Petrography and C and O Isotope Geochemistry of Crinoids, Burlington-Keokuk Formation, Southeastern Iowa and Western Illinois

K. A. Kohrt, W. J. Meyers, G. N. Hanson

Petrographic observations and C and O isotopic compositions of crinoids from the Mississippian (Osagean) Burlington-Keokuk formation constrain the compositional evolution of early phreatic zone pore waters. Crinoid skeletal calcites exhibit a different luminescence than intraskeletal calcite cements, implying that these two phases were precipitated from fluids with different redox potentials. Regional isotopic variations of petrographically identical crinoids suggest pore water isotopic compositions change, while redox potentials remain constant.

A positive covariance is found between ^dgr¹⁸O and ^dgr¹³C of bulk crinoids at each of 10 localities in the study area. The covariant trends fan out from the estimated Osagean marine calcite composition (^dgr¹⁸O = -2.19 ^pmil PDB, 2s = 0.72; ^dgr¹³C = 4.45 ^pmil PDB, 2s = 0.62) intersecting an invariant ^dgr¹⁸O (-4.5 ^pmil PDB) meteoric calcite line at ^dgr¹³C values that increase at localities distal from proposed meteoric recharge. This covariance results from physical mixing of crinoid skeletal calcite and intraskeletal calcite cements that are interpreted to have precipitated from mixed evolved phreatic zone pore waters.

Increasing mean ¹³C values of bulk crinoids at localities away from proposed sites of meteoric recharge suggests enrichment of pore water ¹³C due to increasing rock-water interaction is downflow areas of the aquifer. Increasing linear covariance at localities down the hydrologic gradient is attributed to convergence of fluid end members to similar total dissolved carbon concentrates.

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