ANATOMY OF A MUD VOLCANO ON THE COLORADO RIVER DELTA, LAKE POWELL, HITE, UTAH

Recent drop in Lake Powell water levels has exposed features such as mud lakes (salsas), small craters, and sediment volcanoes on the Colorado River delta near Hite, Utah. These features developed by the escape of shallow-produced, bacterially generated methane gas (CH₄) and pressurized fluids. Characterizing the internal features of sediment volcanoes helps to distinguish methane-generated volcanoes from seismic-generated volcanoes. A mud volcano situated on the heavily mudcracked sediments of the Colorado River delta was trenched in order to study the internal structures and sample for grain size and organic matter content analyses. The cone exhibits a near-circular shape of ~2 m dia. with a 25 cm dia. and 7 cm deep crater located near the center. The cone, up to 12 cm tall, is comprised of 8 laminations of fine silt (mean = 6.0-6.5 phi), each 1-1.5 cm thick. The laminations are graded, contain ripple cross lamination, soft sediment deformation (SSD) and scour marks. Below the strata of the cone, large downward tapering sediment-filled fissures extend ~6 inches into the lower deltaic sediments. These features, interpreted as mudcracks, are distinguished from conduits by their “V” shape, and laminated fill, similar in grain size and appearance to the laminations present in the cone. Directly beneath the crater of the cone is a 10-cm wide conduit, likely arising from the mud ejecting source, that is filled with non-laminated sediment (5.5 phi) slightly coarser than the surrounding delta sediment (6.5 phi). The organic matter of the sediment from the conduit is similar to that of the delta muds, mudcracks fillings, and cone laminae, all ~1.5-2.0%. The mudcracks that extend into the deltaic sediment beneath the cone clearly existed before the volcano and the laminated sediment within them indicates their fill came from mud volcano eruptions. Although the large mudcracks indicate exposure of the delta surface before development of the mud volcano, the cone’s external symmetry and internal features of SSD, scours, graded bedding, and cross ripple laminations indicate the volcano was deposited subaqueously in a very low-energy environment. The consistent thickness of the cone and mudcrack laminae suggest cyclical eruptions of similar volume of sediment from the main vent.