Calcareous Nannofossil Taxonomy and Biostratigraphy of the Toarcian-Lower Bajocian Colle Di Sogno Section (Lombardy Basin, Southern Alps, Italy)

Abstract

Calcareous nannofossil biostratigraphy was conducted in the Toarcian-lower Bajocian interval at Colle di Sogno (Lombardy Basin, Southern Alps, Northern Italy) where the type-section of the Sogno Formation consisting of pelagic marly limestone, marlstone and marly claystone was established. Semiquantitative analyses of calcareous nannofossil assemblages allowed to achieve a high-resolution biostratigraphy based on several biohorizons, including zonal/subzonal markers and additional events. The NJT5 to NJT9 Zones of the standard nannofossil zonation established for the Mediterranean Province were identified. The biostratigraphy obtained at Colle di Sogno was compared to published nannofossil events calibrated with ammonite zones in sections from SE Spain, S France, Portugal and N Algeria. This assessment evidenced some discrepancies in the succession of events of the standard zonation and, furthermore, resulted in the revision of the age of a few datums. Moreover, some new/additional nannofossil biohorizons are proposed as subzonal markers. A morphometric analysis of the genus Watznaueria was conducted to identify diagnostic features for unambiguous species identification. The species W. colacicchii and W. contracta are distinguished on the basis of the coccolith width/central area width ratio, whereas W. britannica britannica is discriminated by the size - as in the original definition of the species - relative to the new subspecies W. britannica minor. The new species W. gaetanii differs from all other Watznaueria taxa by the bridge ultrastructure. In the Toarcian-Aalenian interval a progressive increase in size of Watznaueria specimens is paralleled by the progressive closure of the central area and the modification of the central area structure passing from a cross (W. colacicchii and W. contracta) to a double-button bridge (W. gaetanii) to a single-button bridge (W. britannica). These intrageneric evolutionary innovations accelerated in the Aalenian under stable paleoceanographic conditions and an oligotrophic regime.

On the occurrence of rare nannoliths (calcareous nannofossils) in the Early Jurassic and their implications for the end‐Triassic mass extinction

“…Similar‐looking specimens have been documented in the Toarcian of Italy as Rucinolithus sp. by Casellato & Erba (2015), and demonstrated to represent isolated spines of Mitrolithus elegans in distal view in Sinemurian limestones of Iran by Abdi et al . (2022).…”

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

“…Previous studies indicate that the LO of L. hauffii is either at the Callovian (Bown & Cooper, 1998; Mattioli & Erba, 1999; Young et al, 2022) or at the late Oxfordian stage, with the subject needing further investigation (Casellato, 2010). Considering these discrepancies in the biochronostratigraphic position of the LO of L. hauffii , we suggest further studies in the Lusitanian Basin to calibrate this event better.…”

Pleurotomariida (Gastropoda) from the upper Anisian platform carbonates of the Dolomites (Southern Alps, Italy): systematics, palaeobiogeography and Triassic recovery

“…and Proarcestes sp. These species indicate the Illyrian (upper Anisian), Nevadites secedensis Zone (= Nevadites Zone of and Mietto et al 2003) (cf. Brack et al 2005Balini et al 2010;Jenks et al 2015) for the whole sampled part of the succession (see also Blendinger 1994, De Zanche et al 1995and Russo et al 2000 and confirm, as suggested by De Zanche et al (1995) and in contrast to Assereto (1969), the absence of the underlying Aploceras avisianum Subzone, Hungarites Zone sensu (= Reitzites reitzi Zone of Brack et al 2005) in the studied area.…”

“…These species indicate the Illyrian (upper Anisian), Nevadites secedensis Zone (= Nevadites Zone of Mietto & Manfrin 1995 and Mietto et al . 2003) (cf. Brack et al .…”