ABSTRACT

Thin, regularly spaced, subparallel seams of noncalcareous material in essentially horizontal, Oligocene shell calcarenites are shown to be post-depositional in origin, and to have formed by intrastratal solution in a manner similar to stylolites, except that interpenetrating stylolitic columns did not develop. It is suggested that some of the cement in the calcarenite was derived by initial local pressure solution at grain contacts, and that the carbonate material dissolved during the formation of the seams provided the balance (between [1]/[5] and [3]/[5] of the total cement in the rock).

Regular seams were found to be absent in rocks with a low silt and clay content and a carbonate percentage of less than about 60 percent. However, seams were also absent in rocks with a high silt and clay content and a carbonate content as high as 80 percent. A relationship between original porosity of the calcarenite and the development of seams was deduced.

It is suggested that their generally horizontal attitude was determined by the pressure of overlying strata, and by grain fabric of the rock. In a few cases where seams are inclined, giving the appearance of cross-bedding, they are parallel to the preferred grain orientation in the calcarenite. No satisfactory explanation can be offered for the regular spacing of the seams, which on weathering give rise to conspicuous flagstones.