Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
AAPG Annual Convention, Denver, Colorado, June 36, 2001
Translating Down a Basement Ramp in the
Martin P. A. Jackson,1 Michael R. Hudec,1 Res Fraenkl,2 Wytse Sikkema,2 Leonor F. Binga,3 and Joaquim C. Da Silva3
The 25-to-85-km-wide Monocline Province is located on the Angolan continental slope. It formed by basinward translation of cover across basement steps strongly reactivated by Miocene coastal uplift. While sliding basinward, Aptian salt and its overburden were draped as a gentle monocline above the basement steps (Atlantic Hinge Zone). One or more bathymetric scarps typically form at the upper hinge of the monocline. Strata onlapping these scarps were translated progressively seaward down the monocline, trailing a discontinuous onlap surface that climbs landward to the present-day scarp. The onlap surface becomes younger landward, typically in discrete jumps coinciding with small anticlines or diapirs.
The downdip width of the onlap surfaces across the Monocline Province records the magnitude of translation of cover over the basement step. Downdip translation since the Miocene varies along strike between 18 and 28 km. Small anticlines and diapirs were translated down the monocline as if on a conveyor belt. The basinward flanks of the anticlines and diapirs are onlapped, whereas landward flanks are generally concordant. This onlap asymmetry reflects their former position at the top of the monocline during onlap against the scarp. Where translation was impeded above basement steps, the cover shortened locally, squeezing and reactivating diapirs and tightening small folds.
Three types of onlap minibasin were also translated. (1) Large, landward-tilted onlap basins above the monocline limb formed by translation across the basement steps. (2) Small dish onlap basins between reactivated diapirs and small anticlines formed by shortening above basement steps. (3) Basinward-thinning wedge onlap basins formed landward of the monocline.
2Shell E&P Angola; Rijswijk, The Netherlands.
3Sonangol, Luanda, Angola.