Fluvial to aeolian transition in a lowstand wedge: sedimentology of the Lower Troncoso Member (Cretaceous) in central Neuquén Basin, Argentina.

VEIGA, G.D.

Congreso; III Congreso Latinoamericano de Sedimentología; 2003

The development of non-marine wedges in the centremost part of the Neuquén Basin (west-central Argentina) during periods of relative seal level fall is a common scenario in its Mesozoic history. The Cretaceous Lower Troncoso Member of the Huitrín Formation represents one of these non-marine wedges. It comprises fluvial and aeolian deposits that constitute one of the most important hydrocarbon reservoirs in this part of Argentina. Even when this unit shares some of its main characteristics with the previously accumulated lowstands it stands for some features that make it unique, suggesting different basin conditions than the ones governing the Jurassic and Lower Cretaceous fill in this area. The aims of this work are: a) to define the sedimentary environments developed during the accumulation of this unit, b) to analyse the lateral and vertical variability of these deposits, and c) to asses the main basin-scale factors that lead to its accumulation in order to build a framework to predict the behaviour of these deposits in the subsurface. This unit sharply overlies shallow-marine deposits across a major erosion surface interpreted as a high order sequence boundary. The basal deposits of this units have been accumulated in incised valleys towards the southern margin. The upper boundary is characterised also by a sharp contact with marine deposits (evaporites in this case) of the Upper Troncoso Member across a major flooding surface. The sedimentary fill of this unit comprises two distinctive facies associations. The basal portion is dominated by coarse-grained sandstones in lenticular bodies with a conspicuous erosive basal surface. They intercalate with massive or laminated fine-grained sandstones and mudstones that show desiccation cracks and rootlets. These deposits have been interpreted as fluvial channels and fine-grained vegetated floodplains developed due to recurrent floods. Channel deposits may have a simple fill mostly composed of coarse-grained, horizontally-stratified sandstones that grade upwards into fine-grained, cross-laminated sandstones and siltstones with current ripples. These were interpreted as the product of confined flood events. Channel deposits may have also a more complex fill, characterized by cross-bedded sandstones that represent channel fills and bars accumulated in a well developed fluvial system with more stable channels. Also bimodally sorted, fine- to medium-grained sandstones, with horizontal lamination or small-scale cross-stratification (sets up to 40 cm) are present. They were interpreted as aeolian sand sheets and small dunes developed primarily as the reworking of fluvial sediments and preserved as relicts due to fluvial erosion. Sharply overlying the fluvial facies association an aeolian succession is developed. These facies also have a more widespread distribution, as they directly overlie shallow-marine deposits of the previous sequences in more marginal areas and are not restricted to those places where the fluvial facies were deposited. This association is dominated at the base by an up to 10 m-thick succession mainly composed of small-scale (up to 40 cm thick) sets of cross-stratified, well sorted, fine- to medium-grained sandstones. These deposits are interpreted as the development of small aeolian dunes probably related to an undersaturated environment. Towards the top of the unit, large-scale cross-stratified sets (up to 10 m thick) of medium-grained sandstones are present reaching a total thickness of more than 30 m. They show wind-ripple lamination and the combination of grainfall and grainflow laminae, suggesting the development of more complex, slipfaced draas. Even when the upper portion of the unit is characterised by the development of an aeolian system in the whole studied area, the characteristics of these deposits vary considerably according to the position in the basin that they occupy. In those sector where the aeolian unit reaches its maximum thickness, this interval is characterised by amalgamated sets up to 10 m thick, bounded by 1st order surfaces that represent the development of transverse dunes with no record of interdunes. Towards more marginal areas, the system is replaced by superimposed sets that represent the development of complex longitudinal dunes with extensive interdune areas where small amount of aeolian deposits have been accumulated. Also isolated small aeolian dunes have been identified in marginal areas. The boundary between the two distinctive aeolian sequences is marked by the presence of an up to 1m thick conglomerate that represents an episode of important fluvial erosion. Towards the south, two events of aeolian accumulation have been identified for the upper portion, separated by a conspicuous surface with abundant rootlets that is also overlain by fine-grained deposits accumulated under subaqueous conditions. These lateral variations may reflect the interplay between sand supply and wind regime in different positions of the erg. Also the intercalation of subaqueous deposits suggest periods of contraction of the erg in erg-margin positions. Sudden flooding of the entire basin lead to the accumulation of marine evaporates directly on top of the aeolian deposits. This also produced the preservation of the original dune topography of up to 20 m and occurred together with the reworking and deformation of the uppermost aeolian interval in a shallow marine environment. The vertical evolution of the unit reflects a gradual increase in the relative importance of aeolian activity. This could be related to a decrease in fluvial activity (related to a reduction in rainfall) or to the gradual development of more effective winds that can transport and accumulate the available sand. However, in the basal portion of then unit aeolian deposits were identified intercalated between the fluvial facies, suggesting that wind activity was already important. Therefore the amount of aeolian accumulations could have been related to sand availability in a wet environment where water table was relatively high and consequently reducing the amount of sand available for transport. During dryer periods, when water table was well below the surface, sand availability increased considerably with the subsequent reworking of fluvial deposits. An overall water table lowering could have lead to the definite installation of an aeolian system, that at the beginning accumulated only small sets in a relatively undersaturated environment and then allowed the complete establishment of a major erg when all the sand in the system became available. This transition can be related to a progressive climate change and the development of dryer conditions, leading to the abandonment of fluvial sedimentation and to an overall water table lowering. It has been stated that a warm climate maximum has developed during Aptian/Albian times, evolving form a cold maximum in the Upper Jurassic. The transition from cold to warm climate might be the responsible for the vertical evolution recorded in the Lower Troncoso Member. But an important factor to be taken into account for the accumulation of this unit is that deep marine conditions that characterised the sedimentation in central Neuquén basin during periods of relatively high sea level are not going to be reached again in the basin evolution. This contrasts with other lowstand periods and suggests that a reconfiguration of the basin could have occurred, especially in the connection between the Neuquén engulfment and the proto-Pacific ocean to the west. This is evidenced by the development of restricted conditions and the accumulation of evaporates during the subsequent relative sea level rise. The reorganization of this connection may be related to the growth of the volcanic arc that limits the basin to the west, and associated with changes in subduction rates occurred during the transition from a back-arc/retro-arc environment to the foreland setting that will characterise the latest stages of evolution of the basin. This might have affected the accumulation of the Troncoso Member since the basin could have been completely isolated from the Pacific ocean during this period, generating a negative hydrological balance and resulting in a considerable water table lowering. Once the water table was low, an increase in sand availability occurred and the activity of strong winds (that were already active in the area) produced the development of the Troncoso erg. Furthermore, a growth in the volcanic arc and the development of higher elevations in the boundary between the basin an the open marine setting could have also affected the climate locally, generating a topographic barrier to the west, where moisture is left behind and therefore considerably reducing rainfall in the basin.