bluuebunny/crossref_metadata_2025 huggingface dataset:
The development of the Gorleben salt dome (northwest Germany) based on quantitative analysis of peripheral sinks
Authors: Zirngast Max
Published: April 2008

The structure of the most important stratigraphic horizons in the Gorleben salt dome area, northwest Germany, has been studied using reflection-seismic data and drilling results. Epeirogenic, tectonic and halokinetic movements in the area can be reconstructed on the basis of structure-contour maps, isopach maps, and palaeostructure-contour maps. For this purpose the increase of sediment volume in the rim synclines was calculated in three dimensions and compared with the results of the ‘normal’ subsidence. The additional subsidence in the rim synclines was used for calculation of salt flow by volume. By budgeting the salt movements, the flow rate of salt into the diapir and the salt loss by erosion and leaching were determined. The movement of the salt from the Late Buntsandstein (Röt) until today is depicted in 13 successive stages in three-dimensional diagrams using computer programs.
Salt flow by aggrading and prograding overburdens
Authors: Koyi H.
Published: April 2008

The mechanisms responsible for segmentation of salt sheets and their emplacement into higher stratigraphic levels are not separable and act simultaneously. Three sets of centrifuge models with strongly planar anisotropic (anisotropy, δ, the ratio between the effective viscosities in pure and simple shear, ranging between 4.8 and 16.4) microlaminate overburdens are used to study the effect of aggradation and progradation on segmentation and emplacement of allochthonous salt sheets.

In the first set of models, a tabular buoyant source layer was overlain by tabular anisotropic overburden simulating aggradation. During centrifuging, the underlying ductile source layer was segmented into individual wall-like diapirs by the subsiding blocks formed due to extension and faulting of the overburden. The extensional zone in these models started at the spreading edge (free face) of the model and migrated backwards.

In the second set of models, a tabular buoyant source layer was overlain successively by wedges of anisotropic overburdens simulating progradation. During centrifuging, the buoyant layer was displaced from the back of the model, where loading was higher, towards the free face in the front of the model, where overburden units were thinner. Overburden units extended at the back and the middle of the model while contractional structures dominated at the front, where asymmetric diapirs formed overhangs that spread ‘basinward’ to form ‘salt’ sheets.

In the third set of models, a wedge-shaped buoyant source layer was overlain successively by wedges of anisotropic overburden simulating progradation. The overburden wedge created a lateral pressure gradient ranging from 144 Pa at the back of the model to 80 Pa at the front when deformed in the centrifuge. In these models, as in the second set of models, the underlying buoyant mass was displaced ‘basinward’ by the subsiding thicker overburden units at the back of the model. Contractional structures dominated the deformation at the leading edge of the wedge.

Comparison of model results suggests that progradation (as in the second and third sets of models) loads underlying ‘salt’ differentially, displaces it downdip and segments it. As it segments at the back, the ‘salt’ flows laterally to areas of lower loading by intruding through the thinner overburden units and forming secondary ‘salt’ sheets at the front. On the other hand, aggradation of uniform overburden segments a buoyant sheet into two-dimensional salt walls or stocks, as in the first set of models.
Numerical modelling of stratal architectures resulting from differential loading of a mobile substrate
Authors: Cohen Harvey A. , Hardy Stuart
Published: April 2008

A numerical model is presented in which a viscous layer flows in response to differential loading by deltaic sedimentation. No density effects are cconsidered; the mobile layer and overlying sediments deform solely in response to differential thicknesses of sedimentary overburden. Modelling results include the following key features: (1) sedimentary depocentres are asymmetric and spoon-shaped, with flat-lying shelf strata grading laterally into foreset slope strata at a prograding shelf-slope break; (2) the locus of deformation is controlled by the position of the shelf-slope break; and (3) a displaced bulge of mobile substrate migrates seaward faster than the shelf-slope break progrades. Using viscosities characterisitic of natural salt (η = 1016to 1019Pa s), the sensitivity of the model to several parameters is examined. Increasing the mobile layer thickness increases the rate of substrate withdrawal in a manner similar to decreasing its viscosity. A shorter sediment transport distance results in steeper depositional slopes and faster substrate withdrawal. Comparison with a Miocene example from offshore Texas demonstrates that structures qualitatively attributed to differential loading can be successfully modelled using reasonable values for sediment supply, relative sea-level rise, and substrate thickness.
Temperature dependence of thermal anomalies near evolving salt structures: importance for reducing exploration risk
Authors: Petersen K. , Lerche I.
Published: April 2008

The temperature distribution through time in the subsurface influences hydrocarbon generation. It is therefore of interest to model the spatial variation of temperature through time and the causes of the variation.

The thermal conductivity of salt is a factor of two to three times higher than that of typical sediments. Salt structures often display large vertical relief and so provide low thermal resistance paths for heat conduction to the surface, with focusing through salt at the expense of surrounding sediments. The focused heat re-enters the sediments near the salt structure apex so that local sediments are warmer than remote sediments at equal depth. Sediments close to salt base, and in secondary rim synclines, are cooler than remote sediments at equal depth. Around salt sheets, supra-salt (sub-salt) sediments are warmer (cooler) than remote sediments at equal depth.

Decreasing salt thermal conductivities with increasing temperature are shown to influence thermal anomaly patterns around salt bodies, while temperature anomalies around a laterally moving salt sheet are calculated to exemplify the relative effects of salt shape, depth of burial and heat flow. The spatial temperature history is vital for modelling timing and location of earlier onset of oil generation and maturation above salt structures, and of delay in conversion of trapped oil to gas in deeper sediments compared to the regional regime. The importance of modelling the thermal history in order to reduce exploration risk near salt structures is emphasized.
Salt diapirism with simultaneous brittle faulting and viscous flow
Authors: Poliakov Alexei N. B. , Podladchikov Yuri Yu. , Dawson Ethan Ch. , Talbot Christopher J.
Published: April 2008

In this paper we show for the first time how sedimentary rocks above a salt diapir can deform in either a brittle or a viscous way depending on the stress state and strain rates. Most existing models for salt tectonics incorporate only one or other of these deformation mechanisms resulting in different conclusions. Taking as an example the controversial problem of buoyant salt diapirism, we demonstrate how diapirs can grow in an overburden which may deform by both creep and faulting. We argue that salt movement can take place under low differential stresses because sediments may deform by pressure-solution creep. These zones of diffuse deformation co-exist with faults where differential stresses are high enough to reach the yield limit at which sediments fail. We demonstrate two conditions under which buoyant diapirs can cause substantial faulting in their overburden without the application of tectonic forces. Buoyant diapirs can fault overburdens with high viscosity contrast (μ
overburden
/μ
salt
> 10
2
) if the topographic relief they induce in the surface is removed by rapid erosion.
A summary of Paleogene sequence stratigraphy in northwest Europe and the North Sea
Authors: Neal J. E.
Published: April 2008

Sequence stratigraphic analysis of Paleogene central North Sea well-log, seismic and biostratigraphic data recognizes patterns of cyclic sedimentation seen in the physical stratigraphy and biostratigraphy. Numerous authors have documented cyclic sedimentation resulting from relative changes in sea level in northwest Europe, but interregional integration of these observations with North Sea subsurface data is lacking in the literature. Presented here is a chronostratigraphic correlation framework for the Paleogene of northwest Europe, built by integrating subsurface and outcrop data using sequence stratigraphic first principles. Biostratigraphic data from many sources is ordered with the composite standard method. Graphic correlation of this data documents certain correlations and helps suggest previously unrecognized ties.

Paleogene North Sea sediments record five major regressions and their intervening major transgressions. Overprinting this low frequency signal are 19 higher frequency sequence cycles that control lithofacies distribution. In northwest Europe, western basins (London-Hampshire, Paris and Belgian) have shallow marine to non-marine settings which reveal basinward and landward facies shifts that indicate sea level changes. The biostratigraphy of these shallow water deposits is linked to deep water central North Sea biostratigraphy by correlating through deeper water deposits outcropping in Denmark that have been tied to western basin stratigraphy. Using this biostratigraphic framework, key bounding surfaces are correlated between basins using sequence stratigraphic principles. Depositional sequences are recognized onshore that are completely sediment starved in the North Sea. The mixing of low and high frequency sea level signals requires that all of northwest Europe be studied to recognize the ‘true’ signal. Final correlations resolve 30 depositional sequences with five long-term sea-level changes that can vary from one sub-basin to another.
Forward and reverse stratigraphic modelling of Cretaceous-Tertiary post-rift subsidence and Paleogene uplift in the Outer Moray Firth Basin, central North Sea
Authors: Nadin P. A. , Kusznir N. J.
Published: April 2008

The Paleogene stratigraphy in the Outer Moray Firth is part of the North Sea post-rift sedimentary sequence that was deposited in a thermally subsiding basin following Late Jurassic rifting. Previous studies have drawn attention to anomalous uplift and departure from the McKenzie (1978,
Earth Planetary Science Letters
,
40
, 25–32) post-rift subsidence trend in the early Paleocene followed by an accelerated phase of basin subsidence in the early Eocene before returning to the normal post-rift subsidence. 2D forward and reverse stratigraphic modelling incorporating first-order global sea-level variations have been used to determine the timing and magnitude of departures from McKenzie post-rift thermal subsidence in the early Paleogene for the Outer Moray Firth. Reverse post-rift modelling, consisting of flexural backstripping, decompaction and reverse thermal subsidence, has been used to produce restored post-rift sections from present-day stratigraphy, which have been constrained by palaeobathymetric data. Forward syn-rift and post-rift structural and stratigraphic modelling uses the flexural cantilever model of rift basin formation and has been constrained by syn-rift structural data, present-day stratigraphy and palaeobathymetric markers. Forward and reverse modelling show that observed Cretaceous and Tertiary stratigraphy of the Outer Moray Firth was generated by the combined effects of inherited Late Jurassic syn-rift accommodation space, post-Jurassic rift thermal subsidence, sediment supply, long-term eustasy and an additional transient uplift event in the Paleocene. The much thicker Tertiary compared with Cretaceous can be explained by sediment infilling of starved Cretaceous palaeobathymetry. Reverse and forward post-rift modelling predict regional Paleocene uplift in the Outer Moray Firth of the order of 375–390 m (375 m reverse modelling, 390 m forward modelling), followed by
c.
160 m of rapid Eocene subsidence, both superimposed on post-rift thermal subsidence following Late Jurassic rifting. Regional Paleocene uplift in the Outer Moray Firth is attributed to dynamic uplift associated with the development of the Iceland plume.
Early Paleogene offshore igneous activity to the northwest of the UK and its relationship to the North Atlantic Igneous Province
Authors: Ritchie J. D. , Hitchen K.
Published: April 2008

Offshore occurrences of Early Tertiary igneous rocks to the northwest of Britain are allocated to seven major categories using criteria such as geographical location, structure, age and genetic relationship. The categories are: (1) Faeroe Plateau Lava Group; (2) North Rockall Trough-Hebrides Lava Group; (3) central igneous complexes; (4) volcaniclastic rocks; (5) Faeroe-Shetland Intrusive Complex; (6) the Minch; and (7) Wyville-Thomson Ridge. An igneous chronology is presented based on radiometric age dating (mainly K-Ar whole rock), biostratigraphical data and seismic relationships.

Most landward central complexes (i.e. Skye, Rhum, Ardnamurchan, Mull, Arran) pre-date those which are more oceanward (i.e. Erlend, Darwin, St Kilda, Rockall). Exceptions to this trend (i.e. Rosemary Bank, Anton Dohrn, Hebrides Terrace) are probably genetically linked to crustal thinning in the Rockall Trough. The age of the Faeroe Plateau Lava Group, and especially the contiguous Rockall Trough-Hebrides Lava Group, are not particularly well constrained.

The main intrusive phase on, and to the southeast of, the Faeroe Islands (i.e. Faeroe-Shetland Intrusive Complex) was a relatively late stage magmatic event, coeval with the opening of the North Atlantic between Greenland and NW Europe.
An integrated stratigraphy for the Paleocene and Eocene of the North Sea
Authors: Mudge David C. , Bujak Jonathan P.
Published: April 2008

A high-resolution sequence stratigraphy has been developed for the Paleocene and Eocene rocks of the North Sea Basin, using electric log data and biostratigraphy. A total of nine stratigraphic sequences has been defined, together with a further 11 subsequences. The sequences are bounded by high-gamma log markers representing maximum condensation surfaces that can be readily correlated over the whole basin. Biostratigraphic analysis has allowed the recognition of 37 last occurrence or top abundance dinocyst bioevents that have also been correlated with the microfaunal succession. Integration of the sequence stratigraphy and biostratigraphy demonstrates that a consistent relationship exists between individual sequence boundaries and bioevents, suggesting that these surfaces are time lines.

The usefulness of developing an accurate stratigraphic framework is illustrated by a series of maps showing the lithofacies distribution for individual sequences on a regional scale. At the prospect or field level, it can be used in conjunction with seismic data to map the more subtle Tertiary hydrocarbon plays in the North Sea Basin.
An integrated palynological-palynofacies approach to the zonation of the Paleogene in the Forties-Montrose Ridge area, central North Sea
Authors: Wood Susan E. , Tyson Richard V.
Published: April 2008

A palynological and palynofacies study of late Paleocene and early Eocene sediments of the central North Sea has been undertaken using 467 core samples from 11 wells on the Forties-Montrose Ridge. Conventional dinoflagellate cyst (dinocyst) biostratigraphy gives poor precision within the narrow (
c.
2.5 million years) time interval studied. Furthermore, dinocysts are often present in only very low numbers and are frequently poorly preserved. An alternative local zonation scheme is proposed on the basis of downwell variations in quantitative palynofacies and palynomorph occurrence data, and on angiosperm pollen ratios. Integration of these three sets of data indicates a potential sevenfold subdivision of the mid-Sele Formation (S2) to upper Lista Formation (L3) interval in this area, with up to five divisions within the Forties Sandstone Member. This composite zonation scheme appears to work reasonably well for the studied wells, and preliminary comparisons with independent biostratigraphic data from two of our wells indicate a partial correlation with the dinocyst zones.
Chronostratigraphic framework for the Thanetian and lower Ypresian deposits of southern England
Authors: Ali Jason R. , Jolley David W.
Published: April 2008

Magnetostratigraphic and palynomorph data are used to assess the timing and nature of the late Paleocene-earliest Eocene depositioanl sequences of SE England. Ormesby Clay Formation mudstones in east Norfolk are the oldest upper Paleocene deposits (C26r) of southern England. The base of the type Thanet Sand Formation (= start of the Thanetian Stage) is
c.
0.65 million years younger (C26n). A subaerial erosion surface separates the Thanet-Ormesby package from the Lambeth Group sediments. The duration of the hiatus (>0.5 million years) separating the two units can be calculated from the magnetostratigraphic data; C25n is missing from the stratigraphic record across almost all of southern England. However, the Upnor Formation in central London contains a record of C25n. A hiatus of
c.
0.4 million years separates the late Paleocene Lambeth Group from the early Eocene Harwich Formation. The Lambeth Group, Harwich Formation and lower London Clay Formation were all deposited during C24r. The start of Chron C24n.3n is positioned at the base of Division B of the London Clay Formation.
Upper Paleocene-Lower Eocene dinoflagellate cyst sequence biostratigraphy of southeast England
Authors: Powell A. J. , Brinkhuis H. , Bujak J. P.
Published: April 2008

Detailed study of the aquatic palynomorph assemblages, particularly dinoflagellate cysts, from the type-Thanetian and related sections in southeast England has enabled a detailed biostratigraphic and sequence biostratigraphic analysis to be carried out. The base of the Thanetian Stage at Pegwell Bay lies above the base of the
Alisocysta margarita
(Ama) biozone; the overlying type-Thanetian is assigned to the
Apectodinium hyperacanthum
(Ahy) and
Apectodinium augustum
(Aau) biozones. The base of the Ypresian succession is drawn at the base of the Harwich Formation at Lower Upnor corresponding to the base of the
Glaphyrocysta ordinata
(Gor) biozone. The
Wetzeliella astra
(Was) biozone is characteristic of the lower London Clay Formation at Herne Bay. Five Thanetian and three Ypresian sequences are identified through consideration of dinoflagellate cyst palaeoecology and sedimentological evidence. Deepening and shallowing trends enable transgressive and highstand systems tracts to be identified. The maximum flooding surfaces are identified and correlated into the North Sea Basin (Central Graben) as a series of eight primary condensed sections, two of which (characterized by the
Areoligera
and
Apectodinium
acmes) are of a second-order scale (the other six are third-order). Lowstand system tracts are represented onshore by a series of seven unconformities (type 1 sequence boundaries) with the amount of missing time below biozonal resolution. A single type 2 sequence boundary is also evident in the Ypresian succession at Wrabness. The unconformities may be correlated into more distal locations in the North Sea Basin where lowstand sandstone deposition is characteristic.
Recognition of Chron C25n in the upper Paleocene Upnor Formation of the London Basin, UK
Authors: Ellison Richard A. , Ali Jason R. , Hine Nicolette M. , Jolley David W.
Published: April 2008

Lithostratigraphic studies of four borehole cores drilled through the late Paleocene Lambeth Group (Upnor, Woolwich and Reading Formations) and basal London Clay Formation of central London have been supplemented with palaeomagnetic, calcareous nannoplankton and palynological data. The Woolwich and Reading Formations and the lower London Clay Formation are reversely magnetized and were deposited during the early part of Chron C24r. The first record of both NP9 and Chron C25n, hitherto missing from the Paleogene record in southern England, has been identified in the Upnor Formation (formerly the Woolwich Bottom Bed). It provides a key reference marker for linking events associated with the Paleocene-Eocene boundary (positioned within Chron C24r) to the type area of the internationally recognized Thanetian and Ypresian Stages.
The ‘Oldhaven magnetozone’ in East Anglia: a revised interpretation
Authors: Ali J. R. , Hailwood E. A. , King C.
Published: April 2008

A palaeomagnetic study has been carried out on the type Harwich Formation and basal London Clay Formation in the Ispwich area of East Anglia. Both units were deposited during the middle part of geomagnetic Chron C24r. The normal polarity magnetization previously identified in the upper part of the Harwich Formation at Wrabness (Townsend & Hailwood 1985,
Journal of the Geological Society, London
,
142
, 1–27), and called the ‘Oldhaven magnetozone’ by these authors, is now regarded as a recent overprint. The new information is critical in linking the Paleocene-Eocene boundary to NW Europe where the internationally recognized early Eocene Ypresian stage stratotype is defined.
The earliest Eocene sediments of eastern England: an ultra-high resolution palynological correlation
Authors: Jolley David W.
Published: April 2008

The acritarchs, dinoflagellate cysts, algae, pollen and spores from the Harwich Formation sediments in 28 sections from southeastern England have been quantitatively studied to assist in the stratigraphic correlation of sedimentary units. Application of published zonations based on dinoflagellate cyst zonations does not enable a subdivision of the Harwich Formation to be proposed; to address this problem, quantitative palynological data has been studied to define associations which have been grouped into association sequences. Thirteen subdivisions of the Harwich Formation palynofloras have been achieved, enabling an accurate correlation of units with an
c
. 38 500 year duration across southeastern England. This correlation has identified two major depositional sequences, deposited during separate periods of rising relative sea level during the 54.5–54.0 Ma period. Application of the association sequence technique to three equivalent sections from the North Sea Basin and one from the eastern margin of the Wyville-Thompson Ridge, has allowed a comparison of the southeastern England onlap model with depositional sequence models proposed for the outer Moray Firth by other authors. Correct identification of the relationship of the deposits in southeastern England to the Dornoch, Balder and Sele Formations of the North Sea, and identification of the stratigraphic level of tephra layers −17 and +19, enables comments to be made about the position of the Paleocene-Eocene boundary in the North Sea.
The upper Paleocene-lower Eocene stratigraphic record and the Paleocene-Eocene boundary carbon isotope excursion: implications for geochronology
Authors: Aubry Marie-Pierre , Berggren William A. , Stott Lowell , Sinha Ashish
Published: April 2008

The late Paleocene carbon isotope excursion which was first identified at ODP Site 690, is currently regarded as the best means for an exact correlation at a specific point in space and time between deep sea and continental stratigraphies. Yet its position relative to biostratigraphic datums in several outcrop and deep sea sections is apparently inconsistent. Based on the integration of magnetostratigraphic, biostratigraphic (calcareous nannofossils and planktonic foraminifera) and carbon isotopic data from the North (DSDP Sites 549 and 550) and South (ODP Site 690) Atlantic Ocean, we show that in order to understand the temporal relationships and sequence of late Paleocene to early Eocene biotic and isotopic events, it is necessary to construct a composite reference section. We infer from this that in addition to the well documented ‘classic’ carbon isotope excursion which occurs in mid-Biozone NP9, a younger isotopic excursion, yet undocumented, occurs in lower Biozone NP10. The 55 Ma age estimate in Chron C24r (0.66) used as a calibration in the construction of the latest geomagnetic polarity time scale in DSDP Hole 550 lies at an unconformity (separating stratigraphic levels low in Zone NP9 from low in Zone NP10) with the result that the position of the NP9/NP10 zonal/chronal boundary is higher/younger in Chron 24r. Because the sections recovered from DSDP Sites 550 and 690 probably do not overlap over the upper NP9-lower NP10 zonal interval, and because the former section is unconformable at the NP9/NP10 zonal boundary, there is currently no means of establishing satisfactorily a numerical chronology for Chron C24r, which means that the age estimates for the upper Paleocene to lowermost Eocene biostratigraphic and isotopic events await documentation of bio-, magneto-, and isotopic stratigraphy in continuous stratigraphic section(s).
Early Eocene palaeoceanography and palaeoclimatology of the eastern North Atlantic: stable isotope results for DSDP Hole 550
Authors: Charisi Stella D. , Schmitz Birger
Published: April 2008

High-resolution oxygen and carbon isotope records for benthonic and planktonic foraminifera have been established through the early Eocene at DSDP Site 550, Goban Spur in the North Atlantic. Benthonic stable isotopic records from Site 550 (estimated palaeodepthc.4000 m) represent the first documentation of actual deep water properties during the early Eocene.

At Site 550 in the earliest Eocene (end of NP10 Zone), a 0.5‰ vertical δ18O (i.e. thermal) gradient between subsurface and bottom water was eliminated, probably reflecting upward displacement of subsurface water by intermediate and deep water. Thereafter vertical isothermal conditions prevailed for the rest of the early Eocene. This is consistent with earlier findings suggesting unusually low vertical and latitudinal temperature gradients in the early Eocene ocean. A comparison of the isotopic records from Site 550 and nearby DSDP Site 401 (palaeodepth 1800 m), reflecting intermediate water properties, confirms the isothermal nature of the water column in the eastern North Atlantic. This is in contrast to earlier findings for the Antarctic region (ODP Sites 689 and 690) in the early Eocene, where supposedly warm bottom waters originating in low latitudes were overlain by cooler intermediate waters originating in high latitudes. Both the oxygen and the carbon isotopic records for Sites 550 and 401 indicate that the eastern North Atlantic region was influenced by a single source of intermediate-to-deep water through most of the early Eocene. The benthonic δ18O and δ13C values for Site 550 are generally low compared with coeval sections worldwide, indicating that during most of the early Eocene the North Atlantic was dominated by warm and nutrient-enriched deep water formed in a low latitude region.

The earliest stages of the long-term cooling trend, which characterizes the second part of the Paleogene, can be traced by a 2°C decrease in bottom water temperatures across the NP13–NP14 transition at both Sites 550 and 401.
High-resolution sequence stratigraphy of a mid-Cretaceous estuarine complex: the Woburn Sands of the Leighton Buzzard area, southern England
Authors: Wonham J. P. , Elliott T.
Published: April 2008

The Woburn Sands Formation (Aptian-Albian) is a sand-rich, tide-dominated estuarine deposit up to 120 m thick, which is extensively exposed in the Leighton Buzzard area, southern England. Previous studies have interpreted the depositional processes and environments of these strata, but have not satisfactorily resolved their lithofacies complexity. Application of sequence stratigraphical methods and concepts has identified key surfaces (sequence boundaries, tidal and wave ravinement surfaces) which provide a useful framework for correlation of the succession and for understanding the complex facies architecture.

Two regional-scale erosional surfaces identified in the succession are interpreted as erosional unconformities or sequence boundaries. These surfaces resulted from fluvial erosion during periods of relative sea-level fall, followed by extensive erosional modification by tidal currents during the early phases of relative sea-level rise. The depositional sequences defined by the erosional unconformities are dominated by the deposits of incised valleys. The valley-fills mainly comprise sand-dominated tidal current facies which occur as multistorey estuarine channel complexes. In the upper parts of the sequences thin, locally preserved units of shelf facies overlie wave ravinement surfaces. Over part of the study area the upper erosional unconformity erodes directly into the underlying incised valley-fill producing a composite sequence.
Sequence-stratigraphical interpretation of organic facies variations in marine siliciclastic systems: general principles and application to the onshore Kimmeridge Clay Formation, UK
Authors: Tyson Richard V.
Published: April 2008

The occurrence and stratigraphical distribution of source rock potential in distal fine-grained facies is strongly related to the prevailing palaeo-oxygenation regime, especially during the deposition of the condensed section (CS). The CS will tend to correspond to organic facies B-AB, with good to excellent source-rock potential, only when conditions are dysoxic-anoxic, and organic facies D-CD, with poor source rock potential, when conditions are oxic. The applicability of the Creaney and Passey organic facies model is limited by its assumption that palaeo-oxygenation remains constant within the basinal facies. The CS commonly shows decimetre (Milankovitch)-scale redox cyclicity characterized by total organic carbon (TOC) and organic facies variations whose amplitude increases as sediment accumulation rates decrease. The greater frequency of dysoxic-anoxic conditions in the CS may be related to changes in accommodation space via the relationship between bottom water volume and the rate of deoxygenation, as observed in modern settings.

The onshore Kimmeridge Clay diverges from the classic Exxon basin model as it was deposited in an intra-shelf setting where focusing and ponding of clayey sediment resulted in the basinal sections being both thicker and more complete. However, the best source rock intervals still coincide with the maximum flooding surfaces. The long-term Late Jurassic (shallow to deep to shallow) sea level curve appears to influence the mean (oxic to dysoxic-anoxic to oxic) palaeo-oxygenation regime in the basin and the background trend in sediment accumulation rate; it is expressed in a symmetrical increase in TOC to 4–6 wt% from background values of about 1 wt%. The superimposed decimetre- to metre-scale redox cycles appear to be controlled by Milankovitch climatic controls on water mass stratification. The relative spacing of these cyclic organic-rich interbeds decreases during relatively condensed (maximum flooding surface) intervals associated with the short-term sea-level curve, and the interval TOC values also increase and broaden. Intervals deposited during or shortly after lowstands are less organic-rich and have fewer and more widely spaced organic-rich beds. The individual organic-rich beds also appear to be condensed relative to adjacent mudstones; palynofacies and TOC versus hydrogen index data suggest the origin of these beds is due more to cyclically decreased dilution and redox conditions rather than enhanced palaeoproductivity.
Sedimentological and geochemical controls on ooidal ironstone and ‘bone-bed’ formation and some comments on their sequence-stratigraphical significance
Authors: Macquaker J. H. S. , Taylor K. G. , Young T. P. , Curtis C. D.
Published: April 2008

Ironstones and ‘bone-beds’ are both formed in shallow marine settings where the prevailing sediment accumulation rates are very low. A review of published work shows that both sediment types cap coarsening upward successions and are commonly developed in mudstonedominated settings. Both can be shown to contain similar sedimentary structures (e.g. rip-up clasts, disarticulated shell debris) and a diverse assemblage of ichnogenera. However, the early diagenetic assemblages within each sediment type are both different from each other and from the mudstones within which they are enclosed. In ooidal ironstones, the cement assemblage comprises glaucony, berthierine and siderite; in ‘bone-beds’ it comprises early apatite, glaucony and later calcite and pyrite; in the mudstones it is predominantly pyrite. It is argued that the physical environment in which the ironstones and ‘bone-beds’ formed was similar, whereas their diagenetic environments were rather different. In particular, the characteristic cement assemblage of ironstones was produced in suboxic pore waters which were subjected to intense Fe reduction, whereas the characteristic cement assemblage of ‘bone-beds’ was produced in suboxic pore waters where there was limited Fe reduction. As a consequence of these chemical and stratigraphical constraints it is believed that ‘bone-beds’ formed on marine flooding surfaces and ironstones formed either at sequence boundaries, major flooding surfaces or maximum flooding surfaces.
Sequence stratigraphical analysis of late ordovician and early Silurian depositional systems in the Welsh Basin: a critical assessment
Authors: Woodcock N. H. , Butler A. J. , Davies J. R. , Waters R. A.
Published: April 2008

Sequence stratigraphical concepts are applied to a 55 km long transect through the uppermost Ordovician and lower Silurian rocks of the Welsh Basin and the adjoining Midland Platform. The study focuses on sedimentary rocks deposited during the Llandovery epoch (about 439–430 Ma).

An early Llandovery slope apron of hemipelagite and laterally supplied mudstone turbidites shows the influence of eustatic sea-level changes. Two complete depositional sequences are recognized. Transgressions and highstands on the platform were accompanied by laminated hemipelagite deposition in the basin, recording anoxic bottom waters in a stratified watermass. The intervening regressions generated unconformities on the platform and produced bioturbated basinal sediments in a more oxic environment.

By contrast, the extent and timing of sandstone-turbidite and debrite systems in the basin are strongly affected by tectonic activity. Easterly derived late Llandovery facies relate directly to faulting and submarine mass wasting along the basin margin. Southerly derived late Llandovery to mid-Wenlock sandstone-lobe systems were the products of tectonic uplift in extrabasinal source areas and were partially confined within intrabasinal tilted fault blocks. Subsidence analysis confirms a basinal stretching event in late Llandovery (Telychian) time and shows that it also affected the adjacent Midland Platform. The depositional systems of this period constitute elements of a dual-sourced depositional sequence, within which eustatic effects are masked by the strong influence of relative base-level changes attendant to tectonism.

The architecture of the early Silurian Welsh Basin illustrates the complexities of applying sequence stratigraphical models where there is an interleaving of several depositional systems, each modulated by a different mix of eustatic, tectonic and input controls.
Carbon isotopes and sequence stratigraphy
Authors: Mitchell S. F. , Paul C. R. C. , Gale A. S.
Published: April 2008

The carbon isotope (δ
13
C) stratigraphy of the late Lower Cenomanian to early Lower Turonian is presented for three sections (Folkestone in the Anglo-Paris Basin, Wünstorf in the Lower Saxony Basin and Speeton in the Cleveland Basin). The similarity between these isotope curves suggests that they were controlled by synchronous, global processes and can be used for high resolution correlation. Furthermore, sequence stratigraphic analysis of this interval reveals that each isotope excursion is associated with a sequence boundary and/or onlap surface. This is also demonstrated for the whole of the Cenomanian for the section at Speeton. We show that most (if not all) δ
13
C excursions are synchronous within the limits of current stratigraphic resolution.

We interpret the increase in background δ
13
C values as representing an increase in the area of ocean floor (specifically continental shelf) available for burial of marine organic carbon caused by the mid-Cretaceous rise in eustatic sea-level. Thus background δ
13
C values may provide an independent method for estimating past eustatic sea-levels. We interpret the sharp δ
13
C excursions as reflecting more rapid changes in the carbon cycle particularly the rate of burial of organic carbon within sediments, and/or of storage in deep and intermediate water masses, at times of rapidly changing sea-level. Carbon excursions may be useful in locating sequence boundaries when other criteria are obscure or lacking.
‘Base profile’: a unifying concept in alluvial sequence stratigraphy
Authors: Quirk David G.
Published: April 2008

The sequence stratigraphy of sedimentary strata is governed by the creation and removal of accommodation space. However, current sequence stratigraphic models do not properly account for changes in accommodation space in the alluvial environment. In order to rectify this, a concept is proposed here called base profile. Base profile can be used to describe and explain the deposition and removal of fluvial sediments analogous to the way the term base level (relative sea-level) is used in the sequence stratigraphic analysis of coastal and marine strata. Base profile is the surface measured relative to a chronostratigraphic datum within a drainage basin to which rivers would regrade were conditions to remain constant. It represents the continental extension of base level. Changes in accommodation space occur if base profile rises or falls as a result of (a) subsidence or uplift, (b) variations in sediment supply, (c) variations in river discharge, (d) eustatic fluctuations, and (e) progradation or retrogradation of the coast. During a rise in base profile, coarse-grained siliciclastic sediment is trapped on the alluvial plain due to fluvial aggradation and mostly only fine-grained sediment will reach the marine environment. A fall in base profile is associated with fluvial degradation and maximum sediment input to the marine environment. Base profile forms a simple basis for describing, correlating and interpreting alluvial strata. Simple models are developed here using base profile which show how different continental and marine processes can lead to the deposition of laterally extensive, coarse-grained fluvial strata.
The application of physical measurements to constrain reservoir-scale sequence stratigraphic models
Authors: Bryant Ian D.
Published: April 2008

Sequence and parasequence boundaries are characterized by lateral extents that are typically greater than development well spacings and frequently more extensive than individual oil and gas fields. These surfaces result from shifts of facies belts that generate laterally extensive changes over widespread areas. Commonly only facies interpretations of lithological signatures (e.g. hardgrounds) and biostratigraphic signatures (e.g. faunal changes) are used to identify these boundaries in both surface outcrops and subsurface reservoirs. However, a wide variety of physical measurements may also be made, especially in the subsurface, that may be used as additional criteria to recognize surfaces with sequence stratigraphic significance and hence to infer reservoir architectures.

Palaeomagnetic reversal stratigraphies may assist in establishing a chronostratigraphic framework within which to identify significant hiatuses and to constrain sequence stratigraphic interpretations, of both outcrops and subsurface reservoirs. In reservoirs, sequence and parasequence boundaries may be detected by a variety of other methodologies, all of which rely upon the spatial coherence of changes across these significant stratal surfaces. The geochemistry of both formation waters, and hydrocarbons, may show significant changes across these surfaces as a consequence of original variations in pore-water chemistry and/or the inability of diffusive mixing processes to equilibrate chemical compositions across such laterally extensive barriers to vertical communication. Similarly, formation pressure profiles, measured after depletion from initial reservoir pressures, frequently show significant discontinuities across these surfaces.

Seismic data record the varying strengths of reflections that emanate from geological interfaces as a consequence of varying contrasts in acoustic impedance across geological surfaces. These surfaces (seismic reflectors) are commonly assumed to be of chronostratigraphic significance. It follows that spatially coherent patterns of variation in the physical properties of sediments between these surfaces may serve to delineate the location of facies belts (systems tracts) during the period of geological time represented by the sediments between reflectors. In favourable circumstances these patterns are imaged with a high degree of spatial precision by 3D reflection seismic data. The integration of the geometry of reflections with variations in reflection strength or other seismic attributes, may therefore serve to define both the chrono- and lithostratigraphic framework of reservoir sequences.

It follows that qualitative inferences regarding reservoir architecture that are derived from sequence stratigraphic principles may be tested, refined and given improved spatial definition by the careful integration of diverse measurements with conceptual sedimentological models.
Critical application of high resolution sequence stratigraphic concepts to the Rough Rock Group (Upper Carboniferous) of northern England
Authors: Hampson Gary J. , Elliott Trevor , Flint Stephen S.
Published: April 2008

The Rough Rock Group (Upper Carboniferous, Namurian G
1
) of northern England comprises fluvio-deltaic strata exhibiting high frequency cyclicity. Key elements of high-frequency sequences are recognized in these strata, including lowstand braided fluvial incised valley fills and their correlative interfluve palaeosols. The geometry of these incised valley fills was controlled by the local, synsedimentary tectonic setting. Regionally-extensive coals overlie these incised valley fills and their interfluve palaeosols and are interpreted as initial flooding surface correlatives. These coals are in turn overlain by shales containing faunal-concentrate condensed horizons (marine bands) which are interpreted as maximum flooding surfaces. A previously unrecognized marine band which overlies the Sand Rock Mine Coal and underlies the Rough Rock Flags is documented, implying a major revision of the published stratigraphy and sequence stratigraphy. These sequence stratigraphic elements define four high frequency siliciclastic sequences, which are integrated into a sequence stratigraphic framework for the Rough Rock Group. This framework allows sequence architecture, palaeogeographies and stacking patterns to be described in detail and also provides a basis for identifying potential controls on these variables.
Response to high frequency sea-level change in a fluvial to estuarine succession: Cenomanian palaeovalley fill, Bohemian Cretaceous Basin
Authors: Uličný David , Špičáková Lenka
Published: April 2008

A high resolution sequence stratigraphic analysis of large-scale exposures in the Pecínov quarry (west-central Bohemia, Czech Republic) revealed a complex record of high frequency sea-level change in fluvial to estuarine deposits of late middle to late Cenomanian age. Within the investigated third-order sequence, four parasequences make up a transgressive systems tract and the highstand systems tract is represented by a single parasequence. The parasequences formed on a slowly subsiding basin margin in response to incremental eustatic sea-level rise. The parasequences, composed successively of fluvial, tide-dominated fluvial, supratidal marsh, tidal flat, and estuarine ebb tidal delta deposits, show extensive channelling and erosive features both within the parasequences and at the parasequence boundaries. Many of the channels incise into the underlying parasequences and superficially resemble sequence boundaries. However, sedimentological evidence indicates that the erosion took place on marine-flooding surfaces and their updip equivalents, due to landward translation of highly erosive environments. The most intense erosion was caused by subtidal currents.

Because of the low subsidence rate, the preservation of parasequences was greatly affected by the channelling at most parasequence boundaries. Lateral and vertical extent of parasequences vary over distances of tens of metres. Architectural analysis of sedimentary bodies proved essential in sequence stratigraphic interpretation of the complicated and highly erosive fluvial to tide-dominated estuarine facies succession.
Interbasinal correlation of the Cenomanian Stage; testing the lateral continuity of sequence boundaries
Authors: Owen David
Published: April 2008

Sequence stratigraphic analysis of the Aquitaine, Münster and Paris Basins has shown that many of the major discontinuities found can be traced with good biostratigraphic confidence between the basins, and through a variety of sedimentary, tectonic and palaeobiogeographic regimes. Up to five sequence boundaries appear to be synchronous between the basins when account is taken of the biostratigraphic shortcomings. An extra sequence boundary near the Lower-Middle Cenomanian boundary in the Münster Basin may be a tectonic disruption as all evidence for this relative sea-level fall is from the northern, active margin of the basin, where it is associated with mass flow deposits. Notable features are the absence of lowstand deposits in these epieric, hemipelagic settings and the expression of sequence boundaries as hardgrounds. Multiple transgressive surfaces are common in deeper water settings, and surfaces of maximum flooding are very poorly developed. However, deep water deposits do preserve more accurate evidence of sea-level change, due to increased lithological and biostratigraphical completeness and fewer controls on sedimentation.
Compaction as a primary control on the architecture and development of depositional sequences: conceptual framework, applications and implications
Authors: Hunt Dave , Allsop Tim , Swarbrick Richard E.
Published: April 2008

A conceptual model is developed integrating the compaction process into a sequence stratigraphic framework, and incorporating an understanding of the ways that carbonate platforms respond to sea-level changes. The application of this model to a range of well-constrained examples allows examination of the compaction process within a high resolution temporal framework. This approach helps to gain a better understanding of the compaction process in the shallow subsurface. Conversely, the recognition of unconformities ‘enhanced’ by compaction-induced differential subsidence illustrates the dynamic and interactive role played by compaction during sequence development. It is this aspect of the compaction process, as a control of accommodation development, facies patterns and ultimately sequence architecture, that is the focus of interest here. Examples of compactionally ‘enhanced’ unconformities show compaction to be a dynamic process that can act as a primary control of sequence architecture and development. It is clear that compaction is a much underestimated process in extant sequence stratigraphic models.