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Regional subsurface analysis in southwest Texas indicates that the Olmos Formation (Gulfian) was deposited by a complex of wave- and fluvial-dominated delta systems in two depocenters. Sediment influx was from the north and northwest. Five deltaic subunits, A through E, were deposited in the western depocenter. Three other deltaic wedges (F, G, H) formed the second depocenter farther east in present-day Frio and LaSalle Counties. Subsidence was greater in the western half of the Maverick basin where thickest (1,300 ft; 395 m) deltaic sediments were deposited. Lower Olmos strata represent a succession from wave-reworked, strike-elongate deltas of subunit A, similar to those of the underlying San Miguel Formation, to fluvial-dominated, dip-elongate deltas of subunits B and . Extensive (1,200 mi2 or 3,100 km2 in Texas) aggradational floodplain deposits of B and C are characterized by diverse electric-log patterns; variation in log character is a response to complex depositional facies on the delta platform. Downdip, toward the Cretaceous shelf edge, delta-plain facies merge with upward-coarsening delta-front sandstones.
Uppermost subunits D and E were deposited by a prograding barrier-island system in an interdeltaic embayment marginal to high constructive deltas of the eastern depocenter. Lagoonal and fluvial-channel deposits are recognized from cores. Eastward migration of deposition was accompanied by an abrupt change of depositional style in the western depocenter from deltaic to coastal-interdeltaic.
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