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 110 For permission to copy, contact [email protected] | |Volume © 2012 4 Geological | Number Society2 | LITHOSPHERE of America Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/4/2/110/3049965/110.pdf by guest on 27 September 2021 Landscape evolution in the Western Transverse Ranges | RESEARCH 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 sedi mentary 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 forma tion 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