Normally and Reversely Graded Beds Within Large-Scale Foresets of Oolitic Lake-Margin Bench Sequence--Shoofly Oolite of Southwestern Idaho: ABSTRACT

Abstract

Pliocene lacustrine oolites composing the lower part of the Glenns Ferry Formation crop out in a 40-km northwest-southeast-trending belt along the southwestern margin of the Snake River plain. Near Oreana, Idaho, where this limestone reaches 40 m in thickness, the oolite occurs as three progradational sequences, each consisting of thinly coated ooids in foresets beds up to 20 m thick; these beds are abruptly overlain by thickly coated, massive, burrowed oolite. Foresets beds, each 5 to 15 cm thick, dip basinward to the northeast at 30°; they exhibit both coarsening- and fining-upward trends. Reversed grading (coarsening upward) occurs high in each foreset unit, but the beds become normally graded (fining upward) near the base. The Shoofly Oolite was deposited as three progradational bench sequences which built lakeward during short periods of stillstand in a longer transgressive phase of Lake Idaho. As such, each bench sequence is analogous to a Gilbert delta which extended laterally along the lake margin, but was fed by littoral sands which became coated during transport on the bench platform. Deposition of foreset beds by grain flow on the upper parts of the bench slope, and by fluidized sediment flow on the lower parts of the bench slope, resulted in the formation of reversely graded beds near the bench platform and normally graded beds near the base of the slope. During periods of rising lake level, but prior to the deposition of a subsequent bench sequence, abandoned bench platforms were extensively urrowed and winnowed by waves. As a result, deposition of cortical laminae on platform ooids continued, and the massive oolites which now cap each progradational bench sequence were formed. End_of_Article - Last_Page 536------------