Preliminary geologic map of the Los Angeles 30' x 60' quadrangle, Southern California

Abstract

A geologic-mapping project sponsored jointly by the U.S. Geological Survey and the California Geological Survey San Bernardino 30' x 60' quadrangle (Morton and Miller, 2003), which adjoins the Los Angeles sheet on the east. The modified nomenclature results in some local generalizations of the map. In addition, the Quaternary surficial deposits have been relabeled according to standardized SCAMP nomenclature where data permit. The bedrock units of the area are commonly described in two principal groups: 1) Basement rocks-early Cretaceous and older, crystalline metamorphic and igneous rocks; and 2) The superjacent sequence of late Cretaceous and Tertiary strata. The greatest contrast in basement character is at the boundaries of the San Gabriel Mountains-Soledad Basin block. The San Gabriel Mountains form a basement massif that includes components of Proterozoic, Paleozoic, and pre-middle-Cretaceous Mesozoic metamorphic and plutonic rocks. These are the oldest basement rocks in the Los Angeles quadrangle and appear to represent old continental crust at the western margin of the North American craton, and which has been thrust over Jurassic oceanic crust of a different metamorphic facies than is found in adjacent blocks. In the eastern Los Angeles Basin, to the south of the San Gabriel Mountains, and in the Santa Monica Mountains, to the west of the San Gabriel Mountains, the basement rocks are metasedimentary and metavolcanic rocks of Jurassic age, that were probably deposited on a Jurassic oceanic crust, and accreted to the margin of the North American craton; they have been intruded by early Cretaceous granitic bodies of the same age as similar rocks in the San Gabriel Mountains, but the closeness of their relationship is not clear. The basement rocks of the western Los Angeles Basin also are associated with a Jurassic oceanic crust; however, their metamorphic character is significantly different, including blueschist facies schists that represent metamorphism in a subduction zone. The basement in the Topatopa Mountains block is unknown, but in the Santa Ynez Mountains, a few miles to the west, the basement is Franciscan Formation, including schists very much like those of the western Los Angeles Basin basement rocks. The superjacent sequence consists of Upper Cretaceous and Tertiary sedimentary and volcanic strata that rest unconformably on the crystalline basement. The regional unconformity at the base of the Upper Cretaceous is expressed only in the central Santa Monica Mountains, where the Trabuco Formation overlies basement rocks. Another regional unconformity, at the base of the Paleogene section, can be seen in the western Santa Monica Mountains and the Simi Hills where the Simi Conglomerate overlies the Upper Cretaceous sequence. The middle Miocene disruption of the pre-middle Miocene sequence and the formation of more localized basins is expressed by more restricted unconformities at the bases of latemiddle Miocene and younger strata and extreme variations in thickness of late Miocene and younger units....

Hydrogeomorphic Recovery and Temporal Changes in Rainfall Thresholds for Debris Flows Following Wildfire

“…The Fish and Station Fire burn scars are located in close proximity and share the same regional climate, vegetation community (chaparral), and topographic characteristics. The SGM are composed primarily of Mesozoic granitic and Proterozoic metamorphic rock (Yerkes & Campbell, 2005), which is exposed on the hillslopes as outcrops or as highly weathered saprolite (Staley et al, 2014). A combination of minimal soil cohesion and steep slopes promotes dry ravel following fire, which loads channels with fine hillslope sediment and helps to fuel debris flow activity (e.g., DiBiase & Lamb, 2019;Florsheim et al, 1991;Lamb et al, 2011).…”

Hillslope Morphology Drives Variability of Detrital 10Be Erosion Rates in Steep Landscapes

“…For each watershed, an effective elevation was calculated from pixel‐by‐pixel elevation and latitude scaling, and no topographic shielding correction was applied (DiBiase, 2018). Spatially variable quartz delivery was accounted for using petrographic descriptions in 1:100,000 scale geologic maps (Campbell et al., 2014; Morton et al., 2006; Yerkes & Campbell, 2005).…”

“…We also used geologic maps (Campbell et al., 2014; Morton et al., 2006; Yerkes & Campbell, 2005) to compile a map of Quaternary landslide deposits expected to have the largest influence on 10 Be concentrations (Figure S1 in Supporting Information ). For each 10 Be sample watershed, we calculated the ratio of landslide deposit area to total watershed area.…”

“…Relief broadly mimics rock uplift rate patterns that have been sustained since the 5–7 Ma initiation of thrusting along the Sierra Madre‐Cucamonga Fault Zone, inferred from low temperature thermochronology and geologic constraints (Blythe et al., 2000; Buscher & Spotila, 2007; Matti & Morton, 1993; Spotila et al., 2002). Bedrock geology is dominated by highly fractured igneous and metamorphic basement rocks (Campbell et al., 2014; Morton et al., 2006; Neely & DiBiase, 2020). Mean annual rainfall varies from 200 to 1,200 mm yr −1 , with the driest areas on the northern flank and wettest areas at high elevations (http://prism.oregonstate.edu).…”

“…The contribution to hillslope erosion by deep‐seated bedrock landslides is less clear. Landslide deposits ranging from 10 4 –10 6 m 2 in areal extent are common throughout the San Gabriel Mountains (Campbell et al., 2014; Morton et al., 2006), and dating of deposits from two of the largest landslides (Crystal Lake landslide and Cow Canyon landslide) indicates storage timescales ranging from 9 to 40 Kyr (Del Vecchio et al., 2018; Scherler et al., 2016). The superposition of hillslope erosion processes in the San Gabriel Mountains is expressed as a patchwork of soil and bare bedrock and variable hillslope morphology that is thought to reflect the history of stochastic mass wasting (Heimsath et al., 2012) and base‐level fall (DiBiase et al., 2015).…”