bluuebunny/crossref_metadata_2025 huggingface dataset:
Sedimentology and structure of the Scotland Group, Barbados
Authors: Pudsey C. J. , Reading H. G.
Published: May 2008

The lower to middle Eocene Scotland Group is about 1700 m thick at outcrop and can be divided into: (A) The Lower Scotland Formation consisting of (1) the Walkers Member (500 m): graded turbidite sands and clays, arranged in sand-dominated and clay-dominated packets; (2) the Morgan Lewis Member (200 m): almost all clay, with two sand intervals; (B) the Upper Scotland Formation consisting of (3) the Murphys Member (about 350 m): a thickening and coarsening upward sequence including slumped beds; (4) the Chalky Mount Member (0–250 m): coarse and conglomeratic sands with large slide blocks and minor interbedded muds; (5) the Mount All Member (about 500 m): coarse and fine turbidite sands and clays, much disturbed and displaying no overall sequence. The group was deposited by mass-flow processes in a deep marine environment, probably in a deep-sea trench or a submarine fan.

Current flow was from the SW and mineral composition supports derivation from South American metamorphic rocks. Clay mineralogy shows an upward trend from mainly smectite to mainly kaolinite.

An ENE structural trend is present throughout the island and is anomalous in the present plate-tectonic setting adjacent to a north-south trending island arc. Deformation increases in complexity up the succession; this may be due to sedimentary slumping, partly a result of increasing disturbance by rising mud diapirs. This locally culminated in extrusion of hydrocarbon-rich mud—the overlying Joes River Formation. The Barbados Ridge probably comprises clastic sediments deposited both in a trench and on the Atlantic Ocean floor.
Subduction and tectonics on the continental margin off northern Spain: observations with the submersible
Cyana
Authors: Malod Jacques-André , Boillot Gilbert , Lepvier Claude , Mascle Georges , Taugourdeau-Lantz Josette , Capdevila Raymond , Dupeuble Pierre-Alain , Müller Carla
Published: May 2008

Short-term latest Mesozoic-early Cenozoic convergence between the Iberian and European plates transformed the passive north Spanish margin into an active margin with southward subduction of the oceanic crust of the Bay of Biscay. This margin became passive again when subduction ceased in late Eocene or Oligocene time. Nine dives with the submersible
Cyana
allowed us to sample the rocks outcropping on the Le Danois Bank slope. A Mesozoic sequence includes shelf limestones of the Dogger to the Lower Cretaceous, and Lower Cretaceous detrital sediments (sandstones, conglomerates, pelites); it overlies a typical Palaeozoic Asturian basement.

The main objective of the diving campaign was to study the structure of these strata in order to elucidate deformation of the continental wall of an active margin during its early stages of evolution, before the formation of an accretionary prism. The occurrence in Lower Cretaceous pelites of a weak schistosity in a reversed fold limb gives evidence of compressive structures, probably northward-verging overfolds. Additional evidence of compressive tectonics is the dislocation of quartz grains and dolomite crystals in Lower Cretaceous sandstones and Jurassic dolomites. These observations suggest that the continental slope in the Le Danois Bank area has a predominantly imbricate structure. Deformation and uplift of the northern Spanish margin appear to be related to the early stages of subduction of the European plate under Iberia.
Subduction in the Hellenic Trench: probable role of a thick evaporitic layer based on Seabeam and submersible studies
Authors: Le Pichon X. , Huchon P. , Angelier J. , Lybéris N. , Boulin J. , Bureau D. , Cadet J.P. , Dercourt J. , Glaçon G. , Got H. , Karig D. , Mascle J. , Ricou L.E. , Thiebault F.
Published: May 2008

A Seabeam survey of the Hellenic Trench in 1978 was followed by a submersible survey in 1980. Fifteen dives were completed between 1500 and 3000 m covering 58 km on the bottom and obtaining 48 samples. Thus, the Hellenic Trench is probably now the most intensively studied trench as previous work included seismic reflection, dredging, coring, bottom photography, side-scan sonar and drilling. Because of the high sedimentation rate and the relatively shallow depth, we did not expect outcrops except along faults and in canyons. Active tectonics with numerous faults and folds have been observed. But we have also discovered vertical cliffs with hard rock exposure over a depth range of 1000–1500 m. In this paper, we discuss the nature of the hard rock outcrops observed both on the inner wall and on two hills that we interpret as diapirs. We conclude that at least part of these hard rocks are probably Messinian evaporites and concentrate on the tectonic role of this evaporitic layer in the Hellenic subduction. It is likely that the evaporites do not subduct but tend to pile up in the trench, to form a floating evaporitic basin.
Detailed tectonic trends on the central part of the Hellenic Outer Ridge and in the Hellenic Trench System
Authors: Kenyon N. H. , Belderson R. H. , Stride A. H.
Published: May 2008

Extensive new sonograph coverage in the Eastern Mediterranean allows a re-assessment of the complex relief outside the Hellenic Arc. The trenches to the SE and SW of Crete are similar in their plan view appearance, each having predominantly sinuous or curved structures that are interpreted as the surface expression of major thrusts. Thrusting, possibly imbricate, is also suggested on the central, shallowest part of the Hellenic Outer Ridge where interlocking convex-outward, curved structures occur, along with other faults and folds. These are thought to relate to décollement-type displacement within or beneath evaporite sediments. The structural style deduced from the morphology is consistent with a continuing outward radial push of the Hellenic Arc to the SW, S and SE, rather than with a migration to the SW and strike-slip on the SE side of the arc as proposed by other workers.
The structure of the Calabro-Sicilian Arc: result of a post-orogenic intra-plate deformation
Authors: Wezel Forese Carlo
Published: May 2008

Morphotectonic and geological data show that this arc constitutes a post-orogenic ring-like system concentric round the Marsili ‘backarc’ basin. Peripheral concentric features comprise the ‘Aeolian andesitic ring’ and the arcuate troughs and ridges characteristic of the ‘forearc’ region. The Hyblean horst-like plateau and the Strait of Sicily represent the block-faulted foreland of the Sicilian segment of the Calabro-Sicilian Arc. The present shape of the arc has largely resulted from Plio-Quaternary vertical tectonic movements, superimposed on compressive structures caused by previous phases of tectonic deformation. The effect can be visualized as an outward-moving topographic wave producing inversion of tilting movements. This kinematic process (termed ‘krikogenesis’) is postulated as being related to midplate domal upwarp, dilatation and cauldron-like collapse of the Marsili block in the core of the arcuate system. Volcanicity appears to be associated with various stages of this geological history. Rather than originating by collision of two rigid plates, present-day morphological, volcanic and seismic configurations seem to be due to interaction of intra-plate ‘arches’, activated by local mantle movements.
Deformation of the Makran accretionary sediment prism in the Gulf of Oman (north-west Indian Ocean)
Authors: White Robert S.
Published: May 2008

The offshore Makran of Pakistan and Iran in the Gulf of Oman forms the seaward margin of a folded and faulted accretionary sediment wedge which extends several hundred kilometres inland across the onshore Makran. Sediment is being scraped off the oceanic Arabian plate as it subducts shallowly northward beneath the Eurasian plate. The submerged front of the accretionary prism has been traced along the entire width of the Makran continental margin, from the Strait of Hormuz in the west to the boundary with the Indian plate in the east. The structure of a 70 by 90 km portion of the deformed sediment belt was mapped in detail by a grid of continuous seismic reflection profiles. This survey demonstrates that the folds are laterally continuous and are well lineated, that the deformed belt appears to be migrating southward at a rate of about 10 km every million years and that folding of the initially undisturbed abyssal plain sediments occurs in the southernmost, or ‘frontal’ fold. Mapping of the three-dimensional structure of the frontal fold shows that it systematically increases then decreases in height along strike over a distance of about 80 km. Folding is initially restricted to the uppermost 2½ km of the sediment pile, and there is local evidence of small-scale shale diapirism. The frontal fold is then incorporated into the accretionary prism by uplift along a thrust fault which flattens out within the sedimentary section, possibly in overpressured shales. There is little subsequent deformation of the uplifted material or of the sediment which infills the basins between the fold ridges until some 70 km north of the frontal fold when further rapid uplift occurs, eventually elevating the sediment above sea-level. I attribute this second phase of uplift to major imbricate thrust faulting extending down to the oceanic basement; most of the shortening and thickening of the sediment wedge occurs in this region. The relatively simple pattern of uplifted folds and intervening basins in the frontal 70 km of the accretionary wedge results from a balance of compressive and normal stresses on the thick sediment section. This simple deformational style of the seaward part of the accretionary prism can be traced eastwards until it becomes chaotically disrupted by the consumption of a basement ridge which protrudes to just beneath the seafloor.
The Franciscan Complex of northernmost California: sedimentation and tectonics
Authors: Aalto K. R.
Published: May 2008

The Franciscan Complex consists, from east to west, of a belt of foliated (textural zone 2 and 3) rocks deformed beneath the Coast Range thrust (CRT), a belt of broken formation (BF) and a belt of sheared olistostromes (SO) consisting of conglomerate, greywacke, limestone, chert, greenstone, plutonic and glaucophane schist blocks in chiefly argillaceous matrix. South of the Klamath River, zone 2 and 3 rocks are of an outer fan or basin-plain facies and have overthrust midfan unfoliated (textural zone 1) BF in which suprafan lobes are preserved as megaboudins. To the north, foliated rocks are in gradational contact with similar BF and two episodes of thrusting were confined to the CRT. Blocks of chert, gabbro and metavolcanic rock were tectonically transported along thrust faults. Greywacke and conglomerate of the SO belt are inner fan deposits. The BF-SO contact is gradational, delineated by the westward appearance of greenstone, chert and plutonic blocks increasing progressively in size and/or abundance. Olistostrome units are intercalated with isoclinally folded, recycled sediments. BF and SO greywackes are commonly feldspathic litharenites with mean parameters (Dickinson 1970):
Q
= 54.6,
C/Q
= 0.28,
F
= 21.6,
P/F
= 0.97,
L
= 23.8 and
V/L
= 0.82. Sediments were deposited in a trench slope basin and intercalated with olistostromes derived from elevating portions of the accretionary basement. Progressing upwards in the local sequence north of the Klamath River, the gradational SO-BF transition, decrease in overtuned beds and a retrograding fan sequence reflect basin filling, diminished faulting and lessened relief concomitant to basin elevation.
Sedimentation, metamorphism and tectonic accretion of the Franciscan assemblage of northern California
Authors: Blake M. C. , Jayko A. S. , Howell D. G.
Published: May 2008

The Yolla Bolly terrane of the Franciscan assemblage in northern California is seemingly a typical subduction complex, having undergone penetrative deformation and metamorphism to the high pressure-low temperature blueschist facies. Detailed mapping combined with sedimentological analysis has enabled us to: (1) reconstruct a probable palaeosedimentary environment, (2) analyse the interaction during and after subduction between deformation and metamorphism, and (3) speculate on subsequent deformational history including tectonic accretion of the terrane to North America.

Rocks of the Yolla Bolly terrane consist of three thrust-fault-bounded lithological units: a lower unit of disrupted mudstone and thin-bedded sandstone (broken formation) containing scarce volcanic and radiolarian chert horizons, a middle unit predominantly of thick-bedded to massive sandstone (metagreywacke) that includes several horizons of radiolarian chert, and an upper unit of mudstone and thin-bedded sandstone (broken formation) with numerous intrusive and extrusive volcanic rocks plus rare radiolarian chert. Radiolarians from all three units are of the same age (Tithonian to Valanginian) and together with the sedimentological data, suggest that the rocks represent a continent-derived submarine fan, deposited in a complex transform graben possibly similar to the present-day Gulf of California or basins of the California Continental Borderland, rather than a trench setting.

Metagreywacke containing lawsonite and aragonite yields radiometric ages of approximately 110 Myr and indicate that these rocks were subducted to depths of 20–30 km about 30 or 40 Myr after they were deposited. Shortly after subduction, the rocks were probably involved in a collision that imbricated and tectonically returned the subduction complex to the surface.
Deformation of partly dewatered and consolidated Franciscan sediments near Piedras Blancas Point, California
Authors: Cowan Darrel S.
Published: May 2008

Bedded sequences of turbidite sandstone and mudstone and associated chaotic, polymict pebbly mudstone record a post-depositional, pre-metamorphic deformation. Sandstone layers were extended and locally disrupted and fragmented and most clasts in pebbly mudstone were deformed into oblate ellipsoids. On the scale of outcrops and hand-specimens, extension in sandstone was accomplished by: (1) mesoscopically ductile pinch-and-swell behaviour and boudinage; (2) extreme necking, which locally resulted in extension fractures or brecciation; and (3) sets of parallel shear fractures inclined to bedding. On smaller scales, ductile changes in shape were accommodated by both intergranular flow of sand-rich sediment and slip along zones typically 0.1 mm or less in thickness. Microfracturing, granulation, and cataclasis of clastic grains are less commonly observed, even adjacent to microfaults. Mudstone was also mesoscopically ductile but generally it flowed by displacements on penetrative, anastomosing surfaces of slip and was transformed locally into ‘scaly clay’. These Franciscan sediments probably contained appreciable pore fluids when they were deformed, since the deformation of unconsolidated sands by homogeneous, intergranular flow rather than widespread cataclasis is fostered by high interstitial fluid pressures. More localized discontinuities, such as extension fractures, shear fractures, and microfaults, formed where sand had more completely dewatered and consolidated. Thus, material response may partly reflect water content and consolidation history. The axially symmetric extension recorded by small-scale structures suggests that sediments were deformed in a shallow-level, laterally unconfined environment on the trench slope, perhaps in response to vertical shortening and horizontal elongation that accompanied gravitational collapse and spreading.
Initiation and evolution of the Great Valley forearc basin of northern and central California, U.S.A.
Authors: Ingersoll Raymond V.
Published: May 2008

The late Mesozoic Great Valley forearc basin of northern and central California evolved from a residual forearc basin formed on top of young oceanic crust to a composite forearc basin resting on both oceanic and continental crust. Depositional environments preserved in outcrop along the west side of the Sacramento Valley evolved from deep oceanic floor and slope in the Late Jurassic to basin plain in the Early Cretaceous to submarine fan complexes in the Late Cretaceous to slope and shelf in the Palaeogene. The forearc basin widened and enlarged, and was supplied with voluminous sediment primarily derived from the coeval magmatic arc to the east. However, significant quantities of ophiolitic detritus (chert, and sedimentary, mafic volcanic and mafic metavolcanic lithic fragments) occur in the lower Great Valley strata, thus indicating erosion of ‘tectonic highlands’ formed during arc-arc collision immediately preceeding initiation of the late Mesozoic subduction regime.

The Great Valley forearc owes its geometry and history to the peculiarities of subduction initiation and to the shapes of the continental margin and magmatic arcs extant before initiation of the Great Valley subduction phase. The residual forearc developed on oceanic crust that had formed previously by spreading behind an east-facing intraoceanic arc. After this east-facing arc collided with the west-facing continental-margin arc, a new trench formed west of the suture belt. The shape of the new continental margin was irregular, so that a wide residual forearc basin formed in the Great Valley area, whereas no residual forearc basin formed to the north (Klamath area) or possibly, to the south (southern California borderland area that has been dislocated by Neogene strike-slip motions). The northern coastal promontory (Klamath area) provided much of the ophiolitic detritus within the lower part of the Great Valley strata. Sediment derived from the Klamaths was transported southward directly into the forearc basin, as well as westwards and then southwards into the trench west of the forearc basin. The Great Valley forearc basin is preserved and displayed so well today due to its mode of initiation (residual forearc formed in a previously backarc area) and its mode of termination (northward migration of a triple junction that converted a convergent margin into a transform margin).
Cretaceous-Palaeogene Flysch Zone of the East Alps and Carpathians: identification and plate-tectonic significance of ‘dormant’ and ‘active’ deep-sea trenches in the Alpine-Carpathian Arc
Authors: Hesse Reinhard
Published: May 2008

The Flysch Zone of the East Alps is interpreted as a Cretaceous-Palaeogene deep-sea trench fill that accumulated over a period of about 70 Ma when the subduction zone associated with the trench was dormant. In Aptian-Albian time the trench had an elongate, at least 100 km (possibly 200 or 300 km) long basin plain located below the calcite compensation level; sediment input was from two lateral sources (small acute-angled fan or broad feeder channel exposed in the Falknis- and Tasna Nappe; submarine canyon facies in the area north of Salzburg). The existence of a basin plain has also been proved for the Campanian by the recognition of sheet-like turbidites that are continuous for 50 km. The small number of lateral sediment sources is attributed to the existence of slope basins that intercepted the downslope movement of turbidity currents. The lack of syndepositional deformation and of volcanism and volcaniclastic detritus show that sediment accumulation in the trench was not accompanied by active subduction.

This 500 km long and 10 (in the east perhaps 80 or 100) km wide trench of the East Alps was connected with the 1000 km long Carpathian trench whose western segment was also not associated with subduction until Oligocene time. The East Carpathian flysch, however, experienced continuing deformation and (relative) eastward migration of the depositional trough from Albian to Oligocene time. Huge Neogene volcanic centres of the Inner Carpathians are associated with the final diastrophic phases both in the West and East Carpathians, whereas in the East Alps there is no volcanism because subduction does not seem to have reached a great enough depth. The East Alpine-West Carpathian segment of this trench system may represent a major Cretaceous-Palaeogene transform fault zone separating Europe and its bordering seas in the north from the Cretaceous Penninic-Pannonian plate in the south. The latter has been eliminated almost entirely from the surface due to Late Cretaceous-Palaeogene subduction at its southern boundary nearly juxtaposing the European plate (Helvetic and Ultrahelvetic domains) with the Austro-Alpine/South Alpine (plus Adriatic?) plate. Postulated strike-slip motion in the west-east trending segment of the trench was followed by convergent motion, when southward subduction (of the European plate) resumed in late Eocene-Oligocene time.

Combination of an eastward facing trench with active subduction and a dormant trench along a transform fault zone suggests an analogy between the Alpine-Carpathian Arc and the modern West Atlantic loop arcs in the Caribbean and South Scotia Seas. The geodynamic link between these loop-arc structures could be the westward drift of major lithospheric plates between which smaller plates remain relatively stationary because they are ‘sheltered’ behind westward-dipping subduction zones that are more or less anchored in the asthenosphere.
The anatomy of a Lower Palaeozoic accretionary forearc: the Southern Uplands of Scotland
Authors: Leggett J. K. , McKerrow W. S. , Casey D. M.
Published: May 2008

The Southern Uplands of Scotland consists of an accretionary prism which had a prolonged (over 45 Ma) development off the SE coast of ancient North America (Laurentia) during the late Ordovician and Silurian. To the north, a forearc basin occupied the Midland Valley. An arc massif of older metamorphic rocks in the Grampian Highlands, and capping calc-alkaline volcanics, supplied much of the sediment to the trench. In its early development (late Ordovician) the accretionary complex incorporated slivers of ocean-floor material as well as thick turbidites into a steep lower trench slope. Later, in the Silurian, a trench slope break emerged shedding sediment north into the Midland Valley forearc basin. By that time only turbidites and black shales were being accreted. The Southern Uplands is dominated by coherent strata, and, despite intense imbrication, is devoid of mélange. We conclude that the accretionary prism was the result of high sediment input to the trench and very slow, oblique, subduction of the Iapetus Ocean eastwards below Scotland. Décollement surfaces developed during accretion preferentially utilised a highly incompetent black shale layer near the base of the subducting sequence. Prior to the inception of accretion (in the Caradoc), we postulate sediment subduction, and possibly subduction erosion, during the Cambrian and early Ordovician.
Sedimentology, volcanism, structure and metamorphism of the northern margin of a Lower Palaeozoic accretionary complex; Bail Hill-Abington area of the Southern Uplands of Scotland
Authors: Hepworth Barry C. , Oliver Grahame J. H. , McMurtry Michael J.
Published: May 2008

NE-SW faults in the Bail Hill-Abington area of the Northern Belt of the Southern Uplands define blocks up to 3.2 km wide. The strata, folded and locally overturned, young predominantly to the NW but successive blocks to the SW contain progressively younger sequences. Analogous configurations occur in modern accretionary margins. The oldest rocks are Arenig basalts, dolerites, cherts and brown mudstones underlying red shales, possibly Llanvirn, and black fossiliferous shales and cherts of Llandeilo and Caradoc age.

Trench sediments overlying pelagic sequences represent a range of depositional mechanisms. Rudites and associated fine-grained lithologies of lateral origin relate to a lower trench slope canyon system, whilst axially transported sands, originating on the lower trench slope, were deposited by turbidity currents and related flows.

The Bail Hill Volcanic Group (Upper Llandeilo) represents a mildly alkaline seamount in the Iapetus Ocean, with volcanic activity spanning the transition from pelagic plate to trench sedimentation before accretion.

Faults, initially low-angle thrusts, and bedding were rotated through the vertical within the accretionary complex, pre-dating or accompanying slaty cleavage development. Soft sediment deformation, two fold phases and a kink-band set are recognized. Imbricate fault zones located in incompetent pelagic sequences are tentatively equated with tectonic mélanges of other accretionary complexes.

Index minerals, illite crystallinity and ‘vitrinite’ reflectance establish metamorphic grade as a zeolite to prehnite-pumpellyite facies.
Sedimentary facies associations within subduction complexes
Authors: Underwood Michael B. , Bachman Steven B.
Published: May 2008

Sedimentation patterns within modern subduction zones are complex and variable, and do not necessarily follow models of submarine fan sedimentation. Environmental reconstructions within ancient subduction complexes should follow modern analogues as closely as possible and consider several criteria, including turbidite facies associations, vertical depositional cycles, regional palaeocurrent patterns, and in many cases, structural style and sandstone petrology.

Important variables in trench sedimentation include the volume and texture of sediment entering the trench and the distribution of major sediment sources, especially large submarine canyons. Sediment transport in the trench is commonly longitudinal, although locally, such as at the mouth of a submarine canyon, flow may be at a high angle to the continental margin.

Patterns of trench-slope sedimentation depend largely upon the topography of the slope. In general, coarse sediment is either trapped behind tectonic ridges (within slope basins) or bypasses the slope via submarine canyons. Background sedimentation is dominated by hemipelagic settling. Current directions are commonly at a high angle to the margin, but longitudinal flow may occur within large elongate slope basins. Major facies associations include submarine-canyon, slope, mature-slope-basin, and immature-slope-basin.
Evolution of the slope landward of the Middle America Trench, Nicoya Peninsula, Costa Rica
Authors: Lundberg Neil
Published: May 2008

The Nicoya Peninsula of Costa Rica represents an uplifted portion of the trench slope break landward of the Middle America Trench, and is composed of the Nicoya Complex and its sedimentary cover. The Nicoya Complex is upper Mesozoic oceanic crust, showing the effects of continued igneous activity and deformation probably due to the Late Cretaceous initiation of subduction in an oceanic region. The sedimentary cover reflects uplift and progressive deformation throughout the evolution of this intra-oceanic arc-trench system.

Sediments of the Nicoya Complex are open-ocean deposits, comprising locally derived sedimentary breccias made up of recycled Nicoya Complex material as well as more typically pelagic radiolarian cherts, black shales and deep-water limestones. The basal unit of the sedimentary cover is the mainly hemipelagic Campanian Sabana Grande Formation, which was probably deposited on the juvenile trench slope. In the Sabana Grande Formation, basal siliceous mudstones give way up-section to the foraminifer-rich calcareous mudstones, representing the passage of the sediment surface up relative to the CCD. Interbedded conglomerates and sedimentary breccias reflect recycling of Nicoya Complex material, apparently by uplift and erosion of the forearc basement. The Sabana Grande Formation is overlain by the ?Campanian to Palaeocene Rivas and Las Palmas Formations which consist of thick, mainly volcanogenic, thin- to thick-bedded turbidites and massive and/or pebbly sandstones characteristic of mid-fan facies associations. These rocks were probably deposited in a forearc basin situated landward of a structural high. In contrast, Palaeocene turbidites which crop out to the SW and seaward of the proposed high are thin-bedded, associated with redeposited hemipelagic mudstones, and interpreted as trench slope deposits. This thin-bedded turbidite/hemipelagic mudstone unit is in turn overlain by Palaeocene or Eocene calcareous mudstones and Eocene siliceous mudstones, both of which are interpreted to represent continued deposition on the trench slope, respectively above and below the CCD. These ?Palaeocene and Eocene mudstones are overlain unconformably by Eocene to ?Quaternary shallow-water clastic and carbonate deposits, reflecting rapid Eocene tectonic uplift. The general trend of shallowing in depositional environments through time is paralleled by an overall decrease in intensity of structural deformation up-section, indicating progressive deformation in this forearc terrane.
Lichen weathering of minerals: implications for pedogenesis
Authors: Wilson M. J. , Jones D.
Published: May 2008

The weathering of primary rock-forming minerals beneath crustose lichens is due principally to the activity of the fungal component in excreting extracellular organic acids, particularly oxalic acid. This results in extensive surface etching of the minerals, the conversion of some minerals to siliceous relics, the formation of poorly ordered weathering products and the crystallization of oxalates at the rock/lichen interface and within the lichen thallus. Interpretation of these features is facilitated by the fact that they can be simulated experimentally. The relevance of lichen weathering to the pedogenic weathering predominant in soils under a cool temperature climate is discussed.
Porewater reactions in the unsaturated zone with special reference to groundwater quality in England
Authors: Spears D. A.
Published: May 2008

Porewater composition is an important control on the reactions leading to the formation of residual deposits. Although the mobility of porewater means it is not retained in ancient residual deposits, porewater movement through the unsaturated zone is a key factor in aquifer recharge. Porewater composition provides a sensitive means of monitoring reactions, and if infiltration has been operative over sufficient period of time whole rock changes may also be detected. Reactions involving different sedimentary fractions (resistate, hydrolysate and precipitate) are considered for infiltration into the Permo-Triassic sandstones and the Chalk of England. Externally derived elements may be identified and their behaviour in the unsaturated zone described. The examples considered are rainwater Na and Cl, fertilizer NO
3
and sewage effluent Cu, Zn and Ni.
A review of experimental weathering of basic igneous rocks
Authors: Cawsey David C. , Mellon Paul
Published: May 2008

Attempts to reproduce and accelerate in the laboratory the effects of weathering on minerals are reviewed, together with some observational evidence of actual weathering effects. Laboratory experiments confirm the importance of water as the main agent of chemical weathering and that the progress of weathering is controlled by drainage, oxidation and pH. Some practical applications of accelerated weathering to engineering problems are briefly described, including a short description of the authors’ own work. The relative importance of chemical and physical weathering processes is considered briefly.
Kaolinisation and the formation of silicified wood on late Jurassic Gondwana surfaces
Authors: Wopfner H.
Published: May 2008

In some parts of Australia and Africa intense kaolinisation took place on late Jurassic land surfaces, transforming considerable thicknesses of bedrock into masses of pure kaolin and quartz. Overlying these surfaces occur fluviatile, kaolinitic sandstones of late Jurassic or earliest Cretaceous age. They contain an abundance of silicified wood, including whole trunks of trees. The formation of silicified wood has previously been attributed to hot and arid desert environments, but in the examples described here, silicification proceeded under constantly high groundwater levels, requiring humid and probably warm climatic conditions. Silica was liberated by the kaolinisation of silicate minerals. Humic acids, produced during the initial decay of the fossil timber provided the low pH microenvironment for the fixation of SiO
2
.
Kaolinitic weathering profiles in Brittany: genesis and economic importance
Authors: Esteoule-Choux J.
Published: May 2008

In Brittany, kaolinitic weathering profiles are developed on a variety of sedimentary, metamorphic and igneous rocks. In many places, laterite is associated with ironstone and/or silicified horizons. These profiles developed during the Cretaceous and Eocene, under hot wet climates: some of them are important commercial deposits.
The origin and occurrence of Devon Ball Clays
Authors: Vincent A.
Published: May 2008

Valuable deposits of two types of kaolinitic clay (China Clay and Ball Clay), are found in the South-west of England.
China Clay
is comparatively coarse grained, well-ordered kaolinite derived from granite. Kaolinite particles are also derived from weathering of other country rocks, but such particles are fine-grained, disordered kaolinite. The properties of the two types of kaolinite are different. The two sedimentary
Ball Clay
basins of Devonshire occur in association with the Sticklepath Fault system. The properties of the various seams depend on the provenance of the kaolinite particles and the complex sedimentology of the two basins.
The Ayrshire Bauxitic Clay: an allochthonous deposit?
Authors: Monro S. K. , Loughnan F. C. , Walker M. C.
Published: May 2008

Since the original study of the Ayrshire Bauxitic Clay by Wilson in 1922, new sections have been examined in both quarries and boreholes. It has also been suggested that the Ayrshire Bauxitic Clay is, in effect, a typical flint clay comparable with those of North America, South Africa, Australia and elsewhere. A re-examination of the outcrop and borehole evidence has been undertaken with a detailed chemical, mineralogical and petrographical study of the core of a bore that penetrated both the Ayrshire Bauxitic Clay and associated altered basalts. The study presents a body of evidence which suggests that, in part at least, the Ayrshire Bauxitic Clay is allochthonous in origin.
A low level laterite profile from Uganda and its relevance to the question of parent material influence on the chemical composition of laterites
Authors: McFarlane M. J.
Published: May 2008

A 78 foot (24 m) ‘low level’ laterite profile from Tira, in Busia District, Uganda, is described. Mineral and chemical progressions, from the parent rock through saprolite and into the laterite, indicate its residual development. The chemical compositions of the residuum and the parent material are compared with further data from Uganda and other published results in order to assess the importance of the Fe, Al and quartz content of parent rocks in determining the chemical composition of a laterite. Quartz-richness of parent material does not appear to be disadvantageous for bauxite formation. Neither is Al-richness advantageous. This leads to the conclusion that the absence of bauxite in Uganda may be attributed to unfavourable leaching conditions rather than, as formerly believed, to the absence of suitable parent rocks.
Geochemistry of a nickeliferous laterite profile, Liberdade, Brazil
Authors: Esson J.
Published: May 2008

The 23 samples studied represent a complete vertical profile, 11 m thick, from topsoil to bedrock. Multiple bedrock samples were collected to obtain good estimates of bedrock variability and bulk composition. Nineteen of the 22 major and trace elements studied fall into five behavioural groups within each of which all element pairs show strong positive correlations: (1) Al, Ti, V, Nb and Zr are very highly enriched (factors 20–65 relative to bedrock) in horizon B; (2) Fe, Cr, Sc, Cu and Ce are also strongly enriched (factors 4–40) in horizon B, but show slightly greater enrichment (factors 10–50) in horizon C; (3) Ni, Y, La, Nd and Zn are characterized by concentrations (enrichment factors 8–90) in horizon C of 5–10 times their levels in horizon B; (4) Mn and Co are intermediate between (2) and (3); (5) Si and Mg are strongly depleted in all soil horizons. A mass-balance model, based on quantitative Ti retention, gives relative wt% losses over the profile as a whole of MgO 99.5, Na
2
O 99.3, CaO 98.5, SiO
2
98.3, K
2
O 96, NiO 91, CoO 89, MnO 88, Cr
2
O
3
83, Fe
2
O
3
79, Sc 78, Zn 75, La 69, Ce 65, Al 59, V 57, Nb 40, Nd 19, Y 12, TiO
2
0. Zr and Cu show
ca
. 10% relative gains, indicating stronger retention than Ti.
In situ
leaching of an estimated serpentinite thickness in excess of 200 m is required to produce 11 m of residual laterite.
Post-depositional reddening of late Quaternary coastal dune sands, north-eastern Australia
Authors: Pye K.
Published: May 2008

Deeply weathered quartz sand dunes of late Quaternary age occur at Cape Bedford and Cape Flattery on the seasonally humid tropical east coast of North Queensland, Australia. Many dunes have deep podzolic profiles with reddened B horizons up to 8 m thick. The red colour is produced by iron oxides and hydroxides which, together with kaolinite and minor gibbsite, form interstitial matrix and cutans on quartz grains. The red sands are generally uncemented, but nodular ironstone layers are present in some sections. Radiocarbon dating has shown that one red podzol profile near Cape Flattery has formed in the last 7500 C-14 years. Progressive podzolization has removed much of the iron pigment from the oldest dunes in the area, which are >48 000 C-14 years old. These dunes are degraded and contain subsurface horizons which are cemented by humate.
Origin of red beds in a moist tropical climate (Etruria Formation, Upper Carboniferous, UK)
Authors: Besly B. M. , Turner P.
Published: May 2008

The Etruria Formation is a Westphalian mudstone-dominated red bed succession, which is interdigitated with, and overlies coal-bearing paralic sediments in Central England. Sedimentological studies indicate that the formation mainly comprises swamp and muddy alluvial plain deposits. The distribution of red beds shows that the red coloration formed in areas of improved drainage soon after deposition, and was associated with the formation of complex palaeosols. The occurrence of coals and humid climate floras within the red bed succession suggests that the onset of oxidizing conditions was not the result of climatic change. There are no fundamental differences in the mineralogy of associated grey and red beds: both have distinctive residual characteristics, and consist mostly of kaolinite and quartz. The alluvium was derived from intensely weathered source areas, and was reddened during and soon after deposition by the dehydration of detrital ferric hydroxides and the oxidation of ferrous iron associated with organic material. Palaeomagnetic results support the view that reddening took place shortly after deposition, when the area lay in close proximity to the Carboniferous equator. The Etruria Formation thus provides an excellent example of the genesis of red beds as muddy alluvium under moist tropical climatic conditions.
Environment of silcrete formation: a comparison of examples from Australia and the Cologne Embayment, West Germany
Authors: Wopfner H.
Published: May 2008

Silcretes are subdivided into three main groups, depending on whether their matrix has been formed by: I crystalline quartz growth; II cryptocrystalline quartz aggregation or III formation of cristobalitic-tridymitic silica (opal CT). Groups I and II silcretes are usually associated with intense kaolinisation and formed in low-pH environments, where Al
2
O
3
solubility exceeds that of SiO
2
. Palludal regimes in moist/warm climates provide such conditions. Group I silcretes are purely diagenetic, whereas pedogenic processes play an essential role in the formation of Group II silcretes. Group III silcretes, composed primarily of cristobalite lepispheres, developed primarily under oxidising conditions in arid, alkaline environments. These genetic models, originally developed from observations on Australian silcretes, are applied with Tertiary silcretes of the Cologne Embayment in western Germany. SEM investigations of grain surfaces and fabrics of these so-called ‘lignite quartzites’ show that they are comparable with the Australian examples, with the exception that cristobalite-lepispheres are not exclusive to Group III silcretes, but occur also in porcellanitic interlayers of Group I silcretes, where the parent material consists of very fine grained siliciclastics.
Silcrete in Western Australia: geomorphological settings, textures, structures, and their genetic implications
Authors: van de Graaff W. J. E.
Published: May 2008

Tertiary silcrete and laterite are extensively preserved in Western Australia in low-relief landscapes. Silcrete is best developed on topographic highs developed on fluviatile sands which overly radiolarites or deeply kaolinised claystones. Laterite occurs topographically lower and is generally developed on fine-grained bed rock. Lateral transition of silcrete to laterite indicates that these duricrusts form a soil catena. Cone structures in the silcretes indicate strongly evaporitic, dry conditions, whereas solution pipes in duricrust profiles indicate humid conditions. The silcrete/laterite catena thus indicates a strongly seasonal, hot, and humid climate, with rainfall probably in excess of 1000 mm yr
−1
.
Geochemistry of weathering profile silcretes, southern Cape Province, South Africa
Authors: Summerfield M. A.
Published: May 2008

Silcrete of Cenozoic age associated with deep weathering profiles occurs on residual surfaces along the coastal belt of southern Cape Province. Petrographic and geochemical evidence indicates loss of aluminium and enrichment of silica and titanium during silcrete formation. Silica released locally within the weathering profile was apparently precipitated in a zone of restricted drainage close to the water-table where a low pH environment allowed the removal of aluminium and the migration and concentration of titanium. Silcrete formation probably occurred in a humid tropical or subtropical environment with minimal local relief.
Pliocene channel calcrete and suspenparallel drainage in West Texas and New Mexico
Authors: Reeves C. C.
Published: May 2008

The recent discovery of two calcium carbonate-cemented drainage channels in the Pliocene Ogallala section of the Southern High Plains, Texas and New Mexico, indicates that Ogallala drainage channels were commonly cemented with calcium carbonate, due to leaching of calcrete from adjacent Ogallala sands. Parallel drainage lines near Lovington, New Mexico, originally thought to represent calcrete incisement along ancient dune swales, are now interpreted as an example of recent drainage between a suspenparallel drainage pattern in bas-relief.
Ancient duricrusts and related rocks in perspective: a contribution from the Old Red Sandstone
Authors: Parnell John
Published: May 2008

Calcretes are abundant in continental Devonian (Old Red Sandstone) rocks, occurring both within the sequence and at the basal unconformity. Basal limestones in the Orcadian Basin also include algal deposits and fissure linings, all of which can be considered as a single phase of carbonate precipitation and growth. A prominent silcrete in Orkney replaces both rudites and chert-bearing lacustrine laminites. Silcrete and calcrete are examined in a wider context of chemical sedimentation in lacustrine rocks. The effects of weathering processes in these rocks may be combined or confused with those of algal processes. A silicified plant-bearing sequence at Rhynie displays the same diagenetic features as the Orkney silcrete. Limited metalliferous and bituminous enrichments are recorded in the largest ORS duricrusts. A basal sequence at Dalroy exemplifies the possible confusion between weathering concentrations and mineral deposits trapped at an unconformity surface. Barite is a common component of some duricrusts. Duricrust formation is part of a range of sedimentary replacement and cementation phenomena which may be difficult to distinguish in ancient rocks.
A process-response model for the formation of pedogenic calcretes
Authors: Klappa Colin F.
Published: May 2008

The compositional and fabric evolution of pedogenic calcretes is viewed as a process-response model involving inorganic and organic soil-forming processes on the one hand, and diagenetic, rock-forming processes on the other; the resulting battle being expressed as six evolutionary stages: (1) preparation of host material; (2) soil formation and horizon differentiation; (3) accumulation of calcium carbonate; (4) profile development; (5) induration; and (6) reworking. In nature, some stages may be repeated or reversed; others may be omitted. Emphasis is given to the role of organic activity which determines largely the morphology of pedogenic calcrete profiles and which controls the direction and extent of profile development.
Stable isotope abundances in calcretes
Authors: Talma A. S. , Netterberg F.
Published: May 2008

About 300 published and unpublished measurements of carbon and oxygen stable isotope ratios of calcretes are compared in an attempt to define general trends. The δ
13
C values of the carbonates range from − 12 to + 4‰ PDB. On a global scale the main factor responsible for the average
13
C content of calcrete in an area appears to be the
13
C content of the dominant plants in the region. No correlation could be found between
13
C contents and radiocarbon age. The carbonate δ
18
O values range from − 9 to +3‰ PDB. On a global scale, the factors discussed as possibly responsible for the
18
O content of a calcrete are the
18
O content of the local rainfall, temperature, and the extent to which the water underwent free surface evaporation before or during calcrete formation. Little or no correlation was found between
13
C or
18
O contents and rainfall, temperature, or evaporation rate on a world basis. Some relation between the
18
O content of calcrete and rain on a regional basis was confirmed. The range of
13
C and
18
O values suggest a diversity of origins and environments of formation.
A Geotechnical classification of calcretes and other pedocretes
Authors: Netterberg F. , Caiger J. H.
Published: May 2008

Authigenic calcareous accumulations within regoliths can be simply classified for geotechnical purposes as calcareous soils, calcified soils, powder calcretes, nodular calcretes, honeycomb calcretes, hardpan calcretes, and calcrete boulders and cobbles. Each of these categories represents a particular stage in the growth or weathering of a calcrete horizon and possesses a significantly different range of geotechnical properties. A similar classification can be applied to other pedocretes.
Karstic residual fluorite-baryte deposits at two localities in Derbyshire
Authors: Shaw R. P.
Published: May 2008

Various karst processes may rework primary mineralization producing secondary ore deposits in a variety of karstic cavities both on the surface and underground. Two surface localities, on Bonsall Moor, near Matlock, and near Castleton are filled with sediments containing locally derived fluorite and baryte clasts, in sufficient quantity to be worked as ore deposits. The associated clastic sediments are of Pleistocene fluvioglacial origin.
Cenozoic pedogenesis and landform development in south-east England
Authors: Catt John A.
Published: May 2008

Soils are useful stratigraphically because they mark land surfaces of significant duration. They also provide valuable palaeo-environmental evidence complementary to that obtained from sedimentary successions. In south-east England both buried and unburied soils are broadly divided on field morphology and petrographic characteristics into those formed during the Flandrian Stage and others (palaeo-argillic soils) dating from the Cromerian and later interglacial stages. Even where the latter are derived from pre-Quaternary deposits, as on the Chalk dipslope, their profile characteristics and stratigraphic relationships to remnants of early Quaternary marine deposits suggest development during the Quaternary rather than in the Tertiary. Although most major landscape features were probably inherited from the late Tertiary, the entire land surface of south-east England suffered significant Quaternary erosion, and there are no definite relics of Tertiary pedogenesis.