March GEOLOGY and GSA TODAY media highlights

The mystery of the pre-Grand Canyon Colorado River - Results from the Muddy Creek Formation

Joel L. Pederson, Department of Geology, Utah State University, Logan, Utah 84322-4505, USA.

The Colorado River’s integration off the Colorado Plateau remains a classic mystery in geology, despite its pivotal role in cutting Grand Canyon and in the region’s landscape evolution. The upper paleodrainage apparently reached the southern plateau during Miocene time (between 23 and five million years ago), and recent work supports the longstanding idea that the river was superimposed over the Kaibab uplift by this time. Once off the plateau, the lower river integrated with the Gulf of California by downstream basin spillover starting about five or six million years ago. An unknown link remains: the history of the river in the Grand Canyon region in Miocene time. One of the viable hypotheses put forward by previous workers - that the Muddy Creek Formation represents the terminal deposits of the paleo–Colorado River in the Basin and Range northwest of Grand Canyon - is tested in this paper. Results indicate instead that local drainages and the paleo-Virgin River are the likely sources of this sediment. The remaining hypothesis - that the paleo-upper Colorado River dissipated and infiltrated in the central-western Grand Canyon area - has modern analogs, provides a potential source for extensive Miocene spring and evaporite deposits adjacent to the southwestern plateau, and implies a groundwater-driven mechanism for capture of the upper drainage.

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