FROM OUTCROP TO PALEOGEOGRAPHY – RECONSTRUCTING THE PERMIAN HERMIT FORMATION AND RELATED ROCKS, COLORADO PLATEAU

Paleogeographic reconstructions represent the ultimate synthesis of geologic data and interpretation of Earth history. Meaningful paleogeography can be executed only after careful interpretation of numerous local outcrops. The Lower Permian Hermit Formation and broadly coeval deposits of the Organ Rock Formation and Cutler Group formed on an extensive, flat, alluvial and coastal plain between the Ancestral Rocky Mountains and seas flanking southwestern North America. Extensive continental redbed sedimentation in deposits 100-200 m thick suggests regional steady subsidence and rapid sediment accumulation where the depositional surface remained mostly above sea level. Depositional environments were varied and complex. Fluvial systems included bedload and mixed-load streams, mostly unconfined, but locally incised. Caliche-dominated floodplains with ubiquitous loess plains and eolian dunes document an arid setting; caliche nodules sourced abundant intraformational conglomerate. Possible tidal deposits occurred near coastal reaches. The red slope- and ledge-forming deposits contrast in color and origin with overlying and underlying light-colored, cliff-forming eolian sandstone.

Detailed regional correlation and establishment of stratigraphic sequences is not yet possible, however, correlation of regional patterns aids in paleogeographic reconstruction. Mixed-load and bedload streams transported gravel and sand off the flanks of the Ancestral Rocky Mountains but evolved downstream into systems that transported mixed loads of sand and mud. In contrast, more distal stream channels were mostly sand-choked, probably because of extensive input of eolian sand and loess, and formed as both lateral- and downstream-accreting alluvial ridges and macroforms. Local conglomerate-filled arroyos contrasted with the broad, sandy dryland rivers. Minor flaser-bedded sandstone suggests tidal influence.

Extensive sheets of vertically stacked and intercalated floodplain and channel deposits suggest that external controls including climate, tectonics, and eustacy weighed heavily on the continental architecture of the Lower Permian deposits. Dunes, rivers, and tidal flats shifted across the Colorado Plateau in response to these controls.