VOIDS: SMALL-SCALE, GAS-GENERATED SOFT SEDIMENT DEFORMATION STRUCTURES FOUND IN DELTAIC MUDS OF THE LAKE POWELL DELTA, GLEN CANYON NATIONAL RECREATION AREA, HITE, UTAH

Small-scale voids produced by gas emanating from the subsurface have rarely been described as sedimentary structures found in either modern sediment or sedimentary rocks. The delta of the Colorado River in Lake Powell provides an excellent recently accessible locality for examining soft sediment deformation structures generated by escaping methane.

Large-scale structures, including craters, salses, and mud volcanoes are present on the modern Lake Powell delta and are attributable to methane and water escape. The void structures were found in excavations in the craters and the banks of the Colorado River. The voids are irregular- to sigmoidal- to biconvex shape, 1 to 3 mm wide, up to ~3.0 cm long and ~ 1 cm deep. The voids crosscut and are restricted to cm-scale, laminated silty clays to clay graded beds. In cross-section the void’s long axes have an average dip of 20° and are typically rimmed by reddish halos. The tops of the voids are wider than their bases. Sides of voids display near vertical trending lineations and asperities. Size and density of voids increase towards the sediment surface and closer to the vents in small craters. Voids are consistent with geometries and orientations predicted by gas migration experiments in muds. In addition the vertical migration of the gas in the experiments deforms the surrounding sediment similar to that found in the voids reported here. Reddish oxidation rims observed around the small voids in the Lake Powell muds mark the former presence of a methane-oxidizing bacteria community that consumed the methane. If voids are recognized in modern sediment then early methane generation in the sediments can be inferred. If a reaction rim is present as suggested by the red halos, the probable consumption of the methane by bacteria is the most likely cause.