FORMATION AND DIAGENESIS OF CHAROPHYTE MOUNDS IN THE MIDDLE MIOCENE BARSTOW FORMATION, MUD HILLS, CALIFORNIA

Field and petrographic relationships, cathodoluminescence, and minor-element and oxygen and carbon isotopic composition define the depositional and diagenetic history of a lacustrine limestone unit in the middle Miocene Barstow Formation in the Mud Hills. The unit is dominated by 1- to 3-m-thick, 3- to 5-m diameter organically constructed mounds that formed by the extracellular calcification of the green alga Chara. The mounds are bounded by porous, coarse-grained siliciclastic sediment deposited in a fan delta. Primary calcification consists of alternating bands of dull-orange luminescent prismatic calcite and bright-orange luminescent micrite. Bladed and sparry calcite cements in the framework porosity of the mounds have orange luminescence similar to that of the micrite. Moderate levels of Mn (mean 1400 ppm) and low Fe (mean 470 ppm) in primary calcite indicate lake waters were only weakly oxic during formation of the mounds owing to high productivity in the shallow nearshore environment. The d¹⁸O of the charophyte fabric ranges from –5 to –9 permil (VPDB), and the d¹³C ranges from +1 to –2 permil (VPDB). Assuming a temperature of formation of ~20°C, the d¹⁸O of the lake water ranged from –3.5 to –6.5 permil (SMOW). The high values of the water, compared to probable d¹⁸O Miocene meteoric values of –8 to –10 permil (SMOW), indicate that the lake water underwent moderate to significant evaporation. The d¹³C values show only small variations in the proportions of dissolved atmospheric CO₂ and dissolved CO₂ from organic decomposition. Early prismatic and blocky cements in the mounds and micrite cement in the overlying sandstone have a range in d¹⁸O similar to the mounds, but d¹³C compositions that range from –1 to –9 permil (VPDB). The carbon and oxygen values suggest that the cements formed from infiltration of meteoric water, which underwent variable evaporation in the shallow vadose zone, as well as addition of significant amounts of dissolved CO₂ from organic decomposition. The cement textures suggest that the sandstone was cemented in the vadose zone, whereas primary porosity in the mounds was filled in the phreatic zone. High Mn (mean 9500 ppm) and Fe (mean 2300 ppm) levels in the pore-filling calcite in the mounds indicate that meteoric waters were strongly reduced by organic decomposition in the shallow subsurface.