The Ayn Formation of the Mirbat Group, Dhofar, Oman
Philip A. Allen, Ruben Rieu, James L. Etienne, Albert Matter, Andrea Cozzi, 2011. "The Ayn Formation of the Mirbat Group, Dhofar, Oman", The Geological Record of Neoproterozoic Glaciations, Emmanuelle Arnaud, Galen P. Halverson, Graham Shields-Zhou
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Glacial deposits are found in the Ayn Formation and Shareef Formation of the Mirbat Group close to Mirbat in Dhofar, southern Oman. The Mirbat Group is most likely a correlative of the Abu Mahara Group of the Huqf Supergroup of northern Oman. The Ayn Formation, the main subject of this chapter, comprises <400 m of mainly coarse-grained glaciogenic deposits, ponded in 2- to >8-km-wide N- to NW-oriented palaeovalleys eroded into crystalline basement, with few or no deposits preserved on intervening palaeohighs. The Shareef Formation occurs as thin, lenticular, erosional remnants beneath the unconformably overlying Cretaceous. The Ayn Formation is overlain by a thin (<3 m), discontinuous cap carbonate that passes from carbonate-cemented talus on the basin margin to stromatolitic carbonate on palaeohighs and resedimented gravity flows on palaeovalley flanks.
The Ayn Formation is younger than its youngest detrital zircons and the youngest late plutons in crystalline basement, constraining it to <c. 720 Ma, but its exact age is unknown. The detrital zircon population comprises exclusively Neoproterozoic sources, suggesting derivation from the juvenile Neoproterozoic crust of the Arabian area.
The composition of fine-grained matrix in glaciogenic diamictite units and of non-glacial mudstones, plotted using the chemical index of alteration (CIA), suggests strong variations in the intensity of palaeoweathering on contemporary land surfaces between the mechanical weathering-dominated Ayn Formation, and the chemical weathering-dominated overlying Arkahawl Formation, which supports the notion of major glaciation followed by rapid climatic transit as basin margins were flooded and buried with sediment during post-glacial transgression. The carbon isotopic ratio (δ13C) of the post-glacial carbonate is strongly variable from −3.5‰ to +5.8‰, whereas carbonate fissures in the underlying basement range between +4.1‰ and +5.7‰.
Two independent palaeomagnetic studies have yielded low palaeomagnetic latitudes for the Mirbat Group.
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In recent years, interest in Neoproterozoic glaciations has grown as their pivotal role in Earth system evolution has become increasingly clear. One of the main goals of the IGCP Project No. 512 was to produce a synthesis of newly available information on Neoproterozoic successions worldwide similar in format to Hambrey & Harland’s (1981) Earth’s pre-Pleistocene Glacial Record. This Memoir therefore consists of a series of overview chapters followed by site-specific chapters. The overview chapters cover key topics including the history of research on Neoproterozoic glaciations, identification of glacial deposits, chemostratigraphic techniques and datasets, palaeomagnetism, biostratigraphy, geochronology and climate modelling. The site specific chapters for 60 successions worldwide include reviews of the history of research on these rocks and up-to-date syntheses of the structural framework, tectonic setting, palaeomagnetic and geochronological constraints, physical, biological, and chemical stratigraphy, and descriptions of the glaciogenic and associated strata, including economic deposits.