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Triassic strata of the Dockum Group in Texas comprise two major upward-fining alluvial-lacustrine depositional sequences.
The two sequences are represented by the (1) Santa Rosa-Tecovas, and (2) Trujillo-Cooper Canyon Formations. The second sequence
is much thicker than the first, and occupies a greater geographic part of the Dockum basin. Each sequence of alluvial and
lacustrine sediment accumulation is characterized by sediment derivation from a different source terrain. The unconformable
relationship between the two depositional sequences, the change in mineralogical composition and presumed source areas between
these units, differences in paleocurrent orientation between units, and evidence for intervening episodes of local deformation
indicate that the sequences are of tectonic origin. These strata are not the product of a single sediment dispersal system,
such as the centripetally-drained lacustrine delta complex previously envisioned for the Dockum basin. Both Dockum sequences
are comprised largely of two typical alluvial facies associations; stream channel facies, and overbank flood-plain facies,
that are similar to those described in nearly all fluvial deposits. In addition, the Dockum Group contains a peculiar lacustrine
facies that accumulated in local flood-plain depressions, and probably resulted from subsidence over areas of subsurface salt
dissolution. Vertebrate fossil assemblages are found in all three Dockum facies associations. Five fossiliferous sites in
the Dockum are discussed in the context of these three depositional settings. The Dockum tetrapod diversity is reviewed in
a hierarchical phylogeny with remarks on the history of collection, stratigraphic distribution of genera, and their taxonomic
status. The stratigraphic ranges of tetrapod taxa do not support the recently proposed successive Otischalkian, Adamanian,
Revueltian, and Apachean biochrons within the Dockum Group. Instead, a few index fossils provide a broad framework for correlation
of Late Triassic nonmarine strata of the Dockum with the Carnian and Norian Alpine marine stages. 相似文献
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Paul F. Hudak 《Environmental Geology》2005,47(2):283-289
This study investigated distributions of alpha and beta activity in the Ogallala and Dockum Aquifers of northwest Texas, USA. The survey included 319 samples from the Ogallala Aquifer and 48 samples from the Dockum Aquifer. The Ogallala Aquifer produced slightly higher median alpha (7.2 versus 6.1 pCi/L) and beta (7.7 versus 7.0 pCi/L) levels. However, a higher percentage of samples from the Dockum Aquifer exceeded drinking water standards, and the maximum alpha observation from that aquifer was 197 pCi/L, more than 13 times the 15-pCi/L standard. Approximately 4.4% of alpha observations from the Ogallala Aquifer and 16.7% of observations from the Dockum Aquifer exceeded the standard. Fewer observations, 0% from the Ogallala Aquifer and 6.3% from the Dockum Aquifer, exceeded the 50-pCi/L level of concern for beta activity. There were significant positive correlations between alpha and beta activity in each aquifer. However, there were no significant correlations between radioactivity and well depth, other than a weak inverse correlation between beta activity and well depth in the Ogallala Aquifer. Uranium deposits in sandstone and shale of the Dockum Aquifer, and in calcrete and silcrete of the Ogallala Aquifer, and overlying lacustrine sediments, likely influence radioactivity patterns observed in this study. 相似文献
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