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Coal-bearing Wilcox strata near Uvalde in south Texas are the deposits of a tide-dominated delta. The delta of the Klang and Langat Rivers, Malaysia, provides a modern analog for these strata. Five facies have been identified from a study of core and well logs: (1) lignite, (2) underclay, (3) interbedded sand and mud with lenticular, wavy, and flaser bedding, (4) ripple-laminated or cross-bedded sand, and (5) greenish, very strongly bioturbated sand. On the Klang-Langat delta, the modern equivalents of these facies are (1) peat formed in freshwater swamps, (2) root horizons developed beneath the peat (3) interbedded sand and mud deposited on tidal flats, (4) channel sands, and (5) shallow marine sand and mud.
Tidal flat deposits are the most abundant type of sediment on the Klang-Langat delta and in the coal-bearing Wilcox strata. The tidal flats of the modern delta are crossed by small tidal creeks and by larger tidal streams. The tidal channels are cut into tidal flat sediments and separate peat-forming areas. Channel sands in the Wilcox are cut into tidal flat deposits and form washouts in the lignite. Two types of channel-fill sand are present in the Wilcox, sands 5-15 ft (1.5-4.5 m) thick and sands more than 30 ft (9 m) thick. The thinner sands, deposits of small tidal creeks, have sharp, erosive bases, fine upward and pass into interbedded sand and mud. The thicker sands have sharp tops as well as sharp bases and show no grain-size trends; they are fills of larger tidal streams.
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