FISHMAN, NEIL S., U.S. Geological Survey, Denver Federal Center, Denver, CO

ABSTRACT: Large-Scale Fluid Movement in a Cambrian Paleoaquifer--Evidence from the Mt. Simon Sandstone, Illinois and Indiana

Siliciclastic rocks of the Cambrian Mt. Simon Sandstone, Illinois and Indiana, have undergone a complex alteration history related to movement of fluids of varying composition over large distances. Understanding the nature, timing, and distribution of alterations is crucial to identifying the source(s) and migration pathways of fluids including basinal brines or those containing hydrocarbons.

Authigenic quartz, the principal cement (15-20%) in the formation (data from 15 cores), occurs as euhedral or interpenetrating overgrowths. Preliminary fluid inclusion data suggest that the overgrowths formed at temperatures >60 degrees C. K-feldspar (K-spar), another authigenic cement, is variable in abundance (0-10%) and occurs as euhedral overgrowths on detrital feldspars and, locally, as small rhombic crystals in pores (all authigenic K-spar is partially dissolved). Variability of authigenic K-spar abundance appears to be controlled, in part, by the occurrence of detrital feldspar suggesting detrital grains served as nucleation sites. K-spar rhombs appear devoid of detrital cores, suggesting detrital hosts were not required for nucleation. Additional authigenic minerals include minor illite, chlorite, Fe-oxides, anhydrite, calcite, pyrite, and fluorite.

Abundant authigenic quartz in the Mt. Simon implies widespread fluid migration (preliminary mass-balance studies suggest an external source for much of this quartz). Differing morphologies of authigenic K-spar and evidence of dissolution (released K may have been incorporated into illite) indicate that fluids of differing compositions migrated through this paleoaquifer at different times. This evidence of large-scale fluid movement in the Mt. Simon implies that any hydrocarbons or basinal brines that passed through the formation might have had dispersal patterns similar to those of fluids from which the authigenic cements precipitated.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.