Oak Grove (Limestone) Member
Nelson, W.J., P.H. Heckel and J.M. Obrad, 2022, Pennsylvanian Subsystem in Illinois: Illinois State Geological Survey, Bulletin (in press).
Wanless (1931, charts, p. 184, 192) applied the name “Oak Grove member” to interbedded limestone and shale in the upper part of the Liverpool cyclothem overlying coal No. 2 (Colchester). Wanless wrote (p. 186), “The marine limestone above coal No. 2 is divided into several beds by thick partings of shale. This series of limestone bands provides the most striking evidence of widespread uniformity in sedimentation. Each of the limestone bands and intervening shale beds possesses distinctive lithologic and faunal characters and occurs in the same order and with nearly the same thickness from Mercer County southeast to Fulton County and from there southeast to Adams County [all in western Illinois].”
The unit was named for Oak Grove School, a one-room school that formerly stood northwest of Lewistown in Fulton County, Illinois.
Wanless (1939) variously called the unit “Oak Grove marine member,” “marine horizon,” or “limestone and shale bands.” Wanless (1957) called the unit “Oak Grove beds” and separately described 14 components. Kosanke et al. (1960, p. 35) formalized the name as Oak Grove Limestone Member, applying the lithologic designation “because its most distinctive elements are limestone and it occurs at a position in the cyclical sequence normal for a limestone.” In the present report, use of the term “limestone” is considered optional because shale is an important constituent.
In an unpublished manuscript, Wier (1961) proposed the name Velpen Limestone Member for the limestone overlying the Colchester Coal in Pike County, Indiana. Wier acknowledged that his Velpen is the same unit as the Oak Grove, as used by Wanless (1957). Despite acknowledging equivalence to the Oak Grove, Shaver et al. (1970, 1986) formalized usage of Velpen Limestone in Indiana. Oak Grove clearly has priority over Velpen, and the latter name is hereby abandoned.
The correlative unit in the Midcontinent is the Ardmore Limestone, which was named for the unincorporated village of Ardmore in Macon County, north-central Missouri (Gordon 1896, p. 20). Although the name “Ardmore” has priority over “Oak Grove,” widespread usage of the latter name for nearly a century argues for retention of Oak Grove in the Illinois Basin.
The section is in a small ravine north of the site of Oak Grove School, about 4 mi (6 km) northwest of Lewistown in the SW¼ SE¼ sec. 6, T 5 N, R 3 E, Fulton County.
Wanless (1957, geologic section 22, p. 197). As shortcomings, the Mecca Quarry Shale is absent at this site and the Colchester Coal is not exposed.
The present condition of the outcrop is unknown.
Cut bank of Spoon River at Wolf Covered Bridge, NW¼ NE¼ sec. 13, T 10 N, R 3 E, Knox County, Illinois. Built by Jacob Wolf in 1831, the original bridge burned in 1994 and has been replaced by a replica (http://www.knoxcountyscenicdrive.com/wolf-covered-bridge.html).
The section illustrated here (Figure 4-25) is from Smith et al. (1970, p. 15). This section illustrates the multilayered aspect of the Oak Grove Member together with the underlying Mecca Quarry, Francis Creek, and Colchester Coal Members.
Core from Indiana Geological Survey borehole SDH-306, drilled east of Winslow in sec. 2, T 2 S, R 7 W, Pike County, Indiana. This core was nominated as the principal reference section for the Velpen Limestone Member, but that name is hereby abandoned in favor of Oak Grove Member. The limestone is 0.6 ft (18 cm) thick in the depth interval of 136.2 to 136.8 ft (41.5 to 41.7 m) in this core (Figure 4-7).
Hasenmueller and Ault (1991).
In addition to the core description, gamma-ray and resistivity logs for SDH-306 are on file at the Indiana Geological Survey and can be downloaded from the Indiana Geological Survey website. The hole ID number is 115871. Core from SDH-306 resides at the Indiana Geological Survey core storage facility in Bloomington, Indiana.
The Oak Grove Member is here considered to embrace both the limestone and marine fossiliferous shale lying between the Mecca Quarry Shale and Purington Shale Members. Thus, the Oak Grove is analogous to marine limestone units such as the younger Hanover, St. David, and Brereton Members, all of which overlie black, phosphatic shale members in cyclothems of the Carbondale Formation. However, in some areas, particularly in western Illinois, the Oak Grove comprises multiple beds of shale and limestone, several of which retain distinctive lithology and fossils across broad areas (Figure 4-25).
Wanless (1964) and his student Wright (1963, 1965) developed a more elaborate conceptualization, in which the Oak Grove of western Illinois contains equivalents of marine limestone beds overlying the Wheeler and Bevier Coal Beds of the Midcontinent Basin (Figures 4-26 and 4-27). They based this proposal on similar, distinctive lithology and fossil content (brachiopods and mollusks) of limestone layers in the two regions. Wanless and Wright further regarded the Colchester, Wheeler, and Bevier Coals as occupying a single depositional sequence, the Liverpool cyclothem. Without explicitly refuting Wanless and Wright, Smith (1970, figure 2) presented a chart in which the Oak Grove Limestone is entirely older than the Bevier Coal and its equivalents.
The Wanless–Wright model is untenable because the Colchester, Wheeler, and Bevier Coals clearly occupy separate sedimentary cycles, each containing underclay (paleosol), regionally extensive coal, local marine limestone, and deltaic clastics. The Wanless–Wright model implied a paradox: continuous marine sedimentation on the slowly subsiding Western Shelf while soil formation, terrestrial vegetation growth, and peat accumulation took place in the more rapidly subsiding Midcontinent and Illinois Basins. Co-occurrence of certain fossils in key beds of different basins is not germane because the named genera (pelecypod Cardiomorpha and brachiopods Desmoinesia and Linoproductus) are long ranging. They reflect similar environments, not contemporaneous ages.
Extent and thickness
In western Illinois (chiefly Fulton, Knox, and Schuyler Counties), where the Oak Grove comprises interbedded shale and limestone, the member is commonly 10 to 15 ft (3 to 4.5 m) thick. The member is less developed in the deeper part of the basin, where it is commonly reduced to a few inches (centimeters) of limestone or calcareous shale. David A. Williams (Kentucky Geological Survey, written communication, 2017) has observed this lithology in several drill cores from western Kentucky.
In western Illinois, the Oak Grove may comprise as many as 14 alternating beds of shale and limestone, each having distinctive fossils and lithologic features (Wanless 1957; Nance 1970). The Wolf Bridge section illustrates this very well (Figure 4-25).
Nance (1970) described small mounds or bioherms of Oak Grove Limestone at a strip mine in western Illinois. These mounds were composed largely of crinoidal grainstone, with abundant phylloid algal fragments in the upper half. The Colchester Coal undulated strongly, with Francis Creek Shale confined to troughs and Mecca Quarry Shale thinning on highs. Oak Grove bioherms overlay the coal highs (Figure 4-24). Nance (1970) surmised that mounds initiated as current or wave ridges that became sites for algal build-ups.
Outside of western Illinois, the Oak Grove has been less thoroughly studied. Generally, the unit consists of a single thin (≤1 ft, 30 cm) limestone layer directly overlying the Mecca Quarry Shale, or of a somewhat calcareous, fossiliferous shale interval containing a few thin interbeds, concretions, or lenses of micritic limestone.
Both contacts of this member vary from sharp but conformable to gradational and arbitrary. The upper contact tends to be vague.
Well log characteristics
Limestone layers of the Oak Grove are generally too thin to register on geophysical logs.
Wanless (1958) tabulated fauna of the Oak Grove on a bed-by-bed basis. The abundant, diverse marine assemblage includes foraminifera (but not fusulinids), corals, bryozoans, brachiopods, scaphopods, gastropods, cephalopods, pelecypods, annelids, trilobites, crustaceans, ostracods, crinoids, and fish remains. Von Bitter and Merrill (1998) reported conodonts from the Oak Grove and neighboring strata.
Age and correlation
The Oak Grove Member is directly correlative with the Ardmore or Verdigris Limestone2 in the Midcontinent Basin. As a point of similarity, the Verdigris in southeastern Kansas comprises three widely traceable limestone beds separated by shale (Howe 1956). In the Appalachian Basin, the Columbiana Limestone overlying the Lower Kittanning coal bed is an Oak Grove equivalent.
Environments of deposition
The Oak Grove apparently records fairly deep-water deposition, below the storm wave base yet within the photic zone. Widespread alternation of thin carbonate and mud layers implies minor fluctuations of water depth and sediment influx affecting large areas.
2 Both names are still in use. Gentile and Thompson (2004) and Pope (2012) used Ardmore, whereas Heckel (2013) used Verdigris.
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