Showing papers in "Sedimentology in 2002"

A comprehensive approach to the study of methane-seep deposits from the Lincoln Creek Formation, western Washington State, USA

Abstract: A comprehensive approach using palaeontology, petrography, stable isotope geochemistry and biomarker analyses was applied to the study of seven small methane-seep carbonate deposits. These deposits are in the Oligocene part of the Lincoln Creek Formation, exposed along the Canyon and Satsop Rivers in western Washington. Each deposit preserves invertebrate fossils, many representing typical seep biota. Authigenic carbonates with δ 1 3 C values as low as -51‰ PDB reveal that the carbon is predominately methane derived. Carbonates contain the irregular isoprenoid hydrocarbons 2,6,11,15-tetramethylhexadecane (crocetane) and 2,6,10,15,19-pentamethylicosane (PMI), lipid biomarkers diagnostic for archaea. These lipids are strongly depleted in 1 3 C (δ 1 3 C values as low as -120‰ PDB), indicating that archaea were involved in the anaerobic oxidation of methane. Small filaments preserved in the carbonate may represent methanotrophic archaea. Archaeal methanogenesis induced the formation of a late diagenetic phase, brownish calcite, consisting of dumbbell-shaped crystal aggregates that exhibit δ 1 3 C values as high as +7‰ PDB. Clotted microfabrics of primary origin point to microbial mediation of carbonate precipitation. Downward-directed carbonate aggregation in the seeps produced inverted stromatactoid cavities. Large filaments, interpreted as green algae based on their size, shape, arrangement and biomarkers, imply that deposition occurred, in places, in water no deeper than 210 m.

Avulsions, channel evolution and floodplain sedimentation rates of the anastomosing upper Columbia River, British Columbia, Canada

Abstract: Ages of channels of the anastomosing upper Columbia River, south-eastern British Columbia, Canada, were investigated in a cross-valley transect by C-14 dating of subsurface floodplain organic material from beneath levees. The avulsion history within the transect was deduced from these data, and morphological stages in channel development were recognized. Additionally, floodplain sedimentation rates were established. The new data demonstrate that the upper Columbia River is a long-lived, dynamic anastomosing system. Results show that anastomosis at the study location has persisted since at least 2700 cal. years BP, with avulsions occurring frequently. At least nine channels have formed in the studied cross-valley transect within the past 3000 years. Channel lifetimes from formation to abandonment appear to be highly variable, ranging from approximately 800 to 3000 years. Log jams provoking avulsions and/or silting up of old channels are proposed as reasons for this variability. Long-term average floodplain sedimentation rates appear to be significantly lower than previously proposed by Smith (1983, Int. Assoc. Sedimentol. Spec. Publ., 6, 155-168). A long-term (4550 years) average of 1.75 mm per year (after compaction) was based on C-14 dates, whilea short-term sedimentation rate of 0.8 mm was determined for a single, relatively small, seasonal flood in 1994 using sediment traps. However, short-term sedimentation rates vary considerably over the floodplain, with levees aggrading up to four times faster than floodbasins. Channels of the upper Columbia River anastomosed reach follow a consistent pattern in their development, with each stage being characterized by different morphology and processes. Channel evolution comprises the following succession: (1) avulsion stage, in which a crevasse splay channel deepens by scour and levee sedimentation; (2) widening and deepening stage, in which bank slumping and bed scouring dominates; (3) infilling stage, in which either channel narrowing (bank accretion) or channel shallowing (bed accretion) takes place; and (4) abandonment stage, in which the residual (remnant) channel is filled exclusively by silt, clay and organic material. Vertical stacking (super-imposition) of active channels on recent channel-fill sand bodies is a notable feature of the upper Columbia River, which suggests that reoccupation of residual channels is a common process.

Stratigraphic evolution and preservation of aeolian dune and damp/wet interdune strata: an example from the Triassic Helsby Sandstone Formation, Cheshire Basin, UK

Abstract: New and previously published models of wet aeolian system evolution form a spectrum of types that may be explained in terms of aeolian dune dynamics, rate of water table rise and/or periodicity of interdune flooding This is illustrated with an example from the Mid-Triassic (Anisian) Helsby Sandstone Formation, Cheshire, UK Lenses of damp and wet interdune strata exhibit an intertonguing, transitional relationship with the toe-sets of overlying aeolian dune units This signifies dune migration that was contemporaneous with water table-controlled accumulation in adjacent interdunes Downwind changes in the geometry and facies of the interdune units indicate periodic expansion and contraction of the interdunes in response to changes in the elevation of the groundwater table and episodic flooding, during which accumulation of dune strata continued relatively uninterrupted This contrasts with other models for accumulation in wet aeolian systems where interdune flooding is associated with a cessation in aeolian bedform climbing and the formation of a bypass or erosional supersurface Architectural panels document the detailed stratigraphy in orientations both parallel and perpendicular to aeolian transport direction, enabling a quantitative three-dimensional reconstruction of genetically related aeolian dune and interdune elements Sets of aeolian dune strata are composed of grainflow and translatent wind-ripple strata and are divided by a hierarchy of bounding surfaces originating from oblique migration of superimposed dunes over slipfaceless, sinuous-crested parent bedforms, together with lee-slope reactivation under non-equilibrium flow conditions Silty-mudstone and sandstone interdune units are characterized by wind ripple-, wavy- and subaqueous wave ripple-laminae, desiccation cracks, mud flakes, raindrop imprints, load casts, flutes, intraformational rip-up clasts and vertebrate and invertebrate footprint impressions and trackways These units result from accumulation on a substrate that varied from dry- through damp- to wet-surface conditions Interdune ponds were flooded by either fluvial incursions or rises in groundwater table and were periodically subject to gradual desiccation and reflooding Red silty-mudstone beds of subaqueous origin pass laterally into horizontally laminated wind-ripple beds indicating a progressive transition from wet- through damp- to dry-surface conditions within a single interdune

Identifying causes of ground‐penetrating radar reflections using time‐domain reflectometry and sedimentological analyses

Abstract: Ground-penetrating radar (GPR) is a geophysical technique widely used to study the shallow subsurface and identify various sediment features that reflect electromagnetic waves. However, little is known about the exact cause of GPR reflections because few studies have coupled wave theory to petrophysical data. In this study, a 100- and 200-MHz GPR survey was conducted on aeolian deposits in a quarry. Time-domain reflectometry (TDR) was used to obtain detailed information on the product of relative permittivity (er) and relative magnetic permeability (lr), which mainly controls the GPR contrast parameter in the subsurface. Combining TDR data and lacquer peels from the quarry wall allowed the identification of various relationships between sediment characteristics and erlr. Synthetic radar traces, constructed using the TDR logs and sedimentological data from the lacquer peels, were compared with the actual GPR sections. Numerous peaks in erlr, which are superimposed on a baseline value of 4 for dry sand, are caused by potential GPR reflectors. These increases in erlr coincide with the presence of either organic material, having a higher water content and relative permittivity than the surrounding sediment, or iron oxide bands, enhancing relative magnetic permeability and causing water to stagnate on top of them. Sedimentary structures, as reflected in textural change, only result in possible GPR reflections when the volumetric water content exceeds 0AE055. The synthetic radar traces provide an improved insight into the behaviour of radar waves and show that GPR results may be ambiguous because of multiples and interference.

Spatial variation of overbank aggradation rate and its influence on avulsion frequency

Abstract: River avulsions are commonly considered to be driven by the aggradation and growth of alluvial ridges, and the associated increase in cross-valley slope relative to either the down-channel slope or the down-valley slope (the latter is termed the slope ratio in the present paper). Therefore, spatial patterns of overbank aggradation rate over stratigraphically relevant time scales are critical in avulsion-dominated models of alluvial architecture. Detailed evidence on centennial- to millennial-scale floodplain deposition has, to date, been largely unavailable. New data on such long-term overbank aggradation rates from the Rhine–Meuse and Mississippi deltas demonstrate that the rate of decrease of overbank deposition away from the channel belt is much larger than has been supposed hitherto, and can be similar to observations for single overbank floods. This leads to more rapid growth of alluvial ridges and more rapid increase in slope ratios, potentially resulting in increased avulsion frequencies. A revised input parameter for overbank aggradation rate was used in a three-dimensional model of alluvial architecture to study its effect on avulsion frequency. Realistic patterns of avulsion and interavulsion periods (� 1000 years) were simulated with input data from the Holocene Rhine River, with avulsions occurring when the slope ratio is in the range 3–5. However, caution should be practised with respect to uncritical use of these numbers in different settings. Evidence from the two study areas suggests that the avulsion threshold cannot be represented by one single value, irrespective of whether critical slope ratios are used, as in the present study, or superelevation as has been proposed by other investigators.

Aeolian/fluvial interactions and high-resolution sequence stratigraphy of a non-marine lowstand wedge: The Avilé member of the Agrio Formation (Lower Cretaceous), central Neuquén Basin, Argentina

Abstract: Accumulation within the unconformity-based Hauterivian Avile´ Sandstone oftheNeuque´nBasin,Argentina,wascharacterizedbyacloseinteractionbetweenfluvialandaeolianprocessesdevelopedafteramajorrelativesea-leveldropthatalmostcompletely desiccated the entire basin and juxtaposed these non-marinedeposits on shallow- and deep-marine facies. Aeolian deposits within the Avile´Memberincludedune(A1)andsandsheet(A2)unitsthatcharacterizethelowerpart of the unit. Fluvial deposits comprise distal flood units (F1) interbeddedwith aeolian dune deposits in the middle part of the succession, and low- (F2)and high-sinuosity (F3) channels associated with floodplain deposits (F4)towards the top. The internal characteristics of the aeolian system indicate thatits accumulation was strongly controlled by water-table dynamics, with thedevelopment of multiple horizontal deflation super surfaces that truncate dunedeposits and form the basal boundary of flood deposits and sand sheet units. Along-term wetting-upward trend is recorded throughout the entire unit, with anincrease in fluvial activity towards the top and the development of a morepermanent fluvial system overlying a major erosion surface interpreted as asequence boundary. The upward increase in water-table influence might berelated to relative sea-level rise, which controlled the position of the water tableand allowed the accumulation of tabular aeolian units bounded by horizontaldeflation surfaces. This high-frequency, eustatically driven process actedtogether with a long-term climatic change towards wetter conditions.Keywords Aeolian/fluvial interactions, Argentina, Avile´ Sandstone, Creta-ceous, Neuque´n Basin, water table.INTRODUCTIONSeveral authors have described intertonguingbetween aeolian and fluvial deposits in differentbasins, suggesting that the interaction betweenthese two processes is common in both modernenvironments (Langford, 1989) and the rockrecord (Langford & Chan, 1989; Herries, 1993).TheLowerCretaceous(Hauterivian)Avile´ Sand-stone Member of the Agrio Formation is excep-tionally well exposed in the central part of theNeuque´n Basin. It consists of aeolian and fluvialdeposits intercalated between ammonite-bearingshales of the Lower and Upper Members of theAgrioFormation(Fig. 1).Ithasbeeninterpretedasa lowstand wedge produced by a major relativesea-level drop (Legarreta & Gulisano, 1989).Although the Avile´ Sandstone has been dis-cussed in terms of its depositional environments(Veiga & Vergani, 1993) and aeolian processes

Turbidite depositional architecture across three interconnected deep‐water basins on the north‐west African margin

Abstract: The Moroccan Turbidite System (MTS) on the north-west African margin extends 1500 km from the head of the Agadir Canyon to the Madeira Abyssal Plain, making it one of the longest turbidite systems in the world. The MTS consists of three interconnected deep-water basins, the Seine Abyssal Plain (SAP), the Agadir Basin and the Madeira Abyssal Plain (MAP), connected by a network of distributary channels. Excellent core control has enabled individual turbidites to be correlated between all three basins, giving a detailed insight into the turbidite depositional architecture of a system with multiple source areas and complex morphology. Large-volume (> 100 km3) turbidites, sourced from the Morocco Shelf, show a relatively simple architecture in the Madeira and Seine Abyssal Plains. Sandy bases form distinct lobes or wedges that thin rapidly away from the basin margin and are overlain by ponded basin-wide muds. However, in the Agadir Basin, the turbidite fill is more complex owing to a combination of multiple source areas and large variations in turbidite volume. A single, very large turbidity current (200-300 km3 of sediment) deposited most of its sandy load within the Agadir Basin, but still had sufficient energy to carry most of the mud fraction 500 km further downslope to the MAP. Large turbidity currents (100-150 km3 of sediment) deposit most of their sand and mud fraction within the Agadir Basin, but also transport some of their load westwards to the MAP. Small turbidity currents (< 35 km3 of sediment) are wholly confined within the Agadir Basin, and their deposits pinch out on the basin floor. Turbidity currents flowing beyond the Agadir Basin pass through a large distributary channel system. Individual turbidites correlated across this channel system show major variations in the mineralogy of the sand fraction, whereas the geochemistry and micropalaeontology of the mud fraction remain very similar. This is interpreted as evidence for separation of the flow, with a sand-rich, erosive, basal layer confined within the channel system, overlain by an unconfined layer of suspended mud. Large-volume turbidites within the MTS were deposited at oxygen isotope stage boundaries, during periods of rapid sea-level change and do not appear to be specifically connected to sea-level lowstands or highstands. This contrasts with the classic fan model, which suggests that most turbidites are deposited during lowstands of sea level. In addition, the three largest turbidites on the MAP were deposited during the largest fluctuations in sea level, suggesting a link between the volume of sediment input and the magnitude of sea-level change.

Anatomy of shelf deltas at the edge of a prograding Eocene shelf margin, Spitsbergen

Abstract: Although shelf-edge deltas are well-imaged seismic features of Holocene and Pleistocene shelf margins, documented outcrop analogues of these important sand-prone reservoirs are rare. The facies and stratigraphic architecture of an outcropping shelf-edge delta system in the Eocene Battfjellet Formation, Spitsbergen, is presented here, as well as the implications of this delta system for the generation of sand-prone, shelf-margin clinoforms. The shelf-edge deltas of the Battfjellet Formation on Litledalsfjellet and Hogsnyta produced a 3–5 · 15 km, shelf edge-attached, slope apron (70 m of sandstones proximally, tapering to zero on the lower slope). The slope apron consists of distributary channel and mouth-bar deposits in its shelf-edge reaches, passing downslope to slope channels/chutes that fed turbiditic lobes and spillover sheets. In the transgressive phase of the slope apron, estuaries developed at the shelf edge, and these also produced minor lobes on the slope. The short-headed mountainous rivers that drained the adjacent orogenic belt and fed the narrow shelf, and the shelf-edge position of the discharging deltas, made an appropriate setting for the generation of hyperpycnal turbidity currents on the slope of the shelf margin. The abundance of organic matter and of coal fragments in the slope turbidites is consistent with this notion. Evidence that many of the slope turbidites were generated by sustained turbidity currents that waxed then waned includes the presence of scour surfaces and thick intervals of plane-parallel laminae within turbidite beds in the slope channels, and thick spillover lobes with repetitive alternations of massive and flatlaminated intervals. The examined shelf-edge to slope system, now preserved mainly below the shelf break and dominated by sediment gravity-flow deposits, has a threefold stratigraphic architecture: a lower, progradational part, in which the clinoforms have a slight downward-directed trajectory; a thin aggradational zone; and an upper part in which clinoforms backstep up onto the shelf edge. A greatly increased density of erosional channels and chutes marks the regressive-to-transgressive turnaround within the slope apron, and this zone becomes an angular unconformity up near the shelf edge. This unconformity, with both subaerial and subaqueous components, is interpreted as a sequence boundary and developed by vigorous sand delivery and bypass across the shelf edge during the time interval of falling relative sea level. The studied shelf-margin clinoforms accreted mostly during falling stage (sea level below the shelf edge), but the outer shelf later became estuarine as sea level became re-established above the shelf edge.

Ecological succession, palaeoenvironmental change, and depositional sequences of Barremian–Aptian shallow‐water carbonates in northern Oman

Abstract: Barremian and Aptian shallow-water carbonate facies (uppermost Lekhwair, Kharaib and Shuaiba Formations) are described from outcrops in northern Oman Based on facies analysis and bedding pattern, three orders of depositional sequences are defined (third to fifth order) and correlated between sections Over the course of three third-order sequences, covering the Barremian to Lower Aptian, a third-order depositional pattern is documented that consists of a succession of three distinct faunal assemblages: discoidal orbitolinids and calcareous algae were deposited during early transgression; microbialites and microencrusters dominate the late transgressive to early highstand facies; and a rudist- and miliolid-dominated facies is typical of the highstand This ecological succession was controlled largely by palaeoenvironmental changes, such as trophic level and clay influx, rather than sedimentological factors controlled by variations in accommodation space Orbitolinid beds and carbonates formed by microbialites and microencrusters seem to be the shallow-water carbonate response to global changes affecting Late Barremian to Aptian palaeoclimate and palaeoceanography

Preservation of aeolian genetic units by lava flows in the Lower Cretaceous of the Paraná Basin, southern Brazil

Abstract: The Lower Cretaceous geological record of the intracratonic Parana Basin in southern Brazil comprises a thick succession of aeolian sandstones and volcanic rocks. The intercalation between aeolian sandstone and volcanic floods allowed the preservation of distinct aeolian genetic units. Each genetic unit represents an accumulation episode, bounded by supersurfaces, that coincides with the base of lava flood events. The entire package can be subdivided into a Lower Genetic Unit, which corresponds to aeolian sandstones preserved below the initial lava flows (Botucatu Formation), and an upper set of genetic units, which comprises interlayered aeolian deposits and lava floods (Serra Geral Formation). The Lower Genetic Unit is up to 100 m thick. Its base is composed of ephemeral stream and aeolian sand sheet deposits that are overlain by cross-bedded sandstones whose origin is ascribed to simple, locally composite, crescentic and complex linear aeolian dunes. Aeolian accumulation of the lower unit was possible as a result of the existence of a wide topographic basin, which caused wind deceleration, and a large sand availability that promoted a positive net sediment flux. The Upper Genetic Units comprise isolated sand bodies that occur in two different styles: (1) thin lenses (<3 m thick) formed by aeolian sand sheets; and (2) thick sand lenses (3–15 m) comprising cross-bedded cosets generated by migration and climbing of simple to locally composite crescentic aeolian dunes. Accumulation of the aeolian strata was associated with wind deceleration within depressions on the irregular upper surface of the lava floods. The interruption of sedimentation in the Lower and Upper Genetic Units, and related development of supersurfaces, occurred as a result of widespread effusions of basaltic lava. Preservation of both wind-rippled topset deposits of the aeolian dunes and pahoehoe lava imprints indicates that lava floods covered active aeolian dunes and, hence, protected the aeolian deposits from erosion, thus preserving the genetic units.

Preservation of cross‐strata due to the migration of subaqueous dunes: an experimental investigation

Abstract: This experimental investigation examined the controls on the geometry of cross-sets formed by subaqueous dunes. A range of steady, unidirectional flow conditions spanning the field of dune existence was investigated, andaggradation rate ranged from 0 mm s - 1 to 0.014 mm s - 1 . Data from an ultrasonic depth profiler consist of high-resolution temporal and spatial series of bed profiles from which dune height and length, migration rate and the depth of trough scour were measured. Cross-set thickness and length were measured from sediment peels. The size and shape of dunes from an equilibrium assemblage change continuously. Individual dunes commonly increase in height by trough scouring and, occasionally, by being caught-up by the upstream dune. Both types of behaviour occur suddenly and irregularly in time and, hence, do not appear to depend on dunes further upstream. However, dune climbing or flattening is a typical response of dunes that disappear under the influence of the upstream dune. All types of behaviour occur at any flow velocity or aggradation rate. Successive dune-trough trajectories, defined by dunes showing various behaviours, affect the geometry of the preserved cross-sets. Mean cross-set thickness/mean dune height averages 0.33 [′0.7), and mean cross-set length/mean dune length averages 0.49 [′0.08), and both show no systematic variation with aggradation rate or flow velocity. Mean cross-set thickness/mean cross-set length tends to decrease with increasing flow velocity and Froude number, therefore allowing a qualitative estimation of flow conditions. Quantitative analysis of the temporal changes in the geometry and migration rate of individual dunes allows the development of a two-dimensional stochastic model of dune migration and formation of cross-sets. Computer realizations produced stacks of cross-sets of comparable shape and thickness to laboratory flume observations, indicating a good empirical understanding of the variability of dune-trough trajectories. However, interactions among dunes and aggradation rates of the order of 10 - 2 mm s - 1 should be considered in future improved models.

Dolomitization of the Capitan Formation forereef facies (Permian, west Texas and New Mexico): seepage reflux revisited

Abstract: Interpretation of seepage reflux dolomitization is commonly restricted to intervals containing evaporites even though several workers have modelled reflux of mesosaline brines This study looked at the partially dolomitized forereef facies of the Capitan Formation to test the extent of reflux dolomitization and evaluate the possible role of the near-backreef mesosaline carbonate lagoon as an alternative source of dolomitizing fluids The Capitan Formation forereef facies ranges from 10% to 90% dolomite Most of the dolomite is fabric preserving and formed during early burial after marine cementation, before and/or during evaporite cementation and before stylolitization Within the forereef facies, dolomite follows depositional units, with debris-flow and grain-flow deposits the most dolomitized and turbidity-current deposits the least The amount of dolomite increases with stratigraphic age and decreases downslope Within the reef facies, dolomite is restricted to haloes around fractures and primary cavities except where the reef facies lacks marine cements and, in contrast, is completely dolomitized This dolomite distribution supports dolomitization by sinking fluids Oxygen isotopic values for fabric-preserving dolomite (δ 1 8 O = 09 ′ 10‰, N = 101) support dolomitization by sea water to isotopically enriched sea water These values are closer to the near-backreef dolomite (δ 1 8 O = 21 ′ 07‰, N = 48) than the hypersaline backreef dolomite (δ 1 8 O = 36 ′ 09‰, N = 11) Therefore, the fabric-preserving dolomite is consistent with dolomitization during seepage reflux of mainly mesosaline brines derived from the near-backreef carbonate lagoon The occurrence of mesosaline brine reflux in the Capitan Formation has important implications for dolomitization in forereef facies and elsewhere First, any area with a restricted carbonate lagoon may be dolomitized by refluxing brines even if there are no evaporite facies present Secondly, such brines may travel significant distances vertically provided permeable pathways (such as fractures) are present Therefore, the absence of immediately overlying evaporite or restricted facies is not sufficient cause to eliminate reflux dolomitization from consideration

Slump-initiated and controlled syndepositional sandstone remobilization: an example from the Namurian of County Clare, Ireland

Abstract: Remobilization of sandstones can dramatically reconfigure original depositional geometries and results in very unusually shaped sandstones, which resemble little, if any, of the original geometry. A number of deformational sandstone bodies, dykes and volcanoes from the upper part of the Carboniferous Ross Formation are described, which offer the opportunity to examine a suite of field-scale reconfigured sandstones. These structures are located in close proximity to the Ross Slide, which outcrops along a 2-km section on the northern coast of the Loop Head Peninsula, County Clare, Ireland. Dome- and ridge-shaped deformational sandstone bodies, dykes and volcanoes are interpreted to be the product of remobilization of a turbiditic sandstone. Liquification and remobilization were triggered by translation, cessation and loading of the underlying turbiditic sandstone by the Ross Slide. Deformational sand body, dyke and volcano development occurred in an asynchronous fashion with deformational sand bodies formed during slump translation. Sand dykes and volcanoes developed after the cessation of slump movement. During slump translation, the minimum principal stress (σ 3 ) was orientated vertically and the slump behaved in a 'ductile' manner. After slump arrest, the minimum principal stress was oriented horizontally, and the unit regained shear strength to behave in a 'brittle' manner. The relative change in rheological states with changing applied shear stress is indicative of thixotropic-like behaviour within the slump mass. Ridge-shaped deformational sand bodies are aligned parallel to slump folds, and their morphology is inferred to be controlled by compressional slump deformation associated with heterogeneous cessation of slump movement that was initiated by frontal arrest of the translating mass.

Siliciclastic and volcaniclastic influences on equatorial carbonates: insights from the Neogene of Indonesia

Abstract: In active tectonic areas of humid equatorial regions, nearshore shallow-water environments are commonly sites of near-continuous siliciclastic influx and/or punctuated volcaniclastic input. Despite significant clastic influence, Neogene carbonates developed in SE Asia adjacent to major deltas or volcanic arcs, and are comparable with modern mixed carbonate–clastic deposits in the region. Research into delta-front patch reefs from Borneo and fore-arc carbonate platform development from Java is described and used to evaluate the effects of siliciclastic and volcaniclastic influx on regional carbonate sedimentation, local changes in carbonate-producing biota and sequence development. Regional carbonate development in areas of high siliciclastic or volcaniclastic input was influenced by the presence of antecedent highs, changes in the amounts or rates of clastic input, delta lobe switching or variations in volcanic activity, energy regimes and relative sea-level change. A variety of carbonate-producing organisms, including larger benthic foraminifera, some corals, coralline algae, echinoderms and molluscs could tolerate near-continuous siliciclastic or volcaniclastic influx approximately equal to their own production rates. These organisms adopted various ‘strategies’ for coping with clastic input, including a degree of mobility, morphologies adapted to unstable substrate inhabitation or shedding sediment, and shapes adapted to low light levels. Local carbonate production was also affected by energy regime, clastic grain sizes and associated nutrient input. Clastic input influenced the inhabitable depth range for photoautotrophs, the zonation of light-dependent assemblages and the morphology and sequence development of mixed carbonate–clastic successions. This study provides data on the dynamic interactions between carbonate and non-carbonate clastic sediments and, when combined with information from comparable modern environments, allows a better understanding of the effects of siliciclastic and volcaniclastic influx on carbonate production.

Electrical resistivity ground imaging (ERGI): a new tool for mapping the lithology and geometry of channel-belts and valley-fills

Abstract: Efforts to map the lithology and geometry of sand and gravel channel-belts and valley-fills are limited by an inability to easily obtain information about the shallow subsurface. Until recently, boreholes were the only method available to obtain this information; however, borehole programmes are costly, time consuming and always leave in doubt the stratigraphic connection between and beyond the boreholes. Although standard shallow geophysical techniques such as ground-penetrating radar (GPR) and shallow seismic can rapidly obtain subsurface data with high horizontal resolution, they only function well under select conditions. Electrical resistivity ground imaging (ERGI) is a recently developed shallow geophysical technique that rapidly produces highresolution profiles of the shallow subsurface under most field conditions. ERGI uses measurements of the ground’s resistance to an electrical current to develop a two-dimensional model of the shallow subsurface (<200 m) called an ERGI profile. ERGI measurements work equally well in resistive sediments

Sub-wavebase cross-bedded grainstones on a distally steepened carbonate ramp, Upper Miocene, Menorca, Spain

Abstract: Cross-bedded grainstones on carbonate ramps and shelves are commonly related to the locus of major wave energy absorption such as shorelines, shoals or shelf breaks. In contrast, on the Early Tortonian carbonate platform of Menorca (Balearic Islands), coarse-grained, cross-bedded grainstones are found at a distance from the palaeoshoreline where they were deposited below the wavebase. Excellent exposures along continuous outcrops on the sea cliffs of Menorca reveal the depositional profile and three-dimensional distribution of the different facies belts of the Tortonian ramp depositional system. Basinward from the palaeoshoreline, fan deltas and beach deposits pass into 5-km-wide gently dipping bioturbated dolopackstone (inner and middle ramp), then into 12-20°-dipping dolograinstone/rudstone clinobeds (ramp slope) and, finally, into subhorizontal fine-grained basinal dolowackestone to dolopackstone (outer ramp). In this Miocene example, coarse-grained grainstones exist in five different settings other than beach deposits: (1) on the middle ramp, where cross-bedded grainstones were deposited by currents roughly parallel to the shoreline at 40-70 m estimated water depth and are interbedded with gently dipping bioturbated dolomitized packstones; (2) on the upper slope, where clinobeds are composed mostly of in situ rhodoliths and red-algae fragments; (3) on the lower slope, as small-scale bedforms (small three-dimensional subaqueous dunes) migrating parallel to the slope; (4) at the transition between the lower slope and the outer ramp, where mollusc-rich and rhodolithic rudstones and grainstones, interbedded in dolomitized laminated wackestones containing abundant planktonic foraminifera, infill slide/slump scars as upslope-backstepping bodies (backsets); (5) at the toe of the slope, where coarse skeletal grainstones indicate bedform migration parallel to the platform margin, induced by currents at more than 150 m estimated water depth. This Late Miocene example also illustrates how changes in intrabasinal environmental conditions (nutrients and/or temperature) may produce changes in stratal patterns and facies architecture if they affect the biological system. Two depositional sequences compose the Miocene platform on Menorca, where a reef-rimmed platform prograded onto an earlier distally steepened ramp. The transition from the ramp to the reef-rimmed platform was effected by an increase in accommodation space caused by ecological changes, promoting a shift from a grain- to a framework-producing biota.

Quantitative analysis of variations in depositional sequence thickness from submarine channel levees

Abstract: Thickness variations across-levee and downchannel in acoustically defined depositional sequences from six submarine channel-levee systems show consistent and quantifiable patterns. The thickness of depositional sequences perpendicular to the channel trend, i.e. across the levee, decreases exponentially, as characterized by a spatial decay constant, k. Similarly, the thickness of sediment at the levee crest decreases exponentially down the upper reaches of submarine channels and can be characterized by a second spatial decay constant, λ. The inverse of these decay constants has units of length and defines depositional length scales such that k−1 is a measure of levee width and λ−1 is a measure of levee length. Quantification of levee architecture in this way allowed investigation of relationships between levee architecture and channel dimensions. It was found that these measures of levee e-folding width and levee e-folding length are directly related to channel width and relief. The dimensions of channels and levees are thus intimately related, thereby limiting the range of potential channel-levee morphologies, regardless of allocyclic forcing. A simple sediment budget model relates the product of the levee e-folding width and e-folding length to through-channel volume discharge. A classification system based on the quantitative downchannel behaviour of levee architecture allows identification of a ‘mid-channel’ reach, where sediment is passively transferred from the through-channel flow to the levees as an overspilling flow. Downstream from this reach, the channel gradually looses its control on guiding turbidity currents, and the resulting flow can be considered as an unconfined or spreading flow.

Palaeoenvironmental significance of rounded pyrite in siliciclastic sequences of the Late Archaean Witwatersrand Basin: oxygen‐deficient atmosphere or hydrothermal alteration?

Abstract: Petrographic and sulphur isotope studies support the long-held contention that rounded grains of pyrite in siliciclastic sequences of the Late Archaean Witwatersrand Supergroup originated as placer grains. The grains areconcentrated at sites where detrital heavy minerals are abundant within quartz-pebble conglomerates and quartzose sandstones. Depositional sites with abundant pyrite are: (1) within the matrix of bar-type, clast-supported conglomerates; (2) on scoured or winnowed surfaces; and (3) on stratification planes. The grains are internally compact or porous, with truncation of internal structure at outer margins indicating fragmentation and rounding of pyritic source-rocks during erosion and sediment transport. A large range in textures reflects source-rock lithologies, with known varieties linked to sedimentary-hosted diagenetic pyrite, volcanic-hosted massive sulphide deposits and hydrothermal pyrite. Laser ablation sulphur isotope analysis of pyrite reveals a broader range in δ 3 4 S values (- 5.3 to +6.7‰) than that of previously reported conventional bulk-grain analyses (- 1 to +4‰). Rounded pyrite from the Steyn Reef has significant variation in δ 3 4 S values (- 4.7 to +6.7‰) that establishes heterogeneous sulphur compositions, with even adjacent grains having diverse isotopic signatures. The heterogeneity supports a placer origin for rounded pyrite. Euhedral pyrite and pyrite overgrowths which are undoubtedly authigenic have restricted δ 3 4 S values (- 0.5 to + 2.5‰), are chemically distinct from rounded pyrite and are probably the products of metamorphism or hydrothermal alteration. The placer origin of rounded pyrite indicates that pyrite was a stable heavy mineral during erosion and transport in the early atmosphere. Its distribution in three sequences (Witwatersrand Supergroup, Ventersdorp Contact Reef and Black Reef), and in other sequences not linked to Witwatersrand-type Au-U ore deposits, implies deposition of redox-sensitive detrital heavy minerals during the Late Archaean. Consequently, rounded grains of detrital pyrite are strong indicators of an oxygen-poor atmosphere. While not confirming a placer origin for gold in Witwatersrand Au-U ore deposits, the palaeoenvironmental significance of rounded pyrite negates its link to hydrothermal mineralization.

Internal structure of mixed‐sand‐and‐gravel beach deposits revealed using ground‐penetrating radar

Abstract: Mixed-sand-and-gravel beaches are a distinctive type of coarse-clastic beach. Ground-penetrating radar (GPR) and photographic records of previous excavations are used to investigate the stratigraphy and internal sedimentary structure of mixed-beach deposits at Aldeburgh in Suffolk, south-east England. The principles of radar stratigraphy are used to describe and interpret migrated radar reflection profiles obtained from the study site. The application of radar stratigraphy allows the delineation of both bounding surfaces (radar surfaces) and the intervening beds or bed sets (radar facies). The deposits of the main backshore berm ridge consist of seaward-dipping bounding surfaces that are gently onlapped by seaward-dipping bed sets. Good correspondence is observed between a sequence of beach profiles, which record development of the berm ridge on the backshore, and the berm ridge's internal structure. The beach-profile data also indicate that backshore berm ridges at Aldeburgh owe their origin to discrete depositional episodes related to storm-wave activity. Beach-ridge plain deposits at the study site consist of a complex, progradational sequence of foreshore, berm-ridge, overtop and overwash deposits. Relict berm-ridge deposits, separated by seaward-dipping bounding surfaces, form the main depositional element beneath the beach-ridge plain. However, the beach ridges themselves are formed predominantly of vertically stacked overtop/overwash units, which lie above the berm-ridge deposits. Consequently, beach-ridge development in this progradational, mixed-beach setting must have occurred when conditions favoured overtopping and overwashing of the upper beachface. Interannual to decadal variations in wave climate, antecedent beach morphology, shoreline progradation rate and sea level are identified as the likely controlling factors in the development of such suitable conditions.

Recent environmental changes in Laguna Mar Chiquita (central Argentina): a sedimentary model for a highly variable saline lake

Abstract: Laguna Mar Chiquita, a highly variable closed saline lake located in the Pampean plains of central Argentina, is presently the largest saline lake in South America ( 6000 km 2 ). Recent variations in its hydrological budget have produced dry and wet intervals that resulted in distinctive lake level fluctuations. Results of a multiproxy study of a set of sedimentary cores indicate that the system has clearly recorded these hydrological variations from the end of the Little Ice Age ( AD 1770) to the present. Sedimentological and geochemical data combined with a robust chronology based on 2 1 0 Pb profiles and historical data provide the framework for a sedimentary model of a lacustrine basin with highly variable water depth and salinity. Lake level drops and concurrent increases in salinity promoted the development of gypsum-calcite-halite layers and a marked decrease in primary productivity. The deposits of these dry stages are evaporite-bearing sediments with a low organic matter content. Conversely, highstands are recorded as diatomaceous organic matter-rich muds. Average bulk sediment accumulation rose from 0.22 g cm - 2 year - 1 in lowstands to 0.32 g cm - 2 year - 1 during highstands. These results show that Laguna Mar Chiquita is a good sensor of high- and low-frequency changes in the recent hydrological budget and, therefore, document climatic changes at middle latitudes in south-eastern South America. Dry conditions were mostly dominant until the last quarter of the twentieth century, when a humid interval without precedent during the last 240 years of the lake's recorded history started. Thus, it is an ideal system to model sedimentary and geochemical response to environmental changes in a saline lacustrine basin.

Facies architecture of a submarine fan channel–levee complex: the Juniper Ridge Conglomerate, Coalinga, California

Abstract: The Upper Cretaceous Juniper Ridge Conglomerate (JRC) near Coalinga, California, provides a rare, high-quality exposure of a submarine channel to overbank transition. The facies architecture of the JRC comprises a thick, predominantly mudstone sequence overlain by a channellized conglomerate package. Conglomeratic bounding surfaces truncate successions of interbedded turbiditic sandstones and mudstones both vertically and laterally. Thick-bedded, massive sandstones are interbedded with conglomerates. Facies architecture, palaeocurrent indicators, slump features, sandstone percentages and sandstone bed thickness trends lead to the interpretation that these elements comprise channel and overbank facies. A vertical sequence with conglomerate at the base, followed by thick-bedded sandstone, and capped by interbedded turbiditic sandstone and mudstone form a fining-upward lithofacies association that is interpreted as a single channel-fill/overbank system. Three similar lithofacies associations can be related to autocyclic processes of thalweg migration and submarine fan aggradation or to allocyclically driven changes in sediment calibre.

The Sis palaeovalley: a record of proximal fluvial sedimentation and drainage basin development in response to Pyrenean mountain building

Abstract: The Sis conglomerate body represents the Middle Eocene to Oligocene transfer-zone trunk palaeovalley fill of the Sis fluvial system, a drainage system established within the Pyrenees during Late Palaeocene times. The spatial stability of the fluvial transfer zone (active for at least 38 My), and hence the longevity of its aggradational palaeovalley component (>19·5 My), was controlled by its location between long-lived pre-existing structures. Coarse-grained fluvial facies dominate the palaeovalley fill, with alluvial fan facies shed from its defining marginal structures. The detailed sedimentology of very proximal fluvial facies deposited within the dominantly erosional realm of an active mountain belt has rarely been documented before because of their poor preservation potential. The Sis conglomerate body contains a robust internal stratigraphy with stratigraphic units defined by distinct bounding surfaces, across which there are pronounced changes in facies and provenance. These mark the reorganization of the headward portions of the Sis fluvial system during the evolution of the Pyrenean Axial Zone antiformal stack. Major changes in discharge resulted, demonstrating the highly variable nature of even mountain belt-scale fluvial systems when viewed on timescales of several to tens of millions of years. Provenance details indicate that initial unroofing of Hercynian granitoids, situated within the Pyrenean Axial Zone, occurred around 54·5 Ma (early Ypresian) immediately before the first significant exhumation event within the drainage basin of the Sis fluvial system. This is earlier than previously constrained by apatite fission track studies. Rock uplift accelerated in the Lutetian and Bartonian with the initial aggradation of the palaeovalley fill (the Cajigar and Cornudella Formations and Sis One and Two Members). This became marked in the Priabonian (Sis Three and Four Members), with significant activity on local structures including the Morreres backthrust. An increase in basement-derived clasts and a headwater decapitation event also indicate pronounced Axial Zone antiformal stack development at this time. Axial Zone development intensified further in the Oligocene with the deposition of the Collegats Formation and the switch in the main depositional loci of the system from the Tremp-Graus thrust-sheet-top basin to the Ebro Basin to the south.

The Middle–Late Jurassic forced regression and disconformity in central Portugal: eustatic, tectonic and climatic effects on a carbonate ramp system

Abstract: Successions across the Middle-Upper Jurassic disconformity in the Lusitanian Basin (west-central Portugal) are highly varied, and were probably developed on a large westward-inclined hangingwall of a half-graben. The disconformity is preceded by a complex forced regression showing marked variations down the ramp, and provides an example of the effects of rapid, relative sea-level falls on carbonate ramp systems. In the east, Middle Jurassic inner ramp carbonates ('Candeeiros' facies) are capped by a palaeokarstic surface veneered by ferruginous clays or thick calcretes. In the west, mid-outer ramp marls and limestones ('Brenha' facies) are terminated by two contrasting successions: (1) a sharp-based carbonate sandbody capped by a minor erosion surface, overlain by interbedded marine-lagoonal-deltaic deposits with further minor erosion/ exposure surfaces; (2) a brachiopod-rich limestone with a minor irregular surface, overlain by marls, lignitic marls with marine and reworked non-marine fossils and charophytic limestones, with further minor irregular surfaces and capped by a higher relief ferruginous erosional surface. The age ranges from Late Bathonian in the east to Late Callovian in the west. This disconformity assemblage is succeeded by widespread lacustrine-lagoonal limestones with microbial laminites and evaporites ('Cabacos' facies), attributed to the Middle Oxfordian. Over the whole basin, increasingly marine facies were deposited afterwards. In Middle Jurassic inner-ramp zones in the east, the overall regression is marked by a major exposure surface overlain by continental sediments. In Middle Jurassic outer-ramp zones to the west, the regression is represented initially by open-marine successions followed by either a sharp marine erosion surface overlain by a complex sandbody or minor discontinuities and marginal-marine deposits, in both cases capped by the major lowstand surface. Reflooding led to a complex pattern of depositional conditions throughout the basin, from freshwater and brackish lagoonal to marginal- and shallow-marine settings. Additional complications were produced by possible tilting of the hangingwall of the half-graben, the input of siliciclastics from westerly sources and climate change from humid to more seasonally semi-arid conditions. The Middle-Late Jurassic sea-level fall in the Lusitanian Basin is also recorded elsewhere within the Iberian and other peri-Atlantic regions and matches a transgressive to regressive change in eustatic sea-level curves, indicating that it is related in part to a global event.

The importance of breaching as a mechanism of subaqueous slope failure in fine sand

Abstract: Large bank failures, comprising up to 10 6 m 3 of sediment, are common features along steep channel banks in estuaries and large rivers that consist of clean, fine sands, and are mostly assumed to be generated by sudden liquefaction of large masses of very loosely packed sand. Another less commonly recognized type of failure is manifested by the gradual retrogression of a very steep wall, steeper than the angle-of-repose. Instead of the voluminous surging plastic sediment-water flow, or hyperconcentrated density flow (sensu Mulder & Alexander, 2001) generated by liquefaction, this type of failure, known as breaching by dredging companies and hydraulic engineers, produces a sustained quasi-steady, turbidity current. To date, sedimentologists have not recognized the process of breaching as such. In this paper, it is suggested that breaching may be the origin of many thick, massive sand layers known from ancient deposits from various environments, notably in some turbidite successions. Possible differences in the sedimentary structure of the deposits produced by breach failures vs. liquefaction slope failures ( = liquefaction flow slides) can be deduced from a knowledge of the sediment transport processes initiated by the failure. A field study is presented on some poorly structured beds in the Eocene Vlierzele Sands in Belgium, which are supposed to have originated from liquefaction failures, but are reinterpreted to be the products of breaching. It is postulated that the local steep slope disturbance required to initiate an active breach can be produced by a small liquefaction slope failure ( = liquefaction flow slide failure) or local erosion by river or tidal channel flow at the initial stage of the failure event.

Tertiary syntectonic carbonate platform development in Indonesia

Abstract: Cenozoic tropical carbonate sedimentation was strongly influenced by local and regional tectonics in SE Asia. This paper outlines the evolution of the syntectonic Eocene to middle Miocene Tonasa Formation of South Sulawesi, evaluating controls on sedimentation, facies distribution and sequence development. Development of a facies model for this Cenozoic tropical carbonate platform provides a meaningful analogue for similar, less well-studied SE Asian carbonates, which commonly comprise targets for hydrocarbon exploration. This study also has considerable implications for the study of syntectonic carbonates, controls on carbonate sedimentation, carbonate platform development in backarc areas and SE Asian tectonics. Detailed facies mapping, logging, petrographic and biostratigraphic analyses indicate that the Tonasa Formation was deposited initially as part of a transgressive sequence in a backarc setting. By late Eocene times, shallow-water carbonates were being deposited over much of South Sulawesi forming a widespread (100-km long) platform area. Shallow-water sedimentation continued unabated in some areas of the platform until the middle Miocene. Elsewhere, active normal faulting resulted in fault-block platforms, with local subaerial exposure of footwall blocks and the formation of basinal graben in adjacent hangingwall areas. Platform-top facies were aggradational and dominated by larger benthic foraminifera. Low-angle slopes, particularly hangingwall dip slopes, were characterized by the development of ramps. Faults, controlled in part by pre-existing structures, were periodically active and formed steep escarpment margins. Variable regional subsidence strongly influenced the development of the Tonasa Carbonate Platform, whereas platform-wide effects caused by regional eustacy have not been identified. Computer modelling of the Tonasa Platform confirms that the accommodation space and sedimentary geometries observed can be produced by block faulting and regional subsidence alone. Modelling also reveals that regional subsidence and extension, oblique to the main stretching direction, were low on the margins of the backarc basin. Shallow-water accumulation rates for this foraminifera-dominated tropical carbonate platform were an order of magnitude lower than those for modern warm-water platforms dominated by corals or ooids.

Flow structure of turbidity currents

Abstract: A two-dimensional numerical model is used to describe the flow structure of turbidity currents in a vertical plane. To test the accuracy of the model, it is applied to historical flows in Bute Inlet and the Grand Banks flow. The two-dimensional spatial and temporal distributions of velocity and sediment concentration and non-dimensionalized vertical profiles of velocity, turbulent kinetic energy and sediment concentration are discussed for several simple computational currents. The flows show a clear interaction between velocity, turbulence and sediment distribution. The results of the numerical tests show that flows with fine-grained sediment have low vertical and high horizontal gradients of velocity and sediment concentration, show little increase in flow thickness and decelerate slowly. Steadiness and uniformity in these flows are comparable for velocity and concentration. In contrast, flows with coarse-grained sediment have high vertical and low horizontal velocity gradients and high horizontal concentration gradients. These flows grow considerably in thickness and decelerate rapidly. Steadiness and uniformity in flows with coarse-grained sediment are different for velocity and concentration. The results show the influence of spatial and temporal flow structure on flow duration and sediment transport.

On the frequency distribution of turbidite thickness

Abstract: The frequency distribution of turbidite thickness records information on flow hydrodynamics, initial sediment volumes and source migration and is an important component of petroleum reservoir models. However, the nature of this thickness distribution is currently uncertain, with log-normal or negative-exponential frequency distributions and power-law cumulative frequency distributions having been proposed by different authors. A detailed analysis of the Miocene Marnoso Arenacea Formation of the Italian Apennines shows that turbidite bed thickness and sand-interval thickness within each bed have a frequency distribution comprising the sum of a series of log-normal frequency distributions. These strata were deposited predominantly in a basin-plain setting, and bed amalgamation is relatively rare. Beds or sand intervals truncated by erosion were excluded from this analysis. Each log-normal frequency distribution characterizes bed or sand-interval thickness for a given basal grain-size or basal Bouma division. Measurements from the Silurian Aberystwyth Grits in Wales, the Cretaceous Great Valley Sequence in California and the Permian Karoo Basin in South Africa show that this conclusion holds for sequences of disparate age and variable location. The median thickness of these log-normal distributions is positively correlated with basal grain-size. The power-law exponent relating the basal grain-size and median thickness is different for turbidites with a basal A or B division and those with only C, D and E divisions. These two types of turbidite have been termed ‘thin bedded’ and ‘thick bedded’ by previous workers. A change in the power-law exponent is proposed to be related to: (i) a transition from viscous to inertial settling of sediment grains; and (ii) hindered settling at high sediment concentrations. The bimodal thickness distribution of ‘thin-bedded’ and ‘thick-bedded’ turbidites noted by previous workers is explained as the result of a change in the power-law exponent. This analysis supports the view that A and B divisions were deposited from high-concentration flow components and that distinct grain-size modes undergo different depositional processes. Summation of log-normal frequency distributions for thin- and thick-bedded turbidites produces a cumulative frequency distribution of thickness with a segmented power-law trend. Thus, the occurrence of both log-normal and segmented power-law frequency distributions can be explained in a holistic fashion. Power-law frequency distributions of turbidite thickness have previously been linked to power-law distributions of earthquake magnitude or volumes of submarine slope failure. The log-normal distribution for a given grain-size class observed in this study suggests an alternative view, that turbidite thickness is determined by the multiplicative addition of several randomly distributed parameters, in addition to the settling velocity of the grain-sizes present.

Sedimentological characteristics, heavy metal distribution and magnetic properties in subtidal sediments, Ria de Pontevedra, NW Spain

Abstract: The rias of NW Spain are coastal ecosystems of high biological productivity and great economic importance. They are intensively exploited by man for fish and shellfish. There are a number of important centres of population and industrial activity along their margins, which serve as sources of contamination. In this context, it is desirable to achieve the best possible understanding of the physico-chemical processes that control spatial and temporal variations in the geochemical, mineralogical and sedimentological characteristics of near-surface sediments in the Ria de Pontevedra and, in particular, the distribution and mobility of heavy metal contaminants. Thus, adequate environmental planning can be achieved for this site and understanding gained for comparable contexts. Core samples were examined from the inner, middle and outer parts of the ria. Grain-size distributions reflect the presence of two main populations, one dominated by silt and clay, derived mainly from terrestrial sources, and the other by fine sand to coarse silt, which is derived mainly from continental shelf and ria mouth sources. Mineralogical analysis shows an abundance of terrestrial intensive-weathering products near the ria head, a dominance of shelf-derived sediment towards the mouth and the presence of several diagenetic minerals whose nature varies with location within the ria. In the inner ria, the near-surface sediments are slightly enriched in Pb, Cu and Zn from anthropogenic sources. These sediments are fine grained and have a high organic content; hence, they have a higher potential to sorb contaminants than the coarser grained, less organic-rich sediments of the mid and outer ria. The estimated sedimentation rates for the fine-grained organic-rich sediments from the inner part of the ria are about 1 mm year–1. The dominant authigenic minerals in the inner ria are iron sulphides, whereas in the mid and outer ria, iron silicates and oxyhydroxides are more important. These differences in authigenic iron mineralogy are clearly reflected by the magnetic properties of the sediments.

Neogene evaporites in desert volcanic environments: Atacama Desert, northern Chile

Abstract: The Upper Miocene and Pliocene evaporite deposits of the Atacama Desert of northern Chile (Hilaricos and Soledad Formations) are among the few nonmarine evaporites in which aridity not only formed the deposits, but has also preserved them almost unaltered under near-surface conditions. These deposits are largely composed of displacive Ca sulphate and halite together with minor amounts of glauberite, thenardite and polyhalite. However, at the base and top of these deposits, there are also beds of gypsum crystal pseudomorphs that originally formed as free-growth forms within shallow brine bodies, rather than as displacive sediments. The halite is present as interstitial cement, displacive cubes and shallow-water, bottom-growth chevron crusts. Most of the calcium sulphate is presently anhydrite, pseudomorphous after gypsum, that was the primary depositional sulphate mineral. The secondary anhydrite formed under early diagenetic conditions after slight burial (some metres) resulting from the effect of strongly evolved pore brines. The anhydrite has been preserved without rehydration during late diagenetic and exhumation stages on account of the arid environment of the Atacama Desert. Both the Hilaricos and the Soledad Formations contain geochemical markers indicating that these Neogene evaporites had a largely non-marine origin. Bromine content in the halite is very low (few p.p.m.), indicating neither a sedimentological relation with sea water nor the likelihood of direct recycling of prior marine halites. Moreover, the d 34 S of sulphates (+4AE5& to +9&) also reflects a non-marine origin, with a strong volcanic influence, although some recycling of Mesozoic marine sulphates cannot be ruled out. d 34 S of dissolved sulphate from hot springs and streams in the area commonly displays positive values (+2& to +10&). Leaching of oxidized sulphur and chlorine compounds from volcanoes and epithermal ore bodies, very common in the associated drainage areas, have been the main contribution to the accumulation of evaporites. The sedimentary and diagenetic evolution of the Hilaricos and Soledad evaporites (based on lithofacies analysis) provides information about the palaeohydrological conditions in the Central Depression of northern Chile during the Neogene. In addition, the diagenesis and exhumation history of these evaporites confirms the persistence of strongly arid conditions from Late Miocene until the present. A final phase of tectonism took place permitting the internal drainage to change and open to the sea, resulting in dissolution and removal of a significant portion of these deposits. Despite the extensive dissolution, the remaining evaporites have undergone little late exhumational hydration.