Grain-shape variation in the St. Peter Sandstone; a record of eolian and fluvial sedimentation of an early Paleozoic cratonic sheet sand

Fourier grain-shape analysis and scanning electron microscopy was employed in a sedimentological study of the St. Peter Sandstone from southeastern Minnesota. Grain-shape analysis indicated that two grain-roundness types are mixed in various proportions in all the samples studied: the first type is a population of extremely smooth and well-rounded grains, whereas the second is a population of irregular, angular grains. Scanning electron microscope examination of quartz surface textures revealed that the first grain-roundness type was created during a stage of eolian transport and abrasion, whereas the second grain-roundness type is relatively unabraded and was probably transported in less abrasive fluvial environments. As a result, the latter grain-roundness type still retains "relict overgrowths" on grain surfaces from the source of the St. Peter. The relative proportions of these two grain-roundness types define alternating units of abraded sand-rich and unabraded sand-rich beds. Upon consideration of the thickness of these beds, and of the formation as a whole, it is proposed that the bulk of the sediment in the St. Peter was transported from coastal eolian and fluvial-deltaic sources into the deeper waters of the inner shelf by a combination of shoreface and river-mouth bypassing mechanisms.--Modified journal abstract.