Abstract Avalonian sections in the Saint John area, southern New Brunswick, have long contributed to global understanding of Cambrian chronostratigraphy. A tuffaceous bed in the Ratcliffe Brook Formation (RBF) in the Somerset Street section dated at c. 531 Ma has traditionally been considered to post-date small shelly fossils attributed to the Watsonella crosbyi Zone in the Hanford Brook section. A fine-grained tuffaceous bed approximately 8 m stratigraphically lower in the Somerset Street section yields a chemical abrasion isotope dilution–thermal ionization mass spectrometry zircon age of 532.3 ± 0.3 Ma; a tuffaceous carbonate unit in the lower RBF in Hanford Brook gives an age of 531.5 ± 0.3 Ma. Crystal and crystal-lithic tuff beds near the top of the RBF yield ages of 520.3 ± 0.3 Ma (in Hanford Brook) and 519.1 ± 0.3 Ma (in Ratcliffe Brook). The new ages confirm the correlation between the Somerset Street and Hanford Brook sections based on acritarchs and make the association of small shelly fossils in the Hanford Brook section younger than 531 Ma. This result is relevant to ongoing discussions on the age of the base of undefined Cambrian Stage 2. The radiometric ages also support a young age for the upper part of the RBF, perhaps extending into Epoch 2.

Abstract The Iberian Peninsula has some of the most extensive Cambrian outcrops in Europe ( Lotze 1961 c ), including a diverse, continuous record of fossils and facies, and is thus a fundamental source of biostratigraphic information for the Cambrian System and its intercontinental correlations. Most exposures of Iberian Cambrian rocks occur in the Iberian Massif, but they are also known from the Pyrenees, the Catalonian Coastal Ranges and the Iberian Ranges (Fig. 3.1 ). Many exposures are geographically isolated and/or show tectonic boundaries, and facies changes are common, and these characteristics have led to a profuse stratigraphic nomenclature (see Fig. 3.2 ; Zamarreño 1983; Liñán et al . 1993 a ). Following Lotze (1961 c ), however, the Cambrian sequence can be overviewed as a diachronous Lower to Middle Cambrian carbonate sequence sandwiched by silici-clastic successions (Fig. 3.2 ). The lower of the siliciclastic units is entirely Lower Cambrian, whereas the upper unit ranges from upper Lower or Middle Cambrian to Upper Cambrian (Fig. 3.2 ). The Lower Cambrian series has been subdivided into the Corduban, Ovetian, Marianian and Bilbilian stages, and the Middle Cambrian series subdivided into the Leonian, Caesaraugustan and Languedocian stages (Fig. 3.2 ). The Precambrian/Cambrian boundary stratotype was erected by the International Subcommission on Cambrian Stratigraphy (ISCS) at the Fortune Head section in eastern Newfoundland (Canada) with the first appearance datum (FAD) of Phycodes (= Trichophycus) pedum ( Landing 1994 ). This FAD coincides with behavioural changes, increased