THE LITHOLOGY OF THE ALAMOS CANYON SILTSTONE MEMBER AND THE FORMATION OF DOLOMITE, RIDGE BASIN FORMATION, CALIFORNIA

For several million years in the late Miocene, the Ridge Basin of southern California (33°42’40.00”N, 116°14’40.95”W) alternated between lacustrine facies (Peace Valley Formation) and terrestrial facies (Ridge Route Formation). The Alamos Canyon Siltstone, ~1800 m in thickness, was deposited ~5.5 Ma as the last lacustrine member of the Peace Valley Formation. It is primarily distinguished from the underlying Posey Canyon Shale by grain size. A coarse-resolution study was conducted to determine how the lake evolved during this last depositional phase. Most of the described outcrops of the Alamos Canyon Siltstone (ACS) are submerged beneath Pyramid Reservoir and thus sampling focused on five discrete subsections that had adequate exposure. The bottom (1835 to 1815 m), middle (732 to 481 m), and top (0 to 63 m) exposures are mostly silt, indicating deposition in calm waters. Two sandstone sections (487 to 481 m and 1096 to 1069 m depth) have cross-bedding and clearly indicate higher energy deposition, perhaps indicating tectonic uplift and associated turbidite and debris flows, or the progradation of streams during lower lake levels. Minerology and stable-isotope geochemistry of the lacustrine facies suggest that the lake was anoxic and brackish during the entire depositional history of the ACS. Dolomite is the primary carbonate in the top and bottom subsections. The δ¹⁸O_dol values are positive for both units but increase towards the top, suggesting an increase in residence time and related evaporative concentration of ¹⁸O due to the possible closing of the basin. In contrast a marked shift from positive to negative δ¹⁸C_dol values toward the top implies a shift in the mode of formation with microbial activity playing a more significant role in the breakdown of organic matter as expected of anoxic conditions.