GROWTH RATE AND REACTION TO SEDIMENTATION EVENTS OF THE UPPER ORDOVICIAN HEAD-FORMING CORAL CYATHOPHYLLOIDES FROM SOUTHEASTERN INDIANA

The head-forming coral Cyathophylloides occurs in southeastern Indiana in the basal part of the Saluda Member of the Whitewater Formation. These coral heads are embedded in a cyclic set of lithologies. This paper reports attempts at growth ring analysis. Coral heads in this study were collected from Madison Indiana, and Buckner, Kentucky.

The Madison locality, contains more and larger heads at two horizons, each capping a succession of marine shell bed limestones and in turn buried below deposits of siltstone. This suggests that corals grew in clear water during periods of maximum starvation, and then were smothered with terrigenous material. Centimeter-scale siltstone laminae are intercalated with the bases of these coral heads, and sometimes leave muddy layers along certain growth horizons within the coral heads, suggesting that growth was interrupted by episodic depositional events.

The Saluda just above the coral horizons contains desiccation cracks and thin laminations indicative of a sabkha-type tidal-flat environment, suggesting that these corals grew in relatively shallow water. In many cases, the coral heads are overturned, suggesting the effects of large storms, and supporting the shallow-water interpretation.

By analyzing growth rings, seasonality, growth rates, and ages of coral heads might be interpreted and the information applied to stratigraphic analysis. Coral heads are large and unlikely to have been transported, so the age of entire heads could constrain the minimum time required to accumulate the thickness of sediment that buried them. At a more detailed scale, the seasonality of burial events should be evident from the positions of muddy zones within the coral heads. This seasonality is related to the sedimentological mechanisms by which silt reached that part of the basin.

Preliminary data suggest coral growth rates in the vicinity of about a centimeter of year. Given that the largest heads reach up to a meter tall, but the majority are about 30 to 60 cm in diameter, we estimate coral head ages averaging 30 to 60 years and not exceeding 100 years. Sedimentation rates for silts appear to be on the order of centimeters per year. Without apparent means of correlating one coral head record to another, it is difficult to say how long corals grew during each cycle, but a few hundred years is plausible.