Abstract: Probable High-Amplitude Glacial-Eustatic Origin for Pennsylvanian Cyclothems Requires Modification of Applied Sequence-Stratigraphic Classification and Terminology

John A. French, Philip H. Heckel

Much of the current sequence-stratigraphic classification and terminology is based on the assumptions that 1) a highstand systems tract is deposited as an aggradational-to-progradational unit during the late stages of sea level rise; 2) most of the succeeding lowstand systems tract is deposited after an abrupt fall of relative sea level, mainly downdip of the topographically lowest shoreline; and 3) there is little or no deposition on the progressively more exposed, and in places incised, regressive surface. The possibility that complex and volumetrically important stratal assemblages (forced-regressive systems tracts) can be deposited on the ultimately subaerially exposed surface during relative falls of sea level has not been widely considered until recently.

One of the best examples of depositional sequences dominated by strata formed during sea level falls are Pennsylvanian cyclothems, particularly as developed in midcontinent North America. Because these normally less than 10-m thick, cyclothemic depositional sequences were likely controlled by glacial-eustatic fluctuations on the order of 100 m, and because the deepest water lithofacies generally occurs near the base of each sequence, the majority of each sequence likely represents a forced-regressive systems tract deposited as the excess accommodation space was progressively reduced during sea level fall. On the preserved lower to middle shelf, only the middle of the thin, condensed, dark, phosphatic core shale is equivalent to the thicker highstand systems tract (now eroded) that pre umably existed near the highest stand of shoreline north of Iowa. Subsequent sea level fall forced carbonates to step basinward, either smoothly, or in a series of pulses interrupted by minor sea level rises, or in a series of abrupt basinward shifts; the resulting unit is the shallowing-upward, regressive limestone. The volumetric importance of regressive deposits in glacial-eustatic successions is underscored by the observation that Pleistocene glacial buildups (i.e., sea level falls) appear to occupy most of the time during these recent cycles of glaciation. During lowstand, carbonate (and locally siliciclastic) progradation continued and formed a lowstand wedge, until terminated by rapid sea level rise. Glacial-eustatic forced-regressive systems tracts in carbonate facies regimes are therefore shallowing-upward units (i.e., regressive limestones) that laterally link the updip highstand systems tracts with downdip lowstand systems tracts.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994