Biostratigraphic Resolution Research Articles

An offshore transgressive–regressive mudstone-dominated succession from the Sinemurian of Skåne, Sweden

A Sinemurian mudstone-dominated succession was exposed until recently in the Gantofta quarry in Skåne, southern Sweden. The deposits are placed in the Döshult and Pankarp Members of the Sinemurian–Aalenian Rya Formation. Similar facies of the same age are widespread in the Danish Basin where they constitute the F-Ib unit (F-I member) of the Fjerritslev Formation. The Gantofta succession thus represents the easternmost extension of the environment characteristic of the Fjerritslev Formation and is essentially the only locality where it has been possible to study the facies of this formation in outcrop. Sedimentation seems to have taken place under relatively quiet tectonic conditions except for the possible fault-control of the basin margin. The lower part of the Gantofta section is of Early and early Late Sinemurian age. It represents the upper part of the Döshult Member and consists of muddy, lower shoreface sandstones, abruptly overlain by dark, bioturbated, fossiliferous mudstones with thin storm siltstones and sandstones. They are overlain by the Upper Sinemurian Pankarp Member which comprises red-brown, restricted marine calcareous mudstones with an upwards increasing number of storm siltstones and sandstones reflecting general shallowing and shoreline progradation. The succession spans the greater part of two simple sequences with a distal sequence boundary located at the boundary between the Döshult Member and the Pankarp Member. The exposed part of the lower sequence includes a thick transgressive systems tract and a very thin highstand systems tract. The upper sequence is represented by an undifferentiated transgressive and highstand systems tract. An Early Sinemurian sea-level rise, a late Early Sinemurian highstand, an early Late Sinemurian fall and a Late Sinemurian minor rise and a major fall are recognised. Nearby boreholes show evidence for an end-Sinemurian – Early Pliensbachian major rise. This evolution corresponds well with trends recorded in the subsurface Fjerritslev Formation of the Danish Basin. Comparison with published European and British Jurassic sea-level curves show similar overall trends, but exhibit differences in the precise ages of sequence boundaries and maximum flooding surfaces. This may reflect poor biostratigraphical resolution of the Gantofta section, differences in sequence stratigraphic interpretation, real differences in the age of sequence stratigraphic key surfaces, or the basin marginal position of Gantofta in the Fennoscandian Border Zone.

Lower Aptian carbon isotope stratigraphy from a distal carbonate shelf setting: the Cau section, Prebetic zone, SE Spain

A δ 13C curve is reported for the latest Barremian to Early Aptian at a section located in the Prebetic zone (Cau section, SE Spain). The studied section records a hemipelagic succession of dark shales, deposited on a distal carbonate ramp with a high subsidence rate, adjacent to shallow carbonate environments. The integrated biostratigraphy of the section is based on ammonites, planktonic foraminifera and calcareous nannofossils, and it has allowed an accurate dating of the succession. The δ 13C curve presented shows a distinctive evolution, leading to the recognition of three major excursions, as well as a subdivision into eight segments, which represents an improvement of the current biostratigraphic resolution. The correlation, both isotopic and biostratigraphical, with other well resolved sections is very accurate even at the higher resolution attained. Correlation with sections with low-resolution biostratigraphic characterisation from shallow platform limestones also gives good results, which supports the effectiveness of carbon isotope stratigraphy as a correlation tool.

Temporal–spatial reconstruction of the early Frasnian (Late Devonian) anoxia in NW Africa: new field data from the Ahnet Basin (Algeria)

Anoxic conditions were widespread in NW Africa during the early Frasnian (Late Devonian) that resulted in deposition of organic-rich shales and limestones with total organic carbon (TOC) values of up to 14%. Organic richness and thickness of these sediments vary laterally, and organic-rich vs. organic-poor facies boundaries are likely to have been diachronous. A precise temporal–spatial reconstruction of this anoxic phase in NW Africa is complicated because the organic matter in outcrops is largely oxidised and biostratigraphic resolution in boreholes is generally low due to the lack of recoverable conodonts. This contribution is based on eight outcrop sections at the margin of the central Algerian Ahnet Basin, where detailed spectral gamma-ray measurements were carried out using a handheld instrument. The pre-weathering organic richness in Frasnian outcrop sections is approximated using the characteristic uranium enrichment in the anoxic facies that, based on well studies, is positively correlated with the total organic carbon content. Conodont biostratigraphic results from these sections suggest that the uranium-enriched interval (the anoxic interval) at the basin margin is most common in the basal Frasnian conodont Biozones 1–2, confirming previous results from the Anti Atlas in Morocco. In three of the eight localities studied the basal Frasnian has not been deposited and the Frasnian here commences with distinctly younger uranium-enriched intervals, including Zones 4–11. Well data from the eastern Algerian Berkine Basin is interpreted to indicate a significantly longer anoxic phase there. Million-year-scale diachroneities of the Frasnian anoxia, therefore, clearly exist across the North Africa shelf. It is assumed that the palaeorelief might have been a major factor in controlling the onset, duration, and intensity of anoxia in the region.

Advances in Scottish graptolite biostratigraphy: an introduction

Graptolites were colonial organisms that first appeared in the Cambrian. In early Ordovician times they migrated from the benthos into the plankton, becoming some of the most significant zooplankton of the Lower Palaeozoic. Planktonic graptolites radiated quickly, reached an acme of variation in the late Ordovician and Silurian, then declined into extinction by the mid-Devonian. The benthic graptolites followed them to extinction in the Carboniferous. Planktonic graptolites underwent rapid evolution, recorded in their widespread and relatively durable, colonial skeletons. Their evolutionary variation through time affords important biostratigraphical data, but there was also latitudinal or palaeogeographical control on the distribution of different assemblages. In terms of British Caledonian geology, the Ordovician and Silurian periods saw the growth and closure of the Iapetus Ocean with contrasting graptolite faunas on its northern, tropical margin (Scotland) and southern, temperate margin (England and Wales). As the Ocean narrowed during the early Silurian, faunas became more cosmopolitan, but major differences are apparent in the Ordovician, where graptolite biostratigraphy has long been bedevilled by problems of correlation. Despite these difficulties, for the Lower Palaeozoic in any one region, graptolite faunas allow a remarkably detailed level of biostratigraphical resolution. Biostratigraphy depends empirically on observing similar successions of faunal assemblages in various places. It can be effective only if based on exact identifications, continuous taxonomic refinement and nomenclatural consistency. Systematic, detailed fossil collecting from a wide range of localities is an essential prerequisite. Graptolites provide a particular challenge taxonomically as the preserved remains are of a colony, not of . . . P. Stone and J. D. Floyd contribute by permission of the Executive Director, British Geological Survey (NERC).

Late Miocene calcareous nannofossil biotratigraphy and astrobiochronology for the Mediterranean region

Research Article| January 01, 2003 Late Miocene calcareous nannofossil biostratigraphy and astrobiochronology for the Mediterranean region I. Raffi; I. Raffi 1Department of Earth Science, University “G. d’Annunzio”, Via dei Vestini 31, 66013 Chieti, Italy, e-mail: raffi@unich.it; cristina.mozzato@unipd.it Search for other works by this author on: GSW Google Scholar C. Mozzato; C. Mozzato 1Department of Earth Science, University “G. d’Annunzio”, Via dei Vestini 31, 66013 Chieti, Italy, e-mail: raffi@unich.it; cristina.mozzato@unipd.it2Department of Geology, Paleontology, and Geophysics, University of Padova, Via Giotto 1, 35137 Padova, Italy, e-mail: eliana.fornaciari@unipd.it; domenico.rio@unipd.it Search for other works by this author on: GSW Google Scholar E. Fornaciari; E. Fornaciari 2Department of Geology, Paleontology, and Geophysics, University of Padova, Via Giotto 1, 35137 Padova, Italy, e-mail: eliana.fornaciari@unipd.it; domenico.rio@unipd.it Search for other works by this author on: GSW Google Scholar F. J. Hilgen; F. J. Hilgen 3Department of Geology, Faculty of Earth Sciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, The Netherlands, e-mail: fhilgen@geo.uu.nl Search for other works by this author on: GSW Google Scholar D. Rio D. Rio 2Department of Geology, Paleontology, and Geophysics, University of Padova, Via Giotto 1, 35137 Padova, Italy, e-mail: eliana.fornaciari@unipd.it; domenico.rio@unipd.it Search for other works by this author on: GSW Google Scholar Micropaleontology (2003) 49 (1): 1–26. https://doi.org/10.2113/49.1.1 Article history received: 16 Oct 2001 rev-recd: 21 Feb 2003 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 I. Raffi, C. Mozzato, E. Fornaciari, F. J. Hilgen, D. Rio; Late Miocene calcareous nannofossil biostratigraphy and astrobiochronology for the Mediterranean region. Micropaleontology 2003;; 49 (1): 1–26. doi: https://doi.org/10.2113/49.1.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu nav search search input Search input auto suggest search filter All ContentBy SocietyMicropaleontology Search Advanced Search Abstract Results are presented of a quantitative biostratigraphic study of the calcareous nannofossils from the upper Miocene Metochia section (Gavdos Island, Greece) located in the eastern Mediterranean. The study allows the completion of the calcareous nannofossil Zonal Scheme for the Mediterranean Neogene. Parts of the scheme have already been proposed earlier for the Pliocene-Pleistocene (Rio et al. 1990b) and for the latest Oligocene to early late Miocene (Fornaciari and Rio 1996, Fornaciari et al. 1996).The Metochia section proved to be a suitable section for high-resolution biostratigraphic studies. It has a detailed planktonic foraminiferal biostratigraphy and a robust chronology based on the astronomical calibration of the sedimentary cycles (Krijgsman et al. 1995, Hilgen et al. 1995). Age estimates for the nannofossil biohorizons recognized in the section were obtained through direct correlation to this astrochronology. Five out of seventeen biohorizons were selected for biostratigraphic purposes and to define the (sub)zonal boundaries. They are (in stratigraphic order): LO (Last Occurrence) of Discoaster hamatus Martini and Bramlette 1963, equivalent to the base of Subzone MNN10a, dated at 9.53 Ma; FCO (First Consistent Continuous Occurrence) of Discoaster pentaradiatus (Tan 1927) Bramlette and Riedel 1954, equivalent to the base of Subzone MNN10b, at 9.37 Ma; PB (Paracme Beginning) of Reticulofenestra pseudoumbilicus (Gartner 1967) Gartner 1969, equivalent to the base of Zone MNN11a, at 8.76 Ma; FO (First Occurrence) of Amaurolithus spp. (A. primus (Bukry and Percival 1971) Gartner and Bukry 1975), equivalent to the base of Subzone MNN11b, at 7.42 Ma, and FO of Nicklithus amplificus (Bukry and Percival 1971) Raffi Backman and Rio 1998, equivalent to the base of Subzone MNN11c, at 6.69 Ma. We compared our biochronology with data available from other Mediterranean sections (Sprovieri et al. 1996, Negri et al. 1999, Negri and Villa 2000, Bonomo 2001), and with the nannofossil biochronology from low-latitude oceanic reference sections (Raffi et al. 1995, Backman and Raffi 1997). Our results evidence differences and similarities: some of the recently proposed events in the Tortonian-Messinian (e.g., FO and FCO of Reticulofenestra rotariaTheodoridis 1984, FO of Amaurolitus delicatus Gartner and Bukry 1975) proved not to be reliable and do not improve the biostratigraphic resolution by calcareous nannofossils in the time interval under consideration. On the other hand, the five selected biohorizons can easily be reproduced elsewhere in the Mediterranean, and are considered useful for correlations to the extra-Mediterranean realm. You do not currently have access to this article.

Middle Eocene to Middle Miocene planktonic foraminiferal biostratigraphy for internal basins (Monferrato and northern Apennines, Italy)

Abstract A planktonic foraminiferal zonal scheme is proposed for the internal basinal successions of the Monferrato and Northern Apennines (late Bartonian-early Serravallian). This scheme is based on a synthesis of quantitative analyses performed on the foraminiferal assemblages from 30 stratigraphic sections, cropping out over a distance of 300 km, between the Piedmont and the Northern Apennines. The proposed zonal scheme represents the synthesis, with minor changes, of the biozonations proposed by Novaretti et al. (1995) and Mancin and Pirini (2001) with the addition of unpublished data from some new sections, spanning a broader temporal and geographical range. With respect to the previous regional and standard biozonations, this scheme shows a better biostratigraphical resolution resulting from the replacement of missing or unreliable standard bioevents with new “reproducible” and reliable regional biohorizons. These new events are: LO of muricate species, which identifies the top of the Acarinina spp....

Late Miocene calcareous nannofossil biostratigraphy and astrobiochronology for the Mediterranean region

Results are presented of a quantitative biostratigraphic study of the calcareous nannofossils from the upper Miocene Metochia section (Gavdos Island, Greece) located in the eastern Mediterranean. The study allows the completion of the calcareous nannofossil Zonal Scheme for the Mediterranean Neogene. Parts of the scheme have already been proposed earlier for the Pliocene-Pleistocene (Rio et al. 1990b) and for the latest Oligocene to early late Miocene (Fornaciari and Rio 1996, Fornaciari et al. 1996).The Metochia section proved to be a suitable section for high-resolution biostratigraphic studies. It has a detailed planktonic foraminiferal biostratigraphy and a robust chronology based on the astronomical calibration of the sedimentary cycles (Krijgsman et al. 1995, Hilgen et al. 1995). Age estimates for the nannofossil biohorizons recognized in the section were obtained through direct correlation to this astrochronology. Five out of seventeen biohorizons were selected for biostratigraphic purposes and to define the (sub)zonal boundaries. They are (in stratigraphic order): LO (Last Occurrence) of Discoaster hamatus Martini and Bramlette 1963, equivalent to the base of Subzone MNN10a, dated at 9.53 Ma; FCO (First Consistent Continuous Occurrence) of Discoaster pentaradiatus (Tan 1927) Bramlette and Riedel 1954, equivalent to the base of Subzone MNN10b, at 9.37 Ma; PB (Paracme Beginning) of Reticulofenestra pseudoumbilicus (Gartner 1967) Gartner 1969, equivalent to the base of Zone MNN11a, at 8.76 Ma; FO (First Occurrence) of Amaurolithus spp. (A. primus (Bukry and Percival 1971) Gartner and Bukry 1975), equivalent to the base of Subzone MNN11b, at 7.42 Ma, and FO of Nicklithus amplificus (Bukry and Percival 1971) Raffi Backman and Rio 1998, equivalent to the base of Subzone MNN11c, at 6.69 Ma. We compared our biochronology with data available from other Mediterranean sections (Sprovieri et al. 1996, Negri et al. 1999, Negri and Villa 2000, Bonomo 2001), and with the nannofossil biochronology from low-latitude oceanic reference sections (Raffi et al. 1995, Backman and Raffi 1997). Our results evidence differences and similarities: some of the recently proposed events in the Tortonian-Messinian (e.g., FO and FCO of Reticulofenestra rotaria Theodoridis 1984, FO of Amaurolitus delicatus Gartner and Bukry 1975) proved not to be reliable and do not improve the biostratigraphic resolution by calcareous nannofossils in the time interval under consideration. On the other hand, the five selected biohorizons can easily be reproduced elsewhere in the Mediterranean, and are considered useful for correlations to the extra-Mediterranean realm.

AN INTEGRATED CALCAREOUS PLANKTON BIOSTRATIGRAPHIC SCHEME AND BIOCHRONOLOGY FOR THE MEDITERRANEAN MIDDLE MIOCENE

The relative position of 30 main bioevents pertaining to calcareous nannofossils and planktonic foraminifera was identified in the time interval between 13.75 Ma and 10.50 Ma, based on the quantitative study of the those microfossils in three Mediterranean sections spanning the late Langhian – lower Tortonian stratigraphic interval. The events were correlated to the astronomic target curve using a cyclostratigraphic approach, resulting in a very detailed biostratigraphic and biochronologic subdivision of the interval. The zonal scheme proposed by Fornaciari et al. (1996) was adopted for the calcareous nannofossils, but three subzones were identified in the MMN7 Zone. For the planktonic foraminifera reference is made to the zonal scheme recently proposed by Foresi et al. (1998), slightly modified in order to increase its biostratigraphic resolution. The age of all the zonal boundaries is reported.

Devonian miospore successions of Western Gondwana: update and correlation with Southern Euramerican miospore zones

The composition and main biohorizons of Devonian miospore successions of Western Gondwanan regions are reviewed and compared to corresponding miospore zones of Southern Euramerica. The stratigraphic distribution of the most significant taxa is recorded in the main investigated basins of North Africa, Saudi Arabia and Brazil. With the exception of certain forms geographically restricted to Western Gondwana, most of the stratigraphically useful taxa are common to both Southern Euramerica and Western Gondwana. Successions of Western European-defined zones can be recognized and correlated in Western Gondwana regions on the basis of these common index species. Many Devonian miospore biozones defined in Western Europe are well represented in Western Gondwana, whereas some remain unrecorded or poorly characterized. The biozones may alternate with intervals with poor biostratigraphic resolution which can be attributed to various causative agents.

ABSTRACT: Sequence Stratigraphy in the Central Sumatra Basin, Indonesia: A Statistical Approach to Enhanced Biostratigraphic Resolution

ABSTRACT: Sequence Stratigraphy in the Central Sumatra Basin, Indonesia: A Statistical Approach to Enhanced Biostratigraphic Resolution

Tectonic controls on the deposits of a foreland basin: an example from the Eocene Corbières–Minervois basin, France

ABSTRACT During the Eocene in the Corbières–Minervois foreland basin, southern France, there was a transition from marine carbonate to fluvial–lacustrine sedimentation. This evolution took place in six depositional sequences, the first controlled by a eustatic rise or flexural downwarping, then following under compressive tectonic conditions. The second to the fourth sequences show marine to marshy, mainly carbonate sediments with a transgressive–regressive evolution, while the last two comprise terrigenous and carbonate continental sediments. The tectonic evolution is marked by blind fault‐propagation folds which deformed the basin during the Ilerdian–Cuisian. A paroxysmal compressive tectonic phase occurred at the Bartonian when the ancient blind thrusts started to emerge. A model for the evolution of the basin is presented, involving the northward propagation of structural culminations, which focused shallow water or emergent conditions, and structural lows in which deeper water sedimentation took place. The diachronous migration of these structural zones can be constrained from the high biostratigraphic resolution of the foreland basin fill.

Quartz silt in mudrocks as a key to sequence stratigraphy (Kimmeridge Clay Formation, Late Jurassic, Wessex Basin, UK)

Sequence‐stratigraphic interpretation of mudrocks is often carried out using proxy indicators for grain size or by consideration of other aspects of lithology thought to relate to sea‐level change, such as organic‐matter content. An alternative stratigraphic analysis, based on direct estimation of quartz‐silt content, was carried out on a major Late Jurassic mudrock (and oil source rock), the Kimmeridge Clay Formation of the Wessex Basin, Dorset, UK. The new data, generated by backscatter SEM, X‐ray and image analysis, show decametre‐scale stratigraphic patterns that are incompatible with many previous sequence‐stratigraphic interpretations based on gamma‐ray logs or visual lithofacies and biofacies description. Correlation with a basin‐margin section in the Boulonnais, northern France, indicates that silt‐rich intervals in basinal facies are coeval, within the limits of biostratigraphic resolution, with shallow‐water sand‐rich packages on the margin. Variation in silt content in the Kimmeridge Clay therefore appears to be a record of relative sea‐level change of at least regional extent. It is suggested that analysis of silt content offers the most reliable basis for generation of a regional sequence stratigraphy in basinal mudrocks. A revised relative sea‐level curve for the Wessex Basin Kimmeridgian and early Tithonian is presented based on this premise.

Lower Cretaceous turbidites of the Moray Firth: sequence stratigraphical framework and reservoir distribution

Lower Cretaceous depositional systems of the Moray Firth are influenced by eustatic sea-level oscillations which have been dramatically overprinted by two major phases of pulsed tectonism, the Late Cimmerian and Austrian. The biostratigraphical resolution obtained has allowed the timing and differentiation of distinct tectonic/sequence boundaries, some of which are utilized as important seismo-stratigraphic markers. The construction of detailed facies maps for individual sequences has, in parallel, allowed an insight into the tectonic history of the main source areas during the Early Cretaceous.

Calcareous nannofossil biostratigraphy, biochronology and paleoecology at the Tortonian/Messinian boundary of the Faneromeni section (Crete)

A detailed calcareous nannofossil analysis was performed on 180 samples from the astronomically dated Faneromeni section to identify the most important calcareous nannofossil events across the Tortonian/Messinian (T/M) boundary in the Mediterranean and to date and correlate them precisely to the previously observed sections. Here we confirm the reliability of the genus Amaurolithus, the successive entries of Amaurolithus primus, Amaurolithus aff. amplificus and Amaurolithus delicatus, have been dated at 7.430, 7.388 and 7.250 Ma, respectively. The first occurrence and the first common occurrence of Reticulofenestra rotaria are further confirmed bioevents, dated at 7.421 and 7.264 Ma. These events improve the biostratigraphic resolution in the T/M boundary interval, at least for the Mediterranean. Quantitative analysis revealed marked changes in the calcareous nannofossil assemblage associated with the sapropels as previously observed in other sections of the same time interval. A decrease in the total number of coccoliths, but an increase in abundance of the genera Rhabdosphaera and Helicosphaera characterize sapropels. The observed fluctuations are comparable to the previous findings in Neogene Mediterranean sapropel sediments and suggest that similar mechanisms were responsible for their formation during this interval. The possible influence of preservation on nannofossil assemblages is discussed.

Upper Cretaceous planktonic foraminiferal biostratigraphy from NW Turkey: calibration of the stratigraphic ranges of larger benthonic foraminifera

Planktonic and larger benthonic foraminifers of the Upper Cretaceous (Santonian–Maastrichtian)-Lower Tertiary (Danian) rock units from north, northwest and central Anatolian fore-arc basins have been investigated in order to improve the biostratigraphic resolution of this time interval. Total abundance and diversity of planktonic foraminifers vary from rare (the sequence from the Haymana region) to high (the sequences from the Cide, Caycuma, Hanonu and Yenikonak regions) and preservation is poor to moderate due to lithologic variation. First and last appearances of the planktonic foraminifera evaluated as major bio-events have been used to establish the biostratigraphic framework. Planktonic foraminiferal zonation from bottom to top of the succession consists of zones defined by Dicarinella concavata, Dicarinella asymetrica, Globotruncanita elevata, Globotruncana ventricosa, Radotruncana calcarata, Globotruncanella havanensis, Globotruncana aegyptiaca, Gansserina gansseri, Abathomphalus mayaroensis and Morozovella pseudobulloides. Ten Orbitoides and Lepidorbitoides species have been identified. The different phylogenetic development stages of Orbitoides and Lepidorbitoides populations and other larger benthonic foraminifers, Pseudosiderolites vidali, Siderolites calcitrapoides, Siderolites denticulatus, Omphalocyclus macroporus, Cideina sozerii, Hellenocyclina beotica, and Clypeorbis mamillata have been calibrated with the planktonic foraminiferal zonation established in the same successions. Hence, an integrated zonational scheme composed of planktonic foraminifers and larger benthonic foraminifers has allowed a detailed stratigraphy of these successions to be erected. Copyright © 1999 John Wiley & Sons, Ltd.

Calcareous nannofossil biostratigraphy of the M. del Casino section (northern Apennines, Italy) and paleoceanographic conditions at times of Late Miocene sapropel formation

A detailed quantitative calcareous nannofossil analysis has been performed on 138 samples from the astronomically dated Monte del Casino section with the aim to identify and precisely date the most important calcareous nannofossil events across the Tortonian/Messinian boundary in the Mediterranean, and to unravel paleoceanographic conditions at times of sapropel formation during the Late Miocene. From the biostratigraphic perspective, the genus Amaurolithus provides three successive first occurrences (FOs): A. primus, A. cf. amplificus and A. delicatus, dated at 7.446, 7.434 and 7.226 Ma, respectively. Other bioevents include the base and top of the `small reticulofenestrids' Acme, dated at 7.644 and 6.697 Ma, and the FO, FCO and LO of R. rotaria, dated at 7.405, 7.226 and 6.771 Ma. These events appear to be useful in improving biostratigraphic resolution in the Tortonian–Messinian boundary interval, at least for the Mediterranean. Quantitative analysis revealed changes in the calcareous nannofossil assemblage associated with the sapropels. The observed fluctuations suggest a single mechanism for sapropel formation in the Mediterranean during the late Neogene. Sapropels are characterized by a decrease in the total number of coccoliths, interpreted mainly as a reduction in calcareous nannofossil production due to increased siliceous plankton production during spring blooms; and an increase in reworked specimens, interpreted to reflect enhanced continental input via river run-off. An increase in abundance of the genus Rhabdosphaera can be explained by opportunistic behavior at the end of the spring bloom when nutrient levels start to become impoverished. As far as sea surface water temperature indicators are concerned, warm water D. pentaradiatus shows positive fluctuations in sapropels while cooler water D. intercalaris and C. pelagicus show negative fluctuations.

Microfossil diachronism in southern Norwegian North Sea chalks: Valhall and Hod fields

Abstract An integrated late Cretaceous calcareous microfossil framework for the southern Norwegian chalks is presented based on detailed study of 13 wells (many with core) and circum-North Sea outcrops, in addition to many cosmopolitan events recognized in reference sections in Tunisia, North America and Europe. Previous biozonations for the North Sea chalks: (1) lack the biostratigraphic resolution needed in production geology; (2) are largely founded on benthic foraminifera and acme events; (3) fail to recognize numerous stratigraphic discontinuities within the North Sea chalks; and (4) are poorly calibrated. Application of this new integrated framework cautions towards the use of certain acme and benthic foraminifera events as chronostratigraphic horizons both within the basin and from the basin into circum-North Sea outcrops. An additional complicating factor is palaeogeographic segregation between low and high latitudes, which intensified during the middle Campanian and became problematic within the North Sea basin itself during the late Maastrichtian. A final consideration is redeposition, which can lead to erroneous correlations between wells, but may also affect reservoir quality. Established North Sea biozonations have not recognized this primary palaeoenvironmental signal and massive redeposition across major lithostratigraphic units, which has resulted in the correlation of diachronous facies rather than isochronous horizons. This could lead to complications in operational applications, such as biosteering.

MIOCENE QUANTITATIVE CALCAREOUS NANNOFOSSIL BIOSTRATIGRAPHY FROM SOUTHERN APENNINES FOREDEEP DEPOSITS AND MEDITERRANEAN DSDP SITE 372

A quantitative biostratigraphic study based on calcareous nannofossil assemblages was carried out in nine Miocene calcareous and siliciclastic foredeep sections, cropping out in the outer part of the Southern Apennines and generally ascribed to the external Irpinian units. Several biozonal events were recognised by means of quantitative analyses, according to the biostratigraphic schemes of Fornaciari & Rio (1996) and Fornaciari et al. (1996) for the lower and middle Miocene and of Theodoridis (1984) for the upper Miocene. In the lower and middle Miocene interval some biohorizons such as the first common occurrence of Helicosphaera walbersdorfensis and last common occurrence of Calcidiscus premacintyrei are not always detectable in the studied sections and the first common occurrence of Calcidiscus macintyrei appears to be a better biohorizon in the MNN6b/7 than the last common occurrence of C. premacintyrei . New data on abundance patterns of selected calcareous nannofossils have been collected, thus improving the biostratigraphic resolution of the zonal schemes: a paracme interval of small Reticulofenestra pseudoumbilicus was noted within Zone MNN4b and MNN5a; the beginning and the end of the paracme are useful events in the studied sections for stratigraphic correlations. Moreover the occurrence of R. pseudoumbilicus has been recorded from Zone MNN2b on upwards, fairly below the FCO of the species that defines the base of Zone MNN6b. The distributions observed in the on-land sections are consistent with those recognised in DSDP Site 372 located in the western Mediterranean Sea, confirming their potential biostratigraphic utility within the Mediterranean region.

Discontinuity surfaces on a shallow-marine carbonate platform (Berriasian, Valanginian, France and Switzerland)

Discontinuities in sedimentation are commonly expressed as surfaces in outcrop sections and are due to rapid and substantial environmental changes. On shallow-marine carbonate platforms most such surfaces represent hiatuses below biostratigraphic resolution, and detailed analysis is necessary to identify and evaluate the environmental change involved. Surfaces in nine sections of the Lower Cretaceous of the French and Swiss Jura platform are characterized on the basis of eight universally applicable criteria (geometry, lateral extent, morphology, biological activity, mineralization, facies contrast, diagenetic contrast, and biostratigraphy). Nine different surface types are distinguished by their common features and environment of formation. All of them are related to environmental changes in the form of subaqueous erosion, subaerial exposure, subaqueous omission, or changes in texture and facies. The distribution of surface types in the studied sections shows that condensation and exposure-related surfaces tend to occur repetitively in certain intervals. Calibrated by biostratigraphy, these surface zones can be correlated across the platform from proximal to distal positions. In comparison with the global sequence-stratigraphic framework (Hardenbol et al. 1997) most exposure zones correlate with third-order sequence boundaries; condensation zones fall in between. In the studied sections, third-order eustatic sea-level drops appear to be represented rather by zones of small-scale discontinuities than by widespread and well-marked single sequence boundaries. This is explained by the superposition of high-frequency, low-amplitude sea-level fluctuations on a larger-scale sea-level trend under greenhouse conditions. The lateral extent of the surface zones varies through time and indicates important changes in platform morphology. Changes in local subsidence rate indicated by variable thicknesses of the deposits in comparison with second- and third-order sea-level trends suggest an evolution of the French Jura platform from a ramp-type morphology in the late Middle Berriasian to a flat-topped platform in the Late Berriasian. The Early Valanginian again is characterized by increased differential subsidence and well-marked platform morphology. This study demonstrates that: (1) small-scale and short-lived discontinuities can reflect large-scale variations of relative sea level; (2) on shallow platforms characterized by small topographic variations and lateral facies changes, third-order sequence-stratigraphic surfaces are not necessarily expressed by one widespread single surface, but by zones of surfaces indicating repeated environmental changes; surface zones can serve as an additional tool for correlation and interpretation of platform evolution; and (3) small-scale discontinuities form an integral part of the stratigraphic record and should receive the same attention as the sedimentary deposits they delimit.

New uppermost Cambrian U–Pb date from Avalonian Wales and age of the Cambrian–Ordovician boundary

A crystal-rich volcaniclastic sandstone in the lower Peltura scarabaeoides Zone at Ogof-ddû near Criccieth, North Wales, yields a U–Pb zircon age of 491∓1 Ma. This late Late Cambrian date indicates a remarkably young age for the Cambrian–Ordovician boundary whose age must be less than 491 Ma. Hence the revised duration of the post-Placentian (trilobite-bearing) Cambrian indicates that local trilobite zonations allow a biostratigraphic resolution comparable to that provided by Ordovician graptolites and Mesozoic ammonites.