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The Guayabo Group is an oolitic ironstone-bearing delta complex which accumulated in the southwestern part of the Maracaibo Basin in late Miocene-early Pliocene(?) time. The 1,000 to 1,700-m-thick deposit comprises the paralic to fluvial Cúcuta, Cornejo, and Urimaco Formations. These are characterized by combinations of seven different vertical sequences of sediment. Sequence 1 consists of channel sandstones containing small quartz and chert pebbles, and burrow-mottled mudstones, deposited on alluvial plains with braided and low-sinuosity rivers. Sequence 2 is made up of color-mottled mudstones and thick channel sandstones which accumulated on the upper delta plain where channelways were more sinuous. Sequence 3 consists of thin, narrow channel sandbodies deposited in crevasses at high angles to major distributaries, and associated drab mudstone enclosing thin sandy lentils. Sequence 4 includes elongate channel sandstones with complex internal geometry and abundant carbonaceous fragments, inter-bedded with burrowed drab mudstone, which accumulated along distributaries and bar fingers in very shallow water. Sequence 5 is carbonaceous, muddy sheet and channel sandstones and mudstones, as well as local lignite, which accumulated in interdistributary marshes. Sequence 6 is made up of carbonaceous mudstone grading upward through laminated siltstone and mudstone to prograding sheet sandstones deposited along a microtidal shoreline. Repeated decrease in detrital influx led to local development of thin oolitic ironstones in marginal embayments during stillstand or the initial stage of renewed transgression. Sequence 7 includes drab mudstone and discontinuous sheet sandstones formed as distal splays in embayments where thin oolitic ironstones developed locally during interruptions of detrital influx.

During early and middle Cenozoic time, intermittent deformation and uplift of the Cordillera Central of Colombia produced three thick conglomerates in the Andean foredeep and repeated regional uplift and erosion of the more stable craton in northeast- ern Colombia and adjacent Venezuela. In late Oligocene and early Miocene time, waning uplift of the Cordillera Central and subdued uplift of basement blocks in northeastern Colombia and adjacent Venezuela were accompanied byfine-graineddetrital sedimentation both in the foredeep and on the fractured craton to the northeast. In late Cenozoic time major deformation and uplift produced coarse-grained deposits in the Andean foredeep and in marginal basins around the basement blocks. Detritus shed eastward from the Santander Massif in the Cordillera Oriental accumulated in the low-energy Guayabo delta complex. Initial paralic deposits composed of combinations of sequences 3 to 7 constitute the Ciicuta Formation. The succeeding Cornejo Formation consists ofmore varied combinations of sequences 2 to 7 in nonmarine to paralic sediments with associated marginal marine oolitic ironstones. Increased uplift of the massif supplied coarser fluvial deposits of the Urimaca Formation (sequences 1 to 6) which prograded eastward across the southwestern corner of the Maracaibo Basin. Late Pliocene and Pleistocene regional uplift led to excavation of the area by northward-flowing drainagesystems.

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