Floodplain Muds Research Articles

Mud redeposition during river incision as a factor affecting authigenic 10Be/9Be dating: Early Pleistocene large mammal fossil‐bearing site Nová Vieska, eastern Danube Basin

ABSTRACTThis study examines the suitability of the authigenic 10Be/9Be dating method to the dating of the deposits of an incising river, taking as an example the Nová Vieska river terrace, which accumulated during the neotectonic inversion of the Danube Basin (western Slovakia). The succession was formed by a wandering river with minor preservation of proximal floodplain muds. The frequent occurrence of mud intraclasts reflects significant input of eroded material from underlying, older successions. The ages of 13 authigenic 10Be/9Be dating samples formed three groups: (1) samples from below the base of the river terrace yielded dates of ~4.13–3.70 Ma (including uncertainties); (2) muddy intraclasts from the river terrace gave an age range of ~2.79–1.96 Ma; and (3) in situ muddy layers had ages in the range of ~1.91–1.39 Ma. The large mammal fossil assemblage from channel thalweg deposits yielded a biostratigraphic age of ~3.6–2.2 Ma, matching the age of intraclasts, and thus emphasising the redeposited origin of those fossils. The relatively wide range of authigenic 10Be/9Be dating ages is interpreted as a result of the redeposition of mud from older strata on three scales: decimetre‐scale intraclasts, millimetre‐scale rip‐up clasts mixed into the newly formed beds, and formation of two authigenic rims with different age and 10Be/9Be records around individual particles. Considering these observations, an age range of in situ layers of ~1.91–1.39 Ma is proposed as the depositional age of the river terrace, with the most probable age falling within the most recent part of this interval. The effect of redeposition is thus shown to be potentially limiting to the application of authigenic 10Be/9Be dating to incising rivers, and stands in marked contrast to aggrading river settings, where redeposition of older sediments is limited and the degree of 10Be/9Be variability is low.

The Architectural Surfaces Characteristics of Sandy Braided River Reservoirs, Case Study in Gudong Oil Field, China

The architecture analysis of the different orders sedimentary bodies is of great significance to the efficient development of oil and gas fields. In order to investigate the effects of the architectural interfaces on reservoir quality and heterogeneity, this study takes the Gudong oil field as a case to investigate the architectural characteristics of hierarchical bounding surfaces using detailed descriptions of core and wireline logs. Architectural models from the 7th-order to the 3rd-order are analyzed, and the developmental characteristics of the 5th-order braided river, 4th-order single sandstone, and 3rd-order accretion are summarized. The interlayer between two braided rivers is floodplain mud deposition, with poor physical properties, stable thickness, and strong blocking capacity. Two models of interlayers are found in the 4th-order deposition. The first interlayer is between the braided filling channel and midchannel bar, which is composed of generally fine-grained sediments with calcium cementation and poor physical properties. The second interlayer is a transformation belt between two midchannel bars and is generally composed of gravel-scoured deposition with penetration capability. The 3rd-order surfaces are defined as the surfaces of accretions within midchannel bars. Two models of interlayers are also found in the 3rd-order surfaces of accretions. The paleocurrent of the sandy braided river is reconstructed by synthesizing the core data, well logging data, and production performance data. A total of 1 fluvial system (7th-order), 2 compound braided rivers (6th-order), 11 braided rivers (5th-order), 41 midchannel bars (4th-order), and 96 accretions (3rd-order) are developed in the study area.

The geomorphology and Late Quaternary sedimentary record of the Salado/Juramento fluvial megafan, Central Andes foreland basin (Chaco Plain, Argentina)

This research provides geomorphological and sedimentary data on the Salado-Juramento fluvial megafan, the second largest megafan (204,747 km2) of the Chaco aggradational plain, generated by the Salado-Juramento River in NE Argentina. It comprises one of the most extensive depositional systems of the Central Andean foreland basin system in South America, also being among the largest megafans on Earth. Despite this, it has not been previously studied from the perspective of its geomorphology, stratigraphy and dynamic evolution. An integrated approach focusing on the analysis of morpho-sedimentary processes, the resulting landforms and the reconstruction of the Late Quaternary evolution is presented here. The applied methods combine geomorphological mapping from remote sensing images analysis, assessment of landforms and processes, and field data. Historical documents were also analysed. Insights into the near sub-surface stratigraphy were obtained from analysis of research boreholes (up to 30 m depth), integrated with groundwater borehole information. The Fluvial Megafan of the Salado-Juramento Geomophological System comprises four geomorphological units: 1) the middle-proximal area of the megafan, 2) the distal area of the megafan, 3) the complex fluvio-aeolian belt of the Golondrinas-Calchaquí, and 4) the present fluvial belt of the Salado. Thirteen geomorphological sub-units were defined and mapped, representing the complexity of sub-environment interactions. Amalgamated braided channel deposits and aggradation lobes with limited floodplain muds are common in the proximal area, while mixed-load, low-energy, narrow, shallow, and sinuous palaeochannels and associated floodplain marsh deposits characterize the distal area, with groundwater discharges and hydromorphic soils. The megafan construction was dominated by short-term sedimentation and channel avulsions to overbank areas. Fan-shaped depositional lobes of the proximal area and sandy channel deposits and sand sheets linked to terminal splays that are covering mud-flat deposits (floodplains, including palaeosols) in the middle area suggest a high-flow regime. Neotectonics favoured subsidence, avulsion, and prolonged flooding events, with high potential of preservation of an extensive Late Quaternary wetland unit, ponds, lakes, soils, and loessic deposits in the distal megafan. Aggradation and progradation patterns have been conditioned by tectonic control regulating the interaction between the megafan and its axial trunk river (Paraná). Furthermore, the Late Quaternary megafan sequences would also have responded to climatic changes, primarily related to strong hydroclimatic seasonality linked to changes in the South American monsoon system strength. Seasonal high sediment load of the formative river linked to the characteristics of the Andean basin (38,000 km2) and its climatic patterns are considered as the main control on megafan dynamics over at least the Upper Quaternary. Morpho-stratigraphical data of the megafan presented here and the first interpretation of its Quaternary evolution is the key to support socio-economic decisions on aquifer exploitation, land uses, and infrastructure planning.

The tectono-sedimentary evolution of a major seismogenic zone with low slip rate activity: A geochronological and sedimentological investigation of the Dobrá Voda Depression (Western Carpathians)

Seismically active fault zones receive a great deal of attention due to their potential for quantification of seismic hazards. Zones with low slip rates pose a challenge, however, since their poor topographic expression is related to difficulties in the quantification of fault movement. This study focuses on the Dobrá Voda Depression, an area with the highest level of seismic activity in the Western Carpathians. The Quaternary tectono-sedimentary evolution of the small intramontane basin was investigated with the use of facies analysis of cores, dated with the use of cosmogenic nuclide depth profiles (10Be, 26Al and 36Cl), together with 26Al/10Be burial dating and radiocarbon dating. A set of archived boreholes and geoelectric survey data was used for the correlation of results with those from new boreholes across the depression. Four facies associations were distinguished: (FA1) Colluvial deposits that comprise subaerial debris flows and mudflows; (FA2) Fluvial deposits with high sediment supply: accommodation ratio, composed mostly of sandy-gravelly channel fill facies; (FA3) Fluvial deposits with low sediment supply: accommodation ratio, consisting mostly of floodplain muds, overbank heterolithic facies and minor sandy-gravelly channel fills; and (FA4) Swamp deposits, which are mostly made up of peat. Geochronological results suggest that the studied part of FA3 was deposited before 1.0 Ma due to a rise in the base level following a major incision event. Overbank-dominated deposits of FA3 covered an incision surface, resulted in a difference of ca. 65 m of elevation of these strata, which represents the minimal thickness of FA3. The second phase of incision was related to reactivation of Miocene normal faults resulting in further topographic differentiation. The initiation of fault activity is recorded by the deposition of colluvial FA1 before ca. 250 ka. FA2 accumulated between ca. 160 and 100 ka, mostly at the toes of slopes bounding the fault scarps on the basin margins. The last documented phase of evolution represents an increase of accommodation, which was connected to the deposition of Holocene peat in swamps as well as floodplain muds of FA4 above FA2. The observed settings imply that variation between incision and accumulation in a scale of hundreds of thousands of years is characteristic for low relief tectonically active zones. The presented research demonstrates the significance of sedimentological analysis for reconstruction of tectonic evolution in areas with low slip rate activity.

Decoupled geomorphic and sedimentary response of Po River and its Alpine tributaries during the last glacial/post-glacial episode

The complex geomorphic and sedimentary evolution of the central Po Plain (northern Italy) during the last 30 ky was reconstructed through the integration of stratigraphic, geomorphological, geochemical and radiocarbon data. A key element of the Late Pleistocene stratigraphy is a 20 km-wide channel-belt sand body, with its top at a depth of ~13 m, fed by the Po River. Whereas the southern boundary of the Po fluvial channel belt coincides with a sharp lithological contact with floodplain muds, its northern boundary is an erosional (sand-on-sand) surface that was traced tentatively in the subsurface with the aid of sediment provenance (Po versus Alpine) proxies and radiocarbon data. Stratigraphic and geomorphological features testify to a decoupled sedimentary and geomorphic response of the Alpine and Po River systems to climate change in the last 30 ky. Contemporaneous Po River incision and Alpine rivers aggradation occurred at the onset of the Last Glacial Maximum (LGM). In contrast, Po River aggradation and Alpine rivers entrenchment took place during early deglaciation. The Holocene stratigraphy records the overall aggradation and northward migration of the Po River, with the consequent erosion of distal Alpine LGM deposits and formation of a fluvial scarp parallel to the Po River course. Late Pleistocene and Holocene climate change influenced river dynamics controlling (i) the balance between sediment supply and water discharge, through glacier and vegetation dynamics, and (ii) the rate of sea-level fall/rise. Apennine, Alpine and Po river systems responded in distinct ways to climate forcing due to the influence of local factors. Lithology, drainage area and mean elevation of river catchments, as well as river and valley gradients, determined the type of sedimentary response (e.g., aggradation versus degradation). This study shows that a combined geomorphic and stratigraphic approach focusing on the geometric relations between exposed and buried features can provide valuable information about the evolution of a fluvial system and its controlling factors.

Assessing the recharge of a coastal aquifer using physical observations, tritium, groundwater chemistry and modelling

Assessing recharge is critical to understanding groundwater and preventing pollution. Here, we investigate recharge in an Australian coastal aquifer using a combination of physical, modelling and geochemical techniques. We assess whether recharge may occur through a pervasive layer of floodplain muds that was initially hypothesized to be impermeable. At least 59% of the precipitation volume could be accounted for in the shallow aquifer using the water table fluctuation method during four significant recharge events. Precipitation events <20mm did not produce detectable aquifer recharge. The highest recharge rates were estimated in the area underneath the floodplain clay layer rather than in the sandy area. A steady-state chloride method implied recharge rates of at least 200mm/year (>14% of annual precipitation). Tritium dating revealed long term net vertical recharge rates ranging from 27 to 114mm/year (average 58mm/year) which were interpreted as minimum net long term recharge. Borehole experiments revealed more permeable conditions and heterogeneous infiltration rates when the floodplain soils were dry. Wet conditions apparently expand floodplain clays, closing macropores and cracks that act as conduits for groundwater recharge. Modelled groundwater flow paths were consistent with tritium dating and provided independent evidence that the clay layer does not prevent local recharge. Overall, all lines of evidence demonstrated that the coastal floodplain muds do not prevent the infiltration of rainwater into the underlying sand aquifer, and that local recharge across the muds was widespread. Therefore, assuming fine-grained floodplain soils prevent recharge and protect underlying aquifers from pollution may not be reasonable.

Quantification of a Distributive Fluvial System: The Salt Wash DFS of the Morrison Formation, SW U.S.A.

Abstract Recent analysis of modern aggradational continental sedimentary basins reveals that sedimentation patterns are dominated by distributive fluvial systems (DFSs). The Salt Wash Member of the Late Jurassic Morrison Formation has previously been described as a fan-shaped fluvial system. This study characterizes facies variations across the Salt Wash DFS to quantitatively test predicted trends in conceptual DFS models. Notable proximal-to-distal trends include a change in total thickness of the fluvial succession from 174 m to 40 m, and an average grain size from coarse sand to silt, while the percentage of sand decreased from 70% in the proximal region to 8% in the distal region. The proportion of amalgamated channel-belt deposits decreased from 67% to 0%, while floodplain facies and lacustrine deposits increase (38% to 94% and 0.1% to 7% respectively). A downstream decrease in average channel-belt thickness (15 m to 3.8 m, from thickest to thinnest) and average story thickness (7.7 m to 2.3 m, from thickest to thinnest) is also recorded. Significant downstream changes in deposit architecture were also noted, with proximal regions dominated by stacked channel-belt deposits with a high degree of amalgamation. Distal deposits are dominated by floodplain muds and sheet sandstones and sparse ribbon channels, with little to no amalgamation of channel deposits. This study provides quantified information for an ancient DFSs with the aim of providing a dataset that can be used for objective comparison between different DFSs, as well as providing numerical data to aid resource exploration and modelling efforts.

Facies associations of the northern Dandiero Basin (Danakil depression, Eritrea, including the Pleistocene Buya homo site)

The Dandiero Basin is located on the northern part of the 300 km-long Danakil depression. The geologic succession filling the Dandiero Basin is up to 1000 m thick and consists of three synthems, inascending order: the Early-to-Middle Pleistocene Maebele Synthem of fluvio-lacustrine origin; the Late Pleistocene Curbelu Synthem of alluvial deposits and the Late Pleistocene to Holocene Samoti Synthem alluvio-eolian sand. This study and map (1:5000 scale) focuses on the Maebele Synthem exposed in a 3.8 km2 area near Mt. Alat. Regionally, the Maebele Synthem consists of six lithostratigraphic units, in ascending order: (1) the Bukra Sand and Gravel, (2) the Alat Formation, (3) the Wara Sand and Gravel, (4) the Goreya Formation, and (5) Aro Sand, (6) Addai Fanglomerate. The Bukra Sand and Gravel is about 150–200 m thick and made up mainly of fluvial sand. The Alat Formation is 70–100 m thick and consists of alternating fluvial, lacustrine and deltaic deposits. Fluvial deposits consist of sand-filled channels that occur as amalgamated or isolated bodies within floodplain mud. Lacustrine sediments consist of mud, whereas deltaic deposits are represented by sandy shallow-water and Gilbert-type deltas. The Wara Sand and Gravel (250 m thick) is made up of fluvial sand. The Goreya Formation (50 m thick) consists of lacustrine, deltaic and fluvial deposits. The fluvial deposits consist of sand-filled channels the lacustrine sediments consist of mud with subordinate limestone, and the deltaic deposits are represented by sandy shallow-water deltas. The Aro Sand consists of fluvial sand up to 120 m thick. The Addai Fanglomerate (250–300 m thick) consists of coarse-grained alluvial fan.

Land use and land cover changes over a century (1914–2007) in the Neyyar River Basin, Kerala: a remote sensing and GIS approach

Land use and land cover change, perhaps the most significant anthropogenic disturbance to the environment, mainly due to rapid urbanization/industrialization and large scale agricultural activities. In this paper, an attempt has been made to appraise land use/land cover changes over a century (1914–2007) in the Neyyar River Basin (L=56 km; Area = 483.4 km2) in southern Kerala – a biodiversity hot spot in Peninsular India. In this study, digital remote sensing data of the Indian Remote Sensing satellite series I-D (LISS III, 2006–2007) on 1:50,000 scale, Survey of India (SOI) toposheet of 1914 (1:63,360) and 1967 (1:50,000) have been utilized to map various land use/land cover changes. Maps of different periods have been registered and resampled to similar geographic coordinates using ERDAS Imagine 9.0. The most notable changes include decreases in areas of paddy cultivation, mixed crops, scrub lands and evergreen forests, and increases in built-up areas, rubber plantations, dense mixed forests, and water bodies. Further, large scale exploitation of flood plain mud and river sand have reached menacing proportions leading to bank caving and cut offs at channel bends. Conservation of land and water resources forms an important aspect of ecosystem management in the basin.

Freshwater recharge into a shallow saline groundwater system, Cooper Creek floodplain, Queensland, Australia

Freshwater lenses have been identified as having penetrated the shallow regional saline groundwater beneath the Cooper Creek floodplain near Ballera (south-west Queensland). Piezometers were installed to evaluate the major-element chemistry along a floodplain transect from a major waterhole (Goonbabinna) to a smaller waterhole (Chookoo) associated with a sand dune complex. The floodplain consists of 2-7m of impermeable mud underlain by unconsolidated fluvial sands with a saline watertable. Waterholes have in places scoured into the floodplain. The transect reveals that groundwater recharge takes place through the base of the waterholes at times of flood scour, but not through the floodplain mud. Total dissolved solids rise with distance from the waterhole and independently of the presence of sand dunes. Stable water isotopes (δ2H and δ18O) confirm that recharge is consistent with, and dependant on, monsoonal flooding events. Following floods, the waterholes self-seal and retain water for extended periods, with sulfate-δ34S and δ18O isotopes suggesting bacterial reduction processes within the hyporheic zone, and limited interaction between the surface water and groundwater during no-flow conditions. The area occupied by the freshwater lenses (TDS <5000 mg/L) is locally asymmetrical with respect to the channel flow direction, extending down gradient along distances of ∼300 m.

A diverse Upper Jurassic dinosaur ichnofauna from central-west Portugal

Mateus, O. & Milan, J. 2009: A diverse Upper Jurassic dinosaur ichnofauna from central-west Portugal. Lethaia, Vol. 43, pp. 245–257. A newly discovered dinosaur track-assemblage from the Upper Jurassic Lourinha Formation (Lusitanian Basin, central-west Portugal), comprises medium- to large-sized sauropod tracks with well-preserved impressions of soft tissue anatomy, stegosaur tracks and tracks from medium- to large-sized theropods. The 400-m-thick Lourinha Formation consists of mostly aluvial sediments, deposited during the early rifting of the Atlantic Ocean in the Kimmeridgian and Tithonian. The stratigraphic succession shows several shifts between flood-plain mud and fluvial sands that favour preservation and fossilization of tracks. The studied track-assemblage is found preserved as natural casts on the underside of a thin bivalve-rich carbonate bed near the Tithonian–Kimmeridgian boundary. The diversity of the tracks from the new track assemblage is compared with similar faunas from the Upper Jurassic of Asturias, Spain and the Middle Jurassic Yorkshire Coast of England. The Portuguese record of Upper Jurassic dinosaur body fossils show close similarity to the track fauna from the Lourinha Formation. □Dinosaur tracks, Lusitanian Basin, Portugal, skin impressions, Upper Jurassic.

Fragmentary Evidence of Great-Earthquake Subsidence during Holocene Emergence, Valdivia Estuary, South Central Chile

A reconnaissance of Holocene stratigraphy beneath fringing marshes of the Valdivia estuary, where an M 9:5 earthquake caused 1-2 m of regional coseismic subsidence in 1960, shows only fragmentary evidence of prehistoric coseismic sub- sidence. In most of the 150 hand-driven cores that were examined, a distinct uncon- formity separates 0.5-1.5 m of late Holocene tidal and floodplain mud, peat, and sand from underlying middle Holocene subtidal mud and sand. At the Las Coloradas site, where stratigraphy is best preserved, two A horizons of marsh and meadow soils abruptly overlain by sand and mud probably record coseismic subsidence shortly fol- lowed by tsunamis. The amount of subsidence during the earthquakes proved difficult to reconstruct with a diatom transfer function because of differences between modern and fossil diatom assemblages. Maximum 14 C ages on macrofossils from the two A horizons at the Las Coloradas site of 1.7-1.3 ka and 2.7-1.7 ka allow correlation of the younger horizon with either of two of six 14 C-dated A horizons buried by tsunami sand or post-tsunami tidal sand 200 km to the south at Maullin, and with a lake-wide mass wasting event in Lago Puyehue, 100 km to the southeast. Tidal records of prehistoric coseismic subsidence at Valdivia are scarce because of a sea-level fall of 3-8 m over the past 6000 years, erosion of marsh and meadow soils during subsidence-induced flooding of the estuary, and largely complete land-level recovery during cycles of coseismic subsidence and postseismic uplift. Online Material: Diatom species data from the Las Coloradas site, Valdivia estuary.

Quaternary fluvial and eolian deposits on the Belan river, India: paleoclimatic setting of Paleolithic to Neolithic archeological sites over the past 85,000 years

Archeological sites in the bedrock Belan Valley at the southern margin of the Ganga Plains of India have unearthed Paleolithic to Neolithic artifacts and the first known evidence for rice cultivation. We present a sedimentological and paleoclimatic analysis for Belan sections, incorporating new luminescence and radiocarbon dates and a compilation of previous research. Some 20 m of strata are exposed in fluvial terraces, commencing with pedogenic and channel calcretes linked to groundwater ponding on the underlying bedrock. Overlying alluvium deposited from mixed-load meandering rivers yields dates between 85±11 and 72±8 kyr before present (BP), implying sustained fluvial activity during Marine Isotope Stage 5 and later; these strata contain Middle Paleolithic artifacts. Thin reworked gravels with Upper Paleolithic artifacts are dated at ∼21–31 kyr BP, and may represent declining alluviation and floodplain gully erosion during reduced monsoonal activity around the Last Glacial Maximum (LGM). Younger channel fills contain shell-rich eolian sand, and mounds of shelly sand lie inland from the river. Five OSL dates from the sands span 14 to 7 kyr BP, corresponding to a period of climatic instability that includes the Younger Dryas as the monsoon intensified following the LGM. Although suggesting more arid phases, the source-bordering eolian material has a small volume and the grains are partially bleached, indicating local wind action. Overlying floodplain muds reflect renewed alluviation, after which the river incised during peak monsoon flow. The Mesolithic settlement of Chopani-Mando spans a period of reduced monsoon activity and climatic instability following the LGM. Subsequent Neolithic settlements were probably established under stronger monsoon conditions suitable for the development of agriculture. Mesolithic habitation may have ended when a nearby bedrock channel was abandoned as the reinvigorated Belan cut a new course, along which Neolithic settlements were later established.

Upper-Flow-Regime Mud Floodplains, Lower-Flow-Regime Sand Channels: Sediment Transport and Deposition in a Drylands Mud-Aggregate River

Abstract Dryland rivers in which fine sediments travel as aggregates are increasingly recognized in modern and ancient fluvial systems. Fowlers Creek, Australia, is an ephemeral dryland mud-aggregate river whose sediments provide insights into the dynamics of mud-aggregate floodplains, the origin of massive mudrocks from arid depositional environments, and the nature of planar bedding. Fowlers Creek's flow conditions were inferred from relationships between landforms and the sediment texture, bedforms, and sedimentary structures remaming after flow ceases. Floodplain muds, consisting of fine sand and sand-size mud aggregates, are distributed over the floodplain in suspension. As flow decelerates they are deposited as bedload. The shallow depth, high sediment load, and low aggregate particle density promote flow conditions ranging from high in the lower flow regime to upper flow regime, producing coexisting ripples, scours, flat beds, and clay layers. With time, visible signs of aggregate structure are lost, leaving a massive cohesive mud; consolidation is not achieved by burial. Aggregates reappear when muds reenter fluvial transport. In unchanneled reaches, sheetflows deposit sediment with a pervasive horizontal fabric. Channel sediments (coarse sands and gravels) are a minor component of Fowlers Creek's deposits. Widespread lower-flow-regime conditions produce planar beds and 2-D dunes, usually deposited without internal stratification or in horizontal laminae. Lower-flow-regime planar bedding is also observed in fine silty sands. Near the close of flow, rapid shallowing may move channel conditions from lower to upper flow regime, or from lower to higher positions in the lower flow regime bedform stability spectrum, leading to unusual bedform associations. In the rock record sediments from a river like Fowlers Creek would be characterized by structureless gravelly sands (channel facies), massive red mudstones (floodplain facies), and sediments with horizontal fabric but poorly expressed bedding (sheetflow facies).

Threshold-dominated fluvial styles in an arid-zone mud-aggregate river: The uplands of Fowlers Creek, Australia

Fowlers Creek is a mud-aggregate fluvial system. Floodplain muds dominate the river's deposits and consist of silt, fine to very fine quartzose sand, and clay. Up to ∼ 80% of the silts and clays are bound into sand- and silt-sized aggregates and comprise a substantial component (> 42%) of the floodplain muds. Mud-aggregate sediments behave like sands during transport, and as a result, muds can be deposited under conditions of greater flow velocity than would otherwise be the case. Newly deposited floodplain muds are loose and easily entrained, but older floodplain muds are cohesive, and the distribution of modern and older floodplain muds influences erosion patterns across Fowlers Creek. In the lower order streams of the Fowlers Creek uplands, alternate reaches of shallow rectangular channels and unchannelled floodplains collectively form discontinuous ephemeral streams. These landform sequences consist of gullies, coalescing downstream to arroyos, which terminate in distributary intermediate floodouts. At Fowlers Creek, floodouts are preferentially located at tributary junctions, reflecting their origin during very large floods. At floodouts, low slope and high vegetation density promote sheetflow infiltration and landform stability. Their efficiency in retaining runoff make floodouts drought refugia; they are an important ecological element in this arid area. The higher order channel of the mid-uplands is a mobile, low-sinuosity, single-thread arroyo, incised into wide muddy unstable floodplains. Fluvial processes are dominated by episodic flood-driven channel avulsion, and variability in stream energy and boundary resistance contributes to a non-equilibrium fluvial style. Frequent reach-scale channel relocation is accompanied by the burial of the abandoned channel in floodplain muds and both erosion and aggradation in downstream floodplains.

A large-mammal site in a meandering fluvial context (Fonelas P-1, Late Pliocene, Guadix Basin, Spain): Sedimentological keys for its paleoenvironmental reconstruction

The Fonelas P-1 large-mammal site is located stratigraphically in the youngest unit (Late Pliocene–Pleistocene) of the endorheic fill of the Guadix Basin (Betic Cordillera). At the site location this unit is represented by fluvial sediments of the basin's axial drainage system. The general succession of the site consists of stacked fining-upward cycles, basically beginning with gravel and/or sand filling a meandering channel and concluding with floodplain mud and carbonate. Each of the cycles is made up of four facies associations determined by sedimentological analysis of the sections measured in the paleontological excavation of the site. These associations correspond to sedimentation in definite areas of the fluvial system: (A) filling of a sinuous channel at maximum activity, (B) levee deposits, (C) abandoned channel and (D) floodplain. In the cycle in which the site is located there is a fifth association (E), where the main fossil concentration is found (Fonelas P-1). This is a facies bioturbated by mammals moving over sediments of the abandoned channel association (C). By space–time reconstruction of the fluvial environments, we can infer that the morphological depression caused by a meander abandonment due to chute cut-off was used as a hyena den. The animal settlement ended when overflow from the laterally migrating active channel began to bury the abandoned meander with floodplain sediments.

No Change in Fluvial Style Across a Sequence Boundary, Cretaceous Blackhawk and Castlegate Formations of Central Utah, U.S.A.

Nonmarine sequence stratigraphic models are based largely on studies of fluvial units in the Cretaceous Western Interior of North America, including the Blackhawk and Castlegate formations of central Utah. These models suggest that fluvial units should show a transition from mudstone-prone, isolated meander-belt deposits of the transgressive and highstand systems tract into amalgamated braided-stream sandstones of the lowstand systems tract. These mudstone-rich fluvial strata are largely assumed, rather than documented, to represent the deposits of isolated meander-belt deposits, despite recent work that suggests that there is no simple relationship between the proportion of preserved floodplain mud and fluvial style. There are numerous studies documenting the braided character of the sandstone-rich Castlegate Formation, but there are no corresponding studies that document the internal fluvial facies architecture within the Blackhawk Formation to support the interpretation that the Blackhawk comprises the deposits of a meandering-stream system. We test the validity of the sequence stratigraphic model by comparing dimensions and styles of associated bar and channel deposits, as well as cross-stratal thicknesses, between the Blackhawk and Castlegate formations along Salina Canyon in Utah. Data compiled from 24 measured sections and 6 photomosaics show that the Blackhawk Formation comprises isolated sandy channel-belt sheet sandstones, 5–8 m thick, contained within thick floodplain mudstones, whereas the Castlegate Formation consists of about 80 m of amalgamated sandy channel-belt sheets, 4–7 m thick, with only minor mudstone. The average height of cross-sets is 13 cm in the Castlegate Formation and 14 cm in the Blackhawk Formation and formative dune height is estimated to be 38 cm for the Castlegate Formation and 45 cm for the Blackhawk Formation. Average bar-accretion thicknesses are 2 m in both the Blackhawk and Castlegate formations. Corresponding water depths are estimated to be 2.5–4.1 m for the Blackhawk and 2.3–3.8 m for the Castlegate. The correlation of master bedding planes in the Blackhawk Formation shows overlapping lens-shaped bar deposits and channel fills. Bar tops appear to dip in several directions, indicating both lateral and downstream accretion of mid-channel braid bars. While there is some indication that Castlegate rivers were slightly shallower, the differences do not suggest a major change in fluvial style across the sequence boundary; both are braided. The lack of change in scale of channels also is not compatible with a hypothesized increase in aridity between the Blackhawk and the Castlegate. The difference in fluvial architecture between the two formations is interpreted to reflect changes in accommodation that were likely tectonic in origin. This supports that idea that preservation of thick successions of fine-grained overbank material is not a function of the plan-view channel geometry.

Discontinuity-Bounded Alluvial Sequences of the Southern Gangetic Plains, India: Aggradation and Degradation in Response to Monsoonal Strength

Discontinuity-bounded late Quaternary sequences in the southern Gangetic Plains (Himalayan Foreland Basin) reflect floodplain aggradation and degradation in response to forcing by the powerful Southwest Indian Monsoon. The major Himalayan and cratonic rivers in this area occupy narrow, incised valleys and do not inundate the adjoining broad interfluves, which display soil development, small plains-fed rivers, lakes, eolian deposits, and badland ravines. However, these areas formerly experienced active alluviation because thick floodplain muds underlie the interfluve. An age model suggests that interfluve areas near the major rivers aggraded periodically between about 27 ka and 90 ka (Marine Oxygen Isotope Stages 3–5). They subsequently degraded or accumulated sediment only locally, probably reflecting decreased monsoonal precipitation around the Last Glacial Maximum (Marine Isotope Stage 2), when major river valleys moved to an underfit condition. Increased precipitation during the 15 to 5 ka period of monsoon recovery probably increased discharge and promoted incision and widespread badland formation. Discontinuities at deeper levels reflect earlier degradation phases, and are marked by reworked gravel lenses, thin channel bodies, and carbonate-cemented surfaces. Carbonate veins (infilled joints) that cut strata below but not above one discontinuity suggest some local tectonic influence. There is no evidence that sea-level fluctuations influenced stratal patterns in this elevated, inland region. Discontinuity-bounded sequences linked to climate change may characterize alluvial successions in many continental settings, especially along the cratonic margins of foreland basins, where subsidence rates are modest.

Monitoring Yearly Changes and Their Influence on Electrical Properties of the Shallow Subsurface at Two Sites Near the Rio Grande, West Texas

We present the results of a case study that monitored changes in the electrical properties of soils on a bi-monthly basis between August 1999 and November 2000 at two sites located along the banks of the Rio Grande, west Texas. One site was located within an abandoned channel of the Rio Grande and showed relatively homogeneous layering, while the second site exhibited complex interfingerings of crevasse splay (sands and silts) and floodplain muds. Repeated EM-31 ground conductivity measurements at both sites showed that conductivities within the upper [Formula: see text] of soil could change by up to a factor of 2 within a year’s time. DC resistivity studies suggested up to a factor of 5 change in resistivity values within the upper [Formula: see text] of soil. These changes appear to be related to remobilization of salt in the soil following rainfall or irrigation events. These dramatic fluctuations make it difficult to image deeper ([Formula: see text] depth) conductivity changes in the subsurface.

A dynamic model to predict responses of millets (Echinochloa sp.) to different hydrologic conditions for the Illinois floodplain–River

AbstractMillets grow on floodplain mud flats exposed when seasonal floods recede, and the seeds of this plant are an important food source for waterfowl during their spring and autumn migrations in the Mississippi Flyway. Productivity of millets along the Illinois River has declined because of unnaturally frequent floods that inundate the mud flats and drown the plants during the summer growing season. These small floods are caused by operation of the navigation dams on the main channel and by alterations of the floodplain and tributary watersheds and channels. Predictive models are needed to evaluate the most cost‐effective combination of approaches for restoring plant productivity. We developed a moist‐soil plant model that simulates millet growth on 1 m2 in response to daily water levels during the summer growing season. The model responds to daily water depth, flood timing (within the growing season), and flood duration, and was qualitatively verified using historical (1938–1959) water levels and plant coverage for three areas along the Illinois River. In the absence of untimely floods, the model predicts net above‐ground primary productivity of ∼500 g m−2 yr−1 and plant heights of up to 130 cm by the end of the growing season. As expected, growth declines with decreasing land elevation or with more frequent flooding (or a shorter duration of the dry period) at the same elevation. A dry period of &gt;85 days is required to achieve at least 50% of maximum production during the growing season, which is somewhat longer than the 70‐day recommendation based on reported field observations. The model predictions of plant success or failure agree with historical observations, indicating that water regime is a major factor limiting plant success. Copyright © 2004 John Wiley &amp; Sons, Ltd.