Speed Member
  

JBD & RHS

North Vernon Limestone,

Devonian System


Type section and use of name: The name Speeds Member (of the Sellersburg Limestone) was given by Sutton and Sutton (1937, p. 326) to 1.5 feet (0.5 m) of shaly fossiliferous limestone that was exposed beneath the Silver Creek Limestone Member of Butts (1918) and above the Jeffersonville Limestone at Speed's Quarry near Sellersburg (Clark Military Grants 131 and 132, Clark County, Ind.). This unit (called the Speed Member here) presently makes up a lower part of the North Vernon Limestone. (See the North Vernon article for the evolution of North Vernon nomenclature affecting the Speed Member.)

Description: Characteristically, the Speed Member consists of gray granular (poorly sorted) shaly thin-bedded fossiliferous limestone. The thin-bedded, platy appearance on exposure is partly due to many brachiopod shells. This unit, however, is a facies of the Silver Creek Member, so that characteristic Silver Creek lithology (gray dense and argillaceous to sublithographic limestone) is interbedded with characteristic Speed lithology in some places.

The Speed unconformably overlies the Jeffersonville Limestone, including the Vernon Fork Member in some places. It is overlain variably by its intimate facies, the Silver Creek Member, conformably, and by the Beechwood Member (upper North Vernon), unconformably.

The Speed is recognized both along the Devonian outcrop area in southern Indiana and in the subsurface, especially where it has been cored in a few places, but its subsurface parameters have not been defined. It is completely eroded eastward, but in its facies relationship with the Silver Creek Member, the Speed has these changing thicknesses: (1) a depositional zero at the Falls of the Ohio in Clark County, where the Silver Creek is about 15 feet (4.6 m) thick (Powell, 1970, p. 26); (2) 1.8 feet (0.5 m) in the Sellersburg Stone Co. quarry (about 10 miles [6 km] north of the Falls), Clark County, where it underlies 15.6 feet (4.8 m) of the Silver Creek (Patton and Dawson, 1955, p. 16); (3) nearly 9 feet (2.7 m) in the exposure along Big Camp Creek east of Deputy in Jefferson County (about 36 miles [58 km] north of the Falls), where it makes up all the North Vernon below the Beechwood Member but has some intercalated Silver Creek lithology (Shaver, 1974a, p. 4); and (4) 2.6 feet (0.8 m) in the Berry Materials Corp. quarry at North Vernon (about 50 miles [80 km] north of the Falls), Jennings County, where it makes up all the pre-Beechwood part of the very thin North Vernon Limestone (6.5 feet) (2 m) but without intercalated Silver Creek lithology (Droste and Shaver, 1978a, p. 405). (See also Dawson, 1941.)

Correlation: Speed rocks had been correlated with part of the Hamilton Group of New York even before their naming (Whitlatch and Huddle, 1932) on the basis of the brachiopod Stropheodonta demissa. Conodonts from the upper Silver Creek equivalent of the Speed identify the Icriodus latericrescens latericrescens Zone (Orr, 1971, p. 20) as is noted in the Silver Creek article. This zone has been thought to be early Givetian in age and is also found in the Skaneateles Formation (upper lower Hamilton) of New York. According to Dale Sparling (1983 and written communication, February 23, 1984) and on the basis of studies cited to Seddon (1970) and to Curt Klug, the latter to be published in the Transactions of the Wisconsin Academy of Science, Arts, and Letters, a Givetian age for the Speed and Orr's (1971) conodont zone is by no means assured; an Eifelian age is also possible. (See under "Silver Creek Member" for a listing of the midwestern correlatives of the Speed-Silver Creek interval.)






 
 
 
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