SEDIMENTOLOGY OF THE NEOPROTEROZOIC BECK SPRING - KINGSTON PEAK TRANSITION IN THE KINGSTON RANGE, CALIFORNIA

The Neoproterozoic Kingston Peak Formation (KPF) is known for its diamictites. The KPF overlies the Beck Spring Dolomite (BSD) that contains early fossil forms and microbialites. This study aims to test lateral correlation and understand the transition between those units and focuses on the basal 4 meters of the KP1 subunit of the KPF as it appears in the Kingston Range, California. We described and correlated sedimentary rocks at and immediately overlying the contact and analyzed the sedimentary structures, textures and sequences.

We have found only three locations in the northern Kingston Peak Range where the contact can be observed. At each location we measured sections about 4 meters long across the contact. The top of the BSD in all locations contain microbial laminations, oncoids and pisoids and sheets of chert overlain by a peloidal and brecciated wackestone. Thin sections from about 15 cm below the contact to about 50 cm above the contact shows microbial laminations with some peloids or ghosts of peloids and abundant chert. The base of the KPF is composed of millimeter to centimeter-scale alternating clastic-rich (A) and dolomite-rich (B) thick laminae and thin beds. A-beds are dolomitic silty mudstone with abundant muscovite. B-beds are recrystallized silty dolostones with peloid ghosts. A few of the beds show faint current ripple cross bedding and lenticular structures. A to B bed contacts are sharp. At all stratigraphic levels, clastics have immature textures and composition. XRD and thin section analysis show a trend of decreasing of dolomite up-section. Measurement and analysis of individual alternating bed thickness show cyclical patterns and a gradational increase in bed thickness to massive sandstones.

Mineralogical patterns, pisoid-rich layers, and bed thickness patterns can all be correlated among the three sections. The pattern of increasing clastic grains and increasing clastic bed thickness upsection suggest clastic influx during progradation that progressively overwhelmed carbonate production. This progradational setting could have occurred on a tide dominated delta or on a prograding tidal flat. The BSD-KPF contact in the northern Kingston Range most likely represents a sequence boundary as evidenced by a change in lithology from dolostones to clastics.