H. B. Willman and T. C. Buschbach
The Matson Member of the Joachim Dolomite (Templeton and Willman, 1963, p. 61), which overlies the Defiance Member, is named for Matson, St. Charles County, Missouri, just east of the type section.
The type section of the Matson Member is located in the north bluff of the Missouri Valley (NE NE NE 4, projected, 44N-2E), where the member is 29.8 feet thick.
Extent and Thickness
The Matson Member persists throughout the area of the Joachim. In Illinois it is exposed only at West Point Landing in Calhoun County, where it is 21 feet thick. It is about 30 feet thick in the outcrop area, but in subsurface in Illinois it commonly is 15-40 feet thick.
The Matson Member differs from the other Joachim Members in being relatively pure, brown, finely porous dolomite that generally appears to be massive but is finely laminated, the laminae separated by thin films of dark brown clay. Locally it contains a few thin beds of the lithologic types common in the Defiance and Augusta Members. The Matson appears to be largely algal in origin.
In the Matson Member ostracodes occur locally in the basal beds, and a marine fauna with corals, brachiopods, and trilobites has been observed in the basal beds at one locality near Zell, Missouri.
TEMPLETON, J. S., and H. B. WILLMAN, 1963, Champlainian Series (Middle Ordovician) in Illinois: Illinois State Geological Survey Bulletin 89, 260 p.
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