The Ediacaran Hiyam Formation: A zoom through the diagenetic and structural complexity of the metamorphic Hi₂ Member, Saih Hatat Dome, Oman Mountains

This is the first diagenetic study of the metamorphic upper Hiyam Member “Hi₂” of the Saih Hatat Dome. Despite metamorphism, texture and paragenesis of the Hi₂ Member are so well preserved that it is possible to unravel its diagenetic evolution. We applied lithological and structural field methods, transmission and cathodoluminescence microscopy to establish the paragenetic sequence and vein-forming deformation. Our work reveals a complex diagenetic and structural history, including several alternating dolomitization, calcite precipitation, silicification and fracturing events. Most of the replacive dolomite and silica are linked to early diagenesis under intertidal to sabkha and restricted marine as well as to terrestrial conditions. Evaporation of sea water concentrated Mg²⁺ and triggered extensive early dolomitization while silica-bearing fluids caused local silicification. Various generations of dolomite and calcite cements pervasively occluded macropores, micropores and veins. Late burial is reflected by pressure solution seams/stylolites, non-ferroan to weakly ferroan saddle-type dolomite cement and non-ferroan blocky and drusy calcite cement, filling veins and macropores. The various generations of calcite cements (e.g., in veins) are considered to have formed during burial or possibly meteoric conditions. The latter potentially resulted from tectonic uplift, while late dolomite cement displays hydrocarbon residues from coeval oil migration. A later minor oil migration event has also been recognized. Initial fracture/fissure formation is most likely related to folding during the Early Cambrian Angudan Orogeny (open NW-SE fractures) and to divergence during the Late Paleozoic Pangean/Neo-Tethys rifting (open NW-SE fractures). Metamorphism impacted the Hi₂ Member, resulting in phyllite formation, phyllosilicate crystallization and ductile shearing. Metamorphism occurred during shallow Late Cretaceous subduction. Striking lithological similarities between the Hi₂ Member and that of the upper Kharus Formation of the Jabal Akhdar region could indicate their lateral equivalence.