Lithostratigraphic Framework Research Articles

Response of a carbonate platform to the Cenomanian-Turonian Drowning and OAE 2: a case study from the Adriatic Platform (Dalmatia, Croatia)

Abstract Global perturbations during the Cenomanian–Turonian boundary (CTB) interval and the Oceanic Anoxic Event 2 (OAE 2) represent one of the most extensively studied past environmental changes. To explore the response of various carbonate-platform depositional systems to such major environmental perturbations, strata of the intra-Tethyan Adriatic carbonate platform (sensu stricto) from the island of Brac (Adriatic Sea, Croatia) provide excellent exposures and a previously well-established Upper Cretaceous lithostratigraphic framework. Within this context, this study integrated lithostratigraphy, biostratigraphy, and chemostratigraphy to describe a drowned-platform succession underlain and overlain by peritidal carbonates. Carbon-isotope stratigraphy of this succession revealed a shift towards positive δ13C values that reached +4 to +5% VPDB, and represent the CTB interval excursion plateau. We observed variations in thickness of the drowned-platform successions and explained them by three superimposed mechanisms: (1) diachronous drowning of platform relief; (2) intra-platform redeposition of parts of the successions by various mass-gravity transport processes (indicating enhanced instability due to increasing accommodation space related to the late Cenomanian platform drowning and synsedimentary tectonics); and (3) migration of major carbonate factories during the recovery of shallow-platform environments. The results indicate that the CTB interval event caused unusual increase in accommodation space on the carbonate platform enabling open-marine influence and synsedimentary redeposition. However, widespread organic-rich black shales reported from coeval strata of other regions have not been documented from the platform-top successions to date, and were probably accumulated in deeper (anoxic) settings below the rising sea level.

Multiple cycles of wave-dominated estuarine deposits in low-accommodation settings, Cretaceous J sandstone, northwestern Nebraska

The J sandstone comprises less than 46 m (151 ft) of sandstone-dominated strata within the mudrock-dominated lower Upper Cretaceous succession of northwestern Nebraska. The unit is a prolific hydrocarbon producer in this region (Denver-Julesburg Basin), but its lithostratigraphic and sequence-stratigraphic framework, critical for reservoir characterization and mapping, is poorly known. We have achieved an improved understanding of depositional history and sequence stratigraphy by describing and correlating cores and wireline logs from wells within Sioux, Dawes, and Box Butte counties, Nebraska, and Niobrara and Goshen counties, Wyoming. Coals, paleosols, fluvial or inner estuarine sandstones, estuarine mudstones, fluvial conglomerate, shoreface sandstone, and reworked volcanic fallout (bentonite) lithofacies were identified. Trace fossil assemblages representing stressed expressions of the Skolithos and Cruziana ichnofacies are common. These lithofacies are arranged vertically into three erosionally based cycles, each less than 28 m (92 ft) thick, and each grade upward from fluvial or inner estuarine sandstones into estuarine mudstones and in turn into shoreface sandstones. The lateral and vertical stacking patterns of the lithofacies are complex, however, and the upper cycles appear to fill space eroded into the underlying ones. Northeast-southwest–elongate isochore trends appear in all three cycles. Lithofacies transition downdip from inner estuarine sandstones in the northeast to estuarine basin mudstones and shoreface sandstones toward the southwest. Detrital mineralogy indicates an easterly (cratonic) provenance for the entire unit. Our data suggest that the J sandstone in northwestern Nebraska accumulated in wave-dominated estuarine settings, as part of a long-lived transgressive systems tract. The unit as a whole occupies a complexly incised landscape cut during a third-order lowstand ca. 98 Ma. Coeval cycles of similar magnitude throughout the Western Interior suggest that the three cycles represent eustatic fluctuations. The highest quality reservoirs occur at the base of the unit in inner estuarine lithofacies in the central and southwestern parts of the study area.

Lithostratigraphic and tectonic framework of Jurassic and Cretaceous Intermontane sedimentary basins of south-central British Columbia1This article is one of a series of papers published in this Special Issue on the theme of New insights in Cordilleran Intermontane geoscience: reducing exploration risk in the mountain pine beetle-affected area, British Columbia.

The south-central Intermontane belt of British Columbia has a complex architecture comprising late Paleozoic to Mesozoic volcanic and plutonic arc magmatic suites, marine and nonmarine clastic basins, high-grade metamorphic complexes, and accretionary rocks. Jurassic and Cretaceous clastic basins within this framework contain stratigraphy with hydrocarbon potential. The geology is complicated by Cretaceous to Eocene deformation, dismemberment, and dislocation. The Eocene to Neogene history of the southern Intermontane belt is dominated by non-arc volcanism, followed by Pleistocene to Recent glaciation. The volcanic and glacial cover makes this a difficult region to explore for resources. Much recent work has involved re-evaluating the challenges that the overlying volcanic cover has historically presented to geophysical imaging of the sedimentary rocks in this region in light of technological advances in geophysical data collection and analysis. This paper summarizes the lithological and stratigraphic framework of the region, with emphasis on description of the sedimentary units that have been the targets of hydrocarbon exploration.

A contribution to regional stratigraphic correlations of the Afro-Brazilian depression – The Dom João Stage (Brotas Group and equivalent units – Late Jurassic) in Northeastern Brazilian sedimentary basins

Abstract The Dom Joao Stage comprises an interval with variable thickness between 100 and 1200 m, composed of fluvial, eolian and lacustrine deposits of Late Jurassic age, based mainly on the lacustrine ostracod fauna (although the top deposits may extend into the Early Cretaceous). These deposits comprise the so-called Afro-Brazilian Depression, initially characterized as containing the Brotas Group of the Reconcavo Basin (which includes the Alianca and the Sergi Formations) and subsequently extended into the Tucano, Jatoba, Camamu, Almada, Sergipe, Alagoas and Araripe Basins in northeastern Brazil, encompassing the study area of this paper. The large occurrence area of the Dom Joao Stage gives rise to discussions about the depositional connectivity between the basins, and the real extension of sedimentation. In the first studies of this stratigraphic interval, the Dom Joao Stage was strictly associated with the rift phase, as an initial stage (decades of 1960–70), but subsequent analyses considered the Dom Joao as an intracratonic basin or pre-rift phase – without any relation to the active mechanics of a tectonic syn-rift phase (decades of 1980–2000). The present work developed an evolutionary stratigraphic and tectonic model, based on the characterization of depositional sequences, internal flooding surfaces, depositional systems arrangement and paleoflow directions. Several outcrops on the onshore basins were used to build composite sections of each basin, comprising facies, architectural elements, depositional systems, stratigraphic and lithostratigraphic frameworks, and paleocurrents. In addition to that, over a hundred onshore and offshore exploration wells were used (only 21 of which are showed) to map the depositional sequences and generate correlation sections. These show the characteristics and relations of the Dom Joao Stage in each studied basin, and they were also extended to the Gabon Basin. The results indicate that there were two main phases during the Dom Joao Stage, in which distinctive sedimentary environments were developed, reflecting depositional system arrangements, paleoflow directions were diverse, and continuous or compartmented basins were developed.

Lithostratigraphy of the Upper Oligocene – Miocene succession of Denmark

This paper presents a revised lithostratigraphic scheme for the uppermost Upper Oligocene – Miocene succession of Denmark. The marine Oligocene Brejning Clay Member is upgraded to formation status and includes the Sydklint Member and the Øksenrade Member (new). The shallow marine and deltaic deposits of mainly Early Miocene age are included in the Ribe Group (new) while the fully marine Middle and Upper Miocene clay-rich deposits are referred to the Måde Group (new). The Ribe Group is subdivided into 6 formations: the Vejle Fjord Formation is revised and includes the Skansebakke Member, the Billund Formation (new) includes the Addit and Hvidbjerg Members (new), the Klintinghoved Formation is redefined formally and includes the Koldingfjord Member (new), the Bastrup Formation (new) includes the Resen Member (new), the Vandel Member is a new member in the Arnum Formation (revised), the Odderup Formation is redefined and includes the Stauning Member (new) and the coalbearing Fasterholt Member. The Måde Group is subdivided into the Hodde, Ørnhøj (new), Gram and Marbæk (new) Formations. Subdivision of the Upper Oligocene – Miocene succession into two groups, the Ribe and Måde Groups, is compatible with the North Sea lithostratigraphic framework where they correlate with the upper part of the Hordaland Group and the Nordland Group, respectively.
 The revised lithostratigraphic framework correlated in three dimensions provides rigorous constraints on the palaeogeographic interpretation of the Late Oligocene – Miocene period. Three major deltaic units (Billund, Bastrup and Odderup Formations) prograded from the north and north-east into the North Sea Basin during the Early – early Middle Miocene. Delta progradation was punctuated by deposition of marine clay and silt associated with minor transgressive events (Vejle Fjord, Klintinghoved and Arnum Formations). During the Middle–Late Miocene, marine depositional conditions dominated (Hodde, Ørnhøj and Gram Formations). A fourth and final progadational event (Marbæk Formation) commenced in the latest Tortonian heralding the emergence of present-day Denmark (including the North Sea sector).

Seismic imaging of Paleogene sediments of Kachchh Shelf (western Indian margin) and their correlation with sea-level fluctuations

The sedimentary history of Kachchh offshore (central western Indian margin), especially since the eruption of the Deccan Traps (∼65 Ma), has remained scantily studied despite an area with promising resource potential. Of late, new marine surveys combined with industrial drilling along the Kachchh shelf are beginning to elucidate the depositional history of this region. Here, we attempt to synthesize interpretation of new offshore seismic data, along with borehole information and long-term sea-level variations to provide a coherent sedimentological and lithostratigraphic framework over the past ∼65 Ma in this area. Based on integrated interpretation of information derived from complementary sources, Paleogene successions of Kachchh shelf are investigated. Six major seismic facies are identified and attributed to post-Cretaceous sea-level fluctuations. Each lithological unit mapped on the seismic sections suggests coeval sedimentation in the area. Variable geometry and seismic characters of the identified facies are noted and attributed to the evolutionary history of this region. Recognized seismic horizons are distributed uniformly and are inferred to have regional significance. Apparently, the sea-level fluctuations in the past have played a major role over geodynamic contributions in terms of sediment deposition in this region. The incessant sea-level changes have resulted in transgression–regression type sediments throughout the area. Regular but low levels of tectonism are inferred from the seismic sections in the form of faults. Each sediment unit reflects dual signatures of eustatic sea-level changes as well as structural activities. Integration of lithologs with seismic data also suggests that the area was dominated by carbonate sedimentation during the Paleogene. The most important aspect of this study is to provide sub-surface images that illustrate stratigraphic details of the Paleogene sediments with significant resource potential.

A holostratigraphic approach to the chalk of the North Sea Eldfisk Field, Norway

Abstract A biostratigraphic review, conducted on 34 wells from the chalk of the Eldfisk Field, Norwegian Central Graben, has been integrated with petrophysical, geophysical and sedimentological information resulting in a revised lithostratigraphic framework for the chalk on this structure. Chalk of Danian to Turonian age is divided into five formations: the established Ekofisk Formation of Danian age and Tor Formation of Maastrichtian age, together with a new three-fold division of the Hod Formation, namely the Magne Formation of Campanian age, the Thud Formation of Santonian age and Narve Formation of Coniacian to Turonian age. This work demonstrates the application of this three-fold division of the Hod Formation. Internal field specific subdivisions of all formations are also presented for the Eldfisk Field. This lithostratigraphic framework is applied across the Eldfisk Field, together with the recognition of erosional features, unconformities, areas of non‐deposition, reworking and lateral changes in biofacies. The results have also allowed recognition of the following regionally synchronous tectonic phases for the first time on a Norwegian chalk structure: Stille's Ilsede phase (Late Turonian–Coniacian) and Wernigerode phase (Late Santonian–‘earliest’ Campanian), Mittel–Santon phase (Middle Santonian) of Niebuhr et al. and Reidel's Peine phase (‘latest’ Early Campanian), together with un-named phases of ‘latest’ Campanian, intra Mid Maastrichtian and (previously unrecognized?) intra Danian age. Evidence for these tectonic phases is compared with work from Denmark, Germany and the Anglo-Paris Basin. An innovative approach to mapping lateral biofacies (principally water depth) variations has been applied using the microfaunal database. This enhances understanding of the timing of structural phases when integrated with time lines generated by nanoplankton data. Biofacies proxies for silica content in the sediment may also correlate with changes in reservoir quality. Biofacies interpretations have also facilitated the identification and mapping of allochthonous bioclastic rich debris flow deposits. The fully calibrated biostratigraphic, lithostratigraphic and tectonostratigraphic frameworks presented can be applied to chalk structures regionally.

The limitations of Quaternary lithostratigraphy: an example from southern Ireland

Abstract The lithostratigraphy of Quaternary extra-glacial (cold-climate) sediments is described from five sites along the south coast of Ireland. These sediments are compared to a regional-scale lithostratigraphic framework. This comparison enables consideration of the applicability of standard lithostratigraphic principles and practices to the Quaternary record. It is concluded that a strict lithostratigraphic approach is not favourable towards interpreting the age or depositional process/environment of Quaternary sediments, either in isolation or as part of a sedimentary succession. The concepts of time transgression and lateral equivalence are useful interpretive tools that can support lithostratigraphic procedures as applied to the Quaternary record.

A shift from lithostratigraphic to allostratigraphic classification of Quaternary glacial deposits

The bedrock of the northern halves of North America and Europe is covered by Quaternary glacial deposits, forming a surficial overburden that is relatively thin, nonlithified, litho logically variable on a small scale (in terms of grain-size, mineralogy, texture, fabric, structure, and color), and often has a well-preserved depositional topography. These geologically unique characteristics and the fact that the glacial overburden was long considered to be of only restricted economic value have caused it to be treated differently in geological research from the older, regularly lithified strata. Due to the striking geomorphology of these glacial deposits, their investigation has also been incorporated into physical geography research. Thus, the segregation of the Quaternary research community into different schools of geology and geography has created multiple classification approaches and has caused the formal stratigraphic classifications successfully applied in pre-Quater nary geology to be applied less regularly to Quaternary glacial strata. This has led to inefficient use of Quaternary geological data for scientific and socio-economic purposes. The few currently existing national Quaternary stratigraphic frameworks are based on lithostratigraphy. These are poorly suited for describing deposits in glaciated shield areas in particular; we propose a classification for such areas based on the combined use of allostratigraphic and lithostratigraphic data, with alloformations as the fundamental units and lithostratigraphic units filling out the framework where appropriate. This classification would provide a hierarchical framework for gla ciogenic deposits that could potentially support stratigraphic information systems, databases, and digital spatial models more effectively than the traditional lithostratigraphic frameworks.

Geochemistry of the Neoarchaean mafic volcanic rocks of the Geita area, NW Tanzania: Implications for stratigraphical relationships in the Sukumaland greenstone belt

Geochemical data are presented for a suite of mafic volcanic rocks from the Geita area in the Sukumaland greenstone belt (SGB) of northwestern Tanzania with the aim of constraining their petrogenesis, tectonic setting and to assess a possible genetic link with mafic volcanic rocks from the Rwamagaza area also from the SGB previously reported by [Manya, S., Maboko, M.A.H., 2003. Dating basaltic volcanism in the Neoarchaean Sukumaland greenstone belt of the Tanzania Craton using the Sm–Nd method: implications for the geological evolution of the Tanzania Craton. Precambrian Research 121, 35–45] and [Manya, S., 2004. Geochemistry and petrogenesis of volcanic rocks of the Neoarchaean Sukumaland greenstone belt, northwestern Tanzania. Journal of African Earth Sciences 40, 269–279]. Mafic volcanic rocks from the two locations in the SGB show similar geochemical and Nd-isotopic compositions. Trace element and Nd-isotope compositions are consistent with their generation from a depleted MORB mantle (DMM) source which had been metasomatised by a subduction component in a late Archaean back arc setting at ∼2823 Ma. These findings are at variance with the previously proposed lithostratigraphical framework in the SGB which postulated an inner arcuate belt dominated by lower Nyanzian mafic volcanic rocks and an outer belt dominated by upper Nyanzian chemical sedimentary rocks, rare felsic flows and shales. The presence of mafic volcanic rocks flanking the outer belt which are of similar composition and age as those of the inner belt suggests that mafic volcanics in the SGB form discontinuous patches of rock distributed throughout the belt and separated by intervening granites. Furthermore, they corroborate previous evidence that both the rocks of the inner and outer belt formed more or less coevally and the subdivision of the volcano-sedimentary package of the SGB (and other greenstone belts of the Tanzania Craton) into a lower mafic volcanic dominated unit and an upper felsic volcanic and BIF dominated unit is not stratigraphically valid.

A Revised Stratigraphy for the Pliocene and Lower Pleistocene Deposits of the Lower Rhine Embayment

AbstractThe Plio-Pleistocene succession in the Lower Rhine Embayment was subjected to a thorough revision of existing stratigraphic concepts. The deposits were studied at key sites in the type area near Venlo and in the large open-cast mine Hambach in the southern part of the Lower Rhine Embayment by means of sedimentological, petrographical, as well as palaeo- and rockmagnetic methods.The work has yielded improved insights of the drainage pattern and the Late Pliocene and Early Pleistocene depositional history of the region. As a result, a new comprehensive lithostratigraphical framework has been established.Study of the succession at Hambach showed the occurrence of deposits of the Rhine, Meuse and a local river in the Lower Pleistocene part of the succession. Paleo- and rockmagnetic studies of the deposits marking the transition from Pliocene to Pleistocene indicate that the Gauss-Matuyama magnetic reversal occurs several meters above the top of the Reuver Clay at Hambach.The study of the Lower Pleistocene succession in the type area has confirmed the recently new established lithostratigraphic framework of the Netherlands. As a consequence, the previous Dutch lithostratigraphic system which forms the basis of the chronostratigraphic subdivision of the Pliocene and Early Pleistocene of NW Europe has been proven to be inappropriate and should be abandoned. This chronostratigraphic framework is based on the interpretation of palynological data and was first established in the Netherlands during the 1960s. The new lithostratigraphic concept has revealed numerous contradictions with the chronostratigraphic framework. Based on these results it is proposed to abandon the chronostratigraphic subdivision of the Early Pleistocene in northwestern Europe.

The confluence area of Rhine, Meuse, and Belgian rivers: Late Pliocene and Early Pleistocene fluvial history of the northern Lower Rhine Embayment

AbstractThe fluvial history of the northern Lower Rhine Embayment shows interplay of three main river systems: Rhine, Meuse and smaller rivers draining the central and northern part of Belgium.The Pliocene and Early Pleistocene (pre-)Rhine and Meuse river systems had their conjunction in the southern part of the Roer Valley Graben between Aachen and Jülich. Despite slight differences in the heavy-mineral assemblages the lithological composition of the Pliocene deposits of the three river systems shows close resemblance and therefore they cannot be mapped separately. However, due to a marked change of the petrographical composition the Upper Pliocene and Lower Pleistocene deposits of the Rhine are easily recognised and as a result Rhine and Meuse deposits can be mapped separately upstream of their confluence.The Lower Pleistocene deposits of Rhine, Meuse and the Belgian rivers show a clear interrelationship. They are bounded by two regional well-mapable unconformities and are preserved in from west to east changing lithostratigraphical sequences. Revision of the lithostratigraphical schemes in Germany and the Netherlands and the better defined lithostratigraphical position of Meuse deposits in Germany now strongly constrain the correlation of the various fluvial deposits. As a result existing reconstructions of the fluvial deposition and tectonic history of the southern Roer Valley Graben can be evaluated and re-adjusted.It is concluded that the main course of the Meuse was aligned through the so-called East Meuse valley during the larger part of the Early Pleistocene. Available pollen data do not conflict with this conclusion. At the same time the Rhine ceased to enter the southern part of the Roer Valley Graben. Instead, the Meuse accumulated here a series of deposits derived from the East-Meuse valley. Simultaneously, the Belgian rivers filled available accommodation space in the Roer Valley Graben of the southern Netherlands. The conclusions are based primarily on the revised lithostratigraphical framework. In general they simplify the picture of fluvial and tectonic behaviour of the area.

Introduction of the Boxtel Formation and implications for the Quaternary lithostratigraphy of the Netherlands

AbstractApplication of the traditional lithostratigraphic framework to subdivide the Middle- and Upper-Quaternary locally-derived fine-grained deposits in the Netherlands is problematic. Deposits of many formations cannot be distinguished from each other based on lithological characteristics and stratigraphie position alone. To overcome this problem, we present a new, well-defined lithostratigraphy for these deposits, based on detailed research in the central part of the Roer Valley Graben. This area contains an up to 35 m-thick sedimentary record of Middle- and Upper-Quaternary sand, loam and peat deposits. These have mainly been formed by aeolian and small-scale fluvial processes and have been preserved as a result of tectonic subsidence. The traditional lithostratigraphic subdivision of these deposits into three formations (Eindhoven Formation, Asten Formation and Twente Formation) was based on a combination of litho-, bio- and chronostratigraphic evidence and the presumed widespread presence of a horizon of organic-rich interglacial sediments of Eemian age. To avoid intermingling of criteria regarding lithological characteristics, genesis and age, we now incorporate all fine-grained sediments into the new Boxtel Formation. The implications for the lithostratigraphic framework in other parts of the country are explored and discussed. Eight lithostratigraphic members are introduced that describe the most characteristic parts of the formation. To fully illustrate the sedimentary sequence in the Roer Valley Graben, two new members are defined here. The Best Member incorporates alternating floodloam deposits and sandy aeolian deposits in the lower part of the Boxtel Formation. The Liempde Member includes reworked aeolian loess and sandy loess deposits (‘Brabant loam’) that occur in the upper part of the sedimentary sequence.

Late Neogene continental stages in NW Europe revisited

Pollen records from fluvial deposits in The Netherlands have traditionally been used to establish continental stages for the Neogene in NW Europe. These stages are still widely used across Europe for stratigraphic correlations and palaeoclimatic studies. Despite their extensive use, the validity of this continental correlation scheme for the Plio- and Pleistocene has never been tested in the area where they were originally proposed: the Roer Valley Graben (RVG). The fluvial and deltaic depositional setting in this area is controlled by a complex pattern of tectonic and climatic processes, which strongly complicates the definition of stages and sub-stages of regional or supra-regional significance. Here, the original palynological data used to define the Praetiglian, Reuverian, Brunssumian, Susterian, and ‘Linne’ (sub-)stages are assessed and compared to new borehole data. With a multivariate (Correspondence Analysis) approach we show that most of these stages cannot be distinguished by numerical analysis of the pollen assemblages. A tripartition of pollen assemblages is possible, which results in a cooler Pleistocene cluster, warm-temperate to sub-tropical Pliocene cluster, and a warm and relatively dry Late Miocene to Early Pliocene cluster. However, no further stages, sub-stages or clear climatic phases can be consistently recognized in these data. A detailed lithostratigraphic framework of the study area allows a correlation of the sections independent of the pollen data. Results confirm the numerical analyses and furthermore point to a strong lithological (depositional) bias on the pollen assemblages. Variations in the abundance of Sequoia and Taxodium, originally used to define the Brunssumian/Reuverian transition and Brunssumian A–C sub-stages, are shown to represent mostly local changes. The number and absolute percentage of the fluctuations in Sequoia and Taxodium abundances depend largely on the sample resolution and the site location within the RVG. Our data assessment indicates that long-distance chronostratigraphic correlations based on the original continental Neogene stages likely are invalid. Furthermore, the palaeoclimatic reconstructions that have been based on the RVG successions are largely over-interpreted. The present palaeoclimate reconstructions from the RVG reflect variations in the highly-dynamic fluvial depositional setting, taphonomic effects and environmental conditions, including climate. A sequence-stratigraphic approach combined with the analysis of interfingering marine sediments is potentially better suited to establish absolute ages and associated climatic conditions of the Neogene deposits within the RVG.

Lithostratigraphy of the Palaeogene – Lower Neogene succession of the Danish North Sea

As a result of a lithological, sedimentological and biostratigraphic study of well sections from the Danish sector of the North Sea, including some recently drilled exploration wells on the Ringkøbing–Fyn High, the lithostratigraphic framework for the siliciclastic Palaeogene to Lower Neogene sediments of the Danish sector of the North Sea is revised. The sediment package from the top of the Chalk Group to the base of the Nordland Group is subdivided into seven formations containing eleven new members. The existing Våle, Lista, Sele, Fur, Balder, Horda and Lark Formations of previously published lithostratigraphic schemes are adequate for a subdivision of the Danish sector at formation level. Bor is a new sandstone member of the Våle Formation. The Lista Formation is subdivided into three new mudstone members: Vile, Ve and Bue, and three new sandstone members: Tyr, Idun and Rind. Kolga is a new sandstone member of the Sele Formation. Hefring is a new sandstone member of the Horda Formation. Freja and Dufa are two new sandstone members of the Lark Formation. Danish reference sections are established for the formations, and the descriptions of their lithology, biostratigraphy, age and palaeoenvironmental setting are updated.

Colophon, contents, abstract, introduction, geological setting, previous work and material and methods

As a result of a lithological, sedimentological and biostratigraphic study of well sections from the Danish sector of the North Sea, including some recently drilled exploration wells on the Ringkøbing–Fyn High, the lithostratigraphic framework for the siliciclastic Palaeogene to Lower Neogene sediments of the Danish sector of the North Sea is revised. The sediment package from the top of the Chalk Group to the base of the Nordland Group is subdivided into seven formations containing eleven new members. The existing Våle, Lista, Sele, Fur, Balder, Horda and Lark Formations of previously published lithostratigraphic schemes are adequate for a subdivision of the Danish sector at formation level. Bor is a new sandstone member of the Våle Formation. The Lista Formation is subdivided into three new mudstone members: Vile, Ve and Bue, and three new sandstone members: Tyr, Idun and Rind. Kolga is a new sandstone member of the Sele Formation. Hefring is a new sandstone member of the Horda Formation. Freja and Dufa are two new sandstone members of the Lark Formation. Danish reference sections are established for the formations, and the descriptions of their lithology, biostratigraphy, age and palaeoenvironmental setting are updated.

A New Lithostratigraphic Framework for the Cretaceous Colorado Group in the Cold Lake Heavy Oil Area, East-Central Alberta, Canada

The Albian to Santonian Colorado Group in the heavy oil area of Cold Lake, east-central Alberta represents a relatively condensed section of shale-dominated sedimentation within the Western Interior Sea. These shales form the cap rock to the underlying Clearwater and Grand Rapids formations that are exploited for bitumen extraction. Two cores covering the entire Colorado Group provide a unique opportunity for establishment of a stratigraphic reference in an area that has received attention only for its heavy oil-bearing Mannville Group. Based on sedimentology, geochemistry, micropaleontology, nannofossils, and wireline log data, the Colorado Group was subdivided into the Joli Fou, Viking, Westgate, Fish Scales, Belle Fourche, Second White Specks, and Niobrara formations. The Niobrara Formation was subdivided further into the Verger Member, informal Cold Lake member, and First White Specks Member. Because of this region’s small accommodation space and distance to sources of coarse clastic sediment, disconformities are indicated lithologically by bioclastic layers and missing biozones. Foraminiferal subzones revealed two erosional boundaries associated with the Viking Formation. Faunal and floral evidence coupled with wireline log correlations suggest that the Middle to Upper Turonian Carlile Formation, as described from southeastern Alberta, is missing. That extends the Middle Turonian to Coniacian unconformity, as recognized in central Saskatchewan, westwards into Alberta.

Invited comment on Wray & Gale's ‘The palaeoenvironment and stratigraphy of the Late Cretaceous Chalks’: Proceedings, Vol. 117, 2006, pp. 145–162

Despite the context of this reply we would wish to echo the tributes to Jake Hancock expressed elsewhere in this volume. His contribution in papers over many years (e.g. 1961, 1972, 1975, 1991, 2000) is considerable and, in many cases, still relevant. We would also like to thank Wray and Gale for demonstrating the breadth of research into the understanding of the Chalk and its environment of deposition. Our major concern is with the inaccuracy, incorrect assumptions and misrepresentations implied and stated by Wray & Gale (2006) in their discussion of the modern Chalk lithostratigraphical framework applied to British Geological Survey (BGS) maps and demonstrated in many published documents. This reply gives the BGS an opportunity to restate in brief the lithostratigraphical framework for the Chalk Group of England and to deal with specific points raised by Wray and Gale. Our reply is prompted in part by the implications in Wray & Gale that the geological mapping of the Chalk Group in England is carried out in a haphazard and uncontrolled manner without scientific rigour and is of ‘little value’. This implication is refuted absolutely. In fact, it is a test of the robustness of the framework that a large number of field geologists can apply the scheme across southern England and provide such a powerful predictive tool for the practical benefit of the nation. A further verification of the framework’s pertinence, over and above its application to pure scientific endeavours, is its widespread use in applied geological studies. For example, the framework provides the foundation for investigations into the structure of the Chalk, its engineering characteristics and hazards and, perhaps most importantly of all, provides the key and new impetus to studies, including predictive modelling, of the hydrogeology of the UK’s largest and most important aquifer. As stated by Wray & Gale, the development of the regional Chalk lithostratigraphies in the late 1970s to the mid-1980s (Wood & Smith 1978; Mortimore 1983, 1986; Jarvis & Woodroof 1984; Robinson 1986) made it apparent that the traditional tripartite scheme embodied by Jukes-Browne & Hill (1903, 1904) did not delineate fully the lithological variation in the Chalk. This inadequacy led to the development of a more detailed and broadly applicable framework by BGS. The modern lithostratigraphical framework builds upon the expertise of a large number of field geologists. It was presented in a model (Bristow et al. 1997) that was modified at a workshop of the UK’s Chalk experts in 1999. It belongs to all those who contributed and signed up to it, and is the ‘agreed lithostratigraphical framework’ (Rawson et al. 2001). The outline framework with minor additions at member level is given in Figure 1. The full framework report for the Chalk Group of the UK, approved by the Geological Society of London’s Stratigraphy Commission (GSLSC), appears in Hopson (2005) and is available (in pdf format) as a free download from the BGS website (http:// www.bgs.ac.uk) as a joint BGS/GSLSC publication.

Contribution to a geodynamic reconstruction of the Anti-Atlas (Morocco) during Pan-African times with the emphasis on inversion tectonics and metallogenic activity at the Precambrian–Cambrian transition

New geochronological analyses (U–Pb SIMS zircon ages) have yielded ages of 552 ± 5 Ma for the Bou Madine rhyolitic dome (Ougnat, eastern Anti-Atlas), 543 ± 9 Ma for the Tachkakacht rhyolitic dyke (Saghro–Imiter, eastern Anti-Atlas), and 531 ± 5 Ma for the Aghbar trachytic sill (Bou Azzer, central Anti-Atlas). Inherited zircon cores from the Aghbar trachytic sill and from the Bou Madine rhyolitic dome have been shown to be of Statherian age (ca. 1600–1800 Ma) and Palæoproterozoic (>2100 Ma) age, respectively, suggesting that a significantly older protolith underlies the Pan-African rocks in the Central and Eastern Anti-Atlas. Granodiorites and rhyolites from the Saghro–Imiter area have similar low 87Sr/ 86Sr (0.702–0.706) and 143Nd/ 144Nd (0.5116–0.5119) initial ratios, suggesting a mixture of mantle and lower crust sources. This can also be inferred from the low 187Os/ 188Os ratios obtained on pyrite crystals from the rhyolites. A recently published lithostratigraphic framework has been combined with these new geochemical and geochronological data, and those from the literature to produce a new reconstruction of the complex orogenic front that developed at the northern edge of the Eburnian West African craton during Pan-African times. Three Neoproterozoic magmatic series can be distinguished in the Anti-Atlas belt, i.e., high-K calc-alkaline granites, high-K calc-alkaline to shoshonitic rhyolites and andesites, and alkaline-shoshonitic trachytes and syenites, which have been dated at 595–570, 570–545 and 530 Ma, respectively. The accretion of the Pan-African Anti-Atlas belt to the West African super continent (WAC) was a four-stage event, involving extension, subduction, moderate collision and extension. The calc-alkaline magmatism of the subduction stage was associated with large-scale base metal and gold mineralisation. Metallogenic activity was greatest during the final extensional stage, at the Precambrian–Cambrian boundary. It is characterised by world-class precious metal deposits, base–metal porphyry and SEDEX-type occurrences.

A revised lithostratigraphy for the Palaeogene – lower Neogene of the Danish North Sea

Intense drilling activity following the discovery of the Siri Field in 1995 has resulted in an improved understanding of the siliciclastic Palaeogene succession in the Danish North Sea sector (Fig. 1). Many of the new wells were drilled in the search for oil reservoirs in sand bodies of Paleocene–Eocene age. The existing lithostratigraphy was based on data from a generation of wells that were drilled with deeper stratigraphic targets, with little or no interest in the overlying Palaeogene sediments, and thus did not adequately consider the significance of the Palaeogene sandstone units in the Danish sector. In order to improve the understanding of the distribution, morphology and age of the Palaeogene sediments, in particular the economically important sandstone bodies, a detailed study of this succession in the Danish North Sea has recently been undertaken. An important aim of the project was to update the lithostratigraphic framework on the basis of the new data. The project was carried out at the Geological Survey of Denmark and Greenland (GEUS) with participants from the University of Aarhus, DONG E&P and Statoil Norway, and was supported by the Danish Energy Agency. Most scientific results cannot be released until September 2006, but a revised lithostratigraphic scheme may be published prior to that date. Formal definition of new units and revision of the lithostratigraphy are in preparation. All of the widespread Palaeogene mudstone units in the North Sea have previously been formally established in Norwegian or British wells, and no reference sections exist in the Danish sector. As the lithology of a stratigraphic unit may vary slightly from one area to another, Danish reference wells have been identified during the present project, and the lithological descriptions of the formations have been expanded to include the appearance of the units in the Danish sector. Many of the sandstone bodies recently discovered in the Danish sector have a limited spatial distribution and were sourced from other areas than their contemporaneous counterparts in the Norwegian and British sectors. These sandstone bodies are therefore defined as new lithostratigraphic units in the Danish sector, and are assigned Danish type and reference sections. There is a high degree of lithological similarity between the Palaeogene–Neogene mudstone succession from Danish offshore boreholes and that from onshore exposures and boreholes, and some of the mudstone units indeed seem identical. However, in order to acknowledge the traditional distinction between offshore and onshore stratigraphic nomenclature, the two sets of nomenclature are kept separate herein. In recent years oil companies operating in the North Sea have developed various in-house lithostratigraphic charts for the Paleocene–Eocene sand and mudstone successions in the Danish and Norwegian sectors. A number of informal lithostratigraphic units have been adopted and widely used. In the present project, these units have been formally defined and described, maintaining their original names whenever feasible, with the aim of providing an unequivocal nomenclature for the Palaeogene – lower Neogene succession in the Danish sector. It has not been the intention to establish a sequence stratigraphic model for this succession in the North Sea; the reader is referred to the comprehensive works of Michelsen (1993), Neal et al. (1994), Mudge & Bujak (1994, 1996a, b), Michelsen et al. (1995, 1998), Danielsen et al. (1997) and Rasmussen (2004).