An official website of the United States government
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.
more »
« less
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.
more »
« less
- Award ID(s):
- 1713893
- 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
More Like this
-
-
The upper Tonian ChUMP (Chuar-Uinta Mountains-Pahrump) strata of the southwestern U.S.A. are hypothesized to be regional correlatives and to record a time of rift basin evolution commencing at ca. 770 Ma in western Laurentia (modern-day coordinates). We test this correlation using U-Pb chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) on detrital zircon grains from basal units within these successions. ChUMP units yield CA-ID-TIMS maximum depositional ages (MDA) between 775 and 766 Ma: the Chuar Group of AZ has an MDA of 770.1 ± 0.5 Ma (n = 1) and an additional young zircon mode at 775.7 ± 0.3 Ma (n = 11); the Uinta Mountain Group of northern UT has an MDA of 766.3 ± 0.5 Ma (n = 5) and contains a second young mode at 775.1 ± 0.7 Ma (n = 3); and the basal Horse Thief Springs Formation of the middle Pahrump Group CA has an MDA of 775.4 ± 0.7 Ma (n = 3). The ca. 775 and 770 Ma grains are interpreted to be from zircon-bearing mafic sources related to the 770–778 Ma Gunbarrel Large Igneous Province of Yukon and NW U.S.A. The 766 Ma population was either derived from the Mt Rogers complex of eastern Laurentia or could have come from conjugate margins that were in the process of rifting away, such as Tasmania. The CA-ID-TIMS dates on the Chuar Group in Grand Canyon anchor a Bayesian age model for evaluating late Tonian Earth systems. Faster sediment accumulation rates (80 + 150/-44 m/My) in the lower Chuar Group are consistent with the inception of an extensional basin related to Rodinia breakup; slower rates in the upper Chuar Group (25 + 12/-5 m/My) record are associated with relatively deeper water sedimentation and concomitant organic carbon burial during marine transgression. The model also constrains the timing of several biological events recorded in the Chuar Group, including eukaryovorous predation (>767 Ma), the first appearance of vase-shaped microfossils (∼741 Ma), and the ranges of Cerebrosphaera globosa (=C. buickii; 800–743 Ma) and Lanulatisphaera laufeldii. (766–740 Ma), both proposed as possible marine index fossils for late Tonian time. Finally, the model can also be used to search for stratigraphic evidence of a purported glaciation at ca. 751 Ma.more » « less
-
Rapid sediment accumulation rates (SAR) in a fan delta situated on the rapidly uplifting footwall of the Taormina normal fault in NE Sicily preserves a rare record of earthquakes and base level change for a tightly coupled source to sink system. We use this sedimentary archive to reconstruct the kinematics and slip history of the fault and further an understanding of how tectonic forcing across various scales are encoded in stratigraphy. A revised luminescence-based age model indicates that ~82 m of the Pagliara fan-delta foreset facies was deposited in ~11 ka at a mean SAR of ~0.74 cm/yr during MIS 7. Syn-depositional terrestrial cosmogenic nuclide (TCN) determined paleoerosion rates of 0.91±0.12 mm/yr and 1.31 ±0.61 mm/yr are similar to published modern erosion rates for the Pagliara basin of 0.97 ±0.11 mm/yr. At the stratigraphic scale, a time series of magnetic susceptibility (c) sampled at 1 m intervals in the foresets displays four ~2,800 yr / 20 m-thick cycles of growing c, bounded by sharp decreases that do not coincide with changes in sediment texture. The c of the low-grade metamorphic bedrock in the source is 20-100 times weaker than the c of rubified soils mantling the hillslopes, which is comparable to the c of the delta sediments. We propose that large, bedrock-cored landslides quasi-periodically deliver weak c sediment to the delta that dilutes a c signal otherwise dominated by the stripping of soil-mantled hillslopes. We propose that centennial-scale recurrence interval earthquakes are most capable at triggering a basin-scale landslide only after channel incision has increased relief of hillslopes to the threshold condition, which requires millennia to achieve. At the landscape scale of delta geometry and location, the Pagliara delta accumulated in a hanging wall basin that has since been inverted. We reconstruct the history of base level fall for the delta from an inversion of fluvial topography and apportion that record to its rock uplift, delta deposition, and eustatic components. We show that footwall uplift has been unsteady over the past 600 ka ranging from -1 to 3 mm/yr. The integration of our stratigraphic- and landscape scale observations furthers our understanding of the natural hazards related to normal fault earthquakes and their impact on sediment dynamics in this steep, active tectonic setting.more » « less
-
Abstract Development and evaluation of models for tectonic evolution in the Cascadia forearc require understanding of along-strike heterogeneity of strain distribution, uplift, and upper-plate characteristics. Here, we investigated the Neogene geologic record of the Klamath Mountains province in southernmost Cascadia and obtained apatite (U-Th)/He (AHe) thermochronology of Mesozoic plutons, Neogene graben sediment thickness, detrital zircon records from Neogene grabens, gravity and magnetic data, and kinematic analysis of faults. We documented three aspects of Neogene tectonics: early Miocene and younger rock exhumation, development of topographic relief sufficient to isolate Neogene graben-filling sediments from sources outside of the Klamath Mountains, and initiation of mid-Miocene or younger right-lateral and reverse faulting. Key findings are: (1) 10 new apatite AHe mean cooling ages from the Canyon Creek and Granite Peak plutons in the Trinity Alps range from 24.7 ± 2.1 Ma to 15.7 ± 2.1 Ma. Inverse thermal modeling of these data and published apatite fission-track ages indicate the most rapid rock cooling between ca. 25 and 15 Ma. One new AHe mean cooling age (26.7 ± 3.2 Ma) from the Ironside Mountain batholith 40 km west of the Trinity Alps, combined with previously published AHe ages, suggests geographically widespread latest Oligocene to Miocene cooling in the southern Klamath Mountains province. (2) AHe ages of 39.4 ± 5.1 Ma on the downthrown side and 22.7 ± 3.0 Ma on the upthrown side of the Browns Meadow fault suggest early Miocene to younger fault activity. (3) U-Pb detrital zircon ages (n = 862) and Lu-Hf isotope geochemistry from Miocene Weaverville Formation sediments in the Weaverville, Lowden Ranch, Hayfork, and Hyampom grabens south and southwest of the Trinity Alps can be traced to entirely Klamath Mountains sources; they suggest the south-central Klamath Mountains had, by the middle Miocene, sufficient relief to isolate these grabens from more distal sediment sources. (4) Two Miocene detrital zircon U-Pb ages of 10.6 ± 0.4 Ma and 16.7 ± 0.2 Ma from the Lowden Ranch graben show that the maximum depositional age of the upper Weaverville Formation here is younger than previously recognized. (5) A prominent steep-sided negative gravity anomaly associated with the Hayfork graben shows that both the north and south margins are fault-controlled, and inversion of gravity data suggests basin fill is between 1 km and 1.9 km thick. Abrupt elevation changes of basin fill-to-bedrock contacts reported in well logs record E-side-up and right-lateral faulting at the eastern end of the Hayfork graben. A NE-striking gravity gradient separates the main graben on the west from a narrower, thinner basin to the east, supporting this interpretation. (6) Of fset of both the base of the Weaverville Formation and the cataclasite-capped La Grange fault surface by a fault on the southwest margin of the Weaverville basin documents 200 m of reverse and 1500 m of right-lateral strike-slip motion on this structure, here named the Democrat Gulch fault; folded and steeply dipping strata adjacent to the fault confirm that faulting postdated deposition of the Weaverville Formation. Based on these findings, we suggest that Miocene rock cooling recorded by AHe ages, accompanying graben formation, and development of topographic relief record early to middle Miocene initiation of underplating or “subcretion” in the southern Cascadia subduction zone beneath the southern Klamath Mountains.more » « less
-
ABSTRACT Buried channel sand bodies are important reservoirs of subsurface water and energy resources, but their arrangement and interconnectedness are difficult to predict. The dominant process that distributes channels and their sediments in alluvial basins is river avulsion, which occurs when a channel seeks a new location on the adjacent floodplain. Floodplain sedimentation, incision, and channel levee growth influence channel pathfinding during avulsion, and should control key aspects of the stratigraphic arrangement of channel bodies, including compensational (spatially and temporally even) deposition, stratigraphic completeness, and facies distributions; however, this impact has been difficult to isolate in natural and experimental basin fills. To test how different avulsion pathfinding parameters influence stratigraphic architecture, we use a numerical model of a fluvial fan to produce synthetic fluvial stratigraphy under seven different runs with progressively more complex channel pathfinding rules. In the simplest models where pathfinding is set by a random walk, the channel rapidly changes position and avulsions spread across the fan surface. The corresponding deposit is dominated by channel facies, is relatively incomplete, and the compensation timescale is short. As rules for pathfinding become more complex and channels can be attracted or repulsed by pre-existing channels, lobe switching emerges. Deposits become more diverse with a mix of channel and floodplain facies, stratigraphic completeness increases, and the compensation timescale lengthens. Previous work suggests that the compensation timescale is related to the burial timescale and relief across the depositional surface, yet we find that compensation approaches the burial timescale only for model runs with high morphodynamic complexity and relatively long topographic memory. Our results imply that in simple systems with limited degrees of freedom, the compensation timescale may become detached from the burial timescale, with uniform sedimentation occurring quickly relative to long burial timescales.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.2110/jsr.2020.60
