A Vertical Pattern of Variation in the St. Peter Sandstone--Fourier Grain Shape Analysis

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ABSTRACT Fourier grain shape analysis of quartz sand grains from a vertical section of the St. Peter Sandstone at St. Paul, Minn., has detected a vertical pattern of non-random variation in the relative proportions of 2 shape families. The shape families consist of mature, highly abraded sand, and irregularly shaped, immature, sand respectively. Changes in the proportion of mature and irregular sands define large-scale (7, 19 foot) zones characterized by a decrease upwards in irregular sand content, and within each zone smaller scaled units of interbedded mature and irregular sands which form cycles approximately 1 foot thick. In light of studies of Recent shelf environments (Brown and Ehrlich, 1980) the large-scale zones are interpreted as a record of individual transgressive phases with a resultant movement of the littoral energy fence (and concomittantly the shoreline) away from the area. Two and, perhaps, three such zones occur in the uppermost 30 feet studied. If their repetition is due to regressive pulses, this corroborates Sloss' (1963) observations that the middle Ordovician transgression was interrupted by numerous partial regressions. The small-scale cycles within each zone are interpreted to record the breaching of the littoral energy fence; each thin layer of irregular sand is likely storm-generated. Detection of such a detailed record in an otherwise homogeneous, featureless orthoquartzite ind cates that the Fourier grain shape technique, applied on a regional scale to the St. Peter Sandstone, may reveal a clearer record of its accumulation.