SEDIMENTOLOGICAL INDICATORS OF PALEOPRECIPITATION IN PERMIAN-TRIASSIC REDBEDS AND EVAPORITES

Precipitation is one of the most important aspects of Paleozoic climate, yet is difficult to reconstruct. Advances in isotope geochemistry have provided insight into deep-time climate systems, including many aspects of paleoprecipitation. However, more robust results can be obtained through multi-proxy reconstructions using quantitative methods in conjunction with qualitative indicators based on high-resolution sedimentary observations.

Field and petrographic observations of modern continental redbed and evaporite environments reveal sensitivity to precipitation, and lack thereof, at differing temporal and spatial scales. Precipitation and drought events are recorded in specific sedimentary textures and sedimentary structures. The comparative sedimentology approach allows for recognition of those same sedimentary textures and structures in ancient red beds and evaporites. In this way, careful sedimentary observations can yield interpretations of past drought and precipitation events at various timescales.

We present several examples from Permian-Triassic redbeds and evaporites. For example, the Red Peak Formation of the Chugwater Group in Wyoming is characterized by its striking red color, laterally persistent beds, cyclicity, and lack of fossils or traces. Detailed measured sections suggest that the distinctive bedding is the result of variations in system moisture. Cycles are defined by displacive evaporite units that grade upward into thin graded beds. Displacive evaporites indicate deposition on a mud flat in which climate was dry enough to make groundwater briny. The thin graded beds formed as sheet flood deposits. The succession strongly suggests an arid to semi-arid climate with discrete, occasional heavy rainstorms.

We propose that quantitative geochemical measurements of precipitation-sensitive components of the rock record be used with detailed observations of the sedimentology of the same rocks to best unravel paleoprecipitation.