Correlation of low-latitude radiolarian-bearing pelagic lithofacies around the Jurassic–Cretaceous boundary

Calpionellids have been considered as the stratigraphically most important fossils to define the Jurassic–Cretaceous boundary and, after the formal vote in 2016, the base of the Calpionella Zone (Alpina Subzone) was accepted as the primary marker for the Berriasian Stage. The occurrence of calpionellids is restricted to micritic cherty limestone known as the Maiolica (or Biancone) limestone from classical localities in the Southern Alps. The first appearance of true calpionellids in the Upper Tithonian Crassicollaria Zone coincides with the base of the Maiolica limestone which is located below but close to the Jurassic–Cretaceous boundary. Radiolarians allow us to study a longer continuous interval around the boundary since they are common in the Maiolica-type limestone and also in the underlying lime-poor to lime-free siliceous deposits. A distinct facies change in Tithonian pelagic successions is best known from the Alpine–Mediterranean region but is not limited to this region and is not everywhere manifested in a change from chert- to carbonate-dominated lithologies. Starting from the western North Atlantic (DSDP Site 534), greyish red calcareous claystone is replaced by nannofossil-radiolarian limestone and chalk at this level. In the western Tethys including the Alps, Carpathians, Apennines and Dinarides–Hellenides, bedded radiolarian chert (or highly siliceous Rosso ad Aptici limestone) is overlain by the Maiolica-type limestone. Eastwards in the Neotethys, the proportion of silica in the Upper Tithonian to Lower Cretaceous deposits increases significantly. From the Kocali Complex in eastern Turkey to the Yarlung-Zangbo Suture Zone in southern Tibet, the deep-water Jurassic and Cretaceous successions are lime-free radiolarites. Where the Jurassic–Cretaceous transition is well exposed and tectonically undisturbed, e.g. in the Zagros Mountains in Iran, siliceous claystone prevails in Upper Jurassic radiolarites but only pure bedded chert characterizes the Upper Tithonian–Lower Cretaceous interval. An identical Upper Tithonian–Lower Cretaceous radiolarite facies with no clay and no carbonate admixture occurs in the distal sections of the Hawasina Nappes in Oman; coeval sections of topographically higher areas of the same depositional basin are characterized by Maiolica-type micritic limestone. The sections in Oman are important for the comparison of different Upper Tithonian–Lower Cretaceous facies within a single basin but are less suitable to study vertical facies changes because a tectonically-induced stratigraphic gap occurs just below the Upper Tithonian deposits. If we continue further east from the Neotethys to the western Pacific (ODP Leg 129, Site 801) the Jurassic–Cretaceous sediments are also fully siliceous and the proportion of clay varies; the Lower Tithonian radiolarite contains a considerable amount of clay whereas in the Upper Tithonian the amount of clay is reduced. A clearly marked intra-Tithonian facies change is globally synchronous and can be traced from the Atlantic across the Alps and the Himalayas to the Pacific. It is reflected in a drastic increase of pelagic carbonate and in decrease of clay content. In deeper-water successions that are devoid of carbonate, the decrease of clay content is considerable and easy to recognize. Higher in the successions, no obvious facies change is recorded at the Jurassic–Cretaceous boundary. According to the so far established radiolarian zones (for a review on Mesozoic zonations see Gorican et al., 2018), the transition between the different facies corresponds to the boundary between two radiolarian zones, namely between the Unitary Association zones 12 and 13 of Baumgartner et al. (1995), and between the Pseudodictyomitra primitiva Zone and the Pseudodictyomitra carpatica Zone of Matsuoka (1995). The Jurassic–Cretaceous boundary is indistinct and lies within the Unitary Association Zone 13 and within the Pseudodictyomitra carpatica Zone. The intra-Tithonian boundary is thus more pronounced in facies change and in faunal break than the Jurassic–Cretaceous boundary as it is currently defined.