Life at the continental–marine interface: palaeoenvironments and biota of the Alcobaça Formation (Late Jurassic, Central Portugal), with a formal definition of the unit appended

Franz T Fürsich,Colm S Pierce,Berta Lopez-Mir,Simon Schneider,Winfried Werner

doi:10.1007/s12549-021-00496-x

Abstract

The Kimmeridgian Alcobaça Formation of the Lusitanian Basin forms a mixed carbonate–siliciclastic unit between basinal deposits of the Abadia Formation, and fluvial–terrestrial strata of the Lourinhã Formation. This study presents >2.5 km of detailed logs of nine outcrop sections of the Alcobaça Formation in its type region. Eight of these sections encircle the Caldas da Rainha Diapir, which was a prominent, emergent, passive salt diapir during the time of deposition. Palaeoenvironments of the unit form a complex mosaic of low- to high-energy, carbonate- or siliciclastic-dominated shallow shelf settings; coastal embayments and lagoons; and coastal plains with rivers, lakes and playas. In the strata, abundant microfauna is often joined by a rich macrofauna, usually dominated by bivalves. Locally, corals, calcareous sponges or oysters form meadows or patch reefs. These autochthonous to parautochthonous remnants of former communities are assigned to 35 benthic macrofaunal associations. The integration of palaeoecological analysis of these associations with microfaunal and sedimentological data provides constraint on their salinity ranges, which range from slightly hypersaline to freshwater. Frequent temporal and spatial salinity fluctuations are attributed to variations in relative sea-level, salt tectonics or climate. The NNE-trending Caldas da Rainha Diapir induced pronounced facies differentiation. Predominantly, non-marine siliciclastic facies in the northwest and carbonate to siliciclastic, marine to brackish facies in the southwest are contrasted by shallow-marine carbonate facies east of the diapir. Comprehensive exposure and well-preserved fossils make the Alcobaça Formation an excellent showcase to demonstrate how biofacies analysis can help to disentangle the interplay of climate changes, sea-level fluctuations and salt tectonics. Based on the improved characterisation of the unit, the Alcobaça Formation is formally defined, and seven members are established.

Highlights

History of researchThe term ‘Couches d’Alcobaça’ was coined by Choffat (1885: 5, 18) for a succession of lignites from Fervença near Alcobaça (Fig. 3), including a horizon with brackish-water invertebrate fossils in its upper part
We provide a brief overview on the 35 benthic associations/assemblages identified in the Alcobaça Formation, many of which apparently lived in salinity regimes deviating from normal marine values
The undisturbed base of the Alcobaça Formation is only exposed in the Fonte Santa section, where the unit overlies clean, thick-bedded limestones of the middle Oxfordian Montejunto Formation

Summary

Introduction

The term ‘Couches d’Alcobaça’ was coined by Choffat (1885: 5, 18) for a succession of lignites from Fervença near Alcobaça (Fig. 3), including a horizon with brackish-water invertebrate fossils in its upper part. Alternating marine and freshwater deposits occurring at Batalha (Fig. 3) and Ourém (Fig. 1) (the latter containing freshwater molluscs and plant litter) were assigned to this unit. Choffat (1885) further mentioned abundant trigoniid bivalves from marine intercalations in the lignites of the Couches d’Alcobaça. In the same study, Choffat (1885) introduced the term ‘Couches à Lima alternicosta’ for deposits in the surroundings of Torres Vedras, Cesareda and Sobral da Lagoa, which he regarded as a potential southern equivalent of the Couches d’Alcobaça. Modified from Alves et al (2002)

Methods

Findings

Discussion

Conclusion