QUATERNARY CALCAREOUS DURICRUSTT, IN AL QASSIM PROVINCE, SAUDI ARABIA

The calcareous duricrust is one of the important factor in shaping the landscape, in Al Qassim Province, Saudi Arabia, as it is protecting the underlying older rocks from denudation processes throughout the Quaternary period. Two types of duricrust can be recognized. The first covers most of the Paleozoic sandstones, and the Triassic mixed siliciclastic-carbonates. This type of duricrust is grey to brown in color, and occurs as scattered nodules with a thicknesses that varies from a few centimeters to decimeters. The second type covers the Upper Permian carbonates and clays of the Khuff Formation, and ranges in thickness from 60 cm to 160 cm. It consists of white massive caliche, overlain by white, yellow patchy duricrust capped by brown, nodular porous duricrust with black manganese oxides as pore filling.

The development of the duricrust is controlled by three factors: the climatic changes, type of underlying rocks, and tectonics. The repeated changes from arid to humid climate is the main controlling factor, in generation of duricrust. During humid periods descending water carries the dissolved carbonates downward as bicarbonates, where the carbonate rich solution are cerried upwards by capillary action during the arid periods.

It is evident that the limestones and marls possibly affect the development of calcareous duricrust, more than the sandstones and dolostones, since the former have supplied the calcium carbonates to form duricrust. The tectonic factor manifested in faults in the study area have provides the topographic lows, which enabled preservation of runoff water and inturn more water infiltration can rise later by capillarity carrying calcium carbonate rich solutions.

The development of calcareous duricrust is mainly attributed to the Pleistocene, Early Holocene alternating humid and arid periods. Two humid periods are documented, the oldest phase of which is between 9000 and 8000 years B. P., while the younger one is between 7000 and 5500 years B. P.