The Paleogene record of Himalayan erosion: Bengal Basin, Bangladesh
Yani NajmanM. J. BickleMarcelle K. BouDagher‐FadelAndrew CarterEduardo GarzantiMaxence PaulJ.R. WijbransE. WillettGretchen D. OliverR. ParrishS. H. AkhterR. AllenSergio AndóEmdad ChistyLaurie ReisbergGiovanni Vezzoli
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Paleogene
Diachronous
BENGAL
Detritus
Diachronous
Extensional tectonics
Tectonic subsidence
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Abstract The early Cenozoic topography of the northern Tibetan plateau remains enigmatic because of the paucity of independent paleoelevation constraints. Long‐held views of northward propagating deformation imply a low Paleogene elevation, but this prediction is speculative. We apply flexural modeling to reconstructed Paleogene isopach data obtained from the Qaidam basin, which requires a larger topographic load in the Qilian Shan and a smaller load in the Eastern Kunlun Shan. Incorporating knowledge of proto‐Paratethys marine incursions in the Paleogene Qaidam basin, we infer a topographically low (0.4–1.0 km) Eastern Kunlun Shan and a higher (0.4–1.5 km) Qilian Shan during the Paleogene. This implied paleo‐relief contrasts with previous predictions and suggests more recently, Neogene surface uplift in the Eastern Kunlun Shan has been more significant than in Qilian Shan, highlighting diachronous growth of the northern Tibetan plateau. The low‐moderate paleoelevation implies a warmer and more humid climate in Northern Tibet during the Paleogene.
Paleogene
Diachronous
Neogene
Isopach map
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Diachronous
Magnetostratigraphy
Molasse
Neogene
Passive margin
Mountain formation
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Abstract We describe a late Miocene to early Pliocene axial drainage system in the East Carpathian foreland, which was an important sediment supplier to the Black Sea and the Dacian Basin. Its existence explains the striking progradation of the northwest Black Sea shelf prior to the onset of sediment supply from the continental-scale Danube River in the late Pliocene to Pleistocene. This axial drainage system evolved due to the diachronous along-strike evolution of the Carpathians and their foreland; continental collision, overfilling, slab breakoff, and subsequent exhumation of the foreland occurred earlier in the West Carpathians than in the East Carpathians. After overfilling of the western foreland, excess sediment was transferred along the basin axis, giving rise to a 300-km-wide by 800-km-long, southeast-prograding river-shelf-slope system with a sediment flux of ∼12 × 103 km3/m.y. Such late-stage axial sediment systems often develop in foreland basins, in particular, where orogenesis is diachronous along strike. Substantial lateral sediment transport thus needs to be taken into account, even though evidence of these axial systems is often eroded following slab breakoff and inversion of their foreland basins.
Diachronous
Progradation
Neogene
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Late Permian Karoo Basin tectonics in South Africa are reflected as two fining-upward megacycles in the Balfour and upper Teekloof formations. Foreland tectonics are used to explain the cyclic nature and distribution of sedimentation, caused by phases of loading and unloading in the southern source areas adjacent to the basin. New data supports this model, and identifies potential climatic effects on the tectonic regime. Diachronous second-order subaerial unconformities (SU) are identified at the base and top of the Balfour Formation. One third-order SU identified coincides with a faunal turnover which could be related to the Permo-Triassic mass extinction (PTME). The SU are traced, for the first time, to the western portion of the basin (upper Teekloof Formation). Their age determinations support the foreland basin model as they coincide with dated paroxysms. A condensed distal (northern) stratigraphic record is additional support for this tectonic regime because orogenic loading and unloading throughout the basin was not equally distributed, nor was it in-phase. This resulted in more frequent non-deposition with increased distance from the tectonically active source. Refining basin dynamics allows us to distinguish between tectonic and climatic effects and how they have influenced ancient ecosystems and sedimentation through time.
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Tectonic phase
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We have constrained the time-space migration of the Zagros foredeep basin by performing Sr isotope stratigraphy on 31 samples of marine macrofossils from Neogene sediments now exposed in the Zagros mountain belt in southwest Iran. Our results show that these deposits (represented mainly by the Mishan Formation) are strongly diachronous, with ages ranging between 17.2 ± 0.2 and 1.1 ± 0.1 Ma. These deposits are older in the west (Dezful region) and become progressively younger towards the south and the south-east (Fars region). Our results show that the marine foredeep was replaced by a fluvial sedimentary environment between ca. 14 and 12 Ma in the western sector, while this occurred between ca. 8 and 1 Ma in the eastern sector, becoming younger towards the south. These results enable us to show that the foreland basin migrated perpendicular to the orogen at rates of between 17.5 and 50 mm year−1 throughout the Neogene, exceeding migration rates in the Alps, Pyrenees, Apennines and Himalayan foreland basins. The sporadically elevated rates in the Zagros appear to be related to times when major widely spaced pre-existing basement faults became reactivated. Finally, our results, when combined with published data, have enabled us to establish a new chronostratigraphic diagram for the Neogene portion of the Zagros foreland basin. Our study highlights that foreland basins are extremely dynamic settings where depocentres and palaeoenvironments may change rapidly in both time and space in relation to migrating deformation.
Neogene
Diachronous
Basement
Isopach map
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In the Italian Southern Alps, the Lombardic and Venetian chains and related foreland are separated by the Lessini-Berici-Euganei foreland block, which is unaffected by the Neogene-Quaternary shortenings. However, the Veneto-Friuli alluvial plain to the east of this foreland block has been affected by a poly-phase evolution since the Mesozoic era and represents the foreland of three surrounding chains. The area was affected by several flexural cycles related to the diachronous build-up of the External Dinarides to the east (Late Cretaceous-Late Eocene), the Eastern Southern Alps to the north (Late Oligocene-Quaternary), and the Northern Apennines to the southwest (Middle Miocene-Quaternary). The last two chains are currently active, although at different rates. The western margin of the foreland is marked by the Schio-Vicenza fault, which divides the deformed foreland (Veneto-Friuli alluvial plain) from the undeformed foreland (Lessini and Berici Mountains and Euganei Hills). The aim of this work is to unravel the architecture and evolution of this boundary using 2D seismic sections and deep wells acquired by ENI for hydrocarbon exploration. Approximately 1,000 km of seismic lines were interpreted and 10 wells were used to calibrate the seismic interpretation. Seven seismic sections that are sub-orthogonal to the main faults were selected to obtain geological cross-sections through a depth conversion process. The collected data display a complex buried fault system (Schio-Vicenza fault system) that extends with a NW-SE trend from the foot of the Prealps to the Po Delta. In the cross sections, the movement of the fault shows a vertical component with down-throw of its eastern side (hanging wall block). Some faults display a Mesozoic extensional displacement in accordance with the Mesozoic basin and swell architecture of the area. In addition, the Pliocene throw increases from the southeast to the northwest.Therefore, the Schio-Vicenza fault system can be interpreted as an inherited Mesozoic structure that reactivated during the Neogene shortening of the area. In particular, this fault system appears to have been mainly active during the Pliocene-Quaternary flexural cycle that is related to the Northern Apennines subduction, which is when the fault system accommodated a scissor movement between the Veneto-Friuli foreland and the Lessini-Berici-Euganei block.
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Neogene
Alluvial fan
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The Venetian basin evolved in two distinct stages: (a) from the Chattian to the Langhian it behaved as a foreland basin of the Dinaric range and evolved under rather weak tectonic control, due to low rates of thrust propagation. Positive features related to the growth or rejuvenation of structural highs acted as a focus for the localization of shelf sand ridges during transgressive stages, while major regressive stages are marked by progradation of significantly diachronous sandstone bodies (during the late Aquitanian and the late Burdigalian) and by evidence of partial basin isolation and faunal endemism. Such events can be correlated with major discrete pulses of thrust activity in the outer Dinarides. From Chattian to Langhian an important 'external' source of clastics was represented by the axial zone of the Alpine range (Austroalpine and Penninic units) subjected to rapid uplift and denudation; (b) from Serravallian onwards a drastic change in the tectonic framework of the Venetian basin occurred: the axis of subsidence shifted in position and trend, and the basin was thereafter incorporated in the South Alpine kinematic system. The latter evolved in a context of oblique convergence which resulted in a migration of thrusting and of foreland subsidence around the South Alpine compressional belt from the west (Lombard foredeep) toward the east (Venetian foredeep) during the time span from late Oligocene to Recent. This shifting pattern was accommodated by wrenching along the Insubric (Periadriatic) lineament and along sets of conjugate transverse faults which segmented the Southern Alps in a number of sub-areas with significantly diachronous tectono-sedimentary evolution. On a larger scale, the South Alpine and Apenninic domains show a close correspondence in timing of major events and in stepwise eastward shifting of deformation and foredeep subsidence, suggesting an evolution within the same megashear system. From Serravallian onwards an imbricate stack of overthrusts rapidly 'prograded' SSE-wards in the eastern South Alpine area and shed a huge amount of clastics into the Venetian basin. Unconformity-bounded sequences developed in response to discrete pulses of thrust progradation, with localized angular unconformities and along-strike differential subsidence resulting from the interplay of thrust pulses with strike-slip motion along transverse faults. Although tectonic control is pervasive, especially in the second stage of basin evolution, the effect of major eustatic events can still be recognized in the trend of sedimentation.
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Molasse
Massif
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Orogenesis plays a crucial role in creating, modifying, and disrupting sediment routing fairways. Reconstructing past source to sink systems is a critical step to unravel the geological history, mandatory to disentangle tectonic and climatic forcings, and to understand the perturbations that affected the overall system. The Paleocene-Eocene period is a debated early stage of the early Pyrenean orogeny, characterized by the piggy-back style tectonic partitioning of sub-basins as the deformation front propagates towards the southern foreland. An important issue is to understand how sedimentary systems reacted and reorganized in response to this dynamic scenario. and whether the stratigraphic and sediment provenance results align with the envisioned scenario.In this paper we aim at contributing to the paleogeographic, sedimentologic and tectono-sedimentary evolution of the South-Pyrenean foreland basin by reviewing the chronostratigraphic framework of the basin infill in its south-central sector (the Ager sub-basin in the Serres Marginals thrust sheet). We built five new magnetostratigraphic sections, which together encompass most of the the Paleogene record, aimed to best reconcile magnetostratigraphic data with the defined biostratigraphic framework of the region including marine Shallow Benthic Zones (SBZ biozones) and continental mammalian localities (MP levels). Detailed trends in subsidence show the development and evolution of the foreland depozones, from forebulge to foredeep and wedge-top setting, relative to the successive emplacement of the Montsec and Serres Marginals thrust sheets.A correlation with the eastern portion of the foreland basin (Lower-Middle Pedraforca and Cadí Thrust Sheets) and adjacent northern Graus-Tremp basin (Montsec Thrust Sheet) was feasible and seeks to contribute to the tectono-sedimentary evolution of the South-Pyrenean foreland in the light of a source to sink approach. Our proposal includes a new paleogeographic evolution of the area in a series of paleogeographic maps from Late Thanetian to Late Cuisian times.
Diachronous
Paleogene
Tectonic subsidence
Magnetostratigraphy
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