The calcareous duricrust in al-Hasa, Saudi Arabia, covers a rugged topography developed on sandy limestones of Miocens and Pliocene age. It is typically gray or reddish brown and concretionary, and it has been divided into three types: very immature, immature, and mature. The very immature crust rests on the youngest topographic surfaces and is the youngest, thinnest, and least thoroughly developed. The immature crust rests on somewhat older surfaces, and is older, thicker, and more thoroughly developed. The mature crust rests on the oldest topographic surface and is the oldest, thickest, and most thoroughly developed.

The crusting process involved (1) the recrystallization of original calcite in the parent sandy limestone, (2) the introduction of additional calcite either from above or below, and (3) the replacement of quartz and feldspar by calcite. The CaCO3 content of the three crust types varies directly with their age.

The duricrust appears to have evolved progressively, though intermittently, during the very late Pliocene, Pleistocene, and early Holocene. Differences in the thickness of the three crust types are attributed to differences in the climate under which they formed, and differences in their degree of development are attributed to differences in their age.

Calcareous duricrust is recognized widely as an indicator of semiarid climate, and under certain circumstances, it provides evidence of climatic changes. The relations in al-Hasa suggest the possibility of dating and correlating crusted surfaces by the character of their crust in regions where surfaces and crust have developed alternately over the same period of time.

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