Origin of marine oo͏̈ids and grapestones

The cortex of marine ooids and agglutinating carbonate of grapestones consists of aragonitic particles, both rods and granules, formed through biomineralization by burrowing thallophytes, likely blue-green algae. The formation of the ooid cortex hinges on grapestone aggregation, where algae bind sedimentary grains using threads and mucilage, followed by endomucilage calcification. Voids between grains are filled with aragonitic rods in algal mucus, eventually encasing the grapestone nucleus with an ooid cortex. While grapestone formation requires stable sediment, ooid development necessitates sufficient grain movement to prevent binding. Optimal conditions for growth include warm, shallow, well-lit sea water supersaturated with CaCO3, with aragonite secretion by algae possibly being a physiological response to this environment. A genetic connection is proposed linking ooids and grapestones to stromatolites and oncolites. Additionally, aragonite secretion by thallophytes may significantly contribute to aragonitic mud formation. The biocalcification of primitive algae is crucial for marine shallow water calcareous sediment formation. Environmental physicochemical differences explain facies differentiation into ooid sand, grapestone aggregate sand, and lime mud. Three conditions can inhibit ooid or grapestone formation: nutrient-rich skeletal grains, deposition in deep sea water, and cold sea water with low CaCO3 saturation. Micr