ANALYSIS OF PETROFABRICS IN THE JURASSIC TUTTLE LAKE FORMATION NEAR GRASS LAKE; MOUNT TALLAC ROOF PENDANT, EL DORADO COUNTY, CA

Three-dimensional R_f/phi analysis of petrofabrics in exposures of the Jurassic Tuttle Lake Fm in the southernmost Mt Tallac roof pendant reveals an interesting pattern of generally subhorizontal oblate fabrics. Our analysis was conducted in the vicinity of Grass Lake in the Desolation Wilderness Area of Eldorado National Forest. Here, the Tuttle Lake Fm is characterized by a weakly metamorphosed sequence of debris flows of matrix-supported volcaniclastic breccia and conglomerate. These rocks are cut by at least five sets of Jurassic dikes, the Keiths Dome pluton, and an array of predominantly sinistral ductile shear zones and brittle faults. We hypothesized that penetrative fabrics observed in the Tuttle Lake Fm were associated with either subduction-related convergence or with the intrusion of local plutons. To test these hypotheses, we measured petrofabrics in at least three near-orthogonal faces containing clasts with discernable boundaries at 22 locations in the area between Grass Lake and the Keiths Dome pluton. Outlines of at least 60 clast boundaries were traced from each face onto clear plastic overlays. Photo-registration marks and the strike and dip of each face were also recorded onto overlays. Photographs of overlays were adjusted, rectified, and reoriented in Adobe Photoshop. The software program EllispeFit (Vollmer, 2017) was used to conduct the R_f/phi analysis of the clasts in each tracing and to compile mean fabric ellipsoids at each location. Our results reveal a range of fabric intensities (E = 0.15-0.79) and symmetries (v = -0.43-0.71), and weakly oblate to moderately prolate shapes (k = values 0.28-3.05). When bedding is restored to horizontal, oblate fabrics appear to be roughly consistent with vertical flattening associated with deposition and compaction. Two locations exhibit stronger prolate fabrics with subhorizontal long (X) axes that trend N-S, and subhorizontal short (Z) axes that trend E-W. The X axes of these two samples correlate poorly and with trends of dikes, shear zones, and faults in the Grass Lake area. Oblate fabrics are likely either related to deposition and compaction or are associated with the intrusion of a pluton at depth. The slight SW-NE distribution of the short (Z) axes of these fabrics may support the latter hypothesis.