Transgressive coastal systems (2nd part): geometric principles of stratal preservation on gently sloping continental shelves

Abstract

This study focuses on the causes and mechanisms of coastal-lithosome preservation during transgressions driven by roll-over processes of barrier migration. Using the Shoreface Translation Model, a large range of idealised coastal settings was simulated to identify the environmental conditions of stratal preservation. Preservation occurs within two broad categories of experimental conditions. The first category relates to transgressive phases evolving under relatively constant conditions in which stratal preservation takes place only if the coastal barrier experiences positive net sediment supplies. The resulting deposits show tabular geometries, have poorly differentiated internal architectures and tend to extend continuously with quite uniform thickness upslope across plain regions of the shelf. In the second category, by comparison, deposits are thicker and stratal preservation is more localised. Moreover preservation occurs as an adaptive morpho-kinematic response to environmental perturbations due to variations in: (1) the ratio of sediment supply (Vs) to accommodation generated by sea-level rise (SLR); (2) the substrate topography; (3) the morphology of the barrier profile. More specifically, changes of the ratio Vs /SLR, where SLR is an approximate surrogate for added accommodation space, directly promotes growth of the barrier (Vs /SLR >> 0) and its subsequent drowning (Vs /SLR➝0). The topographic variations of the substrate may include minor irregularities as well as sudden changes in gradient that afford other types of preservation, such as local fills and residual littoral packages. Finally, barrier-profile changes inducing stratal preservation may include the reduction in barrier width and depth of surf base as well as the increment in shoreface concavity and shoreface length. Simplified methods are given for relating the geometry of preserved deposits to rates of sea-level rise and sediment supply over different shelf slopes, and for identifying the position of the shoreline at specific times. Holocene evolution of some coastal deposits from the Tuscan shelf (Italy) is presented in a morpho-kinematic reconstruction to illustrate the geometric relationships for stratal preservation.