The Middle Jurassic palynostratigraphy of the northern Lusitanian Basin, Portugal

Abstract

A composite largely Middle Jurassic succession spanning the Toarcian-Aalenian transition to the lowermost Bathonian exposed at Cabo Mondego and São Gião in the northern Lusitanian Basin, western Portugal, was examined palynologically. The 129 samples are correlated to ammonite biozones (ABs) spanning Pleydellia aalensis to Zigzagiceras zigzag. The Cabo Mondego succession comprises the type section of the Cabo Mondego Formation and spans virtually the entire interval studied. This is a significant interval because it includes the Global Stratotype Section and Point (GSSP) and the Auxiliary Stratigraphical Section and Point (ASSP) for the Bajocian and Bathonian stages respectively. The Cabo Mondego Formation largely yielded relatively abundant palynomorph associations in the 68 2 productive samples recovered. By contrast, the Póvoa da Lomba Formation at São Gião only includes the Toarcian-Aalenian transition; the 21 productive horizons produced sparse assemblages. The uppermost Toarcian to lowermost Bajocian is characterised by a low diversity dinoflagellate cyst association, typified by Nannoceratopsis. Above this is a markedly more diverse assemblage, dominated by the family Gonyaulaceae. This influx, in the Witchellia laeviuscula AB, represents a global evolutionary radiation which may be linked to sea level rise. The lower Bajocian yielded significant proportions of gonyaulacacean forms with epicystal and multiplate precingular archaeopyles, representing morphological experimentation as part of this radiation. The trend of increasing dinoflagellate cyst diversity continued at the Bajocian-Bathonian transition, particularly sexiform tabulate gonyaulacaceans with single plate precingular archaeopyles. The Middle Jurassic dinoflagellate cyst associations of the Lusitanian Basin are significantly less diverse than coeval palynobiotas from eastern and northern Europe, and the Arctic. The Toarcian Oceanic Anoxic Event (T-OAE) profoundly inhibited cyst-forming dinoflagellates in this depocentre, and the recovery was protracted. Hence the T-OAE may have suppressed dinoflagellate cyst diversity well into the Middle Jurassic. This phenomenon may have been exacerbated by the absence of typically Arctic taxa through latitudinal controls and/or global cooling during the early Aalenian. These low levels of dinoflagellate cyst species richness may also be related to the palaeogeography of the Lusitanian Basin. This relatively isolated deepwater depocentre close to the Proto Atlantic, may have precluded extensive biotal exchange with the widespread shelfal areas of the western Tethys. The absence of Dissiliodinium giganteum in the Lusitanian Basin is consistent with this scenario. The pollen and spores observed in this study are typical of Middle Jurassic assemblages worldwide. Araucarian pollen, largely Callialasporites, diversified and became prominent during the Aalenian.

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

“…The taxa Ctenidodinium cornigerum , Gonyaulacysta jurassica subsp. adecta ( Gonyaulacysta adecta in Riding et al, 2022) and Pareodinia ceratophora , are more abundant in the Bathonian and Callovian, but may also occur through the Upper Jurassic, especially in the Oxfordian (Borges et al, 2011; Correia et al, 2019; Feist‐Burkhardt & Wille, 1992; Jan du Chêne et al, 1985; Riding & Thomas, 1992; Riding et al, 2022; Smelror, 2021). It is well established that Meiourogonyaulax and Sentusidinium occur from the Bajocian onwards, and various species span the later Mesozoic (Riding & Thomas, 1992; Smelror, 2021; Wood et al, 2016).…”

Reply to the discussion and comments of Azerêdo et al. (2023) and Schneider et al. (2023) on the paper by Magalhães et al. ‘Middle Jurassic multi‐scale transgressive–regressive cycles: An example from the Lusitanian Basin’, The Depositional Record, 9, 174–202

“…It is also vital to highlight that the abundance of organisms depends on environmental context, mainly the physical-chemical conditions of ecosystems The taxa Ctenidodinium cornigerum, Gonyaulacysta jurassica subsp. adecta (Gonyaulacysta adecta in Riding et al, 2022) and Pareodinia ceratophora, are more abundant in the Bathonian and Callovian, but may also occur through the Upper Jurassic, especially in the Oxfordian (Borges et al, 2011;Correia et al, 2019;Feist-Burkhardt & Wille, 1992;Jan du Chêne et al, 1985;Riding et al, 2022;Smelror, 2021;Riding & Thomas, 1992).…”

“…For instance, the low abundance could be justified due to poor preservation or local palaeoenvironmental factors. In addition, most of the publications referenced in the reply (Borges et al, 2011;Correia et al, 2019;Jan du Chêne et al, 1985;Riding et al, 2022;Riding & Thomas, 1992;Smelror, 2021) do not contain any record of C. cornigerum in the Oxfordian. It is important to highlight that C. cornigerum is considered a typical species of Bajocian-Bathonian transition (Correia et al, 2019;Feist-Burkhardt & Monteil, 1997;Feist-Burkhardt & Wille, 1992;Jan du Chêne et al, 1985;Riding & Thomas, 1992;Wiggan et al, 2017) or Callovian (Borges et al, 2011(Borges et al, , 2012, in addition to two occurrences in the Callovian according to the software Palynodata Inc and White (2008).…”

“…In addition, most of the publications referenced in the reply (Borges et al, 2011;Correia et al, 2019;Jan du Chêne et al, 1985;Riding et al, 2022;Riding & Thomas, 1992;Smelror, 2021) do not contain any record of C. cornigerum in the Oxfordian. It is important to highlight that C. cornigerum is considered a typical species of Bajocian-Bathonian transition (Correia et al, 2019;Feist-Burkhardt & Monteil, 1997;Feist-Burkhardt & Wille, 1992;Jan du Chêne et al, 1985;Riding & Thomas, 1992;Wiggan et al, 2017) or Callovian (Borges et al, 2011(Borges et al, , 2012, in addition to two occurrences in the Callovian according to the software Palynodata Inc and White (2008). Thus, the dinocyst association common to the Callovian is corroborated by the LO of C. cornigerum (Borges et al, 2011(Borges et al, , 2012 as well as by the distribution of calcareous nannofossils, here considered not to be younger than Callovian.…”

The impact of the Toarcian Oceanic Anoxic Event (T-OAE) on the radiation of Early Jurassic dinoflagellate cysts in the Lusitanian Basin, Portugal

“…Apparently, the palaeoenvironmental changes associated with the T-OAE were more extensive in the Lusitanian Basin, compared with coeval basins in northern Europe. Correia et al (2018Correia et al ( , 2019 recognized a recovery of dinoflagellate cysts, both in diversity and abundance, in the early Bajocian in the Lusitanian Basin. Hence, the T-OAE in this basin strongly affected the dinoflagellate cyst evolution patterns.…”