Galatia Channel:Delafield Member
Delafield Member (New)
Name and definition
Between the Hanover Limestone and the base of the Springfield Coal is a thick, regionally extensive interval of shale, siltstone, sandstone, and mudstone that typically coarsens upward. The name Delafield Member is hereby proposed in reference to the community of Delafield in Hamilton County, southern Illinois.
The type section of the Delafield Member is the core from the Energy Plus #ME-13 borehole, which was drilled about 1.6 mi (2.5 km) southeast of Delafield (Sec. 31, T4S, R6E, Hamilton County, County No. 25463). The member occupies the depth interval from 1,048.3 to 1,123.2 ft (319.5 to 342.4 m) in the core, and is 74.9 ft (22.8 m) thick (Figure 6, Appendix). The entire core from the Energy Plus borehole is in permanent storage at the Illinois State Geological Survey (ISGS) Samples Library. The core description, a gamma ray-density log, and other data on the borehole are on file at the ISGS Geologic Records Unit and available via the ISGS website.
Thickness and distribution
Weiner (1961) produced a map showing the thickness of what is essentially the Delafield Member in the Illinois Basin. Wanless et al. (1963, Figure 13) and Wanless et al. (1970, Figure 4) published more legible versions. As reproduced here (Figure 7), the map shows that the Delafield Member thins westward from maximum values of more than 98.4 ft (30 m) in southwestern Indiana and southeastern Illinois. The greatest known thickness is 124.7 ft (38 m) in eastern Wayne County, Illinois. The Delafield thins to less than 16.4 ft (5 m) in much of western and northern Illinois, although the detailed pattern undoubtedly is more complex than shown on Figure 7.
The Delafield consistently coarsens upward. Dark gray, sideritic clay-shale that contains abundant nodules and bands of siderite is at the base. This grades upward through silty shale to siltstone and fine-grained sandstone. Shale, siltstone, and sandstone are commonly interlaminated in the upper Delafield. Structures include planar, wavy, ripple, and cross-lamination, along with slumped lamination. Definite neap–spring tidal bundles were displayed in one of the cores examined. At the top, siltstone grades upward to claystone or silty mudstone having paleosol features (Springfield underclay). The upward-coarsening profile is evident on electric and other geophysical logs (Figure 5).
The base of the Delafield Member is the top of the Hanover Limestone or, where the Hanover is absent, the top of the Excello Shale. This contact is sharp or gradational within an interval a few centimeters thick. The top of the Delafield is the base of the Springfield Coal or the base of the new Galatia Member. The contact to the coal is generally conformable but sharp, whereas the contact to the Galatia Member is erosive. Where the Galatia Member is absent, the underclay of the Springfield Coal is part of the Delafield Member.
Fossils are scarce in the Delafield Member, but they record a transition from near-normal marine conditions at the outset of deposition to increasingly brackish water through time. Articulate brachiopods, crinoid fragments, and other marine forms occur at the base. Above this are found rare pectenoid pelecypods, linguloid brachiopods, a single nautiloid cephalopod, and plant fragments. Burrows are present but not common. The type core contains the trace fossils Teichichnus and Conostichus, which have marine affinities.
The Delafield reflects a rapidly prograding shoreline. Deposition apparently took place in a complex of deltas and shoaling bays; normal marine salinity rapidly gave way to brackish water. At the end of Delafield deposition, terrestrial clastics essentially filled the Illinois Basin as marginal marine sedimentation gave way to emergence and soil formation (underclay of Springfield Coal).