Sedimentary evolution through a 2 Ma. Lowstand System Tract: Responses to Tectonic Activity and Eustatic Sea-Level Change in the Mulichinco Formation, Lower Cretaceous, Argentina

SCHWARZ, E.; HOWELL, J.A.

Congreso; 16th International Sedimentological Congress; 2002

International Association of Sedimentologists

The Valanginian Mulichinco Formation was deposited in the small, semi-enclosed depocentre of the Neuquén Basin (western Argentina), immediately after a secondorder sea-level fall. Excellent outcrops together with good biostratigraphic coverage allowed the detailed examination of contemporaneous non-marine and marine deposits through a 2 Ma sequence. Sixteen facies associations have been recognised. The interpretation of these illustrates the evolution of sedimentary environments at different points within the basin and at different stages within the evolution of the subsequent sequence. This distribution of depositional environments together with the identification of key surfaces, stratal patterns and stacking patterns has resulted in a compressive sequence-stratigraphic framework that illustrates the depositional response to eustatic and tectonic changes in small basins. The base of Mulichinco Formation is represented by an Intravalanginian uncorformity interpreted as a second order sequence boundary, related to tectonic activity. Both fluvial/tidal influenced- and storm dominated shallow marine sediments were deposited on previous deep marine mudstones during the early stages of this sequence (early/late LST). During the late LST, fluvial processes decreased and tidal/ and storm dominated conditions prevailed. The complexity of the LST is illustrated by the variability in the facies and reflects the variety of processes affecting the basin during that time. This degree of variability has not been previously documented and its occurrence may be attributed to: 1) the small size of the basin (less than 200 km long and 80 km width, 2) the irregular basin topography created by tectonic inversion and sea level drop and, 3) differences in sediment supply along the basin margin, also possibly related to the irregular, tectonic relief. Deepening of the marine environments marked the beginning of transgression (early TST) in the north, while towards the southernmost margin of the basin, braided fluvial deposits pass to meandering fluvial systems, characterised by more sinuous and isolated channel deposits. The maximum-flooding interval is represented by clastic starvation associated with the deposition of oyster banks in the north and central areas of the basin and tidal estuarine valley fill successions in the south. The middle to upper part of the sequence (late TST and HST) show first aggradational and ultimately, weak progradational stacking of marine parasequences. Consequently, from the early TST to HST the sequence evolution resembles more conventional third order sequences developed elsewhere, which have been interpreted as being eustatically driven. It is proposed that in the Neuquén Basin, as in similar small intra-continental, semi-enclosed basins, eustatically driven sequences are modified by tectonics. Most importantly, the influence of these tectonics is most apparent during periods of restricted accommodation (early to late LST), whilst during periods of greater accommodation the systems respond in a more conventional manner.