EARLY CENOZOIC EROSION RATES OF THE BLACK HILLS, SOUTH DAKOTA

The Laramide uplift of the Black Hills area commenced at ~64 Ma and the landscape was eroded to approximately the present topography by ~37 Ma. The average denudation rate for the core of the uplift during the early Cenozoic Era was determined by estimating the thickness of rocks eroded over this time interval. In the Central Black Hills, where the Proterozoic basement is now exposed, ~2,463 m of rocks were eroded, including ~2,286 m of Paleozoic and Mesozoic sedimentary strata and ~177 m of Precambrian crystalline rock. The interval of erosion, beginning in the early Paleocene Epoch and lasting until the late Eocene Epoch, was approximately 27 Ma. Thus, assuming a constant rate of uplift and erosion, early Cenozoic denudation averaged ~0.0912 m/ka, a rate similar to that determined by Lisenbee and DeWitt (1993), based primarily on stratigraphic relationships within sedimentary strata of the adjoining Powder River Basin. The early Cenozoic denudation rate of the Black Hills is similar to the modern (albeit pre-dam) Mississippi River denudation rate, estimated at 0.096 m/ka.

An alternative exhumation history is possible if the rate of erosion as a function of lithology is considered. For the Black Hills uplift three lithologic units, Mesozoic shale and sandstone (MZ), Paleozoic carbonate, sandstone and minor shale (MZ), and Precambrian metamorphic and igneous rock (P€) are utilized. The relative erosion rates of the three units, estimated from 1) rock strength, 2) weathering characteristics and, 3) the topographic relief of Black Hills drainage basins having lithologies similar to the three units, are interpreted to be (MZ: PZ: P€) 1 : 0.311 : 0.253. Accordingly, using a piece-wise function, the denudation chronology would be:

--from 64 to 51.41 Ma, erosion of ~1,836 m of MZ unit at 0.145 m/ka,

--from 51.41 to 41.80 Ma, erosion of ~450 m of PZ unit at 0.0417 m/ka,

--from 41.80 to 37 Ma, erosion of 177 m of P€ rock at 0.037 m/ka.