DEDUCING GEOMORPHIC PROCESS AND THE CHARACTER OF PALEOCLIMATE FROM SURFICIAL GEOLOGY AND GEOCHRONOLOGY BASED ON OPTICALLY STIMULATED LUMINESCENCE (OSL) DATING (Invited Presentation)

During the Late Holocene, in the central Coast Ranges of California, periods of drought or low precipitation lasting 1 to 2 thousand years were punctuated by periods of intense rainfall and floods lasting a few hundreds of years. It is possible to make this conclusion because of the smallness of the error inherent to our OSL dating, which allows correlating, with some certainty, stream-terrace alluvium ages to variations in effective moisture inferred from lake levels, alpine-glacier advances, and historical records. In the context of these precise correlations, interpretations of stream-process response to climate based on terrace morphostratigraphy are unequivocal and reveal the character of paleoclimate.

Bitter Creek (BC) is within the contractional orogeny at the northwest end of the “Big Bend” of the San Andreas fault (SAF). Observations on the effect of large storms in 2014 and 2015 indicate the predominance of debris-flow processes in the channel of BC during a prolonged drought, but the fill terraces that constitute the single paleofluvial level in BC consist mostly of fluvial sediment. The OSL age of the fluvial sediment is 1790 to 2600 years with a median at 2195 years, which correlates to a highstand of Soda Lake at 2250 calibrated radiocarbon years before present (Calybp), and a glacial advance in the Big Pine drainage of Sierra Nevada at 2200 Calybp. Terrace morphostratigraphy and field observations indicate that the channel of BC degrades during relatively dry times when debris flow predominates. Aggradation and channel backfilling occur when rainfall is sufficient to dilute and thereby preclude debris flow in the channel of BC.

Gillis Canyon is in the Temblor Range and within the topographic expression of the SAF. The morphostratigraphy of the fill terraces composing the single paleofluvial level in Gillis Canyon indicates aggradation during 2 or more floods. This conclusion is supported by correlation of OSL ages to historical records and to the terraces of BC. The OSL age of the 1 to 1.5 meter thick bed that forms the terrace treads is 140 ± 20 years, which encompasses the three largest floods recorded in the history of California, and is centered within 2 years of the largest flood recorded in the history of California. The OSL age from the lower part of the outcrop is 1,960 ± 190 years.