A study of deep-water deposition: Constraints on the sedimentation mechanics of slurry flows and high-concentration turbidity currents, and the facies architecture of a conglomeratic channel-overbank system.

摘要

The goal of this project is to use outcrop, petrographic, and geochemical data to constrain the sedimentation mechanics of subaqueous sediment-gravity flow deposits at a wide range of scales. The Cretaceous Juniper Ridge Conglomerate (Coalinga, California) provides a world-class exposure of a submarine channel to levee transition. Detailed measured sections, correlations, and oblique photomosaic mapping illuminate the facies architecture of this channel/overbank system and provide the basis of a detailed chronostratigraphic model. Exposures of the Pennsylvanian Jackfork Group (Arkansas) yield insights into the sedimentation of thick-bedded, structureless, mud-poor sandstones. Petrographic grain size was determined for 146 thin sections from detailed vertical sampling transects through mud-poor sandstone beds. These beds show normal, inverse, and no grading. Normal and inverse-graded beds can be distribution-, coarse-tail, or fine-tail graded, resulting in a classification of eleven grading styles. These grading styles can be explained by variation in near-bed concentration during the evolution of a sediment-gravity flow and provide detailed insight into the deposition of mud-poor sandstones. Petrographic sampling of mud-rich sandstones of the Jackfork Group, along with detailed measured sections and polished slabs, indicate that they were deposited by flows transitional between cohesionless (Newtonian) turbidity currents and cohesive (non-Newtonian) debris flows. These deposits are termed 'slurry beds,' and tend to show a statistically significant vertical sequence that includes, from base to top: heterogeneously structured mud-rich sandstone, mud-clast breccia, and mudstone. Conceptual dynamic models are proposed to explain the evolution of a typical slurry flow and the deposition of the mud-rich sandstone. Alternating light and dark bands comprise an enigmatic primary sedimentary structure type within mud-rich sandstones. Petrographic and geochemical analyses of bands from the Jackfork Group and the Cretaceous Britannia Formation (North Sea) show that dark bands are more clay-rich and finer-grained than light bands. Three models are proposed as interpretations of the sedimentation mechanics of bands: an autocyclic floc disaggregation model, a critical sediment fallout rate model, and a two-layer/roll wave model. The latter two models are preferred and suggest that band deposition occurs under unsteady, surging flow conditions near the base of a slurry flow.