THE EFFECTS OF ACTIVE TECTONICS AND CLIMATE CHANGE ON STRATH TERRACE FORMATION—AN EXAMPLE FROM ARROYO SECO, CENTRAL CALIFORNIA COAST RANGE

As many as twenty individual strath terraces rise nearly 500 m above the active drainage of Arroyo Seco, a major tributary of the Salinas River on the east side of the Santa Lucia Range, central California Coast Ranges. Geologic mapping and dating of the terrace deposits allow for the study of climate-controlled landscape evolution in a tectonically active region. Terrace formation is driven by (1) movement on the Reliz Fault that bounds the range front on the west side of the Salinas Valley, and (2) base-level perturbations caused by climate-driven sea level fluctuations.

Broad continuous terrace surfaces occur at and adjacent to the Arroyo Seco drainage, and older terraces are preserved as much smaller remnants at higher elevations. The terraces were once active floodplains that were eroded into the bedrock and are now capped by gravel deposits as thick as 2m. Each terrace level defines the longitudinal profile of the stream. Abandonment of the old floodplain indicates a change in stream equilibrium in response to changes in the base level. In Arroyo Seco, both mountain uplift by fault movement and base level changes, driven by climate change, contributed to the timing of terrace formation. If the timing of abandonment of the floodplain is driven by climate change, ages should reflect climate trends that alter stream equilibrium.

Terrace ages were originally estimated based on soil development and regional correlations to dated deposits. Eight Infrared stimulated luminescence (IRSL) and optically stimulated luminescence (OSL) ages were determined for seven different terrace units. These ages are approximately 1-2 ka, 8-10 ka, 35 ka, 45-50 ka, >65 ka and >120 ka. Determined ages are consistent with the estimated terrace ages based on soil development, clay content, and degree of cementation. Arroyo Seco aggradation was synchronous with previously documented regional, climatically driven aggradation that elsewhere in California led to strath terrace formation and alluvial deposition. Terrace treads tend to represent periods of climate-change induced aggradation events during interglacial cycles. Termination of aggradation can be climate or tectonically induce and is caused by base-level fall.