Sedimentary Geology, 56 (1988) 207-260 207 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands Pennsylvanian to Jurassic eolian transportation systems in the western United States FRED PETERSON u.s. Geological Survey, MS 939, Denoer, CO 80225-0046 (U.S.A.) (Received December 1, 1986; revised and acceptedFebruary 3, 1987) Abstract Peterson, F., 1988. Pennsylvanian to Jurassic eolian transportation systems in the western United States. In: G. Kocurek (Editor), Late Paleozoic and Mesozoic Eolian Deposits of the Western Interior of the United States. Sediment Geol., 56: 207-260. The direction of sediment transport in eolian sandstones of Permsylvanian to Jurassic age was interpreted from crossbedding resultants (vector means) obtained from studies of eolian rocks in the western U.S., supplemented by data from the few eolian units of eastern North America. These were compiled from the published or unpublished (theses) hterature, from unpublished fidd data contributed by colleagues, or from measurements made for this study. In addition, new paleogeographic maps were compiled to evaluate the influence of geographic features on the atmospheric circulation patterns that are inferred from the crossbedding studies. Regionally, the crossbedding indicates northeasterly, northerly, or northwesterly winds (present coordinates) from Pennsylvanian through most of Middle Jurassic time. A rather abrupt change in wind directions occurred in late Middle Jurassic time (late part of the CaUovian Age) when westerly wind patterns developed. By the Late Jurassic the winds shifted to southwesterly. Calculations of the consistency factor (vector mean strength) made from region-wide analyses of the resultants indicate fairly unidirectional winds from the Permsylvanian through the Early Jurassic. Middle Jurassic circulation was more varied, judging from crossbedding studies in the lower part of the Entrada Sandstone. Crossbedding in Upper Jurassic eolian rocks of Wyoming and South Dakota yielded a random pattern but Upper Jurassic rocks farther south on the Colorado Plateau and adjoining areas show a return to a fairly unidirectional pattern. Comparing the resultants with their reconstructed paleogeographic setting shows surprisingly little influence of major geographic features on overall circulation patterns. However, the greatest amount of local variation occurred at or near highly indented shorelines where the temperature contrast between land and water produces local wind currents that may vary appreciably from regional circulation patterns. Although they do not cause noticeable horizontal deflections in wind patterns, small and low topographic highs appear to be able to promote the development of a dune field if a source of sand is available and if streams do not enter the growing dune field. Introduction The abundant, widely distributed, and well- studied eolian sandstones of Pennsylvanian, Per- mian, Triassic and Jurassic age in the western U.S. (Figs. 1-3) offer an opportunity to reconstruct near-surface wind patterns over broad areas dur- ing fairly narrow time intervals. In addition, sedimentologic, paleogeographic, paleomagnetic 0037-0738/88/$03.50 © 1988 Elsevier Science Publishers B.V. and paleotectonic studies have evolved sufficiently to reconstruct the distribution of environments reasonably well and to determine the latitudinal position of North America during this span of time. Thus, the wealth of information available from studies of these rocks allows one to ap- proximately reconstruct ancient paleogeographies and regional wind patterns, to compare and con- trast these patterns with predictions from theoreti-208 ••,Primary Study Area CANADA Nova Scotia 0 Mi , ICO Fig. 1. Index map showing study areas. cal circulation models, and to postulate reasons for deviations. The present study is primarily a compilation of data obtained from sedimentologic and crossbed- ding studies of Pennsylvanian-Jurassic eolian sandstones in the Western Interior of the United States (Fig. 3) that were made by numerous workers, cited in the appendix, or by me. Ad- ditional data were also gathered from the scarce eolianites in eastern North America (Fig. 1) for completeness and to compare them with theoret- ical circulation patterns in other parts of the conti- nent. The eolian data are superimposed on new paleogeographic maps constructed from studies by me or other workers credited in the text. Interpre- •Olympia •Eugene !20" _ ! WASH. • Boise CANADA i0: '3 " . . / 0 Helena ,/ ~'Q~ ~I~-oT Bismark • S.DAK. Black ~H IlIa • Pierre Slngatsee RaRae MEV. A Jackson Mtna. East Humboldt ,~-~Ran geUTAH I Salt Lake Cheyenne • • Denver Monument Upwarp ~'1 / / ARIZ. • Santa Fe Pacific Ocean ~.._MA ,~ ' \ 100 200 300 -!,00 500 K!~'I Phoe t Yuma SNH CR MEXICO TEX. Fig. 2. Map showing localities in the western U.S. that are discussed in this report. B = Baboquivari Mountains; BM = Black Mountain; BMA = Big Maria Mountains; CH = Canelo Hills; CR = Cobre Ridge; S = Sierrita Mountains; SNH = Sil Nakya Hills; SR = Santa Rita Mountains.tations in this paper are confined primarily to generalizations about the atmospheric circulation patterns, the relationship of the circulation pat- terns to geographic features, and interpretations of upwind source areas for the eolian deposits. Additional interpretations concerning the process- es responsible for the circulation patterns are re- served for another paper (Parrish and Peterson, this volume) where the data can be evaluated in the context of global atmospheric circulation mod- els. Methods Crossbedding dip vectors obtained from an- cient eolian sandstones are here considered an excellent reflection of ground-level wind currents. Although there may be local deviations from the regional wind pattern (Rubin and Hunter, 1983; G. Kocurek, pers. commun., 1986), the con- sistency suggests that, in general, this is negligible when a fairly large number of measurements are made through as great a vertical sequence of strata as possible. Notable exceptions are the Middle
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