Interpreting the Neoproterozoic Glacial Record: The Importance of Tectonics

Abstract

The breakup of Rodinia between about 750 and 610 Ma created rifted margin basins along opposing paleo-Atlantic and paleo-Pacific margins of Laurentia. Sedimentary processes and stratigraphic successions were strongly controlled by tectonic subsidence and sediment supply. This relationship creates systematic changes in relative water depths during the rift cycle giving rise to a distinct fill stratigraphy. Lowermost synrift facies successions are dominated by coarse grained sediment, derived from faults and tectonically-uplifted topography, deposited by sediment gravity flows. Poorly-sorted debrites (diamictites) interbedded with sediment gravity flows such as turbidites, are a common component. These facies are often mistaken for poorly-sorted ice sheet deposits (tillites). Rift-related diamictites are generated regardless of paleolatitude requiring independent evidence of glaciation. Thick shales (principally slope turbidites) that overlie diamict-bearing synrift facies are widely regarded as recording postglacial 'glacioeustatic' flooding but instead record rapid thermal subsidence. The distribution of glacial and glacially-influenced marine strata on a simplified paleogeographic reconstruction of Rodinia, indicates that ice was widespread but regional in scope and likely related to strong tectonic uplift on the rifted margins of Laurentia. The Snowball Earth model draws heavily on fluctuations in carbon isotopes as a record of global glaciations that shut down hydrologic and biologic activity. Sedimentary and biological records instead indicate open marine conditions with a weak glacial influence and an uninterrupted hydrosphere and biosphere. Neoproterozoic glaciation was likely widespread but regional in extent, strongly controlled by tectonics and diachronous in its timing as Rodinia progressively broke apart.