MICROBIALITE AND MUDSTONE: ALTERNATING CARBONATE AND SILICICLASTIC DEPOSITION ON THE MARGIN OF A MIOCENE LAKE, MUD HILLS, CALIFORNIA

Formation of interbedded microbial limestone and siliciclastic mudstone in a portion of the Middle Member of the Miocene Barstow Formation in Owl Canyon, Mud Hills, California defines the history of lake expansion and contraction in this area. Six detailed sections, measured approximately 15 m apart, were made of a 12-m-thick portion of the limestone interval to define the pattern of deposition. Some limestone beds are tabular, with minor "pinch-and-swell" variations along their extent and thicknesses ranging from 0.05 to 0.4 m. Others are lenticular, up to 2 m thick and 20 m wide over their thickest portion. The margins of the lenses laterally grade into thinly interbedded limestone and mudstone. The thick lenticular beds stack vertically in the sequence. The soles of many of the beds have mudcrack casts. The limestone consists of wavy, irregular laminations of micrite, with common parting surfaces formed by mudstone laminae. Small domal stromatolites occur in some of the thicker tabular beds and in the thickest portions of the lenticular beds. The wavy bedding, irregular lamination, and presence of stromatolites indicate that all of the limestone is microbial biolithite. Mudcracks on the soles of the limestone beds indicate that limestone deposition was initiated in microbial mats at the shoreline of the lake, following a period of exposure. As mats developed, mud episodically washed into the area, covering the mats. However, wave agitation at the lake margin was able to move mud offshore and restore photic conditions needed for continued mat development. Periods of lake expansion ended mat development, possibly because the mats were submerged below wave base and turbidity from suspended mud limited the photic zone. On subtle topographic highs, microbialite development continued longer because these areas remained above wave base. Here, the thickest portions of the lenticular limestone developed. The vertical stacking of these beds suggests that underlying limestone maintained subtle topographic highs for initiation of microbialite formation during subsequent cycles of deposition. During episodes of lake expansion as wave base moved landward, mud was deposited in this area of the lake. Lake contraction ended mud deposition and began a new cycle of microbialite formation.