GEOMETRY OF SMALL-SCALE FLUID/GAS CONDUITS: LAKE POWELL, HITE UTAH

Understanding the geometries of the shallow-generated gas and fluid escape mud volcano conduits is poor. Recently produced fluid and gas escape features on the Lake Powell delta provided a unique opportunity to examine the surface and subsurface structures.

Lake Powell filled to the maximum depth in the early 1980’s. Subsequent severe droughts caused a rapid drop in the lake’s depth initiating rapid progradation of the Colorado River delta southward past Hite, Utah. Delta top muds overlie progradationally stacked prodelta sands that cover organic-rich high-stand muds followed by delta top muds. Rapid incision of the Colorado River into the prodelta and delta plain sediments breached the confined aquifer composed of the Cedar Mesa Sandstone. The release of fluid pressure generated upward bending of the delta plain muds and the subsequently developed of mud volcanoes spewing over-pressured water and methane from the organic-rich high-stand deposits. As fluid over-pressuring quelled methane, degassing became the predominant phenomenon.

Hundreds of collapse caldera-like features are ubiquitous on the exposed delta surface. The study focuses on a smaller caldera with no apparent vent, similar in geometry and form to active salses, water-filled craters having vents of methane to the south. Surface features included evidence of microbial mats – gas bubble collapse and thin-film cracks, complex arcuate desiccation cracks, micropits, pellet mounds, rill-like features on the wall, and burrows with pellets on the top margins. Trenches in the crater exposed, mud filled vents consistent of: 1) sharp-walled and downward tapering, 2) arcuate-shaped, bifurcating-upward conduits, and 3) small open, irregular margined, vertical chambers decreasing in abundance towards the surface. Conduits cross cut homogenous mud and start in the upper several centimeters of the underlying stratified prodelta sands. The newly identified features are vital in recognizing the early seal breeching that permits the expulsion of gas from potential lacustrine reservoirs.