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MONTANA GEOLOGICAL SOCIETY FIELD CONFERENCE & SYMPOSIUM GUIDEBOOK TO SOUTHWEST MONTANA
LOWER TRIASSIC STRANDLINE DEPOSITS IN THE TENDOY RANGE NEAR DELL, MONTANA
Examination of the Lower Triassic Dinwoody, Woodside, and Thaynes Formations exposed in Little Water Canyon and Hidden Valley provide evidence for interpretation of the environments responsible for their deposition.
Petrographic study, combined with paleontologic evidence and sedimentary structures, indicate the presence of the Triassic strandline in close proximity to the Tendoy Range.
The Dinwoody Formation was deposited under shall, clear, well-circulated nearshore marine conditions with low to moderate wave and current action. Sedimentary structures indicate emergent flow, intermittent subaerial exposure, and variable current velocities.
Silty, algal-laminated, dolomitic limestone and interbedded siltstone, sandstone, and minor gastropod-pelletal packstone characterize the Woodside Formation. The fenestral algal laminae are similar to the LLH-S and LLH-C microlaminae described by Logan et al. (1964). The abundance of planar algal laminae indicate deposition within an intertidal zone along a coastline protected from wave action.
Petrographic examination of the fossiliferous packstone, grainstone, and wackestone of the Thaynes Formation indicate these rocks have undergone extensive diagenesis in an active freshwater phreatic environment Deposition took place in a shallow, subtidal to intertidal normal marine setting with moderate wave action. The presence of local erosion surfaces and solution cavities suggest the alternation of a protected lagoonal setting with shoaling conditions.
All Lower Triassic strata contain varying amounts (5-50%) of angular to subangular silt-sized quartz and muscovite which may have been delivered by eolian process from an and red bed terrane immediately inland.
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