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Batt, Liselle1
(1) University of Idaho, Moscow, ID

ABSTRACT: Preliminary High Resolution Sequence Stratigraphy of upper Mississippian (Chesterian) Age Mixed Siliciclastic and Carbonate Facies from South-central Idaho to Southwest Montana

Upper Mississippian (Chesterian) age strata in south-central Idaho and southwest Montana provide a rich record of mixed carbonate and siliciclastic deposition in the western United States during a time of global paleoclimatic and regional tectonic change. Deposition occurred during the latter stage regression of the Kaskaskia first order sequence and may record higher order signatures related to tectonic subsidence and glacio-eustacy. To decipher the sequence stratigraphy of these Chesterian units, we integrated lithostratigraphic, chemostratigraphic and biostratigraphic data obtained from nine measured sections (30m to >500 m thick) across south-central Idaho and southwestern Montana. Our preliminary detailed cross-section forms a regional transect perpendicular to the paleoshoreline of the westward facing foreland basin. This cross section provides insights concerning the regional paleogeography and its controls on the observed cyclicity (i.e. sedimentation rate, sediment loading, lithospheric flexure). Recognizable updip (eastward) stratigraphic trends include a decrease in stratigraphic thicknesses, and coeval increases in siliciclastic content, non-marine facies and sub aerial exposure surfaces. Overall, the cross section depicts an early Chesterian wedge of thick, eastwardly pinching shelf carbonate successions that transitions during latest Chesterian into thinner cyclic intervals of prograding mixed siliciclastic and shallow water carbonates. A tentative relative sea level curve accompanies the cross section. Forthcoming biostratigraphic and chemostratigraphic data from this study will permit refinement of this preliminary curve and lead to improved understanding of the timing of initial icehouse conditions during the Late Chesterian.

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