Large-scale connectivity of fluvio-deltaic stratigraphy

Abstract

Multiscale simulation of fluvio-deltaic stratigraphy was used to quantify the elements of the geometry and architectural arrangement of sub-seismic-scale fluvial-to-shelf sedimentary segments. We conducted numerical experiments of fluvio-deltaic system evolution by simulating the accommodation-to-sediment-supply (A/S) cycles of varying wavelength and amplitude with the objective to produce synthetic 3-D stratigraphic records. Post-processing routines were developed in order to investigate delta lobe architecture in relation to channel-network evolution throughout A/S cycles, estimate net sediment accumulation rates in 3-D space, and extract chronostratigraphically constrained lithosomes (or chronosomes) to quantify large-scale connectivity, that is, the spatial distribution of high net-to-gross lithologies. Chronosomes formed under the conditions of channel-belt aggradation are separated by laterally continuous abandonment surfaces associated with major avulsions and delta-lobe switches. Chronosomes corresponding to periods in which sea level drops below the inherited shelf break, that is, the youngest portions of the late falling stage systems tract (FSST), form in the virtual absence of major avulsions, owing to the incision in their upstream parts, and thus display purely degradational architecture. Detailed investigation of chronosomes within the late FSST showed that their spatial continuity may be disrupted by higher-frequency A/S cycles to produce “stranded” sand-rich bodies encased in shales. Chronosomes formed during early and late falling stage (FSST) demonstrate the highest large-scale connectivity in their proximal and distal areas, respectively. Lower-amplitude base level changes, representative of greenhouse periods during which the shelf break is not exposed, increase the magnitude of delta-lobe switching and favour the development of system-wide abandonment surfaces, whose expression in real-world stratigraphy is likely to reflect the intertwined effects of high-frequency allogenic forcing and differential subsidence.