Trans-Avalonian green–black boundary (early Middle Cambrian): transform fault-driven epeirogeny and onset of 26 m.y. of shallow-marine, black mudstone in Avalonia (Rhode Island–Belgium) and Baltica

Abstract

The Avalonia microcontinent has diagnostic terminal Ediacaran–Ordovician lithostratigraphy, depositional sequence architecture, and igneous activity that extends for 2000+ km reflecting epeirogeny related to the Avalonian transform fault. Avalonia records an abrupt early Middle Cambrian (late Wuliuan) change from green, purple, or light grey to overlying black, dark grey, and brown facies in platform and off-platform areas (Meguma, North Wales). This change within one trilobite zone marks onset of ca. 26 m.y. of shallow-marine anoxia/strong dysoxia lasting into the Ordovician with Hatch Hill oxygenated mid-water zone (OMZ) onlap onto the shelf. A Bakken model (new, based on the middle Paleozoic Bakken Formation) is applied to shallow-shelf–shoreline organic-rich mud deposition. Erosion of greenish Avalonian depositional sequence (Ads) 7 was followed by Ads 8 tilting, volcanism, debris flows, and bentonite deposition on a cryptic unconformity in SE Newfoundland. The early Middle Cambrian age of the Ads 7–8 boundary is obscured by referring the lower Manuels River Formation and Cristallinium cambriense Zone to the younger Drumian Stage. Ads 8 has thin ashes in coterminous British and North American Avalonia where erosion and subaerial exposure with caliche development preceded onlap of upper Middle or Upper Cambrian Ads 9 black muds and sands. The green–black change emphasizes Avalonian unity; it precludes multiple Avalonian “micro-terranes” or assigning parts of Avalonia to West Gondwana or “Ganderia” (the Little River, Brookville, and Bras d'Or “terranes” are part of the Avalonian marginal platform). Coeval green–black transitions and similar later Cambrian faunas show comparable paleoenvironments in Avalonia and Baltica.