Late Ordovician Paleogeography and Carbonate Ramp Development in Central Idaho

E. A. Measures

Development of a relatively narrow carbonate ramp containing an intra-ramp basin indicates significant differences between the Late Ordovician paleogeography of central Idaho and Nevada. Measured sections of Upper Ordovician carbonates (Saturday Mountain Formation/Fish Haven Dolomite) located along a transect perpendicular to depositional strike in central Idaho indicate that the ramp was approximately 100 km wide, in contrast with a shelf approximately 400 km wide reported in Nevada. The carbonate ramp lithofacies in Idaho are cyclical and grade from inner platform bioturbated dolomud-dolowackestones to middle-platform bioturbated and from storm laminated dolowacke-dolopackstones to outer platform bioturbated dolomudstones and laminated argillaceous dolomudstones with tr nsported bioclastic debris. A ramp profile was apparently maintained throughout the platform's evolution inasmuch as facies contain no resedimented slope or margin deposits associated with distally steepened ramps or rimmed shelves, although poorly developed bioclastic beds may indicate initiation of distal steepening. The inner platform sequence shallows upward, indicating sedimentation equal to or greater than subsidence or sea level rise. The middle platform contains the thickest stratigraphic section reflecting downwarping of this area to form an intra-ramp basin. Middle platform sedimentation was approximately equal to subsidence or sea level rise in order to maintain the ramp profile. The outer platform sequence deepens upward indicating that sedimentation was exceeded by subsidence or sea level rise. A complex interplay of subsidence, carbonate deposition lag time, and relative sea level fluctuations were responsible for the overall geometry and cyclical carbonates within the ramp.

AAPG Search and Discovery Article #91040©1987 AAPG Rocky Mountain Section Meeting, Boise, Idaho, September 13-16, 1987.