DISTAL RECORD OF TURBULENT TECTONIC TIMES – PENNSYLVANIAN-PERMIAN DEPOSITION ON SOUTHERN AND WESTERN COLORADO PLATEAU

Although the Colorado Plateau lies immediately adjacent to the Ancestral Rocky Mountains (ARM), only modest volumes of course detritus were deposited on the Plateau and only sand and mud reached distal areas to the south and west. However, parts of the Pennsylvanian and most of the Permian rocks are silisiclastic units that record massive influxes of sedimentation across the Colorado Plateau. Rough calculations of volume of sediment indicate that much sand, silt, and mud was derived from sources other than ARM. Detrtal zircon analyses confirm Appalachian and Gondwanan contribution for Permian sandstone and siltstone units. Paleogeographical constraints and paleocurrent data coupled with detrital zircon analyses suggest that large Appalachian rivers flowed NW across central North America, became desiccated along the western margin of the craton, and were deflated by northerly winds to supply huge volumes of eolian sand and loess southward to strata deposited on the Colorado Plateau. Coincidentally, sand, silt, and mud derived from ARM spread south and west and was mixed with Appalachian-derived detritus. The Plateau was a broad, low-lying, region mantled by eolian dunes, sluggish alluvial systems, and broad loess plains. During marine highstands, shallow marine carbonate and mudstone tongues penetrated across much of the region.

The geometry and distribution of Pennsylvanian-Permian sedimentary units described above document complex flexuring of the crust across the region. In some areas, rapid, short-lived subsidence is followed by periods of non-deposition and erosion; sedimentation in adjacent basins is commonly non-synchronous. Overall, sediment influx appears to be the most dominant control, at least during the Permian, as rates of continental sedimentation accumulation generally matched combined rates of subsidence and sea-level rise.