Abstract

AZEREDO, Ana C. et al. Stratigraphical study of a new outcrop of the Cabaços Formation (Oxfordian) at the Serra do Bouro region (Caldas da Rainha). Comunicações Geológicas [online]. 2010, n.97, pp.5-22. ISSN 1647-581X.

A recent stratigraphical study in the Serra do Bouro region (Caldas da Rainha, central Lusitanian Basin, Portugal) has allowed recognition of the Cabaços Formation (Oxfordian) at outcrop, for the first time in this area. Previously, within the spatial range of the 1:50.000 geological sheet of the Caldas da Rainha region, the Cabaços Formation was known to crop out only very locally to the south at Amoreira and at some localities eastwards (Serra dos Candeeiros), though at the basin scale many other outcrops exist. This Formation is the first lithostratigraphical unit of the Upper Jurassic in the basin, overlying a regional disconformity that separates it from the Middle Jurassic shallow marine successions. The Cabaços Formation is composed of a range of facies types, developed in a complex mixture of depositional environments (terrestrial, fresh/brackish lacustrine, restricted-marine varying from brackish to hypersaline), interpreted as a series of pericontinental and coast marginal shallow lakes. It consists mainly of charophyte - and ostracod rich limestones, argillaceous/ferruginous limestones and marls, associated with pedogenic limestones, lignitic clays, coals, microbial laminites and locally sandstones. The fossil content also commonly includes gastropods, bivalves, plant remains and, consistently in the middle part of the unit, the dasycladacean Heteroporella lusitanica (Ramalho,1970), which is attributed to the Middle Oxfordian in Portugal and is only known in the Cabaços Formation. The Cabaços Formation is overlain by the Montejunto Formation, well dated by ammonites as Middle to Upper Oxfordian. In spite of the small dimension of the field section studied at Serra do Bouro, and the strong local tectonism, field characterization and sampling of 10 levels (some discontinuous) was made possible: these are mostly clayey-detrital limestones (locally breccias), marls, sandstones and breccia sandstones, all usually ferruginous, making up a total of c.7 m thick succession. The analysis of the limestone macro/microfacies and of the loose ostracods obtained from the marly levels have shown the unequivocal affinity of these deposits with the Cabaços Formation, well known in the basin. The limestones and marls contain charophytes and ostracods, commonly abundant, gastropods, bivalves and, at the top of the section, H. lusitanica. In addition, the ostracod fauna, typical of fresh-brackish waters, is composed of species that, in the Lusitanian Basin, have been only recognized in the Oxfordian deposits of the Cabaços Formation, except for two species of Timiriasevia.

Keywords : Cabaços Formation; microfacies; ostracods; Serra do Bouro; Lusitanian Basin; Portugal.

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