Recognition criteria,characteristics and implications of the fluvial to marine transition zone in ancient deltaic deposits (Lajas Formation,Argentina)

The seaward end of modern rivers is characterized by the interactions of marine and fluvial processes, a tract known as the fluvial to marine transition zone, which varies between systems due to the relative strength of these processes. To understand how fluvial and tidal process interactions and the fluvial to marine transition zone are preserved in the rock record, large‐scale outcrops of deltaic deposits of the Middle Jurassic Lajas Formation (Neuquén Basin, Argentina) have been investigated. Fluvial–tidal indicators consist of cyclically distributed carbonaceous drapes in unidirectional, seaward‐oriented cross‐stratifications, which are interpreted as the result of tidal modulation of the fluvial current in the inner part of the fluvial to marine transition zone. Heterolithic deposits with decimetre‐scale interbedding of coarser‐grained and finer‐grained facies with mixed fluvial and tidal affinities are interpreted to indicate fluvial discharge fluctuations (seasonality) and subordinate tidal influence. Many other potential tidal indicators are argued to be the result of fluvial–tidal interactions with overall fluvial dominance or of purely fluvial processes. No purely tidal or tide‐dominated facies were recognized in the studied deposits. Moreover, fluvial–tidal features are found mainly in deposits interpreted as interflood (forming during low river stage) in distal (delta front) or off‐axis (interdistributary) parts of the system. Along major channel axes, the interpreted fluvial to marine transition zone is mainly represented by the fluvial‐dominated section, whereas little or no tide‐dominated section is identified. The system is interpreted to have been hyposynchronous with a poorly developed turbidity maximum. These conditions and the architectural elements described, including major and minor distributary channels, terminal distributary channels, mouth bars and crevasse mouth bars, are consistent with an interpretation of a fluvial‐dominated, tide‐influenced delta system and with an estimated short backwater length and inferred microtidal conditions. The improved identification of process interactions, and their preservation in ancient fluvial to marine transition zones, is fundamental to refining interpretations of ancient deltaic successions.     

How Do Surficial Lithic Assemblages Weather in Arid Environments? A Case Study from the Atacama Desert,Northern Chile

Archaeological sites composed only of surficial lithics are widespread in arid environments. Numerical dating of such sites is challenging, however, and even establishing a relative chronology can be daunting. One potentially helpful method for assigning relative chronologies is to use lithic weathering, on the assumption that the most weathered artifacts are also the oldest. Yet, few studies have systematically assessed how local environmental processes affect weathering of surficial lithics. Using macroscopic analyses, we compared the weathering of surficial lithic assemblages from seven mid‐to‐late Holocene archaeological sites sampled from four different microenvironments in the Atacama Desert of northern Chile. Changes in polish, texture, shine, and color were used to establish significant differences in weathering between two kinds of locations: interfluves and canyon sites. Lithics from interfluve sites were moderately to highly weathered by wind and possessed a dark coating, whereas canyon lithics were mildly weathered despite greater exposure to moisture, often lacked indications of eolian abrasion, and lacked dark coatings. Our results show that lithic weathering can be used as a proxy for relative age, but only after considering local environmental factors. The power of such chronologies can be improved by combining archaeological, paleoenvironmental, geomorphological, and taphonomic data.     

Sediment distribution and depositional processes along the fluvial to marine transition zone of the Mekong River delta,Vietnam

The area of coastal rivers with a combination of fluvial, tidal and wave processes is defined as the fluvial to marine transition zone and can extend up to several hundreds of kilometres upstream of the river mouth. The aim of this study is to improve the understanding of sediment distribution and depositional processes along the fluvial to marine transition zone using a comprehensive dataset of channel bed sediment samples collected from the Mekong River delta. Six sediment types were identified and were interpreted to reflect the combined action of fluvial and marine processes. Based on sediment‐type associations, the Mekong fluvial to marine transition zone could be subdivided into an upstream tract and a downstream tract; the boundary between these two tracts is identified 80 to 100 km upstream of the river mouth. The upstream tract is characterized by gravelly sand and sand and occasional heterolithic rhythmites, suggesting bed‐load supply and deposition mainly controlled by fluvial processes with subordinate tidal influence. The downstream tract is characterized by heterolithic rhythmites with subordinate sand and mud, suggesting suspended‐load supply and deposition mainly controlled by tidal processes with subordinate fluvial influence. Sediment distributions during wet and dry seasons suggest significant seasonal changes in sediment dynamic and depositional processes along the fluvial to marine transition zone. The upstream tract shows strong fluvial depositional processes with subordinate tidal influence during the wet season and no deposition with weak fluvial and tidal processes during the dry season. The downstream tract shows strong coexisting fluvial and tidal depositional processes during the wet season and strong tidal depositional processes with negligible fluvial influence during the dry season. Turbidity maxima are present along the downstream tract of the fluvial to marine transition zone during both wet and dry seasons and are driven by a combination of fluvial, tidal and wave processes.     

Partitioning between “wedge-top” river- and wave-dominated successions: an example from the late Tortonian — early Messinian Terravecchia Formation (NW Sicily)

Detailed field sedimentological and facies analyses have been performed in the Terravecchia Formation cropping out in NW Sicily, in order to differentiate and describe, for the first time, wave- and river-dominated shallow-marine (deltaic) siliciclastic successions. The latter were deposited filling syntectonic basins, developed between the late Tortonian and early Messinian time, within the wedge-top depozone of the Sicilian Foreland Basin System. It has been observed that river-dominated successions, recording the deposition of small fan-deltas are characterized by fining- to coarsening upward, transgressive-regressive sequences which were mostly deposited filling relatively narrow and often oversupplied basins. These basins were probably located in a proximal sector of the wedge-top depozone, closer to emergent sectors of the chain and probably sheltered from the main marine areas. Wavedominated successions, on the other hand, are characterized by upward fining, mostly transgressive sequences which were deposited filling wider basins open to major marine regions and located in a more distal position of the wedge-top depozone. The documented partitioning between river- and wave-dominated successions, as a function of the position of the sedimentary basin within the wedge-top depozone, is coherent with data from analyses of the deformational patterns of the Terravecchia Formation in this study area. Furthermore, the data here presented could be considered as a preliminary database for future characterization and analysis of siliciclastic reservoirs from Sicilian outcrop analogues.     

The “Camporeale wedge-top Basin” (NW Sicily; Italy) in the frame of the Late Miocene Sicilian Foreland Basin System; Inferences from the Upper Tortonian–Lower Messinian Terravecchia Formation

Since the latest Oligocene–earliest Miocene the building of the Sicilian fold and thrust belt has been accompanied by development of a “peripheral” foreland basin system which migrated toward the foreland. In north-western Sicily, the sedimentary record of the foreland basin system migration is represented by a stratigraphic succession made up of several lithostratigraphic units, bounded by regional unconformity surfaces, deposited recording at least four main sedimentary phases, each characterized by the development of different types of syntectonic basins.     

Adaptive POD model reduction for solute transport in heterogeneous porous media

We study the applicability of a model order reduction technique to the solution of transport of passive scalars in homogeneous and heterogeneous porous media. Transport dynamics are modeled through the advection-dispersion equation (ADE) and we employ Proper Orthogonal Decomposition (POD) as a strategy to reduce the computational burden associated with the numerical solution of the ADE. Our application of POD relies on solving the governing ADE for selected times, termed snapshots. The latter are then employed to achieve the desired model order reduction. We introduce a new technique, termed Snapshot Splitting Technique (SST), which allows enriching the dimension of the POD subspace and damping the temporal increase of the modeling error. Coupling SST with a modeling strategy based on alternating over diverse time scales the solution of the full numerical transport model to its reduced counterpart allows extending the benefit of POD over a prolonged temporal window so that the salient features of the process can be captured at a reduced computational cost. The selection of the time scales across which the solution of the full and reduced model are alternated is linked to the Péclet number (P e), representing the interplay between advective and dispersive processes taking place in the system. Thus, the method is adaptive in space and time across the heterogenous structure of the domain through the combined use of POD and SST and by way of alternating the solution of the full and reduced models. We find that the width of the time scale within which the POD-based reduced model solution provides accurate results tends to increase with decreasing P e. This suggests that the effects of local-scale dispersive processes facilitate the POD method to capture the salient features of the system dynamics embedded in the selected snapshots. Since the dimension of the reduced model is much lower than that of the full numerical model, the methodology we propose enables one to accurately simulate transport at a markedly reduced computational cost.     

Estimating building exposure and impact to volcanic hazards in Icod de los Vinos,Tenerife (Canary Islands)

Principal and subsidiary building structure characteristics and their distribution have been inventoried in Icod, Tenerife (Canary Islands) and used to evaluate the vulnerability of individual buildings to three volcanic hazards: tephra fallout, volcanogenic earthquakes and pyroclastic flows. The procedures described in this paper represent a methodological framework for a comprehensive survey of all the buildings at risk in the area around the Teide volcano in Tenerife. Such a methodology would need to be implemented for the completion of a comprehensive risk assessment for the populations under threat of explosive eruptions in this area. The information presented in the paper is a sample of the necessary data required for the impact estimation and risk assessment exercises that would need to be carried out by emergency managers, local authorities and those responsible for recovery and repair in the event of a volcanic eruption. The data shows there are micro variations in building stock characteristics that would influence the likely impact of an eruption in the area. As an example of the use of this methodology for vulnerability assessment, we have applied a deterministic simulation model of a volcanic eruption from Teide volcano and its associated ash fallout which, when combined with the vulnerability data collected, allows us to obtain the vulnerability map of the studied area. This map is obtained by performing spatial analysis with a Geographical Information System (GIS). This vulnerability analysis is included in the framework of an automatic information system specifically developed for hazard assessment and risk management on Tenerife, but which can be also applied to other volcanic areas. The work presented is part of the EU-funded EXPLORIS project (Explosive Eruption Risk and Decision Support for EU Populations Threatened by Volcanoes, EVR1-2001-00047).     

Theoretical analysis of X-ray absorption near-edge structure in forsterite, Mg2SiO4-Pbnm, and fayalite, Fe2SiO4-Pbnm, at room temperature and extreme conditions

We calculated the forsterite Mg K-edge and the fayalite Fe K-edge X-ray absorption spectra both for the M 1 and M 2 sites and for the overall edge by using the one-electron multiple-scattering theory. The validity of the theoretical model is well illustrated by comparison of calculations with experimental data at the Mg K-edge of MgO (periclase) and at the Mg and Fe K-edges spectra of forsterite and fayalite. Starting from these results at room conditions, we calculated the Mg and Fe K-edges X-ray absorption spectra of forsterite and fayalite at low and high temperatures and at high pressures as well. Variations of fine structures occur mostly in the intermediate multiple scattering (IMS) regions and as a result of the applied pressure. In order to demonstrate the capability of XAS to lead to deeper knowledge of structure relevant to Earth's upper mantle we also attempted calcuating the high-P edge for Fe ²⁺ in low-spin using a different occupation of valence electrons. If a change in spin state really occurs in fayalite, our simple model shows that XAS would evidence it easily even with low resolution.