EPICONTINENTAL SEDIMENTATION ON THE PACIF IC SIDE OF COLOMBIA TAKES PLACE ON AN ACTIVE MARGIN

The continental shelf is very narrow and steep to the north, and broad er to the South. This shelf is structuraly influenced by the Andes. Th is area is mainly covered with terrigenous deposits, i.e. muds and san ds which are recent on the internal shelf and relict on the external s helf. The sediments, supplied by rivers, reflect the intense erosion o f the Andean backland. This erosion can only be reduced in situ by the vegetation cover. Clays are the main component of the fine fraction r eaching the sea. Three different clay assemblages are observed: 1) an Andean (illite-chlorite) assemblage, reflecting the proximity of the c ordillera; 2) a mid-American assemblage (smectite), which supplies the North Pacific Margin through the Choco counter-current; 3) an Equator ial assemblage (smectites) which is brought to the South by the Colomb ian current.