Fluvial Variability within Lowstand Deposits, Neuquén Basin, Argentina.

VEIGA, G.D.; SPALLETTI, L.A.; COLOMBO-PIÑOL, F.

Delft, Países Bajos

Congreso; 8th International Conference on Fluvial Sedimentology; 2005

TU Delft

The Neuquén basin is located in west-central Argentina and Chile and it is a large triangular basin that during most of its history (Jurassic-Early Cretaceous) evolved as a backarc basin in the south-western margin of Gondwana. It was closely associated with a volcanic arc to the west and two cratonic areas to the NE and SE. The basin was connected with the paleo-Pacific Ocean trough gaps in the arc and its sedimentary record consists of a series of transgressive-regressive cycles associated both with eustatic oscillations and a complex tectonic history. Most of the sedimentary record in the central part of the basin is characterised by rather uniform deep marine sedimentation. However, several non marine lowstand deposits were developed as a result of sudden relative sea level falls triggered by a combination of eustatic movements and tectonic reactivation. Excellent exposures across the central and southern Neuquén Basin allowed the study of lateral and vertical variability within one of these lowstands (Kimmeridgian). These deposits have been accumulated due to a major relative sea level drop induced by an Upper Jurassic tectonic inversion that affected several of the fault systems developed during the previous (Triassic-Early Jurassic) synrift stage. Contrasting with relatively uniform transgressive and highstand accumulation, these lowstand deposits show important differences in thickness, facies and vertical organization in different sectors of the basin. In the western part of the basin, Kimmeridgian deposits constitute an up to 450 m-thick succession. It is dominated at the base by coarse-grained deposits (fluvial conglomerates and sediment gravity flow deposits) associated with the development of semiarid alluvial fans fed from the west (associated with the growth of the volcanic arc) and from the south (associated with the tectonic inversion of the Huincul synrifts). Towards the east and north, there is a systematic downstream change to ephemeral stream deposits and fine-grained playa deposits. A gradual change up sequence towards fine-grained deposits associated with an increase in accommodation and to wetter conditions is also recorded. In contrast, towards the southern and eastern margins of the basin the thickness of the unit decreases considerably (up to 40 m). The base of the unit is composed of coarse-grained channel fill deposits, associated with sheetflood and fine-grained distal facies. The key difference in this area rises from the vertical evolution of the studied interval that records the evolution to a dry aeolian system. The fundamental variations observed within these particular lowstand deposits seem to be closely related to the influence of some of the major structures in the basin that might have been reactivated during the main (Upper Jurassic) inversion. However, transgressive deposits developed on top show again a rather uniform distribution in the whole basin, without major control from the inverted structures. Consequently, facies and stratigraphic organization within lowstand deposits, especially in this tectonically-induced wedge, may be much more complicated than facies distribution during highstand and transgressive periods. Therefore, models based on the distribution of marine deposits in the basin may not predict the variability observed during lowstand periods.