Alluvial Fill Research Articles

Medieval accumulation in the Upper Dniester river valley: The role of human impact and climate change in the Carpathian Foreland

Research in the valleys of Upper Dniester, Strvjaž and Stryj Rivers, in the Eastern Carpathian Foreland, documents the occurrence of 3–4 Holocene floodplain terraces and several alluvial fills dating back to the Late Vistulian and the Atlantic, as well as the periods 3500–3000 BP (1850–1250 cal. BC), 2200–1700 BP (360 cal. BC–390 cal. AD), 5th–6th, 10th–12th and 14th–16th centuries AD. Flood phases distinguished in the Upper Dniester drainage basin correlate well with previously established phases of enhanced fluvial activity in the valleys of Upper Vistula, the Wisłoka and the San in the Western Carpathian Foreland. An episode of tree fall during the 5th–6th century floods is the most conspicuous and is linked to the beginning of a wet climate phase (1500–1350 cal. BP). Subsequent phases of greater fluvial activity, from the 10th to the 12th and from the 14th to the 16th centuries, record increasing human activity, as well as correlation with wet and cool climate phases (the first half of the 11th century and the beginning of the Little Ice Age). Fluvial accumulation processes along the longitudinal profile of the Upper Dniester Valley were affected not only by human activity and more humid climate phases, but also by diverse neotectonic movements.

Exploration for a cave by magnetic and electrical resistivity surveys: Ayvacık Sinkhole example, Bozdağ, İzmir (western Turkey)

Geophysical survey techniques have been successfully applied to near-surface cave detection in karstic terrains. We used magnetic and electrical resistivity surveys to delineate the karstic structure of the Ayvacık Sinkhole, which may be considered to be a vertical cave. The magnetic-total-field-anomaly map helped reveal the metamorphic and sedimentary units in the study area. The total-horizontal-gradient map, which was based on a calculated pseudogravity anomaly, successfully identified the contact between the limestone unit and the cave system. Using these results, we positioned and carried out a vertical electrical sounding (VES) survey with a Schlumberger array along a line that consisted of 11 stations. The VES data were then processed using a 1D global optimization technique, which used a genetic algorithm and a 2D linearized least-squares algorithm. The results were generally in good agreement with each other, and together they pointed out three geologic layers: (1) an overburden layer ([Formula: see text]), (2) an approximately 25-m-thick alluvial fill (100–316 Ωm), and (3) a limestone unit (316–3162 Ωm); and also suggested the existence of a high-resistive anomaly ([Formula: see text]), possibly a karstic cave, located at the depth of approximately 40 m. Also, the results suggested that the buried limestone unit had an undulating karstic topography including a probable pinnacle structure. A synthetic modeling study was carried out, and it validated the reliability of the results. Finally, our findings indicated that the geophysical survey techniques used here were successful in detecting a cave located deep enough to make human exploration difficult.

A mid to late Holocene history of floodplain and terrace reworking along the middle Delaware River valley, USA

This study tests and refines the traditional floodplain development model for the partly confined middle Delaware River valley, which has shown that the main channel was relatively stable and flanked by a 6000–8000year old, vertically accreting alluvial terrace. The Holocene alluvial processes and history presented here in 6 fluvial phases were reconstructed using morphostratigraphy, 36 profile descriptions, 332 grain size analyses, and 82 14C ages from soil trenches, auger borings, and archaeological excavations. Fluvial phases I–III largely validate previous reconstructions showing a late Pleistocene (I: >10.7ka) braided stream transition into an early Holocene wandering stream with prolonged floodplain stability (II: 10.7–8ka), followed by early–middle Holocene erosion and then deposition (III: 8–5ka). The early and middle Holocene changes in alluvial stratigraphy and sedimentology broadly correlate with climatically derived Holocene subdivisions, suggesting climate change partly controls alluvial response along the middle Delaware River valley. This study documents for the first time a middle Holocene episode of channel incision occurring sometime between 6.0 and 5.0ka. Although the results reconfirm that the majority of alluvial landforms are composed of vertical accretion deposits, we present here new evidence of oblique, abandoned channel, and lateral accretion deposits inset to similar landforms with different formation histories (i.e., polycyclic terrace development), depicting a river valley that has experienced floodplain and terrace reworking. The majority of floodplain and terrace reworking occurs during the late–middle and late Holocene phases IV–VI (5.5–0ka), following the middle Holocene incision event. These phases demonstrate floodplain reworking processes in the form of channel abandonment, stripping, flood channeling, and convex bank erosion. The subsequent space filled rapidly with evidence of multistory soil formation, and eventually resulted in alluvial fill terraces with heights comparable to older surfaces. The most recent fluvial phase, VI (1.0–0ka), shows markedly faster sedimentation rates and coarser flood deposits associated with peak prehistoric population and the onset of Euroamerican settlement. The results presented here contribute newly documented evidence of erosion and depict a river valley that has undergone middle Holocene incision and floodplain and terrace reworking since the late–middle Holocene. Stream modelers, fluvial geomorphologists and geoarchaeologists working in the NE USA should consider floodplain reworking processes when examining alluvial history and predicting changes in stream dynamics.

The nature, timing and controls of the Quaternary development of the Rio Bergantes, Ebro basin, northeast Spain

Geomorphological, sedimentological and geochronological analyses have been undertaken to decipher the effects of Quaternary environmental change on the development of the Rio Bergantes, Ebro basin, northeast Spain. Field mapping and profiling using aerial photography and airborne LiDAR data have identified a sequence of six river terraces. Age control, provided by luminescence dating, has revealed two major phases of valley floor aggradation that appear to have occurred at ~183–130ka (Terrace BT1, elevated ~25m above the modern river channel) and ~111–26.8ka (BT2, ~18–20m above the modern river channel), punctuated by ~20m of incision in the intervening period. Following these two phases of large-scale aggradation, the Bergantes river system appears to have adjusted to a relatively sediment-limited regime and responded to environmental changes through a series of smaller cut and fill cycles. This resulted in four lower elevation terraces (BT3–BT6) that are inset within the older (BT1 and BT2) valley fills. These lower terraces have been temporally constrained to ~24.5–14.2ka (BT3); 10.6–9.8ka (BT4); 8.3–6.2ka (BT5) and ~3ka (BT6). Field sedimentological and stratigraphic analyses of the alluvial fills indicate the influence of tributary and hill slope derived sediment supply during the aggradation of BT1 and, to a lesser extent BT2. This contrasts with the predominantly trunk river derived deposits which make up the fill of BT3–BT6, when the hillslopes and tributaries in the study area appear to have been relatively de-coupled from the trunk river. Data presented in this paper suggest that the Bergantes river system has responded to, and has preserved, a record of environmental changes operating both in synchrony with, and at a higher frequency than, Milankovitch-driven glacial–interglacial cycles. Major net aggradation phases (those associated with relatively large amounts of fluvial sediment e.g. BT1 and BT2) appear to coincide with Pleistocene glacial conditions in MIS 6, and parts of MIS 5 and MIS 4, when the climate on the Iberian Peninsula was cool and dry. Intervening phases of net valley incision appear to coincide with warmer and moister climates in Pleistocene interglacials. The younger terraces (BT3–BT6) are characterised by much smaller amounts of fluvial sediment supply. Dating control places their development during stadial (aggradation) and interstadial (incision) events of the Late Pleistocene and Holocene. Some phases of aggradation appear to coincide with North Atlantic Heinrich Events, and also with other securely dated phases of aggradation elsewhere in the Mediterranean basin.

Dynamic properties of low velocity alluvial deposits influencing seismically-induced shear strains: the Grottaperfetta valley test-site (Rome, Italy)

The presence of peats and high compressibility inorganic clays within alluvial fills on the left-side tributaries of the Tiber River, close to Rome’s historic center in Italy, is well documented in literature. Nevertheless, few literature data exist until now on the dynamic properties of these deposits by undisturbed bore-hole samples. The Galba test-site was set up to characterise dynamic properties of the alluvial deposits by using lab-tests as well as to derive velocity profiles by seismic noise measurements. These were performed in the Giustiniano Imperatore area located in the Grottaperfetta valley, about 2 km south of Rome’s historic centre. The alluvial deposits filled a paleovalley excavated in the bedrock during the Wurm glacial (18–20 ky). The stratigraphic setting of the alluvial body was reconstructed along three geological cross-sections by means of the available logs; seven lithotecnical horizons can be distinguished within the alluvial body, some tens of meters thick, based on both log-stratigraphic data and in-site geotechnical tests. These horizons include peaty layers (T) and high compressibility inorganic clays (AGI), which characterise the alluvial deposits in the Grottaperfetta valley. They do not have direct correlation with the alluvial horizons which constitute the alluvial body of the main Tiber valley in Rome’s historical centre. These alluvial horizons which are distinguished and characterised at the Galba test-site can be regarded as typical of other lateral valleys of the Tiber River in Southern Rome. They are characterised by the presence of similar high compressibility clayey deposits as well as peaty layers up to some meters thick. Undisturbed samples were also obtained at the Galba test-site for dynamic testing via resonant column and cyclic torsional shear tests. In order to attribute dynamic properties to the alluvial body at the Grottaperfetta valley, an extrapolation process was performed based on a detailed engineering-geology model of the alluvial body which was reconstructed along three transversal geological sections of the valley using bore-hole data. Three subsoil profiles, considered representative of the geological setting along the three reconstructed transversal cross sections, were analysed by seismic noise measurements performed specifically to derive S-waves velocity profiles. The results obtained show a very low velocity (<180 m/s) for the layers T and AGI. 1D modelling of seismic shaking was performed by the code Shake91, in order to evaluate the influence of the low-velocity strata on maximum shear strains induced within the alluvial deposits under the maximum expected seismic action. The results of the numerical modelling indicate that the AGI and the T layers play a key role in: (i) concentrating the maximum shear strain along the subsoil profiles, even though the volumetric threshold is never exceeded; (ii) increasing the maximum shear strain along the subsoil profiles; (iii) causing the resonance frequency of the alluvial fill to assume an almost constant value (about 1Hz) which is quite similar to that measured in the main Tiber River valley, despite a significant change in thickness of the alluvial body along the Grottaperfetta valley.

Alluvial archives of the Nochixtlan valley, Oaxaca, Mexico: Age and significance for reconstructions of environmental change

A reconnaissance of the Nochixtlan valley confirms that thick alluvial deposits of the late Quaternary are both well preserved and well exposed along the heavily incised modern streams. Their geometry and age were ascertained by logging representative cutbank exposures and radiocarbon dating buried palaeosols. The last four cut-and-fill cycles recorded cover the period since 14,000BP, but older alluvial insets are also present. Contrary to previous reconstructions, the streams have had an arroyo-type morphology throughout the late Quaternary. The climatic oscillations at the transition from the Pleistocene to the Holocene seem to be expressed in the alternating formation of clayey organic-rich cumulic A horizons, and the precipitation of secondary carbonates. Rapid though intermittent aggradation set in after 10,300BP in response to the establishment of a rather dry, warm and strongly seasonal climate and of open canopy forest or scrub on slopes. The unstable arroyo floodplains offered a favorable niche for the establishment of secondary vegetation and foragers exploiting annual and heliophilous plants. After 4000BP sedentary farmers cleared the valley of its natural vegetation increasing sediment transfers and the frequency of local cut-and-fill cycles. They modified streams intentionally by building long flights of cross-channel agricultural terraces known as lama-bordos, the remains of which are preserved in many cutbanks. The oldest specimen dated goes back to ca. 3000BP. A remarkably synchronous incision swept through the fluvial system close to 800BP, possibly as a result of a marked climatic shift or valley-wide changes in land use. The widespread gullying of the latest pre-Conquest settlements and fields suggests the introduction of grazing or the collapse of hillside terraces as cause, but coeval alluvial fills have not yet been positively identified.

Historic range of variability in geomorphic processes as a context for restoration: Rocky Mountain National Park, Colorado, USA

ABSTRACTEvaluation of historic range of variability (HRV) is an effective tool for determining baseline conditions and providing context to researchers and land managers seeking to understand and enhance ecological function. Incorporating HRV into restoration planning acknowledges the dynamic quality of landscapes by allowing variability and disturbance at reasonable levels and permitting riverine landscapes to adapt to the physical processes of their watersheds. HRV analysis therefore represents a practical (though under‐utilized) method for quantifying process‐based restoration goals. We investigated HRV of aggradational processes in the subalpine Lulu City wetland in Rocky Mountain National Park to understand the impacts of two centuries of altered land use and to guide restoration planning following a human‐caused debris flow in 2003 that deposited up to 1 m of sand and gravel in the wetland. Historic aerial photograph interpretation, ground penetrating radar surveys, and trenching, coring, and radiocarbon dating of valley‐bottom sediments were used to map sediment deposits, quantify aggradation rates, and identify processes (in‐channel and overbank fluvial deposition, direct hillslope input, beaver pond filling, peat accumulation) creating alluvial fill within the wetland. Results indicate (i) the Lulu City wetland has been aggrading for several millennia, (ii) the aggradation rate of the past one to two centuries is approximately six times higher than long‐term pre‐settlement averages, (iii) during geomorphically active periods, short‐term aggradation rates during the pre‐settlement period were probably much higher than the long‐term average rate, and (iv) the processes of aggradation during the last two centuries are the same as historic processes of aggradation. Understanding the HRV of aggradation rates and processes can constrain management and restoration scenarios by quantifying the range of disturbance from which a landscape can recover without active restoration. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Reconstructing migration phases of meandering channels by means of ground-penetrating radar (GPR): the case of the Obra River, Poland

The main goal of the study was to compare the potential of conventional and geophysical methods (ground-penetrating radar; GPR) to reconstruct the migration phases of a meandering lowland riverbed: the Obra River in western Poland. The purpose was to verify that the migration phases can be distinguished using the GPR measurement providing near-continuous imaging of alluvial structures and to discuss differences between the spatial extent of the migration phases distinguished using geologic data and GPR surveys. Historical maps from the eighteenth and the nineteenth century were analyzed to study transformations of the Obra River bed pattern. One hundred twenty tube sample borings were undertaken along five profiles to recognize the floodplain lithology. Granulation coefficients were determined using the method of moments to distinguish lithofacies of the alluvial deposits. The GPR measurement was performed using a georadar MALA ProEx equipped with shielded 100 and 250 MHz antennae. The GPR images were compared to geologic profiles to determine the depth scale and accuracy of the measurements. The depth scale of the GPR images was determined on the basis of the correlation of the radar reflections with the geologic profiles. Ten organic sediment samples were collected to determine the age of the migration phases. Point bars with layering of sand deposits, organic sediment fills, erosion surfaces, and gravel beds were distinguished using the 250 MHz antenna. However, some of sedimentary structures (e.g., inserts of fine sands in the organic deposits) were beyond the detection range of the antennae. The glacial/alluvial sediment interface was detected using the 100 MHz antenna which enabled the determination of the thickness of the alluvial fill. Traces of a period of early development of the Obra valley and three migration phases of the Obra River bed were distinguished. The research indicated that the Obra was a meandering river at least 7,000 years before present and that during the last 3,000 years, channel islands were formed within the studied section of the valley. The GPR method allowed for the analysis of the spatial variability of alluvial deposits and helped to describe how meander bends were formed. The presented GPR images illustrate the spatial extent of the migration phases in a more detailed way than using only sedimentary information. However, geologic data are necessary for interpretation of the GPR images. It was shown that the GPR method gives useful results in spite of rough land surface and dense vegetation.

Climatically controlled Late Quaternary terrace staircase development in the fold- and -thrust belt of the Sub Himalaya

The alluviation and incision history of Late Quaternary terrace staircases of Yamuna River and its tributaries in the Sub Himalaya, between the Main Boundary Thrust (MBT) and the Himalayan Frontal Thrust (HFT), were studied to understand interlinkages between tectonics and climate in their genesis. We documented five levels of terraces (T1 to T5), cutting across numerous tectonic plains, ranging in elevation from 65–80, 40–44, 28–30, 8–12 and 3–4m from the present day river bed, deposited by both glacier-fed perennial and piedmont-fed ephemeral streams. Temporal and spatial distribution of these terraces, coupled with absolute ages based on optically stimulated luminescence (OSL) chronology documents five distinct fluvial accretions separated by incision events spanning between Marine Isotope Stage (MIS) 3 and MIS 1. Two major accretion phases occur in the Late Pleistocene (>37 to 24ka and >15 to 11ka) and three minor phases in the mid- to late Holocene (7–4ka; 3–2ka; and <2ka). The Phase-I aggradation is thicker and occurs as alluvial fill in a pre-existing valley, whereas younger accretion sequences are cut-in-fill and rest over remnant deposits. The periods of terrace formation are coeval with the Indian Summer Monsoon (ISM) oscillations, however abrupt termination of T1 to T3 terraces in the Himalayan mountain front reflects tectonic upheaval along HFT after their development. This study outlines the climatic control on the terrace genesis in the tectonically active Himalaya on the scale of 103–104 years.

Determining the depositional pattern by resistivity–seismic inversion for the aquifer system of Maira area, Pakistan

Velocity and density measured in a well are crucial for synthetic seismic generation which is, in turn, a key to interpreting real seismic amplitude in terms of lithology, porosity and fluid content. Investigations made in the water wells usually consist of spontaneous potential, resistivity long and short normal, point resistivity and gamma ray logs. The sonic logs are not available because these are usually run in the wells drilled for hydrocarbons. To generate the synthetic seismograms, sonic and density logs are required, which are useful to precisely mark the lithology contacts and formation tops. An attempt has been made to interpret the subsurface soil of the aquifer system by means of resistivity to seismic inversion. For this purpose, resistivity logs and surface resistivity sounding were used and the resistivity logs were converted to sonic logs whereas surface resistivity sounding data transformed into seismic curves. The converted sonic logs and the surface seismic curves were then used to generate synthetic seismograms. With the utilization of these synthetic seismograms, pseudo-seismic sections have been developed. Subsurface lithologies encountered in wells exhibit different velocities and densities. The reflection patterns were marked by using amplitude standout, character and coherence. These pseudo-seismic sections were later tied to well synthetics and lithologs. In this way, a lithology section was created for the alluvial fill. The cross-section suggested that the eastern portion of the studied area mainly consisted of sandy fill and the western portion constituted clayey part. This can be attributed to the depositional environment by the Indus and the Kabul Rivers.

Stratigraphy of alluvial fills and phases of the Holocene floods in the lower Wisłok River valley, SE Poland

Stratigraphy of alluvia and chronology of the Holocene flood events were studied on the basis of 50 radiocarbon dates as well as sedimentological and palynological analyses made on 20 depositional sequences of sediments in the lower section of the Wislok River valley. These sites evidence the occurrence within the 4–5 m and 6–7 m high levels of the floodplain of separate alluvial fills and palaeochannel systems, recording phases of increased Wislok activity and frequent floods dated at: the Allerod-Younger Dryas Phase, 9,900–9,600 BP, 8,500–8,000 BP, 6,600– 6,100 BP, 5,400–5,000 BP, 4,400–4,000 BP, 2,100–1,700 BP, as well as the 11–13th centuries AD and the middle of the 18th century AD. These periods are in accordance with the phases documented earlier within the Upper Vistula River drainage basin, particularly in the for-mountainous section of the Wisloka River valley, Vistula River valley downstream of Krakow , as well as in the Lower San River valley.

Electrical resistivity in support of geological mapping along the Panama Canal

Dredging and widening of the Panama Canal is currently being conducted to allow larger vessels to transit to and from the Americas, Asia, and Europe. Dredging efficiency relies heavily on knowledge of the types and volumes of sediments and rocks beneath the waterway to ensure the right equipment is used for their removal. To aid this process, a waterborne streaming electrical resistivity survey was conducted along the entire length of the canal to provide information on its geology. Within the confines of the canal, a total of 663 line-kilometers of electrical resistivity data were acquired using the dipole–dipole array. The support of the survey data for dredging activities was realized by calibrating and qualitatively correlating the resistivity data with information obtained from nearby logged boreholes and geological maps. The continuity of specific strata was determined in the resistivity sections by evaluating the continuity of similar ranges of resistivity values between boreholes. It was evident that differing geological units and successions can have similar ranges of resistivity values. For example, Quaternary sandy and gravelly alluvial fill from the former river channel of the Chagres River had similar resistivity ranges (generally from 40 to 250 Ω m) to those characteristic of late Miocene basalt dikes (from 100 to 400 Ω m), but for quite different reasons. Similarly, competent marine-based sedimentary rocks of the Caimito Formation were similar in resistivity values (ranging from 0.7 to 10 Ω m) to sandstone conglomerate of the Bohio Formation. Consequently, it would be difficult to use the resistivity data alone to extrapolate more complex geotechnical parameters, such as the hardness or strength of the substrate. A necessary component for such analyses requires detailed objective information regarding the specific context from which the geotechnical parameters were derived. If these data from cored boreholes and detailed geological surveys are taken into account, however, then waterborne streaming resistivity surveying can be a powerful tool. In this case, it provided inexpensive and highly resolved quantitative information on the potential volume of loose suctionable material along the Gamboa Sub-reach, which could enable large cost savings to be made on a major engineering project involving modification of one of the most important navigable waterways in the world.

The geoarchaeology of route systems in northern Syria

AbstractLinear valleys, termed hollow ways, form a distinctive feature of the North Syrian Bronze Age landscape, but few have been described in detail or excavated. This paper examines the geoarchaeology of hollow way route systems in northern Syria at three scales of analysis: (1) from satellite imagery, at which scale it is possible to examine patterns of hollow ways over very large areas; (2) in the field, where cleaned sections supply ground control and dating evidence for specific features or hollow ways; and (3) using soil micromorphology to investigate the formation processes evident in the hollow way fills. We also use ethnographic analogy to extend the interpretation of the pattern of hollow ways. Finally, we examine the role of these features in the drainage network as well as their contribution to the development of alluvial fills of the Khabur tributaries. The three sections examined demonstrated that the Brak hollow ways were incised into the landscape, probably in the third millennium B.C. or slightly earlier. The fills show evidence of episodic flow and low‐energy soil wash with weak soil development over the last 4000 years. © 2010 Wiley Periodicals, Inc.

A Preliminary Investigation into the Relationship between Long-Period Seismic Noise and Local Fluctuations in the Atmospheric Pressure Field

Summary The displacement response of an elastic half space to a plane pressure wave is examined in order to establish the conditions under which sources of this type can contribute significantly to the long-period seismic noise field. The study is restricted to pressure waves which propagate at velocities well below the seismic wave velocities characteristic of the half space. The numerical studies indicate that pressure waves with amplitudes of 100 pbar or more can contribute significantly to the long-period vertical background noise observed at the surface, provided that the detectors are located on sections of alluvial fill or poorly to moderately indurated sandstones and shales whose thicknesses are greater than about a kilometre. These same waves can also create significant tilt noise on long-period horizontal seismographs located at or near the surface, regardless of the rock type. The seismic disturbances created by pressure waves decay rapidly away from the surface. Therefore, it appears that it may be possible to eliminate the effects of atmospherically generated noise by placing the detectors at moderate depths.

Widespread aggradation in the mountainous catchment of the Alaknanda–Ganga River System: timescales and implications to Hinterland–foreland relationships

Cut-and-fill type fluvial terraces are ubiquitous in the Lesser Himalayan zone of the Alaknanda–Ganga (Ganges) rivers system, which flows perpendicular to Himalayan lithotectonic units and traverses a steep climatic gradient. Lithofacies analysis of the sedimentary sequences of cut-and-fill terraces indicated that the valley aggradation took place via (1) channel bar development and excess sediment supply, (2) debris flows composed of mixed rounded to sub-rounded lithoclasts, resulting from episodic high intensity rainfalls in the upper catchment or (3) debris flows or rockfalls generated by local landslides. The luminescence chronology indicates that valley aggradation took place in two phases: ∼49 –25 ka and 18 –11 ka. The incision of the fill started soon after 11 ka. Paleoclimatic records from marine sediments indicate that the aggradation and incision in the Alaknanda–Ganga River has oscillated in-phase with global climatic variations. Glaciation –deglaciation processes in the upper catchment produced huge amounts of sediment between 63 and 11 ka, which was fluvially transferred to the lower valley via several cycles of erosion and deposition, leading to extensive aggradation. The climatic amelioration at ∼11 ka and the completion of deglaciation processes led to increased fluvial discharge and decreased sediment supply, conditions conducive for incision of the alluvial fills. Records from the Indo-Gangetic plain and the Ganga Delta demonstrate that the phase of aggradation was regional but that incision in the foreland started at least 2 –3 ka later, after 7 ka. Bedrock incision rates, as calculated from dated alluvial covers of terraces that are separated by bedrock steps, are spatially variable and fall within the range of rates reported from across the Himalaya. These estimated rates, however, are higher than the basin average erosion rates calculated using isotopic mass balance in riverbed sediments. This study suggests that during the last 50 ka river dynamics in the Himalayas were dominated by monsoon variability and the role of tectonic activity was limited to bedrock incision in few reaches only.

Early maize pollen from Chaco Canyon, New Mexico, USA

Pollen concentrations containing abundant Zea mays pollen grains are AMS radiocarbon dated 3940 ± 40 to 2450 ± 40 14C years BP. The maize pollen is from two prehistoric woodrat (Neotoma) middens that occur in fractures in the south-facing sandstone cliff at Chaco Culture National Historical Park. The diameters of the Archaic-age maize pollen grains are significantly larger than Puebloan and modern maize pollen. The size distributions of the earliest Zea grain populations are not normal, suggesting the possibility that more than one variety of maize is represented by the pollen. The occurrence of large numbers of maize pollen grains as well as pollen from weedy plants indicates the nearby presence of an Archaic cornfield, now buried in alluvial fill adjacent to the cliff. It was also found that the AMS radiocarbon ages of the pollen concentrations differ significantly from the age of twigs from the same woodrat middens. Because of the strong age differences of components of woodrat middens, pollen assemblages should be dated independent of plant macrofossils.

Mount Augustus Geology and Geomorphology

AbstractMount Augustus (Burringurrah) at 1105 m AHD, is a prominent elongate inselberg, underlain dominantly by resistant sandstone of Middle Proterozoic age (1.6 billion years old). It is located midway between Meekatharra and Carnarvon in semi‐arid Western Australia. The Mount Augustus Sandstone and overlying sediments were folded and tilted about 1 billion years ago, providing the template for the modern Mount Augustus. It is essentially a structural landform. Its shape broadly conforms to the underlying asymmetrical anticline, which has a steep northern limb and a gentler dipping southern limb. The anticline plunges at a shallow angle to the southeast. The mountain is the core of the original fold and it stands some 700 m above the surrounding Gascoyne planation surface. Other rock types present on Mount Augustus include metamorphic and igneous rocks, which preclude it from being termed a monolith. Weathering, fluvial erosion and sedimentation have modified the original bedrock structures. Fluted bedrock provides evidence of rapid runoff from bare bedrock surfaces. A joint system has influenced the orientation of a step‐like sequence of numerous scarplets on slopes. The weathered rock is red, but the coloured rock is only a centimetre deep on most surfaces. The surrounding alluvial, gibber and sand plains shallowly bury the eroded bedrock plains of the Gascoyne planation surface. Former alluvial fills have been ferricreted in the lower reaches of some channels and are currently being eroded. Mount Augustus has often been compared with Ayers Rock, and unwarranted claims have been made that it is the biggest rock in the world and the world's biggest monolith.

Holocene Inset Fills of the Odra River in the Racibórz Basin

Holocene Inset Fills of the Odra River in the Racibórz Basin Geomorphic and lithological research conducted in the Odra River valley between Koźle and Krapkowice led to recognition of the postglacial development of the upper Odra River valley floor, for the first time, on the basis of a larger number of absolute datings. The formation of the inset alluvial fills by a meandering river started during the Early Holocene at the latest; however, the absence of a clearly developed system of large Late Vistulian palaeomeanders is notable within the analysed reach of the valley. Mid- and Late Holocene alluvial series were deposited by a river that exhibited a tendency to decrease in channel sizes. Nevertheless, large meanders were formed again in historical times. Oxbow fills in the Odra River valley are dominated by mineralogenic deposits. Their sedimentation can be correlated with the accretion of overbank and colluvial sheets in the valley floor. In the light of radiocarbon chronology, a relationship between Neolithic and subsequent phases of settlement in the catchment and the intensive deposition of these sediments cannot be ruled out.

Glacier and landslide feedbacks to topographic relief in the Himalayan syntaxes

Despite longstanding research on the age and formation of the Tibetan Plateau, the controls on the erosional decay of its margins remain controversial. Pronounced aridity and highly localized rock uplift have traditionally been viewed as limits to the dissection of the plateau by bedrock rivers. Recently, however, glacier dynamics and landsliding have been argued to retard headward fluvial erosion into the plateau interior by forming dams and protective alluvial fill. Here, we report a conspicuous clustering of hundreds of natural dams along the Indus and the Tsangpo Rivers where these cross the Himalayan syntaxes. The Indus is riddled by hundreds of dams composed of debris from catastrophic rock avalanches, forming the largest concentration of giant landslide dams known worldwide, whereas the Tsangpo seems devoid of comparable landslide dams. In contrast, glacial dams such as river-blocking moraines in the headwaters of both rivers are limited to where isolated mountain ranges intersect the regional snowline. We find that to first-order, high local topographic relief along both rivers corresponds to conspicuously different knickzones and differences in the type and potential longevity of these dams. In both syntaxes, glacier and landslide dams act as a negative feedback in response to fluvial dissection of the plateau margins. Natural damming protects bedrock from river incision and delays headward knickpoint migration, thereby helping stabilize the southwestern and southeastern margins of the Tibetan Plateau in concert with the effects of upstream aridity and localized rock uplift.

Late Quaternary loess landscape evolution on an active tectonic margin, Charwell Basin, South Island, New Zealand

Loess deposits constitute an important archive of aeolian deposition reflecting wider patterns of glacial atmospheric circulation, and more localised interactions between riverine source areas, loess trapping efficiency and geomorphic controls on erosion rate. Conceptual models have been formulated to explain the coeval evolution of loess mantles and associated landscapes (loess landscape models) but none apply to areas of tectonically induced base-level lowering. This study uses an age sequence of alluvial fill terraces in the Charwell Basin, north-eastern South Island New Zealand, which straddles the transpressive Hope Fault, to investigate geomorphic controls on loess landscape evolution in an active tectonic region. We hypothesize that the more evolved drainage networks on older terraces will more effectively propagate base-level lowering by way of a greater areal proportion of steep and convex hillslopes and a smaller proportion of non-eroding interfluves. Eventually, as the proportion of interfluves diminishes and hillslope convexity increases, terraces shift from being net loess accumulators to areas of net loess erosion. We investigate the nature of erosion and the geomorphic thresholds associated with this transition. Morphometric analysis of alluvial terraces and terrace remnants of increasing age demonstrated geomorphic evolution through time, with a decrease in extent of original planar terrace tread morphology and an increase in frequency of steeper slopes and convexo-concave land elements. The number of loess sheets and the thickness of loess increased across the three youngest terraces. The next oldest (ca. 150 ka) terrace remnant had the greatest maximum number of loess sheets (3) and loess thickness (8 m) but the loess mantle was highly variable. A detailed loess stratigraphic analysis and the morphometric analysis place this terrace in a transition between dominantly planar, uniformly loess-mantled landforms and loess-free ridge and valley terrain exemplified by the oldest terrace remnant. Variations in thickness and preservation of loess sheets demonstrated spatially and temporally variable erosion during loess accumulation. To test our hypothesis of loess persistence we calculated critical steady-state hillslope curvatures from a soil transport model, calibrated for the study area, above which the uppermost loess sheet (L1, max. thickness 1.8 m) should be completely eroded. We compared loess distribution mapped in the field to values of slope curvature calculated from topographic surveys and found two of three critical curvature values had acceptable predictive ability. Where predictions failed this is probably due to transient responses related to active steam incision. Because all critical curvature values predicted presence of loess on the oldest terrace remnant where there is none we conclude that important factors other than morphometric ones are important in determining loess distribution in loess landscapes in active tectonic regions. These may include internal changes to regolith affecting erodibility, or vegetation or topographic interaction with wind patterns affecting loess trapping.