Tectonics and alluvial sedimentation of the upper Oligocene/lower Miocene Vasquez Formation, Soledad basin, southern California ERIC D. HENDRIX* | Department of Earth and Space Sciences, University of California, Los Angeles, California 90024 RAYMOND V. INGERSOLL ABSTRACT esses in both sub-basins. Erosional dissec- have been designated Vasquez Rocks sub-basin, tion of Mint Canyon Ridge allowed physical Texas Canyon sub-basin, and Charlie Canyon The upper Oligocene/lower Miocene Vas- interconnection of the two depocenters at this sub-basin (Jahns and Muehlberger, 1954; quez Formation marks the earliest sedimenta- time. Muehlberger, 1958). In each sub-basin, the Vas- tion in the Soledad basin, central Transverse Charlie Canyon sub-basin, the northern- quez Formation is either faulted against, or non- Ranges, southern California. The Vasquez most Vasquez depocenter, displays no clast conformable upon, igneous and metamorphic consists primarily of alluvial sediments and suites or alluvial megacyclicity to suggest basement rocks, which form prominent structur- presently appears as outcrops in three geo- close affinities with the other two depocen- al blocks between each pair of depocenters graphically restricted, fault-bounded sub- ters. A sequence of easterly derived braid- (Fig. 1). The San Andreas fault separates the basins. The two southern sub-basins (Vas- plain sediments low in the section is overlain Soledad basin from the Mojave block to the quez Rocks and Texas Canyon) shared by a 1,600-m, upward-coarsening, alluvial east; the San Gabriel fault defines the boundary similar tectonic and depositional histories; the sequence. This sequence reflects probable between the Soledad and Ventura basins. northernmost sub-basin (Charlie Canyon) inception of the San Francisquito fault, uplift The Vasquez Formation provides a useful appears to have had a distinct history. of a marine-sedimentary/quartz-monzonite/ framework within which to evaluate Soledad The Soledad basin originated as a predom- quartz-diorite source terrane, and northward basin evolution; it comprises both the oldest and inantly orthogonal rift during the latest Olig- progradation of an alluvial-fan system. the thickest sequence of sediments in the basin ocene. Incipient subsidence was concentrated Post-Oligocene clockwise rotation of the (Fig. 2). A thick volcanic interval in the Vasquez in the southeastern region of the basin, as Soledad basin by as much as 40 degrees is Rocks sub-basin has yielded potassium-argon debris-flow deposits accumulated as small, indicated by paleomagnetic data of other ages averaging -25-24 m.y. (Crowell, 1973; thick, alluvial fans draining local source areas workers. Restoration of the Soledad basin to V. Frizzell, personal commun., 1985); these adjacent to Vasquez Rocks sub-basin. Intense its Oligocene orientation indicates that south- volcanics occur low in the Vasquez stratigraphic rifting, volcanism, and rapid subsidence pro- east-northwest extension caused rifting and sequence and thus provide constraints on the duced the half-graben geometry of this sub- heralded Vasquez deposition in small, rapidly maximum age of Vasquez strata. An early(?) basin, as a source area rose to the south/ subsiding basins. Neither compressional de- Miocene (Arikareean) vertebrate assemblage southeast across the active Soledad fault. formation nor strike-slip deformation seems occurs in the nonmarine Tick Canyon Forma- Coeval displacements along the Vasquez to have been a significant factor during Vas- tion, which is separated from the Vasquez by a Canyon and Pelona faults led to the asym- quez sedimentation. The sedimentary and tec- pronounced angular unconformity (Durham metric-graben geometry of Texas Canyon tonic history of the Soledad basin is and others, 1954; Woodburne, 1975; Ehlert, sub-basin, as abundant detritus derived from consistent with a plate-tectonic model involv- 1982). This radiometric and biostratigraphic an eastern/southeastern source interfingered ing extension in the North American plate control implies that Vasquez sedimentation oc- with small, debris-flow-dominated fans along north of the unstable Mendocino triple curred principally between 25 and 21 m.y. B.P., the Pelona fault margin. Periodic faulting and junction. with a cumulative sedimentation rate of 1.4 m/ source-area uplift, followed by tectonic 1,000 yr (Hendrix, 1986). quiescence and erosion, produced thick, INTRODUCTION upward-fining alluvial megacycles in both OBJECTIVES AND METHODS sub-basins. The Soledad basin is part of the central OF STUDY Major tectonic uplift in the ancestral San Transverse Ranges province, Los Angeles Gabriel area led to drainage-system enlarge- County, southern California (Bailey and Jahns, There are three salient objectives to this study. ment, increased water discharge into the 1954). The Vasquez Formation is a thick allu- The first is to synthesize sedimentary lithofacies depositional systems, and deposition by hyper- vial sedimentary unit which crops out in three and paleodispersal data in order to reconstruct concentrated-flood and braided-fluvial proc- geographically separate, fault-bounded zones or depositional environments and Vasquez paleo- "sub-basins" within the Soledad basin (Fig. 1) geography. Although both Muehlberger (1958) •Present address: Leighton & Associates, Inc., 1151 (Muehlberger, 1958; Bohannon, 1976). From and Bohannon (1976) presented preliminary Duryea Ave., Irvine, California 92714. south to north, these sub-basins or depocenters environmental analyses of Vasquez sediments, Geological Society of America Bulletin, v. 98, p. 647-663, 22 figs., June 1987. 647 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/98/6/647/3434641/i0016-7606-98-6-647.pdf by guest on 25 September 2021 648 HENDRIX AND INGERSOLL Figure 1. Generalized geolog- ic map of Soledad basin area, depicting adjacent lithologic units: gn, Precambrian gneisses; an and ga, Precambrian anorth- osite and gabbro; gd, Triassic Lowe Granodiorite series; gr, Cretaceous granitic to quartz- monzonitic plutonics; ps, Pelona Schist; sf, San Francisquito Formation. Vasquez Formation stippled: 1, Vasquez Rocks sub- basin; la, eastern volcanic field; 2, Texas Canyon sub-basin; 3, Charlie Canyon sub-basin. there has been no previous comprehensive, ac- others, 1981; Luyendyk and Hornafius, 1987); 8000 C ft S 1 A I C tualistic evaluation of Vasquez depositional sys- however, these rotations are accounted for in the F M. T I tems. The second objective is to discern tectonic regional tectonic summary of the Soledad basin. o style during Soledad basin evolution and Vas- MINT o quez sedimentation, with particular attention to VASQUEZ ROCKS SUB-BASIN CANYON m distinguishing "pure" extension from strike-slip F M. z and compressional tectonism. The final objec- The largest and southernmost of the three m tive is to re-evaluate the sub-basin correlations Vasquez depocenters, Vasquez Rocks sub-basin, 6000 • I SCON F O RM IT Y ? proposed by Muehlberger (1958) and to define features the Vasquez type section as defined by TICK CANYON FM. genetic relationships, if any, among the three Sharp (1.935) in Escondido Canyon. Other pre- m sub-basins. vious work in this sub-basin includes the map- A N G U IA R a UNCONFORMITY •o Lithofacies analysis involved detailed mea- ping of Irwin (1950), Muehlberger (1958), surement of stratigraphie sections; where possi- Oakeshott (1958), and Bohannon (1976). The ble, measured sections were correlated and Soledad fault is the southern boundary of both compared along strike across sub-basins, in this sub-basin and the Soledad basin proper, and order to document lateral facies and/or thick- it is expressed as a high-angle structure dipping VA SQ U E Z ness changes. Compositional clast counts of toward the Vasquez Rocks depocenter (Muehl- berger, 1958) (Fig. 3). This fault juxtaposes c O Vasquez conglomerates and breccias were per- •o O formed using a one-hundred-point grid on the Vasquez strata and mid-Proterozoic anorthosite, z F M. 3s o outcrop. Three counts (one hundred clasts per gabbro, metapyroxenite, and syenite of a large u m count) were made at each stratigraphie locality stratiform intrusion in the San Gabriel Moun- z (Hendrix, 1986). Sandstone samples were thin- tains. Other basement rocks exposed south of 2000 m sectioned and point-counted using the Gazzi- the fault include the Upper Triassic Lowe intru- Dickinson method (Ingersoll and others, 1984) sive series, featuring distinctive epidote- and hornblende-bearing granitic lithologies. Creta- VoI can i c for comparison to clast-count data. Paleocurrent data were measured in the field, corrected for ceous granitic plutons, in turn, cut all older in- Interval bedding attitude (Ragan, 1973), and analyzed trusive rocks of the San Gabriel/Soledad region; utilizing circular statistical methods (Royse, all above-named lithologies also crop out north 25 myBP K - A r 1970). Discussions of paleoflow directions and of the Soledad fault, along the eastern margin of I paleogeography throughout the text do not re- the Vasquez Rocks sub-basin (Oakeshott, 1958; Figure 2. Composite Soledad basin strati- flect palinspastic corrections for post-Oligocene Silver, 1971; Ehlig, 1981). graphie section. clockwise rotation of this terrane (Terres and The Vasquez has an aggregate thickness of Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/98/6/647/3434641/i0016-7606-98-6-647.pdf by guest on 25 September 2021 TECTONICS AND ALLUVIAL SEDIMENTATION, CALIFORNIA