Distal Settings Research Articles

Sedimentary characteristics of Cenozoic strata in central-southern Ningxia, NW China: Implications for the evolution of the NE Qinghai–Tibetan Plateau

Cenozoic sedimentary deposits in central-southern Ningxia province, NW China are an important record of Tertiary tectonic events along the evolving Qinghai–Tibetan Plateau’s northeast margin. Shortly after the onset of the Indo-Eurasia collision to the south, a thrust belt and adjoining foreland basin began to form during 40–30 Ma. The Eocene Sikouzi Formation developed in a distal setting to this basin, in normal fault-bound basins that may have formed in a forebulge setting. Subsequent deposition of the Oligocene Qingshuiying Formation occurred during a phase of apparently less intense tectonism and the previous underfilled foreland basin became overfilled. During the Early Miocene, contractional deformation was mainly distributed to the west of the Liupan Shan. This resulted in deformation of the Qingshuiying Formation as indicated by an unconformity with the overlying Miocene Hongliugou Formation. The unconformity occurs proximal to the Haiyuan Fault suggesting that the Haiyuan Fault may have begun movement in the Early Miocene. In the Late Miocene, thrusting occurred west of the southern Helan Shan and an unconformity developed between the Hongliugou and Qingshuiying Formations proximal to the the Cha-Gu Fault. Relationships between the Miocene stratigraphy and major faults in the region imply that during the Late Miocene the deformation front of the Qinghai–Tibetan Plateau had migrated to the Cha-Gu Fault along the western Ordos Margin, and the Xiang Shan was uplifted. Central-southern Ningxia was then incorporated into the northeast propagating thrust wedge. The driving force for NE propagation of the thrust wedge was most likely pronounced uplift of the northeastern plateau at the same time. Analysis of the sedimentary record coupled with consideration of the topographic evolution of the region suggests that the evolving fold-and-thrust belt experienced both forward-breaking fold-and-thrust belt development, and out-of-sequence fault displacements as the thrust wedge evolved and the foreland basin became compartmentalised. The documented sedimentary facies and structural relationship also place constraints on the Miocene-Recent evolution of the Yellow River and its tributaries.

Evidence for pervasive mud volcanism in Acidalia Planitia, Mars

We have mapped 18,000+ circular mounds in a portion of southern Acidalia Planitia using their sizes, shapes, and responses in Nighttime IR. We estimate that 40,000+ of these features could occur in the area, with a distribution generally corresponding to the southern half of the proposed Acidalia impact basin. The mounds have average diameters of about 1 km and relief up to 180 m and most overlie units mapped as Early Amazonian. High resolution images of mound surfaces show relatively smooth veneers, apron-like extensions onto the plains, moats, and concentric circular crestal structures. Some images show lobate and flow-like features associated with the mounds. Albedo of the mounds is generally higher than that of the surrounding plains. Visible and near-infrared spectra suggest that the mounds and plains have subtle mineralogical differences, with the mounds having enhanced coatings or possibly greater quantities of crystalline ferric oxides. Multiple analogs for these structures were assessed in light of new orbital data and regional mapping. Mud volcanism is the closest terrestrial analogy, though the process in Acidalia would have had distinctly martian attributes. This interpretation is supported by the geologic setting of the Acidalia which sits at the distal end of the Chryse–Acidalia embayment into which large quantities of sediments were deposited through the Hesperian outflow channels. In its distal position, Acidalia would have been a depocenter for accumulation of mud and fluids from outflow sedimentation. Thus, the profusion of mounds in Acidalia is likely to be a consequence of this basin’s unique geologic setting. Basinwide mud eruption may be attributable to overpressure (developed in response to rapid outflow deposition) perhaps aided by regional triggers for fluid expulsion related to events such as tectonic or hydrothermal pulses, destabilization of clathrates, or sublimation of a frozen body of water. Significant release of gas may have been involved, and the extensive mud volcanism could have created long-lived conduits for upwelling groundwaters, providing potential habitats for an in situ microbiota. Mud volcanism transports minimally-altered materials from depth to the surface, and mud volcanoes in Acidalia, therefore, could provide access to samples from deep zones that would otherwise be inaccessible. Since the distal setting of Acidalia also would favor concentration and preservation of potentially-present organic materials, samples brought to the surface by mud volcanism could include biosignatures of possible past or even present life. Accordingly, the mounds of Acidalia may offer a new class of exploration target.

Alpine peridotites from the Ligurian Tethys: an updated critical review

We present an updated overview of the petrogenetic evolution of upper mantle peridotites exposed in the Alpine–Apennine ophiolites; the latter represent fragments of Jurassic oceanic lithosphere derived from the Ligurian–Piedmontese basin. This synthesis is based on the results of recent multidisciplinary studies of these peridotite massifs in the Eastern, Central and Western Alps, Ligurian Alps, Northern Apennines and Alpine Corsica, and has been developed within the framework of the geodynamic evolution of the Ligurian–Piedmontese basin between Europe and Adria. The Ligurian–Piedmontese basin began forming through continental rifting during the Triassic. As the continental lithosphere was progressively stretched and thinned, the underlying asthenosphere underwent adiabatic upwelling and decompressional partial melting. The subsequent crustal break-up during the Jurassic led to the development of the non-volcanic passive margins of Europe and Adria, and to the exhumation and exposure of the sub-continental lithospheric mantle on the seafloor. The rifting stage of the basin was characterized by percolation through diffuse porous flow of asthenospheric mid-ocean ridge basalt (MORB) melts via the continental lithospheric mantle. Melt–rock interaction caused depletion/re-fertilization of the lithospheric mantle and generated the extreme compositional heterogeneity of the upper mantle peridotites, which were later exposed on the seafloor at distal settings of the basin. Depleted spinel peridotites, which were previously interpreted as refractory residua after asthenosphere partial melting, are now recognized as reactive peridotites formed by melt–rock interaction. MORB melts migrating through the sub-continental lithospheric mantle along the axial zone of the extending system modified the upper mantle peridotites significantly and were entrapped by interstitial crystallization.

Litho‐ and chronostratigraphy of the Late Weichselian in Vendsyssel, northern Denmark, with special emphasis on tunnel‐valley infill in relation to a receding ice margin

Lithostratigraphy and chronostratigraphy of samples from 18 deep boreholes in Vendsyssel have resulted in new insight into the Late Weichselian glaciation history of northern Denmark. Prior to the Late Weichselian Main advance c. 23–21 kyr BP, Vendsyssel was part of an ice‐dammed lake where the Ribjerg Formation was deposited c. 27–23 kyr BP. The timing of the Late Weichselian deglaciation is well constrained by the Main advance and the Lateglacial marine inundation c. 18 kyr BP, and thus spans only a few millennia. Rapid deposition of more than 200 m of sediments took place mainly in a highly dynamic proglacial and ice‐marginal environment during the overall ice recession. Mean retreat rates have been estimated as 45–50 m/yr in Vendsyssel with significantly higher retreat rates between periods of standstill and re‐advance. The deglaciation commenced in Vendsyssel c. 20 kyr BP, and the Troldbjerg Formation was deposited c. 20–19 kyr BP in a large ice‐dammed lake in front of the receding ice sheet, partly as glaciolacustrine sediments and partly as rapid and focused sedimentation in prominent ice‐contact fans, which make up the Jyske Ås and Hammer Bakker moraines. In the northern part of central Vendsyssel, at least four generations of north–south orientated tunnel valleys are identified, each generation related to a recessional ice margin. This initial deglaciation was interrupted by a major re‐advance from the east c. 19 kyr BP, which covered most of Vendsyssel. An ice‐dammed lake formed in front of the ice sheet as it retreated towards the east; the Morild Formation was deposited here c. 19–18 kyr BP. Related to this stage of deglaciation, eight ice‐marginal positions have been identified based on the distribution of large tunnel‐valley systems and pronounced recessional moraines. The Morild Formation consists of glaciolacustrine sediments, including the sediment infill of more than 190 m deep tunnel valleys, as well as the sediments in recessional moraines, which were formed as ice‐contact sedimentary ridges, possibly in combination with glaciotectonic deformation. The character of the tunnel‐valley infill sediments was determined by proximity to the ice margin. During episodes of rapid retreat of the ice margin, tunnel valleys were quickly abandoned and filled with fine‐grained sediments in a distal setting. During slow retreat of the ice margin, tunnel valleys were filled in an ice‐proximal environment, and the infill consists of alternating layers of fine‐ to coarse‐grained sediments. At c. 18 kyr BP, Vendsyssel was inundated by the sea, when the Norwegian Channel Ice Stream broke up, and a succession of marine sediments (Vendsyssel Formation) was deposited during a forced regression.

Palynofacies, organic geochemistry and depositional environment of the Tartan Formation (Late Paleocene), a potential source rock in the Great South Basin, New Zealand

Detailed palynofacies analysis of sidewall core samples taken from below, within and above the Tartan Formation (Thanetian, Late Paleocene, 58.7–55.8 Ma), a potential source rock in the epeiric Great South Basin, shows that the formation is characterised by very high percentages of degraded brown phytoclasts, rare marine algae and amorphous organic matter and thereby represents a mix of terrestrial and marine kerogen. The results indicate that the formation was deposited in a marginally marine (hyposaline), proximal environment under bottom conditions that varied from anoxic to oxic along a nearshore–offshore transect. Samples from the upper part of the underlying Wickliffe Formation indicate deposition in a marginal to normal marine, proximal environment under anoxic to oxic bottom environments. The lower part of the overlying Laing Formation was deposited in an open marine, relatively distal setting under anoxic to oxic bottom environments. The palaeodepositional changes observed through this sequence may be best explained as a result of base-level changes in a relatively shallow-water sea. The Tartan Formation was deposited in the central, southern and eastern parts of the Great South Basin during a peak regression in the Thanetian that terminated an overall mid- to Late Paleocene aggradational to regressive trend in the basin and gave way to a latest Paleocene–earliest Eocene transgression. Its earliest and thickest known occurrence is in the Pakaha-1 well, in the centre of the basin. The formation is absent from proximal offshore wells and absent to very thin in onshore Otago drill cores and outcrops, most probably due to sediment bypass or subaerial erosion. The Tartan Formation is characterised by a combination of high organic carbon contents and 13C enrichment relative to the enclosing formations; C org and TOC values are mostly in the range 3.7–17.1% (mean 8.0%) and δ 13C values, −21.4 to −15.8‰, up to c. 9‰ heavier than in the under- and overlying formations. For sidewall core samples, there is a strong correlation between TOC and δ 13C values (R 2 = 0.91). The heavy δ 13C values can only partly be attributed to the global Paleocene Carbon Isotope Maximum, and indicate that other, organic matter source and/or regional depositional factors affected carbon fractionation in the Tartan kerogen. The Tartan Formation displays very good-excellent petroleum generative potential (up to 31 mg HC/g rock), but HI values presented herein range only from 157 to 268 mg HC/g TOC (mean 203 mg HC/g TOC), indicating the potential is primarily for gas, with only minor oil. The relatively poor HI values are attributed to the large influx of degraded woody material, rendering the Tartan Formation less oil-prone than previously inferred. The Tartan Formation is biostratigraphically coeval with the Waipawa Formation of the New Zealand East Coast Basin, which is a similar source rock unit also enriched in 13C. Samples from the Waipawa Formation taken 1000 km north of the Great South Basin share palynofacies and geochemical characteristics with the Tartan Formation, indicating that a base-level fall also controlled the deposition of the Waipawa Formation in the East Coast Basin. The size of the area affected by the base-level fall and the lack of evidence for contemporaneous large-scale tectonic uplift imply that the base-level fall was caused by eustasy. This suggests that the Tartan and Waipawa Formations may be more extensive in New Zealand offshore basins than previously thought.

Depositional record of historic lahars in the upper Whangaehu Valley, Mt. Ruapehu, New Zealand: implications for trigger mechanisms, flow dynamics and lahar hazards

Mt. Ruapehu, in the central North Island of New Zealand, is one of the most lahar-prone volcanoes in the world. Since historic observations began in 1861 AD, more than 50 individual lahars have been recorded in the Whangaehu valley alone, the natural outlet to the summit Crater Lake. These lahars have been triggered by a variety of mechanisms, including explosive eruptions that displaced Crater Lake water over the outlet or ejected it onto the snow-clad summit area of the volcano; rain-remobilisation of tephra deposits on steep slopes; displacement over the outlet as a result of syn-eruptive changes in lake bathymetry; and lake break-outs from Crater Lake following impoundment of excess water behind temporary barriers of tephra and/or ice emplaced over the outlet. However, only 9 lahar deposits can be distinguished in the upper Whangaehu valley on sedimentological, stratigraphic, geomorphic and petrological grounds, and these are skewed towards either the largest or the most recent flows. In some cases magnitude can be reconstructed from deposit geometry, with the largest lahars producing the highest level terraces, the coarsest deposits, and crossing drainage divides into normally inactive channels. This under-representation of historic events reflects the low preservation potential of unconsolidated deposits in a steep alpine environment, and the overprinting and recycling effect of large magnitude lahars that rework material down to bedrock and effectively reset the stratigraphic record. Development of magnitude-frequency relationships for Ruapehu lahars therefore requires the identification of lahar deposits in proximal, medial and distal settings in order to ensure that the full range of events is represented.

Oceanic spreading center–generated basaltic crust and associated sulfidic and carbonate-rich hydrothermal deposits in the Archean (ca. 3 Ga), North Spirit Lake greenstone belt, Ontario, Canada

Detailed mapping of a portion of the North Spirit Lake greenstone belt reveals a basal basaltic volcanic unit (ca. 3 Ga) with a komatiitic component and strong major and trace element geochemical affinity with typical mid-ocean ridge basalts (e.g., N-MORB). The overlying transitional unit includes basaltic tuffs, which have close geochemical affinity to the underlying basalts, and to N-MORB. Interbedded with and overlying the tuffs is a thin, laterally continuous sulfide-rich unit with chert interbeds, which is in turn overlain by interbedded carbonate, chert, and iron-rich sedimentary rocks. The chemical sedimentary rocks of the transitional unit display rare-earth element (REE) patterns and positive Eu anomalies consistent with derivation from fluids associated with submarine hydrothermal vent fields. The sulfide unit is interpreted to represent proximal hydrothermal vent deposits derived from black smoker activity near a spreading center. The overlying carbonate and chert mounds and beds are interpreted as hydrothermal deposits formed at lower temperatures in a more distal setting relative to the spreading center. The carbonate deposits may have formed in an environment analogous to that of the Lost City hydrothermal field (Kelley et al., 2001), where abundant chimneys, dendrites, and beds of carbonate are produced ∼15 km from the mid-Atlantic rift axis. The basal volcanic unit consequently is interpreted as ocean-floor basalts formed at an Archean spreading center, with the basaltic tuffs representing the last volcanic emanations, and with the chemical sediments representing hydrothermal-vents deposits both proximal and distal to the spreading center. The capping siliciclastic unit likely is part of an accretionary wedge, possibly representing ocean closure after a protracted seafloor spreading episode. The sedimentary rocks were derived, in part, from a weathered tonalite-trondhjemite-granodiorite source. The supracrustal rocks of the North Spirit Lake greenstone belt provide an unusually clear window into some tectonic activity on the primitive Earth, including evidence for the existence of ocean-floor basalts, proximal and distal hydrothermal activity, and possibly a protracted cycle of ocean opening and destruction.

Widespread syn‐sedimentary deformation on a muddy deep‐water basin‐floor: the Vischkuil Formation (Permian), Karoo Basin, South Africa

ABSTRACTThe ∼380‐m‐thick mudstone–siltstone‐dominated Vischkuil Formation represents the initiation phase of a 1.3‐km‐thick prograding basin floor to slope to shelf succession that marks a significant increase in the rate of siliciclastic sediment supply to the early Karoo Basin in the Permian. In the upper Vischkuil Formation three well exposed, widespread (∼3000 km2) 10–70‐m‐thick intervals of deformed strata are encased within undeformed sediments. Such chaotic mass movement deposits that are mappable over areas comparable with seismic‐scale mass transport deposits are commonly associated with submarine slope settings. However, the surrounding lithofacies and the correlation of distinctive marker beds indicate that these deformation intervals developed in a distal low gradient basin floor setting. The deformed intervals comprise a lower division of tight down‐flow verging folds dissected by thrust planes that sole out onto a highly sheared décollement surface that are interpreted as slides. The lower divisions are overlain by an upper division of chaotic lithofacies with large contorted clasts of sandstone supported by a fine‐grained matrix interpreted as a debrite. The juxtaposition of these lithofacies, the distribution of thickness of the divisions, and their close kinematic relationships indicate that the emplacement of the debris‐flows triggered and drove the underlying slide, in a low‐gradient distal setting. Individual beds in the deformed intervals can be mapped laterally into undeformed strata indicating limited movement of the slide. Therefore, widespread zones of syn‐sedimentary deformation in deep‐water settings do not necessarily indicate a slope setting and should not be used as single criterion to determine depositional setting. When associated with major debrites they may be developed on a flat basin floor.

Late Permian Sedimentation in the Sverdrup Basin, Canadian Arctic: The Lindstrom and Black Stripe Formations

Abstract Contrary to a long-held belief, Upper Permian strata are present in the Sverdrup Basin of Arctic Canada, and a nearly continuous conformable record with the overlying Blind Fiord Formation occurs in the basin central area. Upper Permian strata are dominated by spiculitic chert, the bulk of which is part of a Wuchiapingian unconformity-bounded transgressive-regressive sequence, one of four sequences that encompass the Middle Permian to Lower Triassic interval. Two new stratigraphic units are here formally erected to better account for Upper Permian strata in the Sverdrup Basin. The Lindstrom Formation is a unit of white to pale grey spiculitic chert that represents sedimentation on a shallow, open marine, wave- and storm-dominated ramp. The Black Stripe Formation, which is in part correlative with the Lindstrom Formation, comprises black to dark grey spiculitic chert associated with siliceous shale and siltstone of slope to basinal origin. The upper part of the Black Stripe Formation comprises a deepening-upward succession of black cherty siltstone and siliceous shale that recorded Changhsingian sedimentation. The basal Blind Fiord contact which coincides with the Latest Permian Extinction event in the Sverdrup Basin is marked by an unusual abundance of pyrite, a hallmark of the extinction horizon around the world. A hiatus associated with maximum flooding and condensation exist across the Black Stripe-Blind Fiord contact in distal settings. However, a seemingly rapid, yet progressive carbon isotopic shift suggests minimal stratigraphic interruption. The Late Permian age of the Lindstrom and Black Stripe formations is substantiated by conodont biostratigraphy and carbon isotope chemostratigraphy. Late Changhsingian conodonts were recovered from the base of the Blind Fiord Formation, while Hindeodus parvus, the species that marks the base of the Triassic at the GSSP in China, and associated conodont assemblages, occurs a few tens of metres above the base of the Blind Fiord Formation. Black shale in the Lindstrom and correlative Black Stripe Formation may have a source potential as shown by high TOC values and preliminary RockEval analysis.

Late Quaternary alluvial fans and paleosols of the Kangra basin, NW Himalaya: Tectonic and paleoclimatic implications

Abstract The paper describes Late Quaternary pedosedimentary sequences of the alluvial fans from the Kangra basin of NW Himalayas for tectonic and paleoclimatic implications. In the proximal part of the Kangra basin three coalescing alluvial fans, namely Rait-Rihlu fans (~ 65 km2), Kangra fans (~ 200 km2), and Palampur fans (~ 170 km2) from west to east evolved due to reactivation of longitudinal and transverse faults and climatic changes during the Late Quaternary. The fans are characterised by subsidence of Rait-Rihlu fans, uplift of Kangra fans and tilting of Palampur fans. The thick (~ 90 m) pedosedimentary sequences exposed along the rivers characterise the dominant formative processes over the fans. The stream flow sediments dominate the Rait-Rihlu fans, whereas the debris flow sediments dominate the Kangra and Palampur fans. The fan sequences are also marked by the formation of strongly developed paleosols on loess (L1–L3 loess paleosols) and weakly developed paleosols on fluvial deposits in response to the tectonics and climate change in NW Himalayas. Formation of the loess in proximal and distal settings of the alluvial fans is related to the cool–dry climates during the advance of glaciers in the adjoining areas at 78 ka, 44 ka, 30 ka and 20 ka when climate was approaching towards the last glacial maxima. Field characters, micromorphology, grain size and clay minerals of these loessic deposits suggest strong pedogenesis of the loess during a warm–humid climate after the retreat of glaciers in the NW Himalayan region and are likely related to marine isotope stages (MIS4/5 in proximal and MIS2/3 in distal). The loess-paleosols show some degree of syndepositional pedogenesis in upper horizons and are accretionary in nature. Weakly expressed pedogenic features in fluvial and debris flow deposits suggest rapid sedimentation over unstable surfaces related to the reactivation of Main Boundary Thrust (MBT) and other regional faults.

Depositional model and stratigraphic architecture of rift climax Gilbert-type fan deltas (Gulf of Corinth, Greece)

Facies, depositional model and stratigraphic architecture of Pleistocene giant Gilbert-type fan deltas are presented, based on outcrop data from the Derveni–Akrata region along the southern coast of the Gulf of Corinth, Greece. The common tripartite consisting of topset, foreset and bottomset [Gilbert, G.K., 1885. The topographic features of lake shores: Washington, D.C., United States Geol. Survey, 5th Annual Report, 69–123.] has been identified, as well as the most distal environment consisting of turbidites, and is organised in a repetitive pattern of four main systems tracts showing a clear facies and volumetric partitioning. The first systems tract (ST1) is characterised by the lack of topset beds and the development of a by-pass surface instead, thick foresets and bottomset beds, and thick well-developed turbiditic systems. This systems tract (ST1) is organised in an overall progradational pattern. The second systems tract (ST2) is characterised by a thin topset and almost no foreset equivalent. This systems tract is not always well-preserved and is organised in an overall retrograding trend with a landward shift in the position of the offlap break. The offshore is characterised by massive sandy turbidites. The third systems tract (ST3) is characterised by small-scale deltas prograding above the staked topsets of the giant Gilbert-type fan delta. Those small Gilbert-type fan deltas are generally organised in a pure progradation evolving to an aggradational–progradational pattern. In the distal setting of those small Gilbert-type fan deltas, almost no deposits are preserved on the remaining topography of the previous Gilbert-type fan delta. The fourth systems tract (ST4) is characterised by continuous vertically aggrading topsets that laterally pass into aggrading and prograding foresets. Bottomsets and distal turbiditic systems are starved. This fourth systems tract (ST4) is organised in an overall aggrading trend. These giant Gilbert-type fan deltas correspond to the Middle Group of the Corinth Rift infill and their stratigraphic development was strongly influenced by evolving rift structure. They record the migration of the depocenter from the rift shoulder to the rift axis in four main sequences from ca. 1.5 to 0.7 Ma, related to the migration of fault activity. It is worth noting that the maximum paleobathymetry was recorded during the final stage of the progradation of the Middle Group, suggesting that the rift climax was diachronous at the scale of the entire basin. The rapid (< 1 Ma) structural and sedimentological evolution, the migration of fault activity as well as the youth of the Corinth Rift, are probably exceptional factors allowing the characterisation of such diachronism.

Dimensions and architecture of late Pleistocene submarine lobes off the northern margin of East Corsica

AbstractSandy lobe deposits on submarine fans are sensitive recorders of the types of sediment gravity flows supplied to a basin and are economically important as hydrocarbon reservoirs. This study investigates the causes of variability in 20 lobes in small late Pleistocene submarine fans off East Corsica. These lobes were imaged using ultra‐high resolution boomer seismic profiles (&lt;1 m vertical resolution) and sediment type was ground truthed using piston cores published in previous studies. Repeated crossings of the same depositional bodies were used to measure spatial changes in their dimensions and architecture. Most lobes increase abruptly down‐slope to a peak thickness of 8 to 42 m, beyond which they show a progressive, typically more gradual, decrease in thickness until they thin to below seismic resolution or pass into draping facies of the basin plain. Lobe areas range from 3 to 70 km2 and total lengths from 2 to 14 km, with the locus of maximum sediment accumulation from 3 to 28 km from the shelf‐break. Based on their location, dimensions, internal architecture and nature of the feeder channel, the lobes are divided into two end‐member types. The first are small depositional bodies located in proximal settings, clustered near the toe‐of‐slope and fed by slope gullies or erosive channels lacking or with poorly developed levées (referred to as ‘proximal isolated lobes’). The second are larger architecturally more complex depositional bodies deposited in more distal settings, outboard more stable and longer‐lived levéed fan valleys (referred to as ‘composite mid‐fan lobes’). Hybrid lobe types are also observed. At least three hierarchical levels of compensation stacking are recognized. Individual beds and bed‐sets stack to form lobe‐elements; lobe‐elements stack to form composite lobes; and composite lobes stack to form lobe complexes. Differences in the size, shape and architectural complexity of lobe deposits reflect several inter‐related factors including: (i) flow properties (volume, duration, grain‐size, concentration and velocity); (ii) the number and frequency of flows, and their degree of variation through time; (iii) gradient change and sea floor morphology at the mouth of the feeder conduit; (iv) lobe lifespan prior to avulsion or abandonment; and (v) feeder channel geometry and stability. In general, lobes outboard stable fan valleys that are connected to shelf‐incised canyons are wider, longer and thicker, accumulate in more basinal locations and are architecturally more complex.

Proglacial sediment trapping in recently formed Silt Lake, upper Lillooet Valley, Coast Mountains, British Columbia

AbstractThe sedimentology of proglacial Silt Lake was assessed by lake sediment coring and monitoring of lacustrine processes during a late‐summer period of high glacier melt to characterize sediment delivery from the heavily glacierized catchment and investigate the sediment trapping dynamics of this upland lake. A complete varve chronology was established for a distal basin of the lake which was exposed by Lillooet Glacier retreat between 1947 and 1962. The varve record showed decreasing sedimentation rates in the basin while the glacier retreated, and as the lake became free of ice contact in the early 1970s. Although recession has continued over recent decades, and glacier proximity to the lake has, therefore, continued decreasing, lacustrine sedimentation rates are now accelerating due to changing basin morphometry caused by delta progradation. Over shorter time scales, lake sedimentation patterns respond to changing runoff conditions, including late‐summer glacier melt intensity, intra‐annual flooding events, diumal runoff fluctuations, and within‐lake turbidity currents. Turbidity currents included quasi‐regular flows during high diurnal discharges and an episodic flushing of temporarily stored sediment from the sandur or delta at a time of low stage. Suspended sediment yield to Silt Lake is estimated to exceed 103 Mg km−2 a−1, a magnitude that surpasses previous local and regional yield estimates for the glacierized headwaters of the Lillooet River valley. Since Silt Lake currently traps a significant prooportion of that upland sediment supply, and the trapping efficiency of the basin has been variable at decadal time scales, the formation and continued development of Lilt Lake has likely had a significant influence on downstream sediment delivery. Lacustrine sediment‐based proxies of long‐term hydroclimatic variability being developed in glacially distal settings should include provisions for dynamic sediment trapping effects in upstream water bodies that often form in the active proglacial environment. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Fingerprinting facies of the Tuff IF marker, with implications for early hominin palaeoecology, Olduvai Gorge, Tanzania

The top of Tuff IF marks the upper boundary of the Plio-/Pleistocene Bed I succession exposed in Olduvai Gorge, NE Tanzania, a tephrostratigraphic interval that is well known for remarkable Oldowan archaeological and vertebrate fossil assemblages, including early hominins. Geochemically, Tuff IF is characterized by a relatively consistent silica-undersaturated trachytic to phonolitic composition that relates both in terms of numerical ages (1.79 Ma) and compositional constraints to Olmoti volcano, located 22–38 km E of Olduvai Gorge. Measured Tuff IF sections are characterized by a thick unwelded pyroclastic flow in proximal settings and a succession of surges and ashfalls that interfingers with fluvio-lacustrine deposits of the Olduvai Basin in the medial to distal settings. Only in those areas with a relatively low topographic gradient, in distal reaches and beyond the toe of an Olmoti-sourced volcaniclastic fan, did the Tuff IF marker develop a typical threefold subdivision comprising: (1) primary surges and minor fallout, (2) reworked pumice units as lateral correlatives of proximally emplaced pyroclastic flows and (3) a succession of mass flows in conjunction with aeolian and fluvially reworked units. The pyroclastic marker unit is thus highly heterogeneous with regard to its vertical and lateral facies architecture and this has immediate effects on its preservation potential and in situ burial of fossils and stone artifacts. Even though the volcanically-related environmental perturbations were probably more severe at the eastern lake margin, the evidence for at least temporary freshwater sources and trees suggests environments conducive to hominin activities, but not during emplacement of Tuff IF pyroclastic flows and surges. This inference is supported by the presence of rich Oldowan stone artifact assemblages immediately preceding and following the deposition of Tuff IF, but only extremely sparse archaeological traces from one site in Tuff IF, restricted to the upper, fluvially reworked portion of Tuff IF. Tuff IF facies record the maximum of a regressive (drying) cycle that correlates with regional marine arid indicators. An ecological crisis covering ∼ 2000–3500 a of time during Tuff IF deposition resulted not only from the lethal effects of explosive volcanism but also from its coincidence with a pronounced period of climate induced drought. These combined effects appear to have made at least the eastern basin uninhabitable by hominins and other vertebrates for most of the time during which Tuff IF accumulated.

Oxidative forcing of global climate change: A biogeochemical record across the oldest Paleoproterozoic ice age in North America

Carbon isotope compositions of organic matter in fine-grained siliciclastic units deposited above and below glacial diamictite at the base of the ca. 2.45–2.22 Ga Huronian Supergroup in Ontario, Canada were studied to constrain relationships between profound fluctuations in the exogenic carbon cycle and dramatic climate changes at the beginning of the Proterozoic Eon. In both drill core and outcrop sections the organic matter preserved in proximal lithofacies, dominated by coarse-grained sand and silt, are enriched in 13C relative to distal lithofacies, dominated by argillites. In the drill core, sand-dominated lithofacies of the McKim Formation beneath the glacial diamictite of the Ramsay Lake Formation have a narrow range of δ 13C values (− 28.4 to − 26.0‰ V-PDB), but organic matter in argillite-dominated lithofacies of the outcrop section ∼ 40 km to the southeast is somewhat more 13C-depleted with values ranging from − 34.5 to − 26.4‰. Similarly, sand-dominated lithofacies of the Pecors Formation above the glacial diamictite in the drill core section with δ 13C values of ca. − 28‰ are notably 13C-enriched relative to argillite-dominated lithofacies, which record values as low as − 40.5‰. The sand-dominated lithofacies of the Pecors Formation in the outcrop sections have δ 13C compositions ranging from − 34.4 to − 27.9‰. The isotopic differences appear to be unrelated to organic carbon abundances, so we suggest that these are controlled by environmental differences in proximal and distal settings. The strong 13C-depletion in the organic-lean McKim and Pecors argillites, especially in the drill core section of the Pecors Formation, is consistent with significant biological methane production and oxidative recycling by methanotrophs both before and after the ice age in shallow-water environments stratified with respect to oxygen. The rise of atmospheric oxygen and subsequent enhanced biogeochemical methane cycling in shallow-water settings likely contributed to unstable climate conditions during the Paleoproterozoic glacial epoch.

The Early Aptian Grünten Member: Description of a new lithostratigraphic unit of the helvetic Garschella Formation

Early Aptian sediments mostly referred to as “upper Orbitolina beds” are known from a relatively small number of outcrop areas throughout the distal part of the Helvetic Zone of Switzerland, Austria and Germany. These sediments are here formally defined as a new, basal member of the Garschella Formation; the Grunten Member. Equivalent and correlatable sediments also exist in the Vercors region of the French Dauphinee Zone. The historical type section (holostratotype) of the Grunten Member is situated on the Grunten Mountain in southern Germany. A new type section (lectostratotype) for the Grunten Member is chosen in the better suited Bauen-Brisen area of Central Switzerland and an additional reference section (hypostratotype) is defined near the Rawil Pass in the Bernese Oberland of Switzerland. In relatively proximal settings, the Grunten Member overlies Early Aptian limestones of the Urgonian Carbonate Platform (Schrattenkalk Formation), documenting its demise and early “drowning”. In relatively distal settings, it overlies the contemporary hemipelagic sediments of the Drusberg and Mittagspitz Formations. In complete successions, the Grunten Member is in turn overlain by the basal, phosphoritic Luitere Bed of the Brisi Member (Garschella Formation) documenting the continuing “drowning” of the Urgonian Carbonate Platform in the Late Aptian. The Grunten Member essentially consists of a single stratigraphic sequence, beginning with a marly base and gradually passing to crinoidal limestones at its top. Rare ammonite finds as well as sequence stratigraphic correlations suggest a late Early Aptian age (parts of the Deshayesi and Furcata Ammonite Zones). In the relatively proximal reference section of Rawil, the Grunten Member contains two phosphoritic horizons. Phosphateenriched horizons are also known from other proximal sections of the Grunten Member.

Sea level influence on the nature and timing of a minibasin sedimentary fill (northwestern slope of the Gulf of Mexico)

This study focuses on the sedimentary fill of basin 4, the termination of the Brazos-Trinity minibasin slope system in the northwestern Gulf of Mexico. Results from multistratigraphic analyses of 15 giant piston cores provided (1) important information regarding the nature (hemipelagic versus gravity-induced mud and sand deposits) and the timing of the sedimentary fill; (2) some key chronostratigraphic constraints for the evolution of this system; and (3) strong links between well-known cycles of sea level change to clearly imaged deposits in the fill of basin 4. Gravity-flow–induced sedimentation in basin 4 occurred and increased in importance during the stepwise sea level regression that developed between 115 and 15 ka and clearly ceased just prior to the meltwater spike in the Gulf of Mexico dated at about 14 ka. The onset of gravity-induced deposition in basin 4 is dated at marine isotope stage (MIS) 5d (115 ka). This finding implies that sandy turbidity currents reached this distal setting as a consequence of a higher frequency sea level fall within a time of general high sea level (MIS 5). An interval of hemipelagic sedimentation lasting from 90 to 45 ka illustrates cessation of gravity-induced deposits in basin 4. Turbidite sandy deposits resumed in mid-MIS 3 and increased toward MIS 2 (approximately from 30 to 15 ka). The largest proportion of reservoir-grade sandy sediment was deposited during the maximum sea level lowstand of the last glacial maximum, consistent with the prevailing view of sequence-stratigraphic models for deep-water deposition.

Riparian landscapes and human habitat preferences during the Hoxnian (MIS 11) Interglacial

AbstractThe archaeological, environmental and geological data from Hoxnian Interglacial (Marine Isotope Stage (MIS) 11) sites from Britain are examined to elucidate the type of habitats that humans preferred during this temperate episode. The conclusion is that humans avoided lacustrine situations, but did make use of the full range of resources that fluvial environments provide. This model is strengthened by the examination of other non‐archaeological Hoxnian sites. The problem of archaeological visibility in lacustrine sediment sequences is also discussed and methods of identifying other evidence of human presence are suggested that may offset the deficiencies in the lithic record. These include presence of cut‐marked bone, micro‐debitage and possibly charcoal in fine‐grained sediments deposited in distal settings. The reasons for human selection of fluvial situations are discussed. It is concluded that these environments provide a greater diversity of animal, plant and lithic resources, but also are major route‐ways through the landscape. Patterns of human site use are identified, which seem to be triggered by local changes in hydrology and drainage, themselves possibly caused by regional changes in climate. Finally, Lower Palaeolithic sites on the interfluves are discussed. Although they lack environmental or dating evidence, it is tentatively suggested that they were used during cooler episodes, when more open conditions prevailed. Copyright © 2006 John Wiley &amp; Sons, Ltd.

Synthesis of the tectonic-sedimentary evolution of the Mesozoic-Early Cenozoic Pindos ocean: evidence from the NW Peloponnese, Greece

Abstract The tectonic development of the western part of the Pindos ocean in southern Greece is exemplified by the mountainous Pindos thrust belt in the NW Peloponnese. A Late Triassic-Early Cenozoic succession exposed within imbricate thrust sheets records a range of deep-water siliciclastic, redeposited carbonate and siliceous sediments, which in general become more distal oceanwards towards the east. Igneous rocks, locally dated as Triassic, occur within a mélange that is entrained beneath and within the Pindos thrust stack; these igneous rocks and related sediments are interpreted as remnants of a continent-ocean transition zone. ‘Immobile’ element geochemistry is explicable by rifting of a compositionally heterogeneous subcontinental mantle, possibly related to pre-existing Hercynian subduction, although coeval Triassic subduction cannot be excluded based on evidence from this area alone. Localized, ‘enriched’ basalts are interpreted as fragments of oceanic seamounts formed in a relatively distal setting. Late Paleocene-Early Eocene (locally Mid-Eocene) siliciclastic turbidites, derived from the north, record the latest deposition prior to incorporation of the sedimentary succession into a westward-migrating accretionary wedge during post-Early Eocene time in the NW Peloponnese. Structural restoration of the well-ordered thrust stack indicates a minimum of 201 km (55%) of shortening at an average rate of 5.8 mm a −1 . As the Pindos allochthon approached the Apulian continent, the Gavrovo-Tripolitza foreland underwent flexural upwarp during the Mid-Eocene, followed by collapse to create a foreland basin by the Late Eocene. This basin was infilled with generally upward-thickening and -coarsening deep-water turbiditic sediments of Late Eocene-Early Oligocene age. The foreland was, in turn, overthrust by the Pindos accretionary prism during post-Early Paleocene time, and was then imbricated and thrust over the Ionian foreland basin to the west by Pliocene time.

Pleistocene coralline algal build-ups ( coralligène de plateau ) and associated bioclastic deposits in the sedimentary cover of Cutro marine terrace (Calabria, southern Italy)

Abstract Carbonate build-ups mainly constructed by encrusting coralline red algae are currently developing on Mediterranean soft bottoms, at depths ranging from 20 m to 160 m. They are usually referred to as ‘ coralligène de plateau ’. Few fossil examples of these bioconstructions have been described in the literature and their evolution in the context of a stratigraphic cycle has never been modelled in detail. Cutro marine terrace (Calabria, southern Italy) preserves mid-Pleistocene deposits assigned to Marine Isotope Stage (MIS) 7 or MIS 9. Extensive algal build-ups representing the deepest unit of the succession occur in the outer and central portion of the terrace, interpreted as the most distal setting from the palaeo-shoreline. Two studied sections, Vrica and Telegrafo, showed that the solid biogenic framework grew over a basal rhodolithic layer, which was stabilized by the binding activity and overgrowth of non-geniculate Corallinales (calcareous red algae). Therefore, these bioconstructions represent a rare fossil example of coralligène de plateau . At the Telegrafo section, Titanoderma pustulatum has been identified as the major rhodolith component. The build-ups are dominated by T. pustulatum , associated mainly with Mesophyllum spp. and locally with Lithophyllum stictaeforme . Rhodalgal bioclastic deposits are found in lateral contact with the build-ups. The two facies developed together under a hydrodynamic regime where phases of sedimentation from storm-driven currents alternated with phases of calm conditions. They were deposited during a single stratigraphic cycle beginning with the generation of a ravinement surface during the transgressive systems tract (TST) and ending with the burial of the coralligène by well-sorted shoreface bioclastic sands. Optimal and extensive growth of the coralligène took place during the highstand systems tract (HST).