--> --> Sequence Stratigraphy of Lower Madison Strata in the Greater Williston Basin Area

AAPG Rocky Mountain Section Meeting

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Sequence Stratigraphy of Lower Madison Strata in the Greater Williston Basin Area

Abstract

The Madison 2nd-order sequence is a late Devonian to early Carboniferous sequence that was originally defined in Wyoming outcrops, where it is bounded by major, regional, subaerial unconformities. The basal unconformity beveled the Williston basin area during Famennian exposure forming a low-relief surface. Following unconformity development, a slow transgression occurred, and this led to shallow-water deposition of Bakken and Bakken-equivalent sediments in partially connected basins. In the greater Williston basin area, a 2nd-order maximum flooding surface separates underlying Bakken and equivalent stratal units (Englewood, Cottonwood Canyon) that were deposited in separate basins, from overlying Lodgepole stratal units that represent sediments deposited after maximum flooding. The 2nd-order transgression corresponded with a rapid, 100-meter relative sea-level rise that inundated western North America with open-marine Lodgepole deposition. Regionally, the maximum flooding surface may overlie: 1) a stylolitic contact, 2) a transgressive lag deposit, or 3) a heterogeneous, glauconitic, skeletal-intraclastic grainstone, packstone and wackestone unit. In western basin-flank outcrops (Little Rocky Mountains, Big Snowy Mountains and Little Belt Mountains), the maximum flooding surface is overlain by dark, thinly bedded, argillaceous, deep-water mudstone and skeletal mudstone. Carbonate bioherms grew locally in the sediments overlying the maximum flooding surface. The dark mudstone transitions upward and landward (Bighorn Mountains, Black Hills) into skeletal wackestone. These deposits are overlain by five cycles that are defined in landward areas by vertical transitions from a skeletal-oolitic grainstone to a peloidal mudstone. The cycles parallel biozones, marker beds and sequence boundaries. Cycles are progradational into the Williston basin and aggradational in central Montana. In the Williston basin, the basal skeletal-oolitic unit (Scallion) transitions seaward to a crinoid-dominated packstone and wackestone facies that transitions further seaward to a clinoform-toe skeletal wackestone and mudstone-dominated facies that downlaps on the maximum flooding surface.