Limitations in Paleocurrent Analysis of Trough Cross-Stratification: ABSTRACT

Abstract

More than 50% of cross-stratification is trough type. Its complex geometry accounts for large dispersion in End_Page 844------------------------------ many paleocurrent data. In simple cases, trough-sets yield bimodal histograms bisected by trough axes (which must be distinguished from other bimodal causes). Asymmetry of troughs or predominance of readings from trough ends produce more complex histograms, which commonly are statistically random (e.g., St. Peter Sandstone; certain Cambrian sandstones, Wisconsin; Meridian Sandstone, Mississippi). Trough-axis plunge azimuths provide a superior paleocurrent indicator; Hamblin's Franconian data and new data from Wisconsin show dispersions half as great as published results for cross-sets only. But oppositely plunging troughs associated in single outcrops and even doubly plunging single troughs discovered in Wisconsin may becloud trough-axis distributions. Double plunges probably reflect oth oscillatory (wave?) flow and unidirectional current flow, which produced complex, coalescing, elongate dune forms between which doubly-plunging troughs formed. Trough cross-stratification has no environmental significance. Long-standing eolian interpretations reflect early recognition of only 1 possible modern analogue, whereas subaqueous dunes with amplitudes up to 65 ft have been known for a century. Cambrian sandstones with complex trough cross-stratification probably reflect submarine dunes affected both by current and oscillatory flow like those of Georges Banks. Paleoslope has little if any influence on orientations of eolian and most shallow-marine cross-stratification; records of rare storms may mask average conditions. Therefore, independent dispersal indicators (e.g., pebble or mineral trains) should be sought. End_of_Article - Last_Page 845------------