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-l ithification folds, foliations, and lineations formed by slumping of t urbidite sequences in the Cretaceous Pigeon Point Formation, Californi a, provide a useful comparison with strain and microstructures develop ed in lithified and tectonically deformed turbidites. Our results indi cate the following: (1) multiple generations of folds, cleavages, and lineations can develop prior to any post-lithification tectonic deform ation; (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 2 0-40% shortening or volume loss), with the intensity of alignment part ly controlled by the percent of quartz and feldspar grains; (4) the fa bric ellipsoids in sand-rich layers largely reflect deposition and slu mping: pre- and post-slump compactions did not occur, in sand-rich uni ts but did align clay particles in mud-siltstone units, and (5) intra- grain microstructures in quartz and feldspar (e.g., undulose extinctio n, subgrains) are inherited or recycled features rather than represent ing effects of post-lithification strains. Our data also suggest that prelithification slumping occurred by pervasive grain rotation and gra in boundary sliding in saturated sands with some local movement of mat erial along bedding horizons. A likely model for the folding and assoc iated fabrics is that buckling and fold-hinge flattening drove fluid e xpulsion, which in turn caused local grain-scale realignment, transpos ition of bedding, and the development of an axial planar cleavage in t he hinge zones. Continued fluid flow was channeled by the clay-silt la yers in fold limbs and axial plane cleavege in fold hinges.