PALEOENVIRONMENT AND PALEOTECTONICS OF THE EARLY PROTEROZOIC HOMESTAKE BASIN, NORTHERN BLACK HILLS, SOUTH DAKOTA

A large block of early Proterozoic crust has been explored through extensive mine workings of the Homestake mine in the northern Black Hills. Detailed stratigraphic and geochemical studies of metavolcanic and metasedimentary rocks indicate a possible ancient backarc basin with abundant biologic activity. The history of this basin begins with an early phase of oceanic basaltic volcanism represented by metamorphosed tholeiitic basalt and volcaniclastics found at the base of the Poorman Formation. These volcanics are associated with tectonic thinning and initial basin development. Later, during a long period of basin quiescence, the remaining Poorman metasediments were deposited that include a complex succession of lithologies. These include dirty dolomite, banded carbonate-rich claystone and siltstone, marl, iron formation, carbonaceous pyrrhotite-bearing siliceous exhalite, and interbedded tuffs. Poorman metasediments are interpreted as chemical precipitates dominated by Ca and Mg carbonates admixed with fine-grained terrigenous detrital material that constitute hemipelagic basin fill followed by input from seafloor volcanic exhalative activity and minor volcanic ash. This depositional period was followed by a transition to Fe and Mg carbonate chemical precipitation and iron formation near the top of the Poorman Formation and into the Homestake Formation. Multiple horizons of carbonate facies iron formation interlayered with marl are abundant throughout the Homestake Formation. Finally, at the same time that tectonism was rejuvenated in the area, deep marine fans encroached into the basin that formed the superjacent Ellison Formation. The Ellison is a metaclastic sequence dominated by feldspathic litharenite with abundant shale, siltstone, and tuffaceous units. Abundant carbonaceous material in many metasediments and volcanic exhalites in this succession may indicate the activity of both phototrophic organisms in surface waters and chemotrophic ones associated with volcanic exhalative activity at the seafloor.