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

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.

Discussion on ‘Middle Jurassic multi‐scale transgressive–regressive cycles: An example from the Lusitanian Basin’, by Magalhães et al., Depositional Record, 2023, 9(1), 174–202

“…However, the Middle Jurassic section at Baleal is also to the west of the suggested 'fence' and it only exposes marine carbonate. A similar model was presented by Fürsich et al (2022) for the Alcobaça Formation in the Consolação sub-basin (as those authors call it), but of broader scope and referred to a later, different stage of the LB's history and configuration, in Kimmeridgian times. Magalhães et al (2023) invoke a very local explanation dubiously in middle Jurassic times.…”

“…The Alcobaça Formation, recently formalised by Fürsich et al (2022), is an important unit of the Upper Jurassic of the LB (Figure 2). Fürsich et al (2022) provided an extensive literature overview, in which the formation is thoroughly described, illustrated and correlated within the basin (including the Consolação section); they presented facies analysis, macropalaeontological and micropalaeontological taxonomic and palaeoecological data and palaeogeographical interpretations. However, this seminal work is not mentioned by Magalhães et al (2023).…”

“…However, this seminal work is not mentioned by Magalhães et al (2023). The Alcobaça Formation is a mixed siliciclastic‐carbonate succession; its age is constrained by macrofossil and microfossil data and strontium isotope values, and has been confidently considered to be mainly of Kimmeridgian age, although possible slight age extensions of its base and top have been discussed (Fürsich et al, 2022; Kullberg & Rocha, 2014; Leinfelder, 1986; Manuppella et al, 1999; Schneider et al, 2009; Werner, 1986).…”

“…Highly simplified lithostratigraphic scheme of the Triassic‐Upper Jurassic interval for the central‐southern Lusitanian Basin, representing only the main broad lithologies and units under informal designations, for easier understanding of the text for discussion purposes. For additional lithostratigraphic information, see for instance: Wilson et al (1989), Leinfelder and Wilson (1998), Azerêdo (2007), Kullberg and Rocha (2014) and Fürsich et al (2022). …”

“…The paper by Magalhães et al (2023) concerns the Jurassic coastal outcrop between Consolação and São Bernardino beaches, in the western‐central Lusitanian Basin (LB), Portugal (Figure 1). The authors used a sequence stratigraphic approach, defining multi‐scale cycles and, based on nannofossils and dinoflagellate cysts (dinocysts) data, they re‐assigned the section to the Middle Jurassic ‘Candeeiros Formation’; formerly the section had been established as part of the Upper Jurassic Alcobaça Formation (namely Fürsich et al, 2022; Leinfelder, 1986; Manuppella et al, 1999; Schneider et al, 2009; Werner, 1986). Although the sedimentology and sequence analysis were detailed, we consider their assessment to be incorrect and oversimplified including, for example, the assigned age of the unit and the top of the section.…”