Andrea Cozzi

Neoproterozoic snowball Earth under scrutiny: Evidence from the Fiq glaciation of Oman

The Fiq Member of the Ghadir Manqil Formation, Huqf Supergroup, Oman, is composed of ∼1.5 km of glacial rainout diamictites, mass-flow deposits, turbiditic sandstones, and slope and shelf sandstones and siltstones, overlain by a classic <10-m-thick transgressive cap dolostone. Paleocurrents, facies distribution, and subsurface imagery show that the Fiq Member was deposited in a series of extensional half-grabens and grabens. Facies associations represent proximal glaciomarine, distal glaciomarine, nonglacial sediment gravity flow, and nonglacial shallow-marine siliciclastic depositional environments and processes. The member is divided into seven units that can be correlated across the rift basin. Although the facies mosaic is complex, cycles of relative sea-level change, most likely driven by glacio-eustasy, show that glacial advance and retreat during the glacial epoch was strongly pulsed. Only the final (youngest) diamictite is overlain by a well-developed cap carbonate. It is unlikely that there was any prolonged and substantial shutdown of the hydrologic cycle during deposition of the Fiq Member. On the basis of sedimentological and stratigraphic data, the Neoproterozoic glacials of Arabia were more like the familiar oscillatory glaciations of the Pleistocene than those required by the snowball Earth hypothesis.

A composite stratigraphy for the Neoproterozoic Huqf Supergroup of Oman: integrating new litho-, chemo- and chronostratigraphic data of the Mirbat area, southern Oman

The Huqf Supergroup in Oman contains an exceptionally well-preserved and complete sedimentary record of the Middle to Late Neoproterozoic Era. Outcrops of the Huqf Supergroup in northern and central Oman are now well documented, but their correlation with a key succession in the Mirbat area of southern Oman, containing a sedimentary record of two Neoproterozoic glaciations, is poorly understood. Integration of lithostratigraphic, chemostratigraphic and new U–Pb detrital zircon data suggests that the Mirbat Group is best placed within the Cryogenian (c. 850–635 Ma) part of the Huqf Supergroup. The c. 1 km thick marine deposits of the Arkahawl and Marsham Formations of the Mirbat Group are thought to represent a stratigraphic interval between older Cryogenian and younger Cryogenian glaciations that is not preserved elsewhere in Oman. The bulk of detrital zircons in the Huqf Supergroup originate from Neoproterozoic parent rocks. However, older Mesoproterozoic, Palaeoproterozoic and even Archaean zircons can be recognized in the detrital population from the upper Mahara Group (Fiq Formation) and Nafun Group, suggesting the tapping of exotic sources, probably from the Arabian–Nubian Shield.

Evolution of a terminal Neoproterozoic carbonate ramp system (Buah formation, Sultanate of Oman): Effects of basement paleotopography

The Buah Formation is a carbonate unit of late Neoproterozoic age, exposed in the Jabal Akhdar of northern Oman and the Huqf region of central Oman. The Buah Formation is interpreted as a distally steepened storm-dominated ramp, containing inner ramp tidally influenced oolitic shoal and back-shoal lagoon fades and storm-dominated mid ramp facies. A slope break separates mid-ramp facies from outer ramp facies characterized by accumulation of redeposited carbonates on gently inclined slopes (1-2 degrees). Combined facies stacking patterns and outcrop correlation via δ 1 3 C profiles suggest that Buah carbonates formed as a highstand systems tract characterized by a general upward shallowing of facies and strong lateral progradation. On a regional scale, Buah facies distributions across Oman were controlled by the presence of basement highs defining areas of reduced subsidence, which promoted lateral progradation of the Buah carbonate ramp for tens of kilometers, while the faster-subsiding areas became sites of deep-water deposition.

Facies patterns of a tectonically-controlled Upper Triassic platform-slope carbonate depositional system (Carnian Prealps, Northeastern Italy)

SummaryUpper Triassic (Middle-Upper Norian) shallow-water carbonates of the Dolomia Principale and its deep-water counterparts (Forni Dolomite) have been studied in the Carnian Prealps (northeastern Italy). The Dolomia Principale was a storm-dominated carbonate platform; in the Mt. Pramaggiore area, along a well-preserved 3.5 km-long platform-to-basin transition, the inner platform facies of the Dolomia Principale, characterized by m-scale shallowing upward cycles, give way seaward to open marine storm-dominated shallow subtidal lagoon deposits with frequent hardgrounds and evidence of microbial stabilization of the bottom sediment. The margin of the Dolomia Principale platform was colonized by meter-scale stromatolites and serpulid-microbial mounds that thrived due to the local highly stressed environment, characterized by drastic salinity fluctuations and turbid waters, that excluded the Upper Triassic coral-sponge communities. The Forni Dolomite slope-basin complex was characterized by an upper slope facies with debris flows, megabreccias, turbidites and serpulid-microbial mounds. The lower slope and basinal facies show thinning and fining trends. After restoring the original geometry of the slope, the depositional angles of the clinoforms range between 11 and 36 degrees, reflecting closely the coarse-grained character of the Forni Dolomite slope complex, which can be interpreted as a slope apron that, as a model, can be extended to steeply inclined carbonate slopes. The onset of synsedimentary extensional tectonics at the Middle-Late Norian boundary affected the platform-slope depositional system via: 1) localized inner platform collapses and the formation of an intraplatform anoxic depression at Mt. Valmenone, 2) a switch from platform lateral progradation during the Middle Norian to vertical aggradation in the Late Norian, reflected in an increase in platform relief, steeper foreslope angles and coarser-grained slope facies, and 3) controlling the spatial orientation of the margin of the Dolomia Principale.

From global geology to hydrocarbon exploration: Ediacaran–Early Cambrian petroleum plays of India, Pakistan and Oman

Abstract The vast amount of new lithostratigraphic, chemostratigraphic and geochronologic data from the Huqf Supergroup (Sultanate of Oman) has established it as the Cryogenian (850–635 Ma) and Ediacaran (635–542 Ma) reference section for the Neoproterozoic of the Middle East Region. A direct litho- and chemostratigraphic comparison of the Huqf Supergroup of Oman with the supposed time-equivalent succession of the Marwar Supergroup in western Rajasthan (India) reveals remarkable affinities in facies evolution and chemostratigraphic signature through time. Ara Group equivalent strata are also found in the Salt Range Formation of Pakistan, which shows an almost identical repetition of evaporites and carbonates with six to seven basin refreshening–desiccation cycles, comparing well with the A0–A6 Ara Group stratigraphy of the South Oman Salt Basin. These similarities bring out a consistent picture of a cratonic setting for the Marwar Supergroup of West Rajasthan, changing into a more open marine setting towards Pakistan until Oman, suggesting an assembly of Oman–Pakistan and India ‘terrains’ prior to being accreted to the Arabian shield sometime after 650 Ma. From a petroleum exploration perspective, key success factors when pursuing the Late Neoproterozoic plays in the Salt Basins of India, Pakistan and Oman are source rock maturity, charge preservation and seal integrity.

Geology and hydrocarbon potential of Neoproterozoic–Cambrian Basins in Asia: an introduction

In 2005, the Maghreb Petroleum Research Group (MPRG), University College London, initiated a major research programme focused on the relatively poorly understood Neoproterozoic petroleum systems of the world. A series of research projects were undertaken to understand the generation and entrapment of hydrocarbons in this unique geological time interval, which is dominated by several episodes of global glaciations and post-glacial transgressions, coupled with basin development and rifting on a more local scale (Craig et al. 2009). The research started with a field-based study of the Neoproterozoic sequences in North Africa (Libya, Morocco and Mauritania) and northern India (Rajasthan and Jammu & Kashmir). A series of international conferences, with field excursions/workshops, were run in parallel with the research programmes. The first of these was held at the Geological Society of London in November 2006 and the proceedings were published in 2009 in Geological Society London, Special Publication 326, entitled, ‘Global Neoproterozoic Petroleum Systems: the emerging potential in North Africa’ (Craig et al. 2009). The second international conference was held at the University of Jammu in 2008 with a focus on the Neoproterozoic petroleum systems of Asia, including India, Pakistan, Oman, China and Siberia (Bhat et al. 2008) (Fig. 1). This current volume contains some of the papers presented at the Jammu conference, in addition to new research on the geology and hydrocarbon potential of the Neoproterozoic–Cambrian basins of Asia. A third and concluding conference and an associated third Geological Society Special Publication will focus on the Neoproterozoic petroleum systems in regions of the world not covered in the previous volumes (mainly North and South America, western and southern Africa and Australia) and will complete the project to provide a global synthesis of the Neoproterozoic petroleum systems. The present volume contains fifteen papers covering the Neoproterozoic petroleum systems of India (Ojha, Ram, Kumar and Majid et al.), Pakistan and (Jamil & Sheikh and Siddiqui), Oman (Cozzi et al.), China (Turner) and Siberia (Howard et al.). The remaining five papers concentrate on various aspects of Neoproterozoic geology and palaeobiology, including stratigraphy (Tewari) and tectonics (Mishra & Mukhopadhyay) of the NW Himalaya, salt tectonics in Oman (Smith), acritarchs in Oman (Butterfield & Grotzinger) and the palaeobiology of the Vindhyan succession in central India (Sharma & Shukla). This Introduction provides a synthesis of the key conclusions in a palaeogeographic context, but for details the reader is referred to the relevant articles in this volume.

Geology and Hydrocarbon Potential of Neoproterozoic-Cambrian Basins in Asia

This volume provides a comprehensive overview of the geology and hydrocarbon potential of the major Neoproterozoic–Cambrian basins of Asia from Oman, across the Middle East and the Indian Subcontinent, to China and SE Siberia, along with new research on the region. Many of these areas (e.g., Oman, Bikaner–Nagaur Basin in India, South China and SE Siberia) host prolific Neoproterozoic–Cambrian petroleum systems with giant to supergiant fields. Three key elements: (1) tectonic stability, (2) relatively late phase of hydrocarbon generation and (3) presence of an effective evaporite seal, seem to be critical for the development of effective Neoproterozoic–Cambrian petroleum systems. These key elements appear of less consequence for the development of ‘unconventional’ hydrocarbons, and the future prospectivity in many of these basins may lie in the exploration for, and production of, shale gas and shale oil directly from the thermally mature, organic-rich source rocks. This volume follows on from the successful Geological Society SP326 on African basins and documents hydrocarbon exploration potential of basins in Asia that until now were poorly understood and documented.

Chapter 19 The Ayn Formation of the Mirbat Group, Dhofar, Oman

Abstract 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.

Chapter 20 The Abu Mahara Group (Ghubrah and Fiq formations), Jabal Akhdar, Oman

Abstract The Abu Mahara Group (c. 725–<645 Ma) of the Huqf Supergroup in the Jabal Akhdar of northern Oman hosts two glacial successions in the Ghubrah and Fiq formations, separated by the <50-m-thick volcanogenic Saqlah Member. The >400-m-thick Ghubrah Formation is dominated by distal glaciogenic rainout diamictites, laminites and turbiditic siltstones, whereas the <1.5-km-thick Fiq Formation exhibits a cyclical stratigraphy of proximal and distal marine glaciogenic facies, and non-glacial sediment gravity flow and shallow marine facies. The Fiq Formation is overlain by a transgressive, isotopically light carbonate known as the Hadash Formation. A tuffaceous ash interbedded with glacial diamictites of the Ghubrah Formation in Wadi Mistal has yielded a U–Pb zircon age of 713.7±0.5 Ma. The Fiq Formation contains detrital zircons as young as 645 Ma. The use of the CIA (Chemical Index of Alteration) shows the Fiq Formation to be climatically cyclic, with alternations of high and low chemical weathering of contemporary land surfaces driven by phases of glaciation and deglaciation. The transgression into the post-glacial Masirah Bay Formation is marked by a major increase in chemical weathering.

Comment: Oman Chronostratigraphy (Comment on “Geochronologic constraints on the chronostratigraphic framework of the Neoproterozoic Huqf Supergroup, Sultanate of Oman” by Samuel A. Bowring, John P. Grotzinger, Daniel J. Condon, Jahandar Ramezani, Mark J. Newall and Philip A. Allen, American Journal of Science, v. 307, p. 1097–1145.)

Bowring and others (2007) presented a compilation of high-precision U-Pb zircon ages (both detrital and from ash beds) of the exceptionally well preserved Late Neoproterozoic-Early Cambrian Huqf Supergroup of Oman. Given the general lack of precise geochronological data of the Late Neoproterozoic, the data of Bowring and others (2007) represent an important contribution to the understanding and time calibration of this period of Earth history, which records some highly enigmatic features such as the occurrence of low-latitude glaciations, extreme perturbations of the carbon cycle and the evolution of metazoans. The work by Bowring and others (2007) puts the Huqf Supergroup on the map as one of the geochronologically best constrained sections of its age and makes it an invaluable comparison for other Late Neoproterozoic successions around the world. Given the global significance of the Huqf Supergroup of Oman, we would like to clarify two important issues that arise from the Bowring and others (2007) article: (1) an inconsistency in the correlation presented by Bowring and others between glaciations recorded in northern and southern Oman and (2) the highly speculative inference of a long-lived unconformity at the Khufai-Shuram boundary.