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The Paola Limestone (Missourian) of the Mid-Continent region is the basal carbonate member of the Iola Formation (Kansas City Group). The Paola is a thin (1 to 3 ft; .3 to .9 m) massive layer of bioturbated, fossiliferous (algae, crinoids, and foraminifers) calcilutite containing abundant phosphatic nodules. This distinctive limestone is, according to previous investigators, correlative from Nebraska, southward, into northeastern Oklahoma. The Paola Limestone is overlain (in ascending order)
by the Muncie Creek Shale and Raytown Limestone.
In a NE-SW outcrop trend across Allen County, Kansas, the Paola Limestone forms the initial substrate on which a phylloid algal buildup developed within the Raytown Limestone. The Paola consists of three distinctive carbonate microfacies (described below). Microfacies 2 overlies microfacies 1; this microfacies association occurs only beneath the phylloid algal buildup. Both exhibit petrographic features indicative of submarine lithification. Northeastward, away from the phylloid algal buildup, microfacies 1 and 2 change abruptly into microfacies 3.
Microfacies 1 is a moderately bioturbated pyritized calcilutite with Archaeolithophyllum crusts, Hikorocodium, Tetrataxis, Tuberitina, and low-spired gastropods. This microfacies has a highly irregular (scoured) upper surface that is encrusted by Nubecularia, Archaeolithophyllum lamellosum, and bryozoans and locally penetrated by borings.
Microfacies 2 consists of profusely bioturbated, matrix-supported, crinoidal-fusulinid biocalcarenite. Large, bean-shaped, algaloid concretions of Nubecularia and Archaeolithophyllum lamellosum are common accessory components. The large, ramose burrow networks are infilled with microcrystalline dolomite and scattered phosphate nodules; small rugose corals also occur in the burrow fills.
Microfacies 3 is a crinoidal-pelletoidal biocalcarenite containing Archaeolithophyllum crusts. Composita, oncolites, productid brachiopods, small gastropods, fenestrate bryozoans, brachiopod and echinoid spines, Nubecularia-encrusted bioclasts, ostracods, and neomorphosed pelecypods shells are accessory components. Baroque dolomite occurs as a filling within phylloid algal blades. Bioturbation textures are present, but sparse, relative to microfacies 1 and 2.
Prior to lithification, the hardground (microfacies 1) was bioturbated; following lithification it was scoured, encrusted, and bored. The lithification of microfacies 1 is inferred to have occurred in a submarine environment because: (1) it contains a fauna of encrusting marine organisms and (2) petrographic features indicative of subaerial exposure are lacking. Microfacies 2 is interpreted as a firm ground. Microfacies 3 represents a normal, shallow marine subtidal environment.
The recognition of ancient hardgrounds allows a more thorough understanding of the sedimentologic, paleoecologic, and diagenetic histories of carbonate sequences. Submarine diastems also have potential as chronostratigraphic markers.
Because petroleum accumulations are commonly associated with diastems, an awareness of these features could provide insights for the location of some obscure hydrocarbon traps. Additionally, hardgrounds can create intraformational permeability barriers; the recognition of such reservoir heterogeneities is essential for optimum hydrocarbon recovery. Detailed petrographic analysis is a prerequisite to the location and understanding of ancient hardground sequences.
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