Biostratigraphic Horizons Research Articles

Renouvellement des ammonites en fosse vocontienne a la limite Valanginien-Hauterivien

A study based on more than 12,000 ammonites found along a one-hundred-meter-thick continuous pelagic section at La Charce (Drôme), together with comparisons with other west-European deep-water sections, allows to describe the very steps of the turnover affecting the Neocomitids, which spanned the depositional time of the late lowstand and transgressive systems tracts in the uppermost Trinodosum zone, before the maximum flooding in the Radiatus zone. We suggest that migrations in open marine environments as a response to climatic changes (from the warm lowstand to the cool transgression) were the main driving mechanism of the evolution of the fauna, a transformation which is rooted in the late lowstand but accelerated during the transgression. Many biostratigraphic horizons are individualized in the upper Valanginian.

Radiocarbon dating the Devensian Lateglacial in Britain: New evidence from Llanilid, South Wales

AbstractRadiocarbon dates are described from a section through Lateglacial and early Flandrian sediments at Llanilid, Mid‐Glamorgan, South Wales. Comparisons between age determinations on the alkali soluble (humic) and alkali insoluble (humin) organic fractions from 12 biostratigraphic horizons reveal the extent of contamination by both older and younger carbon residues. The Llanilid time‐scale suggests that for the Lateglacial, the earliest organic sediments date from around 13 200 yr BP, the early Interstadial Juniperus maximum occurred at ca. 12 400‐12 500 yr BP with a marked decline some 200 years later, the main Betula phase lasted only from ca. 11 700 to 11 400 yr BP and the end of the Interstadial occurred around 11 100 yr BP. The beginning of the Flandrian dates from ca. 10 000 yr BP, the Juniperus maximum occurred approximately 200 years later, the expansion of birch woodland began around 9600 yr BP, while the first hazel arrived in the area at ca. 9300 yr BP. These age determinations are discussed in the context of radiocarbon dates from comparable biostratigraphic horizons in western Britain and the dating of Lateglacial events in the ocean core records from the North Atlantic.

New Monoplacophora (Mollusca) from Late Cambrian and Early Ordovician of Missouri

Fourteen new species and six new genera of the molluscan class Monoplacophora are described from the Upper Cambrian Potosi and Eminence formations and the Lower Ordovician Gasconade Formation of the Ozark Uplift of Missouri and some new biostratigraphic horizons are introduced. A new superfamily, the Hypseloconellacea nom. trans. Knight, 1956, and a new family, the Shelbyoceridae, are named. The genus Proplina is represented by five new species: P. inflatus, P. suttoni from the Cambrian Potosi Formation, P. arcua from the Cambrian Eminence Formation and P. meramecensis and P. sibeliusi from the Lower Ordovician Gasconade Formation. A new genus and species in the subfamily Proplininae, Ozarkplina meramecensis, is described from the Upper Cambrian Eminence Formation. Four new monoplacophoran genera in the superfamily Hypseloconellacea and their species are described, including: Cambrioconus expansus, Orthoconus striatus, Cornuella parva from the Eminence Formation, and Gasconadeoconus ponderosa, G. waynesvillensis, G. expansus from the Gasconade Formation. A new genus in the new family Shelbyoceridae, Archeoconus missourensis, is described from the Eminence Formation and a new species of Shelbyoceras, S. bigpineyensis, is described from the Gasconade Formation.

Age of biostratigraphic horizons within the Ordovician and Silurian systems

Summary Three samples that have a bearing on the age of horizons within the Ordovician and Silurian systems, two previously dated by the conventional K-Ar method and one by the 40 Ar/ 39 Ar total-fusion method, have been reanalysed using the 40 Ar/ 39 Ar age-spectrum method. Conventional K-Ar and total-fusion 40 Ar/ 39 Ar ages can always be questioned because of the relative ease with which the K-Ar system can be disturbed, either thermally or chemically (i.e. Dalrymple & Lanphere 1969; Clauer et al. 1982). The 40 Ar/ 39 Ar age-spectrum method has the potential for identifying disturbed K-Ar systems (i.e. Berger 1975; Harrison & McDougall 1980). The authors feel that the age-spectrum data from these samples are significant because the previous results for these samples have been questioned in recently proposed Palaeozoic time-scales because of a possible disturbance of the K-Ar isotopic system (i.e. Gale et al. 1979, 1980; Gale 1982).

Postglacial Environmental History of Rannoch Moor, Scotland III. Early-and Mid-Flandrian Pollen Stratigraphic Data from Sites on Western Rannoch Moor and Near Fort William

This final paper in the present series on the Postglacial environ- mental history of Rannoch Moor describes pollen-stratigraphic investigations at two sites in the lee of the mountains that delimit the western margins of Rannoch Moor, and two sites at a lower altitude to the west of those mountains near Fort William. Radiocarbon dates were obtained from the basal horizons in two of the sites, but the significance of these age determinations is difficult to assess as one set is somewhat younger than dates from similar biostratigraphic horizons at other sites on Rannoch Moor, while the second series appears to have been considerably affected by hard-water error. The pollen-stratigraphic data suggest a measure of synchroneity in the timing of deglaciation from the Loch Lomond Advance maximum between Rannoch Moor and the valleys to the west, and that immediately following ice wastage, Empetrum heath and juniper scrub became established in both areas prior to the expansion of birch woodland. Follow- ing the spread of Corylus into the western Grampian Highlands, marked differ- ences developed in forest composition, with pine and birch woodland covering large areas of the Rannoch plateau, while birch, hazel and oak forests character- ized the lower ground to the west. The arrival of alder in large numbers coincided with the onset of blanket peat growth, the latter being largely responsible for the decline in the woodlands. The principal findings of the Rannoch Moor project are discussed in a concluding section to the paper.

Pleistocene biostratigraphy, chronostratigraphy and paleocirculation of the southeast pacific central water core RC 11-220

An 11-m core, RC 11-220, raised from a depth of 2,950 m in the unstudied Southeast Pacific Central Water of the Tuamotu Ridge (14° 49′S 139° 58′W) received paleontologic and paleoecologic analysis. Eight biostratigraphic horizons were recognized (6 planktonic foraminiferal, 1 coccolithophorid and the discoaster extinction) defining the core as continuous through the basal Pleistocene. The Globorotalia truncatulinoides (d'Orbigny) datum was approached but not encountered. The core has an estimated average sedimentation rate of 0.60 cm/10 3 years. Extrapolation from two paleomagnetic datums (Brunhes/Matuyama, ca. 0.69 Ma and the top of the Olduvai, ca. 1.61 Ma) indicates the age of the core bottom to be ca. 1.87 Ma. Based on the evidence recorded in RC 11-220 and our knowledge of the circulation patterns at the core site today, the following paleocirculation model is proposed. The total carbonate and coarse fraction data indicate a relatively constant depositional pattern and productivity. In addition, judging from the faunal dominance and the distributional pattern of key species such as Globigerinoides ruber (d'Orbigny), Globigerinoides sacculifer (Brady) and Globigerinita glutinata (Egger), a minor shift in the salinity regime is suggested while temperature remained relatively constant. On the average, salinity was slightly higher during the first approximate million years and was somewhat reduced during the last million years B.P. The major conclusion of this study however is that the character of the surface South Central Pacific Waters remained relatively constant throughout the Pleistocene.

Upper Precambrian of the Soviet Northeast

We propose a scheme for correlating upper Precambrian deposits of the Okhotsk, Omolon, and Taygonos massifs and the Kolyma rise, based on marker biostratigraphic and lithologic horizons. The Riphean section is subdivided into three complexes, the lower and upper of which are terrigenous, and the middle, terrigenous-carbonate. The Yudoma complex consists of sandstones that pass upward through the section into dolomites. We compare the sections in the Soviet Northeast with the Yudoma-Maya downwarp. The Oroygk, Nyuchala, and Uyutak suites are correlated with the Totta suite; the Chebukulakh series and the Zarosshiy and Doribi suites are correlated with the upper part of the Maya series (the Malgina, Tsipanda, and Lakhanda suites), the Spiridonova, Turaus, Ol'dyani, and Sibega suites are the equivalent of the Uy series, and the Venkem and Korkodon, Troynoy, and Marevaya suites are correlated with the Yudoma suite.—Authors.