Palustrine Carbonates Research Articles

Sedimentology, faunal content and pollen record of Middle Pleistocene palustrine and lagoonal sediments from the Peri-Adriatic basin, Abruzzi, eastern central Italy

A 25 m-thick outcrop section exposed at Torre Mucchia, on the sea-cliff north of Ortona, eastern central Italy, comprises a rare Middle Pleistocene succession of shallow-water and paralic sediments along the western Adriatic Sea. An integrated study of the section, including facies and microfacies analyses, and characterization of paleobiological associations (mollusks, fishes, ostracods, foraminifers and pollen), enable a detailed reconstruction of the paleoenvironmental and paleoclimatic conditions during deposition. The shallow-water deposits include a transgressive, deepening- and fining-upward shoreface to offshore-transition facies succession overlain by a regressive shoreface-foreshore sandstone body with an erosive base and a rooted and pedogenically altered horizon at the top that imply deposition during sea-level fall. This forced regressive unit is overlain by paralic strata forming a transgressive succession comprising palustrine carbonates and back-barrier lagoonal mudstones. The palustrine carbonates exhibit some of the typical features encountered in palustrine limestones deposited within seasonal freshwater wetlands (marl prairies). Following the sea-level rising trend, the freshwater marshes were abruptly replaced by a barrier-lagoon system that allowed deposition of the overlying mud-rich unit. Within these deposits, the faunal assemblages are consistent with a low-energy brackish environment characterized by a relatively high degree of confinement. The pollen record documents the development of open forest vegetation dominated by Pinus and accompanied by a number of mesophilous and thermophilous tree taxa, whose composition supports a tentative correlation with Marine Oxygen Isotope Stage 17. The new pollen record from Torre Mucchia improves our understanding of the vegetation development in the Italian Peninsula during the Middle Pleistocene and sheds new light on the role played by the most marked glacial periods in determining the history of tree taxa.

Rapid regional surface uplift of the northern Altiplano plateau revealed by multiproxy paleoclimate reconstruction

The central Altiplano is inferred to have experienced ∼2.5±1km surface uplift between ∼10 and 6 Ma, while the southern Altiplano experienced a similar magnitude of surface uplift that began earlier, between ∼16 and 9 Ma. To properly constrain the along strike timing of the Altiplano plateau surface uplift, it is necessary to know how and when the northernmost part of the Altiplano plateau evolved. We reconstruct the paleoclimate and infer the corresponding paleoelevation from the Miocene–Pliocene deposits of the Descanso–Yauri basin (14–15°S) in the northernmost part of the Altiplano plateau using 4 different proxies, including carbonate clumped isotope composition (i.e., Δ_(47) values), carbonate δ^(18)O_c, leaf wax δD_(wax) and pollen assemblages from paleosol, lacustrine and palustrine carbonates and organic-rich sediments. The isotopic signatures reflect past climate conditions of mean annual air temperature (Δ_(47)) and meteoric water isotope values (δ^(18)O_c, δD_(wax)). Our results show that the northernmost plateau remained at low elevation (0.9±0.8 to 2.1±0.9km) until late Miocene time (∼9 Ma) characterized by ∼15 °C warmer than modern temperature (mean annual air temperature of 23±4°C, 2σ), low elevation vegetation and precipitation signature with reconstructed □ δ^(18)O_(mw) (VSMOW) of −8.3±2.0‰(2σ) from carbonate (δ^(18)O_c) and −8.6±1.8‰(2σ) from leaf wax (δD_(wax)). Modern elevations of 4 km were not reached until 5.4±1.0Ma, as indicated by a negative shift in δD_(wax) (VSMOW) from −143.4±12.8‰(2σ) to −209.2±21.1‰(2σ) between 9.1±0.7 and 5.4±1.0Ma. The timing of surface uplift of the northernmost Altiplano is consistent with the evidence for late Miocene surface uplift of the central Altiplano (16–19°S) between 10 and 6 Ma, and indicates that regional scale uplift in the northern–central plateau significantly postdates the onset of surface uplift in the southern Altiplano (19–22°S) between ∼16 and 9 Ma. These results are consistent with piecemeal removal of the lower dense lithosphere, combined with possible lower/middle crustal flow from south to north in the plateau acting as the main mechanisms for the formation of the Altiplano plateau.

Sedimentologic study of palustrine continental carbonate deposits of the Tajerouine area, NW Tunisia

Les calcaires palustres d'âge quaternaire (Pleistocene moyen et superieur), objet de cette etude, se trouvent au niveau du massif El Gara Tajerouine (Nord-Ouest de la Tunisie). L'etude de ces depots a differentes echelles, macroscopique (description lithologique) et microscopique (petrographie et mineralogie), montre qu'ils ont une structure tabulaire tout en reposant en discordance angulaire sur la serie marno-calcaire de l'Aptien-Albien. Ces depots montrent egalement des variations laterales et verticales de facies : conglomerats, calcaires et stromatolithes. Enfin, l'epaisseur des couches tend aussi a augmenter progressivement d'est en ouest et vers le sud de la zone d'etude. En raison de fluctuations de la nappe phreatique, ces carbonates ont subi des periodes d'inondation et d'emersion periodiques provoquant de nombreuses modifications par une pedogenese d'emersion plus ou moins profonde (nodulisation, traces de racines, pseudo-microkarst), celles-ci influant sur les cimentations au cours de la diagenese (alternances de ciment phreatique et vadose) qui justifient l'appellation des calcaires palustres.

Climatic and halokinetic controls on alluvial–lacustrine sedimentation during compressional deformation, Andean forearc, northern Chile

AbstractThe Salar de Atacama forms one of a series of forearc basins developed along the western flank of the Central Andes. Exposed along the northwest margin of the basin, a salt‐cored range, the Cordillera de la Sal, records the Mid‐Miocene to recent sedimentological and structural development of this basin. Sediments of the Mid‐Miocene Vilama Formation record the complex interaction between regional/local climate change, halokinesis and compressional deformation. This study reveals how these factors have controlled the facies development and distribution within the Salar de Atacama. Detailed sedimentary logging, cross‐sections and present day geomorphology through the northern Cordillera de la Sal have been used to establish a lithostratigraphy, chronostratigraphy and the regional distribution of the Vilama Formation. The Vilama Formation documents an increase in aridity with a hiatus in sedimentation from Mid‐Miocene to 9 Ma with initial uplift of the Cordillera de la Sal. From 9 Ma to 8.5 Ma deposition of a meandering fluvial system is recorded followed by a rapid decrease in sedimentation till 6 Ma. From 6 to 2 Ma, the deposition of extensive palustrine carbonates and distal alluvial–mudflat–lacustrine demonstrates the existence of an extensive lake within the Salar de Atacama. Post 2 Ma, the lake decreased in size and braided alluvial gravels associated with alluvial fans were widespread through the region suggesting a final shift to hyperarid conditions. By comparing the Vilama Formation with similar age facies throughout northern Chile and southern Peru, several shifts in climate are recognized. Climate signatures within northern Chile appear to be largely diachronous with the last regional event in the Mid‐Miocene. Since that time, humid events have been restricted to either Precordillerian basins or the Central Atacama. Within the Central Atacama, the final switch to hyperarid conditions was not till the earliest Pleistocene, much later than previously estimated within the region.

Types of soft-sediment deformation structures in a lacustrine Ploužnice member (Stephanian, Gzhelian, Pennsylvanian, Bohemian Massif), their timing, and possible trigger mechanism

The succession of Stephanian C lacustrine and fluvial facies of the Ploužnice member (Semily Formation) paleolake in the Krkonose Piedmont Basin (northern Czech Republic) preserved in borehole Sm-1 shows five types of soft-sediment deformation structures (SSDS): (1) sediment injections (clastic dikes), (2) load structures and ball-and-pillow structures, (3) water-escape structures (discontinuous laminations, deformed wavy bedding, recumbent folding, and dish or pillar structures), (4) deformations by growth of carbonate and silica minerals during diagenesis, and (5) bioturbation. Bioturbations disturbing mechanical SSDS suggest that soft-sediment deformations may occur syndepositionally or soon after deposition. The discussed mechanical SSDS are developed in a lacustrine environment, most being in lower shoreface and offshore facies. The mechanical SSDS found in the Ploužnice lake deposits occur in all lacustrine facies associations (290 SSDS horizons in a 67-m-thick succession). The cumulative thickness in cm of mechanical SSDS horizons per meter of thickness (ratio cm SSDS/m) is the highest in lower shoreface facies where it reaches from 50 up to 59.7 cm SSDS/m. Offshore facies association reaches 44 cm SSDS/m. Upper shoreface facies associations have 25.9 and 26.0 cm SSDS/m, while nearshore/mudflat facies associations preserve from 22.5 to 20.5 cm SSDS/m, and in palustrine carbonate, 13.5 cm mechanical SSDS/m was found. SSDS in fluvial facies are rare (2.9 cm SSDS/m). The distribution of SSDS in the Sm-1 borehole shows clear relationships to sedimentary facies and processes such as density flows or fluctuation of water level. The relationships of particular structures indicate a relative timing of formation which is as follows: sediment deposition was followed by the formation of mechanical SSDS, then by bioturbation, and finally by deformations due to early diagenetic growth of carbonates and silica. The distribution of SSDS in vertical sections and their direct relationship to sedimentary facies point to endogenic rather than external trigger mechanisms such as seismic activity. The main endogenic trigger mechanisms responsible for the origin of SSDS included a Stephanian semi-humid seasonal climate, basin morphology with relatively steep gradients, and elevated source areas, which created a conducive environment for rapid and repeated deposition of sheetfloods and hyperconcentrated flows and turbidites causing syndepositional loading.

Calcretes and palustrine carbonates in the Oligo-Miocene clastic–carbonate unit of the Farafra Oasis, Western Desert, Egypt: Their origin and paleoenvironmental significance

The origin and paleoenvironmental significance of the calcretes and palustrine carbonates in the Oligo-Miocene clastic–carbonate unit that outcrops at the eastern sector of the Farafra Depression (Western Desert, Egypt) have been discussed based on field and petrographic investigations. The calcretes–palustrine carbonates assemblage occurs above a siliciclastic/distal alluvial-floodplain facies. The calcretes represent the transition from the underlying siliciclastic/distal alluvial-floodplain facies to the overlying palustrine carbonates. The calcrete-host rocks are muddy sandstones and sandy mudrocks.This study reveals the occurrence of groundwater calcretes with an upward gradational maturity pattern, ranging from incipient to nodular and to massive calcretes. The calcretes micromorphological analysis suggests that they were originated in vadose and phreatic diagenetic environments by groundwater through evaporation, degassing with no biological activity. The palustrine carbonates are also recognized above the calcrete horizons. They occur in the form of micritic limestones displaying different features that indicate their modifications during pedogenesis and subaerial exposure. These features include clotted-peloidal texture, fenestral fabric, mottling, pseudo-brecciation, desiccation cracks, pseudomicrokarst, root traces and silicification of the lime mud. The calcretes–palustrine carbonates assemblage records a progressive decrease in the terrigenous supply and a continuous rise of the groundwater table associated with local subsidence in a semi-arid to sub-humid climate. A model is suggested for the development of calcretes and palustrine carbonates in the study area.

Paleoenvironnement and paleoclimate reconstruction for the early to middle Miocene from stable isotopes in pedogenic carbonates (Digne-Valensole basin, southeastern France)

Abstract Stable isotope (C and O) analyses were carried out on pedogenic nodules from carbonate-rich paleosols from early to middle Miocene strata in the Digne-Valensole foreland basin (southeast France). Paleosols from three sections corresponding to several paleoenvironments were sampled in order to investigate the local influence on the isotope record of the depositional sites: a deltaic floodplain (Châteauredon section, BCR), an inland floodplain (Saule Mort section, SM) and a shallow lacustrine system (Pont d’Aiguines section, PA). According to their biostratigraphy, they correspond to the same stratigraphic interval. Paleosols are regularly distributed along the three sections. They have developed either on floodplain alluvium or on palustrine carbonate mud. They are characterized by fersiallitic red soils with a calcic horizon, typical of a xeric soil moisture conditions with a marked seasonality. Carbonate nodules sampled at depths of 0.3 to 1 m below the preserved top soil are considered suitable for paleoecological interpretation. The cross-plot of the δ18O and δ13C values shows well-distributed data indicating that the pedogenic nodules were not affected by any major recrystallizations during burial. Their distribution indicates climate conditions similar to those of modern Mediterranean soils, characterized by carbonates precipitating from enriched meteoric water. A refined stratigraphic attribution of all the sections is based on the carbon isotope chemostratigraphy developed on the Châteauredon section by correlation with marine carbon isotope data [Bialkowski et al., 2006]. All sections extend from very early Aquitanian to middle Serravallian. No major hiatus was identified except for the one associated with a highly karstified surface in the Pont d’Aiguines section. The hiatus duration, estimated to 1 Ma, is interpreted as the record of some deformation at the front of the Digne thrust sheet during the Langhian. Most carbon isotope values are heavier than −8‰, suggesting a water-stressed C3 vegetation growing in a semi-arid environment. These semi-arid conditions are found in all sections during early Aquitanian and late Burdigalian to early Serravallian (Middle Miocene climatic optimum, MMCO), indicating similar environmental conditions between the coastal sites (BCR and PA sections) and the inland SM section. From middle Aquitanian to late Burdigalian, data from the coastal sites indicate typical C3 vegetation whereas xeric conditions characterized the inland site, suggesting distinct local ecosystems. The interpretation of the oxygen isotope record is more complex due to the different interactions during carbonate precipitation. In the early Aquitanian, the concomitant decrease in the pedogenic δ13C and δ18O isotope values is interpreted as a decrease of the water-stressed environmental conditions due either to a change in precipitation patterns resulting in low evaporation or to a fall in temperature. For the Langhian, the high δ13C values indicate xeric conditions throughout the area, and the high δ18O values at the coastal site suggest an increase in the combined effect of temperature and evaporation. Evidence of deformation related to the foreland basin evolution during this period is supported by the great differences in the oxygen isotope record of the different sections. The observed changes in the environmental conditions of the Digne-Valensole basin agree with the climate fluctuations identified on the basis of paleobotanical assemblages and mammal teeth isotope records from western European continental basins and of marine data. Our results show that a study of paleosols and the isotope composition of the pedogenic carbonate of the early to middle Miocene in the Digne-Valensole basin provide valuable information concerning the regional paleotopographic evolution of the foreland basin, as well as the distinct paleoecological systems that developed between the inland and the coastal sites and the regional paleoclimatic trends that correspond to those identified in other European continental basins. These reconstructions based on paleosols, the dry-end member of the climatic record, are complementary to those based on the paleofloras, mostly obtained from the wet-end member.

Paleoenvironmental clues archived in non-marine Pennsylvanian–lower Permian limestones of the Central Appalachian Basin, USA

Nonmarine limestones are a key component of the upper Middle Pennsylvanian through lower Permian succession in the Appalachian Basin. Previous interpretations of their environments of deposition range from brackish coastal mudflats to hydrologically open freshwater lake complexes with peat-forming vegetated marshes to semi-closed or closed, possibly saline, shallow lakes developed on a distal alluvial plain.Ostracode wackestones/packstones, some exhibiting laminations, and with fish debris, articulated ostracode shells and phosphatic clasts define the lower portions of limestone beds or benches, passing upward into ostracode–peloidal wackestones/packstones and intraclastic–skeletal–peloidal packstones/grainstones. Desiccation features and rooting structures, which are developed in the upper portions of the limestones, record subaerial exposure, pedogenic alteration and desiccation. Pseudomicrokarst and caliche-like vadose and early diagenetic phreatic cements suggest a seasonally dry subhumid to semi-arid regional climate. Many of the sedimentologic and diagenetic features of the limestones are characteristic of palustrine carbonates, which coupled with their stratigraphic relation to paleo-Vertisols, siliciclastics, and coals, indicate that they formed in broad seasonal wetland–pond complexes that developed on distal regions of a low-gradient, distal alluvial plain under seasonally dry subhumid to semi-arid climates. Integration of Sr isotopic compositions of shark teeth with previously published stable isotope compositions of the limestones suggests that these purely continental environments were hydrologically semi-closed to closed systems. Repeated stacking of these features at the bed- to limestone bench-scale defines repeated shallowing upward, drying cycles at the 103 to 104yr-scale, which were likely climate-driven.

Palustrine carbonates and pedogenic calcretes in the Çal basin of SW Anatolia: Implications for the Plio-Pleistocene regional climatic pattern in the eastern Mediterranean

Abstract Sedimentologic and stable isotopic studies in the Cal basin-fill provide new insights on the late Neogene–Quaternary climatic pattern in the eastern Mediterranean. Long-term subsidence and terrestrial sedimentation in the basin beginning in the late Miocene are recorded in alluvial fan and fluvial deposits overlain by Pliocene palustrine carbonates. An erosional unconformity separates the Pleistocene siliciclastic and calcrete deposits above as a single sedimentary unit resting on top of floodplain fines and restricted to the basin center. The stable isotopic analysis of the palustrine carbonates and pedogenic calcretes displays a wide range of values (− 7.98

Abrupt transition from wet Pliocene into dry Pleistocene proved by faunal and isotopic composition of the palustrine carbonates and pedogenic calcretes in the Çal-Karahallı basin of SW Anatolia

Abrupt transition from wet Pliocene into dry Pleistocene proved by faunal and isotopic composition of the palustrine carbonates and pedogenic calcretes in the Çal-Karahallı basin of SW Anatolia

Development and lithogenesis of the palustrine and calcrete deposits of the Dibdibba Alluvial Fan, Kuwait

A model is proposed for the development of the Quaternary palustrine carbonate–calcrete association, which occurs as hard crust capping low hills at a distal flood plain of Al Dibdibba alluvial fan located at southwestern Kuwait. Field occurrence, detailed petrographic investigation and geochemical analysis revealed that a single cycle of groundwater calcrete with vertical gradational maturity pattern was developed. This represents a continuous sedimentological cycle during which flood sheet conditions prevailed with intermittent periods of humid and arid conditions. Subsequently, calcitic micrite was continuously precipitated from small, shallow, local, isolated and short lived ponds fringed by freshwater marshes with abundant charophyte meadows. The latter were developed as a result of flooding scattered depressions by groundwater supersaturated with respect to calcite due to rise of groundwater table. The deposition of two facies of carbonate muds, namely; biomicrite and pelintraclasts skeletal micrites was followed by a drought phase which witnessed desiccation of the fresh water ponds and significant drop in groundwater level. A sequence of pedogenic and diagenetic processes acted on the deposited carbonate muds are manifested by: (a) desiccation cracks, (b) micrite neomorphism, (c) infilling of root burrows and some cracks by aeolian siliciclastics, (d) karstification, (e) marmorization, (f) calcretization of root burrow infill and development of pseudo-rhizocrete, (g) calcite cementation and mineral authigenesis, and (h) silcretization. These processes are responsible for the development of hard palustrine carbonate crust. At the advent of aridity, the whole system of Al Dibdibba alluvial fan was subjected to deflation. This resulted in reversing the paleotopography of the hard crusted palustrine depressions into carbonate capped domal hills.

Sedimentology, diagenesis and ichnology of Cretaceous and Palaeogene calcretes and palustrine carbonates from Uruguay

The Cretaceous (Mercedes Formation) and Paleogene (Queguay Formation) deposits cropping out in W and S Uruguay comprise two terrestrial limestone units that are very rich in trace fossils. The study of these units permits to propose a sedimentological model for palustrine limestones and calcretes in which the distribution of different types of trace fossils is considered. The study units include three main types of deposit: lacustrine limestones, palustrine limestones and calcretes. The lacustrine limestones are relatively homogeneous and contain gastropods, charophytes and ostracods, but no trace fossils. They were deposited in a relatively more perennial lacustrine environment. The palustrine limestones include four different facies: desiccated mudstones, nodular limestones, granular limestones and gravel-sheets. The desiccated mudstones indicate a lesser degree of pedogenic modification and the granular limestones a higher degree. The gravel-sheets are an indication of the reworking of previous limestones deposits during low lakewater levels. Most of the palustrine limestones (except the gravel sheets) contain the same bioclasts as the lacustrine limestones plus a variety of trace fossils such as Rebuffoichnus sciuttoi, Fictovichnus gobiensis and different ichnospecies of Celliforma. The calcretes are either massive (groundwater) or laminar. The massive calcretes are sandy limestones made up of a carbonate matrix and cements. The laminar calcretes (root mats), which contain alveolar septal structures, occur as centimetre-thick layers and can be seen in all types of deposit. They contain the same trace fossil association as the palustrine limestones; the massive calcretes are poorer in such fossils. The distribution of trace fossils in these environments is under strong facies control and provides good evidence of subaerial exposure and semi-arid climates. All the limestones are partially replaced and cemented by opal and quartz, but in all cases the primary structure is preserved. Silicification occurred under groundwater meteoric conditions. Overall, the limestones facies (calcretes, palustrine and lacustrine) and their lateral distribution likely reflect the existence of wetland environments in semiarid to sub-humid climates. These climatic conditions were interrupted by a period of increased precipitations, probably the Early Eocene Climatic Optimum.

Palaeoenvironmental evolution of Lake Gacko (Southern Bosnia and Herzegovina): Impact of the Middle Miocene Climatic Optimum on the Dinaride Lake System

In the Early to Middle Miocene, a series of lakes, collectively termed the Dinaride Lake System (DLS), spread out across the north-western part of the Dinaride–Anatolian continental block. Its deposits, preserved in numerous intra-montane basins, allow a glimpse into the palaeoenvironmental, palaeobiogeographic and geodynamic evolution of the region. Lake Gacko, situated in southern Bosnia and Herzegovina, is one of the constituent lakes of the DLS, and its deposits are excellently exposed in the Gračanica open-cast coal-mine. A detailed study of the sedimentary succession that addresses facies, sediment petrography, geophysical properties, and fossil mollusc palaeoecology reveals repetitive changes in lake level. These are interpreted to reflect changes in the regional water budget. First-order chronologic constraints arise from the integration of radio-isotopic and palaeomagnetic data. 40Ar/39Ar measurements on feldspar crystals from a tephra bed in the upper part of the sedimentary succession indicate a 15.31±0.16Ma age for this level. The reversed magnetic polarity signal that characterises the larger part of the investigated section correlates to chron C5Br of the Astronomically Tuned Neogene Timescale. Guided by these chronologic data and a detailed cyclostratigraphic analysis, the observed variations in lake-level, evident as two ~40-m and seven ~10-m scale transgression–regression cycles, are tuned to ~400-kyr and ~100-kyr eccentricity cycles. From the tuning, it can be inferred that the sediments in the Gacko Basin accumulated between ~15.8 and ~15.2Ma. The economically valuable lignite accumulations in the lower part of the succession are interpreted to indicate the development of swamp forests in conjunction with lake-level falls corresponding to ~100-kyr eccentricity minima. Pedogenesis, rhizoliths and palustrine carbonate breccias in the upper part of the section reveal long-term aridity coinciding with a ~400-kyr eccentricity minimum. Eccentricity maxima are interpreted to trigger lake-level high-stands. These are accompanied by eutrophication events caused by enhanced denudation of the surrounding basement and increased detrital input into the basin. The presented age model proves that Lake Gacko arose during the Middle Miocene Climatic Optimum and that the optimum climatic conditions triggered the formation of this long-lived lake.

Does burial diagenesis reset pristine isotopic compositions in paleosol carbonates?

Sedimentological study of early Oligocene continental carbonates from the fluvial Dagshai Formation of the Himalayan foreland basin, India resulted in the recognition of four different types namely, soil, palustrine, pedogenically modified palustrine and groundwater carbonates. Stable oxygen and carbon isotopic ( δ 18O and δ 13C) analyses of fabric selective carbonate microsamples show that although the pristine isotopic compositions are largely altered during deep-burial diagenesis, complete isotopic homogenization does not occur. δ 18O and δ 13C analyses of ~ 200 calcrete and palustrine carbonates from different stratigraphic horizons and comparison with δ 18O of more robust bioapatite (fossil vertebrate tooth) phase show that dense micrites (~ > 70% carbonate) invariably preserve the pristine δ 18O value (mean) of ~ − 9.8‰, while altered carbonates show much lower δ 18O value ~ − 13.8‰. Such inhomogeneity causes large intra-sample and intra-soil profile variability as high as > 5‰, suggesting that soils behave like a closed system where diagenetic overprinting occurs in local domains. A simple fluid–rock interaction model suggests active participation of clay minerals to enhance the effect of fluid–rock ratio in local domains during diagenesis. This places an upper limit of 70% micrite concentration above which the effect of diagenetic alteration is minimal. Careful sampling of dense micritic part of the soil carbonate nodules, therefore, does provide pristine isotopic composition and it is inappropriate, as proposed recently, to reject the paleoclimatic potential of all paleosol carbonates affected by burial diagenesis. Based on pristine δ 13C value of − 8.8 ± 0.2‰ in soil carbonates an atmospheric CO 2 concentration between ~ 764 and ~ 306 ppmv is estimated for the early Oligocene (~ 31 Ma) Dagshai time. These data show excellent agreement between two independent proxy records (viz. soil carbonate and marine alkenone) and support early Oligocene survival of the Antarctic ice sheet.

Evidence for the development of the Andean rain shadow from a Neogene isotopic record in the Atacama Desert, Chile

Varying ages from Triassic to Pliocene have been proposed for the onset of hyperaridity in the Atacama Desert. The exact timing for the initiation of hyperaridity is critical for determining potential causes, which range from regional effects of global cooling to Andean uplift above elevations conducive to extreme rain shadows. Analysis of the stable isotopic composition of lower Miocene–Quaternary (21–0.015 Ma) palustrine and lacustrine carbonates in the Calama Basin reveals extreme changes in their oxygen and carbon isotopic composition during the Miocene. Limestone δ 18O values increased by ∼ 5‰ from middle to late Miocene, ranging from − 5.5‰ at 12 Ma to − 1‰ at ∼ 6 Ma. Carbon isotopic values increase by 9‰ over the Neogene, from average values of − 3‰ at 21 Ma to + 3‰ at 12 Ma, and reaching a maximum of + 6‰ at 5 Ma. The increase in oxygen isotopic values occurred over a time span in which the catchment area of the basin experienced significant uplift, causing the δ 18O value of precipitation to become more negative. We attribute the shift towards higher δ 18O values to enhanced evaporative enrichment both of soil water or snow prior to infiltration, and within shallow lakes or wetlands prior to carbonate precipitation. The large increase in δ 13C values was likely caused by a transition from a vegetated landscape influenced primarily by soil-respired CO 2 to a landscape largely devoid of vegetation and influenced by atmospheric and volcanic CO 2. Isotopic values of palustrine carbonates therefore indicate that hyperaridity commenced in the Calama Basin during the middle to late Miocene, in agreement with other paleoclimatic records from the basin. The cause for the onset of this climate change is thought to be due to the development of a strong Andean rain shadow associated with the uplift of the Andes to mean elevations > 2 km.

A FOSSIL EVERGLADES-TYPE MARL PRAIRIE AND ITS PALEOENVIRONMENTAL SIGNIFICANCE

Located at the interface between land and sea, marl prairies are sensitive to changes in water balance and useful recorders of climate and sea-level changes. Palustrine carbonate in marl prairies precipitates in temporary, barely flooded grasslands within microbial mats. Despite the special mode of carbonate production, descriptions of the sedimentary facies are exceptional and cursory because marl prairies are so far reported only from the recent of the Everglades (Florida, USA), where they produce an unspectacular calcite mud. We present a Pleistocene Everglades-type marl prairie from coastal Tanzania as the first fossil example. The unique preservation and high productivity (two times higher than in the Everglades) of the periphyton community in this marl prairie is due to increased calcification of coccoid and filamentous cyanobacteria. The excellent preservation allows us to characterize a marl prairie facies in great detail for the first time. Facies analyses of the sediments reveal a transition from tidal to terrestrial settings that started at ca. 44 14 C ka in response to eustatic sea-level fall and coastal tectonic uplift. The resultant drop of the groundwater table triggered the development of the marl prairie. The decline of the marl prairie was initiated at ca. 33 14 C ka due to the onset of the Last Glacial Aridity Maximum in equatorial East Africa.

Paleoclimatic implications (Late Cretaceous–Paleogene) from micromorphology of calcretes, palustrine limestones and silcretes, southern Paraná Basin, Uruguay

Abstract Sedimentologic and petrographic analyses of outcroping and subsurface calcretes, palustrine carbonates, and silcretes were carried out in the southern Parana Basin (Uruguay). The aim of this work is to describe the microfabric and interpret the genesis of these rocks through detailed analyses, since they contain significant paleoenvironmental and paleoclimatic evolution information. The main calcrete and silcrete host rock (Mercedes Formation) is represented by a fluvial thinning upward succession of conglomerate and sandstone deposits, with isolated pelitic intervals and paleosoils. Most of the studied calcretes are macroscopically massive with micromorphological features of alpha fabric, originated by displacive growth of calcite in the host clastic material due to evaporation, evapotranspiration and degassing. Micromorphologically, calcretes indicate an origin in the vadose and phreatic diagenetic environments. Micrite is the principal component, and speaks of rapid precipitation in the vadose zone from supersaturated solutions. The abundance of microsparite and secondary sparite is regarded as the result of dissolution and reprecipitation processes. Although present, brecciated calcretes are less common. They are frequent in vadose diagenetic environments, where the alternation between cementation and non-tectonic fracturing conditions take place. These processes generated episodes of fragmentation, brecciation and cementation. Fissures are filled with clear primary sparitic calcite, formed by precipitation of extremely supersaturated solutions in a phreatic diagenetic environment. The micromorphological characteristics indicate that calcretes resulted from carbonate precipitation in the upper part of the groundwater table and the vadose zone, continuously nourished by lateral migration of groundwater. The scarcity of biogenic structures suggests that they were either formed in zones of little biological activity or that the overimposed processes related to water table fluctuations produced intense recrystallization completely obliterating the biogenic fabric. Limestone beds containing terrestrial gastropods are geographically restricted. Situated at the top of the calcrete successions, they exhibit brecciated and peloidal-intraclastic textures but lack lamination, edaphic structures, aggregates and vertical rhizoliths. This indicates they correspond to low-energy palustrine deposits, generated in shallow, local and ephemeral ponds developed in topographic depressions. When water table levels dropped, the palustrine deposits were exposed. This favours the presence of terrestrial gastropods, seeds and insect nests. The combination of calcretes and palustrine carbonates indicates periods and areas with a reduced clastic input and a predominantly semiarid climate, with well-defined humid and dry seasons. Characteristics of the later developed massive and nodular horizons of silcretes, such as, preservation of the internal structure of the host rock, the small areal extent, the formation of massive lenses, the complex pore infillings and the lack of a columnar upper section, indicate that they were generated from groundwaters. Every silcretized horizon shows different positions of the groundwater table and relates to the dissection of landscape. The age of calcretization and silcretization is bracketed between the Late Cretaceous (Campanian–Maastrichtian) and the Early Eocene. Paleoclimate indicates changing conditions from warm and humid at the end of the Cretaceous (Mercedes Formation) to semiarid and seasonal during Paleocene (groundwater calcretes and palustrine deposits) and subtropical and seasonal in the early Eocene (Asencio Formation).

Die Fazies lakustriner und palustriner Karbonate aus dem Miozän von Langenaltheim/Südliche Frankenalb

Die Fazies lakustriner und palustriner Karbonate aus dem Miozän von Langenaltheim/Südliche Frankenalb

Occurrence of cyclic palustrine and calcrete deposits within the lower Pliocene Hagul formation, East Cairo district, Egypt

A depositional model of the lower Pliocene Hagul formation, which is exposed in the East Cairo district (Egypt), is proposed with more than 10 depositional cycles recognized. Field occurrence, detailed petrographic investigation and geochemical analysis revealed that the sediments within each cycle are the result of three sequential sedimentological processes: (1) alluvial sedimentation, (2) calcretization, and (3) precipitation of palustrine carbonate. It was concluded that Hagul formation has been deposited within the distal part of an alluvial plain during three successive climatic conditions: a humid climate during which alluvial sediments were deposited, a semi-arid climate with episodic precipitation which was favorable for pedogenic calcrete development, and a sub-humid climate during which groundwater level was gradually elevated and groundwater calcrete accumulated. Rising groundwater level continued until shallow wetlands covered the area and palustrine limestone was precipitated. Variations in the thickness and the nature of the host sediment, calcrete and palustrine limestone cycle suggest that each of the sedimentation processes varied from cycle to cycle.

Palaeoenvironmental significance of palustrine carbonates and calcretes in the geological record

Interest in palustrine carbonates and calcretes has increased over the last 20 years since they contain significant environmental information. Much of the work performed in this area has focused on either of two types of terrestrial carbonate—palustrine carbonates or calcretes (pedogenic and groundwater)—yet their simultaneous study shows there may be a gradual transition from one form to the other, revealing the interplay between pedogenic, sedimentary, and diagenetic processes. Three main factors control the formation of these carbonates: the position of the water table, the host rock, and the period of sub-aerial exposure. In pedogenic calcretes, precipitation of carbonate takes places mostly in the vadose zone above the water table, and within a previous host rock or sediment. In groundwater calcretes, the precipitation of carbonate also occurs within a previous host rock and around the groundwater table. In palustrine carbonates, however, the precipitation of lime mud occurs in a lacustrine water body. Palustrine carbonates necessarily form on previous lacustrine mud, whereas both types of calcretes may form on any type of sediment or soil. The sub-aerial exposure time needed to form palustrine carbonates may by relatively short (even a season), whereas pedogenic calcretes need more time (several years to millions of years). Groundwater calcretes do not form on the topographic surfaces, so there is no need of sub-aerial exposure. However, stable surfaces favour the development of thick groundwater calcretes. Small fluctuations in the water table cause gradual transitions of these three types of terrestrial carbonates and the subsequent mixture of their characteristic features, causing difficulties in the interpretation of these carbonates. The formation of these carbonates is controlled by palaeoenvironmental factors. Both commonly form in semi-arid climates. Arid climates are also suitable for calcretes, but sub-humid conditions are more suitable for palustrine carbonates. More indications of climatic conditions may be obtained through the analysis of the d18O content of both calcretes and palustrine carbonates, and from the depth of the horizon containing carbonate nodules in pedogenic calcretes. Vegetation is also important in the formation of these types of carbonates. Data on the prevailing vegetation can be obtained from the analysis of the micro and macrofabric as well as from the d13C signal of the primary carbonates, which, in pedogenic carbonates, has also been used to estimate atmospheric pCO2 during the Phanerozoic. These terrestrial carbonates are widely distributed on floodplains and distal areas of alluvial basins. Their presence and characteristics can be used as indicators of aggradation, subsidence or accommodation rates, and therefore as indicators of different tectonic regimes. Even though the study of these carbonates has notably increased in recent years, much less is known about them than about marine carbonates. Presently, there is much emphasis on obtaining a general model for sequence stratigraphy in terrestrial basins, with a need to include the carbonates analysed in this paper.