Climate-controlled landscape evolution in the Western Transverse Ranges, California: Insights from Quaternary geochronology of the Saugus Formation and strath terrace fl ights

Duane E. DeVecchio1, Richard V. Heermance2, Markus Fuchs3, and Lewis A. Owen4 1EARTH RESEARCH INSTITUTE, UNIVERSITY OF CALIFORNIA, SANTA BARBARA, CALIFORNIA 93106-9630, USA 2DEPARTMENT OF GEOLOGICAL SCIENCES, CALIFORNIA STATE UNIVERSITY, NORTHRIDGE, CALIFORNIA 91107, USA 3DEPARTMENT OF GEOGRAPHY, JUSTUS-LIEBIG-UNIVERSITY GIESSEN, 35390 GIESSEN, GERMANY 4DEPARTMENT OF GEOLOGY, UNIVERSITY OF CINCINNATI, CINCINNATI, OHIO 45221, USA

ABSTRACT

The Las Posas and Ojai Valleys, located in the actively deforming Western Transverse Ranges of California, contain well-preserved fl ights of strath terraces and Quaternary strata (i.e., Saugus Formation) that when numerically dated elucidate the tectonic, geomorphic, and fl uvial histories that sculpted the landscape since ca. 140 ka. This study includes 14 new optically stimulated luminescence and 16 new terrestrial cosmogenic nuclide ages from the late Pleistocene to Holocene that record two regional aggradation events and four intervals of strath terrace formation. Geochronologic data indicate that terrestrial Saugus strata in the Las Posas Valley (Camarillo Member) prograded over marine deposits at ca. 125 and 80 ka and are as young as 60–25 ka, which is an order of magnitude younger than the youngest Saugus strata elsewhere in Southern California. These results highlight the need for precise dating of Saugus strata where identifi ed and utilized to assess rates of tectonic deformation. Based on its compositional character, thickness, stratigraphic relations, and inferred ages, the Camarillo member of the Saugus Formation is correlated with sediments of the Mugu aquifer identifi ed in subcrop throughout the Ventura Basin and thus provides a new regional chronostratigraphic subsurface datum. The aggradation of these sediments and similar deposits in the study area between 13 and 4 ka is subsequent to the transition from humid to semiarid climate correlating to the end of the ultimate and penultimate glacial maximums. Aggradation is inferred to have resulted from increased sediment supply in response transient vegetative conditions and consequent hillslope destabilization. Similar to aggradational events, strath terrace cover sediments ages correlate to dry warm climate intervals, indicating straths in Southern California were cut at ca. 110–100 ka, 50–35 ka, 26–20 ka, and 15–4 ka. These results support recent mathematical and experimental models of strath formation, where increased sediment fl ux and decreased water discharge enhances lateral erosion rates and inhibits vertical incision. Subsequent incision and strath terrace formation is inferred to occur during intervening wet climate intervals. The correlation of strath terrace ages and aggradational events with environmental changes that are linked to global climate indicates that climate rather than tectonics exhibits fi rst-order control of depositional, denudational, and incisional processes in the Western Transverse Ranges. Moreover, these results provide a chronostratigraphic framework that allows these landforms to be regionally correlated and used to assess rates of active tectonics where geochronologic data are unavailable.

LITHOSPHERE; v. 4; no. 2; p. 110–130; GSA Data Repository Item 2012077. doi: 10.1130/L176.1

INTRODUCTION which are capable of numerically dating a variety of Earth materials over time scales of 105 yr, detailed chronologies can now be developed for In actively deforming regions, landscape evolution is controlled by regions that have previously lacked late Quaternary geochronological data the interplay between tectonics and climate, which modulate deposi- (e.g., National Research Council, 2010). This study provides insight into tional, incisional, and denudational processes (e.g., Whipple and Tucker, the processes governing landscape evolution and develops a chronologic 1999; Anders et al., 2005; Wobus et al., 2006; Kamp and Owen, 2012). framework for estimating ages of regional geomorphic features that can Consequently, quantifi cation of the age of both erosional landforms and be used for quantifying tectonic and geomorphic processes where site- depositional strata provides critical insight into the effects of past climate specifi c geochronology is otherwise unavailable. variability and active faulting of Earth’s surface. Furthermore, an under- The Western Transverse Ranges of Southern California are an ideal standing of the effects of climatic and/or tectonic changes on erosion and location to study the driving forces of landscape evolution because the aggradation is critical to modeling all aspects of the landscape linked region is easily accessible and has a well-documented late Pleistocene pre- to rock, ocean, and atmospheric systems, and for quantifying local and cipitation (e.g., Kirby et al., 2006), sedimentation (Kennett et al., 1995), regional earthquake hazard (e.g., Rockwell, 1983; Rinaldo et al., 1995; vegetation (Heusser, 1995), and tectonic history (e.g., Yeats, 1988a). In Whipple et al., 1999; Roering et al., 2007). With increasingly widespread addition, the need for precise rates of active faulting in Southern Cali- usage of Quaternary geochronological techniques, such as optically stim- fornia necessitates geochronological investigation of deformed landforms ulated luminescence (OSL) and terrestrial cosmogenic nuclides (TCNs), and sedimentary deposits. However, few ages based on geochronological

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data of landscape features in this region exist. Therefore, this study focuses Strath terraces are common along rivers within the Western Trans- on numerical dating of late Pleistocene aggradational deposits, including verse Ranges, are shown on numerous geologic maps (e.g., Kew, 1924; the Saugus Formation, and bedrock strath terraces preserved in the Las Rockwell et al., 1988; Dibblee, 1992a, 1992b), and are widely used in Posas and Ojai Valleys (Fig. 1). These study areas are actively deform- fl uvial and tectonic studies (e.g., Rockwell et al., 1984). However, the ing (Rockwell et al., 1984; Yeats, 1988b; DeVecchio et al., 2012) and are processes that control their formation are still debated (e.g., Bull, 1991; characterized by different rates and styles of tectonic deformation, and, Pazzaglia et al., 1998; Hancock and Anderson, 2002; Wegmann and Paz- therefore, they are ideal for comparing the effects of local tectonics and zaglia, 2009; Finnegan and Dietrich, 2011). Strath terraces represent global climate oscillations on the depositional and denudational processes ancient and now-abandoned river fl oodplains that formed during periods that control landscape evolution. of valley-bottom widening by lateral planation. Bull (1990) suggested The Saugus Formation is of particular interest because it is a region- that lateral erosion and strath cutting are the dominant processes when ally extensive Pliocene–Pleistocene shallow-marine and terrestrial streams are in dynamic equilibrium, where river discharge and sediment sedimentary deposit that is deformed across numerous active faults load are in balance. Recent quantitative fi eld studies and conceptual and and folds within the Western Transverse Ranges (Fig. 1) (e.g., Hershey, numerical models predict they form in response to balanced or oscillating 1902; Kew, 1924; Yeats, 1988b). The formation is commonly cited sediment supply (Formento-Trigilio et al., 2003; Hancock and Anderson, as the youngest unit involved in tectonic deformation and has been 2002, respectively), elevated sediment supply (Fuller et al., 2009), and/or reported in numerous consulting reports and paleoseismic trench logs, periods of stable base level (Pazzaglia and Gardener, 1993). Experimental and its inferred upper age limit (200–500 ka) is typically utilized for results indicate that during high sediment supply, transient fl uvial deposits assessing rates of tectonic deformation (e.g., Weber et al., 1976; Yeats, inhibit incision by armoring the channel (e.g., Sklar and Dietrich, 2001), 1983; Yeats et al., 1988; Huftile, 1991; Huftile and Yeats, 1995, 1996; which leads to a wider band of active erosion and lateral channel erosion Baldwin et al., 2000; Meigs et al., 2003; Geosoils, Inc., 2007). There- (Finnegan et al., 2007). As experimental and mathematical models con- fore, the age of the Saugus Formation is critical for evaluating rates of tinue to unravel the complexities in the processes controlling strath forma- deformation on numerous active structures in the Western Transverse tion, fi eld-based studies are required to compare local driving mechanisms Ranges. Unfortunately, the Saugus Formation is poorly defi ned, highly with predicted forcings. By focusing on high-precision numerical dating variable, and regionally diachronous (Blackie and Yeats, 1976; Levi of these denudational landforms, in addition to aggradational deposits, and and Yeats, 1993; Levi et al., 1986; Yeats, 1983). Thus, the upper age of correlation of these features to the well-documented climate proxy records the Saugus is poorly known. of Southern California, we illuminate the effects of water discharge and

119° 00“ W 118° 30“ W Califo

Ojai Valley San Gabriel Mts. rnia 34° 30 N

San Cayetano F.. “ Mw6.6 N

Area of Figure 4B Santa Susana Mts. San Gabriel F. Oak Ridge F. Sierra Madre F.

Ventura F. Simi Valley Ventura SantaSanta SSusanausana F.F. Verdugo Mts. Simi fault zone Santa Santa Clara San M w 6.7 Barbara River Area of Figure 4A Fernando Channel Oxnard Valley Port Plain Hueneme Santa Monica Mts. Saugus Fm. Point Hollywood F. Strath terraces Mugu Los Angeles 34° 00 Santa Clara River Basin Malibu Coast F. 0 km 20 Reverse fault “ Santa Monica F. N Strike-slip fault 119° 00“ W 118° 30“ W

Figure 1. Simplifi ed index map of the principal active faults in Southern California with respect to the study areas (black boxes) draped over a topographic hillshade image. The east-west structural grain of this part of California defi nes the Western Trans- verse Ranges tectonic province, with the Santa Monica fault delineating the southern boundary. Darker-gray areas show the distribution of previously mapped Pleistocene Saugus Formation, which was deposited by the ancestral Santa Clara River that drains the San Gabriel Mountains. Note that many of the faults deform the Saugus Formation, which is typically the youngest

unit deformed by faulting in the region. Focal mechanisms show the approximate epicenter locations for the 1971 Mw 6.6 San

Fernando–Sylmar, and 1994 Mw 6.7 Northridge earthquakes (lower-hemisphere projections), which both deform Saugus strata. Dashed polygons show the locations of selected group of strath terraces in the province that are locally faulted and folded.

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Weber et al. (1976) Pressler (1929) Jakes (1979) Kew Bailey (1951) Rockwell et al. Huftile (1991) Hanson Geologic Geologic Las Posas area Dibblee Yeats et al. (1924) This Study (1984); Dibblee Huftile and et al. Period Epoch (1992a, 1992b) (1994) (1990) Yeats (1995) (2003) Lithologic Units and Formations Aquifers Recent Alluvium Alluvium (Qa2) Recent Alluvium Shallow Holocene (Active lagoonal, beach, river, and floodplain, and alluvial deposits) (Active alluvium) Older Alluvium Terrace deposits Older Alluvium (Qa1) Older Alluvium Oxnard (Deformed beach, river, floodplain, (Deformed river (Incised and gently (Deformed river and terrace deposits) deposits) folded fluvial deposits) deposits) Saugus Formation Saugus Mugu Upper (Qs2, Qs3, and Qs4) Formation Pleistocene Saugus Formation Saugus Saugus Formation Saugus Upper and (Terrestrial fluvial) Formation (Terrestrial and marine sand Las Posas Sand Formation Lower

Quaternary (Terrestrial) and gravel) (Shallow marine sand, Hueneme San Pedro Formation thickening westward) (Marine clays and sand Las Posas Sand Fox and terrestrial sediment) Lower (Shallow Canyon Pleistocene Santa Barbara Formation marine sand) Grimes (Shallow marine sand) Canyon Pico Formation Fernando GroupAbsent Fernando Group Non-water Pliocene (Marine siltstone and sandstone) bearing Modelo Formation Monterey Modelo Monterey Siquoc and Monterey Formations (Marine mudstones) Formation Formation Subsurface only Miocene Conejo Volcanics Rincon and Vaqueros Formations

Tertiary (Terrestrial and marine extrusive and intrusive igneous rocks) Oligocene/ Sespe Formation Eocene (Sandstone and cobble conglomerate)

Figure 2. Stratigraphic nomenclature table illustrating the diversity of local lithologic naming of Pliocene–Pleistocene to Holo- cene strata in the Los Angeles and Ventura Basins. Ojai Valley nomenclature is shaded gray.

river sediment load on depositional and erosional processes that sculpted chronology, and fi ssion-track dating (see Yeats, 1988b, and the refer- the landscape in response to past climate variability. ences therein). However, these strata are diachronous (Yeats, 1977), vary widely in thickness along strike, and lack chronostratigraphic markers GEOLOGIC SETTING near the top of the section (Yeats, 1988a, 1988b). Consequently, the upper age limits for deformed strata within the Western Transverse Regional Structural and Stratigraphic Framework Ranges are poorly defi ned. The name Saugus Formation is loosely applied to regionally extensive The California Borderlands west of the San Andreas fault have expe- (Fig. 1), shallow-marine, and terrestrial strata that are predominately con- rienced a multiphase tectonic evolution following cessation of subduc- formable with underlying Pliocene–Pleistocene deep-marine strata (e.g., tion during early Miocene time, including microplate capture, clockwise Yeats et al., 1988; Levi and Yeats 1993). The Saugus Formation is up to rotation in excess of 90°, and syntectonic volcanism (e.g., Kamerling and 2000 m thick in the Santa Clara River Valley and thins to the north and Luyendyk, 1979; Luyendyk et al., 1980; Nicholson et al., 1994; Weigand south toward each of the study regions (Fig. 3). In the Santa Clara River et al., 2002), as well as transpressional deformation related to the “Big Valley and on the Oak Ridge–South Mountain uplift (Fig. 4A), the Saugus Bend” of the San Andreas fault (e.g., Wright, 1991; Dolan et al., 1995; Formation is dominated by metamorphic and plutonic clasts originating Shaw and Suppe, 1996; Yeats, 1988a). Thick sections of volcanic and volcaniclastic rocks of the Conejo volcanics and deep-marine siliceous mudstones (Monterey Formation) accumulated in middle Miocene trans- Figure 3. Composite Ojai Santa Oak Las Posas tensional fault-bounded basins, with the Oligocene Sespe Formation on stratigraphic columns Valley Paula Ridge Valley 0 the upthrown side (Kew, 1924; Durrell, 1954; Ehrenspeck, 1972; Jakes, from four locations in Qa Shallow and Oxnard aquifers Qa 1979; Williams, 1983; Ingersoll, 2001). Extensional deformation of the the Western Trans verse 100 Mugu Qs Ranges from north QTlp region continued until ca. 5 Ma, at which time development of the “Big Qs to south illustrating 200 Qs Qs Bend” of the San Andreas fault resulted in transpressional deformation Ts Pliocene–Pleistocene 500 and inversion of Miocene basins (e.g., Crowell, 1976; Wright, 1991). Pre- stratigraphic relations Qs Qs Hueneme vious studies in the Western Transverse Ranges indicated that the rates of and thickness variabil- 600

QTlp illo shelf illo

ity of the Saugus For- Thickness (m) tectonic uplift due to Pliocene–Pleistocene shortening varied from ~1 to 700 Qs 15 mm/yr (e.g., Lajoie et al., 1979; Rockwell et al., 1984; Yeats, 1988a; mation (shaded). Note QTp Huftile and Yeats, 1995). that the Saugus For- 800 Camar mation (shaded gray) QTp m Following Miocene transtension and volcanism, deep-marine basins thins signifi cantly to 900 T were fi lled by a progradational sequence of Pliocene–Pleistocene deep- Fox Canyon the north and south 2000 marine turbidites, shallow-marine sandstone and mudstone, and terrestrial of Santa Paula, Califor- sandstone and conglomerate (Kew, 1924; Winterer and Durham, 1958; nia, which is located in 2100 QTp Qs the Santa Clara River Ts Weber et al., 1976; Yeats, 1965, 1977). In the Las Posas and Ojai Val- 2200 leys, these strata have been given a variety of formational and local names Valley. Generalized col- QTp umn location names are shown within the study areas on Figure 4. Impor- based on stratigraphic position and inferred correlations to strata in the tant aquifer unit names are shown between the Santa Paula and Oak Ridge Los Angeles Basin (Fig. 2). stratigraphic columns. Qa—alluvium; Qs—Saugus Formation; QTlp—Las Regionally, the age of Pliocene–Pleistocene strata shown in Figure 2 Posas Sand; QTp—Pico Formation/Fernando Group; Tm—Monterey/ is locally defi ned by biostratigraphy, magnetostratigraphy, tephro- Modelo Formation; Ts—Sespe Formation.

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119° 00′ W TpTp QsQs A Qa2Qa2 SaSantanta ClaraClara N QTlpQTlp River ValleyV Valllleyeey TsTs Santa TmTm QsQs Oak Ridge fault Susana fault Santa TpTp Paula TmTm Qs1Qs1 TsTs Qs1Qs1 Ventura OakOaOak RiRRidRidgeidgdgee/ /S//SouthSououuthuttthh MountainMMounouountnntainttainain upliftuuppliliftft TmTm BBiBigg MoMouMountainuntatainin fault Qa2Qa2 QTlp Tp Tcv-TtTcv-Tt Santa Clara TmTm Qt1Qt1 97 ± 7 ka River Qs1Qs1 102 ± 13 ka 13 ± 1 ka Qa2Qa2 Qa1Qa1 TpTp 5 ± 1 ka Qs1Qs1 Qt1Qt1 Qs1Qs1 Location Qs1Qs1 Qs1Qs1 MM Qt1Qt1 of Fig. 8A Location Qs1Qs1 Moorpark TsTs of Fig. 9 Qa1Qa1 Qs1 QTlpQTlp WestWeWestst LLasasa Posas PPosaPasososasa ValleyValley EastEastst LLasaass PosasPPooossaass VaValleyValleley Qt2Qt2 Qa1 Qt2 QsQs TsTs 25 ± 2 ka 34° 16 N ′ Qt2Qt2 CamarilloCamarillo foldfold beltbelt 27 ± 4 ka 125 ± 9 ka Qt2Qt2 Qa2Qa2 Qs3Qs3 Qt2Qt2 ′ 78 ± 6 ka Calleguas TsTs 141 ± 10 ka Tcv-TtTcv-Tt N 34° 16 Qa2Qa2 Creek QlsQls Qs2Qs2 QTlpQTlp Qs2Qs2 Optically stimulated Qs2Qs2 Tcv-TtTcv-Tt luminescence age Cosmogenic nuclide age Qs3Qs3 Qs3Qs3 QTlpQTlp Qs2Qs2 TsTs 84 ± 7 ka Western extent QTlpQTlp Reverse fault Radiocarbon age of the Qa2Qa2 Qa2Qa2 Qt2Qt2 85 ± 6 ka Qs3Qs3 Simi fault zone Quatenrary Tertiary Tcv-TtTcv-Tt Younger strath Camarillo Qt2 terrace Tp Pico Fm. 23 ± 1 ka Active fluvial and Qa2 alluvial deposits Qt1 Tm Monterey Fm. Oxnard Plain Qs4Qs4 Older strath terrace Conejo Volcanics and Qa1 Incised fluvial and Qs(1-4) Saugus Fm. Tcv-Tt km 119° 00′ W alluvial deposits Topanga Fm. Qls Landslide complex QTlp Las Posas Sand Ts Sespe Fm. 0 1 2 3 4 5

119° 15′ W TToTopatopaopapattoopapa MMountainsouountntainsns TcTcd B Tcd Tj N Tcd Tmama QQs TcwTcw Tcww QfQ Ventura Qf River Tcd Qa Ts Qss TcwT Tss Qt5bQt5bb Qf llowerowewerOjr OOjOjaijaaii VValleValleyaalleylley QsQ Qs TTsSanS Cayetano fault 35 ± 10 ka QfQ QsQ TcTcw Ojai Tm/Tm/TsqTsqT Qt6QtQt6ct6c Tr/Tr/TvqTvqT QtQQt5att5a5a Arroya Parida fault Qt6QQt6aa Qs Qf Sisar Qs Qf Ts Creek 7 ± 4 ka BlBlackaca k upupperupppeerrOj Ojai OjOjaiai ValleyValley 34° 25 Qt5b Qt6b N

″ Qss QQtQt4tt44 Location MoMountainsuntains Tr/Tr/Tvq/Tvqvqq of Fig. 6 30 TTmTm/Tm/TsqTsqsqq Tr/TTr/Tvqrr/TvqTvqv ′

Qt6c ′ 30 Qt6bt6bb Tr/Tr/TvqTvq Qt6aQt6Qt6aa QQs ″ N

34° 25 Ts QQtQt5Qt5at5t5a Qss Qt5Qt5a5a TTm/Tm/TsqTsTsqq Tr/TTr/TvqTvqTv Qt6bQt6bb Tm/Tm/TsqTsq Qt6aQt6a TmTTm/Tsqm/m/TsqTsTssqq Oak View SSuSulphurulplphhuur MMoMountainoununtatainin Tpp faults Qt6bQt6bb Quaterary Tertiary Qa Active fluvial deposits Qt6aQtQt6att666a Qt5b Strath terrace Monterey and Sisqu0c Fms., Matilija Sandstone Tm/Tsq undifferentiated Tma Oak Qf Alluvial Fan Qt6a Strath terrace Tr/Tvq Rincon and Vaquros Fms., Tj Juncal Shale View San undifferentiated Antonio Qls Landslide complex Qt6b Strath terrace Ts Sespe Fm. Creek TpTp Strath terrace Reverse fault Qt4 Qt6c Strath terrace Tcw Coldwater Sandstone km Cosmogenic nuclide age Qt5a Strath terrace Qs Saugus Fm. Tcd Cosy Dell Shale 01 23

Figure 4. Geologic maps of the two study areas showing the distribution of Tertiary bedrock units, including the Saugus Formation, and numerical ages (white boxes). (A) The Las Posas Valley and Camarillo fold belt (hollow white polygon), south of the Oak Ridge/South Mountain uplift, are the main areas of focus for this study. Pliocene–Pleistocene bedrock mapping was compiled from Yeats (1988b), Dibblee (1990, 1992a, 1992b), and modifi ed in this study. Two distinct late Pleistocene strath terrace levels are shown (Qt1 and Qt2). A third discontinuous remnant intermediate terrace level is pres- ent, but it is too small to be depicted here, yet is shown in Figure 7A. MM—Moorpark mammoth site. (B) Geologic map of the Ojai Valley. Tertiary bed- rock mapping was compiled from Dibblee (1990, 1992b), with modifi ed late Pleistocene strath terrace levels of Rockwell et al. (1984) shown along the Ventura River. Note that the nomenclature of Rockwell uses ascending numbers for increasing terrace age, which is the reverse for the Las Posas Valley.

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from the San Gabriel Mountains, refl ecting deposition by the ancestral Much of the existing geologic mapping within the Las Posas Valley Santa Clara River (e.g., Kew, 1924; Yeats et al., 1994; Yeats, 1988a; Levi area is built upon regional-scale mapping by Kew (1924), which accu- and Yeats, 1993). rately illustrates most of the Oligocene to Pleistocene bedrock rela- The Saugus Formation is Quarternary in age (Fig. 2). Levi and Yeats tions. In the Camarillo fold belt, Jakes (1979) correlated post-Miocene (1993) utilized magnetostratigraphy to estimate a lower age of 2.3 Ma and shallow-marine and terrestrial sandstone, siltstone, and conglomerate to upper age of 0.5 Ma of the Saugus in the Santa Susana Mountains (Fig. 1). the Saugus Formation. In contrast, Pressler (1929) and Dibblee (1990, The lower age limit on Saugus strata to the west is unknown; however, 1992a, 1992b) differentiated the marine and nonmarine Saugus Forma- amino acid racemization (AAR) data from overlying terrace deposits near tion, assigning the terrestrial sediments to the Saugus Formation and Ventura suggest an upper age of ca. 200 ka (Lajoie et al., 1979, 1982; Yer- the Las Posas Sand to underlying shallow-marine sediments (Fig. 4A). kes et al., 1987). Because few Quaternary numerical ages exist within the Our study utilizes the nomenclature of the latter; however, we further region, the upper age of the Saugus Formation and other correlated sedi- subdivide the terrestrial sediments of the Saugus Formation into chro- mentary strata (Fig. 2) is commonly utilized for estimating the timing and nostratigraphic units, Qs1 through Qs4 (oldest to youngest) based on rates of deformation on numerous active faults and folds in the region our geochronology investigation. Although the upper age of Pleistocene (e.g., Yeats, 1988a; Huftile, 1991; Baldwin et al., 2000). Therefore, these sedimentary rock in this region is poorly defi ned, a biostratigraphic age studies and other unpublished consulting reports give the impression that of 850–780 ka at the Moorpark mammoth site (MM on Fig. 4A) indi- deformation began synchronously everywhere west of the San Gabriel cates that Qs1 strata north of the Camarillo fold belt were deposited Mountains at 200 ka. during the early to middle Pleistocene (Wagner et al., 2007). A regional unconformity separates the Saugus Formation from over- During the early Pleistocene, the Camarillo fold belt occupied what lying sediments that constitute the upper aquifer system (Figs. 2 and 3) Yeats (1965) referred to as the Camarillo shelf, where most of the late in the Santa Clara River Valley and on the south fl ank of the Oak Ridge Tertiary sediments (Monterey Formation) were removed and Pliocene uplift, as well as offshore (Turner, 1975; Greene et al., 1978; Yeats, 1988a; deep-marine sediments (Pico Formation and Fernando Group) did not Hanson et al., 2003). The Mugu aquifer sediments have a thickness of accumulate. Consequently, Pleistocene strata in the Camarillo fold belt ~60 m, and the sediments are differentiated from the Saugus Formation are signifi cantly thinner (~80 m) compared to those to the west in the in the Las Posas Valley by being less indurated, fi ner grained, having a Oxnard Plain and to the north, where thicknesses exceed 1 km (Fig. 3; local source at South Mountain (Turner, 1975; Hanson et al., 2003) and an Jakes, 1979; Yeats, 1988b). Although the entire Pleistocene section upper age limit of 75–60 ka (Dahlen, 1992) (Table 1). thickens westward along the axis of the Camarillo fold belt, the Sau- gus Formation maintains a near-uniform thickness of 40−60 m where Local Structure and Stratigraphy of the Study Areas observed in outcrop (McNamara et al., 1991; Lopez, 1991; Kile et al., 1991; this study). Las Posas Valley Overlying the Saugus Formation, the uppermost Pleistocene to Holo- The Las Posas Valley is situated in the topographic low between the cene sedimentary units are composed of a thin veneer of dissected and Oak Ridge–South Mountain uplift to the north and the Santa Monica weakly deformed surfi cial deposits termed Quaternary older alluvium Mountains to the south, with the Camarillo fold belt bounding its southern (Qoa of Dibblee, 1992a) or Quaternary terrace deposits (Qt of Kew, extent (Fig. 4A). The Camarillo fold belt consists of several active south- 1924). These previously mapped older alluvial and fl uvial deposits are verging anticlinal folds that characterize the western extent of the Simi composed of geomorphic and geologic units, including two terrace lev- fault zone, which is described, in detail, in DeVecchio et al. (2012). In els (Qt1 and Qt2) and an older Quaternary alluvial unit (Qa1) identifi ed addition to deformation along the Simi fault zone, the Las Posas Valley is in this study (Figs. 4A and 5). No detailed study of these landforms being actively uplifted in the hanging wall of the Oak Ridge fault, a reverse and sediments has been conducted; however, a paleoseismic investiga- fault estimated to have a late Quaternary slip rate of 5.9–12.5 mm/yr tion undertaken on the southernmost Qt2 exposure described a younger (Yeats, 1988a). alluvial deposit that unconformably overlies Saugus strata and has a Regional correlation and lithostratigraphic nomenclature of geologic calibrated radiocarbon age 23,000 ± 1000 cal. yr B.P. (Buena Engineers, units in the Las Posas Valley are summarized in Figure 3 (unshaded). 1987) (Fig. 4A; Table 1).

Table 1. Compilation of existing age estimates discussed in text Name Location Age (ka) Geochronological technique Reference Las Posas Sand Camarillo fold belt 450–750 Amino acid racemization Wehmiller (2010, personal commun.) Saugus Formation Santa Susana Mountains 2300–0.5 Magnetostratigraphy Levi and Yeats (1993) Saugus Formation Near Ventura >200 Amino acid racemization Lajoie et al. (1982) Saugus Formation Las Posas Valley 850–780 Biostratigraphy Wagner et al. (2007) Mugu aquifer Offshore Ventura 60–75 Marine isotope stage correlation Dahlen (1992) Qt2 Camarillo fold belt 23 ± 1 Radiocarbon Buena Engineers (1981) Qt6c Ojai Valley 94 ± 13* Relative displacement Rockwell et al. (1984) Qt6b Ojai Valley 56 ± 10* Relative displacement Rockwell et al. (1984) Qt6a Ojai Valley 40* Radiocarbon† Rockwell et al. (1984) Qt5b Ojai Valley 32 ± 1* Radiocarbon/correlation# Rockwell et al. (1984) Qt5a Ojai Valley 16–22* Radiocarbon/correlation# Rockwell et al. (1984) Qt4 Ojai Valley 8–12* Relative displacement Rockwell et al. (1984) *Calibrated radiocarbon ages of Rockwell et al. (1984). Calibration was conducted using online calculator (www.calpal.de). †Based on average of two calibrated radiocarbon ages 39,117 ± 1188 and 41,587 ± 2101 cal yr B.P. #Calibrated radiocarbon ages from samples collected from terraces south of study area near Ventura, California, and correlated to the Oak View area.

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Composite topographic profile

110 Ojai Valley Las Posas Valley 100 Qt1 90 80 Figure 5. Composite topographic profi le of the Ojai and Las Qt6c (94 ± 13 ka) Posas Valleys showing the relative strath terrace elevations 70 with respect to the active river channel—the Ventura River 60 Qt6b Saugus and Sespe and Calleguas Creek, respectively. Age assessments and 50 (56 ± 10 ka) elevations for the Ojai Valley are from Rockwell et al. (1984). Formations 40 Qt6a Elevations from the Las Posas Valley were taken from a 3-m (ca. 40 ka)

Normalized elevation above base level (m) digital elevation model. 30 Qt5b Qt2 (32 ± 1 ka) 20 Qt5a Qa1 (ca. 16–22 ka) 10 Saugus Formation 0 0 1 2 3 4 5 6 7 8 9 Distance (km)

Ojai Valley METHODS OF INVESTIGATION The Ojai Valley is a complex, structurally controlled intramontane basin bounded by the Topatopa Mountains to the north and Black/Sulfur Geologic Mapping Mountains to the south (Fig. 4B). The lower Ojai Valley closes to the east, where the San Cayetano fault overrides the Arroyo Parida fault. We focused our geologic mapping on late Pleistocene strata previ- The San Cayetano fault is a north-dipping active reverse fault that Yeats ously interpreted as Saugus Formation, Las Posas Sand, strath terraces, (1981) suggested fl attens at depth, where it joins with a regional mid- and Quaternary older alluvium. Because the Saugus Formation in the crustal detachment. In contrast, the Oak View faults, which offset several Las Posas and Ojai Valleys has been mapped and described in detail strath terrace levels and the Saugus Formation along the Ventura River (e.g., Jakes, 1979; Yeats, 1988b, Huftile, 1992; Dibblee, 1987a, 1987b, (Fig. 4B), were interpreted by Rockwell et al. (1984) to be a series of 1992a, 1992b; McKay, 2011), the focus of mapping of the Saugus For- fl exural slip faults related to synclinal folding that do not root into high- mation was carried out in the Camarillo fold belt. There, mapping and strength pre-Miocene bedrock. Fault-slip rates near the Ojai Valley on numerical dating focused on the contact between the Las Posas Sand the San Cayetano fault are estimated to be ~1.05 mm/yr (Rockwell et and the Saugus Formation exposed along most of the folds within the al., 1988), whereas slip rates on the Oak View faults are estimated to be Camarillo fold belt to characterize the nature of the contact and the between 0.3 and 1.1 mm/yr (Rockwell et al., 1984). compositional variability of the two different geologic units (Fig. 4A). Similar to the Las Posas Valley, late Pleistocene strata in the Ojai Val- Stratigraphic relations described in outcrop were then compared with ley have been given a variety of different names (Fig. 2, shaded). This data from more than 50 subsurface wells compiled by Jakes (1979) and study utilizes the nomenclature of Huftile (1991, 1992), who referred Hanson et al. (2003) in the Las Posas Valley and Camarillo fold belt to late Pleistocene conglomeratic units that are unconformable with the (fi g. 4B of DeVecchio et al., 2012). underlying Pliocene–Pleistocene strata, as Saugus Formation. The Sau- Due to extensive agricultural use and urbanization on strath terraces gus is compositionally unique in the Ojai Valley, containing predomi- in the Las Posas Valley, Qt1 and Qt2 were mapped based on geomor- nantly clasts from the Eocene Coldwater and Matilija Sandstone that phic expression and altitudinal continuity using a high-resolution (3 m) compose much of the Topatopa Mountains to the north (Fig. 4B; Huftile, digital elevation model (DEM). In the Ojai Valley, we relied upon ter- 1991). In contrast to the Las Posas Valley and Santa Clara River Valleys, race mapping, nomenclature, and age estimates of Rockwell (1983) and the Saugus Formation is angularly discordant with the underlying Plio- Rockwell et al. (1984) (Table 1). cene and older strata, indicating that deformation of Sulfur Mountain is younger than deformation to the south (Huftile, 1991; Huftile and Yeats, Geochronological Approaches and Technique 1995) or that the base of the Saugus Formation is younger to the north. Estimated from uplift rates, Rockwell et al. (1984) provided an age esti- We used optically stimulated luminescence (OSL) and terrestrial mate of ca. 100 ka for the oldest strath terrace that is cut into the Saugus cosmogenic nuclide (TCN) to constrain the absolute ages of Saugus Formation, which places an upper age limit on the formation. Formation strata and strath terraces in the Las Posas and Ojai Valleys. The seminal work on soil chronosequences by Rockwell (1983) in We combined these new data with preexisting chronologies to chronicle the Ojai Valley defi ned six late Pleistocene to early Holocene strath ter- the upper Pleistocene terrestrial depositional and erosional history of race levels, named Qt6c, Qt6b, Qt6a, Qt5b, Qt5a, and Qt4 (oldest to these areas. Next, we outline a brief primer on these techniques and their youngest) (Figs. 4B and 5). The chronosequence is tied to four radiocar- applicability to dating Quaternary sediments and landforms in the West- bon dates from three terraces, two from the Qt6a terrace (40,000 cal yr ern Transverse Ranges. B.P.) near Oak View, one from a terrace near Ventura correlated to Qt5b (32,000 ± 1000 cal yr B.P.), and a lower terrace near Ventura correlated Optically Stimulated Luminescence to Qt5a (15,000–20,000 cal yr B.P.) (Rockwell et al., 1984). Strath ter- We utilized OSL dating on quartz mineral separates and infrared stim- races are variably deformed by the Oak View faults and deeply incised ulated luminescence (IRSL) dating on feldspar mineral separates. The by the Ventura River (Fig. 5). luminescence signal derived from OSL and IRSL records the elapsed time

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since the sediment was last exposed to sunlight during transportation and/ or as it is deposited and, therefore, represents the depositional age (e.g., Aitken, 1998). Due to the relatively high radiogenic dose rate (>2.6 Gy/ka) of sedi- ments in the Camarillo fold belt, rapid saturation of luminescence signal

in quartz OSL samples limited the maximum resolvable age to ca. 45 ka E ELT (Table 2). Where the quartz luminescence signal was saturated or near sat- D uration, age estimations are only minimums; therefore, we utilized IRSL (Gy)** on feldspar mineral separates at these sites. Because feldspar saturates at higher doses than quartz (Aitken, 1998), IRSL analyses of feldspar sepa- rates are capable of resolving a greater burial duration and thus older ages. (Gy/ka)

For a more thorough discussion of the OSL and IRSL technique employed DR Total in this study and examples of quartz and feldspar luminescence character- istics, see DeVecchio et al. (2012). # Cosm Cosmogenic Nuclides DR TCNs, such as beryllium-10 (10Be), are isotopes that form from the (Gy/ka) interaction of cosmic particles with elements in Earth’s atmosphere and on (1994). surface. 10Be forms from the interaction of cosmogenic particles with S84) datum. a§

quartz (Lal, 1991). TCN production decays exponentially with depth as W the penetration of cosmic rays attenuates below the ground surface. Thus, Δ production of TCNs is negligible a few meters below Earth’s surface. Fur- thermore, 10Be is radiogenic, with a half-life of ~1.3 × 106 yr (Nishiizumi

et al., 2007), and thus it is not present in buried rocks more than a few mil- K 40 lion years old. The concentration of 10Be in rocks or sediments at Earth’s (%) confi dence level. confi surface is a function of the production rate (known) and the time (what we σ wish to determine) the rocks or sediments have been exposed to cosmic rays, modeled for a particular latitude and altitude. Knowing these key 10 Th 232

components, we can calculate exposure ages based on measured Be con- (ppm) centrations summarized in Table 3 (see Appendix A for expanded table1). Either in situ boulder ages or depth profi les, outlined herein, were used to defi ne surface ages within the Las Posas and Ojai drainages. Exposure dating. Exposure dating was undertaken on fi ve boulders U 238 on the Qt6a surface in the Ojai Valley (Fig. 4B). Samples were 2 cm (ppm) thick and collected from the top of each boulder. Shielding corrections were made for each sample from fi eld measurements and calculated using the CRONUS online calculator (Balco et al., 2008). All boulder samples were prepared at the University of Arizona (UA) cosmogenic laboratory. Sandstone samples were crushed to 250–500 µm grain size

for processing. Approximately 50 g aliquots of quartz grains were sepa- ), incorporating the error from beta source estimated at about ±5%. E rated from the sample following the method of Kohl and Nishiizumi OSL Qs2 1.48 ± 0.11 8.25 ± 0.41 2.52 ± 0.10 1.15 ± 0.07 0.17 ± 0.01 3.42 ± 0.25 Min. 129 (1992). Prior to dissolution, samples were analyzed for purity on the OSLOSL Qs2 Qs2 ± 0.05 1.18 4.29 ± 0.17 ± 0.03 1.49 2.57 ± 0.11 4.89 ± 0.04 1.15 ± 0.07 2.69 ± 0.23 0.16 ± 0.01 1.15 ± 0.07 2.85 ± 0.21 0.14 ± 0.01 Min. 129 2.96 ± 0.26 Min. 137 Method Unit of the D

inductively coupled plasma–optical emission spectrometer (ICP-OES) n-1 σ † †† †† ††

in the Department of Hydrology at UA. Samples were spiked with an ), dose rates (DR) and age determinations are expressed at the 1 − − E

10 9 15 16 -notation (weight wet sample/weight dry sample).

in-house ultrapure beryllium carrier ( Be/ Be ~2 × 10 ± 3 × 10 ), (ka) Δ and a blank was processed along with the samples to account for back- ground contamination in the laboratory. Samples were dissolved and run ) and error (1 through chromatography columns to extract the beryllium hydroxide, E which was later combusted over a Bunsen burner for 20 min to produce beryllium oxide. Targets were packed with a mixture of niobium pow- der and the beryllium oxide sample in accordance with procedures from Location* Age Lawrence Livermore National Laboratories (LLNL) Center for Accel- eration Mass Spectrometry (AMS). AMS ratios from LLNL were con- Easting Northing verted to concentrations and exposure ages calculated using the online TABLE 2. ANALYTICAL RESULTS FROM OPTICALLY STIMULATED LUMINESCENCE GEOCHRONOLOGY FROM THE LAS POSAS VALLEY AND CAMARILLO FOLD B THE LAS POSAS VALLEY FROM LUMINESCENCE GEOCHRONOLOGY STIMULATED FROM OPTICALLY RESULTS ANALYTICAL 2. TABLE All aliquots in saturation. Only a minimum age can be assessed. Uncertainties in the equivalent doses (D Estimated water content given in the Cosmic doses and attenuation with depth were calculated using the methods of Prescott Stephans (1982) Hutt † § # ††

59 306417 3789099 84 ± 713 323608 IRSL 3791538 141 ± 10 319672 Qs2 IRSL 3798155 Las Posas Sand 0.69 ± 0.07 13 ± 1 2.71 ± 0.03 1.71 ± 0.05 7.82 ± 0.04 OSL 2.03 ± 0.04 2.54 ± 0.23 1.15 ± 0.07 1.15 ± 0.07 0.15 ± 0.01 Qa1 0.14 ± 0.01 2.44 ± 0.15 3.79 ± 0.28 344.35 ± 12.1 3.15 ± 0.06 318.4 ± 10.8 ± 0.09 11.80 2.04 ± 0.17 1.15 ± 0.07 0.15 ± 0.01 3.25 ± 0.25 40.97 ± 3.21 3 306417 3789099 >45

246 310474 37923868 306417 3789099 >45 10 310474 35 ± 10 379238612 323595 OSL 78 ± 6 3791611 33343214 125 ± 9 3797975 IRSL 317585 Qs2 IRSL 97 ± 7 3793613 311397 Qs2 IRSL 1.59 ± 0.04 27 ± 4 3797343 Qs3 5.14 ± 0.04 IRSL 4.91 ± 0.05 5 ± 1 2.45 ± 0.21 Qt1 10.81 ± 0.05 0.69 ± 0.07 1.15 ± 0.07 OSL 2.22 ± 0.20 Qt2 2.46 ± 0.05 0.14 ± 0.02 1.15 ± 0.07 3.05 ± 0.03 2.72 ± 0.05 2.87 ± 0.24 0.17 ± 0.01 Qa1 9.04 ± 0.06 1.15 ± 0.07 1.33 ± 0.03 98.55 ± 27.31 4.19 ± 0.29 2.75 ± 0.17 0.15 ± 0.01 4.13 ± 0.03 326.7 ± 7.0 1.15 ± 0.07 1.89 ± 0.07 3.07 ± 0.19 2.56 ± 0.19 0.15 ± 0.01 12.40 ± 0.10 386.48 ± 11.6 1.15 ± 0.07 2.25 ± 0.20 4.13 ± 0.28 0.20 ± 0.01 1.15 ± 0.07 399.9 ± 15.0 2.87 ± 0.28 0.17 ± 0.01 78.18 ± 9.99 3.64 ± 0.28 17.41 ± 1.07

1 318247 3791107 >45 **Average equivalent dose (D **Average 711 318247 3791107 85 ± 6 332000 Geodetic System 1984 (WG using World Mercator (UTM) coordinates system zone 11, Transverse *Locations are given in Universal 3795598 IRSL 25 ± 2 Qs2 IRSL 4.75 ± 0.05 Qt2 7.32 ± 0.03 2.63 ± 0.16 2.71 ± 0.05 1.15 ± 0.07 7.93 ± 0.05 0.15 ± 0.01 2.39 ± 0.16 4.27 ± 0.28 1.15 ± 0.07 365.1 ± 10.3 0.17 ± 0.01 3.25 ± 0.25 80.23 ± 2.09 1GSA Data Repository Item 2012077, three alternative CRONUS Calculator re- Sample Number sults for cosmogenic radionuclide exposure and depth profi les analyses utilizing alternative erosion rates, and one expanded geochronology table, is available at www.geosociety.org/pubs/ft2012.htm, or on request from [email protected], Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301-9140, USA.

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†† (yr) 9804 9188 9718 11640 10678 15242 (11810) (11002) (10196) (13022) (24642) (15184) Apparent age error (yr) age** (53011) (29545) (39078) (47430) (67032) (42431) Apparent ) σ (1 Be error 10 Be 10 concentration Be ) (atoms/gram) 9 Be concentration σ 10 (1 error Be/ 10 Be 9 datum. Be/ 10

r Be GEOCHRONOLOGY Laboratories or at the University of Arizona, respectively. Laboratories or at the University of e 10 i s are shown in Appendix Table 2 (see text footnote 1). Table Appendix s are shown in r es are shown in Appendix Table 2 (see text footnote 1). Table Appendix es are shown in r a AMS values larger error that completely encompasses its duplicate 6a-2L. See text for details. C (mg)

e B (g) Quartz # Rate (cm/k.y.) Denudation factor Shielding s § n o u M n o i t a l l (atoms/g*yr) a p Production Rate S (cm) Thickness TABLE 3. ANALYTICAL RESULTS OF TERRESTRIAL COSMOGENIC NUCLIDE TERRESTRIAL OF RESULTS ANALYTICAL 3. TABLE (m) 370 2 5.32 0.20 0.9995 0–0.1 37.8593 0.4379 5.44E-13 1.25E-14 4.18E+05 1.48E+04 2 see Appendix Table 227227 10 10 5.09 5.09 0.20 0.9994 0.20 0.9994 0–0.1 36.7800 0–0.1 0.4153 43.8300 5.26E-14 1.81E-15 0.4153 5.38E-14 3.73E+04 1.50E-15 1.76E+03 3.21E+04 see appendix table 2 1.02E+03 see appendix table 2 370 10 5.32 0.20 0.9995 0–0.1 56.1900 0.2229 1.23E-12 2.79E-14 3.23E+05 1.13E+04 2 see Appendix Table 227 10 5.09 0.20 0.9994 0–0.1 38.6800 0.3866 5.16E-14 1.62E-15 3.22E+04 1.19E+03 see appendix table 2 227 10 5.09 0.20 0.9994 0–0.1 44.9907 0.3760 6.03E-14 1.83E-15 3.15E+04 1.24E+03 see appendix table 2 370 10 5.32 0.20 0.9995 0–0.1 40.4700 0.4364 2.86E-13 6.59E-15 2.04E+05 7.24E+03 2 see Appendix Table 227 10 5.09 0.20 0.9994 0–0.1 46.0739 0.3760 3.42E-14 1.16E-15 1.65E+04 9.13E+02 see appendix table 2 370370 10 10 5.32 5.32 0.20 0.9995 0.20 0.9995 0–0.1 38.4900 0–0.1 0.4379 44.6000 9.37E-14 2.54E-15 0.2231 1.56E-13 6.93E+04 4.25E-15 2.71E+03 4.98E+04 2 see Appendix Table 2.00E+03 2 see Appendix Table Elevation type profi le profi profi le profi profi le profi profi le profi le profi profi le profi profi le profi profi le profi le profi profi le profi Sample g n i h t r † o N g n i t s a E 290200 3813531 boulder 227 2 4.66 0.19 0.9989 0–0.006 41.6793 0.4400 3.70E-13 3.76E-14 2.61E+05 3.29E+04 49174 §§ Apparent error is based on Cronus calculator results (0 erosion model). In parentheses, 6mm/ka erosion model. Depth profi le age Apparent error is based on Cronus calculator results (0 erosion model). In parentheses, 6mm/ka model. Depth profi Sample 6a-2a was not used for calculation of terrace age because this sample run in duplicate at two laboratories and has a Locations are given in Universal Transverse Mercator (UTM) coordinates system zone 11, using World Geodetic System 1984 (WGS84) using World Mercator (UTM) coordinates system zone 11, Transverse Locations are given in Universal Production rates are from the CRONUS calculator and script of Hidy et al. (2010). Denudation rates show the ranges used in models. Please see text for details. **Apparent age is based on Cronus calculator results (0 erosion model). In parentheses, 6 mm/ka erosion model. Depth profi le ag **Apparent age is based on Cronus calculator results (0 erosion model). In parentheses, 6 mm/ka model. Depth profi † § # †† §§ *”L” or “A” at end of name indicates where the sample was run on accelerator mass spectrometer (AMS); Lawrence Livermore 6a-3 290200 3813531 boulder 228 2 4.66 0.19 0.9989 0–0.006 19.9700 0.4226 1.43E-13 4.25E-15 2.02E+05 1.67E+04 37952 6a-5L 290200 3813529 boulder 228 2 4.66 0.19 0.9989 0–0.006 39.9017 0.4140 2.44E-13 5.68E-15 1.69E+05 2.06E+04 32485 6a-6a 290234 3813532 boulder 228 2 4.66 0.19 0.9989 0–0.006 56.6335 0.3000 3.87E-13 6.05E-14 1.37E+05 2.57E+04 25661 Qt1-01 333432 3797975 depth Qt5b-1L 292153 3812042 depth Qt5b-2L 292153 3812042 depth 6a-2L 290200 3813531 boulder 227 2 4.66 0.19 0.9989 0–0.006 19.7800 0.4172 1.56E-13 3.65E-15 2.20E+05 1.70E+04 41369 Qt1-02 333432 3797975 depth Qt5b-3L 292153 3812042 depth Qt1-03 333432 3797975 depth Qt5b-4L 292153 3812042 depth Qt1-04 333432Qt1-05 3797975 333432 depth 3797975 depth Qt5b-5L 292153 38120426a-1a depth 290216 3813527 boulder 228 2 4.66 0.19 0.9989 0–0.006 49.3600 0.4300 3.71E-13 3.50E-14 1.85E+05 2.62E+04 34730 6a-2a Sample* Location

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CRONUS calculator (Balco et al., 2008). Surface age was calculated as Pebbles were described and then crushed to the 250–500 µm size fraction the weighted mean of the fi ve samples. Boulder inheritance and post- for processing as per the boulder samples. Qt5a samples were prepared at depositional boulder erosion are considered in our calculations and are UA and analyzed at LLNL in August 2008. Samples Qp1 01, 03, and 05 discussed in the results section. were processed at UA and analyzed at LLNL in December 2008. Samples Depth profi les. We used depth profi les to constrain the ages of the Qt1 Qt1–02, 04, and 06 were processed at the University of California, Santa and Qt5a surfaces in Moorpark and Ojai, respectively. TCN depth pro- Barbara, cosmogenic laboratory and at California State University, North- fi les provide an effective method for determining the age of abandonment ridge, and run at LLNL in August 2010. of fl uvial and marine terraces (e.g., Hancock and Anderson, 1998; Hidy Depth profi le ages were determined from two methods: simple linear et al., 2010). The 10Be concentration theoretically should decrease with regression and Monte Carlo simulations. The underlying assumption for depth along an exponential curve that represents the production in the near both methods is that production of TCNs decreases exponentially with surface since deposition. For young (~10,000 yr), pristine terrace surfaces, depth as a function of sediment density (ρ), depth below the surface (z), muonogenic production at depth is insignifi cant, and the concentration and the absorption mean free path (Λ; ~160 g/cm2 for these samples). The within the deepest samples represents the “inheritance” acquired prior to equation for decrease in production rate (P) at any depth below the surface deposition in the deposit. (z) is based on the following equation (Lal, 1991): Samples were collected from the terrace surfaces along an incised (−ρ Λ) arroyo in Moorpark and along a road cut at the Villanova School in Pz(())= P0 e z/ . (1) Ojai, California. At each site, we excavated >75 cm into the vertical face (Fig. 6A) to ensure samples had not accumulated TCNs from the side- P(0) is the surface production rate scaled for altitude and latitude. wall. Samples were collected from 10 cm intervals spaced ~40 cm apart The linear regression method fi ts a line to our data points via the fol- (Fig. 6A). At least 30 pebbles (1–5-cm-diameter clasts) were analyzed to lowing methodology. Nuclide concentration C (atoms) at any depth z is avoid any bias in the profi le due to migration of sand through the soil the product of the production rate at that depth (Eq. 1) and time (yr B.P.) profi le. Clast-size distributions are shown in Appendix A (see footnote 1). plus the 10Be concentration within the sample prior to deposition (called

Qt5a depth profile Linear regression (35 cm erosion) A C 8 0 Original Qt5a surface y = 29849x + 21649 5b-1 Top of pit (-0.31 m or -0.51 m) 6 age = 5.6 ± 4 ka 0.5 5b-2 Pre-pit 5b-1 5b-3 excavation 5b-1 1.0 surface 5b-3 5b-4 4 Depth (m) Strath 5b-5

1.5 5b-5 atoms/g) 4 Road Sample pit 2 2.0 Be concentration 10 (×10 SCALE 0 Active Vaqueros Formation Siltstone 1 m 0 0.1 0.2 0.3 0.4 0.5 0.6 rill e(–zρ/Λ)

B 0 Model fit D Best-fit model Linear regression (51 cm erosion) (6.5 ± 3.1 ka) 8 y = 38327x + 21649 - 50 6 age = 7.2 ± 3.5 ka

4

Sample location atoms/g) 4 Depth (cm) σ - 100 with 2 error bars Be concentration 2 10 (×10 Best-fit model solutions 0 0 0.1 0.2 0.3 0.4 0.5 4 4 2 x 10 4 x 10 e(–zρ/Λ) Concentration (atoms/g) Figure 6. Cosmogenic nuclide data from Qt5a depth profi le. See Figure 4B for location. (A) Schematic sketch of sampling strategy, and view of depth profi le showing the lack of AV soil horizon of Rockwell (1983) from the terrace sediments. (B) Results from analysis of 100,000 Monte Carlo simulations after Hidy et al. (2010). Modal result indicates a preferred age of 6.5 ka. This value is between the simple linear regression results (C, D). See Table 3 for production rates and model parameters and text for method details.

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nuclide inheritance, I). We assume that, on average, each amalgamated Discussion section in DeVecchio et al., 2012). However, partial bleaching sample has the same transport history. The concentration of any sample of the sample would lead to an age overestimate. Due to the saturation of can thus be written as: the OSL signal in all other quartz mineral separates from Qs2, and chro- nostratigraphic inconsistency between samples 3 and 4, it seems prob- Cz( ) =×( AgePz[]) + I. (2) able that sample 4 was contaminated by younger quartz grains that were translocated downward by biological processes such as roots or burrowing Further substitution of Equation 1 into Equation 2 leads to: organisms, which led to the anomalously young age of the sample. The interpretation that sample 4 is an age underestimate is further supported []−ρ Λ Cz( ) =×( AgeP[]0 ez/ ) + I. (3) by all of the IRSL ages from the base of Qs2, which indicate that these strata are well beyond the saturation of quartz mineral separates, including Equation 3 defi nes a line with a slope of Age times P(0) (known). Therefore, IRSL sample 5, which was collected from the same stratigraphic horizon we can regress a line through our data points plotted with C(z) on the y-axis as sample 4 (Table 2). and e(–ρz/Λ) on the x-axis. Errors are based on 2σ linear regression errors of Due to saturation or near saturation of quartz samples, we utilized the line fi t. This method assumes no erosion of the surface, and it does not IRSL on feldspar mineral separates from the same sample sites where account for errors in other parameters such as production rate or half-life. quartz separates were collected to determine the depositional age of the The second, more quantitative, analysis is after the method of Hidy et base of the Qs2 deposits (Fig. 4A). IRSL samples 5–7 gave ages of 84 al. (2010). This method includes a rigorous analysis of all errors (i.e., pro- ± 7 ka, 78 ± 6 ka, and 85 ± 6 ka, indicating that the base of the Saugus duction rate, erosion or infl ation of the surface, and 10Be half-life) included Formation in the central and western part of the Camarillo fold belt is in the analysis and provides a specifi ed number (100,000 for our analysis) ca. 80 ka in age (Table 2; Fig. 4A). In the eastern part of the Camarillo of probable fi ts at 2σ confi dence. The Monte Carlo simulation was run for fold belt, a slightly older IRSL age of 125 ± 9 ka (sample 8, Table 2) was both the Qp-1 and Qt5 surfaces in Moorpark and Ojai, respectively. Only obtained from the base of the Saugus Formation (Qs3). A sample col- the data from the Qp-1 Moorpark surface provided a result within the 2σ lected ~30 m down section from sample 8 from near the base of the Las confi dence limits. For the Qt5 surface in Ojai, the data do not provide a Posas Sand gave a stratigraphically consistent age of 141 ± 10 ka (Fig. 4A; result at 2σ confi dence, but nonetheless we show the best-fi t profi le of our sample 9, Table 2). data for comparison with the linear regression model. Stratigraphy and Age of Qt1 Terrace in the Las Posas Valley RESULTS The Qt1 strath is cut on the south limb of the Oak Ridge anticline, Stratigraphy and Age of the Saugus Formation and Las Posas extends for >10 km in an east-west direction (Fig. 4A), and is the highest Sand in the Camarillo Fold Belt terrace level in the Las Posas Valley (Fig. 5). The planar surface of Qt1 is well preserved and is easily mapped on topographic maps and in the fi eld The basal contact of the Saugus Formation (Qs2) with the underly- (Fig. 8A), and it presently dips 2°–3° southwestward, which is, in part, likely ing Las Posas Sand is exposed in most folds in the Camarillo fold belt due to tectonic tilting. The strath is beveled onto tilted Oligocene Sespe and (Fig. 4A) and is easily identifi able. The two formations are both unlith- Pleistocene Saugus Formation (Qs1) strata (Fig. 4A). With the exception of ifi ed yet easily distinguished based on color, composition, and sedimen- the easternmost remnants of the Qt1 strath, sediments are absent; therefore, tary structures (Fig. 7). The Las Posas Sand is composed of interbed- we mapped the strath surface. However, where terrace cover sediments are ded greenish mud, and thinly cross-bedded, white quartz-rich sand with present, they are as thick as 3.5 m (Fig. 8A) and characterized by one to two abundant well-rounded pebbles composed of plutonic, metamorphic, and depositional units of trough cross-bedded sand and pebble gravel composed metavolcanic clasts (Fig. 7C). Bivalve hash is ubiquitous in exposures of igneous, metamorphic, volcanic, and sedimentary rocks. of the Las Posas Sand and can be identifi ed even in engineered urban A single IRSL sample and six TCN samples (one depth profi le) were landscapes. In contrast, the Qs2 and Qs3 strata are composed of thickly collected from the same exposure in the eastern part of the study area, bedded (>2 m), reddish, massive, pebble-bearing silt with clasts that are where the strath cover sediments were well exposed and the terrace sur- dominantly composed of Miocene Monterey Shale and Conejo volcanic face showed no evidence of degradation (Figs. 4A and 8A). Results from clasts, with less common granitic and metamorphic clasts. Paleosols with the IRSL analysis showed very good luminescence properties, with bright carbonate-cemented rhizoliths are common (Fig. 7E). The original thick- luminescence signals, very good growth curve properties, and small errors nesses of Qs2 and Qs3 strata are not known and may have been variable of the individual recycling points, and gave an age of 97 ± 7 ka (sample across the Camarillo fold belt. However, where the base and apparent top 10, Table 2). We utilized the lower four points of the Qt1 depth profi le, are exposed within folds of the Camarillo fold belt, terrestrial strata have a which linear regression determined to be a very good fi t, and gave an age near uniform thickness of 40–60 m. of 98 ± 5 ka (Fig. 8B; Table 3). We excluded the surface sample due to the Our initial OSL geochronology investigation of the basal age of Qs2 poor fi t, assuming that it may have been affected by postdepositional pro- strata using OSL on quartz mineral separates was inconclusive. With the cesses such as bioturbation or slope wash from adjacent topography, and exception of sample 4 (Table 2), all OSL analyses indicated the strata were assumed no change in the surface over time (e.g., no erosion or infl ation). saturated with respect to the luminosity signal, indicating that the onset Monte Carlo simulations after Hidy et al. (2010) of these four samples of Saugus deposition occurred prior to 45 ka (time of quartz OSL satura- provided an age of 102 ± 13 ka (Fig. 8C). Larger errors result from our tion). The apparent younger age of sample 4 (35 ± 10 ka) is inconsistent input parameters, which include variable sediment density, minor changes with all other OSL samples from Qs2, including the bed that sample 3 was in surface erosion (0–5 cm), and variable erosion rates (0–0.4 cm/k.y.). taken from, which lies stratigraphically above the former. Sample 4 also Additional Monte Carlo simulations of the points using larger erosion/ − had a high equivalent dose (DE) scatter (28% of the mean) compared to infl ation uncertainty estimates for the surface (+0.1 to 0.5 m, respec- all other samples in this study (mean = 10%; Table 2), which is an indica- tively) produced similar best-fi t and mean age profi les, but with larger tion of insuffi cient bleaching or contamination by younger sediments (see resulting errors (Appendix B [see footnote 1]).

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Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/4/2/110/3049965/110.pdf by guest on 27 September 2021 DEVECCHIO ET AL. South C C s pss ps p Rhizoliths pss Pebble sandstone cf Carbonate in fault zone rb Red clay bed ss Sandstone Mudstone ms Sandstone with ssms mudstone interbeds smss ss k 6 ± e 85 8 ± 6 ka s (D) Carbonate-cemented bivalve hash excavated from the from hash excavated bivalve (D) Carbonate-cemented e the strath that truncates the deformation zone. The strath The strath zone. the deformation that truncates the strath Qs3 and the Las Posas Sand in the Camarillo fold belt. (A) belt. fold Sand in the Camarillo Qs3 and the Las Posas d stimulated luminescence (IRSL) sample site location near sample site luminescence (IRSL) d stimulated p pl ssms Las Posas Sand s L SL S R IRSL sample site IRSL on z F o or h d ss ed ms ed ds h Soil horizon So b b Santa Rosa fault zone a as pe p s h s c s e ss ss e v ve iv i al ma ss v s ss ms b ds Prismatic peds Pr Massive silt bed M Ma Massive silt bed Ma M ed pe p Undated strath fragment hs h nt c a o me ds z ss ss ce h pe p rh r d c e- t e Prismatic peds Pr P t re ve v ma on on is rb covered co r rb Prismatic peds Pr Carbonate-cemented bivalve hash Ca Carbonate rhizoliths Ca s ms E D . Although it could not be dated, we estimate an age of 37 ka (see discussion for details). (B) Contact between terrestrial Q3 terrestrial (B) Contact between details). (see discussion for of 37 ka an age estimate we Although it could not be dated, . s 2 ms cf cf cf f cf cf b r rb rb F F b rb

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va d n Qs3 paleosols i Qs North Cross-bedded sand, pebble, and bivalve shell fragments BiBivalve hash Cr an A B C is fragmented and discontinuous and covers less than 100 m less than 100 and discontinuous covers is fragmented Figure 7. Photographic collage highlighting several of the important key distinguishing sedimentological characteristics of the characteristics distinguishing sedimentological key of the important highlighting several collage Photographic 7. Figure Note 2012). et al., (see DeVecchio zone of the Santa Rosa fault splays and central on the northern mosaic of deformation Photo strata and the marine Las Posas Sand. (C) Cross-bedded granitic sand of the Las Posas Sand containing abundant shell fragments. sand of the Las Posas granitic (C) Cross-bedded Sand. Las Posas and the marine strata Las Posas Sand near the upper contact. (E) Close-up photo of stacked paleosols characteristic of Qs3 and Qs2 units. (F) Infrare of Qs3 and Qs2 units. paleosols characteristic of stacked (E) Close-up photo Sand near the upper contact. Las Posas 2). Table the base of Qs2 (sample 7,

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A Stratigraphy and Age of the Qt2 Terrace in the Las Posas Valley

South Mountain/ The Qt2 terrace level is exposed along both sides of Calleguas Creek Oak Ridge uplift in both Las Posas Valley and within the Camarillo fold belt (Fig. 4A). The Qt1 surface TCNTCN dedepthpthp prprofile:rofiof e:e Qt2 strath dips less than 1° westward, parallels the profi le of Calleguas 10211002±2 ± 13 ka Creek, and is cut into the Sespe, Las Posas Sand, and the Saugus Forma- tion (Qs1, Qs2, and Qs3). Local faulting has variably uplifted the strath IRSL:RSL:L within the study area (see fi g. 12 in DeVecchio et al., 2012). The surface Qt1Q aalluvialuvu ialal cocoverver ~3.5~3.5 m 979 ± 7k7 kkaa has been extensively urbanized, and few exposures of the terrace gravels persist. Where present, sand and gravel beds are thin (<3 m) and contain ce Qt1 ererosionosion sursurfacefac clasts composed of Monterey Shale and volcanic and plutonic rocks. Two quartz OSL ages were collected from Qt2 cover sediments in OOligocenegocgoceneenn SespeSeespepe two different locations separated by 15 km (Fig. 4A). Sample 11, col- FormationFormatm ionon lected from eastern part of the study area, gave an age of 25 ± 2 ka, while the downstream sample gave an age of 27 ± 4 ka (Table 2, sample 12). Both OSL ages are within error of previous radiocarbon results of (23,000 ± 1000 cal yr B.P.; Table 1). B Linear regression 4 Best-fit model Stratigraphy and Age of the Qa1 Fill in the Las Posas Valley (97.8 ± 4.8 ka) The Qa1 deposit forms a broad geomorphic surface in the West Las 3 Posas Valley and fi lls paleodrainages cut into the south fl ank of the Oak Ridge uplift, forming fl at-fl oored valleys that extend to within 150 m of

atoms/g) the range crest (Fig. 9). These sediments are currently being incised by 5 2 modern drainages and have an exposed thickness of ~15 m (Fig. 5). Based Depth profile regression Be concentration on subsurface data of Hanson et al. (2003), Qa1 deposits do not exceed (×10 10 (lower four points) 20 m in the Las Posas Valley. Where exposed, the unit is composed of 1 massive greenish silt and sand and contains fragments of Monterey Shale, which are exposed along the crest of the Oak Ridge uplift (Fig. 4A). 0 Two OSL samples were collected and dated from the Qa1 surface 0 0.2 0.4 0.6 0.8 1 (Fig. 4A). Due to greater incision of the surface toward the East Las Posas (–zρ/Λ) e Valley, sample 13 was collected from a deeper stratigraphic position below the Qa1 surface, which was likely near the base of the deposit and C 0 Model fit gave an age of 13 ± 1 ka (Table 2). Sample 14 was collected 9 km to the west from within the upper 1 m of the Qa1 surface and gave an age of 5 Inheritance ± 1 ka (Fig. 4A). ca. 2.9 ka Best-fit model (102 ± 13 ka) - 100 South Mountain Sample location Depth (cm) with 2σ error bars Best-fit model solutions - 200 5 ± 1 ka

5 5 5 2 × 10 4 × 10 6 × 10 Concentration (atoms/g) Top of Qa1 surfacesurrface Figure 8. Sample location and TCN data from Qt1 in the Las Posas Valley. See Figure 4A for location. (A) Photograph of the sample site location showing the location of the IRSL sample and TCN profi le. The uppermost TCN depth profi le sample was excluded, see text for discussion. TCN— terrestrial cosmogenic nuclide. (B) Age interpretation based on traditional linear fi t to data assuming an exponential decay of production rates (5.52 atoms/g/yr for this site) with depth. Gray shaded area shows 2σ error on Figure 9. View of the top of Qa1 fi ll deposits on the south fl ank of South linear fi t. (C) Age results from analysis of 100,000 Monte Carlo simula- Mountain (view to the north). See Figure 4A for location. Note the broad tions after Hidy et al. (2010). See Table 3 for production rates and model fl at-fl oored valley composed of Qa1 deposits that locally extend from the parameters and text for method details. Linear fi t and Monte Carlos simu- west Las Posas Valley northward to within 150 m of the crest of South lations suggest an age of ca. 100 ka for the Qt1 surface. Mountain. Qa1 deposits fi ll incised paleodrainages on the uplifting Oak Ridge hanging wall and structural lows north of the Camarillo fold belt.

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Exposure Ages for Qt6a Terrace Boulders in the Ojai Valley the error range of 6a-2a. The weighted mean for the fi ve boulders, assum- ing no erosion, is 36 ± 3.5 ka (Fig. 10C). This result based on zero erosion Six boulders from the Qt6a terrace were sampled for exposure dating represents the minimum boulder age. A second age was determined for the in order to constrain the stabilization ages for the surface. All the boulders boulders by assuming 26 cm of erosion from the surface. This 26-cm-ero- stand >0.5 m tall above the surface of the grassy terrace tread (Fig. 10A). sion model corresponds to an erosion rate of 6 mm/k.y. and represents an The boulders are made up of very hard, Eocene sandstone that outcrops arbitrary, but plausible, erosion amount for boulders sitting at the surface north of the terraces in the Topatopa Mountains (Fig. 4B), and they are during the late Pleistocene and can account for the obvious rounding of interpreted to have been rapidly exhumed and transported from a nearby the boulders. For example, such a rate would account for 26 cm of erosion hillslope source area in the Topatopa Mountains. These boulders were over 42,000 yr and is within the range of the rates interpreted for boulder likely deposited during fl oods or by debris fl ows while the Ventura River spallation from fi res (Zimmerman et al., 1994). The resulting weighted was laterally abrading the underlying bedrock. As such, we interpret these mean age for the surface is 43.4 ± 5.7 ka. Thus, the age of 36 ± 3.5 ka boulders to have little to no inheritance, although the absolute inheritance should be considered a minimum age for the surface. value is unconstrained. The boulders have the same composition and are similar in size to boulders found in the active channel and fl oodplain of Depth Profi le Ages from the Qt5a Terrace in the Ojai Valley the Ventura River (Fig. 10B), although the terrace boulders are much more rounded than those in the active channel. The rounded nature of the boul- For the Qt5a data in the Ojai Valley, the data do not fi t an exponen- ders may be due to in situ erosion or spallation since they were deposited tial curve (Fig. 6B). We can still use the data, however, because they do (e.g., Zimmerman et al., 1994), although there is no evidence for boulder decrease with depth. We added 35 cm and 51 cm to the profi le because toppling or movement since deposition. there has been some erosion of the surface (Figs. 6C and 4D). A value Six numerical dates were obtained from fi ve boulders on the surface of 35 cm of erosion was determined based on reconstructing the rounded and are shown on Figure 10C. Boulder samples 6a-1a, 6a-2a, and 6a-6a edge of the road cut (Fig. 6A), and 51 cm was reconstructed by adding were run on the acceleration mass spectrometer at the UA Accelerator the A soil horizon back onto the surface based on detailed soil description Mass Spectrometry Laboratory (Table 3). Samples 6a-2l, 6a-3l, and 6a-5l from the same surface at another location by Rockwell (1983). A simple were run at LLNL Center for Acceleration Mass Spectrometry (CAMS). linear regression for the Qt5 data provides ages of 5.6 ± 5.4 ka (2σ error) Samples 6a-2l and 6a-2a are aliquots from the same boulder and have over- and 7.2 ± 7 ka (2σ error) for 35 and 51 cm, respectively. Large errors are lapping age errors (Table 3). We chose to use the age and error for 6a-2l for due to the poor linear fi t of the data, but this nonetheless places the terrace our calculations because it has a smaller error, and the average falls within surface as Holocene in age.

80 A C Samples run at LLNL 26 cm constant erosion model 70 Samples run at UA 43.4 ± 5.7 ka (2σ error)

14 60 uncalibrated C age from sediment 9 m below strath (Rockwell et al., 1984) 50

40

30

Be model age (ka) 0 cm erosion model

10 20 36.0 ± 3.5 ka (2σ error)

10

0 B 6a-1a 6a-2 (l&a) 6a-3l 6a-5l 6a-6a Sample names

AcActivetive channelchannne ofof thethh VenturaVVentuurra RiRRivervev r Figure 10. TCN surface data from boulders on the Qt6a strath. (A) View of AbAbandonedanddoned boulder 6a-5. Notice the roundness of the boulder, suggesting postdeposi- bobboulderu ded r tional spallation. (B) Active Ventura River channel (right) and young (<250 yr) terrace showing modern example of boulder sampled from the Qt6a surface. (C) Data points show age values with 2σ error for all samples assuming no erosion of the surface. Age in zero-erosion model is weighted mean of all samples except 6a-2 (l) (see text for explanation). LLNL—Lawrence Livermore National Laboratories; UA—University of Arizona.

FFloodplainooo dplaainn

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The Monte Carlo approach described here was applied to the tech- Uplift of Oak Ridge hanging wall at 500–200 ka (Yeats, 1988a) pro- nique of Hidy et al. (2010) and yields a best-fi t age of 6.5 ± 4 ka. The duced no change in depositional environment or sediment composition solution, however, does not fi t within a 2σ error envelope and is only an preserved in the Las Posas Sand accumulating on the Camarillo shelf approximation, although this result does fall within the range of the linear (Fig. 12C). The absence of any lithologic change in the Las Posas Sand regression fi ts. subsequent to tectonic uplift to the north likely refl ects recycling of Santa Clara River sediments from the unlithifi ed Qs1, now being eroded from DISCUSSION the Oak Ridge hanging wall. In addition, longshore currents may have car- ried Santa Clara River sediments to the shelf through the isolated shallow- Effects of Southern California Paleoclimate on Landscape Evolution marine embayment (Fig. 12C). We interpret the absence of any volcanic material derived from the Santa Monica Mountains to indicate that fairly We compared the geochronology results to late Pleistocene to Holo- low local erosion rates and sediment input to the shelf were overwhelmed cene climate proxy data and local sedimentation rates in Southern Califor- by the volume of sediment from the Santa Clara River and/or from erosion nia. The paleoclimatology of Southern California is moderately well con- of unlithifi ed Qs1. strained through fi eld-based geologic studies (Bull, 1991), paleolimnology Erosion of the eastern part of the Camarillo shelf during the middle (Kirby et al., 2006, 2007; Bird and Kirby, 2006), pollen records and precip- Pleistocene is indicated by the relatively young basal age of the Las Posas itation proxies (Heusser, 1995, 1998), sedimentation rates tied to isotope Sand compared to the older AAR age farther west (Fig. 12C). Erosion stratigraphy (Kennett, 1995), and eustatic sea-level estimates (Chappell et likely occurred ca. 140 ka in response to relatively low sea level (115 al., 1996). These records are graphically illustrated in Figure 11 and were below msl), correlating to the penultimate glacial maximum at the end of used to infer the effects of late Pleistocene climate oscillation on the evo- marine oxygen isotope stage (MIS) 6 (Fig. 11). Marine transgression dur- lution of depositional and erosional processes that have sculpted the land- ing Termination II at ca. 130 ka is indicated by the OSL age of 141 ± 10 ka scape of the Western Transverse Ranges. For example, aggradation events from the base of the Las Posas Sand (Fig. 11). Although it is not known in the San Gabriel Mountains, Mojave Desert, and central California are whether Qs1 sediments had previously prograded across the eastern part widely recognized across the Pleistocene-Holocene transition beginning of the shelf and were eroded away during the middle Pleistocene, the con- ca. 15 ka and continuing until ca. 4 ka (Fig. 11; e.g., Melton, 1965; Ponti, formable contact between the Las Posas Sand and Qs3 strata to the west 1985; Wells et al., 2003; Reneau et al., 1990; Bull, 1991; Harvey et al., suggests Q1 did not prograde across the central and western Camarillo 1999). In the Las Posas Valley, this aggradation event is recorded by accu- shelf. We suspect that the central and western part of shelf did not expe- mulation of 20 m of Qa1 sediments between ca. 13 ka and 5 ka (Table 1). rience signifi cant erosion during the penultimate glacial maximum due Aggradation is inferred to be the result of transient vegetative conditions to a down-to-the-west Pleistocene topographic break in the shelf, which in response to warmer, drier climate during the termination of the Last corresponds to the location of a late Miocene fault (Fig. 12C) (DeVecchio Glacial Maximum (Termination I) (Fig. 11) and consequent moderate hill- et al., 2012). slope destabilization (i.e., Bull, 1991). This is supported by an interval of After a relatively short interval (~10 k.y.) of marine deposition on the 13 k.y. of rising eustatic sea level (18–5 ka) and pollen records from the eastern shelf, Qs2 sediments began prograding westward, and by ca. 80 ka, Santa Barbara Channel, which indicate a fl ora change from pine forest Saugus strata had completely capped the Las Posas Sand (Fig. 12D). through oak/juniper woodland to chaparral, delineating the change from We do not have enough numerical data to determine whether westward relatively wet, cool Pleistocene climate to dry, warm, semiarid Holocene progradation of terrestrial sediment was continuous from ca. 125 ka to climate (Fig. 11). Although slightly out of phase with other environmental 80 ka or whether progradation of Qs2 and Qs3 represents discrete depo- changes, sedimentation rates in the Santa Barbara Channel increase from sitional events. However, what is surprising is that all four numerical ages 1.4 mm/yr to 1.9 mm/yr from ca. 20 to ca. 10 ka, suggesting links between from the bases of Qs2 and Qs3 indicate that the transition from marine deep-marine sedimentation, terrestrial aggradation, and increased river to terrestrial sedimentation was everywhere coincident with periods of sediment loads (Fig. 11). Unfortunately, no published proxy records for relatively high sea level (MIS5e and MIS5a, respectively; Fig. 11). This Southern California precipitation exist for the period from 65 ka to 150 ka; strongly argues against eustatic sea-level forcing of terrestrial prograda- therefore, we inferred precipitation over this interval (Fig. 11), and, where tion. Accompanying the change in depositional environment, there was a needed, these inferences are discussed more extensively below. change in sediment composition, from a predominantly plutonic source to a local source at Oak Ridge and the Santa Monica Mountains, which could Saugus Formation Deposition in the Las Posas Valley and suggest a tectonic control. For example, uplift of the Santa Susana Moun- Camarillo Fold Belt tains is inferred from a synchronous change from a plutonic source to a local source and rapid progradation of the Saugus Formation in the Simi Situated on the Camarillo shelf during the Pleistocene (Yeats, 1965), Hills (Saul, 1975; Treiman, 1982) (Fig. 1). Such a tectonic model in the the depositional and erosional environments of the Camarillo fold belt Camarillo fold belt, however, is not preferred for two reasons. First, if uplift evolved differently than regions to the north (Fig. 12A). The lack of accu- of the Oak Ridge did not begin until ca. 125 ka, shortening rates would be mulation and/or preservation of Qs1 in the Camarillo fold belt is arguably in excess of 20 mm/yr, which is not supported by geologic (Yeats, 1993) the most signifi cant difference due to its importance to assessing rates of or geodetic surveys across the region (Donnellan, et al., 1993; Argus et al., active tectonics. Uplift of the Santa Susana Mountains northeast of the 1999). Second, uplift of the Oak Ridge hanging wall could not explain the Camarillo fold belt occurred at ca. 800 ka (Saul, 1975; Treiman, 1982), synchronous appearance of volcanic clasts coming from the Santa Monica by which time, progradation of Santa Clara River sediments (Saugus sedi- Mountains at that time. Therefore, we suggest that the change in sediment ments) from the San Gabriel Mountains had reached to the Las Posas Val- composition and progradation of terrestrial strata across the Camarillo ley area, as indicated by deposition of Qs1 at the Moorpark mammoth shelf were the result of a climate-driven increase in denudation of topo- site (Fig. 12B). Yet, shallow-marine sediments in the central part of the graphic highs and increased sedimentation rates in topographic lows. Camarillo shelf continued to be deposited until 450–750 ka, based on Using the well-documented aggradation event in response to Ter- AAR analysis of shells from the Las Posas Sand (Fig. 12B; Table 1). mination I as an example (see previous section), we infer that climate-

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Age (ka) 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 MISS 2 MIS 3 3c MIS 4 MIS 5 MIS 6 Erosion of 0 TerminationT ation 1 3a3a the eastern Eustatic 5e Camarillo -50 5a 5c sea level shelf 5b 5d5 TeTerminationrmination 2 -100 MMISIS 1 Chappell et al. (1996) (m/below m.s.l) Sedimentation 2.0 rate in the 1.5 Santa Barbara 1.0 (m/ka) Channel 0.5 Kennett (1995) ConiferC forest 0 Pollen record 20 from the JuniperJunip / Oakak woodlanwoodland 40 Santa Barbara pollen) Channel OakO / Chaparralhaparral Huesser (1995) 60 (% Non arboreal Inferred from pollen Inferred record of Heusser cool drierdrier drierdrier drydry (1995) using the and wet wetter precipitation wetter technique of wet Huesser (1998)

Pollen record warmwarm drydry wetwet Santa Barbara Southern aandnd (<50<50 (~100~100 cm/yr)cm/yr Channel core California drydry cm/yr)m/yr) Heusser (1998) precipitation s e y and l Paleolimnology c

y San Bernardino y

temperature y c r r

y County (Kirby et al., dry d reme dry d r wet t proxies d 2006, 2007; variably wetness extreme ex no record increasing wet-storm wet-stormy Bird and Kirby, 2006) wet- wet-dry cycles Aggradation Mts. vs. San GaGabriel Mts. ert Degradation Mojaveave DeDes Bull (1991) Las Posas InferredInfer Qt2Qt2 terraceterra age Valley Qa1Qa1 Qs3Qs3 Qt11 Las Posas Sand chronology Qs2Qs

Terrace age

Ojai a estimates and 4 t

t5a t6 nomenclature after

chronology Qt4 Q

Qt5aQ Qt6c Qt6aQ

Qt5b Qt6b Rockwell et al. (1984)

Total probability 0.05 density curve of terrace ages only 0.045

0.04 Composite Gray bars are intervals of semiarid climate Camel Plot of 0.035 numerical and inferred 0.03 terrace ages 0.025

and Probability depositional Total probability events density curve of 0.02 all terrace ages and depositional events 0.015

0.01

0.005

all age data 0 0 10 20 30 40 50 60 70 80 90 100 110 120 130140 150 160 Age (ka) Figure 11. Geochronology, paleoclimate proxy data, and sedimentation data in Southern California. Hillslope vegetation from ca. 140 ka to the pres- ent is inferred from pollen records from the Santa Barbara channel (Heusser, 1995). Precipitation proxy data from the past 65 k.y. are inferred from paleolimnology data from Baldwin and Elsinore Lakes in Southern California (Kirby et al., 2006; Bird and Kirby, 2006), and pollen record of Heusser (1995). See text for discussion of inferred precipitation from 140 ka to 60 ka and for links between the disparate data sets. All numerical ages from this study and terrace ages of Rockwell et al. (1984) were plotted using “Camel plot” geochronology script downloaded and methodology described at: http://cosmognosis.wordpress.com/2011/07/25/what-is-a-camel-diagram-anyway. Bold gray dotted and solid lines show total probability distribu- tion functions for all numerical data and just strath terrace ages, respectively. Shaded bars correlate to intervals of relatively dry climate inferred from vegetation changes and paleolimnology studies. Note that most strath terrace levels and aggradational events (Qs3, Qs2, Qa1) are well correlated to intervals of dry climate. MIS—marine oxygen isotope stage.

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Santa 500–130 ka Santa A Early Pleistocene C Susana Susana Diverted Mts. Deep marine deposition Mts. Santa Clara (Pico Formation) River Rivers incise Big Mountain Uplift of Oak Ridge anticline begins into Qs1

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Shallow marine

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l Las Posas Sand km topographic feature km

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l deposited by the ancestral l

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Santa Monica Mountains

l Las Posas Sand entire Camarillo shelf topographic feature

Figure 12. Schematic diagram illustrating the stratigraphic evolution of the Las Posas region from the early Pliocene until ca. 75 ka. Darkly shaded areas represent topographic features during the different time slices, whereas lightly shaded areas represent modern topographic features that have not yet developed. (A) Shallow-marine sediments accumulate on the Camarillo shelf, while thick accumulations of Pliocene–Pleistocene marine strata accumulate elsewhere. (B) Santa Susana Mountains begin uplifting. Terrestrial Saugus Formation (Qs1) progrades across the Santa Clara River Valley and future location of the Oak Ridge uplift, while to the south and west marine deposition continues. Black star illustrates the approximate location of the Moorpark mammoth site, whereas hollow star shows the location of amino acid racemization (AAR) age estimate on fossil bivalve. (C) Uplift along the Oak Ridge fault isolates the Las Posas Valley from the Santa Clara River. At ca. 130 ka, Las Posas Sand accumulates on a shallow embayment between the Santa Monica Mountains and the South Mountain–Oak Ridge uplift. (D) Progradation of Qs2 and Qs3 across the shelf occurs between ca. 125 ka and ca. 75 ka. During this interval, a single large strath (Qt1) is cut on the south fl ank of the Oak Ridge hanging wall.

driven aggradation in response to similar environmental changes Saugus Correlation in the Camarillo Fold Belt and Las Posas Valley occurred following the end of the penultimate glaciation ca. 125 ka. Figure 11 clearly shows a change from conifer forest through oak/juni- Based on the conformable contact between Qs2 and Qs3 with the under- per woodland to chaparral at both glacial-interglacial transitions; there- lying Pleistocene Las Posas Sand in the Camarillo fold belt, terrestrial sed- fore, we infer a similar change from semihumid to semiarid climate iments in the Camarillo fold belt are lithostratigraphically correlative with conditions following Termination II. We suggest that progradation of the upper Saugus Formation, as previous researchers have suggested (e.g., Q2 sediments at 125 ka was the result of climate-controlled environ- Kew, 1924; Pressler, 1929; Jakes, 1979; Dibblee, 1992a). Yet, signifi cant mental changes that resulted in destabilized hillslopes and consequent differences in age and composition warrant additional scrutiny. The oldest increased local erosion rates and elevated river sediment loads. This Saugus stratum in the Camarillo fold belt (125 ka) is 85–375 k.y. younger is supported by a contemporaneous 10 k.y. period of 5× increase in than the well-cited upper age limit of the Saugus Formation (200–500 ka). sedimentation rate, from 0.3 m/k.y. to 1.6 m/k.y., in the Santa Barbara Additionally, in the southern Camarillo fold belt, sediment identifi ed as Channel (Fig. 11). Although somewhat less clear, progradation of Qs3 Saugus Formation (Qs4, Fig. 4A) in consulting reports (Earth Systems beginning at ca. 80 ka was, similarly, contemporaneous with a decrease Southern California, 2005; Geosoils Inc., 2007) has an upper age limit in nonarboreal pollen in sediment cores, suggesting drying of the that is an order of magnitude younger (ca. 25 ka; DeVecchio et al., 2012). atmospheric regime and a potential for transient hillslope conditions, Furthermore, the Saugus Formation in the Camarillo fold belt does not which may have led aggradation at that time. These results, together have a San Gabriel source and therefore is compositionally different from with aggradation of Qa1 at the Pleistocene-Holocene transition, suggest Qs1 in the Las Posas Valley (Fig. 4A). Although formally defi ning a new that climate likely exhibited a fi rst-order control on the evolution of stratigraphic member of the Saugus Formation is beyond the scope of this Pleistocene stratigraphy within the actively deforming Western Trans- paper, we, however, use the name “Camarillo member” of the Saugus verse Ranges. The upper age of Qs2 and Qs3 is based on correlation of Formation when referring to Qs2 and Qs3, which are exposed within the these deposits with similar strata mapped throughout the Ventura Basin, Camarillo fold belt, to avoid temporal and genetic inferences associated which is discussed below. with Saugus strata elsewhere in the Ventura Basin.

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A compelling similarity exists between the Camarillo member and the during the late Pleistocene ranged from 6 to 0.3 m/m.y. (Zimmerman et sediments of the Mugu aquifer identifi ed in countless subsurface wells al., 1994). Although the Wyoming boulder composition and climate are (Fig. 3). Within the Santa Clara and Las Posas Valley groundwater basins, different than in Southern California, we know of very few other examples the upper aquifer system is composed of the Mugu, Oxnard, and Shallow of in situ boulder erosion rate studies, and we use the maximum erosion aquifers, which unconformably overlie the lower aquifer system, consisting rate to provide a range for the Qt6a boulders. This erosion-corrected age of the Hueneme and Fox Canyon aquifers (Fig. 3) (Hanson et al., 2003). overlaps newly calibrated 14C ages (~40,000 cal yr B.P.; Table 1) from the The lower aquifer system has a Santa Clara sediment source and is cor- same surface (Rockwell et al., 1984). related to the Saugus Formation of Kew (1924) (Fig. 2), with the Fox Surface Qt5 in the Ojai Valley yielded a depth profi le that did not fi t an Canyon aquifer characterizing the lower Saugus member, the Las Posas exponential curve (Fig. 6B; Appendix E [see footnote 1]). This could be Sand of Dibblee (1990, 1992a, 1992b) (Hanson et al., 2003). The upper due to the low number of clasts (<50) per sample (for discussion of this aquifer system, including the Mugu, has a local source south of the Oak problem, see Hidy et al., 2010), or because of the variable clast sizes used Ridge fault and a Santa Clara River source north of the fault (Hanson et in the analysis (1–5 cm). Nonetheless, linear regression and Monte Carlo al., 2003), indicating accumulation of the upper aquifer sediments after simulation of the data suggest a Holocene age for the terrace formation. the uplift of the Oak Ridge hanging wall (<200 ka, post-Qs1 Saugus). The cosmogenic age of ca. 6 ka from terrace surface Qt5 in the Ojai Valley Similar to the Camarillo member of the Saugus Formation, the Mugu is problematic because it is much younger than the previously reported aquifer sediment consists of interbeds of sand, gravel, and silt that have a age range for the Qt5 surfaces (16–32 ka; Table 1; Rockwell et al., 1984). near-uniform thickness (40–60 m) and that accumulated during a period This surface (preserved at the Villanova Preparatory School adjacent to of oscillating highstand sea level (Turner, 1975; Greene et al., 1978; San Antonio Creek in Ojai, California), although correlated to Qt5b in the Dahlen et al., 1990; Dahlen, 1992). Finally, an offshore marine terrace Ventura River Basin (Rockwell, 1983), may in fact be correlative with the that is correlated to highstand sea-level events at 75 ka or 60 ka (Fig. 9 of Qt4 (8–12 ka) surface observed by Rockwell et al. (1984). This age dis- Dahlen, 1992) overlies the Mugu aquifer, suggesting that the age of the crepancy highlights the complexity in the local timing of strath formation Mugu sediments is similar to that of the Camarillo member (<200 ka to within discrete drainage basins. 75 ka). Therefore, we suggest that the Camarillo member of the Saugus Formation exposed in the Camarillo fold belt is represented by the Mugu Strath Terrace Formation aquifer sediments in the subsurface. The main difference between the sediments of the Mugu aquifer and the One of the most challenging aspects of fi eld-based studies of strath ter- Camarillo member is that the base of the Mugu is defi ned by a late Pleis- races is the interpretation of the genetic relationship between the terrace tocene angular unconformity (Turner, 1975; Dahlen, 1992), whereas the cover sediments and underlying strath surface. Most terrace deposits in the Camarillo member is concordant with the Las Posas Sand in the Camarillo study areas are thin (one channel depth) and assumed to have been depos- fold belt. We suggest that this difference has simply to do with the local ited during strath cutting (Gilbert, 1877; Mackin, 1948). In this case, the timing of deformation in the Ventura Basin. Specifi cally, deformation in sediments represent the tools responsible for cutting the strath, and hence the Santa Clara River Valley and in the Las Posas Valley (Oak Ridge hang- the age of the sediments overlaps with the timing of valley-bottom widen- ing wall) is older than the Mugu sediments, in contrast to deformation ing. However, because straths might take 103–104 yr to develop (Hancock in the Camarillo fold belt, which postdates the Mugu/Camarillo member and Anderson, 2002), progressive lateral migration of the channel across sediments. It seems likely that the regional Pleistocene unconformity at the the strath would result in the replacement of older deposits with younger base of the Mugu aquifer in the Ventura Basin is the same unconformity alluvial fi ll (e.g., Wegmann and Pazzaglia, 2009). Therefore, multiple ter- observed at the base of the Las Posas Sand on the eastern Camarillo shelf, race ages from a single terrace level should represent the minimum inter- correlating to lowstand sea level during the penultimate glacial maximum. val of strath formation (e.g., Wegmann and Pazzaglia, 2002). Age results Therefore, we suggest a lower age of 140 ka for Mugu aquifer sediments from Qt2 in the Las Posas Valley, which was dated in three locations along throughout the Ventura Basin, which provides a new chronostratigraphic a 20 km profi le of Calleguas Creek, indicate a relatively short period of datum from which to estimate the timing and rates of deformation from strath cutting (~9 k.y.) when geochronological confi dence intervals are subsurface data in this part of the Western Transverse Ranges. considered (Fig. 11). However, all numerical ages are within error, sug- gesting the strath cover sediments may record a specifi c event at 23 ka Cosmogenic Geochronology of Strath Terraces (Fig. 11), such as incision and abandonment of the strath. In either case, geochronologic results from terrace deposits in the study area appear to New TCN data from both the Las Posas and the Ojai Valleys provide place close temporal constraints on the minimum age of strath cutting. quantitative age estimates for strath terrace formation. The Qt1 depth pro- The mechanism for strath cutting is more problematic to interpret from fi le age in the Las Posas Valley agrees well with age results from IRSL our data. Montgomery (2004) showed that in areas of weak bedrock and indicating the surface has an age of ca. 100 ka (Table 2). This age overlaps where extreme wet-dry cycles existed, lateral stream erosion rates are the age estimate of 94 ± 13 ka for Qt6a, the highest preserved terrace sur- equal to or greater than incision rates (mm/yr to cm/yr). Similarly, the for- face above the Ventura River (Rockwell et al., 1984). Although we cannot mation of strath terraces in northern Italy appears to be strongly infl uenced be certain that these extensive preserved terrace remnants correlate, the by bedrock type, which consequently affects the volume of sediment in similar ages within different tectonic settings suggest a climatic origin for the channel (Wegmann and Pazzaglia, 2009). These results are supported these broad erosional surfaces. by numerical and mathematical models that suggest changes in sediment Surface exposure ages from boulder samples collected on terrace Qt6a and water supply have causative effect on the formation of strath terraces along the Ventura River suggest a surface age of 36 ± 3.5 ka to 43.4 ± 5.7 ka, (i.e., Hancock and Anderson, 2002; Finnegan et al., 2007). The new geo- based on zero erosion and maximum erosion rate estimates (6 mm/k.y.) for chronology presented in this study combined with Southern California the boulders (Table 3; Appendix C [see footnote 1]). The actual erosion rate climate proxy records enables us to test assumptions related to the effects for the boulders on Qt6a is unconstrained. A study of moraine boulders in of river sediment load and discharge driven by climate variability on the Wyoming, however, determined that fi re-induced erosion rates of boulders formation of straths.

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Numerical ages from this study were obtained from four different bed- surface in the Ojai Valley (Fig. 11). The simplest conclusion that explains rock terrace levels in the Las Posas Valley (Qt1 and Qt2) and the Ojai both why the Qt2 has no age equivalent and why the Qt5b surface is out Valley (Qt6a and Qt5a) and are combined with six terrace levels of Rock- of phase with a dry climate interval is that the Qt5b radiocarbon age of well et al. (1984) and shown on Figure 11. Within the resolution of the Rockwell et al. (1984) overestimates the age of the surface by 5–7 k.y. data, no good correlation exists between beveling of strath terraces and Radiocarbon age overestimates are not uncommon because charcoal can stable eustatic base level (e.g., Pazzaglia and Gardener, 1993), with terrace be stored in hillslope deposits for thousands of years and remobilized and ages correlating to periods of rising and falling sea level. Over the past incorporated in the terrace sediments. This seems particularly probable in 60 k.y., however, strath terrace ages are reasonably well correlated to rela- our conceptual model of strath formation, where there is strong evidence tively dry and/or stormy climate intervals characterized by extreme wet- for signifi cant mobilization of stored hillslope sediments during the transi- dry cycles, compared to intervening periods when the climate is inferred tion from wet to dry periods. Therefore, we suspect that the Qt5b terrace to have been wetter and more stable (Fig. 11; Heusser, 1998; Bird and is coeval with the Qt2 terrace, which formed during a semiarid climate Kirby, 2006; Kirby et al., 2006). If the age of Qt6c from the Ojai Valley interval at ca. 25 ka (Fig. 11). Alternatively, Qt5b may represent a complex is coeval with Qt1, as suggested previously, the highest terrace in both response terrace (Bull, 1991) or may have resulted from pulsed evacuation study areas are, similarly, correlated to an inferred relatively dry climate of hillslope and colluvial hollow sediments (Reneau et al., 1990) due to interval ending ca. 100 ka. These observations support models that suggest transport-limited erosional process (see following discussion). decreased river discharge plays an important role in establishing equilib- The dearth of strath terraces in the Las Posas Valley is likely a refl ec- rium between sediment load and river transport capacity and consequent tion of low preservation due to the local bedrock type (e.g., Garcia, 2006), strath formation (i.e., Bull, 1979; Hancock and Anderson, 2002). or it may be the result of its relative proximity to the high topography. Any Placing fi eld constraints on river sediment load during strath cutting straths developed in the past 65 k.y. would likely have been cut along Cal- is diffi cult and remains a signifi cant obstacle to evaluating the effect of leguas Creek, which parallels the structural grain of the region and fl ows changing sediment supply on strath formation in the Western Transverse through a synclinal valley between the Oak Ridge anticline and Camarillo Ranges. The ages of the Qt1 and Qt2 surface in the Las Posas Valley are fold belt (DeVecchio et al., 2012). Pre-Qt2 erosion surfaces would have contemporaneous with an increase in sedimentation rate in the Santa Bar- been cut onto unlithifi ed Qs2, Qs3, and the Las Posas Sand, which bara Channel from 0.7 mm/yr to 1.7 mm/yr and 1.3 mm/yr to 1.7 mm/ are highly deformed in the Camarillo fold belt and are easily erodible. yr, respectively, suggesting straths are cut during periods of elevated river Although not discussed in detail, a discontinuous and fragmented higher- sediment load (Fig. 11). Similarly, Qt4 in the Ojai Valley is contempo- level terrace in the Camarillo fold belt is exposed along the Santa Rosa raneous with aggradation events in the Las Posas Valley and elsewhere fault zone and shown in Figure 7. However, because there were no alluvial in Southern California that resulted from increased sedimentation rates sediments preserved with which to date the strath, we can only estimate due to destabilized hillslopes following the transition to a drier climate. the age of the surfaces. Based on ~100 m of vertical separation between Although we have no evidence to support increased river sediment load for the strath fragment and Calleguas Creek and an uplift rate on the Santa the remaining terrace levels, we suggest that similar climate and hillslope Rosa anticline of 2.7 ± 0.11 mm/yr (DeVecchio et al., 2012), we estimate conditions following wet to dry transitions could have resulted in similar an age of 37 ± 1 ka, indicating correlation with Qt6a in the Ojai Valley and increases in transient river sediment loads. This assumes that hillslopes an interval of semiarid climate (Fig. 11). Alternatively, aggradation in the are not weathering-limited and thick soils are capable of developing dur- Las Posas Valley rather than strath formation may have been the primary ing intervening wet climate intervals, which typically last between 10 k.y. geomorphic response to climate change due to its proximal position along and 30 k.y. Furthermore, this assumption suggests that hillslope sedi- the fl uvial network compared to the more distal position of the Oak View ments are not transport-limited following the wet-dry transition and that a terraces. This may explain why Qa1 sediments were aggrading and back- signifi cant percentage of the stored hillslope sediments is moved through fi lling incised drainages on the south fl ank of Oak Ridge in the Las Posas the fl uvial network at these times. If these assumptions are valid, it is not Valley while Qt4 was being cut in the Ojai Valley. Similar relationships possible to deconvolve the combined effects of decreased discharge and are observed in Cuyama Valley, north of Ojai in the Western Transverse increased transient river sediment loads on the formation of strath terraces Ranges, where proximal alluvial-fan deposits prograded ca. 45 ka, 25 ka, in the Western Transverse Ranges. and ca. 3 ka (DeLong et al., 2007), which are coeval with strath terraces in Although we are unable to independently evaluate the effects of dis- both the Las Posas and Ojai Valleys and overlapping in age with relatively charge and sediment fl ux on the formation of strath terrace, it is clear that dry climate intervals (Fig. 11). strath formation is the result of climate-controlled effects on the landscape, Although we are confi dent that the combined effects of climate-con- and, therefore, regional correlation of these landforms and use of these trolled decreases in precipitation and high transient river sediments are surfaces as strain markers may be appropriate where numerical data are largely responsible for the processes driving lateral planation and the absent. These results are consistent with previous models suggesting that formation of straths in the Western Transverse Ranges, the model does decreased sediment transport capacity, either by increased sediment fl ux not consider the complexities of hillslope and fl uvial processes and the or decreased discharge, leads to channel armoring, which minimizes verti- formation of complex response terraces (i.e., Bull, 1991). The presence cal incision while enhancing lateral channel erosion (i.e., Bull, 1979; Sklar of multiple terrace levels developed in each of the dry climate intervals and Dietrich, 2001; Hancock and Anderson, 2002; Finnegan, et al., 2007). in the Ojai Valley between 50 ka and 20 ka (Fig. 11) is not surprising With the exception of the Qt5b terrace in the Ojai Valley, the absence of and is likely a manifestation of such dynamic surfi cial processes. It has terrace ages during semihumid climate intervals suggests that incision and been documented that sediment pulses in response to large landslides may terrace formation likely occur when elevated water discharge results in drive local strath formation (Fuller and Perg, 2005; Fuller et al., 2009). In incision (Fig. 11). However, if climate change is the only forcing mecha- this case, transport-limited hillslopes and colluvial hollows may not be nism, then why do the terrace records differ? Specifi cally, only one strath completely evacuated following the wet to dry transition (e.g., Reneau terrace developed in the Las Posas Valley in the past 65 k.y., whereas fi ve et al., 1990), which could result in multiple avulsion events during a sin- terrace levels developed in the Ojai Valley over that same interval. Fur- gle semiarid climate interval. Such a model suggests that transient river thermore, the Qt2 terrace in the Las Posas Valley has no age-equivalent sediment loads may be integral to strath formation and that the effect of

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decreased discharge does little more than affect the stability of hillslopes. Geochronology results of strath terrace dating in the Las Posas Valley Conversely, discharge variation may be exclusively responsible for strath and Ojai Valley were combined with previous age estimates in the Ojai Val- terrace formation, in which case, dry intervals may be punctuated with ley (i.e., Rockwell et al., 1984) and compared to late Pleistocene to Holocene wet periods that enhance river incision rates, enabling a terrace to develop climate proxy data and sedimentation rates in Southern California to assess before the river returns to equilibrium during the remainder of the dry causative links. Correlation of strath terrace ages with climate-controlled interval. In either case, the strong correlation of the strath terrace ages environmental changes enables us to make the following conclusions. with global climate variability in tectonically discrete regimes indicates 3. In the Western Transverse Ranges, valley-bottom widening in climate rather than tectonics is paramount in controlling the formation of response to lateral bedrock planation occurs during relatively dry cli- these landforms. mate intervals. Strath formation is interpreted to be in response to decreased incision (vertical erosion) rates due to reduced precipitation CONCLUSIONS and increased river sediment load, which brings fl uvial discharge and sediment transport capacity into balance. Strath terraces develop during The Western Transverse Ranges tectonic province of California is intervening wet intervals when stream power is increased in response to composed of a discontinuous sequence of late Pleistocene to Holocene either decreased sediment load or increased discharge. However, aggra- sedimentary strata that are being actively deformed along a suite of dation may predominate in proximal alluvial environments during dry east-trending reverse and strike-slip faults. Unfortunately, many of these climatic intervals because of high sediment discharge from hillslopes Quaternary strata fall outside the applicability of many geochronologi- and a lack of upstream drainage basin area. cal techniques, and therefore numerical dates from the youngest strata are 4. TCN exposure dating from terraces in the Ojai Valley agrees with few. Consequently, the timing and rates of Quaternary deformation in the previous age estimates of Rockwell et al. (1984). Similarly, most of Rock- province are poorly defi ned. However, younger geomorphic features such well et al.’s (1984) terrace age estimates agree with our interpretation of as strath terraces and late Quaternary aggradational strata, which are ideal strath formation during dry climate intervals, which argues for the robust- for OSL and TCN geochronology, provide a new opportunity to quantify ness of the original soil chronosequence age estimates of Rockwell (1983). the timing and magnitude of deformation and understand the effects of past climate on the landscape. ACKNOWLEDGMENTS Age constraints on the Saugus Formation and equivalents (e.g., Ojai Conglomerate, Santa Barbara and San Pedro Formation) are needed The initial fi eld work and analytical costs for research in the Las Posas because they are exposed along the entire length of the Western Trans- Valley were funded by the U.S. Geological Survey (USGS) National verse Ranges and are typically the youngest unit involved in deformation. Earthquake Hazard Reduction Program (NEHRP, awards 8HQGR0073 Geologic mapping, stratigraphic correlation, and numerical dating of the and 08HQGR0073) and the Southern California Earthquake Center Saugus Formation in the Las Posas Valley allow us to conclude: (SCEC, award 120044), a consortium of earthquake scientists funded by 1. The Camarillo member of the Saugus Formation in the southern Las the National Science Foundation. Additional support was provided by the Posas Valley and Camarillo fold belt, although lithostratigraphically equiv- University of California, Santa Barbara, in the form of teaching assistant- alent to Saugus strata elsewhere in the Ventura Basin, accumulated signifi - ships to support DeVecchio during his dissertation. Ojai fi eld work and cantly later. In contrast to terrestrial Saugus strata north of the Camarillo fold cosmogenic laboratory analysis was supported by a USGS Mendenhall belt, which have an age range 2300 ka to 200–500 ka (e.g., Levi and Yeats, research fellowship to R. Heermance. Additional funding was provided 1993), the Camarillo member accumulated from ca. 125 ka until ca. 65 ka. by a California State University, Northridge, research grant to Heermance. Hence, the entire Camarillo member is half the age of the youngest cited We thank Villanova Preparatory School and Oak Grove School in the Ojai age for the Saugus Formation in the published literature, and locally sedi- Valley for allowing us permission to sample on their property. We thank ments in the southern Camarillo fold belt identifi ed as Saugus (Qs4) are an Colin Amos, Frank Pazzaglia, and anonymous reviewers for their con- order of magnitude younger (25 ka, DeVecchio et al., 2012). These results structive comments, which greatly improved this paper. are not overwhelmingly surprising because the formation is known to be diachronous. However, it is troubling because tectonic rate estimates based REFERENCES CITED on the inferred upper age of the Saugus Formation (>200 ka) appear both in professional reports and in the published literature. These results exemplify Aitken, M.J., 1998, An Introduction to Optical Dating; the Dating of Quaternary Sediments by the Use of Photon-Stimulated Luminescence: Oxford, UK, Oxford University Press, 267 p. the need for caution when interpreting and correlating young deformed ter- Anders, M.D., Pederson, J.L., Rittenour, T.M., Sharp, W.D., Gosse, J.C., Karlstrom, K.E., restrial strata in the Western Transverse Ranges. Crossey, L.J., Goble, R.J., Stockli, L.J., and Yang, G., 2005, Pleistocene geomorphol- 2. Because the Camarillo member of the Saugus Formation is signifi - ogy and geochronology of eastern Grand Canyon; linkages of landscape components during climate changes: Quaternary Science Reviews, v. 24, no. 23–24, p. 2428–2448, cantly younger than Saugus strata north of the study area, the Camarillo doi:10.1016/j.quascirev.2005.03.015. member likely evolved by different processes. We correlate the Camarillo Argus, D.F., Hefl in, M.B., Donnellan, A., Webb, F.H., Dong, D., Hurst, K.J., Jefferson, D.C., member with the Mugu aquifer sediments encountered in numerous water Lyzenga, G.A., Watkins, M.M., and Zumberge, J.F., 1999, Shortening and thickening of metropolitan Los Angeles measured and inferred by using geodesy: Geology, v. 27, and petroleum wells in the Las Posas Valley, based on similar thickness, no. 8, p. 703–706, doi:10.1130/0091-7613(1999)027<0703:SATOML>2.3.CO;2. lithologic character, local provenance, and overlapping ages. The Mugu Bailey, T.L., 1951, Geology of a portion of Ventura basin: Los Angeles and Ventura Counties, California, scale 1:48,000. aquifer sediments are identifi ed throughout the Ventura Basin in the sub- Balco, G., Stone, J.O., Lifton, N.A., and Dunai, T.J., 2008, A complete and easily accessible surface, suggesting that a regional aggradation event occurred during the means of calculating surface exposure ages or erosion rates from 10Be and 26Al measure- late Pleistocene beginning ca. 125 ka. Aggradation of both the Camarillo ments: Quaternary Geochronology, v. 3, p. 174–195, doi:10.1016/j.quageo.2007.12.001. Baldwin, J.N., Kelson, K.I., and Randolph, C.E., 2000, Late Quaternary fold deformation member and Mugu aquifer sediments appears to have been in response to along the Northridge Hills fault, Northridge, California; deformation coincident with increased sediment fl ux due to destabilized hillslope sediments follow- past Northridge blind-thrust earthquakes and other nearby structures?: Bulletin of the ing the transition from the penultimate glaciation to interglacial MIS5e. Seismological Society of America, v. 90, no. 3, p. 629–642, doi:10.1785/0119990056. Bird, B.W., and Kirby, M.E., 2006, An alpine lacustrine record of early Holocene North Ameri- This correlation provides a new chronostratigraphic datum for estimating can monsoon dynamics from Dry Lake, Southern California (USA): Journal of Paleo- deformation rates from regional subsurface data. limnology, v. 35, no. 1, p. 179–192, doi:10.1007/s10933-005-8514-3.

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