Distal Alluvial Fan Research Articles

Sedimentology and Stratigraphy of the Shenandoah Formation, Western Fan, Jezero Crater, Mars

AbstractSedimentary fans are key targets of exploration on Mars because they record the history of surface aqueous activity and habitability. The sedimentary fan extending from the Neretva Vallis breach of Jezero crater's western rim is one of the Mars 2020 Perseverance rover's main exploration targets. Perseverance spent ∼250 sols exploring and collecting seven rock cores from the lower ∼25 m of sedimentary rock exposed within the fan's eastern scarp, a sequence informally named the “Shenandoah” formation. This study describes the sedimentology and stratigraphy of the Shenandoah formation at two areas, “Cape Nukshak” and “Hawksbill Gap,” including a characterization, interpretation, and depositional framework for the facies that comprise it. The five main facies of the Shenandoah formation include: laminated mudstone, laminated sandstone, low‐angle cross stratified sandstone, thin‐bedded granule sandstone, and thick‐bedded granule‐pebble sandstone and conglomerate. These facies are organized into three facies associations (FA): FA1, comprised of laminated and soft sediment‐deformed sandstone interbedded with broad, unconfined coarser‐grained granule and pebbly sandstone intervals; FA2, comprised predominantly of laterally extensive, soft‐sediment deformed laminated, sulfate‐bearing mudstone with lenses of low‐angle cross‐stratified and scoured sandstone; and FA3, comprised of dipping planar, thin‐bedded sand‐gravel couplets. The depositional model favored for the Shenandoah formation involves the transition from a sand‐dominated distal alluvial fan setting (FA1) to a stable, widespread saline lake (FA2), followed by the progradation of a river delta system (FA3) into the lake basin. This sequence records the initiation of a relatively long‐lived, habitable lacustrine and deltaic environment within Jezero crater.

Multiple ruptures on the North-Sofia fault at Gorni Bogrov from scarp profile and shallow geophysics

The seismic hazard assessment across the densely urbanized region in the Sofia Basin in Bulgaria can be improved through using fault data, in search of which we first aim to attain to a better knowledge on the past earthquake ground ruptures. Currently focused on the North-Sofia fault, we investigated whether, and how, the fluvial system responded to fault displacements at a site located near the village of Gorni Bogrov. At the studied site, a scarp delimits a distal alluvial fan on the upper surface from an alluvial plain on the lower surface. Although the scarp forms a complex structure that resembles a breached relay ramp, its fault origin is questionable, because the basin axial stream could have caused lateral erosion approaching the toe of the fan. The electrical resistivity profiles that we measured imply that recent deposits are associated with fault displacement. A fault located beneath the middle slope juxtaposes Neogene sand layers, covered by an alluvial fan in the footwall, with colluvial deposits laterally merged with deposits from the alluvial plain in the hangingwall. The scarp also contains a non-tectonic component related to lateral erosion affecting mainly the lower slope. Breaks on the scarp profile are interpreted to have originated through erosion repeatedly renewed due to multiple fault displacements in recent times. The interseismic intervals of the North-Sofia fault are inferred to be in order of thousands of years but not longer. The inferred interseismic intervals correspond approximately to those of the adjacent South-Ihtiman and distant Chirpan faults; however, they are noticeably shorter compared to the adjacent Zlatitsa and distant Krupnik faults. It seems likely that faults in a broad region on the Balkans release strain at remarkably different rates, independently from their proximity.

Preservation of Organic Matter in Aqueous Deposits and Soils Across the Mars‐Analog Qaidam Basin, NW China: Implications for Biosignature Detection on Mars

AbstractA variety of aqueous deposits and soils from gullies, fluvial channels, mountain slopes, alluvial fans, playas, polygons, and yardangs have been found on Mars. However, the occurrence of sedimentary organic matter (SOM) in these landscapes under hyperarid and strongly radiative conditions has yet to be fully assessed. With various desiccated landforms analogous to those on Mars, the Qaidam Basin in northwestern China is considered a promising hyperarid Martian analog. Here, aqueous deposits and soils from nine landforms across the extremely dry northwestern Qaidam and from four landforms across the dry southeastern Qaidam were sampled and investigated. Our results reveal that landform and moisture are important controls on organic carbon distribution across the Qaidam Basin, while sedimentary origin has a dominant effect in the extremely dry northwestern Qaidam. Combined with previous studies, low organic carbon contents of soils in the extremely dry northwestern Qaidam suggest possibly low rates of microbial carbon input and of organic carbon degradation in Mars‐like environments. High organic carbon contents of aqueous deposits together with increasing trends along rivers indicate preservation of relic SOM and possibly slow degradation rates of organic matter as compared to their exposure timescale. Moreover, soils from the distal alluvial fan are found to favor the concentration of pedogenic organic carbon as compared to those from other landforms in the dry southeastern Qaidam and from all landforms in the extremely dry northwest. This suggests that subsoils in the distal fans are promising candidate targets for biosignature detection on Mars.

System response of an interlayered deposit with a localized graben deformation in the Northridge earthquake

The Wynne Avenue site in the California San Fernando Valley was analyzed using two-dimensional (2D) nonlinear dynamic analyses (NDAs) with geostatistical subsurface modeling to interpret key mechanisms contributing to a 12-m-wide graben deformation with vertical offsets of 10–20 cm in the 1994 Northridge earthquake. In-situ data from borings and cone penetration tests (CPTs), joined with geological interpretations of the distal alluvial fan deposition, delineate interlayered silty sand lenses within a typically fine-grained soil stratigraphy. The NDAs used the PM4Sand and PM4Silt constitutive models with the FLAC finite difference program. NDAs with uniform properties for distinct soil zones closely reproduced the observed graben. However, stochastic NDAs, modeled with sequential Gaussian simulations (SGS) for conditional random field realizations of critical zones, often obscured the expected graben. The ability of the stochastic NDAs to model the graben was further reduced when SGS was performed collectively over two distinct soil zones. The NDAs provide new insights on the causal mechanisms of liquefaction-induced ground oscillations and lurching, that were not easily deduced from common one-dimensional (1D) liquefaction vulnerability indices (LVIs) and Newmark sliding-block analyses. This case study demonstrates the capabilities of system-level NDAs for modeling localized ground deformation patterns, but also highlights cautionary nuances for their implementation with data-driven stochastic subsurface modeling.

Alluvial fans in the latest phase of the Messinian Salinity Crisis: An example from the Chaabet Et Tabbela Formation, northern Tunisia, Central Mediterranean

Alluvial fan deposits record important information about paleoclimate, tectonic setting, and paleogeography that can be deciphered with detailed sedimentological analysis. The late Messinian Chaabet Et Tabbela Formation, in the western part of the Kechabta Basin in northern Tunisia, shows excellent examples of alluvial fans deposited during the latest phase of the Messinian Salinity Crisis, the Lago Mare event. This exposure provides the opportunity to investigate terrestrial depositional conditions during this period. The Chaabet Et Tabbela Formation was investigated through sedimentological logging and detailed facies analysis at eight locations. Sixteen facies types were identified and interpreted along with variation in clast compositions. These facies were further grouped into four facies associations: (1) proximal alluvial fan, dominated by gravity-driven processes, (2) medial alluvial fan, which mainly comprises a mix of debris flow and high-sediment load streamflow deposits, (3) distal alluvial fan, which consists of laterally continuous sheetflood and subordinate channels deposits, and (4) playa and floodplain, which are made of red mudstones with calcareous paleosols. Clast composition of these fan deposits comprises Triassic, Jurassic, and Cretaceous bedrock suggesting that sediment was sourced from the surrounding Maghrebian mountain chain. Two laterally adjacent alluvial fans were identified along the N-S Messeftine Fault. These fans have prograded during the Late Messinian, with an estimated radius of ca 2 km.The evolution of these alluvial fans is related to a complex interplay of allogenic processes. The uplift related to intra-Messinian tectonic event and climate variation contributed to the formation of these fans during the late Messinian. Although, these deposits are relatively limited in extension, similar continental deposits form synchronously in different marginal Mediterranean basins during the Lago Mare stage, suggesting that the alluvial fans of the Chaabet Et Tabbela Formation should be placed in a context of regionally change climactic and paleogeographic conditions.

The Hoanh Bo Trough‐a landward keyhole to the syn‐rift Late Eocene–Early Oligocene terrestrial succession of the northern Song Hong Basin (onshore north‐east Vietnam)

Located on the northern Vietnam onshore/offshore transition, the Hoanh Bo Trough is an excellently exposed terrestrial Palaeogene sedimentary sequence that may be treated as an analogue for regional interpretations of the sedimentary and structural evolution of the northern Song Hong Basin. The Hoanh Bo Trough lies to the north of the northern Song Hong Basin and to the west of the Beibuwan Basin, the origin and evolution of which are linked with Palaeogene South China Sea rifting. Field and archival well sedimentological observations were made throughout the Palaeogene succession of the Hoanh Bo Trough, and samples were collected for palynological, petrographical, and geochemical analysis. Based on the coexistence of particular lithofacies, proximal alluvial fan, distal alluvial fan, fluvial alluvial plain with channels, alluvial plain and/or lake margin, and lacustrine facies associations were distinguished. Palynological analyses suggest the sedimentary infill of the Hoanh Bo Trough is of the Late Eocene–Early Oligocene age and was deposited in a very warm tropical/subtropical climate. In turn, geochemical results demonstrate that the deposits have intermediate chemical maturity and were probably reworked from older sedimentary rock sources. Moreover, it is suggested to combine the Dong Ho and Tieu Giao formations and synonymize them as the Dong Ho Formation. The sedimentary pattern, age, climatic conditions, and structural evolution of the Hoanh Bo Trough align well with the rift initiation, rift development, and rift termination tectonic system tracts. Moreover, the Hoanh Bo Trough could be treated as a landward keyhole for the offshore basins: for instance, the Kien An Basin in the northern Song Hong Basin.

Paleosols in distal alluvial sequences and their formation mechanisms: Insights from a high-resolution record in a foreland basin during early Miocene (SE France)

The stratigraphic distribution of paleosols provides a framework to investigate the controls on distal alluvial fan architecture. This study focused on paleosol-bound alluvial cycles (PACs) from the southern Digne-Valensole foreland Basin (SE France, early Miocene). We investigated the duration of the paleosol formation and identified the controlling mechanisms on their temporal and spatial distribution. Paleosols formed primarily from fine-grained sediments (65% shale, 16% lime mud). We identified three pedotypes: Calcisols from reddening throughout the weathering profile, Bw horizon overlying Bk horizon; vertic Calcisols with similar features including slickensides; and gleyic Calcisols showing saturated conditions superimposed on well-drained Calcisol features. Duration of Calcisol formation, estimated from the time span of aggrading PACs, was compatible with those from Quaternary Calcisols: median 11 to 29 kyr for maturity Stage II, > 90% of the dataset); range 20–58 kyr for maturity Stage III-IV. The shortest PACs time spans (< 10 kyr) indicate episodic sedimentation in the distal floodplain. Spatially or temporally variable sediment input in the intermediate floodplain prevented the development of paleosols at short time scales, resulting in longer PACs time spans (40 to 70 kyr) in the range of astronomical-related climate cycles. Autogenic processes also controlled the PACs stacking pattern as illustrated by stacked meter scale PACs formed during periods of relative sea-lowering indicative of a regular down-dip slope limiting river entrenchment; or during a period of high base-level in areas of low sediment supply suggesting complete diversion of the flow laterally from the previous course. The age-constrained PACs enabled to document on high-resolution time scales (a few to 100 s of kyr) the interactions between two components of the alluvial architecture, the episodic aggradation and the pedogenic processes that are difficult to determine in fossil alluvial successions. • Paleosol-bound alluvial cycles studied at 10 4 to 10 5 timescales over 3.5 Myr. • Durations of Aquitanian Calcisol formation were comparable to Quaternary ones. • Episodic floodplain aggradation is documented for timescale shorter than 10 kyr. • The potential of paleosol maturity in evaluating climate fluctuations is discussed. • Autogenic controls were identified from stacking pattern of Calcisols.

Tectono-stratigraphic evolution of the northeastern sector of the Ñirihuau basin, North Patagonian Andes, Argentina: Insights from sedimentology and geochronology data of the Ñirihuau Formation

An approximately 3500 m thick stratigraphic section was detailed surveyed in order to study the tectono-sedimentary evolution of the Ñirihuau Formation along the Arroyo Las Bayas, in the northern sector of the Ñirihuau basin, North Patagonian Andes. A total amount of 29 lithofacies were recognized and used to define facies, grouped into six facies associations (FA) that reflect different paleoenvironments: FA1 (Gray to brownish conglomerates), FA2 (Conglomerates and sandstones), FA3 (Tabular mudstones), FA4 (Brownish sandstones and conglomerates with large scale foresets), FA5 (White tuffaceous sandstones) and FA6 (Sandstones and conglomerates with mudstones). The lower section of the unit was interpreted as deposited in a medium to distal alluvial fan or an axial fluvial system developed in the most distal zone of an alluvial fan (FA1 and FA2). The middle section of the unit is characterized by a Gilbert-type delta developed between two lacustrine sequences (FA3 and FA4). In the upper section of the unit, higher energy deposits corresponding to fluvial systems are registered (FA5 and FA6). Deposits from synchronic volcanic events are recorded along the stratigraphic section. Also, a provenance analysis was carried out by using sedimentary petrography and U–Pb zircon geochronology data. The main identified sediment source corresponds to the contemporaneous Miocene volcanic arc. A secondary detrital provenance from older Cenozoic volcanic rocks of the El Maitén Volcanic Belt was also identified, as well as minor contributions from pre-Cenozoic igneous and metamorphic units. U–Pb geochronological data constrain deposition of the middle and upper sections of the Ñirihuau Formation in the Arroyo Las Bayas section to the middle to late Miocene, between 15 and 11.4 Ma (Langhian to Tortonian). Also, a possible middle Miocene age was discussed for the lower section. An integrated tectono-stratigraphic model was finally proposed, with contrasting extensional and synorogenic tectonic stages registered in the lower and upper sections of the Ñirihuau Formation, respectively.

The role of ecotones in the dehalogenation of chloroethenes in alluvial fan aquifers.

The presence of ecotones in transition zones between geological strata (e.g. layers of gravel and sand interbedded with layers of silt in distal alluvial fan deposits) in aquifers plays a significant role in regulating the flux of matter and energy between compartments. Ecotones are characterised by steep physicochemical and biological gradients and considerable biological diversity. However, the link between organic pollutants and degradation potential in ecotones has scarcely been studied. The aim of this study is to relate the presence of ecotones with the dehalogenation of chloroethenes. A field site was selected where chloroethene contamination occurs in a granular aquifer with geological heterogeneities. The site is monitored by multilevel and conventional wells. Groundwater samples were analysed by chemical, isotopic, and molecular techniques. The main results were as follows: (1) two ecotones were characterised in the source area, one in the upper part of the aquifer and the second in the transition zone to the bottom aquitard, where the aged pool is located; (2) the ecotone located in the transition zone to the bottom aquitard has greater microbial diversity, due to higher geological heterogeneities; (3) both ecotones show the reductive dehalogenation of perchloroethylene and trichloroethylene; and (4) these ecotones are the main zones of the reductive dehalogenation of the pollutants, given the more reductive conditions at the centre of the plume. These findings suggest that ecotones are responsible for natural attenuation, where oxic conditions prevailed at the aquifer and bioremediation strategies could be applied more effectively in these zones to promote complete reductive dehalogenation.

A wet climatic terrestrial carbonate record in the middle-upper Pleistocene, north Tunisia

The middle-upper Pleistocene Borj Edouane unit records a singular spectrum of terrestrial carbonates in a shallow depression on Cretaceous rocks in north Tunisia. This study aims to decipher the depositional system and reveal the climatic and paleohydrological conditions of the carbonate facies deposition. They are grouped into two facies associations: (1) a calcrete–palustrine and (2) a composite microbialite-travertine, surrounded by a marginal alluvial association along the western and northern borders of the basin. The calcrete–palustrine association extends along the westernmost south–north strip of the basin, reflecting calcrete development on distal alluvial fan deposits and pedogenic modification (palustrine facies) of shallow fresh-water carbonates. The composite microbialite-travertine, extending along north–south strips in the middle and eastern areas of the basin, consists of microbialite, clastic microbialite–travertine, and travertine associations. Microbialites were developed in fluvio-lacustrine environments within the central strip and pass eastward into the travertines through the clastic microbialite–travertine association. The latter was deposited from currents and in shallow ponds along a bench next to the travertine-depositing springs, linked to a fault system in the eastern margin of the basin. The carbonate depositional system shows an asymmetrical distribution of facies that parallels the paleohydrological asymmetry along the basin. Water supply was restricted in the western part of the basin and abundant in its eastern areas linked to spring waters that kept stable lake levels in the basin center, favoring the genesis of microbialites. The presence of lacustrine carbonates in the studied area indicates higher water availability during the Middle-Late Pleistocene and that climate in North Africa was wetter than today.

Chronology of Miocene terrestrial deposits and fossil vertebrates from Quebrada Honda (Bolivia)

At ~3500 m above sea level in the Eastern Andean Cordillera, Quebrada Honda Basin exposes more than 300 m of middle Miocene sedimentary strata, developed on distal alluvial fan sediments interstratified with volcanoclastic deposits, overlying Paleozoic basement. These deposits are rich in exotic vertebrate fauna adapted to a wetter and warmer climate at lower paleoelevation, as contrasted with the present high-elevation cold and dry Andean conditions. This study provides new magnetostratigraphic and 40Ar/39Ar age control of the Quebrada Honda, Río Rosario and Huayllajara fossiliferous areas, demonstrating that they are 13.1–12.4 Ma. Part of the succession evidences cyclical deposition of calcretes and reddish mudstones which may be a response to precessional orbital-forcing climatic processes. Paleomagnetic measurements show a significant clockwise tectonic rotation related to postdepositional tectonism, associated with evidence of an important uplift after the deposition of Quebrada Honda sediments. This uplift of the Andean Eastern Cordillera was associated with the formation of the Subandean fold-thrust belt and the development of the Amazon and La Plata drainage basins. Proposed uplift rates of >200 m/Ma are compatible with well-established rates of uplift recorded in contemporaneous alpine sections.

Impact of monsoon, vegetation, and landscape on pedogenesis: A case study using organic and inorganic tracers from the Himalayan foreland sediments

The physical and chemical properties of paleosols are widely used to explain their genesis. However, the physical attributes of paleosols subjected to burial cannot be used for paleoenvironmental reconstructions. In contrast, selective pedogenic tracers such as rare earth (REE) and trace elements, and organic proxies such as pristane/phytane (Pr/Ph) ratios, and D/H and 13C/12C ratios of n-alkanes that are resistant to post-depositional processes can provide insight into pedogenic conditions. Although the upper Miocene Siwalik paleosols from the Himalayan Foreland Basin (HFB) have been used for past vegetation and climate reconstructions, little emphasis has been given to understand their formation. In this regard, the Jabbarkhad Siwaliks of the Kangra sub-basin, identified as distal alluvial fan deposits with records of temporal variability in rainfall and vegetation composition, have been used to investigate the combined impact of depositional environment, topography, climate and vegetation composition on paleosol formation.Prominent negative Eu-anomalies in CI-normalised REE patterns along with uniform ratios of refractory elements suggest a dominant and consistent source of sediments from K-rich granitic upper continental crust. A wide variation in the abundance of REE indicates the modification of sediments by pedogenetic processes. In the Jabbarkhad region, oxic (higher Pr/Ph) and well-drained soils favoured the proliferation of grasses. The high root density and organic turnover of grasses facilitated the development of a mature, strongly leached soil (high Ba/Sr) under high rainfall and humid conditions (lower δ2Η values). Less mature (lower Ba/Sr) paleosols were developed in low-lying anoxic (Pr/Ph <1) floodplain areas dominated by C3 trees near the active channel. Comparison of soil maturity from the Gangetic floodplain, a modern analogue of the Siwalik sediments, suggests that the spatial variability of paleosol maturity was controlled by the landscape mosaic of the Siwalik floodplain. Since pedogenesis and vegetation distribution are controlled by floodplain morphology, sampling of paleosols with similar pedogenic and geomorphic characteristics is essential for evaluation of vegetation distribution in temporally separated paleosols.

Facies and paleoenvironments in Paraíba do Sul Deltaic Complex area, North of Rio de Janeiro state, Brazil

The Paraíba do Sul Deltaic Complex (PSDC) is a large Quaternary coastal plain located along Rio de Janeiro's northern coast and forms part of the onshore portion of Campos Basin. The main studies of the PSDC focused on modeling the paleoenvironmental evolution and were based on paleogeographic reconstructions, surface sampling, radiocarbon datings (14C), drillcores from shallow wells (<40 m depth) and results of ground penetrating radar (GPR) surveys. The paucity of deep subsurface data, in particular continuous drillcores, has led to divergent interpretations of not only the age, paleoenvironmental conditions and stratigraphic position of the PSDC deposits, but also of the underlying sedimentary units. In order to improve the knowledge about the evolution of PSDC, this study aims the stratigraphic characterization and paleoenvironmental interpretation by facies analyzes of a deep drillcore (410 m) recovered at the central area of PSDC. Fourteen sedimentary facies, grouped in six facies associations related to a wide variety of depositional paleoenvironments have been identified. The depositional history of the PSDC began with a transgressive event that led to the drowning of an incised valley cutting the continental deposits of the São Tomé Member, which is represented by the distal alluvial fan (DAF) and fluvial (F-I) facies associations in the analyzed drillcore. The valley infill is marked by two Pleistocene depositional sequences representing estuarine (FE-I facies association) and estuarine to open marine (FE-II facies association) paleoenvironments deposited in response to high-frequency base-level oscillations within a major transgressive sequence after the Marine Isotope Stage 6 (Penultimate Marine Transgression - 123.000 years B.P.). This transgression was followed by a regressive phase (normal to forced) that promoted the development of extensive Pleistocene beach-ridge terraces (BR facies association), which are still partially preserved at the surface of the study area. Marine and lagoonal deposits associated with the transgressive period occurred after the Marine Isotope Stage 2 (Last Marine Transgression - 5.100 years B.P.) documented in the previous studies, were not found in the analyzed drillcore, suggesting that these deposits may have discontinuously preserved within the study area or that they were eroded by Holocene fluvial systems represented by F-II facies association.

New Ideas and Critical review of Middle Eocene Gercus Formation, Kurdistan Region, NE-Iraq

In Iraq, Gercus Formation (Middle Eocene) crops out now near boundary between Low and High Folded Zone and generally consisted of red claystone and sandstone with occasionalbeds of conglomerate. Lithologically, it changes toward the southwest to carbonatesiliclastics of either Khurmala or Aliji Formations inside the Low Folded Zone. All previous studies have concluded that the environment of the formation was fluvial and deltaic of continental mollasse facies with possible some shallow marine sediments at its lower part. Conversely, the most recent two studies have assigned two different, opposite environments of the formation, which are deep marine (turbidite facies), and desert environments. The aim of the present study is critical and objective analysis of the previous studies in addition to introduction of new ideas that are opposite to deep marine and aeolian environments. The study gives many evidences and features for justification of the introduced new ideas, which are revealing the actual environment of the formation. This reveal is depended on the field and regional correlation in areas of boundary between High and Low Folded Zone (area of exposure of Gercus Formation) and area of Imbricated and Thrust Zone where its equivalent in age and facies (unit five of Red Bed Series) is exposed. The present study supports neither the deep marine turbidite nor the desert erg system of the previous studies. The present study proves that the Gercus Formation consists of the distal alluvial fan and its proximal part is unit five of Red Bed.

Barremian synrift sedimentation in the Oliete sub-basin (Iberian Basin, Spain): palaeogeographical evolution and distribution of vertebrate remains

A review of the onset of the synrift sedimentation and synsedimentary extensional tectonics of the Oliete sub-basin (northwestern Maestrazgo basin, East Spain) is presented here based on new data acquired after extensive sedimentological, structural and palaeontological analysis of the Barremian Blesa Fm. The lower boundary of the Blesa Fm is a prominent basal synrift unconformity overlying Jurassic units. This formation has been divided into three genetic stratigraphic sequences bounded by sub-basin-wide unconformities. The lower Blesa sequence (LBS) is characterized by distal alluvial to palustrine marls/clays grading upward to palustrine–lacustrine limestones. The LBS is bounded on top by a planar to irregular transgressive, hardened ferruginous surface, locally encrusted by oysters. Above this discontinuity, the middle Blesa sequence consists of oyster-rich limestones and marls deposited in a shallow restricted bay, which grade to distal alluvial and palustrine–lacustrine marls/clays and limestones towards the marginal areas of the basin. The boundary between the middle and upper Blesa sequence (UBS) is a regressive surface outlined by the local presence of an erosive conglomeratic bed. The UBS generally starts with red clays deposited in distal alluvial fan environments, which grade upwards to palustrine and lacustrine carbonates and marls/clays. The local presence of heterolithic alternations of clay with fine-to-medium sandstone and of cross-bedded sandstones indicates the local occurrence of siliciclastic coastal environments in the UBS. The upper boundary of the Blesa Fm is marked by widespread transgression, giving rise to the bioclastic limestones of the Alacon Fm. In the present paper, the stratigraphic position and palaeoenvironmental context of the abundant vertebrate remains found across the defined sequences within the Blesa Formation is reviewed. The results obtained are relevant for a further understanding of the tectosedimentary evolution of the studied basin. Successive stages of evolution are distinguished, including the initial uplift, breakup and erosion of the earlier Jurassic carbonate platform that took place during the Tithonian–Hauterivian; the onset of synrift sedimentation during the early Barremian, which was highly controlled by extensional faulting and differential block subsidence; the homogenization of the basin subsidence accompanied by the incursion of marine waters (sourced from southeastern areas) during the middle part of the Barremian; and the significant fall in base level, of possible climatic origin and also involving significant siliciclastic input in the northern areas of the Oliete sub-basin around the middle part of the late Barremian.

El aluvión del 9 agosto 2015 en Alto Patache, región de Tarapacá, Desierto de Atacama 1

During the 09 August 2015 (strong El Nino year) heavy rains fell in the coastal Atacama Desert, including our Research Station Atacama UC_Alto Patache (cordillera de la Costa, Tarapaca Region). In this study we provide a detailed analysis of the meteorology and hydrogeomorphic response to the 51,6 mm in 6 hours of rain recorded in Alto Patache. In our control basin (7.2 Ha) we estimated a maximum erosion of ~1070 m3, which were partially stored in the distal alluvial fan, where we quantified 637 m3 of material. We es timated a mean water discharge of 11 m3 s-1 at the lower basin. The comparison between the meteorological and morphostratigraphical data allows us to define three phases that best characterize the anatomy of the 9A: Initial, Peak and End. We conclude that Alto Pa tache embraces an overall geomorphic stability that is interrupted recurrently by debris flows linkable to ENSO variability.

Early cretaceous topographic growth of the Lhasaplano, Tibetan plateau: Constraints from the Damxung conglomerate

AbstractConstraining the timing of early topographic growth on the Tibetan plateau is critical for any models of India‐Asia collision, Himalayan orogeny and subsequent plateau development in the Cenozoic. Stratigraphic, sedimentological and provenance analysis of the Lower Cretaceous red‐beds of the Damxung Conglomerate provide new key information to reconstruct the paleogeography and the tectonic evolution of the Lhasa terrane at the time. The over 700‐m‐thick Damxung Conglomerate documents distal alluvial fan to braidplain sedimentation passing upward to proximal alluvial fan sedimentation. Deposition began near sea level, as documented by limestone beds occurring at the base of the unit. Zircon U–Pb dating of interbedded tuff layers constrain deposition age at ca. 111 Ma. Abundance of volcanic clasts, Cretaceous U–Pb ages and Hf isotopes of detrital zircons yielding mainly negative εHf(t) values together with paleocurrent data indicate an active volcanic source located in the North Lhasa subterrane. Pre‐Mesozoic‐aged zircon, recycled quartz and (meta) sedimentary rock fragments increase up‐section, indicating progressive erosional exhumation of the Paleozoic sedimentary/metasedimentary basement. The Damxung Conglomerate thus records a significant uplift and unroofing stage in the source region, implying initial topographic growth on the Lhasa terrane at early Albian time. Early Cretaceous topographic growth on the Lhasa terrane is supported by the stratigraphic record in the Linzhou basin, the Xigaze forearc basin and the southern Nima basin. In contrast, marine strata in the central‐western Lhasa terrane lasted until the early Cenomanian (ca. 96 Ma), indicating diachronous marine regression on the Lhasa terrane from east to west.

Geochemical characterization of the middle and late Pleistocene alluvial fan-dominated infill of the northern part of the Weihe Basin, Central China

Major reorganizations in climate and tectonic regime occurred in East Asia during the Pleistocene, resulting in large-scale environmental changes. In this paper a detailed geochemical and mineralogical record of these changes is presented from a distal alluvial fan sedimentary sequence in the northern Weihe Basin. We established that, in addition to glacial-interglacial variation, there are three distinctly different units deposited over the past 1m.y. These units are the result of variations in the overall tectonic regime in the northern Weihe Basin.Fine-grained detrital minerals were predominantly delivered during colder climatic periods, whereas evaporative minerals were dominantly deposited during the warmer, interglacial periods, probably as a result of strong seasonal contrast. This compositional variation demonstrates the importance of climate control on hinterland erosion, surface runoff, chemical weathering and evaporation.Al-normalized ratios of indicative major elements relative to average loess composition, indicate important variations in sedimentary processes, mostly related to sediment flux. Si-enrichment is an index for past flooding events, while Fe enrichment, just like K and Ti, reflects influx of clays. In contrast, Ca and Mn are strongly enriched throughout the core, associated with the authigenic precipitation of carbonates, especially during interglacial periods.The lower (~1000–690ka) and upper (~330–0ka) units of the core are characterized by relative intense and frequent flooding, which coincided with extensive ponding in the study area. In the middle unit (~690–330ka) increased salinity levels caused by evaporation, as reflected in the high Sr/Ca ratio and dolomite abundance, led to increased carbonate precipitation. Simultaneously, the increased influx of fine sediments indicates increased clay production in the source area as a result of a more intense summer monsoon strength after 600ka.

The Jurassic of the Western Tien Shan: the Central Kyzylkum Region, Uzbekistan

Abstract The Sarbatyr inlier in the Kyzylkum area, Uzbekistan contains rare outcrops of Jurassic (Bajocian–Bathonian) rocks which form part of the Kuduksarbatyr Formation. Six facies are differentiated in the general succession, ranging from conglomerates to mudstones. The coarser sediments were deposited in a distal alluvial fan setting, which interdigitated with nearshore/lagoonal marine sediments rich in fossil fragments and glauconite. The overall succession provides evidence of varying sea-levels over time, with three distinct ‘transgressive’ events being noted. Two global transgressive events have been recognized in the Bajocian with another significant event at the Bajocian–Bathonian boundary. While it is possible that the events recognized within the Sarbatyr succession correspond to these global events, the effects of local tectonic activity must also be taken into account. The re-examination of the sediments of the Kuduksarbatyr Formation extends the marine influence on sedimentation through to the Bathonian (previous interpretations suggested that the area was continental).

Subtle signatures of seeps: Record of groundwater in a Dryland, DK, Olduvai Gorge, Tanzania

AbstractFew proxies exist to identify aridity in the depositional record, although drylands cover ca 30% of the modern continental surface. New exposures in a siliciclastic and carbonate sequence in an arid to hyperarid basin at Olduvai Gorge, Tanzania provide a unique multi‐proxy record of a 1·85 Ma landscape that was exploited by early humans. The 2 m thick sequence of clastics and carbonates that are exposed along a 450 m outcrop records climate change over a single precession (dry‐wet‐dry) cycle. Siliciclastic data (sedimentary structures, grain size, mineralogy) and biological data are combined with data for a 10 to 35 cm thick limestone (stable isotopes, elemental geochemistry, petrography) to generate a depositional facies model for a site DK (Douglass Korongo) on this dry rift basin landscape. This site was situated on a low gradient, distal portion of a volcaniclastic alluvial fan. The clastics are intercalated distal alluvial fan sandy silts and lake clays that accumulated in a low energy environment. Groundwater discharge and the alkaline springs and seeps during wet‐to‐dry change in climate made a freshwater carbonate‐rich environment. Bedded lithofacies (a lime mudstone with fossils) were deposited in shallow standing (spring‐fed) pools, while nodular lithofacies with calcite spherulites indicate permanently saturated ground (seeps). Both environments experienced similar diagenesis, that is, the precipitation of authigenic barite from supersaturated groundwater, desiccation and pedogenesis, and late‐stage calcite precipitation. Compositional and isotopic data suggest that a fresh groundwater‐fed system was available to early humans even during dry intervals of the precession cycle.