The Marble Falls Formation (Lower Pennsylvanian) records normal marine conditions and widespread limestone deposition over the Llano uplift and the adjacent Concho platform in Texas. Examination of depositional environments of the outcropping Marble Falls facilitates interpretation of the oil-producing Lower Pennsylvanian shelf and shelf-edge carbonate rocks along the western margin of the Fort Worth basin.

Most of the outcropping Marble Falls can be divided into two units separated by an unconformity. Lower Marble Falls is entirely Morrowan. Upper Marble Falls becomes younger westward: Morrowan in the east and Atokan in the west.

Marble Falls deposition began with establishment of an open marine platform centered at the Llano uplift. Incipient calcarenite shoals developed at some slight break in slope. The northeast part of the platform resembled the modern Bahamian Platform, although platform-off platform relief was less than 9 m. Platform margins were approximately coincident with present outcrops on the north and east sides of the Llano uplift.

The upper Marble Falls was deposited primarily as algal buildups and calcarenite shoals and as shale and spiculitic biomicrite in topographic lows. While the older lower Marble Falls Platform was subaerially exposed, deposition continued on the off-platform shelf adjacent to the rapidly filling Fort Worth basin. Progressive subsidence of the old platform allowed these facies to onlap the erosional surface. Strawn deltas simultaneously prograded across the upper Marble Falls shelf from the east. Marine energy levels and depositional relief were less than during deposition of the lower Marble Falls.

End_of_Article - Last_Page 479------------