M.E. Brookfield

The evolution of the great river systems of southern Asia during the Cenozoic India-Asia collision: rivers draining southwards

During uplift of the Tibetan plateau and surrounding ranges, tectonic processes have interacted with climatic change and with local random effects (such as landslides) to determine the development of the major river systems of Asia. Rivers draining southward have three distinctive patterns that are controlled by different tectonic and climatic regimes. In central and southern Afghanistan, the rivers have moderate gradients and fan out from northeastern sources to disappear into arid depressions. Anti-clockwise rotation of southern Afghanistan, caused by differential compression and right-lateral shear, cut the rivers on the north, while increasingly arid conditions developed on the south as arc accretion in the Makran separated sources from the coastal rains. In Tibet and southeast Asia, the rivers are widely separated and have low gradients on the Tibetan plateau, higher gradients as they turn southwards into close and parallel gorges, before they fan out southeast to enter different seas. Differential shear and clockwise rotation between the compressing Tibetan plateau and Southeast Asia determined the great sigmoidal bends of this river system which was accompanied by increasing aridity, with truncation of river systems in the north and river capture in the south. In the Himalaya and southern Tibet, the main rivers have steep gradients where they cut across the Himalayan range and occasionally truncate former rivers with low gradients on the Tibetan plateau to the north. Southward thrusting and massive frontal erosion of the Himalaya caused progressive truncation of longitudinal rivers on the plateau, accompanied by river capture, and glacial and landslide diversions on the south. The drainage history of southern Asia can be reconstructed by restoring the gross movements of the plates and the tectonic displacement, uplift, and erosion of individual tectonic units. Most important changes in drainage took place in Pliocene to Quaternary times.

The Himalayan passive margin from Precambrian to Cretaceous times

Abstract The Himalaya passive margin should be separated into the Indian Himalaya and the Pakistan Himalaya, divided by the Nanga Parbat-Haramosh basement uplift in the northwestern syntaxes. The Indian passive margin shows a complex history involving splitting of microcontinents off the northern Gondwana margin from early Paleozoic times until the Jurassic. In contrast, the Pakistan Himalaya formed part of the stable Indian shelf until separation from Africa started in the Jurassic. Each margin has a distinct stratigraphic history until the mid-Mesozoic after which they show similar histories until the Tertiary. The Himalaya mountains in India consist of three distinct tectonic units juxtaposed during the Neogene by southward thrusting of the northern Indian Precambrian to Mesozoic passive margin. The Lesser Himalaya on the south forms a late Precambrian passive margin usually directly overlain by late Cretaceous to Tertiary clastics: its stratigraphy is closest to that of the Indian Shield. In contrast, the High Himalaya has a thin Ordovician to Carboniferous shelf sequence deposited after early Ordovician deformation and granite intrusion. Above this, thick Permian and Mesozoic shelf sequences mark the separation of continental blocks off the northern Indian margin and the opening of the Neotethys ocean. The North Himalaya formed the slope and basin of this ocean and consists of reactivated Paleozoic gneiss domes overlain by thick Mesozoic sediments which pass, in the northeastern Himalaya, into an enormously thick sedimentary sequence resting on oceanic crust. Paleomagnetic and structural evidence indicates at least 500 km of southward thrusting along the Main Central Thrust between the Lesser and High Himalaya. This thrusting, together with a further 250 km of erosion at the front of the nappes, has removed the entire inner shelf of the Himalaya passive margin and can explain the startling contrast between the High and Lesser Himalaya stratigraphic sequences. But there are still discrepancies, particularly in the eastern Himalaya, where Precambrian basement is juxtaposed with the North Himalaya Mesozoic slope. These discrepancies can be partly resolved by strike-slip movements roughly parallel to the Himalaya which have removed parts of the northern Indian passive margin. Such faults of the requisite orientation, displacement and age occur in southeast Asia where their cumulative displacements can add up to several thousand kilometres. Similar faults, of undoubted pre-Tertiary age, occur in the Pakistan Himalaya. Here, the passive margin shows a much simpler history of Jurassic to Miocene subsidence.

Neoproterozoic Laurentia-Australia fit

The proposed Vendian juxtaposition of western North America and eastern Australia can be tested by comparing sequences on each margin that developed during the major glacial epochs of the Neoproterozoic. The actual fitting of the margins, however, is more difficult, because both margins have been deformed since separation. A fit requires lines or points that can be correlated across the margins. Three major Neoproterozoic transform-fault offsets can be traced from eastern Australia into western North America to give a fairly precise alignment of the respective margins that is compatible with the stratigraphy.

A mid-Ordovician temperate carbonate shelf-the black river and trenton limestone groups of southern Ontario, Canada

Abstract Mid-Ordovician limestones of southern Ontario are usually considered to be tropical shelf limestones and interpreted using facies models derived from Recent tropical carbonate environments. Nevertheless, depositional rates, grain types, faunas, erosion surfaces, mineralogy and geochemistry are more compatible with a temperate or even cold shelf environment. In view of the low latitudes (20°S) indicated by paleomagnetism, this suggests deposition in relatively cold seas, deposition below a tropical thermocline, or that the paleomagnetic data are wrong. Similar features in other Caradocian limestones of eastern North America show that these inferred cooler conditions were widespread. If deposition occurred below a tropical thermocline, most shallow-water Caradocian limestones must be re-interpreted as deeper-water deposits. More likely, in view of the evidence for temperate conditions in shallow water, is that the whole eastern seaboard of North America was affected by cold currents arising from southern polar ice caps, and thus was a temperate shelf environment.

Morphology, faunas and genesis of Ordovician hardgrounds from Southern Ontario, Canada

Abstract We have used associations of different microfacies to define facies (or microfacies associations) which form reasonably well-defined sequences, which we infer, from analogies with recent and ancient carbonate environments, to have been deposited in a shelf environment characterized by small-scale topographic differentiation into shoal, slope and basinal environments. Shoal environments are characterized by typically cross-bedded, well-sorted bioclastic sands, with intershoal areas consisting of interbedded bioclastic sands and heavily bioturbated finer-grained carbonates. Slope and “basinal” environments are typically represented by “proximal” and “distal” cycles respectively. These we compare with deposits of carbonate ramp bypass channels, and with the more thoroughly studied deep-water clastic submarine fans. Many of the strong variations in environmental energy in these proximal and distal cycles can be attributed to migration of channels on the fans and the effect of funnelling of storm surges down the channels. Although hardground morphology and faunas are mostly related to local effects such as intensity of scouring, time of exposure, topographic differentiation of the surface and other factors, differing hardground types tend to be found in different environments. Smooth and rolling hardgrounds occur in the deeper distal environments, where the beds were subject to only slight scour and often limited exposure before renewed sedimentation. Hummocky and undercut hardgrounds are characteristic of the middle parts of proximal cycles, where they developed marginally to the main bypass channel, and in intershoal areas. Both these areas are sites of intermittent sedimentation and moderate turbulence, where cemented beds may be exposed for some time in environments optimal for attached benthos. These hardgrounds usually contain the most diverse hardground biotas. Pebbly and reworked hardgrounds occur in coarse, basal units of proximal cycles, which are interpreted as the grain-flow fillings of the central parts of bypass channels, though isolated examples occur in intershoal areas and in the higher parts of proximal channels. These hardgrounds contain low-diversity faunas, reflecting the stresses imposed by intermittent or constant abrasion; though some contain more diverse faunal assemblages formed after redeposition.

Late Cretaceous emplacement of the Indus suture zone ophiolitic mélanges and an Eocene-Oligocene magmatic arc on the northern edge of the Indian plate

We report three40Ar/39Ar dates (on stratigraphically located samples) of 82±6Ma from a syenite cutting the Indus suture zone ophiolitic melange and about 39 and about 45 Ma from granodiorite intrusions north of the suture zone. Sedimentological observations indicate Eocene to Miocene deposition of coarse clastics by very large braided and meandering streams in a continental back-arc setting. These observations suggest that the ophiolitic melanges of the Indus suture zone were emplaced in the late Cretaceous, shortly after a major change in plate motions in the Indian Ocean: they further suggest that an Andean-type magmatic arc developed on the northern edge of the Indian plate during the Eocene and Oligocene.

The geology and petroleum potential of the North Afghan platform and adjacent areas (northern Afghanistan, with parts of southern Turkmenistan, Uzbekistan and Tajikistan)

The North Afghan platform has a pre-Jurassic basement unconformably overlain by a Jurassic to Paleogene oil- and gas-bearing sedimentary rock platform cover, unconformably overlain by Neogene syn- and post-orogenic continental clastics. The pre-Jurassic basement has four units: (1) An ?Ordovician to Lower Devonian passive margin succession developed on oceanic crust. (2) An Upper Devonian to Lower Carboniferous (Tournaisian) magmatic arc succession developed on the passive margin. (3) A Lower Carboniferous (?Visean) to Permian rift–passive margin succession. (4) A Triassic continental magmatic arc succession. The Mesozoic–Palaeogene cover has three units: (1) A ?Late Triassic to Middle Jurassic rift succession is dominated by variable continental clastics. Thick, coarse, lenticular coal-bearing clastics were deposited by braided and meandering streams in linear grabens, while bauxites formed on the adjacent horsts. (2) A Middle to Upper Jurassic transgressive–regressive succession consists of mixed continental and marine Bathonian to Lower Kimmeridgian clastics and carbonates overlain by regressive Upper Kimmeridgian–Tithonian evaporite-bearing clastics. (3) A Cretaceous succession consists of Lower Cretaceous red beds with evaporites, resting unconformably on Jurassic and older deposits, overlain (usually unconformably) by Cenomanian to Maastrichtian shallow marine limestones, which form a fairly uniform transgressive succession across most of Afghanistan. (4) A Palaeogene succession rests on the Upper Cretaceous limestones, with a minor break marked by bauxite in places. Thin Palaeocene to Upper Eocene limestones with gypsum are overlain by thin conglomerates, which pass up into shales with a restricted brackish-water ?Upper Oligocene–?Lower Miocene marine fauna. The Neogene succession consists of a variable thickness of coarse continental sediments derived from the rising Pamir mountains and adjacent ranges. Almost all the deformation of the North Afghan platform began in the Miocene. Oil and gas traps are mainly in Upper Jurassic carbonates and Lower Cretaceous sandstones across the entire North Afghan block. Upper Jurassic carbonate traps, sealed by evaporites, occur mainly north of the southern limit of the Upper Jurassic salt. Lower Cretaceous traps consist of fine-grained continental sandstones, sealed by Aptian–Albian shales and siltstones. Upper Cretaceous–Palaeocene carbonates, sealed by Palaeogene shales are the main traps along the northern edge of the platform and in the Tajik basin. Almost all the traps are broad anticlines related to Neogene wrench faulting, in this respect, like similar traps along the San Andreas fault. Hydrocarbon sources are in the Mesozoic section. The Lower–Middle Jurassic continental coal-bearing beds provide about 75% of the hydrocarbons; the Callovian–Oxfordian provides about 10%; the Neocomian a meagre 1%, and the Aptian–Albian about 14%. The coal-bearing source rocks decrease very markedly in thickness southwards cross the North Afghan platform. Much of the hydrocarbon generation probably occurred during the Late Cretaceous–Paleogene and migrated to structural traps during Neogene deformation. Since no regional structural dip aids southward hydrocarbon migration, and since the traps are all structural and somewhat small, then there is little chance of very large petroleum fields on the platform. Nevertheless, further studies of the North Afghan platform should be rewarding because: (a) the traps of strike–slip belts are difficult to find without detailed exploration; (b) the troubles of the last 20 years mean that almost no exploration has been done; and, (c) conditions may soon become more favorable. There should be ample potential for oil, and particularly gas, discoveries especially in the northern and western parts of the North Afghan platform.

Paleoenvironments of the mid-Ordovician (Upper Caradocian) Trenton limestones of southern Ontario, Canada: Storm sedimentation on a shoal-basin shelf model

The mid-Ordovician (Caradocian) limestones of southern Ontario were deposited on a shelf undergoing collision with a magmatic arc. Within the general deepening-upwards sequence, shoals and islands complicate the facies patterns. Around these shoals and islands, carbonate sediments can be divided into nine lithotypes reflecting shallow agitated, to deep, quiet marine environments. Many of these lithotypes show good evidence of storm deposition. The lithotypes can be grouped into natural associations which define shoal, intershoal, slope and basinal facies—though the basins were probably less than 100 m deep. The closest recent analogues of these Ordovician environments occur on the Arabian shelf of the Persian Gulf, and on the Sahul shelf of northern Australia which is undergoing collision with the Banda arc. In both these environments, local shoal-basin shelf topography controls the detailed carbonate shelf sedimentation, which on a large scale is controlled by storm and tsunami effects on a seaward sloping ramp. Such shoal-basin and ramp models now seem more suitable in explaining carbonate facies in epeiric seas, than the flat slope models previously proposed. Glacio-eustatic sea-level changes may have controlled the larger aspects of carbonate sedimentation on the Ordovician shelf, as they did and continue to do now, on the recent shelves. Such changes may explain the localization of the variety of Ordovician hardgrounds which we previously described.

Sedimentology, petrography and tectonic significance of the shelf, flysch and molasse clastic deposits across the Indus Suture Zone, Ladakh, NW India

Abstract In the Ladakh area of India, a passive Triassic to Lower Cretaceous continental margin is indicated by Indian-shield-derived clastics on the shelf and Atlantic-type turbidites off the continental margin. Mid-Cretaceous initiation of ocean closing is reflected in Pacific-type flysch and associated island are volcanics, which were initially emplaced over the northern Indian continental margin in late Cretaceous times-resulting in the formation of a fore-deep in which flysch and minor continental molasse accumulated briefly during the late Cretaceous. These transient uplifts were, however, rapidly destroyed for by the latest Cretaceous to latest Palaeocene, uniform carbonate sediments were being laid down over the area. With the early Eocene, the development of a second fore-deep, this time filled with very thick flysch and molasse sediment, indicates a major uplift of the northern Indian margin, which we attribute to the development of an Andean-type magmatic arc on the northern edge of the Indian plate. Uplift and molasse sedimentation in this fore-deep continued through the Oligocene and Miocene, when the collision of India and Asia caused extensive deformation of all the sequences and the shift of molasse sedimentation southwards to the Himalaya foothills and Indo-Gangetic plain.

Problems in applying preservation, facies and sequence models to Sinian (Neoproterozoic) glacial sequences in Australia and Asia

Abstract The Sinian (800–530 Ma) is an extraordinary period of earth history characterized by the transition from simple to complex multicellular organisms and by extreme variations in climate, eustasy and possibly tectonics. Understanding Sinian stratigraphy may help interpret the co-evolution of life and environment during the period. Though Sinian successions are poorly dated and difficult to correlate, preservation, facies and sequence models based on Mesozoic-Recent successions may be used to interpret them. Preservation models (based on basin subsidence) permit estimates of how thick and complete successions can be in different tectonic settings, and how long it took for them to be deposited. Facies models for rift, craton, shelf, slope and basin environments help in identification of these different tectonic settings. Sequence models facilitate interpretation of the interplay of tectonic subsidence, eustasy, rates of sedimentation and climate. Such models, applied to the Sinian successions of Australia and Asia indicate the following. Great shallowing upwards megacycles occur in Sinian successions and each ends with regional glaciations. Deep rift basins formed around 850–800 Ma preserve the oldest megacycles. But these basins soon were filled with rapidly deposited glaciogenic sediments. Relatively complete sequences are found only in deep basins subject to repeated rifting. Cratons and shallow shelves preserve only the youngest megacycles. Older deposits have been removed by younger glacial advances. Deep shelves and slopes preserve one to three megacycles depending on the timing of the start of passive margin subsidence. Megasequences can be correlated based on limited radiometric dating and inferred regional distribution of unique rock types, but these sequences are not developed to the same extent in all blocks. Problems in correlating the four great megacycles recognized elsewhere with those of Asia will persist until the requisite facies analysis and U/Pb dating of interbedded volcanics are done.