Indus Fan Research Articles

When did the Indus River of South-Central Asia take on its “modern” drainage configuration?

Abstract For sedimentary archives to be used as a record of hinterland evolution, the factors affecting the archive must be known. In addition to tectonics, a number of factors, such as changes in climate and paleodrainage, as well as the degree of diagenesis, influence basin sediments. The Indus River delta-fan system of South-Central Asia records a history of Himalayan evolution, and both the onshore and offshore sedimentary repositories have been studied extensively to research orogenesis. However, a number of unknowns remain regarding this system. This paper seeks to elucidate the paleodrainage of the Indus River, in particular when it took on its modern drainage configuration with respect to conjoinment of the main Himalayan (Punjabi) tributary system with the Indus trunk river. We leverage the fact that the Punjabi tributary system has a significantly different provenance signature than the main trunk Indus River, draining mainly the Indian plate. Therefore, after the Punjabi tributary system joined the Indus River, the proportion of Indian plate material in the repositories downstream of the confluence should have been higher than in the upstream repository. We compared bulk Sr-Nd data and detrital zircon U-Pb data from the Cenozoic upstream peripheral foreland basin and downstream Indus delta and Indus Fan repositories. We determined that throughout Neogene times, repositories below the confluence had a higher proportion of material from the Indian plate than those above the confluence. Therefore, we conclude that the Indus River took on its current configuration, with the Punjabi tributary system draining into the Indus trunk river in the Paleogene, early in the history of the orogen. The exact time when the tributary system joined the Indus should correlate with a shift to more Indian plate input in the downstream repositories only. While the upstream repository records no change in Indian plate input from Eocene to Neogene times, a shift to increased material from the Indian plate occurs at the Eocene–Oligocene boundary in the delta, but sometime between 50 Ma and 40 Ma in the fan. Though further work is required to understand the discrepancy between the two downstream repositories, we can conclude that the tributary system joined the Indus trunk river at or before the start of the Oligocene.

Forgotten Islands of the Past: The Archaeology of the Northern Coast of the Arabian Sea

The Indus Delta plays an important role in the archaeology of the northern coast of the Arabian Sea. Little was known of this region until a few decades ago. The first surveys were carried out in the 1970s and were resumed by the present author in the 2010s. They have shown the great potential of the area for the interpretation of sea-level rise and its related human settlement between the beginning of the Holocene and the Hellenistic period. In this territory, several limestone terraces rise from the alluvial plain of the River Indus, which were islands in prehistoric and early historic times. Many archaeological artefacts, along with marine and mangrove shells, have been recovered from their surface and radiocarbon dated. These discoveries help us to follow the events that took place in the region in well-defined periods and interpret some aspects of the prehistoric coastal settlement in relation to the advance of the Indus Fan and the retreat of the Arabian Sea. The following questions are addressed in this paper: who settled these islands, when and why? During which prehistoric periods were mangrove and marine environments exploited? And what were the cultural characteristics of the communities that seasonally or permanently settled some of the present ‘rocky outcrops’?

Tectonic Evolution of a Sedimented Oceanic Transform Fault: The Owen Transform Fault, Indian Ocean

AbstractThe Owen transform fault (OTF) connecting the Sheba and the Carlsberg spreading ridges in the Indian Ocean currently forms the active plate boundary between India and Somalia plates. This 330‐km‐long transform fault is by far the longest transform fault along the India‐Somalia plate boundary and its valley is buried under the thick distal turbidites of the Indus Fan with total thickness ranging from 1,000 to >5,000 m. A new set of seismic reflection and multibeam bathymetric data reveals remarkable transpressive structures along its entire length recorded as folds in the sedimentary cover, eruption of mud ridges at the seafloor, thrusts in the young oceanic lithosphere. Based on a new regional time‐calibration of the seismic reflectors, we show that sediments in the transform valley (post 8.6 Ma) recorded a period of tectonic quiescence until the onset of a transpressive event around 1.5–2.4 Ma that we relate to a minor change in India‐Somalia kinematics not captured by magnetic anomalies. This tectonic regime is still active based on compressive earthquakes and deformation of the most recent sediments. Transpression resulted in the formation of a proto‐median ridge and the coeval propagation of the tip of the Carlsberg Ridge into the Somalian plate. These features are typically encountered at many other transform faults but rarely captured in their very early stage.

Architectural complexities and morphological variations of the indus fan and its elements: Understanding of the turbidite system through seismic characterization

The investigations of the processes occurring in submarine fans improve the understanding of intricacies of deepwater turbidity system and their associated developed architectural elements. The seismic patterns and depositional geometries of the channel levee complexes of the Indus fan have been studied using 2D/3D seismic and well data. The Indus fan is the second largest submarine fan in the world, with a horizontal extent of some 1500 km and a thickness of approx. 9 km. The development source of the Indus fan is divided into two units primarily based on their stratigraphic architecture and depositional characteristics. Firstly, the Eocene-Oligocene to Miocene lower unit that is built of channel lobes, and regionally developed large mass-transport deposits including megaturbidites. The other unit is Miocene and the Quaternary upper part that constitute major channel levee complexes with minor intercalations of locally developed mass transport deposits. The former progressively increase in thickness and width upwards the Eocene-Oligocene to Quaternary. This gave rise to symmetrical and asymmetrical growth patterns as a result of multiple phases of erosion and deposition. Such an architecture is present at an extensive scale primarily in the Plio-Pleistocene channels where the vertical to lateral growth ratio is higher than the channels. A single episode of thick MTDs is recognized by seismic reflection geometries suggesting sea level fluctuations that occurred between the Eocene-Oligocene and the Miocene, and from the Pliocene and the Quaternary channels. Regional confinements drastically affected the development of the Indus fan from the Eocene-Oligocene to Miocene, diminishing from the Pliocene-Quaternary. Sediment zonation occurred owing to the presence of structural confinements that controlled the growth of channel levee complexes and in general the entire Indus fan. Our results based on seismic reflection data of channel levee complexes suggest that increasing but alternating sediment supply from Eocene till Plio-Pleistocene, assisted the active channels and levee growth. Continuous increase in turbidity current size and energy along with sedimentation rates modified (i.e., increased the channel aspect ratio) the channel architecture of the Indus Fan.

ARCHITECTURAL COMPLEXITIES AND MORPHOLOGICAL VARIATIONS OF THE SEDIMENT WAVES OF PLIO-PLEISTOCENE CHANNEL LEVEE BACKSLOPE OF THE INDUS FAN

The architecture of the turbidity current sediment waves exhibits intricate morphologies and patterns on the Indus Fan channel levee backslope. The sediment waves are present on the channel levee of Plio-Pleistocene age and are absent in the deeper sections of the study area. The architecture of channel levee backslope on the Indus Fan is poorly understood. We used seismic interpretation techniques and modelling by utilizing high-resolution seismic data to approach this problem. The morphological variations in wavelength, crest dimensions and potential wave formation patterns suggest the autogenic and allogenic processes associated with wave development. Wavelengths reach up to 1473 m with an average of 486.84 m and the height of the levee ranges between 10 m and 60 m (average 30 m). The angle of the channel levee and dimension of the sediment wave here are independent of each other. Low angle levees have accommodated high dimension sediment waves and vice versa at multiple points downslope. Characteristically, the waves have formed on the outer levee (usually left) of the channels marked by steep margins suggesting that flow overspill caused the development of the waves. Generally, the younger sediment waves followed the patterns of older sediment waves, but the varying trends are often observed in the study area. The patterns of the sediment waves towards the younger sections of the levee indicate the modified and varying architectural style of growth. Sediment waves are generated by downslope turbidity currents. However, the deformation features have also possibly triggered the development of sediment waves.

Zircon U‐Pb Age Constraints on NW Himalayan Exhumation From the Laxmi Basin, Arabian Sea

AbstractThe Indus Fan, located in the Arabian Sea, contains the bulk of the sediment eroded from the Western Himalaya and Karakoram. Scientific drilling in the Laxmi Basin by the International Ocean Discovery Program recovered a discontinuous erosional record for the Indus River drainage dating back to at least 9.8 Ma, and with a single sample from 15.6 Ma. We dated detrital zircon grains by U‐Pb geochronology to reconstruct how erosion patterns changed through time. Long‐term increases in detrital zircon U‐Pb components of 750–1,200 and 1,500–2,300 Ma record increasing preferential erosion of the Himalaya relative to the Karakoram between 8.3–7.0 and 5.9–5.7 Ma. The average contribution of Karakoram‐derived sediment to the Indus Fan fell from 70% of the total at 8.3–7.0 Ma to 35% between 5.9 and 5.7 Ma. An increase in the contribution of 1,500–2,300 Ma zircons starting between 2.5 and 1.6 Ma indicates significant unroofing of the Inner Lesser Himalaya (ILH) by that time. The trend in zircon age spectra is consistent with bulk sediment Nd isotope data. The initial change in spatial erosion patterns at 7.0–5.9 Ma occurred during a time of drying climate in the foreland. The increase in ILH erosion postdated the onset of dry‐wet glacial‐interglacial cycles suggesting some role for climate control. However, erosion driven by rising topography in response to formation of the ILH thrust duplex, especially during the Pliocene, also played an important role, while the influence of the Nanga Parbat Massif to the total sediment flux was modest.

Architectural Complexities and Morphological Variations of the Indus Fan and its Elements: The Understanding of the Turbidite System Through Seismic Characterization

Architectural Complexities and Morphological Variations of the Indus Fan and its Elements: The Understanding of the Turbidite System Through Seismic Characterization

Erosion of the Himalaya-Karakoram recorded by Indus Fan deposits since the Oligocene

Abstract The Cenozoic erosion history of the Himalaya-Karakoram, which is a function of tectonically driven uplift and monsoon climatic evolution in South Asia, remains elusive, especially prior to the Miocene. Here, we present a multiproxy geochemical and thermochronological analysis of the oldest samples available from the Arabian Sea, which we used to investigate the erosion history of the Himalayan and Karakoram orogenic system. The Indus Fan records rapid and sustained erosion of the Himalayan-Karakoram mountains from before 24 Ma (ca. 30) to ca. 16 Ma concurrent with changing provenance from the Indian (Himalayan) and Eurasian plates. Our data, combined with previous studies of younger Indus Fan deposits, indicate that the mid-to-late Cenozoic erosion history of the Himalayan-Karakoram mountains is overall consistent with a vigorous monsoonal climate from the late Oligocene to middle Miocene and with changes in global climate in the late Miocene, whereas erosion and deposition are relatively insensitive to changes in sources and rock erodibility. Although tectonic processes were active throughout, we suggest that the erosional signatures of the Himalayan-Karakoram mountains from the Indus Fan largely preserve a record of climate changes since the Oligocene.

Marine sedimentary records of chemical weathering evolution in the western Himalaya since 17 Ma

AbstractThe Indus Fan derives sediment from the western Himalaya and Karakoram. Sediment from International Ocean Discovery Program drill sites in the eastern part of the fan coupled with data from an industrial well near the river mouth allow the weathering history of the region since ca. 16 Ma to be reconstructed. Clay minerals, bulk sediment geochemistry, and magnetic susceptibility were used to constrain degrees of chemical alteration. Diffuse reflectance spectroscopy was used to measure the abundance of moisture-sensitive minerals hematite and goethite. Indus Fan sediment is more weathered than Bengal Fan material, probably reflecting slow transport, despite the drier climate, which slows chemical weathering rates. Some chemical weathering proxies, such as K/Si or kaolinite/(illite + chlorite), show no temporal evolution, but illite crystallinity and the chemical index of alteration do have statistically measurable decreases over long time periods. Using these proxies, we suggest that sediment alteration was moderate and then increased from 13 to 11 Ma, remained high until 9 Ma, and then reduced from that time until 6 Ma in the context of reduced physical erosion during a time of increasing aridity as tracked by hematite/goethite values. The poorly defined reducing trend in weathering intensity is not clearly linked to global cooling and at least partly reflects regional climate change. Since 6 Ma, weathering has been weak but variable since a final reduction in alteration state after 3.5 Ma that correlates with the onset of Northern Hemispheric glaciation. Reduced or stable chemical weathering at a time of falling sedimentation rates is not consistent with models for Cenozoic global climate change that invoke greater Himalayan weathering fluxes drawing down atmospheric CO2 but are in accord with the idea of greater surface reactivity to weathering.

Seismic stratigraphy and the sedimentation history in the Laxmi Basin of the eastern Arabian Sea: Constraints from IODP Expedition 355

Detailed interpretation of seismic stratigraphic sequences in the Laxmi Basin of the eastern Arabian Sea are presented in this study using closely spaced high resolution multi-channel seismic (MCS) data. Our stratigraphic interpretation is further corroborated using recent drilling results in the Laxmi Basin, derived from the long sediment cores collected during International Ocean Discovery Program (IODP) Expedition 355. Integrated core-log interpretation discussed in the present study, offer important insights about the lithostratigraphic variations in this region. Analyses of multi-channel seismic reflection data reveal five depositional sequences (ranging from Paleocene to Recent) that led to the development of this marginal basin since the Cenozoic period. Regional igneous basement is successfully imaged, which was also validated by deep sea coring during the IODP Expedition 355. In the present study, we primarily focus on the post-rift sedimentation in the Laxmi Basin and its possible mechanisms. Our detailed interpretation in the prevailing tectonic framework of the basin suggests that near-shelf oldest volcaniclastic sedimentation immediately overlying the acoustic basement is linked to the onset of India–Madagascar and India–Seychelles rifting activities during the Late Cretaceous period. Eventually, during the Early-Mid to Late Miocene, the basin received maximum sedimentation dominantly through an extensive mass transport mechanism implying possible large-scale deformation on the Indian shelf. Subsequent sediment input to the basin appears to have been fed variably via the Indus Fan as well as coastal discharge from the Indian mainland. The total sediment thickness in the Laxmi Basin ranges from 1.1 to 3.5 km. New stratigraphic information and sediment isopach maps presented here provide vital information about syn- and post-rift sedimentation pattern in the region and their long term tectonic implications.

Provenance of Cenozoic Indus Fan Sediments (IODP Sites U1456 and U1457)

ABSTRACTProvenance analysis of IODP Expedition 355 cores in the Laxmi Basin sheds new light on the erosional evolution of the Himalayan belt and its western syntaxis during the Neogene and on large-scale mass-wasting and magmatic events that affected the western continental margin of India in the mid-Miocene and early Paleocene. In the cored Laxmi Basin succession, heavy minerals are far less affected by selective diagenetic dissolution than in foreland-basin sandstones exposed along the Himalayan front. Occurrence of euhedral aegirine and apatite in lower Paleocene mudrocks can be tied to alkaline volcanism affecting the adjacent western Indian margin during the late stage of Deccan activity. In the mid-Miocene Nataraja Slide (the second-largest mass-transport deposit reported from passive margins worldwide), dominant carbonate detritus and depleted heavy-mineral suites (including apatite, garnet, and locally augite or rare aegirine) reveal gravitational failure and sliding of the entire succession of carbonate and siliciclastic Paleogene to lower Neogene strata originally accumulated offshore of the Saurashtra margin of western India. Contrary to previous inferences, reworking of Indus-derived detritus by the slide was negligible. The overlying upper Miocene to lower Pleistocene turbidite package has the same feldspatho-litho-quartzose to litho-feldspatho-quartzose signature of modern Indus fluvio-deltaic sand, indicating that amphibolite-facies metamorphic rocks have been widely exposed in the Himalaya–Karakorum orogen since at least the mid-Miocene. Pleistocene nannofossil oozes with planktonic foraminifera at the top of the fan contain a very subordinate litho-feldspatho-quartzose terrigenous fraction including augitic clinopyroxene, suggesting mixing of dominant biogenic debris with minor detritus contributed both by the Indus River and by a river draining western peninsular India, possibly the paleo-Narmada or the paleo-Tapti.

A Reassessment of the Chronostratigraphy of Late Miocene C3–C4 Transitions

AbstractCombining magnetostratigraphic and carbon isotopic data for the late Miocene can provide a temporal framework for an isotopic shift first documented in soil carbonate nodules of northern Pakistan. The shift has been interpreted as a change in vegetation from trees and shrubs (using the C3 photosynthetic pathway) to grassland (using the C4 pathway). The cause of the event has been hotly debated and its timing is close to a shift in carbon isotopes in the marine realm. Further understanding depends critically on chronology. Unfortunately, temporal calibration of the various records published over decades relied on different time scales. To address the lack of a consistent chronology, we have reevaluated the constraints for the carbon isotopic shifts recorded from the Indian subcontinent. These show a diachronous transition ranging in age from about 7.8 Ma in Pakistan to as late at 6 Ma in Nepal. The record from IODP Expedition 355 Site U1457, drilled on the Indus fan shows that the transition in peninsular India began at about 7.2 Ma. Similar records from the African margin saw an earlier shift to C4 dominance starting around 10 Ma and those from Australia and South America transitioned later, during the Pliocene. The diachroneity around the globe does not invalidate pCO2 as a driver, but is consistent with it being one of several drivers of the global C4 expansion.

Miocene C4 Grassland Expansion as Recorded by the Indus Fan

AbstractIn the late Miocene, grasslands spread across the forested floodplains of the Himalayan foreland, but the causes of the ecological transition are still debated. Recent seafloor drilling by the International Ocean Discovery Program (IODP) provides an opportunity to study the transition across a larger region as archived in the Indus submarine fan. We present a multiproxy study of past vegetation change based on analyses of the carbon isotopic composition (δ13C) of bulk organic carbon, plant wax n‐alkanes and n‐alkanoic acids, and quantification of lignin phenols, charcoal, and pollen. We analyze the hydrogen isotopic composition (δD) of plant wax to reconstruct precipitation δD. We use the Branched and Isoprenoid Tetraether (BIT) index to diagnose shifts between terrestrial versus marine lipid inputs between turbidite and hemipelagic sediments. We reconstruct ocean temperatures using the TEX86 index only where marine lipids dominate. We find evidence for the late Miocene grassland expansion in both facies, confirming this was a regional ecosystem transformation. Turbidites contain dominantly terrestrial matter from the Indus catchment (D‐depleted plant wax), delivered via fluvial transport as shown by the presence of lignin. In contrast, hemipelagic sediments lack lignin and bear D‐enriched plant wax consistent with wind‐blown inputs from the Indian peninsula; these show a 7.4–7.2 Ma expansion of C4 grasslands on the Indian subcontinent. Within each facies, we find no clear change in δD values across the late Miocene C4 expansion, implying consistent distillation of rainfall by monsoon dynamics. Yet, a cooling in the Arabian Sea is coincident with the C4 expansion.

Clay-fraction strontium and neodymium isotopes in the Indus Fan: implications for sediment transport and provenance

AbstractReconstructing the provenance of siliciclastic marine sediment is important for understanding sediment pathways and constraining palaeoclimate and erosion records. However, physical fractionation of different size fractions can occur during sediment transport, potentially biasing records derived from bulk sediment. In this study, records of radiogenic Sr and Nd isotopic composition and K/Al ratio of the separated clay fraction, as well as bulk grain size, are presented, measured from deep-sea sediments recovered from International Ocean Discovery Program (IODP) Sites U1456 and U1457 in the Arabian Sea. These new records are compared with published bulk sediment records to investigate the influence of sediment transport on these proxies and to constrain provenance evolution and its relationship to climate variability since middle Miocene time. Correlations between grain size and the bulk sediment isotopic composition confirm that transport processes are influencing the bulk sediment record. This relationship, although present, is not as strong in the clay-fraction isotopic records. Heterogeneity of bulk sediment likely drives differences between bulk and clay records, thought to be largely controlled by sediment transport processes. The isotopic records reveal variations in provenance that correlate with climatic change at 8–7 Ma, as well as an increase in overall provenance variability beginning at c. 3.5 Ma, likely linked to monsoon strength and glacial–interglacial cycles. The clay-fraction records highlight the potential value of measuring proxy records from multiple size fractions to help constrain provenance records as well as investigate sediment transport and/or weathering and erosion processes recorded in deep-sea sediment archives.

Geomorphometric comparison of submarine channel-levee complexes with fluvial river systems: observations from the Indus

An analogy between submarine channels and fluvial rivers has existed for long, especially on the basis of planform and morphometry. Underlying this broad resemblance are the minute disparities that shape and control these systems. In order to observe and quantify the variations between submarine channels and subaerial rivers, we present a first-ever geomorphometric investigation of one single system, where the fluvial river is compared with its offshore counterpart from source-to-sink. With exhaustive data from the submarine fan, parameters like longitudinal profile, width, sinuosity, slope and planform of the Indus Fan channel-levee complex (CLC) are estimated and compared on the basis of the same parameters estimated for the fluvial Indus River. Our new data analyses offers key insights into the variable geomorphometric patterns prevalent from the source of the Indus River until the margins of the submarine Indus Fan. Channel width and sinuosity vary from high-to-low downstream in the submarine system and from low to high in the fluvial basin. Characteristic depositional features of either system are mutually exclusive. Longitudinal profiles of the submarine fan and the river basin do not conform—principally due to the difference in intensity of erosional and depositional processes active in both regions. These differences are primarily attributed to a single-point (canyon-fed) distributary flow and a multi-point (tributary-fed) cumulative flow source system, and density contrasts between river flows and turbidity currents. By quantifying this variation, our attempt is to dissuade the long-standing morphometric analogy between fluvial rivers and submarine channels.

Late Miocene changes in C3, C4 and aquatic plant vegetation in the Indus River basin: evidence from leaf wax δ13C from Indus Fan sediments

AbstractVegetation changes in the Indus River basin within the past 10.8 million years were investigated based on the analysis of n-fatty acids and their carbon isotopes in sediments from IODP Site U1457 in the Laxmi Basin of the Arabian Sea. The δ13C of long-chain n-C32 fatty acid shifted from −34 to −22 ‰ from 10 to 6.3 Ma, while the δ13C of mid-chain n-C24 fatty acid was nearly constant at around −23 to −22 ‰ over the same period. This large difference in the δ13C values suggests that the mid-chain fatty acids reflect the contribution of aquatic vascular C3 plants. Before 6.3 Ma, the average chain length of n-fatty acids and the δ13C values of long-chain fatty acids were negatively correlated, suggesting that the δ13C values reflected the relative abundance of terrestrial C3 versus aquatic C3 plants in the Indus River basin and western India. After 5.8 Ma, the average chain length was variable, but the δ13C values remained nearly the same, suggesting that the δ13C values reflected heavier δ13C values of both aquatic C3 and C4 plants. A three-end-member model calculation suggests that terrestrial C3 plants were replaced by C4 plants in the Indus River basin and western India from 9.7 or 8.2 to 6.3 Ma. Aridification in those areas during the late Miocene period may have driven the replacement of terrestrial C3 plants by C4 plants. An episodic increase in the abundance of terrestrial plants around 8 Ma is attributed to elevated precipitation by regionally enhanced moisture transport.

Slowing rates of regional exhumation in the western Himalaya: fission track evidence from the Indus Fan

AbstractWe use apatite fission track ages from sediments recovered by the International Ocean Discovery Program in the Laxmi Basin, Arabian Sea, to constrain exhumation rates in the western Himalaya and Karakoram since 15.5 Ma. With the exception of a Triassic population in the youngest 0.93 Ma samples supplied from western Peninsular India, apatite fission track ages are overwhelmingly Cenozoic, largely <25 Ma, consistent with both a Himalaya–Karakoram source and rapid erosion. Comparison of the minimum cooling age of each sample with depositional age (lag time) indicates an acceleration in exhumation between 7.8 and 7.0 Ma, with lag times shortening from ∼6.0 Myr at 8.5–7.8 Ma to being within error of zero between 7.0 and 5.7 Ma. Sediment supply at 7.0–5.7 Ma was largely from the Karakoram, and to a lesser extent the Himalaya, based on U–Pb zircon ages from the same samples. This time coincides with a period of drying in the Himalayan foreland caused by weaker summer monsoons and Westerly winds. It also correlates with a shift of erosion away from the Karakoram, Kohistan and the Tethyan Himalaya towards more erosion of the Lesser and Greater Himalaya and Nanga Parbat, as shown by zircon U–Pb provenance data, and especially after 5.7 Ma based on Nd isotope data. Samples younger than 5.7 Ma have lag times of ∼4.5 Myr, similar to Holocene Indus delta sediments.

Integrating heavy-mineral, geochemical and biomarker analyses of Plio-Pleistocene sandy and silty turbidites: a novel approach for provenance studies (Indus Fan, IODP Expedition 355)

AbstractA multidisciplinary mineralogical, geochemical and biomarker study of Indus Fan sediments cored during International Ocean Discovery Program (IODP) Expedition 355 to the Laxmi Basin was carried out to define the different compositional signatures of sand, silt and clay. Upper Pliocene – lower Pleistocene turbidites from sites U1456 and U1457 were selected as the best candidates for this study. The integrated dataset presented here was obtained by coupling traditional and innovative bulk-sediment and single-mineral techniques on the same samples. Turbiditic deposits mostly consist of medium to fine silt, including rich and diverse heavy-mineral assemblages. Such a fine grain size forced us to push the limits of high-resolution quantitative heavy-mineral analysis down to as low as 5 μm. Heavy-mineral analysis allowed us to establish a Himalayan origin of the detritus in the studied turbidites. Heavy-mineral concentrations are higher in channel-fill than in overbank deposits. Mineralogical and geochemical data concur in revealing that fast-settling ultradense minerals such as zircon are preferentially concentrated in channel-fill deposits, whereas the top of overbank deposits are notably enriched with slow-settling platy phyllosilicates. Biomarker analysis represents a most suitable complementary technique that is able to investigate the provenance signature of the finer sediment fraction, largely consisting of clay. This technique allowed us to identify a largely terrigenous origin of organic matter at Site U1456 and an open marine origin at Site U1457. The latter site lies closer to the Laxmi Ridge, where thermal maturity increases with depth to reach the early oil window (127°C at c. 320 m below the seafloor).

A late Pleistocene sedimentation in the Indus Fan, Arabian Sea, IODP Site U1457

AbstractThe intensity of turbidite sedimentation over long timescales is driven by sea-level change, tectonically driven rock uplift and climatically modulated sediment delivery rates. This study focuses on understanding the effect of sea-level fluctuations and climatic variability on grain-size variations. The grain size and environmental magnetic parameters of Arabian Sea sediments have been documented using 203 samples, spanning the last 200 ka, obtained from International Ocean Discovery Program (IODP) Site U1457. Grain-size end-member modelling suggests that between ~200 and 130 ka there was an increase in the coarse silt fraction caused by sediment transport following reworking of the Indus Fan and development of deep-sea canyons. The sediment size and enhanced magnetic susceptibility indicate a dominant flux of terrestrial sediments. Sedimentation in the distal Indus Fan atc. 200–130 ka was driven by a drop in sea level that lowered the base level in the Indus and Narmada river systems. The low sea-stand caused incision in the Indus delta, canyons and fan area, which resulted in the transportation of coarser sediment at the drilling site. Magnetic susceptibility and other associated magnetic parameters suggest a large fraction of the sediment was supplied by the Narmada River during ~200–130 ka. Since ~130 ka, clay-dominated sedimentation is attributed to the rise in sea level due to warm and wet climate.

Variations in δ13C values of sedimentary organic matter since late Miocene time in the Indus Fan (IODP Site 1457) of the eastern Arabian Sea

AbstractA 1108.6 m long core was recovered at Site U1457 located on the Indus Fan in the Laxmi Basin of the eastern Arabian Sea during IODP Expedition 355. Shipboard examinations defined five lithologic units (I to V) of the lower Paleocene to Holocene sedimentary sequence. In this study, δ13C values of sedimentary organic matter (SOM) confirm the differentiation of the lithologic units and further divide units III and IV into two subunits (1 and 2). Based on the underlying assumption that the SOM is decided primarily by a mixture of marine and terrestrial origins, δ13CSOM values at Site U1457 provide information on the terrestrial catchment conditions since late Miocene time. Low δ13CSOM values from late Miocene to late Pleistocene times are similar (c. −22.0 ‰) for the most part, reflecting a consistent contribution of terrestrial organic matter from the catchment areas characterized by dominant C3 land plants. Significantly lower δ13CSOM values (c. −24.0 ‰) in Unit III-2 (∼8 to ∼7 Ma) might be due to a greater input of C3 terrestrial organic matter. The increase in δ13CSOM values at ∼7 Ma and the appearance of high δ13CSOM values (c. −18.0 ‰) within Unit III-1 (∼7 to ∼2 Ma) indicate that C4 biomass overwhelmed the terrestrial catchment environment as a result of enhanced terrestrial aridity in the Himalayan foreland. The three-end-member simple mixing model, estimating the relative contributions of SOM from terrestrial C3 and C4 plants and marine phytoplankton, supports our interpretation of the distribution of C3 and C4 land plants in the terrestrial catchment environment.