The late Miocene eolian record at the eastern margin of the Puna Plateau, NW Argentina: Evidence of upper-tropospheric paleocirculation

The Puna–Altiplano Plateau of the Central Andes is the second-highest plateau in the world (after Tibet), with a mean elevation of 4000 m.a.s.l. and an arid to hyperarid climate. Uplift of the Puna–Altiplano Plateau has affected lower-level atmospheric circulation, acting as a barrier to humid easterly winds from the Amazon basin and favoring an across-strike precipitation gradient resulting in a humid climate towards the east of the plateau and an arid to hyperarid climate in the orogen's interior. In the modern climate, the Bolivian High anticyclone regulates upper troposphere circulation, but little is known about the high-altitude tropospheric circulation of the past. This work focuses on the eolian record of the San Antonio de los Cobres basin along the eastern border of the Puna Plateau, NW Argentina, with the aim of analyzing its origin and thus elucidating the late Miocene winds. The eolian deposits are constrained by 7.8 Ma (K/Ar and U/Pb) and 6.4 Ma (U/Pb) ignimbrites at the nearly basal and upper contacts, respectively. Based on stratigraphic, sedimentological, and provenance analysis of the eolian units, we have identified three main facies associations (FAs): FA1) cross-stratified sandstones with large- to small-scale tabular, planar cross-bedding and with trough cross-stratification; FA2) sandstones with planar to low-angle stratification associated with thinly laminated ripple sandstone strata; FA3) medium- to coarse-grained massive sandstones associated with pebbly to bouldery, matrix-supported conglomerates and clast-supported conglomerates. The lateral and vertical facies assemblages indicate a dune field confined to topographic depressions dominated by transverse dunes with straight and sinuous crestlines that laterally grade into sandsheets associated with ephemeral streams. Paleoflows, lithotypes, and grain-size determinations indicate a persistent north-northwest provenance and wind velocities of 24–38 km/h (with maximum velocities of 55–75 km/h). The results of our analysis coupled with data from previous studies indicates that, for at least the last ca. 8 Myr, the winds have been blowing constantly from the north-northwest with an intensity similar to the present. This implies that the paleo-atmospheric circulation had a similar pattern to the present-day one. Therefore, we conclude that the upper-troposphere circulation in the Puna Plateau of NW Argentina was already regulated by the Bolivian High anticyclone during the Miocene, generating constant north-northwesterly winds.

Paleoenvironmental insights from the deposition and diagenesis of Aptian pre-salt magnesium silicates from the Lula Field, Santos Basin, Brazil

ABSTRACT Since the discovery of giant Aptian pre-salt reservoirs in Brazilian margin basins, the study of lacustrine carbonates has drawn great attention from the scientific community. Comparatively, minor attention was given to the characterization and genesis of the Mg-silicates (e.g., stevensite, kerolite) which are commonly associated with these carbonates. A systematic petrological study was performed in the Aptian Barra Velha Formation (BVF) within distinct structural compartments of the giant Lula Field in the Santos Basin, in order to recognize the patterns of primary formation and diagenetic alteration of these Mg-silicates. Mg-silicates occur as peloids, ooids, intraclasts, and fine-grained laminated deposits, either mixed in variable proportions with other particles, such as carbonate bioclasts and volcanic rock fragments, or constituting specific intrabasinal deposits. In the BVF interval, clay peloids and laminated deposits are associated with spherulitic and fascicular calcite aggregates, as substrate and hosts for these precipitates. Ooids are interpreted as formed at the sediment–water interface by the nucleation of concentric envelopes on the surface of particles (heterogeneous nucleation), through repeated rolling under gentle wave and current action. Laminated deposits, interpreted as precipitated directly from the water column (homogeneous nucleation) in highly supersaturated and low-hydrodynamic-energy environments, constitute extensive deposits in the BVF. Peloids were probably formed in intermediate energy conditions. Some ooidal arenites show porosity from the dehydration and contraction, and/or the dissolution of ooids. In some rocks, these pores are filled with fibrous calcite, while the remaining Mg-silicates are replaced by dolomite, calcite, or silica. A similar diagenetic pattern occurs in the laminated deposits, where magnesite and dolomite fill shrinkage pores formed along their characteristic wavy laminae. Owing to their elevated solubility, most of the Mg-silicates were dissolved, or intensely replaced by calcite, dolomite, or silica. The detailed petrologic analysis indicates that the original volumes of Mg-silicates were substantially larger, and that their deposition was widespread in the basin, including on structurally high areas. The types and intensity of diagenetic alteration of the Mg-silicate deposits are distinct for each structural compartment, being more intense towards the highs and closer to the overlying evaporites, which imposed a strong influence on reservoir quality.

How distinctive are flood-triggered turbidity currents?

ABSTRACT Turbidity currents triggered at river mouths form an important highway for sediment, organic carbon, and nutrients to the deep sea. Consequently, it has been proposed that the deposits of these flood-triggered turbidity currents provide important long-term records of past river floods, continental erosion, and climate. Various depositional models have been suggested to identify river-flood-triggered turbidite deposits, which are largely based on the assumption that a characteristic velocity structure of the flood-triggered turbidity current is preserved as a recognizable vertical grain size trend in their deposits. Four criteria have been proposed for the velocity structure of flood-triggered turbidity currents: prolonged flow duration; a gradual increase in velocity; cyclicity of velocity magnitude; and a low peak velocity. However, very few direct observations of flood-triggered turbidity currents exist to test these proposed velocity structures. Here we present direct measurements from the Var Canyon, offshore Nice in the Mediterranean Sea. An acoustic Doppler current profiler was located 6 km offshore from the river mouth, and provided detailed velocity measurements that can be directly linked to the state of the river. Another mooring, positioned 16 km offshore, showed how this velocity structure evolved down-canyon. Three turbidity currents were measured at these moorings, two of which are associated with river floods. The third event was not linked to a river flood and was most likely triggered by a seabed slope failure. The multi-pulsed and prolonged velocity structure of all three (flood- and landslide-triggered) events is similar at the first mooring, suggesting that it may not be diagnostic of flood triggering. Indeed, the event that was most likely triggered by a slope failure matched the four flood-triggered criteria best, as it had prolonged duration, cyclicity, low velocity, and a gradual onset. Hence, previously assumed velocity-structure criteria used to identify flood-triggered turbidity currents may be produced by other triggers. Next, this study shows how the proximal multi-pulsed velocity structure reorganizes down-canyon to produce a single velocity pulse. Such rapid-onset, single-pulse velocity structure has previously been linked to landslide-triggered events. Flows recorded in this study show amalgamation of multiple velocity pulses leading to shredding of the flood signal, so that the original initiation mechanism is no longer discernible at just 16 km from the river mouth. Recognizing flood-triggered turbidity currents and their deposits may thus be challenging, as similar velocity structures can be formed by different triggers, and this proximal velocity structure can rapidly be lost due to self-organization of the turbidity current.

Composite microbialites: Thrombolite, dendrolite, and stromatolite associations in a modern environment, Pozo Bravo lake, Salar de Antofalla, Catamarca Puna, Argentina

ABSTRACT Pozo Bravo is a high-altitude Andean lake that harbors modern microbialites thriving in hypersaline conditions in the Salar de Antofalla, one of the driest sites on Earth and located in the Puna region of Catamarca, northwest Argentine. Due to the lake physiography, microbialites are restricted to a narrow belt following Pozo Bravo lake variations. Microbialites exhibit a wide range of external morphologies including domal, discoidal, tabular, and horseshoe-like bioherms which vary considerably in size, as well as large biostromal terraces. As documented by other studies on modern microbialites, external morphology appears to be mainly the product of the environmental setting. In Pozo Bravo lake, high evaporation rates and hypersalinity (driven by high temperature and strong winds), water-level fluctuations, and lake-bottom topography are major controlling factors. The distinctive feature of Pozo Bravo microbialites is their internal structure, showing a gradual transition from a thrombolitic core to dendrolitic structures and to a sharply overlying stromatolitic layer within a single microbialite. We suggest that these various microbialite textures represent a gradual change within an environmental gradient based on lake-level variations, and the influence of these environmental factors on biological activity, mainly by cyanobacteria and diatoms. The study of this site is particularly relevant given that it represents an active system where progressive changes in microbialite type (from thrombolites to dendrolites and stromatolites) are recorded, providing an excellent natural laboratory to study these textural changes from a mechanistic perspective, and it may provide insights for better understanding of the microbialite geological record. In addition, given that these systems are threatened by human activities (mining of lithium-rich brines), its study and preservation are necessary.

Defining bounding surfaces within and between eolian and non-eolian deposits, Lower Jurassic Navajo Sandstone, Moab Area, Utah, U.S.A.: Implications for subdividing erg system strata

ABSTRACT A model-independent, sequence stratigraphic approach is used to define bounding surfaces in the Navajo Sandstone in order to identify an architectural hierarchy of genetically related sedimentary packages and the surfaces that bound them across multiple scales of both eolian and non-eolian components of an erg system. Seven bounding surfaces and eight depositional units are defined, from small to large scale. A lamina-deviation surface bounds wedge- and tabular-shaped sets of laminae and/or laminasets, separating those that have different angle orientations on the dune slipface. A bed-deviation surface bounds a succession of beds (crossbeds) that lie at different angles or orientations to bedding above, below, or adjacent to it. A bedset-deviation surface is curved, inclined, and/or wavy and irregular that bounds bedsets and their internal stratification patterns; that is, bed-deviation surfaces, and lamina-deviation surfaces. A simple surface is gently inclined with or without small, concave or convex segments that bound beds and bedsets. A composite surface is horizontal with or without concave, curved, or irregular portions of that surface. A complex surface is laterally extensive (∼ 1–10+ km) that regionally bounds and truncates underlying conterminous and interfingered eolian and non-eolian strata. An amalgamated surface is a regionally extensive (∼ 10 to 100s km) mappable unconformity, merged unconformities, and their laterally equivalent conformable surface that can exhibit local to regional pedogenic modification, lags, and significant (meters to 10s m) paleotopographic relief. The genetically related sedimentary packages typically bounded by like or higher-rank surfaces are defined as laminae, laminasets, bed, bedsets, and simple, composite, complex, and amalgamated units. Field relationships of strata and surfaces are key to reconstructing the interactions between eolian and non-eolian deposits and the processes they represent at the local, regional, and basin scale. This classification scheme can be applied to erg-system strata to fully integrate changes in diverse facies within and between contiguous deposits.

Fluid-inclusion petrography in calcite stalagmites: Implications for entrapment processes

ABSTRACT Fluids trapped in speleothems have an enormous potential in frontier fields of paleoclimate and paleohydrological research. This potential is, however, hampered by diverse scientific and technical limitations, among which the lack of a systematic methodology for genetically characterizing fluid inclusions is a major one, as these can have different origins, and thus, the trapped fluid (usually water), different meanings. In this work, we propose a systematic petrological classification of fluid inclusions, based on: 1) the temporal relation between fluid inclusions and the host calcite, 2) the spatial relation between fluid inclusions and the “crystallites” and crystals aggregates, and 3) the phases (water, air) trapped inside fluid inclusions. The first criterion allows dividing fluid inclusions in two main categories: primary and secondary, whose identification is critical in any research based on trapped fluids. The other two criteria allow the definition of eight types of primary and four types of secondary fluid inclusions. Primary fluid inclusions contain the drip water that fed stalagmites at the time of crystal growth, and can be intercrystalline, i.e., located between adjacent crystallites, or intracrystalline, i.e., with the fluid trapped within crystallites. We differentiate six main types among the intercrystalline fluid inclusions (elongate, thorn-shaped, down-arrow, interbranch, macro-elongate, and bucket) and other two among intracrystalline inclusions (pyriform and boudin). In primary inclusions, water is the main phase, while gas is much less abundant. The presence of gas could be related to slow drip rates or degassing in the cave, but also to later leakage due to changes in temperature and humidity often occurring during inadequate handling of speleothem samples. Secondary fluid inclusions were clearly related to younger water inlet through stratigraphic disruptions or unconformities. They are formed after water infiltration, but sealed before the renewed crystal growth. We differentiate four main types of secondary inclusions: interconnected, rounded, triangular, and vertical fluid inclusions. The identification of primary and secondary fluid inclusions in speleothems is a key for interpretation in paleoclimate studies. Integration of petrological results allow establishment of three different genetic scenarios for the formation of fluid inclusions, whose identification can be relevant because of their predictive character.

Effects of sea level and upwelling on development of a Miocene shallow-water tropical carbonate ramp system, Ponce, Puerto Rico

ABSTRACT A Miocene (Langhian–Tortonian, ca. 15–10 Ma) tropical ramp system exposed in southern Puerto Rico is characterized by shallow-water facies consisting of heterozoans, red algae, large benthic foraminifera (LBF), and corals, which occur as isolated corals, segment- and cluster-type reefs, and reworked accumulations. Photozoan association components are limited to corals (Montastraea, Porites, Goniopora, and Agaricia) and LBF (amphisteginids, soritids, gypsinids, miliolids) that have been documented to tolerate elevated nutrients, turbidity, and cooler water conditions. Similar shallow-water carbonate systems are found throughout the Caribbean, and this regional development is thought to have resulted from the well-documented upwelling in the Caribbean during the Miocene. Sea-level fluctuations also exerted a major control on facies distributions and shifts in the Puerto Rico ramp, including a vertical facies pattern that occurs in each of three sequences. Basal parts of sequences, deposited during sea-level rises, are dominantly composed of mollusks, echinoderms, red algae, LBF, bryozoans, and solitary corals that formed in low-energy seagrass-bed environments with local associated higher-energy shoal environments. Coral facies occur only in upper parts of sequences and formed in shallow-water, low- to high-energy environments closely associated with seagrass beds during late highstands and sea-level falls. A similar vertical facies pattern occurs in time-equivalent sequences elsewhere around the Caribbean. Strontium-isotope age data indicate two sequence boundaries reflecting sea-level falls formed at about 12.3 Ma and 11.1 Ma. Correlation with time-equivalent unconformities in other well-dated areas in the Caribbean and to sea-level lows on eustatic curves suggests a global signature for sequence development. The connection between the Caribbean and the Pacific along the Central American Seaway (CAS), impacted by local tectonic episodes and sea-level fluctuations during the Miocene, affected nutrient influx and upwelling in the Caribbean, which may be reflected in the vertical facies pattern in shallow-water carbonate sequences. Times of restricted connection during sea-level falls and lows resulted in reduced nutrients and upwelling, which may have been more conducive to coral development. Time-equivalent tropical carbonate systems in the Mediterranean and Indo-Pacific show similarities to those in the Caribbean, indicating influence of global processes (cooling, temperature gradients, oceanographic circulation). Differences between areas indicates the importance of local and regional controls, which in the Caribbean was dominantly the opening and closure of the CAS.

Coarse-grained meandering distributive fluvial system of the basal Cedar Mountain Formation, U.S.A.

ABSTRACT The analysis of downstream changes in ancient fluvial systems can better inform depositional models for foreland-basin systems. Herein we analyze the basal deposits of the Early Cretaceous Cedar Mountain Formation of Utah to better understand the variety of fluvial deposits present and to develop a depositional model for the Sevier foreland basin. We also evaluate the long-held interpretation of a braided origin for these deposits and document numerous examples of point-bar deposition in highly sinuous meandering rivers by analysis of large (20 to 60 km2) plan-view exposures. These plan-view exposures allow comparisons between planform and cross-sectional geometries. The study utilizes outcrop data, virtual outcrop models, and satellite imagery to develop a facies model and analyze the architecture of channel bodies in the Buckhorn Conglomerate and Poison Strip Sandstone of the Cedar Mountain Formation. We document downstream (west to east) decreases in lateral channel migration, sinuosity, channel amalgamation, grain size, and percent of fluvial channel facies (conglomerate and sandstone). Fluvial channel deposits occur arranged into larger stratal bodies: multistory–multilateral channel bodies that are dominantly composed of clast-supported conglomerate in the west to a mix of multistory, multilateral, and isolated channel bodies composed of matrix-supported conglomerate in the east. The median width of highly sinuous point bars is similar across the field area (344 m to 477 m), but the inclusion of narrower (median = 174 m), low-sinuosity bar elements in the east indicates an overall reduction in lateral channel migration and sinuosity downstream. Net-to-gross values range from 100% in much of the western outcrops to as low as 38% in the east. Paleocurrent analysis reveals a transverse (west to east) paleoflow for the study interval that merges with axial (south–north) paleoflow near the Utah–Colorado state line. We estimate 104 m3/s-scale discharge and 106 kilometer-scale drainage area for axial rivers based on paleohydraulic analysis which represents a significant part of the Early Cretaceous continental-scale drainage. The observed downstream trends in lateral channel migration, sinuosity, channel amalgamation, grain size, and net-to-gross for the basal Cedar Mountain Formation are consistent with expected trends for sinuous single-thread distributive fluvial systems and are similar to observed trends in the Jurassic Morrison Formation. Medial (Buckhorn Conglomerate) to distal (Poison Strip Sandstone) zones are preserved and span the forebulge to backbulge depozones of a foreland-basin system. Postulated deposits of the proximal distributive fluvial system have been removed during erosion of the foredeep depozone. The easternmost Poison Strip Sandstone and coeval Burro Canyon Formation represent deposits of an axial system at which western-sourced distributive fluvial systems end. Distributive fluvial systems dominate modern foreland basins, and this study suggests that they may constitute a significant proportion of ancient successions.

Ichnofabric analysis as a tool for characterization and differentiation between calcareous contourites and calciturbidites

ABSTRACT The Eocene–Miocene Cyprus paleoslope system records complex deep-marine sedimentation comprising background vertical settling of autochthonous pelagic–hemipelagic particles (chalks) which were punctuated by calcareous bottom currents (contourites) and gravity flows (calciturbidites). The Eocene Lefkara Formation at the Petra Tou Romiou beach section (Cyprus) shows the incidence of deep-marine bottom currents and distal turbiditic episodes in a context of pelagic–hemipelagic sedimentation. Trace-fossil analysis of this section, using an ichnofabric approach (i.e., ichnodiversity, Bioturbation Index, Bedding Plane Horizontal Index and crosscutting relationships), was conducted to precisely describe the paleoenvironmental conditions of this complex setting. Ichnofabric analysis allow the characterization and differentiation of sporadic turbiditic events that disrupted both pelagic–hemipelagic and contourite deposition. Calciturbidite intervals show ichnofabrics consisting of postdepositional U-shaped traces (i.e., Arenicolites isp., ?Diplocraterion isp.,) and vertical borings typical of consolidated substrates. High-energy sandy contourite deposits are dominated by horizontal deposit-feeder traces and the development of ichnofabrics with Planolites isp., and Thalassinoides isp. The record of ichnofabrics with slightly deformed Planolites in the interbeds of sandy contourites or in the transition between the facies reveals variations in sedimentation in the bi-gradational contourite succession, and can potentially act as an indicator of depositional hiatus.

The initiation of the Mesoproterozoic Bylot basins (Nunavut, Arctic Canada) as recorded in the Nyeboe Formation, Fury and Hecla Group

ABSTRACT Reconstructing Precambrian sedimentary environments over broad cratonic regions often relies on a combination of facies, structural, and provenance analyses. The Mesoproterozoic (ca. 1270–1090 Ma) Fury and Hecla Group, exposed on the Melville Peninsula and northern Baffin Island (Nunavut, Canada), is considered broadly correlative with strata of the Borden, Hunting–Aston, and Thule basins (together referred to as the Bylot basins). We present the results of updated mapping and the first high-resolution sedimentologic and stratigraphic analysis of the lowermost unit in the Fury and Hecla Group, the Nyeboe Formation. The Nyeboe Formation comprises five distinct facies associations: alluvial to fluvial, eolian-backshore, marine-intertidal, marine foreshore to shoreface, and marine-offshore. Thin mafic units are interbedded within the marine shoreface facies and are interpreted to represent volcanic flows. Lateral relationships between facies associations are complex, but generally, facies associations transition from a terrestrial environment at the base to a nearshore marine environment at the top, indicating a transgression. Considering both the along-strike and -dip thickness trends, the presence of mafic volcanic rock units, and possible syndepositional fault orientations crosscutting the deposits, we infer that the Fury and Hecla Group was deposited in a regime of crustal thinning in a half-graben setting. Our results from the Nyeboe Formation suggest a lithostratigraphic correlation to the Nauyat and Adams Sound formations of the Borden Basin. Therefore, this study establishes a geodynamic link between the opening of the Fury and Hecla Basin to the other Bylot basins and contributes to the understanding of a large late Mesoproterozoic intracontinental-basin system.