101 Paleontology (Posters) II - Biostratigraphy, Taphonomy, Ichnology

Monday, 1 November 2010
8:00 AM - 6:00 PM
Hall D

OSTRACODES, MICROFACIES AND MAGNETIC SUSCEPTIBILITY OF THE LOWER GIVETIAN IN THE TYPE-LOCALITY

Paper number: 101-10
Program: Technical Sessions
Time: 8:00 AM – 6:00 PM

Authors

Abstract

For the study of the Lower Givetian in the type-locality at Givet, 184 samples were collected for the study of ostracodes, and 475 for the petrography and the magnetic susceptibility (MS), from the upper part of the Hanonet (HNT) Fm to the top of the Terres d'Haurs (TRH) Fm, including the Trois-Fontaines (TRF) Fm. The four investigated sections are located along the ramparts of a historic entrenched military camp built during the XVIIth century at the Mont d'Haurs. More than 9,000 ostracodes were extracted by the hot acetolysis method and approximately 100 species are recognized. The abundance and diversity of ostracodes are extremely variable, linked to the environmental conditions. Ostracodes belong exclusively to the Eifelian Mega-Assemblage and are indicative of shallow marine, semi-restricted and lagoonal environments. In the TRF Fm, the monospecificity indicative of semi-restricted environmental conditions is frequent and the level rich in Leperditicopida indicative of real lagoonal conditions is 40 m thick. Only 15 samples the majority of which collected in the TRF Fm, are barren of ostracods.

Microfacies organization reveals that TRF Fm consists of a protected shallow lagoon with environments from the back-reef area to the continental plain and that HNT and TRH Fms are characterized by open marine environments. In the first formation, the organisms are endemic (algae, ostracods), in the others they are diversified and abundant. The sedimentary system shows the evolution of a biohermal (stromatopores and corals) mixed ramp (HNT Fm) to a restricted carbonate platform (TRF Fm) and finally to a carbonate ramp (TRH Fm). This evolution is probably related to synsedimentary tectonism and block faulting.

MS values range between –2.42 x 10-9 and 3.0 x 10-7 m³/kg. The MS curve presents numerous fluctuations corresponding to sedimentary regressive or transgressive cycles. The comparison between the microfacies and MS curve is particularly well established in the TRF Fm. MS values are always very weak or even negative in biostromal units. Strong MS values are recorded in the lagoonal sediments of the TRF Fm. The MS values decrease at the end of the TRF Fm during a transgressive event and then increase towards moderately high MS values (< 1.5 x 10-7 m³/kg) in the TRH Fm. Ferromagnetic and paramagnetic minerals control the MS signal.