Abstract: Lacustrine Facies Model for Non-marine Sequences within Cyclothems in the Pennsylvanian (Upper Freeport Formation, Appalachian Basin)

Blas L. Valero Garces, Elizabeth Gierlowski-Kordesch, William A. Bragonier

The Upper Freeport Formation (Upper Allegheny Group, Middle Pennsylvanian) is one of the earliest nonmarine cycles in the Appalachian basin and contains carbonates, siliciclastics, and coal. A detailed facies analyses of 25 limestone cores along with detailed subsurface data from the Upper Freeport Formation in western Pennsylvania (Armstrong and Indiana Counties) identified a large alluvial-wetland complex drained by an alluvial system surrounded by extensive wetlands containing adjacent densely vegetated swamp areas and freshwater, carbonate-producing lakes. These lakes were small in size (several square km), shallow, stratified and connected by surface and ground waters. Carbonate production was not triggered by evaporative concentration but by biogenic algal production in a sedime t-starved system. Carbonates were continually being recycled, both physico-chemically and biologically. Siliciclastic wedges and predominance of reworked and traction-deposited carbonates favor a current-dominated, open lacustrine environment. Sequential analysis does not support climate forcing but a lateral migration of the subenvironments responding to the dynamics of the depositional system.

Although time intervals involved in the generation of cyclothems is unknown, comparison with modern deltaic complexes, indicates time-spans far below those associated with allocyclic forcing mechanisms, like tectonics and eustacy. Autocyclic processes seem to better explain the evolution of these lacustrine/alluvial systems. Small topographic differences, changes in local drainage patterns, and fluvial dynamics governed the arrangement of subenvironments to a large extent. The unique combination of climate, tectonics, and eustatic level simply created a place where lake sediments could collect for a limited period of time.

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