A KARST CONNECTION MODEL FOR GRAND CANYON, ARIZONA

The Colorado River was presumably blocked by the Kaibab arch before its integration through Grand Canyon at ~5.5-6 Ma. How the Colorado River crossed this arch is essential to any model of canyon evolution. The three "classic" models of ancestral Colorado River evolution - McKee et al's (1964), Hunt's (1969), and Lucchitta's (1984) - have major problems. In this paper we propose a new model for the connection of the eastern and western Grand Canyon, one that applies and extends Huntoon's (2000) hydrologic-basin model back to >6 Ma, before integration of the Colorado River through Grand Canyon. In essence our model proposes that water falling on the Kaibab Limestone surface of the southern Marble Platform descended vertically via sinkholes, collapse features, joints, fractures, and breccia-pipe structures to the Redwall aquifer in the area of the present Confluence of the Little Colorado River. This Redwall aquifer karst water then went under the Kaibab arch barrier as it pursued a westward path along the steepest hydraulic gradient to discharge at a structural low in a headward-eroding western Grand Canyon. A karst-aquifer hydrologic connection was established first between the eastern and western Grand Canyon, then collapse and incision of the canyon followed this subterranean route. Evidence that such a process is happening today are the collapse structures of Loughlin (1983) in the Blue Springs area, and the sinkholes Ah Hol Sah, Black Abyss, and Indian Pit in the northeastern part of Marble Canyon. These three sinkholes provide vertical flow routes down to the Redwall aquifer, joining water discharging from the Kaiparowits hydrologic basin at the Colorado River along the Fence Springs/Eminence graben system. Projecting this process back in time and spatially southward, we propose that at ~6 Ma a large sinkhole or sinkholes existed at the Confluence (similar to the 150 m-wide and 40 m-deep Ah Hol Sah). Little Colorado River water, then flowing north to southern Utah, became pirated down this collapse sinkhole(s), thus causing a reversal of drainage (barbed tributaries) in Marble Canyon. Headward erosion then proceeded up Marble and Little Colorado Canyons from this "kingpin" Confluence location.