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Thesis (M.S.)—University of Nebraska—Lincoln, 1966. Department of Geology.


Copyright 1966, the author. Used by permission.


The Thaynes Formation, which accumulated in the Early Triassic miogeosyncline of southeastern Idaho, is a fossiliferous, marine transgressive deposit.The drab beds of the Thaynes are conformable between two red bed units, the Woodside Formation below and the Lanes tongue of the Ankareh Formation above.

In the miogeosyncline the Thaynes is predominantly limestone, but it grades into siltstone, sandstone, dolomite, and limestone on the shelf to the east.Still further east, the Thaynes intertongues with red siltstones and standstones of the Chugwater Formation.

The Thaynes is divided into seven members (in ascending order): the lower limestone, lower black limestone, tan silty limestone, upper black limestone, sandstone and limestone, Portneuf Limestone, and Timothy Sandstone.In the area studied, the Timothy is not present and the Portneuf is present only in the western part.Correlation of the members from the miogeosyncline onto the shelf is tentative, and is based on lithologic similarities and interpretated facies changes.

Minor oscillations are superimposed on the generally transgressive Thaynes.The lower part of each of the members contains sandstones and siltstones, which suggests uplift in the source areas and (or) subsidence in the miogeosyncline.The upper part of each member is mainly limestone and dolomite which suggests stability in the source areas and (or) uplift in the miogeosyncline or general quiescence.

The fauna of the near-shore limestones and dolomites in the Thaynes is composed of common fragmented pelecypods and echinoderm fragments, and algal-coated grains, fragmented gastropods, brachiopods, and cephalopods that are rare.The fossils become less fragmented and gradually increase in abundance toward the miogeosyncline.On the west the assemblage is suggestive of off-shore, open marine deposition.Very shallow water and intertidal environments are suggested by pellets, colites, and fragmented fossils in carbonates of the study area.Petrographic studies indicate that many carbonates lack gran supported textures with the voids filled with micrite, suggesting quiet water deposition.

Based on 25 model analyses, the sandstones and siltstones are arkoses that are moderately sorted to well-sorted, with subangular to subrounded grains.The high feldspar content and lack of shale beds suggest a semi-arid to arid climate in the source areas.Source areas were probably northeast and southeast of the study area.The sorting and rounding of the fine-grained clastic material suggests low relief in source areas and (or) an extended period of current action in the depositional state.Concentration of dolomite toward the east suggests that it is environmentally controlled.A semi-arid to arid climate probably prevailed in the near-shore, littoral, and lagoonal environments that are believed to have existed during deposition of the Thaynes.

Advisor: M. Dane Picard