DEPOSITIONAL TRENDS OF THE PRAIRIE DU CHIEN GROUP (LOWER ORDOVICIAN) ALONG THE WESTERN EDGE OF THE TWIN CITIES BASIN, MINNESOTA: POSSIBLE EVIDENCE OF SYN-TO-POST-DEPOSITIONAL BASIN FORMATION

Preliminary mapping of Paleozoic bedrock in Hennepin County, MN, as part of a County Geologic Atlas program, reveals a possible connection between depositional trends in the Prairie du Chien Group (Lower Ordovician) and reactivated Mesoproterozoic Midcontinent Rift System (MRS) faults associated with the development of the Twin Cities basin. Structural elevation contours of the stratigraphic top of the Upper Cambrian Jordan and Wonewoc Sandstones disclose several NE-SW-trending faults in western Hennepin County. Aeromagnetic and gravity data show these Paleozoic faults directly overlie preexisting MRS faults. Structural maps also highlight a regional structural low, known as the Twin Cities basin (TCB), just east of these faults and centered near southeastern Hennepin County.

Facies of the Prairie du Chien change from the TCB center towards the MRS-fault-bounded western edge. A westward increase in sand content likely reflects progressively shallower water deposition in that direction. This trend parallels a westward thinning of the Prairie du Chien, from ~145 feet to absent, where the Middle Ordovician St. Peter Sandstone unconformably overlies the Jordan Sandstone. Drill cuttings indicate the unconformity is overlain by an apparent lag of variably thick, iron-cemented sandstone with possible Prairie du Chien intraclasts. This may indicate syn-to-post-depositional subaerial exposure and weathering of the Prairie du Chien prior to St. Peter deposition, signifying TCB development during the Early Ordovician.

Changes in thickness of the Prairie du Chien have also been noted in Washington County, MN, and into western Wisconsin overlying a preexisting MRS horst, suggesting syndepositional development. Previous studies proposed the TCB 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 depositional trends of the Prairie du Chien Group across Hennepin County may also be linked to these orogenic far-field stresses.