CARBONATE PHYTOHERM MOUNDS IN THE MIDDLE MIOCENE BARSTOW FORMATION, MUD HILLS, CALIFORNIA

The occurrence of a 5-m-thick limestone unit at the base of the Middle Member of the Barstow Formation a sequence otherwise dominated by conglomerate and pebbly sandstone indicates a temporary cutoff of clastic deposition that could be related to synsedimentary faulting. The lacustrine limestone unit in the Mud Hills crops out only on the north side of the basin between the eastern end of the range and Owl Canyon (where it is terminated by a fault). The steep north side of the asymmetric detachment basin contains common normal faults thought to be contemporaneous with the early stages of deposition of the Barstow Formation. The most prominent lithology of the limestone unit is phytoherm biolithite, i.e., a constructional mound developed by calcification of macrophytes (the taxonomy of which has not yet been determined). The phytoherms are elliptical to circular in area, with diameters of ranging from 3 m to 5 m, and maximum thickness ranging from 1 m to 3 m. Successive growth layers that drape over the tops of the mounds demonstrate that the phytoherms formed positive constructional features on the lake floor. The internal molds of the stems, most commonly filled by calcite cement, occur in clumps and range in diameter from 0.3 to 1.0 mm. Calcification consists of alternating layers of bladed calcite cement and micrite. As concentric coatings on individual stems were added, they coalesced to form a rigid, current-resistant biolithite. Framework porosity, now partly to completely filled by sparry calcite and/or chalcedony, formed 20% to 50% of the rock. Between the mounds, biosparite and biointrasparite were deposited. These units onlap and interfinger with the mound margins. None of the rock types in the limestone unit contain detrital siliciclastic grains, indicating a complete shutoff of siliciclastic sediment into this area. When influx of sand and gravel ceased, lush plant growth developed along the shallow lake margin, and plants underwent calcification as they grew. Burial by coarse-grained clastic sediment ended limestone formation. The under- and overlying sand and gravel were deposited in a fan delta environment. The most likely cause of the diversion of clastics away from this area is synsedimentary normal faulting in the source area or upper fan.