Shallow-water Limestones Research Articles

Paleocene-earliest Eocene Larger Benthic Foraminifera and Ranikothalia-bearing carbonate paleo-environments of Costa Rica (South Central America)

Here we present a biostratigraphic and systematic revision of American Ranikothalia, based on outcrop collections form Costa Rica end westernmost Panama and on type collections of the Naturhistorisches Museum Basel. Paleocene-lowermost Eocene Ranikothalia-bearing shallow-water limestones occur in three distinct palaeogeographic settings along the Southern Central American convergent margin: 1. The Barra Honda Limestone (Tempisque Basin, Costa Rica) is, according to 87Sr/86Sr-ratios and biostratigraphy of mid Selandian to late Thanetian (? earliest Ypresian) age. Tectonic uplift and temporary demise of a Middle American Arc segment during the Selandian allowed the formation of the pure carbonates of the Barra Honda Limestone. The upper well-bedded subunit exposes a higher energy facies, in which Neodiscocyclina spp. and Ranikothalia catenula group (redefined here) are locally abundant. Open marine facies with Morozovella velascoensis interfinger in a small scaled facies pattern in the topmost part. Tectonic subsidence and resumed arc activity resulted in rapid eutrophication/drowning. 2. Herradura – Quepos carbonate shoals on oceanic seamounts. The Herradura and Quepos terranes may represent the earliest seamounts formed by the Galapagos hotspot plume tail activity, >1000km to the SW of their present-day position. Pillowed lava flows and subaerial volcanic breccias occur with Upper Palaeocene-lowermost Eocene shallow-water limestones rich in Ranikothalia gr. catenula, Neodiscocyclina spp. and coralline algae. 3. Burica Peninsula – carbonate shoal on an accreted oceanic plateau. An Upper Paleocene carbonate shoal established on part of the Inner Osa Igneous Complex after its accretion to the western edge of the CLIP (Caribbean Large Igneous Province) and its possible emersion. In situ rhodoid limestones of the Río Palo Blanco (W-Panama) contrast with redeposited material in turbidites and debris flow breccias, interbedded with siliceous pelagic limestones and arc-derived mudstones. A morphometric study on 150 axial sections of Ranikothalia spp. from all studied settings reveals a morphologic continuum between megalospheric (A) forms and microspheric (B) forms previously described as several species, here grouped as the R. gr. catenula. We conceive that these co-occurring morphotypes belong to one biological species. The separation of this group into 2 or 3 genera seems highly artificial, since many samples contain transitional forms between the “sub-evolute” microsphreric forms “Caudrina†soldadensis (R. catenula soldadensis herein) and the involute “Chordoperculinoides†bermudezi (R. c. bermudezi herein). Neither from previous studies nor from our material we can conclude on a biostratigraphic relevance of the various morphotypes. In particular, we cannot confirm a stratigraphic succession from flatter (R. c. soldadensis) to more robust (R. c. bermudezi) forms. Shape and shell thickness distribution of morphotypes of the Ranikothalia gr. catenula can be interpreted in terms of hydrodynamic energy and light availability in their habitat, in comparison with observations made on living LBF. This interpretation matches the hydrodynamic energy deduced from the microfacies and the abundance of photosynthetic red algae in our thin sections. Flat lenticular delicate A-forms (R. c. catenula) occur together with delicate B-forms (R. c. soldadensis) both in shallow protected and deeper off-shore calm water areas. The most robust B-form R.c. bermudezi may be a good indicator of shallow, high energy palaeo-environments. Possible Paleoceme dispersal routes for Amerucan Ranikothalia are discussed.

Los depósitos clásticos pos-Laramide de la Sierra de Guanajuato: Implicaciones de su composición en la evolución tectono-sedimentaria y paleogeográfica

Este artículo presenta los resultados del estudio sobre la sedimentación, los ambientes sedimentarios, la evolución tectono-sedimentaria y paleogeográfica de los depósitos clásticos pos-Laramide y pre-vulcanismo de la Sierra Madre Occidental en la Sierra de Guanajuato. Los depósitos clásticos rojos analizados corresponden al Conglomerado Guanajuato y al Conglomerado Duarte del Eoceno que fueron depositados en la parte media y distal de abanicos aluviales. Los depósitos estudiados están constituidos de clastos de calizas, granito, andesita, rocas sedimentarias, diorita y piroxenita, que indican la erosión de bloques levantados del complejo basal de la Sierra de Guanajuato (Terreno Guerrero). El análisis composicional y petrográfico de clastos de calizas permitió reconocer calizas de cuenca y calizas de plataforma somera provenientes del complejo basal de la Sierra de Guanajuato (Cuenca Arperos). Las calizas de plataforma somera presentan fragmentos esqueletales de bivalvos, braquiópodos, gasterópodos, equinodermos y foraminíferos bentónicos del Berriasiano-Valanginiano. Los clastos de caliza de plataforma se interpreta que podrían proceder de facies someras limítrofes de la cuenca Arperos, cuyos afloramientos originales actualmente no afloran en la Sierra de Guanajuato.

Fasciculate Kleopatrinid corals from the Bashkirian (late Carboniferous) of Sardar Formation (Ozbak-Kuh Mountains, East-Central Iran)

In the East-Central Iran, the Sardar Formation (upper Visean–Moscovian) consists of shallow-water limestone with intercalations of shale containing rugose corals, tabulate corals and brachiopods. Ten sections were sampled in the Ozbak-kuh Mountains, from north to south. Among the rugose corals, an assemblage of fasciculate Kleopatrinidae has been collected. The latter contains the species: Paraheritschioides antoni antoni, P. antoni minor, P. gracilis and two new species for the genera Fomichevella and Heintzella. Heintzella is described from Iran for the first time. However, its age, as determined by conodonts and foraminifers, is early to middle Bashkirian (early late Carboniferous). The most similar, time-equivalent faunal associations are that of the Ellesmere Island, Sverdrup Basin in Arctic Canada, Alexander terrane and Brooks Range in southeastern Alaska and eastern Klamath terrane in northern California, where similar tropical warm water conditions have been identified during the Bashkirian in the northern hemisphere. During these times central Iran block and Northern provinces, characterized by a dominant carbonate facies and more diversified colonial coral faunas.

Anisian Strelovec Formation in the Robanov kot, Savinja Alps (Northern Slovenia)

A detailed sedimentological features of the Slatinski plaz section with a transition from the deep-water Anisian (Illyrian) Strelovec Formation to the Ladinian shallow-water Contrin Formation have been presented. The Strelovec Formation is divided into five different lithostratigraphic units that are characterised by dark hemipelagic and pelagic thin-bedded limestones and dolomitic mudstones that are frequently intercalated with deposits of gravity-mass flows, slump and slide to fine-grained low-density turbidity origin. The Strelovec Formation was deposited in a hydrodynamically quiet, pelagic deeper-water anoxic environment, most probably on a gentle platform slope. In the upper part of the formation, the presence of olistolith blocks of shallow-water limestones indicates the closing of the prograding shallow-water platform wedge. Massive dolostones of Unit 6 mark the complete filling of the basin and the beginning of the shallow-water Sedimentation of the Contrin Formation in the Early Ladinian.

The Homerian (late Wenlock, Silurian) carbon isotope excursion from Perunica: Does dolomite control the magnitude of the carbon isotope excursion?

The δ13Ccarbrecords from two geographically close sections of the shallow-water Kozel Limestone Member (late Wenlock Motol Formation; Perunica microplate) significantly differ in the magnitude of the Homerian carbon isotope excursion as well as in their dolomite content. The present paper tests a hypothesis as to whether a difference of about 2‰ in the magnitude of the δ13Ccarbanomaly is caused by the different content of dolomite, which could be enriched in both13C and18O relative to coexisting calcite, as has been suggested by experimental data. The new data obtained reveal that the δ18O composition of calcite and dolomite was probably controlled by the pore-fluid composition during limestone diagenesis and that both carbonates seem to be close to equilibrium in oxygen isotope composition. On the other hand, the δ13C values of dolomite are similar to those of calcite, and thus the carbon isotope composition of both carbonates was probably determined by the precursor carbonate composition. Moreover, the values of δ13Cdolomite/δ13Ccalciteratios as well as their variability suggest that both calcite and dolomite did not reach equilibrium in their carbon isotope composition. Whole-rock, mineralogical, and C and O isotope data clearly show that dolomite is not the cause for the differences in magnitudes of the δ13C records observed between dolomite-bearing and dolomite-lacking shallow-water limestone successions. The question as to whether the observed differences in the δ13C records of the studied sections across the Homerian carbon isotope excursion were controlled by the dependence of seawater composition on water depth and (or) proximity to shoreline or if the δ13C values were later affected by secondary processes during limestone diagenesis is still unsolved.

Age and provenance of mica-schist pebbles from the Eocene conglomerates of the Tylicz and Krynica Zone (Magura Nappe, Outer Flysch Carpathians)

Abstract During the Łate Cretaceous to Palaeogene, the Magura Basin was supplied by clastic material from source areas situated on the northern and southern margins of the basin, which do not outcrop on the surface at present. The northern source area is traditionally connected with the Silesian Ridge, whereas the position of the southern one is still under discussion. A source area situated SE of the Magura Basin supplied the Eocene pebbly para-conglomerates containing partly exotic material. The studied clastic material contains fragments of crystalline rocks, and frequent clasts of Mesozoic to Palaeogene deep and shallow-water limestones. Numerous mica schists, scarce volcanites and granitoids as well as gneisses, quartzites and cataclasites were found in the group of crystalline exotic pebbles. Monazite ages of “exotic” mica-schist pebbles from the Tylicz, Zarzecze and Piwniczna-Mniszek sections document the Variscan 310±10 Ma age of metamorphic processes. The provenance of these exotic rocks could be connected with a remote source area located SE of the Magura Basin, which could be the NW part of the Dacia Mega Unit. The idea is strongly supported by palaeotransport directions from the SE, the absence of material derived from the Pieniny Klippen Belt, the presence of shallow water limestones, typical facies of the Median Dacides belt and metamorphic age distribution proved by monazite dating.

Planktonic foraminifera and calcareous nannofossils record in the upper Campanian-Maastrichtian pelagic deposits of the Malatya Basin in the Hekimhan area (NW Malatya, eastern Anatolia)

The uppermost Cretaceous (upper Campanian-Maastrichtian) pelagic successions from the Malatya Basin (NW Malatya, eastern Anatolia) were studied by 688 samples, which were collected from five stratigraphic sections in the Hekimhan area. The pelagic deposits conformably overlie rudist bearing shallow-water limestones and are overlain conformably by Maastrichtian dolomites and unconformably by Paleocene-Eocene deposits, respectively.The pelagic successions in the Hekimhan area comprise the Kösehasan Formation at the base and the Zorbehan Formation at the top and reach up to 1100 m in thickness. The Kösehasan Formation rests over the neritic rudist-bearing limestones of the Güzelyurt Formation along a sharp contact and consists mainly of flysch-type sandstone-mudstone alternation with complete and partial Bouma sequences. The carbonate content of abundant planktonic foraminifera and nannoplankton-bearing 980-m-thick succession increases upwards and the formation passes gradually to the clayey limestones and marlstones of the Zorbehan Formation to the top. Occurrences of nannoplankton Lithraphidites quadratus Bramlette and Martini and Micula praemurus (Bukry) in the first beds of the Kösehasan Formation indicate that the age of the Kösehasan Formation and overlying Zorbehan Formation is of late Maasthrichtian. Another late Maastrichtian taxa Cribrosphaerella daniae Perch-Nielsen and Arkhangelskiella maastrichtiana Burnett are observed from the lowermost part of the succession. Maastrichtian planktonic foraminifera such as Contusotruncana walfischensis (Todd) and Globotruncanita pettersi (Gandolfi) were recorded through the successions. Although planktonic foraminifera are diverse and abundant particularly in the Kösehesan Formation, index late Maasthrichtian species were not encountered. Campanian and Santonian-Campanian planktonic foraminifera, e.g. Radotruncana calcarata (Cushman) and Globotruncanita elevata (Brotzen), obtained particularly from the lower part of the succession and calcareous nannofossils such as Broinsonia parca parca Bukry, Reinhardtites anthophorus (Deflanre) and Eiffellithus eximius (Stover) are interpreted as reworked from older strata. Trace fossils are common throughout the succession.Rareness of planktonic foraminifera and nannoplankton in the uppermost part of the succession (Zorbehan Formation) indicates maximum shallowing of the latest Maastrichtian sea in this part of the basin. Rare echinoids, bivalves and ammonites are observed in that part of the sequence.The obtained data indicate that sediment accumulation rate of the pelagic deposits is rather high and about 27.5 cm/ky for this part of the basin. Changes in thickness of the formations along short distances in the five stratigraphic sections analysed in this study should be related to the diachroneity of the depositional and erosional events.

STRATIGRAPHIC DISTRIBUTION OF THE LARGER BENTHIC FORAMINIFERALOCKHARTIAIN SOUTH TIBET (CHINA)

This paper reveals the stratigraphic applicability of the larger benthic foraminifer (LBF) Lockhartia from the Paleocene to Lower Eocene of the eastern Neo-Tethyan Ocean outcropping in South Tibet. During the Paleocene, faunas of the western and eastern Neo-Tethys show distinct differences: The western area is dominated by coralgal assemblages, while in the east, LBF predominate the so-called “ Lockhartia Sea”. Shallow-water limestones of Tingri, Gamba and a completely new section close to the village of Guru in the eastern Neo-Tethyan Ocean reveal the stratigraphic ranges of seven species of Lockhartia : L. conditi, L. haimei, L. hunti, L. praehaimei, L. retiata, L. roeae and L. tipperi . Five interval biozones, called “ Lockhartia biozones”, are defined for the Paleocene to Lower Eocene based on the stratigraphic distribution of these species. The biozones are used to facilitate regional correlations of the three sections and to compare them with the well-established shallow benthic zones (SBZ) used for biostratigraphy of shallow-water environments. The stratigraphic distribution of Lockhartia suggests an earlier evolution of some species in the eastern Neo-Tethyan Ocean compared to the western region as a result of different latitudinal positions of the ocean basins.

Bedding-parallel stylolites in shallow-water limestone successions of the Apulian Carbonate Platform (central-southern Italy)

Bedding-parallel stylolites typically represent the product of chemical compaction (overburden weight-induced pressure solution) experienced by carbonate successions during their burial history, when bedding is still horizontal. Due to their common occurrence in carbonate rocks, with lateral extents that can exceed 1 km, bedding-parallel stylolites are of special interest for the hydrocarbon industry because they may affect the regional fluid flow in the subsurface. Aimed at assessing the development and distribution of bedding-parallel stylolites in shallow-water, platform limestone successions, field and laboratory studies were carried out on Cretaceous limestones originally pertaining to the Apulian Carbonate Platform realm and now exposed in three distinct Italian locations: Maiella Mountain, Gargano Promontory and Murge Plateau. Results point to a prominent role played by the geological characteristics of limestones on development and localization of bedding-parallel stylolites within shallow-water, platform limestone successions. In particular, bedding-parallel lamination and fine rock grain size, co-occurring in stromatolitic limestones, determined there laterally more extensive and closely spaced stylolites than in the associated calcilutites and calcarenites. Large fenestral pores, which are ubiquitous in stromatolitic limestones, represent rock heterogeneities able to influence the roughness of individual stylolites. Laboratory measurements revealed that the permeability of the studied Cretaceous limestones is very low (<10 μD). Pilot tests suggest that bedding-parallel stylolites in stromatolitic layers are not barrier to fluid flow but may represent pathways through low-permeability, platform limestone successions in the subsurface.

Neotethys seawater chemistry and temperature at the dawn of the end Permian mass extinction

The end of the Permian was a time of great death and massive upheaval in the biosphere, atmosphere and hydrosphere. Over the last decades, many causes have been suggested to be responsible for that catastrophe such as global warming, anoxia and acidification. The Gyanyima limestone block was an open ocean seamount in the southern Neotethys at subtropical latitude, and it affords us insight into open-ocean oceanographic changes during the end of the Permian. After careful screening using multiple tests, we reconstructed carbonate/seawater curves from the geochemical data stored in pristine brachiopod shell archives from the shallow water limestone of the Changhsingian Gyanyima Formation of Tibet. The reconstructed strontium isotope curve and data for the late Changhsingian are relatively invariant about 0.707013, but in the upper part of the succession the values become more radiogenic climaxing at about 0.707244. The 87Sr/86Sr curve and trend are similar to those observed for the Upper Permian succession in northern Italy, but dissimilar (less radiogenic) to whole rock results from Austria, Iran, China and Spitsbergen. The Ce/Ce* anomaly results ranging from 0.310 to 0.577 for the brachiopods and from 0.237 to 0.655 for the coeval whole rock before the event, and of 0.276 for whole rock during the extinction event, suggest normal redox conditions. These Ce* values are typical of normal open-ocean oxic water quality conditions observed in modern and other ancient counterparts. The biota and Ce* information clearly discounts global anoxia as a primary cause for the end-Permian biotic crisis. Carbon isotopes from brachiopod shells and whole rock are relatively invariant for most of the latest Permian interval, which is in stark contrast to the distinct negative carbon isotope excursion observed near and about the event. Estimates of seawater temperature at shallow depth fluctuated from 22.2 to 29.0°C up to unit 8–2, and then gradually rise from 29.7°C in unit 8–13 to values exceeding 35°C at a stratigraphic level about 120ky before the Permian–Triassic boundary, and just before the onset of the extinction interval. This dramatic increase in seawater temperature has been observed in global successions from tropical to mid latitude and from restricted to open ocean localities (e.g., northern Italy, Iran). The brachiopod archive and its geochemical proxies from Tibet support the paradigm that global warming must have been an important factor of the biotic crisis for the terrestrial and marine faunas and floras of the late Paleozoic world.

A revised Mississippian lithostratigraphy of County Galway (western Ireland) with an analysis of carbonate lithofacies, biostratigraphy, depositional environments and palaeogeographic reconstructions utilising new borehole data

An integrated study of borehole data and outcrop of Mississippian (late Tournaisian to late Viséan) rocks in Co. (County) Galway, western Ireland has enabled a more detailed geological map and lithostratigraphy to be constructed for the region. Several carbonate formations have been distinguished by microfacies analysis and their precise ages established by micropalaeontological investigations using foraminifers and calcareous algae. In addition, palaeogeographic maps have been constructed for the late Tournaisian, and early to late Viséan intervals in the region. The oldest marine Mississippian (late Tournaisian) deposits are recorded in the south of the study region from the Loughrea/Tynagh area and further south in the Gort Borehole; they belong to the Limerick Province. They comprise the Lower Limestone Shale Group succeeded by the Ballysteen Group, Waulsortian Limestone and Kilbryan Limestone Formations. These rocks were deposited in increasing water depth associated with a transgression that moved northwards across Co. Galway. In the northwest and north of the region, marginal marine and non-marine Tournaisian rocks are developed, with a shoreline located NW of Galway City (Galway High). The central region of Co. Galway has a standard Viséan marine succession that can be directly correlated with the Carrick-on-Shannon succession in counties Leitrim and Roscommon to the northeast and east as far as the River Shannon. It is dominated by shallow-water limestones (Oakport, Ballymore and Croghan Limestone Formations) that formed the Galway-Roscommon Shelf. This facies is laterally equivalent to the Tubber Formation to the south which developed on the Clare-Galway Shelf. In the southeast, basinal facies of the Lucan Formation accumulated in the Athenry Basin throughout much of the Viséan. This basin formed during a phase of extensional tectonics in the early Viséan and was probably connected to the Tynagh Basin to the east. In the late Viséan, shallow-water limestones of the Burren Formation extend across much of the southern part of the region. They are characterized by the presence of rich concentrations of large brachiopod shells and colonial coral horizons which developed in predominantly high-energy conditions. These limestones also exhibit palaeokarstic surfaces and palaeosols which formed during regressive conditions of glacio-eustatically controlled cyclicity. Locally, slightly deeper water, lower energy conditions developed on the shelf with the formation of rare bryozoan-rich mud-mounds. Deep-water basinal facies were maintained in the central and southeastern parts of the region between the two shelves with the persistence of the Lucan Formation. Active syn-sedimentary faulting influenced deposition in the Viséan and interfingering of basinal sediments with slumps and shallow-water shelf carbonates are recognized.

Carbonate formation of the Lower Carboniferous in Central Part of Volga-Ural Basin

Carbonate rocks of the Lower Carboniferous (Tournaisian stage) of the central Volga-Ural basin (the eastern portion of the East European platform) are of practical scientific interest to geologists, particularly because they contain large reserves of oil. Although such layers have been studied, various questions pertaining to development of sedimentation schemes for the rocks have not been answered. We have attempted to resolve these by studying a wealth of drill core materials. The study involved structural and genetic analysis of rocks and facies reconstructions. The rocks are mainly represented by different types of shallowwater limestone. The thickness of coeval layers and their lithological structures changes from well to well within an oilfield, primarily due to the different environments of sedimentation during the Tournaisian stage. Therefore, to identify the characteristics of carbonate sedimentation, we have studied the sequences of different types of limestone and analysed their thickness. As a result, we have developed principle schemes of sedimentation for shallow-water carbonate rocks of the Tournaisian stage. This may help in the predictive search for reservoir rocks in the region of study.

Stromatoporoid Beds and Flat-Pebble Conglomerates Interpreted as Tsunami Deposits in the Upper Silurian of Podolia, Ukraine

Abstract Tsunami deposits are currently a subject of intensive studies. Tsunamis must have occurred in the geological past in the same frequency as nowadays, yet their identified depositional record is surprisingly scarce. Here we describe a hitherto unrecognized example of probable palaeotsunamites. The Upper Silurian (Pridoli) carbonate succession of Podolia (southwestern Ukraine) contains variously devel-oped event beds forming intercalations within peritidal deposits (shallow water limestones, nodular marls and dolomites). The event beds are represented by stromatoporoid and fine-grained bioclastic limestones, in some places accompanied by flat-pebble conglomerates. The interval with event beds can be traced along the Zbruch River in separate outcrops over a distance of more than 20 km along a transect oblique to the palaeoshoreline. The stro-matoporoid beds have erosional bottom surfaces and are composed of overturned and often fragmented massive skele-tons. The material has been transported landward from their offshore habitats and deposited in lagoonal settings. The flat-pebble conglomerates are composed of sub-angular micritic clasts that are lithologically identical to the sediments forming the underlying beds. Large-scale landward transport of the biogenic material has to be attributed to phenomena with very high energy levels, such as tropical hurricanes or tsunamis. This paper presents a tsunamigenic interpretation. Morphome-tric features of redeposited stromatoporoids point to a calm original growth environment at depths well below storm wave base. Tsunami waves are the most probable factor that could cause their redeposition from such a setting. The vastness of the area covered by parabiostromal stromatoporoid beds resembles the distribution of modern tsunami deposits in offshore settings. The stromatoporoid beds with unsorted stromatoporoids of various dimensions evenly distributed throughout the thickness of the beds and with clast-supported textures most probably represent deposition by traction. In some sections, the stromatoporoids are restricted to the lowermost parts of the beds, which pass upwards into bioclastic limestones. In this case, the finer material was deposited from suspension. The coexistence of stromatoporoid beds and flat-pebble conglomerates also allows presenting a tsunami interpretation of the latter. The propagating tsunami waves, led to erosion of partly lithified thin-layered mudstones, their fragmentation into flat clasts and redeposition as flat-pebble conglomerates.

An overview of the Cretaceous stratigraphy and facies development of the Yazd Block, western Central Iran

The Cretaceous successions of the Yazd Block, the western of three structural blocks of the Central-East Iranian Microcontinent (CEIM), have been studied using an integrated approach of litho-, bio- and sequence stratigraphy associated with litho-, bio- and microfacies analyses. The Cretaceous System of that area is in excess of 5km thick and a generalized relative sea-level curve can be inferred from the facies and thickness development. This curve can be subdivided into two transgressive–regressive megacycles (TRMs), separated by a major tectonic unconformity in the Upper Turonian. TRM 1 comprises the Early Cretaceous to Middle Turonian, TRM 2 the Coniacian to Maastrichtian. TRM 1 starts with up to 1500-m-thick conglomerates and sandstones covering Palaeozoic–Triassic basement rocks, metasediments, or Upper Jurassic–lowermost Cretaceous granites. The basal tectonic unconformity, related to the Late Cimmerian event (Jurassic–Cretaceous boundary interval), shows a pronounced palaeo-relief that is levelled by the basal siliciclastic formations. Sparse biostratigraphic data from calcareous intercalations in the upper part of these strata indicate a Hauterivian to Barremian age. The Aptian facies development is marked by the onlap of thick-bedded, micritic carbonates with abundant orbitolinid foraminifera and rudists representing a large-scale shallow-marine carbonate platform system that fringed the Yazd Block in the north and west. These platforms are up to 1000m thick and drowned during the middle to Late Aptian, followed by up to 1500-m-thick basinal marly sediments of Late Aptian to mid-Late Albian ages, representing the maximum relative sea-level during TRM 1. During the latest Albian–Middle Turonian, a gradual shallowing is indicated by progradation of shallow-water skeletal limestones separated by marl tongues, representing a carbonate ramp system. Strata of TRM 2 overlie older units along a regional angular unconformity and indicate tectonic stability and lowered subsidence rates as shown by widespread uniform Coniacian–Maastrichtian facies and thickness development. Above a thick basal conglomerate, the Coniacian–Campanian facies is characterized by shallow-water limestones of a large-scale epeiric platform. Relative sea-level peaked in the Late Campanian, followed by Maastrichtian infilling. The Cretaceous succession is truncated along a tectonic unconformity at the base of the Palaeocene. The major tectonic unconformities recognized at the base of the succession (Late Cimmerian Event), in the Late Turonian, and in the Cretaceous–Palaeogene boundary interval are evidence of significant tectonic activity of the Yazd Block. Their formation is probably related to plate tectonic processes in response to the opening and closure of ocean basins surrounding the CEIM.

Dolomitization of Lower Cretaceous Peritidal Carbonates By Modified Seawater: Constraints From Clumped Isotopic Paleothermometry, Elemental Chemistry, and Strontium Isotopes

Abstract This study investigates one of the few examples of dolomitization of Lower Cretaceous shallow-water limestones from the southern Tethys carbonate platform from outcrops on the Haushi-Huqf High in central-east Oman. Mud-dominated peritidal carbonates rich in microbial mats are replaced by fine crystalline dolomite along at least 60 km in the lower 10 m of the Jurf Formation. Two, meter-thick beds in the overlying Qishn Formation are also dolomitized over lateral distances of one to two kilometers. The stratabound geometry and petrographic relations of the dolomite with all other diagenetic phases indicate that the dolomite precipitated early. The presence of only rare anhydrite relicts suggests that seawater was below gypsum saturation during most of the dolomitization, supporting the hypothesis that Cretaceous slightly evaporated water (δ18Ofluid values range from 2.8‰ to 3.5‰ SMOW) can affect pervasive dolomitization. Despite being composed of peritidal facies with features suggesting high salinity, most of the carbonate succession was not dolomitized, suggesting that the presence of microbial mats exerted a major control on the distribution of dolomite, and that salinity is only a minor control on dolomitization. Clumped-isotope results indicate that the early formed dolomite re-equilibrated with fluids at 44 ± 3°C in a shallow-burial setting. The elevated iron content (on average 6011 ppm) and 87Sr/86Sr (on average 0.70782) with respect to Cretaceous seawater suggests that the burial fluids interacted with Permian clay and feldspars of the Gharif Formation that underlies the Cretaceous carbonates. The vertical gradients of radiogenic Sr and the occurrence of small volumes of coarse crystalline dolomite in stylolites and fractures suggests that burial fluids were driven vertically and laterally by differential compaction, probably during maximum burial in the Late Cretaceous.

Identification of an Archean marine oxygen oasis

The early Earth was essentially anoxic. A number of indicators suggest the presence of oxygenic photosynthesis ∼2700–3000 million years (Ma) ago, but direct evidence for molecular oxygen (O2) in seawater has remained elusive. Here we report rare earth element (REE) analyses of ∼2800 million year old shallow-marine limestones and deep-water iron-rich sediments at Steep Rock Lake, Canada. These show that the seawater from which extensive shallow-water limestones precipitated was oxygenated, whereas the adjacent deeper waters where iron-rich sediments formed were not. We propose that oxygen promoted limestone precipitation by oxidative removal of dissolved ferrous iron species, Fe(II), to insoluble Fe(III) oxyhydroxide, and estimate that at least 10.25μM oxygen concentration in seawater was required to accomplish this at Steep Rock. This agrees with the hypothesis that an ample supply of dissolved Fe(II) in Archean oceans would have hindered limestone formation. There is no direct evidence for the oxygen source at Steep Rock, but organic carbon isotope values and diverse stromatolites in the limestones suggest the presence of cyanobacteria. Our findings support the view that during the Archean significant oxygen levels first developed in protected nutrient-rich shallow marine habitats. They indicate that these environments were spatially restricted, transient, and promoted limestone precipitation. If Archean marine limestones in general reflect localized oxygenic removal of dissolved iron at the margins of otherwise anoxic iron-rich seas, then early oxygen oases are less elusive than has been assumed.

Early Pliensbachian (Early Jurassic) C-isotope perturbation and the diffusion of the Lithiotis Fauna: Insights from the western Tethys

High-resolution carbonate carbon-isotope stratigraphy of Lower Jurassic marine shallow-water limestones are compared with the distribution of the Lithiotis Fauna on the Trento Platform (Southern Alps, Italy). The Lithiotis Fauna is the first example of globally distributed mound-building bivalves in the geological record and experienced global diffusion in the Early Jurassic. A set of carbonate carbon-isotope excursions of 2–3‰, illustrating three distinct negative shifts followed by positive rebounds, are recorded in the isotope stratigraphy and can be correlated with the global negative δ13C shift of the Sinemurian–Pliensbachian boundary Event (S–P Event) and to the subsequent phase of C-isotope perturbation that characterized the lower Pliensbachian. In the studied stratigraphic sections, the S–P Event likely triggered eutrophic conditions illustrated by the presence of organic-rich facies and by fossil associations characteristic of poorly oxygenated waters. After the eutrophic phase, the amelioration of environmental conditions was marked by a positive ~3‰ rebound of the δ13Ccarb values, and by the occurrence of marine stenotypic faunas. On the Trento Platform, the stabilization of the δ13Ccarb values coincided with the appearance of the Lithiotis Fauna that subsequently became widely distributed in the entire range of platform environments and thrived in the late Pliensbachian when metric-scale bivalve mounds were generated. During the same time, the maximum proliferation of the Lithiotis Fauna is recorded both in the Tethyan and Panthalassa regions. Hence, the reported relationships between the δ13Ccarb data and the distribution and ecological characteristics of the genera contained in the Lithiotis Fauna suggest that the S–P Event and its aftermath, possibly coupled with the undergoing coeval continent reorganization that led to the opening of the Hispanic Corridor, could have set the stage for the rapid diffusion of these unusual bivalves across many parts of the globe.

The Coniacian–Campanian Latium–Abruzzi carbonate platform, an example of a facies mosaic

This paper presents the results of a high-resolution analysis of Upper Cretaceous shallow-water limestones in the northeast sector of the Lepini Mountains (Central Apennines, Italy) that belong to the Latium–Abruzzi platform domain. The studied succession is entirely referred to as the “Accordiella conica & Rotorbinella scarsellai Biozone”. The analyzed Coniacian–Campanian succession is primarily characterized by three lithofacies associations (LF-A, LF-B, LF-C) deposited on an open shelf. The intertidal and shallow-subtidal environments are characterized by mudstone to wackestone and laminated bindstone (LF-C), whereas in the low to moderate energy environments of the inner shelf there developed rudist biostrome (rudist pillarstone) and rudist rudstone to floatstone (LF-A). A lithofacies association dominated by cross-bedded grainstone (LF-B) represents the reworking of bioclastic grains (rudist fragments) derived from the areas of the shelf colonized by rudist biostrome; lime-sand shoals related to storm channels passed into submarine dunes in an open-shelf setting. Correlation of the five investigated stratigraphic sections shows how the recognized LF are laterally associated to form a facies mosaic over a few hundred meters. The stratigraphic architecture shows five intervals (I–V) each of which is dominated by one or two LF. Interval I is intensely dolomitized. The following intervals (II and III) record a gradual increase in hydrodynamic energy as evidenced by the presence of rudist biostromes passing upward into cross-bedded grainstone. An increase in mud-supported textures in the interval IV suggests more restricted conditions, which were terminated by a period of emergence. More open-marine conditions in the final interval (V) are shown by the dominance of LF-A and LF-B.

First record of lowermost Cretaceous shallow-water limestones in the basement of the Transylvanian Depression (Romania)

Triassic, Upper Jurassic and upper Lower Cretaceous sedimentary formations were previous- ly studied from the Transylvanian Depression basement, but the presence of lowermost Cretaceous (Berriasian-Valanginian) has not been confirmed paleontologically. The carbonate sequence cored from a borehole drilled in the central part of the Transylvanian Depression yields microfossil assemblages dominated by benthic foraminifera. These new data unequivocally document the presence of characteristic Berriasian-Valanginian taxa in these deposits.

Contribution to the biostratigraphy of the Lower Cretaceous in eastern Serbia: Upper Hauterivian Orbitolinidae from the Kamenica 1 section, Kurilovo anticline

Upper Hauterivian deposits in the Kurilovo area, Kamenica 1 section, NE of Niš, are described based of abundant and diverse orbitolinids. So far, the interval was assigned to the Barremian–Aptian on the geological map. Such a new age assignment results from the first detailed study carried out on the orbitolinid fauna contained in the Lower Cretaceous (upper Hauterivian) shallow-water limestones of eastern Serbia. The upper Hauterivian is documented on the basis of two key stratigraphic markers, specifically Valserina primitiva and Paleodictyoconus beckerae.In addition to these late Hauterivian index fossils, the studied section bears orbitolinids having a larger stratigraphic distribution: Cribellopsis neoelongata, Cribellopsis thieuloyi?, Montseciella glanensis, Orbitolinopsis debelmasi, Orbitolinopsis cf. debelmasi, Orbitolinopsis sp., Paleodictyoconus cuvillieri, Paleodictyoconus cf. cuvillieri, Paleodictyoconus cf. beckerae, Paleodictyoconus cf. actinostoma, Paleodictyoconus sp., Paracoskinolina? jourdanensis, Paracoskinolina cf. hispanica, Urgonina alpillensis, Valserina sp. The microfossil assemblage includes other foraminifers such as Charentia cuvillieri, Mayncina bulgarica, Nautiloculina cretacea, Pfenderina globosa, Pseudocyclammina cf. lituus, Pseudolituonella gavonensis, Ammobaculites sp., Bolivinopsis sp., abundant trocholinids, various miliolids, other foraminifers and sparse algae which will be presented separately.