Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
AAPG Annual Convention, Houston, Texas, April 9–12, 2006
Large Sand Extrusions Affecting Slope Channel
Architecture and Evolution, Mio-Pliocene Monterey and Capistrano Formations,
Stratigraphic columns, photo panels and high-resolution Lidar data are used to address the stratigraphy of a 1 km wide, 50 m thick exposure of the Mio-Pliocene Monterey and Capistrano Formations. The exposure contains (1) a lower unit of Monterey mudstone, (2) an up to 12 m thick middle unit of structureless sandstone masses and (3) an upper sandstone unit composed of laterally and vertically stacked breccia- and sand-filled channels.
Several observations suggest that the sand masses in the middle unit were extruded. First, the lower mudstone unit contains cm to m-scale injections of structureless, poorly sorted sand. Second, facies in the middle sand unit are identical to the injected sand observed within the lower mudstone unit. Third, the middle unit sand masses contain meter-scale angular intraformational mudstone clasts that contain internal sand injections. Fourth, the mudstone of the lower unit located near to the sand masses are topographically low and show distinct variations of strike and dip relative to the regional trend. Fifth, the channels in contact with the sand masses, are deformed and show large-scale syndepositional loading features.
The sand masses in the middle unit are interpreted as sand volcanoes that incorporated mudstone xenoliths during upward sand migration. The volcanoes preceded the arrival of the slope channels and locally accommodate syndepositional deformation of these channels. The fluidization process that causes the sand to migrate upward can be related to (1) Monterey-age hydrocarbon migration, (2) seismicity or (3) change of pressure regime due to shallow mass failure or erosion.