Tectonics and Alluvial Sedimentation of the Upper Oligocene/Lower Miocene Vasquez Formation, Soledad Basin, Southern California

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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

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Figure 1. Generalized geologic map of Soledad basin area, depicting adjacent lithologic units: gn, Precambrian gneisses; an and ga, Precambrian anorthosite 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

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5,500 m in this sub-basin and is exposed as a Basal Vasquez Upsection, the clay content of basal Vasquez southwest-dipping homocline. The sub-basin is breccias decreases as matrix sand content con- cut by numerous left-lateral faults, which post- Three stratigraphic sections were measured comitantly increases. Additionally, internal bed- date initial Soledad fault activity (Fig. 3) within this interval across the sub-basin (sections ding fabric becomes more clast supported (Muehlberger, 1958; Oakeshott, 1958); the most 1, 3, and 8; Fig. 3). The basal Vasquez varies upsection; local clast imbrications and inversely important of these left-lateral faults, in terms of considerably in thickness along strike, from 0 to graded bedding become more common (Fig. sub-basin structural configuration, is the Green 330 m, but in general, the interval is thicker 5A). These sedimentologic features suggest Ranch fault. This fault divides the sub-basin into toward the northwest. The breccias and con- lower primary yield strength during transport, two distinct blocks: the relatively undeformed glomerates of this interval rest nonconformably where the frictional component of shear strength Agua Dulce block, and the complexly folded on Lowe (Parker Mountain) granodiorite near played a more important role than did the cohe- and faulted Tick Canyon block (Fig. 3). The the Soledad fault, but they rest on syenites in the sive component (Rodine and Johnson, 1976). 5,500-m-thick Vasquez section of this sub-basin northern extent of the outcrop area. The principal clast-support mechanism for these is divided into two informal lithostratigraphic In the lower portions of all three sections, the sandy basal Vasquez deposits was probably iner- intervals: lower Vasquez and upper Vasquez; the basal Vasquez consists almost exclusively of tial collision and dispersive pressure, enhanced lower Vasquez is subdivided into the basal, poorly sorted breccia beds, some attaining by the high concentration of large clasts relative volcanic/Tick Canyon and megacyclic units thicknesses of 5 m. These breccias feature clay- to fine-grained matrix (Fisher, 1971; Costa, (Fig. 4). All of these intervals are present in the rich matrices and matrix-supported clast fabrics, 1984; Shultz, 1984). Development of clast im- Agua Dulce block; however, the megacyclic and with common planar, unscoured, lower-bedding brications in some beds implies local turbulent upper Vasquez intervals are absent from the contacts. Maximum clast size exceeds 2 m, and flow conditions and low flow viscosities, as op- Tick Canyon block, presumably due to erosion most clasts are highly angular. These breccias posed to highly viscous, laminar debris flows during development of the Vasquez/Tick resemble high-yield-strength, debris-flow depos- (Enos, 1977). Despite the upsection change of Canyon unconformity. To reduce confusion con- its, where strength during transport was proba- debris-flow character, there are few distinct cerning sub-basin stratigraphie correlations, this bly controlled by cohesion provided by abun- lithofacies changes in the basal interval from nomenclature of informal units differs slightly dant matrix clay (Rodine and Johnson, 1976). southeast to northwest. The only salient change from previous terminology (for example, Hendrix, 1986).

-7 CP MINT CA N YON RIDGE

GNEISS GNEISS o r

GABBRO o T» t o (P LOWE GRANODIORITE

LOWE ANORTHOSITE GRANODIORITE ANORTHOSITE SAN GABRIEL MTS.

GABBRO

Figure 3. General geologic map of Vasquez Rocks sub-basin, depicting locations of measured sections, stratigraphic intervals, local basement lithologies, key faults, and geographic features. Ttc, Tick Canyon Fm.

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METER S FACIES - CLAST SUITES

UPPER VASQUEZ

5000 HYPERCONCENTRATED FLOOD FLOW Massive, Matrix-Supported ANORTHOSITE - GABBRO SUITE Conglomerate Massive, Clast-Supported Conglomerate 4000 MEGACYCLIC Horizontally Stratified r Clast-Supported Conglomerate FOUR ALLOCYCLIC • MEGACYCLES Planar-X-Stratified Figure 4. Generalized Sandy Conglomerate DEBRIS FLOW; stratigraphie section for Vas- BRAIDED FLUVIAL; Planar-X-Stratified Sandstone quez Rocks sub-basin; ar- SHEET FLOW 3000 m rows reflect paleocurrents at Trough-X-Stratified Sandstone selected horizons; lithofacies LOWE D Horizontally Bedded, Laminated and clast suites listed. Thick- GRANOOIORITE to Massive Sandstone SUITE < gm nesses depicted are basin X-Laminated, F.-V. F. Sandstone maxima. 2000- Wavy-Laminated, F. Sandstone VOLCANIC- •p and Siltstone TICK CANYON CD Mudstone EPHEMERAL'LACUSTRINI Silty Carbonate FAN-DELTA 0 BRAIDED FLUVIAL Evaporite LOWE GRANODIOR I T E c Volcanics and Pyroclastics SUITE -TO BASAL DEBRIS FLOW N ANORTHOSITE 1 LOWE SUITES

is the increase in thickness and number of inter- appearance of thick basaltic-andesite units, in subaqueous deposition from suspension, possi- stratified pebbly sandstone beds toward the uninterrupted sequences at least 50 m thick. The bly in local areas created by volcanic ponding of north; these beds reach a maximum thickness of Vasquez volcanics of the Agua Dulce block are fluvial drainages (for example, Oilier, 1967). 60 cm in section 8. Commonly, the lower con- exposed in a sequence which thins to the Due to the presence of volcanics in deposits of tacts of such beds a.re gradational with underly- northwest from 1,300 m near the Soledad fault the Tick Canyon block, the Vasquez in that area ing debris-flow dejrasits; additionally, there are to 850+ m near the Green Ranch fault (Fig. 3). is considered primarily time-equivalent to the few scour structures to suggest that these coarse These volcanics are principally calc-alkaline, volcanic interval of the Agua Dulce block. The sandstones are fluvially reworked debris-flow dominantly hypersthene-normative, subalkaline interpretation of subaqueous, ephemeral-lacus- deposits. These finer-grained units are inter- basaltic andesites (Weigand, 1984). Texturally, trine sedimentation may also be applied to much preted as deposits of "late-stage," turbulent these basaltic andesites vary from vesicular and of the Tick Canyon block. The Vasquez in this debris-flow pulses, which followed the principal amygdaloidal to densely microlitic (pilotaxitic). area is organized into a thick basal breccia debris-flow slurry (Costa, 1984). Plagioclase microlites (An3o_7o) commonly ex- horizon overlain by three distinct volcanic/ Few reliable paleocurrent structures were ob- hibit calcite replacement and albitization in thin sedimentary couplets, each of which becomes served in this interval, probably because original section. This suite becomes more dacitic east- thinner and finer grained westward along strike flow conditions inhibited development of such ward, within the extensive volcanic field east of (Hendrix, 1986) (Fig. 3, sections 6 and 11). structures. Only eleven localities were found in Vasquez Rocks sub-basin (Fig. 3). The mafic The basal breccia horizon of the Tick Canyon the upper part of the interval where clast imbri- volcanics are locally associated with felsic extru- block is exposed best in Tick Canyon itself and cations were obtainable (Fig. 6), yielding a north- sives, including welded vitric rhyodacites and is sedimentologically similar to clay-rich, high- easterly mean paleoflow. rhyolitic, vitric-crystal ash-flow tuffs. yield-strength, debris-flow deposits recognized Clast-count data (Hendrix, 1986) reflect high Thick lenses of coarse conglomerates and in the basal Vasquez interval of the Agua Dulce percentages of anorthosite, gabbro, syenite, and pebbly sandstones are interbedded with the block. Clast counts from these breccias (Bohan- Lowe Granodiorite in breccias from sections 1 Agua Dulce block volcanics. Near the Soledad non, 1976; Hendrix, 1986) reveal anorthosite, and 8. Near the Soledad fault (section 3), how- fault, most of these epiclastic sediments are tex- gabbro, and syenite similar to clasts of the basal ever, most clasts are petrologically identical to turally and structurally identical to low-yield- Vasquez assemblages of the Agua Dulce block, the Lowe Granodiorite, upon which the breccias strength, debris-flow deposits of the basal with which they are correlated. rest nonconformably. This clast assemblage im- Vasquez. Toward the northwest along strike, Ephemeral lacustrine sedimentation in the plies that Lowe Granodiorite was the only base- however, these interbedded rocks become finer Tick Canyon block is exemplified best by a ment lithology exposed south of the Soledad grained and more thinly bedded. Some of the thick sequence of sediments above the second fault during basal Viisquez sedimentation. finer-grained deposits are organized into up- main volcanic horizon in this area. Ephemeral- ward-fining and -thinning cycles reaching 4.5 m lake/fan-delta sedimentation is indicated by sev- Volcanic/Tick Canyon Interval in thickness. The paucity of bedform-migration eral lines of evidence. First, there are two 2- to structures, abundance of normally graded sand- 3-m-thick, vitric-crystal-tuff marker beds within The stratigraphie contact between the basal stone beds, and presence of massive, 15- to 30- this sequence, both of which are relatively free Vasquez and this interval is mapped as the first cm-thick mudstones in these cycles suggest of admixed epiclastic material above their basal

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 Figure 5. Alluvial lithofacies, Vasquez Rocks sub-basin. (A) Inversely graded, clast-supported, debris-flow deposit; basal interval of lower Vasquez; largest clasts are 35 cm in diameter. (B) Symmetrical ripple marks in tuffaceous sandstone, lacustrine shoreline lithofacies, volcanic/Tick Canyon interval. (C) Proximal, low-yield-strength, debris-flow deposits, top of coarse sequence, 3rd megacycle, megacyclic interval of lower Vasquez; height of outcrop -50 m. (D) Channelized cobble conglomerate, megacycle coarse sequence, megacyclic interval of lower Vasquez. (E) Pebbly sheetflow bed with outsized clast, megacycle coarse sequence of lower Vasquez megacyclic interval. (F) Grouped trough cross-stratification (T) above 4-cm-long mudstone ripups (R), braided-midfan lithofacies of fine sequence, megacyclic interval of lower Vasquez.

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present lower in this interval within the Agua U PI» E R Dulce block. MEG ACYCLIC Megacyclic Interval

In the Agua Dulce block, the volcanic- interval/megacyclic-interval contact is drawn as the upper contact of basaltic andesite. The meg- AGUA DULCE acyclic interval consists of four well-organized, BLOCK upward-fining and -thinning alluvial megacycles, which can be traced laterally across the entire sub-basin. Each megacycle has been subdivided into a lower coarse sequence and an upper fine sequence. All four megacycles are present proximal to the Soledad fault, where they have a mean thickness of 525 m (Fig. 3, sections 4, 9, and 12), in contrast to a mean thickness of 250 m to the northwest in the Vas- quez Reeks County Park area (section 2, Figs. 3, 7). This dramatic thickness decrease occurs over a distance of roughly 4 km; only three megacycles are present in the northwestern area Figure 6. Paleocurrent rose diagrams for all stratigraphic intervals of Vasquez Rocks sub- of the sub-basin, due to pre-Tick Canyon For- basin; circled values indicate total number of localities at which measurements were collected; mation deformation and erosion (Fig. 8). standard deviatioiti of samples also given. Proximal to the Soledad fault, both the coarse and fine sequences of the megacycles are highly conglomeratic (Fig. 5C), with bedding units contacts. The absence of erosional truncations or of these thin limestone beds (Kendall, 1984). consistently thicker in coarse sequences. Coarse fluvial scour surfaces in these tuffs implies prob- Finally, there are symmetrical, linear ripple sequences are distinguished by 2- to 5-m-thick, able airfall origin and subsequent deposition marks in tuffaceous siltstones upsection from the clast- to sandy-matrix-supported breccia beds from suspension in a standing body of water fan-delta deposits (Fig. 5B); these deposits are with local clast imbrications, massive internal during periods of low epiclastic input. The interpreted as a lacustrine shoreline lithofacies structure, and commonly nonerosive basal con- second line of evidence includes a suite of (Picard and High, 1981). Westward, these tacts. In beds where erosionally scoured basal coarse-tail-graded sandstone and pebbly sand- ripple-marked siltstones pass gradationally into bedding contacts do exist, clast imbrications are stone beds stratigraphically between the tuff laminated-to-massive mudstones in Tick Can- better developed. Inversely graded basal zones markers. These sandstones are pervasively cal- yon proper, possibly indicating deeper-water are common, suggesting dispersive shear during cite-cemented, with poikilotopic cement com- lacustrine sedimentation in that area. Fresh- transport (Fisher, 1971; Shultz, 1984). Many of pletely surrounding many grains in a fashion water ostracods have been reported from silt- these breccias are interpreted as clast-rich, which implies an eogenetic origin. The sandstones of this interval (P. L. Ehlig, 1986, debris-flow deposits, where inertial, frictional stone beds are organized into 4- to 5-m-thick, personal commun.). forces dominated clast support (Rodine and upward-fining cycles, commonly capped by 10- The ephemeral-lacustrine/fan-delta sequence Johnson, 1976). Some of these clast-supported to 40-cm-thick, laminated or massive mud- passes gradationally upsection into a suite of red, breccias may be sieve deposits (Hooke, 1967), stone beds. The presence of thick mudstones, hematite-cemented sandstones organized into especially the better-sorted beds; however, lack lack of fluvial-reworking features, and coarse- 2- to 3-m-thick, upward-fining cycles. The sand- of good three-dimensional exposures precludes tail-graded structure of the sandstones imply stones, siltstones, and pebble conglomerates of recognition of sieve-lobe geometry. subaqueous suspension sedimentation, possibly these cycles feature grouped trough and planar Fine sequences proximal to the Soledad fault as resedimented, diluted mass flows along the cross-strata resembling braided-fluvial, trans- also feature coarse, low-yield-strength, debris- front of a prograding fan-delta system (for ex- verse-bar/megaripple migration (Cant and flow deposits, but these are more thinly bedded ample, Larsen and Steel, 1978; Gloppen and Walker, 1978). These fluvial deposits signal a than in the coarse sequences. There are also Steel, 1981). The upward-fining cyclicity is period when local sedimentation rate outpaced numerous sandy conglomerates and pebbly sand- nested within larger-scle, upward-coarsening and subsidence rate, thus leading to lacustrine infill- stones interstratified with the debris-flow -thickening sequences, as much as 60 m thick. ing, and subsequent subaerial sedimentation. deposits, and these finer-grained epiclastic beds Similar upward-coarsening sequences have been Paleocurrent readings from braided-fluvial are internally massive, with generally non- described from lacustrine fan-delta sequences cross-strata in the Tick Canyon block suggest erosive bases, horizontal bedding, and uniform (Pollard and other.;, 1982) and possibly repre- northwesterly paleoflow (Fig. 6), which concurs thickness (that is, 50- to 150-cm range). Such sent sedimentary response to base-level fall. The with the general northwestward-fining trends deposits probably resulted from sheetflow proc- third line of evidence for lacustrine paleoenviron- along strike and northwesterly distal lithofacies esses or from rapidly waning currents in broad, ments are the locally stromatolitic, argillaceous pattern seen through the volcanic interval and shallow, alluvial-fan channels (for example, carbonates found upsection from the fan-delta Tick Canyon block sediments. Lowe Granodi- Friend, 1978; Tunbridge, 1981). sandstones. Internal drainage and concentration orite clasts dominate the upper horizons of this As the megacycles are traced northwest across of bicarbonate ions are implied by the presence interval, yet minor gabbro and anorthosite are the sub-basin, distinct lithofacies changes occur

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within both coarse and fine sequences (sections sequences reflect large-scale fan progradation angularity toward the northwest imply downfan 2 and 10; Figs. 3, 7). Coarse sequences feature and migration of upper-fan debris-flow or chan- trends and sediment dispersal in that direction inversely to normally graded, low-yield-strength, nelized-streamflood facies over mid- to outer-fan (Miall, 1970; Bull, 1977). Paleoflow data from debris-flow deposits, similar to those proximal sheetflow and braided-fluvial deposits during the megacyclic interval (Fig. 6) support this to the Soledad fault, yet bed thicknesses in these times of active faulting. Fine sequences reflect model of northwestward sediment transport. mid-basin sequences rarely exceed 2 m. Maxi- rétrogradation of fan facies during periods of mum clast size is less (rarely greater than source-area erosion and diminished faulting. 50 cm), and clast angularity noticeably dimin- METERS SE ishes to the northwest. Some coarse conglomerates feature basal scoured zones (Fig. 5D), which may reflect channelized-streamflood conditions (Bull, 1977). In addition, there are MEGACYCLIC several moderately well sorted, horizontally INTERVAL OF bedded, unchannelized, coarse pebbly sand- LOWER VASQUEZ stones and pebble conglomerates as much as 1.5 m thick interstratified with the debris-flow units. Such beds commonly feature outsized clasts within a finer, pebbly matrix (Fig. 5E) and closely resemble pebbly-sheetflow (Ballance, 1984) or "terminal-fan-sheetflow" deposits (Friend, 1978). Gloppen and Steel (1981) cite large, outsized clasts as being diagnostic of high- energy-sheetflow deposition, as flow compe- tence decreases rapidly in an unchannelized mid- to outer-fan environment. The fine sequences in this "mid-basin" locality are characterized by an abundance of bedform-migration structures (cross-stratification and ripple marks) (Fig. 5F) and by the presence of 4- to 15-m-thick, upward-fining cycles. Many of the cyclic deposits in these sequences are reminiscent of longitudinal-bar, transverse-bar, and bar-top-modification depos-

PROXIMAL SASIN POSITION its of braided-fluvial systems (for example, MID BASIN - DISTAL SECTIONS 4.9.12 Miall, 1978a; Bluck, 1979) and are likely the SECTION 2 • VASQUEZ ROCKS result of autocyclic channel-migration/avulsion processes. Figure 7. Lateral (northwest) thickness and lithofacies changes, megacyclic interval of lower The four megacycles discussed above are too Vasquez, Vasquez Rocks sub-basin. Correlations are depicted for only three megacycles, as the thick to have been generated by any autocyclic, fourth megacycle is absent in the Vasquez Rocks (mid-fan) area due to erosion (see Fig. 4 for steady-state alluvial process (Miall, 1980). The lithologie symbols). only autocyclic process which could have generated cyclicity of this scale is the back-filling of entrenched fanhead channels, avulsion, and whole-scale migration of the locus of sedimentation on the fan surface (Hooke, 1968; Heward, 1978b). If this process had been responsible for the Vasquez megacycles, however, two features should be evident: (1) megacycle fine sequences would represent periods of negligible sedimentation on inactive fan lobes and therefore would not be as thick and (2) inactive fan lobes would have been sites of nondeposition and pedogene- sis (Hooke, 1972; Birkeland, 1974). There is a virtual absence of paleosols, rhizocretions, or other alluvial pedogenic features. The basin- wide extent of these cycles also indicates allocyclic, tectonic control. The four upward-fining megacycles are considered allocyclic, probably related to tectonic events in the ancestral San Gabriel drainage and Figure 8. View north across Vasquez Rocks sub-basin, illustrating three allocyclic megacy- to uplift along the Soledad fault. Coarse cles of the lower Vasquez megacyclic interval.

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Clast-count data from the megacyclic interval 350 < show a dominance: of epidote-rich Lowe Gran- odiorite clasts and also ancillary basaltic-andesite 300 - • • and dacite clasts within the two lower mega- Figure 9. Plot of bed cycles. Gabbro clasts increase in percentage thickness versus maxi- 250 - • 1 e upsection, and anorthosite appears within brec- mum particle size for 40 o R=0.58 200 N-40 cias of the 4th megacycle. Sandstone point hyperconcentrated-flood- " xz •A C+JD • • counts support these data, with abundant flow beds, upper Vas- 150 Lowe-derived detrital epidote in all samples, quez, Vasquez Rocks • • • • plus minor lithic volcanic (Lv) grains. The vol- sub-basin; correlation co- 100 canic detritus probably was derived from nearby efficient is 0.58. • Regression Line • • volcanic sources created during the earlier ex- 50 trusive episode (Hendrix, 1986). HPS. cm Volcanism apparently continued during mega- i. i « . i i.i. 20 40 60 100 140 cyclic-interval sedimentation. There are numer- 120 160 ous vitric crystal, ¡ish-fall tuffs preserved in fine sequences, and a 30-m-thick diabase sill found flow conditions; however, massive bedding, 1.0- sequence -1,600 m thick. Paleoflow data from at the top of the: third megacycle (Hendrix, to 3.3-m bed thicknesses, and a general paucity imbrications reveal a distinct northerly trans- 1986). These volcanic lithologies may imply of bedform-migration structures imply that flow port trend (Fig. 6) (Bohannon, 1976; Hendrix, partial time-equivalence between portions of the may not have been truly turbulent (Costa, 1984; 1986). megacyclic interval and some deposits of the Shultz, 1984). A plot of bed thickness against Tick Canyon block, although no radiometric or maximum particle size (Fig. 9) yields a correla- TEXAS CANYON SUB-BASIN biostratigraphic control exists to confirm this. tion coefficient of 0.58, which is lower than those previously reported for conglomerates of The Vasquez Formation in Texas Canyon Upper Vasquez true debris-flow origin but higher than those sub-basin is exposed as a faulted, southwest- from "classic" fluvial streamflood deposits dipping homocline bounded by the Vasquez Field observatio ns reveal a conformable con- (Bluck, 1967; Steel, 1974; Gloppen and Steel, Canyon fault along the southeast basin margin tact between (a) the uppermost megacyclic in- 1981; Shultz, 1984). Most of the upper Vasquez, and by the Pelona fault along the northwest terval and (b) the conglomerates and breccias of therefore, probably was deposited by "hyper- margin (Fig. 10). Each of these high-angle faults the upper Vasque2 interval. Clast-count data in- concentrated flood flows" (Smith, 1986), transi- dips toward the Texas Canyon depocenter dicate a progressive increase in gabbro and tional between Newtonian flows and those (Remenyi, 1966). Mint Canyon Ridge is the anorthosite between the upper megacyclic inter- which possess finite yield strength. Clast fabrics structural block separating Texas Canyon and val and the upper Vasquez (Hendrix, 1986); of such deposits imply sediment/water ratios Vasquez Rocks sub-basins, and features Pre- therefore, the megacyclic/upper Vasquez contact high enough to effectively dampen original flow cambrian layered to porphyroblastic augen is defined as the stratigraphic horizon at which turbulence (Costa, 1984). gneiss intruded by Cretaceous granitic plutons. conglomerate beds contain greater than 50% The upper Vasquez is an upward-coarsening Across the Pelona fault from the Vasquez out- anorthosite and/or gabbroic clasts. There is no evidence of angular discordance between these 1 O 3 two intervals; therefore, the assignment of upper T km Vasquez sediments to either the Tick Canyon or the Mint Canyon Formations (Oakeshott, 1958; Ehlert, 1982; D bblee, 1984) is considered erroneous. Most upper Vazquez conglomerates contain subangular to sub rounded gabbro, anorthosite, and metapyroxenite clasts within sandy, clast- supported beds with a mean thickness of 1.8 m. Subordinate Lowe Granodiorite clasts are found locally within this interval, but clast counts and sandstone P/F (plagioclase/total feldspar) ratios indicate the gradual exposure of a plagioclase- rich, anorthositic ¡¡ource terrane during the megacyclic/upper Vasquez transition period. Individual conglomerate beds are massive to normally graded, with inverse grading rarely observed. Clast imbrications are locally well developed, as are scoured basal bedding contacts. Some conglomerates pass vertically or Figure 10. General geologic map of Texas Canyon sub-basin, depicting stratigraphic inter- laterally into pebbly cross-stratified sandstones. vals, locations of measured sections, key faults, and local basement lithologies. Ttc, Tick These features suggest turbulent, Newtonian- Canyon Fm.; Tmc, Mint Canyon Fm.

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crops, a distinctive block of Cretaceous quartz METERS FACIES - CLAST SUITES monzonites is thrust over the Pelona Schist 4000 along a narrow mylonite zone (Bohannon, 1976), which apparently represents the Vincent BRAIDED FLUVIAL GN E I SSIC-GRANIT IC SUITE thrust. The Pelona Schist consists primarily of metasedimentary/ metavolcanic quartz-mica- UPPER VASQUEZ albite and actinolite-chlorite schists of latest Cre- 3000 HYPERCONCENTRAT ED taceous to Paleocene reset K-Ar ages (Ehlig, FLOOD FLOW 1981). Most of these basement lithologies also GNEISSIC-GRANITIC & are exposed east of the Texas Canyon depo- SAN GABRIEL SUITES center. 2000 MIDDLE VASQUEZ The Vasquez Formation in Texas Canyon QUARTZ MONZONITE consists of-4,000 m of conglomerates, breccias, SUITE DEBRIS FLOW and subordinate sandstones. Three informal SHEETFLOW stratigraphic subdivisions are recognized: lower, LOWER VASQUEZ middle, and upper (Fig. 11). 1000 MEGACYCLIC; DEBRIS FLOW Lower Vasquez SHEETFLOW

The lower Vasquez is a 1,600-m-thick inter- GNEISSIC-GRANITIC &

val defined by four upward-fining alluvial meg- VOLCANIC SUITES acycles, ranging in thickness from 275 to 500 m 0 (sections 1,2,5,6, 7, and 12; Fig. 10). As in the S LT SS CNG megacyclic interval of Vasquez Rocks sub-basin, these megacycles are basin-wide and are inter- Figure 11. Composite stratigraphic section for Vasquez Formation in Texas Canyon sub- preted as tectonically generated. Coarse se- basin; three key stratigraphic horizons depicted. Details as in Figure 4. quences in these megacycles are up to 75% thicker than associated fine sequences and are characterized by massive, 1- to 3-m-thick breccia beds. Within the lowest megacycle, the breccias events in a rapidly aggrading, mid- to outer-fan suggest an eastern to slightly southeastern source are rich in clay and are dominated by matrix- environment. The megacyclic fine sequences are for lower Vasquez sediments. supported fabrics. Upsection, the breccias are most distinct and better developed close to the The local gneissic breccias along Vasquez Can- progressively sandier, exhibiting clast-supported Pelona fault margin of the sub-basin. Southeast- yon fault ostensibly were shed northward across fabrics and inverse grading. Non-erosional basal ward along strike, the well-developed allocyclic this fault from an ancestral Mint Canyon Ridge contacts and random clast orientations are also megacycles grade into coarse, matrix-supported gneissic positive area (Bohannon, 1976). Most common, yet local clast imbrications occur. breccias along the Vasquez Canyon fault (sec- of the megacyclic lower Vasquez sediments, Sheetlike, poorly sorted pebbly-sandstone beds tion 9; Fig. 10). These thick, poorly bedded however, apparently were derived from a source up to 40 cm in thickness are gradational and breccias are dominated by gneissic lithologies region farther to the east/southeast of the Texas interstratified with many of these breccias. These similar to those of Mint Canyon Ridge. Along Canyon depocenter; these east-derived sediments coarse units are interpreted as debris-flow depos- the Pelona fault, quartz-monzonite-rich breccias interfinger with local quartz-monzonite breccias its, probably of low original yield strength due to interfinger with the megacyclic deposits (section along the Pelona fault. The monzonitic breccias low clay content and viscosities (Rodine and 8; Fig. 10). Local, 3- to 4-m-thick vitric-crystal are massive and poorly sorted, with highly dis- Johnson, 1976). Many of the interbedded sand- tuffs and tuffaceous sandstones are preserved organized to inversely graded clast fabrics; most stones are likely to have been waning-flood, within the fine sequence of the lowest of the beds are 1-3 m in thickness and possibly late-stage, debris-flow deposits. megacycle. represent low-yield-strength, clast-rich, debris- Megacycle fine sequences feature 2- to 6-m- Clast counts from the lower Vasquez reveal flow deposits. thick upward-fining and -thinning cycles con- that gneissic lithologies dominate the popula- taining massive to normally graded sandstone tions (Hendrix, 1986). Granitic clasts, lithologi- Middle Vasquez beds 20-150 cm thick. These cycles commonly cally similar to granites of Mint Canyon Ridge are capped by thick, massive, mudstone beds yet (Fig. 10), are also abundant. Numerous clasts of The lower/middle Vasquez contact in Texas contain few cross-stratified units. The presence basaltic andesite and dacite constitute the most Canyon is defined as the lowest stratigraphic of parting lineations on sandstone bedding distinctive element of the lower Vasquez clast occurrence of well-cemented, quartz-monzonite- planes implies upper-plane-bed flow (Picard and assemblage. Although Bohannon (1976) re- rich red beds along the axis of the sub-basin. The High, 1973); the absence of cross-stratification garded these volcanic clasts as "exotic" (distal 250-m-thick middle Vasquez interval (sec. 4; implies high-energy (sheetflow?) transport source), many are lithologically similar to the Fig. 10) consists of quartz-monzonite-rich brec- (Miall, 1977) and/or rapidly waning flood con- volcanic rocks east of the Vasquez Rocks sub- cias similar to those along the Pelona fault in the ditions (Tunbridge, 1981). Local scour struc- basin (Fig. 3). Sandstone point-counts reveal lower Vasquez interval; however, these deposits tures are present, suggesting that flows were abundant metamorphic, volcanic, and poly- extend across most of the width of the sub-basin. turbulent. Despite the paucity of "classic" crystalline-quartz lithic grains in samples from Additionally, there are abundant, 1- to 2-m- braided-channel features, these upward-fining this stratigraphic horizon (Hendrix, 1986). thick, sheetflow-facies sandstone beds interstrati- cycles are probably autocyclic, related to flood Clast-imbrication paleocurrent data (Fig. 12) fied with the breccias in the axial zone of the

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basin. Gneissic and volcanic clasts are admixed fAUU with quartz-monzonite detritus in this axial zone, as well, suggesting interfingering with the east-derived sediment. An overall fining of grain LOWER size in a direction away from the Pelona fault, coupled with paleocurrent data from planar * I cross-bed foresets (Figs. 11, 12), suggests sediment dispersal southward from a quartz-monzonite source across the Pelona fault. No Pelona Schist detritus has been found in this interval; very likely, the quartz-monzonite terrane com- pletely covered the ancestral Sierra Pelona positive element during Vasquez deposition. An Figure 12. Paleocurrent data from Texas Canyon sub-basin; details as in Figure 6. overall upward coarsening trend in this thin interval attests to the origin of the middle Vasquez deposits as a progradational alluvial-fan wedge (for example, He ward, 1978b), which expanded as a result of intensified activity on the Pelona 320 fault. •• 280 - • Figure 13. Plot of bed • Upper Vasquez 240 - t thickness versus maximum particle size, upper 200 _ m • The middle/upper Vasquez contact is defined EO • • Vasquez hyperconcentrated-flood-flow conglomer- as the lowest stratigraphic occurrence of gray, 160 R-0.61

poorly bedded conglomerates upsection from Bth . N-40 ates, Texas Canyon sub- the quartz-monzonite-rich red beds. More than 120 - basin; correlation coeffi- 2,100 m of poorly exposed, clast- to matrix- • cient is 0.61. 60 tS Regression Line supported conglomerates and subordinate sand- M •• • stones comprise the upper Vasquez interval 40 - • MPS. cm (sections 3, 10, 11, 14, 15, and 16; Fig. 10). • 1 » • i Most conglomerates are horizontally stratified, 0 20 40 60 100 120 140 nonchannelized, and similar in geometry and fabric to the hyperconcentrated-flood-flow de- METERS posits of the upper Vasquez in Vasquez Rocks sub-basin. A plot of bed thickness versus maximum particle size (Fig. 13) yields a correlation coefficient of 0.61, a value transitional between coefficients for debris-flow deposits and fluvial conglomerates laid, down by Newtonian, turbu- METERS 4000 lent flows (Bluck, 1967; Gloppen and Steel, 1981). Paleocurrent data from clast imbrications indicate northerly paleoflow (Fig. 12), similar to 3000 the upper Vasquez of Vasquez Rocks sub-basin. Subrounded to well-rounded gabbro and anorthosite clasts of probable San Gabriel provenance are present in the conglomerates of this 2000 interval, admixed with gneissic, granitic, and volcanic clasts (Hendrix, 1986). Upsection from the conglomeratic lower portion of the upper A A A A AA Vasquez interval, there is a distinctly finer- 1000 AAA A A A A| VOLCANIC grained, 700-m-thick interval. Although possess- ing similar clast populations to those lower in the upper Vasquez interval, it is stratigraphically ä BASAL distinct due to the presence of several upward- fining sequences ( Fig. 10, section 15); it was TEXAS CANYON VASOUEZ ROCKS described separately by Hendrix (1986) for this SUB - BASIN SUB - BASIN reason. These fluvial deposits, composed pre- dominantly of sandstones and sandy conglomer- Figure 14. Stratigraphic correlation between Vasquez sections of Vasquez Rocks and Texas ates, pass gradationally into hyperconcentrated- Canyon sub-basins.

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flood-flow deposits southeastward along strike, ocenter in the Vasquez Rocks sub-basin. The controlling sedimentation in the Vasquez Rocks and interfinger with quartz-monzonite-rich, absence of distinct lithofacies changes north- sub-basin, with the elevation of a major Lowe debris-flow deposits near the Pelona fault. There ward (away from the Soledad fault) and the Granodiorite source south of the fault zone in the is abundant evidence of longitudinal and trans- presence of dramatic differences in clast compo- ancestral San Gabriel highland; (3) incipient de- verse-bar accretion surfaces of probable braided- sitions along strike in the basal Vasquez indicate velopment of a fan system with a downfan-to- fluvial origin (for example, Bluck, 1979); several sedimentation on small fans draining many local the-northwest geometry, with northwest diver- of the upward-fining sequences are capped by basement sources, rather than a single fan system sion of most epiclastic material by volcanic mudstones and thin, concordant carbonate beds. prograding from a southern source. Clast imbri- flows; and (4) development of the half-graben Paleocurrent data from imbrications suggest a cations suggest that an anorthosite/gabbro geometry which probably prevailed in Vasquez return to westward paleoflow during deposition source may have existed inside the present mar- Rocks sub-basin during the remainder of Vas- of the braided-fluvial autocycles, as sediment gins of the basin; this source may have been quez deposition (Fig. 15). No strata correlative was shed longitudinally along the sub-basin axis uplifted along an incipient Soledad fault zone with the basal Vasquez or volcanic/Tick Can- (Fig. 12). (Hendrix, 1986). Southward backstepping of yon interval have been recognized in Texas this fault may have occurred, as extension in the Canyon sub-basin, probably because rifting PALEOGEOGRAPHIC SYNTHESIS: sub-basin progressed (for example, Steel and initiated and was most intense in the southern VASQUEZ ROCKS AND Wilson, 1975); this source likely was buried by region of the Soledad basin. The absence of vol- TEXAS CANYON subsequent volcanic and sedimentary deposits. canics in Texas Canyon and thinning to the The dominance of Lowe Granodiorite clasts in northwest of the volcanics in Vasquez Rocks These paleogeographic reconstructions do not the basal Vasquez proximal to the Soledad fault sub-basin support this hypothesis. reflect palinspastic corrections for clockwise ro- implies that a local granodiorite source existed, The asymmetry of alluvial lithofacies in the tations of the western Transverse Ranges; how- probably southeast across the fault, where Lowe lower Vasquez of Vasquez Rocks and Texas ever, these rotations are addressed in the Granodiorite is presently exposed (Fig. 3). Canyon implies half-graben and asymmetric- regional plate-tectonic synthesis. The Vasquez The volcanic/Tick Canyon interval repre- graben geometries, respectively, for these sub- Rocks and Texas Canyon depocenters shared sents the following: (1) volcanism and sedimenta- basins. The presence of local debris-flow-dom- similar tectonic and sedimentary histories, as in- tion during the principal rifting/basin-widening inated fan deposits along the Pelona fault dicated by stratigraphic correlations (Fig. 14). phase of Soledad basin evolution; (2) establish- suggests that the steepest margin of Texas Can- Incipient subsidence generated an irregular dep- ment of the Soledad fault as the major structure yon sub-basin was the northwest margin (for

Figure 15. Paleogeographic block diagram illustrating Vasquez sedimentation during megacyclic lower Vasquez deposition in Vasquez Rocks and Texas Canyon sub-basins; lithofacies are: Al, high-yield-strength debris-flow; A2, low-yield-strength debris-flow; B, braided-fluvial/midfan; C, sheetflow; El, ephemeral lacustrine; E2, lacustrine fan-delta.

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example, Hooke, 1972; Steel and Aashiem, thickness and lithofacies changes and high sedi- changes, because independent sedimentologic, 1978) (Fig. 15), where steep, relatively small, mentation rates (1.4 m/1,000 yr) imply that the isotopic, and vertebrate paleontologic data do quartz-monzonite-rich fans interfingered with Vasquez fans were small and thick; such fan not indicate such a latest Oligocene/earliest Mi- gneissic, granitic, and volcanic detritus derived geometry was a logical artifact of the narrow, ocene climatic event (Savin and others, 1975; from an eastern source area. technically confined nature of the sub-basins Ingle and others, 1976; M. O. Woodburne, Four upward-fining alluvial megacycles occur (for example, Hooke, 1968; Bull, 1977). Major 1986, personal commun.). Additionally, the in both Vasquez Rocks and Texas Canyon sub- tectonic uplift of the Soledad basin terrane and chemically unstable anorthosite source terrane, basins, generated by fan-system progradation in enlargement of the ancestral San Gabriel drain- which became erosionally unroofed at this time, response to four regional episodes of tectonic age system commenced with upper Vasquez probably would have undergone more intense activity, followed by rétrogradation. Although sedimentation in both sub-basins. Incipient tec- weathering to clay minerals if a more humid clast petrology, sandstone composition, and tonism at this time probably also was responsi- paleoclimate had occurred. No increase in clay paleocurrent data imply separate source areas ble for intensified activity on the Pelona fault, is evident in the upper Vasquez; petrographic and lack of physii^l interconnection between and resultant deposition of the Texas Canyon evidence suggests that most clays in upper Vas- the two depocenters, the allocyclic megacycles middle Vasquez. Correlation of this thin interval quez sandstones were authigenically derived provide a means of establishing probable time- with the lowest upper Vasquez deposits in Vas- from alteration of framework plagioclase grains, equivalence and correlation between the strati- quez Rocks sub-basin is thus implied (Fig. 14). rather than from primary deposition (Hendrix, graphic sections of these two sub-basins Uplift generated the gross upward-coarsening 1986). (Fig. 14). The anxstral Mint Canyon Ridge trends of the upper Vasquez intervals; enlarge- Time-equivalence of the upper Vasquez in provided a barrier between the two sub-basins, ment of the drainage basins led to increased both sub-basins is supported by the following: yet the absence of gneissic detritus in Vasquez water discharge into the depocenters (for exam- (1) similar sedimentologic character of deposits Rocks sub-basin implies that drainages off the ple, Schumm, 1981) and lower sediment/water (hyperconcentrated-flood-flow lithofacies); (2) ridge were asymmetric. The presence of Vasquez ratios of individual depositional events, thus presence of San Gabriel anorthosite and gabbro volcanic clasts in (he lower Vasquez of Texas contributing to the deposition of hyperconcen- clasts in Texas Canyon conglomerates, imply- Canyon implies that rifting and extrusive activ- trated flood flows rather than true debris flows. ing physical interconnection of the two ity had occurred in Vasquez Rocks sub-basin It is likely that this increased water discharge depocenters; and (3) northward paleoflow data prior to Texas Canyon deposition, thereby was not related to regional paleoclimatic from anorthosite/gabbro-bearing conglomerates strengthening the alwve-mentioned correlation. The abundance of sheetflow deposits in the megacyclic fine sequences of both sub-basins implies rapid vertical aggradation and basin sub- VASQUEZ SEDIMENTS sidence (Friend, 1S'78). Additionally, the rapid

SUB - BASINS CONTERMINOUS DRAINAGE BASIN

Figure 16. Paleogeographic block diagram illustrating upper Vasquez sedimentation, Vasquez Rocks and Texas Canyon sub-basins; lithofacies as in Figure 15; plus D, hyperconcentrated flood-flow.

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in both sub-basins. During tectonic uplift and FACIIS - C l AS T SUITES METERS reorganization, the ancestral Mint Canyon 2400 Ridge probably was dissected (Fig. 16), allow- UPPER VASQUEZ ing passage of detritus from Vasquez Rocks sub- DEBRIS FLOW

basin to Texas Canyon sub-basin. The eastern QUARTZ DIORITE SUITE source region, however, continued to supply abundant gneissic and granitic debris, longitudi-

nally dispersed along the axis of the Texas HYPERCONCENTRATED Canyon sub-basin. In addition, erosional rem- FLOOD FLOW nants of Mint Canyon Ridge gneisses projected ARKOSE SUITE above the expanding upper Vasquez fan system, MIDDLE VASQUEZ

as local gneiss breccias appear along the Vas- CHANNELIZED MIDFAN quez Canyon fault margin at this stratigraphie horizon. ARKOSE SUITE We realize that purely lithostratigraphic SHEETFIOW correlations of sedimentary units can be tenuous, especially on a small, bed-by-bed scale; rapid LOWER VASQUEZ DISTAL FLOODPLAIN lithofacies changes inherent in fluvial basins PLAYA make such correlations even more suspect QUARTZITE-METAVOLCANIC (Miall, 1983). The paucity and/or complete ab- SUITE sence of radiometric or biostratigraphic control BRAIDED FLOODPLAIN for the Vasquez Formation necessitates litho- 's LT 'SS ' CN G stratigraphic methods, however. The lithologie similarities discussed above are not fortuitous; it Figure 17. Composite stratigraphie section for Vasquez Formation in Charlie Canyon sub- is likely that large-scale cyclicity or similar litho- basin. Details as in Figure 4. stratigraphic features may be especially valuable for correlations among physically separate but geographically adjacent, unfossiliferous alluvial uous, lenticular beds of massive to planar, The lower braided-fluvial section passes gra- basins, particularly where cyclicity can be un- cross-stratified pebble conglomerates at the dationally upsection into a sequence of thinly equivocally linked to regional tectonism rather bases of these fluvial sequences pass upsection laminated, massive or ripple-marked fine sand- than to geomorphic or climatic processes. into sheetlike, coarse- to fine-grained, moderately stones and mudstones. Coarse-sandstone beds, well sorted sandstones suggestive of longitudinal- up to 25 cm thick, locally punctuate the section, CHARLIE CANYON SUB-BASIN bar, sandflat or transverse-bar deposits (Miall, as do concordant gypsum horizons locally inter- 1978a; Cant and Walker, 1978). Pebbles from stratified within the mudstones. These gypsum The Vasquez Formation attains a thickness of these lower cycles are ubiquitously well rounded horizons increase in thickness and number 2,400 m within the Charlie Canyon sub-basin and dominated by a chert-metarhyolite-granite westward along strike, concomitant with an in- (Fig. 17), within a west-southwest-plunging clast suite (Hendrix, 1986); clasts of arkose are crease in thickness and number of mudstone syncline (Fig. 18). The San Francisquito fault subordinate^ present beds. Paleoflow data, primarily from asymmet- dips at a high angle toward Charlie Canyon sub- basin (Sams, 1964; Königsberg, 1967) and separates Vasquez sediments from the Pelona SAN FRANCISQUITO FM. Schist along the southern margin (Fig. 18). The northern margin of the sub-basin is defined by the Bee Canyon thrust, which brings Upper IE E CANYON Cretaceous-lower Paleocene submarine-fan deposits of the San Francisquito Formation o structurally above the Vasquez (Kooser, 1982). In accordance with the proposal of Bohannon ptv» o? (1976), the Vasquez of Charlie Canyon sub- Tmc basin is subdivided into three intervals: lower, middle, and upper (Fig. 17). 12 8 Lower Vasquez ' QT Z. 'MONIÖ N ITE N This 800-m-thick interval is exposed in the eastern and northern regions of the sub-basin Tmç (sections 1, 3, 4, 5, 6, 10, and 11; Fig. 18). The lower portion of the interval is defined by 3- to 5-m-thick, upward-fining and -thinning sequences of braided-fluvial character. Discontin- Figure 18. General geologie map of Charlie Canyon sub-basin.

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Clast counts reveal important vertical trends within this upward-coarsening sequence. The distal to midfan conglomerates of the middle Vasquez interval are dominated by arkose clasts which superficially resemble rocks of the San Francisquito Formation; these clasts gradually diminish upsection, concomitant with increasing quartz-diorite and quartz-monzonite clasts (Hendrix, 1986). Sandstones from the middle Vasquez feature argillaceous grains of sedimentary provenance, whereas upper Vasquez sandstones ai e dominated by plagioclase, quartz, and potassic feldspars of plutonic origin (Hendrix, 1986). Paleocurrents from numerous clast imbrications reveal a significant northerly paleoflow trend, ostensibly from across the San Francis- quito fault (Fig. 19). As noted in previous Figure 19. Paleocurrent data from Charlie studies (Königsberg, 1967; Bohannon, 1976), Canyon sub-basin; details as in Figure 6. however, no Pelona Schist clasts appear in the Vasquez sediments of Charlie Canyon sub-basin; therefore, it is probable that the Pelona Schist ric ripple marks on bedding planes, suggest (for example, Bull, 1977); the channelized- was either covered by other rocks or emplaced westward transport (Fig. 19). midfan deposits are overlain by coarse, massive by strike-slip faulting subsequent to Vasquez to horizontally bedded conglomerates with well- sedimentation. Middle and Upper Vasquez imbricated clasts (Fig. 20B). These conglomerates closely resemble the hyperconcentrated- Paleogeographic Synthesis The remaining 1,600 m of middle and upper flood-flow deposits of Smith (1986). The Vasquez defines an upward-coarsening, pro- remainder of the Vasquez section consists of The lower Vasquez accumulated on a broad grading-fan sequence (sections 2,7,8,9, and 12; poorly sorted breccias and conglomerates, flood plain, and probably was derived from a Fig. 18). This sequence appears to grade both which coarsen and become highly matrix- relatively distant eastern source. Although Bo- vertically and laterally into the mudstones of the supported upsection (Fig. 17); many of these hannon (1976) considered the San Francisquito lower Vasquez and is defined at its base by hori- breccias display bedding and fabric characteris- fault to have been active during lower Vasquez zontally bedded, moderately well sorted coarse tics of debris-flow deposits (for example, Miall, sedimentation, there are no rapid thickness, sandstones similar to "terminal-fan" sheetflow 1978a; Shultz, 1984). A thick vitric crystal tuff coarseness, or lithofacies changes close to this deposits (Friend, 1978). This sheetflow litho- of probable airfall origin is locally interbedded fault. The gradual westward fining and lithofacies is overlain by pebbly, highly channelized with the upper Vasquez breccias; it is undated facies changes, relatively good sorting of sand- and lenticular sandstones of probable high- but is petrologically similar to tuffs from Vas- stones, and good rounding of conglomerate energy, braided-midfan environments (Fig. 20A) quez Rocks and Texas Canyon sub-basins. clasts suggest that these sediments accumulated

Figure 20. Alluvial lithofacies, Charlie Canyon sub-basin. (A) Channelized-mid-fan lithofacies, middle Vasquez. (B) Well-imbricated clasts in hyperconcentrated-flood-flow lithofacies, upper Vasquez.

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on a broad alluvial braidplain and had a distant LOWER VASOUEZ (QUARTZ I T E , METAVOLCAN ICS, GRANITE , ARKOSE ) eastern source. Additionally, many of the lower DISTAL EASTERN SOURCE Vasquez clasts bear no similarity to local basement lithologies, implying possible distant or "exotic" sources. If the San Francisquito fault was not active during lower Vasquez sedimentation, it certainly X. became active at the onset of middle Vasquez deposition. A thick alluvial-fan system pro- graded northward from a source terrane uplifted across the fault (Fig. 21); this system buried the lower Vasquez braidplain sediments as it pro- graded. As initially proposed by Königsberg (1967), the absence of Pelona Schist debris in the Vasquez may imply that the schist had a (ARKOSE, QTZ. DIORITE, tectonic cover of quartz diorite and quartz mon- QTZ. MONZONITE)

zonite, overlain by San Francisquito sedimen- MIDDLE - UPPER

tary deposits. As the allochthon was eroded, it VASQUEZ SOURCE would have produced an inverted stratigraphy of clasts in the Vasquez conglomerates in the adjacent sub-basins; the presence of quartz- monzonite and San Francisquito-like arkose clasts in Texas Canyon sub-basin also supports this hypothesis. The quartz monzonite exposed in the southwest corner of Charlie Canyon sub- basin (Fig. 18) may be a remnant of this eroded, Figure 21. Paleogeographic block diagram, Charlie Canyon sub-basin; lithofacies as in hypothetical allochthon. Figures IS and 16; plus F, braided flood plain. The absence of either ancestral San Gabriel basement clasts or distinct megacyclicity in the Vasquez deposits of Charlie Canyon sub-basin nate the middle Vasquez suites in Charlie Can- Soledad area originated no earlier than 14 m.y. suggests no physical interconnection or deposi- yon sub-basin. As age control for the Vasquez B.P. and 5 m.y. B.P., respectively, long after tional similarity between this depocenter and Formation, in general, is poorest in Charlie Vasquez sedimentation (Crowell, 1975). Plate Vasquez Rocks or Texas Canyon sub-basins, Canyon sub-basin, chronostratigraphic correla- reconstructions for the eastern Pacific region other than the nonmarine nature and similar age tion between what is called "Vasquez" in this (Powell, 1981; Engebretson, 1982; Glazner and of the sediments. If the ancestral Sierra Pelona sub-basin and the Vasquez of Vasquez Rocks Loomis, 1984) suggest that the northward- was present at this location and was the princi- and Texas Canyon sub-basins remains poorly migrating Mendocino triple junction and incip- pal source area for middle and upper Vasquez constrained. ient San Andreas transform system probably lay detritus in Charlie Canyon, then it seems logical south of the Soledad terrane during initial rifting that it prevented interconnection with Texas TECTONIC IMPLICATIONS OF of the basin (Fig. 22). In fact, uncertainties in the Canyon depocenter to the southeast. Recent THE VASQUEZ FORMATION timing of the initiation of the triple junction are study of central Transverse Ranges basement great enough that a reconstruction involving structures (Powell, 1981), however, requires The association of thick alluvial fill with vol- triple-junction initiation immediately prior to 80-100 km of mid-Tertiary dextral offset along canics is a hallmark of many continental rift Soledad basin formation (26-25 m.y. B.P.) is the San Francisquito fault as part of a much basins, both those related to transtensional or permissible, as opposed to the commonly larger strike-slip fault system, in order to palin- pull-apart tectonics, and those of purely quoted 30-29 m.y. B.P. (Atwater, 1970). These spastically restore basement trends. These dis- orthogonal rifts (Dickinson, 1976). As this uncertainties include corrections for Basin and placements would have placed the original site association exists in Vasquez Rocks sub-basin, Range extension (Ingersoll, 1982; Wernicke and of Charlie Canyon sub-basin far from the other and as Texas Canyon sub-basin apparently others, 1982) and the accuracy of the plate- sub-basins, possibly adjacent to the La Panza shared a tectonically similar history, it is likely circuit method (Engebretson, 1982). Our fa- Range in the southern Coast Ranges. Ehlig and that these two depocenters originated within a vored reconstruction places the Soledad basin Joseph (1977) and Smith (1977) provide con- zone of crustal extension (see Muehlberger, inboard of a convergent margin at 25 m.y. B.P., vincing petrologic, isotopic, and structural evi- 1958; Nilsen, 1984). It is tempting to envision and thus may explain the subalkaline/calc- dence that the La Panza Range quartz mon- the Soledad basin as structurally related to alkaline chemical affinities of Vasquez volcanics zonites may have been the source for the wrench tectonics similar to that which generated (Weigand, 1984). Key sedimentologic features quartz-monzonite clasts in the Charlie Canyon many other Cenozoic basins in southern and of alluvial basins in compressional settings in- Vasquez deposits. Upper Cretaceous arkosic and central California (for example, Blake and oth- clude distinct intraformational unconformities, pelitic rocks unconformably overlie the La ers, 1978). It is clear, however, that much of this syndepositional folds (Miall, 1978b), and canni- Panza Range quartz monzonites; it is possible wrench tectonism accompanied evolution of the balization and reworking of proximal, synoro- that these strata could have been the source for San Andreas transform system (Atwater, 1970). genic fan deposits (DeCelles and others, 1987). the "San Francisquito-like" clasts, which domi- The San Gabriel and San Andreas faults in the None of these features has been observed in the

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long, the alluvial system may have time to Figure 22. Plate-tectonic re-establish equilibrium, resulting in upward- setting of North American fining intervals above each upward-coarsening continental margin during megacycle, thus obscuring tectonic control of rifting of Soledad basin megacyclicity. The evaporitic sulfates and carbon- 25-24 m.y. B.P. PAC, Pacific ates and the lack of well-developed pedogenic plate; NA, North American features in the Vasquez Formation suggest a plate; FAR, Farallon plate; semiarid paleoclimate (for example, Birkeland, MTJ, Mendocino triple junc- 1974). Abundance of debris-flow deposits in the tion; ISAS, incipient San Vasquez indicates that coarse debris accumu- Andreas transform system. lated for sufficient time to reach geomorphic Predicted lithospheric exten- thresholds of instability in drainage channels, sion direction due to unsta- thus implying semiarid paleoclimate (Bull, 1977; ble triple-junction "hole" is Schumm, 1981); therefore, it is unlikely that illustrated by large double climatic effects blurred evidence of tectonic in- arrows. Note position rela- fluences on sedimentation. The high sedimenta- tive to Soledad basin (after tion rates calculated for the Vasquez suggest Powell, 1981; Ingersoll, 1982; dynamic tectonism, rapid subsidence, and fre- Glazner and Loomis, 1984). quent fault displacements (for example, Read- ing, 1980); therefore, megacyclicity probably reflects tectonic style. Sedimentologic data do Vasquez Formation; therefore, we contend that occur in orthogonally rifted alluvial basins not provide evidence for strike-slip tectonism extensional tectonics played the key role in basin (Schwab, 1976; Hempton, 1983). A subordi- during Vasquez sedimentation. In addition, the evolution. nate, but possibly significant characteristic of thick volcanic sequence low in the Vasquez Rocks section seems to favor early magmatic Distinguishing strike-slip or pull-apart basins sedimentation in alluvial depocenters adjacent activity and high geothermal gradient, possibly from non-strike-slip rifts based solely on sedi- to active strike-slip faults is the development of in an active rift zone. Although magmatism also mentologie grounds can be difficult (Reading, stacked, upward-coarsening megacycles (several can be associated with pull-apart basins, such 1980; Hempton, 1983). The most direct lines of hundreds of metres in thickness) (Steel and volcanism is generally a later evolutionary event evidence for characterizing strike-slip tectonics Gloppen, 1980; Hempton, 1983). Such upward rather than an early process (Mann and others, during sedimentation are (1) lithologic mis- coarsening cyclicity results from periodic rapid 1983). matches between sediments and source areas horizontal offsets of alluvial depocenter and and (2) evidence of depocenter migration with source terrane along a strike-slip fault, followed The k inematics of Soledad orthogonal rifting time (Steel and Gloppen, 1980; Crowell, 1982; by progradation of the fan system in response to may be related to upper-lithosphere brittle fail- Hempton, 1983). Documentation of the latter vertical displacements associated with the hori- ure in the North American plate, as the conti- includes recognition of stratigraphic younging in zontal offset. Such upward coarsening mega- nental margin was uplifted above the buoyant, the direction of scurce-terrane migration (Steel cycles appear in several ancient alluvial basins of young, subducting Farallon plate (Nilsen, 1984), and Aashiem, 1978; Crowell, 1982), which proposed strike-slip origin, including the Violin and/or as the unstable Mendocino triple junc- cannot be accomplished with the Vasquez due Breccia of Ridge basin (M. Link, 1985, personal tion evolved (Ingersoll, 1982). Ingersoll's (1982) to poor age control and lack of proper Vasquez/ commun.), the post-Hercynian basins of Spain model predicts northwest-southeast extension in basement onlap exposures. The mismatch condi- (Heward, 1978a, 1978b), and the Devonian the North American plate north of the migrating tion, however, apparently exists along the Hornelen basin of Norway (Steel and Aashiem, triple junction, which concurs with the pre-rota- Soledad fault margin of Vasquez Rocks sub- 1978). In Vasquez Rocks and Texas Canyon tional stress requirements for Soledad basin basin, where anorthositic basement is faulted sub-basins, however, the four alluvial mega- rifting. It is likely that local extension of the against Lowe Granodiorite-bearing breccias of cycles are upward fining; nowhere is there evi- North American plate occurred immediately the megacyclic Vasquez interval, producing dence of stacked, upward-coarsening cyclicity. north of the unstable triple junction, just as in- sinistral separation, in map view. This anomaly, Although the model linking tectonic style and stability may have triggered more regional, however, is explained by the upsection increase alluvial megacyclic sedimentation is tenuous due detachment-fault-related extension in the Mo- of anorthosite clasts and sandstone P/F values in to the paucity of published case histories, there jave block and southern Basin and Range prov- the Vasquez sediments (Hendrix, 1986), re- are ancient examples of upward-fining alluvial ince to the east of the migrating triple junction flecting erosional unroofing of anorthosite from allocycles in basins where evidence of strike-slip (Ingersoll, 1982; Glazner and Bartley, 1984). beneath a mantle of Lowe Granodiorite. It is, deformation cannot be found (for example, Steel The "subducted Mendocino fracture zone" therefore, not neccssary to invoke syn-Vasquez and Wilson, 1975; Mack and Rasmussen, model of Glazner and Loomis (1984) does not strike-slip along the Soledad fault to explain this 1984). We feel that this criterion could prove explain Soledad basin formation after the contact relationship. Continued uplift after potentially useful for delineating ancient strike- 45-degree Miocene clockwise rotation of this erosional exposure of the anorthosite juxtaposed slip basins, after a sufficient database on terrane is palinspastically corrected (for example, the Lowe-bearing units against the Precambrian large-scale, cyclic alluvial sedimentation is Luyendyk and Hornafius, 1987; Terres and basement. Nowhere else adjacent to the Vasquez accumulated. others, 1981). Whole-scale uplift and drainage- depocenters is there any evidence of sediment/ An important codicil for the use of mega- system enlargement during upper Vasquez sedi- source-lithology mismatch. cyclicity in deducing tectonic style relates to mentation, however, may be explained by the High sedimentation rates have been used to basin/source paleoclimate and to the recurrence Glazner and Loomis model, as the terrane south characterize pull-apart basins (Miall, 1978b); interval of marginal-fault displacements. If paleo- of the Vasquez depocenters was uplifted tec- however, rapid sediment accumulation also can climate is too humid or recurrence interval too tonically during fracture-zone passage. Miocene

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Ehlert, K. W., 1982, Basin analysis of the Miocene Mint Canyon Formation, 1980, Cyclicity and the fades model concept in fluvial deposits: Bulletin clockwise rotations occurred later within the southern California, in Ingersoll, R. V., and Woodburne, M. O., eds., of Canadian Petroleum Geology, v. 28, p. 59-80. evolving San Andreas transform system; post- Cenozoic nonmarine deposits of California and Arizona: Society of 1983, Basin analysis of fluvial sediments: International Association of Economic Paleontologists and Mineralogists, Pacific Section, p. 51-64. Sedimentologists Special Publication 6, p. 279-286. Vasquez, pre-Tick Canyon Formation deforma- Ehlig, P. L., 1981, Origin and tectonic history of the basement terrene of the Muehlberger, W. R., 1958, Geology of northern Soledad basin: American tion may have been related to these rotations San Gabriel Mountains, central Transverse Ranges, in Ernst, W. G., ed., Association of Petroleum Geologists Bulletin, v. 42, p. 1812-1844. The geotectonic development of California (Rubey Volume I): Engle- Nilsen, T. H., 1984, Oligocene tectonics and sedimentation: Sedimentary Geol- and/or to passage of the Mendocino fracture wood Cliffs, New Jersey, Prentice-Hall, p. 253-283. ogy, v. 38, p. 305-336. Ehlig, P. L., and Joseph, S. E., 1977, Polka dot granite and correlation of La Oakeshott, G., 1958, Geology and mineral deposits of San Fernando quadran- zone beneath the Soledad terrane in the early Panza quartz monzonite with Cretaceous batholithic rocks north of gle, Los Angeles County, California: California Division of Mines and Miocene. Salton Trough, in Howell, D. G , Vedder, J. G„ and McDougall, K., eds., Geology Bulletin 172,147 p. Cretaceous geology of the California Coast Ranges, west of the San Oilier, C- D., 1967, Landforms of the newer volcanic province of Victoria, in Andreas fault: Society of Economic Paleontologists and Mineralogists, Jennings, J. 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E., 1954, Marine-nonmarine 1978b, Tectonic setting and syndepositional deformation of molasse MANUSCRIPT RECEIVED BY THE SOCIETY MAY 8,1986 relationships in the Cenozoic section of California: California Division and other nonmarine-paralic sedimentary basins: Canadian Journal of REVISED MANUSCRIPT RECEIVED DECEMBER 15,1986 of Mines and Geology Bulletin 170, p. 59-72. Earth Sciences, v. 15, p. 1613-1631. MANUSCRIPT ACCEPTED DECEMBER 31,1986

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