CONTINUED ACTIVITY AND GEOMORPHIC EVOLUTION OF FLUID/GAS ESCAPE STRUCTURES IN MUDDY SURFICIAL BEDS OF THE RECENTLY-EXPOSED HITE DELTA, SOUTHEASTERN UTAH

Dozens of fluid/gas escape pockmarks occur in desiccated, muddy fine-grained topset beds of the composite Hite river delta at the northern end of Lake Powell reservoir in southeastern Utah. The delta surface has been recently exposed (beginning in fall, 2002) due to a 5-year drought that has resulted in a 40 m drop in lake level since the late 1990s, and subsequent incision by the Colorado River of > 10 m. The pockmarks range from 2 to >7 m in diameter and 0.5+ m deep and originated in a shallow subaqueous environment as positive relief features (mud domes and volcanoes, methane springs). They have undergone topographic inversion as they collapse on emergence. Some are water filled (last observed in December, 2004), and display slow-to-vigorously escaping gasses through pit-floor vents.

The 1-2 m thick, highly expandable surficial muds are underlain by several m of coarser heterolithic Colorado River delta beds as well as other laterally extensive muddy beds. Subaerial exposure has resulted in a system of deep (0.4-0.7 m), wide (5-14 cm) crudely hexagonal and orthogonal desiccation cracks on level surfaces which transform abruptly into radial cracks in the pockmark depressions. The radial crack pattern has been in part inherited from the initial subaerial exposure of these features when they had positive relief. These structures formed by gas and fluid expulsion driven by pressure from (1) methanogenic decay of organic material in the delta beds, (2) pore water pressure from locally perched water bodies upslope on the delta surface, and (3) the underlying and laterally adjacent water-saturated Cedar Mesa Sandstone that locally forms the rock walls of the Colorado River gorge.