TERRAIN ANALYSIS AND GEOLOGIC FIELD INVESTIGATIONS USED TO CONSTRAIN DRAINAGE EVOLUTION AND BASIN-FILLING HISTORY WITHIN AND NEAR THE NORTHERN SALINAS VALLEY GROUNDWATER BASIN, CENTRAL CALIFORNIA COAST RANGE

Groundwater models of sedimentary basins can be improved by high-resolution stratigraphy and estimates of hydraulic properties of basin-filling materials. However, such information is difficult to extract from well drillers’ records. Preserved stream terrace deposits within and near sedimentary basins are a record of erosional history that provides a context for down-stream basin aggradation, basin-margin fault-related uplift, and paleoclimate events. We combine terrain analysis of a digital elevation model with traditional geologic field methods to develop a record of the long-term erosional history of the Arroyo Seco drainage, near the Salinas Valley in the Central Coast Ranges of California. Based on data from oil and gas exploration drillholes and water wells, the Salinas Valley is filled with about 10,000-15,000 ft of Tertiary and Quaternary marine and terrestrial sediments.

Arroyo Seco is a perennial stream, one of the largest tributary drainages of the 100-mile-long Salinas River. Arroyo Seco has cut a narrow canyon that opens into a 10-mile-long valley that transects the Santa Lucia Range. Arroyo Seco progrades from an elevation of 945 ft at the canyon mouth to 500 ft where it flows into the Salinas Valley. In the Arroyo Seco valley there is a spectacular sequence of at least six, and perhaps as many as fifteen, strath terraces and strath-terrace deposits. Strath-terrace deposits are as much as about 1,100 ft above the modern drainage; however, younger deposits are 150 to < 3 ft above the modern drainage. The highest (oldest) deposits and their terrace treads record stream erosion and deposition prior to valley incision. A gently sloping, low-relief geomorphic surface northwest of Arroyo Seco records a pre-Arroyo Seco relict landscape above the modern drainage. Remnants of terrace deposits in Arroyo Seco overlie Miocene marine Monterey Formation, and are composed of coarse alluvial gravel less than 10 ft thick. Alluvium transported by Arroyo Seco was deposited across and was cut by the Rinconda and Reliz range-bounding Faults. Valley-side down, reverse movement along the faults resulted in the deposition of an asymmetric, westward-thickening alluvial wedge that provides a long, relatively continuous record of basin aggradation in the Salinas Valley.