PETROGRAPHY, MICROBIOLOGY, AND DIAGENESIS OF HOLOCENE (?) SPRING CARBONATES, SANTA BARBARA COUNTY, CALIFORNIA

A succession of spring-associated carbonate deposits crop out discontinuously near Zaca Lake, in Santa Barbara, California. Circa 1911, the spring was diverted to provide water for a nearby settlement, such that active carbonate formation is currently minor or absent. However, inactive carbonate deposits, the age of which are currently under investigation, are located in the watershed at and above current stream grade in a relatively narrow valley upstream from Zaca Lake having formed on the carbonate-rich Miocene Monterey Formation. Two distinct units of spring-related carbonate deposition have been observed. One unit extends for about 15m upon a steep slope of the valley and averages approximately 1m in thickness. The unit terminates in a 10m nearly vertical cascade facies. Light microscopy analyses show evidence of post-depositional dissolution and recrystallization of the calcite crystal structure. A second unit was deposited discontinuously along a 200m transect, ~ 20m lower than the first unit. Several pool/terrace facies occur along the transect. Light microscopy and SEM analyses reveal that the deposit is composed of primary well-formed calcite crystals, with abundant microbial/algal microfossils. Secondary infill of pore spaces and endolithic microbial traces also occur. The presence of Oocardium stratum –a strictly cold-water (~10°C) microalga—suggests that the carbonates were sourced from ambient temperature water.

The petrographic and geomorphological investigations reveal that: 1) microorganisms are an integral part of the carbonate fabric, but direct mediation of carbonate precipitation by the microbes is equivocal; 2) at least two episodes of growth have occurred as evidenced by differences in the extent of diagenetic transformations; and 3) the presence of extensive carbonate deposits at this site has important implications for much wetter conditions in this region in the recent past and may provide a complimentary climate record to the recently developed lacustrine record in Zaca Lake and the nearby marine record from the Santa Barbara Basin.