Technique for Determining Coccolith Abundance in Shaly Chalk of Greenhorn Limestone (Upper Cretaceous) of Kansas

Abstract

In samples of shaly chalk from the Greenhorn Limestone, coccolith abundance can be determined by means of a newly devised technique. A known volume of shaly chalk is disaggregated ultrasonically and the resulting sludge suspended in 2000 ml of water. A 0.001-ml aliquot of the suspension is then placed on a slide, dried, and covered. Using a polarizing microscope fitted with a mechanical stage, coccoliths are counted and the number on the slide is used to calculate total number in the suspension. Correction is made for portions of the original sample that did not disaggregate. Chief difficulty is in counting specimens that are clustered together on the slide. Duplicate counts were made for most samples, with results suggesting that the technique is valid.

Shaly chalk samples from a Mitchell County locality contain from 5 x 10⁸ to 6 x 10⁹ coccoliths per cubic centimeter. The lowest values are for samples adjacent to bentonite seams. Coccolith abundance in the Greenhorn samples compares favorably with estimates that have been made for Jurassic limestone of Dorset (England) and Franciscan limestone of California, and with some modern shelf deposits.