Middle Jurassic multi‐scale transgressive–regressive cycles: An example from the Lusitanian Basin

Abstract

Multi‐scale transgressive–regressive cycles from the mid‐Jurassic were recognised in the Central Lusitanian Basin, Portugal. These cycles allow the depositional evolution of the basin to be better understood and aid in the construction of stratigraphic sequences composed of three hierarchies. The stacking pattern of high‐frequency transgressive–regressive sequences forms larger clusters that define medium‐frequency transgressive–regressive sequences. Likewise, the stacking pattern of medium‐frequency transgressive–regressive sequences generates two Bathonian–early Callovian low‐frequency transgressive–regressive sequences. Integration of several methods supported the interpretation of facies associations representing clastic deposition in offshore to shoreface environments and carbonate sediments in outer to inner ramp settings. New data from calcareous nannofossils and dinoflagellate assemblages constrained the interval's Bathonian–early Callovian age, thus unveiling the Middle–Upper Jurassic disconformity and filling the Middle Jurassic stratigraphic record gap in the Central Lusitanian Basin. This study may be helpful for similar successions in Tethyan domains and comparable depositional settings elsewhere.

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

“…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).…”

“…The Lourinhã Formation is diachronous, mostly assigned a latest Kimmeridgian/Tithonian age, with some authors proposing a locally different age interval (namely Kullberg & Rocha, 2014;Mateus et al, 2017;Taylor et al, 2014). The Consolação Fault is the northern limit of the section studied by Magalhães et al (2023) and the São Bernardino beach is the southern limit. They describe the section as (p. 4):…”

“…3.1 | The so called 'Candeeiros Formation' Magalhães et al (2023) refer to the Candeeiros Formation. The term 'Candeeiros' has been used in works on the LB, as an informal name coined by industry for a broad package of mostly shallow-water carbonate deposits from the Middle Jurassic, in parallel with the term 'Brenha', used for the more distal marl-limestone facies of the Lower and Middle Jurassic (namely Wilson et al, 1989; Figure 2).…”

“…In the following sections we use the Candeeiros and Brenha to refer to the two broad concepts. Magalhães et al (2023) refer to Taylor et al (2014) to help constrain their section; this warrants discussion. Taylor et al (2014) described the Lourinhã Formation in this region of the LB and they say at p. 143: "The study area …comprises two … coastal outcrops of the Lourinhã Formation close to … Peniche … These outcrops are approximately 7 km apart.…”

“…Magalhães et al are here implying the units Candeeiros and Lourinhã, but Taylor et al (2014) wrote explicitly that at São Bernardino the Lourinhã Formation overlays the Alcobaça Formation; they only refer to the Candeeiros Formation at Baleal. Magalhães et al (2023) incorrectly mixed up the Taylor et al (2014) description of the two outcrops; and further state ' … corresponds to the Middle-Upper Jurassic disconformity'. This straight statement that the top of the studied succession at São Bernardino corresponds to the Middle/ Upper Jurassic disconformable boundary is objectively incorrect.…”