From Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin
(www.beg.utexas.edu).
For more information, please contact the author.

Houston Geological Society and Petroleum Exploration Society of Great Britain Second International Symposium
"Africa: New Plays—New Perspectives"
Houston, 3–4 September 2003

Structural Restoration of Minibasins Translating Down a Basement Ramp in the Deepwater Kwanza Basin, Angola

Martin P. A. Jackson and Michael R. Hudec

The 25-to-85-km-wide Monocline Province is located on the lower continental slope of the Kwanza Basin. The significance of this province lies in its accurate record of basinward translation of the cover during gravity spreading. The timing and amount of translation strongly constrain estimates of updip extension and downdip shortening in the basin as a whole.

While sliding basinward, thin Aptian salt and its overburden were draped as a broad, gentle monocline above several basement ramps (the largest of which is the Atlantic Hinge Zone). Bathymetric paleoscarps formed in the upper hinge of the monocline above the basement ramps. Strata onlapping these paleoscarps were buried and translated progressively seaward down the monocline, as if on a conveyor belt lubricated by salt. The downdip width of the onlap surfaces across the Monocline Province records the magnitude of translation of cover over the basement ramp. Translation since the Miocene ranges along strike from 18 to 28 km.

A wide variety of stratal patterns formed during translation. Structural restoration and forward modeling show that the geometry was affected by the sedimentation rate, translation rate, number and geometry of ramps, and position of salt structures. In its simplest form, the onlap surface gradually climbs landward through progressively younger strata until surfacing at the modern scarp. Typically, the pattern is more complex. Periods of faster sedimentation buried the paleoscarp; this temporarily ended onlap and created discrete jumps in the landward climb of the onlap surface. Where two adjoining basement ramps are present, each created its own onlap surface, so that two onlap surfaces were obliquely stacked. Small anticlines were also translated down the monocline on the tectonic "conveyor belt." The basinward limbs of the anticlines were onlapped, whereas their landward flanks are generally concordant to overlying strata. This onlap asymmetry reflects their former position at the top of the monocline. Diapirs were translated seaward along with the rest of the encasing cover. Their transport was temporarily impeded above basement ramps. There the diapirs were squeezed by continued translation of cover on their landward side. Impedance caused the onlap surface to jump abruptly to younger strata as the onlap surface crossed the diapir.

Three types of onlap minibasins were translated seaward. (1) Large, landward-tilted onlap basins formed seaward of the scarp above the monocline limb by translation across the basement ramps. (2) Small dish onlap basins between reactivated diapirs and small anticlines formed by shortening above basement ramps. (3) Basinward-thinning wedge onlap basins formed landward of the monocline.