Occurrence of recent dedolomite crust on pedogenic calcrete in Al-Jabal Al-Akhdar, Northern Oman

Abstract

Rusty dedolomitized crust occurs in fractured pedogeniccalcrete horizon within terra rossa soil contained in shallow phytokarstic cavities in the calcareous rocks exposed at Al-Jabal Al-Akhdar Mountain, Oman. Two lithotypes were recognized: (a) dedolomitizeddetrital dolomite and (b) dedolomitizedauthigenic dolomite. The first is light brownish, has an undulating sharp contact with the nodular calcretes and consists of euhedral to subhedral moderately to poorly sorted dolomite grains floating in a micritic groundmass. The second is relatively thicker, dark brown, and consists of idiotopic mosaic of moderately sorted well euhedral dolomite crystals that reach up to 200 um in size. Microscopic examination of stained thin sections indicated that most of both detrital and authigenic dolomite crusts have been partially or completely dedolomitized. XRD analysis indicated that both types of crusts mostly consist of calcite and dolomite with traces of quartz, where abundance of calcite reflects their dedolomitization. The chemical composition of these crusts reflects their mineralogy. The paragenetic events of the crust formation are discussed. The availability of dolomitic source rocks and the present prevailed semiarid climate characterized by intermittent wet and dry periods have played a significant role in its development. This work presents a unique mode of occurrence of dedolomitized dolomite associated with recent pedogeniccalcretes.

Keywords

• Al-Jabal Al-Akhdar
• calcrete
• crust
• dedolomite
• Oman.