Potassic alteration of Cambrian-Ordovician sandstones and Precambrian basement rocks of the North American midcontinent
Duffin, Michael Eugene
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https://hdl.handle.net/2142/21663
Description
Title
Potassic alteration of Cambrian-Ordovician sandstones and Precambrian basement rocks of the North American midcontinent
Author(s)
Duffin, Michael Eugene
Issue Date
1990
Doctoral Committee Chair(s)
Hay, Richard L.
Department of Study
Geology
Discipline
Geology
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Geochemistry
Language
eng
Abstract
In Hole UPH-3 the Mt. Simon Sandstone contains three distinct diagenetic zones: an upper K-feldspar overgrowth zone, an underlying sericite zone, and a lowermost feldspar zone. Uppermost Precambrian basement exhibits potassic alteration similar to the overlying Mt. Simon Sandstone. Sericite in the basement rock was likely formed during the same episode as in the Mt. Simon Sandstone, based on similar $\delta\sp{18}$O values. A K/Ar date of 394.6 $\pm$ 6 Ma (Early Devonian) was obtained from authigenic K-feldspar of the basal Mt. Simon Sandstone. K/AR dates of 271.3 $\pm$ 4.6 Ma (Permian), 254.4 $\pm$ 4.6 Ma (Permian), and 214.1 $\pm$ 4.6 Ma (Triassic) were obtained from authigenic sericite of the Mt. Simon Sandstone. Similar potassic diagenesis of Mt. Simon Sandstone in UPH-3 occurs on a regional scale in the Upper Midcontinent area. Here, two widespread zones of K-feldspar and sericite occur. A K/Ar date of 276 $\pm$ 10 Ma (Permian) was obtained on sericite from one locality in Iowa, which is separated by 300 km from Hole UPH-3. The episodic K diagenesis of the Mt. Simon Sandstone is attributed to regional migration of basinal brines caused by groundwater recharge on uplifted arches.
A K/Ar date of 448 $\pm$ 16 Ma (Late Ordovician) was obtained for wholly authigenic K-feldspar from Lower Ordovician Prairie du Chien sandstones in Hole UPH-3. This date if coeval with a Late Ordovician K/Ca date of authigenic K-feldspar from basal St. Peter Sandstone in Wisconsin, suggesting that there may have been a regional episode of K-feldspar authigenesis in the Upper Mississippi Valley during Late Ordovician time.
Authigenic K-feldspar occurs in alteration profiles in uppermost Precambrian basement rocks at several localities of the continent. The K-feldspar has been identified in eight states. It is monoclinic or triclinic by X-ray diffraction, of end-member composition, and may compose 63% of rock volume in altered samples. A sample of wholly authigenic K-feldspar from altered basement in southern Illinois gives a K/Ar date of 549 $\pm$ 18 Ma (Early Cambrian). This date is in agreement with Early Cambrian Rb/Sr dates for potassic alteration of uppermost Precambrian basement in Ohio. Dated authigenic K-feldspars from both Ohio and Illinois give identical $\delta\sp{18}$O values of 17.5%, suggesting formation from similar fluids. Concordancy of both dates and $\delta\sp{18}$O values suggest that the K-feldspar formed during an episode of potassic alteration during Early Cambrian time that affected much of continental North America.
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