The Alloway Clay Member of the middle Miocene Kirkwood Formation (New Jersey) contains an interval unlike any described in the literature. This interval, which has a total areal extent of about 10 sq mi and ranges in thickness from 1 to 10 ft, is composed chiefly of kaolinite, with many of the kaolinite grains exceeding 0.2 mm (200 µ) in size. The individual platelets are so large that, for years, their kaolinitic nature was unrecognized and the material was mistakenly described as "a micaceous talclike clay." The formation of the macrokaolinite, and the simultaneous enrichment in kaolinite of the clays which lie beneath it, are thought to be the result of diagenetic transformation of previously deposited marine illite and montmorillonite clays to kaolinite by upward leaching (dialysis) of groundwater. Primary kaolinite, and possibly some of the converted material, was enlarged subsequently (or concurrently) by lateral epitaxy.

The macrokaolinite, and the Kirkwood Formation in general, are thought to reflect humid, subtropical climatic conditions existing in the region during the time of deposition of the Kirkwood. The presence of gibbsite, lepidocrocite, and goethite in the clay fraction of the sediments and the conversion of part of the Alloway Clay Member to an opaline-cemented orthoquartzite by silica-rich groundwater, possibly derived by the above transformation process, are additional evidence for such conditions. Hence, the boundary between the humid, subtropical and temperate zones must have been as far north as southern New York during the middle Miocene rather than in northern Delaware where it commonly is placed.

End_of_Article - Last_Page 724------------