The Pampatar Formation (middle Eocene) represents the first episode of deposition of the forearc basin of Bonaire. The origin and evolution of this basin is related to the tectonic evolution of the south margin of Caribbean plate and north margin of South America in the Cenozoic.

The type section of this formation is exposed in the eastern side of Margarita Island, near Pampatar city, in the state of Nueva Esparta (Figure 1). The principal section of this formation is characterized by fining-upward siliciclastic sequence, approximately 900 m thick. The base of this section is in discordant contact with a Cretaceous basement (Campos and Guzmán, 2002). Several authors interpret this formation as having been deposited on a slope and floor basin under gravity flows, and the facies interpretation made in this study suggests the same hypothesis.

The study of Pampatar Formation near Pampatar city allows us to define five facies for this formation. The facies analysis suggests that each facies was deposited in a different position of the same genetic event. These deposits were under a strong tectonic activity, and this tectonic activity was responsible for creating the accommodation necessary for these facies. Most of facies observed are related with the progressive transformation that underwent the gravity flows while they were transported inside the basin. These gravity flows are characterized by a wide range of grain sizes, which have been segregated inside the basin according to flow efficiency (Mutti et al., 1999).

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Figure Captions Figure 1. Geologic sketch map of the southern margin of the Caribbean Plate as location map for Margarita Island, Venezuela (after Bartolini et al., 2003). Guyana shield terrane to the south of, and Caribbean Plate terranes to the north of, plate-margin terranes, including Margarita Island terrane. Figure 2. Clast-supported conglomerate with mixed matrix composed of sand and little mud (facies G1). This conglomerate changes transitionally into a cobble conglomerate and microconglomerate with sandy matrix (facies G2). These conglomerates are overlain by coarse to fine sandstones (facies S1 and S2). Figure 3. Well-bedded successions of thin claystone, siltstone, and very fine sandstone. The claystone is dominant at the base of the sequence whereas the sandstone and siltstone are dominant at the top of the sequence. This facies was deposited by traction and fallout from turbidity flows. This facies is known as classic turbidites. Figure 4. Coarse to medium sandstone. It is tabular, massive, with parallel lamination in the middle and at the top of the bed (facies S1). This facies is in abrupt contact with pelagic claystones interbedded with very fine sandstones and coarse siltstones (facies T). Figure 5. Thin to very thin beds composed of claystones, siltstones, and very fine sandstones. The sandstones show undulated lamination and climbing ripples. These structures are evidence of traction and fallout process in turbidity currents. Facies The proximal facies of Pampatar Formation corresponds to clast-supported conglomerates (50% to 80% of clasts), with mixed matrix composed of sand and little mud (facies G1, Figure 2). They were deposited by friction from a hyperconcentrated density flow. This hyperconcentrated density flow underwent a transformation downslope due to a progressive mixing with ambient fluid, and inability to transport coarse clast. This transformation is evidenced by the deposit of conglomerates and microconglomerates with sandy matrix; these rocks can show normal gradation and erosive bases (facies G2, Figure 2). The transformation of hyperconcentrated density flows in density flows results in deposition by friction or traction-fallout processes of the sandy facies. Theses facies are represented by coarse to fine sandstones (facies S1 and S2, Figures 2 and 3). These facies were deposited in the middle of the facies system, and they are identified by their lithotypes and sedimentary structures. The flow underwent a transformation inside the basin; from density flow in turbidity flow. The distal facies is due to the progressive deacceleration of this flow and resultant deposition by traction and fallout. This facies is represented by pelagic clays interbedded with very fine sandstone and coarse siltstone with normal gradation, cross, undulate, and parallel lamination, ripples, and deformation structures (facies T, Figures 3, 4, and 5). References Campos, G., and O. Guzmán, 2002, Estratigrafía secuencial y sedimentología de las facies turbíditicas del flysch Eoceno de la Isla de Maragarita, Estado Nueva Esparta, Venezuela: Tesis de grado,. Universidad Central de Venezuela. 186 p. Giunta, Giuseppe, Michele Marroni, Elisa Padoa, Luca Pandolfi, 2003, Geological constraints for the geodynamic evolution of the southern margin of the Caribbean Plate: AAPG Memoir 79, p. 104-125.   Mutti, E., Roberto Tinterri, Eduard Remacha, Nicola Mavilla, Stefano Angella, and Luca Fava, 1999, An introduction to the analysis of ancient turbidite basins from an outcrop perspective. AAPG Continuing Education Course Note Series No. 39, 61p. Return to top.