The Neuqu6n Basin of Argentina and central Chile contains a near-continuousLate Triassic-Early Cenozoic succession deposited on the eastern side of the evolving Andean mountain chain. It is a polyphase basin characterized by three main stages of evolution: initial rift stage; subduction-related thermal sag; and foreland stage. The fill of the basin records the tectonic evolution of the central Andes with dramatic evidence for baselevel changes that occurred both within the basin and along its margins. The record of these changes within the mixed siliclastic-carbonate succession makes the basin an excellent field laboratory for sequence stratigraphy and basin evolution. The 4000 m-thick fill of the basin also contains one of the most complete Jurassic-Early

This study examines the sedimentary response to a tectonically driven relative sea-level fall that occurred in the Neuquén Basin, west-central Argentina, during the late Early Valanginian (Early Cretaceous). At this time the basin lay behind the emergent Andean magmatic arc to the west. Following the relative sea-level fall, sedimentation was limited to the central part of the Neuquén Basin, with the deposition of a predominantly clastic, continental to shallow marine wedge on top of basinal black shales. This lowstand wedge is called the Mulichinco Formation and consists of a third-order sequence that lasted about 2 Myr and contains high frequency lowstand, transgressive, and highstand deposits. Significant variations in facies, depositional architecture, and internal organization of the sequence occur along depositional strike. These variations are attributed mainly to tectonic and topographic controls upon sediment flux, basin gradient, fault tilting, and shifting of the depocentre through time. These controls were ultimately related to asymmetrically distributed tectonic activity that was greater towards the magmatic arc in the west. The superposition of fluvial deposits directly upon offshore facies provides unequivocal evidence for a sequence boundary at the base of the Mulichinco Formation. However, the Mulichinco sequence boundary is marked by shallow, low erosional relief and widespread fluvial deposition. The surface lacks prominent valleys traditionally associated with sequence boundaries. This non-erosive sequence boundary geometry is attributed to the ramp-type geometry of the basin and/or rapid uplift that limited stratigraphic adjustment to base-level fall. Significant along-strike facies changes and a lowrelief sequence boundary are attributes that may be common in tectonically active, semi-enclosed basins (e.g. shallow back-arc basins, foreland basins).

The Tithonian-Berriasian Vaca Muerta, Carrín Curá and Picú n Leufú formations in the southern Neuquén Basin were deposited on a tidally dominated, mixed carbonate-siliciclastic ramp. Basinal, outer, middle, shallow and back ramp facies associations are recognized and a sequence stratigraphic analysis reveals that the ramp record consists of three shallowing-upwards sequences (Ti1, Ti2 and Ti3) set within a lower-order progradational cycle. A higher order of cyclicity is superimposed on to the middle (Ti2) sequence. The majority of the ramp facies belong to the transgressive and highstand systems tracts; however, at the base of Ti2, a lowstand systems tract is identified, characterized by a basal unconformity and an abrupt basinward shift of the shallow marine lithofacies. Transgressive systems tracts were characterized by slow sedimentation rates and rapid sea-level rises that affected carbonate productivity. Highstand systems tracts show the greatest carbonate productivity and an increased progradation rate on account of a reduction in accommodation space generation. Palaeogeography played a major role in the development of the depositional systems. Partial isolation from the Pacific Ocean reflecting the growth of the Andean magmatic arc and geographic restriction due to tectonic inversion in the central part of the basin resulted in a meso-macrotidal regime that produced a tidally dominated sedimentary record in the shallow and back ramp environments. Coeval anoxic conditions in the central part of the Neuquén Basin favoured distal ramp and basinal black shale deposition during episodes of relative sea-level rise.

Most of the present knowledge of shallow-marine, mixed carbonate-siliciclastic systems relies on examples from the carbonate-dominated end of the carbonate-siliciclastic spectrum. This contribution provides a detailed reconstruction of a siliciclastic-dominated mixed system (Pilmatu e Member of the Agrio Formation, Neuqu en Basin, Argentina) that explores the variability of depositional models and resulting stratigraphic units within these systems.The Pilmatu e Member regressive system comprises a storm-dominated, shoreface to basinal setting with three subparallel zones: a distal mixed zone, a middle siliciclastic zone and a proximal mixed zone. In the latter, a significant proportion of ooids and bioclasts were mixed with terrigenous sediment, supplied mostly via along-shore currents. Storm-generated flows were the primary processes exporting fine sand and mud to the middle zone, but were ineffective to remove coarser sediment. The distal zone received low volumes of siliciclastic mud, which mixed with planktonic-derived carbonate material. Successive events of shoreline progradation and retrogradation of the Pilmatu e system generated up to 17 parasequences, which are bounded by shell beds associated with transgressive surfaces. The facies distribution and resulting genetic units of this siliciclastic-dominated mixed system are markedly different to the ones observed in present and ancient carbonate-dominated mixed systems, but they show strong similarities with the products of storm-dominated, pure siliciclastic shoreface-shelf systems. Basin-scale depositional controls, such as arid climatic conditions and shallow epeiric seas might aid in the development of mixed systems across the full spectrum (i.e. from carbonate-dominated to siliciclastic-dominated end members), but the interplay of processes supplying sand to the system, as well as processes transporting sediment across the marine environment, are key controls in shaping the tridimensional facies distribution and the genetic units of siliciclastic-dominated mixed systems. Thus, the identification of different combinations of basin-scale factors and depositional processes is key for a better prediction of conventional and unconventional reservoirs within mixed, carbonate-siliciclastic successions worldwide.

The Valanginian-aged Mulichinco Formation was deposited in the Neuqutn Basin (west-central Argentina) during and immediately after a major fall in sea level, partially triggered by a tectonic inversion pulse. The formation represents a lowstand wedge where excellent outcrops, together with refined biostratigraphic coverage, have permitted the detailed examination of contemporaneous non-marine and marine deposits. Fourteen facies associations were identified in the Mulichinco Formation. They represent accumulation in a variety of environments ranging from gravelly fluvial braidplains to outer-shelf marine settings. Distribution of depositional environments, together with the identification of key surfaces and stratal patterns, has resulted in the identification of early and late lowstand, transgressive and highstand systems tracts. Accordingly, the Mulichinco lowstand wedge comprises one third-order sequence that lasted about 2 Ma and represents a lowstand sequence set. The character of shoreline sedimentation was highly variable along strike within the Mulichinco depositional area and alluvial deposits were not developed within incised valleys. Tectonically derived topography, basin physiography and fault-controlled subsidence are interpreted to have been the main controls on the evolution of the Mulichinco lowstand wedge. The results of this study have important implications for understanding both the history of Neuqutn Basin and illustrating the previously undocumented architectural complexity that may exist within lowstand wedges.

The interplay between carbonate production and siliciclastic input produces mixed systems that typically contain a very high degree of lateral and vertical facies heterogeneity. This heterogeneity complicates the sequence stratigraphic analysis of mixed systems. Outcrop studies facilitate the deciphering of controls and understanding of facies distributions within sedimentary successions. The Picún Leufú Anticline in the Neuquén Basin (Argentina) offers the opportunity to integrate large‐scale depositional architecture with detailed facies descriptions of the shelf to basin successions of the Upper Jurassic – Lower Cretaceous Quintuco – Picún Leufú – Vaca Muerta System. The strata in the system are mixed and range in depositional environments from shallow marine sandstones and limestones to deep basinal shales. These environments are arranged in metre‐scale shallowing upward cycles and cycle sets, with increasing carbonate proportions in regressive hemicycles. Increased input of siliciclastic material from the volcanic arc area occurred during phases of relative sea‐level rise and was controlled by the intensity of along‐shelf currents. The shelf transport was driven by the available accommodation space on the shelf, and therefore was a function of the eustatic sea‐level fluctuations. Within the studied section, a pure carbonate depositional system developed because siliciclastic input was shut down either due to long‐lived highstand settings or a sudden climatic change to more arid conditions. Carbonate–siliciclastic mixing in this setting is a function of siliciclastic dilution of the carbonate sedimentation and differs from the classical reciprocal sedimentation model, which typically includes shut‐off of carbonate production during lowstand periods. In the regional context, the subsurface strata of time‐equivalent reservoirs in the Eastern Neuquén Embayment display strong similarities of architecture, indicating that similar mixing processes occurred along most of the Neuquén Basin.

A detailed architectural and sequence stratigraphic analysis was carried out in the early-post-rift succession of central Neuquén Basin (Middle Jurassic Cuyo Group), integrating outcrop and subsurface information from a 3,000 km 2 area. Sedimentary facies analysis allowed the definition of six marine facies associations, which are grouped in two main depositional systems. During the late Toarcian to early Bathonian a storm-and wave-dominated shoreface to offshore system was developed. This is overlain by a late Bathonian-early Callovian fluvio-deltaic depositional system. The sequence stratigraphic analysis of this succession identified parasequences limited by marine flooding surfaces with little evidence of erosion. Parasequences could be grouped into four parasequence sets that show the evolution of the depositional systems and can be interpreted in terms of shoreline trajectories. In this sense, a log-term transgressive event defined by a complicated retrogradational stacking pattern and spanning for almost 10 My is identified for the older deposits. This succession is eventually replaced by a progradational unit that represents highstand conditions. The Cuyo succession identified in this part of the basin is completed by the development of a regressive surface and the onset of a deltaic depositional system with no parasequence development but indicating an abrupt basinward facies shift. The long-term transgression that dominates the early post-rift succession in this part of the basin is interpreted to represent a period of sustained accommodation creation produced by the combination of thermal subsidence, differential compaction of syn-rift deposits, and eustatic rise under a relatively low sediment supply. Changes in the stacking pattern of retrogradational parasequence sets are thought to be produced by changes in the bathymetry of the topography being flooded, which is a relic of a complex array of footwalls and shoulders generated during the syn-rift stage. Regional changes in thickness and timing for the early post-rift succession could be therefore the result of the relatively passive infill of an inherited topography of the syn-rift, suggesting that most of the accommodation was already created before the onset of the long-term transgressive trend that characterizes the post-rift succession in this part of the basin. The Cuyo Group succession reported here reflects the complexity in terms of depositional styles, sequence stratigraphic patterns, and controls that can be expected during the early-post rift infill of syn-rift structural depressions. Therefore, the results of this study can provide useful lessons for post-rift systems having prolific hydrocarbon production worldwide (e.g., in the Northern North Sea, Norwegian Sea, Thailand, and Argentina and Chile).

No abstract