The Chocolay Group, Lake Superior region, U.S.A.: Sedimentologic evidence for deposition in basinal and platform settings on an early Proterozoic craton

The early Proterozoic Chocolay Group in the Lake Superior region is a cratonal sequence consisting of a basal unit overlain successively by quartzite, dolomite, and slate formations. Two regional depositional environments are described: (1) present-day structural troughs (Marquette, Menominee, and Felch, Michigan) show evidence of having sedimentary-basin precursors; and (2) other areas (Gogebic Range, Michigan) show evidence of sedimentation in platform settings. The basal unit in the Marquette and Menominee troughs is as much as 160 m thick and was probably deposited by alluvial processes. In the Gogebic Range, the basal unit is less than 10 m thick and resembles lag deposits at the base of transgressive sequences. The overlying quartzite contains cross-beds that show two regional sedimentary-vector (paleocurrent) distributions: (1) unimodal low-variance distributions with sedimentary vectors oriented subparallel to local structural trough axes (Marquette, Menominee, and Felch troughs); and (2) polymodal distributions with sedimentary vectors showing greater dispersion about the mean (Gogebic Range). As indicated by many varieties of sedimentary structures (silicified evaporites, cryptalgal structures, intraclast breccias, and so on), the carbonates and interstratified siliciclastics of the Chocolay Group were deposited in subtidal to supratidal conditions. Taylor noted, in 1972, that certain members of the Chocolay Group carbonates thicken toward the center of the Marquette trough, with concomitant facies changes. The uppermost pelite unit of the Chocolay Group was probably deposited below normal wave base. It is concluded that the basins and platforms were formed on a craton undergoing rift-type tectonism.