EARLY CENOZOIC DENUDATION OF THE BLACK HILLS

The Black Hills was formed during the Laramide Orogeny. Uplift and erosion commenced at ~64 Ma and the landscape was eroded down to nearly the present topography by ~37 Ma. The denudation rate of the Black Hills during the early Cenozoic Era was determined by estimating the thickness of eroded rocks over this time interval. In the region of the Central Black Hills, ~2,463 m of rocks were eroded, including ~2,286 m of sedimentary strata and ~177 m of Precambrian rocks. The erosion started during the late Paleocene Epoch and lasted until the late Eocene Epoch, an interval of approximately 27 Ma. Thus the early Cenozoic denudation rate was ~0.0912 m/ka. This rate is nearly the same as the erosion rate for the early Laramide uplift determined by Lisenbee and DeWitt (1993), based primarily on stratigraphic relationships within sedimentary strata of the the Powder River Basin.

A denudation model of the Black Hills uplift uses three units: MZ: ~1,836 m of Triassic to Paleocene shale and sandstone, PZ: ~450 m of Paleozoic carbonates, and P€: ~177 m of Precambrian metamorphic and igneous rocks. The relative erosion rates of the three units was estimated from the topographic relief of Black Hills drainage basins having lithologies similar to the three units, as well as rock strength and weathering characteristics. The three units have relative erosion rates of 1 : 0.311 : 0.253. Accordingly, the denudation chronology would be:

----- from 64 to 51.41 Ma, erosion of MZ,

----- from 51.41 to 41.80 Ma, erosion of PZ,

----- from 41.80 to 37 Ma, erosion of P€.

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.