CIG   05423
CENTRO DE INVESTIGACIONES GEOLOGICAS
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
Application of the allostratigraphic approach to the study of volcano-sedimentary rift sequences: an example from the Jurassic of the Neuquén Basin, Argentina
Autor/es:
D'ELIA LEANDRO
Lugar:
Mendoza
Reunión:
Congreso; XVIII International Sedimentological Congress; 2010
Institución organizadora:
IAS
Resumen:
Allostratigraphic schemes have been used for decades for the understanding and interpretation of many sedimentary basins and hydrocarbon systems in the world. However the difficulties in finding key surfaces and the intrinsic complexities of volcanic successions are obstacles to the application of this methodology when studying the history of volcano-dominated basin-fills. Despite the natural shortcomings when using this methodology in non-marine volcanic and volcaniclastic successions, its uses when devising a qualitative chronostratigraphic scheme was found valuable in the analysis of an extensional basin that have the signature of volcanism in their infill. The Neuquén basin is located on the eastern side of the Andes in Argentina and central Chile, and it constitutes one of the most important hydrocarbon basins of South America. The history of the basin is very complex, involving an early stage of extension (Upper Tr–Lower J) and subsequent stages of inversion and extension during Mesozoic times. The initial configuration of the basin was characterized by the development of isolated deep depressions bounded by normal faults and filled with volcanosedimentary successions. The syn-rift infill is known as Precuyano, which is constituted by different lithostratigraphic units. The study of the syn-rift succession was carried out through geological mapping, and the measuring of stratigraphic and structural sections. Several lithofacies and facies associations were determined. The characteristics and geochemical signatures of volcanic and pyroclastic rocks were studied through the analysis of thin sections and chemical analyses, while the petrographic analyses on the volcaniclatic rocks determined the nature of their procedence and the relationship with the volcanic facies. Discrete depositional units (accumulation units) were defined based on identification of distinct bounding surface, in conjunction with the facies associations. The main framework of accumulation units in the syn-rift includes volcanic (vents, dikes, domes and lava flows), pyroclastic (ignimbrites) and sedimentary units (volcaniclastic alluvial and fluvial systems, coarse-grained and fine-grained delta systems, offshore marine and mixed shallow marine systems). The integrated analysis was performed by ranking the regional key surfaces (discontinuity surfaces) to define an allostratigraphic scheme. Three tectosedimentary units (UTS) were defined. UTS I is an Upper Tr–Lower J continental silicoclastic sequence deposited prior to the onset of volcanism. UTS II is a continental unit mainly defined by its volcanic signature in which composite volcanoes and volcano-tectonic depressions were interpreted. UTS III is represented by mixed marine units with a minor influence of volcanism. As a result, the superimposition of these tectosedimentary units represents a multiepisodic rifting that lasted over 30 ma from Upper Tr to Pliensbachian times. The result of this research enabled the definition of the major volcanic and sedimentary environments within the syn-rift succession and the main tectonic (accommodation space, polarity of sedimentary and volcanic environments, spatial distribution of depocentres and eustatic cycles) and volcanic controls (types of volcanic environments, and quantity and characteristics of the clastic material provided to the depocentres). Compared with the lithostratigraphy, which is characterized by lateral continuity of the effect of an event, the allostratigraphic approach emphasizes the complex interaction between depositional, non-depositional, and erosional events. This enables a more precise identification of the distribution of different environments and their 3D stratigraphic changes along time. Finally, this methodology could be used as lithology predictor to understand the facies distribution and geometries of different stages of volcano-dominated basin-fills and to make a comparison between surface information and subsurface data sets.