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A study of various types of banks, mud accumulations, and organic reefs in the geologic record hopefully permits some generalizations about them. As an example, several ancient organic buildups are compared on the basis of development relative to the photic zone and wave base, their shape or trend, and their biologic potentials for framework construction. These range from Cambrian algal mounds to Siluro-Devonian stromatoporoid-coral masses, to late Paleozoic carbonate mudstone and algal-foraminiferal accumulations, to Mesozoic coral and rudist buildups. Petrographic criteria useful in categorizing organic buildups include sedimentary structures in biohermal cores and "reef walls," an ecologic-morphologic classification of the biota, microfacies of the flanking or interree beds, criteria for recognizing exposure surfaces and intertidal sediments, and special criteria for depth recognition.
Processes of (1) true organic frame-building reef formation, and (2) trapping and binding of fine-grained material by organisms have operated from Cambrian time on. Organic framework construction is known to be important in the middle Paleozoic and in some situations from Jurassic to Holocene times. However, binding and trapping of carbonate mud and silt by organisms account for practically all the late Paleozoic organic buildups and for many of those in the Cambro-Ordovician and Mesozoic. Most organic buildups in the geologic record are in no sense organic frame-built reefs. Further, many began development in downslope positions below wave base, some even below the photic zone. Whether there are modern analogues to such bodies of sediment, and how far downslope such organic buildups an form are questions that are unanswered.
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