Sedimentology,stratigraphy and architecture of the Nicobar Fan (Bengal–Nicobar Fan System), Indian Ocean: Results from International Ocean Discovery Program Expedition 362

Drill sites in the southern Bay of Bengal at 3°N 91°E (International Ocean Discovery Program Expedition 362) have sampled for the first time a complete section of the Nicobar Fan and below to the oceanic crust. This generally overlooked part of the Bengal–Nicobar Fan System may provide new insights into uplift and denudation rates of the Himalayas and Tibetan Plateau. The Nicobar Fan comprises sediment gravity-flow deposits, mostly turbidites, that alternate with hemipelagite drapes and pelagite intervals of varying thicknesses. The decimetre-thick to metre-thick oldest pre-fan sediments (limestones/chalks) dated at 69 Ma are overlain by volcanic material and slowly accumulated pelagites (0.5 g cm⁻² kyr⁻¹). At Expedition 362 Site U1480, terrigenous input began in the early Miocene at ca 22.5 Ma as muds, overlain by very thin-bedded and thin-bedded muddy turbidites at ca 19.5 Ma. From 9.5 Ma, sand content and sediment supply sharply increase (from 1–5 to 10–50 g cm⁻² kyr⁻¹). Despite the abundant normal faulting in the Nicobar Fan compared with the Bengal Fan, it offers a better-preserved and more homogeneous sedimentary record with fewer unconformities. The persistent connection between the two fans ceased at 0.28 Ma when the Nicobar Fan became inactive. The Nicobar Fan is a major sink for Himalaya-derived material. This study presents integrated results of International Ocean Discovery Program Expedition 362 with older Deep Sea Drilling Project/Ocean Drilling Program/International Ocean Discovery Program sites that show that the Bengal–Nicobar Fan System experienced successive large-scale avulsion processes that switched sediment supply between the Bengal Fan (middle Miocene and late Pleistocene) and the Nicobar Fan (late Miocene to early Pleistocene). A quantitative analysis of the submarine channels of the Nicobar Fan is also presented, including their stratigraphic frequency, showing that channel size/area and abundance peaked at ca 2 to 3 Ma, but with a distinct low at 3 to 7 Ma: the intervening stratigraphic [sub]unit was a time of reduced sediment accumulation rates.     

The anatomy of exhumed river-channel belts: Bedform to belt-scale river kinematics of the Ruby Ranch Member,Cretaceous Cedar Mountain Formation,Utah, USA

Many published interpretations of ancient fluvial systems have relied on observations of extensive outcrops of thick successions. This paper, in contrast, demonstrates that a regional understanding of palaeoriver kinematics, depositional setting and sedimentation rates can be interpreted from local sedimentological measurements of bedform and barform strata. Dune and bar strata, channel planform geometry and bed topography are measured within exhumed fluvial strata exposed as ridges in the Ruby Ranch Member of the Cretaceous Cedar Mountain Formation, Utah, USA. The ridges are composed of lithified stacked channel belts, representing at least five or six re-occupations of a single-strand channel. Lateral sections reveal well-preserved barforms constructed of subaqueous dune cross-sets. The topography of palaeobarforms is preserved along the top surface of the outcrops. Comparisons of the channel-belt centreline to local palaeotransport directions indicate that channel planform geometry was preserved through the re-occupations, rather than being obscured by lateral migration. Rapid avulsions preserved the state of the active channel bed and its individual bars at the time of abandonment. Inferred minimum sedimentation durations for the preserved elements, inferred from cross-set thickness distributions and assumed bedform migration rates, vary within a belt from one to ten days. Using only these local sedimentological measurements, the depositional setting is interpreted as a fluvial megafan, given the similarity in river kinematics. This paper provides a systematic methodology for the future synthesis of vertical and planview data, including the drone-equipped 2020 Mars Rover mission, to exhumed fluvial and deltaic strata.     

Field estimate of paleoseismic slip on a normal fault using the Schmidt hammer and terrestrial LiDAR: Methods and application to the Hebgen fault (Montana,USA)

The weathering characteristics of bedrock fault scarps provide relative age constraints that can be used to determine fault displacements. Here, we report Schmidt hammer rebound values (R‐values) for a limestone fault scarp that was last exposed in the 1959 Mw 7.3 Hebgen Lake, Montana earthquake. Results show that some R‐value indices, related to the difference between minimum and maximum R‐values in repeated impacts at a point, increase upward along the scarp, which we propose is due to progressive exposure of the scarp in earthquakes. An objective method is developed for fitting slip histories to the Schmidt hammer data and produces the best model fit (using the Bayesian Information Criterion) of three earthquakes with single event displacements of ≥ 1.20 m, 3.75 m, and c. 4.80 m. The same fitting method is also applied to new terrestrial LiDAR data of the scarp, though the LiDAR results may be more influenced by macro‐scale structure of the outcrop than by differential weathering. We suggest the use of this fitting procedure to define single event displacements on other bedrock fault scarps using other dating techniques. Our preliminary findings demonstrate that the Schmidt hammer, combined with other methods, may provide useful constraints on single event displacements on exposed bedrock fault scarps. Copyright © 2018 John Wiley & Sons, Ltd.     

The interplay between extrinsic and intrinsic controls in determining floodplain wetland characteristics in the South African drylands

Controls on the characteristics of floodplain wetlands in drylands are diverse and may include extrinsic factors such as tectonic activity, lithology and climate, and intrinsic thresholds of channel change. Correct analysis of the interplay between these controls is important for assessing possible channel–floodplain responses to changing environmental conditions. Using analysis of aerial imagery, geological maps and field data, this paper investigates floodplain wetland characteristics in the Tshwane and Pienaars catchments, northern South Africa, and combines the findings with previous research to develop a new conceptual model highlighting the influence of variations in aridity on flow, sediment transport, and channel–floodplain morphology. The Tshwane–Pienaars floodplain wetlands have formed in response to a complex interplay between climatic, lithological, and intrinsic controls. In this semi‐arid setting, net aggradation (alluvium >7 m thick) in the wetlands is promoted by marked downstream declines in discharge and stream power that are related to transmission losses and declining downstream gradients. Consideration of the Tshwane–Pienaars wetlands in their broader catchment and regional context highlights the key influence of climate, and demonstrates how floodplain wetland characteristics vary along a subhumid to semi‐arid climatic gradient. Increasing aridity tends to be associated with a reduction in the ability of rivers to maintain through‐going channels and an increase in the propensity for channel breakdown and floodout formation. Understanding the interplay between climate, hydrology and geomorphology may help to anticipate and manage pathways of floodplain wetland development under future drier, more variable climates, both in South African and other drylands. Copyright © 2016 John Wiley & Sons, Ltd.     

Channel‐planform evolution in four rivers of Olympic National Park,Washington, USA: the roles of physical drivers and trophic cascades

Identifying the relative contributions of physical and ecological processes to channel evolution remains a substantial challenge in fluvial geomorphology. We use a 74‐year aerial photographic record of the Hoh, Queets, Quinault, and Elwha Rivers, Olympic National Park, Washington, USA, to investigate whether physical or trophic‐cascade‐driven ecological factors – excessive elk impacts after wolves were extirpated a century ago – are the dominant drivers of channel planform in these gravel‐bed rivers. We find that channel width and braiding show strong relationships with recent flood history. All four rivers widened significantly after having been relatively narrow in the 1970s, consistent with increased flood activity since then. Channel planform also reflects sediment‐supply changes, evident from landslide response on the Elwha River. We surmise that the Hoh River, which shows a multi‐decadal trend toward greater braiding, is adjusting to increased sediment supply associated with rapid glacial retreat. These rivers demonstrate transmission of climatic signals through relatively short sediment‐routing systems that lack substantial buffering by sediment storage. Legacy effects of anthropogenic modification likely also affect the Quinault River planform. We infer no correspondence between channel evolution and elk abundance, suggesting that trophic‐cascade effects in this setting are subsidiary to physical controls on channel morphology. Our findings differ from previous interpretations of Olympic National Park fluvial dynamics and contrast with the classic example of Yellowstone National Park, where legacy effects of elk overuse are apparent in channel morphology; we attribute these differences to hydrologic regime and large‐wood availability. Published 2016. This article is a U.S. Government work and is in the public domain in the USA     

Testing the utility of structure‐from‐motion photogrammetry reconstructions using small unmanned aerial vehicles and ground photography to estimate the extent of upland soil erosion

Quantifying the extent of soil erosion at a fine spatial resolution can be time consuming and costly; however, proximal remote sensing approaches to collect topographic data present an emerging alternative for quantifying soil volumes lost via erosion. Herein we compare terrestrial laser scanning (TLS), and both unmanned aerial vehicle (UAV) and ground photography (GP) structure‐from‐motion (SfM) derived topography. We compare the cost‐effectiveness and accuracy of both SfM techniques to TLS for erosion gully surveying in upland landscapes, treating TLS as a benchmark. Further, we quantify volumetric soil loss estimates from upland gullies using digital surface models derived by each technique and subtracted from an interpolated pre‐erosion surface. Soil loss estimates from UAV and GP SfM reconstructions were comparable to those from TLS, whereby the slopes of the relationship between all three techniques were not significantly different from 1:1 line. Only for the TLS to GP comparison was the intercept significantly different from zero, showing that GP is more capable of measuring the volumes of very small erosion features. In terms of cost‐effectiveness in data collection and processing time, both UAV and GP were comparable with the TLS on a per‐site basis (13.4 and 8.2 person‐hours versus 13.4 for TLS); however, GP was less suitable for surveying larger areas (127 person‐hours per ha^?1 versus 4.5 for UAV and 3.9 for TLS). Annual repeat surveys using GP were capable of detecting mean vertical erosion change on peaty soils. These first published estimates of whole gully erosion rates (0.077 m a^?1) suggest that combined erosion rates on gully floors and walls are around three times the value of previous estimates, which largely characterize wind and rainsplash erosion of gully walls. Copyright © 2017 John Wiley & Sons, Ltd.     

Plate tectonics in relation to mantle temperatures and metamorphic properties

When plate tectonics emerged and how it has evolved over Earth history are two of the most fundamental challenges in Earth Sciences. These questions are tackled using a holistic approach to analyze tectonic styles in the history of Earth, giving rise to the interpretation of two styles of plate tectonics since the Archean. In these interpretations, there are different styles of deformation and metamorphism between early times dominated by warm subduction, and later times preferring cold subduction.The two styles of plate tectonics are recorded by different properties of regional metamorphism at convergent plate boundaries,which are linked to the differences in mantle temperature between the Archean and Phanerozoic. A transition to modern plate tectonics is recorded by the signature of blueschist facies metamorphism developed in the Neoproterozoic. This is consistent with geological evidence for the operation of ancient plate tectonics since the early Archean. The temporal cooling of the mantle explains the geochemical trends of mantle-derived melts, the likely change from numerous small plates to fewer but larger plates,changes in thickness and preservation of oceanic crust and lithosphere in accretionary and collisional orogens, and led to the oxygenation of the surface environment providing the environments needed to foster life.     

Radiometric dating of the Middle Palaeolithic tool industry and associated fauna of Pin Hole Cave,Creswell Crags,England

Uranium-series dating of derived speleothem suggests that the sediments enclosing a Middle Palaeolithic stone artefact assemblage in Pin Hole Cave probably accumulated after about 64 ka, and ¹⁴C dates indicate a likely age of > 40 ka for the large mammal fauna associated with it. Electron spin resonance data from the fauna conform with these age constraints and are consistent with accumulation between 38 and 50 ka. This evidence supports the view that Britain was recolonised by hominids during Oxygen Isotope Stage 3. Stratigraphically higher stone tool industries demonstrate the local presence of both early Upper and late Upper Palaeolithic cultures. © 1998 John Wiley & Sons, Ltd.     

Soil constraints on Northwest Yucatán,Mexico: Pedoarchaeology and Maya subsistence at Chunchucmil

The soils and subsistence of ancient Maya Chunchucmil in northwestern Yucatán are the focus of this paper. Today and historically, the population and crop yields here have been very low. Archaeological field work, however, has shown the Late Classic site to be highly populated with densely packed walled mound and field groups. It is enigmatic that this high ancient Maya population existed in a region of meager crop and soil potential. This enigma is addressed by investigating contemporary Maya agriculture, geoarchaeological evidence, and soil potential for intensive agriculture. The local Maya soil classification of kancab and boxluum synthesizes the Alfisols, Inceptisols, and Mollisols described here. The major soil limitations are shallowness, broad areas with no soil, insufficient water holding capacity, and variable deficiencies in phosphorous, potassium, and zinc. Evidence for intensive agriculture and alternative crops can be seen in widespread field walls compartmentalizing the landscape, sascaberas, and preliminary phosphate fractionation signatures. © 1998 John Wiley & Sons, Inc.