Holocene deformed sediments of the southern San Joaquin Valley, Kern County, California

Sediments exposed in exploratory trenches and drill holes at Kern Lake consist of interbedded sand, silt, and clay, thickly bedded sand, and clay and clayey silt representing deltaic, fluvial, and central lacustrine depositional environments. The upper 60 in (150 cm) of sediments exposed in trenches and drill holes indicate that ancient Kern Lake was an east-west oriented shallow-water lake. A series of time-progressive paleogeographic maps inticate that the Panama Slough distributary of the Kern River deposited sediment and divided Kern Lake into two bodies of water--Kern Lake and East Kern Lake. East Kern Lake was subsequently filled with sediment and was traversed by the Old South Fork distributary of the Kern River. Three types of sedimentary structures are present within the sediments of Kern Lake. Type 1 structures consist of flame marks, load casts, and deformed clay beds. Type 2 structures are intrusive sand bodies that penetrate into adjacent sediments and are inferred to be subsurface sandblow features. Type 3 structures are irregular-shaped bodies of disrupted sand, silt, and clay and are postulated to be liquefied sediment. Density measurements of sediments sampled inside and outside sedimentary structures indicate that the structures did not compact or densify during formation into higher density configurations than those of the surrounding sediment. The Kern Lake structures were formed during three periods of seismic activity, KL-1, KL-2, and KL-3. Sandblows that formed in Imperial Valley during the Imperial Valley earthquake mainshock (ML = 6.6) on October 15, 1979, Imperial County, California, were excavated and show a definite similarity to Kern Lake structures. Using a centrifuge, experiments were conducted that modeled seismically induced liquefaction of sediment. Experimental results indicate that a minimum horizontal acceleration of 0.20 g is necessary before sandblows will form under laboratory conditions. This g value is similar to that determined empirically (0.13 g) from previous world-wide earthquakes. Faults in the vicinity of Kern Lake capable of generating horizontal accelerations of 0.13 g or greater and possibly causing liquefaction of Kern Lake sediments are: Pleito, White Wolf, San Andreas, Kern Front, Big Pine, Garlock, Santa Ynez, Lockhart, and probably the unknown fault responsible for the 1812 Santa Barbara Channel earthquake. Faults capable of generating horizontal accelerations greater than 0.2 g and considered most likely to have caused liquefaction within the sediments of Kern Lake are: San Andreas, Pleito, White Wolf, Big Pine, and Garlock. Attempts were made to date the sediments and ages of deformation using radiocarbon and paleomagnetic techniques. However, radiocarbon age dates may have been contaminated by petroleum seeps and must be considered unreliable. Type 2 structures may preserve paleomagnetic directions that existed during periods of liquefactions. Paleomagnetic age dates from sediment of a Type 2 structure suggest that deformation period KL-2 occurred between 1420 to 1520 A.D. This period of deformation at Kern Lake occurs during the span of Sieh's (1978a) prehistoric earthquake event V (1400 to 1510 A.D.) along the San Andreas fault and a prehistoric earthquake along the Garlock fault described by Stepp and others (1980). Historical earthquakes that occurred during 1759-1890 which may have caused the youngest deformation period, KL-3 are: 1812 Santa Barbara Channel, 1852 Lockwood Valley, and 1857 Fort Tejon earthquakes.