PRE-LITHIFICATION STRUCTURES, DEFORMATION MECHANISMS, AND FABRIC ELLIPSOIDS IN SLUMPED TURBIDITES FROM THE PIGEON POINT FORMATION, CALIFORNIA

Quantitative fabric, structural, and microstructural analyses of pre-lithification folds, foliations, and lineations formed by slumping of turbidite sequences in the Cretaceous Pigeon Point Formation, California, provide a useful comparison with strain and microstructures developed in lithified and tectonically deformed turbidites. Our results indicate the following: (1) multiple generations of folds, cleavages, and lineations can develop prior to any post-lithification tectonic deformation; (2) individual grains in sandstones have variable axial ratios, but the ratios and orientations of large populations of grains define fabric ellipsoids with small axial ratios (ave. = 1.25: 1.13:1); (3) phyllosilicate grains define moderate flattening fabrics (reflecting 20-40% shortening or volume loss), with the intensity of alignment partly controlled by the percent of quartz and feldspar grains; (4) the fabric ellipsoids in sand-rich layers largely reflect deposition and slumping: pre- and post-slump compactions did not occur, in sand-rich units but did align clay particles in mud-siltstone units, and (5) intra-grain microstructures in quartz and feldspar (e.g., undulose extinction, subgrains) are inherited or recycled features rather than representing effects of post-lithification strains. Our data also suggest that prelithification slumping occurred by pervasive grain rotation and grain boundary sliding in saturated sands with some local movement of material along bedding horizons. A likely model for the folding and associated fabrics is that buckling and fold-hinge flattening drove fluid expulsion, which in turn caused local grain-scale realignment, transposition of bedding, and the development of an axial planar cleavage in the hinge zones. Continued fluid flow was channeled by the clay-silt layers in fold limbs and axial plane cleavege in fold hinges.