Abstract: Pore Water Evolution in the Warsaw Formation, Illinois Basin: Preliminary Results from a Modified Method of Isotopic Study of Various Sizes of Dolomite

Deba P. Bhattacharyya

Ubiquitous compositional zoning, textural aggradation, and other distinctive textural features of dolomites in sediments suggest that the process of dolomitization is progressive, and is controlled by dissolution and reprecipitation reactions under the influence of one or more generations of pore fluids which also continuously evolve during the process. Therefore, conventional isotopic analysis of bulk or micromilled subsample yields only an average value for the sample and cannot truly depict the progressive evolution of the pore fluid within the sample. Moreover, The ^dgr¹³C-^dgr¹⁸O plots of such data from a set of samples from any sedimentary sequence/horizon may show wide scatter or clusters of such averages. Each of the clusters, then, is commonly assigned t a specific process. But, in reality, these clusters may represent convergence of various processes.

In order to elucidate the isotopic evolution of dolomites within a specific stratigraphic horizon, the dolomitized mudstone and grainstone of the Warsaw Formation in the Illinois Basin were studied by a modified method. Initial investigation indicates that the rocks had been invaded by multiple fluids resulting in texturally aggrading, and compositionally zoned dolomites of various sizes from very fine (<5 microns) to very coarse (>0.5 mm). Also, the progression of dolomite grain size is related to temporal progression of the dolomitizing processes. Therefore, the dolomite grains were disaggregated from specific samples using dilute (2%) hydrochloric acid, and individual size fractions were separated by gravity settling in water column. The various size fractions were then analy ed for their C and O isotopic compositions in each sample.

The results from equivalent samples of the Warsaw sequence, located more than 200 km apart, show the following consistent pattern: (1) overall, the dolomites show progressive oxygen isotopic depletion with increasing size except that the 10-25 micron dolomites are enriched in ¹⁸O compared to finer (approx. 5 microns) and coarser (>25 microns) dolomites; (2) dolomites coarser than 50 microns are very highly depleted in ¹⁸O; (3) the degree of ¹⁸O depletion or enrichment, as the case may be, between neighboring size fractions in the size ranges of dolomites <50 microns is relatively small (commonly <1 per mil), whereas it is relatively large (generally >2 per mil) for the size fractions >50 microns; (4) in samples of a specific location the ^dgr¹³C compositions of finer dolomites, especially of those finer than 40 microns, are within the limits of experimental uncertainties (+/-0.1 per mil), but the larger size fractions show appreciable progressive depletion with increasing size (maximum 2 per mil).

The isotopic data and the patterns mentioned above suggest that the earliest (and the smallest) dolomites of the Warsaw Formation were probably formed from sea water trapped in the sediments following which there was a brief period of flushing the system with evaporative brine. Subsequently, as the mixture of connate sea water and evaporative brine gradually evolved, the rocks continued aggrading various dolomites of sizes up to 50 microns. At that point the system probably started getting an increasing influx of hot, strongly C and O isotope-depleted basinal brine from which the coarser dolomites were progressively precipitated. This is significant because the latest, coarse dolomites are associated with smaller pockets of Pb-Zn sulfide minerals in intergranular pore spaces as well a geodic cavities in the Warsaw sequence. Also, the consistency of the isotopic patterns over wide areas suggests a regional scale of the dolomitizing processes in the Warsaw Formation.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994