Calcareous Crusts Research Articles

Microfacies Characterization of Calcareous Crusts and Pleistocene Moghrebian Strata in the Coastal Basin of Tarfaya (Morocco): Paleoclimatic and Paleoenvironmental Implications

Microfacies Characterization of Calcareous Crusts and Pleistocene Moghrebian Strata in the Coastal Basin of Tarfaya (Morocco): Paleoclimatic and Paleoenvironmental Implications

Taphonomy of the fauna and chert assemblages from the Middle Palaeolithic site of Vajo Salsone, Eastern Italian Alps

The Vajo Salsone site is located at an altitude of 376m a.s.l. on the right slope of the Valpantena valley, in the Veneto Pre-Alps, north of the town of Verona. The site was discovered in 2017 during the construction of a truckable road. It is a small and narrow karst cavity filled with Pleistocene sediments that yield abundant Middle Palaeolithic artefacts and faunal remains, often coated by carbonate encrustations. The lithic assemblage shows the predominance of the Levallois method and is characterised by a large number of retouched elements, including foliated points. These artefacts, typical of the Late Middle Palaeolithic period in central and south-eastern Europe, make Vajo Salsone an exceptional case, as their recovery is rather sporadic west of the Balkan Peninsula and south of the Alps. The fauna is dominated by ungulates, the most abundant being the red deer followed by roe deer, chamois and elk. Among the carnivores, wolf is common, while fox and bear are rare. This fauna suggests an environment characterised by open glades and closed forests, with a discontinuous presence of alpine grasslands and wetlands. Taphonomic analysis of the bone surfaces is complicated by the presence of concretions on most of the remains, which also show a high degree of fragmentation. Post-depositional fractures are common, and weathering is light. Only cervid bones show anthropogenic modifications related to hunting and hominin exploitation such as cut marks, scraping, impacts and burnt modifications. Some long bone diaphysis bear marks testifying to the use of these fragments as retouchers for flint artefacts. Such use has been evidenced at other Middle Palaeolithic sites in the area. Furthermore, the good state of preservation of the occlusal surface enamel of the cervid teeth allowed us to carry out meso and microwear analyses which provided data on seasonality and site occupation. With regard to dental wear analysis, the red deer from Vajo Salsone had mesowear suggesting a mixed feeding and grazing diets, whereas microwear analysis showed that the individuals had a diet between mixed feeding and browsing. The taphonomic study of the site has been integrated by X–RFS analysis of the alterations occurred to the lithic artefacts, which include deep patination and the formation of calcareous crusts. All the data presented here are consistent with the hypothesis of a short-distance displacement of the material from a single archaeological deposit possibly related to human occupation during the warm season.

Where did the water come from? Wetlands and shallow lakes in semi‐arid dunefields from South America during the Pleistocene–Holocene transition

ABSTRACTStabilized and active dunes and sand sheet deposits abound in a small lake‐dotted semi‐arid region of the Western Pampean Dunefield, Argentina. Here, a multi‐scale and multi‐proxy study of three sites, across a hydrologic gradient from lakes to a dryland with groundwater levels at more than 25 m depth, analyzes calcareous and ferruginous rhizoliths, calcareous crusts, hypocoatings, pedogenic carbonate and amorphous Mn‐oxide precipitates within blowout dunes. These palustrine‐related features indicate significantly wetter conditions that allowed the development of shallow lakes and expanding wetlands during the Pleistocene–Holocene transition, limited by associated optically stimulated luminescence ages between ca. 14.7 and 11.6 ka. These wetter conditions, also identified in other nearby proxy records, may be associated with a strengthened South American Monsoon System, potentially during the Younger Dryas Chronozone, though other geological, ecological and climatic forcings cannot be ruled out with available data. Such a scenario lacks a modern analogue, since current hydrologic excess, evidenced in the formation of lakes and new rivers, is not observed in the localities which record paleolakes. This study underlines the variable conditions for pronounced hydrologic excess in semi‐arid eolian environments in western Argentina with complex ecological, anthropogenic and climatic linkages.

The Sarmatian (middle Miocene) “petrified forest” of Gramada (NW Bulgaria): role of the calcareous crusts

La « forêt pétrifiée» sarmatienne (Miocène moyen) de Gramada (nord-ouest de la Bulgarie): le rôle d'une succession d'encroûtements calcaires. Les couches du Sarmatien (Serravallien supérieur, Miocène moyen) affleurant à Gramada (Nord-Ouest de la Bulgarie) renferment des cylindres calcaires de dimensions métriques caractérisés par une cavité centrale également cylindrique. Leur étude détaillée montre une structuration à la fois verticale et concentrique. Les différents éléments constitutifs des cylindres résultent de séquences d'encroûtements carbonatés micritiques à plusieurs composants en majorité d'origine algaire (corallinacées) et dans une moindre mesure microbienne. L'absence de faune associée, hormis les ostracodes, est une caractéristique particulière qui pose le problème de l'environnement de dépôt qui s'inscrit dans le cadre de l'évolution de la sédimentation sarmatienne dans l'ensemble de la Paratéthys. Le mode de formation autour d'éventuels supports végétaux (« forêt pétrifiée») est également discuté.

Archaeocyaths: alternatively explained as consortia of siphonous algae and cyanobacteria-like microbes in shallow Cambrian seas

In order to test the systematic position of archaeocyaths, a group of Cambrian, exclusively marine fossils of unclear biological affinity, currently ascribed mostly to sponges (Porifera), we present an alternative explanation supporting the early idea of siphonous green algal affinity of archaeocyaths. The new arguments are based on a study of the morphology, microstructure, and microtaphonomy of early Cambrian (Tommotian and Atdabanian) archaeocyathan specimens from northeastern Siberia. Light microscopic and scanning electron microscopic (SEM) observations revealed that in many specimens the outer, often microlaminated, skeletal fabric grades continuously into the microfabric similar to that of microbial bush-like and dendritic microfossils such as Angulocellularia and Renalcis. The polyhedral to subglobular units characterizing the skeletal microfabric of the studied archaeocyaths and their microbial outgrowths are most similar to recent and fossil mineralized (calcified) colonies of benthic coccoidal cyanobacteria. This means that what had heretofore been interpreted as skeletons secreted by archaeocyathan organisms represent calcareous crusts precipitated by epibiontic cyanobacteria-like microbes, probably associated with a variety of other bacteria, colonizing surfaces and internal spaces of soft-bodied archaeocyathan organisms. Our study shows that a number of the morphological traits of archaeocyaths conserved by the mineralized microbial cover are similar to those of of modern siphonous green algae, like the marine genus Codium Stackhouse (Bryopsidales). Identifiable in well-preserved archaeocyaths are remains of such traits characteristic for Codium species as utricles, medullary threads, juvenile stages (primordia), and coalescence structures. The taxonomic and palaeoenvironmental implications of the recognition of archaeocyaths as possible consortia of siphonous green seaweeds (Chlorophyta) and cyanobacteria-like microbes are briefly discussed.

Bio-mineral needle fiber calcite (NFC) in Tunisian Pleistocene calcretes (topology and crystallization)

The microfacies of Pleistocene calcareous crust in Tunisia revealed the presence of diverse aspects of acicular crystallizations in a porous system. These crystallizations are in the shape of branched filaments, mono or polycrystalline rods which are either smooth or serrated (calcite), and serrated edge needle (calcium oxalate). The genesis of these acicular structures seems to be directly or indirectly linked to the organic activity. In fact, the facies are tied to the biological activity or biochemical interactions, which occur between living organisms and the mineral (calcite), triggering a succession of precipitation and dissolution reactions. These reactions, which are characteristic of vadose environments, have actively contributed to the endurance of calcareous crusts due to premature or delayed diagenesis. The acicular calcite, found in the Tunisian Pleistocene calcretes, reflects regional climates that influence the form and mineralogy of needle fiber calcite.

Morphogenesis and microstructure of concrete-derived calthemites

Distributions of calcareous calthemite deposits have been widely documented, but depositional processes and the architecture of their internal fabrics are not well understood. These concrete degradation products from a case study area in western Canada have external morphologies comparable to calcite speleothems formed in natural limestone caves, but the internal microstructural architecture and mineralogy are markedly different. The mineralogy consists of mostly calcite with secondary halite and minor percentages of trona and portlandite. A novel morphogenetic model explains depositional processes resulting in calcareous crusts that follow fractures of an overlying concrete surface, and how sufficient structural integrity provided by the internal architecture supports attachment areas of tubular soda straws. Interiors of these multi-cm crusts consist of curvilinear calcite laminae arrayed as sub-parallel walls, compartmentalizing water–gas interfaces along variously interconnected conduits and basin-form chambers. Overall porosities of 40–60% or more are prevalent, in contrast to < 1% associated with externally similar drapery-form crusts deposited within natural limestone caves. Several calcite fabrics new to calthemite deposits are described, including dendritic shrubs that coalesce into concentric growth rings along central canals of soda straws.

Geochemistry of calcite-precipitating waters of the European Northeast

The geochemistry of natural calcite-precipitating waters of the European northeast was presented in this paper. The chemical composition and main hydro-chemical type of these surface and underground waters are various, TDS from 239 to 2090 mg/L. The waters are supersaturated with respect to calcite, there are favorable hydrochemical conditions for formation recent continental carbonates (calcareous tufa, charophyta, calcareous crusts on macrophytes) in the conditions of moderate continental climate.

Seeding of fragments of the agarophyte Gelidium lingulatum (Rhodophyta, Gelidiales) for the repopulation of lower levels of wave-exposed, intertidal rocky shores

Gelidium lingulatum is a frequent species at low intertidal levels of wave-exposed, rocky shores in central Chile. It is harvested from natural stands by artisanal fishermen and sold as raw material for the extraction of the valuable phycocolloid agar. Mass cultivation of this species has not been developed. In this study, we describe a device that allows seeding fragments of G. lingulatum to be used to increase biomass or to repopulate areas damaged by natural or anthropogenic disasters. The device consists of a nylon rivet and a mesh-washer that is pushed in perforations drilled on the rocky shore, holding the seaweed fragments appressed to the substratum favoring secondary attachment. Seeding was done on three sites in each of three localities within the Biobio Region (southern Chile), considering also the type of substratum (rock, calcareous crusts, and barnacles). The transplant units were installed in (austral) winter, 2016, and evaluated in spring and summer 2017. Results showed that, except for human intervention, most seeded fragments grew successfully as new patches and growth rates of up to 1.2% day−1 were recorded. Some significant differences were detected among sites and among substrata, but these can be interpreted as differences in the environmental conditions at the small spatial scale where the patch was growing. This device is simple and inexpensive and our results show that it is effective in producing new patches of G. lingulatum, such that it can be easily applied in repopulation or restoration programs.

Geología y paleolimnología del complejo lagunar ?Santa Rosa del Monte?, Provincia de Buenos Aires, Argentina

Tres lagunas integran el complejo, alojadas en cinco cubetas eólicas de diferente edad, que a nivel hídrico normal forman un ambiente único, donde se caracterizaron los aspectos fisiográficos, hidrológicos y geolimnológicos. En los afloramientos se reconocieron cinco unidades litoestratigráficas y tres pedoestratigráficas, que abarcan del Pleistoceno temprano/medio al tardío y del Pleistoceno final al presente. La Formación Ensenada, constituye el sustrato regional, sobre el cual se disponen cuatro unidades eólicas, las Formaciones Buenos Aires, La Postrera II, III, IV, los Geosuelos Sin Nombre, Puesto Callejón Viejo y Puesto Berrondo y, el aluvio reciente. En los lechos se registraron ocho unidades litoestratigráficas: cinco límnicas y tres eólicas. Las límnicas corresponden a los cuatro paleoambientes de la Formación Luján: Miembros La Chumbiada, Lobos, Río Salado y Monte, del Pleistoceno tardío al Holoceno tardío-tardío y el aluvio reciente; las unidades eólicas a las Formaciones La Postrera I y II del Pleistoceno tardío al Holoceno temprano y la IV del Holoceno tardío-tardío. El origen de las primeras cubetas se inició en la etapa seca del ?Bonaerense?, la posterior alternancia climática seco/húmedo generó las otras cubetas. Las etapas secas se caracterizaron por deflación, sabkhas, precipitación de sales y depósitos eólicos. En las húmedas predominó el aluvionamiento y la pedogénesis. Así, el conjunto de depósitos ?posensenadenses? está representado por cinco episodios secos (Glacial Temprano a la Pequeña Edad de Hielo) y cinco húmedos, con acumulaciones límnicas, desde el Miembro La Chumbiada al aluvio reciente y en los afloramientos los suelos sincrónicos S4, S2, S1 y S0.

Microstructure and palygorskite neoformation in pedogenic calcretes of central Morocco

The aim of this research was the micromorphological and mineralogical characterization of calcrete outcrops located in central Morocco, with particular focus on microstructure development and palygorskite formation processes. Two study sites in the Al Haouz Plain (ca 30km west to Marrakech city) were identified, in an area dominated by Quaternary sedimentary formations including layers cemented by carbonate and widespread calcareous crusts at the surface. The calcrete samples were examined by means of polarizing microscopy from thin sections, scanning electron microscopy / energy-dispersive X-ray spectroscopy (SEM/EDX) from undisturbed samples, and X-ray diffraction (XRD) methods. The matrix of the calcrete material cementing the schist fragments composed of abundant calcite was associated mainly with palygorskite, quartz, and to a lesser extent illite/mica, feldspar, and chlorite. SEM observations indicated that palygorskite mainly occurred as fibres, fibre bundles and networks mainly grown on and/or enclosing calcite rhombohedrons. The EDX spectra of the palygorskite fibres exhibited strong silicon (Si) peaks followed by Al, Mg and Fe peaks. The EDX analysis of calcite rhombohedric crystals exhibited a strong Ca peak, followed by weak peaks of Si, Mg, Al and Fe possibly due to nearby or overlapped palygorskite crystals. One main palygorskite neoformation process was described. Palygorskite was authigenically formed (neoformed) during and after precipitation of calcite from percolating soil solutions in a near-surface setting. Peculiar sub-rounded microstructural units, here designated as pedogenic ooids/pisoids (according to the size), were determined in the matrix of the studied calcretes, associated to pendants and clay-rich rims surrounding them.

Establishment of occupation chronicles in Grotte Mandrin using sooted concretions: Rethinking the Middle to Upper Paleolithic transition

Soot marks, witnesses of past human activities, can sometimes be noticed in concretions (speleothem, travertine, carbonated crust, etc.) formed in cavities. We demonstrate here that these deposits, generally ignored in archaeological studies, turned out to be a perfectly suitable material for micro-chronological study of hominin activities in a site. At the Grotte Mandrin (Mediterranean France), thousands of clastic fragments from the rock walls were found in every archaeological level of the shelter. Calcareous crusts containing soot deposits are recorded on some of their surfaces. They appear in thin section as thin black laminae. Microscopic observation of these crusts revealed that they kept track of many occupations. We show that is possible to link them with the archaeological units identified during the excavation. Minimum Number of Occupations (MNO) can be built out of these sooted crusts. MNO are usually high and attest to the cumulative nature of each archaeological unit. They are witnesses of each occupation of hominin groups in each archaeological level of the cave. This study also shows that, in Grotte Mandrin, a very short time separates the first Middle/Upper Paleolithic transitional groups' occupations from those of the last Mousterians. The research perspectives on soot deposits are diversified and raise the possibility of studying multiple aspects of past human life, and in this case, to rethink the Middle/Upper Paleolithic transition, with an unmatched temporal resolution. Sooted concretion analysis provides high temporal resolution archaeology. There is a real possibility of extending this study with chronological implications to cavities of all ages and areas.

Pleistocene (Calabrian) deep‐water corals and associated biodiversity in the eastern Mediterranean (Karpathos Island, Greece)

ABSTRACTDiversified deep‐water corals and associated communities occur in early Pleistocene (early Calabrian; ∼1.7–1.6 Ma) calcareous crusts discovered on the south‐western coast of Karpathos Island (eastern Greece). Apart from abundant and conspicuous solitary and colonial corals (12 species), the accompanying fauna comprises mostly bivalves (10 species), serpulid worms (4 species), and bryozoans (40 species). The growth and deposition of the studied organisms occurred in an upper bathyal environment at water depths around 400–600 m. The calculated mean uplift rate after deposition of the crusts indicates that Karpathos was drowned during the earliest Calabrian and experienced severe uplifts by the late Calabrian. A comparison between these communities and those of the Pleistocene to Recent Mediterranean and north‐east Atlantic shows that a common stock of taxa exist(ed); among them two azooxanthellate colonial scleractinians (Lophelia pertusa and Madrepora oculata) are predominant.

Post-IR IRSL290 dating of K-rich feldspar sand grains in a wind-dominated system on Sardinia

The reliability of a post-IR elevated temperature IRSL (290 °C; pIRIR290) is tested on wind-blown, sand-sized (180–250 μm) K-rich feldspar grains. The pIRIR290 ages were compared with quartz SAR-OSL data, other independent age controls and historical information. Three study areas along the coast of Sardinia (Italy) were selected: the south Alghero coast, the Bue Marino cave (E Sardinia) and the Alghero bay (NW Sardinia). Along the south Alghero coastline a thick dunefield system is widely recognised in the literature to represent the beginning of the last glacial phase (post 80 ka). From a single block sand-sized grains for quartz SAR-OSL and K-feldspar pIRIR dating were collected. The natural quartz SAR-OSL sample lies below the saturation limit of the dose response curve (De < 2xD0) giving a reliable age of 76 ± 6 ka. The fading-uncorrected pIRIR290 age of 73 ± 5 ka is in good agreement with the quartz result. A further test on older samples was carried out on the sedimentary succession at Bue Marino cave, which includes a sandy wind-blown unit, enclosed between two calcareous crusts. U-series dates of crusts constrain the aeolianite formation between ∼130 and ∼86 ka. The quartz SAR-OSL signals for aeolianite samples lies close to saturation and the resulting ages underestimate the independent age control. Instead, uncorrected pIRIR290 ages on K-feldspar extracts point to a formation of the wind-blown unit between ∼100 and ∼80 ka, in good agreement with the U-series data. The bleachability of the pIRIR290 signal was further investigated using samples from a modern coastal barrier system backing the Alghero bay. The dunefield was stabilized by plantation during the 1950s. The quartz SAR-OSL ages span from 2450 ± 170 years to 60 ± 13 years ago, consistent with the known coastal barrier stabilization. The pIRIR290 ages indicate an offset up to ∼1000 years. We can conclude that the pIRIR290 method on sand-sized K-feldspar grains shows great promise for samples at or beyond the quartz OSL age limit but should not be applied to Late Holocene or modern deposits.

Study of Hydro-Saline Characteristics of Soils a Palm Grove in Basin of Ouargla (Northern Algerian Sahara)

Saharan soils are often faced with several problems of development, taking account the hydro-edaphic constraints, mainly of hydric types by water table, mechanical by gypso-calcareous crusts and saline by irrigation waters and upwelling of water table. Our work consists in doing a soil characterization of a palm grove in Ouargla in order to study the constraints hydro-halomorphes. The results show that irrigation water by two plies of Senonian and Mioplcene had a high salinity with a value of 2.83 and 5.10 dS.m-1 respectively. The conduct of irrigation is traditional random of submersion type. The palm grove has a poor drainage with a level of water table 156.67±15.71 cm and salinity of 31.37±34.04 dS.m-1. The drains are open type and their maintenance is not regular. This situation of management of irrigation-drainage promotes the upwelling of water table and the waterlogging in soils. The study of soil profiles shows the existence of mechanical obstruction of gypso- calcareous crusts which limit the entrenchment of the date palms and the leaching of salts. Soil salinity is excessive in profiles with a range of 8.98 ± 4.58 dS.m-1. This accumulation of salts is due to the dynamic ascending and descending of salts respectively under the effect of upwelling of water table and leaching by irrigation. The salinization, the upwelling of water table and the presence of gypso- calcareous crusts recorded in Ouargla testify to a degradation hydro-halomorphe and mechanic of soil which constitute the major constraints in the management of system irrigation-drainage and sustainable agricultural development of the palm groves of the basin of Ouargla. Some hydro-agricultural planning are necessary to apply in the oasis to improve the hydro-mechanical properties of soils in order to reduce their degradation

Cyanobacterial Contribution to Travertine Deposition in the Hoyoux River System, Belgium.

Travertine deposition is a landscape-forming process, usually building a series of calcareous barriers differentiating the river flow into a series of cascades and ponds. The process of carbonate precipitation is a complex relationship between biogenic and abiotic causative agents, involving adapted microbial assemblages but also requiring high levels of carbonate saturation, spontaneous degassing of carbon dioxide and slightly alkaline pH. We have analysed calcareous crusts and water chemistry from four sampling sites along the Hoyoux River and its Triffoy tributary (Belgium) in winter, spring, summer and autumn 2014. Different surface textures of travertine deposits correlated with particular microenvironments and were influenced by the local water flow. In all microenvironments, we have identified the cyanobacterium Phormidium incrustatum (Nägeli) Gomont as the organism primarily responsible for carbonate precipitation and travertine fabric by combining morphological analysis with molecular sequencing (16S rRNA gene and ITS, the Internal Transcribed Spacer fragments), targeting both field populations and cultures to exclude opportunistic microorganisms responding favourably to culture conditions. Several closely related cyanobacterial strains were cultured; however, only one proved identical with the sequences obtained from the field population by direct PCR. This strain was the dominant primary producer in the calcareous deposits under study and in similar streams in Europe. The dominance of one organism that had a demonstrated association with carbonate precipitation presented a valuable opportunity to study its function in construction, preservation and fossilisation potential of ambient temperature travertine deposits. These relationships were examined using scanning electron microscopy and Raman microspectroscopy.

Erratum to: Spatial and seasonal reef calcification in corals and calcareous crusts in the central Red Sea

The online version of the original article can be found under doi: 10.1007/s00338-015-1383-y .

Spatial and seasonal reef calcification in corals and calcareous crusts in the central Red Sea

The existence of coral reef ecosystems critically relies on the reef carbonate framework produced by scleractinian corals and calcareous crusts (i.e., crustose coralline algae). While the Red Sea harbors one of the longest connected reef systems in the world, detailed calcification data are only available from the northernmost part. To fill this knowledge gap, we measured in situ calcification rates of primary and secondary reef builders in the central Red Sea. We collected data on the major habitat-forming coral genera Porites, Acropora, and Pocillopora and also on calcareous crusts (CC) in a spatio-seasonal framework. The scope of the study comprised sheltered and exposed sites of three reefs along a cross-shelf gradient and over four seasons of the year. Calcification of all coral genera was consistent across the shelf and highest in spring. In addition, Pocillopora showed increased calcification at exposed reef sites. In contrast, CC calcification increased from nearshore, sheltered to offshore, exposed reef sites, but also varied over seasons. Comparing our data to other reef locations, calcification in the Red Sea was in the range of data collected from reefs in the Caribbean and Indo-Pacific; however, Acropora calcification estimates were at the lower end of worldwide rates. Our study shows that the increasing coral cover from nearshore to offshore environments aligned with CC calcification but not coral calcification, highlighting the potentially important role of CC in structuring reef cover and habitats. While coral calcification maxima have been typically observed during summer in many reef locations worldwide, calcification maxima during spring in the central Red Sea indicate that summer temperatures exceed the optima of reef calcifiers in this region. This study provides a foundation for comparative efforts and sets a baseline to quantify impact of future environmental change in the central Red Sea.

Facies associations in warm-temperate siliciclastic deposits: insights from early Pleistocene eastern Mediterranean (Rhodes, Greece)

AbstractDiverse, abundant and usually well-preserved communities of skeletal organisms occur in the lower Pleistocene (Gelasian) siliciclastic deposits of the Greek island of Rhodes. Benthic foraminifers, molluscs and bryozoans have been studied in four measured and sampled sections located in the northern part of the island. Among these bottom-dwelling organisms, numerous extant taxa are good environmental indicators and, combined with field observations and sedimentological data, they provide information on the probable conditions in which they developed. The siliciclastic deposits of the Kritika Formation have been divided into 14 different bio- and lithofacies, which have been further grouped into four facies associations corresponding to four different environmental settings: (1) continental to fluviatile; (2) brackish-water (lagoonal/deltaic); (3) infralittoral (0–20 m); and (4) upper circalittoral (depths of 20–40 m, but also down toc.50–60 m). Among the marine facies associations, several characteristic biocoenoses have been recognized: soft-bottoms (fine to coarse sands and gravels); seagrass meadows; biogenic calcareous crusts on drowned beachrock slabs; red algal rhodoliths; and bivalve shell beds. In the studied sections, 13 superimposed genetic sequences have been documented. The repetition of similar facies associations within each sequence suggests: (1) a possibly eustasy-controlled, cyclic sedimentation; (2) a general subsidence of Rhodes during the deposition of the studied facies associations; and (3) a mostly constant range of environmental conditions (i.e. sedimentation rates and temperature) throughout the Gelasian.

Calcareous crusts on exposed Pleistocene limestones: A case study from Grand Cayman, British West Indies

Calcareous crusts, which include laminar (up to 6cm thick) and non-laminar calcretes, have developed on ooid grainstones and skeletal wacke/pack/grain/floatstone in the Pleistocene Ironshore Formation exposed at Morgan's Harbour, Tarpon Springs, and Rogers Wreck Point on Grand Cayman, British West Indies. Non-laminar calcretes, which consist largely of micrite with variably altered skeletal fragments and lithoclasts derived from the host limestone, contain peloids in their basal parts. In contrast, the laminar calcretes are formed of porous laminae and low-porosity laminae. Calcified roots are common in the porous laminae but rare in the low-porosity laminae. The δ18O values of calcretes, from ‐5.22‰ to +0.16‰, have a positive correlation with the δ13C values that vary from ‐10.03‰ to ‐6.41‰ at Tarpon Springs, from ‐11.57‰ to ‐5.43‰ at Morgan's Harbour, and from ‐9.89‰ to ‐8.68‰ at Rogers Wreck Point.The non-laminar calcretes reflect progressive micritization of the bedrock with a basal peloidal zone that passes upward into the micrite zone. In contrast, laminar calcretes are attributed to accretionary build-up dictated by microscale processes that were controlled largely by the interplay of plant roots and their associated microorganisms. The positive correlation between the δ18O and δ13C values from the calcareous crusts indicates that evaporation influenced calcite precipitation. The δ13C values of the laminar calcretes record a change from vegetation dominated by C3 plants that grew in a cool, wet climate, to a C3/C4 vegetation that grew in a hotter, drier climate. The Cayman calcretes resemble those found on the Pleistocene Miami Limestone in Florida, and highlight the possibility that these calcareous crusts may reflect regional climates and may therefore enable regional stratigraphic correlation.