Wealden Sediments Research Articles

Provenance, palaeoclimate and palaeoenvironments of a non-marine Lower Cretaceous facies: Petrographic evidence from the Wealden Succession

Petrographic datasets from sedimentary rocks are very useful for reconstructing their palaeoenvironmental settings especially when field and fossil datasets are unobtainable or inadequate. This study presents the first detailed and comprehensive petrographic study of the four formations constituting the non–marine Lower Cretaceous of the Weald Sub-basin of south-east England and employed petrographic descriptions to reconstruct their palaeoenvironmental settings. >120 samples were subjected to petrographic (QEMSCAN®, SEM, and thin section) analyses, which revealed that the framework composition of sediments is largely quartz-dominated (quartz arenites and subarkose) and has minor amounts of feldspar and lithics. The nature of the quartz arenites suggests that the Wealden successions were likely sourced from a stable interior craton/passive platform within a continental block and possibly from recycled flat-lying platforms such as low-lying granitic and gneissic bedrocks. The nature, maturity and the compositions of the sandstones suggest palaeoclimatic conditions were probably mildly hot and moist at the massifs and these perhaps favoured intense chemical weathering. The significant quantity of micas suggests that the Wealden sediments largely experienced very insignificant degrees of turbulence and agitation en-route depositional sites, thus yielding unwinnowed sediments. The presence of micaceous minerals, poor winnowing effect, and the general absence of diagenetic glauconite validate non-marine depositional environments. The palaeoenvironmental changes that occurred in the Lower Cretaceous in the Weald Sub-basin reflect (at least in part) palaeogeographic changes in NW Europe.

The biogenic methane potential of European gas shale analogues: Results from incubation experiments and thermodynamic modelling

Core samples from the Lower Jurassic Posidonia Shale (Lower Toarcian), Wealden black shales (both from the Lower Saxony Basin, northern Germany), and from the Cambrian–Ordovician Alum Shale (southern Sweden) have been investigated for their biogenic gas potential in incubation experiments. The sample set represents all stages of thermal maturity (from immature to overmature). Methane and to a higher degree carbon dioxide are generated from all samples by added microorganisms. In general, overmature samples (Rr>1.5%) show lowest gas generation rates. The highest generation rates for produced methane (up to 89μmolg−1day−1) and carbon dioxide (139μmolg−1day−1) were detected in immature Alum Shale samples, when different hydrocarbon-degrading inocula where added; whereas the lowest methane production rates were detected in Wealden sediments. The lightest δ13C values for produced methane and carbon dioxide have been detected in Posidonia Shale samples of oil window maturity. Such samples are some of the most sensitive for gas generation and exhibit the strongest decreases of hydrogen indices after incubation.Hydrogeochemical modelling of methane generation, fate and behaviour showed that methane formed during the early diagenesis got lost from Posidonia Shale, the mainly investigated shale in this study. As the mature Posidonia Shale is according to the incubation experiments still sensitive for gas generation, a production scenario of generically generated methane due to oil degradation is simulated. The basic conditions resemble the conditions in the Antrim Shale in the Michigan Basin. Gas production due to dewatering and pressure decrease results in an increase of the gas volumes coupled to a progressive enrichment of carbon dioxide in the produced gas. Moreover, calcite scaling may be the consequence during progressive production similar to the Antrim Shale in the Michigan Basin. Such calcite precipitation has been demonstrated to predominantly occur in natural samples at oil window maturity.

Constraints on palaeoenvironments in the Lower Cretaceous Wealden of southern England, from the geochemistry of sphaerosiderites

Abstract: Sphaerosiderites are millimetre-scale concretions of FeCO 3 that form predominantly in waterlogged environments and are of use in palaeoenvironmental and palaeohydrological studies. We present petrographic, elemental and stable-isotopic (δ 13 C, δ 18 O) data from sphaerosiderites collected from the Lower Cretaceous Wealden sediments of southern England. The sphaerosiderites are composed of very pure FeCO 3 , with only small amounts of Ca, Mg and Mn present, suggesting that they are well preserved and were precipitated from fresh groundwater. δ 13 C values display a relatively large range from about −5 to −30‰ (VPDB). In contrast, δ 18 O values are relatively invariant (1σ = 0.83‰), with an average of −3.01‰ (VPDB). The relationship between carbon and oxygen suggests that the sphaerosiderites faithfully record palaeo-groundwater δ 18 O. Likely δ 18 O values for groundwater were calculated using a published, experimentally derived siderite–water fractionation equation and plausible assumed values for palaeotemperature. The average estimate was −4.9‰ (SMOW). The Wealden sphaerosiderite oxygen-isotope values are comparable with other early Cretaceous data from similar palaeolatitudes but lower than estimated Holocene values for equivalent latitudes. This observation may indicate greater rainfall at 30°N during the Cretaceous relative to the Holocene and also suggests relative stability of palaeoclimatic conditions during the early Cretaceous at this palaeolatitude.

The palaeoenvironment associated with a partial Iguanodon skeleton from the Upper Weald Clay (Barremian, Early Cretaceous) at Smokejacks Brickworks (Ockley, Surrey, UK), based on palynomorphs and ostracods

In 2001 a partial skeleton of an Iguanodon was discovered in the Upper Weald Clay (Barremian, Early Cretaceous) at Smokejacks Brickworks near Ockley, Surrey, UK. When the dinosaur was excavated, a detailed stratigraphic section was logged and 25 samples taken for palynological and micropalaeontological (ostracod and megaspore) analysis, including a detailed sample set of the dinosaur bed itself. Qualitative and quantitative analysis of the palynoflora revealed rich and well-preserved non-marine assemblages of pollen and spores, including early angiosperms, and freshwater green algae. Four types of angiosperm pollen are described and assigned to the genus Retimonocolpites Pierce, 1961, but left in open nomenclature. Some marine elements such as dinoflagellate cysts are identified as the result of reworking of Middle and Upper Jurassic sediments. The pollen/spore assemblages depict a vegetational change from principally gymnosperm-dominated assemblages at the base to principally pteridophyte-dominated assemblages at the top of the section. The dinosaur bed shows a pteridophyte-dominated assemblage, with a significantly high amount of the freshwater green alga Scenedesmus novilunaris He Cheng-quan et al., 1992. Samples close to the dinosaur bed yielded the first useful ostracod finds from Smokejacks Brickworks: well-preserved assemblages containing Cypridea clavata (Anderson, 1939), Damonella cf. pygmaea (Anderson, 1941), Stenestroemia cf. cressida Anderson, 1971 and Stenestroemia sp. A, and fragments and damaged valves of a thin-shelled ostracod, possibly belonging to Mantelliana Anderson, 1966. Those identified as Cypridea clavata show a wide range of morphological variety and in opposition to Anderson's (1967, 1985) taxonomic scheme, which would assign them to up to five different taxa, they are considered to be intraspecific variants of a single species. The possibilities and limitations of age determination of the Wealden sediments using palynomorphs and ostracods are discussed; distinct forms of early angiosperm pollen, together with the ostracod fauna, are consistent with an early Barremian age. Pollen and spores are discussed in terms of their parent plants and the reconstruction of vegetation and palaeoclimate. Palynology and ostracods give evidence for temporary freshwater conditions at the time when the Iguanodon died and the carcase was buried.

Termite borings in Early Cretaceous fossil wood, Isle of Wight, UK

A pellet-filled boring in fossil wood is described from the Early Cretaceous Wessex Formation (Barremian), Isle of Wight. The cylindrical boring, approximately 1 cm in diameter, is filled with carbonaceous pellets with a hexagonal shape, preserved within a matrix of pyrite. Features of the boring suggest that it was made by termites that bored into the wood, either when the tree was alive or in the early stages of decay on the forest floor. This evidence of termite activity complements previous records of termite wing fossils and faecal pellets in Wealden sediments and is evidence for social behaviour in Wealden insects. This is one of the oldest records of termite borings in wood.

Early Cretaceous environments

A one day meeting of the British Sedimentological Research Group (BSRG), convened by K. G. Taylor and A. H. Ruffell, was held at the Postgraduate Research Institute for Sedimentology, University of Reading on 23 June 1992. The meeting set out to consider what is known about palaeoenvironments of the NW European early Cretaceous and to present current areas of new research that may significantly expand our understanding of this topic. Over 50 people attended this multidisciplinary meeting, coming from the fields of sedimentology, geochemistry, palaeontology and tectonics. There were 13 speakers and five poster presentations. Financial assistance from the BSRG to speakers from overseas is gratefully acknowledged. The Institute is also thanked for the use of their facilities. After a brief welcome by the conveners, D. J. Batten (Aberystwyth University) gave a talk reviewing his work on the palynology of NW European Wealden sediments. In this he stressed how palynological data, integrated with other available evidence, can contribute significantly to interpretations of early Cretaceous environments and should be utilized more by sedimentologists. In particular, the total recoverable organic matter in a sample should be studied, not just the identification of miospores and dinoflagellate cysts. J. Eyers (Open University) described how detailed sedimentology, grain-size analysis and gamma-ray logging have been used to correlate the Lower Cretaceous of the Bedfordshire Province. Using isopachyte maps, she was able to show that basement structures, reactivated as a result of North Sea rifting, were a major control on sedimentation. She also tentatively compared a combined

Non-marine oolitic ironstones in the Lower Cretaceous Wealden sediments of southeast England

AbstractOolitic ironstones are described from the early Cretaceous non-marine Wealden sediments of southeast England. The recognition of non-marine oolitic ironstones in the geological record is rare and, therefore, warrants further study. The oolitic ironstones described take two forms, named here type-I and type-2 ironstones.Type-1 ironstones contain pisoids and ooids of berthierine together with sandstone fragments and detrital quartz grains. The pisoids (up to 0.5 cm in size) vary from subspherical to highly irregular. The smaller ooids (up to 1 mm in size) are generally ellipsoidal but strongly asymmetrical forms are also present. The form of these pisoids and ooids suggest that mechanical accretion was not the dominant mechanism controlling their formation. It is proposed that this ironstone type formed from the reworking and redeposition of local soil material.Type-2 ironstones, of which only one unequivocal example has been studied, is composed of iron oxide ooids set in a detrital matrix. The ooids are most commonly regularly ellipsoidal and exhibit a decrease in iron at their centres. It is proposed here that the ooids suffered post-depositional iron depletion at their centres, in a similar fashion to that proposed for the recent Lake Chad oolites. There is no unequivocal evidence as to the origin of the ooids.This study is important in that it shows that different ironstones can be formed by different processes essentially within the same environment. Comparison of non-marine oolitic ironstones with the better-developed marine examples should prove a valuable exercise.

Phosphatic concretions in the Wealden of South-East England

Phosphatic concretions are reported from several locations in the Wealden sediments of South-east England. The occurrence of such concretions in the Wealden sediments has not previously been recognised and in some cases in the past they may have been mistaken for siderite concretions. The predominant mineral in these concretions, as determined by X-ray diffraction analysis, is francolite, a hydroxy-carbonate apatite. Both primary concretions from the Weald Clay and reworked examples in the Hastings Beds have been recognised.

A remarkable new early cretaceous trilete spore: with observations on sporoderm function and stratigraphic significance

A new trilete spore is described from the early Cretaceous Wealden sediments of southern England. Ultrastructural investigation using scanning electron microscopy has revealed a complex sporoderm structure, totally unlike any previously described form. This fossil material provides some evidence of the mode of spore wall development. The functional significance and the stratigraphic potential of this new spore in the correlation of Wealden sediments is discussed.

Sedimentary succession and palaeoenvironments within a fault-controlled basin: The ‘wealden’ of the Santander area, northern Spain

The so-called Wealden sediments are no less than 2000 m thick in Santander and north Burgos (northern Spain). The succession rests on an extensive scoured and channeled surface over marine carbonates of Early Callovian age, and ends with the incoming of the Urgonian marine strata and equivalents (Lower Aptian). Two major sedimentary cycles (‘megacylothems’) and the beginning of a third are recognized. Following a widespread break in the succession, each megacyclothem opens with a coarse-grained clastic interval of fluviatile origin and then grades up into finer-grained clastics and carbonates recording fluviatile, lacustrine and shallow-marine conditions. The succession accumulated in an elongated fault-bounded basin trending E—W. Detritus came from source areas lying to the south and west of the depositional area, which was at times connected with the sea to the east and possibly to the north. The development of the basin was controlled by the reactivation of Late Hercynian faults acting contemporaneously with sedimentation. The frequency of faulting varied considerably, periods of relative calm alternating with periods of strong activity. These determined the characteristics of the successive megacyclothems.

Continental sedimentation, soil genesis and marine transgression in the basal beds of the Cretaceous in the east of the Paris Basin

ABSTRACTIn the eastern Paris Basin, on the boundary of two French ‘départements’, Marne and Meuse, the Cretaceous deposits begin with Wealden facies assigned to the Valanginian. This is overlain by marine Hauterivian and Lower Barremian.Several techniques have been used for studying these deposits: light‐microscope, scanning electron microscope, grain‐size analysis, chemical and mineralogical analysis.Environmental interpretations of the various facies are based on sedimentological, geochemical and pedological criteria (fossil soils are present). Clay minerals are dominantly illite and kaolinite. In the soil horizons kaolinite is dominant. A mixed‐layer illite‐smectite appears at the base of the transgression. Under marine influence a more stable illite dominant suite is formed.The Wealden sediments appear to have been deposited in a very mature flood‐plain. Part of the sequence was subjected to weathering in situ by soil processes under a warm climate with repeated wetting and drying. Paraconformably above, the marine Hauterivian is the product of a shallow sea that tended to form lagoons. A true marine environment was established in the Lower Barremian, when mixing homogenized the sediments.

Exkursion in den Malm und den tiefen Wealden am südlichen Deister am 12. Oktober 1956

Exkursion in den Malm und den tiefen Wealden am südlichen Deister am 12. Oktober 1956