Magnitude of Early Pennsylvanian (Morrowan-Atokan) Eustastic Change: Constraints from a Fan-Delta Setting
The lower Fountain Formation is composed of successive continental-marine cycles. Each cycle is characterized by alluvial fan deposits overlain by a marine progradational package. A thin cobble conglomerate marks the base of marine strata and is inferred to represent a transgressive lag. Furthermore, each individual cobble bed can be traced throughout the study area. The above field relationships allow for a minimum calculation of the magnitude of sea level change because: 1) the horizontal distance of transgression can be estimated from the mapped transgressive lag deposits; and 2) the slope of the surface over which transgression progressed can be estimated through paleohydraulic analysis of the fluvial deposits.
Magnitudes of sea level change over six successive cycles were calculated using an estimated depositional slope of 0.005; results suggest 10-18 m of change. A large eustatic component is inferred for these six cycles because each marine package is separated throughout the study area by alluvial packages; conversely, autocyclic cycles of the Fountain Formation have alluvial components that grade laterally into marine strata and are inferred to reflect autogenic processes. Tectonic pulses occur over too long a time frame to have produced these higher-frequency cycles.
Marine strata of the lower Fountain Formation have yielded Morrowan-Atokan conodonts (Idiognathoides sinuatus). Few estimates of the magnitude of sea level change exist for this time period, but incised valley depths suggest 20-45 m of eustatic sea level change and modeling of ice volume and facies models suggest 40-60 m of sea level change. Our estimates are absolute minimums and the method does reflect some sensitivity to variations in slope, such that an order-of-magnitude change in slope results in ~100 m of sea level change. Nevertheless, the range given by our method approaches other estimates and indicates: 1) the Morrowan-Atokan time period experienced relatively modest eustatic sea level change compared to other times during the Pennsylvanian and may suggest lesser ice extent; and 2) this method provides a new method for estimating magnitudes of sea level change.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009