Comment on: Magalhães et al., Middle Jurassic multi-scale transgressive–regressive cycles: An example from the Lusitanian Basin, The Depositional Record, 9, 174–202

Abstract

In a recently published paper, Magalhães et al. (2023) re-studied the coastal section between the villages of Consolação and São Bernardino south of Peniche, central Portugal, and determined a novel Bathonian to early Callovian age for the exposed strata. They identified low-frequency to high-frequency transgressive–regressive sequences, and interpreted the depositional settings as offshore to shoreface for siliciclastic strata, with subordinate carbonate intervals associated with inner to outer ramp settings. Based on these new results, they assigned the studied strata to the Candeeiros Formation. We show that these conclusions are based on misinterpretation and the non-consideration of previously published contradictory data.

The Jurassic sedimentary succession of the Lusitanian Basin is unusually well-exposed along the Atlantic coast, and offers unique opportunities for study. The coastal section between the villages of Consolação and São Bernardino is a prime example, and has been studied repeatedly (Fürsich et al., 2022; Mateus et al., 2017; Taylor et al., 2014; Werner, 1986), but a modern sequence stratigraphic analysis of this succession has never been attempted. As such, the study of Magalhães et al. (2023) generally comes as a welcome addition. However, this study falls short in several aspects. Most significantly, its dissonance with previously published contradictory literature is not taken into account, and the interpretation of the data generated is selective and model-driven. We address these issues in the following sections, drawing on the literature, including a recently published study by the present authors (Fürsich et al., 2022), accessible online approximately half a year before the submission of the manuscript of Magalhães et al. (2023), but not considered in their paper.

Magalhães et al. (2023) characterise the geodynamic evolution of the Lusitanian Basin in sufficient detail, and the literature cited in this section is relevant and up to date. However, despite the sequence stratigraphic focus of their study, the sedimentological evolution of the basin is barely addressed. In the context of a presumed Middle Jurassic age for the strata at Consolação, it would have been essential to focus on the Early to Middle Jurassic evolution of depositional environments in the Lusitanian Basin, but the lithostratigraphic scheme used is outdated, and numerous key studies are not cited. Magalhães et al. (2023) use the oversimplified scheme of largely concordant successive Coimbra, Brenha and Candeeiros formations, which was established by Shell Prospex Portuguesa in the 1970s, but never formally published (Witt, 1977). This scheme has been superseded by much more detailed lithostratigraphy. In its revised, restricted sense, the Coimbra Formation of Moitinho de Almeida et al. (1958) varies greatly in thickness and stratigraphic extent (Azerêdo et al., 2003; Duarte et al., 2022; Soares et al., 1985, 1993). The informal Sinemurian to Callovian Brenha Formation of De Meijer and Witt (in Witt, 1977) is replaced by more than a dozen formations today, documenting the lateral and stratigraphic lithological variability of the succession, as well as the progress in dating these sediments (Duarte, 2007; Duarte et al., 2009; Duarte & Soares, 2002; Kullberg et al., 2013). The informal Middle Jurassic Candeeiros Formation of Matos and Witt (in Witt, 1977; originally Candieiros Formation) is now subdivided into several formal or informal lithostratigraphic units, and the Santo António–Candeeiros Formation is restricted to the Serra dos Candeeiros in the eastern part of the basin (Azerêdo, 2007; Azerêdo et al., 2003, 2020; Martins, 2008). The Lusitanian Basin hosts the Global Stratotype Section and Point (GSSP) for the Toarcian stage at Peniche (Rocha et al., 2016), as well as the GSSP for the Bajocian stage (Pavia & Enay, 1997) and an Auxiliary Stratotype Section and Point (ASSP) for the Bathonian stage at Cabo Mondego (Fernández-López et al., 2009), underlining its global importance. As a result, the Early and Middle Jurassic sedimentological and stratigraphic evolution of the Lusitanian Basin is rather well characterised—but none of these data are reflected in the work of Magalhães et al. (2023). The lithostratigraphic assignment of the dominantly siliciclastic succession at Consolação to the Candeeiros Formation, which even in its informal original, broad definition was characterised as exclusively composed of limestone (Witt, 1977, p. 22), disagrees with the concepts of lithostratigraphy (Murphy & Salvador, 1999).

Magalhães et al. (2023) suggest that the boundary between the Bajocian to Callovian Candeeiros Formation and the overlying late Kimmeridgian to Tithonian Lourinhã Formation at Consolação is a major disconformity. This would imply a hiatus of at least 10 Myr duration, which is unmatched in the Lusitanian Basin during this interval. However, no evidence for this disconformity is presented in figure 14 of Magalhães et al. (2023), where the formation boundary is depicted. Previous studies (Fürsich et al., 2022; Taylor et al., 2014) placed the boundary between the Consolação Member of the Alcobaça Formation and the overlying Lourinhã Formation at exactly the same level, but regarded it as conformable; this view is maintained here. Magalhães et al. (2023) referred to the well-known basin-wide Middle–Upper Jurassic disconformity in the Lusitanian Basin in this context, but failed to acknowledge its nature and stratigraphic extent. As established in detail by Azerêdo et al. (1998, 2002), this disconformity separates limestones or marls of late Bathonian to late Callovian age from middle Oxfordian limestones of the Cabaços Formation. In contrast, siltstones are overlain by sandstones of the Lourinhã Formation at Consolação.

Only 6 km north of Consolação, Middle Jurassic strata of the Cabo Mondego Formation are exposed at Baleal and on Ilha das Pombas, within the Consolação Sub-basin, at the same sub-basin margin (Azerêdo et al., 2020; Camarate França et al., 1960; Ruget-Perrot, 1961). The respective succession comprises brachiopod, belemnite and ammonite-bearing open marine limestones and marls of Bajocian to Bathonian age, deposited on a west-facing distal carbonate ramp (Azerêdo, 1988; Azerêdo et al., 2020). However, the significance of these strongly contrasting deposits, as well as their implications for the alleged Middle Jurassic siliciclastic strata at Consolação, were not discussed by Magalhães et al. (2023).

The biostratigraphy and Sr isotope stratigraphy of the Alcobaça Formation, including the Consolação Member, were revised by Fürsich et al. (2022). Re-calculated Sr isotope ages derived from oyster shells collected from several horizons at Consolação (Schneider et al., 2009) indicate Kimmeridgian ages (Fürsich et al., 2022). Although the study by Schneider et al. (2009) is cited in Magalhães et al. (2023, p. 190; erroneously as Schneider & Fürsich, 2009 throughout the paper), Sr isotope stratigraphy is not discussed, and no arguments are provided to discard the Sr age interpretations. Ammonites from Consolação, that is Mesosimoceras and Ataxioceras, were first mentioned by Werner (1986). The Ataxioceras specimen was reclassified as Parataxioceras cf. oppeli (Geyer) and figured by Fürsich et al. (2022, figure 5e) and is indicative of an early Kimmeridgian age (Platynota Zone). Magalhães et al. (2023) did not discuss these finds, nor did they explain the absence of Middle Jurassic ammonites in alleged open marine Middle Jurassic strata.

Ostracods were studied by Magalhães et al. (2023), but were not utilised for biostratigraphy; their specimens, depicted in figure S-6, are poorly preserved and thus determinable only at genus level. Fürsich et al. (2022) reported a Kimmeridgian age for ostracod assemblages from the Alcobaça Formation, based on well-preserved material mostly determined to species level. The most relevant taxa, including several from the Consolação section, were figured (Fürsich et al., 2022, figure 6n-r). Again, these results are not discussed in Magalhães et al., 2023).

Fürsich et al. (2022) did not emphasise the biostratigraphic importance of Foraminifera, Bivalvia or additional benthic macrofossils, since most of the taxa present at Consolação are not stratigraphic indicators at stage level, and the Late Jurassic age of the succession was not in question. However, numerous horizons of the Consolação section are highly fossiliferous, and their microfossil and macrofossil content was reported in great detail by Werner (1986). Most of these taxa are indicative of a Late Jurassic age and occur at numerous localities in the Lusitanian Basin. Examples of bivalves and other macrofossils from Consolação were discussed and figured in several subsequent studies (Fürsich et al., 2009, 2022; Fürsich & Werner, 1986, 1988, 1989; Schneider et al., 2009, 2010). Most of these studies were not considered by Magalhães et al. (2023), and the results of Werner (1986) were not discussed.

Biostratigraphy and consequently the chronostratigraphic assignment of the Consolação section in Magalhães et al. (2023) is based on nannofossils and dinoflagellate cysts. Nannofossil assemblages are characterised as ‘poorly to moderately preserved’ by the authors (Magalhães et al., 2023, p. 189). According to the Nannotax 3 (2023) database, several of the taxa recorded are long-ranging, and do not exclude a Late Jurassic age (Figure 1). Interestingly, Watznaueria manivitiae is regarded as a synonym of Cyclagelosphaera deflandrei, which has its first occurrence date in the Oxfordian, according to Nannotax 3 (2023), while it is displayed as ranging from the early Bajocian to the top of the Callovian in figure 9 of Magalhães et al. (2023), based on Mattioli and Erba (1999). In contrast, Parhabdolithus liasicus has its last occurrence date in the Toarcian, according to Nannotax 3 (2023) (Figure 1). Thus, the presence of these two taxa can only be explained by reworking of Early and Middle Jurassic nannofossil assemblages, and re-deposition in Oxfordian or younger strata. However, these contradictory age ranges are not discussed by Magalhães et al. (2023). A similar case of nannofossil reworking, documented from contemporaneous early Kimmeridgian strata in the east of the Lusitanian Basin (Turner et al., 2017), shows that re-deposition is not an uncommon issue. The near-absence of autochthonous nannoflora at Consolação can be explained by the shallow shelf to restricted depositional settings represented in the section (Fürsich et al., 2022; see also discussion below), where open marine biota do not occur.

A similar scenario emerges for the dinoflagellate cysts recorded by Magalhães et al. (2023), which allegedly indicate a Bajocian to early Callovian age. Only four of the 14 dinoflagellate taxa listed are determined to species level. Of these, Systematophora penicillata is indicative of a late Oxfordian age according to Williams et al. (2017) (Figure 2), while it is displayed as Bajocian to top Callovian by Magalhães et al. (2023, figure 9). Moreover, the genus Amphorulacysta is recorded from Kimmeridgian to Berriasian strata only, according to Williams et al. (2017) (Figure 2). Again, this pattern can only be explained by reworking of the older, Middle Jurassic to Oxfordian taxa, but the contradictory evidence of their results was not discussed by Magalhães et al. (2023). In the course of our own studies, we analysed palynomorph assemblages of five mudstone samples from Consolação, and had similarly poor recovery. Only two samples contained marine taxa, but only one of them yielded a vaguely age-conclusive assemblage, and we did not publish these data. As for the nannofossils, the rarity of autochthonous Late Jurassic dinoflagellates can be explained by the nearshore, restricted origin of the deposits, where open marine microplankton is scarce.

With regard to sedimentology and depositional environments, the analysis of Magalhães et al. (2023) falls short in several aspects. The authors interpreted the depositional environments as ranging from offshore to nearshore in the case of siliciclastic strata, and as outer to inner ramp in the case of carbonates (Magalhães et al., 2023). Pure carbonates are, in fact, very rare at Consolação (foraminifera-rich grainstones and rudstones at the base of the section; Werner, 1986), and carbonates in general are subordinate. Most carbonates exhibit variable admixtures of clay-sized to sand-sized siliciclastic components, documenting a considerable input of terrigenous material throughout the section. Moreover, they intercalate with siliciclastic layers, and thus were clearly not deposited on an outer ramp. The coral meadows in the lower part of the section are also embedded in mixed siliciclastic-carbonate matrix and alternate with fine-grained siliciclastic strata containing a moderately diverse soft-bottom macrofauna. The latter is dominated by euryhaline bivalves and gastropods, with occasional echinoid spines representing the only stenohaline faunal element. Although clearly marine, both the coral meadows and soft-bottom fauna characterise protected nearshore areas (Fürsich et al., 2022; Werner, 1986) rather than offshore environments as envisaged by Magalhães et al. (2023).

Claystone/siltstone packages are generally regarded as having settled out of suspension offshore by Magalhães et al. (2023), neglecting alternative interpretations such as their deposition in protected bays or lagoons. As sedimentological criteria alone may not be sufficient to identify the precise palaeoenvironment, palaeoecological data can be a useful additional tool. The authors claim that they identified the macrofauna, but they did not document or utilise these data. Altogether, the extreme scarcity of ammonites and belemnites suggests a nearshore setting of the Consolação Member. As demonstrated by Werner (1986), several of the siltstone packages, for example those near the top of the succession, contain a low-diversity bivalve fauna composed of taxa that indicate strongly reduced salinities (see also Fürsich & Werner, 1986), thus contradicting their interpretation as offshore muds.

The misinterpretation of facies types raises serious doubts on the sequence stratigraphic interpretation of the section. For example, the siltstone packages near the top of the succession are interpreted as TST deposits by Magalhães et al. (2023), whereas their brackish bivalve fauna clearly points to a restricted marine setting and RST character. Moreover, despite producing virtual outcrop models of the exposures at Consolação, Magalhães et al. (2023) did not utilise these for illustrating facies architecture or sequence stratigraphy. In the higher part of the sections, in particular, these models would help to depict the channelised geometries of some of these strata—which, however, also contradict the interpretation of Magalhães et al. (2023).

At small scale, sequence stratigraphic interpretation is compromised by the interplay of diapirism and sea-level fluctuations at Consolação (Fürsich et al., 2022). As demonstrated by Davison and Barreto (2020), diapirs in the Lusitanian Basin were active and exposed during most of the Jurassic, and salt tectonics have played a significant role in shaping depositional environments, particularly around the Caldas da Rainha Diapir, which is the most prominent salt structure of the basin (Fürsich et al., 2022). The flank of this emergent diapir also is the source of the sandstones of the Consolação Member, which were shed from the east and north-east. In contrast, Magalhães et al. (2023) identified the Berlengas horst block to the west as the source of the siliciclastic strata deposited at Consolação, which, in their opinion, got trapped by the Caldas da Rainha Diapir. However, they fail to explain how sandstones in the Consolação succession would have been transported in NW-SE direction across the Consolação Sub-basin situated between these highs, perpendicular to the NE–SW directed sub-basin axis.

The article by Magalhães et al. (2023) provides a flawed interpretation of the Jurassic succession at Consolação with respect to its age, depositional environments and sequence stratigraphic architecture. This is partly due to the failure to consider published literature. Furthermore, conflicting sedimentological, biostratigraphic and palaeoecological data have been omitted in their interpretation. The coastal section at Consolação exposes Upper Jurassic shallow to restricted marine siliciclastic strata and subordinate carbonates, which were deposited in open to protected shallow shelf settings, exhibiting increasing freshwater influence up-section. Dinoflagellate and nannofossil assemblages from Consolação are mainly composed of reworked Middle Jurassic taxa, because autochthonous Late Jurassic representatives of these groups are rare in nearshore settings. The small-scale ‘sequences’ identified in the study of Magalhães et al. (2023) are likely modulated by a combination of pulses of diapirism related to the adjacent Caldas da Rainha Diapir, base-level change and autogenic paralic system avulsions. There is compelling evidence from biostratigraphy and strontium isotope stratigraphy for a Late Jurassic, Kimmeridgian age of the coastal section at Consolação, which represents the stratotype of the Consolação Member of the Alcobaça Formation.