Oolitic limestone from the Mississippian of Indiana, USA. Sedimentary rocks form by the solidification of loose sediments. Loose sediments become hard rocks by the processes of deposition, burial, compaction, dewatering, and cementation. There are three categories of sedimentary rocks: 1) Siliciclastic sedimentary rocks form by the solidification of sediments produced by weathering & erosion of any previously existing rocks. 2) Biogenic sedimentary rocks form by the solidification of sediments that were once-living organisms (plants, animals, micro-organisms). 3) Chemical sedimentary rocks form by the solidification of sediments formed by inorganic chemical reactions. Most sedimentary rocks have a clastic texture, but some are crystalline. Limestone is a common sedimentary rock composed of the mineral calcite (CaCO3), which bubbles in acid. Many geologically young limestones are composed of aragonite (also CaCO3). Numerous varieties of limestone exist (e.g., fine-grained limestone/micritic limestone/lime mudstone, coquina, chalk, wackestone, packstone, grainstone, rudstone, rubblestone, coralstone, calcarenite, calcisiltite, calcilutite, calcirudite, floatstone, boundstone, framestone, oolitic limestone, oncolitic limestone, etc.). Most limestones represent deposition in ancient warm, shallow ocean environments. Oolitic limestones are whitish to cream-colored limestones composed of sand-sized (1/16 to 2 mm in size), well rounded, concentrically-layered calcite or aragonite grains called oolites (also known as ooliths or ooids). Oolites form by rolling back and forth on a shallow seafloor, or sometimes on a shallow lake bed, by wave action. Oolites are forming today on the Bahamas Platform and in Great Salt Lake, Utah, USA. The technical geologic term for most oolitic limestones is “oolitic grainstone”. Uncertainty exists about the specifics of the origin of oolites. Some researchers conclude that oolites form by completely inorganic chemical precipitation of CaCO3 from water around some nucleus (a tiny shell or skeletal fragment or sediment grain). Other researchers conclude that the presence of bacterial films on oolite grain surfaces play a significant role in the precipitation of CaCO3 layers. However, the undoubted presence of bacteria does not necessarily indicate a biogenic origin for oolites – bacteria are everywhere. Stratigraphy: Salem Limestone, Middle Mississippian Locality: unrecorded/undisclosed site in southern Indiana, USA (James St. John)
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