Substrate-Debrite-Turbidite Relationships: A Mechanical Boundary Layer Approach to Understand Emplacement Processes and Facies

Butler, Robert W.H.¹, Joris Eggenhuisen¹, Adriana Del Pino Sanchez¹, William McCaffrey¹ (1) University of Leeds, Leeds, United Kingdom

Many submarine sandstones show evidence for significant shear associated with their emplacement – for example strung-out dewatering features, strongly modified depositional bed-forms, asymmetric flames and other injection features. The syn-depositional timing can be established by identifying small-scale stratal growth above the shear structures. Traditionally composite beds, especially sand-debrite couplets and intermediate slurry facies are interpreted as the products of complex gravity flows that segregate their contents during transport. However many outcrop examples illustrate that the slurry and debrite facies pass laterally into intact substrate. Thus these parts of the deposit have not travelled far. Deformation fabrics commonly imply shear parallel to the directions deduced from conventional palaeoflow indicators (e.g. flutes and tool marks). Using workflows from structural geology, these facies and their fabrics can be explained in terms of evolving patterns of strain localization. A mechanical boundary layer description is developed that relates sediment aggradation to deformation partitioning between the gravity flow and its instantaneous substrate. This partitioning behavior can be cyclic during the depositional life of a single flow, building complex layered facies associations. In some situations the vertical sequencing of fabrics and sub-facies does not reflect the temporal sequence in which they developed but rather the localization of the evolution of shear partitioning within the mechanical boundary layer. Deformation partitioning between a gravity flow and its substrate will reflect both the flow dynamics and substrate rheology. The implications for interpreting flow dynamics and the consequent distribution of sandstone facies across basins are developed. Examples will be presented from outcrop and core.