--> Abstract: Early Cambrian Carbonate Buildups and Establishment of the Paleozoic Reef Archetype, by N. P. James; #91012 (1992).

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ABSTRACT: Early Cambrian Carbonate Buildups and Establishment of the Paleozoic Reef Archetype

JAMES, NOEL P., Queen's University, Kingston, Ontario, Canada

Early and middle Paleozoic shallow water reefs are constructed primarily by sponges with a calcareous skeleton and encrusting calcimicrobes, contain growth cavities of variable dimensions, were lithified by often extensive marine cements, and are surrounded by pelamtozoan-rich sediments. Research to date implies that this complex ecosystem appeared quite suddenly in carbonates of the Tommotian Stage (sub-trilobite Cambrian) on the Siberian platform. Small biogenic mounds that locally coalesce into large buildups are composed of archaeocyathans in the form of sticks, bowls, and sheets and encrusted by Renalcis, which grew geostophically down into cavities and locally as an anastomosing mesh between skeletons. Fibrous and micritic early marine cements lithified these structures on the s a floor beneath the living surface. These buildups remain little changed throughout Lower Cambrian time except for increased bioerosion and addition of abundant perireefal sediment with the

evolution of echinoderms. This basic plan also characterizes many Ordovician, Silurian, and Devonian buildups except that the sponges with a calcareous skeleton are stromatoporoids, whose larger and more robust skeletons permitted reefs to grow in higher energy environments. These younger reefs are also distinguished by a progressively more complex fauna of associated taxa, especially corals, brachiopods, and true calcareous algae, but the plan remains unchanged. Downslope biogenic mounds rich in microbial(?) mud and sponge spicules are known from middle Early Cambrian rocks in Australia and although not common until Ordovician time, have the same basic attributes as Ordovician-Devonian mounds. This basic reef structure is present until mid-Triassic time, when, with the evolution of s leractinian corals and associated taxa, reefs assumed a characteristically modern appearance.

 

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)