Carbonate Mud Research Articles

Mesoproterozoic Molar Tooth Structure Related to Increased Marine Oxygenation

AbstractMarine carbonate fabrics are intrinsically related to ocean chemistry, physical processes and biological activity. Molar tooth structure (MTS), a globally distributed structure in Proterozoic carbonate sediments, has been widely studied for more than a century; yet its connections with physical and biological processes remain unclear. Using multiple techniques, we studied ∼1.57 Ga MTS and identified a connection between its occurrence and increased marine oxygenation. In our samples, the matrix surrounding MTS is typically dominated by carbonate mud with early diagenetic dolomite crystals. High I/(Ca + Mg) ratios (up to 4.1 μmol/mol) and negative Ce anomalies (∼0.8) detected in the matrix are consistent with the oxidative removal of inhibitors such as Fe2+ and Mn2+ in the water‐column that permitted carbonate “whiting” mud precipitation stimulated by cyanobacterial photosynthesis. This cohesive but not rigid seafloor carbonate mud was a prerequisite for synsedimentary MTS crack formation. Systematically higher carbon isotope (δ13C) values in MTS microspars, relative to host sediment, support origination of the cracks by methane degassing in the organic‐rich carbonate mud. Low, but non‐zero, I/(Ca + Mg) values of the MTS microspar suggest that the precipitation of the microspar that filled the MTS cracks was triggered by oxidative removal of residual Fe2+ and Mn2+ in porewater through mixing with overlying oxygenated seawater. We therefore propose that MTS formed under moderately oxygenated conditions and that its sporadic occurrence prior to ∼1.2 Ga reflects episodes of pulsed marine oxygenation in an overall anoxic setting.

Evidence of upwelling on the Brazilian continental shelf of Rio Grande do Norte

This study evaluates Foraminiferal assemblages and their relation with grain size, calcium carbonate content, organic matter, and mineralogy of sediment samples collected at the sediment-water interface along a transect on the northern continental shelf of Rio Grande do Norte State, Brazil, adjacent to the city of Areia Branca. The sedimentary sequence of this shelf is represented by lithostratigraphic units of a marine regressive sequence dominated by four facies: Siliciclastic sand, Silicibioclastic Sand, carbonate mud, and Biosiliciclastic sand. The carbonate content ranged from 5.83% to 85% and the organic matter content from 1.16% to 27.05%. Mineralogical characters separated the predominant siliciclastic content (37% to 92%) from the bioclastic content (8% to 63%). We have identified 14 species out of 50 species of Foraminifera, associated to particular depths and sediment types as follows: (1)deeper-water sediments in the middle shelf contain Bolivina striatula, Bulimina marginata, Triloculina trigonula, Pyrgo ringens, Textularia gramen (2)the shallowest sediments in the inner shelf contain Ammonia tepida, Buccella peruviana, Miliolinella subrotunda, and Quinqueloculina patagonica, (3)the central parts of the transect, also in the inner shelf, provide habitats for Quinqueloculina lamarckiana, Textularia earlandi, Buliminella elegantissima, Discorbis peruvianus, and Pyrgo nasuta. The distribution of Uvigerina striata and Buccella peruviana is probably related to colder water temperatures and possibly the occurrence of an upwelling phenomenon for the deepest parts of the area rather than the sedimentological features discussed here.

Physical and Rheological Characteristics of Sediment for Nautical Depth Assessment in Bushehr Port and Its Access Channel

Sedimentation in ports and waterways covered with fine deposits is a significant challenge in harbor management. The top layer of the bed in such areas typically consists of fluid mud, for which dredging is complicated and less efficient. The goal of this paper is to investigate physical and rheological characteristics of sediment for nautical depth assessment in Bushehr Port and its access channel. In this study the fluid mud layer was detected by hydrographic surveys with a dual-frequency echo sounder. Moreover, sediment properties in various parts of the channel and port were analyzed through a comprehensive sediment sampling in the field and complementary laboratory studies, including sediment grain-size analysis and distribution, carbonate and organic matter content, rheometry, and consolidation and settling tests. It was found that water contents and concentration, and clay-size fractions are the most important factors in rheological characteristics of sediment in the study area. The results indicated that the clay-size fraction in the surficial bed was between 18 and 31%, which categorized it as fine and cohesive sediment. In terms of mineralogy, the sediment was mostly carbonate mud with carbonate content between 52.9 and 57.2%. The results showed that the sediment concentration and yield stress in most samples were lower than 1030 kg/m3 and 123 Pascals, respectively. Based on the hydrographic surveys and obtained sediment characteristics, it is concluded that the nautical bottom approach can be practically implemented in the Bushehr Port and its access channel.

More gaps than record! A new look at the Pliensbachian/Toarcian boundary event guided by coupled chemo-sequence stratigraphy

The Pliensbachian/Toarcian boundary (Pl/To) event precedes by ca. 1 Myr the onset of the Toarcian Oceanic Anoxic Event. It corresponds to a second order mass extinction associated with an outstanding collapse of shallow marine ecosystems at global scale. Yet, our knowledge about its exact driver(s) and unfolding is relatively ambiguous due to the numerous hiatuses present in the sedimentary record during this critical time interval. In this study, an integrated carbon isotope chemostratigraphy and sequence stratigraphy approach is applied to two case studies (the upper Pliensbachian in South-East France and the Pliensbachian–Toarcian transition in Morocco) to demonstrate how the major changes in sea-level and sedimentation supply accompanying the Pl/To event led to the formation of ubiquitous, often cryptic hiatal surfaces in the sedimentary record. Hence, as a consequence of strongly progradational stacking pattern during the latest Pliensbachian related to a global sea-level lowstand associated with cold greenhouse climate, proximal settings were characterized by bypass and/or erosion, inducing an incomplete record of the Spinatum chronozone in localities situated in the outer part of sedimentary basins. In the earliest Toarcian, the collapse of the neritic carbonate factory led to a halt of carbonate mud export into the basin, resulting in sediment starvation in most basins characterized by a carbonate-dominated sedimentation regime before the environmental perturbation. Only localities where vigorous siliciclastic sediment supply took over are likely to have a more complete sedimentary record of the immediate aftermath of the carbonate production collapse. This combination of causes explains the ubiquitous incompleteness of the record of the Pliensbachian/Toarcian transition in numerous European localities where the bulk of our current understanding about the Pl/To event derives from. A comparison between the two known most expanded and complete records of the Pliensbachian–Toarcian transition of the Llanbedr (Mochras Farm) core in Wales and Bou Oumardoul n'Imazighn section in Morocco shows that the onset of the environmental perturbations is associated with a positive carbon isotope excursion spanning the Pliensbachian/Toarcian boundary. This is followed by a negative carbon isotope excursion during the earliest Toarcian that coincides with the global collapse of neritic carbonate factory and an ample sea-level fall.

A re-examination of the mechanism of whiting events: A new role for diatoms in Fayetteville Green Lake (New York, USA).

Whiting events-the episodic precipitation of fine-grained suspended calcium carbonates in the water column-have been documented across a variety of marine and lacustrine environments. Whitings likely are a major source of carbonate muds, a constituent of limestones, and important archives for geochemical proxies of Earth history. While several biological and physical mechanisms have been proposed to explain the onset of these precipitation events, no consensus has been reached thus far. Fayetteville Green Lake (New York, USA) is a meromictic lake that experiences annual whitings. Materials suspended in the water column collected through the whiting season were characterized using scanning electron microscopy and scanning transmission X-ray microscopy. Whitings in Fayetteville Green Lake are initiated in the spring within the top few meters of the water column, by precipitation of fine amorphous calcium carbonate (ACC) phases nucleating on microbial cells, as well as on abundant extracellular polymeric substances (EPS) frequently associated with centric diatoms. Whiting particles found in the summer consist of 5-7μm calcite grains forming aggregates with diatoms and EPS. Simple calculations demonstrate that calcite particles continuously grow over several days, then sink quickly through the water column. In the late summer, partial calcium carbonate dissolution is observed deeper in the water column. Settling whiting particles, however, reach the bottom of the lake, where they form a major constituent of the sediment, along with diatom frustules. The role of diatoms and associated EPS acting as nucleation surfaces for calcium carbonates is described for the first time here as a potential mechanism participating in whitings at Fayetteville Green Lake. This mechanism may have been largely overlooked in other whiting events in modern and ancient environments.

The origin of carbonate mud and implications for global climate

Carbonate mud represents one of the most important geochemical archives for reconstructing ancient climatic, environmental, and evolutionary change from the rock record. Mud also represents a major sink in the global carbon cycle. Yet, there remains no consensus about how and where carbonate mud is formed. Here, we present stable isotope and trace-element data from carbonate constituents in the Bahamas, including ooids, corals, foraminifera, and algae. We use geochemical fingerprinting to demonstrate that carbonate mud cannot be sourced from the abrasion and mixture of any combination of these macroscopic grains. Instead, an inverse Bayesian mixing model requires the presence of an additional aragonite source. We posit that this source represents a direct seawater precipitate. We use geological and geochemical data to show that "whitings" are unlikely to be the dominant source of this precipitate and, instead, present a model for mud precipitation on the bank margins that can explain the geographical distribution, clumped-isotope thermometry, and stable isotope signature of carbonate mud. Next, we address the enigma of why mud and ooids are so abundant in the Bahamas, yet so rare in the rest of the world: Mediterranean outflow feeds the Bahamas with the most alkaline waters in the modern ocean (>99.7th-percentile). Such high alkalinity appears to be a prerequisite for the nonskeletal carbonate factory because, when Mediterranean outflow was reduced in the Miocene, Bahamian carbonate export ceased for 3-million-years. Finally, we show how shutting off and turning on the shallow carbonate factory can send ripples through the global climate system.

Cable bacteria activity and impacts in Fe and Mn depleted carbonate sediments

The metabolic activities of cable bacteria enable the oxidation of sulfide to sulfate in anoxic sediment at depth through long distance electron transport coupled to the reductions of O2 and NO3−/NO2− in surface sediment. The spatial separation of oxidation and reduction half reactions requires modification of diagenetic models that assume electron donors and acceptors are coupled with each other locally within sedimentary deposits. In this study, we show that cable bacteria can become established and remain metabolically active in sulfidic, but Fe, Mn, and solid phase sulfide-depleted, carbonate muds from Florida Bay, USA. Sediment surface pH maxima reflecting electrogenic metabolism developed between 1 and 3 weeks in sediment incubated in laboratory microcosms with oxic overlying water. Cable filament cell abundances varied over time, with the increase initially focused within the subsurface, but by the end of experiments, cell abundances increased both in subsurface and oxic surface sediment. The development of cable bacteria activity in carbonate muds demonstrates that long-distance electron transport metabolism can utilize subsurface dissolved sulfide (e.g., H2S) as a sole reductant source for sustained periods without transition to Fe-sulfide and apparently without formation of an intermediate suboxic region associated with Fe and Mn cycling. H2S consumption occurs initially within the O2-NO3− rich oxic – suboxic zone and transitions to a dominant, but not exclusive, subsurface cable bacteria anodic zone. Once established, cable bacteria significantly altered sediment carbonate cycling. Thermodynamic calculations demonstrated that aragonite and hi-Mg-calcites were supersaturated (Ω ∼2.0–3.2) in the surface sediment layer (< 0.25 cm depth) and were undersaturated (Ω ∼ 0.2) in the anodic zone (1–2 cm depth). Hi-Mg-calcites and aragonite likely precipitated in the surface sediment and dissolved in the anodic region, the dissolution confirmed by elevated porewater Ca2+, Mg2+ concentrations. P cycling was coupled to remineralization of organic matter, carbonate mineral cycling, and apparently intracellular accumulation of polyphosphates. Thus, cable bacteria may play an important role in the acquisition of P by seagrasses in carbonate deposits where P is otherwise limited by irreversible adsorption and authigenic mineral precipitation.

Fault-Block Platform Evolution between Late Cretaceous and Early Miocene along the Margin of the Latium-Abruzzi Carbonate Platform (Southern Prenestini Mountains, Central Apennines, Italy)

Since the origin of the carbonate platform concept, the reconstruction of the geometry and the evolution of the margin has been an intriguing topic. The Latium-Abruzzi platform is one of the largest shallow water domains of the Central Apennines, however, the reconstruction of the geometry and evolution of its margin has been classically problematic because the latter has been erased by the out-of-sequence Olevano-Antrodoco thrust system. The investigated area is placed in the Prenestini Mountains, the southernmost portion of the Sabina Domain, where a Cretaceous to Neogene carbonate succession, ascribed the Latium-Abruzzi platform margin, crops out. Stratigraphic and facies analyses showed a Late Cretaceous extensional stage that produced two main fault-block platforms, respectively, South Eastern Prenestini and South Western Prenestini platforms, developed westward of the large Latium-Abruzzi Platform domain. In these platforms, during the Paleocene–Eocene interval, pelagite deposition coincides with the main relative sea-level rise. Instead, during the following falling stage, the seafloor, consisting of the Cretaceous carbonate, was in the wave abrasion zone and the pelagic carbonate mud was swept away or trapped in local depressions or fractures (neptunian dykes), whereas the coarse sediment produced by the erosion of indented and fractured substrate formed polygenic conglomerate accumulation on the Cretaceuos carbonate platform substrate. Successively, an isolated homoclinal ramp, the Guadagnolo Fm, Aquitanian to Serravallian in age, developed on the drowned fault-block platforms, suggesting that during Neogene the articulation of the substrate of the two fault-block platforms had to be limited to host the bioclastic sedimentation of the Guadagnolo Fm and to allow the development of a carbonate ramp depositional profile.

The Middle Triassic vertebrate deposits of Kupferzell (Germany): Palaeoenvironmental evolution of complex ecosystems

In 1977, within 3 months of excavation, a 500 m spanning road-cut near Kupferzell (southern Germany) produced a total of ~30,000 vertebrate remains from the Middle Triassic Lower Keuper. The bulk of the material stems from two temnospondyl amphibians, Gerrothorax pulcherrimus (~70%) and Mastodonsaurus giganteus (~30%), with the pseudosuchian archosaur Batrachotomus kupferzellensis ranging first among the other remains. Analyses of data collected during excavation, supplemented by new fieldwork, provide rich information on the sedimentary setting as well as the development of the fauna and their ecosystems. The sequence consists of: basal coaly mudstones (K1), massive siltstones (K2), green siliciclastic mudstones (K3), yellow to pale brown marlstones (K4), and yellow massive dolostones (K5). The deposits comprise a succession of similar water bodies that were emplaced on a lacustrine to floodplain setting in which carbonate muds dominated. Two main lake systems, with intermittent/periodical marine influence, as well as relatively stark periods of drought, harboured complex vertebrate ecosystems. These were dominated by a relatively high diversity of fishes and temnospondyl amphibians with a lesser, but notable, presence of archosaurs, forming the top predators of the trophic web. The sequence records alternating periods of flooding and desiccation, shaping a relatively complex environmental setting that was likely prone to the presence of life, eventually becoming an exceptional fossil lagerstätte. Kupferzell, together with nearby contemporaneous localities, represent relatively diverse and complex ecosystems (including several top predators) that allow understanding the evolution and palaeoecology of Middle Triassic vertebrate communities, including the groups that diversified during the Mesozoic era.

High-resolution taphonomy and sequence stratigraphy of internally complex, bakevelliid-dominated coquinas from the Aptian Romualdo Formation, Araripe Basin, NE Brazil

The Aptian Romualdo Formation (Araripe Basin, NE Brazil) was deposited in a restricted epeiric sea, during the opening of the South Atlantic Ocean, with a coeval record to that in the Brazilian Pre-Salt succession. The upper part of the Romualdo Formation encompasses the Highstand Systems Tract of a third-order stratigraphic sequence, and contains shell beds interbedded with shale, siltstone, and fine-grained sandstone. Based on distinct taphonomic features (shell sorting, fragmentation, abrasion, rounding, and orientation), sedimentological criteria (shell abundance, type of matrix, sedimentary structures, and presence/absence of quartz grains), and stratigraphic attributes (bed thickness, geometry, contacts), the paleoenvironmental conditions and the bed-by-bed accumulation history of the coquinas is disentangled. Eight carbonate microfacies were recognized, including ostracod carbonate mudstone, bivalve-gastropod wackestone, gastropod-bivalve packstone, bivalve-gastropod grainstone, bivalve-gastropod floatstone, bivalve-gastropod rudstone, bakevelliid floatstone, and bakevelliid rudstone, which are organized in six, cm-thick shell concentrations. Mudstone and wackestone facies mark the settling of carbonate muds without significant bottom currents and wave action, probably below storm wave base, whereas packstone, floatstone, and rudstone, with fragmented, rounded, and oriented shells, were deposited in agitated waters, between the fair-weather and the storm wave bases. Ripple cross-laminated grainstone is interpreted as high-energy facies deposited above fair-weather wave base. The shell beds are internally complex and consist of cm-scale alternations of microfacies differing in packing, sorting, and composition. They tend to fine upwards within dm-thick shell beds, generating shallowing-upward facies sequences. The concentrations, the uppermost one with in situ bakevelliid shells in the top, record high-frequency base level oscillations influenced by eustasy and climate changes. These bioclastic accumulations originated by the superposition of sedimentologic and primary biologic processes (e.g., post-deposition meiofaunal bioturbation). The cm-thick, low-diversity shell beds are excellent examples of multiple-event carbonate deposits generated in a siliciclastic-dominated, restricted epeiric sea, revealing high-frequency cycles in proximal to distal restricted marine settings.

Decoupling of Pore Water Chemistry, Bacterial Community Profiles, and Carbonate Mud Diagenesis in a Land-Locked Pool on Aldabra (Seychelles, Indian Ocean)

Comparative pore water-sediment studies are crucial to trace biogeochemical and early diagenetic interactions between mineral and fluid phases. They are commonly conducted in stable environments with long-time diagenetic reactions between sediment and pore water. In contrast, this work presents a comparative study within the Cinq Cases pool system, a shallow and saline water body, with occasional marine influx and short-time diagenetic reactions between sediment and pore water. Cinq Cases is located on the Aldabra atoll in the western Indian Ocean. Sediments span ca. 3800 years and indicate three environmental stages: (i) An initial, at least temporary anoxic, palustrine environment, including meteoric diagenesis (Unit III), (ii) slow marine flooding, with cyanobacteria and sponge blooms (Unit II), and (iii) lagoon flooding, including oxic conditions within the sediment (Unit I). Such frequent changes in diagenetic environments of small water bodies can be retraced by three different proxies representing partially overlapping but different time scales: (i) sediments reflect ancient processes (ii) pore waters are influenced by recent processes, and (iii) bacterial communities reflect an overlay of ancient and recent processes.

Vertebrate ichnofauna from Sierra de las Cabras tracksite (Late Miocene, Jumilla, SE Spain). Mammalian ichnofauna

Upper Miocene sedimentary deposits of Sierra de las Cabras in the Prebetic Ranges (Jumilla, Región de Murcia, Spain) yield a exceptional fossil site of diverse vertebrate ichnofauna, including mammals and birds. This paper presents a detailed study of those ichnites of that site which correspond to mammals. The footprints are located in stratigraphic surfaces found in a thin marly limestone succession that can be assigned to the latest Tortonian to earliest Messinian. The mammalian record consists of 324 footprints (included in trackways, pairs, groups and isolated ichnites) of diverse vertebrates, including: Hippipeda, Rhynoceripeda, Canipeda, Felipeda, Ursipeda, Rodentipeda, as well possible Suipeda. The ichnites were imprinted on wet and cohesive marly carbonate mud, deposited in a semi-arid wetland with relevant freshwater influx. The fossil site, cleaned and protected some years ago but herein studied by the first time, add together the nearby Hoya de la Sima site (Hippipeda, Pecoripeda, Bestiopeda and Paracamelichnium, and probably proboscidean ichnites) to conform an exceptional record of Late Miocene ichnofauna in southern Iberia.

Temporal and spatial variations in calcium carbonate deposition in a mixed siliciclastic–carbonate deep marine system: the Ediacaran Deeside Limestone Formation, Aboyne, Scotland

The Deeside Limestone Formation (DLF), outcropping in western Aberdeenshire, comprises the eastern margin of a distinctive Upper Dalradian mixed siliciclastic–carbonate system of Ediacaran age which may be correlated southwestwards for more than 400 km to County Donegal in Ireland. A reconstructed stratigraphic column suggests three broad vertical sequences (S1–S3), each comprising a general upward proportionate increase in calcium carbonate with respect to quartz, locally capped by metalimestones (L1–L3). A simple explanation for this upward change lies in the differing physical response of the two heterolithic components under the same hydrodynamic process, with the heavier quartz grains preferentially enriching the bedload and the finer carbonate mud fraction, the suspended load. The three metalimestone intervals are observed only in central/eastern parts of the field area, suggesting a lateral facies variation. The predominance of calcareous–siliciclastic bed-scale compositional mixing within the DLF suggests sedimentation on the Dalradian shelf comprised contemporaneous mixing of the siliciclastic and carbonate fractions prior to subsequent re-mixing during transportation downslope into the deep water, punctuated by pulses of point-sourced siliciclastic input.

Fine-grained distal deposits of a mixed siliciclastic–carbonate marine system: Origin of mud and implications on mixing processes

ABSTRACT Mixed siliciclastic–carbonate sediments in the distal settings of marine systems (i.e., offshore and basin areas) are the result of the complex interaction between production of benthic and pelagic biogenic carbonates and the exportation of terrigenous, and/or carbonate mud from shallow-marine settings. Thus, the origin of mud (particles &amp;lt; 63 mm) of different compositions and the processes responsible for mixing them play a key role in the final distal deposits of mixed systems. Although the understanding of these topics in shallow settings has made significant progress, one of the critical issues is to recognize them in the offshore and basinal settings for understanding the link between shallow-marine and deep-marine areas. In this study, a thick Lower Cretaceous succession (330–650 m) of the Neuquén Basin, composed mostly of mixed (siliciclastic–carbonate) fine-grained marine deposits, was studied through a high-resolution analysis of petrography, SEM analysis, and mineralogy, to define and characterize the origin of carbonate and terrigenous mud as well as to characterize the processes involved in the mixing of terrigenous and carbonate mud on a storm-dominated ramp. Contrary to previous suggestions, most of the carbonate recognized in the basinal setting is composed of silt- to sand-size skeletal remains and micrite (micarbs) and is likely derived from a pelagic biogenic factory combined with carbonate derived from the maceration of benthic organism. Furthermore, micarbs are possibly diagenetic in origin but with a distally produced carbonate precursor. In contrast, the siliciclastic fraction is entirely detrital, being silt-size quartz particles and clay minerals (mainly illite) the most important constituents. Most of the terrigenous mud delivered from shallow areas acts to dilute the distally produced carbonate mud. However, a bloom of carbonate production able to dilute terrigenous supply is recorded, and its origin is likely triggered by the input of waters charged with nutrients from the paleo–Pacific Ocean in addition to nutrients derived from proto-Andes volcanic activity. This demonstrates that the relation between the terrestrially derived siliciclastic components and the biogenically produced carbonates in mixed systems is not linear, but a complex interaction of climate, sea-level, origin of the components, and hydrography of the basin, among others.

Early Mesoproterozoic Ca-carbonate precipitates record fluctuations in shallow marine oxygenation

Oxygenation, a key control on marine life, has a long and varied history. This is well seen in the early Mesoproterozoic. During this critical period, midway between the Great Oxidation (GOE) and Neoproterozoic Oxidation (NOE) events, low marine levels of pulsed oxygenation - likely driven by fluctuations in cyanobacterial productivity - alternated with episodic anoxia. However, since baseline oxygen levels were low, deciphering the scale and variability of redox evolution during this prolonged interval is challenging. This in turn has made it difficult to confidently identify key underlying controls on redox evolution. Combining sedimentological and geochemical methods, we studied shallow ∼1.57 Ga shelf carbonates with contrasting depositional fabrics. Carbonate I/(Ca + Mg) data indicate that (i) carbonate mud precipitated in a weakly oxygenated water column, whereas (ii) seafloor crystal fans formed in more poorly oxygenated seawater, containing Fe2+ and Mn2+. We infer that in (i) weakly oxygenated (hypoxic-oxic) conditions reflect increased planktic cyanobacterial productivity that, in the absence of carbonate precipitation inhibitors (Fe2+ and Mn2+), promoted water column carbonate mud (‘whiting’) precipitation. At the same time, aerobic degradation of settling organic matter lowered seafloor CaCO3 saturation. In contrast, in (ii) more poorly oxygenated seawaters containing Fe2+ and Mn2+ inhibited calcite precipitation but permitted aragonite which formed seafloor crystal fans. Together, these contrasting precipitates indicate relatively rapid pulsed redox fluctuations. These were relatively subtle, but sufficient to result in contrasting carbonate sediments, readily recognizable in the field. We hypothesize that cyanobacterial productivity, possibly linked to the availability of nutrients such as phosphate, was a key influence on water column redox cyclicity during the Mesoproterozoic.

Planktonic Biota Constituents Responses to Global Sea-Level Changes Recorded in the Uppermost Albian to Middle Cenomanian Deep-Water Facies of the Outer Carpathians

Interpretations of sea-level changes over geological time are mainly based on analyses of sediments deposited within stable platforms. One of the criteria for recognizing these changes is the composition of skeleton-bearing protists living in environments close to the shoreline and on the continental shelf. It can be assumed that the source of information about sea-level changes may also be found in assemblages of microfossils redeposited from the shelf to the marginal ocean basin. With such an assumption, this article presents an interpretation of changes in marine plankton communities (radiolarians and planktonic foraminifera), which during late Albian–middle Cenomanian were redeposited with carbonate mud by gravity currents from the peri-Tethyan shelf to the northern, marginal Silesian Basin of the Western Tethys. Planktonic biota identified in sediments deposited by gravity flows and those found in hemipelagic shales separating them, indicate that their composition may be correlated with eustatic changes in the sea level from the late Albian (KAl8) to middle Cenomanian (KCe3).

Lithostratigraphy and microfacies analysis of Avanah Formation (Middle Eocene) in Gomaspan section northeast Erbil City, Kurdistan region, Iraq

Lithostratigraphy and microfacies analysis of the Avanah Formation (Middle Eocene) were studied in the Gomaspan section in the Bina Bawi anticline, northeast of Erbil city, Kurdistan Region, Iraq. The field observations refer that the formation attains 56 m of medium to thick bedded yellow limestone, grey dolomitic limestone and blue marly dolomitic limestone interbedded with thin beds of blue marl and dark grey shale with an interval of sandy limestone in the middle part and thin to medium bedded limestone interbedded with red mudstone. The petrographic study of 29 thin sections of Avanah carbonates revealed that the majority of the matrix is carbonate mud (micrite) with few microspar. The skeletal grains include benthic foraminifera, dasycladacean green algae, ostracods, calcispheres, pelecypods, rare planktonic foraminifera and bryozoa in addition to bioclasts. Non-skeletal grains encompass peloids, oncoids, intraclasts and extraclasts with common monocrystalline quartz. Based on the field observation and petrographic analysis, three different lithostratigraphic units were identified. They are in ascending order: A-Thick bedded dolomitic marly limestone interbedded with shale. B- Bedded interbedded with red mudstone. Depending on detailed microfacies analysis of carbonate rocks, three main microfacies and 12 submicrofacies are recognized. From the sum of all petrographic, facies, textural analyses, it is concluded that Avanah Formation in Gomaspan section, was deposited in shallow marine environment, semi restricted lagoon, in lower and upper parts and open lagoon environment in the middle part interval.

Lithostratigraphy and microfacies analysis of Avanah Formation (Middle Eocene) in Gomaspan section northeast Erbil City, Kurdistan region, Iraq

Lithostratigraphy and microfacies analysis of the Avanah Formation (Middle Eocene) were studied in the Gomaspan section in the Bina Bawi anticline, northeast of Erbil city, Kurdistan Region, Iraq. The field observations refer that the formation attains 56 m of medium to thick bedded yellow limestone, grey dolomitic limestone and blue marly dolomitic limestone interbedded with thin beds of blue marl and dark grey shale with an interval of sandy limestone in the middle part and thin to medium bedded limestone interbedded with red mudstone. The petrographic study of 29 thin sections of Avanah carbonates revealed that the majority of the matrix is carbonate mud (micrite) with few microspar. The skeletal grains include benthic foraminifera, dasycladacean green algae, ostracods, calcispheres, pelecypods, rare planktonic foraminifera and bryozoa in addition to bioclasts. Non-skeletal grains encompass peloids, oncoids, intraclasts and extraclasts with common monocrystalline quartz. Based on the field observation and petrographic analysis, three different lithostratigraphic units were identified. They are in ascending order: A-Thick bedded dolomitic marly limestone interbedded with shale. B- Bedded dolomitic limestone interbedded with shale and marl. C- Thin to medium bedded limestone interbedded with red mudstone. Depending on detailed microfacies analysis of carbonate rocks, three main microfacies and 12 submicrofacies are recognized. From the sum of all petrographic, facies, textural analyses, it is concluded that Avanah Formation in Gomaspan section, was deposited in shallow marine environment, semi restricted lagoon, in lower and upper parts and open lagoon environment in the middle part interval.

Transformation mechanism of muddy carbonate rock by the coupling of bioturbation and diagenesis: A case study of the Cretaceous of the Mesopotamia Basin in the Middle East

The transformation mechanism of muddy carbonate rock by the coupling of bioturbation and diagenesis was studied based on core, cast thin section and physical property data of Cretaceous strata in the Mesopotamia Basin, the Middle East. There are 3 ways of biological transformation of rocks: (1) The living creatures transformed formations mechanically to make the rocks looser and intergranular pores increase. (2) After formation, burrows were backfilled with coarse-grained debris, and then unsaturated fluid infiltrated into the burrows during the penecontemporaneous period, resulting in dissolution. (3) Chemical alteration occurred in abandoned burrows and dolomitization produced a large number of intercrystalline pores. The coupling of bioturbation and dissolution occurred mainly in the penecontemporaneous phase, and was controlled by rock type, sea level decline, burrow abundance, infillings, and water environment etc. As the burrows had better physical properties than the matrix, unsaturated fluid preferentially migrated along the burrows, leading to dissolution and expansion of the burrows first and then dissolution of the matrix. The coupling of bioturbation and dolomitization occurred mainly in the burial phase. The rich organic matter and reducing bacteria in the burrow provided material basis, reducing conditions and alkaline environment for dolomitization. The metasomatism in the penecontemporaneous period had little effect on the physical properties of the burrows. When the burrows were separated from the deposition interface, equimolar metasomatism occurred in the burrows in a closed environment, forming euhedral fine-crystalline dolomite with intercrystalline pores. The transformation degree of bioturbation to muddy carbonate reservoir depends on rock type, density, connectivity, infillings and structure of the burrows. With the increase of the carbonate mud content, the improvement to rock physical properties by bioturbation becomes more prominent. When the burrows are filled with coarse-grained debris or fine-crystalline dolomite, the greater the density, the higher the connectivity, and the lower the tortuosity of burrows, the better the physical properties of the muddy carbonate rocks are.

Characteristics and controls on the distribution of sublittoral microbial bioherms in Great Salt Lake, Utah: Implications for understanding microbialite development

AbstractSide‐scan sonar and Compressed High Intensity Radar Pulse mapping of Great Salt Lake, Utah, linked to reprocessing of acoustic data from bathymetric surveys, has enabled the distribution of microbial bioherms to be assessed. Bioherms occupy an estimated area &gt;700 km2 in the south arm and &gt;300 km2 in the north arm. Distributions vary from statistically dispersed to clustered, and in this latter case, are predominantly located on metre‐scale, fault‐controlled topographic highs, with sediment infilling intervening lows between adjacent offsets. Individual bioherms are circular to oblate and range from centimetres to over 2 m in diameter. In some areas, bioherm heights were measured at more than 1.5 m above adjacent substrate. Sublittoral bioherms are made of aragonite, calcite and minor dolomite precipitated due to physico‐chemical, biologically induced and influenced carbonate mineralization processes in association with microbial mats. Bioherm fabrics vary at the millimetre to centimetre‐scale and consist of leiolitic and clotted peloidal micrite‐grade carbonate, sinuous threads of spherulitic fibrous aragonite crystals, laminated micrite boundstone and internal carbonate mud sediment with peloids and ooids. The identification of factors that influence microbial bioherm occurrence and spatial distribution in Great Salt Lake is limited to a set of collinear physical, chemical and biological variables that are confined to a localised closed system, such as salinity, water depth, wave energy, stable substrate and sediment accumulation. Anthropogenic modifications to Great Salt Lake resulting in increased salinity have exceeded the salinity range in which bioherm‐mediating microbial communities can survive, effectively defining an upper limit of salinity for bioherm microbial community viability. The better understanding of the distribution of microbial bioherms has significant implications for managing and protecting the lake ecosystem and may provide insights into the physical and chemical controls that existed during the formation of fossil microbialites in deep time.