Aydin, Atilla1, Peter Eichhubl1
(1) Stanford University, Stanford, CA
ABSTRACT: Deformation Structures in Poorly Consolidated Clastic Sediments of Varying Grain Size
The structure and texture of deformation structures were analyzed in poorly
consolidated sandstones, conglomerates, and mudstones of Miocene/Pleistocene age in
northern California to assess depositional and diagenetic controls on their mode of
Poorly consolidated arkosic sandstone in the footwall of an active reverse fault contains three sets of deformation bands: two inclined sets of reverse shear bands and a third set of subhorizontal bands that have no apparent shear offset and that were characterized as dilation bands. Within shear bands, elongate grains are rotated parallel to the orientation of the macroscopic band. Grain breakage appears largely restricted to softer lithic grains. Bands are preferentially cemented with clay and iron oxide minerals due to the infiltration of fine grains.
In interbedded sandy conglomerate layers, shear bands are more diffuse in contrast to those in sandstone. Elongate pebbles are preferentially rotated parallel to the shear band direction and slip surfaces are localized within the clayey matrix.
In mudstone shear bands appear as darker bands that are about 1-3 mm wide forming anastomosing arrays in lenticular pattern. These bands are bound on one side by striated slip surfaces.
These observations indicate that deformation bands are a fundamental mode of deformation in poorly lithified sediments involving grain translation, reduction or increase in porosity, and grain rotation, with only minor grain size reduction. Together with the diagenetic attributes, they should act as low permeability elements at present time.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004