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Sedimentology Of The Neoproterozoic Beck Spring - Kingston Peak Transition In The Kingston Range, California

Abstract

The Neoproterozoic Kingston Peak Formation (KPF) is known for its diamictites representing glaciation. Studies in the underlying Beck Spring Dolomite (BSD) describe early fossil forms and microbialites that suggest warmer and shallow water. This study aims to understand the changing depositional conditions between those units and focuses on the transition, specifically in 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 immediately overlying the contact and analyzed the sedimentary structures, textures and sequences. A previous study suggested this unit was probably deposited by storm currents in a shelf setting. So far, we have located and measured 2 sections about 5 meters long that include the KPF-BSD contact. The top of the BSD in both 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 50 cm above the contact show 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) thin beds and thick laminae. A Beds are silty dolostones- silty mudstone with abundant muscovite. B beds are recrystallized silty dolostones with ghost peloids. These beds also show an alternating mineralogy with A beds being more clastic rich and B beds dolomite rich. Alternating bed contacts are sharp. XRD and thin section analysis show a decreasing amount of dolomite trending up-section. Almost all thin sections show many fractures indicating tectonic activities. Measurement and analysis of individual alternating bed thickness show cyclical changes taken over by a rapid increase in bed thickness to massive sandstones. All these features can be correlated between the two sections. The pattern of increasing clastics and their bed thicknesses suggest clastic influx during progradation that progressively overwhelmed carbonate production thus resulting in increasing clastic and decreasing carbonate material up section. This progradational setting could occur on a tide dominated delta or on a prograding shoreface. The BSD-KPF contact at Kingston Range most likely represents a sequence boundary located at the change in lithology from dolostones to clastics.