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The AAPG/Datapages Combined Publications Database

Pacific Section of AAPG


The Geologic Transition, High Plateaus to Great Basin - A Symposium and Field Guide (The Mackin Volume), 2001
Pages 426-426

Brackish-water mollusks from the western margin of the western interior seaway: A tool for sorting out the stacked Cenomanian-Turonian marginal marine strata along the Wasatch Line: Abstract

James I. Kirkland


The “middle” Cretaceous along the “Wasatch Line,” extending from near Coalville, Utah, in the north to Cedar Canyon and the Pine Valley Mountains in the south, is represented by a thick sequence of marginal marine strata deposited near peak sea-level rise on the western margin of a rapidly subsiding foreland basin. Many brackish-water molluscan taxa were named from these strata during the latter third of the 19th century, but were often described from poorly preserved material at poorly documented sites. Though evolutionarily conservative, brackish-water mollusks generally display a great deal of morphologic variation, resulting in a number of previously described species being lumped together. Therefore, little systematic research on these faunas was undertaken after 1900, except for comparing the Utah taxa to fossils described from similar-age strata to the south in Texas and Arizona. Recently these brackish-water taxa have been found to be useful as a proxy for determining ancient substrate conditions, paleoturbidity, and paleosalinity gradients. Field research in the area of southwestern Utah has revealed that abundant, well-preserved molluscan fossils commonly characterize brackish-water strata in this area. Distinct species can be recognized at different stratigraphic levels, indicating the potential for some of these brackish-water taxa to have local biostratigraphic utility.

To test their utility, collections of brackish-water fossils were made at many stratigraphic levels and from different localities. These sites were tied into the standard ammonite biostratigraphy established for the Cretaceous Western Interior Seaway through intertonguing and onlap/offlap relations between the brackish-water facies and marine, ammonite-bearing strata. The type collections of the original taxa from the previous century were borrowed from the Smithsonian Institution and the type localities were revisited and fossils collected when accessible. Many type specimens were originally collected from the now-closed coal mines near Coalville in the lower Turonian Coalville Member of the Frontier Formation, so the “Wasatch” coal sites are no longer accessible.

Results of these studies indicate that several brackish-water species needlessly had been lumped together, and that a number of undescribed forms historically lumped with these species represent new undescribed species. While the systematic research of these collections continues, the preliminary results are clear: a number of brackish-water molluscan lineages reveal speciation rates much higher than would be considered typical of brackish-water taxa. Gastropods of the Craginia coalvillensis-C. whitfieldi and Admetopsis rhomboides lineages appear to have the most potential for biostratigraphy, but other gastropods and a number of bivalves also appear to have potential. Using all of these taxa, two “zones” per substage may be discerned. Although it is unlikely that this system will have utility outside the Colorado Plateau region, it appears to work well there. It is often difficult to correlate the lithologically similar stratigraphic succession of marginal marine Cretaceous strata along the “Wasatch Line” due to poor exposures and structural complexity. The brackish-water taxa found in strata along the “Wasatch Line” belt provide a new tool on which to base these correlations.


Copyright © 2009 by AAPG Pacific Section and Utah Geological Society