Coastal
uplift and erosional unroofing of passive margins renews gravity
spreading and rejuvenates supply of sediments to distal offshore
regions, where they form reservoir facies and enhance hydrocarbon
maturation. We constrain the timing and magnitude of uplift in the
Inner Kwanza Basin, Angola, by combining an analysis of Tertiary
hiatuses with new results from apatite fission-track analysis. Planktonic
foraminiferal biozones in wells from the north of the basin show
five major hiatuses: lower Oligocene (34 Ma), upper Oligocene (29
Ma), lower Miocene (16 Ma), mid-Miocene (12 Ma), and Plio-Pleistocene.
Between hiatuses all Oligo-Miocene strata were marine, which refutes
the idea of a massively elevated (1-2 km) "altiplano"
coastal plateau in the mid-Tertiary. Fission-track results suggest
three regional synchronous thermal events: (1) rift-related extensional
and volcanic event at ~150 Ma, (2) a contractional and volcanic
event at 100-70 Ma, and (3) a mild advective event at 20-10 Ma during
uplift of the continental shelf, which renewed gravity spreading
in the Outer Kwanza Basin.
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More
complex histories are also possible. During rifting, the adjoining
basement was uplifted by >3700 m. Cumulative uplift of the basement
since the mid-Jurassic is >7000 m. Along its eastern rim, the basin
was unroofed by a total of up to 1200 m. In some southern regions,
the Inner Kwanza Basin was unroofed by 500-800 m. In contrast, total
uplift in the center and west of the northern Inner Kwanza Basin was
only a few hundred meters at most. Fission-track results suggest ~15°C
of Neogene heating by rising hot fluids, which enhanced hydrocarbon
maturity. |