Evaporite Units Research Articles

Numerical investigation of salinity in controlling ore-forming fluid transport in sedimentary basins: example of the HYC deposit, Northern Australia

This paper presents the first hydrogeological model that fully couples transient fluid flow, heat and solute transport associated with the formation of the HYC SEDEX deposit in the McArthur Basin, northern Australia. Numerical results reveal that salinity plays an important role in controlling hydrothermal fluid migration. In particular, it appears that it is the distribution of evaporitic units within a given basin, rather than their absolute abundance, that controls the development of free convection. Relatively saline conditions at the seafloor strengthen the thermally-induced buoyancy force and hence promote free convection of basinal solutions; whereas high salinities at the bottom counteract the thermal function of natural geothermal gradient and suppress the development of convective hydrothermal fluid circulation. In the latter case, higher thermal gradients are required to initiate substantial free convective fluid flow. Numerical experiments also suggest the position of an ore body with respect to its vent system may be controlled by the spatial and temporal salinity distributions in the basin. Vent-distal ore formation, a result of exhalation of brines that are denser than seawater and hence can flow away from the vent region, is promoted by moderate salinity at the seafloor and higher salinity in the aquifer. Vent-proximal ore accumulation, a result of pluming upon exhalation of brines less dense than seawater, is favored by the highest salinity conditions occurring near the level of the seafloor.

The Messinian post-evaporitic deposits of the Gafares area (Almerı́a-Nı́jar basin, SE Spain). A new view of the “Lago-Mare” facies

The exceptional exposures of the evaporitic and post-evaporitic Messinian deposits in the Gafares area (NE Almerı́a-Nı́jar basin, SE Spain) make it possible to elucidate those processes affecting this region during the critical period of the Messinian Salinity Crisis. The contact between the pre-evaporitic and the evaporitic units is an erosional unconformity. The transition from the evaporitic to the post-evaporitic units is gradual, since the gypsum beds of the evaporitic unit thin upward and the siliciclastic interbeds concomitantly thicken, thus passing gradually to the overlaying post-evaporitic unit. The presence of Globorotalia miotumida group in the last pelitic intercalation of the evaporitic unit as well as throughout the post-evaporitic one indicates a Messinian age. The post-evaporitic unit consists of a variety of sediments formed in continental and marine settings. Continental facies are characterised by fluvial channel-filled conglomerates and alluvial-plain sands and silts. Marine sediments are represented by silty marls with foraminifers. A river-dominated delta developed in the transitional zone at the NW edge of the Gafares area. Localised fan-delta deposits coming from the ESE also occur in the southeastern part of the study area. Microbial carbonates locally colonised the fan-delta conglomerates. During the deposition of the lower half of the post-evaporitic unit, sedimentation took place in a sheltered embayment, with restricted water circulation and relatively isolated from the Mediterranean Sea by an E–W-trending sill located at the south of the study area. In this palaeogeographic context, freshwater influx from the delta exerted a certain influence on the distribution of microorganisms. Thus, brackish settings developed close to the river outflows that were occupied by caspibrackish ostracod assemblages. The presence of gastropods of the family Cerithiidae as well as fishes attributable to the genus Aphanius, organisms that preferentially inhabit sheltered lagoons, is consistent with these settings. Beyond the freshwater influx, foraminifers dominated the microfossil assemblages. In the upper half of the post-evaporitic unit, microfossil assemblages dominated by planktonic forms are consistently found, thus revealing a better connection of the semi-enclosed Gafares area with the marine domain. This is related to the deepening of the sill due to a progressive rise of the sea level. The continental and brackish deposits (Zorreras-like facies) have been equated with the “Lago-Mare” facies, which were supposed to spread throughout the whole Mediterranean at the end of the Messinian. Nevertheless, in the Gafares area, these deposits change laterally to and interbed with marine deposits. This stratigraphic architecture and facies relationship has two important implications. On the one hand, marine waters at the latest Messinian occupied at least the western Mediterranean, predating the long-claimed early Pliocene deluge. On the other hand, the continental-brackish “Lago-Mare” facies were spatially restricted. As a consequence, neither the presence of the caspibrackish ostracods nor the continental-brackish facies can be plausibly taken to support the hypothesis that widespread “Lago-Mare” facies occupied the whole Mediterranean synchronously at the end of the Messinian. Therefore, this type of facies is expected to be found (albeit with varying degrees of development) in marginal basins at different times but not throughout the Mediterranean.

Deep to shallow lacustrine evaporites in the Libros Gypsum (southern Teruel Basin, Miocene, NE Spain): an occurrence of pelletal gypsum rhythmites

ABSTRACT A number of non‐marine evaporite units composed of primary gypsum were deposited in saline lakes that developed in the southern Teruel Basin (NE Spain) during the Miocene. In the basin depocentre, a continuum of lacustrine evaporite lithofacies influenced by the activity of organisms is displayed. The Libros Gypsum was deposited in a deep lake, in which water stratification became unstable with progressive shoaling. Rhythmites, composed of laminae of pelletal gypsum and laminae of very fine lenticular gypsum crystals mixed with siliceous microorganisms, formed in addition to gypsum turbidites, intraformational gypsum breccias and slump structures. The pelletal laminae originated from the faecal activity of animals (crustaceans?) ingesting gypsum crystallites in the lake water during episodes of maximum evaporation, whereas the laminae of very fine lenticular gypsum mixed with microorganisms accumulated during episodes of relative dilution. In the wide marginal zones of the basin, the Libros Gypsum unit consists of massive to thin‐bedded bioturbated gypsum and thin‐bedded clotted gypsum, which formed in intermediate to very shallow (palustrine) water depths. The bioturbated gypsum lithofacies were produced by the action of diverse organisms, presumably worms and coleopterans, and chironomid larvae to a lesser extent; the massive lithofacies precipitated in very shallow water; and the thin‐bedded lithofacies formed in shallow to deeper settings. The thin‐bedded clotted gypsum is a relatively deep facies that may have diverse origins (e.g. bioturbation, compaction, disruption of soft sediments and early diagenesis). There is a well‐developed metre‐scale cyclicity in the marginal lake sequences, which is not observed in the inner lake deposits. This suggests a depth control in the various lacustrine subenvironments to record cyclic evaporitic processes. The isotopic composition of the gypsum indicates early sulphate‐reducing bacterial activity in the bottom of the lake and suggests that the sulphate was derived from the chemical recycling of Triassic evaporites of the country rocks.

Geochemistry of Quaternary travertines in the region north of Rome (Italy): structural, hydrologic and paleoclimatic implications

In the Tyrrhenian region of central Italy, late Quaternary fossil travertines are widespread along two major regional structures: the Tiber Valley and the Ancona–Anzio line. The origin and transport of spring waters from which travertines precipitate are elucidated by chemical and isotopic studies of the travertines and associated thermal springs and gas vents. There are consistent differences in the geochemical and isotopic signatures of thermal spring waters, gas vents and present and fossil travertines between east and west of the Tiber Valley. West of the Tiber Valley, δ 13C of CO 2 discharged from gas vents and δ 13C of fossil travertines are higher than those to the east. To the west the travertines have higher strontium contents, and gases emitted from vents have higher 3He/ 4He ratios and lower N 2 contents, than to the east. Fossil travertines to the west have characteristics typical of thermogene (thermal spring) origin, whereas those to the east have meteogene (low-temperature) characteristics (including abundant plant casts and organic impurities). The regional geochemical differences in travertines and fluid compositions across the Tiber Valley are interpreted with a model of regional fluid flow. The regional Mesozoic limestone aquifer is recharged in the main axis of the Apennine chain, and the groundwater flows westward and is discharged at springs. The travertine-precipitating waters east of the Tiber Valley have shallower flow paths than those to the west. Because of the comparatively short fluid flow paths and low (normal) heat flow, the groundwaters to the east of the Tiber Valley are cold and have CO 2 isotopic signatures, indicating a significant biogenic contribution acquired from soils in the recharge area and limited deeply derived CO 2. In contrast, spring waters west of the Tiber Valley have been conductively heated during transit in these high heat-flow areas and have incorporated a comparatively large quantity of CO 2 derived from decarbonation of limestone. The elevated strontium content of the thermal spring water west of the Tiber Valley is attributed to deep circulation and dissolution of a Triassic evaporite unit that is stratigraphically beneath the Mesozoic limestone. U-series age dates of fossil travertines indicate three main periods of travertine formation (ka): 220–240, 120–140 and 60–70. Based on the regional flow model correlating travertine deposition at thermal springs and precipitation in the recharge area, we suggest that pluvial activity was enhanced during these periods. Our study suggests that travertines preserve a valuable record of paleofluid composition and paleoprecipitation and are thus useful for reconstructing paleohydrology and paleoclimate.

Diagenesis and fluid mobilisation during the evolution of the North German Basin—evidence from fluid inclusion and sulphur isotope analysis

Samples of diagenetic and hydrothermal minerals from drillcores along the DEKORP 9601 seismic traverse in the North German Basin were investigated for their fluid inclusion and sulphur isotope composition to reconstruct the thermal and chemical evolution of fluid systems during basin evolution. The stages of basin subsidence and subsequent inversion are marked by fluid systems of distinct composition. The basinwide Zechstein evaporite units formed an aquiclude, disconnecting convecting fluids in the underlying Rotliegend volcanics from the overlying units. δ34S ratios of sulphides suggest a sedimentary sulphur source. δ34S ratios of diagenetic and hydrothermal sulphates indicate that sulphur was derived partly from Rotliegend volcanic units and partly from the evaporitic Zechstein.

Foraminiferal record of environmental changes: Messinian of the Los Yesos area (Sorbas Basin, SE Spain)

The Late Messinian of the Sorbas Basin (Betic Cordillera, SE Spain) has been sampled to estimate (1) the paleoenvironmental changes recorded by the benthic foraminifers in pre-evaporitic and evaporitic units, and (2) the impact of the ‘Messinian Salinity Crisis’ on the foraminiferal biodiversity. In the pre-evaporitic unit, foraminifers indicate a circalittoral to bathyal environments and show significant changes in bottom-water oxygenation. Simultaneously, the occurrences of epiphytic damaged foraminifers and the blocks of Porites point out the presence of plant substrates and coral-reefs on the infralittoral zone. The overlying evaporitic unit is characterized by four subaqeous selenitic gypsum layers and three marine fossiliferous intercalations. Each episode of gypsum deposition ends with a desiccation event. Each fossiliferous intercalations traduce a rapid reflooding of the basin by marine waters corresponding to infralittoral to upper bathyal environments with normal marine conditions. During the evaporitic period, the ecological conditions in the water column were intermittently favorable to the development of marine organisms, and moreover, the marginal basins of the Betic Cordillera were not continuously desiccated during the Messinian. The low percentage of extinct species (5%) at the Messinian–Pliocene boundary and the occurrences of benthic foraminifers in the evaporitic unit indicate that the infralittoral to upper bathyal environments have not been completely modified and/or barren during the ‘Messinian Salinity Crisis’, i.e. the ‘Messinian Salinity Crisis’ is not associated with a biological crisis.

Correlation of Late Miocene to Early Pliocene sequences between the Mediterranean and North Atlantic

Ocean Drilling Program (ODP) Site 982 in the North Atlantic contains a complete latest Miocene to early Pliocene section that was tuned to the astronomical timescale by correlating the record of gamma ray attenuation (GRA) bulk density to summer insolation at 65°N and the benthic δ18O signal to orbital obliquity for the interval from 4.6 to 7.5 Ma. The astronomical tuning of the Site 982 record permits a direct bed‐to‐bed correlation to the cyclostratigraphy of Messinian sections in the Mediterranean [Krijgsman et al., 1999a, 2001]. The benthic δ18O signal at Site 982 records a latest Miocene glacial period that lasted from ∼6.26 to 5.50 Ma and consisted of 18 glacial‐to‐interglacial oscillations that were controlled by the 41‐kyr cycle of obliquity. Although the intensification of glaciation at 6.26 Ma may have contributed to the restriction of the Mediterranean, it preceded the depositional onset of the lower evaporite unit at 5.96 Ma by some 300 kyr. The transition from Stage TG12 to TG11 at 5.5 Ma marks the end of the latest Miocene glacial period and precedes the Miocene/Pliocene boundary by 170 kyr. Although benthic δ18O values are relatively low and δ18O of bulk carbonate reaches a minimum at the Miocene/Pliocene boundary at 5.33 Ma, there is no single “event” that would indicate deglaciation and sea level rise as the cause of the reflooding of the Mediterranean. We conclude that glacioeustatic changes alone were not responsible for either the start or end of evaporite deposition during the Messinian, suggesting that tectonic or local climate changes in the Mediterranean region were the dominant cause(s).

A prelimennary study: Celestite-bearing gypsum in the Tertiary Sivas basin, central-eastern Turkey

Celestite-bearing evaporite mineralization is common in the Tertiary evaporitic units of Ulas-Sivas basin, East-Central Turkey.

Modeling regional salinization of the Ogallala aquifer, Southern High Plains, TX, USA

Two extensive plumes (combined area >1000 km 2) have been delineated within the Ogallala aquifer in the Southern High Plains, TX, USA. Salinity varies within the plumes spatially and increases with depth; Cl ranges from 50 to >500 mg l −1. Variable-density flow modeling using SUTRA has identified three broad regions of upward cross-formational flow from the underlying evaporite units. The upward discharge within the modeled plume area is in the range of 10 −4–10 −5 m 3 day −1, and the TDS concentrations are typically >3000 mg l −1. Regions of increased salinity, identified within the Whitehorse Group (evaporite unit) underlying the Ogallala aquifer, are controlled by the structure and thickness variations relative to the recharge areas. Distinct flow paths, on the order of tens of km to >100 km in length, and varying flow velocities indicate that the salinization of the Ogallala aquifer has been a slow, ongoing process and may represent circulation of waters recharged during Pleistocene or earlier times. On-going pumping has had negligible impact on the salinity distribution in the Ogallala aquifer, although simulations indicate that the velocity distribution in the underlying units may have been affected to depths of 150 m after 30 years of pumping. Because the distribution of saline ground water in this region of the Ogallala aquifer is heterogeneous, careful areal and vertical characterization is warranted prior to any well-field development.

Marine to non-marine sedimentation in the upper Miocene evaporites of the Eastern Betics, SE Spain: sedimentological and geochemical evidence

Abstract Several Neogene basins in the Eastern Betics contain a record of thick upper Miocene evaporites. The inner basins—those located distal to the Mediterranean coast—include evaporite units ranging from the late Tortonian to the early Messinian, whereas the marginal basins—those located in the vicinity of the Mediterranean coast—include evaporite units corresponding to the late Messinian that are linked to the “salinity crisis”. The sedimentological and geochemical comparison of the evaporites in the inner and marginal basins enable us to show their differences in the modes of deposition under local, structural controls and differentiated palaeogeography. In the inner basins of the Murcia–Alicante region, a number of evaporite units have been differentiated whose gypsum lithofacies have: (1) variable strontium contents, with mean values (primary gypsum) between 1121 and 2380 ppm; (2) isotopic compositions of sulphur ( δ 34 S) ranging from those characteristic of the Tertiary marine water (+20.3 to +25.5‰) to those characteristic of the Triassic sulphates (+8.0 to +16.0‰), the latter suggesting recycling of the sulphate anion in meteoric waters; intermediate values (+17.4 to +19.6‰) are attributed to mixed mother brines; and (3) strontium ratios ( 87 Sr/ 86 Sr) comprised between 0.70804 and 0.70888; these values range from those of the Tertiary marine water to those exceeding the Triassic sulphates, suggesting additional continental contributions (from country rocks and hydrothermal solutions). The comparison between the sulphur and strontium isotopic compositions of the sulphates, together with the study of the biota present in the marls associated with the gypsum, allows us to distinguish between marine, continental, and mixed units. In the marginal basins of the Eastern Betics, where single evaporite units characterized by selenitic gypsum prevail: (1) the strontium contents are more homogeneous, with mean values (primary gypsum) between 493 and 625 ppm; (2) the isotopic compositions of oxygen ( δ 18 O) and sulphur ( δ 34 S) are comprised between +12.2 and +14.8‰, and between +22.0 and +23.5‰, respectively; and (3) the strontium ratios ( 87 Sr/ 86 Sr) vary from 0.70890 to 0.70895. All these isotopic values reveal a marine origin and the lack of contribution of significant continental water during the late Messinian evaporite deposition in the Betic Strait. These characteristics are similar to those of the Lower Evaporite.

Evaporative systems and diagenetic patterns in the Calatayud Basin (Miocene, central Spain)

This paper concerns the evaporite units, depositional systems, cyclicity, diagenetic products and anhydritization patterns of the Calatayud Basin (nonmarine, Miocene, central Spain). In outcrop, the sulphate minerals of these shallow lacustrine evaporites consist of primary and secondary gypsum, the latter originating from the replacement of anhydrite and glauberite. In the evaporative systems of this basin, gypsiferous marshes of low salinity can be distinguished from central, saline lakes of higher salinity. In the gypsiferous marsh facies, the dominant, massive, bioturbated gypsum was partly replaced by synsedimentary chert nodules and siliceous crusts. In the saline lake facies, either cycles of gypsiferous lutite‐laminated gypsarenite or irregular alternations of laminated gypsum, nodular and banded glauberite, thenardite and nodular anhydrite precipitated. Early replacement of part of the glauberite by anhydrite also occurred. Episodes of subaerial exposure are represented by: (1) pedogenic carbonates (with nodular magnesite) and gypsiferous crusts composed of poikilitic crystals; and (2) nodular anhydrite, which formed in a sabkha. Additionally, meganodular anhydrite occurs, which presumably precipitated from ascending, highly saline solutions. The timing of anhydritization was mainly controlled by the salinity of the pore solutions, and occurred from the onset of deposition to moderate burial. Locally, a thick (>200 m) sequence of gypsum cycles developed, which was probably controlled by climatic variation. A trend of upward‐decreasing salinity is deduced from the base to the top of the evaporite succession.

Gypsum–anhydrite transformations in Messinian evaporites of central Tuscany (Italy)

The Messinian succession of Tuscany (central Italy) contains three evaporitic units. Among the several exposed evaporitic lithofacies, only selenitic gypsum precipitated directly from evaporating brines. All the other facies, nodular microcrystalline gypsum, gypsarenites and gypsum laminites, despite their macroscopic differences, display the same petrographic textures, indicating that they are the product of dehydration of gypsum to give anhydrite which has been successively rehydrated to secondary gypsum. These secondary facies show an entire array of textures ranging from cloudy ameboid (xenotopic) with anhydrite relics, to idiotopic without anhydrite relics, that are here interpreted as a sequence of progressive stages of rehydration. The presence of completely hydrated petrofacies at the core of nodules which display a less hydrated rim suggests that these rocks have undergone at least two cycles of a dehydration–rehydration process. This interpretation is supported by the presence of satin spar veins that are replaced by microcrystalline gypsum. Satin spar itself is considered to be a by-product of anhydrite hydration. The first dehydration–rehydration event affected the entire gypsum deposit, producing a completely hydrated (idiotopic) facies together with satin spar veins; the second affected only veins, fractures and the rims of nodules, turning the first generation of satin spar and idiotopic gypsum into cloudy ameboid gypsum. Sedimentary structures typical of sabkha environments indicate for the youngest formation that the first dehydration and rehydration process occurred syndepositionally. The preservation of primary gypsum facies only at sites with condensed sections, indicate for the oldest two formations that the first dehydration event occurred upon burial. This event has been estimated to have occurred in the earliest Pliocene. After the Early Pliocene, dehydration was favored even at shallow depths, due to an increased heat flow related with the emplacement of local crustal magmatic bodies. Rehydration possibly occurred when these formations were uplifted and exposed to ground and/or meteoric water. The Volterra Basin has undergone alternating subsidence and uplift events, that can account for two dehydration–rehydration processes at least, also driven by alternating circulation, in the tectonic fractures, of fresh and salty water, the latter derived from dissolution of Messinian halite.

Palaeomagnetic chronology of the evaporitic sedimentation in the Neogene Fortuna Basin (SE Spain): early restriction preceding the `Messinian Salinity Crisis'

The magnetostratigraphic study of the evaporitic Rı́o Chicamo section (240 m) in the Neogene Fortuna Basin (Murcia, southeast Spain) has identified the record of five magnetozones. The most probable correlations with the standard geomagnetic polarity time scale (GPTS) imply that the marine evaporitic sedimentation of this basin was not coeval with the Messinian evaporites of the Sorbas Basin (Almeria, southeast Spain) and the Caltanissetta Basin (central Sicily) (assigned to the reverse Chron C3r, late Messinian by Gautier et al., 1994). The marine evaporites and diatomites from the Fortuna Basin are older (late Tortonian to early Messinian) than the evaporites of those basins. The chronological framework for the sedimentation in the Fortuna Basin together with the isotopic data from the sulphates in these evaporitic units indicate the following. (1) Restriction and confinement in the basin initiated as early as uppermost Tortonian, leading to deposition of evaporites under mixed (marine–continental) conditions. (2) The subsequent sedimentation of marine evaporites and diatomites in this basin occurred in a period between the Tortonian and Messinian transition and the early Messinian: the onset of this sedimentation pre-dates similar sediments of restricted marine environments in the eastern Betics basins, and possibly of the western Mediterranean region also. (3) Episodes of restriction and reflooding in the basin would have occurred in response to periodic fluctuations of the oceanic level under local tectonic controls. In the Fortuna Basin, the global base level drop associated with the `late Messinian Salinity Crisis' was recorded by the progradation of alluvial fans leading to thick clastic deposits overlying the youngest evaporites. These observations hint to: (a) the peculiar characteristics and sensitiveness of some of the marginal intramontane basins in the eastern Betics to reflect structural controls framed in the late Neogene global climatic changes; and (b) the diachronism of the beginning of the marine evaporitic deposition in the Mediterranean region linked to the salinity crisis during the Messinian.

Basin infill architecture and evolution from magnetostratigraphic cross-basin correlations in the southeastern Pyrenean foreland basin

Magnetostratigraphy, numerical dating, and facies mapping have been combined to provide the first cross-basin correlation of the Eocene southeastern Pyrenean foreland basin in northeastern Spain. This has enabled (1) depositional systems on the northern and southern margins of the basin to be dated and correlated across the basin and (2) changes in the rates of sediment accumulation in time and space to be evaluated. By sampling correlative intervals at several locations in a transect across the basin, magnetozones were detected that otherwise may have been misinterpreted due to recent magnetic overprinting, or missed by wide sampling intervals in previous magnetostratigraphic surveys. Results of this study indicate that marginal marine strata of the southeastern part of the foreland near the Vic area of Spain are 5 m.y. older than previously thought, the base of the interval (46 Ma) being middle Lutetian in age. By proposing an age of at least 35.68 Ma for the overlying Cardona evaporite unit, the age span of the marine interval in the studied part of the foreland is ~10 m.y. Contrary to earlier interpretations, no major southward migration of the southeastern Pyrenean foreland basin depocenter is recorded by the basin stratigraphy during the initial stages of middle Eocene marine sedimentation in the southern margin of the basin. However, marked basin asymmetry is observed later in its evolution as the top of the marine deposits records northward increase in the thickness of chron C17n.1n (37.47‐36.61 Ma). This asymmetry is also proved by the presence of a larger number of magnetozones within marine strata at the top of the marine interval in the northern sector. These results imply that previous age calibrations and the estimated rates of tectonic processes and sedimentation within the foreland need to be reassessed. The implications for basin infill architecture evolution through time and space, subsidence analysis, and depocenter evolution through time are addressed.

Influence of the Ainslie Detachment on the stratigraphy of the Maritimes Basin and mineralization in the Windsor Group of northern Nova Scotia, Canada

The Late Devonian to Lower Permian Maritimes basin of eastern Canada developed as a late to post-tectonic extensional basin following the Early Devonian Acadian orogenic event and is subdivided into five main lithostratigraphic groups. The principal host to base metal mineralization is the Visean Windsor Group, which is also the only marine-dominated interval within the basin. The Windsor Group is a thick accumulation of evaporites, carbonates, and siltstone, deposited during passive regional subsidence and intracontinental submersion of a tectonically thinned crust. Earlier rift deposits consist of the nonmarine coarse clastic sediments of the underlying Tournaisian Horton Group, and the Horton to Windsor succession defines a typical rift and sag transgressive event. The Macumber and stratigraphically equivalent Gays River Formations are the marine basal carbonate units to the Windsor Group and contain numerous Pb-Zn-Ba occurrences in central and northern Nova Scotia. These carbonate units are overlain by thick evaporite deposits of the Lower Windsor Group, consisting of gypsum, anhydrite, and salt. In northern Nova Scotia the evaporite-carbonate contact at the top of the basal carbonates is the locus of intense shear and brecciation which developed in relation to the Ainslie detachment. This detachment is a stratigraphically controlled regional-scale flat-lying extensional fault that affected the hydrodynamic regime and mineralizing environment in the Carboniferous Maritimes basin. Movement on the detachment has stripped away thick evaporitic units and excised the entire Windsor Group across wide areas, effectively breaching a regional aquiclude. With shearing, permeability was locally enhanced through brecciation, creating a favorable environment for mineralization. Significant thickness variations within underlying coarse clastic rocks and pinch outs of the Horton Group aquifer also focused basin fluids.The detachment is rooted in a master basement normal fault which occurs offshore along the southeast margin of the Gulf of St. Lawrence, with the basement fault displaying up to 10 to 12 km of offset. Structural evidence from onshore exposures in the uplifted or horsted regions of mainland Nova Scotia and Cape Breton Island indicate that much of this movement was transferred horizontally along the Ainslie detachment. Thin-skinned gravitational sliding produced large rafts of allochthonous strata which progressively draped over the edge of basement normal faults during extension, resulting in tectonic salt build-up in the main offshore graben and a salt massif. Buttressing at the front end of the raft within the graben produced seismically imaged large amplitude buckle folds above the detachment.As well as a regionally extensive planar low-strength or low-viscosity layer, large-scale detachment faults require high fluid pressures to sustain motion of the allochthonous sheets. Abnormal fluid pressures are known to occur beneath evaporites, which is the site of nucleation for the Ainslie detachment. Fluid focusing along the detachment is suggested from the structural style, and evidence is provided by widespread synkinematic calcite + or - fluorite-barite-pyrite veins which occur in the calc-mylonite and breccia.

Characterization of gases in sedimentary formations through monitoring during drilling and core leaching (Balazuc borehole, Deep Geology of France Programme)

Continuous analysis of the drilling fluids and core sampling formed a major part of the Ardèche theme of the Deep Geology of France Programme when drilling the Balazuc-1 borehole in 1991. The analyses of gas concentrations in the drilling fluids are compared to those of core leaching. Both data sets show good agreement. Although higher concentrations are obtained in the drilling fluids, the drilling to core ratio shows a relatively constant value. These higher concentrations are due to crushing of the rock during drilling and the influence of the sediments surrounding the borehole. The characterization of the gases with depth indicates 3 different zones: (1) the Jurassic carbonates, with high CH 4 and CO 2 concentrations; (2) the Triassic upper sandstone and middle evaporitic units, with average concentrations of He, CO 2, N 2 and low CH 4; and (3) the Triassic lower sandstone unit and the Carboniferous silts, with high He, N 2 and low CO 2 and CH 4 concentrations. These trends indicate that diffusion is not a major process at Balazuc and that the formations remained relatively independent from each other. Nitrogen and CO 2, which are the main constituents, show a reverse correlation; the high CO 2 zones are related to the carbonate formations. In the Hettangian carbonates, a zone of black shales has preserved a high hydrocarbon content due to its very low permeability whilst in the other formations, any hydrocarbons originally present have escaped. The He record in the continuous measurement mainly reflects the fractured levels and the paleoporous zones. This is correlated to major fluid transfer in the formations of the lower part of the borehole, along the Uzer fault.

Sequence Sequence stratigraphy, sea-level changes and depositional systems in the Cambro-Ordovician of the North China carbonate platform

The Cambro-Ordovician strata in North China were deposited over a very extensive craton, extending some 1500 km east-west and 1000 km north-south. The dominantly shallow-water carbonate succession reaches up to 2000 m in thickness and two megasequences (transgressive-regressive cycles) can be distinguished: Lower Cambrian through Lower Ordovician strata, and Middle through Upper Ordovician strata, separated by a major palaeokarst. The first megasequence consists of nine sequences which are generally 50-150 m in thickness. The Lower Cambrian sediments consist of phosphorites and phosphatic sandstones, deposited during the flooding of the craton. Carbonates, mudrocks and evaporites were deposited in the Early Cambrian under an arid climate, laying the foundation for the subsequent long period of shallow-water carbonate deposition which lasted some 70 m.y. The Middle and lower Upper Cambrian sequences consist predominantly of mudrocks and storm deposits (‘tempestites’) in the lower part and oolitic grainstones and tidal-flat lime mudstones in the upper part; these represent outer-mid and mid-inner-ramp depositional systems, respectively, of the transgressive and highstand systems tracts. These sequences have strong similarities with the ‘Grand Cycles’ in the Cambro-Ordovician successions of North America. In the Upper Cambrian, there is a distinctive unit of glauconitic purple-red mudstone several metres thick which is interpreted as the deposits of the maximum flooding of the first megasequence. In the Upper Cambrian, there was a phase of tilting of the North China Carbonate Platform to the north, and storm deposits, especially intraclastic conglomerates and hummocky cross-stratified grainstones-packstones, were very common at this time. Also common in upper Middle and lower Upper Cambrian strata are stromatolitic-thrombolitic bioherms, several metres in diameter. The upper Upper Cambrian through Lower Ordovician strata are dominantly fine-grained limestones and dolomites deposited in shallow-subtidal and inter- to supra-tidal environments on a low-energy epeiric-sea platform. This part of the succession is the regressive part of the first megasequence, so that overall the platform shows an evolution from platform initiation to platform foundation, to a ramp-depositional system and then an extensive epeiric platform. On a small scale, the succession is composed of metre-scale shallowing-upward cycles (parasequences) arranged into cycle sets of 10–30 m thick. The Middle-Upper Ordovician megasequence consists mostly of shallow-water carbonates with several thick evaporite units. Minor palaeokarsts and palaeosoils separate the sequences, which are composed of metre-scale cycles. Regional uplift affected the North China Platform in the Ordovician and sedimentation did not resume until the Carboniferous. There is a good correlation of the two transgressive-regressive megasequences described here with the global 2nd-order relative sea-level curve for the Cambro-Ordovician.

World‐class sediment‐hosted stratiform copper deposits: Characteristics, genetic concepts and metallotects

Detailed studies of the host lithologies, mineral textures, metal zoning and configurations of numerous sediment‐hosted stratiform copper deposits have established a post‐sedimentary (diagenetic) timing and overprint concept for the genesis of this important mineral deposit type. Metals were probably deposited from low‐temperature oxidised saline brines which circulated across the colour‐distinctive redoxcline between host greybeds (commonly sulfidic ‘black shales’ or marginal‐marine microbial mats) and immature footwall redbeds filling continental rift basins. Adequate sources of ore metals (principally copper ± silver or cobalt) and associated metals (e.g. lead, zinc, cadmium) can be found in the redbeds (or deeper rocks), and the circulation of saline pore fluids to and across the redoxcline may have been induced and focused by normal features of rift geology, such as marginal basin faults, contrasting stratigraphic permeabilities, overpressured pore fluids and anomalous thermal gradients. Besides the identification of major greybed/redbed contacts and the longstanding recognition that stratiform copper deposits are commonly associated with extensive continental redbed and evaporitic units, exploration may be guided by the observation that stratiform copper deposits are restricted in geologic time to continental rift zones, to low palaeolatitudes where evaporites tend to form, and to sediments younger than the apparent oxidation of the Earth's atmosphere at about 2.3 Ga (essentially post‐Archaean time). Thus, successful exploration for stratiform copper deposits will probably depend more and more commonly on the detection and refined reconstruction of redbed/greybed sedimentary environments in zones of crustal extension at low palaeolatitudes post‐dating Archaean time and especially on hypothesised patterns of ore brine circulation in the redbed/greybed package following sedimentation.

Dedolomites associated with karstification. An example of early dedolomitization in lacustrine sequences from the Tertiary Madrid basin, central Spain

A variety of calcite fabrics formed by dedolomitization of middle to upper Miocene continental sediments of the Madrid basin, central Spain. The dedolomites are associated with other carbonates that show abundant diagenetic features. The diagenetic carbonates (Diagenetic Carbonate Zone, DCZ) overlies and grades laterally into lacustrine dolomite (Lower Dolomite Unit, LDU) and evaporites (Evaporitic Unit, EU), and in turn is capped by a paleokarst surface.

Ancient synsedimentary structural control on thrust ramp development: an example from the Northern Apennines, Italy

ABSTRACTThe Umbria‐Marche‐Sabina foreland fold and thrust belt (Northern Apennines, Italy) provides excellent test‐cases for the hypothesis of ancient syndepositional structural features controlling thrust ramp development. The sedimentary cover, Late Triassic to Miocene in age, is made of platform and pelagic carbonates, whose deposition was controlled by significant synsedimentary extension. Normal faulting, mainly during the Jurassic and the Late Cretaceous‐Palaeogene, determined sensible lateral thickness variations within the relative sequences. By late Miocene the sedimentary cover was detached from its basement along a mainly evaporitic horizon, and was deformed by means of eastward‐verging folds and thrusts.In order to locate the points where thrust ramps branch‐off the basal detachment, both line‐length and equal‐area techniques were used in the construction of a balanced cross‐section through three major fault‐related folds in southeastern Umbria. The nucleation of thrust ramps was controlled by the occurrence of Jurassic and Cretaceous‐Palaeogene synsedimentary normal faults. These interrupted the lateral continuity of the evaporitic unit (the Late Triassic Anidriti di Burano Fm.) at the base of the sedimentary cover, and acted as obstacles to the eastward propagation of the thrust system, giving rise to major folds which originated from tip‐line folding processes.Therefore, the inferred relationships between ancient normal faults and late thrusts indicate that synsedimentary tectonic structures and the related lateral stratigraphic variations can be envisaged as mechanically important perturbations, which effectively control the nucleation and development of thrust ramps.