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The Pangaean Megamonsoon: Evidence from the Upper Triassic Chinle Formation, Colorado Plateau

Russell F. Dubiel, Judith Totman Parrish, J. Michael Parrish and Steven C. Good
PALAIOS
Vol. 6, No. 4 (Aug., 1991), pp. 347-370
DOI: 10.2307/3514963
Stable URL: http://www.jstor.org/stable/3514963
Page Count: 24
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The Pangaean Megamonsoon: Evidence from the Upper Triassic Chinle Formation, Colorado Plateau
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Abstract

The Upper Triassic Chinle Formation was deposited at an exceptional time in Earth's paleogeographic and paleoclimatic history. During the Triassic, the supercontinent Pangaea was at its greatest size, in terms of both aggregated continental crust and exposed land area. Moreover, the exposed land was divided symmetrically about the paleoequator between the northern and southern hemispheres. These conditions were ideal for maximizing monsoonal circulation, as predicted from paleoclimate models. The Chinle was deposited between about 5°to 15°N paleolatitude in the western equatorial region of Pangaea, a key area for documenting the effects of the monsoonal climate. This study summarizes sedimentologic and paleontologic data from the Chinle Formation on the Colorado Plateau and integrates that data with paleoclimatic models. The evidence for abundant moisture and seasonality attest to the reversal of equatorial flow and support the hypothesis that the Triassic Pangaean climate was dominated by monsoonal circulation. The Chinle Formation contains continental lithofacies deposited in fluvial channels, crevasse splays, lakes, bogs, marshes, and lacustrine deltas that reflect abundant precipitation and shallow water tables. Paleosols and ichnofossils indicate that water tables and lake levels fluctuated episodically. Interbedded lacustrine carbonates and marginal-lacustrine siltstones and mudstones indicate longer-term but regular, episodic fluctuations in lake level. Fine-scale laminations in lacustrine carbonates suggest a seasonal influx of clastic sediment, and thus precipitation, to the basin. Uppermost Chinle strata consist of lacustrine and marginal-lacustrine mudstones interbedded with minor eolian sand sheets and eolian dunes; thus, the later Triassic reflects continued precipitation, but was marked by more pronounced and extended dry seasons. The Chinle fossil record corroborates the paleoclimatic interpretation based on sedimentologic data. The flora is consistent with moist, rather humid conditions, at least in the vicinity of fluvial channels and ponds, where most of the known plants were preserved by rapid sedimentation and burial below the water table. Invertebrates include fluvial and lacustrine unionid bivalves, gastropods, conchostracans, and ostracodes. Widespread occurrence of large unionids indicates perennial streams and lakes. Freshwater bivalves and gastropods provide evidence for seasonal climates on the basis of their ecological requirements, taphonomic occurrences, and growth banding preserved within unionid bivalve shells. A wide variety of actinopterygian fish are present, as well as isolated taxa of hybodont sharks, dipnoans, and coelacanths. The large coelacanths require the presence of perennial fluvial and/or lacustrine systems. The tetrapod fauna is dominated by aquatic forms, notably metoposaurid amphibians, and phytosaurian archosaurs; terrestrial archosaurs and synapsids are more poorly represented quantitatively. Vertebrate taphonomy and the nature of skeletal preservation also provide evidence of seasonality during Chinle deposition.

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