Archuleta, Bonny J.¹, Michael C. Pope¹, Matthew L. Tremblay²
(1) Washington State University, Pullman, WA
(2) ExxonMobil Exploration Company, Houston, TX

ABSTRACT: Upper Pennsylvanian Cyclothems in East-Central Idaho: A Record of Relative Sea-Level Fluctuations on the Western Margin of Laurentia

Mixed carbonate and siliciclastic cyclothems occur in the Upper Pennsylvanian (Desmoinesian) Juniper Gulch Member of the Snaky Canyon Formation in the Lost River, Lemhi, and Beaverhead Ranges of east-central Idaho. Three stratigraphic sections (400-800 m thick) were measured in detail to determine the record of glacially induced sea-level fluctuations along the shallow water carbonate ramp on the western margin of Laurentia during the Late Pennsylvanian. Each stratigraphic section is generally composed of three distinct units. The lower unit consists of cyclic deep subtidal cherty carbonate and nearshore/eolian sandstone. The middle unit is dominated by Palaeoaplysina mounds, and the upper unit consists of slightly deeper subtidal carbonate facies. The easternmost section is capped by a fourth unit of shallow water evaporite facies. Future conodont and fusulinid biostratigraphy will help determine the frequency of cycles in this unit. Long-term changes in accommodation space are being determined by correlating facies stacking patterns within the measured sections. General facies stacking patterns reveal several smaller relative sea level fluctuations superimposed on an overall increase in relative sea level through unit three, possibly followed by a long-term relative sea level fall into the Permian. The lower cyclic unit suggests deposition during high-amplitude sea-level fluctuations. Similar fluctuations are not evident in the upper units due to either a decrease in the amplitude of sea level fluctuations, or the overall increase of relative water depth that produced downdip deposition of subtidal units.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004