Using Varnish Microlaminations to Provide Minimum Ages on Alluvium Associated with Ground Water Discharge Deposits on an Alluvial Fan at Fenner Gap, Cadiz, CA.

Abstract:

Groundwater discharge (GWD) deposits are situated on three lobes of an alluvial fan at Fenner Gap near Cadiz, CA, between 220-250 m elevations. They are representative of past wetlands that raised base level leading to aggradation upstream on the alluvial fan. This study utilized the varnish microlamination (VML) dating method to provide minimum ages on the alluvium overlying GWD deposits, as well as estimating the age of a remnant older alluvium in Fenner Gap. VML results provide a minimum age of 2.8-4.1 ka on the overlying alluvium at the Chambless GWD deposit; agreeable with previously published OSL dates on the underlying GWD of about 10 ka. A VML age of 8.1 ka was found on the overlying alluvium at the Archer sediments GWD situated on the southern lobe. The oldest remnant alluvium in Fenner Gap is situated < 1 km upstream from the GWD deposits and has a minimum VML age of 17.75 ka. This older alluvium could be indicative of a raise in base level caused by wetlands formed during a ground water highstand associated with the last glacial maximum. These VML minimum age estimates may be too young due to the collection of varnish that may not be the oldest present.