New data on the carbonate microfacies from the upper Cenomanian of the Baixo Mondego region (Beira Litoral, Portugal)..

Abstract

Th e Baixo Mondego region in western Portugal is reknown to expose a large record of middle to upper Cenomanian and lower Turonian platform carbonates with rich fossil assemblages. Th ere is a huge variety of microfacies that suggests a rather complex palaeo-environmental setting. Several workers initiated their study since the 1960s, but the available information now require being complemented with new relevant bio-stratigraphic and palaeo-ecological data. Within this scope, the upper Cenomanian carbonate levels “C” to “J” of the Costa d’Arnes Formation were sampled from stratigraphic sections located between the localities of Salmanha, Vila Verde and Lares. A representative collection of 85 thin sections was then prepared for a micropalaeontological study completed in both the Earth Sciences Department of the New University of Lisbon and the University of Vigo. Subsequently, the following planktonic foraminifers were identifi ed in bioclast-rich, wackstone-packstone, carbonate microfacies: Hedbergella delrioensis, Heterohelix sp., Guembelitria cretacea, Helvetoglobotruncana praehelvetica, Rotalipora cushmani, Whiteinella spp., Dicarinella sp., and Praeglobotruncana delrioensis. Th ey occur together with the benthonic foraminifers: Th omasinella punica, Placopsilina cenomana, Hemicyclammina sigali, Gavelinella sp., Marssonella oxycona, Dorothia sp., and Ammobaculites spp., and forms attributed to Lenticulina, Quinqueloculina and Nautiloculina. Other bioclasts include abundant dasycladacean algae and small fragments of invertebrates, including sponges, bryozoans, bivalves, gastropods, serpulid worms, echinoids, and crinoids. Th ese Tethysian-infl uenced assemblages indicate the presence of a rather distal, mid- to outer-shelf environment with open marine conditions.