James Says:

As part of my Masters research with Dr. Sumner, I am studying molar tooth structures. These are enigmatic calcite-filled veins that are common in Precambrian sedimentary rocks. They comprise large proportions of limestones and dolostones in the Proterozoic, but are very poorly understood. They have been attributed to a range of processes, including earthquakes inducing small injections of surrounding sediment, desiccation inducing evaporite mineral precipitation, soft bodied organisms that live in the sediment, cracking of the sediment induced by earthquakes or variations in salinity, and gas bubbles from organic decay that fracture the sediment pile under pressure. In this study, I have focused on an older Archean example from South Africa, and have tried to explain the nature of the calcite crystals that fill these structures. Previous models call for these crystals to have precipitated in open cracks, to have replaced former organic or crystal structures, or to be injections of surrounding sediment. We have shown that these structures form when calcite precipitates in open cracks in the shallow sediment pile. I have produced a model for this calcite precipitation involving rapid mixing of ocean waters and sediment pore waters in the shallow subsurface. This model has a number of broad implications. Firstly, shallow uncompacted sediments in the Precambrian were firm enough to hold open a crack. Secondly, when mixed, ocean waters and sediment pore waters were highly supersaturated with calcite. Finally, near the end of the Precambrian, something changed in the oceans and caused the demise of molar tooth structures: this was probably a combination of changing water chemistry and sediment rheology.

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Dawn Y. Sumner
Department of Geology
University of California
Davis, CA 95616