| Popis: |
New information from two core holes in the Madre de Dios basin of northern Bolivia suggests that the Paleozoic foreland basin (Altiplano and Cordillera Oriental outcrop areas) had correlative units there. The Pando X-1 and Manuripi X-1 wells were cored to 1968 m and 1542 m, respectively, with the former reaching crystalline basement and the latter terminating in the Devonian. Both cores penetrated the Bala (Tertiary) and Beu (Mesozoic) formations, with the latter occurring disconformably on upper Paleozoic units. Madre de Dios basin stratigraphy is applied. Devonian lithologies include fine- to medium-grained quartz arenite, clay mudstone, and rare conglomerate. Upper Carboniferous lithologies include carbonate wackestone and fossiliferous packstone, anhydrite, clay mudstone, and occasional fine- to medium-grained quartz arenite, in descending rank of abundance. Biostratigraphic results, including palynomorphs, brachiopods, and foraminifera, suggest that terrigenous clastic deposition began in the latest Emsian or earlier and continued through at least Bashkirian time. Devonian clastic depositional units represent muddy shelf, shoreface, prodelta, tidal flat, dropstone, and submarine slump paleoenvironments. It appears that some Lower Carboniferous units were removed by the intra-Carboniferous erosion event. Late Carboniferous units represent delta front and delta plain, evaporative lagoon, lagoon, tidal flat, and carbonate inner platform paleoenvironments. Although basal units reflect influence of the local craton (as a source and basin-limiting feature), middle and upper units display geologic development that parallels coeval units in the Lake Titicaca and Altiplano sequences to the south. These include (1) Upper Devonian dropstone, (2) Carboniferous eolian sandstone, and (3) Upper Carboniferous carbonate. We suggest that these wells document the intracratonic connections between the Upper Amazon, Peruvian, and Bolivian basins. Permian beds may have been removed by pre-Cretaceous erosion (Pando X-1 well). |
