Southwest Section AAPG Annual Convention

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Dockum Group Revisited: Deposition and Tectonic Significance


The Late Triassic Dockum Group is not a petroleum-productive formation, but Dockum burial pushed more strata into the oil window and records structural evolution during a critical time of transition from Permian-aged foreland basin style to thermal subsidence in areas marginal to the Gulf of Mexico.

Deposition is fluvial and lacustrine, but there is significantly different interpretations on the volume and type of lake deposits. According to older work (e.g., McGowen et al. 1977), lakes were large, with delta, delta plain, and basin facies. Lake basin facies have evidence for subaerial exposure, indicating that lakes were dry for extended periods. Lake deposits were interpreted as the central part of a closed Dockum basin which was depositionally poorly connected to age-equivalent Chinle Formation deposits in central New Mexico. More recent work interprets fewer and shallower lakes (i.e., Lehman and Chatterjee 2005). Many of the lake basin and delta facies were reinterpreted as flood plain mudstones and sheet flood deposits, respectively. Drainage was interpreted as a predominantly west-flowing system continuous with Chinle deposition in New Mexico.

In the area with best lateral outcrop continuity (the Palo Duro Canyon area) outcrops show unequivocal lacustrine delta clinoforms, the toes of which interfinger with lacustrine mudstones and the tops of which grade into delta-top sandstones, just as described by Seni (1978). Evidence supporting reinterpretation of Dockum lakes as flood plain deposits is equivocal, and many outcrops interpreted as flood plain mudstones and sheet flood sandstones are better interpreted as lacustrine basin facies and lake delta front sandstones. Many of the peculiar features of the Dockum mudstones are more consistent with lake deposits than with a predominantly subaerial floodplain deposit.

Dockum lakes do not indicate closed basin deposition as proposed by McGowen et al. (1977). Dockum lakes formed in flood basins marginal to rivers flowing to the west. Dockum lakes were selectively deposited over the Dockum basin along the Texas – New Mexico border due to its higher Late Triassic subsidence rate, in a manner similar to that seen on actively subsiding alluvial basins today. The Dockum basin was continuous with the New Mexico Chinle deposits until the late Cretaceous.

Detrital zircon age spectra from the Dockum Group document changing sediment source area, possibly in response to exhumation during initial opening of the Gulf of Mexico (Dickinson et al. 2010). Large-scale structural and erosion patterns over the Eastern Shelf and under the East Texas basin show that uplift was centered on the Ouachita Tectonic Front and that tilting marginal to the uplift predated Dockum deposition. Detrital zircon ages and mineralogical zonation show changing source area in response to this uplift.