Kansas Geological Survey, Current Research in Earth Sciences, Bulletin 252, part 1
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Introduction

Environmental interpretations of carbonate facies in the rock record are commonly driven by equivalent facies occurrences in modern ocean environments. For example, Heterozoan Association carbonates (an association of benthic carbonate particles produced by organisms that are light-independent plus or minus red calcareous algae; James, 1997) are a characteristic component of modern cool-water carbonate shelves where sea-water temperatures are less than 20° Celsius for prolonged periods and influx of terrigenous sediment is relatively low (Lees and Buller, 1972; Nelson, 1988; James, 1997; James and Bone, 2000). In contrast, Photozoan Association carbonates (an association of benthic carbonate particles including light-dependent organisms, and/or nonskeletal particles such as ooids and peloids, plus or minus skeletons from the Heterozoan Association) are characteristic of modern warm, shallow-water tropical carbonate shelves. Heterozoans are an important component in most marine environments, and in addition to cool-water areas, can predominate even in low-latitude, warm-water settings where environmental conditions limit the supply of siliciclastic and pelagic sediments, and where photic zone and nutrient conditions prevent the development of photozoans (James, 1997).

Similarly, interpretations of biosiliceous rocks rich in sponge spicules or sponge body fossils are strongly influenced by the distribution of siliceous sponges in the modern ocean, where they thrive in the deep sea and/or on shallow polar shelves (Gammon et al., 2000). Gammon et al. (2000) point out that siliceous spiculites do not occur on the inner portions of modern shelves, especially those in subtropical and tropical environments.

Mixtures of biosiliceous (particularly those rich in sponge spicules) and heterozoan-dominated carbonate deposits occur in the rock record. The occurrences in the modern of similar facies, as mentioned above, have led to these deposits usually being interpreted as recording cold-water polar or deep basinal conditions (James, 1997). However, a growing body of literature documents biosiliceous and heterozoan carbonate deposits, and specifically sponge-rich deposits, in the rock record that are interpreted to have been deposited in shallow-water middle- to low-latitude environments (e.g. Cavorac and Ferm, 1968; Folk, 1973; Chowns and Elkins, 1974; Geeslin and Chafetz, 1982; Maliva et al., 1989; James and Bone, 2000; Gammon et al., 2000). The increasing documentation of these deposits is aiding in understanding the paleoenvironmental attributes of ancient neritic carbonate/spiculite accumulations. However, as pointed out by Gammon and James (2000), the depositional environments and sedimentology of such shallow-marine deposits are still poorly understood.

Early-Middle Mississippian time was characterized by extensive development of biosiliceous and carbonate accumulations in some areas of North America (Lowe, 1975; Gutschick and Sandberg, 1983). During this time a shallow tropical sea covered most of the southern North American continent and was the site of a broad carbonate platform (Gutschick and Sandberg, 1983). Most of present-day Kansas was part of the platform area. Paleogeographic studies place that area at about 200° S latitude, within the tropical to subtropical latitudinal belt during Early-Middle Mississippian time (Parrish, 1982; Witzke, 1990; Golanka et al., 1994; Scotese, 1999). Osagean deposition in the region was characterized by shallow shelf carbonates deposited on a gently sloping shelf (ramp) to the south (Rogers et al., 1995), with the shelf edge (bordering the Anadarko basin) located near the Kansas-Oklahoma border (Selk and Ciriaks, 1968; Lane and DeKeyser, 1980; Gutschick and Sandberg, 1983). Generally, shelf facies consist of limestones, dolomites, and cherts (Lane and DeKeyser, 1980; Gutschick and Sandberg, 1983). Previous detailed studies of Osagean strata in Kansas have focused on shelf-margin areas where thick accumulations of sponge-rich chert deposits occur (informally termed "chat") and form significant reservoir facies (e.g. Rogers et al., 1995; Colleary et al., 1997; Montgomery et al., 1998; Watney et al., 2001). In comparison, relatively little detail is known about equivalent Osagean deposits in inner shelf (as used by Lane and DeKeyser, 1980) locations in Kansas. The purpose of this paper is to 1) document Osagean biosiliceous and heterozoan carbonate and original evaporite facies that were deposited in a shallow-water, inner-shelf area in Kansas; 2) provide examples in the rock record of similar biosiliceous and heterozoan carbonate deposits to help illustrate that these types of facies are more common in shallow-water, tropical/subtropical settings than commonly thought; 3) show that similar shallow-water biosiliceous and heterozoan carbonate deposits occurred elsewhere in North America during Early-Middle Mississippian time; and 4) use the example from this study along with other documented examples in North America to examine paleoenvironmental conditions that may have controlled deposition of biosiliceous and heterozoan carbonate deposits in this Osagean subtropical/tropical shallow-water, mid-latitude setting.

Recognition of these facies and understanding controls on their distribution has additional significance in that these facies form reservoirs in several areas of Kansas and could be important potential reservoir facies elsewhere in Kansas and North America.

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Kansas Geological Survey
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