Abstract

The damming of the Colorado River at Page, Arizona in 1963 created Lake Powell, at full pool 300 km (180 mi) long and with an original capacity of 3.33 × 10¹⁰ m³ (27 million acre feet [maf]). The lake first reached full pool in 1980, and a delta immediately began to form in the lower reaches of Cataract Canyon. The heavy sediment load of the Colorado River (~15-25 × 10⁶ m³ or ~12,000-20,000 af/yr) and the narrowness of Cataract Canyon resulted in the rapid advance of the Hite delta through the canyon. Low rates of inflow during the drought of 2000-2005 resulted in a drop in lake level of >40 m (131 ft). The Colorado River responded to this drop in base level by incising its channel into the delta and exposing a 60 km (37 mi) length of delta sediments along the incised channel banks. Scores of meter-to-decameter across mud domes formed on the montmorillonite-rich delta beds prior to subaerial exposure, driven by methane gas and pore-water pressure. After subaerial exposure and entrenchment by the Colorado and Dirty Devil rivers, many of these domes remained active for months, indicating a continued influx of water and gas. Subaerial exposure transformed some of the more active domes into steep-sided mud volcanoes. Radial cracks formed on the desiccating flanks of most of the mud volcanoes and domes. Eventually, expulsion of sediment caused most of the domes and volcanoes to collapse and form pockmarks that are typically 0.5 to ~7 m (1.6 to 23 ft) in diameter and 0.1 to 0.5 m (0.3 ft to 1.6 ft) deep. Pore-water pressure from the delta beds and the groundwater from the water-saturated Cedar Mesa Sandstone, combined with in situ-produced methanogenic gas, prolonged mud-dome discharge for >7 months after exposure.

The entire length and breadth of the delta has been altered by repeated episodes of slumping, associated back-stepping arcuate slump scarps, and by lateral spreading, triggered by water saturation of competent and incompetent delta beds and by the removal of lateral support during channel incision. A set of arcuate scarps formed on the lateral spreads of the Dirty Devil delta near its confluence with the Colorado River. The surficial fine-grained beds adjacent to the pockmarks have developed a system of 5 to >15 cm (2 to >6 in) wide, 0.4 to >0.7 m (1.3 to > 2.3 ft) deep hexagonal and orthogonal desiccation cracks. Subsequent movement of lateral spreads has resulted in wide tensional and compressional features that have locally deformed the desiccation cracks.

A non-seismogenic origin of the domes, mud volcanoes, lateral spreads and slumps is favored because these features have evolved over a period of years prior to, during, and after the incision of the Colorado River into its delta. This is a region of low seismicity, but there were recurrent but minor earthquakes (≤ 3.6 magnitude) in the Provo-Price-Cedar City-Kanab region during the period 2003-2006 when most of the features evolved (University of Utah Seismograph Station Quarterly Seismic Summaries at http://www.quake.utah.edu/EQCENT-ER/QUARTERLY/qsum06.htm).