ONCOID GROWTH STRATEGIES IN RESPONSE TO FLUCTUATING DEPOSITIONAL ENVIRONMENTS DURING THE LATE EARLY CAMBRIAN OF EASTERN CALIFORNIA

Analysis of the morphologic features (envelope texture, shape and size) of ~1500-2000 oncoids from the Chambless Limestone and lowermost Cadiz Formation in the Marble Mountains, Eastern California provides insight to Late Early Cambrian changes in carbonate depositional facies and sea level fluctuations. The 37m mixed carbonate and siliciclastic units of the lower Cambrian Chambless Limestone and Cadiz Formation suggest a deepening trend punctuated by short term shallowing events. The general deepening resulted in a decrease of tidal influence and storm events. The shift from shallow water cross-bedded grainstones to alternating bioturbated and partially dolomitized packstone-wackstones to mudstones near the top of the Chambless Limestone and lower Cadiz Formation is reflected in the oncoid shape and internal texture. Oncoids at the base of the Chambless Limestone are typically large (10-30mm), elliptical in shape with well developed growth laminae. The laminae in the oncoids are frequently truncated due to the tumbling in response to tidal and storm action. Oncoids in the middle and upper part of the Chambless Limestone are darker, smaller and more spherical. Internally, growth lamiae are continuous, but less prominent. In some oncoids, laminae are absent. Preliminary aquarium studies, indicate that some forms of cynobacteria form spherical masses with continuous laminae under quiet conditions. Surprisingly oncoids in the basal Cadiz Formation show a marked increase in size and superficially resemble oncoids from the high energy basal facies of the Chambless Limestone, but internally are distinct, characterized by continuous growth laminae typical of quiet conditions. Conditions during the deposition of the basal Cadiz Formation, in contrast to the upper Chambless Limestone, must have been conducive to either more rapid or longer term growth to allow the development of continuous laminar growth. We believe that the observed changes in oncoid morphology reflect a decrease in energy levels associated with broad late Early Cambrian transgression punctuated by brief shallowing events.