Stratigraphic record of translation down ramps in a passive-margin salt detachment

Abstract

Ramp syncline basins filled with synkinematic sediments record the incremental translation history and translation rates of strata transported over basement ramps. On passive margins, movement of strata over a stratabound salt detachment is stratigraphically recorded as an isopach thick on the seaward side of a monoclinal ramp. This growth syncline gradually moves seaward down and then off the monocline, as if on a conveyor belt. This shift creates accommodation space for a new isopach thick to form just landward of the older one. Repetition of this cycle creates a shingled series of isopach thicks within the ramp syncline basin. Kinematic forward models and restored cross sections from the Kwanza Basin, Angola, show that the stratigraphic pattern of ramp syncline basins is most influenced by three factors. (1) The relative rates of translation and aggradation control the curvature of the axial trace of the growth synclines. (2) Bathymetric scarps above basement ramps can create spectacular seaward-dipping onlap surfaces >30 km long downdip; adjoining ramps generate stacked, interfering onlap surfaces. (3) Salt diapirs or anticlines are commonly shortened at the top of the ramp, especially where this coincides with the base of the continental slope. Diapir shortening provides a buffer to absorb sliding from its landward side and impede sliding on its seaward side. This buffering can have a major control on the entire gravity-spreading system containing the diapirs.