Mark A. Woods

Biogeography of the Late Triassic wallowaconchid megalodontoid bivalves

Discoveries of Late Triassic alatoform bivalves (family Wallowaconchidae) in Alaska and in south Asia and Arabia reveal a broad distribution for these tropical bivalves, overturning the concept of these bivalves being endemics of offshore island arc terranes in eastern Panthalassa. They occupied an area extending from central Tethys to the eastern Pacific margin of Panthalassa. The Tethyan wallowaconchids occur in nearshore environments on the north margin of Gondwana, indicating an environmental tolerance for continental shelf settings in addition to their primary occurrence on isolated oceanic islands. The new sites reveal a taxonomic diversity among wallowaconchids. Wallowaconchids present in India are conspecific with Wallowaconcha raylenea, the type species of Wallowaconcha, whereas wallowaconchids in Arabia are an undescribed taxon and wallowaconchids in south-central Alaska are small in size and a different undescribed taxon. All known wallowaconchids are of Norian age and they appear to be a wide-ranging component of Late Triassic tropical biotas.

UK Chalk Group stratigraphy (Cenomanian – Santonian) determined from borehole geophysical logs

Borehole geophysical logs through the Chalk Group of SE England have distinctive profiles that correspond with formational boundaries, recognized by lithological changes in borehole core and mapped by the British Geological Survey on the basis of lithology and topographic features at outcrop. Borehole geophysics graphically demonstrates the validity of the stratigraphy of the Chalk Group used by the British Geological Survey, and can be used to refute recent arguments against lithostratigraphical subdivision of the White Chalk Subgroup. Although intra-formational changes in the detail of borehole geophysical logs may occur between regions, generalized trends can still be used for inter-regional recognition of formations in the subsurface. Much previous work has focused on the utility of borehole geophysics for marker-bed recognition and correlation in the Chalk Group, but little attention has been paid to the more fundamental task of characterizing formations. This approach allows a much greater appreciation of regional Chalk Group stratigraphy than might be evident from available outcrops, and is the first step in the development of three-dimensional digital geological models of the Chalk Group.

The Furongian (late Cambrian) Steptoean Positive Carbon Isotope Excursion (SPICE) in Avalonia

Abstract: The Steptoean Positive Carbon Isotope Excursion (SPICE) of earliest late Cambrian (Furongian) age is identified in England. The excursion is found within a c. 145 m thick siliciclastic succession within the middle and higher part of the Outwoods Shale Formation of Warwickshire, and reaches a maximum δ13Corg amplitude of 4.1‰ at values of −25.6‰. Biostratigraphical data show that the excursion occupies the greater part of the Olenus Biozone, an equivalent of the Glyptagnostus reticulatus Biozone that marks the base of the Furongian and coeval base of the Steptoean in North America. The amplitude of the excursion approaches that recorded in limestone-dominated Laurentian successions, and is greater than that recently documented for organic-rich mudstones of palaeocontinental Baltica in southern Sweden. A minor positive excursion above the SPICE may equate with a similar excursion recognized in Siberia. The SPICE in the Outwoods Shale Formation seems closely linked to the widely recognized early Furongian eustatic sea-level rise. There is no evidence in the English succession for slightly later regression, elsewhere considered coincident with the peak of the excursion and pivotal to some previous models explaining the SPICE. Supplementary material: Analytical results, including total organic carbon (TOC) values for each sample, are available at www.geolsoc.org.uk/SUP18455.

The biostratigraphy of the Gault and Upper Greensand formations (Middle and Upper Albian) in the BGS Selborne boreholes, Hampshire

Three cored boreholes at Selborne proved a greatly expanded Gault and Upper Greensand succession compared to those known elsewhere in the Weald, and the thickest Lower Gault yet documented onshore in the UK. Macrofaunas, microfaunas, marine palynofloras and lithology are used to subdivide the c . 140 m thick Gault and Upper Greensand succession. Dramatic expansion of parts of the Upper Gault and Upper Greensand in the Selborne succession is consistent with previously reported evidence for the influence of block fault movements on sedimentation and erosional winnowing.

The stratigraphy of the Gault Formation (Middle & Upper Albian) in the BGS Arlesey Borehole, Bedfordshire

A cored borehole at Arlesey, Bedfordshire proved, in descending sequence, the Lower Chalk, Cambridge Greensand, Gault, and Lower Greensand. The macro- and microfaunas, together with lithology, are used to subdivide the c . 57 m thick Gault succession. Comparison with the Upper Gault of the Duxford and Little Chishill boreholes indicates a possible tectonic influence on sediment accumulation in the Arlesey area.

The stratigraphy of the Chalk Group (Upper Cretaceous) of the Gipping Valley, near Ipswich, Suffolk, UK

The Chalk outcrops in the Gipping Valley, northwest of Ipswich are of great significance in providing a rare glimpse of the often Drift-covered inland East Anglian Chalk succession. The exposures show an unusually thick development of virtually flintless and virtually marl-free chalk, unlike any chalk seen elsewhere in the UK. Macrofossil and microfossil data show that this chalk belongs to the Marsupites testudinarius and O ffaster pilula macrofossil zones, and is coeval with the Newhaven Chalk Formation of southern England. The Uintacrinus anglicus Zone appears to be absent. This chalk is herein named the Blakenham Chalk, provisionally regarded as a Member of the Newhaven Chalk Formation. Similar, virtually flintless chalk, up to 63.5 m thick, occurs in boreholes in Ipswich, beneath a variable cover of Cenozoic and Quaternary strata. This chalk belongs to the basal Gonioteuthis quadrata Zone, and extends down through the O. pilula, M. testudinarius, Uintacrinus socialis and topmost Micraster coranguinum zones, equivalent to the Newhaven Chalk and topmost Seaford Chalk formations of southern England. At outcrop and in boreholes, the virtually flintless chalk is overlain by regularly flinty chalk, up to 8.5 m thick. Macrofossil data show that this chalk belongs to the lower part of the G. quadrata Zone, and that records of Belemnitella from this interval probably correlate with an acme of this belemnite in the lower G. quadrata Zone of southern England. Microfossil data are more equivocal, but this may reflect unusual environmental conditions. Lithostratigraphically, the flinty chalk succession is correlated with the lower part of the Culver Chalk Formation of southern England. The unusual lithology of the Blakenham Chalk Member may reflect the influence of local structural highs rejuvenated by Late Santonian-Early Campanian (Wernigerode Phase) tectonism.

Lithostratigraphy and biostratigraphy of the Lower Carboniferous (Mississippian) carbonates of the southern Askrigg Block, North Yorkshire, UK

A rationalized lithostratigraphy for the Great Scar Limestone Group of the southeast Askrigg Block is established. The basal Chapel House Limestone Formation, assessed from boreholes, comprises shallow-marine to supratidal carbonates that thin rapidly northwards across the Craven Fault System, onlapping a palaeotopographical high of Lower Palaeozoic strata. The formation is of late Arundian age in the Silverdale Borehole, its northernmost development. The overlying Kilnsey Formation represents a southward-thickening and upward-shoaling carbonate development on a south-facing carbonate ramp. Foraminiferal/algal assemblages suggest a late Holkerian and early Asbian age, respectively, for the uppermost parts of the lower Scaleber Force Limestone and upper Scaleber Quarry Limestone members, significantly younger than previously interpreted. The succeeding Malham Formation comprises the lower Cove Limestone and upper Gordale Limestone members. Foraminiferal/ algal assemblages indicate a late Asbian age for the formation, contrasting with the Holkerian age previously attributed to the Cove Limestone. The members reflect a change from a partially shallow-water lagoon (Cove Limestone) to more open-marine shelf (Gordale Limestone), coincident with the onset of marked sea-level fluctuations and formation of palaeokarstic surfaces with palaeosoils in the latter. Facies variations along the southern flank of the Askrigg Block, including an absence of fenestral lime-mudstone in the upper part of the Cove Limestone and presence of dark grey cherty grainstone/packstone in the upper part the Gordale Limestone are related to enhanced subsidence during late Asbian movement on the Craven Fault System. This accounts for the marked thickening of both members towards the Greenhow Inlier.

A reappraisal of the stratigraphy and depositional development of the Upper Greensand (Late Albian) of the Devizes district, southern England

Three members are recognized within the Upper Greensand Formation of the Devizes district on the basis of outcrop, newly acquired cored borehole and petrographical data. These are, in ascending stratigraphical order, Cann Sand Member, Potterne Sandstone Member and Easterton Sandstone Member. Compared to the imprecise historical subdivisions, the members provide a much clearer indication of lithological variation through the Upper Greensand and this, in turn, provides clues to its depositional development. The biostratigraphy of each member was determined using macrofossils and microfossils. The new biostratigraphical data clarify the relationship of the Potterne Rock to the traditionally named ‘Ragstone’, which caps the Shaftesbury Sandstone in the Shaftesbury district, and suggest that the correlation of the Potterne Rock and ‘Ragstone’ is less straightforward than suggested previously. There are some distinct contrasts with the stratigraphy of the Upper Greensand southwest of Devizes (Shaftesbury and Wincanton districts). Whilst tectonic influences have been demonstrated to affect coeval strata in parts of the eastern Weald, these may not be the dominant control on the Devizes succession, which seems to be influenced more strongly by its palaeogeographical setting with respect to sediment source areas, and the effect this had on the volume and timing of sediment infill. Palaeogeography may also be indirectly responsible for the absence of cherts in the Upper Greensand of the Devizes area, in contrast to their conspicuous development in the Upper Greensand of southwest England and the Weald.

Anomalous Turonian-Campanian Chalk deposition in south Dorset; the influence of inherited pre-Albian structures

The Turonian-Campanian Upper Chalk in the area between Dorchester and Weymouth in Dorset shows an abnormal development of hard chalk beds in the late Coniacian-Santonian Micraster coranguinum to the early Campanian Gonioteuthis quadrata zones. The chalk is also strongly affected by faults representing the surface expression of reactivated underlying pre-Albian structures. This suggests a link between tectonism and Turonian-Campanian chalk deposition, which may be related to the proximity of the south Dorset disturbance.

Assessing sampling of the fossil record in a geographically and stratigraphically constrained dataset: the Chalk Group of Hampshire, southern UK

Taphonomic, geological and sampling processes have been cited as biasing richness measurements in the fossil record, and sampling proxies have been widely used to assess this. However, the link between sampling and taxonomic richness is poorly understood, and there has been much debate on the equivalence and relevance of proxies. We approach this question by combining both historical and novel data: a historical fossil occurrence dataset with uniquely high spatial resolution from the Upper Cretaceous Chalk Group of Hampshire, UK, and a newly compiled 3D geological model that maps subsurface extent. The geological model provides rock volumes, and these are compared with exposure and outcrop area, sampling proxies that have often been conflated in previous studies. The extent to which exposure area (true rock availability) has changed over research time is also tested. We find a trend of low Cenomanian to high Turonian to Campanian raw richness, which correlates with, and is possibly driven by, the number of specimens found. After sampling standardization, an unexpected mid-Turonian peak diversity is recovered, and sampling-standardized genus richness is best predicted by rock volume, suggesting a species–area (or ‘genus–area’) effect. Additionally, total exposure area has changed over time, but relative exposure remains the same. Supplementary materials: A locality list, abundance matrix and all correlation and modelling results are available at https://doi.org/10.6084/m9.figshare.c.3592208.