Palaeoecologic significance of Late Jurassic trace fossils from the Boulonnais, N France

Abstract

Late Jurassic invertebrate trace fossil assemblages studied at outcrops along the coastline of the Boulonnais (northern France) are described and their palaeoenvironmental interpretations are discussed. The ichnofauna shows a high diversity (36 ichnospecies). Predominant forms are Spongeliomorpha suevica, S. nodosa, Treptichnus, Asterosoma, Rhizocorallium irregulare, Rh. jenense, Diplocraterion, Teichichnus, Rosselia, Skolithos, Cochlichnus, Gyrochorte, and Bolonia lata. Following SEILACHER’s (1967) archetypic classification the trace fossils belong to the Skolithos and Cruziana ichnofacies. In a 120 m succession of storm-dominated, nearshore, marine clastics of Kimmeridgian-Tithonian age nine frequent palaeoichnocoenoses and one exceptional palaeoichnocoenosis are identified. Their distribution is mainly controlled by hydrodynamic energy. These energy levels represented by the host rocks may differ from what the trace fossils indicate. It can be shown that a large proportion of deposit-feeding structures, such as Teichichnus, Rhizocorallium irregulare, and Asterosoma, indicating quiet and stable conditions, although occurring in host sediments indicating high energy conditions. This contradictory observation is attributed to non contemporaneous formation of the sediments and the trace fossils. In a sequence stratigraphic interpretation of the Gres de la Creche at Le Portel, parasequence boundaries are detected with the help of completely bioturbated horizons. The study of trace fossil assemblages thus gives a more detailed and accurate picture of former environmental conditions than sedimentologic methods alone.