AUTHIGENIC ULTRAFINE CLAYS FROM A MIOCENE PALEOLAKE: PALEOHYDROLOGY IN THE BARSTOW BASIN, CALIFORNIA, USA

Preliminary mineralogical and geochemical analyses of ultrafine (<0.1µm) clays extracted from lacustrine claystones of the Middle and Upper Members of the Barstow Formation, in southern California, provide important evidence of the paleolimnology of the lake that occupied the basin ~16-13 million years ago. All of the ultrafine clays are smectitic and illitic, with little to no interstratification. A Middle Member brown claystone unit was characterized by elevated magnesium content in ultrafine clay minerals, with a mean Octahedral Cation Index (OCI=Mg/(Al+Fe)) of ~1.5 and comparitively low F content (~0.5 wt.%), whereas clays from an Upper Member green claystone have a lower average OCI at ~1.1, and higher F at 1.2 wt %. X-ray diffraction measurements of [060] peaks indicate that greater magnesium content is associated with larger octahedral layer d-spacing, demonstrating that the Mg occupies an octahedral position, not merely in the interlayers. These measurements also indicate the possibility of multiple authigenic 2:1 clay mineral phases. Such mineralogical and geochemical variations in different units are consistent with varying degrees of clay diagenesis in saline, alkaline waters, with more Mg indicating higher salinity and alkalinity, and likely a smaller paleolake. High F in the clays with lower Mg content is surprising, because greater F is typically found in more evaporatively concentrated waters that produce Mg-rich clays; this suggests that an alternative source of aqueous F may have been active, such as hydrothermal springs. The bulk mineralogy of these claystones variably includes dolomite, low-Mg calcite, analcime, and authigenic K-feldspar. These observations support the reconstruction of paleolake Barstow as one that was generally saline and alkaline, but that had episodes of fresher water conditions. Future detailed analysis will be carried out on clays from horizons throughout the lacustrine phases of the Barstow Formation. Such analyses from Barstow Basin, and in similar continental basins elsewhere, offer valuable paleolimnologic data for reconstructing North American terrestrial paleoenvironments, with important implications for Neogene paleoecology and paleoclimate.