Proposal of a reference section and point for the Cambrian Series 2–3 boundary in the Mediterranean subprovince in Murero (NE Spain) and its intercontinental correlation

Abstract

The classical lower-middle Cambrian boundary is approximately equivalent with the boundary of the Cambrian Series 2 and 3, which is now in the process of definition by the International Subcommission on Cambrian Stratigraphy. Currently, there are two oryctocephalid trilobite species first appearance data (FAD) that are suggested as possible markers of this level: Ovatoryctocara granulata Tchernysheva, 1962 and Oryctocephalus indicus (Reed, 1910), respectively. Until now neither of these two species has been recorded in the Mediterranean subprovince or Baltica. As a result, in these regions a level potentially correlating with either the FAD of Ovatoryctocara granulata or Oryctocephalus indicus has to be located using the available bio-, isotope-, and event-stratigraphy data.Carbon isotope signatures reveal global oceanic perturbations adjacent to the Cambrian Series 2 and Series 3 (former lower-middle Cambrian) boundary. This event, first called the Valdemiedes Event (it was defined in Murero, NE Spain; Liñán et al., 1993a), caused a noticeable extinction of trilobites, mostly olenellids and protolenids (which were replaced in some regions by paradoxidids), as well as an almost complete extermination of some other early Cambrian marine animals. Both isotope and palaeontological evidence show that the disappearance of olenellids in Laurentia, protolenids in Siberia and the appearance of the first Acadoparadoxides in Western Gondwana, Baltica, Avalonia, and Siberia, which was formerly thought to be a series of diachronous regional perturbations, were nearly contemporaneous events. Features of a global ecological crisis and faunal replacement being taken together with a pronounced global d 13 C org negative excursion would serve as the best indicator of the lower boundary of the Cambrian Series 3 and Stage 5 in the Mediterranean subprovince and is correlatable with the Ovatoryctocara granulata FAD. Another d 13 C org negative excursion followed by a positive shift occurs at a slightly younger level, which coincides with a transgression maximum and roughly is coeval with the Oryctocephalus indicus FAD, also proposed as a possible base of the Cambrian Series 3.

Comment on: Álvaro, J. J., Esteve, J. & Zamora, S. 2019. Morphological assessment of the earliest paradoxidid trilobites (Cambrian Series 3) from Morocco and Spain [Geological Magazine]

“…Most probably, A. ovatopyge is the species described from Spain as Hydrocephalus cf. harlani (Green, 1834) by Dies et al (2004), Liñán et al (2008) and Gozalo et al (2013).…”

“…Lotze (1961) and Sdzuy (1961) indicated that the interval A7-A15 1702 G Geyer et al in the Rambla de Valdemiedes 1 section includes nine different fossiliferous levels in the 17þ m succession, and subsequent studies indicate structural complications as well (e.g. Liñán & Gozalo, 1986;Gozalo et al 2013). The two cranidia of Sdzuy (1958) are preserved in sandstone with a somewhat different lithology and obviously do not come from the same bed.…”

A blind trilobite with Baltic affinities from Cambrian Series 3 of the Iberian Chains, Spain, and its stratigraphical and palaeobiogeographical significance

“…The Badulesia tenera Zone has been correlated with the lower Ptychagnostus gibbus Zone (Sdzuy 1971(Sdzuy , 1972Sdzuy et al 1999;Fletcher 2005Fletcher , 2006Gozalo et al 2007Gozalo et al , 2013. This correlation is supported by the presence of Bailiaspis tuberculata Lake, 1940 in the Badulesia tenera Zone of the Iberian Chains and the P. gibbus Zone of England (Chirivella Martorell et al 2014).…”

The oryctocephalid trilobite zonation across the Cambrian Series 2-Series 3 boundary at Balang, South China: a reappraisal

“…The Mediterranean area lacks unfortunately cosmopolitan trilobites and correlation needs to be based on other trilobites (see Gozalo et al . , ). Very few oryctocephalids have been found.…”