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 often difficult to extract from well driller's 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.  Here we combine terrain analysis of a digital elevation model with traditional geologic field methods and age-dating techniques 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.  The Salinas Valley is drained by the 100-mile long Salinas River.  Infiltration from the river and its tributaries is the source of irrigation water in this productive agricultural region.  Irrigation water is pumped from the primary aquifers at depths of 180 and 400 ft (Durbin and others, 1978).  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 that include up to 2,000 ft of saturated alluvium.

The Arroyo Seco is a perennial stream, one of the largest tributary drainages of the Salinas River, and is the major source of recharge from infiltration in stream-channel deposits (Figure 1).  Arroyo Seco cut a narrow canyon that opens into a 10-mile long valley that transects the Santa Lucia Range.  The 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 strath terraces and strath-terrace deposits (Figure 2).  Strath-terrace deposits are as much as about 1,000 ft above the modern drainage; however younger deposits are 150 to < 3 ft above the modern drainage.  A longitudinal profile shows the vertical distance between the treads of each terrace.  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 (Figure 1), 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.  The timing of formation and isolation of these strath deposits was controlled by both active range-front faulting as well as by paleoclimatic events.  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.  This material also provides a long, relatively continuous record of basin aggradation in the Salinas Valley.

References

Durbin, T.J., Kapple, G.W., and Freckleton, J.R., 1978, Two-dimensional and

three-dimensional digital flow models of the Salinas Valley ground-water

basin, California: U.S. Geological Survey Water-Resources Investigation

78-113, 134 p.