In the Precambrian world, devoid of higher organisms except near its end, carbonate sediments formed by a variety of abiotic and microbial processes, with patterns of deposition determined by tectonic, eustatic, and climatic processes. These ancient rocks demonstrate that the fundamental tenets of carbonate production and accumulation were initiated early in earth history, with the basic attributes of carbonate sedimentation well established by Neoproterozoic time.
The broad temporal patterns of Precambrian carbonate facies composition and disposition parallel the long-term evolution of the earth’s oceans and atmosphere. Archean and Paleoproterozoic carbonates commonly contain abundant sea-floor precipitates, whereas the Neoproterozoic record is dominated by clastic-textured facies and abundant carbonate mudstones; Mesoproterozoic carbonates are transitional. Mesoproterozoic and early Neoproterozoic carbonates also contain abundant quantitites of the enigmatic molar-tooth structure. Grainstones, dominated by giant ooids with centimeter-scale diameters, are characteristic of many Neoproterozoic carbonates. Texturally unusual carbonates, featuring a reprise of Archean-style sea-floor precipitates, often cap glacial deposits of middle Neoproterozoic age.
The influence of biology on sediment texture is best expressed in the history of Precambrian reefs. Archean through Mesoproterozoic reefs are dominantly stromatolite-based. Lamination textures reveal the progressive shift from in situ precipitation of aragonite and calcite encrusting the sea floor in Archean through Paleoproterozoic stromatolites to textures consistent with accretion of loose sediment through trapping and binding in Neoproterozoic stromatolites. This trend is interpreted to reflect the progressive decrease of abiotic factors and the concomittant increase of benthic microbial mats on controlling stromatolite growth. Neoproterozoic reefs witness the appearance of more complex textures that likely involve the participation of calcified microbes and noncalcified higher algae in colonizing the seafloor, increasing its surface complexity and resulting in highly porous frameworks for the first time in geologic history. Terminal Proterozoic thrombolitic reefs additionally contain the first calcified metazoans.
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Carbonate Sedimentation and Diagenesis in the Evolving Precambrian World - Precambrian carbonates are usually regarded at the simple cousins of the sedimentary realm, composed of stromatolites and dolostones, texturally not challenging and commonly altered beyond recognition by the vagaries of time, diagenesis and metamorphism. However, these carbonates that formed deep in time are commonly exquisitely preserved and contain within them a record of the evolving young earth. SEPM Special Publication 67 explores these aspects. Resulting from a 1997 SEPM/CSPG symposium entitled? Precambrian Carbonates,? these 18 papers demonstrate the importance of understanding these rocks, since within them is contained a record of the early ocean, atmosphere, and biosphere.