A KARST CONNECTION MODEL FOR THE COLORADO RIVER CROSSING UNDER THE KAIBAB ARCH, GRAND CANYON, AZ, USA (Invited Presentation)

The Colorado River did not carve the entire Grand Canyon. It only carved the canyon after ~5.5 Ma when its eastern side was connected to its western side across the Kaibab arch. Geologists still do not agree on: (1) What kept the Colorado River from arriving at the mouth of Grand Canyon until ~5.5 Ma? and (2) How did the Colorado River cross the Kaibab arch? This talk will only cover a “karst connection model” for how the Colorado River may have flowed under the Kaibab arch to connect its two sides. Key to this model is a proposed breccia pipe that would have stoped up from its base in the paleokarstic Mooney Falls Member of the Redwall-Muav aquifer, so as to allow for solution-collapse on the surface of the pipe; descent of water (base-level fall) from Kaibab Limestone level down to the top of the Redwall Limestone; setup of a deep phreatic loop under the arch to its western side; and then the karst-piracy capture of both the Little Colorado and Colorado Rivers. The major knickpoint convexity and incision-rate change that occurs from the Confluence of the Little Colorado with the Colorado River to Lee’s Ferry may support a karst-connection model and its required major base-level fall. Also, this model may explain the 150 yr-long puzzle of why the Colorado River flows in a southwest direction for ~400 km (from Grand Junction) but then suddenly turns west at the Confluence: karst water would have followed the most favorable hydraulic route under the arch from recharge to discharge regardless of strata or faults, and this initial route would have determined the path that the Colorado River has followed ever since.