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Annual monsoon rains recorded by Jurassic dunes


Pangaea, the largest landmass in the Earth's history, was nearly bisected by the Equator during the late Palaeozoic and early Mesozoic eras. Modelling experiments and stratigraphic studies have suggested that the supercontinent generated a monsoonal atmospheric circulation that led to extreme seasonality1,2,3, but direct evidence for annual rainfall periodicity has been lacking4. In the Mesozoic era, about 190 million years ago, thick deposits of wind-blown sand accumulated in dunes of a vast, low-latitude desert at Pangaea's western margin5,6,7. These deposits are now situated in the southwestern USA. Here we analyse slump masses in the annual depositional cycles within these deposits, which have been described for some outcrops of the Navajo Sandstone8. Twenty-four slumps, which were generated by heavy rainfall, appear within one interval representing 36 years of dune migration. We interpret the positions of 20 of these masses to indicate slumping during summer monsoon rains, with the other four having been the result of winter storms. The slumped lee faces of these Jurassic dunes therefore represent a prehistoric record of yearly rain events.

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Figure 1: Pangaea during deposition of the Navajo Sandstone in the Early Jurassic.
Figure 2: Annual cycles within ancient dune cross-stratification in lower Navajo Sandstone at Coyote Buttes, Arizona.
Figure 3: Parts of two successive annual depositional cycles.
Figure 4: Slump mass and enclosing strata.
Figure 5: The properties of a continuous sequence of 36 annual cycles within 6-m-thick cross-strata of set A.


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This work was supported by the Schultz Chair of Stratigraphy at the University of Nebraska.

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Correspondence to David B. Loope.

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Loope, D., Rowe, C. & Joeckel, R. Annual monsoon rains recorded by Jurassic dunes. Nature 412, 64–66 (2001).

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