Proximal Deposits Research Articles

Heavy Minerals in Shelf Sediments off Fujian-Zhejiang Coast of the East China Sea: Their Provenance and Geological Application

To examine the composition and source variations of heavy minerals in the Fujian-Zhejiang mud belt (FZMB) over the past few hundred years, heavy minerals in 150 surficial sediment samples and two sediments cores collected from the southern FZMB were identified and analyzed. The results show that the mineral assemblage of hornblende-magnetite-epidote-chlorite is dominant in the study area. Based on the heavy mineral contents, the study area is divided into two mineral zones, namely a near-shore muddy sediment zone (zone I) and an offshore mixed sediment zone (zone II). The sediments from zone I contains relatively abundant metallic minerals with proximal sediments from the Oujiang River and the Minjiang River as the primary component. The sediment from zone II has relatively low content for minerals from the near-shore materials and is significantly affected by the Oujiang River sediments, and by the flaky minerals from the Yangtze River to a certain extent. The characteristics of heavy minerals in the cores may be affected by a variety of factors. Before the 20th century, under the influence of strong East Asian winter monsoon (EAWM), the sandy sediment in FZMB is significantly affected by the Oujiang River and the Yangtze River sediments, and relatively unaffected by near-shore terrigenous matter. In the 20th century, as the intensity of the Zhejiang-Fujian Coastal Current (ZFCC) decreased, the influence of the Minjiang coastal sand enhanced. Since the 1980s, as the collective effect of relatively weak EAWM and frequent storm surges and typhoons, the impact of the Minjiang River sediments on the FZMB sediments has increased considerably.

Long-term drivers and impacts of abrupt river changes in managed lowlands of the Adige River and northern Po delta (Northern Italy)

We analysed the millennial-scale evolution of the alluvial systems of the Po and Adige rivers around the city of Adria, through the analysis of remote sensing images, digital terrain models, historical maps, and the execution of manual cores across the main alluvial ridges. Selected organic samples were radiocarbon dated.Results show that floodplain sedimentation dominated the area until 4.3–4.0 ka cal BP, followed by aggradation of a major alluvial ridge of the Po River (Adria ridge) until 2.6 ka cal BP. Due to a major upstream avulsion, the Po River shifted tens of kilometers to the south, and the minor Tartaro River started flowing through ancient Adria bringing stable geomorphic conditions. In the 10th century AD, the opening of two crevasse channels on the right bank of the Adige River, the Castagnaro and Malopera rivers, initiated the aggradation of the alluvial ridge of the Tartaro River at Adria. This happened during the Medieval Climate Anomaly, in a period of enhanced flooding recorded in Northern Italy, in the Alps and the Mediterranean basin. Extensive peat sedimentation followed since the 11th century, indicating a decrease of overbank flooding and silting by the Tartaro River due to concurrent early management of the crevasses, proximal sediment accumulation, and favorable regional climatic conditions. Fluvial sedimentation started again in the early 15th century, due to the reactivation of the Castagnaro and Malopera crevasses. This happened at the beginning of the Little Ice Age in a period of recurrent flooding of the Adige River, but human-induced modification of channel width and depth may have concurred. Overall aggradation along the Tartaro ridge led to the deposition of up to 4 m of sediment, and the complete burial of an early Medieval church in the city of Adria. The end of this sedimentary event predates the 17th century, and relates to efficient river management actuated by the Venetian Republic.Our investigation highlights how a major avulsion of a large river such as the Po can abruptly change the environmental conditions of large stretches of alluvial plain, allowing the development of an urban settlement on the former floodplain. It further shows that such favorable setting can be suddenly modified by the opening of crevasse channels tens of kilometers away, if water and sedimentary connectivity is allowed. Adria community was resilient to flooding and silting and always attempted to manage the crevasses, but its efforts were counterbalanced by fluctuating climatic conditions.

Compositional variations in deep-sea gravity-flow deposits. A case study from the Voirons Flysch (Voirons-Wägital complex, Chablais Prealps, France)

Grain-size control is frequently invoked to explain the compositional variation of detrital sediments in fluvial sediments. A similar possibility is here investigated in the fossil deep-sea gravity-flow deposits from the Voirons Flysch (Voirons-Wägital complex, Chablais Prealps). About 270 samples of diverse depositional settings were analysed using the framework composition, including cement and porosity, Mutti facies and grain-size measurements. Six lithofacies have been identified from cluster analysis. They correspond to channel (L1 to L3) and lobe (L4 and L5) depositional settings and to reworked turbidites (L6). Our results emphasize the close relationship between petrography and grain-size which alters the detrital signal from extrabasinal sources. The abundant polycrystalline component of proximal deposits is progressively removed while skeletal grains and additional intrabasinal grains become more abundant in distal deposits. Alongside, porosity is reduced by the simultaneous increase of cement which constrains subsequent diagenetic modifications. Hence, proximal depositional settings keep most of the inherited signal from extrabasinal sources whereas distal deposits will be homogenised. When plotted in provenance-analysis diagrams, the successive depositional settings show distinct fields possibly corresponding to different tectonic settings of the Dickinson model. This compositional variation thus creates a bias in provenance interpretation resulting from both grain-size control and flow mechanisms. Finally, our results emphasize that petrographic data must be handled with care in provenance analyses, especially when comparing various deep-sea gravity flow systems with different grain-size ranges.

The Origin of Fine Sediment Determines the Observations of Suspended Sediment Fluxes Under Unsteady Flow Conditions

AbstractField observations in a wide range of environments have shown that sediment availability is a major control on the suspended sediment observations in streams. Here we examine, via laboratory experiments, how the amount of proximal in‐channel fine sediment storage relative to the upstream fine sediment distal supply influences the observations of suspended sediment concentrations in streams. Experiments under idealized conditions in a laboratory flume with different ratios of proximal and distal sediment supplies were conducted under a varying flow regime. In addition, the role of the sediment particle size of the supplied sediment on suspended sediment observations was explored. The combinations of proximal and distal sediment supply result in multiple responses of the channel bed and sediment quantity within the channel bed, and the responses adjust through aggradation and degradation. The signature of sediment concentration observed at the upstream section of the channel, given by the distal supply, differs from the downstream observations of the total conveyed sediment (distal and proximal), as shown by an in‐phase analysis of sediment concentration‐discharge plots. Furthermore, we show that nonuniform sediment mixtures may result in a change in the direction of the hysteresis observed between sediment concentration and discharge (i.e., from a clockwise hysteresis to a counterclockwise hysteresis). We also demonstrate that the ratio between sediment distal supply and proximal sediment availability modulates the magnitude of the aggradation/degradation processes in the channel reach and thus the joint observations of sediment concentration and discharge.

Downstream evolution of the thermochronologic age signal in the Brahmaputra catchment (eastern Himalaya): Implications for the detrital record of erosion

The Namche Barwa massif in the eastern Himalayan syntaxis is characterized by very rapid exhumation and provides a significant proportion of the sediment flux carried by the Brahmaputra River. We present new detrital zircon fission-track (ZFT) and muscovite 40Ar/39Ar (MAr) data from modern sediments of rivers draining the eastern Himalaya. The cooling-age populations for both thermochronometers contain a characteristic <2 Ma signature related to the rapid exhumation of Namche Barwa, which can be traced hundreds of kilometers downstream from their source into the Brahmaputra foreland, despite dilution from downstream tributary catchments. To estimate present-day erosion in the catchments, we apply a mixing model based on linear inversion of the binned age distributions. The inversion predicts relative erosion rates in the syntaxial region that are an order of magnitude higher than those in upstream catchments, and about twice as high as those in the southern Himalayan catchments, consistent to first order with previous estimates of erosion-rate patterns in the eastern Himalaya. A comparison of the observed downstream evolution of age distributions with a sediment-flux model suggests that the ZFT signal can be explained by dilution from Himalayan tributaries only, whereas the MAr signal is also affected by selective sequestering (possibly through winnowing) of micas as they are transported downstream. Nevertheless, thermochronological ages <2 Ma provide a diagnostic signal of syntaxial exhumation in the sedimentary record of the eastern Himalaya; this study suggests the most robust signal to be recorded in the most proximal deposits with respect to the syntaxis.

Spatter matters \u2013 distinguishing primary (eruptive) and secondary (non-eruptive) spatter deposits

Spatter is a common pyroclastic product of hawaiian fountaining, which typically forms vent-proximal ramparts or cones. Based on textural characteristics and field relations of spatter from the 1969 Mauna Ulu eruption of Kīlauea, Hawai’i, three spatter types were identified: (1) Primary spatter deposited as spatter ramparts and isolated cones during the peak of episode 1; (2) Late-stage spatter comprising dense, small volume, vent proximal deposits, formed at the end of episode 1; (3) Secondary spatter preserved in isolated mounds around tectonic ground cracks that we interpret to have formed by the disruption of overlying lava. We propose that not all spatter deposits are evidence of primary magmatic fountaining. Rather, deposits can be “secondary” in nature and associated with lava drain-back, disruption, and subsequent ejection from tectonic cracks. Importantly, these secondary pyroclastic deposits are difficult to distinguish from primary eruptive features based on field relations and bulk clast vesicularity alone, allowing for the potential misinterpretation of eruption vents, on Earth and in remotely sensed planetary data, thereby misinforming hazard maps and probabilistic assessments. Here, we show that vesicle number density provides a statistically-robust metric by which to discriminate primary and secondary spatter, supporting accurate identification of eruptive vents.

Sedimentology and Depositional Environment of part of Lokoja Formation Exposed at Felele Junction, Southern Bida Basin

The exposed Campanian sedimentary sequence at Felele junction around Lokoja has been mapped, logged and sampled. The total thickness of the section is approximately 21.48m. The sediments consist of conglomeratic sandstones, coarse sandstones and mudstones. Grain size analysis performed on nineteen sandstone samples revealed the average values of mean, sorting, skewness and kurtosis as 0.6, 1.03, 0.53 and 0.76 respectively. These results show that the sandstones are coarse grained, poorly sorted, strongly fine skewed and platykurtic. This characteristic is typical of fluvial depositional environment. The plots of skewness against sorting, sorting versus kurtosis and mean against sorting suggest a fluvial depositional environment. Petrographic analysis of selected sandstones reveals the presence of quartz, feldspars (plagioclase and microcline) and mica (biotite and muscovite). Quartz (especially monocrystalline) dominates followed by feldspar with minor muscovite and biotite. Heavy mineral study revealed the presence of non-opaque mineral suites suggesting that the sediments are derived from the basement complex of igneous and metamorphic rocks. The heavy minerals observed include staurolite, rutile, kyanite, zircon and sphene. The samples are dominated by stauronite which signifies low grade metamorphism in the area. Integration of sedimentological and petrographical characteristics in this study suggest that the sandstone samples are derived from mixed origin and deposited in fluviatile environment under high energy setting not too far from the source. Hence it is a proximal deposit.

The Rio Doce Group revisited: An Ediacaran arc-related volcano-sedimentary basin, Araçuaí orogen (SE Brazil)

The Araçuaí orogen developed between the São Francisco and Congo cratons, during the Western Gondwana amalgamation in Late Neoproterozoic time. Located in the eastern Araçuaí orogen, the Ediacaran Rio Doce magmatic arc includes a large amount of plutonic rocks (the G1 supersuite, ca. 630-580 Ma) spatially associated with a supracrustal succession, the Rio Doce Group (RDG). This group, poorly studied until recently, encompasses four formations (from base to top): Palmital do Sul, Tumiritinga, São Tomé, and João Pinto. The Palmital do Sul and Tumiritinga formations mostly include pelitic schists to paragneisses, interbedded with metavolcanic and meta-volcaniclastic rocks, representing proximal deposits in relation to the volcanic arc front. Pyroclastic deposits represented by lapilli-rich meta-tuffs with scattered volcanic bombs denote explosive volcanism of dacitic to rhyolitic composition. Zircon U-Pb data provide magmatic crystallization ages of 595 ± 13 Ma and ca. 584 ± 5 Ma for the volcanic protoliths. The São Tomé Formation mainly includes psammitic to pelitic meta-turbidites with lenses of calcsilicate rocks, indicating relatively distal marine deposits with a maximum depositional age of ca. 594 Ma and a striking arc-related lithochemical signature. The João Pinto Formation mostly includes quartz sandstones and micaceous to feldspathic sandstones, with a maximum sedimentation age of ca. 620 Ma, representing fluvial to continental shelf environments related to the late stages of the Rio Doce arc development. The Rio Doce Group can be correlated with other orogenic supracrustal successions (i.e., parts of the Andrelândia Group) found in the region covered by the Rio Doce arc, connecting the Araçuaí and Ribeira orogens.

Mud-capped dredge pits: An experiment of opportunity for characterizing cohesive sediment transport and slope stability in the northern Gulf of Mexico

Mud-capped dredge pits (MCDPs), defined as sand borrow sites from paleo river channels with cohesive sediment-capped walls, are used for northern Gulf of Mexico (NGoM) coastal restoration projects due to lower cost than more distal restoration resources. MCDP evolution, including infilling and pit wall slope stability, is poorly understood in relation to sandy borrow areas. We present and interpret bathymetric data from Sandy Point MCDP, which was constructed in 2012 about 20 km northwest of Southwest Pass, the most active Mississippi River distributary, in water depths of ∼11 m. Repeat bathymetric surveys show the pit is infilling ∼54 cm (or volumetrically 200,000 m3) per year, which is on the same order as seasonal-scale and an order of magnitude higher than the decadal-scale vertical accretion rates observed seaward of Southwest Pass. This infilling rate is higher than predicted based solely on excavated volume when compared with other dredge pits in sandy substrate, sediment-starved settings. MCDP infill is volumetrically dominated by far-field (here defined as river plume or shelf bed resuspension) sediment, with only ∼9% of pit infill from pit wall failure. Geomorphic comparison of Sandy Point MCDP walls with Southwest Pass mudflow gully walls shows that the MCDP is oversteepened relative to natural depression features in this setting; the convergence of MCDP and mudflow gully gradients towards a common value (4–5°) may represent an “angle of repose” in lieu of decadal-scale forcings. Findings from this experiment of opportunity show that MCDPs (1) are effective sediment traps, (2) have steep but relatively laterally stable walls (3) do not represent significant risk to infrastructure in a 3-year time scale, and (4) are predicted to infill within about 2 decades provided a proximal sediment source exists.

Stratigraphy, distribution, and evidence for mafic triggering of the ca. 8.5 ka Driftwood Pumice eruption, Makushin Volcano, Alaska, U.S.A

Makushin Volcano on Unalaska Island, Alaska, threatens the Aleutian's largest population centers (Unalaska and Dutch Harbor), yet its eruption mechanisms are poorly known. This study presents a detailed stratigraphic and geochemical investigation of Makushin's most recent highly explosive event: the ca. 8.5 ka Driftwood Pumice eruption.The Driftwood Pumice has measured thicknesses of over 2.5 m, and isopach reconstructions estimate a total deposit volume of 0.3 to 1.6 km3, indicating a VEI 4–5 eruption. Proximal deposits consist of normally-graded, tan, dacitic to andesitic pumice, capped by a thinner dark layer of lower-silica andesitic scoria mixed with abundant lithic fragments. This stratigraphy is interpreted as an initial vent-clearing eruption that strengthened into a climactic ejection of pumice and ash and concluded with vent destabilization and the eruption of somewhat more mafic, gas-poor magma.Within the pumice, geochemical trends, disequilibrium mineral populations, and mineral zonation patterns show evidence of magma mixing between a bulk silicic magma and a mafic melt. Euhedral high-Ca plagioclase (An68–91) and high-Mg olivine (Fo69–77) phenocrysts are in disequilibrium with trachydacitic glass (65–68 wt% SiO2) and more abundant sodic plagioclase (An34–55), indicating the former originally crystallized in a more mafic melt. Tephra whole rock compositions become more mafic upwards through the deposit, ranging from a basal low-silica dacite to an andesite (total range: 60.8–63.3 wt% SiO2). Collectively, these compositional variations suggest magma mixing in the Driftwood Pumice (DWP) magma reservoir, with a systematic increase in the amount of a mafic component (up to 25%) upward through the deposit. Olivine-liquid and liquid-only thermometry indicate the mafic magma intruded at temperatures ~140–200 °C hotter than the silicic magma. Diffusion rates calculated for 5–7 μm thick, lower-Mg rims on the olivine phenocrysts (Fo60 rim vs Fo76 bulk) suggest that the eruption occurred several days to weeks following the mafic injection into a dacitic reservoir. Based on this timing, we infer that the mafic intrusion provided a thermal pulse that initiated convection and volatile exsolution, and ultimately resulted in the DWP eruption. Unalaska's Holocene stratigraphy includes multiple light-dark ashfall couplets with physical and geochemical similarities to the DWP, suggesting that magma mixing may be a common eruptive trigger at Makushin Volcano.

Eruption Sequence of the 46 ka Caldera-forming Eruption of Shikotsu Volcano, Inferred from Stratigraphy of Proximal Deposits at South of Lake Shikotsu, Japan

Eruption Sequence of the 46 ka Caldera-forming Eruption of Shikotsu Volcano, Inferred from Stratigraphy of Proximal Deposits at South of Lake Shikotsu, Japan

MedVolc-Ph: A database of mineralogical phases from volcanic deposits in the central Mediterranean area as a tool for tephrostratigraphy

Databases facilitate the rapid access of crucial information, consequently these tools are vital to the various disciplines within the Earth sciences that study volcanic rocks. In the central Mediterranean, archives of data describing the bulk and glass compositions of volcanic rocks exist, but those resources are not structured as searchable databases or, if they are, they are too general to be useful for specific problems (e.g. correlation of tephra layers in the Tertiary-Quaternary stratigraphy). Here we present a new, open access database that contains information describing (mainly) mineral phases and (minor) glass compositions of distal and proximal deposits from the Miocene to the present day. The MedVolc-PH database can be searched using single or multiple queries, facilitating the correlation or study of deposit(s) with the requested characteristics. This manuscript describes the database scheme and highlights how the database can be queried through user-friendly examples of central Mediterranean tephra correlations.

A revised Triassic stratigraphic framework for the Arctic Alaska Basin

The Triassic Shublik Formation and the Triassic–Jurassic Otuk Formation are partially age-equivalent lithostratigraphic units that were deposited in the Arctic Alaska Basin (AAB). The Shublik Formation represents proximal deposition within the basin, with episodic siliciclastic input, whereas the Otuk Formation was deposited in the distal part of the basin, with significant intervals of mudstone and chert. Both the Shublik and Otuk Formations have significant intervals of organic-rich mudstone, and the Shublik is a major source rock for northern Alaska hydrocarbon accumulations such as Prudhoe Bay. The revised stratigraphic framework presented herein, based on the integration of lithostratigraphy and biostratigraphy, correlates intervals within these two formations, as well as the Ivishak Formation and the Karen Creek and Sag River Sandstones (which underlie and overlie the Shublik). This stratigraphic framework provides a basis for comparison of proximal and distal parts of the AAB through the Triassic, thus allowing for a more robust understanding of the spatial and temporal variability of lithology and organic richness within this basin. Five transgressive–regressive sequences are defined in the Shublik, based on lithostratigraphy and better age constraints provided by the revised stratigraphic framework. These sequences are age-correlative and recognized in other Arctic basins, implying that they have regional, and perhaps global, significance.

A Quaternary aminostratigraphy constraining chronology of depositional environments in the North Sea Basin

The aminostratigraphy of up to 1000m of glacial and interglacial sediments in the North Sea Basin (NSB) is compiled from multiple boreholes sites and dated using strontium isotope (Sr) analysis to provide a chronological framework extending throughout the Quaternary. The Quaternary aminostratigraphy within the NSB can be divided into five amino zones (AZ), AZ1 (youngest) to AZ5 based on alle/Ile ratios which are used to correlate depositional events recorded in sedimentary facies from multiple borehole sites. The new relative and absolute chronology also ties to seismostratigraphy and multi-generational tunnel valley fill. Early Pleistocene sediments produce AZ5 ratios with Sr ages around 1.4–1.5Ma deposited in marine to glacimarine conditions. Sediments with AZ4 ratios are constrained by Sr ages and magnetic polarity to 1.3–0.9Ma, spanning the eccentricity to axial tilt insolation periodicity change of the Mid Pleistocene Transition (MPT). Some coarse-grained sedimentary units with AZ4 ratios may be linked to ice proximal deposition during the MPT. Sediments with AZ3 ratios are poorly chronologically constrained to between 1.1 and 0.26Ma. At least one episode of diminished coarse clasts and increased foraminiferal diversity during this Mid Pleistocene time suggests a progression from ice proximal to warmer, possibly interglacial conditions. Furthermore, the stratigraphically deepest generation of tunnel valleys cut strata with AZ3 ratios indicating subglacial conditions at least once during the Mid Pleistocene. Sediments with AZ2 ratios corresponding to MIS7-5 are tied to seismic units including a thick (up to 40m) deformation till, suggesting glacial incursion into the NSB during MIS6 followed by tunnel valley cutting of the this till, thought to be an MIS6 event also. The geometry and cross-cutting complexities of such tunnel valleys precludes further correlation of this ice extent. The youngest aminostratigraphic zone, AZ1, is recognised in all boreholes and well constrained to MIS4-2 and the youngest set of tunnel valleys, attributed to a single or multiple glacial advances at this time.

Proximal lava drainage controls on basaltic fissure eruption dynamics

Hawaiian basaltic eruptions commonly initiate as a fissure, producing fountains, spattering, and clastogenic lava flows. Most fissures rapidly localize to form a small number of eruptive vents, the location of which may influence the subsequent distribution of lava flows and associated hazards. We present results from a detailed field investigation of the proximal deposits of episode 1 of the 1969 fissure eruption of Mauna Ulu, Kīlauea, Hawai‘i. Exceptional preservation of the deposits allows us to reconstruct vent-proximal lava drainage patterns and to assess the role that drainage played in constraining vent localization. Through detailed field mapping, including measurements of the height and internal depth of lava tree moulds, we reconstruct high-resolution topographic maps of the pre-eruption ground surface, the lava high-stand surface and the post-eruption ground surface. We calculate the difference in elevation between pairs of maps to estimate the lava inundation depth and lava drainage depth over the field area and along different segments of fissure. Aerial photographs collected during episode 1 of the eruption allow us to locate those parts of the fissure that are no longer exposed at the surface. By comparing with the inundation and drainage maps, we find that fissure segments that were inundated with lava to greater depths (typically 1–6 m) during the eruption later became foci of lava drainage back into the fissure (internal drain-back). We infer that, in these areas, lava ponding over the fissure suppressed discharge of magma, thereby favouring drain-back and stagnation. By contrast, segments with relatively shallow inundation (typically less than ~ 1 m), such as where the fissure intersects pre-eruptive topographic highs, or where flow away from the vent (outflow) was efficient, are often associated with sub-circular vent geometries in the post-eruption ground surface. We infer that these parts of the fissure became localization points for ongoing magma ascent and discharge. We conclude that lava inundation and drainage processes in basaltic fissure eruptions can play an important role in controlling their localization and longevity.

Aerosol delivery during invasive mechanical ventilation: a systematic review

BackgroundThis systematic review aimed to assess inhaled drug delivery in mechanically ventilated patients or in animal models. Whole lung and regional deposition and the impact of the ventilator circuit, the artificial airways and the administration technique for aerosol delivery were analyzed.MethodsIn vivo studies assessing lung deposition during invasive mechanical ventilation were selected based on a systematic search among four databases. Two investigators independently assessed the eligibility and the risk of bias.ResultsTwenty-six clinical and ten experimental studies were included. Between 30% and 43% of nominal drug dose was lost to the circuit in ventilated patients. Whole lung deposition of up to 16% and 38% of nominal dose (proportion of drug charged in the device) were reported with nebulizers and metered-dose inhalers, respectively. A penetration index inferior to 1 observed in scintigraphic studies indicated major proximal deposition. However, substantial concentrations of antibiotics were measured in the epithelial lining fluid (887 (406–12,819) μg/mL of amikacin) of infected patients and in sub-pleural specimens (e.g., 197 μg/g of amikacin) dissected from infected piglets, suggesting a significant distal deposition. The administration technique varied among studies and may explain a degree of the variability of deposition that was observed.ConclusionsLung deposition was lower than 20% of nominal dose delivered with nebulizers and mostly occurred in proximal airways. Further studies are needed to link substantial concentrations of antibiotics in infected pulmonary fluids to pulmonary deposition. The administration technique with nebulizers should be improved in ventilated patients in order to ensure an efficient but safe, feasible and reproducible technique.

Dispersal pathways in the early Messinian Adriatic foreland and provenance of the Laga Formation (Central Apennines, Italy)

The early Messinian Laga Formation represents a turbidite complex deposited in the Late Neogene foreland basin system of the growing Apenninic chain. While the stratigraphy and physiography of the Laga Basin are well known, the source of its sediments is contentiously claimed to be either recycled Apenninic or primary Alpine. Furthermore, a shift in paleocurrent was proposed as a marker of provenance change around 6Ma. By combining double-dating of detrital zircons (fission-track and U-Pb dating) with compositional analyses, the sedimentary provenance of the lower Laga arenites and differences between the proximal channelized and distal lobe facies are addressed. Due to sediment sorting processes, the lobe facies shows a reduced heavy mineral spectrum relative to the channelized facies. Hence, proximal deposits reflect their hinterland lithologies better than their distal counterpart and should be preferred in provenance analyses. The petrographic composition of the Laga units implies a major metamorphic source combined with an additional dolomite and carbonate source. No compositional difference spanning the shift in paleocurrents is observed, which therefore likely reflects the evolving topography of the foreland due to syn-sedimentary tectonics. Detrital zircon fission-track data reveal youngest age populations at ~16–17Ma and lag times in the range of 9 to 11Ma that can be related to modern fission-track ages observed in the Central Alps. The two major 238U/206Pb age populations, centered at 277.5 and 37.5Ma, represent (post-)Variscan events and the Paleogene magmatic activity in the Central Alps, specifically the Adamello complex. The Central and Southern Alps are thus inferred as the major source for the early Messinian Laga arenites. The pathways of the sediments from the Alps to the Laga Basin crossed the Alps-Apennines foreland and passed on the outer Apenninic wedge-top along elongated and tectonically controlled basins and channels that entered the basin from the north and northwest. The transfer could have been direct along the Apenninic depozones. Additionally, late Tortonian sediments from the Alps could have been temporarily stored on top of the Apenninic wedge, e.g. in the Marnoso-arenacea Basin, and then seamlessly cannibalized into the Laga Basin.

The last deglaciation of Peru and Bolivia

The tropical Andes of Peru and Bolivia are important for preserving geomorphic evidence of multiple glaciations, allowing for refinements of chronology to aid in understanding climate dynamics at a key location between hemispheres. This review focuses on the deglaciation from Late-Pleistocene maximum positions near the global Last Glacial Maximum (LGM). We synthesize the results of the most recent published glacial geologic studies from 12 mountain ranges or regions within Peru and Bolivia where glacial moraines and drift are dated with terrestrial cosmogenic nuclides (TCN), as well as maximum and minimum limiting ages based on radiocarbon in proximal sediments. Special consideration is given to document paleoglacier valley localities with topographic information given the strong vertical mass balance sensitivity of tropical glaciers. Specific valley localities show variable and heterogeneous sequences ages and extensions of paleoglaciers, but conform to a generally cogent regional sequence revealed by more continuous lake sedimentary records. There are clear distributions of stratigraphically older and younger moraine ages that we group and discuss chronologically. The timing of the local LGM based on average TCN ages of moraine groups is 25.1 ka, but there are large uncertainties (up to 7 ka) making the relative timing with the global LGM elusive. There are a significant number of post-LGM moraines that date to 18.9 (± 0.5) ka. During the Oldest Dryas (18.0 to 14.6 ka), moraine boulders date to 16.1 (± 1.1) ka, suggesting that glaciers either experienced stillstands or readvances during this interval. The Antarctic Cold Reversal (ACR; 14.6 to 12.6 ka) is another phase of stillstanding or readvancing glaciers with moraine groups dating to 13.7 (± 0.8) ka, followed by retreating ice margins through most of the Younger Dryas (YD; 12.9 to 11.8 ka). During the early Holocene, groups of moraines in multiple valleys date to 11.0 (± 0.4) ka, marking a period when glaciers either readvanced or paused from the overall trend of deglaciation. The pattern of glacial variability during the Late Glacial after ~14.6 ka appears to be more synchronous with periods of cooling in the southern high latitudes, and out-of-phase with the overall deglacial trend in the Northern Hemisphere. While insolation and CO2 forcing likely drove the general pattern of deglaciation in the southern tropical Andes, regional ocean-atmospheric and hypsometric controls must have contributed to the full pattern of glacial variability.

Syneruptive sequential fragmentation of pyroclasts from fractal modeling of grain size distributions of fall deposits: the Cretaio Tephra eruption (Ischia Island, Italy)

In this work we use fractal statistics in order to decipher the mechanisms acting during explosive volcanic eruptions by studying the grain size distribution (GSD) of natural pyroclastic-fall deposits. The method was applied to lithic-rich proximal deposits from a stratigraphic section of the Cretaio Tephra eruption (Ischia Island, Italy). Analyses were performed separately on bulk material, juvenile, and lithic fraction from each pyroclastic layer. Results highlight that the bulk material is characterized by a single scaling regime whereas two scaling regimes, with contrasting power-law exponents, are observed for the juvenile and the lithic fractions. On the basis of these results, we infer that the bulk material cannot be considered as a good proxy for deducing eruption dynamics because it is the result of mixing of fragments belonging to the lithic and juvenile fraction, both of which underwent different events of fragmentation governed by different mechanisms. In addition, results from fractal analyses of the lithic fraction suggest that it likely experienced a fragmentation event in which the efficiency of fragmentation was larger for the coarser fragments relative to the finer ones. On the contrary, we interpret the different scaling regimes observed for the juvenile fraction to be due to sequential events of fragmentation in the conduit, possibly enhanced by the presence of lithic fragments in the eruptive mixture. In particular, collisional events generated increasing amounts of finer particles modifying the original juvenile GSDs and determining the development of two scaling regimes in which the finer fragments record a higher efficiency of fragmentation relative to the coarser ones. We further suggest that in lithic-rich proximal fall deposits possible indications about the original GSDs of the juvenile fraction might still reside in the coarser particles fraction.

Crevasse splay processes and deposits in an ancient distributive fluvial system: The lower Beaufort Group, South Africa

Up to 12% of the mud-prone, ephemeral distributive fluvial system stratigraphy in the Permo-Triassic lower Beaufort Group, South Africa, comprises tabular fine-grained sandstone to coarse-grained siltstone bodies, which are interpreted as proximal to distal crevasse splay deposits. Crevasse splay sandstones predominantly exhibit ripple to climbing ripple cross-lamination, with some structureless and planar laminated beds. A hierarchical architectural scheme is adopted, in which ~1m thick crevasse splay elements extend for tens to several hundreds of meters laterally, and stack with other splay elements to form crevasse splay sets up to 4m thick and several kilometers in width and length. Paleosols and nodular horizons developed during periods, or in areas, of reduced overbank flooding are used to subdivide the stratigraphy, separating crevasse splay sets. Deposits from crevasse splays differ from frontal splays as their proximal deposits are much thinner and narrower, with paleocurrents oblique to the main paleochannel. In order for crevasse splay sets to develop, the parent channel belt and the location where crevasse splays form must stay relatively fixed during a period of multiple flood events. Beaufort Group splays have similar geometries to those of contemporary perennial rivers but exhibit more lateral variability in facies, which is interpreted to be the result of more extreme fluctuations in discharge regime. Sharp-based crevasse splay packages are associated with channel avulsion, but most are characterized by a gradual coarsening upward, interpreted to represent progradation. The dominance of progradational splays beneath channel belt deposits may be more characteristic of progradational stratigraphy in a distributive fluvial system rather than dominated by avulsion processes in a trunk river system. This stratigraphic motif may therefore be an additional criterion for recognition of distributive fluvial systems in the ancient record.