Brian A. Hampton1,
Brian K. Horton1
(1) Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA
Abstract: Late Eocene-Oligocene fluvial deposystems in the north-central Altiplano plateau, Corque syncline, Bolivia
The Corque syncline of the north-central Altiplano plateau contains the thickest exposure (~6000 m) of late Eocene-Oligocene non-marine strata in the Bolivian Andes. Measured stratigraphic sections, facies analyses, and paleocurrent determinations indicate that the Huayllamarca Formation (Potoco Formation equivalent) records deposition in ENE-flowing fluvial systems derived from a long-lived, western sediment source (present day Western Cordillera). The Huayllamarca Formation coarsens upward and contains four distinct lithofacies associations. (1) A lacustrine (playa lake) facies characterized by thin-bedded (<20 cm) finely laminated siltstone, fine sandstone and gypsum. (2) A floodplain facies containing thin (20-30 cm) horizontally laminated tabular mudstones and fine sandstones with minor bioturbation. (3) A sand-dominated channel system exhibiting width-to-depth ratios of 50-150 m x 0.3-3 m and low erosional relief on basal scour surfaces of 10-35 cm. Channel fill consists of laterally accreted trough-cross beds (0.5-1 m) and ripple cross-stratified and horizontally laminated sandstone that had undergone soft-sediment deformation. Mudstone intraclasts are present at the base of trough-cross beds as collapsed bank (levee) deposits. Crevasse splay events are represented by tabular deposits that coarsen upward from mudstone to laminated fine sandstone. Periods of non-deposition in overbank deposits are characterized by thin calcareous nodular paleosols within tabular beds of mudstone and sandstone. (4) A braided channel system characterized by multistorey channels (1-3 m thick) filled with trough cross-bedded sandstone. The significant stratigraphic thickness and limited basal scour along bed contacts indicate high sediment accumulation rates and rapid aggradation, possibly due to the early development of the Andean foreland basin system.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana