PROTRACTED DEVELOPMENT OF THE MIXED LAYER: IMPLICATIONS FOR SUBSTRATE EVOLUTION AND EXCEPTIONAL PRESERVATION IN EARLY PALEOZOIC MARINE SHELFAL ENVIRONMENTS
Data were collected from units characterized by heterolithic facies considered representative of marine shelfal settings and binned into three temporal intervals: 1) lower–middle Cambrian, 2) upper Cambrian–middle Ordovician and 3) upper Ordovician–middle to upper Silurian. A variety of metrics, including ichnofabric index, bed thickness, depth of bioturbation and trace fossil taphonomy and paleoecology were employed to track the global pace of mixed layer development. Bedding planes from all intervals are characterized by dense, exceptionally preserved trace fossil assemblages. Mean ichnofabric index (ii 2; ii 2; ii 3) and bed thickness (1.16 cm; 3.30 cm; 4.31 cm) values for each interval indicate that although infaunalization was well-advanced in marine shelfal settings by the early Cambrian, mixed layer development was a protracted process. Ichnological data documenting global-scale suppression of mixing challenge current assumptions that mixing occurred with the first appearance of three-dimensional burrows and hold important implications for the advent of modern-style biogeochemical cycling.