Climate-induced fluvial dynamics in tropical Africa around the last glacial maximum?

The Greenland and East and West Antarctic ice sheets are assessed as being the source of ice that produced an Eemian sea level 6 m higher than present sea level. The most probable source is total collapse of the West Antarctic Ice Sheet accompanied by partial collapse of the adjacent sector of the East Antarctic Ice Sheet in direct contact with the West Antarctic Ice Sheet. This conclusion is reached by applying a simple formula relating the “floating fraction” of ice along flowlines to ice height above the bed. Increasing the floating fraction lowered ice elevations enough to contribute up to 4.7 m to global sea level. Adding 3.3 m resulting from total collapse of the West Antarctic Ice Sheet accounts for the higher Eemian sea level. Partial gravitational collapse that produced the present ice drainage system of Amery Ice Shelf contributes 2.3 m to global sea level. These results cast doubt on the presumed stability of the East Antarctic Ice Sheet, but destabilizing mechanisms remain largely unknown. Possibilities include glacial surges and marine instabilities at the respective head and foot of ice streams.     

Lithologic and structural controls on fluvial knickzones in basalts of the Paraná Basin,Brazil

Knickzones are common features along rivers on the basaltic plateaus of the Paraná Basin. According to current conceptual models, knickpoints are formed in massive basalts that have a high density of vertical joints. Vesicular–amygdaloidal basalts and those with horizontal joints tend to form reaches of low slope due to their lower resistance to erosion. However, field surveys revealed complexities in this general relationship. The research presented here sought to verify the controls on the genesis of knickzones in this type of geological environment. We studied a 61 km-long mixed bedrock–alluvial river. The longitudinal profile of the river was surveyed on a topographic map with 5 m contour intervals. Tectonic lineaments oriented transverse to the channel and longitudinal lineaments in which the river lies were identified from maps. A detailed field survey of the lithologic characteristics of the riverbed was also performed. The results show that knickzones may form in any litho-structural zone in the flood basalts. On the other hand, low slope zones are predominantly sculpted into vesicular–amygdaloidal basalts, which are less resistant to erosion. The fracture densities of vesicular–amygdaloidal basalts are similar in low slope zones and in knickzones (4.86 and 4.93 m/m², respectively). This indicates that knickzones in this type of basalt are not caused by higher resistance to erosion. Approximately 60% of the 18 knickzones identified are associated with tectonic lineaments, irrespective of the structural characteristics of the basalts. Vesicular–amygdaloidal basalt and/or basalt with horizontal joints allow the fastest knickzone migration and aid in the formation of convexities. Knickpoints in these basalts do not migrate, but erosion in the pools advances downstream and breaks the bedrock steps, thus increasing the slope. Massive basalt with vertical joints causes slower migration, and its presence at convexities indicates local uplift. Convex segments are only formed upstream of faults.     

The igapó of the Negro River in central Amazonia: Linking late-successional inundation forest with fluvial geomorphology

Despite important progress on Amazonian floodplain research, the flooded forest of the Negro River “igapó” has been little investigated. In particular, no study has previously focused the linkage between fluvial geomorphology and the floristic variation across the course of the river. In this paper we describe and interpret relations between igapó forest, fluvial geomorphology and the spatial evolution of the igapó forest through the Holocene. Therefore, we investigate the effect of geomorphological units of the floodplain and channel patterns on tree diversity, composition and structural parameters of the late-successional igapó forest. Our results show that sites sharing almost identical flooding regime, exhibit variable tree assemblages, species richness and structural parameters such as basal area, tree density and tree heights, indicating a trend in which the geomorphologic styles seem to partially control the organization of igapó's tree communities. This can be also explained by the high variability of well-developed geomorphologic units in short distances and concentrated in small areas. In this dynamic the inputs from the species pool of tributary rivers play a crucial role, but also the depositional and erosional processes associated with the evolution of the floodplain during the Holocene may control floristic and structural components of the igapó forests. These results suggest that a comprehensive approach integrating floristic and geomorphologic methods is needed to understand the distribution of the complex vegetation patterns in complex floodplains such as the igapó of the Negro River. This combination of approaches may introduce a better comprehension of the temporal and spatial evolutionary analysis and a logic rationale to understand the vegetation distribution and variability in function of major landforms, soil distributions and hydrology. Thus, by integrating the past into macroecological analyses will sharpen our understanding of the underlying forces for contemporary floristic patterns along the inundation forests of the Negro River.     

Changes in water chemistry along the newly formed High Arctic fluvial–lacustrine system of the Brattegg Valley (SW Spitsbergen,Svalbard)

Changes in water chemistry along the High Arctic fluvial–lacustrine system located in Wedel Jarlsberg Land in the SW Spitsbergen (Svalbard) were investigated during the summer season of 2010 and 2011. The newly formed river–lake system consists of three lakes connected with the Brattegg River. The first bathymetric measurements of these lakes were made by the authors in 2010. The Brattegg River catchment represents a partly glaciered Arctic water system. The studied lakes are characterized by low mineralization and temperature of water. The value of the electrolytic conductivity (EC) ranges from 30.2 to 50.5 μS cm^?1 and the temperature of surface water from 1.5 to 7.8 °C. The temperature increase takes place downstream starting from Upper Lake to the outflow from Myrktjørna Lake. The waters of lakes have higher temperatures than the stream. The predominant ions are HCO₃ ^? (up to 16.5 mg L^?1), Cl^? (6.66–8.53 mg L^?1), Ca²⁺ (2.40–4.45 mg L^?1) and Na⁺ (2.65–3.36 mg L^?1). The highest values of ammonium and DOC found in the lowest Myrktjørna Lake seem to be related to the presence of aquatic organisms and also birds. From the group of 10 analyzed microelements, increased concentrations of aluminum, up to almost 500 μg L^?1, are present in the lakes’ water. Water isotopic composition ranges for δ¹⁸O and δ²H, from ?10.6 to ?10.9‰ and from ?70.8 to ?72.3‰, respectively. The vertical zonality of lake waters is manifested in a decrease in the temperature and increase in EC and chemical elements concentrations.     

Tectonic controls on the evolution of the Andean Cenozoic foreland basin: Evidence from fluvial system variations in the Payogastilla Group,in the Calchaquí, Tonco and Amblayo Valleys,NW Argentina

The stratigraphic and sedimentological characteristics of the Payogastilla Group represent important tectono-sedimentary constraints on the evolution of the Andean foreland basin in northwestern Argentina. This nonmarine unit unconformably rests on top of the post-rift deposits of the middle Eocene Lumbrera Formation of the Santa Bárbara Subgroup (Salta Group). Eocene-Pliocene paleoenvironmental changes are a direct result of the tectonic settings and accommodation space. Sequential stratigraphic analysis of the paleoenvironment of the Los Colorados Formation strata indicates the presence of three third-order sequences. Each sequence comprises a low-accommodation systems tract (LAST) and a high-accommodation systems tract (HAST). Substantial tectonic activity from the middle to upper Miocene is represented by Angastaco Formation strata that contain a shallow, gravel-braided fluvial system associated with gravity flows, with thicknesses of 4550 m (Calchaquí River) to 1500 m (Tonco). This activity marked the depocenter of the Angastaco basin. The development of a basal unconformity and the erosion of the Los Colorados Formation suggest a renewed uplift of the source area. Changes in the fluvial systems indicate an increase of the accommodation space.To obtain better temporal constraints on the basin evolution, new U–Pb ages on zircons from five pyroclastic airfall and two sedimentary levels were determined. A substantial environmental change in the upper Miocene (10–5 Ma) is associated with three episodes of tectonic uplift that are reflected in variations in the sedimentation rates of the Palo Pintado Formation. A reactivated Pliocene tectonic uplift is recorded in alluvial fans that originated from the east.     

High-resolution aquifer analog of fluvial–aeolian sediments of the Guarani aquifer system

The Guarani aquifer system (GAS) represents one of the biggest aquifers in the world and is the most relevant groundwater resource in South America. For the first time, by combining field and laboratory measurements, a high-resolution aquifer analog model of fluvial–aeolian sediments of the GAS in São Paulo State (Brazil) is constructed. Three parallel sections of frontal outcrops, 28 m × 5.8 m, and two parallel sections of lateral outcrops, 7 m × 5.8 m, are recorded during open-pit mining of sandy sediments and describe in detail the three-dimensional distribution of the local lithofacies and hydrofacies. Variations of hydraulic conductivity, K, and porosity, n, are resolved on the centimeter scale, and the most permeable units of the fluvial–aeolian facies association are identified. The constructed aquifer analog model shows moderate hydraulic heterogeneity and a mean K value of 1.36 × 10^?4 m/s, which is greater than the reported range of K values for the entire GAS in São Paulo State. The results suggest that the examined sedimentary unit constitutes a relevant portion of the GAS in São Paulo State in the context of groundwater extraction and pollution. Moreover, the constructed aquifer analog is considered an ideal basis for future numerical model experiments, aiming at in-depth understanding of the groundwater flow and contaminant transport patterns at this GAS portion or at comparable fluvial–aeolian facies associations.     

The role of fluvial sediment supply and river-mouth hydrology in the dynamics of the muddy,Amazon-dominated Amapá–Guianas coast,South America: A three-point research agenda

The morphology and sediment dynamics of the 1500 km-long coast of South America between the mouths of the Amazon and the Orinoco Rivers are largely dependent on the massive suspended-sediment discharge of the Amazon, part of which is transported alongshore as mud banks. These mud banks have an overwhelming impact on the geology, the geomorphology, the ecology and the economy of this coast. Although numerous field investigations and remote sensing studies have considerably enhanced our understanding of the dynamics of this coast over the last three decades, much still remains to be understood of the unique functional mechanisms and processes driving its evolution. Among the themes that we deem as requiring further attention three come out as fundamental.The first concerns the mechanisms of formation of individual mud banks from mud streaming on the shelf off the mouth of the Amazon. An unknown quantity of the fluid mud generated by offshore estuarine front activity is transported shoreward and progressively forms mud banks on the Amapá coast, Brazil. The volume of each mud bank can contain from the equivalent of the annual mud supply of the Amazon to several times this annual sediment discharge. The mechanisms by which individual banks are generated from the Amazon turbidity maximum are still to be elucidated. Areas of research include regional mesoscale oceanographic conditions and mud supply from the Amazon.The second theme is that of variations in rates of migration of mud banks, which influence patterns of coastal accretion. Research emphasis needs to be placed on the analysis of both regional meteorological-hydrodynamic forcing and distant Atlantic forcing, as well as on the hydrology of the large rivers draining the Guyana Shield. The rivers appear to generate significant offshore deflection of mud banks in transit alongshore, through a hydraulic-groyne effect. This may favour both muddy accretion on the updrift coast and downdrift mud liquefaction with probably lessened muddy deposition.The third theme concerns sand supply by the Guiana Shield rivers. The rare sand deposits are important in providing sites for human settlements and routes and for nesting by marine turtles. The limited presence of sand bodies on this coast may reflect ‘mud blanketing’, a hypothesis that requires verification through high-resolution seismic analyses of shelf deposits and coring operations. The large Guiana Shield rivers, especially in Surinam and Guyana, have supplied sand for the construction of significant bands of cheniers, probably enhanced by the afore-mentioned downdrift hydraulic-groyne effect on hindered mud deposition. In all the three themes of this future research agenda, two central elements are the sediment input of the rivers of the Amazon basin, starting with the massive mud supply from the Amazon catchment itself, followed by sand inputs by the Guiana Shield rivers and their river-mouth effects on mud banks.     

Paleobiogeoclimatic scenarios of the Late Quaternary inferred from fluvial deposits of the Quadrilátero Ferrífero (Southeastern Brazil)

The Quadrilátero Ferrífero is an important mineral province in Southeastern Brazil and has one of the largest iron ore reserves in the world. Previous work in this region has indicated that the formation of fluvial successions with duricrusts coincided with drier/cooler climatic phases alternating with moister/warmer periods during which the formation of fluvial successions without duricrusts occurred. For the construction of this proposal, ages of fluvial sediments obtained through Optically Stimulated Luminescence (OSL) were associated with data from the literature on paleobioclimatic scenarios. Therefore, using these observations as a starting point, this paper aims to investigate evidence of bioclimatic oscillations obtained directly from the fluvial successions and discuss its influence on the geomorphogenis of local river valleys. For this purpose, phytolith, carbon isotope and granulometric analyses were carried out, as well as dating of sediments using OSL and of soil organic matter through radiocarbon. The results show that in the oldest depositional succession (DS1 — about 34ka) the predominant phytoliths are those of bulliform polyedric, elongate, acicular and globular granulate types and δ¹³C values are typical of C3 plants. On the other hand, despite having a similar phytolith assemblage (abundance of bulliform polyedric, elongate, bulliform cuneiform, acicular, globular psilate and bilobate flat/concave types), the fluvial successions associated with significant conglomeratic duricrusts (DS2 and DS3) present a dominance of δ¹³C values characteristic of C4 plants. The Bi index indicates water stress in all the successions, and the Ic index suggests decreasing temperatures with depth in DS3. Thus, the three fluvial successions indicate a savanna-like environment, but depositional successions DS2 (∼27ka) and DS3 show drier/cooler climatic conditions when compared to DS1 and to the present-day regime. Both scenarios evolved under conditions of the Last Glacial period, but DS2 and DS3 were formed closer to the Last Glacial Maximum, and therefore under the strong influence of the lower temperatures during this period. These drier/cooler conditions in steep valleys with unprotected hillslopes may have been decisive for the formation of relatively thicker layers of gravel and sand, which later became duricrusts. The results indicate that climate has also played an important role in the regional hydrosedimentological dynamics, given the variations in vegetation influencing the abandonment of fill terraces and formation of nested floodplains.     

The sedimentology of an ephemeral fluvial–aeolian succession

Ephemeral fluvial systems are commonly associated with arid to semi-arid climates. Although their complex sedimentology and depositional settings have been described in much detail, depositional models depicting detailed lateral and vertical relationships, and interactions with coeval depositional environments, are lacking compared to well-recognized meandering and braided fluvial systems. This study critically evaluates the applicability of current models for ephemeral fluvial systems to an ancient arid fluvial example of the Lower Jurassic Kayenta Formation of the Colorado Plateau, USA. The study employs detailed sedimentary logging, palaeocurrent analysis and photogrammetric panels across the regional extent of the Kayenta. A generic model that accounts for the detailed sedimentology of a sandy arid ephemeral fluvial system (drawing upon both ancient and geomorphological studies) is developed, along with analysis of the spatial and temporal interactions with the aeolian setting. Results show that the ephemeral system is dominated by laterally and vertically amalgamated, poorly channelized to sheet-like elements, with abundant upper flow regime flat beds and high sediment load structures formed between periods of lower flow regime conditions. Through interaction with a coeval aeolian system, most of the fluvial deposits are dominated by sand-grade sediment, unlike many modern ephemeral fluvial systems that contain a high proportion of conglomeratic and/or finer grained mudstone and siltstone deposits. During dominantly fluvial deposition, high width to thickness ratios are observed for channelized and sheet-like elements. However, with increasing aridity, the aeolian environment becomes dominant and fluvial deposition is restricted to interdune corridors, resulting in lower width to thickness ratio channels dominated by flash-flood and debris-flow facies. The data presented here, coupled with modern examples of ephemeral systems and flood regimes, suggest that ephemeral flow produces and preserves distinctive sedimentological traits that can not only be recognized in outcrops, but also within core.     

Delimiting a saline water zone in Quaternary fluvial–alluvial deposits using transient electromagnetic: a case study in Punata,Bolivia

The semiarid Punata alluvial fan is located in the central part of Bolivia. The main activity of this region is the extensive agriculture, and groundwater is the main water supply. Local villagers who use groundwater reported that in some places groundwater has a salty taste. In order to investigate the origin of this problem, several TEM soundings were performed in the study area, and they were complemented with ERT surveys. The results show top layers with resistivity values ranging from 30 to 200 Ωm and a bottom layer with resistivity values ranging from 1 to 20 Ωm, which might be interpreted as the main aquifer and a layer with high clay content, respectively. Between the top and bottom layer, a transition zone with saline water has been identified, with resistivity values ranging from 0.1 to 1 Ωm. The origin of this closed-basin brine might be a product of the evaporation of paleolakes during the lower Pliocene, where saline clays were deposited. This study demonstrated the effectiveness of TEM sounding for mapping very low resistivity zones such as saline water.     

Internal waves in the near-bottom thermocline and microdeformations in the Earth’s crust in the continent-ocean transition zone

The results of synchronous measurements of temperature variations in a near-bottom thermocline, as well as microdeformations of the Earth’s crust and atmospheric pressure pulsing, recorded on-shore with the help of a laser strainmeter and laser nanobarograph, are presented. A string containing 20 thermosensors spaced at 0.5 m was used; it was placed by an anchored buoy in a place with 21-m depth and 500 m away from the shore. A good correlation between microdeformations and atmospheric pressure variations was observed for periods longer than 6 h. Quantitative estimates and spectral analysis via the Gilbert-Huang method for investigation of nonstationary and nonlinear processes lead to the conclusion that, on temporal scales from tidal to several minutes, the predominant way of formation of microdeformations in the Earth’s crust can be breaking of internal waves in a thermocline that leads to shallow water (i.e., in the zone of “internal breakers”).     

Mercury pollution in the atmosphere-soil-biology ecosystem along the trunk road in Guilin

Based on the mercury distribution in soil, atmosphere and plant in seven sections beside the trunk road, some advances are obtained： （1） Mercury content of fuel oil of petrol station in Guilin is 3.66-9.74 ng/g, higher than that reported by other countries （0.7-3.2 ng/g）. The higher the number of petrol, the lower the content. Number 90 petrol widely used is the highest （9.74 ng/g）. （2） Mercury content in soil is commonly 0.30-0.80 μg/g near the road, （0.30-2.60 μg/g） besides the street, both are higher than the average of Guilin background value （0.204 μg/g）. Mercury content in vegetable near the road is generally 0.025-0.055 μg/g, higher than that far away from the road （0.022-0.038 μg/g）. The content in cinnamon leaves on both sides of the street is 0.06-0.16 μg/g, also higher than that far away from the street （0.051-0.067 μg/g）. Guilin Highway atmospheric mercury content is usually 7-12 ng/m^3, higher than the mean background value （4.4-6.6 ng/m^3）. Atmospheric mercury content near rural roads is below the level on urban roads. The atmospheric mercury content increases apparently in traffic-intensive stations and intersections. （3） The average concentration of mercury in vegetables is mostly 0.025-0.055 μg/g. Mercury content in various tissues of vegetable follows the order of leaves （mostly 0.044）.09 μg/g）〉roots （mostly 0.02-0.05 lag/g）〉stems （mostly 0.01-0.04 μg/g）. （4） The distribution of mercury in soil, plants and atmosphere near the road is of certain similarity, with a single peak, nearly symmetrical. As far away from road, the content reduces gradually, which shows the release of exhaust gas mercury is the main source of atmospheric mercury pollution.     

The “Donauplatin”: source rock analysis and origin of a distal fluvial Au-PGE placer in Central Europe

The mineral assemblage and the sedimentological characteristics of the “Donauplatin” (Danubian fluvial placer containing platinum-group elements (PGE) and gold (Au)) are described for the first time in connection with upstream reference placer deposits near the potential source area in tributaries of the River Danube/Donau. Granulometric and morphometric data have been obtained using the CCD-based CAMSIZER technique. The platinum-group minerals (PGM; iridium, osmium, unknown iridium-osmium-sulfide, ruthenium-osmium-iridium alloys, platinum-iron alloys, iridium-bearing platinum, sperrylite) have been derived from ultramafic magmatic rocks, probably belonging to the ophiolitic series in the Tepla Barrandian unit of the Bohemian Massif. The Au-Pd-Cu compounds in the placer originated from dynamo-metamorphogenic processes in a sulfur-deficient environment at the SW edge of the Bavarian Basement. Gold in the “Donauplatin” has been reworked from a “secondary” or intermediate repository of lateritic gold (Boddington-type). Its primary source is supposed to be of orogenic origin. Provenance analyses of the associated non-heavy minerals point to high-pressure metamorphic rocks, igneous rocks (monazite) and high-temperature metamorphic rocks (750° to 850°C, zircon morphology). Garnet compositions indicate that meta(ultra)basic igneous rocks, calc-silicate rocks and skarns prevailed over paragneisses in the provenance area. Extraterrestrial processes creating the well-known Ries impact crater in the environs of Nördlingen during the Miocene have a minor share in the PGE budget by delivering molybdenum-ruthenium-osmium-iridium alloys and iridium solid-solution series (s.s.s.) minerals. Judging by the heavy mineral suites, Saxothuringian source rocks of the NE Bavarian Basement connected with the Donau River via the Naab River drainage system have not contributed to the element budget of the “Donauplatin” under study. Stream sediments which have been derived from this provenance area are characterized by low-temperature (LT) crystalline rocks and a considerable proportion of pegmatitic and metabauxitic material lacking in the Holocene sediments of the “Donauplatin”.     

Changes in the Abundance and Species Composition of Phytoplankton in the Last 150 Years in the Southern Black Sea

Changes in total abundance and in species composition of phytoplankton in the last 150 years in the southern Black Sea were investigated through a paleoecological study of two sediment cores. The results show changes in the species composition and a marked increase in total abundance of siliceous protists after around 1960. In core 22 (42°13.534′ N/36°29.555′ E), the dominating species before 1960 were Cyclotella choctawhatcheeana and Thalassiosira oestrupii. In core 25 (42°6.212′ N/36°37.460′ E), the dominating species before 1960 were T. oestrupii, Cyclotella meneghiniana, C. choctawhatcheeana, and Pseudosolenia cf. calcar-avis. Core 22 was located in closer proximity to the rim current than core 25, and the differences in total abundance between the cores could be related to differences in local nutrient loading prior to 1960. After around 1960, both cores changed to a community dominated by C. choctawhatcheeana. The changes in total abundance and species composition after around 1960 could be related to the increased nutrient loading from the Danube River into the rim current after the late 1960s. The results also showed changes in both total abundance and in species composition of dinoflagellate cysts. The dominating dinoflagellate cysts recorded were Lingulodinium polyedrum, Polykrikos schwartzii, and Spiniferites spp.     

Evidence for an early land use in the Rhône delta (Mediterranean France) as recorded by late Holocene fluvial paleoenvironments (1640–100 BC)

The overall objective of this paper is to describe the late Holocene (1640–100 BC) sedimentary and biological evolution of the Rhône–delta–plain, to interpret the sedimentary facies and palynofacies as the result of the effects of fluvial dynamic fluctuations and relative sea level change and to evaluate the paleohydrological constraints in the development of the land use and settlements of the Camargue. Focus is made on the upper part of V_III core drilled on NE of the Vaccarès lagoon. By combining sedimentology, palynology, magnetic susceptibility and archeological data, this study allowed to identify the superposition of three types of paleo-environments (marsh, fluvial floodplain, levee/crevasse splay). This sequence indicates a gradual extension of fluvial environments between the end of the second millennium BC and the 1st century BC. The variability of fluvial dynamic is evident during this period with important flood events which contrast with periods of low flow. Pollen record can be a good marker of the fluvial dynamic variability. The expression of the riparian tree pollen grains in the coarser floodplain deposits could correspond to increased fluvial influence and probably to erosion of riverbank during flood events. The local plants are associated to the low energy sedimentary environments. Focuses are made on the relations between the evolution of the environment and land use. The development of the cereal culture in the floodplain of the Rhône delta has been demonstrated between 1640–1410 and 100 BC. The last alluviation of the Rhône perturbs the research of the archaeological sites in the central part of the delta but the existence of the rural villages from the first part of the first millennium BC is highly possible.     

An ichnofossil assemblage from the fluvial deposits of the Upper Pliocene–Pleistocene Pinjor Formation (Siwalik Group), northwestern Himalayas,India: Palaeoenvironmental implications

Continental (fluvial) strata of the Pinjor Formation (Siwalik Group), northwestern Himalayas, India, contain an invertebrate trace fossil assemblage containing Planolites beverleyensis, Palaeophycus isp., Scoyenia gracilis, Taenidium barretti and other undifferentiated traces. The traces are found in an \(\sim \)26 m thick interval of alternating pinkish red siltstone, which is intercalated with mudstone, and thickly-bedded buff and greenish coloured sandstone. These sediments are interpreted as the deposits of floodplains and channel-bars of fluvial environments and low-energy overbank floodplain deposits. The trace fossils studied here are the first well documented ichnofossil assemblage from the vast, late Cenozoic Siwalik depositional system. They are not only of palaeoenvironmental significance, but they add to the growing ichnofossil database in facies of fluvial origin and should be an impetus to further ichnological studies of the Siwalik Group.     

Change in the Geodynamic Settings in the Pacific‒Eurasia Transition Zone at the End of the Early Cretaceous

Doklady Earth Sciences - An attempt to provide a new explanation for the change in the geodynamic regime in the Early Cretaceous is presented below. It accounts for data on a number of global...     

Fluvial shoal water deltas: pre-vegetation sedimentation through the fluvial–marine transition,Lower Cambrian,English Channel region

Lower Palaeozoic fluvial systems tend to be more sand-prone than those of later eras and the nature of coastal environments less certain. Field studies are presented that characterize the fluvial to marine transition over a distance of 80 km, in the Lower Cambrian of the Cotentin Peninsula, northern France. The sedimentary rocks are divided into six facies associations which represent deposition in proximal fluvial, distal fluvial, delta plain, delta front, pro-delta and offshore carbonate bank environments. The basin fill is sandstone-dominated and subdivided into three stratigraphic intervals. A 200 to 300 m thick basal interval contains very coarse-grained fluvial sandstones deposited during a relative sea level lowstand. An overlying interval, 250 to 1500 m thick, is a facies mosaic. Fluvial strata in the north-west pass laterally south-east into deltaic and shallow marine pro-delta sediments. The delta front deposits show repetitively stacked, upward-coarsening parasequences, 8 to 10 m thick, which reflect the repeated progradation of lobate, fluvially-dominated deltas onto a shallow marine shelf. The deltas formed following marine transgression and accumulated during a period of gradually rising relative sea level. An upper unit, 130 m thick, containing offshore stromatolitic and oolitic limestones, caps the study interval and represents deposition during a relative sea level highstand. The fluvial and delta distributary channel sandstones of the middle unit contain <1% mudstone. The cohesionless substrate determined that deltaic distributaries were predominantly braided in character and subject to common bifurcations which resulted in an ordered diminution of channel size and competence in a seaward direction. Terminal distributary channels show evidence of migratory levées and mouth-bars and consistently delivered fine to medium-grained sand to the delta front. The study highlights an example of pre-vegetation deltaic sedimentation that was hydraulically organized and predictable, despite being fed by braided fluvial systems with high levels of peak discharge.