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Title:Relative timing of dolomitization and silica cementation in the Cambrian Potosi and Eminence Dolomites in the Illinois Basin, USA
Author(s):Eickhoff, Brooke
Advisor(s):Fouke, Bruce W.
Department / Program:Geology
Degree Granting Institution:University of Illinois at Urbana-Champaign
Illinois Basin
Quartz cement
Silica Cement
Relative Timing
cloudy-centered, clear-rimmed (CCCR)
Paragenetic sequence
Abstract:The Late Cambrian Potosi and Eminence Dolomites of the Illinois Basin are currently being considered as targets for subsurface carbon capture, transport and storage (CCS) because of their extensive regional distribution and high intrinsic porosity and permeability. Drill core and outcrop samples collected from the center and margin of the Illinois Basin have identified multiple events of subsurface dolomitization and quartz cementation within dissolution voids (vugs). A detailed petrographic study has therefore been undertaken to reconstruct the complex history of post-depositional subsurface physical and chemical alteration (diagenesis) of the Potosi and Eminence Dolomites. Analysis of the Potosi and Eminence Dolomites was completed at two locations within the Illinois Basin, which included: (1) Basin Center - core samples from depths of 1376-1388 m (4513-4553 ft) retrieved from the Illinois State Geological Survey (ISGS) Midwest Geological Sequestration Consortium (MGSC) Illinois Basin-Decatur Project (IBDP) CO2 verification well in Decatur, Illinois; and (2) Southwestern Basin Margin – outcrops near Potosi, Missouri. The presence of columnar stromatolites and soft sediment burrows observed in the IBDP core, as well as previously described cryptozoan algal structures and trilobites, suggest original deposition in shallow marine environments. Both the Potosi and Eminence Dolomites are pervasively recrystallized, massively bedded dolomitized limestones, which are indistinguishable in outcrop, hand sample and thin section analyses. A paragenetic sequence was established in this study via the integrated application of sedimentologic, stratigraphic, and high-resolution plane-light (PL) and cathodoluminescence-light (CL) petrography techniques. The original shallow marine limestone deposits have been pervasively replaced with 250-550 μm-diameter euhedral dolomite crystals that are cloudy-centered, clear-rimmed (CCCR). Each dolomite crystal exhibits a dark CL mauve colored core surrounded by bright red concentrically CL-zoned crystal overgrowths. Both the core and outer zones of these crystals exhibit dissolution voids, which are either filled with a non-CL replacement calcite or a bright orange CL concentrically zoned replacement dolomite. Columnar, euhedral 250 μm to 1 cm-long quartz cement crystals grow as drusy rims within dissolution voids. These are composed of seven CL-defined generations of quartz cements, which include: (1) 190-210 μm-thick CL dark maroon cement; (2) 95-175 μm-thick CL bright red cement with many CL bright defined zones; (3) 30-50 μm-thick dark maroon cement; (4) 200-225 μm-thick cement that is dark maroon in color with some light red zones; (5) 85-100 μm this dark red cement with no zonations; (6) 80-105 μm-thick dark maroon cement; and (7) 50-100 μm-thick dark maroon cement with some hexagonal quartz crystals protruding into the pore space. Paragenesis concludes with hairline fractures that cross-cut all previous diagenetic events. The above paragenetic sequence established for the Potosi and Eminence Dolomites has been correlated with previously published burial histories and paragenetic reconstructions for other Lower Paleozoic strata of the Illinois Basin. This process has permitted relative constraints to be placed on the timing and source waters of dolomitization, dissolution and silica cementation in the Potosi and Eminence Dolomites. Quartz cements in the Cambrian Mt. Simon Sandstone and the overlying Cambrian Eau Claire Sandstone yield early-to-late stage cementation δ18O values (28-18‰ VSMOW), which suggest they were derived from Si-rich source waters during subsidence and associated pressure solution. This suggests that regional quartz cementation took place at 38 to 103oC at burial depths of 1 to 2.4 kilometers at approximately 450 to 250 years before present (Hyodo et al., 2012; Pollington et al., 2011). The paragenetic sequence for the Potosi and Eminence Dolomites established in this study indicates that dolomitization and dissolution occurred after deposition but before quartz cementation. When evaluated within the context of previously published Illinois Basin burial history and quartz cementation, this implies that dolomitization and dissolution of the Potosi and Eminence Dolomites occurred at temperatures of approximately 25 to 38oC, burial depths of approximately 0 to 1 km, and at an age of approximately 501 to 450 million years before present (Hyodo et al., 2012; Pollington et al., 2011; Miller, 2011; Makowitz and Lander, 2006; Fishman, 1997; Pitman et al., 1997). Small dissolution voids and replacement dolomites are common in the CCCR dolomite rhombohedra. This implies that dolomitization and dissolution of the Potosi and Eminence Dolomites both took place soon after deposition in relatively shallow burial environments. This is also consistent with previously published interpretation of dolomitization events in other Paleozoic strata of the Illinois Basin.
Issue Date:2013-05-24
Rights Information:Copyright 2013 Brooke Eickhoff
Date Available in IDEALS:2013-05-24
Date Deposited:2013-05

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