Cretaceous Chalk Research Articles

The multistage structural development of the Upper Weichselian Jasmund Glacitectonic Complex (Rügen, NE Germany)

Glacitectonic deformation in the Quaternary caused the tectonic framework of large-scale folds and displaced thrust sheets of Maastrichtian (Upper Cretaceous) chalk and Pleistocene glacial deposits in the southwestern Baltic Sea area. A wide spectrum of methods has been compiled to unravel the structural evolution of the Jasmund Glacitectonic Complex. The analyses of digital elevation models (DEM) suggest a division into two structural sub-complexes – a northern part with morphological ridges striking NW–SE and a southern part with SW–NE trending ridges. Geological cross sections from the eastern coast (southern sub-complex) were constructed and restored using the software Move™ and the complementary module 2D Kinematic Modelling™. The final geometric model of the southern sub-complex shows a small-scale fold-and-thrust belt. It includes three different orders of architectural surfaces (see PEDERSEN, 2014): erosional surfaces and the decollement (1st order), thrust faults (2nd order), and beds outlining hanging-wall anticlines as well as footwall synclines (3rd order). Thrust faults of the southern structural sub-complex are mainly inclined towards south, which indicates a local glacier push from the S/SE. The glacitectonic structures have a surface expression in form of sub-parallel ridges and elongated valleys in between. Geomorphological mapping and detailed landform analyses together with the structural investigations provide an insight into the chronology of sub-complexes formation. The northern part of the glacitectonic complex is suggested to have been formed before the southern one, considering the partly truncated northerly ridges and their superimposition by the southern sub-complex. Although there is a high number of scientific publications on the glacitectonic evolution of Jasmund, these presented models often lack a consistent theory for the development integrating all parts of the 100 km2 large complex. Therefore, the combination of all results leads to a more self-consistent genetic model for the entire Jasmund Glacitectonic Complex.

The paradoxical ichnotaxonomy of Thalassinoides paradoxicus: a name of different meanings

Thalassinoides paradoxicus (Woodward, 1830) is a prevalent and frequently reported ichnotaxon, but given its early erection and subsequent revisions, it now is poorly understood and lumps together three different kinds of trace fossils having distinct characteristics. A clear definition (i.e., diagnosis) of each individual component is crucial for ichnotaxonomic stability and the application of these ichnotaxa in environmental and evolutionary interpretations. Originally described and figured as the zoophyte Alcyonia (soft-bodied coral) from the Upper Greensand (Lower Cretaceous) of southern England over 200 years ago, it became the type of Spongia paradoxica Woodward, 1830. This type was mostly ignored by subsequent workers, and instead branched burrows from the Upper Cretaceous Chalk were regarded of being representative for S. paradoxica, a form that became typified with a neotype as Thalassinoides paradoxicus Kennedy, 1967. Furthermore, irregularly branched firm- to hardground burrows from the same strata were also called T. paradoxicus but rather belong to Balanoglossites triadicus Magdefrau, 1932. A taxonomic discussion of the involved ichnotaxa is presented, resulting in the erection of the new ichnogenus Lamellaecylindrica, including L. paradoxica (Woodward, 1830), and the differentiation of the ichnospecies Thalassinoides paradoxicus Kennedy, 1967 and Balanoglossites triadicus Magdefrau, 1932. Each of these ichnotaxa is a common constituent of a specific lithofacies and environment.

Thermal evolution of the intracratonic Paris Basin: Insights from 3D basin modelling

The thermal evolution of the Paris Basin (PB) has been widely studied using 1D, 2D and, more rarely, 3D thermal models. It is well documented that the PB experienced higher temperatures in the past compared to what is currently observed. However, a quantitative analysis of the main processes and parameters that affect the temperature distribution, at the basin scale and over time, is still not available. In this study, through basin modeling which accounts for the main processes of the thermal evolution of sedimentary basins, we analyze and quantify the role of the different geological mechanisms by discriminating the causes of abnormal temperatures during the Late Mesozoic. This is done with a 3D basin model built from base Moho to present-day topography using the TemisFlow® basin modelling software. The model includes thermal processes within an evolving upper crust defined by three main structural domains. Each crustal sector presents radiogenic heat production, conductivity and thickness values which are used as input parameters to reproduce the paleo- and present-day basal heat flow variations observed in the basin. The model calculates heat flow through time in both, crust and sedimentary column where the crust is coupled with the geological evolution of the basin. This approach allows estimating the eroded thickness during the main Tertiary uplift event and therefore the maximum temperature in the Late Cretaceous. The model is constrained by different types of paleo-thermo-chronometers and by 52 wells that are regionally distributed over the entire basin, resulting in a new regional thermal history of the PB. The amount of missing section in the Cretaceous chalk which mainly affected the eastern part of the basin is increased by up to 500m compared with previous studies and constitutes the key controlling factor of the temperature evolution. This new regional thermal history of the Paris Basin may be important for further analysis of the HC generation from the Lower Jurassic Toarcian source-rock and bring new insights into the geothermal potential of the basin.

A brief review of the Late Cretaceous “Oceanic Red Beds” of the North Sea offshore region

Late Cretaceous red chalk units of “deep” marine origin were recognised and published from the North Sea Basin almost thirty years ago (King et al., 1989), however it is only now that we are beginning to recognise their oceanographic, depositional and climatic significance. Their stratigraphic occurrence is redefined and an additional red unit is proposed. The palaeoceanographic implications of these red units are considered in the light of recent studies on the origins and occurrences of Cretaceous Oceanic Red Beds (CORB’s) and of the red horizons in the onshore Cretaceous Chalk of North West Europe.

Organofacies composition of Upper Jurassic – lowermost Cretaceous source rocks, Danish Central Graben, and insight into the correlation to oils in the Valdemar Field

Abstract The organic-rich marine shales of the Upper Jurassic - lowermost Cretaceous Farsund Formation are the main hydrocarbon source rocks for the Cretaceous chalk reservoir of the Valdemar Field (Danish North Sea). Geochemical analyses of oil extracts from core samples across the greater Valdemar Field show substantial heterogeneity of the reservoir oils, suggesting contribution from kitchen areas with source rocks of variable organofacies and thermal maturity. In order to better understand the source rock facies that charged the field, a refined organofacies characterization of the Upper Jurassic - lowermost Cretaceous marine shale source rock succession (Farsund Formation) in the Danish Central Graben was established. This organofacies characterization is based on integration of petrographic and geochemical data and demonstrates large variations in both the regional and local distribution of the organofacies at the sequence level in the shales within the Danish Central Graben. In addition, the organofacies characterization shows that the respective sequences in the succession may be represented by several organofacies at the same time. Part of the explanation to the heterogeneity in the oil types accumulated within the Valdemar Field may lie in the presence of different organofacies within the uppermost sequences (Vol-3 to Ryaz-1) in the three kitchen areas. Each organofacies presumable generates different types of oils and the variation in the relative proportions of the different organofacies likely determines the composition of the generated oils. One dimensional (1-D) maturity modelling at three pseudo-well locations, each representing a potential Farsund Formation kitchen, shows that all kitchens are in the main oil window and that mainly oil was expelled from the Volg-3 to Ryaz-1 sequences. Modelling indicates that (i) average values of TOC, HI, thickness and the kinetic model are the main controls on generation and expulsion of hydrocarbons; and (ii) by increasing these parameters, expulsion could also occur from the lower part of the succession.

The Mungo Field, Blocks 22/20a and 23/16a, UK North Sea

Abstract The Mungo Field is a mature producing asset located in the UK Central North Sea. Discovered in 1989 and brought on production in 1998, it is the largest field within the Eastern Trough Area Project (ETAP). Production occurs via a normally unattended installation and is tied back to the ETAP Central Processing Facility. It is a pierced, four-way dip closure against a salt diapir. Light oil is present within steeply dipping Late Paleocene sandstone and Early Paleocene–Late Cretaceous chalk intervals. The vertical relief of the salt stock is around 1500 m TVDSS and top of the salt canopy lies at about 1350 m TVDSS. The Paleocene sandstones (Forties Sandstone Member of the Sele Formation, Lista Formation and Maureen Formation) make up the primary reservoir and have been extensively developed in three phases since 1998 under water injection and natural depletion. The sandstones were deposited as deep-water turbidite complexes (submarine fans with local channels) on and around the flanks of the rising salt diapir. More recently, successful stimulation of the Chalk Group, coupled with re-evaluation of core and well-log data, has indicated that economic production rates could also be achieved from the underlying fractured chalk reservoir.

A unique Lower Paleocene shelf-sea chalk in the eastern U.S. Gulf coastal plain (Clayton Formation, western Alabama): Implications for depositional environment, sea-level dynamics and paleogeography

The deposition of carbonate oozes in shelf-sea settings so prevalent during the Cretaceous generally ceased by the onset of the Paleogene in response to sea-level fall. However, the Lower Paleocene (Danian) Clayton Formation exposed at Moscow Landing in western Alabama, U.S.A., contains a thin chalk bed—the Clayton chalk—that records a later, final phase of shelf-sea carbonate ooze deposition in the U.S. eastern Gulf coastal plain. Sedimentologic character, ichnology, and foraminiferal assemblages indicate deposition in an outer shelf setting, comparable to that for Cretaceous chalks exposed in the Gulf region (i.e., Campanian Demopolis Chalk), at depths of 90 m or more, during a brief transgressive pulse peaking circa 63.8–63.2 Ma. This transgression occurred ~1 Myr after the maximum expansion of the Danian Cannonball Sea in the North American western interior, presumably reflecting different controls on sea-level dynamics (eustasy vs. Laramide tectonism and isostasy). Although regional paleogeography for the Early Paleocene is poorly constrained, deposition of the Clayton chalk suggests that the eastern Gulf seaway at times extended much further north than implied by the presently mapped updip limit of preserved Paleocene marine strata. The apparent geographic restriction of this unique Paleocene shelf-sea chalk indicates that relative sediment starvation and associated carbonate ooze deposition may have been influenced by local structural controls operating along the eastern margin of the Mississippi embayment.

Adaptive morphologies of the brachiopod fauna from Danian coral mounds at Faxe, Denmark

Only few of the brachiopod species that lived in the Late Cretaceous Chalk Sea of northern Europe survived the mass extinction at the Cretaceous–Paleogene boundary. However, a rich brachiopod fauna rapidly evolved from the survivors and most of the vacant niches were filled in by the middle Danian when a large cool-water, subphotic coral-bryozoan mound complex was developed at Faxe in the eastern part of the Danish Basin. The brachiopod fauna of the complex comprises 17 species dominated by micromorphic species of similar types as those known from the Upper Cretaceous, Maastrichtian chalk from the same region. All species are found in the coral mound facies and six of the species are restricted to this facies. Large-sized species are rare both in terms of species numbers and abundance. One species is adapted to a life between the branches of the cool-water hexacorals Dendrophyllia candelabrum and Oculina becki. Secondarily free-living species are close to absent. This is in contrast to the Maastrichtian chalk fauna where this group includes 10 species (~24% of the total brachiopod fauna). The otherwise great similarity in morphological adaptations in the brachiopods from the Maastrichtian chalk and the Danian coral mounds is remarkable and somewhat surprising as the latter almost lacks soft substrates. The Danian brachiopod fauna essentially represents a continuation of the Maastrichtian fauna, but in a very different habitat. The similarity between the two faunas is explained by the abundance of small, hard substrates, notably bryozoans in both environments. The brachiopods from Faxe are one of the very few ancient examples of a rich brachiopod fauna from a coral mound environment.

Combined high-precision ∆48 and ∆47 analysis of carbonates

High-precision analysis of the excess abundance (relative to the stochastic distribution) of mass 48 isotopologues in CO2 evolved from acid digestion of carbonates (∆48) has not been possible until recently due to the relatively low natural abundance of 18O. Here we show that the 253 Plus™ gas source mass spectrometer equipped with Faraday cups and 1013 Ω resistors can perform combined ∆47 and ∆48 analyses on carbonates with external reproducibilities (1SD) of 0.010 ‰ and 0.030 ‰, respectively.~10 mg aliquots of five carbonate reference materials (ETH 1, ETH 2, ETH 3, ETH 4, and Carrara) are digested with phosphoric acid at 90 °C using a common acid bath. The evolved CO2 is purified using an automated gas preparation system (including cryotraps and a GC) and analyzed for its ∆47 and ∆48 compositions using the dual inlet system of a 253 Plus™ gas source mass spectrometer. Raw ∆47 and ∆48 values are finally normalized to the Carbon Dioxide Equilibrium Scale (CDES).In ∆47, CDES 90°Cvs. ∆48, CDES 90°C space, calcite reference materials Carrara, ETH 3 and ETH 4 agree with the equilibrium curve for calcite after adding semi-empirically determined 90 °C acid fractionation factors of 0.196 ‰ (for ∆47) and 0.136 ‰ (for ∆48) to theoretical ∆63 and ∆64 data. Agreement between measured and theoretically expected ∆48, CDES 90°C highlights the accuracy of our high-precision clumped isotope analytical setup. Combined analysis of the abundances of mass 47 and mass 48 isotopologues in CO2 evolved from acid digestion of natural carbonates has excellent potential for the determination of accurate and highly precise paleotemperatures as well as for the identification of rate-limiting kinetic processes involved in biomineralization. A formation temperature of 15(±2) °C is obtained on the 95 % confidence level for the Upper Cretaceous chalk sample ETH 3.

The white cliffs of the Upper Cretaceous Chalk Formations at Flamborough Head, north-east England

The white cliffs of the Upper Cretaceous Chalk Formations at Flamborough Head, north-east England

Evolution of contourite systems in the late Cretaceous Chalk Sea along the Tornquist Zone

AbstractBased on integration of seismic reflection and well data analysis this study examines two major contourite systems that developed during the late Cretaceous in the southern Baltic Sea. The evolution of these Chalk Sea contourite systems between the Kattegat and the southern Baltic Sea started when Turonian to Campanian inversion tectonics overprinted the rather flat sea floor of the epeiric Chalk Sea. The Tornquist Zone and adjacent smaller blocks were uplifted and formed elongated obstacles that influenced the bottom currents. As a consequence of the inversion, the sea floor west of the Tornquist Zone tilted towards the north‐east, creating an asymmetrical sub‐basin with a steep marginal slope in the north‐east and a gentle dipping slope in the south‐west. A south‐east directed contour current emerged in the Coniacian or Santonian along the south‐western basin margin, creating contourite channels and drifts. The previously studied contourite system offshore Stevns Klint is part of this system. A second, deeper and north‐west directed counter‐flow emerged along and parallel to the Tornquist Zone in the later Campanian, but was strongest in the Maastrichtian. This bottom current moderated the evolution of a drift‐moat system adjacent to the elevated Tornquist Zone. The near surface Alnarp Valley in Scania represents the Danian palaeo‐moat that linked the Pomeranian Bay with the Kattegat. The previously studied contourite system in the Kattegat represents the north‐western prolongation of this system. This study links previous observations from the Kattegat and offshore Stevns Klint to the here inferred two currents, a more shallow, south‐east directed and a deeper, north‐west directed flow.

Geographically widespread 13C‐depletion of grazing caddis larvae: A third way of fuelling stream food webs?

Abstract Stream ecosystems are supported by both green (i.e. based on grazing) and brown (i.e. detritus) food webs, whereas methane‐derived carbon is not considered generally to be important; here, we add circumstantial evidence for this potential third way. Grazing cased‐caddis (Trichoptera) larvae in the family Glossosomatidae can be very abundant in springs and headwaters and frequently have much lower stable carbon isotope ratios (i.e. they are depleted in the heavier 13C stable isotope) than the biofilm (epilithon) on the upper surfaces of the stones on which they live, and which is their presumed diet. Evidence for similar isotopic depletion in other lotic invertebrates is currently limited, however; even for glossosomatids it has been observed so far only in some streams draining the southern English cretaceous chalk and in a few headwaters in northern California. If this phenomenon proves to be more widespread, among streams or taxa, it could imply a more general underpinning of stream food webs by isotopically light carbon derived from methane and accessed via consumers feeding on methanotrophic bacteria. Here, we sampled 58 stream sites to examine whether caddis larvae are also 13C‐depleted in streams draining other geologies. We focused mainly on carboniferous limestone and sandstone, as well as on further chalk streams representative of most of the British chalk aquifer: together, these new sites covered an area of almost 90,000 km2, around three times greater than that surveyed previously. At all 58 sites methane gas was supersaturated relative to the atmospheric equilibrium, and at 49 of them larvae were conspicuously 13C‐depleted (from −17.5‰ to −3.6‰) relative to the bulk epilithon (components of which we know can oxidise methane). Although still most pronounced on chalk, this phenomenon was geographically and geologically much more widespread than shown previously and suggests methane‐derived carbon could indeed play a prominent role in stream food webs (i.e. the third way).

Bryozoan limestone experience—the case of Stevns Klint

Bryozoan limestone is a formation of Danian limestone, generally found underlying the younger Copenhagen formation and above the Cretaceous chalk. The mineralogy between the Danian and Cretaceous formations is similar, resulting in similar mechanical responses of the matrix material. The mineralogy of the mentioned formations generally includes calcareous minerals with variable contents of flint (siliceous materials) and irregularly a small content of other materials. The distinctive feature of the matrix material of these formations is primarily the type of porosity reflecting the originating organisms and depositional history. Our database includes Bryozoan limestone data collected on major infrastructure projects between Sjælland (Denmark) and Skåne (Sweden), such as Malmø Citytunnel, as well as from a number of smaller investigation campaigns. The database includes borehole registrations, geological descriptions, classification tests and basic strength tests. Further knowledge collection includes results of wireline logging, advanced laboratory tests etc. In the latest campaign on the Bryozoan limestone, the site and material are investigated for the design of the rock anchors at the UNESCO site of Stevns Klint. The article presents a part of the interpretation of the mechanical behavior of Bryozoan limestone aided by the use of the existing database.

Colony growth strategies, dormancy and repair in some Late Cretaceous encrusting bryozoans: insights into the ecology of the Chalk seabed

Bryozoans are among the most common macrofossils in the Late Cretaceous Chalk. They include many species that encrusted hard substrates, notably echinoid tests, forming habitat islands on the Chalk seabed. The growth strategies adopted by these bryozoans, as well as the occurrence of reparative structures, provides evidence of the conditions experienced by bryozoans and other benthic animals during the accumulation of this unique pelagic sediment deposited over large areas of the continental shelf. Here, we use historical material in the Natural History Museum, London, to provide qualitative evidence that whereas available substrates, including irregular echinoids, were long-lasting, most individual bryozoan colonies were probably short-lived. Some cheilostome species produced heavily calcified polymorphic zooids at the outer edges of the colony that persisted after loss of the feeding autozooids and became the source of regenerative colony growth. Short-term (possibly annual) periodicity is suggested in the benthic environment experienced by encrusting bryozoans, which may have possibly been a result of cyclical variations in dinoflagellate food supply and/or swamping by unpalatable and potentially poisonous coccolithospheres.

Palaeoecology and survival strategies of an impoverished benthic chalk community in a mid-Danian deep shelf setting

Upper Cretaceous chalk is characterized by a rich and diverse benthic fauna followed by a poor benthic fauna in the lowermost Danian immediately after the K/Pg extinction event. In the Early to mid Danian diversity increased and rich bryozoan - and cold water coral mound ecosystems were established in localized areas. In the contemporary mid Danian Dalbyover deep shelf chalk, the benthic fauna is markedly impoverished. The Danian macrofauna from bulk samples at Dalbyover comprises 22 species and several bryozoan morphogroups, and is of low diversity and density compared to latest Maastrichtian and mid Danian shelf faunas. Average Fischer Alpha Index is 2.34 and Shannon Index 1.38 with minor stratigraphic variations. Guild analysis of the macrofauna indicates that the vast majority of species show special adaptations to survival at and in the uppermost few centimetres of the soft substrate with dominance of root-attached species attaching to small substrates and free lying suspension feeders. Further, the morphology of the serpulid tubes and the occurrence of crinoids in situ reflect a stable substrate with little overturning by the infauna. The benthic foraminiferal assemblages are dominated by low trochospiral and planispiral epifaunal morphogroups, whereas elongated forms with an infaunal mode of life are rare. The combined macrofauna and foraminiferal data indicate deposition in a relatively stable environment where food was a limiting factor. For the macrofauna, it is evident that species occurring in small numbers are long ranging and also known from the Cretaceous, whereas species occurring in high numbers are known solely from the Danian. Evidently, the Mesozoic survivors had the strategy to survive in this special chalk environment where only few Danian species were successful.

Late Cretaceous to Miocene and Quaternary deformation history of the Chalk: Channels, slumps, faults, folds and glacitectonics

Late Cretaceous Chalk sedimentation history across the British Isles included (i) fault controlled uplift and subsidence in Northern Ireland and the Inner Hebrides and (ii) uplift along the lines of en echelon folds in Southern Britain and northern France. Synsedimentary slump folds and downslope displacement structures are compared with penecontemporaneous interbed slides and later tectonic folds and faults. Compressional strike-slip tectonic processes at Flamborough Head, Yorkshire, illustrate intra-Chalk slump beds in a half-graben setting. Progressive ‘growth’ of structures characterises early downslope slump folding, interbed sliding and some listric faulting. Sheet-flints replacing slide shear planes and early fractures provide evidence for early movements. Availability of open-slopes or the depth of burial under which the range of structures developed is reflected in the degree of disruption and fragmentation of chalk and flint. Fragmentation provides clues to the timing of events and origin of the Late Campanian Altachuile Breccia (Northern Ireland) and the Coniacian Hope Gap slides (Sussex). Fragmentation and formation of sheet flints together help distinguish intra-Chalk tectonics from Quaternary glacitectonic structures.The role of marl seams, high porosity chalk beds and hardgrounds on bed-sliding, décollement zones and disruption of chalk blocks from bedrock in glacitectonics is discussed. Chalk formations with marl seams develop a special style of fracturing related to early interbed sliding and pore-fluid escape structures. Marl-seams are shown to be primary sedimentary features and not the products of post depositional pressure-solution. More than any other formation the Late Santonian – Early Campanian Newhaven Chalk contains extensive sheet-flints and shows great lateral variation in thickness and lithology across the fold belts of southern England and northern France.

Well flow and dilution measurements for characterization of vertical hydraulic conductivity structure of a carbonate aquifer

The paper aims to characterize vertical variations in horizontal hydraulic properties in a fractured carbonate aquifer, the Cretaceous Chalk in East Yorkshire, UK. Two approaches are used: an inverse model of well flow applied to flow logs of pumped open wells, and open well dilution testing. In this case study, transmissivity in the unconfined part of the aquifer is dominated by the highly permeable zone of water table fluctuation, where carbonate dissolution has occurred enhancing fracture aperture; a similar enhanced permeability zone is present at the top of the aquifer where it is confined beneath glacial deposits, although periglacial physical weathering during Quaternary cold periods, rather than carbonate dissolution, is responsible. The aquifer is also shown to contain deeper permeable horizons of stratigraphic origin, which are better developed in the unconfined section.

The role of biogenic silica in the formation of Upper Cretaceous pelagic carbonates and its palaeoecological implications

During the Late Cretaceous due to high global sea-level the coastlines were shifted far towards the land and pelagic carbonates dominated over siliciclastic sedimentation. In most parts of the epicontinental European Basin, during that time, the huge amounts of pelagic chalk were formed, while in the NE part of the European Basin similar carbonates with biogenic silica (opoka) were deposited. The main mineralogical component of both lithofacies is calcium carbonate, which is derived from coccoliths (skeletal remains of planktonic algae called coccolithophorids). The opoka is harder than chalk and contains a significant amount of biogenic silica. Detailed petrographic studies of the opoka and its comparison with chalk show the role of biogenic silica in the formation of both lithofacies deposited in an epicontinental sea. The studies indicate that the difference between two lithofacies are primary in origin and are triggered by the presence of a significant amount of sponge spicules occurring only in opoka. The spicules contribute in opal-A input to the calcareous-mud-precursor, which during diagenesis recrystallized into more stable opal-CT, responsible for the macroscopic difference between opoka and chalk. A different scenario occurs in chalk lithofacies, where the sponge spicules are rare and opal-A comes from radiolarian skeletons. The primary difference in compositions of the calcareous-precursor of opoka and chalk are discussed in light of palaeoecological factors during deposition (depth, rate of sedimentation and nutrient influx from currents). The calcareous ooze is considered the modern equivalent of the Cretaceous chalk, but its mineralogical composition is more similar to the opoka mud-precursor.

Late Cretaceous stratigraphy, sediments and structure: Gems of the Dorset and East Devon Coast World Heritage Site (Jurassic Coast), England

Extraordinary, long-distance litho-marker beds such as the Lewes and Shoreham Tubular Flints and associated marl seams and fossils, recognised in cliff exposures and cliff-fall boulders, are keys to unlocking the stratigraphy and tectonic structures in the Late Cretaceous of the Dorset and East Devon Coast World Heritage Site (Jurassic Coast). Durdle Cove is a special gem exposing the Lewes and Seaford Chalk stratigraphy where new marker beds are identified and sediments and tectonic structures provide clues to timing of movements that produced a Late Cretaceous pericline which grew into a Miocene monocline along the line of the underlying Purbeck Reverse Fault. During ‘inversion’ along this fault some Late Cretaceous Chalk formations were in part or completely ‘lost’ (e.g. Middle Turonian New Pit Chalk Formation) and others were condensed (e.g. Late Santonian and Early Campanian Newhaven Chalk Formation). Excavation of the A354 road cutting at the Lower Bincombe Farm, has greatly added to the stratigraphical records of Late Cretaceous fossils in South Dorset, especially Coniacian and Early Campanian inoceramid bivalves and the various stratigraphically specific forms of the Late Santonian to Early Campanian echinoid fossil Echinocorys scutata spp. not recorded before in this coastline. The very large bivalve fossil Platyceramus sp. provides clues to chalk sea-floor environments.

Seismic geomorphology and origin of diagenetic geobodies in the Upper Cretaceous Chalk of the North Sea Basin (Danish Central Graben)

AbstractKilometre‐scale geobodies of diagenetic origin have been documented for the first time in a high‐resolution 3D seismic survey of the Upper Cretaceous chalks of the Danish Central Graben, North Sea Basin. Based on detailed geochemical, petrographic and petrophysical analyses, it is demonstrated that the geobodies are of an open‐system diagenetic origin caused by ascending basin fluids guided by faults and stratigraphic heterogeneities. Increased amounts of porosity‐occluding cementation, contact cement and/or high‐density/high‐velocity minerals caused an impedance contrast that can be mapped in seismic data, and represent a hitherto unrecognized, third type of heterogeneity in the chalk deposits in addition to the well‐known sedimentological and structural features. The distribution of the diagenetic geobodies is controlled by porosity/permeability contrasts of stratigraphic origin, such as hardgrounds associated with formation tops, and the feeder fault systems. One of these, the Top Campanian Unconformity at the top of the Gorm Formation, is particularly effective and created a basin‐wide barrier separating low‐porosity chalk below from high‐porosity chalk above (a Regional Porosity Marker, RPM). It is in particular in this upper high‐porosity unit (Tor and Ekofisk Formations) that the diagenetic geobodies occur, delineated by “Stratigraphy Cross‐cutting Reflectors” (SCRs) of which eight different types have been distinguished. The geobodies have been interpreted as the result of: (i) escaping pore fluids due to top seal failure, followed by local mechanical compaction of high‐porous chalks, paired with (ii) ascension of basinal diagenetic fluids along fault systems that locally triggered cementation of calcite and dolomite within the chalk, causing increased contact cements and/or reducing porosity. The migration pathway of the fluids is marked by the SCRs, which are the outlines of high‐density bodies of chalk nested in highly porous chalks. This study, thus, provides new insights into the 3D relationship between fault systems, fluid migration and diagenesis in chalks and has important applications for basin modelling and reservoir characterization.