Playa Lake Deposits Research Articles

The Cretaceous palaeodesert of the Sanfranciscana Basin (SE Brazil): A key record to track dissolved evaporites in the West Gondwana

Extensive palaeodeserts are known from the intracontinental basins of South American Platform, simultaneously to volcanic activity and thick salt beds developed during the rifting phase of Brazilian coastal basins. Based on palaeoenvironmental conditions and modern analogues, it would be expected that Cretaceous evaporites were frequently described in the intracontinental basins of West Gondwana, however, such deposits are rarely mentioned. This work evaluated the recognition and role of evaporitic-related features as palaeoclimatic/palaeoenvironmental proxies, through outcrop-based facies, X-ray diffraction and petrographic analyses of the Areado Group (Lower Cretaceous), Sanfranciscana Basin, southeastern Brazil. The Areado Group stands out as a remarkable record of evaporitic-siliciclastic interactions, consisting of alluvial, lacustrine and aeolian deposits. The stacking pattern reflects the progressive drying-up of the Sanfranciscana Basin during Cretaceous times. Regardless salt dissolution, the Areado Group comprises pseudomorphs after evaporites, evaporitic moulds, sand patch-fabric, and replication of the microtopography of efflorescent crusts. Preservation of evaporitic features in the Areado Group was controlled by the unique geological context of the Sanfranciscana Basin, which was submitted to syndepositional/eodiagenetic cementation and pseudomorphism, along with rapid burial caused by the progradation of aeolian dunes. Equivalent sedimentary structures and morphological aspects also occur in other Cretaceous palaeodeserts of intracontinental basins throughout Brazil. Timing of salt dissolution and isovolumetric/non-isolumetric replacement are important controlling factors on the geological record of evaporitic-siliciclastic systems that undergo post-depositional modifications. The database provided here indicates that playa lake deposits are commonest than traditionally represented in the non-marine Cretaceous record of the West Gondwana and other similar settings worldwide.

Palaeoenvironmental reconstruction of a lower to middle Permian terrestrial composite succession from the Catalan Pyrenees: Implications for the evolution of tetrapod ecosystems in equatorial Pangaea

Tetrapod diversity in Permian terrestrial ecosystems of southwestern Europe is poorly recorded by bone specimens, but it is better represented by an important tetrapod ichnological record that is relevant to our understanding of vertebrate communities in equatorial Pangaea. Herein, two tetrapod ichnoassociations from three new ichnosites, within Cisuralian and Guadalupian volcanosedimentary terrestrial successions (the Lower Red Unit and the Upper Red Unit deposits) of the Castellar de n'Hug sub-basin (Catalan Pyrenees, NE Iberian Peninsula) are presented. Tetrapod ichnology in combination with stratigraphic and facies analyses permit a characterisation of these ecosystems. The sedimentary deposits show an evolution from fluvial meandering systems to playa-lake floodplains, denoting increased aridification and seasonality under a monsoonal regime. The ichnofossil record shows how tetrapod assemblages shifted from a prevalence of non-amniotes and eureptiles to a dominance of therapsids, eureptiles and parareptiles. The reported ichnotaxa are distributed in two ichnoassociations. The first preserves Batrachichnus, Dromopus, Hyloidichnus, Characichnos and an indeterminate tetrapod morphotype, whereas the second is defined by the presence of Dromopus, Hyloidichnus, Brontopus antecursor and Pachypes. The stratigraphically older ichnoassociation, preserved in volcaniclastic and fluvial deposits, presents features of the early Erpetopus biochron, whereas the stratigraphically younger one, preserved in playa-lake deposits, is constrained to the Brontopus sub-biochron. Biostratigraphic and magnetostratigraphic data suggest a late Cisuralian to middle Guadalupian age for the whole succession. The ichnoassemblage highlights a palaeobiogeographical connection between the Pyrenean Basin and other peri-Tethyan basins, which allows us to expand our knowledge about the palaeoecology and palaeobiodiversity of the Cisuralian to Guadalupian terrestrial ecosystems.

The Oligocene – Early Miocene erg in the western Andean basins: Patterns during the transition from eolian to fluvial sedimentation

This paper analyzes an extended eolian field developed during the Oligocene-Early Miocene along the eastern flank of the Andes. This eolian episode, herein named OMEB (Oligocene-Miocene Eolian Belt), is recorded in several units, including Mariño, Pachaco, Cuculí, Vallecito, El Crestón, Tambería, Angastaco, Quiñoas, Las Chacras, and Vizcachera Formations. Eolian accumulation occurred in five environments: 1. Sand sheet and extradune, 2. Dune field, 3. Dune and draas, 4. Deflationary eolian fields, and 5. Wadi and fore-eolian areas. While sand sheet deposits are composed of horizontally laminated and low-angle cross-bedded sandstones, dune and draas environments comprise giant- and large-scale cross-bedded sets and dune fields by stacked sets of cross-bedded sandstones. In contrast, the deflation field environment exhibits low-relief irregular surfaces carved in dune and extradune environments. Finally, the wadi and fore-eolian deposits correspond to the eolian dune interbedded with fluvial conglomerates and muddy sediments deposited mainly by low-energy fluvial currents on the interdunes. Towards the end of the Early Miocene, the OMEB declined in almost all the foreland area, and fluvial and lacustrine sediments replaced eolian deposits. Although the stratigraphic expression of this change was different along the region, three principal patterns of change are recognized: 1. Different degrees of fluvial incisions, 2. Fluvial replacement of eolian deposits without evidence of significant incision, and 3. interbedding of eolian, fluvial, and occasionally playa-lake deposits. In the first pattern, conglomerates forming lenticular channels abruptly incise into the dune deposits. In pattern 2, fluvial channels are dominantly sandy, and incision surfaces are poorly defined. The third pattern, contrasting with the two previously described, results from the recurrent interstratification of eolian, fluvial, and playa lake deposits. This type of eolian-fluvial transition was studied in detail in the Tambillos creek (Northeastern Sierras de Los Colorados), where a sequential model for the eolian fluvial transition using the position of the water table is proposed. This model allows defining four primary states: 1. Low water table, 2. High water table (below the surface), 3. High water table (at or slightly above the surface) with partial flooding and damming, and 4. Runoff for fluvial channels.

Late Quaternary aeolian environments, luminescence chronology and climate change for the Monahans dune field, Winkler County, West Texas, USA

• Thermal transfer luminescence dating indicates Monahans aeolian record spans 550 ka. • Chronostratigraphic analyses identified at least six aeolian depositional periods (ADP). • Prominent ADP occur at 545 to 475, 300 to 260, 70 to 45, and post 16 ka. • A Playa lake deposit dated to 235 to 195 ka identified adjacent to Monahans dune field. • ADPs occur preferential during stadials, when the Laurentide ice is <80 % maximum volume. Dune fields on the Southern High Plains such as the Monahans in West Texas are archives of Quaternary environmental variability. Stratigraphic analyses and sixty-one optically stimulated luminescence (OSL) ages from seven Geoprobe cores and one section from the Monahans reveal a ∼ 550 ka old aeolian sedimentary record with seven carbonate/argillic paleosols and a playa-lake margin deposit. OSL ages on quartz-grains from aeolian sediments by two protocols, single aliquot regeneration (SAR) and thermal transfer (TT), yield congruent ages between 50 and 250 ka, and the oldest ages of ca. 550 ka, potentially minima. This chronostratigraphic analysis and finite-mixture modeling of the OSL-age distribution identify-four aeolian depositional periods (ADP) at 545 to 475, 300 to 260, 70 to 45, and post 16 ka and possibly-two additional ADPs 460 to 420 ka and 350 to 320 ka. Playa lake deposits identified west of the Monahans and correlative to carbonate-rich paleosols indicate that wetter conditions prevailed during interglacial MIS 7, 235 to 195 ka. Another wetter period, 25 to 16 ka, with the formation of Lake King in the adjacent Rio Grande Valley is correlative with a pedogenically-modified <2 m-thick aeolian sand. This study underscores that there may be multiple climatic states, during glacials and interglacials, associated with wetter conditions. In turn, the thickest, preserved aeolian deposits are associated with transitional climate periods, penecontemporaneous with stadials, when the Laurentide ice sheet was <80 % of the last glacial maximum volume, with precipitation-bearing zonal circulation shifted northward and weakened meridional moisture flux.

Early diagenetic calcite replacement of evaporites in playa lakes of the Quiricó Formation (Lower Cretaceous, SE Brazil)

Evaporites are abundant in modern playa lakes, where efflorescence, inorganic and/or bioinduced precipitation coexist with siliciclastic deposition. Overall, the sensibility of salts to slight physicochemical changes results in partial to complete dissolution of evaporites in surface and shallow subsurface. However, in some cases, salt minerals are pseudomorphically replaced, providing a snapshot to depositional settings and diagenetic processes of evaporite-bearing successions. This is recorded in the Quiricó Formation (Lower Cretaceous), Sanfranciscana Basin, southeastern Brazil. This studied succession portrays a playa lake system subdivided in saline flat and saline lake deposits, whose diagenesis was marked by ubiquitous cementation and replacement reactions. Petrographic and geochemical techniques were undertaken to characterise primary and secondary textures, mineralogy and distribution of mudstone- and sandstone-hosted pseudomorphs. The pseudomorphs frequently underlie the stratigraphic contact of playa lake-aeolian beds. Pseudomorphs occur as coalescent nodules and as chaotically-distributed chevron crystals, composed of inequicrystalline calcite mosaics. Nodular pseudomorphs have cubic/sub-cubic “ghost” inclusions and displacive contacts with the matrix, whereas the contacts of chevron pseudomorphs and the matrix are undisturbed. Both types exhibit morphological replication of precursor minerals and non-luminescent euhedral relicts. Stable isotopic signatures are covariant and range between −1.6–0.6 ‰ δ13C and 1.4–2.5 ‰ δ18O for nodular pseudomorphs, and −2.2 to −1.5 ‰ δ13C and 0.3–1.3 ‰ δ18O for chevron pseudomorphs. Nodular crystals reflect subaerial and intrasedimentary crystallisation of Ca-sulphates in saline flats, whereas chevron-shaped pseudomorphs indicate bottom-growth halite in shallow lakes. Textural evidences from pseudomorphs and host lithologies constrain the diagenetic overprint to surface and shallow burial settings (syndepositional/eodiagenetic stages). Despite multiple generations of calcite, the constant isotopic signatures of pseudomorphs reflect water chemistry fractionation in a closed-lake basin. Widespread calcitisation of playa lake sediments was caused by calcium-rich groundwaters, influenced by limestones from the Precambrian basement. The playa lake deposits record the onset of aridification processes that culminated in the Thermal Cretaceous Maximum.

Myriapod divergence times differ between molecular clock and fossil evidence: U/Pb zircon ages of the earliest fossil millipede-bearing sediments and their significance

ABSTRACT Molecular clock calculations suggest a late Cambrian (~ 500 Ma) divergence of myriapod classes. Yet, the earliest myriapods only appear in the latest Silurian (~425 Ma). 75 million years later; though correlation with the standard marine-based geological time scale is difficult. We radiometrically dated (U/Pb method) zircons in sediments at 3 sites in the UK associated with the supposed earliest millipedes. Kerrera, with only extinct kampecarid myriapods from temporary playa lake deposits in a semi-arid intermontane basin is 425 Ma old. Ludlow, with kampecarid myriapods and the first trigonotarbid arachnids (with book lungs), from transported material in ripple troughs at the base of a semi-arid delta complex, is 420 Ma. Cowie, with the supposed earliest air-breathing millipede (with spiracles), from a temporary playa lake in an intermontane basin complex, is 414 Ma old. The more diverse land arthropod fauna of the Rhynie Chert hot spring deposit, also in an intermontane basin, is 407 Ma old. A rapid radiative evolution from simple intermontane lake margin communities through more evolved lowland communities took only about 20 Ma to reach complex forest grade communities by the Middle Devonian Givetian (about 385 million years).

Myriapod divergence times differ between molecular clock and fossil evidence: U/Pb zircon ages of the earliest fossil millipede-bearing sediments and their significance

ABSTRACT Molecular clock calculations suggest a late Cambrian (~ 500 Ma) divergence of myriapod classes. Yet, the earliest myriapods only appear in the latest Silurian (~425 Ma). 75 million years later; though correlation with the standard marine-based geological time scale is difficult. We radiometrically dated (U/Pb method) zircons in sediments at 3 sites in the UK associated with the supposed earliest millipedes. Kerrera, with only extinct kampecarid myriapods from temporary playa lake deposits in a semi-arid intermontane basin is 425 Ma old. Ludlow, with kampecarid myriapods and the first trigonotarbid arachnids (with book lungs), from transported material in ripple troughs at the base of a semi-arid delta complex, is 420 Ma. Cowie, with the supposed earliest air-breathing millipede (with spiracles), from a temporary playa lake in an intermontane basin complex, is 414 Ma old. The more diverse land arthropod fauna of the Rhynie Chert hot spring deposit, also in an intermontane basin, is 407 Ma old. A rapid radiative evolution from simple intermontane lake margin communities through more evolved lowland communities took only about 40 Ma to reach complex forest grade communities by the Middle Devonian Givetian (about 385 million years).

Rhaetian (Late Triassic) Milankovitch Cycles in the Tethyan Dachstein Limestone and Laurentian Passaic Formation Linked by the g2-g5 Astronomical Metronome

Milankovitch cycles have long been suspected of having been recorded in the shallow marine Lofer cyclothems of the Rhaetian Dachstein Limestone of the European Northern and Southern Calcareous Alps. However, sufficient evidence has not yet been forthcoming to make a compelling case for Milankovitch control on the cyclothems. At the same time, there is overwhelmingly strong evidence that Milankovitch cycles were influencing the paleoclimates of Rhaetian Laurentia and the wet-dry playa lake deposits of the Passaic Formation (Newark Basin, USA). The strongest cycles in these deposits, the McLaughlin cycles, have been linked to the metronomic g2-g5, or 405 kyr orbital eccentricity cycle. Thus, Milankovitch cycles were operating throughout the Rhaetian, and being global in nature, likely played a role in the genesis of the coeval Lofer cyclothems. The Dachstein Limestone in the Julian Alps, relatively undisturbed by tectonics, exhibits strong Milankovitch-like cyclicity, including evidence for g2-g5 cycling. The shallow marine Dachstein and continental Passaic formations provide evidence for complementary forcing mechanisms and active aquifer-limno-eustasy: low sea level in Lofer exposure facies is reflected in wet Passaic playa lake deposits, and high sea level in Lofer subtidal facies in dry Passaic playa lake deposits. The Dachstein and Passaic formations are further linked by the g2-g5 metronome; a provisional correlation is proposed, subject to future validation by additional, as yet undeveloped chronostratigraphic constraints.

Sedimentology and U-Pb dating of Carboniferous to Permian continental series of the northern Massif Central (France): Local palaeogeographic evolution and larger scale correlations

The Carboniferous to Permian volcanic-sedimentary succession shown by the LY-F core from the Lucenay-lès-Aix area, in the northern part of the Massif Central, has been studied in order to obtain both landscape reconstructions (sedimentological analyses) and geochronological constraints (U-Pb dating on zircon and apatite). The lowermost part of the core consists mainly of lacustrine deposits with Gilbert-type delta and volcaniclastic-rich fan delta deposits including several altered volcanic ash layers (tonstein). In contrast, in the uppermost part of the core, playa-lake deposits dominate. LA-ICP-MS U-Pb analyses were performed on both zircon and apatite grains from interbedded tonsteins. This coupled U-Pb dating approach allows to assess potential reworking of volcanic material or the occurrence of non-volcanic grains, such as xenocrysts, in order to provide better evaluations for the depositional ages of the tonsteins. These investigations reveal that sedimentation took place between the late Gzhelian and the late Sakmarian (i.e., between c. 301 and 290 Ma). This sedimentary succession can therefore be compared to those from adjacent basins for which geochronological constraints are available (i.e., Autun and Lodève basins, resp. located north and south of the Massif Central). This study provides a reference section for future comparisons with similar sections from other Carboniferous to Permian basins, in France as well as elsewhere in Europe.

Reservoir interpretation of Rotliegend seismic reflections in the central part of the Mid-Polish Trough

The search for gas reservoirs in the deep central parts of the Mid-Polish Trough remains an open question because of fragmentary knowledge of the area caused by the small amount of available borehole data and poor seismic imaging of sub-Zechstein formations on vintage seismic data.2D seismic data acquired in 2011 reveals seismic events related to strongly indurated sandstone intercalation found within fine-grained playa-lake deposits in the axial part of the Mid-Polish Trough. Seismic full-wave modelling has been carried out along recorded seismic 2D line in order to analyse seismic signatures from potential reservoir layers of variable porosity and gas saturation. This has allowed to evaluate the possibility of use of seismic methods to detect potential reservoir sandstones located in the deep parts of the basin below a thick Zechstein-Mesozoic cover, in which the actual spatial distribution of Rotliegend formations is only hypothetical.The analyses were focused on impact of porosity changes and variations in content of minerals with high bulk modulus on the P-wave velocity and seismic response. 2D seismic modelling have revealed that under specific geological conditions, playa formation may contain a sandstone layer – with a porosity of approx. 5% and acoustic impedance values not distinctly different from those observed for the surrounding rocks. As the result sandstone reflections will be undetectable through seismic methods.Promising results have been obtained by comparison of synthetic sections against a seismic profile recorded in the other deep part of the Mid-Polish Trough. The similarity of seismic signatures on synthetic section calculated for layer with porosity of 9% and observed on actual section implies that these events are related to a porous sandstone layer. This interpretation is confirmed by borehole data showing a thin layer of aeolian sandstone with 11% porosity.

The volcano-sedimentary evolution of a post-Variscan intramontane basin in the Swiss Alps (Glarus Verrucano) as revealed by zircon U–Pb age dating and Hf isotope geochemistry

The Late Palaeozoic Glarus Verrucano basin (GVB, Glarus Alps, eastern Switzerland) formed as an intramontane graben in the aftermath of the Variscan orogeny. Its fill, the Glarus Verrucano, consists of immature alluvial fan and playa lake deposits with intercalated bimodal volcanics (basalts and rhyolites). Despite its importance for local and regional geology, no modern sedimentologic or stratigraphic studies on the GVB exist. By means of sedimentologic and geochronologic studies, we reconstruct the volcano-sedimentary evolution of the GVB: it developed at the Carboniferous/Permian boundary and experienced a first (bimodal) volcanic phase around 285 Ma. For the same time, indications for temporarily humid climate in the otherwise rather arid Early Permian are demonstrated (e.g. pyrite-bearing sandstones). During the Middle and Early Late Permian, increasing aridity is indicated by playa deposits, fanglomerates and subaerial ignimbrites, which mark a second (silicic) volcanic phase at 268 Ma. The detrital zircon age spectra are dominated by Late Variscan ages and thus demonstrate that older sedimentary and metamorphic rocks once forming the Variscan nappe edifice were already mostly eroded at that time. Finally, some larger-scale speculations are given which could indicate a causal connection between the widespread tectono-magmatic Mid-Permian Episode and the local development of the Glarus Verrucano basin.

A paleoclimatic review of southern South America during the late Paleozoic: A record from icehouse to extreme greenhouse conditions

This paper provides a review of the Late Mississippian to Permian paleoclimatic history for southern South America based on lithologic indicators, biostratigraphic information, and chronostratigraphic data. The region is divided into three major types of basins: 1. Eastern intraplate basins (e.g., Paraná Basin), 2. Western retroarc basins (e.g., Paganzo Basin) and 3. Western arc-related basins (e.g., Río Blanco Basin). Four major types of paleoclimatic stages are recognized in these basins: 1. glacial (late Visean–early Bashkirian), 2. terminal glacial (Bashkirian–earliest Cisuralian) 3. postglacial (Cisuralian–early Guadalupian), and 4. semiarid–arid (late Guadalupian–Lopingian). The glacial stage began in the late Visean and continued until the latest Serpukhovian or early Bashkirian in almost all of the basins in southern South America. During the Bashkirian–earliest Cisuralian (terminal glacial stage), glacial deposits disappeared almost completely in the western retroarc basins (e.g., Paganzo Basin) but glaciation persisted in the eastern basins (e.g., Paraná and Sauce Grande Basins). A gradual climatic amelioration (postglacial stage) began to occur during the earliest Permian when glacial deposits completely disappeared across all of South America. During this interval, glacial diamictites were replaced by thick coal beds in the Paraná Basin while north–south climatic belts began to be delineated in the western basins, which were likely controlled by the distribution of mountain belts along the Panthalassan Margin of South America. Towards the late Permian, climatic belts became less evident and semiarid or arid conditions dominated in the southern South America basins. Eolian dunes, playa lake deposits, and mixed eolian–fluvial sequences occur in the Paraná Basin and in the western retroarc basins. Volcanism and volcaniclastic sedimentation dominated along the western margin of South America at that time. The stratigraphic record obtained in southern South America supports a long duration transition from icehouse to extreme greenhouse conditions.

Controls on marine–erg margin cycle variability: aeolian–marine interaction in the mid‐Cretaceous Iberian Desert System, Spain

AbstractThe interaction between aeolian dunes of the Iberian erg and Tethys waters during the mid‐Cretaceous led to a variety of sedimentary facies associations such as subtidal deposits, aeolian dunes, playa lakes, coastal lakes with tidal creeks, and marshes and lagoonal embayments with tide‐influenced delta deposits. Facies associations are organized in several stacked cycles. Every cycle is defined by a sand‐drift surface separating playa lake deposits below from aeolian dune deposits above, followed by a transgressive surface separating aeolian deposits below from shallow marine deposits above. The latter are covered by playa lake deposits and finally topped by the next sand‐drift surface. Similar marine–erg margin cycles have been explained previously in terms of high‐order relative sea‐level variations (normally controlled by glacioeustasy). The studied erg margin developed during the mid‐Cretaceous which is considered to be the archetypal example of a polar ice‐free greenhouse period. This manuscript presents an alternative explanation for the cycles as caused by climate‐induced variations in the aeolian sediment supply. In this scenario, orbitally induced latitudinal shifts of the boundary between climate belts led to alternating periods of increased and decreased precipitation in the highland catchments of the back‐erg system (Variscan Iberian Massif). When climate belts shifted to lower latitudes, back‐erg highlands were under the influence of the Northern Warm Humid Belt. While arid conditions prevailed in the lowland central‐erg, increased precipitation in the highlands and the consequent recharge of ground water led to a rise of the phreatic level in the erg system; that diminished wind erosion and windblown sediment input to the fore‐erg, so that the desert margin contracted. With ongoing basin subsidence this favoured the transgression. The increased desert‐ground water flux favoured permanent coastal lakes in the fore‐erg margin and the local development of vegetation. In places where such lakes (lagoons) were connected to the sea, tidal channels were active, as seen nowadays along the desert coast of Qatar (Persian Gulf). When climate belts moved to higher latitudes, the catchment areas in the back‐erg Iberian Massif were under the influence of the Northern Hot Arid belt and a decrease of precipitation led to a drop of the phreatic level, allowing deflation of dunes and exposed wadi channel floors, and the formation of desert pavements and deflation lags in the back‐erg area; this favoured the import of large volumes of windblown sand and dust, forcing progradation of the erg and consequently a retreat of the coastline. The sand fraction accumulated in climbing aeolian dunes and dust was trapped in extensive playa lake systems. Another long‐term allocyclic control is formed by active extensional tectonics that enhanced the creation of accommodation space, especially in the first cycle. A high ground water table sustained by subsurface water supply from the Variscan Iberian Massif generated coastal fresh/brackish lakes in which tide‐influenced meandering channels and salt marshes developed. Erg margin cycles thin upward in response to a decrease of the rate of basin subsidence, suggesting that transgressions led to deep erosion of the underlying aeolian dune sands.

Diagenesis of Paleozoic playa-lake and ephemeral-stream deposits from the Pimenta Bueno Formation, Siluro–Devonian (?) of the Parecis Basin, central Brazil

The Parecis Basin is a large intracratonic rift located in central Brazil and filled with Paleozoic carbonate, evaporite and siliciclastic sediments. The occurrence of gas seeps has recently attracted significant exploration interest by the Brazilian petroleum agency and by Petrobras. The continuously cored PB-01-RO well provided the first opportunity to study the depositional environments, diagenetic evolution and hydrocarbon potential of the largely unknown sedimentary successions of the Parecis Basin. The cored lithologies, belonging to the Siluro–Devonian (?) Pimenta Bueno Formation, are interpreted as deposited in playa-lake and ephemeral-stream environments. The deposits display a strong facies control on the diagenetic mineral assemblages and evolution. Diagenetic minerals in the ephemeral-stream deposits include eogenetic hematite and smectitic clay coats and quartz cement, and the mesogenetic process includes precipitation of sulfates (anhydrite and barite) and carbonates (calcite, dolomite and kutnahorite–ankerite–huntite), followed by partial dissolution of these carbonates and sulfates, and of feldspar grains. Telogenetic processes include the precipitation of hematite and kaolinite within secondary pores, and the replacement of anhydrite by gypsum. A second burial phase and mesodiagenesis is indicated by the precipitation of discrete K-feldspar crystals within moldic pores after dissolved feldspars, and by the illitization of etched, telogenetic kaolinite. The playa-lake deposits show early diagenetic dolomitization of lime mud, precipitation of anhydrite nodules and extensive silicification. The anhydrite nodules were replaced by gypsum and chalcedony during telodiagenesis. Potential source rocks are locally represented by organic shales. The fluvial sandstones show fair reservoir quality and limited compaction, as indicated by their intergranular volume, suggesting that the succession has undergone moderate burial. Potential seals for hydrocarbon accumulations are provided by the evaporites and playa-lake mudstones. The presence of an active petroleum system in the basin, as indicated by the gas seeps, suggests a good hydrocarbon exploration potential for the Parecis Basin, gas and stresses the importance of studying the depositional and diagenetic evolution of its sedimentary successions.

Syndepositional tectonics recorded by soft-sediment deformation and liquefaction structures (continental Lower Permian sediments, Southern Alps, Northern Italy): Stratigraphic significance

The Lower Permian succession of the Central Southern Alps (Lombardy, Northern Italy) was deposited in fault-controlled continental basins, probably related to transtensional tectonics. We focussed our study on the stratigraphic record of the Lower Permian Orobic Basin, which consists of a 1000 m thick succession of prevailing continental clastics with intercalations of ignimbritic flows and tuffs (Pizzo del Diavolo Formation, PDV) resting on the underlying prevailing pyroclastic flows of the Cabianca Volcanite. The PDV consists of a lower part (composed of conglomerates passing laterally to sandstones and distally to silt and shales), a middle part (pelitic, with carbonates) and an upper part (alternating sandstone, silt and volcanic flows). Syndepositional tectonics during the deposition of the PDV is recorded by facies distribution, thickness changes and by the presence of deformation and liquefaction structures interpreted as seismites. Deformation is recorded by both ductile structures (ball-and-pillow, plastic intrusion, disturbed lamination, convolute stratification and slumps) and brittle structures (sand dykes and autoclastic breccias). Both the sedimentological features and the geodynamic setting of the depositional basin confidently support the interpretation of the described deformation features as related to seismic shocks. The most significant seismically-induced deformation is represented by a slumped horizon (about 4 m thick on average) which can be followed laterally for more than 5 km. The slumped bed consists of playa-lake deposits (alternating pelites and microbial carbonates, associated with mud cracks and vertebrate tracks). The lateral continuity and the evidence of deposition on a very low-angle surface along with the deformation/liquefaction of the sediments suggest that the slump was triggered by a high-magnitude earthquake. The stratigraphic distribution of the seismites allows us to identify time intervals of intense seismic activity, which correspond to rapid and basin-wide changes in the stratigraphical architecture of the depositional basin and/or to the reprise of the volcanic activity. The nature of the structures and their distribution suggest that the magnitude of the earthquakes responsible for the observed structures was likely higher than 5 (in order to produce sediment liquefaction) and probably reached intensity as high as 7 or more. The basin architecture suggests that the foci of these earthquakes were located close to the fault-controlled borders of the basin or within the basin itself.

Cenozoic sediments in the southern Tarim Basin: implications for the uplift of northern Tibet and evolution of the Taklimakan Desert

Abstract Cenozoic sedimentary successions along the southern margin of the Tarim Basin, western China, reach up to 10 km in thickness. The two studied sections, the Yecheng and Aertashi, comprise c . 4.5 km and c . 7.0 km of clastic sedimentary rocks respectively. The base of the Yecheng section has been dated palaeomagnetically to be about 8 Ma. Age control of the Aertashi section is based on 87 Sr/ 86 Sr measurements (for the basal marine bed), together with magnetostratigraphy and regional stratigraphic correlation. The lower part of each section is mainly composed of fine-grained mudstone and fine sandstone, which makes up the Wuqian Group (Miocene). The palaeoenvironment is low-energy, meandering and braided streams. The middle part is composed of red mudstone, sandstone with thin conglomerate beds, which make up the Artux Formation (Pliocene). The palaeoenvironment is a distal- to mid-fan environment. The uppermost part of the section, known as the Xiyu Formation (Plio-Pleistocene), consists of cobble and boulder conglomerate intercalated with massive siltstone lenses, which formed as proximal alluvial fan and aeolian deposits. Neogene red beds passing upward into upward-coarsening conglomerate and debris-flow deposits record the change in palaeoslope related to uplift of the northern margin of Tibetan Plateau. The formation of aeolian dunes at c . 8 Ma, and underlying playa lake deposits (as at Aertashi), may indicate an arid, enclosed basin in the southern Tarim after this time. Sedimentological characteristics, together with grain size distribution and geochemistry of siltstone bands in the Xiyu and Artux Formations, point to an aeolian origin. This indicates that the Taklimakan Desert and the regional climate regime may have been fully developed by the Early Pliocene. The onset of aeolian sedimentation in the southern Tarim Basin coincided with uplift of the northern Tibetan Plateau, as inferred from the lithofacies change. Tibetan Plateau uplift resulted in the shift of sedimentary environments northwards into the southern Tarim Basin, and could well have triggered the onset of full aridity in the Taklimakan region as a whole.

Sedimentary evolution of the Late Eocene Vernet lacustrine system (South-Central Pyrenees). Tectono-climatic control in an alluvial-lacustrine piggyback basin

A complete third-order tectono-sedimentary cycle filled the Vernet lacustrine piggyback basin during the late Eocene. The depositional sequence can be subdivided into five units or systems tracts with distinct architectural frameworks, which developed a progressive unconformity. Sedimentary infilling of the piggyback basin began with retrogradational alluvial fans and fluvio-alluvial deposits being deposited, arranged in two thinning- and fining-upward cycles during the uplifting and emplacement of the southernmost Pyrenean thrusts, and with three thickening- and coarsening-upward progradational floodplain and lacustrine cycles, during the quiescence of the southernmost thrusts, at the same time as the emplacement of several northernmost thrusts of the southern Pyrenees. The facies associations identified represent different depositional environments in a sheetflow-dominated system, including playa-lake deposits, floodplain deposits, sheetflow fluvial deposits and sheet deltas in ephemeral to shallow lacustrine settings. Fourth and fifth order arid/semi-arid climatic cycles can be recognized, driven either by orbital obliquity and precessional forcing (Milankovitch cyclicity), insolation cycles or possibly subprecessional autogenic mechanisms related to the emplacement of northern thrusts and the consequent changes in the drainage basin. Arid periods are recorded by sheetflood fluvial, ephemeral lacustrine or terminal splay deposits, with abundant medium-to-coarse-grained sandstone packages in red-to-grey mudstones, a product of unconfined or low-confined sheetflows or hyperconcentrated flows, with bedload and suspended load, during flood episodes, at times of lake lowstand. Semi-arid periods are recorded as relative highstand lacustrine deposits, including grey-to-white mudstones deposited in prodelta environments and fine-grained sheet sandstone units sedimented in a delta front (with minor channels and extensive sheet lobes) and prodelta environments in shallow lakes, from sheetflood events.

Sedimentology and geochemistry of the middle miocene playa lake evaporites in the Gürün basin (S of Sıvas), central Anatolia, Turkey

The Gurun basin is a half graben filled by alluvial, fluvial and playa-lake deposits of the Gurun Formation accumulated under the N−S direction extensional tectonic regime and volcanic rocks.

The playa environments of the Lodève Permian basin (Languedoc-France).

The Lodeve Permian synrift sequence is represented by thick stacked climate-controled playa cycles which have been constrained by structural, sedimentological, geochemical and paleontological processes. The present paper merges the different results in order to better constrain the dynamics and palaeoecologycal evolution of the playa system from Late Cisuralian to Early Lopingian time. In this depositional environment, thining upwards sheet-flood sequences represent flooding events; they are relayed by ephemeral ponding, suspension settling and limited carbonate precipitation accompanied by large amounts of shallow to subaerial sedimentary structures (current and wind ripples, desiccation cracks, rain drops, etc.). The aridity of the climate contrasts with the surprising abundance of the fauna encountered into carbonate-rich silty-clay playa-lake deposits. Ephemeral pools are colonized by numerous shellfish: Choncostraca, Triopsids, and Insects, which led to intense burrowing of the overbank deposits (Scoyenia facies). Tetrapods are common and dominated by several types of reptiles after footprints: Pelycosauria, Parareptilia and Lepidosauria in the Rabejac Fm, and Mammalian reptiles (Therapsida) on the top of the fluvial deposits of the Salagou Fm near La Lieude farm. In addition, elements of a weighty herbivorous Pelycosaurian and a small amphibian Tupilakosaurid, only described in the Triassic before, have been also found in the Upper part of the Permian series of this sector.

Sheetflow fluvial processes in a rapidly subsiding basin, Altiplano plateau, Bolivia

AbstractAlthough facies models of braided, meandering and anastomosing rivers have provided the cornerstones of fluvial sedimentology for several decades, the depositional processes and external controls on sheetflow fluvial systems remain poorly understood. Sheetflow fluvial systems represent a volumetrically significant part of the non‐marine sedimentary record and documented here are the lithofacies, depositional processes and possible roles of rapid subsidence and arid climate in generating a sheetflow‐dominated fluvial system in the Cenozoic hinterland of the central Andes. A 6500 m thick succession comprising the Late Eocene–Oligocene Potoco Formation is exposed continuously for &gt;100 km along the eastern limb of the Corque syncline in the high Altiplano plateau of Bolivia. Fluvial sandstone and mudstone units were deposited over an extensive region (&gt;10 000 km2) with remarkably few incised channels or stacked‐channel complexes. The Potoco succession provides an exceptional example of rapid production of accommodation sustained over a prolonged period of time in a non‐marine setting (&gt;0·45 mm year−1 for 14 Myr). The lower ≈4000 m of the succession coarsens upward and consists of fine‐grained to medium‐grained sandstone, mudstone and gypsum deposits with palaeocurrent indicators demonstrating eastward transport. The upper 2500 m also coarsens upward, but contains mostly fine‐grained to medium‐grained sandstone that exhibits westward palaeoflow.Three facies associations were identified from the Potoco Formation and are interpreted to represent different depositional environments in a sheetflow‐dominated system. (i) Playa lake deposits confined to the lower 750 m are composed of interbedded gypsum, gypsiferous mudstone and sandstone. (ii) Floodplain deposits occur throughout the succession and include laterally extensive (&gt;200 m) laminated to massive mudstone and horizontally stratified and ripple cross‐stratified sandstone. Pedogenic alteration and root casts are common. (iii) Poorly confined channel and unconfined sheet sandstone deposits include laterally continuous beds (50 to &gt;200 m) that are defined primarily by horizontally stratified and ripple cross‐stratified sandstone encased in mudstone‐rich floodplain deposits.The ubiquitous thin‐sheet geometry and spatial distribution of individual facies within channel sandstone and floodplain deposits suggest that confined to unconfined, episodic (flash) flood events were the primary mode of deposition. The laterally extensive deposition and possible distributary nature of this sheetflow‐dominated system are attributed to fluvial fan conditions in an arid to semi‐arid, possibly seasonal, environment. High rates of sediment accumulation and tectonic subsidence during early Andean orogenesis may have favoured the development and long‐term maintenance of a sheetflow system rather than a braided, meandering or anastomosing fluvial style. It is suggested here that rapidly produced accommodation space and a relatively arid, seasonal climate are critical conditions promoting the generation of sheetflow‐dominated fluvial systems.