CHAPTER 6 GEOCHEMICAL-SEQUENCE STRATIGRAPHY

Transcription

1 152 CHAPTER 6 GEOCHEMICAL-SEQUENCE STRATIGRAPHY 6.1 SEQUENCE STRATIGRAPHY Stratigraphy is the geological science related to the study of rock strata, their succession, lithology, processes of deposition, fossil contents, physical evidences of the global and regional geological event, positions in time and distribution of economic resources. Sequence stratigraphy is a sub discipline of stratigraphy, the latter being defined as the historical geology of stratified rocks. There have been many definitions of sequence stratigraphy over the years, but perhaps the simplest is the sub division of sedimentary basin fills into genetic packages bounded by unconformities and their correlative conformities (Emery and Myers 1996). Sequence stratigraphy is used to provide a chronostratigraphic framework for the correlation and mapping of sedimentary facies and for stratigraphy prediction. Sequence stratigraphy invites for re-look the stratigraphy succession in terms of lateral and vertical relationship of rock record. The sequence stratigraphy is an integrated science yields optimum results by integration of multiple proxy datasets of the sedimentary basin. The integration of seismic, outcrop, core, well-log, biostratigraphy and geochemistry provides the basin sedimentary deposition system formed in response to the T-R cycles, eustacy with time and space. Each data set provides different insights toward the identification of stratal stacking patterns and the identification of sequence surfaces. The geochemical element

2 153 concentrations and composition is directly response to the geological history. The comparison and correlation of geochemical constitutions with biotic assemblages is vital clues to decipher the sequence stratigraphic frame work of Cauvery Basin. Sloss et al (1949) used the term sequence for a stratigraphy unit bounded by regional sub-aerial unconformities and initiated the discipline of sequence stratigraphy. Mitchum et al (1977) revised the definition of a sequence to a stratigraphy unit composed genetically related strata bounded at the top and bottom by unconformities or their correlative conformities, with the inclusion of the concept that sequence boundaries can be extended basin ward along correlative conformities, this definition provides for a sequence that potentially can be delineated over an entire basin and not limited to basin margin, where unconformities are common. 6.2 SEQUENCE STRATIGRAPHY TERMINOLOGY The reference of the terminology used in sequence stratigraphy is needed for understanding the concepts of sequence stratigraphy, its importance and applications in sedimentary basin analysis (Van Wagoner et al 1987). A sequence is a genetically related depositional unit bounded by unconformities or correlative conformities (Sloss 1963). Sequences consist of stacked facies succession, each of which shows gradual upward change in facies character indicating progressive shift in the local depositional environment. A stratigraphy sequence represents one or more cycles of sedimentation through transgressive/regressive processes within a specific time interval between regional unconformities. A sequence is therefore unconformity bounded i.e., form in response to relative falls in sea level.

3 154 Accommodation is a term for relative sea-level can be thought of as the space in which sediments can fill, defined at its base by the top of the lithosphere and at its top by the ocean surface. Basin-ward shift in facies: In cross-section, a shifting of all facies towards the center of a basin and this is a lateral shift in facies, such that in vertical succession, a basin ward shift in facies is characterized by a shift to shallow facies. Bed: A layer of sedimentary rocks or sediments bounded above and below by bedding surfaces. The bedding surfaces are produced during periods of non-deposition or abrupt changes in depositional conditions, including erosion. Bedding surfaces are synchronous when traced laterally; therefore, beds are time-stratigraphy units. Bed set are two or more superposed beds characterized by the same composition, texture, and sedimentary structures. Thus, a bed set forms the record of deposition in an environment characterized by a certain set of depositional processes. Conformity: Bedding surface separating younger from older strata, along which there is no evidence of sub-aerial or submarine erosion or nondeposition and along which there is no evidence of a significant hiatus. Unconformities (sequence boundaries) and flooding surfaces (parasequence boundary) will pass laterally into correlative conformities, commonly in deeper marine sediments. Depositional System: Three-dimensional assemblages of litho facies genetically linked by active processes (modern) or inferred (ancient) processes and environments (Fisher and McGowan 1967).

4 155 Figure 6.1 Depositional system of Exxon model Eustatic Sea Level: Global sea level, which changes in response to changes in the volume of ocean water and the volume of ocean basins. Marine Flooding Surface: The surface separating younger from older strata, across which there is evidence of an abrupt increase in water depth. Surface may display evidence of minor submarine erosion and forms in response to an increase in water depth. Maximum Flooding Surface: Marine flooding surface separating the underlying transgressive systems tract from the overlying highstand systems tract. This surface is marks the deepest water facies within a sequence. Parasequence: A relatively conformable succession of genetically related beds or bed sets bounded by flooding surfaces or their correlative surfaces. Systems Tract: A systems tract is defined as a linkage of contemporaneous depositional systems defined by stratal geometries at bounding surfaces, position in the sequence, and internal parasequencesstacking pattern.

5 156 Lowstand Systems Tract (LST): Systems tract overlying a type 1 sequence boundary, overlain by a transgressive surface, and characterized by a progradation to aggradational parasequence set. Figure 6.2 Lowstand systems tract (Van Wagoner et al 1990) Transgressive Systems Tract (TST): A systems tract bounded below by the transgressive surface and above by the down lap surface or maximum flooding surface. Parasequences within the transgressive system tract back step in a retrogradational parasequence set. The systems tract progressively deepens upward as successively younger parasequences step farther landward. Figure 6.3 Transgressive systems tract (Van Wagoner et al 1990)

6 157 Highstand Systems Tract (HST): Systems tract overlying a maximum flooding surface, overlain by a sequence boundary, and characterized by an aggradational to progradational parasequence set. Figure 6.4 Highstand systems tract (Van Wagoner et al 1990) Transgressive Surface: The first major flooding surface across the shelf within a sequence. This surface defines the top of the LST. The surface is parasequence set boundary separating an overlying retrogradational parasequence set from an underlying progradational or aggradational parasequence set. Unconformity: The surface separating younger from older strata, along which there is evidence of sub aerial erosion/truncation or sub aerial exposure or correlative submarine erosion in some areas, indicating a significant hiatus. Forms in response to a relative fall in sea level. 6.3 APPLICATION OF SEQUENCE STRATIGRAPHY The publication of AAPG memoir 26 in 1977 initiated a tremendous interest in understanding sedimentary architectures of the rock record, which consists of unconformity and/or correlative conformities bound sequences and vertical successions with similar lateral variations. Sequence

7 158 stratigraphy is born out of such efforts. Sequence stratigraphy has come of age, and it is tool for exploration of mineral and oil resources. It is essential for regional and global exploration studies, for seismic evaluation and reservoir evaluation. It provides the architecture of a depositional sequence like the rate of sediment supply, rate of subsidence/accommodation volume and relative sea level change. It has developed into a powerful, predictive facies analysis tool for both the hydrocarbon industry and exploration stratigraphic studies of academic work. 6.4 OUTCROP SEQUENCE STRATIGRAPHY OF ARIYALUR-PONDICHERRY SUB-BASIN The Albian-Maastrichtian sediments are exposed around Ariyalur area. These strata have been divided into three groups, Uttatur, Trichinopoly and Ariyalur (Blanford 1862), consisting of seven formations in stratigraphy order viz. Dalmiapuram, Karai, Garudamangalam, Sillakkudi, Kallankurichchi, Ottakovil and Kallamedu (Tewari et al 1996a, 1996b, Sundaram et al 2001). 6.5 CAMPANIAN-MAASTRICHTIAN OUTCROP-ARIYALUR GROUP The Ariyalur Group is represented in stratigraphic order by Sillakkudi, Kallankurichchi, Ottakovil and Kallamedu Formations. The Sillakkudi Formation is comprises of sandstones which are well exposed along the Mettol(N :E ),Nochikkulam (N : E ) and Vayalpadi (N :E ).The sedimentological parameters of the sandstones indicate that the sandstones are mineralogically and texturally immature and poorly sorted. The sandstone outcrop at Vayalpadi is a quartzarenite with calcareous cement and 15% clay. The

8 159 abundance of acid plagioclase indicates that the rapid deposition of these sediments derived from a nearby granitic/gneissic provenance. The feldspar angular grains indicate the short distance transportation history from a nearby provenance. The Campanian sandstone sequence of Sillakkudi Fm is topped by a conglomerate bed exposed at a Kaller stream section (N : E ). The pebbly grain stone with abundant sub-angular to angular, pink to white quartz and feldspar pebbles and cobbles characterize the conglomerate bed. The Kallankurichchi Formation is unconformably overlies the Sillakkudi Formation. The presence of Kaller conglomerate between the Sillakkudi and Kallankurichchi Formation indicates a break in the deposition during the Late Campanian. The Kaller conglomerate unit is non-calcareous indicating a sub-aerial depositional regime. The Kallankurichchi Formation consists of dominantly marine carbonates. The lower most ferruginous limestone is a fine to medium-grained massive biomicrite, rich in benthic foraminifera. The lower arenaceous limestone directly overlies the ferruginous limestone. It is yellowish, massive, highly compact, and rich in silica. The Gryphaea limestone overlying the lower arenaceous limestone is reddish brown in colour, fine- to medium-grained and rich in carbonate. The upper arenaceous limestone unit is rich in terriginous influx and directly overlies the Gryphaea limestone. The Ottakkovil sandstone contains macrofossils and has conformable upper and lower contacts. The cross-lamination structures in the overlying Kallamedu Formation indicate fluvial channel deposits, and mark the termination of the Cretaceous in Ariyalur area.

9 Sequence Surfaces of Campanian-Maastrichtian Sediments The lower sequence boundary traced between outcrop Campanian sandstone lithosection; Sillakkudi sequence and carbonate lithosection; Kallankurichchi sequence. Kilpalvur rests over Archaean and contact between Sillakkudi sandstone and Garudamangalam is conformable. The top sandstone becomes fluvial and is topped by conglomerate bed (Kaller conglomerate Kallankurichchi sequence), which marks the upper sequence boundary of Campanian-Maastrichtian. Based on the petrofabric character and their relative position in the basin the Sillakkudi Formation has been assigned as transgressive systems tract deposits within sequence stratigraphic framework (Figure 6.5). The Kaller conglomerate bed which separates Sillakkudi and Kallankurichchi Formations, represent sequence boundary which is characterized by a relative sea level fall at the end of Sillakkudi Formation. The base of ferruginous limestone is interpreted as a transgressive surface, and is marked by the presence of smaller benthic foraminifera (Nagendra et al 2002) assemblages indicating first marine flooding within Kallankurichchi sequence. The transgressive systems tract is composed of upward parasequences represented by ferruginous limestone, lower arenaceous limestone units which correspond to intermittent flooding events characterized by retrogradational parasequence stacking pattern. Both macroand microfossil assemblages, their frequency and preservation of microfossil tests indicate a gradual upward increase in bathymetry. The abundant giant sized Gryphaea suggests their full life mortality in a tranquil environment, observed during stand still conditions occurring in maximum flooding event. The Gryphaea limestone unit is defined as the maximum flooding surface.

10 161 Figure 6.5 Sequence deposition models of Campanian-Maastrichtian sediments The highstand systems tract is composed of a parasequence represented by upper arenaceous limestone litho unit. The possible cause for the development of HST appears to be the result of basin rise due to advected reunion mantle plume, leading to major KTB regression (Raju et al 1994). Rich in silica content and reduced micro and macrofossil contents suggests relative shallowing towards the top of upper arenaceous limestone of Kallankurichchi sequence. The upper arenaceous limestone is followed by Ottakkovil sandstone and Kallamedu sandstone stratigraphically. At TANCEM mine the upper contact of sequence boundary which is eroded and is marked as the contact between upper arenaceous limestone and sandstone of Miocene age (Cuddalore sandstone). The upper sequence boundary was caused by a regional regression at the end of upper arenaceous limestone during Late Maastrichtian. This unconformity surface has been identified across the KTB in Cauvery Basin (Raju et al 1994). The Kallankurichchi

11 162 Formation is represented as a 3 rd order depositional sequence bounded by unconformity, relatively thicker TST and thinner HST, with MFS. Based on these studies a sequence model has been proposed for TANCEM mine section of Kallankurichchi sequence (Figure 6.5) Geochemical-sequence Stratigraphic Framework Geochemical classification of terriginous sedimentary rocks are proposed by many author based on major, trace, rare earth elements and Carbon and Oxygen isotope composition and successfully classified as the sequence stratigraphic frame work. Using the geochemical sequence diagram of Robaszynski et al. (1998) and Jarvis et al. (2001) the Campanian (Sillakkudi Formation) and the Maastrichtian (Kallankurichchi Formation) sediments are proposed the sequence stratigraphy frame work of transgressive system tract, sequence boundary, maximum flooding surfaces and highstand system tract. Petrographically, the Campanian sandstones are derived from the transgressive system tract, the Maastrichtian ferruginous limestone and arenaceous limestone are derived between the transgressive system tract and the biostromal were identified as the maximum flooding surfaces and the highstand system tract. The geochemistry of Campanian sandstone are having the major elements variation of Mn ( to %), Ca( %), CaCO 3 ( %), Si( %), Ti( %), Al( %) and trace element Sr ( ppm), Zr ( ppm) and the correlation between Mn/Al ( ), Ti/Al( ), Si/Al( ), Zr/Al ( ) and Sr/Ca ( ) and 13 C isotope ( to PDB) and 18 O isotope (-7.70 to PDB). The Maastrichtian limestone (Kallankurichchi Formation) geochemistry variation are; Mn ( to %), Ca ( %), CaCO 3 ( %) Si ( %), Ti ( %), Al ( %) and trace element Sr ( ppm), Zr ( ppm) and the correlation between Mn/Al (

12 ), Ti/Al ( ), Si/Al ( ), Zr/Al ( ) and Sr/Ca ( ) and 13 C isotope (-3.20 to PDB) and 18 O isotope (-7.80 to PDB) Manganese Sequence Stratigraphy The Mn chemostratigraphy shows remarkably consistent trends with respect to the sequence stratigraphic model (Jarvis et al 2001). The sequence boundary which is apparently overlain by sediments with high (but falling) rather than low (Rise) Mn values. The Campanian-Maastrichtian sediments Mn range from the to % of sandstone and limestone sample range from to % (Table 6.1, Figure 6.6). The higher values in sandstone and the lower values in limestone, indicates the Campanian-Maastrichtian sea-level changes. The Ti, Zr, and Si maxima at the sequence boundary rather, than the Transgressive system tract. The Mn/Al ratios are lower in the upper arenaceous limestone whereas higher values in the ferruginous limestone which forms the Campanian-Maastrichtian unconformity beds. The higher concentration of Mn and higher ratios of Mn/Al indicates the sea-level fall. The low values of Mn and Mn/Al occur in the highstand system tract of arenaceous bed. The limestone facies was probably deposited in marine water to clearly reflect the relatively small changes of sea-level for the rise and fall in the Kallankurichchi Formation. The transgressive surface of sequence is placed at a level of sharply increasing Mn values occurring on the sequence boundary of sandstone bed, in a prominent thin grey bed that overlie an omission surface. The higher content of Mn increases in the transgressive system tract. Transgressive system tract is indicates the sea-level rise promoting the increasing organic matter and Mn flux to the seafloor.

13 Table 6.1 Geochemistry of Campanian sediments (Sillakkudi Formation) Location Elements Mn % Ca % CaCO3 Si % Ti % Al % Sr ppm Zr ppm Ti/Al Si/Al Mn/Al Sr/Ca Zr/Al 13 C 18 O KLC SK-III SK-V PR-II PR-II PR-III PR-IV PR-V SK-IV SK-VI PR-I SK-II SK-XI PV KP VP-I KN KR TM SB

14 Table 6.1 (Continued) Elements Location Mn % Ca % CaCO3 Si % Ti % Al % Sr ppm Zr ppm Ti/Al Si/Al Mn/Al Sr/Ca Zr/Al 13 C 18 O SK-IX SK-XII KD KK SK-X Sk-VII SK-I MM MM-I KL KY KLC- Kaller conglomerate, SK-III- Sillakkudi, SK-V- Sillakkudi, PR-II- Peraiyur, PR-II.I- Peraiyur, PR-III- Peraiyur, PR-IV- Peraiyur, PR-V- Peraiyur, SK-IV- Sillakkudi, SK-VI- Sillakkudi, PR-I- Peraiyur, SK-II- Sillakkudi, SK-XI- Sillakkudi, PV- Puthuvettakudi, KP- Karupur, VP-I Vayalpadi, KN- Kannanur, KR- Karaipadi, TM- Timmur, SB- Sattambadi. SK-IX- Sillakkudi, SK-XII- Sillakkudi, KD- Kadur, KK- Kovandamkurichchi, SK-X- Sillakkudi, SK-VII- Sillakkudi SK-I- Sillakkudi, MM- Mel Mattur, KL- Kulapadi, KY-Kiliyapattu 165

15 Table 6.2 Geochemistry of Maastrichtian sediments (Kallankurichchi Formation) Elements Location Mn % Ca % CaCO3 Si % Ti % Al % Sr Zr Ti/Al Si/Al Mn/Al Sr/Ca Zr/Al 13 C 18 O UA UA UA UA GRY GRY GRY GRY GRY GRY LA LA FR FR FR FR FR FR FR FR FR

16 167 Figure 6.6 Stratigraphy, Mn, Sr/Ca and carbon-isotope curves for the Campanian-Maastrichtian sediments (Robaszynski et al 1998 and Jarvis et al 2001)

17 168 Figure 6.7 Stratigraphy; Carbon-isotope, Mn, Si/Al and Ti/Al, Campanian-Maastrichtian sediments (Robaszynski et al 1998 and Jarvis et al 2001)

18 Figure 6.8 Carbonate, Al, Si/Al, Ti/Al and Zr/Al chemostratigraphy of Campanian-Maastrichtian sediments (Robaszynski et al 1998 and Jarvis et al 2001) 169

19 Figure 6.9 Idealized geochemical trend and their relationship to stratigraphic sequences and eustatic sea-level (Robaszynski et al 1998 and Jarvis et al 2001) 170

20 171 The maximum flooding surface is placed at the Mn maximum on the smoothed profile (Figure 6.6), which occured at the biostromal bed. The long-term decrease in Mn is eliminated in the Mn/Al profile (Figure 6.6). The major trends identified on the Mn profile are displayed by Mn/Al, the major trend connected with the Campanian-Maastrichtian boundary. The long-term trends in Mn/Al ratios appear to reflect the sequence stratigraphy better. The Mn flux increased with rising sea-level. The Mn reaching a maximum around the maximum flooding surface. The thick sediments in highstand system tract at upper arenaceous limestone suggests that increased carbonate dominated sedimentation, which might have been reduced the Mn flux by limiting the effectiveness of the diagenetic manganese pumped during sea-level changes Carbonate Sequence Stratigraphy The carbonate profile shows the sequence stratigraphy model with rising values from transgressive system tracts, with maximum flooding surfaces that plateau through the overlying highstand system tracts. This correspondence is to be expected because carbonate ratios are one of the criteria used by Robaszynski et al (1998) and Jarvis et al (2001) to define their sequence stratigraphic frame work in basinal sequences. In coastal areas additional sediment accumulation can result from depositional processes that are largely controlled by relative shifts in paleo-shoreline position (Preston et al 2007). The lithification of coastal sandstone/siltstone deposits can be achieved by the precipitation of carbonate cements within pore spaces of the strata. Taylor et al (2000) noted preferential sheet like cementation of strata exclusively beneath marine flooding surfaces. The sandstone indicates the cementation at sequence boundaries was initiated by marine transgression and continued through the sediment burial process. Multiple scenarios have been proposed to favor the preservation of systems tracts in the stratigraphy record (Gillette and Lockley 1989, Lockley and Hunt 1995, Lockley and Peterson

21 ). The systems tract sites seem to exhibit preservation characteristics consistent with deep footprints acting as a shelter from erosive currents and a trap of sediments in transport (Lockley 1991, Jenkyns et al 1994, Gatesy et al 1999, Paik et al 2001). The stable isotopic composition of δ 13 C and δ 18 O Campanian sandstone suggests precipitation during progressive burial from evolved marine fluids. The carbonate and other mineralizing solutes were derived, increasingly with burial, from decarboxylation of organic matter and clay mineral transformations in adjacent siltstones. It has been documented that CO 2 produced during decarboxylation can lead to the dissolution/replacement of precursor calcite by stable dolomite (Sundaram and Yin 1994). The carbon isotope stratigraphy of the Campanian sediments shows that, the systematic variation that enables detailed correlation on an intercontinental scale, despite the absence of unequivocal interregional biostratigraphic markers. The positive δ 13 C is a typical marine limestone signature (Moss and Tucker 1996), the negative δ 13 C in this Campanian sandstone, attributed as an inheritance from the original carbonate sediment. The Al trend is close to a mirror image of Ca (Figure6.8). The transgressive surface and maximum flooding surfaces are picked out by positive digressions in Ti/Al (Figure 6.7) whereas sequence boundaries generally exhibit low Ti/Al contents; Zr/Al shows similar trends. It is notable that in most cases peaks are defined by several samples and consequently represent relatively long-term changes in sediment composition, rather than reflecting mineralogical and/or chemical variation associated with sediments overlying individual omission or erosion surface. The increased Ti/Al ratios in deep sea sediments correlated with increased current energy and detrital supply. The high Ti/Al ratios in Campanian sediments are derived from the increased current energy and siliciclastic supply, and the low values of Maastrichtian sediments are derived

22 173 from the decreased current energy and absence of siliciclastic supply. The silica occurs in a much wider range of sediment constituents than Ti and Zr, including the detrital quartz and clay minerals. The high Si/Al ratios declining from the base of Campanian and high Si/Al ratios at transgressive surfaces and maximum flooding surfaces (Figure 6.7) correlate with increased detrital quartz. The correlation between the Ti and Si peaks is consistent with Ti residing in titanomagnetite inclusion in detrital quartz, of Campanian sediments (Montgomery 1994). The Campanian Maastrichtian sea-level curve of Sundaram et al (2001), Miller et al (2005), Watkinson et al (2007) and Nagendra et al (2010) is widely used as the reference for assessing eustatic influences on regional sequence stratigraphy and sea-level change. The long-term eustatic curve rises steadily through the Campanian Maastrichtian and peaks in the Late Maastrichtian, when a Phanerozoic sea-level maximum is indicated. The short-term curve shows early, mid and upper Maastrichtian minima followed by intervals of rapid sea-level rise. Similar trends are observed in the δ 13 C curve (Figure6.9), but the low stratigraphic resolution of the Miller et al (2005) composite curve makes it unfeasible to test these relationships without any rigor. The largest positive Mn digression occurs closer to the Campanian- Maastrichtian boundary, an interval representing a major anoxic event in the oceans; this anoxic event indicates the accumulation of organic rich sediments, a global near the positive δ 13 C digression in marine carbonates and organic matter, and an occurrence of major biotic turnover. The Mn/Al ratios are low at the base of Campanian sandstone and Maastrichtian limestone and increase erratically to a maximum in the Lower Maastrichtian, and the sandstone have the digression of δ 13 C. The high Mn content controlled by the increasing detrital flux associated with the increasing faunal assemblage. The

23 174 smaller digressions of middle Maastrichtian δ 13 C peaks, which have a maximum, shifted the δ 13 C 1.1 PDB by biostromal, this biostromal not associated with extensive ocean anoxia, the increasing of marine organic matter has been bring in to observed isotopic digressions. The digressions of Campanian-Maastrichtian boundary and the Maastrichtian coincide with low Mn and Mn/Al values. The low values of carbon isotope digressions of the Campanian- Maastrichtian boundary and Late Campanian events follow periods of sealevel fall. The Campanian isotope and eustatic sea-level profiles are remarkably dissimilar, with a long-term fall in δ 13 C associated with falling eustatic sea-levels, and Maastrichtian isotope and eustatic sea-level profiles are remarkably similar, with a long-term rise in δ 13 C associated with rising eustatic sea-levels Strontium Stratigraphy The Sr/Ca of sandstone ( ) and limestone ( )) are tabulated in Tables 6.1 and 6.2 and shown in Figure 6.6. The maximum values of limestone occur in upper part of highstand system tract (HST; upper arenaceous limestone) and the overlying transgressive systems tracts, and the maximum values of Campanian sandstone Sr/Ca ratios closer to the sequence boundaries. Sr/Ca ratios fall through transgressive systems tracts attain minimum values in the upper TST, before rising through the highstand. The rising sea-levels during transgression promoted renewed aragonite deposition and falling seawater Sr/Ca (Mabrouk et al 2007). This was reversed by the development of mature carbonate platform systems with lower aragonite accumulation rates during the highstand environment.

24 175 The current geochemical data (Renard 1985, 1986, Stoll and Schrag 2001, Steuber 2002, Steuber and Veizer 2002) suggest that Sr/Ca ratios rose progressively through the Mid-to Late Cretaceous, a period of generally rising eustatic sea-level (Hancock and Kauffman 1979, Haq et al 1988, Hancock 1993), sea-level cannot be the main forcing mechanism for long-term Sr/Ca variation. The long-term trend is best explained by a decreasing contribution of aragonite to the formation of carbonate platforms (Steuber 2002). Sr/Ca values of the samples and interpretation 6.6 SUBSURFACE SEDIMENT GEOCHEMICAL-SEQUENCE STRATIGRAPHY Geochemistry The geochemistry of Campanian subcrop sediments (SS-1; m) major elements variation of Mn ( to %), Ca ( %), CaCO 3 ( %), Si ( %), Ti ( %), Al ( %) and trace element Sr ( ppm), Zr ( ppm) and the correlation between Mn/Al ( ), Ti/Al ( ), Si/Al ( ), Zr/Al ( ) and Sr/Ca ( ) and 13 C isotope ( to PDB) and 18 O isotope (-7.70 to PDB). The Maastrichtian sediment geochemistry variation of Mn ( to %), Ca ( %), CaCO 3 ( %) Si ( %), Ti ( %), Al ( %) and trace element Sr ( ppm), Zr ( ppm) and the correlation between Mn/Al ( ), Ti/Al ( ), Si/Al ( ), Zr/Al ( ) and Sr/Ca ( ) and 13 C isotope (-3.20 to PDB) and 18 O isotope (-7.80 to PDB).

25 176 The Campanian-Maastrichtian sediment section of Cauvery Basin comprises of shale and silty shale in well SS-1, siltstone, shale and calcareous shale in well SS-2 and SS-3 consists of silty shale, shale and fossiliferous shale. Based on geochemical variation in clastic input, Robaszynski et al (1998) proposed three depositional sequences for the Campanian- Maastrichtian sediments Manganese Sequence Stratigraphy The increasing Mn concentration in the SS-1 and SS-2 sequence through the Campanian sediment (sequence-c) (Figure 6.10) has been deciphered the increasing Mn supply, and relatively irregular background Mn/Al ratios indicates the correlation with carbonate content The decreasing Mn content in the SS-1 and SS-3 sequence through the Late Campanian-Early Maastrichtian sediments (Figure 6.10) (sequence- A and B) has been interpreted as resulting from the decreasing detrital Mn supply (Jarvis et al 2001), as indicated by an inverse correlation with carbonate content and relatively constant background Mn/Al ratios. Carbonate/shale ratios and the Mn flux increased with rising sea-level, with Mn reaching a maximum around each maximum flooding surface (Figure 6.10), before decreasing through the overlying highstand systems tract, representing a period of relative constant carbonate supply.

26 Table 6.3a Geochemistry of Maastrichtian sediments (SS-1; m) Elements Depth (m) Mn % Ca % CaCO3 % Si % Ti % Al % Sr Zr Ti/Al Si/Al Mn/Al Zr/Al Sr/Ca 13 C 18 O

27 Table 6.3b Geochemistry of Campanian sediments (SS-1; m) Elements Depth (m) Mn % Ca % CaCO3 % Si % Ti % Al % Sr Zr Ti/Al Si/Al Mn/Al Zr/Al Sr/Ca 13 C 18 O

28 Table 6.4a Geochemistry of Maastrichtian sediments (SS-2; m) Elements Depth (m) Mn % Ca % CaCO3 % Si % Ti % Al % Sr Zr Ti/Al Si/Al Mn/Al Zr/Al Sr/Ca 13 C 18 O

29 Table 6.4b Geochemistry of Campanian sediments (SS-2; m) Elements Depth (m) Mn % Ca % CaCO3 % Si % Ti % Al % Sr Zr Ti/Al Si/Al Mn/Al Zr/Al Sr/Ca 13 C 18 O

30 Table 6.5a Geochemistry of Maastrichtian sediments (SS-3; m) Elements Depth (m) Mn % Ca % CaCO3 % Si % Ti % Al % Sr Zr Ti/Al Si/Al Mn/Al Zr/Al Sr/Ca 13 C 18 O

31 Table 6.5b Geochemistry of Campanian sediments (SS-3; m) Elements Depth (m) Mn % Ca % CaCO3 % Si % Ti % Al % Sr Zr Ti/Al Si/Al Mn/Al Zr/Al Sr/Ca 13 C 18 O

32 Figure 6.10 Stratigraphy, Mn, Sr/Ca and carbon-isotope curves for the Campanian-Maastrichtian sediments (SS-1, SS- 2 and SS-3) (Robaszynski et al 1998 and Jarvis et al 2001) 183

33 Figure 6.11 Stratigraphy; Carbon-isotope, Mn, Si/Al and Ti/Al, Campanian-Maastrichtian sediments (SS-1, SS-2 and SS-3) (Robaszynski et al 1998 and Jarvis et al 2001) 184

34 Figure 6.12 Carbonate, Al, Si/Al, Ti/Al and Zr/Al chemostratigraphy of Campanian-Maastrichtian sediments (SS-1, SS- 2 and SS-3) (Robaszynski et al 1998 and Jarvis et al 2001) 185

35 Figure 6.13 Idealized geochemical trend and their relationship to stratigraphic sequences and eustatic sea-level (SS-1, SS-2 and SS-3) (Robaszynski et al 1998 and Jarvis et al 2001) 186

36 187 Increasing Mn in the transgressive systems tract might relate to increased productivity during sea-level rise promoting an increased organic matter associated particulate Mn flux to the seafloor. The maximum flooding surface is generally a well developed omission surface, indicating reduced sedimentation. Therefore, high Mn contents might be caused by lower rates of sedimentation, with increased efficiency of Mn redox cycling leading to elevated Mn contents in the sediment. Increased carbonate sedimentation rates during the highstand may have reduced the Mn flux by limiting the effectiveness of the diagenetic manganese. The lowest Mn occurs at Campanian-Maastrichtian boundary of subsurface sediments (Figure 6.10).The level of closer to the positive δ 13 C digressions defining the Campanian-Maastrichtian boundary. The highest Mn in the curves deciphers the negative δ 13 C digression. This coincidence has led to suggestions that the two peaks are genetically related, high levels of Mn in the sediments reflecting the irregular increase of the oxygen minimum of Mn rich waters from the basin margins (Jenkyns et al 1991, Pratt et al 1991, Mabrouk 2007) during the Campanian- Maastrichtian oceanic anoxic event. The superimposed on the SS-1 sections shows the long-term trend and one short-term cycles, SS-2 section shows two short-term cycles and SS-3 sections three short-term cycles with maxima that do not correlate with either Al and Ca (Figures 6.11 and 5.8). The maxima of most of these cycles occur around the maximum flooding surface defined by Robaszynski et al (1998). Long- term trends in Mn/Al ratios to reflect the sequence stratigraphy better. Transgressive system tract display sharply rising Mn values that each maxima around maximum flooding surface and before declining again through the highstand system tract. The transgressive surface of sequence A and B in SS-1 is placed at a level of sharply rising Mn values occurring the Late Campanian and Maastrichtian boundary, in a prominent thin rising Mn that is

37 188 overlies an omission surface. Mn is highly insoluble under oxidizing conditions and is incorporated into marine sediments as oxyhydroxides (Calvert et al 1996, Jarvis et al 2001) associated with sinking organic matter, carbonate and detrital particles. Mn oxyhydroxides are dissolved during suboxic Mn reduction Carbon Isotope The carbon stable isotope profiles for three subsurface samples (SS-1, SS-2 and SS-3) are correlated in Figure The biostratigraphic control ; calcareous nannofossil, foraminifera and macrofossil evidence (Burnett 1990, McArthur et al 1992, 1993, Clauser 1994, Wood et al 1994, Robaszynski et al 2000, Jarvis et al 2002, Mabrouk 2003, 2004) of the Campanian-Maastrichtian sections, is consistent with the proposed isotope correlation. The δ 13 C isotope stratigraphy of the Campanian sandstone shows the systematic variation that enable detailed correlation on an intercontinental scale, despite the absence of unequivocal interregional biostratigraphy markers (Figure 6.13). It is notable that the subsurface sediments are shows the negative δ 13 C isotope excursions of the Campanian-Maastrichtian boundary, and SS-2 and SS-3 sections shows the negative δ 13 C isotope excursions of Mid-Campanian and Mid-Maastrichtian events both follow of major sea-level fall. The Campanian δ 13 C isotope and eustatic sea-level profiles are remarkably similar with a long-term fall in δ 13 C isotope associated with falling eustatic sea-levels. The long -term eustatic curve rises steadily through the Campanian-Maastrichtian peaks, when a Phanerozoic sea-level maximum is indicated. The short-term curve shows the SS-1 section Late Campanian- Early Maastrichtian and Late Maastrichtian minima followed by intervals of rapid sea-level rise, in SS-2 sections short-term curve shows the mid, late Campanian-Early Maastrichtian boundary minima indicates the sea-level

38 189 rises, in SS-3 sections short-term curve shows the mid Campanian and late Maastrichtian minima indicates the sea-level rises. The Al profile is close to a mirror image of Ca (Figure 6.12). Transgressive surface and maximum flooding surface are marked by positive digression in Ti/Al (Figure 6.12), whereas sequence boundary generally exhibit low Ti/Al contents; Zr/Al shows similar trends. It is notable that most cases represent relative long-term changes in sediment composition, rather than reflecting mineralogical and/or chemical variation associated with sediments overlying individual omission or erosion surface Strontium Stratigraphy The Sr/Ca profile for the subsurface sections is presented in Figure The Sr/Ca profiles shows the three short-term cycles that broadly correspond to the depositional sequences. The Sr/Ca maxima extent the SS-1 and SS-3 section shows the lower parts of highstand system tract and the overlying transgression system tract in Maastrichtian age. The SS-2 litho section Sr/Ca maxima shows the lower parts of highstand system tract and the overlying transgression system tract in Mid-Campanian age, with the Sr/Ca maxima closer to sequence boundary. The Sr/Ca ratios SS-1 and SS-3 litho sections fall through the transgressive system tract, before rising through the highstand system tract. In SS-2 litho sections Sr/Ca ratios shows the sharp decrease to Late Campanian to Maastrichtian. The relationship between the SS-1 and SS-3 litho sections Sr/Ca profile consistent with the sea-level change forcing the short-term records, in SS-2 section Sr/Ca profile consistent with the sea-level change forcing the long-term through the Late Campanian to Maastrichtian. Falling sea-levels during highstand and transgressive system tracts points to exposure of carbonate shelf and pulses of aragonite-derived Sr to the oceans. Rising sealevels during transgression promoted renewed aragonite deposition and falling

39 190 seawater Sr/Ca (Mabrouk et al 2007). This was reversed by the development of mature carbonate platform systems with lower aragonite accumulation rates during the highstand. The Sr/Ca ratios rise progressively through the Mid to Late Cretaceous, a period of rising eustatic sea-level (Hancock and Kauffman 1979, Haq et al 1988, Hancock 1993, 2000). The long-term trend is best explained by a decreasing contribution of aragonite to the formation of carbonate platforms (Steuber 2002, Mabrouk 2007). An additional factor might be the decline in shallow water carbonate platform versus epicontinental chalk sea areas accompanying eustatic sea-level rise.