Shallow-water Carbonate Research Articles

Large isotopic variability at the micron-scale in ‘Shuram’ excursion carbonates from South Australia

Ediacaran-aged (635–541 million years ago) marine sediments contain a large negative carbon isotope (δ13C) excursion, in which carbonate δ13C values reach −12‰ (VPDB). Known as the ‘Shuram’ excursion, many workers have interpreted this δ13C record as an unprecedented perturbation to the global carbon cycle, leading to speculation about a causal connection to the broadly contemporaneous rise of animal life. Others have interpreted the δ13C signal as a product of diagenesis, thereby minimizing its relevance for understanding the evolution of metazoans. Here, we present SEM imaging and in-situ δ13C and δ18O values measured by secondary ion mass spectrometry (SIMS) to assess these competing hypotheses in the Wonoka Formation of South Australia. Our results from the minimum of the excursion show that rounded sedimentary grains of calcite have δ13C values between −12.8 to −10.6‰ and δ18O values between −17.8 to −15.5‰ (VPDB). Euhedral dolomite that appears to have grown unimpeded in open sedimentary pore spaces also is present. These early-stage dolomites are interpreted as early authigenic in origin and have δ13C values that reach +5‰, requiring a formation fluid with a substantially different δ13C composition from basin waters or bulk sediment. Together, these results provide little evidence for the hypothesis that a late diagenetic overprint has generated the ‘Shuram’ excursion in the Wonoka. Instead, they suggest the presence of a large carbon isotopic gradient in the surface environment, with shallow waters capable of precipitating carbonates with very low δ13C (down to −12‰) and deeper shelf and/or marine pore waters generating carbonates with positive carbon isotope values (up to +5‰). Because negative isotope excursions of similar magnitude are found in widely dispersed Ediacaran basins, it is likely that this gradient was characteristic of shelf environments of this period and that a still-unknown global process led to the ‘Shuram’ excursion in shallow water carbonates.

U-Pb and Re-Os geochronology tracks stratigraphic condensation in the Sturtian snowball Earth aftermath

AbstractThe snowball Earth hypothesis predicts a strong hysteresis resulting in discrete multi-million-year glaciations followed by globally synchronous deglaciation. Here we present new U-Pb zircon and Re-Os sedimentary rock geochronology and Os isotope chemostratigraphy from post-Sturtian sequences in south China to test the synchroneity of deglaciation. High-precision chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb zircon dates refine the minimum age of deglaciation to 660.98 ± 0.74 Ma, which is ∼2 m.y. older than previously reported. We also provide a new maximum age constraint on the onset of the Marinoan glaciation of 657.17 ± 0.78 Ma. A global compilation of new Os isotope chemostratigraphy reveals a large and systematic trend to unradiogenic values over <1 m of stratigraphy. Together, these data indicate that the Mn-carbonates in south China are not cap carbonates that formed as a response to post-snowball alkalinity, but are authigenic carbonates that formed millions of years after deglaciation. Sturtian cap carbonates tend to be absent or more condensed than their younger Marinoan counterparts. We suggest that this reflects a combination of enhanced accommodation space in early Cryogenian underfilled rift basins, stronger hysteresis, larger ice volume, and/or higher CO2 levels needed for deglaciation of the longer Sturtian glaciation. Further, our findings indicate that the apparent diachroneity of deglaciation can be explained readily as a consequence of stratigraphic condensation, itself due to the large post-Sturtian glacioeustatic transgressive sequence that outpaced shallow-water carbonate deposition.

Changhsingian brachiopod communities along a marine depth gradient in South China and their ecological significance in the end-Permian mass extinction

Diversity indices (dominance and evenness) and ecological spatial structure (lifestyles and relative abundances) are important features of Changhsingian brachiopod communities prior to the end-Permian mass extinction (EPME) and could predict temporal and spatial extinction patterns during the EPME. In South China, Changhsingian brachiopod communities show higher diversity than other contemporaneous brachiopod communities in the world and have been reported from a variety of sedimentary environments. In this paper, brachiopods from 18 sections in South China were selected to divide communities and compare their ecological structure. Based on the results of network analysis, cluster analysis and quantitative data from the selected sections, we show that Changhsingian brachiopod communities in South China can be categorized into three assemblages along a marine depth gradient: the Neochonetes–Fusichonetes–Paryphella Assemblage from the shallow-water clastic-rock facies, Spinomarginifera–Peltichia–Oldhamina Assemblage from the shallow-water carbonate platform facies and Fusichonetes–Crurithyris Assemblage from the deep-water siliciclastic intracontinental basin facies. Compared with communities from carbonate platform facies, the communities from siliciclastic facies were characterized by high dominance, low evenness and low lifestyle diversity, which might be important biotic factors leading to earlier extinctions. After the extinction began in all environments, the whole earliest Triassic brachiopod community was first dominated by Fusichonetes and then by Crurithyris. These patterns of domination and replacement could be explained by morphological and ecological advantages. The domination of these two genera, which were already adapted to the oxygen and food-limited deep-water habitat, indicates that the cooler deep-water environment might have been a relatively less stressed habitat after the beginning of the EPME. This suggests that global warming might be the main trigger among the previously proposed synergistic environmental stresses, while anoxia might not, at least for the beginning of EPME.

Paleoecology and taxonomy ofSchoenaster carterensis, a new encrinasterid ophiuroid species from the Upper Mississippian (Chesterian) Slade Formation of northeastern Kentucky, USA

AbstractSchoenaster carterensisnew species, is an asteroid-like ophiuroid (Echinodermata) from Upper Mississippian (Chesterian) shallow-water carbonates in the Ramey Creek Member of the Slade Formation in northeastern Kentucky. First described in the 1860s from Lower and Middle Mississippian rocks,SchoenasterMeek and Worthen, 1860 is not a well-known fossil genus, but the 39 specimens in this collection permitted further definition of the genus and extended its range by ca. 17 Ma into Late Mississippian (Chesterian) time. The number of specimens also permitted differentiation of growth stages based on average arm length and showed that arm length, disk perimeter, and disk area are interrelated in statistically significant ways. Although replaced by chert, the specimens are nearly intact due to rapid burial as rare constituents in habitat communities distributed among four once-contiguous habitats, including shoal, shoal margin, transitional, and basinal. Most of the ophiuroids were concentrated on firm grounds or hardgrounds in shoal and transitional environments, concentrations that probably reflect substratum stability and the ability to support the ophiuroid's generalist feeding strategy. Many fossil ophiuroid species are known from only a few specimens, severely limiting interpretations about their detailed taxonomy, individual variation, and ecology. In contrast, the greater number of specimens and extensive knowledge of geologic occurrence in this study permitted detailed interpretations regarding the taxonomic, intraspecific, and ecologic attributes of this species, which might be useful in the study of other fossil ophiuroids.UUID:http://www.zoobank.org/ffd945d8-63ac-4c38-a2d3-8647558dbbf0

Brittle vs. ductile strain during the synorogenic exhumation of HP-LT rocks: An example from the Lungro-Verbicaro Unit mylonites (northern Calabria, Italy)

We present a study on the mylonites exposed in the Cirella area along the Tyrrhenian coast of northern Calabria (southern Italy), in order to understand the role of ductile and brittle deformation on the exhumation process of these rocks. The analyzed shear zone is located in the footwall of a regional tectonic contact between metaophiolites (Ligurian Complex) on the top, and the continental Lungro-Verbicaro Unit (Adria plate) at the bottom. The latter consists of a metasedimentary succession with Triassic shallow-water carbonates at the base of it, evolving upward to Jurassic-lower Miocene slope to deep basin carbonates and foredeep siliciclastic rocks. Both oceanic and continental successions are characterized by high-pressure and low-temperature (HP-LT) metamorphism. The analyzed mylonites show different degrees of strain, ranging from protomylonites to ultramylonites, the latter characterized by well-marked foliation and stretching lineations. Several micro- and mesoscale progressive deformation structures, including two fold sets, S-C’ structures are present in these rocks, and a late oblique foliation, indicating a NW sense of shear. A successive shortening stage produced thrust faults and related folds, indicating a tectonic transport toward SE. We associate the mylonitic deformation with the activity of the overlying detachment fault, which is responsible for the synorogenic exhumation of the Lungro-Verbicaro Unit during the Miocene time. However, the ductile strain, recorded by the studied mylonites, accounted for a small displacement during the exhumation of this tectonic slice. In fact, the calculated displacement achieved by the Lungro-Verbicaro Unit, during exhumation, is much larger than the one estimated considering only the ductile zones activity. This evidence suggests that, synchronously with ductile strain, brittle deformation occurred along the detachment fault located at the top of the thrust sheet. In addition, a severe erosion due to the tectonic exhumation of the HP-LT rocks (in the late Miocene) produced a large amount of clastic sediment that filled the extensional basins widespread in northern Calabria.

Photic-zone euxinia and anoxic events in a Middle-Late Devonian shelfal sea of Panthalassan continental margin, NW Canada: Changing paradigm of Devonian ocean and sea level fluctuations

Abstract The latest Eifelian – Frasnian strata of the Mackenzie Valley, NW Canada, provide an excellent archive of paleoceanographic signals imprinted in oxic and anoxic facies deposited in close proximity. Fondoformic black-shale strata preserve fingerprints of four global anoxic events (Kacak, Frasnes, Middlesex, and Rhinestreet), which receives confirmation with δ13Corg data. The discovery of 2,3,6- and 3,4,5-trimethyl aryl isoprenoids (biomarkers of green sulfur bacteria) at and between the levels of anoxic events contributes to the growing evidence of photic-zone euxinia as a common state of oceanographically open and semi-restricted shelfal basins of the Middle Devonian – Early Mississippian; a condition impossible under present-day vigorous thermohaline circulation, but consistent with models of greenhouse ocean depicting drastic slowdown in watermass turnover, reversals of deep ocean circulation, greatly expanded oxygen minimum zones, and profoundly changed nutrient flows. The rocks under study were deposited in an oceanographically open basin with fluctuating chemocline as attested by unstable presence of gammacerane in GCMS spectra and co-occurrence of signatures of water-column euxinia and pyritized hyalosponge spicules indicating episodes of weak bottom oxygenation. Shallow-water carbonate banks in the same basin show signatures of reduced hydrodynamic activity and do not record sea level changes in excess of several meters, which lines up with the shortage of evidence for high-amplitude base-level fluctuations in coeval strata worldwide. This supports discarding changes in sea level as the principal control over Devonian anoxic events and instead suggests pulsatory expansions of thick, semi-continuous oxygen minimum zones of the greenhouse ocean. Sea level fluctuations could still be involved as non-glacial (thermal and aquifer?) eustatic transgressions of a very modest amplitude.

Resilience of marine invertebrate communities during the early Cenozoic hyperthermals

The hyperthermal events of the Cenozoic, including the Paleocene-Eocene Thermal Maximum, provide an opportunity to investigate the potential effects of climate warming on marine ecosystems. Here, we examine the shallow benthic marine communities preserved in the late Cretaceous to Eocene strata on the Gulf Coastal Plain (United States). In stark contrast to the ecological shifts following the end-Cretaceous mass extinction, our data show that the early Cenozoic hyperthermals did not have a long-term impact on the generic diversity nor composition of the Gulf Coastal Plain molluscan communities. We propose that these communities were resilient to climate change because molluscs are better adapted to high temperatures than other taxa, as demonstrated by their physiology and evolutionary history. In terms of resilience, these communities differ from other shallow-water carbonate ecosystems, such as reef communities, which record significant changes during the early Cenozoic hyperthermals. These data highlight the strikingly different responses of community types, i.e., the almost imperceptible response of molluscs versus the marked turnover of foraminifera and reef faunas. The impact on molluscan communities may have been low because detrimental conditions did not devastate the entire Gulf Coastal Plain, allowing molluscs to rapidly recolonise vacated areas once harsh environmental conditions ameliorated.

Anatomy and Jurassic evolution of a Hercynian basement high (Caloveto High - Calabria, Southern Italy)

The Caloveto area in North-East Calabria hosts a stratigraphic succession which documents the evolution from shallow water carbonates to deeper-water pelagic and hemipelagic deposits, bearing ample evidence for severe tectonic control on Jurassic and Lower Cretaceous sedimentation. Geological mapping indicates that the shallow-water carbonates, also with coral assemblages, formed a narrow fringe around a high of the Hercynian basement, made of lowgrade metamorphic rock, which remained emergent throughout the Pliensbachian and became an intrabasinal high which stood proud of the Longobucco basin, hosting thin pelagic sedimentation through most of the Jurassic. An extensional phase in the Toarcian disrupted and foundered the benthic carbonate factory, whose drowning isdocumented by a change to Rosso Ammonitico-type deposits. This resulted in a complex network of neptunian dykes (also intruding the basement) and in-situ breccias. Starting in the late Toarcian, the rugged submarine topography was gradually leveled by onlapping marls, radiolarites and pelagic limestone, which sealed the Toarcian fault zones producing a diverse array of unconformities. Colonization by microbial communities characterizes the initial stages ofmarine sedimentation around the cores of Paleozoic basement, as banded polychrome microbialites and “swollen” phyllites, a result of the microbially-influenced/induced displacive growth of calcium carbonate crystals along split cleavage planes, constitute an unexpected field tool for identifying basin margins. Silicified marginal zones in the shallow water limestone characterize, in analogy withother Tethyan regions, the onlap unconformities of chert-rich basinal units on the submerged carbonate fringes.

Seawater paleotemperature and paleosalinity evolution in neritic environments of the Mediterranean margin: Insights from isotope analysis of bivalve shells

The first step of ice-sheet build-up on Antarctica at the Eocene-Oligocene boundary was followed by a phase of climate instability culminating during the Miocene with a warming event called the Middle Miocene Climate Optimum (MMCO), that ended with a marked cooling phase identified as the Middle Miocene Climate Transition (MMCT). While numerous benthic foraminifera δ18O and Mg/Ca data have been used to capture the global climate evolution during this interval of climate and ice-sheet instability, geochemical records from shallow-water carbonates still remain scarce. Yet such records are crucial to capture the diversity of regional environmental responses to global climate changes, and thus to better understand the behavior of our climate system during this critical interval. In this work, we test the potential of a multi-proxy approach (δ18O, clumped isotope (Δ47), strontium isotopes (87Sr/86Sr)) applied to bivalves recovered from the Liguro-Provencal Basin and Rhodano-Provencal basin (Northwestern of the Mediterranean Sea) to reconstruct the evolution of shallow seawater temperature and salinity in this region over the latest Oligocene to Middle Miocene interval (~10 Myrs). Our results highlight a local cooling in the northwestern Mediterranean Sea during the MMCO that contrasts with the warming observed in other regions, with seawater temperatures inferred from Δ47 analyses in the 13–18 °C range. These cool seawater temperatures recorded in the studied region during the MMCO are much cooler than those recorded in the open oceans, but are in agreement with the proliferation of bryozoan observed in the Castillon-Du-Gard area during this interval. Low bivalve δ18O values (−3.24‰ in average) are recorded during this episode are interpreted to reflect enhanced freshwater inputs, lowering local salinity and seawater δ18O. Such enhanced freshwater inputs point to a phase of enhanced hydrological cycle in the studied region, possibly linked to the global mid-Miocene warming event. Bivalve 87Sr/86Sr data mostly fall within the global seawater Sr isotope reference curve, but remain compatible with enhanced freshwater inputs during the Aquitanian and during the Langhian, as these inputs were probably not important enough to induce a large departure from the seawater curve at a regional scale.

Chapter 5 Interpretation of the origin and evolution of the Arabian Intrashelf Basin and the development of the Dhruma Atash, Tuwaiq and Hanifa sequences

Abstract This is the first of two chapters in which the data presented in the first four chapters of this Memoir are reviewed and interpreted. A summary cross-section across the basin from the Saudi outcrop to Abu Dhabi and the Tethyan margin is used to summarize the regional setting and sequences as interpreted in this Memoir. The key points illustrated by the summary cross-section are stated and discussed in the first part of this chapter. Two different interpretations for the eastern margin of the intrashelf basin are reviewed. In the first of these interpretations, the margin with the deeper waters of the Tethys Ocean shelf is adjacent to the intrashelf basin rim in Abu Dhabi, whereas in the second, preferred in this Memoir, a broad, shallow Tethyan shelf platform (200–300 km wide) extends from the intrashelf basin rim to the Tethys continental shelf edge. The implications of Late Jurassic exposure and erosion on the adjacent Tethyan shelf are discussed. The development of the Arabian Intrashelf Basin during the Callovian–late Oxfordian (the Tuwaiq Mountain Formation, the source rock and the Hanifa intervals and associated sequences) is interpreted and discussed with illustrations, including step-by-step facies maps. The interpretation integrates depositional, eustatic and tectonic factors in the evolution of the intrashelf basin. The interpretation in this Memoir is that the Arabian Intrashelf Basin began with isostatic and extensional subsidence on top of a broad Dhruma Atash Member platform, which had largely filled the accommodation space up to the wave base and to near sea-level. It developed fully into an intrashelf basin during the deposition of the Callovian Tuwaiq sequence, with rising sea-levels coincident with a productive shallow water carbonate factory resulting in a rim of shallow water carbonate. An end-Callovian to early Oxfordian lowstand terminated the Tuwaiq sequence on the basin rim. During the lowstand, restricted conditions in the basin deposited the rich Lower Hanifa source rock as a lowstand systems tract. As more normal conditions returned in a subsequent sequence, the source rock facies graded upwards into Hanifa reservoir facies, which partially filled the basin. Hanifa progradation was terminated by another lowstand during which a subaqueous gypsum/anhydrite marker bed was deposited in at least part of the remnant basin. Earlier interpretations of these sequences are also discussed.

Chapter 6 Jubaila–Arab–Hith sequences

Abstract During deposition of the Jubaila-Arab-Hith sequences, the most important Arab-D reservoir, the Arab-C to A reservoirs and the Arab and Hith anhydrite seals were formed. The Kimmeridgian Jubaila-Arab-D transgression (MFS J70) restored and expanded the intrashelf basin geometry. The restored basin was filled by deeper subtidal Jubaila facies, which graded upwards to the Arab-D reservoir facies. In the late stages the Arab-D facies included both basin rim shoal facies and at least one large isolated shoal surrounded by ‘lagoonal’ intrashelf basin remnants interconnected by accessway channels. This differentiation was facilitated by local structural growth of a few metres. The Arab-D reservoir is overlain by the Arab-D anhydrite, deposited initially as gypsum in broad salinas, which blanketed the intrashelf basin region, forming the first major seal. By the end of Arab-D anhydrite deposition, the entire intrashelf basin area had been filled and transformed into a broad evaporite platform. Atop this platform, the Arab-C to -A (MFS J80-100) alternating shallow water carbonate and anhydrite (initially gypsum) intervals were deposited, ending regionally with the thick Hith Anhydrite Formation. The depositional setting for each of the main Jubaila-Arab-Hith sequences is reviewed and illustrated. The evolution of Jubaila-Arab-D reservoir models is discussed. Different interpretations of Jubaila-Arab-D-Hith sequences are compared, including the issue of whether the Arab carbonate-evaporite depositional sequence boundaries should be taken at the top of the carbonate intervals or the top of the anhydrites. Examples are included which illustrate how easterly palaeowind directions were an important factor controlling Arab-D facies variation. A scenario is presented to show how the Late Jurassic Tethys shelf uplift and low angle unconformity coupled with westward structural tilt were important. An alternating balance developed between the evolving land barrier and eustatic sea-level change during deposition of the Arab carbonate–evaporites and the Hith Anhydrite. Carbonates were deposited when the barrier was inundated and evaporites when the barrier was exposed and the area restricted and hypersaline. A major late Tithonian flooding event (MFS J110) formed the top-Hith Manifa carbonate reservoir and brought open marine carbonate mud-rich deposition to the area, continuing without hiatus into the Early Cretaceous.

New data on Pseudolituonella Marie 1954, emended herein, benthic foraminifera from the Cenomanian of the Sarvak Formation of SWIran (Zagros Zone) - Phylogenetic considerations

The benthic foraminifera Pseudolituonella reicheli Marie is widely reported from Cenomanian shallow-water carbonates of the Caribbean (Mexico) area and the Neotethyan realm. Its wall-structure is classified as imperforate, homogeneous microgranular. Specimens from the Cenomanian Sarvak Formation of southwestern Iran (Zagros Zone) yield the presence of a pseudo-keriothecal wall-structure an observation that has already been obtained from the Eocene P. robineti Gallardo Garcia and Serra-Kiel in Serra-Kiel et al. With the “reappearance” of an equivalent morphological trait and the double test size after twomainmass-extinctions (C/T andK/Pg), P. robineti might be an example of Cope`s rule. Further comments on the suprageneric classification, other species of Pseudolituonella and phylogeny are provided.

Taxonomic revision of Dictyoconus? vercorii Arnaud-Vanneau, 1980 (Foraminifera, Orbitolinidae) from the Lower Cretaceous (lowermost Barremian–lower Aptian) of southeastern France, Switzerland and Spain

The new genus Vanneauina belonging to the family Orbitolinidae is described herein from Lower Cretaceous shallow-water carbonates of southeastern France. Its type-species, here referred to as Vanneauina vercorii (Arnaud-Vanneau, 1980) comb. nov., has been described originally as belonging with some reservations to Dictyoconus Blanckenhorn. Vanneauina gen. nov. differs from Dicytoconus above all by its simple exoskeleton (lacking rafters), triangular-shaped primary beams, pillars laterally fusing at the base and top, and a simple and slightly eccentric embryo. So far, Vanneauina is a monospecific genus. In its type-area, it is reported from the lowermost Barremian (upper part of the hugii zone) to the lower Aptian (lower part of the weissi zone). After the present taxonomic revision, there is no representative of Dictyoconus in the orbitolinid assemblages from the upper Hauterivian to lower Aptian interval in the Urgonian type area of southeastern France and Switzerland.

Carbonate ecosystem and environmental evolution during the Valanginian–Barremian: Discussion on possible controlling factors in the Apennine Carbonate Platform record (Italy)

Shallow-water carbonate records are often characterised by low stratigraphic resolution and poor chronostratigraphic calibration despite abundant and diversified fossil content. We present here a stratigraphic study of a Valanginian–Barremian carbonate platform section, exposed in the southern Apennines (Italy). In the San Lorenzello section, we produced a high-resolution dataset integrating sedimentology, biostratigraphy, cyclostratigraphy and sequence stratigraphy to investigate how neritic biofacies responded to palaeonvironmental and palaeoclimatic changes in the time interval after the Weissert event. Fossil assemblages and lithofacies characteristics allowed the reconstruction of the paleoenvironmental history of the section, which is part of a shallow carbonate platform influenced by waves activity, storm events and sea-level changes. The latter induced cyclic emersions of this flat-topped subtropical platform, witnessed by karstification and pedogenesis. Investigations lead to a refinement of the previous biostratigraphy, also producing a close agreement between the biostratigraphic and C-isotope position of the Valanginian–Hauterivian boundary. Numerous green algae and benthic foraminifera bioevents have been identified and six biozones have been defined and anchored to the southern Tethys biozonal schemes.We used biostratigraphy and previous C-isotope stratigraphy to correlate local to global stratigraphic events and to discuss controlling factors influenced this carbonate platform system. An eustatic control on the hierarchical organization of the superbundles, also interpreted in terms of depositional sequences, is evidenced. Most Trangressive/Regressive Facies Trends (T/RFTs) boundaries have been correlated with the “global” Sequence Boundaries and to the Cretaceous eustatic curve. A correspondence between calcareous algae and benthic foraminiferal turnover phases and long-term eustatic events is also shown.

Comment on “Does specialization imply rare fossil records of some benthic foraminifera: Late Palaeocene examples from the eastern Neo-Tethys (Meghalaya, NE India)” by Suman Sarkar [Palaeogeography, Palaeoclimatology, Palaeoecology 514 (2019) 124–134.

Sarkar (2019: Does specialization imply rare fossil records of some benthic foraminifera: Late Palaeocene examples from the eastern Neo-Tethys (Meghalaya, NE India) Palaeogeogr., Palaeoclimatol., Palaeoecol. 514, 124–134) uses the “first occurrences” of two large benthic foraminifera (LBF), namely Haymanella paleocenica Sirel and Keramosphaera iranica Rahaghi, in the upper lower Paleogene (Thanetian) Lakadong Limestone of NE India for far-reaching speculations on autecology and specialization as well as paleobiogeography and migration patterns in the central and eastern Neotethyan realm. [Note: The authors requested post-submission that all instances of “paleocenica” made in the Abstract and Introduction be altered to “elongata”.] We believe that the true occurrences of such taxa in Late Paleocene shallow-water carbonates from NE India are not only speculative but irrelevant because they are based on incorrect identifications. On the basis of such documentation, Sar19 claimed that the sparse occurrences of both species can be correlated to their specialist behavior. Sar19 also added a more specific interpretation of the paleoecology of H. elongata and K. iranica relating them to the upper and lower parts of the upper photic zone, respectively. This conclusion is, in our point of view, not supported by the data and also conflicts with a bulk of publications supporting the opinion that the autecology of LBF must be interpreted by means of comparative anatomy with recent counterparts along with taphonomy and a comprehensive facies analysis of the host carbonates.We will show that the taxon identified by Sar19 as Keramosphaera iranica instead is assigned to the Orduella spherica Sirel, whereas the identification of Haymanella paleocenica (elongata) from NE India remains unverified. We will also show how, contrary to what is claimed by Sar19, the paleoecological model proposed is not substantiated by data. Finally, we will argue that, in the Paleocene and lower Eocene of central and eastern Neo-Tethys, the identification of a biogeographic region must be regarded with a focus on the faunal exchanges established within the so-called ‘Lockhartia Sea’.

Sea level, carbonate mineralogy, and early diagenesis controlled δ13C records in Upper Ordovician carbonates

Abstract Stratigraphic variability in the geochemistry of sedimentary rocks provides critical data for interpreting paleoenvironmental change throughout Earth history. However, the vast majority of pre-Jurassic geochemical records derive from shallow-water carbonate platforms that may not reflect global ocean chemistry. Here, we used calcium isotope ratios (δ44Ca) in conjunction with minor-element geochemistry (Sr/Ca) and field observations to explore the links among sea-level change, carbonate mineralogy, and marine diagenesis and the expression of a globally documented interval of elevated carbon isotope ratios (δ13C; Hirnantian isotopic carbon excursion [HICE]) associated with glaciation in Upper Ordovician shallow-water carbonate strata from Anticosti Island, Canada, and the Great Basin, Nevada and Utah, USA. The HICE on Anticosti is preserved in limestones with low δ44Ca and high Sr/Ca, consistent with aragonite as a major component of primary mineralogy. Great Basin strata are characterized by lateral gradients in δ44Ca and δ13C that reflect variations in the extent of early marine diagenesis across the platform. In deep-ramp settings, deposition during synglacial sea-level lowstand and subsequent postglacial flooding increased the preservation of an aragonitic signature with elevated δ13C produced in shallow-water environments. In contrast, on the mid- and inner ramp, extensive early marine diagenesis under seawater-buffered conditions muted the magnitude of the shift in δ13C. The processes documented here provide an alternative explanation for variability in a range of geochemical proxies preserved in shallow-water carbonates at other times in Earth history, and challenge the notion that these proxies necessarily record changes in the global ocean.

Lower Jurassic benthic foraminiferal assemblages from shallow-marine platform carbonates of Mallorca (Spain): stratigraphic implications

The stratigraphic distribution of larger benthic foraminifera and other microfossils, as analysed in thin-sections, has been investigated in three stratigraphic sections from the island of Mallorca (Son Maina, S’Heretat and Cuevas de Arta). These stratigraphic sections represent deposition in a Lower Jurassic shallow-water carbonate platform succession (Es Barraca Member, Soller Formation) of the Llevant Mountains. The microfossil assemblages contain abundant benthic foraminifera and calcareous algae, including the typical Liassic dasycladalean alga Palaeodasycladus mediterraneus (Pia) and the microproblematica Thaumatoporella parvovesiculifera (Raineri), which provide good age constraint of the succession. Three consecutive biozones have been recognized based on the stratigraphic distribution of foraminifera, including the evolutionary bioseries Mesoendothyra—Lituosepta—Orbitopsella spp., which are documented for the first time in the Balearic Basin: Biozone A (interval Zone) is characterized by the occurrence of small Siphovalvulina spp., Mesoendothyra sp. and rare Lituosepta ancestors. Biozone B (lineage Zone), whose lower boundary is defined by the first occurrence of Lituosepta recoarensis Cati and its top by the first appearance of Orbitopsella aff. primaeva (Henson). The uppermost part of biozone B shows an interval with transitional morphotypes between Lituosepta and Orbitopsella, indicating that the change between biozones B and C1 is gradual (B/C1 transition). This interval is characterized by the presence of very primitive forms of Orbitopsella. Finally, Biozone C1 (lineage Zone) is defined by the first occurrence of Orbitopsella aff. primaeva together with L. recoarensis. The upper boundary of the Biozone C1, which is defined by the first occurrence of O. praecursor s.l., is not recorded in the studied sections of the Es Barraca Member. This biostratigraphic zonation is consistent with a Sinemurian age for the Es Barraca Member with its top most likely not extending into the Pliensbachian. The proposed biostratigraphic scheme is comparable with those established for other western Tethyan margins such as the High Atlas of Morocco, the Southern Italian Alps or the Dinaric Alps of Slovenia and Croatia. The reconstruction of a depositional transect across the studied sections shows the progressive loss of the upper biozones towards the northeast, demonstrating the existence of significant hiatuses in some sections of the Llevant Mountains domain. These hiatuses point to an intra-lower Pliensbachian (Carixian) early stage of platform fragmentation, with areas affected by erosion or non-deposition, and areas with marine marly limestone sedimentation followed by deltaic siliciclastic progradation on the north-western part of Mallorca.

Intensified Ocean Deoxygenation During the end Devonian Mass Extinction

AbstractThe end‐Devonian mass extinction (~359 Ma) substantially impacted marine ecosystems and shaped the roots of modern vertebrate biodiversity. Although multiple hypotheses have been proposed, no consensus has been reached about the mechanism inducing this extinction event. In this study, I/Ca ratio of carbonate was used to unravel the changes in local oxygen content of the upper water column during this critical interval. The Devonian‐Carboniferous boundary was recorded in two shallow water carbonate sections in south China. I/Ca values at both locations show a clear decline in the Middle and Upper Siphonodella praesulcata conodont zones, which coincides with a positive shift in carbonate carbon isotope composition and a negative shift in nitrogen isotope composition. These results suggest that deoxygenation was intensified during this critical interval, which likely influenced shallow water habitats, lending support to the notion that oxygen deficiency likely was a direct mechanism impacting the end‐Devonian mass extinction.

Geological and structural map of the southeastern Pag Island, Croatia: field constraints on the Cretaceous - Eocene evolution of the Dinarides foreland

The sedimentary succession exposed in the Northern Dalmatia Islands mainly consists of Cretaceous to Neogene shallow water carbonates, folded and imbricated within the External Dinarides thrust belt. During Cretaceous times, carbonate sediments were deposed on a heterogeneous, tectonically-influenced carbonate platform, which was then uplifted and eroded, as evidenced by a regional unconformity embracing the Late Cretaceous and Paleocene. Sedimentation resumed during the Eocene, when the area was part of the foreland basin of the Dinaric belt. With our geological and structural map of the southeastern Pag Island at the 1:25,000 scale, we refined the stratigraphic and structural setting and the tectono- sedimentary evolution of the area.

Structural diagenesis of shallow platform carbonates: Role of early embrittlement on fracture setting and distribution, case study of Monte Alpi (Southern Apennines, Italy)

Bed-perpendicular, opening-mode diffuse fractures are common features in carbonate rocks because they become brittle during the first stages of diagenesis. Early-fractures could be independent of tectonics and form a background structural network from sub-millimeter to 10's of centimetres scale. This study focuses on the outcrop-to-micro scales structural, stratigraphic, and petrographic characterization of Lower Cretaceous, shallow-water, tight limestones of the Inner Apulian Platform exposed along the axial sector of the southern Apennines fold-and-thrust belt, Italy. This work aims at understanding the formation of early diagenetic fractures during the first stages of sediment lithification, and on their impact on subsequent deformation mechanisms associated to polyphase tectonics. The results of field structural surveys, of petrographic and cathodoluminescence analyses of representative samples unravel the main structural-diagenetic processes in shallow-water carbonates. We document that early brittle fractures were intrinsically related to the host rock pore type and cementation processes, and summarize the overall time-dependant evolution of the structural assemblages in a conceptual model. This model includes all abutting and crosscutting relationships among the various structural elements from field to thin-section observation, and form the now days fracture network.