Penecontemporaneous Erosion Research Articles

The stratigraphy of the Middle Chalk (Upper Cretaceous) succession at Mundford, Norfolk, UK

Four continuously cored boreholes and 82 observation shafts were drilled over an area of c. 45km2 at Mundford, Norfolk in 1966–67 as part of a geological and geotechnical investigation of for a large (2.4km diameter) proton accelerator and attendant experimental target areas that was to be commissioned by the Committé Européan pour Recherche Nucleaire (CERN). The site is a low-relief area underlain by Middle Chalk and the oldest part of the Upper Chalk in a region where the stratigraphy of the Chalk was poorly known prior to the investigation. The shafts enabled the unweathered succession to be examined in detail, both vertically and laterally, and proved numerous lithological marker beds, mostly flint bands and marl seams. Some were restricted to the site area; others remained unchanged over a much larger area. Penecontemporaneous erosion channels in the Middle Chalk that probably resulted from minor tectonic activity locally cut out some of the marker beds. The Mundford succession falls within the northern part of the Transitional Chalk Province. A few of the marker beds, notably the thicker marl seams, have been correlated with marker beds in the Southern and Northern Provinces. Some of the on-site names for marker beds were used to illustrate the geological succession and structure in the geotechnical report on the proposed site. As a result, the site subsequently became the de facto standard for the Middle Chalk stratigraphy of the region. A description of the full succession is included here for the first time.

Settlement strategy in Symplectophyllum (Cnidaria, Rugosa)

1. IntroductionThe larger solitary corals of the Early Carboniferous in eastern Australia – Amygdalophyllum Dun & Benson, 1920, Merlewoodia Pickett, 1967 and Symplectophyllum Hill, 1934 – have broad dissepimentaria characterised by an outer zone of large and irregular lonsdaleoid dissepiments, axial of which is a zone of naotic dissepiments, with regular dissepiments only the innermost zone. By contrast, the septa of the inner part of the corallite are thickened and braced by tabulae and regular dissepiments, and the axial structure is robust (except in Merlewoodia). Specimens of these genera are usually recovered as decorticated individuals, having undergone some penecontemporaneous erosion. Because of the zone of lonsdaleoid dissepiments, the outer part of the corallite is fragile, and little erosion is needed for decortication. Occasionally however, notably at a locality on “Pinaroo Plain” station at Caroda, near Bingara, New South Wales, specimens of Symplectophyllum are commonly found in growth position within larger colonies of the tabulate genus Syringopora, the growth direction of the two species being subparallel. More rarely they may be associated with branching lithostrotionids such as Cionodendron Benson & Smith, 1923 or Pickettodendron Denayer & Webb, 2015. The dynamics of this association form the subject of this article.2. Localities and ageTwo localities are the principal sources of the present material. The first, which has provided the largest number of spec

Palaeoclimate indicators (clay minerals, calcareous nannofossils, stable isotopes) compared from two successions in the late Jurassic of the Volga Basin (SE Russia)

AbstractA study of clay mineral and calcareous nannofossil abundances in late Jurassic–early Cretaceous sediments from the Volga Basin, SE Russia, is presented. From these results, we are able to compare some general patterns of mineralogical and palaeontological change for the Volga Basin to the palaeoclimate models developed for northern Europe and beyond. The two successions examined comprise calcareous mudstones with black organic‐rich shale horizons, overlain by a series of phosphatic silty sands. Clay mineralogical results show a progressive decrease in kaolinite and the concomitant increase of smectite and illite through the middle Volgian, followed by an abrupt increase in kaolinite in the late Volgian. The clay mineral evidence suggests increasing aridity at the end of the Jurassic, similar, in part, to many western European successions. Because of differential settling of clay minerals, superimposed upon this possible climatic signature is likely to be the effect of relative sea‐level change. Calcareous nannofossil analysis from a single section reveals a shift through the middle Volgian from low nutrient, warm water assemblages dominated by Watznaueria to cooler surface water and high nutrient assemblages dominated by Biscutum constans. These observations suggest that increased aridity is also associated with climatic cooling. Black shales are associated with increased productivity, higher sea levels and increases in smectite content. Hence, periods of low (chemical) hinterland weathering during semi‐arid conditions are paradoxically associated with relatively nutrient‐rich waters, and organic‐rich shales. Comparison of published carbon and oxygen stable isotope results from this and other sections to the clay mineral and nannofossil data confirms the palaeoclimatic interpretation. This study significantly improves the published biostratigraphically constrained clay mineral database for this time period, because other European and North American successions are either non‐marine (and thus poorly dated), absent (through penecontemporaneous erosion) or condensed. Copyright © 2002 John Wiley & Sons, Ltd.

Microatoll microbialites of Lake Clifton, Western Australia: Morphological analogues ofCryptozoön proliferum Hall, the first formally-named stromatolite

The Linnaean nameCryptozoon proliferum Hall was proposed in 1883 for a previously undescribed life-form preserved in spectacular exposures of Cambrian limestones in New York State, USA. It is now recognised that these are exposures of stromatolitic microbialites, laminated organosedimentary structures formed from interaction between a benthic microbial community (BMC) and the environment. Microbialites are neither fossil organisms nor trace fossils. These complex structures are the products of dissipative, self-organising systems involving a BMC, the external environment and the accreting microbialite. Functionally analogous BMCs of different species compositions may build similar structures in similar environments in quite separate periods. The type exposures ofCryptozoon proliferum show objects composed of complex, concentric rings, up to a metre in diameter, that have grown laterally without any restriction other than that provided by neighbouring structures. They are not the relicts of domes truncated by penecontemporaneous erosion or Pleistocene glaciation, but depositional forms in which upward growth was restricted. Analogous modern structures occur on a reef platform along the north east shore of hyposaline Lake Clifton, Western Australia. These are tabular thrombolitic microbialites that vary lakeward across the reef platform from low, compound structures to discrete, concentric structures up to 50 cm high. The Lake Clifton forms are, in turn, morphological analogues of microatolls found on coral reef platforms. Coral microatolls are coral colonies with flat, dead tops and living perimeters in which upward growth is constrained by the sea surface. In shallow water they form circular rims of laterally growing coral around a dead centre. In deeper water they form coral heads that develop flat tops on reaching sea level. It is concluded that both the tabular microbialites of Lake Clifton and the type exposures ofCryptozoon proliferum are analogous to coral microatolls in both form and origin-structures that have been able to grow laterally, but in which upward growth is restricted by subaerial exposure. Similar microatoll microbialites have been described from other modern environments, including Great Salt Lake, Utah, USA and Stocking Island, Exuma Cays, Bahamas. Ancient examples may include some of the “tufa” deposits of the Basal Purbeck Formation in Dorset, UK, as well as the coalesced domal bioherms of the Upper Cambrian Arrinthrunga Formation of the Georgina Basin, Central Australia, and the “washbowl” structures described from the Batsfjord Formation of the Varanger Peninsula, north Norway. Progress towards a reliable interpretation of ancient microbialites depends on an understanding of modern environments in which analogous structures are forming. This study of microatolls has demonstrated that other sessile life forms may create colonial ecomorphs that, used cautiously, can serve as analogues for understanding the factors controlling the growth and form of microbialites. The surprising lack of pre-Pleistocene examples of microatolls recorded to date has simply been due to their lack of recognition in the geological record. They occur in sequences from the Proterozoic onwards, and provide powerful environmental indicators of ancient reef platforms on which biological growth was adjusted to contemporary sea level.

Radial dikes of Lyttelton Volcano — their structure, form, and petrography

Abstract Radial dikes of Lyttelton Volcano, Banks Peninsula, New Zealand, are mainly hawaiite, with basalt, mugearite, and trachyte next in abundance. Spatial distribution of compositional types is not regular, one interpretation being that the proportion of trachyte increased with time. Analysis of dike numbers suggests there were 2000 near the volcanic centre, and <500 at the level of the eroded crater rim. Dikes have a blade shape with the long axis horizontal. Flow directions are dominantly along that axis with significant deviations up to 90° either side. Dike traces are marked by bends and offsets of as much as 125 m. Most dikes are <1.5 m thick, but they range up to 20 m. Thickness changes slowly horizontally outwards, but quickly vertically; many dikes thicken upwards to the point where they feed flows and domes, or have been cut off by penecontemporaneous erosion. Dikes played a major role in the construction of the volcano, probably a continuous role. A modern analogue is Etna.

Palaeobathymetric interpretation of wave ripple-marks in a Ludlovian grainstone of Gotland

Abstract A ripple-marked grainstone bed in the upper Hemse Beds is described externally and internally. Most of the ripple-mark crests are flat, and some are concave. The entire surface is covered by traces, most of which are biogenic. It is concluded that the ripple train was generated by oscillatory wave action, and that the flat and concave crests were caused by penecontemporaneous erosion. A reconstruction of the original ripple form is made and the mean grain size is estimated. A graph is constructed which enables an interpretation of the set of ripple-marks in terms of wave conditions and absolute water depth. It is concluded that the ripples were formed by waves with a period of about 11 s and a near-bottom water-particle orbital diameter of about 0.9 m, at a water depth of about 10 m or less.

Late Cretaceous-Tertiary stratigraphy, structure, and tectonic history of the area between Whareama and Ngahape, eastern Wairarapa, New Zealand

Abstract A NNE-striking belt of Late Cretaceous-Early Tertiary sediments in the area between Whareama and Ngahape consists of a sequence, from base to top, of: sandstone, conglomerate, and minor siltstone (Glenburn Formation); thin-bedded sandstone and mudstone (Te Mai Formation); siltstone (Whangai Formation); black siltstone (Waipawa Formation); and glauconitic sandstone (Huatokitoki Formation). The belt is faulted against indurated, strongly deformed Early Cretaceous sandstone and siltstone (Pahaoa Group) in the west, and overlain unconformably by Late Tertiary (Miocene) sediments in the east. Shallow intrusions of dolorite and basalt, and submarine basalt flows, comprising the Ngahape Volcanic Complex (new), occur mostly in the upper part of the sequence. Volcanic activity was probably restricted to late Maastrichtian-early Paleocene (Haumurian-Teurian) time. The Late Cretaceous-Early Tertiary formations are considered to have been deposited in inner shelf or marine delta (Glenburn), tidal strait (Te Mai), and shallow restricted basin (Whangai and Waipawa) environments. Major faults, including the new Motuwaireka Fault, in the area mostly trend NNE. Significant discoveries with respect to faults are: repeated movement (late Cretaceous, Pliocene, and Recent?) on Adams Fault, forming the western boundary of the belt; a reverse component of movement on the low-angle (20–40°) west-dipping Pukeroro Fault; and strike-slip (dextral) movement on the ENE-trending Kaumingi Fault. Faulting and folding took place during deposition of Late Cretaceous-Early Tertiary sediments, resulting in penecontemporaneous erosion and slumping. Major tectonic upheaval occurred in post-late Paleocene time, probably in the early Eocene.

On the geochemistry and origin of the D Tree, Wonarah, and Sherrin Creek phosphorite deposits of the Georgina Basin, northern Australia

The early Middle Cambrian phosphorites of the Georgina Basin were deposited in shallow nearshore marine environments varying from lagoonal, estuarine, littoral, to intertidal. Some deposits show penecontemporaneous erosion by stream channels in addition to subaerial weathering during periods of regression. There are three distinct types of phosphorite: mudstone phosphorite, replacement phosphorite, and pelletal phosphorite, the latter two were formed by diagenetic phosphatization of carbonate skeletal sands, bioclastic and micritic limestones, and dolomites. By comparison, the mudstone phosphorite, which predominates in the three deposits studied, shows little textural evidence of such an origin and has been accepted as an orthochemical sediment. However, this paper proposes a diagenetic origin identical to the other types.Some fifteen known deposits occur over a distance of about 1,000 km along the periphery and insular portions of the basin. The geochemistry of two of these deposits, D Tree and Sherrin Creek, which lie 75 km apart on the eastern edge of the basin, has been studied and compared with that of the Wonarah deposit, which lies within the basin 200 km to the west. Fifty-eight analyses reveal four element associations: apatite group (Ca, P, F), clay group (Al, K, Ti, Cr, Zr), heavy minerals group (Fe, Mn, Zn, Ni, Cu), and a leached group. Although the elements Na, S, CO 2 , Y, La, Ce, Sr, U, and Th correlate with apatite in the little-leached Sherrin Creek deposit, they exist as a separate leached group, or correlate with clays, in the more leached D Tree and Wonarah samples. Ba, V, Cu, and Pb do not behave to a consistent trend. The strongest correlations in the leached deposits are at a lower level of significance than at D Tree.Intense Cenozoic weathering extends beyond the maximum known depth of phosphorite. It, together with certain though unquantifiable Cambrian subaerial weathering effects, has been responsible for leaching, increasing in the order Sherrin Creek, D Tree, and Wonarah. Uranium, Th, La, Ce, Y, Ba, Sr, Na 2 O, SO 3 , and CO 2 are leached from the system to the extent of 45 to 80 weight percent. Comparison of the composition of leached and unleached phosphorites suggests there were discernible differences in the geochemistry of waters in the different embayments at the time the phosphorites were formed.

Estuarine sedimentation in the Eocene of southern England

ABSTRACTA temporary section in the Cuisian Bagshot Beds, which has been mapped in detail, displayed estuarine sediments with interlayered sands and muds, fine sands, channel‐fill sands and intraformational (mainly mud clast) conglomerates. The facies show rapid lateral and vertical changes in grain size and bed form and a restricted suite of trace fossils including Ophiomorpha nodosa and Arenicolites sp.The sequence is shown to have been deposited in a subtidal channel where tidal, wave and fluvial processes were dominant at different times. The following points are considered to be characteristic of sedimentation in this environment: (i) correlation is difficult and facies predictability is low; (ii) there are frequent lateral facies changes from the channel to the subtidal bank environment; (iii) pene‐contemporaneous erosion removes considerable amounts of sediment; (iv) load structures may be exposed, eroded, buried and reactivated; and (v) muddy layers and bioturbated horizons offer similar resistance to penecontemporaneous erosion.

Cap des Rosiers Formation at Grosses Roches, Quebec—deposits of the mid-fan region on an Ordovician submarine fan

Resedimented deposits in the Ordovician Cap des Rosiers Formation near Grosses Roches, Quebec, are arranged in fining-upwards sequences, each with conglomerate at the base, and argillaceous sandstones, or interbedded sandstones and shales at the top. The conglomerates are primarily cobble-grade, and are poorly sorted; graded bedding and clast imbrication are poorly developed, and internal stratification is uncommon. They pass upwards and laterally to pebbly sandstones and sandstones, most of which are massive, but which contain cross-stratification, parallel stratification, or dish structure in places. The sandstones are overlain by finer-grained sandstones with many load-deformation and water-escape structures and at the top of some sequences there are sandstone and siltstone turbidites. The presence of large sediment slabs and soft clasts in the conglomerates and sandstones, and of the abundant shale chips in the argillaceous sandstones, provides evidence of penecontemporaneous erosion. Load casts, flame structures, dish structures, and other water-escape structures indicate relatively high rates of deposition. At the base of the section, 10–12 m of conglomerate and sandstone beds in the lowermost sequence onlap against the eroded pelites of a channel margin. The exposed bases of four other fining-upwards sequences are erosional, but no onlapping relationships are observable, and the eroded sediments are coarser grained than the pelites at the very base of the section.The sequences are considered to represent the back-filling of a channel-mouth in the mid-fan region of a submarine fan, and subsequent accumulation of fining-upwards lobes beyond the front of a principal channel which continued to supply sediment. The superposition of conglomerate-bearing fining-upwards sequences is interpreted as a result of fluctuations in supply from a tectonically active source.

Analytical study of turbidites, Otadai Formation, Boso Peninsula, Japan

ABSTRACTOn the Boso Peninsula, southeast of Tokyo, individual turbidite layers have been traced over about 38 km with the aid of tuff marker‐beds.Sandstones are generally much more variable in thickness than the interbedded mudstones, showing lenticular shapes with great lateral continuities up to 30 km or more. But the latter have greater extents than the former, forming extensive sheets with a slight upslope thickening. Most sandstones show vertical and lateral grading, while mudstones gradually coarsen upslope. Sandstones have a variety of sedimentary structures typical of turbidites, whereas mudstones are virtually devoid of them throughout. Sandstones contain intensely abraded shallow‐water fauna in contrast with mudstones with well‐preserved deep‐water fauna. Thus, mudstones are mostly of hemipelagic origin, and turbiditic mudstones, if present, are of negligible amounts.In general, individual sandstone beds are much more variable in thickness on the upstream side than on the downstream side. This is probably because of the existence of some minor channels and the adjacent levee‐like embankments on the upslope side. At the base of thicker sandstone beds local penecontemporaneous erosion has usually taken place, giving rise to a few minor channels bordered by small levee‐like embankments on both sides. The downslope side of the largest channel is commonly flanked by the greatest embankment that is coincident with the thickest part of each sandstone layer. Sandstones in the minor channels and levee‐like embankments contain a lot of mud lumps torn out of the substratum, which decrease in size and amount and improve in roundness with increasing distance from the channels. In general, thicker sandstones show Bouma abcde sequences around the thickest part. Bouma d‐division and convolution are developed preferentially on the downstream side of the thickest part. With decreasing bed thickness laterally, the complete depositional sequences arc successively replaced by Bouma bcde, then cde, and de sequences downslope, while by Bouma bc/c and then c/e sequences upslope.The facies transition of the horizon studied probably represents a section extending from the downslope end of the channelized portion of a suprafan to the outer portion with a smooth surface.

Subsurface Expansion Megapolygons in Upper Jurassic Dolostone (Kimmeridge, UK)

ABSTRACT Megapolygons with thrusted margins occur within the lowermost dolostone of the Kimmeridge Clay Formation (Upper Jurassic), at the type locality in southern England. Unlike similar features previously described, these structures did not develop at, or near, the sediment surface but at a considerable depth of burial. They formed at a time when the adjacent shales were already compacted but prior to tectonic jointing and faulting. Features distinguishing them from similar near-surface structures include sigmoidal thrust planes, complete lack of penecontemporaneous erosion, absence of internal sediments and association with localized deformation of surrounding, compacted sediments. Diagenetic growth of dolomite is proposed as the mechanism that gave rise to expansion.

Discussion of Ordovician transgressive ash-flow tuffs of Snowdonia, N. Wales

D r R. V. B eavon writes questioning submarine welding of ash-flow tuffs as suggested by Francis &amp; Howells (1973) . The irregular basal contacts, and associated soft-sediment structures below mildly transgressive ash-flow deposits, were probably caused by pre-eruption uplift, tilting and superficial slumping of wet sediment during and immediately preceeding ash-flow eruption, emplacement and consolidation. Beavon (1963) has described the emplacement of a grossly transgressive (unconformable) ash-flow deposit, preceeded by a volcano-tectonic uplift that induced significant gravity gliding of an older tuff sheet and its weak basement of watery sediment. The resultant landslide (or lahar) may have been partly contemporaneous with one of the most voluminous ash-flows in Snowdonia. Smaller scale slump structures, analogous in age and origin to those described by Francis &amp; Howells (1973) have been described in thin welded tuffs that were subjected to penecontemporaneous erosion ( Beavon 1963 , p. 486). If we imagine eruptive mechanisms consisting of (1) volcano-tectonic uplift, (2) eruption and emplacement of ash-flow and (3) welding of ash-flow during volcano-tectonic subsidence ( cf. Smith &amp; Bailey 1968 ), operating, to a degree commensurate with the volume of effusions, in an extensive near-shore tidal flat, then the Welsh Ordovician characteristic of subaerial welded tuffs interbedded with marine sediments should be regarded as the norm. If the interpretation offered herein is correct, then it may be possible to recognize important eruptions where the volume of welded tuff was reduced by penecontemporaneous erosion. Perhaps the authors will comment on the possibility of stratigraphic correlation on the basis of the degree of

Shallow-Water Sedimentation as illustrated in the Upper Devonian Baggy Beds

Summary The Baggy Beds (440 m thick) form a transgressive sequence of shallow-water marine and delta-like sediments of Upper Famennian age between the continental Pickwell Down Sandstone and the overlying neritic Pilton Beds (which straddle the Devonian-Carboniferous boundary). The Baggy Beds are divided into lower and upper divisions (the lower division containing the bulk of the sandstones) and a number of members. There is a strong diversity of facies, and nine main facies are recognized; they show little evidence of cyclicity but their stratigraphical relationships are important for their interpretation. Penecontemporaneous erosion, principally attributed to wave-action, was often intense, and has contributed more than any other factor to the overall aspect of the marine facies. The bulk of the sediment comprises the shales, siltstones, and very fine sandstones of the Diplocraterion yoyo facies group, which display all gradations from non-life (letal) graded siltstones and shales to intensely bioturbated sandstones, in which the trace-fossil D. yoyo is generally conspicuous. The Reynard facies comprises graded sets of fine to very fine sandstones, the upper portions of which commonly show oscillation ripple or are bioturbated. The sets are intercalated singly in the D. yoyo facies, or form cosets (11 per cent of Baggy Beds). These two facies, which yield a small neritic fauna, are considered to represent deposits of the delta-front platform, or a like off-shore environment, with the cosets of Reynard facies for the most part representing a nearer-shore environment. The other facies (with an interpretation of their depositional environment) are as follows: Arenicolites curvatus facies (6 per cent), which include thin, fine to very fine sandstones with A. curvatus and bioturbated sandstones: sub-beach environment. Rough facies (1 per cent), thin sets of shelly, cross-stratified sandstones with Dolabra , and intraformational conglomerate of local derivation: near-shore, shelly, and possibly intertidal channel fills. Gull facies (1 per cent), cross-stratified fine well-sorted sandstones: near-shore submarine dune depositis. Lingula facies (8 per cent), bioturbated and pene-contemporaneously eroded graded siltstones and shales with patches of disarticulated Lingula and thin bellerophontoid limestones: lagoonal or restricted bay environment, possibly brackish. Timber and Cormorant facies (11⋅5 per cent), channel-fill sediments with parallel and cross-stratified fine to very fine sandstones of northerly provenance with much plant debris: distributary fills. (Cormorant subfacies represents deposition mainly under upper flow regime.) Tag facies (5 per cent), multiple channel-fill sediments deposited under lower flow intensity with fine to very fine sandstones and silt-grade sediments, in which small-scale cross-lamination is important: interpretation doubtful, possibly distributary mouth bar deposits, or crevasse fill or the fill of an off-shore barrier inlet. Hoe facies (1⋅5 per cent), shales and siltstones associated with Timber and Tag facies: deposited under conditions of low flow intensity, or in fresh-water lakes. Plant remains and, rarely, Chondrites? parvus . A model is erected (Fig. 27) suggesting that the sediments were deposited in the vicinity of a delta or similar fluvially influenced environment, subject to a moderate tidal range, though largely on the delta-front platform, or relatively inshore on the continental shelf.

Deformation of Lee-Side Laminae in Eolian Dunes

Research Article| February 01, 1971 Deformation of Lee-Side Laminae in Eolian Dunes EDWIN D McKEE; EDWIN D McKEE U. S. Geological Survey, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar JOHN R DOUGLASS; JOHN R DOUGLASS U. S. National Park Service, Yellowstone National Park, Wyoming 83020 Search for other works by this author on: GSW Google Scholar SUZANNE RITTENHOUSE SUZANNE RITTENHOUSE U.S. Geological Survey, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1971) 82 (2): 359–378. https://doi.org/10.1130/0016-7606(1971)82[359:DOLLIE]2.0.CO;2 Article history received: 25 May 1970 rev-recd: 27 Aug 1970 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation EDWIN D McKEE, JOHN R DOUGLASS, SUZANNE RITTENHOUSE; Deformation of Lee-Side Laminae in Eolian Dunes. GSA Bulletin 1971;; 82 (2): 359–378. doi: https://doi.org/10.1130/0016-7606(1971)82[359:DOLLIE]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Processes responsible for structures in sand dunes consist of (l) primary deposition by saltation and creep and by settling from suspension, (2) redeposition accompanying avalanching, and (3) penecontemporaneous erosion. Characteristics of dune structures were examined in the field by introducing marker beds of magnetite at times of sand deposition, thus recording original surfaces and making possible the determination of subsequent changes. Similar structures were examined in the laboratory by testing processes and comparing the resulting structural forms with corresponding natural features.Avalanching in sand is of two types: sand flow and slumping. Deformational structures characteristic of each were recorded in the field and were reproduced in the laboratory. Nine varieties of deformational structures are recognized and described. Analysis of these structures suggests criteria for distinguishing compressional types (lower dune slope) from tensional types (upper dune slope).The analysis of deformational structures also serves to distinguish between forms developed in cohesive sand and those in non-cohesive sand. Since the degree of cohesion is largely a function of the amount of moisture in the sand at the time of avalanching, the deformational structures provide a means for recognizing original dry sand, wet sand, sand crusts, and saturated sand surfaces in ancient deposits. A testing of these criteria was made by comparing laboratory samples with those of dry sand at White Sands, New Mexico, and with those of coastal dunes (probably wet sand) in southern Brazil. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

La subsidence du recif campanien de Vigny [summ.] [with discussion

Abstract Study of the reef complex exposed in the Bois des Roches quarry in the Vigny region, France, shows repeated variations of depositional conditions, successive phases of penecontemporaneous erosion, and ruptures in the peripheral zone and in overlying beds. The age is undoubtedly Campanian (Cretaceous); arguments to the contrary are refuted.

The Stratigraphical Distribution of the Cornbrash : II. The North-Eastern Area

I. I ntroduction I n a previous communication to the Society ( Douglas &amp; Arkell, Part I, 1928 ) we gave a detailed account of the stratigraphical distribution of the Cornbrash in the South-West of England between Oxford and the Dorset coast, describing the more important features of the faunal sequence and giving our reasons for amending the list of brachiopod zones propounded by the late S. S. Buckman (1927, p. 10) . The present paper deals mainly with that part of the Cornbrash outcrop which lies between Oxford and the Humber, running through the counties of Oxfordshire, Buckinghamshire, Bedfordshire, Northamptonshire, and Lincolnshire. Preliminary observations on the outcrop in Yorkshire are also included, but the detailed investigation of that county has not yet been completed. Many of the conclusions arrived at during our study of the South-Western area have been strengthened or confirmed by discoveries made when following the outcrop farther north, and many new facts have come to light which show that our criticism of Buckman's clinal diagram (1927, p. 12), in which he sought to prove extensive penecontemporaneous erosion in the Cornbrash, was fully justified. As before, we have confined our attention chiefly to the brachiopod sequence, but it is hoped that the numerous ammonites collected will be dealt with at some future date. It may be recalled that the exact stratigraphical position of two of the suggested brachiopod zones—those of Tegulithyris bentleyi and Obovothyris stiltonensis, yet remained to be proved. The first-named (Zone 11) was placed by Buckman at the summit of

Societies and Academies

LONDON Geological Society, June 17.—Olaf Holtedahl: Some general structural features of the arctic and adjacent regions. The huge region comprising the Canadian and the Baltic Shields and the areas lying between them represents a sort of structural unit which may be more or less symmetrically divided by two lines, one north and south, the other at right angles to it, crossing each other in the central part of Greenland. Although Baffin Land and the western zone of Fennoscandia had a somewhat different history in ancient geological times, yet each may be regarded as the reflected image of the other, both consisting of mountain ranges with the highest elevation on the side bordering the adjacent deep sea area. In the northward continuation of these two zones, in Elles-mereland and Spitsbergen respectively, and at the north end of Greenland, are other zones where the distribution of the various formations tells of a some-what similar inclination of the earth's crust an inclination away from central Greenland, the previously mentioned structural centre. Thus there is, in a roughly ring-shaped belt, an inclined elevation of recent, probably younger Tertiary, date, and it seems a natural conclusion that this particular elevation has been of fundamental importance in the gathering of snow, which in Quaternary time developed into the modern ice-fields, the centre of which coincides with the above-mentioned structural centre.—James Archibald Douglas and William Joscelyn Arkell: The stratigraphical distribution of the cornbrash: (2) The north-eastern area. Attention has been chiefly confined to the brachiopod zones, and their distribution throughout the area is indicated by a detailed account of many typical exposures. Further readjustment of Buckmans zonal table has been found necessary in respect of the zones of Tegulithyris bentleyi and Obovothyris stiltonensis. The evidence for and against penecontemporaneous erosion is discussed.

The Junction-Bed of the Middle and Upper Lias on the Dorset Coast

The object of this communication is to amplify and correct certain descriptions and sections of the Junction-Bed of the Middle and Upper Lias near Eypesmouth on the coast of Dorset, which I contributed as an appendix to Mr. S. S. Buckman's recent paper on Jurassic chronology. On the Dorset coast the long time-gap between the top of the Domerian, in the Middle Lias, and the base of the Yeovilian, or Bridport Sands, in the Upper Lias, is represented by only a few feet of strata, and these vary greatly from section to section and even within a few yards. These few feet constitute the Junction-Bed, and fossils collected inch by inch in the different exposures show that, although all the hemeræ recognized by Mr. Buckman as filling this gap have not been found, yet representatives of many hemerae occur, some in one exposure and some in another, and always in their appropriate order; so that it is fair to assume that the period was one of continual oscillation and penecontemporaneous erosion, and that the strata present a succession of non-sequences. At least in one exposure, at Watton Cliff, a considerable non-sequence can be directly demonstrated in a single block; here a bed belonging to a definite horizon thins out completely between two beds of respectively earlier and later dates. In order to make clear how much of the supposed complete sequence is represented and when the non-sequences occur, the following table is given, showing all the horizons of this date

Appendix II. NOTE ON THE STRATIGRAPHY OF ROULSTON SCAR

A short visit to Roulston Scar in December, 1917, yielded the following information :— (1) Towards the northern end of the Scar the break between the Calcareous Grits and the underlying Kellaways is sharply defined ; the Corallian consisting of a fine-grained non-ferruginous grit contrasting with the ochre-coloured Kellaways. Towards the southern end, however, this junction is not so readily seen, the lower beds of the Calcareous Grits being largely ferruginous. (2) The top of the Kellaways appears to be everywhere marked by a bed consisting largely of rolled fragments of Belemnites together with smaller nodules ; the nature of this deposit forcibly calling to mind the two beds of rolled fossils and phosphatic nodules in the Gault and pointing to pene-contemporaneous erosion. (3) The general section below the Calcareous Grits is approximately as follows :— Ferruginous sandstone, shrly in parts, crowded with water-worn fragments of Belermnites, apparently belonging to the following species : B. abbreviatus and B. oweni, 4 to 6 inches ; ferruginous sandstone, unfossiliferous, about 1 foot ; ferruginous sandstone, fossils abundant but very badly preserved, Trigonia allied to T. clavellata, Pecten inaequicostatus and P. demissus, Hinnites sp. together with a few fragments of belemnites and unrecognisable fossils, about 10 inches. [From apparently the same bed at the southern end of the Scar two species of ammonites were obtained ; these were sent to Mr. S. S. Buckman, who wrote— “Roughly speaking your Amm. show a Macrocephalites indicating top of the Cornbrash ( macrocephalus zone, base of the ...