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Virgil and lower Wolfcamp sediments on the Eastern shelf were deposited within 3 depositional systems--Bowie fluvial system, Cisco delta system, and Lake Hubbard interdeltaic embayment system--which are the subjects of this report. Complementary shelf-edge bank, slope, and basin systems, to be discussed by William E. Galloway, complete the facies tract.
Bowie fluvial facies compose tabular and belted sandstone bodies characterized by braided and coarse-grained meander-belt sequences and sedimentary structures. Fine-grained meander-belt facies generally becoming finer upward, lateral accretionary point bar deposits, and characteristic sedimentary structures commonly occur high in the Bowie fluvial system, indicating decreasing tectonic activity in source areas. Basal fluvial channel facies normally cut subjacent deltaic facies and may overlie rocks of previous depositional episodes.
Cisco deltaic facies are composed of prograding, prodelta mudstones that become coarser upward, delta front, channel-mouth bar and distributary channel sandstones, and aggradational crevasse splay facies, delta-plain mudstones, and coal or organic-rich clays. Local sandstone bars of destructional origin may fringe the delta facies.
Lake Hubbard interdeltaic embayment facies flank deltaic lobes and are composed of mudstone, thin extensive sheet sandstone, and thin impure detrital coal. Some sheet sandstones reflect strandplain accretion, but more commonly they are reworked delta-front sands.
Deltas prograded rapidly westward across the slowly subsiding Eastern shelf. Sediments from the east supplied crevassing delta lobes until avulsion of over-extended systems occurred. Delta construction, accompanied by deposition of complementary facies within nearby interdelta embayments, restricted open-shelf limestone facies to areas downslope or in areas of abandoned deltas. Channels were cut deeply into subjacent facies as prograding deltas extended fluvial erosion and deposition far across older deltaic facies. Marine processes slowly modified abandoned, compacting, and subsiding deltas. Winnowed sediment was swept into interdeltaic embayment mudflats and strandplains. In the absence of local terrigenous sediment, shelf limestones transgressed upslope and along the coast to coale ce over marsh-stabilized deltaic and interdeltaic areas. All environments could occur simultaneously, shifting with distribution of delta sites to produce repetitive
sequences of thin superposed depositional systems. Deposition of fluvial and deltaic facies was rapid; deposition of destructional and transgressive facies represents most of the time consumed by deposition. Depositional systems shifted westward through time as the eastern flank of the basin was filled.
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