6
J.N. Kellogg and W.U. Mohriak
nantly transgressive marine siliciclastic sequences in most basins, with the
exception of the Santos Basin where phenomenal episodes of massive clastic progradation resulted in overall regression in the Late Cretaceous, with
deposition of continental red beds on the present-day platform. The prograding sequences filled salt-evacuated troughs, particularly in the Cabo
Frio region of the Santos Basin (Mohriak et al. 1995). The massive clastic
progradation resulted in huge offlapping sequences, associated with a peculiar style of salt tectonics characterized by antithetic faults. The Tertiary is
characterized by progradation from the platform towards the deep-water
regions, in offlapping sequences, and also by local post-rift volcanic episodes that may occur both onland and in the offshore regions.
3 The Andean Margin
3.1 Geomorphology and Active Tectonics
The Cocos, Nazca, and Antarctic oceanic plates are converging with the
South American continental plate along the western (Andean) margin of
Central and South America at rates of 70 to 90 mm year- 1 (Fig. 2). The convergence has produced an unstable coastline with the strongest recorded
earthquakes of this century and the greatest relief anywhere in the world,
formed by the Andean mountain range (maximum height over 7,000 m),
which extends for over 8,000 km along the Pacific coast of South America,
and the Peru-Chile Trench (as deep as 8,000 m). East of the Trench, the continental slope rises steeply to a narrow 5- to 120-km-wide shelf. For the first
15 km east of the trench, the slope consists of accretionary complex
sediments stacked against the seaward edge of the continental crust. In
northern Peru the upper slope and shelf are underlain by up to 5,000 m of
Eocene and younger sediments unconformably overlying Mesozoic sedimentary and Paleozoic continental basement rocks. Off northern Chile
(23°S to 33°S) the continental slope steepens, with only 100 m of Tertiary
sediments overlying consolidated Mesozoic and Paleozoic rocks. The continental shelf is narrow or absent, and sediment along the trench axis is
sparse (von Huene 1989). In Peru and northern Chile, the coastal region is
a narrow desert belt with coastal plains, low hills, and marine terraces. The
coastal belt receives almost no rain, but is watered in the subsurface along
river courses by runoff from melting snows of the high Andes. As a result,
the continental shelf, slope, and trench are receiving relatively little sediment input at present. Off south-central Chile, sediment floods the trench
J.N. Kellogg and W.U. Mohriak
nantly transgressive marine siliciclastic sequences in most basins, with the
exception of the Santos Basin where phenomenal episodes of massive clastic progradation resulted in overall regression in the Late Cretaceous, with
deposition of continental red beds on the present-day platform. The prograding sequences filled salt-evacuated troughs, particularly in the Cabo
Frio region of the Santos Basin (Mohriak et al. 1995). The massive clastic
progradation resulted in huge offlapping sequences, associated with a peculiar style of salt tectonics characterized by antithetic faults. The Tertiary is
characterized by progradation from the platform towards the deep-water
regions, in offlapping sequences, and also by local post-rift volcanic episodes that may occur both onland and in the offshore regions.
3 The Andean Margin
3.1 Geomorphology and Active Tectonics
The Cocos, Nazca, and Antarctic oceanic plates are converging with the
South American continental plate along the western (Andean) margin of
Central and South America at rates of 70 to 90 mm year- 1 (Fig. 2). The convergence has produced an unstable coastline with the strongest recorded
earthquakes of this century and the greatest relief anywhere in the world,
formed by the Andean mountain range (maximum height over 7,000 m),
which extends for over 8,000 km along the Pacific coast of South America,
and the Peru-Chile Trench (as deep as 8,000 m). East of the Trench, the continental slope rises steeply to a narrow 5- to 120-km-wide shelf. For the first
15 km east of the trench, the slope consists of accretionary complex
sediments stacked against the seaward edge of the continental crust. In
northern Peru the upper slope and shelf are underlain by up to 5,000 m of
Eocene and younger sediments unconformably overlying Mesozoic sedimentary and Paleozoic continental basement rocks. Off northern Chile
(23°S to 33°S) the continental slope steepens, with only 100 m of Tertiary
sediments overlying consolidated Mesozoic and Paleozoic rocks. The continental shelf is narrow or absent, and sediment along the trench axis is
sparse (von Huene 1989). In Peru and northern Chile, the coastal region is
a narrow desert belt with coastal plains, low hills, and marine terraces. The
coastal belt receives almost no rain, but is watered in the subsurface along
river courses by runoff from melting snows of the high Andes. As a result,
the continental shelf, slope, and trench are receiving relatively little sediment input at present. Off south-central Chile, sediment floods the trench