English abstract
The sandstones of Mahatta Humaid Group (Amin and Miqrat formations) of Cambrian age exposed at Huqf region, Oman have been analysed for petrofacies, provenance, tectonic setting, paleoclimate and diagenetic history. A total of 429 thin sections have been used for this study and were collected from three different areas; Khufai Anticline, Buah Anticline and Mahatta Humaid. Petrographic analysis and modal compositions had been used to reach the goals of the study. Amin Formation sandstones in Khufai Anticline are medium- to coarse-grained, sub-angular to sub-rounded and moderate- to well-sorted. They constitute mainly monocrystalline quartz (av. 32.5%) and chert fragments (av. 38.4%). While, Amin Formation sandstones in Buah Anticline are fine- to medium-grained, sub-rounded to rounded and moderate to well-sorted. They compose of detrital frameworks of monocrystalline quartz (av. 71.4%) and chert fragments (av. 7.5%). Sandstones of both areas showed low content of lithic fragments and lack content of feldspar grains. On the other hand, Miqrat Formation sandstones are fine- to medium-grained in Buah Anticline and medium- to coarse-grained in Mahatta Humaid area, sub-rounded to rounded and moderate- to well-sorted. Sandstones in Buah Anticline and Mahatta Humaid areas contain monocrystalline quartz (av. 50.2%). Sandstones of Mahatta Humaid (av. 3.6%) have higher amount of feldspar content than Buah Anticline (av. 1.1%). They have good amount of mica in their composition and low content of lithic fragments.
Amin sandstones are classified as litharenite to quartz arenite. Moving from Khufai Anticline to Buah Anticline, the sandstones are more mature and richer with quartz grains. Miqrat sandstones are classified as subfeldsarenite to quartz arenite and some of sandstones are classified as sublitharenite. These sandstones were derived from mix provenance including granodiorite of Al-Jobah basement, low and middle metamorphic basement of South Oman and South Yemen and sedimentary rock fragments of underlying Huqf sediments. The identified petrofacies reveal that these sandstones of Mahatta Humaid Group (Amin and Miqrat formations) belong to the continental blocks and recycled orogeny provenances. However, the interpreted recycled orogeny provenance does not fit with the alluvial/ fluvial Amin Formation and aeolian Miqrat Formation sandstones because of their richness in chert grains; chert grains are considered as lithic fragments in the tectonic provenance discriminations diagrams. Therefore, it is recommended that someone should consider the effect of chert in provenance interpretation. The paleoclimate of provenance during Amin sandstones deposition in Khufai Anticline was semi-arid to semi-humid, while it was semi-arid to humid during Amin Buah sandstones deposition. Whereas, the paleoclimate of provenance during Miqart sandstones deposition in Buah Anticline and Mahatta Humaid area was semi-humid to humid.
The main diagenetic processes that affected Amin and Miqrat sandstones composition were dissolution and replacement. The less chemically stable grains e.g. chert, polycrystalline quartz and feldspar grains are dissolved easily by chemical weathering. The replacement of feldspar grains by clay minerals and quartz grains by anhydrite mineral have been observed in the studied sandstone. However, the original mineral composition was recalculated to avoid and reduce the effect of diagenesis on provenance interpretations.
