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|Title:||Petrography, Diagenesis and Depositional Models of the St. Louis Limestone, Valmeyeran (Middle Mississippian), Illinois Basin, u.s.a. (United States)|
|Department / Program:||Geology|
|Degree Granting Institution:||University of Illinois at Urbana-Champaign|
|Abstract:||The Valmeyeran (Middle Mississippian) St. Louis Limestone of the Illinois Basin was studied through eight cores and six field sections. Detailed petrography of 1542 oriented thin sections led to recognition of 16 carbonate microfacies. The vertical succession, reciprocal (lateral and vertical) relationships and correlation coefficients of the microfacies permitted construction of three depositional models.
Model 1 consists of supratidal evaporitic flats (stromatolites, anhydrite and halite), intertidal to subtidal upper ramp (bioclastic calcisilites), subtidal lower ramp (mud-supported to grain-supported biocalcarenites) with a crinoidal-bryozoan baffle zone and, basinal environment (mud-supported biocalcarenites and, bioclastic calcisiltites).
Model 2 shows the following environments; narrow supratidal evaporitic flats (stromatolites), lagoon (bioaccumulated limestones and biocalcarenites with calcisiltite matrix), subtidal to low intertidal bioclastic bar (biocalcarenites to intraclastic biocalcarenites), slope (biocalcarenites with bioclastic to calcisiltite matrix) and basin (mud-supported biocalcarenites and bioclastic calcisiltites).
Model 3 consists of: still narrower supratidal evaporitic flats (stromatolites), lagoon (calcisiltites, bioaccumulated limestones and biocalcarenites with calcisiltite to bioclastic matrix), oolitic bar-to-bank (oolitic biocalcarenites), slope (biocalcarenites with bioclastic to calcisiltitic matrix) and basinal environment (mud-supported biocalcarenites and bioclastic calcisiltites).
Dolomitization by seepage refluxion of sabkha brines (from supratidal flats) is strong in the upper-ramp calcisiltites of model 1, moderate in the lagoonal calcisiltites of model 2 and minimal in those of model 3.
The three depositional models follow one another in time expressing a progradation of the medium to high energy carbonates towards the depocenter. Near the margins of the basin (SE and E), model 1 was active throughout St. Louis time. Away from the basin edge, model 1 (baffle zone) evolved into model 2 (bioclastic bar) which in turn changed to the oolitic bar-to-bank system of model 3 which remained active till the close of St. Louis time. Probably the depocenter corresponded to a continuous deposition of basinal bioclastic calcisiltites.
The diagenetic features observed petrographically, represent changes that took place in the following environments; marine phreatic, marine vadose, undersaturated freshwater phreatic, saturated freshwater phreatic, mixing marine-freshwater phreatic and, burial.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1984.
|Date Available in IDEALS:||2014-12-16|