Gravely and Sandy Facies of Río Guache Formation, Venezuelan Ándes: Evidences of Transformation of Gravity Flows in Deep Marine Water

C. Campos and A. Pilloud
Universidad Simón Bolívar, Departamento de Ciencias de La Tierra, Caracas, Venezuela

The Río Guache Formation has been recognized in the Southeastern flank of the Venezuelan Ándes, specifically at Lara, Portuguesa and Trujillo states. This formation has been interpreted as deposits on a slope under gravity flow and turbidity currents. Its age is uncertain between Maastrichtian and Middle Eocene. (Von Der Osten and Zozaya, 1957; Ramírez, 1968; Bruno Blin, 1989; Campos, 2006). The present work was carried out in the area between Guaramacal and the Anitos river, (Trujillo state), being made the geologic study of 25 localities. Based on the study of 25 localities we could define seven facies, five gravelly and two sandy. These facies were identified by their lithotypes and sedimentary structures. In addition the processes of transport and depositation for each of the different facies were interpreted in order to know the progressive transformations that underwent the gravity flows while they were transported inside the basin. As a result we generated a depositation model for the sandy and gravelly facies at the Río Guache Formation. The sandy and gravelly facies are associated to different positions within a same genetic event. This event corresponds to the debris flows downstream progressive dilution, with a wide range of grain sizes, which have been segregated by the flow efficiency (Mutti et al, 1999).

The gravelly facies consist of gravelly mudrock to sand supported conglomerates, these facies represent the product of the transformation of debris flows in hyperconcentrated density flows, whereas, sandy facies represent the transformation of hyperconcentrated density flows in density flows. A brief description of these facies follows:

§ Facies: R-Df1: Gravelly mudrocks and mud-supported conglomerates. The clasts in these rocks are granules to boulders (< 3 m of diameter) and they are floating in a muddy matrix. The beds of this facies are about 1m to 10 m-thick and are massive. They are the product of cohesive debris flows, being deposited in masse (Figure 1A).

§ Facies R-Df2: Clast-supported conglomerates with muddy matrix. The clasts in these rocks are granules to boulders (< 50 cm of diameter). The beds of this facies have a thickness below 5m. These rocks are massive and they are the product of cohesive debris flows, being deposited in masse (Figure 1B).

§ Facies R-Gs: Sand-supported conglomerates to grain - supported conglomerates with sandy matrix (Figure 1C). The clasts in these rocks are granules to cobbles. The beds of this facies are smaller than 5 m-thick. These rocks are massive or crudely graded and they show basal scours. This facies is the product of hyperconcentrated flows. The main mechanism for deposition of hyperconcentrated flows is frictional freezing resulting from grain-to-grain interaction (Mulder and Alexander, 2001).

§ Facies R-Gl: Conglomerates and micro-conglomerates lenses with erosive bases. These conglomerates are clast-supported; the clasts in these rocks are granules to cobbles (Figure 1D). The beds of this facies are narrower than 50 cm-thick. These rocks are massive or they can show normal or inverse gradation. Elongate clasts have their long axis mostly parallel to the flow direction. This facies was deposited by hyperconcentrated flows, being the main mechanism for deposition the frictional freezing. The preferential orientation of the long axis of the clast suggests their ability to move independently within the flow (Zavala and Olivero, in press).

§ Facies R-Gc: Clast-supported conglomerates with calcite cement. In this facies the muddy matrix is almost absent, and it can be mistaken with pseudomatrix. The clasts in these rocks are cobbles to boulders (< 1 m of diameter). The beds of this facies have thicknesses below 5m. These rocks are massive and they are the product of residual deposits due to the progressive loss of strength of the flow. Therefore the coarsest clats are segregated at the base of the flow (Figure 1E).

§ Facies R-S1: Fine to coarse sandstones. This facies can present erosive bases with pebbles and cobbles. These sandstones are massive or can show normal gradation and parallel lamination in the middle or top of the beds. The beds of this facies have a thickness between 20 cm to 5m. The basal scours with pebbles and cobbles could be product of hyperconcentrated flows. The sandstones were deposited by process of friction and traction - fallout in a density flow (Figure 1F).

§ Facies R-S2: Middle to very fine sandstones. These rocks are tabular, massive or can show parallel and cross lamination in the middle or top of the beds. The beds of this facies have a thickness between 30 cm to 1.7 m. This facies was deposited by processes of friction and traction - fallout in a density flow (Figure 2A and B).

The interpretation of the facies tracts and its depositional model is based on the models proposed by Mutti (1992 and 1999). This author proposes that the different facies occupy predictable positions within a succession of facies. Such succession of facies is called by Mutti “facies tract”. In this model the facies are related genetically and they represent the evolution of submarine gravity flows during its basinward travel.

The group of facies observed in the field makes us to infer that this facies are the product of the progressive dilution of debris flows downstream. The most proximal facies within this set of facies correspond to gravelly mudrock, massive mud-supported conglomerates (facies R-Df1) and clast-supported conglomerates with muddy matrix (facies R-Df2). They were deposited in masse from of cohesive debris flow. This cohesive debris flow undergoes a transformation downslope due to a progressive mixing with ambient fluid, becoming in hyperconcentrated density flow. This flow is deposited by friction, being represented by sand-supported conglomerates to grain- supported conglomerates with sandy matrix (facies R-Gs) as well as conglomerate and micro-conglomerates lenses (facies R-Gl). The progressive lost of strength of the flow result in the segregation of coarser clast at the base of the flow, forming the residual deposits as clast-supported conglomerates with calcite cement (facies R-Gc). The transformation of hyperconcentrated density flows in density flows generates the deposition by friction or traction-fallout processes of the middle and distal facies. Theses facies are represented by fine to coarse sandstones with basal scours (facies R-S1) and middle to very fine sandstones (facies R-S2) (Figure 3).

References

Blin, B., 1989, La Front de la Chaîne Caraïbe Vénézuélienne entre la Serrania de Portuguesa et la région de Tiznados. Thése Doct. Etat, Univ. de Bretagne Occidentale, Brest, 389p.

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Zavala, C. y Olivero, E, (en prensa), Sistemas Turbidíticos de Márgenes Activos y contornitas: Procesos Sedimentarios y Análisis de Facies. En: Ambientes Sedimentarios. Editores Ricardo Astini y Eduardo Piovano. Universidad de Cordoba, Argentina.

Figure 1. A) facies R-Df1, mud-supported conglomerate. The clasts are floating in a muddy matrix. It is the product of cohesive debris flow. B) facies R-Df2, clast-supported conglomerate with muddy matrix. Deposit of a cohesive debris flow. C) facies R-Gs, grain-supported conglomerate with sandy matrix. Deposit of hyperconcentrated density flows. D) facies R-Gl, micro-conglomerate lense with erosive base. It is the product of hyperconcentrated density flows. E) facies R-Gc, clast-supported conglomerate with calcite cement, it is the product of residual deposits due to the progressive loss of strength of the cohesive debris flow. F) facies R-S1, fine to coarse sandstone with erosive base. This sandstone was deposited by density flows.