Paper No. 31
Presentation Time: 8:00 AM-6:00 PM
PORE DISTRIBUTION IN THE UPPER CAMBRIAN EAU CLAIRE FORMATION OF INDIANA - THIN SECTION AND SEM OBSERVATIONS
YAWAR, Zalmai, Geosciences, Indiana University, 1001 E 10th. Street, Bloomington, IN 47405 and SCHIEBER, Juergen, Geological Sciences, Indiana University, 1001 East 10th Street, Bloomington, IN 47405, zyawar@indiana.edu
The Upper Cambrian Eau Claire Formation consists of fine grained sandstones interbedded with varying amounts of mudstones. Based on sedimentary structures and mudstone content the Eau Claire has been divided into five distinct lithofacies, A, B, C, D and E. Lithofacies A is very thin to thin bedded mudstone with small amounts of siltstone and sandstone. Lithofacies B consists of thin bedded sandstone with mudstone interbeds. Lithofacies C consists of thick sandstone beds with mudstone partings (usually less than 0.04 inches thick). Lithology D consists of massive appearing sandstone with rare internal breaks. Lithofacies E consists entirely of sandstone and internal lamination is barely visible. Petrographic thin sections of the Eau Claire show an abundance of fossil debris (trilobites, brachiopods and echinoderms) throughout the mudstones of lithofacies A. The sandy portions of the Eau Claire consist largely of quartz grains and a very small proportion of microcline grains.
Thin section assessment of the sandy portions of the Eau Claire Formation reveals that the sandstone was initially porous, but porosity has been reduced by a number of postdepostional processes. Pore distribution within the sandy part of the Eau Claire Formation is variable. Quartz cement and dolomite cement significantly reduced porosity in many samples. Dissolution of feldspar and calcite has produced porosity but often dissolved calcite has been replaced by quartz. Dolomite rich samples often show reduced porosity compared to quartz rich portions. Despite the burial history of the Eau Claire Formation pressure solution effects are not very high.
Shale-rich portions of the Eau Claire Formation generally shows a firmly packed tight fabric. However, isolated pockets of pores are commonly associated with pressure shadows of larger, compaction resistant grains. Within these pressure shadows, framework texture of phyllosilicates are common that show triangular pores ranging from 0.05 to 0.5 micrometers.
