ARROYO SECO—AN IDEAL LOCATION FOR THE USE OF TERRAIN ANALYSIS AND FIELD OBSERVATIONS TO CREATE A PRELIMINARY QUATERNARY GEOLOGIC MAP

We have combined GIS-based terrain analysis and digital data integration with traditional geologic field methods to develop a Quaternary geologic map of the Arroyo Seco drainage, near the Salinas Valley in the Central Coast Ranges of California. A spectacular sequence of strath terraces are exposed in a 2-4-km-wide swath along the 30-km-long valley. Terraces of paleostreams that eroded the Tertiary Monterey shale are located up to 500 m above the modern drainage, and are capped by rounded fluvial gravel in beds typically <2 m thick. The terraces terminate upstream at a nickpoint above which Arroyo Seco is constrained to a narrow deep canyon in Mesozoic rocks. Arroyo Seco is an ideal location to combine quantitative mapping tools with traditional field methods because of the contrast between steep valley walls and low-angle terrace levels, and because the homogeneous lithology of the substrate Monterey shale contrasts with the capping quartzite-stream gravels derived from the Mesozoic basement. Soils developed on the terraces are dominated by accumulations of clay in B horizons that increase in thickness and percent clay over time.

A color-coded contour map and a slope map of all surfaces <6 degrees were constructed from a 10-m Digital Elevation Model. Map polygons of terrace surfaces were initially developed from the slope map. Geomorphic factors that affect the slope analysis include partial burial of terraces by alluvial fans deposited from tributary drainages and presence of colluvium derived from the steep valley walls. The presence of remnant field stones was used to identify flat-lying terrace surfaces in the field. The terrain-derived and field-identified terrace locations were used to create longitudinal profiles of 11 discrete terraces above the active drainage. In map view, polygons of unique terrace surfaces were created and coded to match the projected terrace profiles. Digital soil maps, including data on clay percent and distribution, were compared to the mapped terrace polygons. These data confirm field observations that higher, older terraces have more clay than the younger, lower terraces. Field soil descriptions and associated OSL dates will be collected to interpret the timing of terrace formation and whether the driving force of terrace formation is predominantly climatic or tectonic.