Gutschick's (1961, 1972, 1983, 1987) detailed, high-quality maps and published descriptions show that bedding and the major faults in the Kentland quarry generally dip steeply to the north and west of the apex of the Kentland dome (Fig. 3). The major fault in the quarry, the Kentland Quarry Fault, is a folded, bedding-parallel, reverse fault between the St. Peter Sandstone and overlying Platteville Group (Fig. 3 and below). The Kentland Quarry Fault and surrounding strata are folded into a steeply northward-plunging (~58°) macroscopic-scale fold train, which controls the principal outcrop pattern of rocks in the quarry. The Shakopee Formation, the oldest unit exposed in the quarry, has been uplifted more than 600 m above its regionally consistent subsurface position (Fig. 2, 3; Gutschick (1972, 1987)).
The > 300 m of Lower-Middle Paleozoic stratigraphy exposed in and drilled near the Kentland quarry (Gutschick, 1972, 1983, 1987) can be subdivided into seven unconformity-bounded depositional sequences (Fig. 2) (Sloss 1963). The Lower Ordovician Shakopee Formation is the oldest unit exposed and represents the top of the Sauk sequence. The overlying Tippecanoe-1 sequence consists of the basal, transgressive St. Peter Sandstone, a medium- to fine-grained, well-sorted, quartz sandstone containing the detrital apatite grains used in our fission-track analysis, and the overlying Middle Ordovician Platteville Group carbonates. The age of the St. Peter Sandstone is known from regional stratigraphic considerations to be approximately 460 Ma. Platteville conodonts in the quarry yielded a Conodont Alteration Index (CAI) of 1.5, indicating that these rocks were heated to < 90°C (Votaw 1980). The Quimby's Mill samples used for the paleomagnetic study of Jackson and Van der Voo (1986) were taken from near the top of the Platteville Group. The overlying Tippecanoe-2 sequence consists of the Late Middle to Late Ordovician Galena and Maquoketa Groups. The youngest strata exposed in the quarry are limestone, dolostone, and shale of the Lower to Middle Silurian Sexton Creek and Salamonie Formations, which constitute the Tippecanoe-3 depositional sequence.
Late Devonian shale occurs as rare fault breccia fragments in the quarry, and Mississippian limestone and shale were mapped in the subsurface surrounding the quarry (Fig. 2, 3; Gutschick 1987); these two units comprise the Kaskaskia sequence. Pennsylvanian channel sandstones of the Absaroka sequence cut into older strata outside the quarry, and 0-25 m of Pleistocene glacial drift unconformably cap the Paleozoic strata throughout the region (Fig. 2, 3; Gutschick (1983, 1987)).
Was this article helpful?