Clay Assemblages Research Articles

Late Cretaceous palaeoenvironments expressed by the clay mineralogy of Cenomanian–Campanian chalks from the east of the Paris Basin

The clay fraction of Cenomanian–Campanian chalks cored at Poigny and Sainte-Colombe, close to Provins (east-south-east of Paris), includes variable proportions of smectitic minerals, illite and kaolinite. The smectitic sediments (which constitute the background of low-terrigenous supply throughout the stratigraphic interval) resulted mainly from the warm, humid climate and high sea level that prevailed during Late Cretaceous in this area. During the Late Turonian, the smectitic sedimentation was interrupted by significant detrital inputs of illite and kaolinite. This reflected tectonic rejuvenation of landmasses coeval with an explosive volcanism expressed by the occurrence of bentonite layers. Comparison with clay assemblages occurring at equivalent stratigraphic intervals in the western part of the Paris Basin reveals great variation in clay mineralogy reflecting either local detrital input or complex basin morphology. Bentonites identified in the Poigny and Sainte-Colombe boreholes include the Southerham, Caburn, Bridgewick and Lewes marls. Cropping out from England to Germany, these clay-rich beds usually show a smectitic clay fraction originating from the submarine weathering of volcanic glass shards. Surprisingly, at Poigny and Sainte-Colombe, the occurrence of authigenic kaolinite characterizes these bentonite layers. Authigenesis of kaolinite in volcanic layers deposited in marine environments is unusual. The formation of kaolinite, which requires a low pH, usually occurs in organic matter-rich continental settings. At Poigny and Sainte-Colombe, kaolinite probably formed shortly after burial in reducing microenvironments. The systematic Cr, V, U and organic carbon enrichment of kaolinite-rich bentonites by comparison with smectite-rich bentonites suggests deposition in oxygen-depleted environments in the deepest part of the Paris Basin.

Clay minerals and sedimentary basin history

Clay minerals in the mud and soil that coat the Earth's surface are part of a clay cycle that breaks down and creates rock in the crust. Clays generated by surface weathering and shallow diagenetic processes are transformed into mature clay mineral assemblages in the mudrocks found in sedimentary basins. During metamorphism, the release of alkali elements and boron from clay minerals generates magmas that are subsequently weathered and recycled, representing the magma-to-mud pathway of the clay cycle. Volcanogenic clay represents an important but hitherto underestimated proportion of recycled clay. Within sedimentary basins, immature clays are transformed to mature and supermature clay assemblages by a series of reactions that generally obey the Ostwald Step Rule. Bedding-parallel microfabric generated by these reactions produce significant changes in the physical properties of deeply buried mudrocks. Clay minerals react to form equilibrium assemblages in 1 × 104 years in some hydrothermal systems, but immature clays may survive for up to 2 × 109 years in mid-continental rift basins. Clay mineral assemblages and the b cell dimension of K-white mica can be used to infer the geotectonic settings of sedimentary basins.

Origin of the clay mineral assemblages in the Germanic facies of the English Trias: application of the spore colour index method

AbstractThe origin of the regional and stratigraphical variation in the Triassic authigenic clay assemblages of England is discussed in relation to new estimates of the palaeotemperatures experienced by their host sediments and a preliminary study by transmission electron microscopy of their microtextural features. Spore colour index measurements, based on the spore type Deltoidospora s.l. occurring in the sediments (Penarth Group) at the very top of the Triassic sequence, give estimated palaeotemperatures ranging from 60–74°C (south Devon) to 89–97°C (northeast Yorkshire). Calculated palaeotemperatures, based on a gradient of 25°C/km, for the main zone of authigenic clay minerals range from 63–77°C to 89–97°C for the top to 71–85°C to 94–104°C for the base. Irregular mixed-layer smectite-chlorite, corrensite and Mg-rich chlorite are associated with calculated palaeotemperatures of 66–86°C, 66–104°C and 75–104°C respectively. The suggestion that elsewhere in the UK corrensite and Mg-rich chlorite were formed at temperatures in excess of 100°C finds no support. Geothermal gradients would have to have been of the order of at least 100–300°C/km to obtain these temperatures within the Triassic sediments; such values are associated typically with high-level magmatic intrusions or geothermal systems of which there is no geological evidence. The balance of evidence suggests that the Triassic authigenic clay assemblages formed by neoformation during the early stages of sediment diagenesis under the influence of variation in the alkalinity of the depositional environments.

Constraints on alluvial clay mineral assemblages in semiarid regions. The Souss Wadi Basin (Morocco, Northwestern Africa)

Illite, chlorite and kaolinite, resulting directly from substratum weathering, dominate the clay assemblages recorded in altered rocks and soils of the Souss Wadi watershed. Nevertheless, the transformation and neoformation of diverse clay minerals, which is mainly controlled by changing climatic, lithological and topographic conditions, enables one to establish three distinctive source areas. High altitude, largely watered zones where chlorite is transformed into vermiculite; middle altitude drier areas where more pronounced seasonal contrast induces the transformation of chlorite into chlorite-smectite mixed layer; finally, semiarid low altitude zones with gentle slopes where the weathering of carbonate-dominated substrata, evaporation and chemical concentration result in smectite and palygorskite neoformation. These minerals, which result from transformation and neoformation, provide suitable environmental indicators and local to zonal tracers of sediment contribution. Nevertheless, the original clay mineral assemblages undergo a significant homogenisation due to the destruction of most of the transformed and neoformed minerals during fluvial transport. As a consequence, inherited clay minerals (illite, chlorite and kaolinite) usually dominate the assemblages recorded in the downstream alluvial sediments. The clay assemblages recorded in different tributary wadi alluvial deposits reflect major sediment contribution from the High-Atlas and the older alluvial plain deposits affected by intensive erosion. On the other side, Anti-Atlas contribution is lesser as a consequence of its wide tableland topography and the widespread outcropping of karstified carbonate rocks, which enhance water infiltration and preclude sediment supply.

Heterogeneous mixed-layer clays from the Cretaceous Greensand, Isle of Wight, southern England

AbstractThe sea-cliffs of the Isle of Wight were deposited during a period of overall sea-level rise starting in the Barremian (Lower Cretaceous) and continuing into the Aptian and Albian. They consist of fluvial, coastal and lagoonal sediments including greensands and clays. Numerous episodes of erosion, deposition and faunal colonization reflect condensation and abandonment of surfaces with firmgrounds and hardgrounds. This study focused mainly on shallow marine cycles where variations in clay mineralogy would not be expected, because overall system composition, sediment source, and thermal history are similar for all the samples in the studied section. Instead we found a wide variety of clay assemblages even in single samples within a 200 m interval.In this interval, distinct clay mineral assemblages were found and can be described as consisting of Al-rich, Fe-richand intermediate Fe and Al compositions withrespect to 2:1 and 1:1 layers in mixed-layer arrangements. Nearly pure glauconite-nontronite clays exist in the <2 µm fraction only when the bulk rock is free of K- and plagioclase feldspar. Conditions favorable to glauconite-nontronite formation are interpreted to result from a hiatus in volcanoclastic sedimentation, thus providing a stable substrate for glauconitization.The Fe-bearing mixed-layer clay assemblages consist of glauconite, nontronite and berthierine-like layers in various proportions with several mixed-layer clays often coexisting in the same sample. In different samples, Al-richand Fe-Mg-rich mixed-layer clays are similar in their content and distribution of 1:1 and 2:1 layers. This suggests that the original clay assemblages were similar and later diagenesis affected certain horizons resulting in substitution of Al by Fe + Mg while preserving the original layer structure and arrangement.Structural formulae for the berthierine-like phase and berthierine-like layers in these mixed-layer clays show their layer cation composition is intermediate between odinite and standard berthierine. The total sum of octahedral cations varies from 5.26 to 5.55 whereas the amount of Fe2+ cations varies from 2.12 to 2.22 per O10(OH)8. A feature of the berthierine-like phase as well as of berthierine-like layers is that they are di-trioctahedral and Fe2+ and Fe3+ are the prevalent cations. Moreover, in these berthierine-like components, the amount of Fe2+ is greater than that of Mg (in contrast to odinite) and Fe3+ cations prevail over Al (in contrast to berthierine). The presence of authigenic ferrous Fe clays and the relationship between glauconite-nontronite and bulk mineralogy has implications for sedimentological processes and geochemical conditions during and shortly after deposition.

Surficial clay assemblage associated with the hydrothermal activity of Bouillante (Guadeloupe, French West Indies)

Surficial clay minerals as indicators of present-day geothermal activity were studied around Bouillante, Guadeloupe, French West Indies. The study is based on widespread sampling (>100 surface samples) in a zone extending 5 km north and 9 km south from Bouillante. Three types of mineral associations were distinguished: (1) dioctahedral smectites (in which the ‘beidellite–nontronite’ behavior predominates), with zeolites, calcite±quartz±kaolinite; (2) ordered illite–smectite mixed-layered clay with adularia, silica±calcite; and (3) kaolinite–smectite mixed-layered clays±halloysite±kaolinite±smectite±silica. The first association characterizes steaming grounds and areas subjected to hydrothermal fluid infiltrations along fractures or breccia zones in marine and subaerial environments. The second association was only identified in the matrix of epithermal breccias. This assemblage appears as a tracer of high-temperature fluids and is a good indicator of the proximity to the geothermal reservoir. The third assemblage was found both within and beyond the bounds of the known geothermal area, suggesting a pedogenic origin. We interpret the occurrence of the three assemblages in the Bouillante geothermal area as a background of clayey tropical weathering (kaolinite–smectite) with a hydrothermal overprint (dioctahedral smectites and illite–smectite).

Tectonic–palaeoenvironmental forcing of clay-mineral assemblages in nonmarine settings: the Oligocene–Miocene As Pontes Basin (Spain)

Two small, alluvial-lacustrine subbasins developed during the early restraining overstep stages of the Oligocene–Miocene As Pontes strike-slip Basin (NW Spain). Later, the basin evolved into a restraining bend stage and an alluvial-swamp-dominated depositional framework developed. The palaeobiological record demonstrates that the Oligocene–Miocene palaeoclimate in NW Spain was subtropical, warm and humid to subhumid. The metamorphic and igneous basin catchment yielded clay assemblages made up by kaolinite, illite and Al-smectite. Illite occurred as an original mineral in the source rock area, whereas kaolinite and Al-smectite resulted mainly from weathering of feldspar and clinochlore, respectively. This detrital primary clay assemblage remained preserved in the colluvial, alluvial fan and shallow lacustrine facies, whose early diagenesis was influenced by diluted, poorly evolved pore waters with neutral to slightly alkaline pH. The original clay assemblage was mildly to strongly transformed under early diagenetic conditions in the lacustrine and swampy environments where significant hydrochemical and Eh–pH changes took place. A fibrous magnesium-rich clay mineral-dominated assemblage (palygorskite and sepiolite) formed in shallow, saline lakes and palustrine zones under the influence of magnesium-rich, alkaline waters. Moreover, kaolinite-enriched assemblages formed in deep lacustrine, swamp and swamp-related alluvial zones under the influence of slightly to highly acidic pore waters. Pore water acidic conditions, characterising environments with organic matter accumulation, led to early diagenetic transformation of Al-smectite into kaolinite. This process was relatively limited in some environments such as organic matter-rich bottoms in meromictic lacustrine zones, whereas it was pervasive in peat-forming swamp zones. The stratigraphic relationships between the diverse clay mineral assemblages in the As Pontes Basin fill demonstrate the coeval development of rather diverse clay compositions. This clearly reflects the influence on clay assemblages of palaeoenvironmental changes forced by the morphological and tectonic evolution of the catchment–basin system. The interplay between climate and tectonic processes in the source areas did not result in major variations of the clay minerals fed into the basin. Conversely, this tectonic-sedimentation interplay influenced the evolution of the drainage and the water balance in the depositional zones, causing a complex environmental–hydrochemical evolution to occur. As a consequence, drastic early diagenetic changes affected the original clay mineral assemblages and resulted in a variety of early diagenetic assemblages. The As Pontes case study emphasises the major influence of palaeogeographical and tectonosedimentary evolution on the clay mineral record in nonmarine depositional systems. The depositional record in As Pontes Basin demonstrates that morphological and tectonically forced environmental and hydrochemical changes can result in variations similar to those forced by low-order climatic changes and that, in some cases, the role of climate can be negligible.

Evolution of Clay Minerals in a Chronosequence of Poldered Sediments Under the Influence of a Natural Pasture Development

AbstractClay minerals appear to change significantly under the influence of pasture development on poldered sediments in the Baie d'Authie area (Somme, France). Cores 40–90 cm deep from recent salt marsh sediments and poldered sediments developing grass pastures since 1737, 1575 and 1158 indicate that the natural mineral suite of kaolinite, mica, illite, and two disordered mixed-layered illite-smectite (I-S) phases common to the sedimentary input changes gradually but significantly in the materials. In the oldest, best-developed profile, there is a dominance of a disordered, illitic I-S in the humic upper part of the profile and a more abundant, more smectitic I-S mineral below. It appears that grass-derived humic materials tend to stabilize closed (collapsed) or illitic behavior in I-S clays. The natural evolution of the sediment (lower part of the profile) is towards a smectitic clay assemblage. Destruction of organic matter of the smectitic I-S minerals by oxidation indicates that this material can significantly modify the physical behavior of the clays keeping the structure open to polar molecules.

Environmental controls on clay mineralogy of an Early Jurassic mudrock (Blue Lias Formation, southern England)

We present a dataset including clay mineralogy, gamma-ray spectrometry, organic matter content and magnetic susceptibility of the Hettangian to lowest Sinemurian successions of Dorset and Somerset, southern UK (Blue Lias Formation, Bristol Channel and Wessex basins). In both areas, the clay assemblages comprise predominantly detrital illite, kaolinite and illite/smectite mixed layers. Clays probably originated from the erosion of the Hercynian massifs, the relative proportions of kaolinite and illite being modulated by arid–humid climatic fluctuations. The organic matter (OM) content (types II to IV) ranges up to 12% in both areas. A clear stratigraphical trend in clay mineral assemblages is apparent in Somerset, whereas in Dorset sharp contrasts between adjacent horizons and a greater dilution by carbonate mask the long-term evolution. Correlations between both areas based on similar vertical trends in clay mineral abundance support the suggestion of a hiatus within the angulata Zone of the Dorset succession. As expected, the kaolinite/illite ratio correlates with the Th/K ratio deduced from gamma-ray spectrometry. However, significant departures from the correlation occur in OM-rich intervals, suggesting that Th may be partly adsorbed on to OM particles. Surprisingly, high magnetic susceptibility correlates with abundant kaolinite, not with Fe-rich clays, indicating either that kaolinite is accompanied by a soil-inherited magnetisable phase (possibly iron oxide) or that illite-rich rocks are more strongly diluted by carbonate than are kaolinite-rich strata.

Greenschist-facies metamorphism of the Burgess Shale and its implications for models of fossil formation and preservation

Metamorphosed silty mudstones of the Burgess Shale and Stephen Shale formations record a poly metamorphic history. An early greenschist-facies event associated with burial by Paleozoic strata produced a nearly ubiquitous bedding-parallel cleavage (S1). Tectonic exhumation during the formation of the southern Canadian Rocky Mountains produced a domainal subgreenschist-facies retrograde overprint in which a high-angle crenulation cleavage (S2) was developed. Whereas all rocks have experienced these two events, the degree of deformation and fossil pre servation varies with position relative to the Cathedral Escarpment. This paleosubmarine cliff resulted in a zone of reduced deformation within adjacent strata by buttressing them during burial and deflecting deformation during orogenesis. Fossil-bearing strata are composed of a typical greenschist assemblage of muscovite–chlorite–quartz–albite, are devoid of clays, and contain an average of 0.28% organic carbon. This typical metamudstone assemblage is consistent with the typical whole-rock composition of these rocks which tends to be richer in K and Al and poorer in Fe relative to the Post-Archean Average Shale. These mineralogical–compositional characteristics suggest that the premetamorphic clay assemblage was likely illite–smectite–kaolinite, with no evidence of highly reactive species such as nontronite or Na-montmorillonite. This is contrary to the required conditions for taphonomic models involving organic preservation due to clay-related suppression of decomposition-related reactions. Metamorphism of the Burgess Shale has also re duced the total organic carbon content to <20% of initial values. This must be considered in any models that involve interpretation of organic carbon in diagenetic processes (e.g., fossil formation and determination of paleoredox conditions).

Syndepositional formation of Fe-rich clays in tropical shelf sediments, San Blas Archipelago, Panama

Solid-phase chemistry of modern carbonate–siliciclastic marine sediments from the San Blas Archipelago, Panama, reveals the occurrence of an Fe–Mg authigenic clay facies that is dominated by a ∼7.1-Å ferric clay mineral. These Fe-rich clays are highly susceptible to HCl-attack and can comprise a large fraction (up to 5.4 wt.% Fe) of the sediment. Some of the clays are clearly forming in the marine environment as they coat grains, infill carbonate micropores, and replace faecal pellets and carbonate shells. The mode of occurrence, structure, and ferric nature of the clay assemblage identifies the facies as verdine, which has odinite, a dioctohedral–trioctohedral Fe 3+-rich 1:1 clay mineral as its key member. The verdine facies in San Blas occurs in suboxic sediments where iron reduction can be a prevalent organic carbon oxidation pathway. Sediment radiocarbon ages show that the San Blas verdine clays formed within the last few thousand years, and in some cases, the clays have been extensively converted into pyrite. The verdine clays most likely grew by incorporating terrestrially derived Fe and Al, seawater Mg, and pore water Si derived from biogenic opal dissolution. Similar clay formation processes may be common in other tropical nearshore environments where sediments are derived from highly weathered lateritic soils and may prove to be an important oceanic elemental sink.

Late Quaternary clay mineral record in Central Lake Baikal (Academician Ridge, Siberia)

We investigated the mineralogical composition of two cores recovered on the Academician Ridge (Central Lake Baikal, Siberia). Sedimentological features show that the cores are unaffected by turbidity currents. However, hemipelagic deposition is not continuous, but intermittently disturbed by syn- or post-sediment reworking (e.g., bioturbation, slumps, faulting). Such modes of deposition are consistent with the complex uplift history of the ridge. Bulk mineralogy suggests that terrigenous sediment supplies are constant through glacial/interglacial stages, and diluted by diatom-rich intervals related to warmer interglacial stages. The core stratigraphy is based on the correlation of the diatom zonation and opal abundance with the marine oxygen isotope reference curve SPECMAP. The ∼8-m cores partly recover the last four interglacial/glacial cycles, i.e., since oxygen isotope stage 8. We test the use of clay minerals as a proxy for paleoclimatic reconstruction. The clays are more weathered during the diatom-rich intervals in agreement with warmer climate conditions. However, the mean clay composition does not change significantly through glacial/interglacial stages. This observation implies that, in the Academician Ridge sediments, a simple smectite/illite ratio (S/I) does not alone provide a reliable indicator of climatic variation. It reflects the complex clay assemblages, especially the smectite group, delivered to Central Lake Baikal. Smectites include primarily illite–smectite mixed layers, made of a mixture of montmorillonite and beidellite. According to their behavior after cation saturation, the illite–smectite mixed layers are primarily transformed smectites, with some neoformed smectites intermittently observed. In addition, Al-smectites occur in minor proportions. We conclude that the S/I ratio has a climatic significance only if it evolves in parallel with the weathering stage of the clays and is confirmed by a change in the composition of the smectites.

Late Eocene–Oligocene oceanographic development at southern high latitudes, from terrigenous and biogenic particles: a comparison of Kerguelen Plateau and Maud Rise, ODP Sites 744 and 689

Detailed investigation (630 samples) of clay mineral assemblages and benthic foraminifer accumulation rates (BFAR) as a proxy for paleoproductivity has been conducted on two late Eocene–Oligocene sequences drilled at intermediate water depths of the Kerguelen Plateau and Maud Rise (Southern Ocean). Distinct changes in clay assemblages and BFAR – that are in general a factor two higher on the Maud Rise compared to the Kerguelen Plateau – unravel a step-by-step evolution of marine and continental Antarctic environments that eventually resulted in permanent ice coverage and dense cold water formation: (1) late Eocene dominance of chemical weathering was associated with intermittent erosion of Antarctic soils and substrates. Main erosional events are recorded at 36 and 34 Ma, prior to the major earliest Oligocene glacial event, and are especially noticeable in the Indian sector of the Southern Ocean; (2) physical weathering progressed in continental areas adjacent to the Maud Rise and intermediate water production intensified in the Indian sector of the Southern Ocean throughout the Eocene–Oligocene transition, as the Tasmanian seaway developed. Environmental changes and a strong 600 kyr lasting peak of paleoproductivity seem closely associated with the earliest Oligocene isotope shift and the accumulation of ice-rafted detritus on the Kerguelen Plateau; (3) aridity and physical weathering progressed in East Antarctica near the Maud Rise (Site 689) at 33.2–33.15 Ma, as polar transfer of heat within the deep ocean increased slightly. This may have prevented extensive ice development during at least part of the early Oligocene; (4) increased productivity, vertical mixing and turnover at Site 744 on the Kerguelen Plateau, followed by a two-step increase of physical weathering on the adjacent continent from 32 to 31.2 Ma, suggest progressive homogenization of climates and circulation as the passageway South of Tasmania widened and deepened; (5) physical weathering reached a maximum in East Antarctic areas adjacent to the Maud Rise and a hiatus developed on the Kerguelen Plateau during the mid-Oligocene Oi2–Oi2a interval (31–26.5 Ma), in possible relation with opening processes of the modern Drake Passage and Scotia Sea and a further step in the development of oceanic circumpolar circulation; (6) near-400-kyr cycles of productivity and clay mineral composition throughout the two sequences suggest that orbital frequencies played a role in the small-scale variations of both continental and marine Antarctic environments of the late Paleogene.

Three-dimensional Distribution of Minerals in the Sediments of Hayonim Cave, Israel: Diagenetic Processes and Archaeological Implications

The mineral components of the sediments that accumulate in an archaeological site constitute a potentially rich source of information on the diagenesis, and in turn the archaeology of the site. This detailed three-dimensional study of the mineral assemblages in mainly the Mousterian sediments of Hayonim Cave incorporates more than 2100 infrared analyses performed on-site during the excavation, as well as diverse analyses in the laboratory. Three major mineral assemblages are identified: the calcite-dahllite (CD) assemblage, the assemblage comprising mainly montgomeryite, leucophosphite and siliceous aggregates (LMVS), and a highly altered sediment in which the clays have broken down and silica was released. The boundaries between these assemblages were mapped in detail. The overall picture is one of extreme heterogeneity with sharp variations occurring over distances of a few centimetres. The relation between the CD and LMVS assemblages shows that it is a product of post-depositional diagenesis, whereas the altered clay assemblage formed beneath an erosional unconformity. The CD and LMVS assemblages were derived primarily from an accumulation several metres thick of ash deposits produced by humans. Ash is thus shown to be a major component of the sediments of this cave. The distribution of the CD assemblage reflects to a large extent the locations of two active springs/seepages in the cave. The distribution of the CD assemblage also faithfully maps the distribution of bones in the cave, showing that their distribution is a function of preservational conditions and not human activities (Stineret al ., 2001). The conditions that produced the erosional unconformity, also resulted in severe alteration of the clays and other mineral components of these sediments. The erosion process and the fact that the thickness of this altered zone decreases towards the centre of the cave, indicates that the diagenetic driving force was probably climatic. The three-dimensional distributions of the mineral assemblages have a direct bearing on the dating of the cave by thermoluminescence and electron spin resonance.

Contrasting clay mineral assemblages in British Lower Palaeozoic slate belts: the influence of geotectonic setting

AbstractRegional differences in clay mineralogy are found in British Lower Palaeozoic slate belts formed during Caledonian terrane amalgamation. Extensional basins in Wales, the northern Lake District and the Isle of Man are characterized by a greater diversity of species in clay mineral assemblages. Slates that evolved in these basins contain both the K- and Na-rich products of the 2:1 dioctahedral reaction series. Pyrophyllite, rectorite and corrensite are sporadically distributed but kaolinite is rarely recorded even in lowest-grade mudstones. In contrast, clay assemblages that evolved in the convergent basins of the Scottish Southern Uplands and northern England generally contain fewer mineral species, and Na-micas and pyrophyllite are rare or absent. Na-bearing clays may have been generated from low-temperature mixing of hydrothermal fluids and seawater in the extensional basins. Such fluids appear to have been unavailable in the convergent basins through lack of volcanic activity in the early stages of basin development.

Microfacies, sequence stratigraphy and clay mineralogy of a condensed deep-water section around the Frasnian/Famennian boundary (Steinbruch Schmidt, Germany)

A multidisciplinary analysis (microfacies, sequential stratigraphy and clay mineralogy) was made on Frasnian/Famennian (F/F) boundary strata of the Steinbruch Schmidt section in Western Germany. Three major microfacies are recognised. Their succession records a shallowing-upward evolution from deep, quiet and poorly oxygenated environments, below the storm wave base, to environments influenced by current activities close to the storm wave base. The Kellwasser Horizons correspond to the deepest microfacies. The shallowest microfacies correspond to fine-grained calcareous tempestites or turbidites coming from a distant shelf of northwest Germany. The sequential pattern through the F/F boundary shows the succession of seven systems tracts. Two sequence boundaries are located just above the Lower Kellwasser Horizon and at the F/F boundary itself. These are underlined by hardgrounds suggesting time gaps. The Kellwasser Horizons correspond to sea-level highstands and the overlying beds record a transition from lowstand to transgressive systems tracts. Illite and kaolinite are the dominant clay minerals associated with mixed layers and traces of chlorite. Illite abundance is maximal during Kellwasser Horizons. Illite and kaolinite were probably inherited from a highly weathered source area although part of the illite is diagenetic. Kaolinite is the second most abundant clay mineral and is particularly well represented (up to 50%) between the Kellwasser Horizons. An unusual clay assemblage of illite and mixed layers is associated with a bentonite layer. Kaolinite increases during times when thin tempestites or turbiditic microbioclastic layers come from a distant shelf during sea-level falls. The kaolinite percentage reaches its maximum at the top of the lowstand systems tract. The high percentage of kaolinite suggests a hot–wet climate and could be related to global warming.

Phyllosilicates and zeolite assemblages in the carbonate periplatform of the Great Bahama Bank: origin and relation to diagenetic processes (ODP Leg 166, Sites 1006 and 1007)

The western flank of the Great Bahama Bank, drilled during ODP Leg 166 at seven sites, represents a prograding carbonate sequence from late Oligocene to Holocene [Eberli et al., Proc. ODP Init. Reports 166 (1997)]. The signatures of the detrital input and of diagenetic alteration are evident in clay enriched intervals from the most distal Sites 1006 and 1007 in the Straits of Florida. Mineralogical and chemical investigations (XRD, TEM, SEM, ICP-MS) run on bulk rocks and on the clay fractions enable the origin and evolution of silicate parageneses to be characterized. Plio–Pleistocene silt and clay interbeds contain detrital clay assemblages comprising chlorite, illite, interstratified illite–smectite, smectite, kaolinite and palygorskite. The greater smectite input within late Pliocene units than in Pleistocene oozes may relate either varying source areas or change in paleoclimatic conditions and weathering intensity. The clay intervals from Miocene–upper Oligocene wackestone sections are fairly different, with prevalent smectite in the fine fraction, whose high crystallinity and Mg contents that point towards an authigenic origin. The lower Miocene section, below 1104 mbsf, at depths where compaction features are well developed, is particularly characterized by abundant authigenic Na-K-clinoptilolite filling foraminifer tests. The authigenic smectite and clinoptilolite paragenesis is recorded by the chemical trends, both of the sediment and the interstitial fluid. This diagenetic evolution implies Si- and Mg-rich fluids circulating in deeper and older sequences. For lack of any local volcaniclastic input, the genesis of zeolite and the terms of water–rock interaction are discussed. The location of the diagenetic front correlates with that of the seismic sequence boundary P2 dated as 23.2 Ma. This correspondence may allow the chronostratigraphic significance of some specific seismic reflections to be reassessed.

Source rock and environmental control on clay mineral associations, Lower Section of Paganzo Group (Carboniferous), Northwest Argentina

The significance of clay mineral assemblages present in the Lower Section of the Paganzo Group (Carboniferous) is considered in this paper. The sequence was deposited in a large (140,000 km 2) foreland basin located in Northwest Argentina. The Lower Section of Paganzo Group is a classic Gondwanic sequence that includes alternating marine and non-marine deposits up to 1200 m thick. The existence of two marine episodes (here named Namurian and Stephanian transgressions) is the basis for subdividing the Lower Section of Paganzo Group into five depositional systems (DS). DS1 makes up the base of deposits and includes conglomeratic diamictites and sandstones of glacial origin. DS2 comprises mudstones, turbiditic sequences and marls of transitional (fjord?) environments, all deposited as a response to the sea level rise occurring during Namurian times. DS3 is mostly made up of fluvial sandstone channels that interfinger with carbonaceous and coal-rich flood basin deposits that originated under warm and humid climatic conditions. The Stephanian transgression, included in DS4, only reached the Precordillera area, where it is marked by mudstones, marls and fine sandstones of nearshore environments. To the east, in the Sierras Pampeanas area, thick coal and kaolinitic beds belonging to high-sinuosity river deposits were deposited. Finally, DS5 comprises a variegated section of sandstones and mudstones of fluvial origin deposited under a seasonally dry climatic regime. Four clay mineral assemblages are defined on the basis of X-ray diffraction analyses of 54 mudstones samples: illite+kaolinite (I+K) , illite+chlorite (I+Ch), kaolinite≫illite (K≫I) and illite, kaolinite and illite/smectite mixed layers (I+K+I/Sm). Because of the low degree of diagenesis, the strong dependence of clay mineral distribution on pre-depositional factors such as source area, sea level changes and paleoclimate can be recognized. In this manner, clay assemblages of the fluvial deposits from the Sierras Pampeanas area were mostly formed by illite and kaolinite, while illite and chlorite dominate in marine intervals that have the Precordillera as their main source area. Paleoclimate seems to have controlled the degree of mechanical and/or chemical weathering, as well as the significant kaolinite (and halloysite) accumulation that took place from feldspar and mica alteration under warm and humid conditions. Mixed layer clays of illite/smectite, illite/vermiculite and chlorite/smectite are generally subordinated phases tied to the degradation of previous clays.

Evolution in Basin Fill Style During the Mesozoic Gondwana Continental Break-up in the Godavari Triple Junction, SE India

Consistent changes are observed in the Mesozoic sedimentary lithofacies and palaeocurrents during the transition from the continental Gondwana (NW-SE trending Chintalpudi Graben - CG) to the coastal Gondwana (NE-SW trending Krishna Godavari Coastal Basin - KGCB) in the Godavari Triple Junction (GTJ). The Mesozoic continental Gondwana sediments include the Kamthi (early Triassic), Kota (Jurassic) and Gangapur (early Cretaceous) Formations. Coastal Gondwana succession comprising the Raghavapuram (early Cretaceous) and the Tirupati (Middle to late Cretaceous) Formations transversely superimpose the continental Gondwana. The Continental Gondwana is mainly cross-stratified sandstone and fossiliferous ( Glossopteris and Ptilophyllum flora) clay assemblage of alluvial environment. Palaeocurrent is towards NW in the Kamthi Formation, WNW in the Kota Formation and centripetal in the Gangapur Formation. The Raghavapuram Formation (KGCB) comprises of grey/carbonaceous clay-sandstone association of SE flowing delta plain distributories with inter-distributory swamps and fossiliferous white clay (a mix of Ptilophyllum flora and invertebrate fauna) of prodelta / shallow marine environment. The succeeding sandstone- conglomerate facies of the Tirupati Formation (Middle to late Cretaceous) with SE palaeocurrent indicates delta progradation whereas the infratrappean clay-limestone beds (mainly invertebrate fauna of Maastrichtian age) point to delta abandonment, prior to the K/T boundary volcanism. Tectono-sedimentary model presented in this communication incorporates that the change from the continental Gondwana to coastal Gondwana had occurred in four stages. During stage-I (late Carboniferous to early Jurassic) the sediments were deposited by a north-westerly flowing alluvial system whose distal source was located in southeast beyond the east coast of India. Stage -II (Middle to late Jurassic) is marked by westerly shift in palaeocurrents attributed to an obstruction caused by an upliftment of the Mailaram high parallel to the NE-SW Eastern Ghat trend. Stage-III (early Cretaceous) is heralded by the onset of deltaic sedimentation in SE due to separation of landmass that supplied detritus to the CG until the end of the Jurassic. During stage-IV, the delta progradation is accentuated by the basement uplift (Eastern Ghat Mobile Belt). It is suggested that the change over from continental Gondwana to coastal Gondwana had occurred over a time span of about 70 my between late Jurassic to Cretaceous and controlled by phase-wise upliftment of the basement as a corollary to the Gondwana continental break-up.

Age, origin and climatic signal of English Mesozoic clays based on K/Ar signatures

AbstractThe K/Ar characteristics of 53 clay assemblages (Triassic–Cretaceous), representing the detrital, volcanogenic and arid-facies clay mineral associations, are interpreted in relation to their mineralogy, chronostratic age and geological origins. The K-bearing mineral components of the 1–2 μm, 0.2–1 μm and <0.2 μm fractions of each clay assemblage together display one of two characteristic patterns of K2O and 40Ar values (the K/Ar signature of the assemblage) on a 40Ar/K2O correlation diagram. Interpretation of the K/Ar signatures indicates that: (1) all of these clay assemblages are apparently unaffected by burial diagenetic illitization; (2) the Jurassic and Cretaceous detrital clay assemblages are derived from the reworking of weathered Caledonian metasediments (420 500 Ma) and weathered kaolin-bearing sediments of Upper Devonian/ Carboniferous age; and (3) the role played by palaeoclimate in developing the pattern of clay minerals in the Mesozoic sediments of England is much less significant than previously believed.