Stratigraphy and depositional setting of the Carrington shale facies (Mississippian) of the Williston Basin

The Carrington shale facies is a radioactive illitic lagoonal shale, apparently deposited behind lime mudstone banks (Waulsortian bioherms), along the eastern margin of the Williston basin during Early Mississippian time. The Scallion subinterval, which is the basal subunit of the Bottineau interval of the Madison Formation in North Dakota, is divisible into six lithologic facies, one of which is the Carrington shale facies. West of the shale facies, on the basin-shelf hinge line, is the lime mudstone facies. Basinward of this facies is the interbedded shale-limestone facies. Stratigraphically above the latter two facies is the sand-silt-shale facies. Overlying a portion of this facies and the Carrington shale facies is the crinoid packstone-grainstone facies. Overlying the remaining portion of the sand-silt-shale facies is the wacke-packstone facies. Following a period of erosion during the latest Devonianearliest Mississippian time, Waulsortian mounds formed along the basinshelf hinge line. These mounds and associated sediments (lime mudstone facies) created a barrier which allowed the deposition of the Carrington shale facies in a restricted environment. The source of the shale was the weathered Precambrian shield to the east. The sand-silt-shale facies was deposited on the basin slope during the deposition of the Carrington shale facies. After the deposition of these two elastic facies, carbonate deposition occurred on the basin shelf and slope in the form of the crinoid packstone-grainstone and wacke-packstone facies respectively. Possible petroleum traps exist where; the Carrington shale facies overlies erosionally truncated Devonian strata, abrupt facies changes occur within the Scallion subinterval, and along the pre Mesozoic subcrop of the Scallion. Conditions necessary for the concentration of uranium beneath or within the Carringtington shale facies may have occurred after the deposition of the shale facies