Apatite as a paleoenvironmental indicator in the Precambrian-mesozoic clastic sequence of the Middle East

Apatite, either detrital or authigenic, or both, occurs in the Precambrian arkoses and Cambrian subarkoses, grits and quartz-arenites in Israel, Sinai, southern Jordan and northwest Arabia. However, no apatite is found in the sandstones that overlie the sub-Carboniferous unconformity (often superceded by a sub-Cretaceous unconformity) throughout the Middle East.

Within the Precambrian-Cambrian sequence, apatite distribution is not uniform and varies between 0–80% of transparent heavy minerals. The detrital apatite was derived from the acid igneous terrain that supplied the sands. Authigenic apatite was formed by recrystallization of the detrital apatite.

Until its abrupt stratigraphic termination, apatite accompanies the ultrastable minerals zircon-tourmalinerutile without displaying, like the semi- and nonstable heavy minerals, vertical trends of gradual disappearnce. Therefore its disappearance cannot be explained by repeated reworking and transportation which, by themselves, are not known to result in the complete elimination of a heavy mineral from an assemblage.

Numerous heavy-mineral studies, and especially the experimental work of Nickel (Contr. Sedimentol.1, 1–68, 1973) have shown that apatite dissolves under conditions of low (< 6) pH, which may develop in well-leached humic soils, pedalfers and laterosols, but which are not known in intrastratal groundwaters. The apatite distribution suggests, together with clay-mineral indications, that pedogenesis of this type developed with the spread of terrestrial floras over the Arabo-Nubian land surfaces during the Late Paleozoic. Various lines of evidence, including paleomagnetic data, indicate that the present-day Near East area was in a temperate-humid zone during the Cambrian. After drifting through subpolar latitudes during the Ordovician-Silurian-Devonian, it moved again into a tropical-humid zone toward the Late Paleozoic. This coincided with the appearance of plant fossils in the clastic section, and the disappearance of apatite. Since the dissolution of apatite is inhibited in the presence of carbonate or Ca²⁺ ions, its removal must already have been completed before the Permian, when carbonate deposition became gradually dominant.