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Sequence Stratigraphy, Sea Level Change and Palaeoenvironments via Chemostratigraphy: Regional to Global Correlations*
By
Amina Mabrouk1, Ian Jarvis2, Habib Belayouni1, Amanda Murphy2, and Richard T.J. Moody3
Search and Discovery Article #40244 (2007)
Posted June 30, 2007
*Adapted from extended abstract prepared for oral presentation at AAPG Annual Convention, Long Beach, California, April 104, 2007.
1Faculte des Sciences de Tunis, Département de Géologie, Université de Tunis El Manar, 2092 Tunis, Tunisia ([email protected])
2School of Earth Sciences & Geography, CEESR, Kingston University, Penrhyn Road, Kingston upon Thames KT12EE, UK ( [email protected] )
3Moody-Sandman Associates, ‘Haslemere’ Gordon Road, Curdridge, Hants S032 2BE, UK. ([email protected])
Fourteen d13C events are recognised in four Cenomanian sections in England and are isochronous with respect to biostratigraphic datums. Events in the Middle and Upper Cenomanian are readily identified in Italy. A d13C correlation of Campanian sections in England, France, and Tunisia is demonstrated using 11 isotope events, which enable calibration of Boreal and Tethyan biostratigraphic schemes.
Carbon isotope profiles predominantly follow eustatic sea-level, with rising d13C accompanying transgression and falling d13C accompanying regression. Significant positive carbon excursions are broadly associated with major transgressive events. Sr/Ca ratios and the Mn contents of pelagic carbonates vary systematically with respect to sea-level change. Increasing Sr/Ca ratios during periods of sea-level fall are attributed to the release of Sr from aragonite dissolution and replacement in subaerially exposed platform carbonates. Falling Sr/Ca ratios accompanied the re-establishment of shallow-water carbonate factories during sea-level rise.
Mn flux increases with rising sea-level, with Mn reaching a maximum around each maximum flooding surface, before decreasing again through the overlying highstand systems tract, representing a period of relative constant carbonate supply.
Correlations between variations in Sr/Ca ratios and Mn content with sequence stratigraphy in the English Cenomanian are consistent with published data from other Mesozoic successions.
Elemental chemostratigraphy provides a means of developing a sequence stratigraphic framework in pelagic carbonate successions. This framework clearly needs to be constrained by geological evidence, but offers a powerful new tool for sequence analysis and correlation.
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Pelagic and hemipelagic sediments are predominantly composed of stable primary low-Mg calcite and are frequently characterised by low-permeabilities. In addition, subaerial exposure horizons and early interaction with meteoric fluids are normally absent due to the exclusively open-ocean setting prevailing during the deposition of such facies. Consequently, relatively uniform compositions and minimal early or late diagenesis, commonly characterise these sediments and validated the wide and successful use of carbon and oxygen stable isotopes (e.g., Scholle and Arthur, 1980; Arthur et al., 1987; Gale et al., 1993; Pearce and Jarvis, 1995; Mitchell et al., 1996; Voigt and Hilbrecht, 1997; Voigt, 1999; Jarvis et al., 2001; Mabrouk et al., 2005) and, to a lesser extent, elemental chemostratigraphy (e.g., Renard, 1986; Jarvis et al., 2001; Stoll and Schrag, 2001; Jenkyns et al., 2002) as a mean for correlating sedimentary barren sequences. In this study, geochemical studies of eight pelagic carbonate successions of Late Cretaceous age in England, France, Italy, and Tunisia (Figure 1) will be used to illustrate and enhance the potential of isotopic and elemental chemostratigraphy for international correlations and the interpretations of sedimentary sequences. Analytical procedures for the determination of whole rock contents in Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, P, Ba, Sr and Zr are presented elsewhere (Murphy, 1998; Jarvis, 2003; Mabrouk, 2003). Results are presented in weight percent oxide for ‘major’ elements and as mg g-1 (parts per million) for ‘trace’ elements. Carbon and oxygen stable-isotope ratios were determined at the University of Oxford, following the method of Jenkyns et al. (1994). Results are reported in the δ notation, in parts per thousand or ‘per mille’ (‰) relative to the Vienna Pee Dee Belemnite (VPDB) international standard. Reproducibility was better than 0.1 ‰ for both carbon and oxygen. Elemental and isotopic numerical data for the Culver Cliff study are reported in Murphy (1998) and for the El Kef study in Mabrouk (2003).
Mitchell et al. (1996) described seven isotope events in the Cenomanian at Speeton, North Yorkshire (Figure 1): an Albian / Cenomanian Boundary Event (ACBE); three Lower Cenomanian events (LCE I – III); two Middle Cenomanian events (MCE I, II); and the Cenomanian / Turonian Boundary Event (CTBE), which were correlated to southern England and northern Germany. A compilation of published and new carbon isotope data (Figure 2) from sections in northern (Speeton), eastern (Trunch), and southern (Culver, Dover) England (Figure 1) demonstrates that these and several additional events provide a basis for detailed correlation of sections throughout the country, despite significant differences in thickness and facies between the northern and southern provinces. Comparison with the positions of biostratigraphic datums (Figure 2) demonstrates that the d13C events are isochronous within the resolution provided by zonal events and marker bed biostratigraphy. In addition to the seven isotope events recognised previously, nine additional correlation levels are shown in Figure 2. Carbon stable-isotope profiles for two Tethyan (Figure 1; El Kef, Tunisia and Bidart, France), and one Boreal site (Trunch, England) are correlated in Figure 3. The biostratigraphic control on these correlations is considerably poorer than that available for the Cenomanian, but calcareous nannofossil, foraminiferal, 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) is consistent with the proposed isotope correlations. The shapes of the three curves are remarkably similar, but absolute d13C values are lower at El Kef. This offset to lighter d13C values probably reflects a primary depletion in seawater on Tethyan carbonate platforms (Jarvis et al., 2002; Mabrouk, 2003). On the other hand, as in the Cenomanian, the carbon isotope stratigraphy of the Campanian shows systematic variation that enables detailed correlation on an intercontinental scale, despite the absence of unequivocal interregional biostratigraphic markers.
Carbon Isotopes and Related Sea Level Changes The Exxon global Mesozoic – Cenozoic sea-level curve of Haq et al. (1987, 1988) 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 Albian – Cenomanian and peaks in the early Turonian, when a Phanerozoic sea-level maximum is indicated. The short-term curve shows early, mid- and upper Cenomanian minima followed by intervals of rapid sea-level rise. Similar trends are seen in the d13C curve (Figure 3), but the low stratigraphic resolution of the Haq et al. (1987, 1988) composite curve makes it unfeasible to test these relationships with any rigour. However, an eustatic curve for the Cenomanian, constrained by detailed ammonite biostratigraphy, has been presented by Gale et al. (2002). In addition, comparison between biostratigraphically well constrained regional sea-level data for North Africa and the Middle East (Lewy, 1990; Lüning et al., 1998) and northern Europe (Hancock, 1989; Hancock, 1993; Niebuhr et al., 2000) and the Campanian d13C profile (Figure 4) indicates that the latter once again broadly follows sea level. It is notable that the two main positive carbon isotope excursions of the Santonian / Campanian Boundary and Mid-Campanian events both follow periods of major sea-level fall. The Upper Campanian isotope and eustatic sea-level profiles are remarkably similar, with a long-term fall in d13C associated with falling eustatic sea-levels, and four carbon-isotope ‘cycles’ and four third-order eustatic cycles. Falling sea-levels in the Late Campanian are not reflected in the regional NW Europe and Egypt curves (Figure 4), although regressions are documented in northern Germany, Spain, West Africa , South America and the US Western Interior (Jarvis et al., 2002). Within the resolution of existing data, therefore, there seems to be a remarkably close correspondence between carbon isotope and eustatic sea-level curves, although further work is necessary to test the temporal relationships and to investigate short-term variations in more detail.
A Sr/Ca profile for the Cenomanian of Culver is shown in Figure 5, plotted against the sequence stratigraphic framework of Robaszynski et al. (1998) with the additional Sequence 5a of Jarvis et al. (2001). The Sr/Ca profile displays seven short-term cycles that broadly correspond to the depositional sequences. Sr/Ca maxima generally span the upper parts of highstands and the overlying lowstand systems tracts, with maximum values close to, but not necessarily coincident with, sequence boundaries. Sr/Ca ratios generally fall through transgressive systems tracts, attain minimum values in the upper TST, before rising again into and through the highstand. By contrast, on the longer term, Sr/Ca ratios remain relatively constant through the Cenomanian (Figure 5) except at the top of the stage in Sequence 6, where there is a sharp decrease to much lower Sr/Ca ratios that continue through the Lower Turonian. The observed relationships between the Sr/Ca profile and the sequence stratigraphy are consistent with sea-level change forcing the short-term Sr/Ca record. Falling sea-levels during late highstands and lowstands led to exposure of carbonate shelves and pulses of aragonite-derived Sr to the oceans. Rising sea-levels during transgression promoted renewed aragonite deposition and falling seawater Sr/Ca. This was reversed by the development of mature carbonate platform systems with lower aragonite accumulation rates during the highstand. Current 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, 2000), so 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). An additional factor might be the decline in shallow-water carbonate platform versus epicontinental chalk sea areas accompanying eustatic sea-level rise.
The overall decrease in Mn contents through the Cenomanian (Figure 5) has been interpreted as resulting from a decreasing detrital Mn supply (Jarvis et al., 2001), as indicated by an inverse correlation with carbonate content and relatively constant background Mn/Al ratios. However, the short-term Mn cycles that equate to individual depositional sequences do not correlate well with either silicate or carbonate contents; for example, most clay-rich lowstand systems tracts display Mn minima. This suggests that Mn supply was tied to the biogenic flux (organic carbon and carbonate), which must have decreased during the lowstand systems tract. Carbonate/clay ratios and the Mn flux increased with rising sea-level, with Mn reaching a maximum around each maximum flooding surface (Figure 5), before decreasing again through the overlying highstand systems tract, representing a period of relative constant carbonate supply. 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 pump. One of the largest Mn peaks at Culver Cliff occurs around the Cenomanian / Turonian boundary (Figure 5), the level of the large positive δ13C excursion defining the CTBE (Figure 2). This coincidence has led to suggestions that the two peaks are genetically related, high levels of Mn in the sediments reflecting abnormal intensification of the oxygen-minimum layer or advection of Mn-rich waters from the basin margins ( Jenkyns et al., 1991; Pratt et al., 1991) during the Cenomanian / Turonian oceanic anoxic event (OAE 2). However, these models are not consistent with current understanding of the behaviour of Mn in the oceans (see Jarvis et al., 2001), and other large Mn peaks at Culver (Figure 5) do not correspond to large positive carbon isotope excursions. Indeed, the positive d13C excursion defining MCE I coincides with a Mn minimum. The Mn maximum associated with the CTBE, therefore, is better explained by the same geochemical processes affecting the entire Cenomanian–Turonian, rather than representing a geochemical anomaly associated with the global oceanic anoxic event.
Idealised stratigraphic relationships between Mn and Sr/Ca in pelagic carbonates, sequence stratigraphic units, and eustatic sea-level change are illustrated in Figure 6. A combination of Mn and Sr data offers criteria to develop sequence stratigraphic schemes in pelagic carbonate successions. Other geochemical data provide additional constraints for sequence stratigraphic interpretation. Based on studies of the Culver Cenomanian, Jarvis et al. (2001) demonstrated that carbonate contents increase in response to rising sea-level, but show sharp decreases at sequence boundaries, and display minima in lowstand systems tracts. Carbonate values increase through each transgressive systems tract, reach a maximum around the maximum flooding surface, and then remain high and constant through the highstand systems tract. In the detrital fraction, silica, titanium, and zirconium to aluminium ratios (Si/Al, Ti/Al, Zr/Al) peak around transgressive surfaces and maximum flooding surfaces, indicating pulses of increased siliciclastic input which are not confined to single omission/erosion surfaces. The sequence stratigraphic framework clearly needs to be constrained by geological evidence, but offers a powerful new tool for sequence analysis and correlation.
The Cenomanian and Campanian studies demonstrate that remarkably consistent relationships exist between carbon isotope profiles and eustatic sea-level curves, with increasing d13C accompanying sea-level rise and decreasing d13C accompanying sea-level fall. The strontium content of sediments also responds to sea-level change. In the Cenomanian, Sr/Ca maxima span the upper parts of highstand and the overlying lowstand systems tracts, with maximum values around sequence boundaries. Sr/Ca values fall through transgressive systems tracts and attain minimum values in their upper parts, before rising again into and through the overlying highstand. Furthermore, manganese exhibits consistent but different relationships to sequences, with minima around sequence boundaries and through lowstands, rising values from the transgressive surfaces through transgressive sequence tracts, maxima around maximum flooding surfaces, and declining values through highstands. The positive Mn anomaly previously associated with the large positive d13C excursion spanning the Cenomanian / Turonian boundary is not tied to the abnormal oceanic conditions accompanying Oceanic Anoxic Event 2, but is related to normal marine processes. Our study demonstrates that the combination of elemental and isotopic studies enables improved regional to global correlation which would better constrain local and regional geological models as well as petroleum system evaluation.
Elemental data for Culver were obtained by AMM during doctoral work funded by the UK Natural Environmental Research Council (CASE award GT4/93/12/G) in collaboration with the British Geological Survey (BGS). AM acknowledges British Council Chevening Scholarship TUN0100022, during which some of this work was undertaken. Research support by BG Exploration & Production for the Tunisia study is gratefully acknowledged. Isotopic analyses were undertaken in collaboration with Dr Hugh Jenykns (University of Oxford); Julie Cartlidge operated the PRISM mass spectrometer at Oxford on which these data were generated. Heather Stoll (Williams College, Massachusetts) kindly provided digital data from her chemostratigraphic studies.
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