Early Diagenetic Concretions Research Articles

Unravelling the fabrics preserved inside early diagenetic concretions: Insights for the distribution, accumulation and preservation of organic-rich mud in the interior of epicontinental basins

Fine-grained sedimentary rocks generally undergo severe mechanical compaction during burial, which complicates the recognition of primary mudstone fabrics and associated sedimentary features. Early diagenetic concretions, however, provide a rare glimpse of primary fabrics because cement filling the pore space prevents the collapse of original grain arrangements. Hand specimens of concretions collected from the basal condensed section of the Late Jurassic-Early Cretaceous Vaca Muerta Formation (Neuquén Basin, Argentina), allow for analysis of sedimentary processes responsible for the dispersal, accumulation and burial of organic carbon-rich sediment in an epicontinental sea. Representative samples from central basin depositional localities were examined by optical, scanning electron microscopy and energy dispersive X-ray spectroscopy. Petrographic observations were complemented with palynological and organic geochemical analyses. Close examination of uncompacted fabrics reveals a significantly more complex and dynamic depositional scenario than previously assumed (suspension settling). Although many of the component grains in the studied samples were originally delivered to the sediment–water interface by suspension settling processes (i.e., marine snow, hypopycnal plumes, pumice rafts), there is substantial evidence of episodic sedimentation controlled by punctuated events of seafloor disturbance and erosion. The common presence of muddy intraclasts indicate that the seafloor was frequently reworked by bottom currents that caused the widespread distribution of organic carbon-rich sediment across distal basin depositional environments. Bottom current circulation supplied oxygen to the sediment–water interface and created suitable conditions for benthic life, contravening the assumption of bottom water anoxia as a prerequisite for organic carbon preservation. The excellent preservation state of freshwater algae (Pediastrum complex) suggests that organic matter contained inside mud composite particles can travel long distances before being deposited in distal depositional settings. Encapsulation protects organic components from mechanical/biogenic degradation and provides an anoxic microenvironment for preventing the oxidation of the organic matter contained inside of mud composite grains. The study shows that organic carbon encapsulation may be an important mechanism for organic carbon preservation in relatively energetic and non-anoxic settings, calling for a critical reappraisal of the processes responsible for the sequestration of organic carbon from the biosphere and its long-term storage in organic-rich mudstone successions.

The Lac au Renard Tephra Cluster: a record of Lochkovian (Lower Devonian) volcanism in the Indian Point Formation, Gaspe Peninsula, Quebec, Canada

Approximately twenty tephra beds, comprising the Lac au Renard Tephra Cluster (new), occur in the Rosebush Cove and Petit Portage members of the Indian Point Formation (Chaleurs Group) on the Gaspe Peninsula of Quebec, Canada. The tephra beds range in thickness from <1.0 cm to 82 cm and occur in a mudrock-dominated sequence with coarser tempestite interbeds. Mineralogically and texturally graded accumulations of phenocrysts mark the bases of the thickest tephra beds. Early diagenetic concretions in one composite tephra preserve pre-compaction fabrics and original hypocrystalline textures with microphenocrysts and devitrified glass shards. The presence of plagioclase, quartz, K-feldspar, biotite, apatite, and zircon suggest a rhyolitic source. The coarseness of the microphenocrysts in the basal accumulations, along with the abundance and thickness of the tephras, suggest that deposition in the Gaspe area was in a proximal position relative to the volcanic source. The zonal graptolites Monograptus praehercynicus and Monograptus aequabilis ssp. from the Petit Portage Member indicate a middle Lochkovian age for the Lac au Renard Tephra Cluster of the Indian Point Formation. Correlation with the tephra cluster that includes the Judds Falls Bentonite in the New Scotland Formation and other possible tephras in the Kalkberg Formation (Helderberg Group) of New York and the Corriganville and Mandata formations of Pennsylvania is likely. The graptolite fauna of the Indian Point and probable correlations to New York may provide additional biostratigraphic constraints on a U–Pb zircon radiometric age determination of 417.6 Ma from New York that has been used to estimate the age of the Silurian–Devonian boundary.

Interactions of A Paleocene River, A Rising Fold, and Early-Diagenetic Concretions

Abstract: The relative rates of sediment accumulation, erosion, and structural uplift determine whether a growing fold develops positive topographic relief, is beveled by antecedent streams, or is buried under thick growth strata. When folds rise in subsiding basins, upward, convergent flow of groundwater through the permeable growth strata that underlie antecedent streams enhances the flux of ions required for concretion growth. Early diagenetic concretions that grow in such alluvial strata may constitute the only clasts larger than sand size available for transport when antecedent streams become erosive. The first reworked concretions deposited by these streams should accurately mark the transition from aggradation to erosion as folds rise into the paths of streams. In this situation, the ability to differentiate reworked from in situ concretions is crucial. The west-vergent Simpson Ridge anticline, a N–S-trending, thick-skinned Laramide structure in east-central Wyoming, separates the larger Hanna basin from the Carbon basin. Near the north nose of this anticline, in situ iron-oxide-rich concretions are abundant in folded Paleocene strata (Ferris Formation) and, just to the east, large, reworked , iron-rich concretions are abundant in younger, more gently dipping conglomerates in the basal Hanna Formation of the backlimb. Smaller reworked concretions are also present near the base of the Hanna Formation at least 7 km south of the anticlinal nose and just east of the fold9s axis. At the anticlinal nose, in situ (non-reworked) concretions up to 3 m × 1 m × 1 m are abundant at the top of an ∼ 7-km-thick sequence of sandstones and siltstones that constitute the Late Cretaceous–early Paleocene Ferris Formation. Reworked concretions are absent in the strata hosting these in situ concretions, but reworked concretionary clasts up to 2 m in diameter are present in exposures of conglomerates in the lowermost Hanna Formation that lie just above the in situ Ferris concretions and southeast of the anticlinal nose. These early-diagenetic concretions were originally cemented by siderite (FeCO 3 ). Oxidation of some small, rinded siderite-cemented clasts took place after their fluvial transport into the Hanna Formation, but abundant angular, un-rinded, iron-oxide-cemented clasts indicate that many large, in situ siderite concretions had resided in the vadose zone before they were entrained. The distribution of reworked concretions and the orientations of crossbeds show that antecedent Hanna streams eroded a swath at least 5 km wide across the rising structure. These streams transported Ferris Formation concretions southeastward into the Carbon basin, and deposited them in a conglomeratic sandstone body in the Hanna Formation. Large calcite-cemented concretions, many with a pipe-like morphology, then grew within Hanna crossbeds. In many cases, these in situ concretions enclose transported, iron-rich concretions, but there is no evidence any calcite-cemented concretions were reworked. The NW–SE alignment of the pipes record southeastward flow of groundwater and thus also provide evidence (together with the orientation of the crossbeds) of the original continuity of the Hanna Formation across the anticline. Reworked concretions reveal the interplay of deposition, diagenesis, and erosion. Due to convergent groundwater flow over growing anticlines, early diagenetic concretions, both in situ and as reworked clasts, are especially likely to be found in growth strata.

Early Jurassic anoxia triggered the evolution of the oldest holoplanktonic gastropod Coelodiscus minutus by means of heterochrony

The tiny gastropod Coelodiscus minutus is superabundant in concretions of the Early Jurassic Posidonia Shale of South Germany which were formed under anoxic or extremely dysoxic conditions. Previous suggestions that C. minutus was a holoplanktonic organism are corroborated based on new evidence from exceptionally well-preserved specimens. The measurements of shell thickness show that the shell of Coelodiscus is very thin (mean 11 µm). In contrast to previous suggestions, the shell of Coelodiscus was not formed in three ontogenetic phases (embryonic, larval and adult shell) but in two phases comprising an embryonic and a secondary shell, the latter forming during an extended larval phase. Hostile conditions on the sea floor, absence or extreme scarcity of epibenthic animals as well as the small size also argue against a benthic life style of this gastropod. Coelodiscus minutus is the oldest known holoplanktonic gastropod. We speculate that Coelodiscus evolved during the Early Jurassic from a benthic precursor, which had a planktotrophic larval development. Probably under the influence of increasing frequency of dysoxic episodes along with hostile benthic conditions, the larval phase was extended neotenously and eventually, a holoplanktonic species evolved. During the Early Toarcian anoxic event, C. minutus was highly abundant in the plankton and dead shells rained down to the anoxic or dysoxic sea bottom. These thin and fragile shells formed an ooze similar to the pteropod ooze in the modern deep sea. The shells were preserved due to the absence or low level of deposit feeding and bioturbation as well as the formation of early diagenetic concretions.

Shapes of organic walled dinoflagellate cysts in early diagenetic concretions—markers for mechanical compaction

Two species of organic walled dinoflagellate cysts: Pareodinia ceratophora Eisenack 1938 (Jurassic) and Deflandrea phosphoritica Deflandre 1947 (Palaeogene) are proposed as markers for determining the mechanical compaction ratio of fine-grained rocks. The near original shapes of these species are obtained from specimens preserved in siderite and calcareous concretions occurring in mudstone host rocks of various ages. An efficient and simple light microscopy examination method of the dinoflagellate cyst height along the microscope optical axis is presented and quantitatively tested on the available material. The differences of the cyst height measurements between specimens preserved in concretions and specimens preserved in the host rock deposits reflect the compression of dinoflagellate cysts most likely due to mechanical compaction of the rocks studied. The mechanical compaction ratios revealed are about 67% for Jurassic mudstones and about 64% for Palaeogene mudstones. Further marker investigations of samples from different outcrops and strata of different ages are recommended.

Reworked marine sandstone concretions: a record of high-frequency shallow burial to exhumation cycles

Concretions, with abundant calcite-dolomite cement-replacement textures originally hosted in shallow-marine sandstones, were reworked into Lower Cretaceous fluvio-deltaic conglomerates and shoreface sandstones (External Zones, Betic Cordillera). A cycle of host sand deposition, early diagenetic concretion formation and concretion reworking is documented: (1) Well-sorted shoreface sandstone deposited. (2) Spherical to ovoid, non-ferroan calcite-cemented concretions formed below flooding surfaces at shallow-burial depths during early eodiagenesis. Non-ferroan calcite cements were precipitated from the bicarbonate derived from seawater and from dissolution of marine bioclasts, as shown by isotope analyses. (3) Concretions were reworked and exposed on the seafloor in a high-energy setting as indicated by the presence of numerous bivalve borings (Entobia ichnofacies), laminated micritic microbial crusts around the concretions, and epilithobionts (oysters, barnacles and corals) on the concretion surface. Concretions also appear as erosional remnants on the floor of channels which were incised into the shoreline sandstone when sea-level fell. (4) The fluvio–deltaic channels were filled with sediment during flooding in the late lowstand of sea-level. (5) The concretions are partly dolomitized, and the presence of siderite, pyrite and barite in the outer part of the concretions precipitated before the dolomite, suggests that the latter formed during shallow burial.

Active hematite concretion formation in modern acid saline lake sediments, Lake Brown, Western Australia

Concretions can provide valuable records of diagenesis and fluid–sediment interactions, however, reconstruction of ancient concretion-forming conditions can be difficult. Observation of modern hematite concretion growth in a natural sedimentary setting provides a rare glimpse of conditions at the time of formation. Spheroidal hematite-cemented concretions are actively precipitating in shallow subsurface sediments at Lake Brown in Western Australia. Lake Brown is a hypersaline (total dissolved solids up to 23%) and acidic (pH ∼4) ephemeral lake. The concretion host sediments were deposited between ∼1 and 3 ka, based on dating of stratigraphically higher and lower beds. These age constraints indicate that the diagenetic concretions formed <3 ka, and field observations suggest that some are currently forming. These modern concretions from Lake Brown provide an example of very early diagenetic formation in acid and saline conditions that may be analogous to past conditions on Mars. Previously, the hematite concretions in the Burns formation on Mars have been interpreted as late stage diagenetic products, requiring long geologic time scales and multiple fluid flow events to form. In contrast, the Lake Brown concretions support the possibility of similar syndepositional to very early diagenetic concretion precipitation on Mars.

Depositional environments of the lower Permian Dwyka diamictite and Prince Albert shale inferred from the geochemistry of early diagenetic concretions, southwest Karoo Basin, South Africa

The upper Dwyka and lower Ecca Groups in the Karoo Basin of South Africa document the climatic and palaeoenvironmental changes associated with the final Permo-Carboniferous deglaciation of the Gondwana supercontinent. The depositional environments of these groups have, until recently, been interpreted on the basis of sedimentological and palaeontological evidence. Here we use the geochemistry of early diagenetic concretions – septarian calcite concretions from the upper Dwyka Group and phosphatic chert concretions and beds from the lower Ecca Group – to infer the depositional environment of these rocks in the southwestern Karoo Basin. δ 18O values (7.8 to 8.9‰ SMOW) suggest that the calcite concretions precipitated from a mixture of meteoric and glacial melt waters rather than Permian seawater. δ 13C values (− 15 to − 3‰ PDB) indicate that the carbon was derived from a mixture of craton-derived calcareous material and organic matter, bacterially degraded in the lower sulphate-reduction to upper methanogenesis zones during early burial diagenesis. The rare-earth element (REE) patterns, Sr concentrations and 87Sr/ 86Sr ratios (0.716–0.737) significantly greater than Permian seawater (0.708), together also support the interpretation that calcite and phosphatic concretions formed in glacial, fresh water sediments.

Paleoenvironment and Taphonomy of the Chanares Formation Tetrapod Assemblage (Middle Triassic), Northwestern Argentina: Spectacular Preservation in Volcanogenic Concretions

Other| October 01, 2001 Paleoenvironment and Taphonomy of the Chañares Formation Tetrapod Assemblage (Middle Triassic), Northwestern Argentina: Spectacular Preservation in Volcanogenic Concretions RAYMOND R. ROGERS; RAYMOND R. ROGERS 1Department of Geology, Macalester College, St. Paul, MN 55105 Search for other works by this author on: GSW Google Scholar ANDREA B. ARCUCCI; ANDREA B. ARCUCCI 2Museo de Paleontologia, Instituto de Anthropologia, Universidad Nacional de La Rioja, Avenida Ortiz de Ocampo 1700, 5300 La Rioja, Argentina Search for other works by this author on: GSW Google Scholar FERNANDO ABDALA; FERNANDO ABDALA 3Laboratorio de Paleontologia, Museu de Ciências e Tecnologia, Pontifícia Universidade Católica do Rio Grande do Sul, Avenida Ipiranga 6681, Porto Alegre (90619-900), Brazil Search for other works by this author on: GSW Google Scholar PAUL C. SERENO; PAUL C. SERENO 4Department of Organismal Biology and Anatomy, University of Chicago, Chicago, IL 60637 Search for other works by this author on: GSW Google Scholar CATHERINE A. FORSTER; CATHERINE A. FORSTER 5Department of Anatomical Sciences, State University of New York, Stony Brook, NY 11794 Search for other works by this author on: GSW Google Scholar CATHLEEN L. MAY CATHLEEN L. MAY 6Institute for Environmental Education, Geological Society of America, 3300 Penrose Place, Boulder, CO 80301 Search for other works by this author on: GSW Google Scholar PALAIOS (2001) 16 (5): 461–481. https://doi.org/10.1669/0883-1351(2001)016<0461:PATOTC>2.0.CO;2 Article history accepted: 30 Mar 2001 first online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation RAYMOND R. ROGERS, ANDREA B. ARCUCCI, FERNANDO ABDALA, PAUL C. SERENO, CATHERINE A. FORSTER, CATHLEEN L. MAY; Paleoenvironment and Taphonomy of the Chañares Formation Tetrapod Assemblage (Middle Triassic), Northwestern Argentina: Spectacular Preservation in Volcanogenic Concretions. PALAIOS 2001;; 16 (5): 461–481. doi: https://doi.org/10.1669/0883-1351(2001)016<0461:PATOTC>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyPALAIOS Search Advanced Search Abstract The enigmatic concretionary exposures that typify the Chañares Formation (Ladinian, northwestern Argentina) long have defied precise paleoenvironmental characterization. Recent work indicates that the formation accumulated in an alluvial-to-lacustrine setting within an active rift basin that received sedimentary detritus from surrounding highlands, as well as copious amounts of volcanic ash. Ash- flow sheets were emplaced presumably as secondary mass flows on alluvial surfaces characterized by small fluvial channels and shallow lakes. Thin bentonite beds intercalated in the Chañares Formation indicate that ash also accumulated via direct airfall, although this mode of emplacement accounts for a very small fraction of the overall section. A shift to widespread lacustrine deposition is recorded by the superjacent Los Rastros Formation, which preserves at least six shallowing-upward hemicycles, five of which commenced amidst explosive volcanic activity as evidenced by intercalated bentonite beds.Volcanism played an important role in the generation and preservation of the Chañares Formation's exceptional tetrapod fossil record. This is especially true of the classic Los Chañares locality, where more than 100 individuals representing a diverse array of taxa (archosaurs, cynodonts, dicynodonts) are entombed in volcanogenic concretions with matrices of relic glass shards diagenetically replaced by calcite. Taphonomic attributes of the Los Chañares locality are consistent with the scenario of mass mortality, and several clues hint at the nature of the event. The killing agent was lethal to a variety of taxa, killed both adults and juveniles, and led to the concentration of taxa that under normal circumstances would tend to dissociate, such as carnivores and their potential prey. It also produced a counterintuitive bias against the preservation of large-bodied taxa, which may have been largely unsusceptible to the death event, or perhaps were excluded from the Los Chañares death assemblage via post-mortem sorting. The spatial arrangement of skeletal material in a small sample of concretions is consistent with the stranding of tetrapod carcasses along a strandline, and it is feasible that volcanism led to catastrophic flooding of the landscape via damming and/or diversion of local drainages. Uncompacted skeletal elements and relic outlines of glass shards indicate that carbonate concretions formed shortly after skeletal material was buried in reworked volcanic ash. The microbial decay of organic matter presumably catalyzed concretion diagenesis. There is no indication that bone hydroxyapatite diffused into the entombing glassy matrix and contributed to concretion formation. Bones entombed within early diagenetic concretions were safeguarded from subsequent destructive pedogenic and/or diagenetic processes, and were incorporated in exquisite quality into the fossil record. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Paleoenvironment and Taphonomy of the Chanares Formation Tetrapod Assemblage (Middle Triassic), Northwestern Argentina: Spectacular Preservation in Volcanogenic Concretions

Other| October 01, 2001 Paleoenvironment and Taphonomy of the Chañares Formation Tetrapod Assemblage (Middle Triassic), Northwestern Argentina: Spectacular Preservation in Volcanogenic Concretions RAYMOND R. ROGERS; RAYMOND R. ROGERS 1Department of Geology, Macalester College, St. Paul, MN 55105 Search for other works by this author on: GSW Google Scholar ANDREA B. ARCUCCI; ANDREA B. ARCUCCI 2Museo de Paleontologia, Instituto de Anthropologia, Universidad Nacional de La Rioja, Avenida Ortiz de Ocampo 1700, 5300 La Rioja, Argentina Search for other works by this author on: GSW Google Scholar FERNANDO ABDALA; FERNANDO ABDALA 3Laboratorio de Paleontologia, Museu de Ciências e Tecnologia, Pontifícia Universidade Católica do Rio Grande do Sul, Avenida Ipiranga 6681, Porto Alegre (90619-900), Brazil Search for other works by this author on: GSW Google Scholar PAUL C. SERENO; PAUL C. SERENO 4Department of Organismal Biology and Anatomy, University of Chicago, Chicago, IL 60637 Search for other works by this author on: GSW Google Scholar CATHERINE A. FORSTER; CATHERINE A. FORSTER 5Department of Anatomical Sciences, State University of New York, Stony Brook, NY 11794 Search for other works by this author on: GSW Google Scholar CATHLEEN L. MAY CATHLEEN L. MAY 6Institute for Environmental Education, Geological Society of America, 3300 Penrose Place, Boulder, CO 80301 Search for other works by this author on: GSW Google Scholar PALAIOS (2001) 16 (5): 461–481. https://doi.org/10.1669/0883-1351(2001)016<0461:PATOTC>2.0.CO;2 Article history accepted: 30 Mar 2001 first online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation RAYMOND R. ROGERS, ANDREA B. ARCUCCI, FERNANDO ABDALA, PAUL C. SERENO, CATHERINE A. FORSTER, CATHLEEN L. MAY; Paleoenvironment and Taphonomy of the Chañares Formation Tetrapod Assemblage (Middle Triassic), Northwestern Argentina: Spectacular Preservation in Volcanogenic Concretions. PALAIOS 2001;; 16 (5): 461–481. doi: https://doi.org/10.1669/0883-1351(2001)016<0461:PATOTC>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyPALAIOS Search Advanced Search Abstract The enigmatic concretionary exposures that typify the Chañares Formation (Ladinian, northwestern Argentina) long have defied precise paleoenvironmental characterization. Recent work indicates that the formation accumulated in an alluvial-to-lacustrine setting within an active rift basin that received sedimentary detritus from surrounding highlands, as well as copious amounts of volcanic ash. Ash- flow sheets were emplaced presumably as secondary mass flows on alluvial surfaces characterized by small fluvial channels and shallow lakes. Thin bentonite beds intercalated in the Chañares Formation indicate that ash also accumulated via direct airfall, although this mode of emplacement accounts for a very small fraction of the overall section. A shift to widespread lacustrine deposition is recorded by the superjacent Los Rastros Formation, which preserves at least six shallowing-upward hemicycles, five of which commenced amidst explosive volcanic activity as evidenced by intercalated bentonite beds.Volcanism played an important role in the generation and preservation of the Chañares Formation's exceptional tetrapod fossil record. This is especially true of the classic Los Chañares locality, where more than 100 individuals representing a diverse array of taxa (archosaurs, cynodonts, dicynodonts) are entombed in volcanogenic concretions with matrices of relic glass shards diagenetically replaced by calcite. Taphonomic attributes of the Los Chañares locality are consistent with the scenario of mass mortality, and several clues hint at the nature of the event. The killing agent was lethal to a variety of taxa, killed both adults and juveniles, and led to the concentration of taxa that under normal circumstances would tend to dissociate, such as carnivores and their potential prey. It also produced a counterintuitive bias against the preservation of large-bodied taxa, which may have been largely unsusceptible to the death event, or perhaps were excluded from the Los Chañares death assemblage via post-mortem sorting. The spatial arrangement of skeletal material in a small sample of concretions is consistent with the stranding of tetrapod carcasses along a strandline, and it is feasible that volcanism led to catastrophic flooding of the landscape via damming and/or diversion of local drainages. Uncompacted skeletal elements and relic outlines of glass shards indicate that carbonate concretions formed shortly after skeletal material was buried in reworked volcanic ash. The microbial decay of organic matter presumably catalyzed concretion diagenesis. There is no indication that bone hydroxyapatite diffused into the entombing glassy matrix and contributed to concretion formation. Bones entombed within early diagenetic concretions were safeguarded from subsequent destructive pedogenic and/or diagenetic processes, and were incorporated in exquisite quality into the fossil record. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Bacterially mediated formation of carbonate concretions in the Oligocene Boom Clay of northern Belgium

ABSTRACT Carbonate concretions in the Oligocene Boom Clay in Northern Belgium grew as a result of bacterially mediated geochemical processes. Concretion distribution patterns are controlled by glacio-eustatic controls on water depth and related sediment detrital carbonate contents. Bacterial remains are present throughout these early diagenetic concretions and can be observed by SEM analysis of etched carbonates. The oxygen isotopic composition (18O 0o/oo) of the calcite in the concretions indicates early diagenetic growth close to the sediment-water interface. Negative carbon-isotope values (13C -17 to -22o/oo) from concretion carbonates, as well as pyrite 34S values of -15 to -45o/oo, suggest precipitation resulting from bacterial sulfate reduction during shallow burial diagenesis. Additionally, organic components in the concretions are characteristic of the remains of sulfate-reducing bacteria.

Acritarchs and microfossils from the Mesoproterozoic Bangemall Group, northwestern Australia.

Three microfossil assemblages occur in the Mesoproterozoic Bangemall Group (1625-1000 Ma) of northwestern Australia, each occupying a different environmental and taphonomic setting. In peritidal environments, benthic prokaryotic filaments and spheroids of matting habit and small size were permineralized by early diagenetic silicification of stromatolitic carbonates. In shallow subtidal environments, benthic filaments of large size and nonmatting habit and planktonic sphaeromorph acritarchs with thin walls and moderate dimensions were compressed in mildly kerogenous shale. In deeper subtidal environments, planktonic megasphaeromorph acritarchs with thick walls were initially entombed in concretionary nodules in highly kerogenous shale and then permineralized by silica during later diagenesis. Taxonomic diversity and numerical abundance evidently decrease offshore. The three assemblages have typical Mesoproterozoic aspects: peritidal benthic habitats were dominated by Siphonophycus-Sphaerophycus-Eosynechococcus-Myxococcoides-Palaeopleurocapsa, shallow subtidal settings were occupied by Siphonophycus-Leiosphaeridia-Pterosphermopsimorpha-Satka, and offshore plankton consisted solely of very large chuarid acritarchs. Because of its taphonomic restriction to mid-intertidal stromatolites, the peritidal assemblage can be equated in microenvironment with a similar assemblage in the Neoproterozoic Draken Conglomerate, suggesting that ecological stasis at the community level can last for intervals up to 900 million years. In the deeper subtidal assemblage, the common chuarid has an unusual mode of preservation, in three dimensions in early diagenetic concretions, revealing that it possesses a thick multilamellate wall. Because of this distinctive ultrastructure, the new genus Crassicorium is erected for these fossils, which are among the oldest indubitable eukaryotes. Very large (34-55 micrometers in diameter) filaments from shallow subtidal habitats are assigned to the emended species Siphonophycus punctatum.

UPb dating of calcite concretions from Cambrian black shales and the Phanerozoic time scale

The UPb dating of carbonate offers considerable potential for accurate and direct dating of sedimentary rocks. Here we show that carbonate concretions from the Upper Cambrian black shales in southern Scandinavia have remarkably high U to Pb ratios ( 238U 204Pb up to 288,000) permitting precise geochronology. A fine-crystalline early diagenetic concretion yields an age of 509.8 ± 5.1 Ma, providing an accurate age for the late Cambrian, whereas a coarse ‘cone-in-cone’ concretion has yielded an age of 478.2 ± 4.9 Ma, defining the final stage of sediment compaction and indicating a protracted dewatering and diagenetic history. These results demonstrate the power of this method for directly calibrating the geological time scale and constraining diagenetic histories.

Origin and timing of siderite and calcite concretions in late Palaeogene non- to marginal-marine facies of the Te Kuiti Group, New Zealand

The Late Eocene-earliest Miocene Te Kuiti Group represents an overall transgressive sequence of formations ranging upwards from non-marine coal measure facies through marginal-marine low-energy shoreline siliciclastic sediments to fully-marine mixed siliciclastic-carbonate deposits. Concretionary structures are common in the lowest formations of the group, being dominantly sideritic in the non-marine to brackish sediment facies (Waikato Coal Measures and Glen Afton Claystone) and calcitic in the more marine-influenced sediments (Rotowaro Siltstone). The geochemistry of the carbonate cements within these early diagenetic concretions records the change from carbonate precipitation from purely meteoric fluids to precipitation from marine fluids, and a shifting source of carbonate carbon across this transition.Siderite precipitation in the Waikato Coal Measures concretions and hardpans was initiated and often largely completed in the methanogenic zone, within the upper 20 m of the sediment pile over a period possibly lasting 100–300 ka. Precipitation continued at a reduced rate to burial depths of 300 m or more when septarian cracking occurred in some siderite concretions and was healed by calcite vein cements that probably derived carbonate from the oxidation of methane ascending from the maturing coal seams directly below.Siderite precipitation in the concretions and hardpans of the overlying brackish-marine Glen Afton Claystone was completed in the topmost few metres of sediment, possibly a few 10 ka. Stable-isotope values near zero support a marine carbonate carbon source, suggested to be from remobilised shell material as evidenced by the common occurrence of shell casts in the formation and the high degree of Mg and Ca substitution in the siderites.The overlying marginal-marine to low-energy marine Rotowaro Siltstone contains calcite concretions precipitated from pore waters enriched in bicarbonate derived from the sulphate-reduction zone, largely within the upper 8 m of sediment, and possibly growing over a few 100 ka. Concretion development continued at a reduced rate to burial depths of several tens of metres and in some cases septarian cracks developed during the later stages of concretion growth. The precipitation of septarian vein calcites with more depleted δ18O values and more enriched δ13C signatures than central concretions cements, may be due to increasing burial temperatures, and the progressive introduction of carbonate sourced from shell dissolution.

Records of Late Permian Surface Temperatures in Continental Gondwana in Isotope Geochemistry of Upper Permian Early Diagenetic Calcite Concretions: ABSTRACT

We present geochemical and isotopic evidence of paleotemperatures from freshwater continental deposits from 55[degrees]S in interior southern Gondwana. Lacustrine shales host spheroidal concretions with abundant septarian cracks. Cement carbonate varies from 65% at the centre to 15% at the edges. Cistraccide remains are preserved. Septarian calcite occurs in dull and bright bands, with three distinct generations of vein-fills. Early-formed cement both in concretions and septarian veins is magnesium-rich whereas, later-formed carbonates are pure calcite. Carbon- and oxygen-isotope ratios (PDB) are: host shales, [delta][sup 13]C= -4.36 to o.77[per thousand], [delta][sup 18]O= -12.73 to -17.12[per thousand]; concretion cements, [delta][sup 13]C=+0.26 to [delta][sup 18]O= -9.34[per thousand]; and vein-fills, [delta][sup 13]C= -7.05 to +1.09[per thousand], [delta][sup 18]O= -8.28 to -18.24[per thousand]. 13C and 18O ratios are depleted from the center of concretions to the periphery, as well as from the centre of veins to the tip. Near-surface cementation is suggested by textural evidence. [delta]18O in the range of -12.636[per thousand] to -8.989 SMOW is calculated for the meteoric palaeowaters, from which average annual surface temperatures of 5-8[degrees]C are inferred. [delta]18O of early-formed cements also yields a mean annual surface temperature of 2-6[degrees]C. Based on continentality and palaeolatitudes of northern Malawi during the Late Permian,more » we propose that mean annual surface palaeotemperatures may have been as high as 10[degrees]C, similar to those found in modern continental temperate climates.« less

Geochemistry of an early diagenetic concretion from the Birchi Bed (L. Lias, W. Dorset, U.K.)

Three concentric growth zones ( calcitic core, dolomitic zone, outer rim) have been recognised in a carbonate concretion from the Birchi Bed (L. Lias, U.K.) of W. Dorset. Each zone has characteristic trace metals, carbonate stable isotopes and biological markers. The organic matter which fuelled the formation of the concretion was derived from the immature parent Shales-with-Beef, however, bitumen trapped within each zone has been affected by the dominant bacterial processes which led to the precipitation of the carbonates. It is suggested that the calcitic core precipitated under sulphate reducing conditions. Here, most biological markers are present in low amounts, possibly as a result of bacterial degradation. The dolomitic zone is characterised by a high abundance of isotopically light phytane which suggests direct input from methanogens. This is consistent with isotopic data for the dolomite which indicates a dominantly methanogenic origin. However, the presence of hop-17(21_-ene, n-alkenoic acids and of the C 32 hopanoic acid indicate concomitant non-methanogenic bacterial activity. In the outer rim where calcites have negative carbon isotopic values, the high concentrations of n-alkenoic acids in the bitumen suggest a non-methanogenic bacterial origin for the carbonate. The study shows the potential of using organic geochemical techniques to study the in situ formation of diagenetic precipitates. Biological markers trapped in the concretion may provide a “snapshot” of sedimentary microbial processes.

Reworked early diagenetic concretions and the bioerosional origin of a regional discontinuity within British Jurassic marine mudstones

ABSTRACTThe Coinstone is a well known hiatus‐concretion level in the Lower Lias (Lower Jurassic, Upper Sinemurian) of Dorset, southern England. It has long been recognized as a layer of bored and encrusted, early diagenetic, clay‐hosted septarian concretions coincident with a biostratigraphic gap of three ammonite subzones. Several different types of concretion of variable complexity can be distinguished, of which two, probably derived from slightly different stratigraphic levels, have been juxtaposed by condensation at the erosion surface. Diagenetic and biological processes occurring before, during and after exhumation on the Jurassic sea‐floor can be recognized. The relative timing of these events can be distinguished, suggesting that initial concretion consolidation, the first generation of septarian cracking, and the precipitation of the first generation of crack‐lining calcite preceded exhumation. These, therefore, probably took place at an early stage, at shallow burial depths within the accumulating sediment pile. The early calcite is brown, UV‐fluorescent and inclusion‐rich, and is similar to the first calcite generations seen in many other clay‐hosted septarian concretions. A generally early diagnetic origin of this material is thus inferred. Observations on crack textures and geometries and the interactions of the post‐exhumation fauna of encrusters, borers and burrowers lend support to previous suggestions that initial cracking in some septarian concretions took place in a stiff rather than a fully rigid concretion body, possibly given coherence by initial growth of some organic substance that was only later replaced by the calcite cements seen in most such concretions today.The burrowing activities of a benthic fauna in muds cause resuspension of sediment and facilitate erosion, even in the absence of high energy physical processes. Regional stratigraphic gaps may be formed as a consequence of sea level rises or falls, or as a response to sediment supply reduction independent of sea level change. Such major episodes of biologically mediated erosion in mudstone sequences may be of more general importance than has hitherto been recognized.

Fossil fish stomachs: A microenvironment for exceptional preservation

Exceptionally preserved fossil fish from the Cretaceous of north east Brazil contain well preserved arthropods and small fish in the alimentary tract. The fish are preserved in early diagenetic concretions and often retain their original three dimensional morphology. A significant number of specimens have soft tissues preserved, including part of the alimentary tract, musculature and gills. In many specimens the stomach retains elements of the fishes’ last meal, including a variety of smaller fishes and arthropods. Although many are disarticulated, a significant number of specimens within the gut remain articulated. Many specimens display exceptional anatomical detail despite having been devoured by predators. Although arthropods can be found outside of the body cavity of the fish, such specimens are not usually so well preserved.

The origin and sedimentary, diagenetic and metamorphic evolution of chlorite–mica stacks in Llandovery sediments of central Wales, U.K.

AbstractChlorite–mica stacks in the Llandovery sedimentary rocks of central Wales have been studied, both within cleaved turbidite mudrocks, and within early diagenetic apatite, carbonate and monazite concretions. Stacks show a graded distribution within turbidite units, indicating an origin as detrital particles. Concretions provided an arrested diagenetic environment which restricted subsequent deformation and so revealed the nature of the stack precursor minerals. These included detrital mica, and also probable pyroxenes, amphiboles and volcanic rock particles, relic textures of which can be seen preserved within the concretions. Diagenetic alteration to chlorite, probably with some white mica intergrowth, occurred, most probably via smectite and then mixed-layer or swelling chlorite intermediates. Subsequent modification, during the Caledonian cleavage-forming episode, determined the present shape and mineralogy of the stacks. Outside concretions, chlorite pseudomorphs were rotated by tectonic compression, crushed at their margins and pulled apart along (001) cleavage and slip planes. Growth of at least two generations of secondary mica infillings has been recognized, and this was probably accompanied by the growth of secondary chlorite. This resulted in barrel-shaped stacks, elongated parallel with cleavage, and with the (001) plane rotated from the bedding towards the perpendicular to the cleavage. Within early diagenetic concretions it is evident that some stacks had formed prior to lithification. However, these have a relatively simple morphology and the original bedding-parallel grain shapes are largely preserved.

Field relationships and stable isotope geochemistry of concretions from James Ross Island, Antarctica

Early diagenetic (precompactional) concretions are abundant throughout the Cretaceous-Tertiary Marambio Group Larsen Basin, Antarctica. Four distinct concretion types are recognised: (1) spherical-subspherical concretions: (2) sheet concretions; (3) fossil-nucleated concretions; and (4) concretionary burrow networks. All concretion types have a micritic to microsparry variably non-ferroan to ferroan calcite cement. Stable isotope analyses show a wide spread in both δ 18O and δ 13C. δ 13C values are typically negative, ranging between –3.38 and –39.15%o (PDB) (usually –16 to 230%). δ 18O ranges between –1.28 and –13.81% (PDB) with most of the values between –5 and –10%. The δ 13C signature is interpreted to represent carbon sourced from sulphate reduction and/or methane oxidation, with minor input from shell dissolution, and is consistent with a shallow burial, early diagenetic origin. A single mudstone hosted concretion has a δ 18O composition indicative of precipitation of carbonate from seawater. The low δ 18O signatures in the sandstone- and siltstone-hosted concretions are possibly due to early diagenetic modification of the pore water composition through volcaniclastic mineral dissolution/reprecipitation reactions and perhaps through input of meteoric water. Concretion distribution is related to (a) changes in sedimentation rate and (b) the dominance of diffusion on concretion cementation.