MACRO- TO MICROFACIES ANALYSES OF COLD-WATER, SPRING-ASSOCIATED CARBONATES FROM SANTA BARBARA, CALIFORNIA: IMPLICATIONS FOR A TERRESTRIAL RECORD OF LOCAL WET EVENTS DURING THE LAST GLACIAL MAXIMUM AND THE HOLOCENE

Two successions of spring-associated carbonates occur within a relatively small catchment (~5km²) near Zaca Lake in Santa Barbara, Southern California and serve as evidence of a wetter climate in the region’s past. One is a fluvial deposit (0-0.5m thick) that occurs along the stream grade bed of a narrow valley draping the carbonate-rich Miocene Monterey Formation. Flow along the valley floor is minimal­ today as a natural spring in the area was boxed and piped about 100 years ago. Petrographically the fluvial carbonates contain microbially influenced carbonate (calcite) fabrics with primary porosity and little evidence of primary pore-filling cements. A second carbonate deposit occurs perched upon a steep slope about 10 m above the fluvial deposit. Carbonates extend for about 15m and are about 1.5 m thick. Petrographic analyses of the perched carbonates reveal evidence of several dissolution and reprecipitation cement phases.

Radiocarbon analyses of modern carbonate (formed within the last 50 years) from the fluvial deposit indicate a long residence time of carbon in the modern aquifer (~9,000 years) likely resulting from a mixture of organic matter-derived carbon and limestone-derived carbon from the Monterey Formation bedrock. Thus the time of carbonate deposition for the fluvial cascade remains to be determined. Optically Stimulated Luminescence (OSL) ages of the perched carbonates suggest at least two episodes of carbonate growth, one with a Last Glacial Maximum (LGM) age of 20,400±2600 years (1 sigma uncertainty), and another with an age of 12,500±1,600 years. Differences in geomorphology and diagenetic fabrics of the perched and fluvial carbonate deposits suggest the fluvial and perched deposits are distinct but could contain some overlap in growth history. The carbonates also likely record an overall drop in the local water table from a high point during deposition of the perched deposits (during and up to at least the LGM) to the present position of the boxed spring orifice (total drop of about 10m) in the bottom of the valley, indicating a transition from a wetter to a drier climate. Alternatively, the carbonates could be a product of a drastic change in artesian flow patterns in the local aquifer resulting from local tectonic activity.