Fluvial Sedimentary Rocks Research Articles

Reconstruction of the Cretaceous continental arc–trench system of the Japanese Islands: a basis for Cretaceous palaeoenvironmental studies

Abstract Spatiotemporal distributions of Cretaceous rocks differ markedly between the SW and NE Japan arcs. However, four parallel zonal arrangements of rocks are recognized broadly throughout both arcs: mostly non-marine sedimentary rocks in back-arc/intra-arc basins; granitic and volcanic rocks in magmatic arcs; predominantly marine and subordinately fluvial sedimentary rocks in forearc basins; and sedimentary rocks of turbiditic and mélange facies in accretionary complexes. These zones constituted a palaeo-Japan continental arc–trench system during the Cretaceous. We describe and correlate 71 Cretaceous back-arc/intra-arc and forearc basinal successions from Kyushu (south) to Hokkaido (north) islands, including a southern Sakhalin and two Kuril Arc (eastern Hokkaido) successions. Stratigraphic ranges and major sedimentary facies are generally similar between the SW and NE Japan arcs, except for the pre-Aptian Lower Cretaceous in Hokkaido of NE Japan, suggesting continuity throughout the two arcs during the Cretaceous. Although Cretaceous strata are sporadically exposed in northern Honshu, NE Japan, interpretation of seismic sections suggests that Cretaceous forearc sedimentary rock measuring several tens of kilometres laterally are developed offshore beneath the present Pacific forearc. In contrast, Cretaceous forearc strata in southern SW Japan are distributed along two narrow belts that may have been deformed by post-Early Miocene tectonism.

Lithostratigraphy of the Serra Geral Formation in the northern portion of the Paraná-Etendeka Igneous Province: A tool for tracking Early Cretaceous paleoenvironmental changes

A detailed lithofacies analysis in Continental Flood Basalt Provinces allows the comprehension of their evolution and paleoenvironmental significance. In the northern portion of the Paraná-Etendeka Igneous Province (PEIP), in Brazil (Uberlândia-Araguari area), a volcanological and stratigraphic approach provided evidence that the basaltic volcanic succession is not monotonous, involving different lava flow morphologies and architecture. The volcanic succession reaches a thickness of ca. 300 m and includes four lithofacies associations: pillow lavas, compound pahoehoe, simple pahoehoe, and sediment-matrix basalt breccia. Pillow lavas, simple pahoehoe, and compound pahoehoe characterize the onset of volcanic activity in the area. Sediment-matrix basalt breccia and thick simple pahoehoe lavas occur in upper stratigraphic levels. Some of these thick igneous bodies can represent invasive lavas or shallow intrusions. In the study area, the common presence of volcanic deposits and structures formed in wet conditions is evidenced by the occurrence of pillow lavas, undulatory columns, and interleaved lacustrine and fluvial sedimentary rocks. These deposits reflect different environmental conditions from those previously described as arid in other portions of the PEIP. Low-temperature post-depositional alteration has affected the volcanic and sedimentary rocks of the study area, obliterating many of the primary sedimentary structures. The subaerial and subaqueous lavas are chemically similar, and compatible with high-TiO2 Pitanga magma type. The Uberlândia-Araguari area and the southern part of the PEIP have a similar stratigraphic style both characterized by the dominance of compound pahoehoe lavas at the base and by an intermittent volcanism, which is evidenced by the intercalation of sedimentary deposits with lavas.

Geology of Aeolis Dorsa alluvial sedimentary basin, Mars

ABSTRACTAeolis Dorsa is a topographic depression, ∼800 km east of Gale Crater, located along the Mars’ dichotomy boundary. This area hosts a set of fluvial sedimentary rocks displaying an exceptional record of depositional environments and fluvial channels patterns that suggest the presence of a large amount of surface and/or subsurface water. We interpreted the plain as an ancient waterlogged environment, a sedimentary basin passing into distal depositional environments. Regional mapping of the area revealed the presence of a large-scale fluvial system that points to a long-term and extensive hydrological cycle. A significant wet period with changing environmental conditions in Hesperian/Amazonian occurred in the study area diverging from the present-day climate. Our map (Main Map) contributes to the understanding of past climatic conditions on Mars. Moreover, it provides an interesting perspective for future missions looking for evidence of present-day and/or past extraterrestrial organisms as the life as we know it needs liquid water.

Detrital zircon U–Pb geochronological and sedimentological study of the Simao Basin, Yunnan: Implications for the Early Cenozoic evolution of the Red River

The paleo-Red River is suggested to have been a continental-scale drainage system connecting the Tibetan Plateau to the South China Sea. However, the evolution of the paleo-Red River is still under debate. This study presents new results from sedimentological analyses and detrital zircon U–Pb geochronologic data from fluvial sedimentary rocks of Paleocene to Oligocene age of the Simao Basin to constrain the nature of the paleo-drainage system of the Red River. The detrital zircon U–Pb results reveal multiple age groups at 190–240 Ma, 260–280 Ma, 450–540 Ma, 1700–1900 Ma and 2400–2600 Ma for the Paleocene to late Eocene Denghei Formation (Fm.), but only one conspicuous peak at 220–240 Ma for the late Eocene–Oligocene Mengla Fm. Provenance analyses illustrate that the former likely had source areas that included the Hoh-Xil, Songpan-Ganzi, northern Qiangtang, Yidun and western Yangtze Terranes, which are consistent with the catchments of the Upper and Lower Jinshajiang Segments, whereas the latter mainly transported material from a limited number of sources, such as the Lincang granitic intrusions west of the Simao Basin. Integrated with available detrital zircon U–Pb geochronologic and paleogeographic data, our study suggests the existence of a paleo-Red River during the Paleocene to late Eocene that was truncated and lost its northern sources after approximately 35 Ma, due to left-lateral strike-slip faulting of the Ailao Shan–Red River and clockwise rotation of the Lanping–Simao Terrane.

Seismic-stratigraphic architecture of the Oligocene-Pliocene Camaná Formation, southern Peruvian forearc (Province of Arequipa)

The Camaná-Mollendo Basin is an active-margin depression ~NW-SE elongated, which is located in the forearc of southern Perú and extends from the Coastal Cordillera to the Perú-Chile Trench. This basin consists of a grabens and half-graben complex, filled with deltaic and fluvial sedimentary rocks of the Oligocene-Pliocene Camaná Formation (~500 m thick). An integration of compiled onshore stratigraphic logs, reinterpreted 2D seismic offshore information, sediment provenance data, and previous zircon U-Pb geochronology on volcanic reworked ash supports a refined tectono-chronostratigraphic framework for the whole Camaná-Mollendo Basin fill. To complete this integration we needed firstly to elaborate a geological reinterpretation of seismic offshore data and highlight their most prominent features (i.e., erosive surfaces). This step allowed establishing a first correlation between onshore and offshore deposits of Camaná Formation by means of their sequence boundaries, giving as result a consistent division for Camaná Formation: (i) “CamA Unit” (coarse-grained deltas) and (ii) “CamB Unit” (fluvial deposits). CamA Unit is further subdivided into three subunits based on minor erosive surfaces (i.e., A1: Oligocene, A2: Early Miocene, and A3: Middle Miocene). CamA reflects prograding geometry (subunits A1 and A2) as well as onlapping geometry (subunit A3). CamB Unit (Late Miocene to Pliocene) consists of high-energy hyperpycnal flows composed of fluvial conglomerates in onshore, which very possibly changes to progradational deltaic in offshore. Each one of these units and subunits extends offshore and preserves similarities in depositional geometry and sequence boundaries with Camaná Formation onshore. Subunits A1 and A2 observed in offshore are grouped in this paper as “A1+A2” (Oligocene to Middle Miocene) because they show similar progradational geometry and it is difficult to differentiate them from each other. A regressive systems tract (RST) represents these subunits. These deposits reach up to ~2.5 km thick, and they are intensely affected by normal faulting associated to pinch-out depositional geometry. Strata of subunit A3 (Middle Miocene) reflect a transgressive systems tract (TST), and blanket the entire basin with fine-grained sediments. These deposits are up to ~1 km thick, being less affected by synsedimentary tectonic and show minor effects of synsedimentary tectonics. Finally, deposition of CamB Unit (Late Miocene to Pliocene) occurred during a new regressive systems tract (TST), which turned to progradational geometry similar to deltaic deposits in offshore, and according to seismic lines they are much less affected by synsedimentary faulting. Stratigraphic boundaries between “A1+A2” and A3, and between A3 and CamB observed in onshore outcrops are used here as tools to differentiate, correlate and predict the main depositional geometries in offshore. High-frequency seismic reflectors represent such boundaries and support divisions and subdivisions within Camaná Formation. These boundaries are also used to define depocentres of Camaná Formation along the entire Camaná-Mollendo Basin, where the thickests are located in the proximity of the large river mouths (e.g., Planchada, Camaná, and Punta de Bombón). Strata of subunits “A1+A2” are considered as potential reservoir for hydrocarbon due to their high rate of sediment accumulation. Deposits of A3 are transgressive and they are considered as potential potential seal rock. Structurally, Camaná-Mollendo Basin is composed of graben and half-graben components ~NW-SE-oriented, typical of a trantensional tectonic regime.

Chemostratigraphic age model for the Tornillo Group: A possible link between fluvial stratigraphy and climate

The expression of global climate trends (106 to 107years) and events (105years) in terrestrial sedimentary sections can only be assessed from long, continuous continental records. Such records are rare due to the paucity of well-dated terrestrial deposits covering millions of years. This study uses isotope chemostratigraphy to develop an age model for fluvial sedimentary rocks of the Tornillo Group (Tornillo Basin, TX) and to decipher the potential expression of global climate variations in this sub-tropical paleo-fluvial system. The carbon isotope ratio (δ13C) of Tornillo Group pedogenic carbonate correlates well with δ13C variations from benthic foraminifera and suggests deposition between ~69Ma and ~52Ma for the studied stratigraphic interval. Higher sediment accumulation rates occurred in the greenhouse period of the early Eocene than during the cooler middle Paleocene. Carbon isotope excursions associated with Eocene hyperthermals were not identified, but their predicted stratigraphic positions coincide with thick sand bodies. We interpret the long-term sediment accumulation rates and rapid shifts in facies distribution as reflecting change in sediment supply in the basin, possibly driven by fluctuations in precipitation seasonality and intensity driven associated with global temperature changes. Carbon and oxygen isotope ratios from pedogenic carbonates display a strong correlation throughout the section, interpreted as an expression of coupling between the hydrological cycle and soil processes. Those observations suggest that global climate variations, along with tectonic and eustatic sea-level changes, may be an important control of stratigraphic variations of the Tornillo Group at multiple timescales.

Structural Reconstruction of the Flin Flon Volcanogenic Massive Sulfide Mining District, Saskatchewan and Manitoba, Canada

The Flin Flon mining district is part of a greenstone belt, the Flin Flon-Glennie Complex, in the Paleoproterozoic Trans-Hudson orogen. Its tectonic history began prior to 1872 Ma with the development of regional folds—the faulted F1 Burley Lake syncline and F2 Hidden Lake syncline—during D1 and D2 intraoceanic accretion of the 1888 Ma Flin Flon arc to other volcanic terranes. Amalgamation of the Flin Flon to Glennie terrane, possibly during D3, produced a W-propagating thrust-fold belt and basins in which fluvial sedimentary rocks were deposited between 1847 Ma and 1842 Ma. As the fold-thrust belt migrated westward, these rocks were incorporated into a stack of E-dipping thrust sheets bounded by NNW-striking thrust faults (1920 fault) and internally folded by W-verging folds (Pipeline, Mud Lake, and Grant Lake synclines). Subsequent D4 collision of the Flin Flon-Glennie Complex with the Archean Sask microcontinent was broadly coeval with but outlasted the emplacement of 1840 Ma Phantom Lake dikes. D4 produced a second truncating fold-thrust system characterized by N-directed thrust faults (Club Lake and Railway faults) and E-trending folds (Flin Flon Creek syncline). These folds were overprinted by two regional cleavages, and the thrust faults were reactivated as oblique-slip shear zones, either late during the same collisional event (D5) or during terminal collision (D6) of the Sask craton and Flin Flon-Glennie Complex with the Superior craton at 1.83 to 1.79 Ga. The Flin Flon volcanogenic massive sulfide ore system was thrust-imbricated during D3 and D4, and ore lenses were stretched parallel to a regional, SE-plunging, stretching lineation that formed during D4 and was later modified during D5.

Paleosol and dolocrete associated clay mineral occurrences in siliciclastic red sediments of the Late Miocene Kömişini Formation of the Tuzgölü basin in central Turkey

The Late Miocene was characterized by high global temperatures and partly to nearly complete desiccation of the Mediterranean Sea throughout the latter part of the Messinian age (the Messinian salinity crisis). The Late Miocene marine Mediterranean Sea area has been widely studied from different perspectives, but terrestrial paleoenvironmental and paleoclimatic records from the Mediterranean area are highly limited in their spatial range. To this end, the paleosols, dolocrete and fluvial sedimentary rocks of the Kömişini Formation in the northwestern part of the Tuzgölü basin are described in detail. Six lithofacies were identified as (A) clast-supported gravel, (B) massive (pebbly) sandstone, (C) massive mudstone, (D) disrupted mud, (E) disrupted conglomerate, and (F) disrupted matrix-supported conglomerate. The siliciclastic facies were interpreted as braidplain, and the distal part of braidplain deposits is associated with an ophiolitic area while the disrupted mud and disrupted conglomerate were identified as three types of paleosols: Inceptisols, Aridisol and Vertisols. The calcretes occur in the paleosols in nodular, powder, and massive forms. Clay minerals include smectite, chlorite, palygorskite, illite, and serpentine. Gypsum, smectite and chlorite are dominant minerals at the base and middle part of the section, whereas palygorskite and illite appear as the prevailing clay minerals in the upper sections. Smectite, illite and palygorskite precipitated in a vadose zone from strongly evaporated alkaline water rich in Si, Ca and Mg and poor in Al at increased temperatures and at an advanced stage of dolocrete and paleosol formation. K, Si, Mg, Al+Fe and Ca are required for smectite, illite, palygorskite and calcification. Given the increase in Ba, Cr, Ni and Co, these cations might have originated from ophiolite, carbonate and volcanic units. As a consequence, the vertical distribution of clay mineral types suggests the variations of paleoclimatic conditions during the deposition, and the relationships among the type of host rock, paleosol and dolocrete, which corresponds to nearly complete desiccation of the Mediterranean Sea.

Provenance and Depositional Ages of Late Paleogene Fluvial Sedimentary Rocks In the Central Rocky Mountains, U.S.A.

Abstract The erosion and basin-filling histories of the central Rocky Mountains (Rockies) are critical to the understanding of landscape evolution in the interior of western U.S.A. Ages and sources of fluvial deposits formed during late Paleogene allow the reconstruction of paleogeography within an improved chronostratigraphic framework. Here we use detrital-zircon and single-grain-zircon U-Pb geochronology to constrain the provenance and depositional ages of upper Paleogene fluvial deposits in the central Rockies. A total of 885 concordant detrital-zircon dates from ten fluvial sandstone samples show major age populations of 24–44 Ma, 920–1326 Ma, 1334–1825 Ma, and 1830–3314 Ma, and minor age populations of 45–218 Ma and 227–708 Ma. The youngest population was derived from distal ignimbrite eruptions in western and southwestern North America. The populations older than 45 Ma were derived directly from local Precambrian basement-cored uplifts and recycled from Paleozoic–lower Cenozoic strata. The maximum depositional ages based on the weighted mean U-Pb ages of the youngest clusters of detrital zircons are generally consistent with the available ash radiometric ages for samples of latest Eocene–Oligocene, a time interval with high ignimbrite eruption flux in western and southwestern North America, suggesting that detrital-zircon U-Pb ages can be used to tightly constrain the depositional ages of these strata when pure ash beds are not present and when the strata contain abundant concurrent magmatic zircons. Sandstone samples of latest Eocene age have more Archean zircons (> 2500 Ma) than samples of Oligocene age in the central Rockies, suggesting that the Archean cores of the Laramide ranges were largely exposed until the Oligocene. By combining these results with previously published detrital-zircon provenance results of late Paleogene eolian sandstones in the same region, we suggest that the gradual burial of the Archean cores during the Oligocene resulted from widespread eolian deposition initiated during the latest Eocene–early Oligocene. Generally eastward-flowing fluvial systems and eolian depositional processes supplied unlithified source sediments for each other and promoted sediment recycling during the Oligocene. Furthermore, the abundance of zircons with 37–25 Ma ages in the late Paleogene deposits shows that the ignimbrite eruptions in western and southwestern North America were most active during this time interval.

Isotopic evidence for biological nitrogen fixation by molybdenum-nitrogenase from 3.2Gyr.

Nitrogen is an essential nutrient for all organisms that must have been available since the origin of life. Abiotic processes including hydrothermal reduction, photochemical reactions, or lightning discharge could have converted atmospheric N2 into assimilable NH4(+), HCN, or NOx species, collectively termed fixed nitrogen. But these sources may have been small on the early Earth, severely limiting the size of the primordial biosphere. The evolution of the nitrogen-fixing enzyme nitrogenase, which reduces atmospheric N2 to organic NH4(+), thus represented a major breakthrough in the radiation of life, but its timing is uncertain. Here we present nitrogen isotope ratios with a mean of 0.0±1.2‰ from marine and fluvial sedimentary rocks of prehnite-pumpellyite to greenschist metamorphic grade between 3.2 and 2.75billion years ago. These data cannot readily be explained by abiotic processes and therefore suggest biological nitrogen fixation, most probably using molybdenum-based nitrogenase as opposed to other variants that impart significant negative fractionations. Our data place a minimum age constraint of 3.2billion years on the origin of biological nitrogen fixation and suggest that molybdenum was bioavailable in the mid-Archaean ocean long before the Great Oxidation Event.

Detrital zircon U–Pb age constraints on Cretaceous sedimentary rocks of Lingshan Island and implications for tectonic evolution of Eastern Shandong, North China

In order to clarify sedimentary environment and tectonic history of the Sulu orogenic belt, we date zircons from four sandstones and one interbedded rhyolite collected on Lingshan Island in East Shandong, North China. U–Pb age distributions of detrital zircon populations from the newly named Lingshandao Formation resemble that of overlying fluvial sedimentary rocks. Age distributions exhibit four main populations of Paleoproterozoic (2.5–2.3Ga), Mesoproterozoic (2.1–1.9Ga), Neoproterozoic (850–700Ma) and Early Cretaceous (138–121Ma) age, accompanied by sporadic occurrences of Archean, Paleozoic and early Mesozoic aged grains. Data indicate a depositional age of ∼121Ma for the Lingshandao Formation, which constrains the maximum depositional age of overlying fluvial sedimentary rocks. The Lingshandao Formation thus more likely correlates to the uppermost Laiyang Group of the Jiaolai basin. Proterozoic age populations indicate a mixture of material derived from the North China (NCB) and Yangtze Block (YZB) basement along with protolith material from the HP–UHP metamorphic units of the Sulu orogenic belt. Proterozoic age populations differ from those of the roughly coeval Jiaolai basin, which represents a terrestrial depositional environment to the north of the Sulu terrane. The YZB appears to be a significant source of detrital zircons, consistent with field observations.

Palynology, Stratigraphy and Geometry of the Pennsylvanian continental Santa Susana Basin (SW Portugal)

The continental Pennsylvanian Santa Susana Basin (SSB) is located along an important N-S strike shear zone (Santa Susana Shear Zone - SSSZ) that separates the Ossa-Morena and South Portuguese Zones (SW Portugal). This shear zone controlled the sedimentation of the basin and probably its post-sedimentary evolution. The lower (basal) unit is mostly conglomeratic and is known essentially from drill-hole data while the upper unit, that covers most of the current surface area, is composed by sandstones, shales and some coal beds. Fluvial sedimentary rocks seem to prevail, but evidences of lacustrine sedimentation can be found in restricted areas within the upper unit. Palynological data indicate that at least part of the sedimentation took place during the Early Kasimovian (Cantabrian) but the sporomorph content of different localities suggest that significantly older sediments (Late Moscovian -Early Bashkirian) may be present. Geometrical, structural, and cartographical evidences indicate that the SSB formed as a pull-apart basin along the SSSZ and was later partially uplifted during latest Varsican and Alpine orogenies.>

Late Cenozoic basalt and gabbro in the subsurface in the Phetchabun Basin, Thailand: Implications for the Southeast Asian Volcanic Province

Fragments of basaltic and gabbroic rocks were obtained in cuttings from 15 exploration wells in the Na Sanun area of the Wichian Buri Sub-basin of the Phetchabun Basin in central Thailand. The samples represent flows and sills in lacustrine and fluvial sedimentary rocks of the Lower to mid-Miocene Wichian Buri Group. Mafic igneous units were identified in the sections based on their typically high-amplitude seismic reflections, confirmed by the examination of several hundred well cuttings and magnetic susceptibility measurements. Cross-sections of the sub-basin were constructed on the basis of previously published subsurface interpretations, seismic and well data, and petrological observations. Basaltic flows A, B, E, and F have ages of ca. 2Ma, 16Ma, 24Ma and 18Ma, based on inferred stratigraphic position. Gabbroic sill C and dioritic sill G are inferred to be correlative at ca. 11.6Ma, and differ petrologically from ca. 12.8Ma gabbroic sill D. Major minerals in both basaltic and gabbroic samples are plagioclase (ca. An50), anorthoclase, and augite, with pervasive alteration to Na- and Ca- zeolite minerals and analcime. Leucodioritic sill G also contains amphibole and high Ti-phlogopite. Overall, the rocks show within-plate tholeiitic to alkalic characteristics, and show similarities to basaltic surface outcrops of similar ages in the Wichian Buri-Lop Buri area. No evidence was seen in the subsurface for the andesitic to rhyolitic rocks of similar ages that occur at surface, but their presence cannot be precluded based on our limited data.

Constraints on Cenozoic regional drainage evolution of SW China from the provenance of the Jianchuan Basin

Detrital zircon U‐Pb geochronology was applied to Cenozoic fluvial sedimentary rocks from the Jianchuan Basin, Yunnan Province, China to constrain the provenance and the nature of paleo‐drainage. Local geology testifies to a large river flowing through the Jianchuan Basin during the Paleogene and this previously has been linked to a paleo drainage system that connected the Qiangtang and Lhasa blocks to the South China Sea. The detrital zircon results from this study do not fit with this model and instead show provenance consistent with a river draining a watershed within the Songpan‐Garze Complex, most likely from the northeast. From the late Oligocene and thereafter zircon provenance records greater contributions from erosion of local sources that surround the basin including the South China Block and Yidun Arc rocks that suggest loss of the northern sources. The timing for these changes overlap with regional deformation related to strike‐slip faulting or displacement by shear strain rather than the later uplift associated with an expanding margin of the Tibetan Plateau.

The Use of the Common Pb Isotope Composition of Detrital K-Feldspar Grains as a Provenance Tool and Its Application to Upper Carboniferous Paleodrainage, Northern England

K-feldspar is a common detrital mineral in sandstones. It contains little U or Th, and hence the common Pb isotope composition of unaltered K-feldspar is that of the source rock. Pb isotope variations in igneous and metamorphic crustal rocks define broad spatial patterns that make the Pb signature of detrital K-feldspar grains a useful provenance tool. However, it is unclear how robust this signal is, and to what extent it can be modified by weathering, transport, burial diagenesis, and/or exposure to hydrothermal fluids. This study reports an evaluation of the technique, using two granite–arkose pairs (from Helmsdale, northern Scotland, and Shap, northwest England) where the composition of the detrital grains can be compared to feldspars in the granite from which they were derived. The results of this test have informed a pilot study using the Pb isotope composition of detrital K-feldspar in Upper Carboniferous fluvial sedimentary rocks of northern England to constrain the sediment routing. Pb isotope compositions have been measured using an in situ high-resolution single-grain technique that can distinguish original composition, heterogeneity, and subsequent alteration within individual sand grains. The results for the granite–arkose pairs show that the common Pb isotope signature of the detrital K-feldspar grains matches the in situ feldspars in the source granites. The signal is independent of grain size (medium-grained sand to granules), but alteration along a paleo-regolith at Helmsdale has resulted in perturbations in Pb composition and an increase in the errors on the analyses. The application of the technique to arkosic Carboniferous (Serpukhovian–Bashkirian) Millstone Grit Group sandstones in the Pennine Basin reveals two discrete populations of K-feldspars that cannot be distinguished petrographically. Matching of the compositions to potential sourcelands to the north and northeast of the basin suggest a far-traveled (> 500 km) source of K-feldspar from the Archean–Paleoproterozoic Lewisian rocks of northwest Scotland or their extension to the north, but also an equally important proximal source emanating from the Southern Uplands Belt of Scotland in what would have been the lower part of the hinterland drainage basin of the Millstone Grit.

A brief history of the Indus River

Abstract The Indus River system is one of the largest rivers on the Asian continent, but unlike the Ganges-Brahmaputra system, the drainage of the Indus is dominated by the western Tibetan Plateau, Karakoram and tectonic units of the Indus Suture Zone, rather than the High Himalaya. The location of the river system relative to the Indus Suture Zone explains the deep exhumation north of that line in the Karakoram, compared with the modest erosion seen further east in Tibet. The modern Indus cuts Paleogene fluvial sedimentary rocks of the Indus Group located along the Indus Suture Zone in Ladakh, northern India. After the final marine incursion within the Indus Group in the early Eocene (<54.6 Ma), palaeo-current indicators changed from a north-south flow to an axial, westward pattern, synchronous with a marked change in sediment provenance involving erosion of South Tibet. The Indus probably was initiated by early Tibetan uplift following the India-Asia collision. The river has remained stationary in the suture since Early Eocene time, cutting down through its earlier deposits as they were deformed by northward folding and thrusting associated with the Zanskar backthrust at c. 20 Ma. The Indus appears to have been located close to its present position within the foreland basin since at least Mid-Miocene time ( c. 18 Ma), and to have migrated only c. 100 km east since Early Eocene time. In the Arabian Sea Paleogene fan sedimentation was significant since at least Mid-Eocene time ( c. 45 Ma). Sediment flux to the mid fan and shelf increased during Mid-Miocene time (after 16 Ma) and can be correlated with uplift of the Murray Ridge preventing sediment flow into the Gulf of Oman, tectonic uplift and erosion in the Karakoram and western Lhasa Block, and an enhanced monsoon triggered by that same uplift. Sedimentation rates fell during Late Miocene to Recent time. The Indus represents 18% of the total Neogene sediment in the basins that surround Asia, much more than all the basins of Indochina and East Asia combined ( c. 11%). Unlike the rivers of East Asia, which have strongly interacted as a result of eastward propagating deformation in that area, the Indus has remained uninterrupted and represents the oldest known river in the Himalayan region.

Coeval sedimentation, magmatism, and fold-thrust belt development in the Trans-Hudson Orogen: geochronological evidence from the Wekusko Lake area, Manitoba, Canada

Lithological and structural mapping in the east Wekusko Lake area of the Flin Flon Belt, Trans-Hudson Orogen, suggested an intimate relationship between magmatism, fluvial sedimentation, and initiation of fold and thrust belt deformation. Conventional U-Pb geochronology of volcanic rocks in fault-bounded assemblages provides a minimum age of 1876 ± 2 Ma for McCafferty Liftover back-arc basalts, and ages of between 1833 and 1836 Ma for the Herb Lake volcanic rocks. A rhyolite which unconformably overlies Western Missi Group fluvial sedimentary rocks has complex zircon systematics. This rock may be as old as about 1856 Ma or as young as 1830 Ma. The sedimentary rocks overlying this rhyolite are locally intercalated with 1834 Ma felsic volcanic rocks, and yield sensitive high resolution ion microprobe (SHRIMP) U-Pb and Pb-evaporation detrital zircon ages ranging from 1834 to 2004 Ma. The Eastern Missi Group is cut by an 1826 ± 4 Ma felsic dyke, and contains 1832-1911 Ma detrital zircons. The dominant source for detritus in the Missi Group was the Flin Flon accretionary collage and associated successor arc rocks. The fluvial sedimentary rocks and the Herb Lake volcanic rocks were essentially coeval, and were then incorporated into a southwest-directed fold and thrust belt which was initiated at about 1840 Ma and active until at least peak regional metamorphism.

Coeval sedimentation, magmatism, and fold-thrust development in the Trans-Hudson Orogen: propagation of deformation into an active continental arc setting, Wekusko Lake area, Manitoba

Early structures in the southeastern part of the Trans-Hudson Orogen (THO), Wekusko Lake area, Manitoba, comprise northwest-verging folds and a set of steep, southeast-dipping faults that occur at a low angle to bedding. Kinematic indicators are rare, however, the vergence of the associated folds supports reverse movement on the faults. These folds and faults formed prior to the peak of regional metamorphism and therefore must correlate with the widely recognised deformational (D2) event which developed pre- to syn-metamorphism and involved development of a regional, south- to southwest-directed fold-thrust belt. The D2 folds and reverse faults in the study area are interpreted to have originally formed as gently dipping, southwest-verging structures, i.e., as inclined folds and thrust faults, which were then rotated into a steep, northwest-verging orientation during later deformation (D3). In the southeastern THO, regional D2 fold and thrust structures are cut by ca. 1839-1830 Ma plutons, indicating that D2 initiated prior to ca. 1839 Ma. Recognition of these structures in the study area, and their relationships to key stratigraphic units, has provided significant insights into the tectonic setting during the regional D2 event. The D2 fold and thrust structures deform ca. 1834 Ma fluvial sedimentary rocks and ca. 1836-1832 Ma volcanic (subaerial) and plutonic rocks with a "continental" arc geochemical signature. These are the youngest known volcanic and sedimentary rocks within the THO. The subaerial deposition of these young rocks was coeval with initiation of D2 deformation in the region. They were then rapidly incorporated into the thrust fault sequence soon after their formation, which suggests that the fold and thrust structures propagated into a zone of active magmatism which must have been located in the foreland to the belt of thrusting and folding.

Sequence stratigraphy and sedimentology of the Neoproterozoic Kuibis and Schwarzrand Subgroups (Nama Group), southwestern Namibia

The Kuibis and Schwarzrand Subgroups of the Nama Group form a succession of shallow-marine and minor fluvial sedimentary rocks that is exposed over much of central and southern Namibia. Ediacaran-type body fossils and the stratigraphic carbon-isotope variability indicate a Vendian age for much of these strata; the Precambrian-Cambrian boundary is in the uppermost part of the Schwarzrand Subgroup. Radiometric dating of abundant volcanic ash beds which span much of the Vendian and extend into the Cambrian part of the section could make the Nama Group the best calibrated reference section for Vendian chronostratigraphy. Here we describe the sedimentology and sequence stratigraphy of exposures of the Kuibis and Schwarzrand Subgroups in southwestern Namibia. We place tighter environmental constraints on the paleontology and geochemistry and we identify seven depositional sequences. The boundaries of the depositional sequences correspond to the more important disconformities in the Kuibis and Schwarzrand Subgroups; their identification is crucial for defining rates of organism evolution and isotopic differentiation.

UPb geochronology and tectonic development of the southern flank of the Kisseynew Domain, Trans-Hudson Orogen, Canada

The southern flank of the Kisseynew Domain, northern Manitoba, underwent polyphase deformation and metamorphism during the Palaeoproterozoic Trans-Hudson Orogen, and represents a lithological, metamorphic and structural transition from greenschist-grade volcano-plutonic rocks of the Flin Flon Domain to amphibolite-grade paragneisses of the central Kisseynew Domain. The timing of geological events has previously been constrained using widely dispersed sample sites, but this study has integrated UPb geochronology with detailed lithological and structural mapping in the Cleunion Lake area. Pink gneisses, interpreted as Missi supracrustal rocks, were deposited after 1855 Ma, but prior to ∼ 1835 Ma, and thus are similar in age to fluvial sedimentary rocks in the lower-grade Flin Flon Domain. All the zircons in the pink gneisses appear to be inherited, or resorbed during later amphibolite-grade metamorphism, and a late Archaean component is also indicated. Corundum-bearing gneisses in the Missi Group and leucosomes in Burntwood River Group rocks are deformed by F 1 and are indicative of an early high- T, low- P metamorphic event (M 1) that may have been synchronous with the intrusion of 1835 to 1827 Ma intrusions, and related to regional crustal extension. The above rocks are folded during a continuous compressional deformation event (D 2) related to terminal collision between the Palaeoproterozoic rocks of the Trans-Hudson Orogen and the Superior Province. Three main fold generations (F 1, F 2, F 3) have been identified, which occurred during D 2. A 1818 Ma granitic vein is approximately synchronous with F 2 and peak metamorphic (M 2) conditions in the Cleunion Lake area. Cooling, uplift, and intrusion of pegmatites during F 3 at ∼ 1800 Ma provides a time limit on the waning stages of compressional deformation. The main phase of compressional deformation (D 2) in the southern flank of the Kisseynew Domain thus lasted for about 30 million years.