High Resolution Chemical Sequence Stratigraphy in Carbonates: Oxfordian Smackover USA Ramp Proxy

Kulwadee L. Pigott, John D. Pigott, Michael H. Engel, and Richard P. Philp
University of Oklahoma, Norman, OK

In comparison to the deposition of principally extrabasinal materials into clastic basins, carbonate deposition is dominantly a product of in situ intra-basinal organic production. Consequently, parasequence comparison of carbonates to the “Sea Slug” paradigm may be limited in its usefulness. For carbonate ramp settings, differentiating vertical from horizontal (allocyclic from autocyclic processes) is especially difficult when facies changes are gradational rather than discrete. Results obtained from a high resolution geochemical study of cores from the Oxfordian Smackover of Alabama USA suggest chemostratigraphy can be a powerful tool for differentiating allocyclic (global) effects from autocyclic (local) effects in carbonate sequence analysis.
Diagnostic elemental signatures for the major parasequences may be separated into detrital, in situ, and oceanic species. Detrital elements Al, Ti, Si, and P are highest in concentration during Lowstand Systems Tracts (LST), and lowest during Highstand Systems Tracts (HST). Antithetically, in situ carbonate productivity elements Ca and Sr are highest during HST, and lowest during LST. As a convenient sequence punctuation mark, Mn is the major oceanic element which increases during the Transgressive Systems Tract (TST), is highest during the Maximum Flooding Surface (MFS), and lowest during the HST-LST boundary.
While the trends of elemental concentrations can discriminate sequence cycles accompanying alternating autocyclic transgressions and regressions, when viewed at subcycle resolution, there are complications owing to the proximity to shoreline input, climatic changes, and/or secular vital effects. For example, the TOC signal represents in situ productivities mixed with terrigenous fluxes spiked by pulsed P nutrient inputs. Such mixing is also indicated by concomitant alternating synthetic and antithetic tracking of organic and inorganic del 13C isotopes. Therefore, source rock quality and quantity in carbonate ramp settings are both a function of allo- and autocylic influences and require careful discrimination of cycle position, terrestrial proximity, and secular position in the geologic record.

AAPG Search and Discovery Article #90072 © 2007 AAPG and AAPG European Region Conference, Athens, Greece