The age of the Tojeira Formation (Late Jurassic, Early Kimmeridgian), of Montejunto, west-central Portugal

Abstract

Precise biostratigraphic dating of the Tojeira Formation (Late Jurassic, Early Kimmeridgian) of the Montejunto section of west-central Portugal, which has yielded important planktonic foraminiferal assemblages, is hindered by poor preservation in the upper part of the section as the lithology shifts from shale to coarser clastics. Assignment was previously made to the Idoceras planula and Sutneria platynota zones based on ammonites. Coccolith and dinoflagellate assemblages described here concur with the Early Kimmeridgian, yet, a finer age constraint is proposed by cyclo-and chemostratigraphical correlation. Peaks in d 13 C org and TOC, if equivalent to maxima in the envelope of clay/carbonate cycles in SE France, imply that the c. 50 m-thick section spans a 0.8-myr interval of the S. platynota through upper Ataxioceras hypselocyclum ammonite zones, with the approximate base of the A. hypselocyclum Zone at c. 15.4 m (level 13). Such stratigraphy provides new insights into the upper part of the formation by interbasinal correlation with other Tethyan records. An extended first occurrence of the dinoflagellate species Dichadogonyaulax? pannea in the S. platynota Zone is also proposed.

Pseudolamarckina pseudorjasanensis Dain, 1967 (Foraminifera) as a Kimmeridgian marker species and its significance for biostratigraphy, palaeoecology and palaeobiogeography

“…The north Portugal forms of P. rjasanensis display all the typical morphological characteristics of the species P. pseudorjasanensis, including a convex spiral side, broad and slightly raised sutures, last chamber extended towards the centre and almost covering the umbilicus and a loop-shaped, umbilical aperture (Stam 1986). Similar specimens were recorded in the upper part of Lower Kimmeridgian strata of the Tojeira Section (north Portugal) associated to the upper part of the ammonite Platynota Zone and/or to the base of the ammonite Hypselocylum Zone (Stam 1986;Turner et al 2017) (Fig. 7).…”

“…Specimens of the genus Pseudolamarckina were found in sub-Mediterranean and peri-Tethyan regions within the uppermost Lower Kimmeridgian strata of Portugal and the Caucasus (Grigjalis 1982;Stam 1986;Turner et al 2017). Similarly, the species P. rjasanensis (= pseudorjasanensis) was identified in the uppermost Lower Kimmeridgian successions of the Tojeira Section (north Portugal), associated to the boundary between the Platynota and Hypselocylum zones.…”

“…The first occurrence of the species P. pseudorjasanensis is recognized in a coeval level, associated to the uppermost Lower Kimmeridgian of sub-Mediterranean and peri-Tethyan (Portugal, Caucasus) to Arctic (Laptev Sea coast, Arctic Alaska and Canada) regions (Fig. 7) (Kuznecova 1979;Grigjalis 1982;Mesežnikov 1984;Stam 1986;Hedinger 1993;Nikitenko 2009;Mitta et al 2012;Nikitenko et al 2013;Nikitenko et al 2015;Turner et al 2017).…”

Sedimentation of the Kimmeridge Clay Formation in the Cleveland Basin (Yorkshire, UK)

“…Characterisation of the sediment facilitates the interpretation of ancient depositional environments, which provide a unique and important insight into global climate and carbon cycle dynamics [24]. However, lateral sediment transport and potentially multiple episodes of sediment remobilisation prior to deposition means that mudstones may record palaeoenvironment conditions from other areas of the ocean with little or no evidence of long-range transport; therefore, [36] reproduced with permission after Atar et al (2019b) [15]. (b) Geological map of the Cleveland Basin (Yorkshire, UK) after Powell (2010) [52], reproduced with permission after Atar et al (2019a) [14].…”

The Middle Jurassic palynostratigraphy of the northern Lusitanian Basin, Portugal

“…These authors only recovered three dinoflagellate cysts (Barrón and Azerêdo 2003, p. 285). Other substantial contributions on the Jurassic palynology of Portugal include Mohr and Schmidt (1988), Bucefalo Palliani and , Oliveira et al (2007), Borges et al (2011Borges et al ( , 2012, Barrón et al (2013), Rocha et al (2016), Correia et al (2017aCorreia et al ( , 2017bCorreia et al ( , 2018 and Turner et al (2017).…”