--> --> Assessing Hypotheses for the Origin of the Giant Lower Niobrara “Channel” in the Southern Powder River Basin, Campbell and Converse Counties, Wyoming

AAPG Rocky Mountain Section Meeting

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Assessing Hypotheses for the Origin of the Giant Lower Niobrara “Channel” in the Southern Powder River Basin, Campbell and Converse Counties, Wyoming

Abstract

A large arcuate southerly trending erosional feature about 130 miles long, 5 to 20 miles wide expanding southward, and 100 to 200 ft deep was cut into the mid-Turonian Sage Breaks Shale (aka Carlile Shale) about 88.5 million years ago near the onset of Upper Turonian Niobrara deposition. This enigmatic feature has been recognized for decades, but its origin and mode of formation continue to be debated. Those who have placed it in a sequence stratigraphic framework invoked large-scale sea level fluctuations with erosional valley fills suggesting that the “channel” formed as a sedimentstarved subaerial fluvial channel cut down into the Sage Breaks Shale during a sea level lowstand. Others have suggested that the lower Niobrara thick fills a two-stage salt-solution feature formed during dissolution of underlying Permian Goose Egg evaporite deposits such that the Niobrara filled a major sag on the sea floor. Less well understood is the suggestion that massive deepwater current erosion scoured the “channel” into the Sage Breaks Shale with the basal Niobrara subsequently accumulating in this sea floor low. Here we assess these three hypothesis on the basis of regional 2-D and 3-D seismic data, hundreds of wireline logs, a few conventional cores, and petrographic study of one core cut across the precise contact between the Sage Breaks and Niobrara. The fluvial channel hypothesis requires that the contact be highly erosional likely with at least a lag of fluvial sands and gravels at the base, a period of subaerial exposure that could have oxidized pyrite to hematite in the Sage Breaks, and evidence for freshwater diagenesis. A recent key core cut in the “channel” reveals none of these features and instead exhibits a coccolithrich Niobrara Marl deposited directly on the marine mudstone of the Sage Breaks Shale with scattered planktonic foraminifers. A two-stage evaporate dissolution origin should be easily observed on regional seismic lines because the Goose Egg evaporites are well-defined marker horizons, but these markers offer no indication of extensive dissolution during Upper Turonian time; instead all Goose Egg evaporate dissolution predates the Lower Cretaceous Mowry Shale. The seismic, core, and petrographic data, however, do support the hypothesis that the Niobrara was deposited on a subaqueously formed unconformity at the top of the Sage Breaks Shale.