Petrographic Evidence for the Recycling of Late Proterozoic - Early Paleozoic Quartzose Arenites, Southeastern Minnesota - Southwestern Wisconsin

Abstract

The Late Proterozoic - Early Paleozoic arenites of southeastern Minnesota - southwestern Wisconsin are all mature, quartzose arenites. The Late Proterozoic Hinckley Sandstone was deposited as a rift fill sandstone. With the transgression of the Sauk Sea during Cambrian time, the continent was flooded. In southeastern Minnesota, the sea entered the Hollandale embayment. During this time of transgression and regressions, the quartzose Cambrian Mt. Simon Sandstone, Eau Claire Formation, Galesville Sandstone, Ironton Sandstone, Franconia Formation, Jordan Sandstone, and the New Richmond Sandstone of the Shakopee Formation were deposited. Following a major regression at the end of Shakopee time, the sea reentered the continent. The Tippecanoe transgression was responsible for the widespread deposition of the St. Peter Sandstone during Middle Ordovician time. All of the arenites of this study are quartzose in nature and highly mature. Almost 95 percent of all the framework grains consist of monocrystalline quartz; potassium feldspar and plagioclase are minor components. Two units, the Eau Claire Formation and the Franconia Formation, contain significantly higher amounts of feldspar, generally 11 to 15 percent. These two quartzose feldspathic units are in general finer grained than the other more quartzose units, an observation noted earlier by other workers. Other detrital minerals include polycrystalline quartz and multicycle monocrystalline quartz. A decrease in the amount of polycrystalline quartz stratigraphically upward in the column suggests that this unstable mineral portion was eliminated by abrasion. Multicycle quartz grains with abraded quartz overgrowths are present in all nine of the units. The presence of an abraded quartz overgrowth indicates that the grain was once previously cemented by quartz, and that through erosional processes, the grain was removed from its original environment of deposition and incorporated into a stratigraphically younger unit. The grain, with the original overgrowth intact, is then recemented with quartz, resulting in a euhedral overgrowth enclosing the abraded one. The amount of unaltered and altered feldspar types can also be used to support the recycling of the quartzose arenites. Peaks of unaltered feldspar occur during Eau Claire and Franconia time, indicating that influxes of fresh sediment may have occurred during these times. Further evidence of the recycling of the quartzose arenites in relation to the feldspar concentrations can be found in the Ironton Sandstone, in which unaltered feldspar of Eau Claire time was reworked and recycled to the point that during Ironton time, the amount of altered feldspar surpassed the amount of unaltered feldspar. A high mineralogical maturity in the arenites is also evidenced by the heavy mineral concentrations. All of the units contain similar heavy mineral concentrations, consisting mainly of rounded zircon, tourmaline, rutile, and garnet. As further evidence of the recycling of the arenites, all nine units also contain tourmaline grains with abraded overgrowths. Abraded tourmaline overgrowths form in the same manner that quartz overgrowths do, and are indicative of recycling. In summary, it has long been speculated that the Late Proterozoic - Early Paleozoic quartz arenites are multicycle in origin because of the presence of clean quartz sand (e.g. Matsch and Ojakangas, 1982, p. 65; Datt and Batten, 1982, p. 283; Datt and Prothero, 1994, p. 236). This study now provides petrographic evidence that erosional processes during the Late Cambrian - Early Ordovician marine transgressions and regressions played a substantial role in the creation of these quartz arenites, thereby recycling grains from older exposed units within the Hollandale embayment.