Vertical Facies Transitions in a Progradational Fluvial-Fan Succession (Mendoza Foreland, Argentina): Depositional Processes and Spurious Climate Signals

Abstract

Facies and architectural changes through alluvial successions are classically interpreted in terms of climatic, tectonic and base-level forcing. Foreland basins in particular accommodate thick continental successions which are hard to decipher in absence of high-resolution chronological and geochemical proxy data. However, recent debates on the role of fluvial (mega)fans for the infill of continental basins force a reconsideration of genetic relationships between stratigraphic units previously linked to allogenic controls. Hypotheses on the volumetric dominance of fluvial-fan deposits in continental records are accompanied by the inference that thick alluvial successions may relate to long-term fan progradation, reflecting vertical stacking of coarse clastic facies associations from proximal high-discharge feeders over relatively fine facies associations from distal, distributary alluvial domains of progressively lower discharge. We discuss the sedimentology of a 1300 m-thick, Oligo-Miocene stratigraphic succession aggraded in an arid setting in the Central Argentinian Foreland. The basal interval consists of a vertical transition from playa deposits up into thick mudstones encasing rare, coarse-clastic channel fills and overprinted by incipient pedogenic cycles, to thick eolian strata from a well-developed erg environment subject to fluvial interference. Further up, a gradual coarsening-upward over hundreds of metres in stratigraphy comprises vertically stacked, alternating, laterally extensive, amalgamated, sandy to gravelly channel fills, alternating to isolated pods of eolian strata and mud-rich overbank deposits. A gentle angular unconformity leads to overlying conglomerates represent highly amalgamated infills of deep channels with more sustained and competent discharge. In spite of marked large-scale sedimentological changes suggestive of climate and hydrologic fluctuations, the overall vertical transition in facies associations is more easily interpreted as continuous progradation of a large fluvial fan supplied by the uplifting Cordillera Frontal. Alluvial facies details show that deposition occurred from ephemeral streams with fluctuating discharges, confirming a persistence of relatively arid climate, as evidenced also by large volumes of associated eolian strata. Proxy data from mudstone units may more accurately record actual climate changes than large-scale sedimentological and architectural trends.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016