Galatia Channel:Hanover Limestone Member: Difference between revisions
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Hanover Limestone Member
The marine limestone member that directly overlies the Excello Shale is called the Hanover Limestone. The Hanover is regionally continuous but locally lenticular. In the deeper part of the basin, the Hanover ranges from fossiliferous shale a few inches (centimeters) thick to limestone averaging around 9.8 in. (25 cm) and rarely exceeding 19.7 in. (50 cm). The usual lithology is dark gray, very argillaceous, fossiliferous lime mudstone and wackestone. Fossils are chiefly brachiopods and echinoderm fragments, along with a few gastropods, bivalves, bryozoans, and ostracods. Shells are commonly unbroken and crinoid stems partly articulated, indicating low depositional energy. The rock may be massive or show indistinct, wavy banding of fossil fragments and shale laminae. On the western side of the basin, the Hanover tends to be thicker (locally more than 9.8 ft [3 m]) and the rock is lighter colored, is less argillaceous, and contains more diverse fossils. The contact with the Excello Shale may be sharp and wavy or gradational.
The Hanover and its fauna record a return to normal marine water circulation, with near-normal salinity and oxygen content. Bottom waters were intermittently agitated, likely below the normal wave base but within the storm wave base. Parts of the basin may have been too deep for carbonate production. Thicker and purer carbonate accumulated in shallower water on the Western Shelf. We interpret limestone deposition as occurring around highstand under a relatively dry, seasonal climate. Conversely, the switch from black shale to limestone might reflect stronger wind-driven circulation under climate otherwise unchanged (Cecil et al. 2003b).
Nelson, W.J., S.D. Elrick, W.A. DiMichele, and P.R. Ames, 2020, Evolution of a peat-contemporaneous channel: The Galatia channel, Middle Pennsylvanian, of the Illinois Basin: Illinois State Geological Survey, Circular 605, 85 p., 6 pls.