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

Abstract

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. Moreover, the new fossil data are too briefly discussed and based on limited data, and the local/regional geology is barely discussed.

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). 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).

If the part of the section described by Magalhães et al. (2023) were Middle Jurassic strata, then the local succession would be: the outcrops of Kimmeridgian Alcobaça Formation at the Consolação section, succeeded southwards by the Middle Jurassic (Bathonian/Callovian) ‘Candeeiros Formation’, in turn overlain by the much later Upper Jurassic Lourinhã Formation, the intervening units locally (and implausibly) not present. No considerations of plausible depositional geometries or regional correlations with nearby formations were made to support the drastic sequence changes implied.

At Cesareda zone, about 9 km eastwards, and at Baleal Peninsula, about 7 km north-westwards (Figure 1), Middle Jurassic carbonate outcrops occur, dated by ammonites from, respectively, Bajocian to Callovian and Bajocian/Bathonian (Azerêdo, 1988, 1993; Guéry et al., 1986; Ruget-Perrot, 1961). Incidentally, Magalhães et al. (2023) cite Azerêdo (1988) but in the references wrongly assign it to her 1993 thesis; the former work is the original study of the Baleal section. These carbonate successions developed within the framework of a healthy carbonate ramp depositional system that prevailed for the whole of the Middle Jurassic in the LB (Azerêdo, 1988, 1993, 1998, 2007; Azerêdo et al., 2014, 2020). Thus, it is highly implausible that a siliciclastic system occurred simultaneously only in that small local section. Magalhães et al. (2023) do address (p. 22) the idea of a siliciclastic influx from the uplifted Berlengas block to the west into their study area, which would be bounded to the east by a topographic high (diapir related), hence siliciclastic input not influencing the more distant carbonate deposition to the east and north (as at Serra dos Candeeiros—MCE, Figure 1). 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.

We do not discuss the sequence stratigraphic approach of Magalhães et al. (2023) nor the criteria to define cycles, and we accept that this could be a positive new contribution to understanding the regional stratigraphy if the correct time frame were used. However, the upper top limit of their Sequence J, underlying the Lourinhã Formation, does not represent the Middle/Upper Jurassic (Callovian/Oxfordian) disconformity; and the micropalaeontological evidence for assigning a Bathonian/Callovian age to the section they studied is not robust (discussed in detail below). In addition, they did not compare their sequence scheme with published cycles for the Middle Jurassic interval of the LB (Azerêdo et al., 2014, 2020).

Magalhães et al. (2023) address three microfossil groups: ostracods, calcareous nannofossils and palynomorphs. They do not identify the foraminifera and algae taxa in the limestones. Evidence from nannofossils and dinocysts are given to justify revising the dating of the succession. Several aspects warrant further discussion, and we view this dating as highly questionable.

Overall, the conclusions of Magalhães et al. (2023) that (i) the Middle/Upper Jurassic disconformity is present in the section they studied; (ii) the studied section is of the Middle Jurassic ‘Candeeiros Formation’; and (iii) the studied succession fills the Middle Jurassic stratigraphic record gap between the Lower Jurassic of Peniche and the Upper Jurassic of São Bernardino, are not convincingly supported. In our view the section belongs to the Upper Jurassic Alcobaça Formation.

The authors declare no conflict of interest.