Arizona Geological Society: New Discovery of Beach Sand, Beachrock, and Tuffa from the Uppermost Portion of the Bidahochi Formation and their Implications for the Development of the Grand Canyon

The formation of the Grand Canyon is a long-standing controversial topic in geology. The literature is littered with hypotheses and conjecture. A river that has cut a canyon through a topographic bedrock highland, as the Colorado River does through the Kaibab Plateau, is termed a transverse drainage. Four mechanisms have been proposed to explain the development of transverse drainages: antecedence, superimposition, piracy, and lake-overflow. In the past 150 years each of these have been proposed for the development of Grand Canyon, but since the mid-1900s only the latter two have been considered viable explanations.

The Bidahochi Formation, which outcrops on the Colorado Plateau in northeastern Arizona, was deposited between roughly 16 and 5 Ma. The lower member of the formation records deposition of mostly silt and clay in a playa lake environment within a broad basin. The character of deposition changes abruptly in the middle member with the eruption of maar volcanoes of the Hopi Buttes, and the upper member records a transition into lacustrine and fluvial-lacustrine conditions ~7 Ma. Deposits of the upper member record the arrival of a substantial fluvial system, almost certainly the proto-Colorado River arriving from the north. Research in the past several decades has constrained the development of the lower Colorado River below Grand Canyon to ~5 Ma. Thus, the Bidahochi Formation is the nexus in time and space between the upper and lower segments of the Colorado River. For a lake to spill over and begin to cut Grand Canyon, the highest lacustrine deposits should be at least close to the elevation needed for spillover. Prior to the new discoveries presented here, the estimated high stand for this lake was about 2,000 m, about 300 m lower than the estimated spillover elevation. This was used as an argument against the spillover mechanism (Dickinson, 2013), although post-Bidahochi erosion could have removed some of the highest lake deposits. 

Recent photointerpretation and field reconnaissance has led to the discovery of higher outcrops of the upper member. Subaerial siliciclastic deposits are dominant with minor lacustrine deposits south of Greasewood, AZ; lacustrine deposits predominate north of Balakai Mesa. The lack of subaerial deposits on Balakai Mesa may have resulted from the capture and diversion of a Chuska Mountains fluvial system that would have supplied sediments to the shoreline. The low sediment input to the Balakai Mesa area allowed for the deposition of first beach sand, then beachrock, and eventually capping tufa recording the rising lake margin. The tufa is complex, with possible algae filament sheaths individually and in coarse concentrations. Sand grains, ooids, and peloids were found deposited within massive to structured micrite and laminated cements that built stromatolite and thrombolite microbial structures. The beach, beachrock, and capping tufa deposits outcrop up to 2,250 m, 50 m shy of the 2,300 m needed for a lake to overtop the Kaibab Plateau. The newly discovered tufa and associated subaqueous facies on Balakai Mesa strongly support a lacustrine setting prior or during lake-overflow across the Kaibab Plateau, leading to the formation of the Grand Canyon.    

Presented by

John Douglas, Ph.D.

Faculty, Paradise Valley Community College, Phoenix, Az.