ABSTRACT: The little-studied third rock sequence of the middle and upper parts of the Bainbridge Group (Upper Silurian) and of the New Harmony Group (Lower Devonian) in the tristate central Illinois Basin is dominated by four kinds of carbonate rocks. (1) Reef rocks, fringing the deeper basin area and as thick as 900 feet, consist of light-colored, poorly sorted, skeletally derived materials that range from within the St. Clair Limestone (lower part of the Bainbridge Group) to the uppermost part of the Bailey Limestone (upper Bainbridge). (2) Very light colored, fairly well sorted high-purity bioclastic carbonate rocks, nearing 300 feet in thickness, are distinguished genetically from reef rocks by their principal distribution in thick, areally broad piles that make up especially the Backbone Limestone (lower and middle parts of the New Harmony Group) and that are concentrated along the eastern margin of the central basin. (3) Variably colored fine-grained impure noncherty to very cherty carbonate rocks make up most of the Moccasin Springs Formation and the Bailey Limestone (Upper Silurian), whose nonreef facies are more than 600 feet thick in places. (4) Light-colored fine- to medium- grained carbonate rocks that are as thick as 1,000 feet and are characterized by replacement chert and a speckled appearance dominate the Grassy Knob and Clear Creek Cherts (lower and upper New Harmony). The Silurian reefs belong to two principal generations, beginning during St. Clair and Moccasin Springs deposition respectively. Undiscovered, probably smaller reefs in the deeper basin are a distinct possibility. Distributions of the Backbone type of sediment and of the chertier New Harmony facies attest to asymmetrical sedimentational development of the deeper basin during Early Devonian time.
Droste, J. B., and Shaver, R. H., 1987, Upper Silurian and Lower Devonian stratigraphy of the central Illinois Basin: Indiana Geological Survey Special Report 39, 29 p., 11 figs.
You may also like:
Keywords: Bainbridge Group, New Harmony Group, St. Clair Limestone, Bailey Limestone, Backbone Limestone, Moccasin Springs Formation, Grassy Knob Chert, Clear Creek Chert, stratigraphy, Silurian, Devonian, Illinois Basin
Can't find what you're looking for? Feel free to contact us directly:
Publication Sales Office
Indiana Geological and Water Survey
420 N. Walnut St.
Bloomington, IN 47404
IGS Return Policy
All sales are final. Materials may not be returned after they have been accepted by the customer. Any discrepencies in mail orders must be reported to IGWS within 10 days of receipt of order.
The Indiana Geological and Water Survey will refund the purchase price and sales tax of products if returned with the original sales receipt within 10 days of receipt of order. Refunds will be issued with the same method of payment as indicated on the original receipt except for cash purchases, which will be refunded by check. Items must be returned in the same condition as purchased. Refunds on digital products and services are not allowed, except for defective media such as CD-ROMs, flash drives, and so on. The customer is responsible for paying shipping costs to return items.