Deformational Structures in Pliocene Bouse Formation Near Vidal, Southeastern California: ABSTRACT

Abstract

Ball-and-pillow, flamelike, and load structures accompanied by convolute laminations in the Pliocene Bouse Formation are thought to be caused by slumping of water-saturated sediments. New structural data suggest that these structures formed by foundering and by lateral movement of unconsolidated sand into hydroplastic End_Page 427------------------------------ mud and silt. Asymmetric ball-and-pillow structures and overturned flamelike structures suggest that movement was toward the southeast. Deformation probably was initiated by earthquake shock. The deformational structures are restricted to the basal member of the Bouse Formation 13 km northeast of Vidal, California. The basal member is divided into a lower unit of mudstone that grades upward to siltstone. The lower unit is conformably overlain by a well-bedded calcareous sandstone. Ball-and-pillow structures, 5 cm to 4.5 m across, consist of mudstone, siltstone, and sandstone. The curvilinear long axes of these structures trend at oblique angles to tectonic strike and plunge at angles steeper than the dip. Overturned flamelike structures of mudstone and siltstone separate the ball-and-pillow structures. Sandstone bedding planes conform to the outline of the ball-and-pillow structures but are undeformed a few centimeters above these structures; hence the ball-and-pillow structures are contemporaneous with deposition of the overlying sandstone. Saclike load casts and elliptical sandstone balls formed concurrently with the ball-and-pillow structures. Sandstone beds are buckled but not displaced by faults that stop within the sandstone unit. End_of_Article - Last_Page 428------------