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 We examined the palaeogeographical distribution of selected Cambrian trace fossils. Astropolichnus hispanicus , Climactichnites , Syringomorpha nilssoni and early examples of Paleodictyon all have a restricted palaeogeographical distribution, probably representing that of their producers. A cosmopolitan distribution is seen in Didymaulichnus miettensis and in early examples of Rusophycus and Dactyloidites . Oldhamia shows a wide distribution in Lower Cambrian deep-sea sediments although that of Oldhamia geniculata is restricted.

Abstract Earliest Cambrian fine-grained sediments appear to have been firm close to the sediment-water interface. As a result there was a high preservational potential of shallow tiers. There is limited evidence for a mixed layer at this time; rather, most of the preserved trace fossils were open burrows. Later in the Cambrian, depth of sediment mixing increased but firmground conditions are still found relatively close to the sediment-water interface. The development of the mixed layer and properties of Cambrian muddy sediments have numerous stratigraphic and ichnological consequences. These include secular trends in the preservation of event beds and shallow-tier trace fossils including Rusophycus and Cruziana .