Interaction and effects of allocyclic processes on the stratal and sedimentological expression of depositional systems: an example from the Bolivian Sub-Andean Foreland basin

Large-scale variations in depositional environments, determined from over 3000 meters of continuous core through rocks ranging in age from 450 to 60 MY, makes the Madre de Dios Basin an excellent place to examine the complex interplay between tectonics, eustasy, climate and sediment supply. The tectonic development begins with a Paleozoic intracratonic setting followed by a late Mesozoic sub-andean foreland basin. The Devonian and Cretaceous marine shale intervals, located on the cratonic side of the Madre De Dios foreland basin, acted as source and seal for petroleum reservoirs, and acted as decollement zones during periods of thrusting. The basin fill includes a world-class Upper Devonian marine source rock with total organic carbon (TOC) content of up to 18%.

Using 7000 kilometers of 2D seismic data eight depositional sequences were identified mapped, and subdivided into fourteen geologic time slices from which paleogeographic maps were constructed. The Siluro-Ordovician age depositional sequence ties unconformably on metamorphic basement (garnet schist). This sequence is divided into a lower package of fluvial/deltaic origin and an upper package of shallow marine carbonates. Four Devonian sequences are identified and consist of facies formed in depositional environments ranging from subaerial to subaqueous deltaic environments and exhibit northeast to southwest progradation. A Late Carboniferous to Early Permian sequence includes facies that are characterized by eolian dune, coastal sabkha, and shallow marine carbonate facies. These facies indicate a major change in climatic conditions compared to the underlying Devonian fluvial/deltaic sequences. Cretaceous sequences are composed mainly of strata of fluvial origin, with the upper Cretaceous sequence containing an incised valley system 10 to 15 kilometers wide and 300 meters deep. Valley fill facies are interpreted to represent low sinuosity, braided fluvial systems emplaced during the end of a relative sea-level fall and initial sea-level rise. The terraced morphology of the valley margin formed in response to multiple cut-and-fill episodes, with basement-involved faulting influencing the location and magnitude of valley incision. Recurrent movements of basement fault blocks, related to migration of the forebulge and pre-existing topography, appear to have controlled the drainage incisement patterns.