Chaetetid buildups in a Westphalian (Desmoinesian) Cyclothem in Southeastern Kansas

Small buildups of chaetetids and associated biota in the Higginsville Limestone Member of the Fort Scott Formation (Desmoinesian, Middle Pennsylvanian) are well exposed in three dimensions in an abandoned quarry in Crawford County, Kansas, Regional studies indicate that the buildups were deposited during a relative sea-level highstand, but within normal wave-base, on the southern flank of the Bourbon Arch. The chaetetid facies consists of a fusulinid wackestone to packstone that encloses the chaetetids. In the lower part of the chaetetid interval, chaetetids are small and tabular, but gradually upsection they grade into larger domical forms that sometimes form tall, ragged columns. These, in turn, coalesce upward to form large masses (boundstone) up to 6 m across. At the top of the bed, the chaetetid masses and surrounding matrix are covered by intertidal to supratidal, coastal, and /or terrigenous facies, indicating a shallowing of sea level to the point that chaetetid growth was terminated. Attachment surfaces for individual chaetetids initially involved partially lithified sediment or shell fragments, whereas within the columns they attached to underlying chaetetids, Multithecopora or algal overgrowths on underlying chaetetids, or thin sediment layers that define growth interruptions within the columns. Apparent maximum relief of chaetetids relative to the seafloor during growth was from 30 to 45 cm. Cross laminations in the matrix and some toppling of chaetetids indicate a moderately high-energy paleoenvironment. Ragged margins of the chaetetids accompanied by overlying thin sediment layers, and periodic overgrowths by the tabulate coral Multithecopora and encrusting algae, indicate growth interruptions instigated by periodic influxes of sediment or other environmental factors. Abundant fossils are present on and among the chaetetids, including brachiopods, bryozoans, rugose corals, crinoids, echinoids, gastropods, encrusting algae, ostracodes, and foraminiferids. Many of them are apparently in life positions, indicating a diverse, stenohaline biota living in this environment. Chaetetid masses are relatively evenly distributed throughout the quarry and, thus, give no indication of an end to the buildup in any direction, or of a windward or leeward orientation. Directional indicators within the rocks and linear trends of the buildups suggest a paleocurrent that moved in a northeasterly direction. Apparently the environment of deposition was relatively uniform, flat, shallow, well-aerated, and open marine, at some distance from shore. At the time of deposition, this area was within 10 degrees of the paleoequator. Changes in rainfall, ocean circulation, mixing, aeration, and nutrient input are reflected in the rocks and by their preserved biota. Black shale represents the least mixing and aeration, the phylloid algal limestone suggests low turbidity and possibly low salinity but well-aerated and nutrient-rich marine conditions, and the chaetetid interval represents normal marine salinity and low nutrient input with the best circulation, mixing, and aeration. Recent studies of growth rates in extant coralline sponges suggest that the chaetetid bed of this study represents a minimum of 10,000 to nearly 20,000 years of deposition, and the largest individual chaetetid in this bed may have lived over 3,000 years. The relatively thin transitional bed between the underlying phylloid algal limestone and the chaetetid bed represents several hundred years of deposition. The limestone bed overlying the chaetetid bed contains a few chaetetids that may have lived up to 400-800 years, and it contains evidence of a tidal cycle of strong and weak tides.