TWO STREAM'S DISSIMILAR RESPONSES TO THE CLIMATE OF THE LATEST PLEISTOCENE, GABILAN MESA, CENTRAL COAST RANGES OF CALIFORNIA

San Lorenzo Creek (SLC) and Pancho Rico Creek (PRC) are Salinas River tributaries draining adjacent watersheds. SLC is the axial drainage of Peachtree Valley, which is a linear, NW trending valley that parallels the nearby San Andreas fault zone and forms the NE edge of the Gabilan Mesa. PRC is the axial drainage of Pancho Rico Valley, which trends SW, and is cut into the Gabilan Mesa. Prior to capturing the upper 60 km² of the SLC watershed sometime in the Holocene, PRC’s watershed was formed entirely in fine-grained, marine sedimentary rocks. The SLC watershed includes Franciscan Complex rocks, and SLC’s gravelly bedload is composed mostly of chert, igneous, and metamorphic rocks.

Two optically stimulated luminescence (OSL) age estimates on SLC basal fill-terrace sediments indicate that vertical accretion in a meandering-stream system was taking place by 26-20 ka. Another OSL age indicates accretion continued until at least 18-22 ka. Three pending OSL age estimates on thalweg and point-bar facies beds will constrain the timing of vertical accretion.

Twenty meter thick fill-terrace sequences along PRC overlie straths and consist of channel facies overlain by debris-flow deposits. Three OSL ages on basal beds indicate deposition occurred when the channel of PRC was filled by debris flows triggered by the stormy climate of the Pleistocene to Holocene transition, between 15.5 and 11.8 ka. We propose a hypothesis to explain why PRC was able to cut straths and erode its bed between 26 and 18 ka, when SLC was aggrading. Our model is based on observations and measurements of clasts on the channels of SLC and a PRC tributary whose catchment is still formed entirely in fine-grained, marine sedimentary rocks. We propose that because weathering of fine-grained, marine sedimentary rocks does not produce significant amounts of gravelly bedload, and/or predominantly produces small caliber sediment that is transported as suspended load, exceeding PRC’s required capacity and burying its channel via prolific debris-flow activity. Also, the role of tectonism cannot be ruled out: PRC flows perpendicular to the SW tilting Gabilan Mesa, and is ideally situated for the effect of tectonic base-level fall to be maximized and sustain relatively high stream power, while the channel slope of the NW-flowing SLC is less affected by SW titling.