Diagenetically Altered Stable Isotope Values from Petrographically Pristine Brachiopods: Lower Devonian Helderberg Group, New York State

P. F. Rush, H. S. Chafetz

Brachiopods in the Devonian Helderberg Group carbonates and overlying Oriskany Sandstone are preserved with either an "unaltered" fabric which exhibits extremely fine preservation of original texture, or as neomorphosed coarsely crystalline bladed calcite. Petrographically pristine-looking as well as obviously neomorphosed brachiopods both display the same diagenetically altered carbon isotopic signatures. The carbon values of all skeletal components, including the petrographically well-preserved brachiopods, show a gradational change with stratigraphic distance beneath a Devonian unconformity. These values range from 2 to 3.5% PDB approximately 40 m below the unconformity to -0.6 to 1.5% PDB immediately subjacent to the unconformity. Superjacent to the unconformity, brac iopods display an isotopic signature of 2.3 to 3.7% PDB, which is similar to values observed well below the unconformity. Diagenesis of the skeletal material must have occurred in contact with a meteoric system during the Siegenian (Lower Devonian) exposure. Of greater significance, petrographically pristine brachiopods have not retained their original carbon signature; instead these brachiopods have equilibrated with lighter meteoric waters without any visible change in their skeletal ultrastructure.

This analysis was performed on the Lower Devonian Helderberg Group of central New York State, an accumulation of paralic to shallow open marine carbonates deposited on a shallow gradient ramp in a slowly subsiding (1.5 cm/1,000 yrs) foreland basin.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.