Depositional Record最新文献

The present study examines the stratigraphy of two sandspits that both appeared and developed since the middle of the 17th century during the Little Ice Age. The Arçay spit is located along the macrotidal Atlantic coast in South-West France with a maximum tidal range of 6.5 m. The Pointe du Banc in the English Channel, North-West France, is located in a hypertidal coastal setting with a tidal range of up to 14 m. The evolution of the two spit systems has been compared using historical maps, ground penetrating radar data and facies analysis and geochronological data from sediment cores. The Pointe du Banc spit developed between 1650 and 1750 CE, in a dominant seaward direction while the main mode of construction of the Arçay spit was in a longshore direction. It is proposed that this difference relates to sediment supply and coastline morphology. At the Pointe du Banc spit, the large tidal range causes a long wind fetch and sustained aeolian sediment supply. Moreover, the spit is located at the apex of a large-scale embayment where sediment transport from north and south converges. Combined, these factors result in a positive sediment budget and seaward shoreline progradation. At the Arçay spit, high wave obliquity results in a large littoral drift and sustained longshore spit construction. At both locations, low gradient shorefaces may have favoured a net landward-directed sediment flux that supply sand to the foreshore.

The Río Alías Strait developed in the Early Pliocene as a narrow marine corridor at the connection of the microtidal Mediterranean Sea and the north-eastern margin of the Almería-Níjar Basin in the eastern Betic Cordillera (South-East Spain). The orientation and topography of the strait were controlled by the transpressive Carboneras and Polopos/South Cabrera fault systems. Ten sedimentary facies occur in the up to 150 m thick mixed biogenic carbonate-terrigenous succession distinguished on the basis of their lithology, components, grain size, stratal geometries and sedimentary structures, which were observed in seven sections at well-exposed outcrops of four sectors. The sedimentary record of the Río Alías Strait reflects the morphological constraints, which conditioned its sedimentary dynamics and facies distribution. Even in this microtidal setting, tidal current amplification through narrow constrictions produced thick accumulations of large cross-stratified bodies up to 15 m thick formed by the opposite migration of three-dimensional simple and compound dunes. The Río Alías Strait reconstruction shows: (1) a very narrow constriction in the central sector from which “constriction-related deltas” (CRDs) formed in the flood downstream (westward) and ebb (upstream) directions and (2) a relatively deep depression (>65 m water depth) separating the eastern and central-east sectors, where tidal current energy was attenuated and dunes were not generated. The closure of the strait resulted from the tectonic uplift of the antecedent upland of Sierra Cabrera at the northern side, which promoted the southward progradation of deltaic systems over the strait. The Río Alías Strait represents the only clear record of a microtidal strait in the Betic Cordillera since the Miocene. The case study presented here improves existing models on the sedimentary dynamics of ancient tidal-dominated straits by expanding the knowledge on their spatial environment variability.

The Selendi Basin developed as a supradetachment basin on the Simav detachment fault in response to the continental extension in western Turkey (Türkiye) and the associated Menderes Massif uplift. Process-based facies analyses were used to define and interpret the depositional evolution of the lacustrine Selendi Basin. The Early-Middle Miocene deposits are composed of stream-dominated alluvial fans, foreshore, shoreface, shoal-water delta and offshore transition facies assemblages, which are laterally and vertically transitional throughout the succession. The alluvial fan and nearshore deposits reveal that the topography of the basin floor and the coastal plain was gently inclined. Shoal-water deltas, 70–250 cm thick, in the delta complex reflect comparable water depths and a relative rise in lake level, ultimately attributable to basin subsidence. Shallow environmental conditions prevailed during the basin's evolution, as evidenced by widespread nearshore deposits along its southern margin. The facies assemblages forming the basin's southern part show a predominantly aggradational stacking pattern, recording a balance between the sediment supply and a steadily growing basin accommodation. The balanced fill deposition and aggradational stacking pattern of the sedimentary facies associations indicate that the rate of basin subsidence and the uplift of the Menderes Massif was relatively consistent. This study shows that the intensity of a synsedimentary fault system, especially one leading to basin opening, plays a crucial role in controlling the architecture and sedimentary facies of the basin succession, both by creating accommodation and by filling it.

Deep-water megabeds are a particular type of sediment gravity flow deposit that are anomalously thick and often of distinctive composition compared to the deep-water strata within which they are embedded. Pure siliciclastic or carbonate megabeds have been widely reported from deep-marine systems. Less documented are carbonate-rich mixed megabeds with abundant carbonate clasts in a siliciclastic matrix, which are embedded in siliciclastic deep-water systems. Here, such examples are reported from outcrops of the Lower Triassic in the West Qinling orogenic belt, central China, with a focus on the character, processes and implications of these carbonate-rich megabeds. Based on regional geology and characteristics of the encasing siliciclastic turbidites and autochthonous micritic limestones, these megabeds are inferred to have been deposited in a deep marine trough. The megabeds are thick (1 to ca 10 m) compared to surrounding beds (commonly less than 1 m), and are of mixed composition, comprising both siliciclastic grains and shallow-water carbonate clasts. These megabeds are commonly characterised by a distinctive bipartite or tripartite vertical succession of facies. A complete (tripartite) sequence consists of a basal clast-supported conglomeratic division (Division I), an intermediate matrix-supported conglomeratic division (Division II), and an upper normally graded and/or laminated sandy division (Division III). These divisions are interpreted to be deposited from evolving debris flows transitioning to turbidity currents during a single flow event, and are the result of flow deceleration and dilution. The megabeds show variability over very short lateral distances (several tens to a few hundred metres), possibly related to surface relief on the debritic portion of the deposit. A new depositional model is proposed for the mixed deep-water system, with frequent siliciclastic turbidite deposition within this elongate basin from axially flowing turbidity currents, and episodic deposition from laterally-supplied carbonate-rich megaflows that eroded and incorporated the substrate during transport.

The regressive surface of marine erosion is a key stratigraphic discontinuity used to identify episodes of forced regression in marine strata. Typically, it marks the base of shallow-marine (e.g. shoreface and deltaic) deposits eroding over relatively deeper-water (e.g. shelf and prodelta) lithofacies. While well-documented in marginal-marine areas, its occurrence in offshore or strait settings dominated by tidal currents is less understood. This study investigates lower Pleistocene outcrops in the Plio-Quaternary Siderno Basin, Calabria, southern Italy, where tidal sand ridges developed in a Mediterranean strait. High-resolution drone images reveal a basinward-dipping basal surface marking the onset of tidally dominated sedimentation. This discontinuity separates underlying shelf fines from overlying cross-stratified, tidal bioclastic/siliciclastic arenites, indicating a prolonged period of marine regression in a strait setting. The stratal architecture of the ridges shows cross-strata aggradation in up-dip sections, transitioning down-dip into balanced aggrading/prograding strata, and further basinward into markedly prograding deposits. These features are interpreted to reflect an initial phase of normal regression, evolving into forced regression. This latter stage is characterised by a progressive deepening of the basal discontinuity, causing more erosional effects on the underlying beds, with a vertical basinward fall of about 60 m over 2 km. Internal foreset geometry (two-dimensional vs. three-dimensional cross strata) and their vertical and lateral repetition indicate stages of equilibrium and disequilibrium for tidal bedforms, reflecting varying current speeds and water depth changes. The regressive surface of marine erosion here is formed by tidal currents, rather than waves, suggesting a new type of sequence stratigraphic discontinuity associated with tide-dominated settings.

This contribution examines the climate variations reflected by a mainly lacustrine succession spanning from 17.73 to 14.0 Ma in north-east Iberia, thus encompassing the Miocene Climatic Optimum (MCO). The study is based on the δ¹³C and δ¹⁸O composition of an array of carbonate facies and marl samples, complemented with sedimentological analysis, illite crystallinity index and magnetic susceptibility data. The onset and ending of the MCO have been detected at ca 17.10-17.06 Ma and 14.56 Ma, roughly equivalent to the boundaries in the marine record, although with relatively short lags. The variability of the data series evidenced changes in humidity and air temperature through the MCO, some of which coincided with similar variations in other records. Specifically, an evolving positive shift in δ¹³C values, from 16.5 to 14.5 Ma, seems to fit the Monterey excursion observed in marine records. Likewise, increases in δ¹⁸O values between 16.8 and 16.5 Ma and between 14.85 and 14.56 Ma in the study succession concurred with warming intervals recorded in palaeosols of Central Europe, emphasising the coincidence with the temperature maximum at ca 16.6 Ma. A general decline in temperature and an increase in humidity are detected from 14.56 Ma, both with steeper trends until 14.41 Ma then more gradual onwards, indicating the beginning of the Middle Miocene Climatic Transition. These results shed light upon the tightly coincidental features between terrestrial and marine records over those time intervals and, more importantly, highlight the earlier warming and the faster cooling experienced by the lake system as compared with the marine record. These findings provide further evidence to help infer palaeoclimate conditions on a much broader reach than the regional context.

Mineralogy and texture of diagenetic phases in the Aptian Pre-Salt Barra Velha Formation are described, quantified and compared by facies and structural setting to understand their spatial and temporal distribution, and to develop predictive concepts for their genesis. This study examined data from eight wells from one oil-field in the Santos Basin. Calcite is the most abundant mineral and occurs with fibro-radial texture as spherulites and shrubs and sparse microcrystalline mudstone. The δ¹⁸O values from calcite spherulites and shrubs suggest water of similar composition and temperature, but they have different δ¹³C values. Mudstones show lower δ¹⁸O, suggesting warmer lake water and/or lower evaporation, whereas δ¹³C values indicate a variable, but commonly strong influence of biogenic CO₂. Dolomite with rhombohedral habit was the first to precipitate, followed by lamellar, saddle and anhedral varieties. Rhombohedral dolomites show a positive δ¹³C-δ¹⁸O correlation and a similar range in values to spherulites and shrubs, suggesting similar lake water. The lamellar dolomite is related to biofilms, whereas anhedral dolomite is attributed to mixing of pore and meteoric waters. Lamellar and anhedral dolomites have similar isotopic values, precipitating after rhombohedral dolomite in slightly warmer and/or less evaporatively concentrated pore water. Saddle dolomite is related to hydrothermal fluids that percolated the formation during early diagenesis. Silica occurs as replacement of primary calcite and Mg-clay, but also as a cement and more rarely as a depositional chert. Both dolomite and silica display a complex range of petrographic textures, many of which are facies dependent. This study focusses on the most important phases of the paragenetic sequence that took place pre-burial and are (1) formation of Mg-clay, calcite spherulites and shrubs, (2) partial dolomitisation of shrubs and spherulites and precipitation of rhombohedral and lamellar dolomites, (3) precipitation of saddle dolomite, matrix and carbonate dissolution and (4) anhedral dolomite and all textures of precipitated or substituted silica.

Facies, microfacies and stable isotope analyses of limestone beds in the northernmost Ñirihuau Basin, North Patagonian Andes, Argentina, document and constrain the past hydrological, sedimentological and climate conditions that prevailed during the deposition of a lacustrine system between ca 15 and 13 Ma. This palaeoenvironment is recorded in the middle section of the Ñirihuau Formation, which holds significance because: (1) It was deposited during a transition from an extensional to a compressional tectonic regime; (2) it spans the Middle Miocene Climatic Optimum and the beginning of the Middle Miocene Climatic Transition; and (3) it contains limestone beds interbedded within a 600 m thick interval of mudstones and siltstones, along with intercalated sandstone and volcaniclastic bodies. Two detailed sedimentary logs were surveyed along the Arroyo Las Bayas, at the western and eastern flank of the David Syncline. Limestones from both stratigraphic sections were sampled as well as isolated limestone beds from two other sites. One facies association was defined and interpreted as a perennial lake associated with a deltaic system and dominated by detrital clastic material. It comprises Facies 1 (Marginal lacustrine) and Facies 2 (Lower delta plain); in both, the presence of grainstones and calcimudstones stands out. Through petrography and cathodoluminescence studies of these continental carbonates, nine microfacies were identified: (a) Intraclastic grainstone, (b) Homogeneous calcimudstone, (c) Silty grainstone, (d) Disrupted micrite, (e) Birds eye micrite, (f) Bioclastic mudstone, (g) Calcimudstone with sparse detrital grains, (h) Fenestral micrite, (i) Stromatolitic boundstone. These indicate mainly bio-induced subaqueous carbonate precipitation and subordinate deposition by tractive flows with short-distance transport on a littoral lacustrine environment. Most of these microfacies exhibit very early diagenesis (eogenesis) effects. These features, and the geochemistry results, indicate that they were deposited in a palaeolake system under temperate to warm and humid conditions.

The export of neritic material from the top of carbonate platforms is a key process in the construction of their slopes. However, our knowledge of the supply pattern of materials from platforms is dominantly based on platforms lying in the euphotic zone during the present sea-level highstand. This is a somewhat biased perspective as through geological time not all platforms were euphotic. The Saya de Malha Bank in the Mascarene Plateau is an example of a modern mesophotic carbonate platform, and as such, its flooding and export patterns differ from those of euphotic ones. Using cores collected on the western slope of the Saya de Malha Bank, the export patterns of the platform since the last glacial maximum were explored. Material on the platform edge is winnowed and transported to the slope by multiple possible processes. The material on the platform is a combination of high and low magnesium calcite as well as high and low strontium aragonite, integrating pelagic and neritic sources. The ratio of these constituents varies over time with changes in the platform production capability as it was flooded and drowned during the Holocene transgression. The material from the platform is transported in both confined flows, mainly during lowstands, and unconfined flows, mainly during late transgression and early highstand. In the present state of the highstand, supply may have diminished, leading to erosion of the canyon shoulders.

In the Early to Middle Miocene, the post-orogenic intramontane lacustrine Sinj Basin that belonged to the Dinarides Lake System evolved in the area of the External Dinarides. A composite 770 m thick stratigraphic column was measured spanning the basin's stratigraphy. Eight facies were differentiated. Four facies are almost entirely composed of freshwater carbonate deposits. Carbonate facies are divided into calcareous mudstone, charophytic micritic limestone, calcisiltite and coquina facies. They are interpreted to belong to a prograding carbonate bench on a gently inclined lake margin. In addition, tuff/clays, carbonate conglomerate, carbonate breccia and coal were differentiated. The tuff/clays are the result of remote volcanic eruptions, while the coarse-grained sediments belong to subaqueous shallow stream channels or were deposited by gravity flows. The coal at the top of the measured succession, mostly of allochthonous origin, was deposited as a fen forest peat, representing the final stage of the lake. The formation of the Sinj Basin might have been triggered by dissolution of Permo-Triassic evaporites, within the mostly carbonate basement but also by breakdown and collapse of Mesozoic and Palaeogene carbonate rocks and coalescence of contiguous sinkholes. The non-tectonic interpretation of the basin genesis is a novel hypothesis explaining the origin of one of the Dinarides intramontane basins and is in contrast to previous considerations that evolution of the Sinj Basin was controlled by strike-slip or extensional tectonics.