A COMPARISON OF DIP ANGLES FROM ANCIENT CROSS-BEDDED SANDSTONES AND MODERN EOLIAN DUNES

Cross-bed dip inclinations of ancient cross-bedded sandstones are often reported as “steep” in the literature, especially if the cross-bed sets are thick, or if the sandstone is presumed to be eolian. Only rarely are data referenced in making this assessment. This preliminary study examines published data from ancient sandstones and compares them with published measurements from modern eolian cross-beds. The goal was to see what differences existed between the two sets of data.

Published dip angles were collected from 13 sandstones (most interpreted as eolian) from 88 localities consisting of 5,242 measurements (Bigarella and Salamuni 1961, Fryberger et al. 2016, Kiersch 1950, Maithel 2019, McKee 1940, McKee 1982 and Reiche 1938). Sandstones included the Botucatú (Brazil); Casper and Tensleep (Wyoming); Cedar Mesa and Navajo (Utah); Coconino, De Chelly, Esplanade, Manakacha, Wescogame, and Tapeats (Arizona); Tacuarembó (Uruguay); and the Wingate (New Mexico). Additionally, 5,785 dip measurements were collected from 76 eolian localities (Bigarella 1972, Bigarella et al. 1969, McKee 1966, McKee and Bigarella 1979). Sets of data ranged from 6 to 676 measurements (mean=67, mode=30, median=34). In all cases, it was assumed authors were reporting representative examples of inclinations without bias. Data were analyzed and plotted with Excel and Grapher.

Mean dips from sandstones, eolian dunes, and weighted means of eolian dunes are similar, with dips of 19.8°, 17.8°, and 19.8°, respectively. The main difference occurred in the spread of the data. Ancient sandstones have a standard deviation of 5.7, while those from modern eolian deposits have a value of 10.1. The data show that the middle two quartiles of sandstone dips cluster between 15.5 - 24.0° and often lack values >30°, while the middle two quartiles of modern eolian dips are more widespread, clustering between 9.0 – 27.0°, with many values >30°. Sandstones that are often considered to have “steep” cross-beds have central tendencies that were not very different from non-eolian sandstones. A surprise was that the set of data from the Coconino, Tapeats, and Wescogame (of the Grand Canyon area) could not be statistically distinguished from one another, despite having different cross-bedding styles, set thicknesses, and paleoenvironmental interpretations.