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P. E. Isaacson
E. Díaz Martínez
Department of Geology
University of Idaho
Moscow, Idaho, U.S.A.
Abstract
Devonian-Permian
data of western Bolivia and adjacent regions are used to construct a paleogeography
of the central Andes. Four phases characterize the sedimentation history.
(1) Shallow marine clastic deposition occurred through the Devonian (Lochkovian-Frasnian),
with an increase in sedimentation in Emsian-Eifelian time. Lithofacies
distribution and sediment thicknesses indicate primarily a western source.
Endemic, high-latitude (>55° S) fauna with several megafaunal originations
in Bolivia also contain organisms characteristic of North Africa and northeastern
United States (Middle Devonian megafaunas and Late Devonian palynomorphs).
(2) Latest Devonian-Early Carboniferous (Famennian-Viséan) sedimentation
is characterized by glaciomarine and fan-deltaic sedimentation. Clasts
are derived from underlying sedimentary units and andesitic, granitic,
and tuffaceous rocks. (3) A middle Carboniferous (Serpukhovian-Bashkirian)
hiatus in sedimentation occurred, its age and duration varying across the
region. (4) Siliciclastic and carbonate deposition occurred in Late Carboniferous-middle
Permian time (Moscovian?-Artinskian). The clastics were derived from a
western source. Carbonate rocks (Copacabana Formation) were deposited
in
situ, in low latitudes ( 25°
S lat). Devonian sedimentation is inferred to have occurred on continental
crust in a foreland setting, with a western magmatic arc. Restoration of
the San Nicolás batholiths (U-Pb zircon, 425 and 394-388 Ma) relative
to the Devonian basins suggests that they may have constituted the magmatic
arc. Intra-arc basins may have existed near present-day coastal Peru. Following
the middle Carboniferous hiatus, sedimentation continued in a back-arc
region, although differentiation of distinct Carboniferous and Permian
basins, the intrusion of plutons inboard of and within the basin along
strike, and extensional faulting in the Late Permian indicate major changes
in the tectonic setting, possibly including a reorientation of the subducting
slab to a low angle. Uncertainties in the tectonic setting interpretation
are introduced by the incomplete stratigraphic record, which is obscured
in the Altiplano and Cordillera Occidental, and by the undefined history
of plate boundary interactions, such as possible postdepositional strike-slip
motion and tectonic erosion along the plate margin. |
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