Addressing mixed facies interpretation difficulties by coupling sedimentary data with ichnofacies and microfossil data: an example from several paralic deposits in Brunei Darussalam.

<p>The geology of a depositional system can mostly be described by looking at sedimentary structures and sedimentary composition. However, in areas of complex shoreline mixed-process system (influenced by fluvial-tides-marine processes), many factors should be put into consideration. In Brunei, the mixed-sediment types occur extensively. The geology is mainly characterized here by thick Neogene siliciclastic facies ranging from fluvial, tidal and marine sediments deposited during periods of deltaic to shelfal setting, affected also by tectonic events. Due to this, differentiating tide and wave dominated facies is often a major challenge in the region.</p><p>In this study, it is emphasized that in order to support interpretations on these transitional facies, specific factors such as ichnofacies and microfossil content can be considered. Pollen and spores are more expected and useful in rather terrestrial systems, whilst in marine environments dinoflagellates, foraminifera and nannofossils could be convenient if preserved. Foraminifera and ichnofossils have the merit to be great indicators of a variety of sub-environments within the complex shallow water system.</p><p>The methods involve standard outcrop logging of fluvial, tidal and shallow marine outcrops, identifying lithology and key sedimentary features including trace fossils. Clay-rich samples were checked for microfossil content. Laboratory work involved extracting organic (pollen, spores, dinoflagellates) and calcareous (foraminifera, nannofossils) microfossils and documented them with light microscope (LM), stereo microscope, and Scanning Electron Microscope (SEM).</p><p>The results revealed that the most common trace fossil assemblages are the <em>Ophiomorpha, Cruziana</em> and <em>Skolithos</em> ichnofacies, and they refer to proximal marine settings. Among the calcareous microfossils recovered were very few coccolithophorids (<em>Sphenolithus abies</em> and<em> Sphenolithus moriformis</em>), which indicate very rare holomarine conditions, while the following benthic foraminifera genera were identified: <em>Ammonia, Nonion, Elphdium, Elphidiella, Quinqueloculina, Ammobaculites,</em> and <em>Trochammina</em>. Each of these genera have specific environmental requirements concerning hydrodynamics, trophic resources, oxygen content, substrate-type and deltaic influence. Results on pollen and spores, mangrove vegetation is marked by <em>Sonneratia</em> and <em>Rhizophora</em>-types, mixed-dipterocarp by <em>Shorea</em> spp., while peat swamp by <em>Verrucatosporites usmensis</em> and <em>Osmunda</em> sp.. Besides few dinoflagellate cysts (<em>Achomosphaera </em>sp., cf. <em>Exosphaeridium</em> sp., cf. <em>Operculodinium</em> sp., gen indet., <em>Lingulodinium? pycnospinosum</em> and <em>Tuberculodinium vancampoae</em>) and two acrtitarch taxa (<em>Cymatiosphaera</em> sp. and <em>Cymatiosphaera </em>cf. <em>nuda</em>) were found. These findings indicate incomplete sets of parasequences with palaeoenvironments of mixed shallow marine conditions. Mangrove pollen retrieved within tidal sediments indicates mangrove-dominated tide-influenced shoreline, while shoreline with diverse ichnofossils show coastal area connected to wave-dominated upper shoreface/ delta front. The calcareous foraminifera and nannofossil differentiate sediments belonging to lower shoreface to offshore/ prodelta deposits.</p>

The Art of Identifying Equatorial Waves

<div> <div> <div> <div> <p>Equatorial waves are synoptic- to planetary-scale propagating disturbances at low latitudes with frequencies from a few days to several weeks. Here this term includes Kelvin waves, equatorial Rossby waves, mixed-Rossby gravity waves, and inertio-gravity waves, which are closely related to linear wave theory, but also tropical disturbances, African easterly waves, and the intraseasonal Madden-Julian Oscillation. These waves can couple with deep convection, leading to a substantial modulation of rainfall. Recent work has shown that equatorial waves are amongst the dynamical features internal to the troposphere with the longest intrinsic predictability and that some models forecast them with an exploitable level of skill at lead times of up to a few weeks.</p> <p>A number of methods have been developed to identify and objectively isolate equatorial waves, both in (usually satellite) observations and in model fields. Most of these rely on (or at least refer to) the adiabatic, frictionless linearized primitive equations or shallow water system on the tropical beta plane. Common ingredients to these methods are longitude-time filtering (Fourier or wavelet) and/or projections onto predefined empirical or theoretical dynamical patterns. This paper aims to give an overview of the different methods to isolate the waves and their structures, to discuss underlying assumptions, to provide a systematic comparison, and to reveal advantages and disadvantages of each method. This way this study helps to optimally choose an approach suited to a given problem at hand and to avoid misuse and misinterpretation of the results.</p> </div> </div> </div> </div>

Inviscid, zero Froude number limit of the viscous shallow water system

In this paper, we study the inviscid and zero Froude number limits of the viscous shallow water system. We prove that the limit system is represented by the incompressible Euler equations on the whole space. Furthermore, the rate of convergence is also obtained.

Viscoelastic flows of Maxwell fluids with conservation laws

We consider multi-dimensional extensions of Maxwell’s seminal rheological equation for 1D viscoelastic flows. We aim at a causal model for compressible flows, defined by semi-group solutions given initial conditions, and such that perturbations propagates at finite speed. We propose a symmetric hyperbolic system of conservation laws that contains the Upper-Convected Maxwell (UCM) equation as causal model. The system is an extension of polyconvex elastodynamics, with an additional material metric variable that relaxes to model viscous effects. Interestingly, the framework could also cover other rheological equations, depending on the chosen relaxation limit for the material metric variable. We propose to apply the new system to incompressible free-surface gravity flows in the shallow-water regime, when causality is important. The system reduces to a viscoelastic extension of Saint-Venant 2D shallow-water system that is symmetric-hyperbolic and that encompasses our previous viscoelastic extensions of Saint-Venant proposed with F. Bouchut.