PRELIMINARY OUTCROP-SCALE RF/PHI PETROFABRIC ANALYSIS OF CONGLOMERATE FROM THE JURASSIC TUTTLE LAKE FORMATION; MOUNT TALLAC ROOF PENDANT, EL DORADO COUNTY, CA

A preliminary 3D Rf/phi analysis of outcrop-scale petrofabrics in near-orthogonal exposures of conglomerate reveals a heterogeneous pattern of penetrative strain within the southernmost Mt Tallac roof pendant. Our analysis was conducted in outcrops of the Jurassic Tuttle Lake Formation near Grass Lake in the Desolation Wilderness Area of Eldorado National Forest. The Tuttle Lake Fm contains a thick sequence of weakly metamorphosed volcaniclastic deposits cut by four sets of intermediate dikes, the Jurassic Keith’s Dome granodiorite pluton, and an array of predominantly sinistral ductile shear zones and brittle faults. Observations of shapes and orientations of clasts in the Tuttle Lake Fm led to the hypothesis was the unit was deformed by the intrusion of the Keith’s Dome pluton. To test this hypothesis, we examined petrofabrics at nine locations along a N-S transect between Grass Lake and the Keith’s Dome pluton. Four near-orthogonal faces containing clasts with discernable boundaries were selected at each location. Outlines of at least 60 clast boundaries (color coded by clast composition) were traced onto clear plastic overlays. Photo-registration marks, location information, 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 EllispeFit computer program (Vollmer, 2015) was used to conduct Rf/phi analysis of the clasts in each tracing and to compile fabric ellipsoids at each location. Our results reveal a range of fabric magnitudes and shapes (weakly oblate to moderately prolate, E 0.15 to 0.81, Nu -0.31 to 0.47). When bedding is restored to horizontal, oblate fabrics appear to be consistent with vertical flattening associated with deposition and compaction. Prolate fabrics record higher strains and subhorizontal long (X) axes that generally trend NE-SW. However, these X axes orientations correlate poorly with each other and with trends of dikes, shear zones, and faults in the Grass Lake area. Given fabrics measured in other Sierran Jurassic roof pendants, it is interesting that the Tuttle Lake Fm has not accumulated a more pervasive penetrative strain. Further study is needed to determine the cause of the fabric heterogeneities and their context in the regional host rock geology.