A MODEL OF HOLOCENE TERRACE PRODUCTION IN THE CENTRAL COAST RANGES OF CALIFORNIA

Terrace morphostratigraphy is used to deduce terrace production by re-incision of backfilled valleys in the central Coast Ranges of California (CCRC). Optically stimulated luminescence (OSL) dates on alluvial deposition measure a highstand of Soda Lake (Eigenbrode, 1999) and a January 1862 storm deposit (Engstrom, 1996).

Deposition of a 1-1.5 m thick bed forming the tread of a terrace which grades downcanyon to the alluvial floor of Gillis Canyon (GC; Temblor Range) yielded an OSL age of 140 ± 20 years (sample collected in 2013). This date is on a fluvially aggraded deposit that was created during a period of continuous heavy rainfall centered in January 1862 and lasting over a month, which caused the largest-magnitude flood recorded in California history. Fluvial sediments in the lower part of the GC terraces sequences yield an OSL age of 1,960 ± 190 years, which coincides with the final part of a of Soda Lake highstand.

Three OSL ages from terrace fluvial sediment (locally interbedded with debris flow deposits) in the canyon of Bitter Creek (CBC; NW. San Emigdio Mountains) overlap at 2,300 to 2,200 years, which coincides with the peak of a highstand of Soda Lake. The present-day valley bottom of the CBC is mantled with debris flow deposits, indicating debris-flow dominated transport.

OSL ages from fluvial-terrace sediment in Palo Prieto Canyon (PPC; Temblor Range) overlap at 7,360 – 6,450 years; a time period for which paleoclimate records in the CCRC are not preserved, but during which a highstand of Pyramid Lake has been documented. What is significant about the PPC terraces is that like the CBC and GC terraces, they are the lowest paleofluvial level in the drainage basin, formed by incision into a backfilled valley, and lack buried soils. Our model proposes that Holocene stream aggradation in the central Coast Ranges is time transgressive; occurring as single episodes of valley backfills when local factors are favorable, precipitation is sufficient to trigger debris flows, and stream discharge is able to rework debris flow sediments into fluvial terraces. In the studied areas, Holocene terraces formed and were preserved within the last few thousand years, and as recently as 1862 A. D.