Palaeoenvironmental control on distribution of crinoids in the Bathonian (Middle Jurassic) of England and France

Bulk sampling of a number of different marine and marginal marine lithofacies in the British Bathonian has allowed us to assess the palaeoenvironmental distribution of crinoids for the first time. Although remains are largely fragmentary, many species have been identified by comparison with articulated specimens from elsewhere, whilst the large and unbiased sample sizes allowed assessment of relative proportions of different taxa. Results indicate that distribution of crinoids well corresponds to particular facies. Ossicles of Chariocrinus and Balanocrinus dominate in deeper-water and lower-energy facies,with the former extending further into shallower-water facies than the latter. Isocrinus dominates in shallower water carbonate facies, accompanied by rarer comatulids, and was also present in the more marine parts of lagoons. Pentacrinites remains are abundant in very high-energy oolite shoal lithofacies. The presence of millericrinids within one, partly allochthonous lithofacies suggests the presence of an otherwise unknown hard substrate from which they have been transported. These results are compared to crinoid assemblages from other Mesozoic localities, and it is evident that the same morphological ad-aptations are present within crinoids from similar lithofacies throughout the Jurassic and Early Cretaceous

Replacements of benthic associations in a sequence stratigraphic framework, examples from Upper Ordovician of Sardinia and Lower Devonian of the Massif Armoricain

Sedimentological, palaeoecological and taphonomic analyses were carried out to establish the impact of relative sea-level variations on benthic faunas in a platform depositional context and to propose a reliable palaeoecological model for different palaeogeographic areas during different periods of the Palaeozoic. One Caradocian-Ashgillian section in Sardinia and one Lower Emsian section in the Massif Armoricain have been selected. The approach combines sequence stratigraphy, taphonomy and delineation of recurrent fossil associations. Several sedimentary facies have been recognized in relation to the distance from the shoreline. The succession consists of the stacking of genetic sequences related to sea-level changes linked to cyclic variations of orbital parameters. Several fossil associations, identified by means of multivariate analyses, are recurrent in similar facies. Eleven associations are recognized. These are the Longvillia, Nicolella, Paucicrura and Iberomena associations in the Upper Ordovician of Sardinia; the Adolfia, Stenorhynchia, Chonetid, Crinoid-Brachiopod, Ostracod-Tentaculitid, Bryozoan and Dacryoconarid associations in the Lower Devonian of the Massif Armoricain. The results of multivariate analyses based on the faunal contents are in good accordance with the clustering based on the sequence stratigraphic analysis. Benthic associations are closely related to relative sea-level variations. When the sea-level fluctuations were gradual and moderate, the association replacement is characterized by reorganization of the taxonomic composition, changes in diversity and trophic webs (association replacement by reorganization). When the sea-level changes rapidly, the association replacement is characterized by renewal of the ecological structure (association replacement by renewal). Palaeoecological changes are recognized to be strongly related to sea-level variations. (c) 2006 Elsevier B.V. All rights reserved

Relations entre les variations des assemblages benthiques emsiens et l’eustatisme dans la coupe de Seillou (Massif armoricain, France)

Palaeoecological study and sequence stratigraphy of the Emsian part of the Seillou section (Le Faou formation, Armorican massif) allows the identification of benthic and pelagic associations, and shows the correlation between the distribution of associations and the sea level variations. A major change in sedimentation and faunas could represent the signature of the Basal Zlichov Event in the Armorican massif. (C) 2001 Academie des sciences / Editions scientifiques et medicales Elsevier SAS

The Late Ordovician glacio-eustatic record from a high-latitude storm-dominated shelf succession: The Bou Ingarf section (Anti-Atlas, Southern Morocco)

Evidences of glaciation at the end of the Ordovician are widespread in western Gondwana Some authors consider the glaciation was restricted to the Hirnantian time, but occurrences of glacial deposits in the Lower Silurian strata of South America indicate that the Gondwana glaciers did not completely disappear after the Hirnantian glaciation. In addition, numerous studies based on palaeoecology, sedimentology, sequence stratigraphy and stable isotopes have suggested that ice sheets formed as soon as the Early/Middle Katian The objective of this work is to investigate the high-frequency eustatic signals in a stratigraphic succession corresponding to the Katian and the Hirnantian (similar to 10 My comprising the uppermost part of the Ordovician) The studied section (Bou Ingarf section >600 m) is located in the Central Anti-Atlas, southern Morocco It shows an almost continuous succession of siliciclastic platform deposits. The lower package, Katian to early Hirnantian in age, corresponds to a shelf succession, within which storm dynamics prevailed The upper package, middle to upper Hirnantian in age, comprises a glaciation-related succession made up of a suite of coastal to flood-dominated fluvio-glacial deposits The latter have essentially filled in large palaeochannels interpreted as subglacial tunnel valleys From high-resolution facies and sequential analysis, a curve of variation of depositional environments was established at very high, high and low frequencies Time calibration was performed based on a high-quality biostratigraphic control mainly derived from chitinozoan biozones through the whole succession. Assuming a constant tectonic subsidence and a bathymetric model (shoreface/upper offshore boundary 30 m, upper offshore/lower offshore boundary: 120 m), changes in facies-based water depths are converted into an eustatic sea-level curve using a one-dimensional backstripping procedure The eustatic sea-level curve shows that the stratigraphic succession is dominated by the stacking of sequences at very high- (40 m) and coeval forced regressions were identified during the Katian They represent glacial episodes of significant extent prior to the Hirnantian, but essentially lacking a glacial record Strata reflecting the Hirnantian glaciation include two differentiated events and an intermediate but important transgression. The second Hirnantian glacial event, the only one associated with subglacial erosion in the Bou Ingarf area, corresponds to the Late Ordovician glacial climax characterised by a continental-scale ice sheet Ice sheets permanently occupying the centre of the Gondwana landmass throughout the Late Ordovician may reconcile moderate Hirnantian eustatic sea-level fall amplitude (4080 m) and palaeoglacial reconstructions that show a Hirnantian ice sheet covering the main part of western Gondwana. (C) 2010 Elsevier B V All rights reserved