Transform margin source–sink clastic deposystems

Abstract

Abstract Shelf to basin floor clastic-sediment-routing models for individual structural parts of a transform margin are presented. Using 3D and 2D seismic, gravity and well data from the Guyana, Coromandal, Romanche, Saint Paul and Zenith–Wallaby Perth margins as examples, and drawing from a wide range of analogues, four discrete tectonic segments are recognized as characteristic of all transform margins: (1) the transform margin sensu stricto ; (2) the local pull-apart segment on the transform margin; (3) the margin segment developed from the narrow horsetail splaying off from the transform; and (4) the margin segment developed from the extensional end of the horsetail structure, although there are other transitional variants specific to each margin. The routing of clastic sediments varies markedly across these segments. The shelfal staging area varies in width between the segments, increasing in width between segments 1 and 4, with wider shelves (10–25 km) characterized by significant sediment sorting through longshore drift, and narrower segments (0–10 km) often fed directly from focused fluvio-deltaic systems. The overall deep-water slope gradient decreases from segments (1) to (4), with sediment runout distance from shelf to basin floor increasing. The seafloor topography along the margin is highly variable, with significant sand sequestration on slopes with intra-slope ramps and terraces found in segments (2)–(4), with rapid downslope transitions from tributary slope gullies to slope channel complexes and then ponded intra-slope frontal splay complexes on the intra-slope terraces. Mass-transport deposits (MTDs) are ubiquitous and found as either regional slope MTDs or local collapses of slope channel complex margins. Stratigraphic and/or combination traps are associated with the ramp–terrace transitions. Local sectors show pinned long-lived intra-slope channel complex and canyon development, with prominent long-lived intra-slope stacking of intra-slope sandbodies. The outboard basin-floor- or trough-confined deep-water frontal splays (lobes), and their head area erosion by slope-originated MTDs, are directly controlled by the deposystems of the inboard slope systems. The lateral and vertical evolution of deep-water slope architecture is directly controlled by the inherited seafloor topography, set up by the underpinning crustal fabric across the four segments.