Abstract

GIUSIANO, Adolfo; DIAZ, Ezequiel  and  ISAACSON, P. Composición y procedencia de areniscas del Devónico medio-Carbonífero superior de la Cuenca Madre de Dios, Bolivia: Pozo Manuripi x-1. Rev. Asoc. Argent. Sedimentol. [online]. 1998, vol.5, n.1, pp.1-8. ISSN 1853-6360.

Late Paleozoic deposits in the Madre de Dios basin (MDB) (northern Bolivia) underlay a sedimentary pile of Mesozoic and Cenozoic deposits reaching 800 m at the Manuripi X-1 well site. There are not Paleozoic outcrops in the area, and all the information available comes from oil exploration wells. At the Pando X-1 well site (Fig. 1), Proterozoic basement was found at a depth of 1968 m. The Manuripi X-1 did not reach this basement, but instead was terminated within the Mid and Upper Devonian mudstones and fine grained sandstones of the Tomachi Formation. This unit is interpreted as shallow clastic shelf environment (Isaacson et al., 1995) (Fig. 2). The Lower Carboniferous Toregua Formation concordantly overlies the Devonian. It is composed of graded sandstones, siltstones and mudstones deposited in a prodelta to delta plain subenvironment. An erosional unconformity separates this unit from the Yaurichambi Formation (Upper Carboniferous) which consists of fine to medium grained, graded sandstones with interbeds of laminated mudstones representing an eolian dune environment. The Copacabana Formation (Upper Carboniferous-Lower Permian) concordantly overlies the Upper Carboniferous, and is composed of limestones, dolostones, fine sandstones and mudstones, deposited in a tidal flat to cabonate shelf environment (Isaacson et al., 1995) (Fig. 2). The sandstones of Copacabana and Yaurichambi Formations vary from fine to medium grained with subrounded and rounded shapes and moderate to good sorting. The Toregua Formation has subangular to rounded, medium to coarse sandstones, and the Tomachi Formation is characterized by fine grained and subangular sandstones (Table1). The sandstones plotted in QmFLt diagram (Fig. 3a, Table 2), using the Folk (1974) compositional fields, are classified as subarkoses. These sandstones are characterized by dominantly Qm with minor Qp, and F (K >P), and correspond to the quartzose petrofacies of Dickinson (1988). The QmFLt and QtFL (Dickinson, 1985) (Fig. 3c, Table 2) plots display that the Toregua and Tomachi Formations are derived from craton interior, but instead the Copacabana and Yaurichambi Formations are plotted at the boundary between craton interior and recycled orogen. QmPK plot (Dickinson & Suczek, 1979) (Fig. 3b, Table 2) indicates clearly that all the samples are derived from a continental block. Paleocurrents taken from dipmeter log, display W-NW direction for sediments of the Yaurichambi Formation and E-NE for the Toregua and Tomachi Formations (Fig. 2). The Tomachi and Toregua Formations sources were the Precambrian rocks from Brazilian Shield that crop out in eastern Bolivia as the San Ingancio Schist and the 1300 to 1000 Ma old Rodonia/Sunsas granite (Litherland et al., 1989). The Yaurichambi and Copacabana Formations derived from granites and sedimentary rocks from the Arequipa Massif (western MDB) as the 390 Ma San Nicolás batholith (Musaka & Henry, 1990), and uplifted late Devonian-early Carboniferous sediments from the Peruvian Sub-basin (Isaacson & Sablock, 1990).

Keywords : Provenance; Sandstone petrography; Devonian; Carfoniferous; Bolivia.

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