ABSTRACT

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 the reef communities, these Chazyan reefs represent assemblages of organisms which are transitional in taxonomic composition and ecologic setting betwen pre-Chazyan and Silurian reefs.

Early Chazyan trepostome (Batostoma) and cyclostome (Cheiloporella) bryozoans built linearly aligned reefs up to ten 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 show an evolution of reef assemblages from a laminar stromatoporoid (Cystostroma) - algal (Solenopora?) 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 assemblage and combine with the alga (Solenopora) to form a different assemblage. This succession of assemblage takes place with no apparent change in habitat.

The Lower Chazyan bryozoan reefs contain more detrital quartz and have more predominant 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 suggests that the Middle and Upper Chazyan reefs also developed in shallow water.

Non-reef sediments vary from carbonate mudstones through well-washed grainstones. Such a spectrum of textures indicates environments of deposition varying from restricted to open circulation, high energy conditions.

Diagenetic processes affecting the rocks, in order of their occurrence, are: 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 aragonitic skeletons and lime mud.