Chazyan (lower middle Ordovician) reefs from the Virginias, Vermont and New York, and Quebec show changes in organic composition through time. In the evolution of reef communities, these Chazyan reefs represent assemblages or organisms which are transitional in taxonomic composition and ecologic setting between pre-Chazyan and Silurian reefs.

Early Chazyan trepostome (Batostoma) and cyclostome (Cheiloporella) bryozoans built linearly aligned reefs up to 10 feet high in shallow, agitated waters. The reef matrix of carbonate mud and skeletal debris differs markedly from the cross-bedded, mud-free skeletal carbonates adjacent to the reefs.

Middle Chazyan reefs shown an evolution of reef assemblages from a laminar stromatoporoid (Cystostroma)--algal (Anthracoporella) composition to an assemblage with a higher percentage of tabulate corals (Billingsaria), sponges (Zittelella), and a different stromatoporoid (Pseudostylodictyon). At the top of the Middle Chazyan, three separate assemblages (stromatolite-calcareous alga-nautiloid, trepostome and cyclostome bryozoans, and stromatoporoid-sponge-coral) are all in close lateral contact with each other and appear to have been contemporaneous. In the Upper Chazyan, the trepostome bryozoans replace the stromatoporoids of the early assemblages and combine with the alga (Anthracoporella) to form a different assemblage. This succession of assemblages takes place with no apparent chan e in habitat.

The Lower Chazyan bryozoan reefs contain more detrital quartz and have more pronounced cross-bedding in adjacent sediments than the younger Chazyan reefs, indicating that the bryozoans existed in more agitated conditions closer to land than the later assemblages. However, close proximity of oolitic and oncolitic carbonates, dislodged and tumbled corals and stromatoporoids, erosional channels and margins cut into the reefs, and the presence of blue, green, and red algae suggest that the Middle and Upper Chazyan reefs also developed in shallow water.

A spectrum of textures in the non-reef sediments, mudstones through well washed grainstones, represents most stages from restricted to open circulation, high energy conditions in their environments of deposition.

The sequence of diagenetic events that affected the limestones is: formation of rim cement in grainstones before and concomitant with pore-filling drusy cementation, dolomitization, lithification of carbonate mud, and finally grain growth in the aragonite skeletons and carbonate mud.

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