Dinámica de la acumulación sedimentaria de los depósitos de cima de cuña (wedge top deposits) de la Formación El Corral, provincia de La Rioja, Argentina

Abstract

The wedge top depozone overlies the frontal part of the orogenic wedge in foreland basin systems. This depozone contains the deposits between exhumed thrust-sheets (inner wedge top or piggy back basins in this work) and the frontal part of the thrust wedge defined by the distal extent of hidden thrust systems (outer wedge top). The outer wedge top is characterized by coarse, synorogenic sediments deposited by piedmont systems in which numerous, progressive unconformities are found. The Corral Formation (Pliocene) near Guandacol, Argentina, is an example for this type of deposit, related to a thin-skinned thrust belt.

Compositional changes and paleoenvironmetal interpretation of El Corral Formation allow to establish a model for the wedge top depozone where variations in clast composition are marked by color changes of the conglomerates.

Conglomerate provenance shows a strong lithological control by the rocks forming the topographic highs, regardless of the structural and lithological complexity of the thrust belt. A topographic high depends on the relationship between uplift and exhumation rates; therefore the uplift of a thrust belt that feeds the foreland basin system can be represented in different ways: 1) if the uplift rate (UR) is less than the exhumation rate (ER), the thrust will not form a topographic high and the eroded material will be directly transported to the foredeep, without forming part of the coarse outer wedge top deposits; 2) if the uplift rate is greater than the exhumation rate, it could develop a surface uplift that will be recorded in the inner and outer wedge top depozones.

If we consider, in addition, periods of tectonic activity and quiescence, the generation of a topographic high (UR greater than ER) will have at least three stages. During the first stage, the uplift of the thrust sheet will have low relief and, therefore, it will be easy for the piedmont facies coming from the inner wedge top to cross over it, feeding the outer wedge top; it will be recorded as a clastic wedge. This stage will generate tabular beds with similar supplies from both systems (thrust sheet and piggy back basin). In a second stage, the uplift has a higher profile, and the passage of sediments from the inner wedge top to the outer wedge top will be through small, confined valleys that cross the orogenic front. Here, the provenance of the outer wedge top conglomerates should be dominated by the uplifting thrust-sheet lithology, and the piggy back supply will only be recorded in the outer wedge top as small lenses. The third stage of the topographic high formation will begin when it reaches enough relief to completely close the piggy back basins in the inner wedge top.