Climate-controlled strath-terrace formation and abandonment during the Late Pleistocene in Southern California

Fluvial strath terraces represent ancient and now abandoned river floodplains that formed during times of lateral channel erosion and valley widening. Both climatically controlled hydraulic conditions and tectonic deformation (e.g. uplift) influence the formation and latter incision of these erosional surfaces, however the specific forcing mechanisms for strath formation (e.g. climate shifts or rapid uplift) are poorly understood. Strath terraces in the Ojai and Las Posas valleys in southern California, which characterize two discrete tectonic regimes but are located within 20 km of each-other, provide the ideal locality to study the timing of terrace formation and abandonment within the context of the well-known paleoclimate record from the adjacent offshore Santa Barbara basin. New in-situ cosmogenic 10Be exposure ages from two major fluvial strath-terraces along the Ventura River, combined with optically stimulated luminescence ages from lag deposits above bedrock erosion surfaces along the Arroyo Las Posas, indicate terrace abandonment (incision) at 23,500±3000 and at 9500± 1500 years before present (y.b.p.). Terrace abandonment ages are coincident with intervals for which local paleoclimate records imply a 2-fold decrease in precipitation from ~100 cm/yr to <50 cm/yr. These observations support the following two-phase model for terrace-formation: 1) Increased coarse-grained sediment supply within the river channels during times of higher precipiation in coastal southern California promoted channel armoring and lateral erosion of the weak sedimentary bedrock rather than transport or aggradation of the more resistant bedload. 2) Decreases in precipitation at ~25,000 and 10,000 years reduced the sediment flux, promoting incision and abandonment of the strath surfaces. These data suggest that changes in precipitation resulting from climate change rather than tectonic or eustatic base level change are the dominant controls on strath formation and abandonment in regions of weak sedimentary bedrock. This study suggests that the ubiquitous erosion surfaces in southern California may be correlative and be valuable temporal geomorphic markers for tectonic studies.