Alluvial Fan Successions Research Articles

Dynamic landscape response to Younger Dryas and earliest Holocene cooling events in the European Eastern Alps (Austria)

In classic loess areas research into dust deposition and concomitant environmental changes has a long history and numerous well-investigated loess-palaeosol sequences provide insights into Late Quaternary landscape responses to climatic change on orbital to centennial time scales. This contrasts with mountain regions, where an understanding of dust deposition under rapidly changing climatic and environmental conditions is much less developed. Here we describe two sediment records from the Austrian Alps that provide rare evidence for intramontane loess accumulation. Dust deposition has been numerically constrained to the Younger Dryas (YD) to earliest Holocene interval, making these sites the first proof of significant intramontane aeolian activity during this time interval in the Eastern Alps.At study site 1, located in the Northern Calcareous Alps, a loess layer 10–20 cm in thickness and laterally exposed over more than 300 m is sandwiched into an alluvial-fan succession. Two optically-stimulated luminescence ages of quartz and four radiocarbon ages of charcoals indicate loess accumulation during the Younger Dryas to earliest Holocene. This laterally extensive loess drape suggests much drier climatic conditions compared to today, which is corroborated by palynological investigations. Macro- and micro-charcoal particles within the loess indicate regional wildfires that might have exaggerated climatically induced sedimentary processes. The second site is a Mesolithic site located ∼30 km southeast of site 1. At site two, redeposited till and pebbly scree are sharply overlain by a few centimetre thick light-grey loess layer, which is capped by the archaeological living floor hosting 14C dated fireplaces suggesting human presence as early as 10.9 ± 0.2 cal ka BP. These radiocarbon ages in combination with single-grain IRSL dating of feldspar at site two suggests concomitant aeolian deposition at site 1 and 2.Recently, a widespread drape of loess deposited immediately subsequent to the LGM was described for the same sector of the Eastern Alps (Gild et al., 2018). The present study expands on these findings and provides evidence for the recurrence of loess deposition also during the YD to earliest Holocene, and suggests a highly sensitive response of the geomorphic system in mountain ranges to abrupt post-LGM climate fluctuations. We conclude that, in addition to moraine records of Alpine glaciers and speleothems that accurately chronicle Late Glacial temperature fluctuations, the spatio-temporal pattern of inner-Alpine loess deposits records the mode and sensitivity of landscape responses to such cooling events. Because such aeolian sediment layers can be redeposited or pedologically overprinted, they are often overlooked, and are thus still severely under-researched.

Impact of faulting in depocentres development, facies assemblages, drainage patterns, and provenance in continental half‐graben basins: An example from the Fanja Basin of Oman

AbstractFault throw gradients create transverse folding, and this can influence accommodation creation and sedimentary routing and infill patterns in extensional half‐graben basin. The Fanja half‐graben basin (Oman) offers an excellent outcrop of an alluvial fan succession displaying cyclical stacking and basin‐scale growth‐fold patterns. These unique conditions allow for an investigation of fault‐timing and accommodation development related to fault‐transverse folding. Our study combines geological mapping, structural analysis, sedimentary logging and correlation, and bulk mineralogical compositions. Mapping reveals that the basin is bounded by a regional‐scale fault, with local depocentres changing position in response to transverse syncline and anticline development ascribed to fault‐displacement gradients. The alluvial Qahlah Formation (Late Cretaceous) is unconformably overlying the Semail Ophiolite, and is in turn overlain by the marine Jafnayn Formation (Late Palaeocene). Facies and stratigraphic analysis allows for subdivision of the Qahlah Formation into four informal units, from base to top: (i) laterite in topographic depressions of the ophiolite, (ii) greenish pebbly sandstones, deriving from axially draining braided streams deposited in the low‐relief half‐graben basin. This green Qahlah grades vertically into the red Qahlah, formed by alluvial fanglomerates and floodplain mudstones, with drainage patterns changing from fault‐transverse to fault‐parallel with increasing distance to the main fault. The red Qahlah can be divided into (iii) the Wadi al Theepa member, found in a western basin depocentre, with higher immaturity and sand: mud ratio, suggesting a more proximal source, and (iv) the Al Batah member, located in the eastern part of the basin. The latter shows better sorting, a lower sand: mud ratio, and more prominent graded sub‐units. It also shows eastward expansion from an orthogonal monocline, ascribed to accommodation developed in a relay ramp. Changes in sedimentary facies and depositional patterns are consistent with differential mineralogical composition. The Green Qahlah is composed of quartz and lithic mafic rock fragments, sourced from the ophiolite and schists of the metamorphic basement. The Red Qahlah is composed of chert and kaolinite sourced from the Hawasina Nappe succession in the footwall of the master fault. These changes in source area are linked to unroofing of fault‐footwalls and domal structures during the extensional collapse of the Semail Ophiolite. The novelty of this study resides in linking sedimentology and fault‐displacement events controlling fault‐perpendicular folding, and its influence on depocentre generation and stratigraphic architecture. This is an approach seldom considered in seismic analysis, and rarely analysed in outcrop studies, thus placing the results from this study among the key outcrop‐based contributions to the field.

Sedimentation of open-framework gravels in alluvial-fan settings: Quaternary Poplar Fan, northwest China

Open-framework gravels (OFGs) form common lithofacies of alluvial fans. However, inferences of their mechanisms of transport and deposition, and of controls on their distribution in alluvial-fan successions, remain difficult, limiting our ability to interpret the environmental significance of such deposits and to predict their distributions in ancient successions. From an applied standpoint, such predictions are important in the development of hydrocarbon reservoirs and remediation of aquifers hosted in alluvial-fan strata. To elucidate our understanding of formative conditions of different types of OFGs, a study has been undertaken on the Quaternary Poplar Fan, in the endorheic Heshituoluogai Basin, China.Based on observations from outcrops and trenches on the Poplar Fan, eight different types of OFGs are identified: these lithofacies types account for 7% of the total deposits of the fan. During high flow stages, OFGs are deposited in association with supercritical sheet-like flow (type 1), unconfined streamflow (type 2), and flow in groundwater-fed channels (type 3). During low flow stages, OFGs are associated with trough cross-stratification (type 4) and sigmoidal cross-stratification (type 5); these types occur over the entire fan. At the fan margins, OFGs are associated with gulleys (type 6), especially at the distal fan toe. Other OFG types are associated with bank collapse (type 7) or intercalated with aeolian deposits (type 8). A model is proposed to account for the distribution of different types of OFGs across the fan, wherein types 1, 2 and 4 seen in the medial fan are dominant. This provides a framework for the identification, prediction and correlation of OGFs in subsurface alluvial-fan successions.

Episodic Sedimentary Evolution of an Alluvial Fan (Huangshui Catchment, NE Tibetan Plateau)

Alluvial-fan successions record changes in hydrological processes and environments that may reflect tectonic activity, climate conditions and changes, intrinsic geomorphic changes, or combinations of these factors. Here, we focus on the evolution of a stream-dominated fan in a tectonic depression of the Xining basin of China, laid down under a semi-arid climate in the northeastern Tibetan Plateau (NETP). The fan succession is composed of three facies associations, from bottom to top: (1) matrix to clast-supported, poorly sorted, planar cross-stratified to crudely stratified sheets of coarse-grained sediments; (2) horizontal laminated sand, laminated layers of reddish fine silt and yellow coarse silt with stacked mounds of sand; and (3) clay-rich deposits with incipient paleosols. The succession shows rapid sediment aggradation from high-energy to low-energy alluvial fans and finally to a floodplain. The dating results using optically stimulated luminescence (OSL) method show that a gravelly, high-energy fan was deposited during MIS 6, after which a low-energy fan, mainly composed of sand and silt, was deposited and finally covered by flood loam during the MIS 6–5 transition and the warmer last interglacial. Stacked sand mounds are interpreted from their sediment structure and grain-size distribution as shrub-coppice dunes in low-energy fan deposits. They may be considered as a response to the interaction of alluvial and aeolian processes in a semi-arid environment.

Evidence of Quaternary and recent activity along the Kyaukkyan Fault, Myanmar

Cenozoic right-lateral shear between the eastern Indian margin and Eurasia is expressed by numerous N-S trending fault systems inboard of the Sunda trench, including the Sagaing Fault. The most easterly of these fault systems is the prominent ∼500 km long Kyaukkyan Fault, on the Shan Plateau. Myanmar’s largest recorded earthquake, Mw 7.7 on 23rd May 1912, focused near Maymyo, has been attributed to the Kyaukkyan Fault, but the area has experienced little significant seismicity since then. Despite its demonstrated seismic potential and remarkable topographic expression, questions remain about the Kyaukkyan Fault’s neotectonic history.In this study we document robust geologic evidence of fault activity along the Kyaukkyan Fault. Field investigation and interpretation of satellite imagery reveal deformation features developed along a fault system mostly indicative of Quaternary dextral strike-slip faulting. Clearly defined tectonic geomorphology, including fault scarps and linear valleys, are used to trace the northernmost and southernmost part of the fault. The fault’s central section is characterised by a complex pull-apart system, whose normal border faults show signals of relatively slow recent activity.Dextral transtensional activity along the Kyaukkyan Fault is recorded by geomorphic features such as sag ponds, shutter ridges, offset and beheaded streams, triangular facets and low-sinuosity mountain fronts. The Quaternary age of activity is demonstrated by short-lived geomorphic features such as wind-gaps, offset and deformed alluvial fans, and even offset of man-made structures. In the Inle basin, alluvial fan successions along the easternmost mountain front reveal a vertical transition from faulted and folded alluvial fan sequences adjacent to pre-Cenozoic flanking ranges, to overlying gravels that appear less deformed. Conversely, a locally buried cross-basin fault system has fresh geomorphic expression even within the uppermost parts of the lacustrine/fluvial basin fill. This may indicate Quaternary migration of dominant fault deformation from sidewall faults to a cross-basin fault system, which is indicative of a mature strike-slip fault system and has been observed in other active strike-slip faults around the world and in analogue models.

Orbital-climate control of mass-flow sedimentation in a Miocene alluvial-fan succession (Teruel Basin, Spain)

Abstract The role of climate change in driving alluvial-fan sedimentation is hard to assess in pre-Quaternary successions, for which detailed chronologies and climate-proxy records cannot be easily established. In the Teruel Basin (Spain), high-resolution (10 4 –10 5 years) chronological and palaeoclimatic information was derived by orbital tuning of Late Miocene mudflat to ephemeral-lake deposits. The semi-arid palaeoclimate made this low-gradient, basinal environment sensitive to thresholds in the local hydrological balance. Basic facies rhythms are attributed to alternating, relatively humid/arid phases controlled by the climatic precession cycle. The lower stratigraphic interval of this reference section interfingers with distal, coarse-clastic beds from a coeval alluvial fan. The consistent interdigitation of debris-flow deposits with distal strata indicative of arid-to-humid climate transitions shows that fan sedimentation was regulated by climate cyclicity. In particular, the largest volumes of terrigenous debris were shed from the fan onto adjacent mudflats during transitions to relatively humid periods with pronounced seasonality, during precession minima. Distal to medial sections within alluvial-fan outcrops also feature prominent, laterally continuous alternations of coarse- and fine-clastic packages. This high degree of architectural organization, uncommon in fan successions, and stratigraphic relationships with the reference section suggest orbitally controlled climate change to have been the forcing mechanism.

High-resolution estimates of rates of depositional processes from an alluvial fan succession in the Miocene of the Ebro Basin, northern Spain

Abstract Obtaining estimates of the rates of sedimentation and frequency of depositional events at high resolution from pre-Quaternary successions is rarely possible, particularly in coarse-grained clastic successions of continental basin margins. In the Miocene strata of the Ebro Basin, northern Spain, basin-marginal deposits interpreted as the products of alluvial fans are part of the continuous aggradational fill of a basin of internal drainage. A palaeomagnetic reversal stratigraphy has been established in basin-centre sediments and correlation across to the basin margin makes it possible to estimate the rate of aggradation within the fan successions to be 90 mm ka −1 . The deposits of one alluvial fan near the village of Linás de Marcuello have been analysed in the context of these rates to provide estimates of the frequency of depositional events. This analysis suggests that deposition on gravel bars approximately 700 mm thick in stream channel deposits occurred at intervals of 70–200 years over a 2.2 myr time period. This is the first time an estimate of the frequency of depositional events has been obtained at such high resolution from a pre-Quaternary alluvial fan succession.

Early Syn-Rift Evolution In the West Cameros Basin (Upper Jurassic, Nw Iberian Range), Spain and Pedogenetic Calcretes In Early Syn-Rift Alluvial Systems (Upper Jurassic, West Cameros Basin), Northern Spain—Discussion

The Brezales Formation, originally defined as the Senora de Brezales Formation (Platt 1986, 1990; see also Martin-Closas and Alonso Millan 1998; Clemente Vidal 2010), comprises a syn-rift alluvial fan succession displaying strong lateral and vertical facies changes. Sacristan-Horcajada et al. (2015) primarily relate these variations to drainage area and basin-scale tectonics. They suggest that “source area lithology can be considered as a constant factor.” In this regard, it is not clear why Sacristan-Horcajada et al. fail to discuss the work of Platt (1995), which showed how sedimentation and clastic supply was strongly controlled by subcrop geology as well as kilometer- and outcrop-scale faulting. Deposition across the area recorded progressive erosion and unroofing of the underlying succession and varying subcrop beneath the complex Cimmerian unconformity. In the south of the basin, a “progradational” (coarsening-upwards) facies trend noted by Sacristan-Horcajada (2015) reflects local reworking of upper Oxfordian sandstones at the unconformity, leading to the deposition of red-bed sandstones in the basal section of the Brezales Formation. Only later did erosion of deeper-lying marine Jurassic limestones result in the deposition of conglomerates. An example occurs at Espejon, where the Brezales Formation is not absent as suggested by Sacristan-Horcajada et al. (2015), but features a basal …

Spatial characteristics of the Pliocene to modern alluvial fan successions in the uplifted sedimentary basins of Almería, SE Spain: review and regional synthesis

Abstract Conventional sedimentological models of alluvial fan sequences tend to be based on fans in subsiding sedimentary basins. Fans in uplifting terrains are rather different. We examined the sequence of fan delta and alluvial fan deposition from the Pliocene to the Late Quaternary in the uplifting terrain of the Neogene sedimentary basins of Almería, Betic Cordillera, SE Spain. There were three phases of fan deposition, each yielding particular morpho-sedimentological characteristics and each having distinctive spatial distributions. The first phase was the Pliocene fan deltas, deposited where the larger river systems entered the receding early Pliocene seas. This phase was followed during the late Pliocene–Early Pleistocene by a phase of extensive alluvial fan deposition extending from the mountain fronts to the basin centres. Continued uplift occurred during the Pleistocene, resulting in incision of the drainage network and dissection of the Plio-Pleistocene fan systems. The third phase, during the Quaternary, involved much smaller mountain front alluvial fans set well below the Plio-Pleistocene fans, except in local areas of tectonic subsidence where extensive fans were formed. The older fans exhibit stacked sedimentary successions with very little evidence of sustained vertical incision. However, the Quaternary fans show much evidence of sustained incision between major aggradational phases, often resulting in inset rather than stacked stratigraphic relationships. We interpret this to imply that, although the tectonic context was the primary temporal control on variations within the older fan successions, within the Quaternary fans the overall tectonic control has been modified by responses to the Quaternary climatic sequence.

Tectonic and climatic controls on fan systems: The Kohrud mountain belt, Central Iran

Late Pleistocene to Holocene fans of the Kohrud mountain belt (Central Iran) illustrate the problems of differentiating tectonic and climatic drivers for the sedimentary signatures of alluvial fan successions. It is widely recognised that tectonic processes create the topography that causes fan development. The existence and position of fans along the Kohrud mountain belt, NE of Esfahan, are controlled by faulting along the Qom-Zefreh fault system and associated fault zones. These faults display moderate amounts of historical and instrumental seismicity, and so may be considered to be tectonically active. However, fluvial systems on the fans are currently incising in response to low Gavkhoni playa lake levels since the mid-Holocene, producing incised gullies on the fans up to 30m deep. These gullies expose an interdigitation of lake deposits (dominated by fine-grained silts and clays with evaporites) and coarse gravels that characterise the alluvial fan sediments. The boundaries of each facies are mostly sharp, with fan sediments superimposed on lake sediments with little to no evidence of reworking. In turn, anhydrite–glauberite, mirabilite and halite crusts drape over the gravels, recording a rapid return to still water, shallow ephemeral saline lake sedimentation. Neither transition can be explained by adjustment of the hinterland drainage system after tectonic uplift. The potential influence in Central Iran of enhanced monsoons, the northward drift of the Intertopical Convergence Zone (ITCZ) and Mediterranean climates for the early Holocene (~6–10ka) point to episodic rainfall (during winter months) associated with discrete high magnitude floods on the fan surfaces. The fan sediments were deposited under the general influence of a highstand playa lake whose level was fluctuating in response to climate. This study demonstrates that although tectonism can induce fan development, it is the sensitive balance between aridity and humidity resulting from changes in the climate regime of Central Iran that influences the nature of fan sequences and how they interrelate to associated facies.

The Plio-Pleistocene fluvio-lacustrine Upper Valdarno Basin (central Italy): stratigraphy and Basin fill evolution

The Upper Valdarno Basin stands out from the Neogene-Quaternary basins of the Northern Apennines given its outstanding fossil mammal record, good quality of natural and artificial outcrops and remarkable chronological control on the basin-fill succession. The present paper aims to summarize the stratigraphic and sedimentological studies focused on the Upper Valdarno Basin during the past decades, and integrate them with recent investigations. The Upper Valdarno Basin is located about 35 km SE of Florence between the Chianti Mountains and the Pratomagno Ridge. It consists of a main asymmetric tectonic depression filled with 550 m of Plio-Pleistocene fluvio-lacustrine deposits (Upper Valdarno Basin s.s.) and a minor basin known as the Palazzolo sub-basin. The Upper Valdarno Basinfill is made of three unconformity-bounded units, named Castelnuovo dei Sabbioni (CSB), Montevarchi (VRC), Torrente Ciuffenna (UFF) synthems, whereas the Palazzolo sub-basin fill consists of the Fosso Salceto (OLC) and Torrente Ciuffenna (UFF) synthems. The Upper Valdarno Basin formed during Late Pliocene because of the tectonic damming of a northeastward flowing drainage. The early phase of basin development is recorded by the accumulation of fluvial gravels in vallive settings, whereas the definitive of these streams damming caused the development of lacustrine conditions at about 3.1 Ma. The accumulation of deltaic sand fed from the SW margin caused the lake filling and stopped the deposition of the CSB Synthem.Before 2.58 Ma, a tectonic phase caused uplift of the basin and partial erosion of the CSB deposits. Deposition of the lower part of the VRC Synthem occurred during a marked basin broadening and accumulation of alluvial fan successions, which were capped by aeolian-reworked alluvial sand deposited at about 2.5 Ma. At about 2.3 Ma, a new deformative phase caused further basin widening, erosion along the SW margin and development of a small lake inthe central areas. Deposition of the upper part of the Montevarchi Synthem started just after this tectonic phase and was characterized by development of axial fluvial drainage and marginal alluvial fans.During the Early Pleistocene (Olduvai Subchron, 1.95-1.78 Ma) a subsidence pulse promoted development of floodplain lakes and swamps in the axial part of the basin, where thick organic-rich mud were accumulated. During late Early Pleistocene the capture of the paleo-Arno River, which started to flow into the basin, caused the development of a marked unconformity. This unconformity was covered by fluvial and alluvial fan deposit in the axial part and along the margin respectively.

Post-last glacial alluvial fan and talus slope associations (Northern Calcareous Alps, Austria): A proxy for Late Pleistocene to Holocene climate change

Near Innsbruck city (Austria, Eastern Alps), following the Last Glacial Maximum (LGM), an alluvial fan-to-talus slope succession was supplied from a carbonate-rock cliff more than 1000 m in height. 234U/ 230Th ages of 9.5 to 9.37 isotope kyrs of diagenetic cements in the alluvial-fan succession suggest that the fan/talus deposit accumulated mainly during late-glacial to, perhaps, early Holocene times. The deepest-exposed interval of the fan succession contains cracked lithoclasts probably fractured by overburden from late-glacial ice; this interval is topped by an intra-sequence unconformity. Following final glacial retreat, and rapid aggradation of the alluvial fan and talus slope, the geomorphic regime changed to erosion, as recorded by fanhead trenching and cutting of fluvial terraces, abandonment and vegetating of scree slopes, and excavation of 'talus flatirons'. The changeover from the accumulation of fan and talus to abandonment and dissection probably took place during the terminal late-glacial interval to perhaps the early Holocene. This erosional regime persists until present. A record of rapid late-glacial to early Holocene accumulation of an alluvial fan/talus deposit followed by: (i) abandonment and vegetation growth, combined with (ii) cutting of intra-sequence unconformities of limited lateral extent, is typical of Alpine mountain-flank deposystems situated at comparatively low altitudes. This record consists of (a) an autocyclic component, that is, progressive lowering of sediment input due to onlap and burial of freshly-deglaciated mountain flanks supplying alluvial fans and talus slopes, and (b) an allocyclic component, that is, deglacial climatic warming and upward rise of an altitudinal range with a maximum number of freeze-thaw cycles ('talus window'), also leading to progressive vegetation-induced hillslope stabilization and lowering of scree production.

Depositional facies, architecture and environments of the Sihwa Formation (Lower Cretaceous), mid-west Korea with special reference to dinosaur eggs

This paper presents detailed facies and architectural analyses and palaeoenvironmental reconstruction of the Sihwa Formation (Lower Cretaceous), mid-west Korea, which comprises an about 3-km-thick non-marine succession containing abundant dinosaur eggshells. Based on constituent facies, bedset geometry, stacking pattern, and bounding surface characteristics, the entire succession can be classified into three architectural elements. Element I occurs along the basin margin and shows a monotonous stacking of tabular or crudely stratified conglomeratic units. It represents alluvial-fan deposits of debris-flow-dominated fan and sheetflood-dominated fan. Element II is characterized by multi-storey, sheet or upward-widening, conglomeratic channel-fills whose maximum thickness exceeds 1.5 m. Each channel-fill unit is encased within red–brown silty sandstones of Element III with sharp erosional bases but diffuse gradational upper boundaries. It consists generally of (1) cut-and-fill deposit (trough cross-stratified or openwork conglomerate) in the lower part and (2) composite low-relief bar deposit (lenticular conglomerate and stratified gravelly sandstone) in the upper part. Shallowness of each cut-and-fill unit, absence of fining-upward lateral accretion package and the predominance of simple-bar deposit collectively suggest deposition from ephemeral braided streams. Element III typically shows a fining-upward stacking of (1) single- or multi-storey small-scale (<1.5 m thick) channel fills with limited lateral extent of less than 15 m, (2) poorly sorted, graded and diffusely bounded silty conglomerates or gravelly siltstones with discontinuous gravel sheets and pockets and (3) homogeneous or graded, red–brown fine-grained deposits with calcretes and burrows in ascending order. Element III is interpreted as channel-margin to floodplain, including crevasse channel fill, crevasse splay and floodplain fines. The entire sequence of the Sihwa Formation can be divided into the alluvial-fan and terminal-fan successions. The alluvial-fan succession displays a progradational stacking pattern and indicates a low rate of basin subsidence in the initial phase of rifting. The upper terminal-fan succession consists of proximal braided stream and distal floodplain deposits in the western and central parts of the basin and is characterized by an axial dispersal pattern and an aggradational stacking. It suggests rapid subsidence of the basin floor during the main phase of rifting. The asymmetrical cross-basin distribution of each architectural element reflects a half-graben structure of the basin with steep-gradient fault-bounded eastern margin (footwall block) and gently sloped, flexural western margin (hangingwall block). The predominance of ephemeral braided-stream deposits along with red–brown fine-grained floodplain deposits with common calcretes indicates arid to semi-arid palaeoclimates. Approximately 140 dinosaur eggs (Faveoloolithidae and Dendroolithidae) were identified mainly from the (gravelly) siltstones and small-scale channel fills of Element III deposits and partly from the cut-and-fill conglomerates of Element II deposits. The eggs commonly retain their original oval shape but are invariably breached and stuffed with the substrate of gravels and silt. They are either isolated or clustered, forming a circular concentration in plan view. The abundant yield of eggs, more than 20 eggs in 5 separate nests from a single depositional unit, suggests a dense population of the parental dinosaurs. The repetitive occurrence in many stratigraphic horizons reflects site preference as a nesting habitat of the near-channel or abandoned channel areas.

Proveniência de arenitos da Formação Cerro da Angélica (Grupo Bom Jardim, Ediacarano do RS) na porção sul da sub-bacia Camaquã central e suas implicações tectônicas

This paper presents provenance data from the lowermost sandstone deposits of the Cerro da Angelica Formation (~605 Ma) of the Bom Jardim Group (Camaqua Basin), which crops out in the Casa de Pedra region, Bage, Rio do Sul State, southern Brazil, integrated with sedimentary facies analysis, based on detailed description of columnar sections. This approach aims to determine the tectonic context and the sense of displacement of the basin-border faults during the deposition of the unit. Four facies associations were identified: (i) distal alluvial fan deposits, (ii) ephemeral stream deposits, (iii) proximal alluvial fan deposits and (iv) eolian dune field deposits. Microscopic provenance analysis revealed stratigraphic changes in the proportion of granitic and basic volcanic sources. In the case of the alluvial fan deposits, there is a progressive increase of the granitic contribution, which is interpreted as the result of a progressive denudation of the volcanic cover and ensuing exposure of the plutonic basement. In the distal alluvial deposits, there is an alternation between the two main sources, which is interpreted as a consequence of change in the contribution from proximal, border-fault derived detritus related to transversal alluvial systems and from distal sources fed by axial alluvial systems. The presence of basic or intermediate volcanic rocks as detritus in the basin reveals volcanic activity before the deposition of the Cerro da Angelica Formation. This interpretation corroborates the model of active rifting for the evolution of the Camaqua Basin, as the main extensional phase (Bom Jardim Group) begins with volcanic activity. The absence of provenance variations in the alluvial fan successions suggests that there was no sin-depositional strike-slip displacement between the source and the deposits, thus confirming previous models of normal faulting during the deposition of the Bom Jardim Group.

The diachroneity of alluvial-fan lithostratigraphy? A test case from southeastern Ebro basin magnetostratigraphy

Alluvial-fan strata contain valuable proxy data for kinematic, climatic and sediment-flux phenomena associated with high-relief source regions. Proper application of this data is dependent upon acquisition of accurate high-resolution chronostratigraphic data, best achieved through magnetostratigraphy. Collection of magnetostratigraphic and other proxy data from composite alluvial-fan sections assumes the chronostratigraphic significance of lithostratigraphy. We present the first test of this assumption with new magnetostratigraphy collected from Paleogene rocks of the southeastern Ebro basin. We report five new magnetostratigraphic sections that collectively sample 850 m of alluvial-fan and associated fluvial strata. One-hundred meters of lithostratigraphic overlap between these sections reveals equivalent magnetic polarities over short along-strike distances (∼200–2000 m). However, lithostratigraphic correlation of these sections with the magnetostratigraphy of continuously exposed strata eight km away reveals discrepancies in magnetic polarity trends and age interpretations. Correlation of our 760 m composite magnetostratigraphic section with the Paleogene geomagnetic polarity time scale suggests that this section spans from ca. 31.3 to 27.7 Ma (C12r–C9n; early to late Oligocene). This correlation implies steady sedimentation rates of ∼12 cm/ka for fluvial facies and ∼32 cm/ka for alluvial-fan facies. Alternative interpretations that force chronostratigraphic correlation with the lithostratigraphically equivalent interval generate unsteady and/or unlikely sedimentation rates in one or both of the stratigraphies and require rejection of multiple well-constrained polarity zones. The absence of significant surfaces of erosion or paleosol development argue against these alternative interpretations and suggest that even in the best exposed study areas, alluvial-fan successions may be diachronous and preclude lateral extrapolation of chronostratigraphic and other proxy data. Interpretations and models based upon data collected from alluvial-fan strata may be tenuous unless supported by sufficient spatial overlap to constrain the relationship between time surfaces and rock surfaces.

Neoproterozoic (Torridonian) alluvial fan succession, northwest Scotland, and its tectonic setting and provenance

The presence of alluvial fan deposits in the lower Neoproterozoic Torridon Group in north-west Scotland illuminates Torridonian basin development at the eastern Laurentian margin. The 450 m thick Cape Wrath Member of the Applecross Formation consists of alluvial fan conglomerate and arkose succeeded by more distal, braidplain feldspathic sandstone. Palaeocurrent data comprising &gt; 2650 measurements on trough cross-bedding are of low variability and show overall eastward flow. The projection upcurrent of regionally divergent flow directions for the lower part of the member indicates a fan of c. 50 km radius with its apex 30 km to the west near a basement (pre-Caledonian) normal fault with downthrow to the east beneath the north Minch Basin. Extensional tectonics controlled deposition of the Applecross Formation. Regional uplift, causing erosion of a youthful topography on the Lewisian Gneiss, was followed by the development of the Applecross extensional basin in two main stages. Uplift of a western source area by movement on basin-bounding normal faults occurred first in the north and caused pediplanation and alluvial fan deposition in the Cape Wrath area, with subsequent uplift of the source area for the main body of the Applecross Formation occurring further to the west and south along the line of the Minch Fault. The bulk of the Applecross Formation was derived from a weathered terrain of felsic crystalline and related supracrustal rocks reaching from the Outer Hebrides region westward for up to c. 250 km onto what are now the continental margins of the North Atlantic. The tectonic events may mark an early phase in the crustal extension that led ultimately to the opening of the Iapetus ocean.

Neoproterozoic (Torridonian) alluvial fan succession, northwest Scotland, and its tectonic setting and provenance

The presence of alluvial fan deposits in the lower Neoproterozoic Torridon Group in northwest Scotland illuminates Torridonian basin development at the eastern Laurentian margin. The 450 m thick Cape Wrath Member of the Applecross Formation consists of alluvial fan conglomerate and arkose succeeded by more distal, braidplain feldspathic sandstone. Palaeocurrent data comprising &gt;2650 measurements on trough cross-bedding are of low variability and show overall eastward flow. The projection upcurrent of regionally divergent flow directions for the lower part of the member indicates a fan of c. 50 km radius with its apex 30 km to the west near a basement (pre-Caledonian) normal fault with downthrow to the east beneath the north Minch Basin. Extensional tectonics controlled deposition of the Applecross Formation. Regional uplift, causing erosion of a youthful topography on the Lewisian Gneiss, was followed by the development of the Applecross extensional basin in two main stages. Uplift of a western source area by movement on basin-bounding normal faults occurred first in the north and caused pediplanation and alluvial fan deposition in the Cape Wrath area, with subsequent uplift of the source area for the main body of the Applecross Formation occurring further to the west and south along the line of the Minch Fault. The bulk of the Applecross Formation was derived from a weathered terrain of felsic crystalline and related supracrustal rocks reaching from the Outer Hebrides region westward for up to c. 250 km onto what are now the continental margins of the North Atlantic. The tectonic events may mark an early phase in the crustal extension that led ultimately to the opening of the Iapetus ocean.

Stable isotope record in mollusca and pedogenic carbonate from Late Pliocene soils of Central Italy

The Late Pliocene (younger than 2.5 Ma) alluvial fan succession of Toppetti Quarry, near Todi, Umbria, Central Italy, includes many palaeosols. Pedogenic carbonate concretions and mollusc remains were separated from 73 palaeosols collected along 150 m of exposure and analysed for their oxygen and carbon stable isotope content. δ 13C and δ 18O of the pedogenic carbonate and land snail shells suggest that no dramatic change in local climate took place. Values of the pedogenic carbonates are similar to other data reported for the Mediterranean region. δ 18O data suggest a mean annual temperature and/or δ 18O similar to modern climatic conditions. Carbon isotope data from both land snails and pedogenic carbonate are consistent with a prevailing type C 3 vegetation. A mixture of forest and C 3 grassland, sustained by a mean annual precipitation between 300 and 1000 mm, mainly in the cool periods of the year, seems to be the most likely scenario. The limited variation shown by the isotope composition of the biogenic and inorganically precipitated carbonate indicates that the environmental variations are referable mainly to change of humidity rather than temperature.

Cycles in Alluvial Fan Successions, Hornelen Basin (Devonian), Norway: ABSTRACT

Cycles in Alluvial Fan Successions, Hornelen Basin (Devonian), Norway: ABSTRACT

Stratigrafia e sedimentologia dei depositi fluviolacustri plio-pleistocenici dell'area nord occidentale del Valdarno superiore (Toscana)

In this paper are exposed the results of statigraphical and sedimentary analysis carried out in the north-western side of the Plio-Pleistocene fluviolacustrine upper Valdarno basin. The deposits of Montevarchi and Torrente Ciuffenna Successions were mapped. These deposits originated during the second and third phases, respectively, of a three-phases sedimentary history. They are separated by an erosional surface extended on the whole basin. According to the existing literature the chronostratigraphic gap extends from the upper part of the Early Pleistocene to the lower part of the Middle Pleistocene. The studied Successions can be classified using the unconformity-bounded stratigraphic units nomenclarure. The Sinthem term is proposed replacing the Succession term. The sedimentary evolution of the studied area is reconstructed starting from the Upper Pliocene. During the deposition of the Montevarchi Sinthem, a fluviolacustrine setting developed in this area. Facies analysis made it possible to recognize coarse-grained fan deltas prograding into a shallow lacustrine environment. The fan delta succession records significant tectonically induced variations of the clastic imput. The lower part of the fan delta succession is characterized by calcareous-arenaceous conglomerates of northwestern provenance. A subsequent rise of the Pratomagno chain lead to a predominance of arenaceous clastic material of eastern provenience. In the fan deltas deposits mammal fossils belonging to the Late Villafranchian and probably to the Early Galerian have been recovered. According ro these data the fluviolacustrine succession developed until the upper part of Lower Pleistocene. The unconformity separating the second and third phase may cover a smaller span of time in the studied area. In the third depositional phase an alluvial setting replaced the fluviolacustrine one previously developed. The alluvial fan succession building up the Torrente Ciuffenna Sinthem, filled the basin during the Middle Pleistocene. The erosion of the northern threshold closing the basin for the most part of the Pleistocene, induced in the Late Pleistocene the beginning of the erosive phase. On the whole the depositional evolution of the studied area was tectonically controlled. The climatic influence over the sedimentation was of lesser importance.