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Title:Subsurface dolomitization and porosity occlusion within Early to Middle Ordovician strata of the Illinois Basin, USA
Author(s):Dwyer, Samantha E.
Advisor(s):Fouke, Bruce W.; Marshak, Stephen
Department / Program:Geology
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Illinois Basin
Shakopee Dolomite
Everton Formation
St. Peter Sandstone
Abstract:Analysis of sedimentological, stratigraphic, paragenetic and geochemical characteristics of samples from the Early Ordovician Shakopee Dolomite and Everton Formation, and the overlying Middle Ordovician St. Peter Sandstone, obtained from drill cores and outcrops in the Illinois Basin indicate that subsurface dolomitization locally reduced primary porosity, and that the degree of reduction changes with location. Specifically, porosity occlusion by dolomitic cementation is pronounced in the Shakopee and Everton on the northern shelf of the basin, and, increases progressively to the south into the deeper part of the basin. Three sample locations were selected for this study based on relative paleoceanographic position along a north-south transect through the Illinois Basin. These include: (1) core from the shallow marine shelf in Stephenson County (UPH well); (2) hand samples from the LaFarge Quarry in LaSalle County; and (3) core from the deep marine basin in White County (Superior Well). Quartz arenites of the St. Peter overlie the Shakopee at the post-Knox unconformity (an erosional surface) in the northern Illinois Basin. The Shakopee is a fine-grained dolomite interlayered with thinly bedded shales and siltstones. In the southern, deeper portions of the Illinois Basin, the St. Peter directly overlies the Everton . Here, the Everton is a near-shore dolomitized marine deposit containing irregular lenses of quartz sands. Previous biostratigraphic correlation studies in the southern end of the basin suggested that the Everton is age-equivalent to both the Shakopee and the St. Peter in the basin, and thus, that the post-Knox unconformity underlies the Everton. My studies of sedimentologic composition and diagenetic alteration suggest, however, that the Everton belongs within the Knox Supergroup instead of the Ancell Group, and thus that the top of the Everton is the post-Knox unconformity. Of note, cementation and lithostratigraphic characteristics at the post-Knox unconformity makes the surface act as an efficient aquitard. A suite of 61 polished thin sections were analyzed petrographically in plane light, polarized light, and with cathodoluminescence (CL). Use of CL allowed identification of an early quartz cement and two dolomite cements (D1 and D2) that are separated from later dolomitization events by a dissolution event. A pink-red CL dolomite (RD1) replaced D1 and D2, which is followed by a third non-luminenscent dolomite cement (D3) and a later anhydrite cement. Two late replacement dolomite cements (RD2 and RD3), which exhibit dark red and yellow-orange luminescence respectively, replaced all of the prior dolomite cements. This observed paragenetic sequence was capped by the precipitation of a late bright blue quartz cement. Covariation modeling of δ18O, δ13C and 87Sr/86Sr is consistent with the dolomitizing fluids having been formed from seawater-derived brines that were partially diluted with meteoric fluids that gained radiogenic Sr from either the Precambrian granite-rhyolite basement or from Paleozoic shales. The various cements, which reflect the consequences of successive diagenetic events, significantly reduced porosity locally. Comparison of thin sections from the three sample localities used for this studies indicates that porosity decreases progressively from north to south in the Illinois basin. Specifically, the Shakopee, Everton and St. Peter deposits have between 10% and 30% porosity in samples from the UHP well, whereas they have only 1-5% porosity in the Superior well. This change suggests that cementation was greater in south, where diagenetic fluids were hotter and contained relatively more dissolved minerals. As these fluids migrated northwards, they progressively lost their mineral content, and thus were less capable of precipitating cements when they reached the northern shelf of the basin. Therefore, strata of the northern portion of the basin are likely better targets for subsurface CO2 sequestration than are those to the south.
Issue Date:2011-05-25
Rights Information:Copyright 2011 Samantha E Dwyer
Date Available in IDEALS:2011-05-25
Date Deposited:2011-05

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