ABSTRACT

A combination of Fourier grain-shape analysis and scanning electron microscopy was employed in a sedimentological study of 317 samples of the St. Peter Sandstone from 3 thick sections of the formation in southeastern Minnesota. The quartz 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 nvironments with little or no residence time in subaerial environments. As a result, the latter grain-roundness type still retains clearly defined "relict overgrowths" on their surfaces, features inherited from the source of the St. Peter.

The relative proportions of these two grain-roundness types vary in a nonrandom manner through the sections, defining alternating units of abraded sand-rich and unabraded sand-rich beds. This alternation, found in beds deposited in a shallow-marine environment, probably reflects alternation in the immediate local source of the marine sand sheet--the first source being coastal dunes and the second, fluvial-deltaic deposits. 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.