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Pennsylvanian Cycles in East-Central Idaho: A Record of Sea-Level Fluctuations on the Western Margin of Laurentia
The Pennsylvanian Snaky Canyon Formation, exposed in the Lost River, Lemhi, and Beaverhead Mountains of east-central Idaho, is comprised predominantly of interbedded subtidal carbonates and nearshore to eolian siliciclastics that developed offshore of the Quadrant sandstone erg of the northern Rocky Mountains. Four stratigraphic sections (400 – 800 m thick) spanning a portion of the Snaky Canyon Formation (Morrowan – Desmoinesian stage) consist of four distinct units. Unit 1 consists of cyclic subtidal cherty carbonate and shoreline to eolian sandstone and offshore Paleoaplysina and algal boundstone. Unit 2 primarily consists of deep subtidal cherty carbonate mudstone with minor amounts of subtidal siliciclastics. Paleoaplysina and algal boundstone and restricted subtidal wackestone are the main constituents of Unit 3, and they interfinger with Unit 4, a shallow water evaporite-rich facies occurring only in the most landward section. Units 1 through 4 represent a 2nd-order depositional megasequence recording one 10–15 million year sea level fluctuation, an overall increase in relative sea level through Unit 2, followed by a long-term sea level fall through Units 3 and 4.
Facies stacking patterns within the 2nd-order megasequence are indicative of high-frequency cycles produced by changes in sea level superimposed on the longer-term megasequence. Facies stacking patterns in Unit 1 indicate deposition during high-amplitude sea-level fluctuations. The mixed eolian siliciclastic and subtidal carbonate cycles within Unit 1 are similar to Pennsylvanian cyclothems documented throughout the western United States. Units 2 and 3 cycles are dominated by subtidal carbonates whose facies stacking patterns appear to reflect a decrease in the amplitude of sea level fluctuations. However, a decrease in the siliciclastic sediment supply to the subtidal carbonate ramp may also explain the differences in facies stacking patterns between these units and may be unrelated to changes in relative or eustatic sea level fluctuations.
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