PRIMARY FABRIC ELLIPSOIDS IN SANDSTONES - IMPLICATIONS FOR DEPOSITIONAL PROCESSES AND STRAIN ANALYSIS

When measuring strains in deformed clastic rocks, geologists usually a ssume one or more of the following: (1) objects were initially circula r; (2) objects were initially non-circular but had uniform orientation s; (3) object populations initially had a fabric, but that this fabric had a symmetrical relationship to bedding; or that (4) initial fabric s are recognizable even after straining. To evaluate these assumptions , we measured 43 fabric ellipsoids in non-strained; poorly sorted sand stones from four depositional settings: DSDP core 174 (fan and abyssal plain deposits off the coast of Oregon); the Cretaceous Great Valley sequence, California (fore-arc deposits); the Cretaceous Pigeon Point Formation, California (accreted and slumped turbidites); and cross-bed ded sandstones from intracontinental basins in California and Australi a. Our results indicate the following: (1) in two-dimensional cuts, in dividual grains have variable but usually small axial ratios (<3/1) an d grains with larger axial ratios are more often, but not always, orie nted at small angles to bedding; (2) averaged ratios and orientations of populations of grains in three dimensions define non-spherical fabr ic ellipsoids, but with small axial ratios (average principal ratios = 1.31:1.14: 1); (3) these fabric ellipsoids show a wide range of shape s; and (4) orientations of fabric ellipsoid XY planes have highly vari able orientations and are not parallel to bedding or cross-bedding. Th ese results indicate that fabric ellipsoids measured in deformed sands tones must be corrected for the presence of primary fabrics when attem pting to calculate strains. However, because of the variable orientati ons and shapes of the primary fabric ellipsoids and lack of relationsh ip to bedding, strains can, at best, only be bracketed by multiplying final ellipsoids measured in strained samples by reciprocal primary fa bric ellipsoids having a variety of orientations and shapes.These data and microstructures in grains and matrix cements also argue against s ignificant compaction during burial in these clast-supported, sand-ric h units and instead suggest that the primary fabrics largely reflect g rain packing processes during deposition, and/or slumping and packing in the case of the Pigeon Point samples.