Alluvial Fan Systems Research Articles

Response of alluvial fan systems to the late Pleistocene to Holocene climatic transition: contrasts between the margins of pluvial Lakes Lahontan and Mojave, Nevada and California, USA

Dated shorelines of late Pleistocene pluvial Lakes Lahontan (Great Basin Desert, northwest Nevada) and Mojave (Mojave Desert, eastern California) provide timelines for the assessment of alluvial fan sedimentation at the lake margins during the late Pleistocene to early Holocene. Two sets of alluvial fan systems have been mapped: the Stillwater fans, feeding Lake Lahontan; and the Zzyzx fans, feeding Lake Mojave. Their contrasting morphologies suggest different responses of the two fan systems to late Pleistocene to early Holocene climatic change. At the time the Stillwater fan systems underwent minimal sedimentation, with the catchment hillslopes apparently stable. The Zzyzx fans experienced major changes in water and sediment supply from the catchment hillslopes. There was a major phase of hillslope debris-flow activity, followed by fanhead trenching and distal fan progradation. Both areas were wetter and colder in the late Pleistocene than they are today, but during the transition to the Holocene the Zzyzx area was more likely to experience intense rains associated with the monsoonal penetration of warm moist tropical air into the Southwest. Vegetation reconstructions for the late Pleistocene to the early Holocene suggest that catchment hillslopes in the Mojave supported a desert shrub vegetation, but those in the Stillwaters supported juniper woodland and grasses at low elevations and pine at higher elevations. Contrasts in hillslope vegetation cover together with storm activity may account for the different responses of the alluvial fans to climatic change during the Pleistocene to Holocene climatic transition. After the falls in lake levels of Lakes Lahontan and Mojave in the early Holocene, both areas underwent aridification, resulting in reductions in hillslope vegetation cover. Increased storm runoff led to fanhead trenching and distal progradation of the alluvial fans. Variations in fan style at that time may relate primarily to base-level conditions resulting from different gradients on the exposed lake shores.

Tectonic and climatic controls on sedimentation during deposition of the Sinakumbe group and Karoo supergroup, in the mid-Zambezi Valley Basin, southern Zambia

Sediments of the Ordovician to Devonian Sinakumbe Group (∼210 m thick) and overlying Upper Carboniferous to Lower Jurassic Karoo Supergroup (∼4.5 km thick) were deposited in the mid-Zambezi Rift Valley Basin, southern Zambia. The Sinakumbe-Karoo succession represents deposition in a extensional fault-controlled basin of half-graben type. The basin-fill succession incorporates two major fining-upward cycles that resulted from major tectonic events, one event beginning with Sinakumbe Group sedimentation, possibly as early as Ordovician times, and the other beginning with Upper Karoo Group sedimentation near the Permo-Triassic boundary. Minor tectonic pulses occurred during deposition of the two major cycles. In the initial fault-controlled half-graben, a basin slope and alluvial fan system (Sikalamba Conglomerate Formation), draining southeastward, was apparently succeeded, without an intervening transitional facies, by a braided river system (Zongwe Sandstone Formation) draining southwestward, parallel to the basin margin. Glaciation followed by deglaciation resulted in glaciofluvial and glacio-lacustrine deposits of the Upper Carboniferous to Lower Permian Siankondobo Sandstone Formation of the Lower Karoo Group, and isostatic rebound eventually produced a broad flood plain on which the coal-bearing Lower Permian Gwembe Coal Formation was deposited. Fault-controlled maximum subsidence is represente by the lacustrine Upper Permian Madumabisa Mudstone Formation. Block-faulting and downwarping, probably due to the Gondwanide Orogeny, culminated with the introduction of large quantities of sediment through braided fluvial systems that overwhelmed and terminated Madumabisa Lake sedimentation, and is now represented by the Triassic Escarpment Grit and Interbedded Sandstone and Mudstone Formations of the Upper Karoo Group. Outpourings of basaltic flows in the Early Jurassic terminated Karoo sedimentation.

Depositional and structural evolution of a foreland basin margin in a magnetostratigraphic framework: the eastern Swiss Molasse Basin

This integrated study of the sedimentology, magnetostratigraphic chronology and petrography of the mostly continental clastics of the Oligocene to Miocene Swiss Molasse Basin underpins a reconstruction of facies architecture and delineates relationships between the depositional evolution of a foreland-basin margin and exhumation phases and orogenic events in the adjacent orogen. A biostratigraphically based high-resolution magnetostratigraphy provides a detailed temporal framework and covers nearly the whole stratigraphic record of the Molasse Basin (31.5–13 Ma). Three transverse alluvial fan systems evolved at the southern basin margin. They are characterized by distinct petrographic compositions and document the exhumation and denudation history of the growing eastern Swiss Alps. Enhanced northward propagation of the orogenic wedge is interpreted to have occurred between 31.5 and 26 Ma. During the period 24–19 Ma, intense in-sequence and out-of-sequence thrusting took place as Molasse strata were accreted to the orogenic wedge. A third active tectonic phase, possibly caused by backthrusting of the Plateau Molasse, probably occurred between ca. 15 and 13 Ma. Fan head migration between 31.5 and 13 Ma is probably controlled by the structural evolution of the thrust front due to Molasse accretion and backthrusting.

Combining Sequence Stratigraphy, Geostatistical Simulations, and Production Data for Modeling a Fluvial Reservoir in the Chaunoy Field (Triassic, France)

Reservoir modeling of the Chaunoy field was performed by combining a sedimentological study, a sequence stratigraphic analysis, geostatistical simulations, and the analysis of production data and fluid-flow simulations. The reservoir corresponds to the distal part of a Middle Triassic alluvial fan system in the Paris basin (France), and is extremely heterogeneous and layered. The reservoir mostly consists of small ribbon channel deposits interbedded with flood-plain and lacustrine mudstones. The channel amalgamation rate varied with cyclic lake-level variations, which directly controlled the reservoir geometry. Within a base-level cycle, during periods of low accommodation, channels were amalgamated, forming highly heterogeneous sand sheets. As the accommodation increased, channels became progressively isolated within flood-plain mudstones. Finally, a lacustrine transgression deposited lacustrine mudstones and induced thin but widespread vertical permeability barriers across the field. As accommodation started to decrease, considerable pedogenetic alteration occurred, as shown by dolocretes and groundwater dolomites. Five cycles that constituted the reservoir layering framework were identified. Geostatistical simulations of lithotype distribution within these units were computed using the truncated random Gaussian function method. Horizontal and vertical lithotype proportion curves and variograms were calculated from well data. Because of the wide well spacing, it was not possible to determine the range of horizontal experimental variograms. Three lithotype realizations were simulated within a high-resolution grid to compare short, medium, and long correlation lengths. After assigning petrophysical properties to the lithotypes and upscaling, fluid-flow simulations were performed for the three realizations. The three flow simulations were then compared to the 10-yr production history of the field. The simulations showed quite a good match regardless of the variogram range, except in the northern part of the field, indicating a problem in the reservoir layering in this area. This relative insensibility of the flow simulation to the correlation length probably is due to the high net pay within the amalgamated channel reservoir units and to the high number of conditioning wells; however, the flow simulation performed with the longest correlation length showed the best fit with the production history.

The Upper Permian Boniches Conglomerates Formation: evolution from alluvial fan to fluvial system environments and accompanying tectonic and climatic controls in the southeast Iberian Ranges, central Spain

The Boniches Conglomerates Fm. (Upper Permian) represents the first episode of sedimentation in the southeast border margin of the Iberian Basin, central Spain. The Iberian Basin commenced development as a rift basin running mainly NW-SE during the Early Permian and underwent its first general extensional period during the Late Permian. The Boniches Fm. was deposited as a small segment (less than 100 km long and 9–10 km wide) of the Iberian Basin that extended in a northeast direction during this period. Detailed mapping and palaeocurrent analysis have shown that the Boniches Fm. is of a curvilinear, wedge-shaped morphology. Sediments close to the southwest main faulted border are of a maximum thickness of 230 m. Clear lateral variation in thickness controlled by the activity of secondary transfer faults perpendicular to the main NW-SE axis may be observed. This network of faults permitted the formation of blocks, limited laterally by the transfer faults. These blocks experienced subsidence and rotation against the elevated southwest footwall block which resulted in the isolation of reduced sub-basins with different sedimentary characteristics. Based on facies and facies association, the Boniches Fm. is thought to have evolved vertically from two main alluvial fan systems in the lowest part of the formation, into fluvial braided systems flowing southeast. As expected, no flow orientations towards the footwall block are observed during the first episodes of the refill. This may be explained by erosion, or simply a lack of sedimentation due to the gentle slope. The change from alluvial fan to fluvial environments is interpreted as a relatively sudden tectonic reorganization of the whole segment favouring new longitudinal fluvial drainage from a transfer zone located to the northwest. Three different reorganization episodes, clearly marked in the Boniches Fm. as ‘main boundary surfaces’ (MBS) separate four members subdivided into 4–5 levels (3–4 m thick). Each level consists of fining-upward successions less than 1 m thick. Tectonics and climatic factors clearly controlled sedimentation. Tectonic activity was probably of greatest importance and was conditioned by the movement of the main NW-SE and NE-SW transfer faults. The role of climate in ancient conglomerate sediments has not been well established. Palaeogeographical, mineralogical, sedimentological and clast analyses suggest that the Boniches Fm. was deposited under humid conditions by running water. The differentiation between ‘proper’ alluvial fans and fluvial environments during the transition period of the Boniches Fm.is not clear since, in many recent alluvial fans, braided fluvial systems are associated with fans.

Late quaternary sediment budget in alluvial-fan–source-basin systems in Japan

Late quaternary sediment budget in alluvial-fan–source-basin systems in Japan

Sistemas deposicionais da Chapada Diamantina Centro-oriental, Bahia

The Mesoproterozoic sedimentary rocks cropping out in the Central-eastern Chapada Diamantina, Bahia, forming a Q-P-C (Quartzite-Pelite-Carbonate)Association, comprise twelve distinct conglomerate, sandstone and carbonate lithofacies. The lithofacies are composed of 1) conglomerates both clast and matrix supported; 2) coarse, medium, fine grained and bimodal, paraiel bedded and trough, tabular and herring bone cross bedded sandstones, 3) graded sandstones and, 4) carbonates, algal laminites and dolarenites. These lithofacies are assembled into alluvial fan, fluvial, desertic, tidal flat, deltaic and marine depositional systems, alternating along the stratigraphic column. The onset of the sedimentation is continental, represented by desertic, alluvial fan and fluvial systems that progressively changes to a deltaic system, typical of a continental margin. A western source area is proposed, based on paleocurrent data. Overlying the deltaic system, occur continental and marine depositional systems with an eastern provenance because of basin tilting, suggested by paleocurrent inversion. The analysis of these systems has allowed the reconstitution of the tectonic evolution of the region, that begins by deposition on a stable cratonic area, followed by the establishment of a foredeep.

Distinctive characteristics of a streamflow-dominated alluvial fan deposit: Sanghori area, Kyongsang Basin (Early Cretaceous), southeastern Korea

In the northwestern part of the non-marine Kyongsang Basin (Early Cretaceous), conglomeratic deposits occur at intervals along the northern basin margin and grade basinward into interbedded sandstone and mudstone. One of these conglomeratic deposits, referred to as the Sanghori deposit, is 150 m thick and characterized by amalgamated sheets of conglomerate and sandstone (facies assemblages 1 and 2) alternating with interbedded sandstone and purple siltstone (facies assemblage 3). Facies assemblage 1 comprises clast-supported, disorganized and organized sheet conglomerates interbedded with massive to stratified sandstones. This assemblage is interpreted as the product of proximal gravel-bed stream deposition. Facies assemblage 2 consists of trough and planar cross-stratified and horizontally stratified sandstones. It represents sandy braided-stream deposits in which sand bars and dunes were common. Sedimentary facies and architecture of the coarse-grained facies assemblages suggest that the gravel-bed and sandy braided streams were wide and shallow, and rapidly aggraded during flash floods. Facies assemblage 3 consists of interbedded purple siltstone and tabular sandstone which show evidence of weak pedogenic modification and bioturbation. These characteristics are interpreted as the result of floodplain processes. Sediments were supplied from the north and dispersed radially, forming a streamflow-dominated alluvial-fan system. The Sanghori alluvial system was most probably fed from a large drainage basin, resulting in the predominance of streamflow deposits and rounded clasts. The alternation of the gravel-bed stream and floodplain deposits was due to autocyclic switching of active fan sectors under the conditions of relatively slow and steady basin subsidence.

Permian (Upper Rotliegend) synsedimentary tectonics, basin development and palaeogeography of the southern North Sea

Abstract Upper Rotliegend sedimentation in the southern North Sea Basin (SNSB) was a response to complex synsedimentary tectonics and cyclical climatic changes. During the early Upper Rotliegend, faulting (?dextral strike-slip) resulted in the formation of a number of isolated sub-basins e.g. the Sole Pit, Silverpit and the Broad Fourteens, which were gradually united as a consequence of the progressive collapse of the Inde-Cleaver Bank High. Basin-margin tectonics controlled the evolution of the basin and the nature and distribution of the various types of alluvial fan and fan delta systems that evolved during pluvial climatic phases. Many of the earlier Upper Rotliegend fans display a systematic lateral displacement along synsedimentary faults. Along the Dowsing Fault Zone, ‘dry-type’ alluvial fans display a progressive northwestward migration with time. Also, fan deltas, sourced from the Texel-Ijsselmeer High, young progressively to the SE along the trend of the main faults. These systematic lateral displacements have been tentatively explained by invoking dextral oblique transfer of the source area relative to the depositional basin, within the context of a strike-slip tectono-sedimentary model. Alternatively, in an extensional model, the distribution pattern could be explained by sourcing the fans from both the footwall scarps and the transfer zones of dip-slip faults. A rapid decline in synsedimentary faulting, and a gradual change to more pluvial climatic conditions, resulted in the development of larger ‘wet type’ fans during the later part of the Upper Rotliegend time interval.

Sedimentology of the upper cretaceous red beds of Angostura Colorada formation in the western sector of the Northpatagonian Massif, Argentina

Detailed sedimentological analysis of the Angostura Colorada Fm. (Upper Cretaceous) in the Comallo Jacobacci Gastre areas of the western sector of the North Patagonian Massif has enabled the definition of three members based on sedimentary structures, paleocurrents, grain size, compositional characteristics and lithofacies associations. These members are: lower (alluvial fan), middle (sandy braidplain) and upper (flood plain) developed within a rift sequence. Compositionally, two alternating source rocks are shown in the geological record, a continental block and recycled orogen provenance for the lower and middle members (Jacobacci area) and magmatic arc provenance for the middle (Gastre area) and upper members. This is interpreted as a progressive magmatism towards the K-T boundary during the rising of the Andean Cordillera to the west. The granitic provenance is attributed to igneous bodies of Paleozoic age to the N and SW of the study area. The smectite-zeolite rich clay mineral association of the upper member is interpreted as product of alteration of thick pyroclastic deposits. The paleogeography of this unit includes two different paleodepositional systems, to the SW an alluvial fan system running from the SW to the NE, to the NE a sandy braided river running from the NW to the SE, perpendicular to the previous one in a wide paleovalley. Later flood plain and lacustrine shaley deposits developed to the east which are equivalent to the Alamitos Fm. This distribution of paleodepositional systems is coherent with the main structural trends within the study area. A late Campanian age is assigned to this unit.

Depositional systems and seismic stratigraphy of a Quaternary basin: north Okanagan Valley, British Columbia

Seismic and core data from north Okanagan Valley, a deep (~700 m of fill), elongate (~3 km wide by 45 km long) basin located in the Cordillera of south-central British Columbia, reveal a thick sequence of fine and coarse elastics. The architecture of the basin fill was delineated using 16 km of high-resolution, reflection seismic profiles, and 30 lithologic logs. Using a depositional systems approach, four systems were identified: subglacial fluvial, glaciolacustrine, alluvial fan, and channel. The subglacial fluvial system consists of a basal suite of compact, stratified to poorly stratified coarse clastics deposited beneath glacial ice under high hydrostatic pressure. Older sediments were almost completely excavated from the basin fill during periods of maximum subglacial flow during Late Wisconsinan glaciation. One of the outcomes of this study is that it lends support to the notion that Late Wisconsinan glaciers were capable of almost totally eroding older Pleistocene basin deposits while depositing thick sequences of subglacial fluvial sediments. During deglaciation, the basin was occupied by a lake in which laminated silt and clay were deposited (glaciolacustrine system). The wedge-shaped alluvial fan system interfingers with the finer clastics of the basin fill. Incised into the upper part of the basin fill are channel sediments forming the channel depositional system. Two stages in the evolution of the north Okanagan basin were identified: in the first stage (~10 ka), lake sediments were rapidly accumulating coeval with the formation of alluvial fans and fan deltas; the second stage shows the present-day architecture of the basin fill.

Genetically related alluvial deposits across active fault zones: an example of alluvial fan-terrace correlation from the upper Quaternary of the southern Po Basin, Italy

Integrated studies of alluvial fan systems and related intramontane valley deposits provide potentially useful tools for understanding the character of alluvial sedimentation and its control in fault-controlled piedmont zones. An example is reported from the uppermost Quaternary alluvial deposits of the northern Apennines and the southern margin of the Po Basin. A distinctive cyclicity characterizes both the outcropping and the buried units in the study area. In intramontane valleys individual cycles consist of regular alternations of sections displaying high and low terrace preservation. In the subsurface, a rhythmic alternation of coarse-grained bodies and laterally extensive pelitic horizons occurs at various scales and represents the basic cyclic motif of the depositional system. The correlation between the terrace sequences and the depositional cycles in the subsurface allows the differentiation of distinct sequences with a characteristic internal architecture. In the lower part of sequences, poorly preserved terrace deposits are laterally correlatable with alluvial fan gravel bodies, whereas highly preserved terrace deposits are correlated laterally with predominantly floodplain sediments in the upper part of sequences. Reliable correlation lines have been constructed using 14C dating. Thrust tectonics and climate exerted a major control on sequence development. Deep valley excavation, with subsequent low terrace preservation, occurred in response to relief rejuvenation during successive thrusting events. On the other hand, decreasing erosion related to periods of minor tectonic activity led to terrace preservation in the intramontane valleys. The coeval episodes of alluvial fan growth and abandonment, recorded at the basin margin, are interpreted to reflect changes in the efficiency of the drainage system, due to variations of tectonic uplift in the adjacent areas. Alternating phases of deglaciation and renewed glaciation, however, are inferred to have played a major role in controlling the amount and type of sediment delivered to the alluvial fan complexes.

Northern Margin of the Gediz Graben: Age and evolution, west Turkey

In Karataş area, four different Plio-Quaternary lithostratigraphic units, up to 226 m total thickness, are exposed. They include,in ascending stratigraphical order, the Lower Pliocene Balçiklidere, Middle-Upper Pliocene Ulubey and Lower-Middle Pleistocene Asartepe formations and (?) Upper Pleistocene- Holocene Kula Volcanics. The Balçıklıdere Formation, which nonconformably overlies the Basement rocks, is made up of stacked fining-upward sequences of conglomerate, sandstone and siltstone,mudstone with calische bands and thought to be deposited on a southerly flowing braid plain. The lacustrinal carbonates of the Ulubey Formation are overlying conformably the braid plain deposits and unconformably the Basement rocks. The Asartepe Formation unconformably overlies the older units and is composed of stacked coarsening and fining-upward sequences of grain/matrix-supported conglomerates, pebbly sandstones and siltstone-mudstones with rare caliche bands which are probably deposited in the proximal to mid-fan and partly distal areas of southwestwards propagating alluvial fan system. The youngest unit is the alkali basaltic lava flows of Kula Volcanics which outpoured, flowed and filled depressions above all older units. Structural elements of the Karataş segment of the Gediz Graben are high-angle extensional faults, namely the Kırdamları fault, Filiztepe fault segments, Karataş fault and Gediz River fault in NW-SE, WNW-ESE, NW-SE and NE-SW directions, respectively. They have controlled the fluvio-lacustrinel depositional settings throughout the tectonostratigraphic evolution of the Karataş segment. Kırdamlari fault, which is the northernmost boundary fault of the Gediz Graben, marks the initiation of subsidence at the northern margin of the Gediz Graben. It is evidenced by the deposition of Balçıklıdere Formation during Early Pliocene. Intracontinental crustal extension in the study area and its neighborhoods is thought to have formed in the Neotectonical evolutionary framework of West Turkey. It might have been caused by both the WSW escapement of the Anatolian Block and northern subduction of the African Plate beneath the West Anatolia to Aegean Sea.

Heavy mineral response to the progradation of an alluvial fan: implications concerning unroofing of source area, chemical weathering and palaeo-relief (Upper Cretaceous Parkstein fan complex, SE Germany)

During the Late Cretaceous, the NE Bavarian Basement was continuously uplifted along the deep-seated “Franconian Line” fault zone. As a consequence of this vertical displacement, a fan sequence was deposited in the adjacent lowlands. In this area Upper Triassic arkoses are unconformably overlain by a lignite-bearing arenaceous series, the Parkstein Formation, representing a meandering to anastomosing fluvial system. These deposits gradually pass upwards into the Red Claystone and Ferruginous Sandstone Member, deposited under strongly oxidizing conditions, as shown by the ubiquitous presence of trivalent Fe compounds and the lack of coal. The succeeding Friedersreuth Formation contains widespread debris flows and was deposited in an alluvial fan system. Alluvial fan progradation ended with the conglomerates of the Hesserberg Formation, which contains boulders as much as 0.5 m in diameter. The pronounced variation in depositional environment and grain-size is (from bottom to top) accompanied by a striking variation in heavy mineral assemblages. Minerals of high chemical and mechanical stability (zircon, tourmaline, rutile) prevail among the detrital heavy minerals of the basal formation, whereas younger formations bear less stable minerals such as members of the amphibole and epidote groups. These stable detrital heavy minerals are well-rounded to rounded. Provenance and palaeo-relief are most decisive for the coexistence of heavy minerals with different stabilities derived from a great variety of source rocks of different ages (Precambrian and Early Palaeozoic gneisses, late Variscan granites, Permo-Carboniferous volcanites, Triassic arkoses). Heavy mineral associations reflect the reverse lithological sequence of the adjacent source area, as a function of the interaction between uplift and erosion along the boundary fault (“unroofing story”). During slow uplift, chemical weathering operative in the peneplained hinterland and on the alluvial plain helped to release the minerals from the parent rocks and to remove labile constituents from the detritus of the clastic fan sediments. With increasing rate of uplift, the intermixing of minerals from the distal basement source and the proximal Permo-Mesozoic platform sedimentary source became less pronounced. Some authigenic minerals (anatase, “leucoxene”, Fe-oxides, Fe-sulphides, Fe-carbonates, Fe-phosphate) in context with the absence of some heavy minerals in the sand fraction that are present in rock fragments (e.g., apatite) help determine inferences about changes in pH values and may help constrain Eh values during fan progradation.

Tectonic controls on Upper Jurassic to Lower Cretaceous reservoir architecture in the Jeanne d’Arc Basin, with some comparisons from the Porcupine and Moray Firth Basins

Abstract Seismic, wireline log and core data from three geographically separate basins in the North Atlantic provide an understanding of the regional dynamics of the basin system, and quantify the sedimentary geometry response to a composite rift episode. The response to the Late Cimmerian rift episode in the Jeanne d’Arc, Porcupine and Moray Firth Basins is broadly comparable, with an initial episode of clastic reservoir facies deposition associated with active growth faulting. This was followed by a mid-rift period where subsidence outstripped sedimentation, resulting in organic-rich source rocks and culminating in a mappable maximum flooding surface. During the final phase of the rift episode, the decrease in subsidence rate resulted in the return of reservoir-quality clastics. The reservoir-scale response to the initial rifting is illustrated in cored sequences from the Jeanne d’Arc Basin. These indicate the development of axial and lateral sandy and conglomeratic braided fluvial and alluvial fan systems, with periods of muddy lacustrine deposition. Pedogenic surfaces offer much potential for detailed correlation and the analysis of sandbody geometry. The onset of anoxic conditions conducive to source rock deposition during the mid-rift phase of Late Cimmerian tectonism, and the preferential development of reservoir facies at the base and top of syn-rift sediment fill, are characteristics of considerable importance to exploration of the North Atlantic borderland rift basins.

High Resolution Sequence Stratigraphy and Reservoir Architecture of Proximal Alluvial Deposits: The Buntsandstein Facies of Central Spain: ABSTRACT

The Buntsandstein facies outcrops along a 12 km long, 150 m thick cuesta near Ayllon (Central Spain). The outcrop study is based on vertical sedimentological sections and continuous photo paneling, and demonstrates the presence of two depositional systems: an alluvial fan system in the lower half of the outcrop, and a straight and braided river system in the upper part of the outcrop. This overall evolution is probably related to base-level fall to base-level rise cycle, in which the reservoir architecture is linked to genetic units stacking pattern: during the base-level fall, the alluvial fan is prograding over sand flat and sandy alluvial plain deposits. Coarse and pebbly proximal sandsheets are interbedded with finer reddish distal deposits. Reservoirs units are laterally continuous, but silty alluvial plain deposits constitute vertical permeability barriers, during base-level stillstand, erosive channels and sandsheets are vertically amalgamated. Reservoirs units are laterally continuous and vertically connected, during the base-level rise, alluvial fan deposits are overlapped by straight river deposits. Reservoirs units are laterally connected but silty argillaceous alluvial plain horizons are preserved, at the end of the base-level rise, braided and straight river deposits are amalgamated. Fully connected, these reservoirs units have a very large lateral extension.more » A lithofacies database is compiled on this outcrop, and variograms, horizontal and vertical proportion curves are completed. Each stage of the base-level cycle is then quantitatively characterized by a specific heterogeneity pattern. The outcrop study will improve the prediction of reservoir extension and architecture in subsurface gas storage of the Paris basin.« less

Evidence of a Precursor in the Neoformation of Palygorskite — New Data by Analytical Electron Microscopy

AbstractThe rocks of the palygorskite deposit at Bercimuel (Segovia, Spain) have been studied by transmission electron microscopy and microanalysis. These rocks correspond to the zone of convergence of two alluvial fan systems that have filled the small basin of the River Riaza and would originally have been composed of illite and quartz silts. Among the accumulations of palygorskite it is possible to observe surrounded particles of micromicas that have undergone dissolution and opening thereby giving rise to disordered illite-smectite mixed-layer clays. This process continued up to the individualization of structural relics formed of units of 1–5 layers. At the same time, the chemical composition was modified with a loss of K and Al and a relative increase in Si and Mg, progressively evolving towards the composition of palygorskite. The paleogeographic position of the deposits, and the climatic conditions (arid environment), appear to be the dominant factors in the neoformation of palygorskite by alteration of the original sediments. The basic mineralogical process could be referred to as ‘early diagenesis’ in the formation of calcretes.

Influence of atmospheric CO2 on the decline of C4 plants during the last deglaciation

CHANGES in atmospheric carbon dioxide concentrations in the past may have caused changes in vegetation type1,2, and it has been suggested2 that the isotopic signature of such vegetation shifts, preserved in palaeosols3, might be used as a proxy for past CO2 variations. But the connection between palaeosol isotopic signatures and atmospheric CO2 concentrations has been difficult to establish, partly because of the unreliability of CO2 proxies and partly because of the difficulty in ruling out other potential causes of vegetation changes, such as climate4. Here we present palaeosol carbon isotope ratios that reveal a shift from C4-dominated grasses to C3-dominated shrubs about 7–9 kyr ago on an alluvial fan system in the Chihuahuan desert, New Mexico. This coincides with a rapid increase in atmospheric CO2 concentration recorded in Antarctic ice cores5–8 and increased aridity recorded by geo-morphic reconstructions9–11 and packrat remains12. Palaeosol oxygen isotope ratios, which depend on temperature and moisture, were relatively constant during the vegetation shift, suggesting that the CO2 change, rather than climate, was the dominant cause. We conclude that the carbon isotope ratios of ancient soils can indeed be used as a proxy for past CO2 changes.

Alluvial fan sedimentation and structure of the southern Molasse Basin margin, Lake Thun area, Switzerland

The Chattian Lower Freshwater Molasse (USM) of the Thun area comprises a thick series of continental sediments which occur in three imbricate thrust sheets (Steffisburg, Zulg-Hombach, Blueme) of the Subalpine Molasse and over wide areas of the autochthonous Plateau Molasse. Facies analysis of outcrops, complemented by conglomerate clast counts, sandstone petrography, heavy mineral analysis and data from two deep exploration wells and seismics permit structural restoration of the thrust sheets, correlation of subalpine and autochthonous USM and consequently the reconstruction of the geometry and facies evolution of the alluvial fan systems. The most complete succession measuring more than 4500 m is preserved in the structurally highest Blueme thrust sheet

Channel migration patterns and related sequences in some alluvial fan systems

The pendular displacement of a channel system, consisting of an area of higher channel density (trunk channel zone) on both sides of which channel density progressively decreases (secondary channel zone), can, under favourable subsidence conditions, lead to the development of alluvial fans. Characteristic sequences are found in these fans, depending on their position in the sedimentary body. A marginal position in the fan is thus recognized by the superposition of fining and thinning upwards (FTU) cycles, the upper part of which is made up of important thicknesses of overbank fines, all the backsets of bars dipping in the same direction (towards the centre of the cone). On the other hand, a central position is characterized by a higher number of FTU cycles, which are incomplete due to erosion of the upper parts (corresponding to the higher concentration of overbank fines), and the backsets in each cycle dip alternately in opposite directions. The displacement of the channel system in a constant direction may be caused by the preferential accumulation of bars on one of the banks of the channel. The change in migratory direction giving rise to the pendular movement is caused by the trunk channel reaching the basin margin or the sedimentation area of an adjoining fan. Palaeogeographic reconstructions of fans using this technique contribute to the analysis of ancient basins: small-radius fans with a high sweep angle (A S) are characteristic of basin margins subjected to a low sediment supply/subsidence ratio (S S /S B), whereas large-radius fans with a low A S characterize periods with a high S S /S B ratio on the basin margin. This model can be applied in economic geology studies, as the location of an ancient cone permits delimitation of the axial strip (with a higher proportion of coarse, highly porous, channelled facies) and the marginal sectors (where thick layers of less porous overbank fines are intercalated). We here present the example of the alluvial fans in the Guadix Fm. (Betic Cordillera, Spain), which lie on a basin margin (ESE) where the S S /S B ratio increased progressively throughout the Lower Pliocene. The detailed analysis of palaeocurrents on the basis of different criteria throughout the Arroyo de Gor section helps to locate more precisely and confirm the palaeogeographical distribution of the Pliocene cone, as deduced from sequential analysis.