Strato-structural evolution of the deep-water Orange Basin: constraints from 3D reflection seismic data
Authors: Maduna Nombuso G. , Manzi Musa S. D. , Jinnah Zubair , Bourdeau Julie E. 
Published: November 2022

Abstract. Deep-water fold-and-thrust belt (DWFTB) systems are
gravity-driven collapse structures often found in passive margin settings
and are comprised of a linked up-dip extensional domain, central
transitional/translational domain, and down-dip compressional domain. Many
Late Cretaceous DWFTB systems occur along the SW African passive margin with
multiple, over-pressurized, seaward-dipping shale detachment surfaces
accommodating gravitational slip. In this study we use 3D reflection seismic
data to constrain the strato-structural evolution of the translational and
compressional domains of a Late Cretaceous DWFTB system and the overlying
Cenozoic deposits in the Orange Basin, South Africa. The stratigraphy and
structure of the Late Cretaceous DWFTB system is shown to have controlled
fundamental sedimentary processes and the stability of the evolving margin.
The compressional domain exhibits large-scale landward-dipping DWFTBs with
thrust faults detaching the main Turonian shale detachment surface at depth
and terminating at the early Campanian surface. A major ∼ 7 km
wide seafloor slump scar reflecting margin instability occurs directly above
a syncline of the same width from the buried DWFTB system's compressional
domain. The translational domain is imaged as a complex region displaying
overprinted features of both extensional and compressional tectonics with
the downslope translation of sediment comprising listric normal and then thrust
and oblique-slip faults distally. Thrust sheets are segmented along strike
by extensive oblique-slip faults which extend from the translational domain
into the down-dip compressional domain. Smaller, localized fold-and-thrust
belts are found directly below the kilometre-scale DWFTB system in the
down-dip compressional domain detaching a lower, Albian shale detachment
surface which corresponds to an older gravitational collapse. The upward
propagation of normal and oblique-slip faults with progressive sedimentation
is hindered by the Oligocene or Miocene stratigraphic markers corresponding
to mass erosional processes in the Cenozoic. A large (∼ 2.3 km
wide), roughly slope-perpendicular Oligocene submarine canyon formed by
turbidity currents is attributed to a major sea-level fall at
∼ 30 Ma. Oceanographic circulation is shown to have held a
significant control on the deposition of mid-Miocene to present-day
sedimentary sequences. Between 1200 to 1500 m water depths along the upper
continental slope well-preserved extensive slope-parallel, sinusoidal
channel-like features occur on the Miocene stratigraphic marker. The
channels are confined within a ∼ 14 km wide zone at the
interface of the upper northward-flowing Antarctic Intermediate Water (AAIW)
and deeper southward-flowing North Atlantic Deep Water (NADW) currents. The
erosive interaction of these oppositely flowing bottom currents combined with
the effects of the Benguela Upwelling System (BUS), all of which formed or
intensified at ∼ 11 Ma, are responsible for the creation and
preservation of the extensive slope-parallel channels. This study shows the
difference in structural styles of the translational and compressional
domains of a Late Cretaceous DWFTB system and the processes responsible for
mass-scale erosion in the Cenozoic.