Carbonate Microfacies, Depositional Environments and Diagenesis of the Galena Group (Middle Ordovician) Along the Mississippi River (Iowa, Wisconsin, Illinois and Missouri), United States
Bakush, Sadeg H.
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https://hdl.handle.net/2142/71140
Description
Title
Carbonate Microfacies, Depositional Environments and Diagenesis of the Galena Group (Middle Ordovician) Along the Mississippi River (Iowa, Wisconsin, Illinois and Missouri), United States
Author(s)
Bakush, Sadeg H.
Issue Date
1985
Department of Study
Geology
Discipline
Geology
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Geology
Abstract
The 1400 samples of vertically-oriented limestone and dolomite and 50 samples of shale and K-bentonites used in this study were collected from six field sections and four cores of the Galena Group.
Detailed petrographic investigation led to recognition of seven distinct carbonate microfacies. Five of these were recognized as fair-weather microfacies and two (5 and 6) were identified as storm-weather microfacies. The vertical succession and correlation coefficients of the microfacies permitted the recognition of an ideal shallowing upward sequence, which in turn was converted into a horizontal depositional model.
The depositional model consists of a shallow open marine subtidal carbonate ramp with the following fair-weather environments and their microfacies (1, 2, 3, 4 and 7) from offshore to onshore: slope (calcisiltite with up to 10% scattered debris of sand-size bioclasts); outer platform (mud- to grain-supported bioaccumulated limestone with calcisiltite matrix, mud-supported biocalcarenite with pelletoidal calcisiltite matrix and grain-supported biocalcarenite with pelletoidal calcisiltite matrix and rare sparite cement); and shoal (grain-supported to pressure welded crinoidal-brachiopodal-bryozoan calcarenite with sparite cement). The storm microfacies deposits (grain-supported biocalcirudite with sparite cement and rare pelletoidal calcisiltite matrix, and grain-supported to pressure welded crinoidal-bryozoan calcarenite with sparite cement and rare calcisiltite matrix) interfered with the fair-weather microfacies in the outer platform environment. The model lacks the more shallow environments.
The storm events display their maximum intensity in the outer platform where they are represented by coarse disorganized calcirudites (microfacies 5). As the storms decrease in intensity landward, less disorganized and graded bedded calcarenites (microfacies 6) are deposited. The Galena Group consists of 25 symmetric and asymmetric depositional cycles divided into at least five successive predominant storm- and fair-weather episodes.
The diagenetic features observed petrographically represent changes in the following environments: marine phreatic; undersaturated freshwater phreatic I; saturated freshwater phreatic I; mixed marine freshwater phreatic; uplifting of the dolomitized Galena limestone; undersaturated freshwater phreatic II; saturated freshwater phreatic II; and burial. (Abstract shortened with permission of author.)
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