PROVENANCE ANALYSIS OF THE PALEOGENE MOUNTAIN MEADOWS FORMATION, BLACK HILLS SOUTH DAKOTA

The Black Hills of South Dakota are the easternmost basin-cored uplift in the Laramide foreland. Uplift occurred during the Latest Cretaceous-Eocene and was coeval with the emplacement of suite of alkaline and peralkaline hypabyssal intrusions. These intrusions were emplaced into Proterozoic metasedimentary rocks and Phanerozoic cratonic strata. Laramide exhumation stripped the Black Hills down to the Precambrian core. The Oligocene? Mountain Meadows Formation is a post orogenic gravel that consists of locally-derived clasts and rests at high elevations indicating that the Black Hills was at least partially buried following the Laramide Orogeny. Here we present detrital zircon data (LA-ICPMS at the Arizona Laserchron Center) for the matrix of the Mountain Meadows Formation near Deadwood, South Dakota (z=143). Here the Mountain Meadows is a crudely stratified, 50 m succession of cobble-boulder gravel that rests on early Paleozoic strata. About 2/3 of the zircons are Paleogene (41-63 Ma) in age, with no distinct peaks evident. These were likely derived locally from the Paleogene intrusive rocks. The balance of the zircon age spectrum is dominated by Precambrian grains, again with no prominent age peaks evident. The Mesoproterozoic grains were likely sourced locally from the underlying upper Cambrian Deadwood Formation. The Archean grains could be derived from the Deadwood as well, or perhaps they were derived from Paleoproterozoic basement rocks. Thus, the Mountain Meadows Formation here appears to be all locally derived, with transport distances of less than 10 km likely. This contrasts with coeval strata that cap the Bighorn uplift to the west, and that occur in the adjacent Great Plains and Powder River Basin, each of which have distal, westerly sediment source areas.