The Bengal Fan is the world’s largest submarine fan deposit by area, and fan sediments preserve a faithful record of Tibetan-Himalayan orogenesis since Early Miocene time. This study uses detrital zircon (U-Th)/He (ZHe) thermochronology, U-Pb and ZHe double-dating, and sediment mixing models to characterize shifts in provenance and erosional history of the Himalaya and Tibet recorded by Bengal Fan sediments from Late Miocene to Late Pleistocene time, with emphasis on the Pliocene-Pleistocene interglacial transition. Zircon grains are collected from sediment cores acquired during IODP Expedition 354 (2015). A total of 157 single zircon grains from 25 samples of sandy and silty turbidites will be analyzed for single ZHe analyses (an average of 8 grains per sample), and ~50 zircon grains will be chosen for double-dating with U-Pb geochronology. In turn, all ZHe results will be compared with large-n (n=300-600) U-Pb age distributions from the same samples. Sediment mixing models will determine potential source regions for individual samples by “unmixing” crystallization and cooling age populations present in each sample. Preliminary results (n=84) show shifts in the range of cooling ages from ~5.9-45 Ma in the Late Miocene, with a single grain at ~233 Ma, to ~2.5-410 Ma in the Late Pliocene, and finally ~0.5-20 Ma in the Early-Middle Pleistocene, with a single grain at ~492 Ma. These results indicate shifts in source contributions from central-East Himalaya in the Late Miocene, to a Lesser and Tethyan Himalaya source in the Late Pliocene, to a predominantly Eastern Tibet and Lhasa terrane source in the Late Pleistocene in tandem with a broad increase in exhumation rates. We attribute these variations in cooling age ranges to change in sediment source terrains for the Ganges and Brahmaputra River system. Ongoing work seeks to further refine sediment mixing models for the Ganges-Brahmaputra-Bengal Fan system in tandem with large-n U-Pb geochronologic efforts.
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Neogene shallow-marine and fluvial sediment dispersal, burial, and exhumation in the ancestral Brahmaputra delta: Indo-Burman Ranges, India
ABSTRACT The stratigraphic record of Cenozoic uplift and denudation of the Himalayas is distributed across its peripheral foreland basins, as well as in the sediments of the Ganges–Brahmaputra Delta (GBD) and the Bengal–Nicobar Fan (BNF). Recent interrogation of Miocene–Quaternary sediments of the GBD and BNF advance our knowledge of Himalayan sediment dispersal and its relationship to regional tectonics and climate, but these studies are limited to IODP boreholes from the BNF (IODP 354 and 362, 2015-16) and Quaternary sediment cores from the GBD (NSF-PIRE: Life on a tectonically active delta, 2010-18). We examine a complementary yet understudied stratigraphic record of the Miocene–Pliocene ancestral Brahmaputra Delta in outcrops of the Indo-Burman Ranges fold–thrust belt (IBR) of eastern India. We present detailed lithofacies assemblages of Neogene delta plain (Tipam Group) and intertidal to upper-shelf (Surma Group) deposits of the IBR based on two ∼ 500 m stratigraphic sections. New detrital-apatite fission-track (dAFT) and (U-Th)/He (dAHe) dates from the Surma Group in the IBR help to constrain maximum depositional ages (MDA), thermal histories, and sediment accumulation rates. Three fluvial facies (F1–F3) and four shallow marine to intertidal facies (M1–M4) are delineated based on analog depositional environments of the Holocene–modern GBD. Unreset dAFT and dAHe ages constrain MDA to ∼ 9–11 Ma for the Surma Group, which is bracketed by intensification of turbidite deposition on the eastern BNF (∼ 13.5–6.8 Ma). Two dAHe samples yielded younger (∼ 3 Ma) reset ages that we interpret to record cooling from denudation following burial resetting due to a thicker (∼ 2.2–3.2 km) accumulation of sediments near the depocenter. Thermal modeling of the dAFT and dAHe results using QTQt and HeFTy suggest that late Miocene marginal marine sediment accumulation rates may have ranged from ∼ 0.9 to 1.1 mm/yr near the center of the paleodelta. Thermal modeling results imply postdepositional cooling beginning at ∼ 8–6.5 Ma, interpreted to record onset of exhumation associated with the advancing IBR fold belt. The timing of post-burial exhumation of the IBR strata is consistent with previously published constraints for the avulsion of the paleo-Brahmaputra to the west and a westward shift of turbidite deposition on the BNF that started at ∼ 6.8 Ma. Our results contextualize tectonic controls on basin history, creating a pathway for future investigations into autogenic and climatic drivers of behavior of fluvial systems that can be extracted from the stratigraphic record.
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- Award ID(s):
- 1713893
- NSF-PAR ID:
- 10289402
- Date Published:
- Journal Name:
- Journal of Sedimentary Research
- Volume:
- 90
- Issue:
- 9
- ISSN:
- 1527-1404
- Page Range / eLocation ID:
- 1244 to 1263
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract The Bengal Basin preserves the erosional signals of coupled tectonic‐climatic change during late Cenozoic development of the Himalayan orogen, yet regional correlation and interpretation of these signals remains incomplete. We present a new geologic map of fluvial‐deltaic deposits of the Indo‐Burman Ranges (IBR), five detrital zircon fission track analyses, and twelve high‐n detrital zircon U‐Pb age distributions (dzUPb) from the Barail (late Eocene–early Miocene), Surma (early–late Miocene), and Tipam (late Miocene–Pliocene) Groups of the ancestral Brahmaputra delta. We use dzUPb statistical tests to correlate the IBR units with equivalent age strata throughout the Bengal Basin. An influx of trans‐Himalayan sediment and the first appearance of ∼50 Ma grains of the Gangdese batholith in the lower Surma Group (∼18–15 Ma) records the early Miocene arrival of the ancestral Brahmaputra delta to the Bengal Basin. Contributions from Himalayan sources systematically decrease up section through the late Miocene as the contribution of Trans‐Himalayan Arc sources increases. The Miocene (∼18–8 Ma) deposition of the Surma Group records upstream expansion of the ancestral Brahmaputra River into southeastern Tibet. Late Miocene (<8 Ma) progradation of the fluvial part of the delta (Tipam Group) routed trans‐Himalayan sediment over the shelf edge to the Nicobar Fan. We propose that Miocene progradation of the ancestral Brahmaputra delta reflects increasing rates of erosion and sea level fall during intensification of the South Asian Monsoon after the Miocene Climate Optimum, contemporaneous with a pulse of tectonic uplift of the Himalayan hinterland and Tibet.
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The thick flysch facies of the Cretaceous to Eocene Chugach-Prince William terrane (CPW) represents a thick accretionary complex that extends approximately 2200 km across southern Alaska, and in the central area is comprised mainly of the Valdez Group and the Orca Group (Fig. 1) (Garver and Davidson, 2015; Davidson and Garver, 2017). The Valdez Group is traditionally viewed as a Campanian to Maastrichtian turbidite deposit with mafic volcanic rocks that have experienced lower greenschist facies metamorphism (Dusel-Bacon, 1991; Gasser et al., 2012). The Orca Group is Paleocene to Eocene turbidite and volcanic deposit that, in most places, has undergone prehnitepumpellyite facies metamorphism (Dusel-Bacon, 1991; Wilson et al., 2012). The relationship between the Valdez Group and the Orca Group is poorly understood (Moffit, 1954). A common hypothesis suggested long ago is that they are stratigraphically related and are a continuous sequence (Capps and Johnson, 1915). Given recent zircon dating, the Valdez Group appears to have maximum depositional ages (MDA) of 75-65 Ma and the deposition of the Orca Group is between 60-50 Ma (Davidson and Garver, 2017). In this case, deformation of the Valdez Group may have occurred 65-60 Ma, just before the deposition of the oldest Orca Group turbidites began. Thus, the youngest strata of the Valdez Group must be older than the oldest strata of the Orca Group. An alternative hypothesis is that the Orca Group formed in a different location and was translated to its current position along strike slip faults after the deformation of the Valdez Group (cf. Plafker et al., 1994). This idea would mean that the ages of the two groups may overlap in age, and the time of juxtaposition of the Orca Group to the Valdez Group is unknown but important. After the deposition of the bulk of the Orca Group was completed, the CPW experienced plutonism by the near-trench Sanak- Baranof Belt (SBB) and the Eshmay plutons (Cowan, 2003; Davidson and Garver, 2017). If a pluton crosscuts two terranes then the age of that pluton is the minimum age that the two terranes were juxtaposed (Coney et al., 1980). The SBB plutons intruded the CPW from 63-47 Ma, with a distinct age progression from 63 Ma to the west to 50-47 Ma to the east (Davidson and Garver, 2017). In Prince William Sound the CPW terrane is also intruded by the Eshmay Suite Plutons (ESP) around 37-41 Ma (Fig. 1) (Johnson, 2012; Davidson and Garver, 2012; Garcia et al., 2019). The Eshamy suite plutons could be explained by high heat flow that melted Orca Group sediments and these melts then mixed in with mantlederived basalts (Johnson, 2012). The ESP stitch the two terranes, as they occur on both sides of the Contact Fault System (Fig. 1) (Davidson and Garver, 2017). A key link between the Orca and Valdez Groups may be conglomerates that occur in the Orca Group. There are five main localities of conglomerates in PWS, and some of the most significant exposures are in eastern and northern PWS. These conglomerates were described by Grant and Higgins (1910) as being near the bottom of the Orca Group stratigraphy, specifically at the basal unconformity. However, Capps and Johnson (1915) described the conglomerates as being at the top of the Orca Group, occurring after and interleaved with basaltic volcanic rocks (cf. Tysdal and Case, 1979). If the Valdez Group is the source of the Orca Group conglomerate clasts, then the two terranes were adjacent at a time earlier than previously known (38-39 Ma) (Davidson and Garver, 2017). Capps and Johnson (1915) proposed that the matrix of the conglomerates and the majority of the clasts were derived from the Valdez Group. They also suggest that a few clasts could be derived from the greenstones of the Orca Group. The provenance of the Orca Group conglomerates is important in our understanding of the relationship between the Valdez and Orca Groups as well as our overall understanding of the Cordilleran tectonics. This study will focus on understanding the Valdez Group and the Orca Group through the study of detrital zircons from sandstone clasts from the Orca Group Conglomerates and the host strata to those conglomerates.more » « less
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Abstract The Late Miocene and Pliocene Quillagua depocenter lake system existed in a forearc basin on the west side of the Andes Mountains in northern Chile, alternating between standing-water and salar conditions. Quaternary incision of the Loa River Canyon resulted in bypass of the prior depositional surface and drainage of groundwater from the abandoned depocenter. Systematic regional geological mapping, 32 new chronological constraints on the strata in the basin, outcrop-scale facies analyses, and geophysical data underpin a revised evaluation of the controls on the lake system. The progressive stages, ages, and causes of the Quaternary destruction of the lake system are reconstructed based on mapped distributions of superficial fluvial sediments, chronological studies of terrace deposits, and landform analysis. The lake system occurred at the junction of small catchments draining the slowly rising western Andean foothills and the large paleo-Loa River catchment draining the Andean volcanic arc, during a time span of intense caldera activity. Small magnitude climate variability affected both the hyperarid low elevation sectors and arid upper sectors of the catchments. By 10 Ma, the regional climate was extremely arid, limiting water and sediment to small amounts, and during the Late Miocene and Pliocene, there was no surface-water outlet to the Pacific. Hydrological variations from 9 to 2.6 Ma led to sediment accumulation in variable lake environments, alternating with long hiatuses. Minor deformation within the Quillagua depocenter shifted the topographic axis and groundwater outlets. Simultaneous headward erosion from the Pacific shore captured the Loa River, which triggered large-magnitude incision that persists today. The progression of surface water environmental change was accompanied by changing composition and amount of surface and groundwater, which determined deposition of primary evaporite minerals, extensive diagenesis, and eventually, complex patterns of dissolution expressed as karst.
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Expedition 359 Scientific Prospectus: Sea Level, Currents, and Monsoon Evolution in the Indian Oceannull (Ed.)International Ocean Discovery Program (IODP) Expedition 359 is designed to address sea level, currents, and monsoon evolution in the Indian Ocean. Seven proposed drill sites are located in the Maldives and one site is located in the Kerala-Konkan Basin on the western Indian continental margin. The Maldives carbonate edifice bears a unique and mostly unread Indian Ocean archive of the evolving Cenozoic icehouse world. It has great potential to serve as a key area for better understanding the effects of this global evolution in the Indo-Pacific realm. Based mainly on seismic stratigraphic data, a model for the evolution of this carbonate bank has been developed, showing how changing sea level and ocean current patterns shaped the bank geometries. A dramatic shift in development of the carbonate edifice from a sea level–controlled to a predominantly current-controlled system is thought to be directly linked to the evolving Indian monsoon. Fluctuations in relative sea level control the stacking pattern of depositional sequences during the lower to middle Miocene. This phase was followed by a two-fold configuration of bank development: bank growth continued in some parts of the edifice, whereas in other places, banks drowned. Drowning steps seem to coincide with onset and intensification of the monsoon-related current system and the deposition of giant sediment drifts. The shapes of drowned banks attest to the occurrence of these strong currents. The drift sediments, characterized by off-lapping geometries, formed large-scale prograding complexes, filling the Maldives Inner Sea basin. Because the strong current swept most of the sediment around the atolls away, relict banks did not prograde, and steady subsidence was balanced by aggradation of the atolls, which are still active today. One important outcome of Expedition 359 is ground-truthing the hypothesis that the dramatic, pronounced change in the style of the sedimentary carbonate sequence stacking was caused by a combination of relative sea level fluctuations and ocean current system changes. Answering this question will directly improve our knowledge on processes shaping carbonate platforms and their stratigraphic records. Our findings would be clearly applicable to other Tertiary carbonate platforms in the Indo-Pacific region and to numerous others throughout the geological record. In addition, the targeted successions will allow calibration of the Neogene oceanic δ13C record with data from a carbonate platform to platform-margin series. This is becoming important, as such records are the only type that exist in deep time. Drilling will provide the cores required for reconstructing changing current systems through time that are directly related to the evolution of the Indian monsoon. As such, the drift deposits will provide a continuous record of Indian monsoon development in the region of the Maldives. These data will be valuable for a comparison with proposed Site KK-03B in the Kerala-Konkan Basin (see Geological setting of the Kerala-Konkan Basin, below) and other monsoon-dedicated IODP expeditions. The proposed site in the Kerala-Konkan Basin provides the opportunity to recover colocated oceanic and terrestrial records for monsoon and premonsoon Cenozoic climate in the eastern Arabian Sea and India, respectively. The site is located on a bathymetric high immediately north of the Chagos-Laccadive Ridge and is therefore not affected by strong tectonic, glacial, and nonmonsoon climatic processes that affect fan sites fed by Himalayan rivers. The cores are expected to consist of a continuous sequence of foraminifer-rich pelagic sediments with subordinate cyclical siliciclastic inputs of fluvial origin from the Indian Peninsula for the Neogene and a continuous paleoclimate record at orbital timescales into the Eocene and possibly the Paleocene.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.2110/jsr.2020.60
