EVIDENCE FOR EARLY ORDOVICIAN REACTIVATION OF MESOPROTEROZOIC RIFT STRUCTURES WITHIN THE PRAIRIE DU CHIEN GROUP: WASHINGTON COUNTY, MINNESOTA

Recent mapping of Paleozoic bedrock in Washington County, MN, as part of a County Geologic Atlas program, better constrains the timing and magnitude of reactivated Mesoproterozoic Midcontinent Rift System (MRS) faults, possibly as part of a regional-scale change in structural configurations that impacted Ordovician depositional patterns. Structure in the Paleozoic bedrock was mapped using the stratigraphic top of the Jordan Sandstone (Upper Cambrian). The structural top of the Jordan was constructed from drill hole and outcrop data. The most prominent feature is a structural high in southern Washington County accommodated along faults with displacements from 25 to 300 feet. Aeromagnetic and gravity data show that this feature directly overlies a thick sequence of uplifted MRS basalts known as the Hudson-Afton Horst. Digital elevation models and isopachs of Paleozoic units reveal significant thickness changes across the horst within the carbonate formations of the Early Ordovician Prairie du Chien Group, suggesting syndepositional reactivation of faults. The Prairie du Chien is much thinner atop the horst (<50 ft) relative to the grabens on either side (~150-250 ft). Much of the thinning is accounted for within the Shakopee Formation (upper Prairie du Chien), as it is nearly absent in areas atop the horst and thickens to over a hundred feet along either side. Folded and brecciated beds in outcrops of the Shakopee are another probable indicator of syndepositional reactivation. Reactivation of the Hudson-Afton Horst may be part of the same structural event triggering the earliest development of the nearby Twin Cities basin (TCB) to the west. The TCB has previously been suggested to have formed via isostatic and/or thermal adjustments accommodated by MRS structures during Middle to at least Late Ordovician time. Several other midcontinent basins and highs (i.e., Michigan Basin, Wisconsin Arch, Hollandale Embayment, etc.) were active roughly contemporaneously and possibly in response to far-field stresses generated during orogenic activity along present-day eastern North America. The reactivation of the horst structure and/or the development of the TCB may also be an early manifestation of these orogenic far-field stresses.