ABSTRACT

The Onondaga Limestone (Middle Devonian) of New York is dominantly fine-grained carbonate. Fossil debris is abundant in eastern and western New York, as well as in the lowermost beds throughout the state. Most skeletal material occurs as broken, angular fragments, although whole shells are present. In rocks where fossil debris is abundant, crinoids, bryozoa, and corals dominate; where fossil debris is subordinate to fine-grained matrix, trilobites, brachiopods and ostrocods are more abundant. Pellets are scarce, whereas intra-clasts and oolites are absent.

The fine-grained carbonate has a mean grain size between 5 and 15 microns. Further, it is poorly sorted (inclusive standard deviation between 0.76 phi and 1.60 phi), contains numerous elongate particles, and is associated with terrigenous silt. These attributes are opposed to those ascribed to "microspar," which is produced by recrystallization of clay-size (4 microns) carbonate, but are considered characteristic of mechanically-deposited carbonate silt. Calcisiltite is the most appropriate term for such material.

Studies of Recent carbonates, show that comminution of carbonate skeletal material yields silt-size carbonate sediment. Abundant fossil debris in the Onondaga, much of which is broken and bored, shows that such a mechanism may account for the bulk of the calcisiltite. Further, most of the calcisiltite is burrowed, probably by organisms searching for nourishment in the mud. Such activity was probably a factor in the distintegration of the shell debris.