LAUSR

Abstract Cavities produced by migrating gas, mainly methane, emanating from the near subsurface are a rarely reported sedimentary structure from either modern or ancient clastic sediments. The delta of the Colorado River in Lake Powell, Glen Canyon National Recreation Area provides an excellent recently accessible locality for examining soft sediment deformation structures generated by escaping gases and fluids. Sub-meter-scale to tens-of-meter-scale structures, including craters, salses, and sand and mud volcanoes are present on the modern Lake Powell delta. Abundant sub-centimeter-scale cavities are found in excavations through crater sediments and exposed along cut-bank exposures of the Colorado River. In cross section the cavities are subvertical, irregular-to sigmoidal-to lenticular-shaped, 1–3 mm short axes, up to ∼3.0 cm long axes, and ∼1 cm intermediate axes. Cavities are developed in cm-scale, laminated silty clay and clay graded beds. The tops of the cavities are wider than their bases. The cavities observed in the Lake Powell delta are interpreted as mode 1 fractures and probably represent a clastic morphologic analog to ‘molar-tooth’ carbonate structures. The characteristics of the cavities, and the geometries and deformation of the surrounding delta-mud host, are consistent with bubbles and fracturing generated by gas migration experiments and theoretical calculations but vary because of inherent heterogeneity of the sediments. Fractures developed as gas migrated into pre-existing cavities causing overpressure in cavities and leading to subvertical mode 1 fracture propagation. Recent sudden decompression related to rapid lake level drops of Lake Powell may have enhanced the release of gas, primarily bacterially produced, leading to the development of these soft sediment deformation structures.