Early Kimmeridgian Research Articles

Distribution de Megaporellaboulangeri Deloffre et Beun, 1986 (algue dasycladale) dans le Jurassique du bassin atlantique marocain

The recent biostratigraphic and micropaleontologic works permitted to specify the distribution of Megaporella boulangeri Deloffre and Beun in the Jurassic of the Moroccan Atlantic basin. The lower limit of the extension of this dasycladale alga, earlier attribued to the Early Kimmeridgian by Deloffre and Beun (1986), is currently placed in the Early Callovian, on the basis of the age provided by Megaporella boulangeri biozone. The Otternstella arabica biozone dates the upper limit of the extension of the species of the Early Oxfordian. Megaporella boulangeri proliferates in the borders of the basin where the internal lagoons facies dominate, however it is absent in the western parts with open sea facies.

Influence of oblique basement strike–slip faults on the Mesozoic evolution of the south‐eastern segment of the Mid‐Polish Trough

ABSTRACTA series of analogue models are used to demonstrate how the multistage development of the Mid‐Polish Trough (MPT) could have been influenced by oblique basement strike–slip faults. Based on reinterpretation of palaeothickness, facies maps and published syntheses of the basin development, the following successive stages in the Mesozoic history of the south eastern part of the MPT were simulated in the models: (1) Oblique extension of the NW segment of the MPT connected with sinistral movement along the Holy Cross Fault (HCF, Early Triassic–latest Early Jurassic). (2) Oblique extension of both NW and SE segment of the MPT, parallel to the HCF (latest Early and Middle Jurassic). (3) Oblique extension of the SE segment of the MPT and much lesser extension of its NW segment connected with dextral movement along the HCF (Early Oxfordian–latest Early Kimmeridgian). (4) Oblique extension of the SE segment of the MPT and much lesser extension of its NW segment connected with dextral movement along the Zawiercie Fault (ZF, latest Early Kimmeridgian–Early Albian). (5) Oblique inversion of the NW segment of the MPT connected with dextral movement along the HCF (Early Albian–latest Cretaceous). (6) Oblique inversion of the SE segment of the MPT along the W–E direction (latest Cretaceous–Palaeogene). The different sense of movements of these two basement strike–slip faults (HCF and ZF) resulted in distinct segmentation of the basin and its SW margin by successive systems of extensional en‐echelon faults. The overall structure of this margin is controlled by the interference of the border normal faults with the en‐echelon fault systems related to successive stages of movement along the oblique strike–slip faults. This type of en‐echelon fault system is absent in the opposite NE‐margin of the basin, which was not affected by oblique strike–slip faults. The NE‐margin of the basin is outlined by a typical, steep and distinctly marked rift margin fault zone, dominated by normal and dip–slip/strike–slip faults parallel to its axis. Within the more extended segment of the basin, extensive intra‐rift faults and relay ramps develop, which produce topographic highs running across the basin. The change in the extension direction to less oblique relative to the basin axis resulted in restructuring of the fault systems. This change caused shifting of the basin depocentre to this margin. Diachronous inversion of the different segments of the basin in connection with movement along one of the oblique basement strike–slip faults resulted in formation of a pull‐apart sub‐basin in the uninverted SE‐segment of the basin. The results of the analogue models presented here inspire an overall kinematic model for the southeastern segment of the MPT as they provide a good explanation of the observed structures and the changes in the facies and palaeothickness patterns.

Application of the wavelet transforms on axial strain calculation in ultrasound elastography

Dinoflagellate cysts from two Middle-Late Jurassic sections in the Wenquan and Yanshiping regions of the Qiangtang Basin comprise 24 genera and 36 species including one new taxon of the genus Tenua. Based on their stratigraphic distribution, six assemblage zones are recognized in ascending order as follows: 1) Tubotuberella egemenii Zone, probably Callovian, in the mid-upper part of the Xiali Formation; 2) Pareodinia ceratophora Zone, probably Oxfordian, in the lower part of the Suowa Formation; 3) Batiacasphaera floralis Zone, probably Oxfordian, in the middle part of the Suowa Formation; 4) Amphorula metaelliptica Zone, probably Early Kimmeridgian, in the upper part of the Suowa Formation; 5) Alisocysta spp. Zone, probably Early Kimmeridgian, in the base of the Xueshan Formation; 6) Tenua wenquanensis sp. nov. -Dichadogonyaulax schizoblata Zone, probably Late Kimmeridgian-Tithonian, in the lower part of the Xueshan Formation. Based on our analysis the Jurassic-Cretaceous Boundary in the Wenquan section is placed higher than previously.

New stratigraphic insights in the ’Late Jurassic’ of the Southern Central North Sea Graben and Terschelling Basin (Dutch Offshore) and related exploration potential

AbstractMiddle Jurassic - Early Cretaceous strata are a target for oil and gas exploration in the Dutch offshore. During the initial stages of the ‘Late Jurassic’ offshore exploration, various oil fields and a few gas fields were discovered of which only one, the F3-FB field, proved to be economically viable. In the Northern Offshore of the Netherlands, latest Middle Jurassic (Callovian) - earliest Cretaceous (Ryazanian) strata are mostly limited to the Dutch Central Graben and Terschelling basins. Outside the Dutch Central Graben and the Terschelling Basin only thin veneers of these strata occur on the fringing highs such as the Schill Grund High and the Step Graben. The geology of this non-marine to shallow marine succession is complex. The combination of lateral facies changes, repetitive log and facies characteristics in time, sea-level and climate changes, salt tectonics and structural compartmentalisation hamper straightforward seismic interpretation and log correlation. The large number of lithostratigraphic units defined in the Stratigraphic Nomenclature of the Netherlands illustrates the complexity of this time-interval.In recent years, new biostratigraphic techniques and newly acquired stratigraphic data led to the identification of a series of events which can be related to the tectonic, climatic, environmental and stratigraphic development of the ‘Late Jurassic’ in the Dutch Central Graben and Terschelling basins. Based on these data, three stratigraphic sequences can be recognized. Sequence 1 (Callovian - earliest Kimmeridgian) records the initiation of the Dutch Central Graben, Sequence 2 (early Kimmeridgian - early Portlandian) that of the initiation of the Terschelling Basin. During sequence 3 (late Portlandian - Ryazanian) the Dutch offshore was draped by a regional transgression. These insights have directly impact on the exploration potential, which is discussed in two play concepts. The first is a strat-trap play in the fluvial/paralic sediments of Sequence 1 in the lows between the graben boundary and salt domes. The second example is the Spiculite play, which comprises a bioclastic sandstone reservoir at the top of a dome with a 4-way dip closure. These two examples highlight the necessity of understanding the paleoenvironment and geography for assessing the future exploration potential.

Sedimentary evolution of the Torrecilla Reef Complex in response to tectonically forced regression (Early Kimmeridgian, Northern Spain)

The early Kimmeridgian Torrecilla Reef Complex in the northern Iberian Basin of Spain consists of a fringing reef composed of eight accretionary units. The first four were deposited along a steep margin. They display down-lapping and off-lapping geometries, and are characterised by poor reef-framework development, large volumes of reworked corals and transported sediment, and limited growth of micro-encrusters. In contrast, deposition of the fifth and younger accretionary units occurred on a shallow platform without a pronounced slope where coral reefs grew in a shallow protected environment. The main features of these reefs are an absence of reef-slope facies, a high proportion of preserved framework elements, relatively low volumes of intra-reef sediment, high proportions of terrigenous material, and abundant micro-encrusters and microbialites. These reefs were protected from storm waves by long-shore sand bars, which also protected a very shallow lagoon during the last stage of sedimentation. The early Kimmeridgian was a period of rising global sea level, a trend apparent across other portions of the Iberian Basin. However, geometry and sedimentary evolution of the Torrecilla Reef Complex is consistent with those of off-lapping reefs that develop during sea-level fall. Thus, we conclude that down-stepping geometries and evolution to progressively shallower environments within the Torrecilla Reef Complex occurred as a result of a tectonically forced regression.

Paleoclimatic control of biogeographic and sedimentary events in Tethyan and peri-Tethyan areas during the Oxfordian (Late Jurassic)

The paleobiogeographical distribution of Oxfordian ammonites and coral reefs in northern and Central Europe, the Mediterranean area, North and East Africa, and the Middle East and Central Asia is compared with the distribution in time and space of the most important lithofacies. Interest in the Oxfordian is focused on changes in facies and in biogeographical patterns that can be interpreted as the results of climatic events. Paleotemperature trends inferred from oxygen isotopes and paleoclimatic simulations are tested against fossil and facies data. A Late Callovian–Early Oxfordian crisis in carbonate production is indicated by the widespread absence of Lower Oxfordian reefal formations. There is a gap (hiatus) in deposition on epicontinental platforms, with Middle Oxfordian deposits resting paraconformably on Upper Callovian, while shales accumulated in adjacent intracratonic basins. Simultaneously, in Mediterranean Tethys, radiolarites accumulated in deep troughs while Rosso Ammonitico facies formed on pelagic swells. However, deposition on swells was also discontinuous with numerous gaps (hiatuses) and sequences that are much reduced in thickness. Middle Callovian deposits are generally overlain by Middle Oxfordian limestones. The dearth of carbonates is consistent with a cooling event lasting about 1 My. By the middle Oxfordian a warming, leading to “greenhouse” type conditions, is suggested on the basis of both biogeographical (mostly coral-reef distribution) and geochemical data. Carbonates spread onto an extensive European platform while radiolarites reached a maximum development in the Mediterranean Tethys. Two distinct latitudinal belts, with seemingly different accumulation regimes, are therefore inferred. Similar latitudinal belts were also present in the late Oxfordian, when carbonates were widespread. The distribution of reefal facies in the late Oxfordian–early Kimmeridgian fits relatively well with GCMs simulations that imply low rainfall in the Tethyan Mediterranean area and slightly higher precipitation in central and northern Europe. Local salinity variations, reflecting more arid or humid conditions, may bias the paleotemperature signal inferred from δ 18O values. Biogeographical and facies distributions, combined with δ 18O values, unravel the ambiguity and support a Late Callovian–Early Oxfordian cooling followed by warming in the later Oxfordian.

The structural and sedimentary evolution of the Arruda and Lower Tagus sub-basins, Portugal

The study discusses the Jurassic structural and sedimentary evolution of the Arruda and Lower Tagus sub-basins, located in the central and southern part of the Lusitanian Basin of Portugal. In the last five decades, thousands of kilometres of 2D seismic reflection lines were acquired for oil exploration. Reprocessing and reinterpretation of over 700 km from the study area was carried out. After the seismic to well ties based on well logs, VSP analysis, synthetic seismograms, revised petrological analysis and stratigraphic reinterpretation of the wells, nine key seismic/geologic horizons were mapped and depth converted. Reinterpretation of the seismic profiles was carried out using outcrop data and reprocessed aeromagnetic and gravimetric data of the study area that was integrated in a GIS environment. Several aspects of the tectonic and sedimentary evolution of the study area were enlightened. From the Triassic until the Late Jurassic, two rift phases were recognised, in agreement with data from other parts of the Lusitanian Basin. The initial rifting phase in the Triassic continued until the Callovian, with three prominent tectonic pulses: a first in the Late Triassic, a second in the Hettangian and a third from the Sinemurian onwards. The latter episode lasted until the Callovian in the Arruda sub-basin, and at least until the Bathonian, in the Lower Tagus sub-basin. During the Middle Jurassic, the Arruda sub-basin evolved to a NW-deepening carbonate ramp. The Lower Tagus sub-basin constituted a platform since the Early Jurassic, limited in the east by the Setúbal-Pinhal Novo fault and in the west, by the Cadafais fault. The third rifting episode occurred in the Late Oxfordian–Early Kimmeridgian, producing a major N–S fault system already recognised by several authors, and another important NW–SE and NE–SW fault systems post-dating the former.

Late Jurassic Paleogeography and Paleoclimate in the Northern Iberian Basin of Spain: Constraints from Diagenetic Records in Reefal and Continental Carbonates

Abstract Sedimentation in the northern part of the Iberian Basin during the early Kimmeridgian was characterized by the development of coral reef complexes of the Torrecilla en Cameros Formation, which were situated along a seaway that connected the Boreal and Tethys domains. Along the northern portion of that seaway, close to the Boreal Sea, early diagenesis of the Torrecilla fringing reef complex was controlled by rising sea level and by local tectonism, leading to alternations of submergence and reefal exposure. While exposed, reef corals were neomorphosed and dissolved. When the corals were submerged, secondary porosity was filled by calcite cements precipitated from marine-derived waters. During the latest stages of reef development, rate of sea-level rise could no longer keep pace with tectonic uplift and, while younger accretionary units continued to be deposited, older units were exposed subaerially, leading to the precipitation of meteoric calcite cement and to the development of a paleosol. This stage of diagenesis also affected the reef complex during late Kimmeridgian times, as the Boreal and Tethys coastlines progressively retreated to the north and southeast, respectively. Progressive retreat of the Boreal Sea and subsequent Late Jurassic to Early Cretaceous rifting that formed the Cameros Basin resulted in the deposition of Tithonian continental carbonates directly on these reefal units. Oxygen isotope compositions of these continental carbonates are 3% more negative than Kimmeridgian meteoric cement, suggesting an evolution in the isotopic composition of meteoric water in response to an increasing continental effect on meteoric precipitation. In contrast, early Kimmeridgian reef complexes of the Torrecilla en Cameros Formation in the southern sectors of the seaway, closer to the Tethys Sea, were exposed during the late Kimmeridgian as the Tethys coastline retreated, leading to dissolution of corals and subsequent precipitation of meteoric calcite cement. Oxygen isotope compositions of these meteoric cements are 2% more negative than time-equivalent meteoric calcites precipitated in the northern Torrecilla Reef Complex. Given the regional paleogeography, this difference suggests that sources of meteoric water must have been different for the northern and southern sectors, and probably reflects southward transport of air masses and water vapor from the Boreal Sea to the northern Torrecilla sector, and northwest transport of water vapor from the Tethys Sea to the southern sectors. Tithonian–Berriasian continental carbonates, which unconformably overlie reef complexes in the southern and northern sectors, exhibit an opposite trend, with the δ18O of continental carbonates in southern sectors being 1.6% more positive than time-equivalent carbonates that overlie the Torrecilla Reef Complex to the north. This shift in the trend of the isotopes between northern and southern sectors is coeval with paleogeographic changes that occurred in response to Late Jurassic–Early Cretaceous rifting of the Iberian Plate and the progressive retreat of Boreal and Tethys coastlines. During this time, continental carbonates in both the southern and northern sectors of the seaway precipitated from meteoric waters that were sourced from the Tethys Sea to the southeast. More negative δ18O values in the northern Torrecilla sector reflect enhanced continental fractionation effects as the influence of the Boreal Sea-derived meteoric waters diminished.

Radioisotopic and biostratigraphic age relations in the Coast Range Ophiolite, northern California: Implications for the tectonic evolution of the Western Cordillera

The Coast Range ophiolite (CRO) in northern California includes two distinct remnants. The Elder Creek ophiolite is a classic suprasubduction zone ophiolite with three sequential plutonic suites (layered gabbro, wehrlite-pyroxenite, quartz diorite), a mafi c to felsic dike complex, and mafi c-felsic volcanic rocks; the entire suite is cut by late mid-oceanic-ridge basalt (MORB) dikes and overlain by ophiolitic breccia. The Stonyford volcanic complex (SFVC) comprises three volcanic series with intercalated chert horizons that form a submarine volcano enclosed in sheared serpentinite. Structurally below this seamount are melange blocks of CRO similar to Elder Creek. U/Pb zircon ages from plagiogranite and quartz diorites at Elder Creek range in age from 165 Ma to 172 Ma. U/Pb zircon ages obtained from CRO melange blocks below the SFVC are similar (166‐172 Ma). 40 Ar- 39 Ar ages of alkali basalt glass in the upper SFVC are all younger at ≈164 Ma. Radiolarians extracted from chert lenses intercalated with basalt in the SFVC indicate that the sedimentary strata range in age from Bathonian (Unitary Association Zone 6‐6 of Baumgartner et al., 1995a) near the base of the complex to late Callovian to early Kimmeridgian (Unitary Association Zones 8‐10) in the upper part. The SFVC sedimentary record preserves evidence of a major faunal change wherein relatively small-sized, polytaxic radiolarian faunas were replaced by very robust, oligotaxic, nassellarian-dominated faunas that included Praeparvicingula spp. We suggest that CRO formation began after the early Middle Jurassic (172‐180 Ma) collision of an exotic or fringing arc with North America and initiation of a new or reconfi gured east-dipping subduction zone. The data show that the CRO formed prior to the Late Jurassic Nevadan orogeny, probably by rapid forearc extension above a nascent subduction zone. We infer that CRO spreading ended with the collision of an oceanic spreading center ca. 164 Ma, coincident with the oldest high-grade blocks in the structurally underlying Franciscan assemblage. We further suggest that the “classic” Nevadan orogeny represents a response to spreading center collision, with shallow subduction of young lithosphere causing the initial compressional deformation and with a subsequent change in North American plate motion to rapid northward drift (J2 cusp) causing sinistral transpression and transtension in the Sierra foothills. These data are not consistent with models for Late Jurassic arc collision in the Sierra foothills or a back-arc origin for the CRO.

Jurassic dinoflagellate cyst stratigraphy of Store Koldewey, North-East Greenland

The Jurassic of Store Koldewey comprises a Middle Jurassic succession towards the south and an Upper Jurassic succession towards the north. Both successions onlap crystalline basement and coarse sediments dominate. Three main lithostratigraphical units are recognised: the Pelion Formation, including the Spath Plateau Member, the Payer Dal Formation and the Bernbjerg Formation. Rich marine macrofaunas include Boreal ammonites and the successions are dated as Late Bathonian – Early Callovian and Late Oxfordian – Early Kimmeridgian on the basis of new collections combined with material in earlier collections. Fine-grained horizons and units have been analysed for dinoflagellate cysts and the stratigraphy of the diverse and well-preserved flora has been integrated with the Boreal ammonite stratigraphy. The dinoflagellate floras correlate with contemporaneous floras from Milne Land, Jameson Land and Hold with Hope farther to the south in East Greenland, and with Peary Land in North Greenland and Svalbard towards the north. The Middle Jurassic flora shows local variations in East Greenland whereas the Upper Jurassic flora gradually changes northwards in East Greenland. A Boreal flora occurs in Peary Land and Svalbard. The characteristic and stratigraphically important species Perisseiasphaeridium pannosum and Oligosphaeridium patulum have their northernmost occurrence on Store Koldewey, whereas Taeniophora iunctispina and Adnatosphaeridium sp. extend as far north as Peary Land. Assemblages of dinoflagellate cysts are used to characterise significant regional flooding events and extensive sequence stratigraphic units.

Cyclostratigraphy, orbital tuning and inferred productivity for the type Kimmeridge Clay (Late Jurassic), Southern England

Three independently measured variables (magnetic susceptibility, photoelectric factor and total gamma-ray) obtained from throughout the type Kimmeridge Clay Fm in Dorset (Southern England) were used to identify regular metre-scale, sedimentary cycles. Spectral analysis demonstrates that for long stratigraphical intervals the cycles are expressed as large-amplitude cycles of 1.87–4.05 m wavelength and smaller-amplitude cycles of around half that wavelength. These cycles are interpreted to record orbital obliquity and precession, respectively. The much larger amplitude of the inferred obliquity cycles compared with the precession cycles may indicate a high-latitude climatic forcing transferred to lower latitudes via sea-level variations. Orbital tuning indicates that the Early Kimmeridgian ( sensu anglico ) lasted at least 3.6 Ma (95 longer-wavelength cycles) and the Late Kimmeridgian at least 3.9 Ma (103 longer-wavelength cycles). The first detailed productivity estimates for the Kimmeridge Clay Fm, on a cycle-by-cycle calculation, indicate that average productivity of the type Kimmeridge Clay (220 g m −2 a −1 ) was less than the average productivity on modern continental shelves. The high average organic carbon content of the type Kimmeridge Clay (3.8% total organic carbon) cannot be attributed to high average productivity. However, the average organic carbon content is consistent with low siliciclastic mineral dilution of organic matter and/or elevated preservation linked to reduced bottom-water oxygenation.

Settlement down again patterns of a carbonated platform following up a sedimentary crisis : example of the middle-late Jurassic boundary in the southeastern part of the Paris Basin

Abstract Following a sedimentary crisis which begins in the late Lower Callovian and spans all the early Oxfordian, the settlement down patterns of a platform with carbonated sedimentation are analysed in a southeastern area of the Paris Basin (fig. 1). Ten lithostratigraphic units (reefal formations, associated bioclastic facies and marly distal lateral facies ; fig. 2) are defined (fig. 3). New ammonite and brachiopod faunas, collected in situ, allow to date accurately the sedimentary units with a precision matching an ammonite subzone of the standard bio-chronostratigraphic scale (fig. 4) of the middle-late Oxfordian (from the Parandieri Subzone, at the base of the Transversarium Zone, to the Planula Subzone, at the top of the Planula Zone). The sedimentologic analysis coupled with the study of the benthic and pelagic faunal communities allow to define twelve type-facies (tabl. I and II). Regrouped into three associations, these characterise depositional environments which occur in succession, following three platform models (fig. 5). As witnesses of the evolution of the accomodation/sedimentation ratio, the resulting time succession of sedimentary bodies shows a depositional dynamics organised into three sequence tracks (fig. 5 and 6) : – a retrogradation phase is characterised by a moderately deep and open platform, dominated by low energy and marly sedimentation, which ranges from the Middle Oxfordian (Plicatilis Zone and Transversarium Zone) to the lowermost late Oxfordian (Bifurcatus Zone) ; – during the late Oxfordian (Bimammatum Zone, from the Semimammatum Subzone to the Bimammatum Subzone) an aggradation phase corresponds to the installation of three successive shallow platforms with contrasted morphology. Indicating the re-initiation of carbonated production, these platforms are well limited and represent high energy shallows with reef buildings, which lateraly grade into dismantling bioclastic faciès, then secondly and more laterally again into low energy and medium deep marly facies ; – the upper part of the late Oxfordian (Hauffianum Subzone, in the uppermost Bimammatum Zone, and Planula Zone) shows the wide extent of a low energy and morphologically very little contrasted distal platform. This one has a high potential of carbonated production characterised by bioclastic, oolitic and micritic facies which illustrate a progradation phase ; such a phase proceeds in the early Kimmeridgian. The collected data and the corresponding phenomenons pointed out on this area of the southeastern border of the Paris Basin are discussed and replaced in the general framework of the sedimentary, biologic, palaeogeographic and palaeoclimatic events henceforth recognised at the middle-late Jurassic boundary on the peri-Tethyan intracratonic domains of western Europe. The demise of the carbonate production seems to be correlated with a global cooling of both marine waters and atmosphere, which is considered as a limiting factor. During the Middle Oxfordian, the re-initiation of carbonate production with the developement of reef buildings should correspond to a large scale warming of the marine waters still observed elswhere on the Russian Platform, in the North Sea and in the Paris Basin. However, the geographic distribution and the chronologic succession of the facies and deduced palaeoenvironments is probably equally related to a synsedimentary tectonic activity which operates as a favorable factor at both a local and regional scale.

Carbon and oxygen isotope composition of Oxfordian–Early Kimmeridgian belemnite rostra: palaeoenvironmental implications for Late Jurassic seas

Stable isotope analyses have been undertaken on Oxfordian–Early Kimmeridgian belemnite rostra derived from three palaeogeographic provinces of Europe: (1) the Boreal–Subboreal Province (Isle of Skye in Scotland); (2) the Submediterranean Province (central Poland and southern Germany); and (3) the Mediterranean Province (Pieniny Klippen Belt in the Western Carpathians). The samples have been screened for possible diagenetic alteration by using cathodoluminescence and trace element analyses and have been found to be largely well-preserved. Calculated palaeotemperatures (10–15°C) and analyses of belemnite habitats suggest that the Boreal–Subboreal belemnites were living in a relatively shallow sea (not deeper than 100–150 m) while the other belemnites possibly inhabited deeper neritic environments. Despite the influence of a vital effect, the δ 13C values of belemnite rostra are interpreted to reflect primary variations in the oceanic carbon isotope signal. The fairly uniform Submediterranean δ 13C values indicate a similar isotope composition of dissolved inorganic carbon (DIC) in the area of sedimentation of sponge megafacies in the deep Tethyan shelves. The slightly lowered δ 13C values of some Mediterranean belemnites might occur due to upwelling providing 12C enriched bottom waters onto the area of the Czorsztyn Ridge in the Oxfordian. The discernibly more positive (1–2.5‰) δ 13C values of Boreal–Subboreal belemnites are related to a high biologic productivity and a high organic matter burial in the partly isolated Boreal Sea. The stated differentiation in carbonate δ 13C values distinctly exceeds present-day differences in isotope composition of seawater DIC.

A Late Jurassic Carbonate Ramp Colonized by Sponges and Benthic Microbial Communities (External Prebetic, Southern Spain)

Abstract During the Middle to Late Oxfordian (locally ranging into the earliest Kimmeridgian), the epicontinental shelf that persisted in the southeastern paleomargin of Iberia (Prebetic Zone) was largely colonized by siliceous sponges (Dictyida, Lychniskida, and Lithistida, in descending order of abundance). Spongiolithic lithofacies (sponge-rich deposits in which buildups are rare and small) represent a marine environment extensively colonized by sponges in the form of large sponge meadows showing associated epibenthic organisms (brachiopods, bivalves, and echinoderms). Marl-limestone rhythmites represent muddy bottoms colonized by scarce endobenthos (bivalves and irregular echinoids). However, the occurrence of patchy sponge bioherms provided a favorable substrate for epibionts and microbial lithoherms (microbialites). The record of meter-scale buildups showing sponge bioherm-microbial lithoherm in marl-limestone rhythmites resulted from the preferred colonization of sponge patches by benthic microbial...

Sequence stratigraphy of the Jurassic of the Danish Central Graben

A sequence stratigraphic framework is established for the Jurassic of the Danish Central Graben based primarily on petrophysical log data, core sedimentology and biostratigraphic data from about 50 wells. Regional seismic lines are used to assist in the correlation of some wells and in the construction of isochore maps. In the Lower Jurassic (Hettangian–Pliensbachian) succession, five sequences have been identified. The Middle Jurassic is subdivided into four sequences that together span the uppermost Aalenian/lowermost Bajocian to the Callovian. In the Upper Jurassic, better well coverage permits greater stratigraphic resolution, and 11 sequences are identified and mapped. On the basis of the sequence stratigraphic correlation and the construction of isochore maps for individual sequences, the Jurassic basin history of the Danish Central Graben can be subdivided into seven discrete phases: (1) Shallow marine and offshore sediments deposited in a prerift basin extending from the North Sea to the Fennoscandian Border Zone (Hettangian–Pliensbachian). (2) Uplift and erosion in association with a Toarcian–Aalenian North Sea doming event. A major hiatus represents this phase in the study area. (3) Terrestrial and marginal marine sedimentation during initial rifting (latest Aalenian/earliest Bajocian – Late Callovian). (4) Early Oxfordian – Early Kimmeridgian transgression during and after a rift pulse. The sedimentary environment changed from coastal plain and marginal marine to fully marine. (5) Regression associated with a cessation or slowing of subsidence during a structural rearrangement that took place in the Late Kimmeridgian during a break in the main rift climax. Shallow to marginal marine sandstones were deposited above an erosion surface of regional extent. (6) Deep-water mudstones deposited in a composite graben with high subsidence rates related to rift pulses (latest Late Kimmeridgian – middle Middle Volgian). (7) Deposition of organic-rich mudstones and turbidite sandstones during the late Middle Volgian – Early Ryazanian. The main basin shallowed, became more symmetrical and experienced a decreasing rate of subsidence, recording the onset of the post-rift stage. A relative sea-level curve is constructed for the Middle–Late Jurassic. It shows close similarity to published eustatic (global) and relative (North Atlantic area) sea-level curves in the latest Bathonian – late Early Kimmeridgian, but differs in the Late Kimmeridgian – Middle Volgian interval, probably due to the high rate of subsidence in the study area.

Microbialite morphology, structure and growth: a model of the Upper Jurassic reefs of the Chay Peninsula (Western France)

During the Early Kimmeridgian, the northern margin of the Aquitaine Basin (Western France) is characterised by a significant development of coral reefs. The reef formation of the Chay Peninsula comprises two main reefal units, in which the microbial structures can contribute up to 70% of framework. The microbial crusts, which played an important role in the stabilisation and growth of the reef body, show the characteristic clotted aspect of thrombolitic microbialites. Corals are the main skeletal components of the build-ups. The bioconstructions of the Chay area are thus classified as coral-thrombolite reefs. Four main morpho-structural types of microbial crusts are distinguished: (1) pseudostalactitic microbialites on the roof of intra-reef palaeocaves; (2) mamillated microbialites, found either on the undersides or on the flanks of the bioherms; (3) reticular microbialites in marginal parts of the reefs and between adjacent bioconstructed units; and (4) interstitial microbialites in voids of bioclastic deposits. Thrombolitic crusts developed on various substrates such as corals, bivalves, or bioclasts. The thrombolites formed a dense, clotted and/or micropeloidal microbial framework, in which macro- and micro-encrusters also occur. Variations in accumulation rate strongly influenced the reef morphology, in particular its relief above the sediment surface. The coalescence of the coral-microbialite patches created numerous intra-reef cavities of metre-scale dimensions. The direction of microbial growth, which defined the macroscopic microbialite forms, strongly depended on the position within the reef framework but was also controlled by water energy, accumulation rate and light availability.

The Scott Field, Blocks 15/21a, 15/22, UK North Sea

Abstract The Scott Field straddles Blocks 15/21 and 15/22 on the southern flanks of the Witch Ground Graben in the Outer Moray Firth Basin, UKCS. The oil field is developed in the highly productive Upper Jurassic Humber Group sandstones of Oxfordian to Kimmeridgian age. The field was discovered in 1983, sanctioned in 1990, and produced first oil in 1993. The field structure, effectively a large southwards tilted fault block, is compartmentalized into a series of four main pressure isolated fault blocks by mid to late Jurassic faulting. The Kimmeridge Clay Formation provides both the top seal and the source of the trapped hydrocarbons. Fluid contact, overpressure and compositional trends suggest that the trap was filled primarily from the north. Some trap-defining faults were already active during the deposition of the reservoir intervals. Well data indicate that the development of accommodation space was technically controlled during this period, with subsidence occurring more rapidly in the western areas of the field. The Scott Field reservoir consists of two major sand packages, the Scott Sandstone Member and the Piper Sandstone Member, bounded above and below by marine flooding surfaces. The late Oxfordian Scott Sandstone Member consists of a westwards prograding marine shoreface sandstone overlain by aggradational and retrogradational back-barrier deposits. Above this, the Mid Shale is a regionally extensive flooding event separating the Scott Sandstone Member from the overlying Piper Sandstone Member. The early Kimmeridgian Piper Sandstone Member consists of stacked mass flow sandstones, overlain by a shoreface/back-barrier system. Lateral facies changes and thickness variations significantly affect reservoir distribution in both Scott and Piper intervals. The best reservoir quality occurs within the coarsest grained, highest energy facies, particularly the shoreface and proximal washover deposits. At the crest of the field, 10400 ft TVDss, multi-Darcy permeabilities and porosities of 20% are common. However, reservoir quality declines progressively downflank due to increased quartz cementation and compaction. The Scott Field currently produces from 23 wells supported by 20 water injectors. Current modelling is aimed at targeting bypassed oil to increase ultimate recovery. The field has presently produced 300 MMSTB of oil from forecast reserves of 440 MMSTB with an estimated ultimate recovery factor of c. 46%.

The biostratigraphic calibration of the Scottish and Outer Moray Firth Upper Jurassic successions: a new basis for the correlation of Late Oxfordian–Early Kimmeridgian Humber Group reservoirs in the North Sea Basin

The combined results of palynological sampling of Scottish onshore sections and offshore wells from the Outer Moray Firth and the re-appraisal of ammonites extracted from subsurface cores enable a revised, unified macropalaeontological–palynological correlation scheme to be proposed for the Late Oxfordian–Early Kimmeridgian (Regulare–Mutabilis ammonite zones) of the North Sea. The results contradict previous, controversial correlations, which were based upon reported ammonite recovery from boreholes in the Witch Ground Graben, a component part of the Outer Moray Firth Basin. It can now be shown that the Upper Jurassic biostratigraphy of Scotland and the North Sea is essentially comparable to that of the English counterpart, suggesting that no faunal provincialism persisted across the former North Sea Dome from uppermost Oxfordian times onwards. Having clarified and resolved previous conflicting views, the application of the revised scheme should aid exploration and production in the Moray Firth Basin through the construction of more accurate reservoir correlations and robust palaeogeographic (play fairway) maps.

The context of lowstand events in the Kimmeridgian (Late Jurassic) sequence stratigraphic evolution of the Wessex–Weald Basin, Southern England

Abstract A depositional sequence stratigraphic framework for the Kimmeridgian Stage of the Wessex–Weald Basin, southern England, is applied in the investigation of the sedimentary and erosional response to proposed lowstand events. This framework recognises 13 sequence boundaries (Km1-Km13) and is based on the integrated assessment of geophysical, sedimentological, palaeontological and geochemical data. Changes in the nature and continuity of Kimmeridgian unconformity surfaces record the effect of the reorganisation of the onshore basins in response to eustasy and tectonic movements. During the Earliest Kimmeridgian, the basin geometry was such that falls in relative sea level generated widespread unconformities. Associated lowstand events are recorded in marginal areas by a number of sand units, derived from a variety of local sources. A tectonically enhanced deepening event during the late Early Kimmeridgian (Eudoxus Zone) led to unconformities being confined in their distribution, to the basin margins. As sea level fell during the Latest Kimmeridgian, a progressive reduction in the depositional area led to an amplification of the effects of lowstand processes at the basin margins where unconformity surfaces are amalgamated. On the most positive areas these surfaces are overprinted by a major interregional hiatus, the ‘Late Cimmerian Unconformity’. The timing and effects of Kimmeridgian lowstand events were intimately linked to the tectonic evolution of the Wessex–Weald Basin. Thus at times when rates and amounts of eustatic change are small (i.e. during ‘greenhouse times’) the tectonic signature of a basin will be the primary signal recorded in sedimentary successions.

Evolutionary Rates of Jurassic Ammonites in Relation to Sea-level Fluctuations

Other| August 01, 2001 Evolutionary Rates of Jurassic Ammonites in Relation to Sea-level Fluctuations JOSÉ SANDOVAL; JOSÉ SANDOVAL 1Departamento de Estratigrafía y Paleontología, Universidad de Granada, 18071- Granada, Spain Search for other works by this author on: GSW Google Scholar LUIS O'DOGHERTY; LUIS O'DOGHERTY 2Facultad de Ciencias del Mar, Universidad de Cádiz, 11510- Puerto Real (Cádiz), Spain Search for other works by this author on: GSW Google Scholar JEAN GUEX JEAN GUEX 3Institut de Géologie et Paléontologie, BFSH-2, Université de Lausanne, CH-1015 Switzerland Search for other works by this author on: GSW Google Scholar PALAIOS (2001) 16 (4): 311–335. https://doi.org/10.1669/0883-1351(2001)016<0311:EROJAI>2.0.CO;2 Article history accepted: 16 Jan 2001 first online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation JOSÉ SANDOVAL, LUIS O'DOGHERTY, JEAN GUEX; Evolutionary Rates of Jurassic Ammonites in Relation to Sea-level Fluctuations. PALAIOS 2001;; 16 (4): 311–335. doi: https://doi.org/10.1669/0883-1351(2001)016<0311:EROJAI>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search nav search search input Search input auto suggest search filter All ContentBy SocietyPALAIOS Search Advanced Search Abstract An analysis is presented of the diversity and faunal turnover of Jurassic ammonites related to transgressive/regressive events. The data set contained 400 genera and 1548 species belonging to 67 ammonite zones covering the entire Jurassic System. These data were used in the construction of faunal turnover curves and ammonite diversities, that correlate with sea-level fluctuation curves. Twenty-four events of ammonite faunal turnover are analyzed throughout the Jurassic. The most important took place at the Sinemurian-Carixian boundary, latest Carixian-Middle Domerian, Domerian-Toarcian boundary, latest Middle Toarcian-Late Toarcian, Toarcian-Aalenian boundary, latest Aalenian-earliest Bajocian, latest Early Bajocian-earliest Late Bajocian, Early Bathonian-Middle Bathonian boundary, latest Middle Bathonian-earliest Late Bathonian, latest Bathonian-Early Callovian, earliest Early Oxfordian-Middle Oxfordian, earliest Late Oxfordian-latest Oxfordian, latest Early Kimmeridgian, Late Kimmeridgian, middle Early Tithonian and Early Tithonian-Late Tithonian boundary. More than 75 percent of these turnovers correlate with regressive-transgressive cycles in the Exxon, and/or Hallam′s sea-level curves. In most cases, the extinction events coincide with regressive intervals, whereas origination and radiation events are related to transgressive cycles. The turnovers frequently coincide with major or minor discontinuities in the Subbetic basin (Betic Cordillera). You do not currently have access to this article.