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Tectonic History of the Black Hills Uplift
Synchronous development of the intra-cratonic Powder River Basin and Black Hills uplift was initiated in the Paleocene as Laramide crustal stresses affected the coastal plain region of a retreating foredeep basin. Sediments stripped from the growing uplift helped fill the adjoining Powder River Basin through the Eocene, and were probably also transported northeast to the Williston Basin.
The Black Hills uplift, which joins the Chadron Arch on the south and the Miles City Arch on the northwest, has the gross character of a broad, doubly-plunging anticline with a maximum structural relief of 9000 ft. In detail, however, it consists of two north-trending structural blocks, the mutual boundary of which marks a zone of recurrent movement, at least in the Phanerozoic. A proterozoic core is exposed in the eastern block due to greater structural relief. On the west, abrupt (up to 5000 ft of structural relief), westerly facing monoclines separate gently west-dipping strata of the basin and uplift for a strike length of 150 mi. They are probably underlain by reverse faults. In contrast, the broadly arched eastern margin contains local, superimposed monoclines with 200 ft or less of structural relief. Three large, west-facing anticlines plunge from both the north and south ends of the eastern block.
A 20 Ma episode of alkalic magmatism associated with the Laramide orogenic event formed dikes, sills, stocks, laccolithic domes, diatremes and ring complexes; composite intrusions formed laccolithic clusters or doming of the basement. These occur in a westerly trending band which crosses the uplift, possibly along a deep basement fault which offsets the boundary of the Archean and Proterozoic provinces.
Oligocene strata of the White River Group covered most of the eroded roots of the uplift as well as the basin. Epeirogenic uplift in the Oligocene, and probably in the Miocene, and Pliocene, resulted in removal of most of these strata.
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