Physiography of the Ventura Region, California

Abstract

The Ventura region is of interest for its great thickness of Tertiary sediments, and for the nearly complete record it affords of events in the Pleistocene. This Pleistocene history may be divided into four major episodes; deposition of 4700 feet of marine sediments, deformation of all the rocks in the region, erosion to a surface of late maturity, designated as the Rincon Surface, and uplift which initiated the present erosion cycle. The late-Pleistocene intermittent uplift is recorded by terraces in the Ventura River valley, and by marine terraces along the coast. Both sets of terraces are warped, and reach their maximum elevation where the most intense deformation of the underlying rocks has occurred. Fossils of birds, plants, vertebrates, and marine invertebrates found on the lowermost of the marine terraces at Carpinteria are believed to have accumulated approximately 30,000 years ago in the waning phase of the last Pleistocene glaciation. The plant fossils may have been introduced by small streams in time of flood; the vertebrates are a tarpit accumulation. Changes in the appearance of the Ventura shoreline during the period of uplift, and at the present time, follow a definite sequence. The recognition of this sequence as part of an erosion cycle applicable to steeply sloping shorelines of emergence is an outgrowth of this study. This new cycle differs materially from the one previously described for gently sloping emergent shorelines. Some evidence is found in the Ventura region favoring a recent eustatic lowering of sea level. The fluctuations of sea level during the last glaciation have also been recorded.The development and preservation of coastal terraces is determined by the nature of the bedrock. The best preserved terraces have been cut in the siliceous shales of the Modelo formation. The marine terraces have a local, rather than regional distribution, and owe their altitude to both vertical uplift and warping. Attempts to correlate marine terraces on this coast with glacial oscillations of sea level are not successful. Deformation, both faulting and warping, has continued in to the present in the Ventura region. The Coast Ranges in this section of Southern California are to be regarded as a growing mountain range.