The Pampatar Formation (Margarita Island, Venezuela): A Result of Gravity Flows in Deep Marine Water
O. Guzmán1 and C. Campos2
1PDVSA, Exploration Management, Caracas, Venezuela
2Universidad Simón Bolívar, Departamento de Ciencias de La Tierra, Venezuela
The Pampatar Formation (Eocene Middle) represents the first episode of depositation of the Fore arc 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 western side of Margarita Island, near Pampatar city, in Nueva Esparta estate. The principal section of this formation is characterized by once fining – upward siliciclastic sequence with 900 m- thick approximately. The base of this section is in discordant contact with a Cretaceous basement (Campos, C and Guzmán, O. 2002). Several authors interpret this formation as 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 helps us to define five facies for this formation. The facies analysis suggests that each facies was deposited in 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 having a wide range of grain sizes, which have been segregated inside the basin according to the flow efficiency (Mutti et al., 1999)
The proximal facies of Pampatar Formation correspond to clast-supported conglomerates (50% to 80% of clasts) with mixed matrix composed of sand and little mud (Facies G1-Figure 1). They were deposited by friction from a hyperconcentrated density flow. This hyperconcentrated density flow undergoes a transformation downslope due to a progressive mixing with ambient fluid, not being able 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 1). The transformation of hyperconcentrated density flows in density flows generates the 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 1 and 2). These facies were deposit in the middle of the facies system, and they were identified by their lithotypes and sedimentary structures. The flow undergoes a transformation inside the basin; it is transformed of density flow in turbidity flow. Due the progressive desaceleration of this flow are deposited by traction and fallout the distal facies. This facies are 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-Figure 2, 3 and 4).
Mutti, E., et al, 1999, An introduction to the analysis of ancient turbidite basins from an outcrop perspective. AAPG Continuing Education Course Note Series No. 39, 61p.
Campos, G. y Guzmán, O. 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 pp.
Figure 1. Class-supported conglomerate with mixed matrix composed of sand and little mud (facie G1). This conglomerate change transitionally to a cobble conglomerate and microconglomerate with sandy matrix (facies G2). These conglomerates are overlain by coarse to fine sandstones (facies S1 and S2).
Figure 2. Well-bedded successions of thin claystone, siltstone an very fine sandstones. The claystone are dominant at the base of sequence while the sandstones 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 3. Coarse to middle sandstone. It’s tabular, massive and show the parallel lamination at the middle and 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).
4. Thin to very thin beds composed of claystones, siltstone and very fine
sandstones. The sandstone shows undulated lamination and climbing ripples. These
structures are evidence of traction and fallout process in turbidity currents.
AAPG Search and Discovery Article #90079©2008 AAPG Hedberg Conference, Ushuaia-Patagonia, Argentina