Unravelling the Neoproterozoic oceanic sedimentary record: Insights from the Mona Complex Ocean Plate Stratigraphy, Wales

Abstract

Ocean Plate Stratigraphy (OPS) represents a sequence of volcanic and sedimentary rocks of the uppermost part of an oceanic plate typically preserved in tectonic stacks of an accretionary wedge. While the characteristics of Phanerozoic OPS are generally well-known, those of Neoproterozoic OPS, especially their sedimentary components, remain underexplored. We combined field observations with petrography and geochemistry (major and trace elements, Sr–Nd isotopes) of sedimentary rocks from the Gwna Group (Mona Complex, Wales, UK) representing a well-preserved example of Neoproterozoic OPS. The rocks were subdivided into five groups: (1) red chert/jasper, (2) carbonate/dolostone, (3) black mudstone, (4) red/green mudstone, and (5) turbiditic/volcaniclastic rocks. The carbonate rock and red chert fill interpillow spaces or form less than 2-meter-thick layers above a basaltic base. These rocks formed as chemical precipitates from hydrothermal fluid (ε_Nd = −2.4 to −1.2) or hydrothermally fluxed seawater (ε_Nd = −8.3 to −7.4) related to magmatic activity on the seafloor. Unlike Phanerozoic OPS, the studied OPS does not include a thick bedded chert sequence resulting from the accumulation of plankton remains. The red, green, and black mudstones were deposited on the ocean floor in a pelagic or hemipelagic environment under oxygenated and anoxic conditions. They were predominantly sourced from distal mature continental crust as exemplified by their low ε_Nd values (−11.3 to −3.5). In contrast, the turbiditic and volcaniclastic rocks (ε_Nd = −1.9 to +0.8) that form most of the Gwna Group were sourced from an adjacent volcanic arc and deposited in a trench. We show that the OPS lithological and compositional changes relate to Neoproterozoic plate motion and increasing proximity to a subduction zone. The OPS of the Gwna Group serves as a crucial paleooceanographic and paleogeographic indicator in the Neoproterozoic.