“OLD FASHIONED” STRATIGRAPHY: THE TEST OF PALEOGEOGRAPHIC RECONSTRUCTIONS OF AVALONIA, “GANDERIA,” AND CADOMIA

Reports on the Avalonia terrane in the Acadian‒Caledonian (A‒C) mountain belt often feature 1) an homage to a complex late Proterozoic basement and siliciclastic-dominated terminal Edicaran‒Ordovician platform cover sequence (E‒O cs) with distinctive macrobiotas; 2) sophisticated zircon/stable isotope work as a prelude to geologic history conclusions; and 3) a disregard of the remarkably consistent E‒O cs along Avalonia. Sedimentary successions reflect unique interactions of factors (i.e., sediment sources, epeirogeny, eustasy, climate), but relevance of the E‒O cs to Avalonia’s geologic history is even dismissed in reports as somehow appearing on separate, later-amalgamated terranes. Emphasizing differences in basement geology has led to naming of “terranes” even within Avalonia. As we documented elsewhere, local E‒O cs reflect Avalonian transtensional history. The NE belt (marginal platform) has a ca. 15 Ma basal succession (terminal Ediacaran rift‒overlying siliciclastic shelf‒middle Early Cambrian quartz tidalite) capped by a 10‒15 Ma hiatus and late Early Cambrian siliciclastics (S. Wales; Burin, SE Nfld; Saint John‒Long Reach‒Beaver Harbour, N. B.). In SE areas (inner platform), the tidalite is unconformably overlain by older sub-trilobitic mudstone, then a 0.5‒6 m terminal subtrilobitic limestone, and unconformably overlying trilobitic mudstone (England; Avalon‒Burin, Nfld; Mira Allochthon and Doctor’s Brook authochthon, N. S.; Cradle Brook, N. B.; eastern MA). Local marginal platform developments include elevated blocks and/or Early‒Middle Cambrian boundary volcanism (Bourinot belt, Cape Breton; Malignant Cove autochthon, N. S.; Beaver Harbour, N. B.). E‒O cs provide little information on provenance of Proterozoic and older basement rocks, but indicate: 1) the Caledonia, Brookville, most of the (composite) New River, the Mira, and Bourinot (of the SE Bras d’Or area) “terranes” are simply Avalonian, no evidence supports assignment of the Brookville and SE New River blocks to “Ganderia;” 2) E‒O cs distinct lithology and biotas through the Cambrian’s first 20 Ma require Avalonia‒tropical Gondwana (Cadomia) separation by the terminal Ediacaran, not Ordovician, 3) E‒O cs lithologic differences with “Ganderia” successions (particularly Gander area) and Laurentian-aspect trilobites (“Ehmaniella”) in “Ganderian” Nfld and Columbia are problems for a “unified” Cambrian “Ganderia.”