Vasquez Formation

Conglomeratic facies of the Vasquez Formation

The Vasquez Formation was described as "Vasquez Series" by Sharp in 1935,[1] and further described by Jahns (1939).[2] The formation crops out in the eponymous Vasquez Rocks, part of the Los Angeles Basin.[3] The formation was deposited in a series of minibasins between the San Gabriel and San Andreas Faults.[4] The Vasquez Formation unconformably overlies Triassic basement of the Mount Lowe intrusive series, and localized the Jurassic syenite occurring in the area.[5] The formation, comprising some rhyodacite and rhyolite, is overlain by the Paradise Springs Formation in the Punchbowl Block and the Tick Canyon strata in the Soledad Basin, separated by an angular unconformity.[6] The formation is considered equivalent to the Plush Ranch and Diligencia Formations.[7]

In the Soledad Basin and the San Andreas Fault Zone, the formation is described by the United States Geological Survey (USGS) as: Early Miocene to Oligocene? yellowish and reddish sandstone, conglomerate, and interbedded andesite-basalt, lying on pre-Tertiary crystalline basement rocks and unconformably below strata of Tick Canyon Formation; total thickness as much as 3,810 metres (12,500 ft). The formation includes numerous beds and lenses of megabreccia, many monolithologic.[2]

The formation represents the stratigraphically lowermost non-marine strata. The Vasquez Formation is composed primarily of bright-red conglomerate and sandstone. The conglomerate exhibits low degrees of rounding and sorting, a muddy matrix, and, commonly, reverse grading. South of Blue Ridge, the Vasquez Formation nonconformably overlies granitoid, although the base of the section is excised by the Blue Ridge Fault along much of its length. Interbedded trachyandesite is present near the base of the section, in places capped by thinly bedded tan limestone.[8] Clasts of the Jurassic granitoid were encountered in the Vasquez Formation.[9]

North of Blue Ridge, the Vasquez Formation also lies depositionally atop granitoid, possibly with some intervening San Francisquito Formation in places. The Vasquez Formation north of Blue Ridge contains lenses of very poorly sorted, very angular, matrix-poor megabreccias, interbedded with Vasquez Formation conglomerate and sandstone. These deposits fit the definitions of "crackle breccia facies" and "jigsaw breccia facies". The sorting, rounding, grading, and matrix within Vasquez conglomerate suggest deposition by debris-flow and hyperconcentrated-flow mechanisms.[10] Where Vasquez Formation conglomerate is dominated by granitoid and volcanic clasts, the sandstone composition is dominated by feldspar, especially plagioclase. While where the Vasquez Formation conglomerate is dominated by sandstone clasts from the San Francisquito Formation, the Vasquez Formation sandstone contains significantly more quartz and sedimentary lithics.[11]

The Vasquez Formation has been interpreted to represent primarily proximal alluvial-fan deposits. The lenses of granitoid crackle and jigsaw breccia in the Vasquez Formation north of Blue Ridge were concluded to be rock-avalanche deposits. The thin interval of thinly bedded limestone atop the interbedded trachyandesite was interpreted as lacustrine, likely the result of ponding against the volcanic flows. The high relative abundance of trachyandesite and rhyolite clasts immediately up section of the trachyandesite flows suggests that the rhyolite clasts are from deposits closely related to, and thus broadly coeval with, the trachyandesite flows. If so, then the 24.4 ± 0.9 Ma age determined by Coffey et al. in 2019 for the rhyolite clasts should approximate the age of the trachyandesite flows, and thus the age of the interbedded strata. Therefore, deposition of the Vasquez Formation in the Punchbowl Block probably began approximately 25–24 Ma or earlier.[10] Zircons found in the overlying Tick Canyon strata and interpreted as coming from the Vasquez volcanics were dated at 25.3 ± 0.1 Ma.[12][13][14]