Abstract Numerical-statistical algorithms are used to model end-member grain-size distributions of pelagic and hemipelagic siliciclastic sediments of the Arabian Sea. The grain-size distributions of sediments from the Oman continental slope, the Owen Ridge, the Pakistan continental slope, and the Indus Fan can be adequately described as mixtures of three end members. The spatial variation in relative contribution of the end members is interpreted in terms of transport processes and provenance. In the western Arabian Sea, deposition is dominated by two end members that represent "proximal" and "distal" eolian dust. A third end member, which dominates the deposits of the middle Indus Fan, represents fluvial mud deposited from low-density turbidity currents (lutite flows). At any given location, the temporal changes in the relative contribution of the end members can be interpreted in terms of climate change. The ratio of contributions of the two eolian end members (i.e., the grain-size distribution of the eolian dust) on the Owen Ridge (NIOP492) reflects the strength of the summer monsoon. Deposition on the upper Indus Fan (NIOP458) is dominated by "distal" eolian dust and fluvial mud. The ratio of contributions of eolian and fluvial sediment reflects continental aridity. The ratio of contributions of the two eolian end members (i.e., the grain-size distribution of the eolian dust) on the upper Indus Fan reflects the strength of the winter monsoon. Our reconstruction of the late Quaternary variations in Arabian Sea monsoon climate corresponds well with interpretations of the loess-paleosol sequences on the Chinese Loess Plateau. In both areas, the bulk of the annual precipitation is confined to the summer monsoon season. Intensification of the summer monsoon during interglacials, which has been identified as the principal control on pedogenesis on the Loess Plateau, also explains increased discharge of Indus River-derived muds to the northern Arabian Sea. Independent evidence for summer monsoon strength, provided by the eolian grain-size record of the western Arabian Sea, fully supports this conclusion. The strength of the summer monsoon thus provides an aridity forcing mechanism for both the Arabian Sea and the Loess Plateau. The grain size of the eolian dust in the northern Arabian Sea and on the Loess Plateau indicates intensified winter monsoons during glacials.

Abstract In the northern Arabian Sea the Arabian, Eurasian and Indian Plates are in tectonic interaction with one another. We present interpretations of multichannel seismic profiles across the Makran sub-duction zone (which is part of the Eurasian-Arabian Plate boundary) and the transtensional Murray Ridge and Dalrymple Trough (which are part of the Arabian-Indian Plate boundary). We distinguish four megasequences in the sedimentary succession, which we correlate over the entire study area. Regional unconformities separate the megasequences and enable us to establish a common history of the region before Late Miocene time (c. 20 Ma). The Early Pliocene (c. 4.5 Ma) reopening of the Gulf of Aden caused a reorganization of the plates and subsequent tilting of the oceanic crust of the Arabian Plate toward the Makran subduction zone. This event is documented by the regional M-unconformity. Since that time, sedimentation on the Oman Abyssal Plain has been permanently separated from the Indus Fan by the Murray Ridge, on the northern end of which there has been no significant sedimentation.

ABSTRACT In well ST-8, Nafud Basin in Saudi Arabia, the pre-Unayzah unconformity (so called Hercynian unconformity) separates the Lower Carboniferous (upper Visean) Berwath Formation, from the Upper Carboniferous (upper Westphalian? and Stephanian) Unayzah C Member. The unconformity (erosion and non-deposition) corresponds to a hiatus in mid-Carboniferous time (Namurian-early Westphalian = Serpukovian-Bashkirian stages; about 327-311 Ma). The Berwath Formation is absent in outcrop, and rarely preserved in the subsurface. It consists of sandstones, carbonaceous shales and limestones deposited in an apparently regional lower coastal plain to marginal marine setting, and has a comparable thickness in two wells located nearly 1,000 km apart: ST-8 (361 ft, 110 m), and Abu Safah-29 (440 ft, 134 m) in the Arabian Gulf. The Berwath Formation is separated from the underlying Jubah Formation by a latest Tournaisian to mid-Visean hiatus, an event that is also recognised in North Africa and Brazil. The subsurface Unayzah C Member (thickness range: 165-740 ft, 50-310 m) and overlying Unayzah B Member (55 ft, 16.8 m thick in reference well Hawtah-1) consist of clastics deposited in alluvial fan systems that are coeval with the glaciogenic: (1) Al Khlata Formation of Oman (upto 2,625 ft, 800 m), (2) Juwayl Member of the Wajid Formation in southwest Saudi Arabia (426 ft, 130 m); and (3) Akbra Shale in north Yemen (426 ft, 130 m). These rock units are assigned to the third glaciation of the Arabian Plate (AP G3), and their palaeodepocentre is located in the Rub’ Al-Khali Basin. The Unayzah C Member was deposited in continental lowlands and channels between uplifted fault blocks and broad arches, and unconformably overlies Proterozoic to Lower Carboniferous formations. The age of the younger and regionally widespread Unayzah B Member is interpreted as Early Permian (Asselian-Sakmarian). The Unayzah A Member in Hawtah-1 consists of the Lower ‘Siltstone’ Submember (135 ft, 41.2 m thick), and Upper ‘Sandstone’ Submember (91 ft, 27.7 m thick). The upper boundary of the Unayzah Formation, the pre-Khuff unconformity, is overlain by the Basal Khuff Clastics. The Unayzah A Member is generally unpalyniferous; recovered palynomorphs in the Lower Submember indicate an Early Permian age. A tectonic compressional event uplifted the extensive NS-trending Arabian fault blocks (e.g. Hawtah and En Nala-Ghawar structures) in mid-Carboniferous time. The duration of the event coincides with the break between the Berwath Formation and Unayzah C Member. The same event caused deformation in the Oman Mountains, where open folds (about 100 m in amplitude, trending N50°E-N70°E) have radiometric ages of 329-321 Ma (i.e. late Visean and Namurian stages). The broad upper Palaeozoic swells (e.g. Central Arabian Arch and Haushi-Huqf Uplift) consist of Proterozoic to Lower Silurian successions that were uplifted without apparent regional faulting, either during the mid-Carboniferous tectonic event or earlier. The third glaciation AP G3 started in southern Arabia at about 311 Ma (or earlier), following the mid-Carboniferous tectonic event.

Abstract The distribution of sand dunes over the southern half of Arabia conforms to the influence of two wind systems: the northern Shamal, which is a strong wind that blows to the SSE down the Persian (Arabian) Gulf and then swings to the SW across the hyperarid Rub al Khali towards North Yemen; and the strong winds of the SW Monsoon system, which were responsible for forming linear dunes that trend north-south in the Wahiba Sands of eastern Oman and SW-NE in the Thar Desert (NW India). In the Thar Desert, the SW Monsoon alternates with the weaker NE Monsoon. The dating of exposures of older dune systems by isotopic, radiometric and optically stimulated luminescence (OSL) analyses has shown that the Shamal was active throughout the latter part of the Quaternary period, and probably as long ago as Mid-Miocene time ( c. 15 Ma). At times of glacial maxima, when global sea level was some 100-120 m or more lower than now, siliciclastic and carbonate grains were deflated from the exposed surface of the Persian Gulf and transported into the NE Rub al Khali within the United Arab Emirates. It is suspected that occasionally the Shamal also transported some quartz sands from the NW onto the exposed narrow continental shelf of SE Arabia, with silt-size particles being carried into the Arabian Sea. The SW Monsoon, on the other hand, was re-established over the coast of SE Arabia several millennia after the last glacial maximum and was fully established near the coast of SE Arabia during the early Holocene interglacial after the atmospheric high-pressure system associated with the glacial period had become weaker. Early during the Holocene interglacial periods when the SW Monsoon dominated, a combination of quartz and carbonate sands was deflated from the exposed continental shelf and transported to the north into the Wahiba Sands. Aeolian activity in the Thar Desert also peaked during this period of transition from full glacial to interglacial conditions. The dune systems of SE Arabia overlie the distal edges of older alluvial fans that in Oman date back at least 350 ka. The sediments of some of these fluvial sequences in Oman reached the Arabian Sea via Wadi Batha, only to be removed by along-shore currents driven by the SW Monsoon. In the Thar Desert, the supply of aeolian sediment is mostly from fluvial sources. Marine sediments from the Arabian Sea between Arabia and Thar record the contrasting effects of the Shamal and the SW Monsoon: the former mostly as a source of wind-blown dust from Arabia and the latter by causing upwelling of nutrient-rich waters leading to organic blooms.