Petrology of the Jeffersonville Limestone (Middle Devonian) of Southeastern Indiana

Abstract

Thin-section, polished-surface, and X-ray analyses of 500 samples collected from the Jeffersonville Limestone at 14 localities in southeastern Indiana are the basis for detailed correlation of carbonate facies and the reconstruction of sedimentary environments. Basal Jeffersonville sediments were deposited on Louisville Limestone (Niagaran) in southernmost Indiana. In south-central Indiana the Geneva Dolomite rose above the sea floor as a low-relief platform trending east-west. The existence of this platform was reflected throughout deposition of Jeffersonville sediments. The Jeffersonville is subdivided into five zones, each with characteristic fauna and carbonate rock types. The lowest, or coral zone, contains three distinct carbonate facies, including (ascending) grain-supported biomicrite, biosparite, and biomicrudite which overlap one another successively from south to north. The northern limit of sedimentation was controlled by the southern margin of the Geneva platform. The lower biomicrite and biosparite contain many upright branching corals, large corals, and both matlike and moundlike stromatoporoids. The upper biomicrudite contains a profusion of solitary corals and branching-coral fragments. These strata accumulated as a coral bank constructed below effective wave base. The Amphipora-zone sediments comprise biosparite, biosparrudite, biolithite, and biomicrudite in southern Indiana which grade northward into pelsparite which was deposited in shallow waters over the Geneva platform. These pelsparites grade northward into laminated dolomite. Regression of the Jeffersonville sea during deposition of upper Amphipora-zone sediments displaced the pelsparite and laminated dolomite facies southward. Fragments of Amphipora and matlike stromatoporoids are the dominant faunal elements. This zone reflects shoaling over the bank. The Brevispirifer gregarius zone contains grain-supported biomicrite in southernmost Indiana which grades northward into mud-supported biomicrite. The laminated beds continued to accumulate in the shallow shelf lagoon developed on the Geneva platform. Periodic exposure of the lagoon flats produced mud cracks. Charophytes grew in fresh- or brackish-water ponds on the flats, supplying oogonia to marine sediments to the south. Brevispirifer gregarius and echinoderms are the dominant faunal elements. Regression of the sea continued throughout deposition of this zone. The fenestrate bryozoan-brachiopod zone is composed of grain-supported biomicrite and biosparite which intertongue northward with the laminated beds of the shelf lagoon. Fenestrate bryozoans, echinoderms, and brachiopods characterize this zone. Corals and stromatoporoids reappear as significant faunal components. A second major transgression of the Jeffersonville sea is indicated by this zone. The Paraspirifer acuminatus zone is the most widespread of all the Jeffersonville zones and consists of grain-supported biomicrite and biosparite. These sediments are linked genetically to the same transgressive phase as the underlying fenestrate bryozoan-brachiopod zone sediments.