Comparative Taphonomy and Sedimentology of Small-Scale Mixed Carbonate/Siliciclastic Cycles: Synopsis of Phanerozoic Examples

Small scale cycles deposited over 10–100 kyr are a common component of Phanerozoic shelfal deposits. A combination of detailed outcrop analysis and data-mining from published literature of cycles largely deposited in greenhouse regimes reveals a series of recurring sedimentological, paleoecological, and taphonomic motifs. In general, each cycle is composed of three to four components: (a) a basal skeleton-rich bed with evidence of condensation and, in some cases mineralization; (b) a medium-dark gray siliciclastic mudstone/shale interval; (c) a calcareous and/or silty mudstone interval with common concretionary, diagenetic overprint. A series of exemplars are highlighted from proximal and distal shelf settings and described using a depositional sequence approach. The cycles studied include examples deposited under greenhouse (Cambrian, Ordovician, Devonian, Jurassic and Cretaceous) and, for comparison purposes, icehouse (Neogene) conditions. The fact that repetitive patterns can characterize deposits that formed over a 500 million year interval is striking. The primary taphonomic moderator in these cycles is rate of sedimentation, which varies exponentially from sediment-starved concentrations to obrutionary deposits. The occurrence of a persistent motif over this time scale suggests that biological innovations, which might be expected to impact upon fossil preservation, have in fact been overprinted by the extremes of sedimentation preserved in these small-scale cycles. Having a skeleton, which is twice as resistant to abrasion, is of little import when sedimentation is dominated by the extremes: instant obrution or condensation.