--> Testing the Distributary Channel Model Against Predicted Changes in Fluvial Reservoir Geometry Transitions From Low to High Accommodation Settings: Upper Pennsylvanian of the Central Appalachian Basin

AAPG Annual Convention and Exhibition

Datapages, Inc.Print this page

Testing the Distributary Channel Model Against Predicted Changes in Fluvial Reservoir Geometry Transitions From Low to High Accommodation Settings: Upper Pennsylvanian of the Central Appalachian Basin

Abstract

Models for distributary channel networks argue that rapidly aggrading systems that are presumably high accommodation will convert from low-accommodation single channel to distributary systems as they encounter the new conditions. If distributary channel systems are also bifurcating systems, it follows that channel size should reduce from low-accommodation to high-accommodation settings. Poorly drained floodplains with a lower energy regime and relatively high water table should record very high accommodation conditions and should particularly be susceptible to developing bifurcating distributary systems. We tested this hypothesis by comparing channel dimensions and lithofacies in high- vs. low- accommodation deposits in the Pennsylvanian strata of the Princess and Conemaugh Formations of eastern Kentucky. A valley complex 5–8 m thick typifies the low-accommodation setting with individual channel belts ranging from 0.75–1.5 m thick. Channels are highly amalgamated, are composed of fine to medium grained sands, and become heterolithic towards the top. Distributary channels, lake strata and smaller tie channels dominate the high accommodation floodplain settings. Distributary channels exhibit lateral accretion elements and are typically 1–3 stories, with individual channels being 0.4–1 m thick. Tie channels are 14–40 cm thick, very fine grained, and cross-cut floodplain lake strata. Thin, discontinuous, fine to very fine grain sand sheets connected to isolated tie channels represent pulses of deposition from channel propagation across the lake as a overbank sheet. Our data indicates a change in channel size, range of channel size, and channel geometry changes with an increase in accommodation state from low to high that is consistent with an upward change to a bifurcating system. This observation confirms the prediction of vertical change in geometry and size/size range of channels with change in accommodation state and supports the hypothesis that channels become bifurcated with sufficiently increased accommodation state. As well, the high accommodation condition is unpredictably well connected through tie channel propagation across the floodplain and distributary channel incision into these tie channel bodies and their overbank sands. Basin-fill models should thus consider the contingency that channel-belt reservoirs can change attributes with aggradation state and need not retain the sizes they possessed in the low-accommodation condition.