Abstract The unusual drainage pattern of the Jinsha River (upper Yangtze) is closely linked to the uplift of the Tibetan Plateau, but when this pattern became established is controversial. In this study we compare the Pb isotopic composition of detrital K‐feldspars from the modern Jinsha River with those from the Plio‐Pleistocene Xigeda Formation, a sequence of fluvial and lacustrine deposits widely distributed along the major rivers draining the Tibetan Plateau, to investigate their relationship and to constrain the drainage evolution of the region. Our results demonstrate that the Jinsha River had achieved its current sediment‐transport pattern by the late Pliocene, indicating a drainage character similar to that operating at present. Our findings not only suggest an upper age limit for formation of the Jinsha River but also shed new light on the evolution of the whole Yangtze system. K‐feldspar grains from the Jinsha River have less radiogenic Pb than those in the Red, Mekong, and Salween Rivers, providing a key test as to whether the Jinsha River was previously connected to the paleo‐Red River. Comparison with published Pb isotopic data shows that these characteristically unradiogenic feldspars are absent in the onshore Cenozoic archive from the Hanoi Basin, suggesting that there has been no connection between the lower Jinsha River and the Red River since the Eocene.
In a source-to-sink sedimentary system, multiple processes have the potential to modify the sediment composition during sediment generation at the source, through transport, deposition and burial. To investigate these issues, a multi-proxy provenance study of deep-water and shallow-marine sandstones from the mid-Carboniferous Clare Basin was undertaken, utilizing zircon and apatite U–Pb geochronology, heavy mineral analysis (including apatite trace element data) and petrography. Data from the deep-water submarine fans show a similar provenance signal to the shallow-marine sandstones. Data from across the Clare Basin stratigraphy indicate that sands were probably and consistently derived from the south and SW, with a portion of the detritus being recycled from Old Red Sandstone basins to the south. The provenance signal, however, shows spatial and temporal variations across the basin that are explained by fluctuations in the amount of recycled sediments and are linked with the evolving Variscan Orogenic Belt. Heavy mineral data from both shallow-marine and deep-water sandstones suggest a basin configuration with a delta system feeding directly into the deep basin. Supplementary material: Figures S1–S4 and Tables S1–S8 are available at https://doi.org/10.6084/m9.figshare.c.5446739
Abstract A multiproxy provenance approach (heavy mineral analysis, U‐Pb zircon geochronology, and Pb isotopic analysis of K‐feldspar) has constrained sediment supply within the Upper Nile drainage system in the Miocene‐Pliocene. Provenance data from sandstones were obtained from three exploration wells, two situated on the north‐eastern margin and one on the eastern flank of the Lake Albert Basin, NW Uganda. Data suggest that high‐grade to low‐grade metamorphic rocks and granitoids have variably supplied the heavy mineral assemblages around the Lake Albert Basin during the Miocene‐Pliocene, with contributions from the isotopically heterogeneous Archean Cratons (including the local Ugandan Craton, Tanzanian, and Congo Cratons) and the Pan‐African rocks (the Mozambique Belt) with possible contributions from the Neoproterozoic and Paleoproterozoic rocks. These data also highlight clear differences between supply to the eastern basin margin, compared with the northeast, which is reconcilable with current models for Miocene‐Pliocene drainage in the region. Supply to northeastern Lake Albert during the Miocene‐Pliocene appears to have been through a proto‐Albert Nile (draining from NE to SW) and from a proto‐Victoria Nile or similarly oriented palaeo‐river systems draining from the east. In contrast, the eastern flanks of the basin were likely supplied via the palaeo‐Nkusi river, tapping local hinterland sources and more distal basement to the far‐east (Mozambique Belt). This study highlights the importance of utilizing a multiproxy approach in provenance analysis as no one signal is capable of distinguishing the different source lands and constraining the evolving drainage patterns.
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.
Beginning by erosion at the source, through transport and intermediate storage, and to final deposition, sediment experiences various processes modifying its mineralogical composition. Among these processes, chemical weathering is governed by climatic conditions, with a cold and arid climate hindering dissolution or replacement of specific silicate mineral phases compared to a hot and humid setting. It is thus theoretically possible to utilise chemical weathering conditions reconstructed from the sedimentary record as a proxy to past climatic conditions. However, chemical weathering intensity, as determined from the final product of a sedimentary cycle, is strongly dependent on the duration of past exposure to weathering conditions, with shorter residence times in the sedimentary transport system resulting in lower chemical weathering intensities. To address these issues, this study interrogates both a modern river catchment (Var River, southeast France) and its offshore sedimentary equivalent (the Var turbidite system), spanning the Upper Pleistocene to Holocene. In the area, five main river tributaries drain various lithologies of the Southern French Alps carrying sediments from a mountainous landscape to the turbiditic system. This sediment delivery has been mainly controlled over the past ca. 70 kyrs by millennial-scale Dansgaard-Oeschger oscillations, with more frequent turbidite activity during the Last Glacial Maximum (LGM). Additionally, previous work on the area highlighted higher denudation rates during the LGM compared to pre- and post-LGM times. This project uses multiple proxies to reconstruct chemical weathering from both active river sandbars in the Var River catchment and from sediments collected in the adjacent turbiditic system. Bulk rock geochemistry data are used to calculate chemical weathering indices such as the traditional Chemical Index of Alteration and more recent alpha indices. Additionally, heavy minerals apatite and tourmaline are employed in the apatite-tourmaline index, a potential indicator of variations in past chemical weathering conditions. This dataset is compared to previously acquired neodymium isotope provenance data and palaeo-denudation rates (10Be). Whilst assessing the robustness of various chemical weathering techniques, this study also intends to shed light on the LGM impact on sediment delivery and chemical weathering in a mountainous landscape.
A rare and unusual large solitary discoidal fossil has been discovered on a paving slab quarried from the cyclothems of the Central Clare Group (Kinderscoutian, Pennsylvanian, Carboniferous), western Ireland. The fossil impression consists of a smooth raised inner discoidal area, surrounded by a slightly lower relief outer ring, ca. 130–135 mm in diameter, with eight prominent equidistant ovoid raised nodes towards the outermost margin. The octoradial body plan of this enigmatic specimen suggests a cnidarian connection and, as it is preserved as a positive hyporelief cast, it is tentatively interpreted as the resting trace of a large benthic anemone, which was either partially or fully infaunal. The discoidal fossil is interesting palaeoecologically; it occurs within the well-known Liscannor flagstone, which consists of thinly bedded, fine-grained sandstone that is extensively covered by prominent, sinuous to meandering, horizontal grazing trails attributed to Psammichnites plummeri. This sedimentary facies likely represents mouth-bar sedimentation on a delta front of a river-dominated delta. The discoidal impression occurs on a portion of the slab where these trace fossils are relatively scarce. Uncertainty surrounds the classification and interpretation of the disc due to its relatively simple morphological form, coupled with a lack of unequivocally diagnostic features — a problem commonly encountered in studies of discoidal fossils from both the Ediacaran and the Phanerozoic.
Research Article| February 01, 2013 Inferring sites of subglacial erosion using the Pb isotopic composition of ice-rafted feldspar: Examples from the Weddell Sea, Antarctica M.J. Flowerdew; M.J. Flowerdew * 1British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK *E-mail: mf@bas.ac.uk. Search for other works by this author on: GSW Google Scholar S. Tyrrell; S. Tyrrell 2School of Geological Sciences, University College Dublin, Belfield, Dublin 4, Ireland Search for other works by this author on: GSW Google Scholar V.L. Peck V.L. Peck 1British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK Search for other works by this author on: GSW Google Scholar Geology (2013) 41 (2): 147–150. https://doi.org/10.1130/G33644.1 Article history received: 18 May 2012 rev-recd: 06 Aug 2012 accepted: 06 Aug 2012 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation M.J. Flowerdew, S. Tyrrell, V.L. Peck; Inferring sites of subglacial erosion using the Pb isotopic composition of ice-rafted feldspar: Examples from the Weddell Sea, Antarctica. Geology 2013;; 41 (2): 147–150. doi: https://doi.org/10.1130/G33644.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The delivery of ice-rafted debris (IRD) from glaciated margins is a function of ice sheet dynamics. Shifts in supply and sourcing of IRD can therefore identify episodes of ice sheet instability; however, records can be difficult to correctly interpret because the subglacial geology of the catchment areas, which controls IRD composition, may be obscured. Importantly, variations can also result from shifts in erosion sites due to changes in the basal ice sheet conditions. This study evaluates where subglacial erosion has occurred in catchments that flow into the southern Weddell Sea, Antarctica, by determining the Pb isotopic compositions of individual ice-rafted feldspars from late Holocene marine sediments. Feldspar compositions match those of rock units inferred (through extrapolation of outcrop, magnetic, and gravity data) to compose areas where ice velocity, bed roughness, and shear stress are high. Significantly, signals from areas where ice velocities are high but bed roughness and shear stresses are low were not recorded, suggesting that there is reduced bedrock erosion in these regions. Major variations in IRD composition in the Weddell Sea can result from changing the loci of subglacial erosion, and do not necessarily correspond with major ice sheet instability. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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