Effect of Permo-Carboniferous climate on illite-smectite, Haushi Group, Sultanate of Oman

The Late Westphalian to Artinskian Haushi Group in the Sultanate of Oman co nsists of the glaciogenic Al Khlata Formation and the Gharif Formation whic h contains marginal marine, coastal plain, and fluvial sediments. The seque nce was deposited during a global-warming event following the Permo-Carboni ferous glaciation of Gondwana. Because of a varied subsidence history, thes e sediments range from the surface in thr SE to almost 5000 m in the NW of the basin. Mixed-layer illite-smectite (I-S) is an important constituent of the <2 mu m size fraction of sandstone and shale samples in both formations at all de pths. Different starting compositions lead to three distinct trends of illi te layers in I-S versus temperature for different sedimentary environments and paleoclimatic conditions. The starting compositions of I-S at the surfa ce range from an ordered I-S in the Al Khlata Formation to smectite-rich in the Upper+Middle Gharif members. Physical, chemical and environmental factors were investigated as causes fo r the different starting compositions of T-S. Both formations share an iden tical burial history, paragenesis, thermal evolution, and source of detrita l material. They differ only in environmental conditions during sedimentati on. Thus, the variation in starting composition of I-S appears to be best e xplained by distinct weathering conditions during sedimentation of the thre e units. Ln particular. the expected low intensity of chemical weathering d uring glaciogenic conditions is marked by the presence of higher amounts of unstable volcanic and sedimentary rock fragments in the Al Khlata Formatio n.