K-AR DATING, X-RAY-DIFFRACTOMETRY, OPTICAL AND SCANNING ELECTRON-MICROSCOPY OF GLAUCONIES FROM THE EARLY CRETACEOUS KURNUB GROUP OF JORDAN

The glaucony of the early Cretaceous Kurnub Group in Jordan has been i sotopically dated using the K-Ar method. The glaucony occurs in an are naceous dolomite unit, referred to here to as glaucony marker unit (GM U), located in the upper part of the Kurnub Group, that persists throu ghout Jordan. The glaucoliths of the heavy fraction are dark green in colour, ovoidal or mammilated, with a mainly cracked smooth surface, w hereas the light fraction glaucoliths are light green in colour, irreg ular in shape and have a rough or porous surface. Both fractions exhib it boxwork and rosette microstructure, whereas lamellar microstructure is restricted to the heavy glaucoliths. X-ray diffraction and chemica l analysis placed the glauconies of the GMU of the Kurnub Group in Odi n and Matter's evolved to highly evolved class corresponding to glauco nitic mica and suggested that they should be well-closed chronometers. On the other hand, petrographic investigation proved these glauconies to be unaltered, whereas the other altered ones are discarded from th e age determination. The unweathered, highly evolved, heavy glauconies that are neither tectonized nor deeply buried best fulfil recommendat ions regarding appropriate samples for K-Ar dating. The apparent age c onstrained, within the analytical uncertainty limits, from the most ev olved glaucony is 96.1 +/- 1.1 Ma and suggests that the GMU is of Albi an age. The other less evolved glauconies, which are still within the evolved to highly evolved class of Odin and Matter, yield a mean appar ent age of 93.6 +/- 1.0 Ma, which is probably slightly younger than th e true depositional age by 2-3 Ma due to genetic and historical uncert ainties, as indicated by the petrographic and sedimentological data. T hus the upper part of the Kurnub Group, where the GMU is located, is o f Albian age. (C) 1998 John Wiley & Sons, Ltd.