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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 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
Carbon
Mitchell et al. (1996)
described seven
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
Carbon
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
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 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
The Upper Campanian
Within the resolution of existing data, therefore,
there seems to be a remarkably close correspondence between carbon
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 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
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 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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