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UT GEO 387H - Parrish and Peterson 1988

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Sedimentary Geology, 56 (1988) 261-282 261 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands Wind directions predicted from global circulation models and wind directions determined from eolian sandstones of the western United States A comparison JUDITH TOTMAN PARRISH * and FRED PETERSON U.S. Geological Survey, MA 971, Denoer, CO 80225-0046 (U.S.A.) U.S. Geological Survey, MS 916, Denoer, CO 80225-0046 (U.S.A.) (Received September 1, 1986; revised and accepted February 3, 1987) Abstract Panfish, J.T. and Peterson, F., 1988. Wind directions predicted from global circulation models and wind directions determined from eolian sandstones of the western United States--A comparison. In: G. Kocurek (Editor), Late Paleozoic and Mesozoic Eolian Deposits of the Western Interior of the United States. Sediment. Geol., 56: 261-282. Wind directions for Middle Pennsylvanian through Jurassic time are predicted from global circulation models for the western United States. These predictions are compared with paleowind directions interpreted from eofian sandstones of Middle Permsylvanian through Jurassic age. Predicted regional wind directions correspond with at least three-quarters of the paleowind data from the sandstones; the rest of the data may indicate problems with correlation, local effects of paleogeography on winds, and lack of resolution of the circulation models. The data and predictions suggest the following paleoclimatic developments through the time interval studied: predominance of winter subtropi- cal high-pressure circulation in the Late Pennsylvanian; predominance of summer subtropical high-pressure circulation in the Permian; predominance of summer monsoonal circulation in the Triassic and earliest Jurassic; and, during the remainder of the Jurassic, influence of both summer subtropical and summer monsoonal circulation, with the boundary between the two systems over the western United States. This sequence of climatic changes is largely owing to paleogeographic changes, which influenced the buildup and breakdown of the monsoonal circulation, and possibly owing partly to a decrease in the global temperature gradient, which might have lessened the influence of the subtropical high-pressure circulation. The atypical humidity of Triassic time probably resulted from the monsoonal circulation created by the geography of Pangaea. This circulation is predicted to have been at a maximum in the Triassic and was likely to have been powerful enough to draw moisture along the equator from the ocean to the west. Introduction Eolian sandstones that postdate the mid- Paleozoic generally are regarded as indicative of arid climate, based on analogy with the distribu- tion of modern eolian sands (e.g., McKee, 1979; * Present address: Department of Geosciences, Gould-Simp- son Building. University of Arizona, Tucson, AZ 85721, U.S.A. see Marzolf, this volume, for an alternative view). In addition, eolian sandstones provide data on paleowind directions. Numerous studies have demonstrated that, in general, regional paleowind directions can be determined from eolian sand- stones (e.g., Opdyke and Runcorn, 1960; Poole, 1964; Bigarella, 1973). Where paleowind direc- tions have been predicted based on general paleogeographic considerations, directions inter- preted from the sandstones have corresponded reasonably well to the predictions (e.g., Poole,262 1964). For this paper, we have predicted paleo- wind directions over the western United States for several time periods, based on maps of atmo- spheric circulation constructed by Parrish (1982) and Parrish and Curtis (1982). These predictions were made independently of the data from eolian sandstones. The correspondence between predict- ed paleowind directions and those determined from the sandstones has numerous implications for both the interpretation of eolian sand deposi- tion and the construction of climate models. We note for the following discussion that this paper brings together two independent bodies of work, the climate models and the geology of the eolian sandstones, in which the European and North American time scales, respectively, were used. It would be confusing and improper to change one or the other, so we have retained the original terminology and cross-reference the time scales using the chart by Van Eysinga (1975). This paper is a companion to the paper by Peterson (this volume). That paper contains sources of data on wind directions; descriptions of the eolian sandstone units, their paleogeographic and geo- logic settings, and correlation; and methods of extracting paleowind directions from eolian sand- stones. This information is not repeated here. Predicted wind directions Predictions of summer and winter atmospheric circulation over Pangaea were published by Par- rish (1982) and Parrish and Curtis (1982) for the Late Carboniferous (Westphalian C-D Age, ap- proximately equivalent to the Desmoinesian), Late Permian (Kazanian Age, approximately equivalent to the Guadalupian), earliest Triassic (Induan Age, approximately equivalent to the late Scythian of Europe), and Early and latest Jurassic (Pliens- bachian and Volgian Ages, respectively). The choice of time intervals was determined by the availability of detailed global paleogeographic maps that included continental positions and loca- tions of shorelines and highlands. These maps were produced by the Paleogeographic Atlas Proj- ect at the University of Chicago (1984; Scotese et al., 1979; Ziegler et al., 1983) and were essential for modeling global circulation because of the geographic controls on circulation patterns. Global circulation models are, in turn, essential for under- standing regional climatic patterns on the scale of the western United States. Wind directions in this paper are extrapolated from the global atmo- spheric circulation patterns predicted by Parrish (1982) and Parrish and Curtis (1982). The follow- ing brief description of their approach is presented for background. General atmospheric circulation The primary concern in the papers of Parrish (1982) and Parrish and Curtis (1982) was to model the major components of atmospheric circulation in order to


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