2.65 x 10-2 A/m N,U vertical horizontal f. DVDP m (AF) Dec Inc MAD NRM 1.43 x 10-2 A/m N,U vertical horizontal Intensity (A/m) x Susceptibility (S.I.) Temperature ( C) W,H E,H Intensity (A/m) x AF (mt) 80 W,H E,H S,D S,D 128

43 Figure 6.7 (previous page): Orthogonal vector component plots of comparative thermal and AF demagnetisation of selected sister specimens. In these examples where AF demagnetisation did not completely remove the remanence, temperatures of over 600 C were need to demagnetise the specimens fully. The high temperature stability of the magnetisation (above the Curie temperature of magnetite) indicates that some maghemite or hematite must be present. However, because AF demagnetisation resulted in very well behaved demagnetisation trajectories (MAD values of less than 4.5) the dominant magnetic mineral is magnetite. Individual samples were only oriented with respect to the up direction, therefore some samples may have been rotated horizontally during cutting which accounts for the variations in declination between sister samples. (especially from DVDP-11) had low unblocking temperatures which is attributed to the presence of large, multidomain grains. Unblocking temperatures and AF demagnetisation spectra in CIROS-2 above 90 metres indicates that magnetite is probably the dominant mineral. However, below 90 metres very low unblocking temperatures and low coercivity minerals indicate that magnetite is not dominant Thermomagnetic analyses Thermomagnetic analyses were conducted on 198 specimens (43 from CIROS-2, 72 from DVDP-10, and 83 from DVDP-11) to determine Curie and Néel temperatures. Nine selected thermomagnetic curves are shown in figure 6.9. In DVDP sediments the dominant Curie temperature in all analyses ranged between 550 C and 580 C. Samples above 124 metres in DVDP-10 and above 167 metres in DVDP-11 display a thermally stable paramagnetic component between 20 C and up to 200 C and in some specimens (e.g. figure 6.9, e) discrete small Curie temperature steps are also seen. Below 124 metres in DVDP-10 and below 190 metres in DVDP-11 a thermally stable high temperature component is recognised in some samples as a linearly decaying slope between 580 C and 700 C (e.g. figure 6.9, d and i). Thermomagnetic alteration was ubiquitous in all samples with the cooling and heating curves having similar shapes. Analyses of specimens from CIROS-2 above 90 metres (e.g. figure 6.9, a) produce similar results to DVDP from the upper portions of the drill cores. The paramagnetic component is ubiquitous and the dominant Curie temperature in all analyses ranges 129

44 Intensity (A/m) x 10-2 Intensity (A/m) x DVDP m DVDP m DVDP m a b c d e f AF (mt) AF (mt) Intensity (A/m) x 10-2 DVDP m h Temperature ( C) Temperature ( C) 6 Mag. sus. (S.I.) Intensity (A/m) x AF T AF T AF g AF Intensity (A/m) x 10-1 T Intensity (A/m) x 10-2 i AF Sample fracturing Intensity (A/m) x AF (mt) AF (mt) DVDP m j Intensity (A/m) x T Temperature ( C) Temperature ( C) Intensity (A/m) x 10-2 Intensity (A/m) x k AF Intensity (A/m) x AF (mt) AF (mt) Temperature ( C) DVDP m l Intensity (A/m) x 10-3 T T Temperature ( C) Intensity (A/m) x DVDP m CIROS m CIROS m m n o p q r AF Intensity (A/m) x T Intensity (A/m) x AF AF (mt) Temperature ( C) AF (mt) Temperature ( C) AF (mt) Intensity (A/m) x T Intensity (A/m) x AF Intensity (A/m) x T Temperature ( C) Figure 6.8: Comparative AF and thermal demagnetisation spectra (black squares) from selected drill core samples. Most DVDP samples lose between 80% and 60% of the remanence at the 40 mt step and thermal demagnetisation of the same samples indicates that sediments demagnetise completely between 500 C and 600 C indicating that magnetite is the dominant magnetic mineral. Samples which did not demagnetise completely at the 100 mt step (e and f) usually carried a remanence above 600 C indicating that maghemite or hematite may also be present. Magnetic susceptibility (open squares) measured after each heating step and shows only minor susceptibility increases during demagnetisation. Samples g and h are from a very coarse sandstone unit with only large magnetic grains. Samples o and p from CIROS-2 hold a remanence to 550 C with a 70% loss of remanence at 40mT indicating that magnetite is probably the dominant magnetic mineral. AF demagnetisation of samples below 90 metres in CIROS-2 (q and r) results in a complete loss of remanence at the 10 mt step and thermal demagnetisation indicates very low unblocking temperatures with all remanence lost at 350 C. It is likely that diagenesis of magnetic minerals has occurred in these sediments. 130

45 between 550 C and 580 C with 0 susceptibility above 580 C. Analyses of specimens below 90 metres (e.g. figure 6.9, b) produces very markedly different results. Sediments have very weak magnetic susceptibility at room temperature ( S.I.) and abrupt thermochemical alteration begins at between 250 C and 300 C resulting in a fold increase in susceptibility. Cooling curves typically have a 100 fold increase in susceptibility which is stable indicating significant alteration of mineralogy. Roberts et al. (1995) conducted thermomagnetic measurements of sediments in DVDP-10 from 126 metres and suggested that the remanence was carried by a mixture of goethite and magnetite. Their conclusions were based on the two peak thermomagnetic behaviour which is also seen in this study (e.g. DVDP m, figure 6.9). However, analyses conducted during this study indicate that this low temperature peak is thermally stable to 700 C and thus cannot be attributed to the presence of goethite which dehydrates to hematite above 400 C (Dunlop and Özdemir, 1997). Thermomagnetic analyses of DVDP samples indicate a dominant Curie temperature of 580 C indicating that magnetite is in all sediments. Sediments in the lower portions of the cores exhibited susceptibilities above 580 C indicating that maghemite, oxidised magnetite, or hematite are also present. Discrete, thermally stable, low temperature steps seen in some DVDP data could be attributed to varying compositions of titanomagnetite where Curie temperatures decrease with increasing titanium content (O Rielly, 1984). CIROS-2 thermomagnetic analyses above 90 metres produced very similar results as in sediments from the upper portions of (above 124 metres in DVDP-10 and above 167 metres in DVDP-11) DVDP cores. Thermomagnetic curves have a paramagnetic component followed by a decay to zero susceptibility between 450 Cand580 C indicating that magnetite is the likely remanence carrier. Below 90 metres thermochemical alteration between 250 C and 300 C indicates the presence of a thermally unstable phase which breaks down to magnetite on heating (the Curie temperature of the alteration product is always 580 C). Possible magnetic minerals which may be responsible for this behaviour are greigite, pyrrhotite, and siderite which all break down to form new magnetic minerals above 300 C (e.g. Ellwood et al. 1986; Roberts 1995). All specimens underwent varying degrees of thermochemical alteration and some analyses were conducted in argon to prevent oxidation. Argon did not prevent oxidation probably because of dewatering clays. In all cases the thermal alteration resulted in the production of more magnetite with Curie temperatures of 580 C. 131

46 Magnetic Susceptibility (S.I.) x10-6 Magnetic Susceptibility (S.I.) x10-6 Magnetic Susceptibility (S.I.) x Temperature ( C) Temperature ( C) a. CIROS m d. DVDP m g. DVDP m Temperature ( C) Magnetic Susceptibility (S.I.) x Magnetic Susceptibility (S.I.) x10 Magnetic Susceptibility (S.I.) x Temperature ( C) b. CIROS m e. DVDP m Temperature ( C) h. DVDP m Temperature ( C) Magnetic Susceptibility (S.I.) x10-6 Magnetic Susceptibility (S.I.) x10-6 Magnetic Susceptibility (S.I.) x c. DVDP m Temperature ( C) f. DVDP m Temperature ( C) i. DVDP m Temperature ( C) Figure 6.9: Thermomagnetic analyses in air of selected CIROS-2 and DVDP samples. In the upper portion of CIROS-2 (a) a significant paramagnetic contribution of susceptibility and dominant Curie temperatures of 580 C indicate that magnetite present. Analyses below 90 metres CIROS-2 (b) indicate weak susceptibilities before heating and significant thermochemical alteration above 250 C. Analyses of DVDP-10 and -11 specimens revealed a paramagnetic component (c, e, f) in sediments above above 124 metres in DVDP-10 and above 167 metres in DVDP-11. The dominant Curie temperature of 580 C is attributed to magnetite and the slope between 580 C and 700 C seen in some samples (d, i) is attributed to maghemite or hematite. Thermomagnetic alteration occurred in all samples and heating samples in argon did not prevent oxidation. 132

47 6.2.3 Hysteresis and IRM analyses Hysteresis and IRM analyses were conducted of 244 specimens (58 from CIROS-2, 69 from DVDP-10, and 117 from DVDP-11) to investigate mineralogy, concentrations, and grainsize of the sediment. Four representative analyses are shown in figure Analyses typically produced well behaved, sinusoidal hysteresis loops. Some loops exhibited wasp-waisted behaviour (Figure 6.10, c) which indicates a mixture of SP and SD grains. Data from each core are plotted on Day plots (Mrs/Ms versus Hcr/Hc, Day et al., 1977; Dunlop, 2002) in figure All data (except one data point in DVDP- 10) plot in the PSD field indicate PSD grains of magnetite. Down core variations in grain-size within the PSD field are apparent and are discussed in detail in chapter 9. IRM analyses for each core are shown in figure Analyses indicate that low coercivity minerals are dominant. 95% saturation is achieved at 300 mt and back field coercivities range between 21 mt and 55 mt. Variation between cores is evident with DVDP-11 having the lowest coercivity mineral assemblage and CIROS-2 having higher coercivities Stability of remanence in sediments Numerous examples exist where reducing conditions in sediments have led to the dissolution of the primary remanence and the precipitation of new non-magnetic minerals or in some cases magnetic minerals. Sulphide reducing conditions, in some cases, are particularly problematic because they can lead to the precipitation of greigite which has a magnetic remanence, behaves much in the same way as SD magnetite, and adopts a magnetic field direction at the time of precipitation (e.g. Roberts and Turner 1993; Rowan and Roberts 2006; Larrasoaña et al. 2007). Carbonate reducing conditions which produce minerals such as siderite can be equally problematic because, although siderite (FeCO 3 ) is paramagnetic it breaks down at high temperatures and can result in spurious magnetisations which are grown during thermal demagnetisation. These spurious magnetisations can lead to incorrect magnetostratigraphies which can complicate the development of accurate age models. Equally problematic is the decomposition of siderite during later stage fluid migration and dewatering. Sagnotti et al. (2005) identified multiple short duration magnetic reversals in a thin interval (4 metres) at the base of the CRP-1 drill core. Detailed magnetic and geochemical studies revealed that authigenic siderite was reduced further 133

48 Magnetisation (mam 2 /kg) Magnetisation (mam 2 /kg) Field (mt) Hc 12 mt Mr 5.41 mam 2 /kg Ms 25.7 mam 2 /kg Hc 16 mt Mr 60.1 mam 2 /kg Ms 208 mam 2 /kg a. DVDP m c. CIROS m Magnetisation (mam 2 /kg) Field (mt) Magnetisation (mam 2 /kg) Hc 14.8 mt Mr 29.6 mam 2 /kg Ms 132 mam 2 /kg Hc 15.9 mt Mr 14.3 mam 2 /kg Ms 60.2 mam 2 /kg b. DVDP m d. DVDP m Field (mt) Field (mt) Figure 6.10: Hysteresis analyses of selected DVDP and CIROS-2 specimens. Analyses results are consistent with magnetite and grain-sizes are variable throughout each core. Mild wasp-waisted behaviour in CIROS m indicates a mixture of SP and SD grains in this specimen. Mr/Ms Day plot -DVDP-10 Day plot -DVDP-11 Day plot - CIROS % 0% 20% 20% 30% 30% SP-S 20% 40% 40% 50% 40% 60% 60% PSD 60% Mr/Ms % 0% 20% 20% 30% 30% SP-S 20% 40% 40% 50% 40% 60% 60% PSD 60% Mr/Ms % 0% 20% 20% 30% 30% SP-S 20% 40% 40% 50% 40% 60% 60% PSD 60% SD-MD 80% SD-MD 80% SD-MD 80% % % 1 Hcr/Hc % % 1 Hcr/Hc % % 1 Hcr/Hc 10 Figure 6.11: Day plots (Day et al., 1977) of magnetite grain-sizes with mixing lines of Dunlop (2002) for New Harbour drill cores. In CIROS-2 only data from above 90 metres are plotted. All data are consistent with PSD magnetite. 134

49 1 a. DVDP-10 IRM 1 b. DVDP-11 IRM Magnetisation Normalised 0 Magnetisation Normalised Field (mt) Field (mt) 1 c. CIROS-2 IRM 1 d. All Cores IRM Magnetisation Normalised 0 Magnetisation Normalised Field (mt) Field (mt) Figure 6.12: Normalised IRM and back field curves from DVDP and CIROS-2 sediments. Low coercivity minerals are dominant with saturation remanence achieved by 300 mt and back field coercivities of between 21 and 55 mt. 135

50 to greigite during late stage, H 2 S fluid migration. The following section discusses the stability of remanence in DVDP and CIROS-2 sediments. Stability of magnetisation in DVDP sediments AF and thermal demagnetisation techniques produced useful demagnetisation data (A, B, and C behaviours) from 69% of DVDP-11 analyses, and 58% of DVDP-10 analyses. Unstable magnetisations are usually confined to diamicts or coarse grained units. Most DVDP samples hold a remanence to at least 580 C and some hold a remanence to above 600 C. Thermomagnetic behaviour indicates a dominant Curie temperature of 580 C with some samples having higher Curie temperatures. AF demagnetisation results in 80% to 60% remanence loss at the 40 mt step and 90% to 100% loss at the 100 mt step indicating the presence of magnetically soft minerals. IRM analyses also indicate the presence of magnetically soft minerals with coercivities of between 21 mt and 55 mt and hysteresis ratios (Mrs/Ms versus Hcr/Hc) are consistent with PSD grains of magnetite. Overall demagnetisation, thermomagnetic, hysteresis and IRM data in DVDP sediments indicate that magnetite is the dominant magnetic mineral with Curie temperatures 580 C and that minor maghemite, hematite or variably oxidised magnetite is present in some specimens. No evidence of diagenetic alteration of magnetic mineralogy was found. EPMA analyses were conducted to investigate the geochemistry of magnetic remanence and to check for signs of diagenesis. Analyses of seven samples from DVDP sediments revealed large grains of rutile and rare, small (<10µm) grains of titanomagnetite or ilmenite (Figure 6.13). Siderite and pyrite were not observed. Stability of magnetisation in CIROS-2 sediments Demagnetisation spectra indicate that in the upper 90 metres of CIROS-2 sediment unblocks at 580 C and that 80% of the remanence is lost by the 40 mt demagnetisation step indicating that magnetite is the likely dominant magnetic mineral. AF and thermal demagnetisation produced similar demagnetisation data (A, B, and C behaviours, not including C2) for 66% analyses. Hysteresis analyses results are consistent PSD grains of magnetite and IRM analyses also indicate magnetically soft mineral such as magnetite is dominant. Thermal demagnetisation of CIROS-2 specimens sometimes 136

51 produced poor results in sandy lithologies which contain large grains with low unblocking temperatures. However, thermomagnetic analyses indicate higher degrees of thermomagnetic alteration between 65 metres and 90 metres and below 90 metres thermomagnetic alteration results up to a 100 fold increase in magnetic susceptibility. Both thermal and AF demagnetisation do not produce useful data and magnetisation of sediment is very weak indicating that diagenetic alteration of the sediment may have occurred. Specimens (92.6 m and 148 m) from CIROS-2 were analysed to investigate the chemistry of the sediment and the lack of remanence below 90 metres (Figure 6.13, table 6.5). Analyses of medium-fine sandstone at 92.6 metres revealed pervasive siderite - ankerite cement between grains and analyses of finer mudstone from 148 metres revealed fine (<10µm) siderite particles in pore spaces. Chemical compositions of siderite were plotted on a ternary (CaCO 3 -MgCO 3 -FeCO 3 ) diagram to reveal variability in compositions of the cement (Figure 6.13). Analyses from 92.6 metres have Fe dominated grouping with minor amounts of Ca and Mg whereas analyses from 148 metres indicate variability with compositions ranging from pure siderite to intermediate phases of ankerite (CaCO 3 -FeCO 3 ) and magnesite (MgCO 3 -FeCO 3 ). Overall analyses from 92.6 metres are less variable than analyses from 148 metres. Table 6.5: EPMA analyses of carbonate cements in CIROS-2. Because the JEOL 8600 electron microprobe is unable to detect carbon, analyses were recalculated by assuming all analyses were perfect and had total counts 100%. Depth Analysis Mg Ca Fe 92.6 m 1, figure % 19.73% 58.61% 92.6 m 4, figure % 23.45% 65.59% 92.6 m % 14.70% 76.99% m 1, figure % 15.77% 73.04% m 2, figure % 12.80% 73.20% m % 1.70% 38.30% m % 2.44% 40.60% m % 8.02% 91.10% m % 5.40% 93.90% m % 4.73% 94.89% m % 17.52% 76.37% Continued on next page

52 Depth Analysis Mg Ca Fe m % 13.83% 76.20% m % 9.46% 88.71% m % 11.03% 87.00% m % 16.76% 71.80% - = analyses not shown in figure 6.13 Discussion of siderite in CIROS-2 Thermomagnetic analyses indicate significant alteration at temperatures above 250 C as indicated by a rapid rise in magnetic susceptibility. Siderite decomposition during heating is a relatively well understood phenomenon (e.g. Ellwood et al., 1986) where siderite decomposes to magnetite at high temperatures (Equation 6.1). 2FeCO O 2 =Fe 2 O 3 +2CO 2 (6.1) Roberts and Weaver (2005) presented evidence for multiply remagnetised sediment where the earliest products of sulphate reduction (pyrite) are overgrown by siderite leaving only the relict microcrystalline structures of pyrite framboids. Only rare iron sulphide framboids were identified in CIROS-2 (Figure 6.13) specimens when compared with the quantity of siderite cement. Relict pyrite framboids were not observed in any specimens. Variability in the composition of carbonate cement may have resulted from variations in fluid composition from which the carbonates precipitated. The upper portion of the core (above 100 metres) is dominated by medium to fine sand where porosity and permeability are high allowing for easy flow and mixing of fluids. Deeper portions of the core are dominated by well cemented diamicts and mudstones which typically have low porosity and permeability thus restricting fluid flow; these sediments may have been the fluid source during compaction and dewatering. It is possible that reducing conditions and migration of carbonate rich fluids through the sedimentary pile resulted in dissolution of Fe rich grains in the lower portions and precipitation of iron rich siderite in coarse grained intervals which occur higher in core (Chapter 5). 138

53 CIROS m KV CIROS m 1 100µm 15 KV CIROS metres CIROS metres x magni cation DVDP m 270 x magni cation 100µm 1 15 KV 160 x magni cation 100µm CaCO3 Calcite MgCO3 Magnesite FeCO3 Siderite Figure 6.13: Back-scattered electron microscope images and analysis spots (red circles) of selected samples from CIROS-2 and DVDP-11. CIROS m: siderite ankerite cement (1 and 4) at grain boundaries (2 = Qtz, 3 = Plagioclase). CIROS m: Disseminated ankerite-siderite cement in pore spaces (1 and 2), DVDP m: rutile (1), feldspar (2), Qtz (3). Ternary diagram of geochemical compositions of carbonate cements from CIROS-2. Analyses from a sand rich interval (92.6 metres) are rich in Fe but have contributions of Mg and Ca. Analyses from 148 metres indicates a dominance of Fe rich, almost pure siderite compositions Summary Overall limited evidence of diagenetic alteration of remanence carrying mineralogy was found in DVDP-10 and -11 sediments. Magnetite and minor quantities of maghemite, oxidised magnetite of hematite are the primary remanence carriers, therefore both cores are suitable for magnetostratigraphic and environmental magnetic studies. 139