ENVIRONMENTS AND PROVENANCE OF REDBEDS OF THE DOG CREEK SHALE (MIDCONTINENT): IMPLICATIONS FOR MIDDLE PERMIAN PALEOCLIMATE IN WESTERN PANGAEA

Middle Permian fine-grained redbeds of the Dog Creek Shale (Oklahoma) and its correlatives in the Midwest are analogous to many Permian units globally. They archive an interesting time in Earth history, but the lack of fossils, and the fine grain size have confounded attempts to refine the age and depositional setting, respectively. New magnetostratigraphic data from Kansas showing reversals during the Dog Creek Shale now constrains the age of deposition to the Guadalupian.

These mudstone-dominant units have long been interpreted to record marine deltaic deposition based primarily on the inferred relationship with fluvial sandstone units southward. However, the uniform grain size, and occurrence of common paleosols, together with the lack of typical deltaic attributes such as hyper/hypo-pycnites, proximal deltaic channels, and large-scale upwardly coarsening trends, complicates this interpretation. Sheet-like tabular siliciclastic units thin northward from the Anadarko foredeep (Oklahoma) onto the Anadarko shelf (Kansas), and undergo lateral facies changes into likely marginal marine evaporites in the Palo Duro Basin (Texas Panhandle). More recent work suggests ephemeral lacustrine environments based on evaporite structures and mudstones overprinted with paleosol features. The fine and uniform grain size, massive bedding, and sheet-like geometry suggest a predominantly eolian origin for the siliciclastic material. Thin dolomite beds pinch out northward into the Hugoton embayment where massive and laminated mudstones and paleosols occur, a trend believed to reflect lack of connectivity of small ephemeral lakes and mudflats. The predominance of eolian-transported material and its capture in a series of lakes indicates the midcontinent formed a major sink for large volumes of atmospheric dust in middle Permian time. Detrital zircon data shed light on possible sources for this dust. Prominent age populations include 300-450 Ma, 550-740 Ma, 1,000-1,300 Ma, 1,600-1,800 Ma, and >2,500 Ma, interpreted to indicate transport from the east and southeast, with fewer grains derived from the west. These data indicate prevailing easterlies, and some westerly and southwesterly winds in western equatorial Pangaea during the transition from the early Permian icehouse to end-Permian greenhouse climates.