--> ABSTRACT: The Bermejo Basin: Unraveling the Contributions of Tectonism, Climate, and Intrinsic Depositional Processes to Non-Marine Sequence Stratigraphy Development, by Anibal E. Fernandez, Teresa E. Jordan; #91020 (1995).

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The Bermejo Basin: Unraveling the Contributions of Tectonism, Climate, and Intrinsic Depositional Processes to Non-Marine Sequence Stratigraphy Development

Anibal E. Fernandez, Teresa E. Jordan

The Bermejo Basin is a non-marine foreland basin located in western Argentina (30° S latitude), at the foothills of the high Andes. Part of the basin is actively accumulating sediments. Tectonism is also active in the associated Precordillera fold and thrust belt. The basin fill comprises up to 9,000 meters of non-marine, eolian, alluvial and lacustrine, sandstones, shales, and conglomerates. Rock ages range from ~ 20 Ma (Lower Miocene) through Holocene sediments. Outcrop information tied to seismic and well data were used to define the sequence stratigraphy of the basin. Age control was obtained through magnetic polarity stratigraphy and absolute chronology techniques. Timing of deformation in the fold and thrust belt determined mainly through cross-cutting relation , was contrasted against the stratigraphic record preserved in the foreland basin.

In the Bermejo strata, the study of surface exposures coupled with subsurface seismic information show no clear evidences of geometrically defined unconformities. During the interval from ~14 to 2.5 Ma for the proximal margin of the basin, clear facies shifts that could be interpreted in terms of sequences are lacking in the extensive surface data. In rocks from 8 to 5 Ma facies shifts occur at ~500,000 year intervals. Major changes in rates of accumulation occur at intervals of roughly 2 to 3 Ma.

Lower Miocene eolian beds together with middle Miocene evaporitic rocks and paleosols with calcic and gypsic horizons indicate arid climatic conditions in the basin at this time. The depositional regime remained overall desertic throughout all the Neogene and into the Holocene. Lacustrine beds with fresh-water ostracods suggest an interval of increased water budget at ~5 Ma. Except for this instance of climatic change, the role of climate as an extrinsic factor was not crucial in controlling the main stratigraphic variations observed in the Bermejo Basin. Changes in facies, architecture, and accumulation rates that occurred in a time scale of up to ~100,000 years are interpreted as the result of variations in intrinsic depositional factors associated with fluvial and distal alluvial f n sedimentary environments.

Tectonism did play a key role in driving tectonic subsidence and defining sharp variations in accumulation rates in the basin at a time scale of 2-3 Ma. Only these documented major changes in rate of accumulation through time are a signature of stratigraphic sequences. This observation agrees with theoretical results provided by Flemings and Jordan's (1990) predictive depositional model for a foreland basin, except that the predicted facies in the model do not agree with the observed field data.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995