Shallow Lacustrine Deposits Research Articles

Whole petroleum system in Jurassic coal measures of Taibei Sag in Tuha Basin, NW China

Based on the latest results of near-source exploration in the Middle and Lower Jurassic of the Tuha Basin, a new understanding of the source rocks, reservoir conditions, and source-reservoir-cap rock combinations of the Jurassic Shuixigou Group in the Taibei Sag is established using the concept of the whole petroleum system, and the coal-measure whole petroleum system is analyzed thoroughly. The results are obtained in three aspects. First, the coal-measure source rocks of the Badaowan Formation and Xishanyao Formation and the argillaceous source rocks of the Sangonghe Formation in the Shuixigou Group exhibit the characteristics of long-term hydrocarbon generation, multiple hydrocarbon generation peaks, and simultaneous oil and gas generation, providing sufficient oil and gas sources for the whole petroleum system in the Jurassic coal-bearing basin. Second, multi-phase shallow braided river delta–shallow lacustrine deposits contribute multiple types of reservoirs, e.g. sandstone, tight sandstone, shale and coal rock, in slope and depression areas, providing effective storage space for the petroleum reservoir formation in coal-measure strata. Third, three phases of hydrocarbon charging and structural evolution, as well as effective configuration of multiple types of reservoirs, result in the sequential accumulation of conventional-unconventional hydrocarbons. From high structural positions to depression, there are conventional structural and structural-lithological reservoirs far from the source, low-saturation structural-lithological reservoirs near the source, and tight sandstone gas, coal rock gas and shale oil accumulations within the source. Typically, the tight sandstone gas and coal rock gas are the key options for further exploration, and the shale oil and gas in the depression area is worth of more attention. The new understanding of the whole petroleum system in the coal measures could further enrich and improve the geological theory of the whole petroleum system, and provide new ideas for the overall exploration of oil and gas resources in the Tuha Basin.

Flood events in the lower reaches of the Yellow River: OSL and radiocarbon dating on the Anshang site

The lower reaches of the Yellow River have witnessed numerous episodes of flooding and considerable shifts in its course. Examining flood records within the Yellow River basin offers invaluable insights into ancient human settlements, agricultural practices, trade routes, and technological advancements of ancient Chinese civilizations. However, dating these fluvial sediments poses a considerable challenge due to either the scarcity of organic material for radiocarbon (14C) dating or the potential incomplete bleaching of sediments for optically stimulated luminescence (OSL) techniques. This study focused on the Anshang site located in Henan Province, we obtained OSL ages from distinct grain size fractions of quartz and cross-verified them with independent acceleration mass spectrometry (AMS) 14C ages. The findings revealed that both coarse quartz grains (CG) and medium quartz grains (MG) underwent complete bleaching in silt-dominated flood deposits and shallow lacustrine deposits. Conversely, the OSL ages of CG and MG quartz were notably overestimated by approximately 3–6 ka in silty clay-dominated flood deposits. Upon comparing the OSL ages with AMS 14C ages, it was evident that bulk organic matter AMS 14C ages aligned with CG OSL ages in shallow lacustrine deposits, but they appeared older than CG OSL ages in flood deposits. However, the AMS 14C ages from plant demonstrated good consistency. Consequently, we successfully identified flood-lacustrine sediments dating back to approximately ∼13 ka, ∼8–3 ka, ∼2.6 ka, ∼1 ka, and the last few hundred years, aligning well with historical documentation and earlier research. The regional evidence presented herein enhances our understanding of the applicability of OSL dating and AMS 14C dating in fluvial-lacustrine sediments. Armed with these new ages, we constructed a reference chronology for archaeological localities situated in the fluvial sediments of the lower reaches of the Yellow River.

Sequence architecture, depositional system evolution and controlling factors in a lacustrine basin: a case study from Changdi Area, Bohai Bay Basin, Eastern China

AbstractThe Paleogene Shahejie and Dongying formations of the eastern Bohai Bay Basin contain >500 m fluvial-deltaic deposits and >600 m deltaic-lacustrine deposits, both of which are important for oil and gas exploration. This study aims to figure out the deposition system distribution within the sequence stratigraphy and predict the exploration target. Sequence boundaries are recognized by unconformities in the seismic profiles and abrupt changes in the well-logging curves (acoustic, spontaneous potential, natural gamma-ray and resistivity), and two second-order and five third-order sequences (SQs1, SQs3 in the Shahejie Formation and SQd1-SQd3 in Dongying Formation) are identified. Four types of depositional system and distribution are determined by well-logging profiles, cores, relative sand content and root-mean-square seismic attributes. The SQs3 sequence is predominantly fan deltas and lacustrine deposits, the SQs1 sequence is predominantly shallow lacustrine deposits, the SQd3 sequence is predominantly braided-river delta, semi-deep lacustrine, and turbidite fan, and the SQd2 sequence is predominantly braided-river delta deposits, whereas the SQd1 sequence is predominantly braided-river deposits. The controlling factors are determined by basin subsidence simulation and fossil data of biological palynology, indicating that tectonic activity is the main controlling factor in SQs3 and SQd3 while climate is the main controlling factor in the SQs1, SQd2 and SQd1 sequences. Combined with the physical properties of sandstone, it is concluded that the beach bar in the SQs1 sequence, turbidite fan in the SQs3 and SQd3 sequences, and thick sandstone in the SQd2 sequence are the important exploration targets.

Early Pleistocene charophyte flora from Dursunlu (Ilgın Basin, Turkey): palaeoecological implications

Here, a charophyte assemblage from lower Pleistocene deposits at the coal mine locality of Dursunlu (Ilgın Basin, Turkey) is described and illustrated for the first time. The assemblage is composed of 11 species recovered from 50 samples taken from an 18 m-thick sedimentary sequence. The base of the sequence is dominated by lignites and organic-rich claystones/siltstones with root marks attributed to palustrine facies. The charophyte assemblage in this palustrine interval is dominated by Chara vulgaris and C. globularis. The upper part of the section is constituted by light grey-yellowish marls, siltstones and a limestone bed related to shallow lacustrine deposits. Two charophyte species dominate this lacustrine interval, Chara hispida and Nitellopsis obtusa, which occurred in intervals in association with Chara pappii, Chara molassica var. notata, Chara sp.1, Chara sp.2, Sphaerochara prolifera, Sphaerochara intricata and Lychnothamnus barbatus var. antiquus. The palaeoenvironmental characteristics and evolution of the Ilgın palaeolake during the Early Pleistocene (Günz glaciation) are inferred from a facies analysis and the ecological requirements of the charophyte assemblages. The results suggest that during this glacial period, the Ilgın Basin was occupied by a very shallow eutrophic lake with a dense palustrine vegetation belt, which evolved to a shallow (2–15 m deep), stable, oligotrophic, alkaline and oligohaline lake. The increase in water availability in the palaeolake can be related to climatic oscillation during the Günz glaciation. Finally, the lake depocentre migrated towards the NW due to tectonism, and these deposits were later subjected to intense pedogenic alteration.

Hydrogeochemical controls and recharge-flow system of groundwater in the lowland of the Fukui-Ohno Basin, central Japan, based on carbon isotopic signatures

• River water mixing and CO 2 dissolution in groundwater was modeled with δ 13 C tracers. • Decomposition of both C3 and lacustrine organic matter was related to CO 2 dissolution. • Modeling identified similar amounts of vertical and horizontal groundwater recharge. Carbon stable isotopic compositions of dissolved inorganic carbon (δ 13 C DIC ) and chemical compositions of groundwater were used to investigate the processes of supply of DIC to groundwater in the lowland area of the Fukui-Ohno Basin and to examine constraints on the groundwater recharge-flow system. The groundwater in the basin is dominantly of Ca-HCO 3 type with relatively high concentrations of Ca 2+ and HCO 3 − and lower concentrations of Mg 2+ ; this groundwater chemistry can be attributed to weathering within the aquifer of mainly anorthite and some pyroxene at partial pressures of CO 2 that are typical of soils. The δ 13 C values of the CO 2 gas initially equilibrated with DIC in the groundwater were calculated to be −28.6‰ to −22.8‰, indicating that the groundwater in the lowland basin has two main sources of carbon: (1) C3 organic matter (δ 13 C ≈ −29‰), distributed mainly in mountains surrounding much of the basin and in aquicludes within the basin; and (2) organic matter in shallow lacustrine deposits within the lowland basin (δ 13 C ≈ −20‰). The results of modeling accounting for these two sources demonstrated that roughly equal proportions (50 ± 20%) of DIC in the groundwater are sourced from lacustrine deposits and C3 organic matter. The distributions of these types of organic matters in and around the lowland basin suggest that there are two major and roughly equal components of groundwater recharge: a vertical component of downward percolation of precipitation in the lowland area and a mainly horizontal component of groundwater flow originating in the mountains around the study area and flowing through aquifers into the lowland basin.

Sedimentary Characteristics Analysis and Sedimentary Facies Prediction of Jurassic Strata in the Northwest Margin of Junggar Basin—Covering the W105 Well Region in the Wuerhe Area

As the northwestern area of the Junggar Basin is a key area for oil and gas exploration, the sedimentary facies of the Jurassic formations in the Wuerhe area has long been a focus of research. The target strata are Jurassic strata, including five formations: the Lower Jurassic Badaowan and Sangonghe, the Middle Jurassic Xishanyao and Toutunhe and the Upper Jurassic Qigu. Disputes over the are sedimentary facies division exist in this area. Considering the W105 well region in this area as an example, the overall sedimentary facies of single-well logging facies is analyzed and then expanded to two cross-sections and characterized. Based on previous studies, a detailed overview of the regional stratigraphy is obtained by well logs and other data. Then, two cross-sections are selected and analyzed. The single-well and continuous-well facies of 10 wells in the sections are analyzed to grasp the sand bodies’ spatial distribution. Finally, a planar contour map of the net to gross ratio is mapped to analyze the sources and the distribution of the sand bodies in each period. The sedimentary facies map is also mapped to predict the sedimentary evolution. The results show that the sedimentary facies of the Badaowan Formation in the study area was an underwater distributary channel of the fan-delta front, and the sand body spread continuously from northwest to southeast. The Sangonghe Formation entered a lake transgression period with a rising water level, at which time shore–shallow lacustrine deposits were widespread throughout the region. The period of the Xishanyao Formation entered a regression period, the northwest region was tectonically uplifted, and the central and southeastern regions facies were dominated by the fan-delta front and shallow lacustrine. During the Toutunhe Formation period, the northwest region continued to uplift and was dominated by delta plain facies. During the period of the Qigu Formation, the thickness of stratigraphic erosion reached its maximum, and the non-erosion area of the study area was mainly deposited by the fan-delta plain. Overall, the Jurassic system in the W105 well area is a fan delta–lacustrine–fan delta sedimentary system.

Sedimentary Environments and Coal Accumulation of the Middle Xishanyao Formation, Jurassic, in the Western Dananhu Coalfield, Turpan-Hami Basin

The Dananhu coalfield, at the southern margin of Turpan-Hami Basin, Xinjiang, has good coal-forming geological conditions. Due to the low research level on the depositional environment and coal accumulation, the comprehensive coal mining is restricted. Based on drilling and sectioning outcrop data, the depositional characteristics, lithofacies, paleogeography, and coal accumulation of the coal-bearing formations in western Dananhu coalfield are analyzed. The middle member of the Xishanyao Formation is the major coal-bearing strata of the Dananhu coalfield, composed of conglomerate, sandstone, siltstone, mudstone, and combustible organic rock, which can be further subdivided into 14 lithofacies according to substance composition, sediment texture, and structure. During the early Middle Jurassic coal-accumulating period, alluvial fan, fluvial plain, fluvial delta, and shallow lacustrine deposits were developed from northeast to southwest in the western Dananhu mining area. Coal accumulation was most developed in the center regions of the study area extending from northeast to southwest, which were primarily concentrated in margin fans, floodplains, interdeltaic bays, and lakeshore environments, especially the transitional zone between the fluvial plain and the delta plain in the west of No. 1 mining area. This coal accumulation was mainly controlled by synsedimentary tectonic subsidence and clastic sediments supply or sedimentary environment, where tectonic subsidence was the primary controlling factor and the source supply was the secondary controlling factor for coal accumulation, which provided theoretical support for the coal prediction and geological exploration in western Dananhu coalfield.

Tectono-climatic controls of the early rift alluvial succession: Plio-Pleistocene Corinth Rift (Greece)

Proximal alluvial sediments represent a useful sedimentary archive to reconstruct the tectono-climatic history of continental rift basins. However, poor dating of coarse fluvial successions usually prevents high-resolution distinction of tectonic and climatic processes, and thus good determination of process rates. This paper presents a dating study of Plio-Pleistocene Kalavryta river system during the early history of the Corinth Rift (northern Peloponnese, Greece) based on magnetostratigraphy and palynology. This river system developed across several active normal fault blocks that are now uplifted along the southern rift margin. The detailed sedimentary record constrains alluvial architectures from the proximal basin to the river outlet where small deltas built into a shallow lake. In four magnetostratigraphy sections the correlation to the reference scale relies on the identification of the Gauss/Matuyama magnetic reversal and biostratigraphic elements. The river system developed between about 3.6 to 1.8 Ma, with sediment accumulation rates (SARs) ranging from 0.40 to 0.75 mm yr−1. SAR is lower in the alluvial fans than in the deltaic system, and higher at the centre of the normal fault depocentres than at the fault tip. By comparison with worldwide Cenozoic SARs, our values are higher but lie in the same range as those determined in coarse alluvial foreland basins. Moreover, in the context of overfilled intra-mountainous rift basins, these rates are minimum values and can be used as a proxy for accommodation rate. Therefore, early rift stratal wedges and growth synclines attest high sedimentation rates and also high rates of tectonic processes. Finally, in the distal river system, floral compositions and changes of vegetation deduced from palynological data are coherent with alternating fluvio-deltaic and shallow lacustrine deposits, which are linked to relative base level variations. Dry/cool climate is preferentially recorded during periods of low lake level, while the warm/moist climate is mainly recorded in prodelta deposits during periods of high lake level. This correlation suggests that, despite the dominant control of active faulting, climate is a key control of syn-rift stratigraphic architectures.

Triassic faunal successions of the Paraná Basin, southern Brazil

The Paraná Basin was filled by a sedimentary package deposited in successive sedimentation episodes related to the tectonic events that hit the SW portion of the Gondwana. The Triassic portion of this package, known worldwide for its continental tetrapod fauna, occurs only in the southern portion of the basin and is represented by 2 s-order sequences: the Sanga do Cabral Supersequence (SCS - Early Triassic) and the Santa Maria Supersequence (SMS - Middle-Late Triassic). The SCS fauna, including temnospondyls, parareptiles (mainly Procolophon), archosauromorphs, putative synapsids, and a number of indeterminate specimens, is traditionally considered Early Triassic and corresponds to the “Procolophon abundant zone” of the Karoo Basin (the upper levels of the Lystrosaurus AZ), in the upper Katberg Formation, which is Induan to early Olenekian in age. The sedimentary environment of the SCS is thought to be a wide alluvial plain, in which small and shallow channels spread northwards into a vast semiarid environment. By its turn, the Middle-Upper Triassic Santa Maria Supersequence is divided into four third-order sequences, from base to top: Pinheiros-Chiniquá, Santa Cruz, Candelária and Mata. Each of these sequences begins with fluvial deposition (low sinuosity rivers) that is overlain by transgressive shallow lacustrine deposits. The first three of these sequences present a very rich record of fossil tetrapods, including four successive faunal associations: Dinodontosaurus Assemblage Zone (Ladinian, within the Pinheiros-Chiniquá Sequence), Santacruzodon AZ (Ladinian/Carnian, in the Santa Cruz Sequence), Hyperodapedon AZ (Carnian) and the Riograndia AZ (early Norian), the latter two respectively at the base and top of the Candelária Sequence. In general, the lower portion of the package (Pinheiros-Chiniquá and Santa Cruz Sequences) was deposited under more basic and dried environmental conditions and are dominated by synapsids, while the top of the section (Candelária Sequence) is characterized by more acid and humid conditions and by a shift in the faunal content, with diapsids as dominating forms and presenting an increase of diversity compared to the lower biozones.

Sedimentary characteristics of hyperpycnites in a shallow lacustrine environment: A case study from the Lower Cretaceous Xiguayuan Formation, Luanping Basin, Northeast China

Hyperpycnites are extrabasinal turbidites related to hyperpycnal flows which form during river floods. Compared to intrabasinal turbidite systems in deep marine or lacustrine settings, hyperpycnites in shallow lakes settings are not understood. In this work, we studied the typical sedimentary characteristics of shallow lacustrine hyperpycnal flow deposits in outcrops of the Luanping Basin, Northeast China. Based on the sedimentary architecture and analysis of the facies associations in these outcrops, the hyperpycnites can be classified into the following three types: (a) bedload‐dominated erosional channel deposits, (b) bedload‐ and suspended‐load‐dominated channel‐to‐lobe transitional deposits, and (c) suspended‐load‐dominated lobe deposits. The erosional channels in proximal locations are filled by mud clasts, parallel‐laminated sandstones, massive sandstones, cross‐bedded sandstones, massive siltstones, and mudstones. The channel‐to‐lobe transitional deposits in a middle location are composed of parallel‐laminated sandstones, cross‐bedded sandstones, mud clasts near internal erosional surfaces, ripple cross‐bedded sandstones, massive siltstones, and mudstones. The lobe deposits in distal locations are composed of ripple cross‐bedded sandstones, massive siltstones, and mudstones. The sedimentary process of the shallow lacustrine hyperpycnal flows shows obvious response to periodic flood energy fluctuations. Gentle slope, small capacity of flood discharges and semiarid paleoclimate may be the possible reason for the observed sedimentary architecture of the shallow lacustrine hyperpycnal flow deposits. Therefore, this study offers new insights into the sedimentology of shallow lacustrine deposits in rift basins.

Depositional sequence response to Paleogene Lake level and Paleoclimate changes in the northern Liaoxi depression, eastern China

Bohai Bay Basin is a Cenozoic rift basin containing abundant oil and gas resources in northeastern China. By combining 3D seismic data and well logs, three composited sequences are identified from regional unconformity relationships, and seven sequences can be further identified from local unconformities. Major depositional systems include fan delta, braided delta, fluvial delta, shallow lake, deep lake, and sublacustrine fan. Through identifying onlap points in the ramp of the Liaoxi depression, a lake level change curve is constructed. Pollen and algae data are used to analyze paleoclimate which shows two large cool-warm cycles in Paleogene Liaoxi depression. Lake level change is controlled by both tectonic subsidence and climate change. Paleoclimate change is the main factor controlling lake level during stable tectonic periods, while tectonic subsidence is the main controlling factor controlling lake level during active tectonic periods. Depositional evolution corresponds with lake level change, which indicates that lake level stays relatively high with deep or shallow lacustrine and delta front deposits developed, and lake level stays relatively low with alluvial fan, braid delta plain, fan delta, and shallow lacustrine deposits developed.

Architecture of tectonic sequences, depositional systems, and tectonic controls of the sedimentary fills of the rift‐related Wenchang Formation in the Lufeng Depression, Pearl River Mouth Basin, China

Different sequence types were formed in various stages associated with tectonic variations and changes of the sediment supply in the rift‐related Wenchang Formation. Based on cores, wire logging, and seismic data, the sequence stratigraphy, depositional systems, and controls on the depositional systems of the Wenchang Formation in the Lufeng Depression were studied. The Wenchang Formation developed braided‐delta, lacustrine, fan‐delta, and nearshore subaqueous fan deposits and represents one first‐order sequence and three second‐order sequences, indicating three stages of tectonostratigraphic evolution. (a) The early syn‐rift sequence (SSQ1) developed fan‐delta and shallow lacustrine deposits. (b) The rift climax sequence (SSQ2) contains the early rift climax systems tract and the middle‐late rift climax systems tract. The early rift climax systems tract formed primarily braided‐delta and shallow lacustrine deposits, whereas the middle‐late rift climax systems tract developed mostly semi‐deep to deep lacustrine deposits. (c) The late syn‐rift sequence (SSQ3) developed braided‐delta and shallow lacustrine deposits. In the SSQ3 transgressive systems tract, retrogradational parasequence sets developed along the southern dip slope because of the weak sediment supply from the Huilu low uplift, whereas westward aggradational–progradational parasequence sets developed along the long axis because of the stronger sediment supply from the Lufeng eastern low uplift. Tectonic activity primarily controlled the early syn‐rift and rift climax deposition; syn‐depositional faults formed the transfer zones and slope break zones. Three types of transfer zones for sedimentary transport into the basin were distinguished, including synthetic approaching transfer zones, strike ramps, and collateral transfer zones. Three types of slope break zones were also recognised, including fault scarp zones controlling fan deltas and nearshore subaqueous fans, fault terrace zones controlling low‐stand fan deltas, and antithetic fault terrace dip‐slope zones controlling braided deltas.

Terrestrial sedimentary responses to astronomically forced climate changes during the Early Paleogene in the Bohai Bay Basin, eastern China

High-resolution measurements of magnetic susceptibility (MS) (8029 samples) and X-ray fluorescence (XRF) elements (4367 samples) were performed on the ~403-m-thick Shahejie Formation in the Bohai Bay Basin, eastern China. The power spectral and fast Fourier transform (FFT) spectrogram analyses of the MS series reveal that the wavelength ratios of the stratigraphic cycles are ~20:5:2:1, which is consistent with the period ratios of astronomical cycles (long eccentricity, short eccentricity, obliquity, and precession). A floating-point time scale (FPTS) was developed by tuning the magnetostratigraphic susceptibility (MSS) zonation to the standard reference zonation (SRZ) established by using the same interval of semi-long eccentricity (200 kyr). The depositional durations of the lower third member (Es3l) and upper fourth member (Es4u) of the Eocene Shahejie Formation were estimated to be 2.03 ± 0.2 Myr and 1.97 ± 0.2 Myr, respectively. The sedimentary accumulation rates (SARs) during different stages were calculated based on the FPTS; the SAR of the deep to semi-deep lacustrine deposits was generally lower than 10 cm/kyr, while that of the shallow lacustrine deposits was higher than 10 cm/kyr. The significant ~1.2-Myr cycles, which were related to the planetary secular frequencies s4-s3, were coincident with the periodic climatic variation in the study area, while the ~0.84-Myr cycles may have been long-period astronomical cycles unique to the Dongying Sag. Furthermore, both of these cycles may have controlled the enrichment of the hydrocarbon source rocks in eastern China.

Integrated overview of the vertebrate fossil record of the Ladruñán anticline (Spain): Evidence of a Barremian alluvial-lacustrine system in NE Iberia frequented by dinosaurs

The Barremian Mirambel Formation (Maestrazgo Basin, Iberian Chain, NE Spain) preserves different types of dinosaur and other vertebrate fossils (skeletal, eggshell and ichnological remains). A total of 31 vertebrate fossil sites and tracksites have been recognized within this unit in the Ladruñán area (Teruel province). Detailed stratigraphic, sedimentological and micropalaeontological analyses have also been performed in the unit. A vertical sedimentary trend from alluvial-dominated facies (meandering river and related overbank areas) to palustrine-lacustrine facies and back has been defined for the Mirambel Formation in this area. The depositional system was located close to the coastline, as indicated by sporadic marine input in the lower part of the unit.Most fossil remains were recovered by surface collection as well as by the usual techniques used for macrovertebrate excavations. The dinosaur record identified comprises ornithopods, theropods and sauropods. Four distinct track-bearing horizons have been identified. The heterolithic nature and aggradation characteristic of the Mirambel Formation are favourable factors for track formation and preservation. The dinosaur tracks consist of convex hyporeliefs or concave epireliefs that record the trackmakers as they frequented lakeshores, alluvial floodplains and fluvial courses. Macrovertebrate bonebeds occur in alluvial settings (poorly-drained floodplains and “ponds”). Microvertebrate concentrations are located in shallow lacustrine deposits. Isolated skeletal elements can be found in a great variety of deposits. Attritional accumulation in a low-energy depositional context is the general pattern of origin for the bone-bearing fossil sites of the Mirambel Formation. As regards the genetic framework, the resulting skeletal assemblages are predominantly the result of physical factors, with sedimentology as a key factor, rather than biological phenomena. Eggshell fragments are frequent throughout the unit but are clearly more common in palustrine-lacustrine deposits. These can be taken to be parautochthonous bioclasts from nearby areas and might be indicative of the preferential affinity of the egg-layers for wetlands and lakeshores.

Dolomitization, gypsum calcitization and silicification in carbonate–evaporite shallow lacustrine deposits

AbstractThis paper describes and interprets the mineral and facies assemblages that occur in carbonate–evaporite shallow lacustrine deposits, considering the importance of the processes pathway (i.e. dolomitization, gypsum calcitization and silicification). The Palaeogene deposits of the Deza Formation (Almazán Basin, central‐northern Spain) are selected as a case study to determine the variety of physicochemical processes taking place in carbonate–evaporite shallow lakes and their resulting diagenetic features. Dolostones are the predominant lithology and are composed mainly of dolomite with variable amounts of secondary calcite (5 to 50%), which mainly mimic lenticular gypsum (pseudomorphs). Five morphological types of dolomite crystal were identified as follows: dolomite tubes, dolomite cylinders, rhombohedral dolomite, spheroidal and quasi‐rhombohedral dolomite, and cocoon‐shaped dolomite. The dolomite cylinders and tubes are interpreted as the dolomitized cells of a widespread microbial community. The sequence of diagenetic processes started with growth of microlenticular interstitial gypsum in a calcareous mud deposited on the playa margin mudflats, and that sometimes included microbial sediments. Immediately following growth of gypsum, dolomite replaced the original calcite (or possibly aragonite) muds, the microbial community and the gypsum. Partial or total replacement of gypsum by dolomite was related mainly to the biomineralization of endolithic microbial communities on gypsum crystals. Later calcitization took place under vadose, subaerial exposure conditions. The development of calcrete in distal alluvial settings favoured the release of silica and subsequent silicification on the playa margin mudflats. Stable isotope compositions of calcite range from −9·02 to −5·83‰ δ13CPDB and −7·10 to 1·22‰ δ18OPDB; for the dolomite, these values vary from −8·93 to −3·96‰ δ13CPDB and −5·53 to 2·4‰ δ18OPDB. Quartz from the cherts has δ18OSMOW values ranging from 27·1 to 31·1‰. Wide variation and relatively high δ18OSMOW values for dolomite indicate evaporitic and closed hydrological conditions; increased influx of meteoric waters reigned during the formation of secondary calcite spar.

Overlapping of pedogenesis and meteoric diagenesis in distal alluvial and shallow lacustrine deposits in the Madrid Miocene Basin, Spain

The Miocene distal alluvial fan and palustrine deposits of the Madrid Basin (Paracuellos de Jarama area) were examined to establish the sequence of its pedogenic–diagenetic processes and the main factors controlling them. In this area the diagenetic processes operated not only on carbonates, as commonly studied, but also in high magnesium clays and opaline cherts. This paper provides a dynamic model for saline–alkaline lake margins that complements the existing freshwater palustrine model. Three sections (BER, PEL and UPC) containing limestones, sepiolite, dolostones and opaline cherts were examined; these represent the entire transition from alluvial fan deposit to lake environment. The alluvial fan deposits (PEL section) show the most complex sequence of processes. After a weak edaphic imprint, the primary mudstone and siltstones are affected by silicification (opaline levels formed mainly by selective replacement of sepiolites) and later dolomitization. Several types of dolomite are present, rounded crystals, spherules and globular bodies being the most characteristic. In the ponds situated at the foot of the alluvial fans or in interfan areas, sepiolite precipitated within free water bodies (BER section) and an intense pedogenesis led to the formation of palustrine sepiolite deposits. Later, the intense silicification of the sepiolite produced lenticular opaline levels that were locally transformed to quartz by aging. In the shallow lakes (UPC section), palustrine limestones and sepiolite were deposited. The carbonates are partially affected by silicification, the sepiolite becoming completely silicified. The opaline levels clearly preserve the structure of the limestones and sepiolite host rocks. All the opaline levels show local aging processes. The silicification processes were different in the three settings due to the interplay of groundwater with sedimentary environments and facies. In the distal alluvial fan sediments of the PEL section, the initial host rock affected by silicification is not preserved due to the later dolomitization that erased both it and its textures. The silicification environment was therefore not easy to determine. In the BER and UPC section, the main silicification took place in groundwater-influenced environments but evidence was seen of cements and silicifications in vadose environments. In all study settings, the overlapping of pedogenic, vadose and groundwater processes was controlled by changes in the position of the water table. Four stages were concluded to explain the transformation from the primary deposits to the later dolomitization process and the formation of vadose cements. The majority of the silicifications seen occurred as responses to changes in the water table position linked to the aggradation of this closed basin. In all the studied environments, the sepiolite deposits were preferentially silicified; their fibrous structure and high absorption capacity helping to retain interstitial fluids. Moreover sepiolite and opal are stable under relatively similar geochemical conditions in arid environments. The dissolution of the sepiolite caused an increase in silica in the interstitial fluids, favouring silicification and ruling out the need of a biogenic source. The replacement of limestones by silica is usually driven by variations in pH and salinity, which are very common in surface environments such as shallow lakes and soils.

Late Miocene to Early Pliocene depositional history of the intramontane Florina–Ptolemais–Servia Basin, NW Greece: Interplay between orbital forcing and tectonics

Upper Miocene and Lower Pliocene shallow lacustrine deposits from the Florina–Ptolemais–Servia Basin in northwest Greece show a distinct m-scale sedimentary cyclicity of alternating marls and lignites or clays, which were shown to be primarily related to precession-induced climate variations ([van Vugt, N., Steenbrink, J., Langereis, C.G., Hilgen, F.J., Meulenkamp, J.E., 1998. Magnetostratigraphy-based astronomical tuning of the early Pliocene lacustrine sediments of Ptolemais (NW Greece) and bed-to-bed correlation with the marine record. Earth Planet. Sci. Lett. 164 (3–4) 535–551; Steenbrink, J., Van Vugt, N., Hilgen, F.J., Wijbrans, J.R., Meulenkamp, J.E., 1999. Sedimentary cycles and volcanic ash beds in the lower Pliocene lacustrine succession of Ptolemais (NW Greece): Discrepancy between 40Ar/ 39Ar and astronomical ages. Palaeogeogr. Palaeoclimatol. Palaeoecol. 152, 283–303; Steenbrink, J., Van Vugt, N., Kloosterboer-van Hoeve, M.L., Hilgen, F.J., 2000. Refinement of the Messinian APTS from sedimentary cycle patterns in the lacustrine Lava section (Servia Basin, NW Greece). Earth Planet. Sci. Lett. 181 (3–4) 161–173]). Three additional sections and drill cores from the same basin are studied using a combination of cyclostratigraphy, magnetostratigraphy and 40Ar/ 39Ar dating. Together with the earlier studied sections, they provide a continuous succession that documents the depositional history of the intramontane Neogene basin fill. Sedimentation in the basin started at around 8 Ma ago with deposition of coarse-grained terrigenous-clastics in an alluvial environment. From ∼ 7.1 to ∼ 6.1 Ma ago, diatomaceous marls were deposited in a lacustrine environment, frequently interrupted by deposition of clays and local xylite-type lignite seams. Hereafter, deposition of alternating coarse- and fine-grained terrigenous-clastic alluvial sediments took place, followed by a gradual shift towards lacustrine sedimentation at around 5.5 Ma. Deposition of alternating fine-grained clastics (silts and clays) and biochemical sediments (carbonates, marls and thin lignites), with episodic periods of sub-aerial exposure continued until ∼ 5.23 Ma. From then onwards up to ∼ 3.9 Ma, regular alternating lignites and marls were formed in a shallow lacustrine environment. Precession-induced fluctuations of regional climate primarily defined the m-scale variations in lithology as observed in all members of succession, whereas the 100- and 400-kyr eccentricity cycle had a clear impact on larger-scale lithological alternations in parts of the succession as well. A marked similarity in the ages of fundamental changes in depositional environments between the Florina–Ptolemais–Servia Basin and the marine basins in the Mediterranean is observed, suggesting (but not necessarily implying) that they have a common, possibly tectonic origin.

Alluvial fan–lacustrine association in the fault tip end of a half-graben, northern Triassic Cuyo basin, western Argentina

The Triassic deposits of Cerro Puntudo in the San Juan province of western Argentina constitute the northernmost exposures of the northern portion of the nonmarine Cuyo rift basin, also known as the Las Peñas-Tamberías half-graben. The local column, with an exposed thickness of approximately 400 m, consists of abundant basal and topmost coarse alluvial fan conglomerates and breccias (facies associations I and II) and a relatively thin (approximately 50 m) intervening sequence of marginal, shallow lacustrine deposits characterized by stromatolitic (domal) limestones, tuffaceous mudstones, and fine-grained sandstones (facies association III). Subaerial exposure in the lacustrine deposits is evidenced by desiccation cracks and brecciation. A very thin (0–6 m), laterally, discontinuous succession of lacustrine deposits with similar characteristics is interbedded with the basal conglomerates. Laterally, this lacustrine interval was eroded by overlying conglomerates. The basal conglomerates commonly show crude normal grading, faint cross-bedding, and b-axis clast imbrication. The predominance of coarse deposits and paleocurrents from NW to SE, indicative of an axial flow pattern, suggest that these exposures correspond to the northern end of the Cuyo basin, which is characterized by a shallow, alluvial fan-encased, carbonate-rich lake margin. This lacustrine interval can be correlated with the thicker lacustrine section exposed to the south at Quebrada del Tigre and Ciénaga Larga along the border fault margin of a nonmarine half-graben. The correlation with these sections suggests that the Cerro Puntudo lacustrine deposits are the shallowest equivalent of more profundal, organic-rich lacustrine sediments exposed in the deepest segment of the border fault margin. These thickness and facies variations are the result of differential subsidence along the border fault margin between the low accommodation, fault tip end represented by the Cerro Puntudo section and the high accommodation, central segment located to the south.

Small Amphibian and Reptile Footprints from the Permian Carapacha Basin, Argentina

This paper contains a taxonomic study of the Permian tetrapod ichnofauna from the Carapacha Basin. Tetrapod traces are analyzed in their environmental context and compared with similar faunas from Europe and North America. This ichnofauna is particularly relevant because of the scarcity of Permian tetrapod tracks from South America and also of Permian tetrapod fossils from Argentina. Ephemeral fluvial and shallow lacustrine deposits compose the sedimentary succession of the basin, which is represented by the Carapacha Formation. Most of the tracks have been collected from the upper member of the formation (Urre-Lauquen Member), mainly from freshwater ephemeral lake deposits as well as from playa-lake mudflats. The deposits of this member have been attributed to the early Late Permian on the basis of a Glossopteris fossil flora. Ichnotaxonomic designations of tetrapod traces are made on the basis of morphologic features that reflect the anatomy of the producer and special attention has been paid to extramorphologic deformations observed in the track assemblage. A total of four footprint ichnotaxa have been recognized, namely Batrachichnus salamandroides (Geinitz, 1861), Hyloidichnus bifurcatusGilmore, 1927, cf. Amphisauropus isp. and cf. Varanopus isp. These track taxa are associated with two forms of vertebrate swimming traces (Characichnos isp. and type A swimming trace) and a possible fish trail. Invertebrate trace fossils include abundant arthropod locomotion traces and Scoyenia isp. The ichnofauna is composed of six tetrapod ichnocoenoses that are dominated by tiny amphibian tracks attributed to Temnospondyli (Batrachichnus and type A swimming trace) and Seymouriamorpha (Amphisauropus), and also contain the footprints of small reptiles, mostly Captorhinomorpha and possibly Pelycosauria (Hyloidichnus and Varanopus). Even if the ichnofauna of the Carapacha Basin is slightly younger than typical examples from the literature of the Early Permian “red bed ichnofacies” (Hunt et al., 1995b), a comparison is made. However, further detailed case studies are needed to formally define this “red bed ichnofacies” and its prospective subdivisions.

Palaeoenvironmental reconstruction of a middle Miocene alluvial fan to cyclic shallow lacustrine depositional system in the Calatayud Basin (NE Spain)

ABSTRACT The middle Miocene sedimentary fill of the Calatayud Basin in north‐eastern Spain consists of proximal to distal alluvial fan‐floodplain and shallow lacustrine deposits. Four main facies groups characteristic of different sedimentary environments are recognized: (1) proximal and medial alluvial fan facies that comprise clast‐supported gravel and subordinate sandstone and mudstone, the latter exhibiting incipient pedogenic features; (2) distal alluvial fan facies, formed mainly of massive mudstone, carbonate‐rich palaeosols and local carbonate pond deposits; (3) lake margin facies, which show two distinct lithofacies associations depending on their distribution relative to the alluvial fan system, i.e. front (lithofacies A), comprising massive siliciclastic mudstone and tabular carbonates, or lateral (lithofacies B) showing laminated and/or massive siliciclastic mudstone alternating with tabular and/or laminated carbonate beds; and (4) mudflat–shallow lake facies showing a remarkable cyclical alternation of green‐grey and/or red siliciclastic mudstone units and white dolomitic carbonate beds. The cyclic mudflat–shallow lake succession, as exposed in the Orera composite section (OCS), is dominantly composed of small‐scale mudstone–carbonate/dolomite cycles. The mudstone intervals of the sedimentary cycles are interpreted as a result of sedimentation from suspension by distal sheet floods, the deposits evolving either under subaerial exposure or water‐saturated conditions, depending on their location on the lacustrine mudflat and on climate. The dolomite intervals accumulated during lake‐level highstands with Mg‐rich waters becoming increasingly concentrated. Lowstand to highstand lake‐level changes indicated by the mudstone/dolomite units of the small‐scale cycles reflect a climate control (from dry to wet conditions) on the sedimentation in the area. The spatial distribution of the different lithofacies implies that deposition of the small‐scale cycles took place in a low‐gradient, shallow lake basin located in an interfan zone. The development of the basin was constrained by gradual alluvial fan aggradation. Additional support for the palaeoenvironmental interpretation is derived from the isotopic compositions of carbonates from the various lithofacies that show a wide range of δ18O and δ13C values varying from −7·9 to 3·0‰ PDB and from −9·2 to −1·7‰ PDB respectively. More negative δ18O and δ13C values are from carbonate‐rich palaeosols and lake‐margin carbonates, which extended in front of the alluvial fan systems, whereas more positive values correspond to dolomite beds deposited in the shallow lacustrine environment. The results show a clear trend of δ18O enrichment in the carbonates from lake margin to the centre of the shallow lake basin, thereby also demonstrating that the lake evolved under hydrologically closed conditions.