Sandstone Distribution and Lithofacies of the Triassic Dockum Group, Palo Duro Basin, Texas

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Date

1988

Authors

Johns, David A.

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Abstract

The Triassic Dockum Group in the Palo Duro Basin comprises numerous interbedded sequences of coarse- to fine-grained terrigenous elastic sedimentary strata. Within the lower Dockum section, four sandstone-dominated progradational sequences are evident. The sandstone in the first unit exhibits a relatively uniform distribution across the basin due to deposition by broad alluvial fans and fan deltas. In contrast, sandstones in the remaining units and the upper Dockum display uneven distribution as a result of deposition by deltaic and fluvial systems. According to net-sandstone trends, sediment sources for Triassic strata in the Palo Duro Basin indicate deposition from the west, north, and east for the basal unit, predominantly from the east for units 2 and 3, and mixed western and eastern sources for unit 4 and the upper Dockum. Net-sandstone distribution patterns and lithofacies trends suggest proximal depositional environments in the present eastern and northeastern Dockum and more distal environments to the west and southwest. Paleodip appears to have been from east to west over most of the Texas side of the basin, possibly changing in the area of the Texas-New Mexico border. Basement structural elements within the basin were subtly active during deposition and influenced the local accumulation of sandstone.

Nine lithofacies in the Dockum can be identified from core samples. Two lacustrine system lithofacies are discernible by claystone, mudstone, and siltstone lithologies. In some instances, these intervals are thick and commonly contain features indicative of subaerial exposure and weathering. Four lithofacies may have been deposited in deltaic environments. Thick siltstone sequences and interbedded siltstone and sandstone sequences represent distal and proximal delta-front deposits, while mudstone was deposited on deltaic mudflats. Interbedded, organic-rich siltstone and sandstone accumulated in swamps on abandoned delta platforms. Two fluvial lithofacies, including delta-distributary-channel fill deposits, are represented by relatively thick sandstone and conglomerate. Fluvial lithofacies encompass bed-load and mixed-load systems. An eolian-flat lithofacies may have formed on distal alluvial-fan sediments.

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