Septarian Concretions Research Articles

Barite-bearing “septarian concretions” (upper Cretaceous – Paleocene) from the Biban area (north Algeria)

The "septarian barite concretions" have garnered significant interest worldwide. This study presents the first detailed investigation of these sedimentary structures in Algeria, with a primary focus on elucidating their genesis through petrography, SEM-EDS, major and trace elements (including REEs), and sulfur isotope analysis. The sampled concretions occur in Upper Cretaceous-Paleocene marl layers from the Biban area, primarily in two main sectors (Koudiat Djebassa and Sidi Ziane), situated within the Southern Tellian Atlas in Northern Algeria (Maghrebides). While these barite concretions exhibit diverse morphologies, our analysis focuses specifically on oblate ellipsoidal concretions. The flattened shape of these concretions may stem from compressed echinoid tests or small, oblate septarian cracked concretions representing the flattened cores. Notably, these concretions feature an outer cortex of radial fibrous barite crystals, measuring centimeters in thickness. Petrographic examination coupled with SEM-EDS analysis suggests that concretion growth occurred from porewaters in unconsolidated, soft sediments at shallow burial depths beneath the sediment-water interface, manifesting in three growth episodes: (i) an initial stage characterized by primarily thin lamellar barite crystals intertwining to form rosette-like structures in concretion cores, (ii) a later stage marked by radial-fibrous barite crystal growth in concretion margins, and (iii) the filling of septarian cracks by barite. The presence of terrigenous sediment, particularly abundant in rosette-like barite crystals, indicates growth within highly porous sediment. Conversely, the radial fibrous outer cortex is related to an effective displacive growth mode. The δ34S values of the barite concretions (ranging from 16 ‰ to 24.4 ‰), consistent with seawater values, which document that seawater was the primary sulfur source. Furthermore, REE geochemistry further supports the seawater origin of diagenetic fluids rather than a hydrothermal one. Concretions are suggested to have formed in organic-rich, fine-grained sediments at shallow depth below the sea floor, in sedimentation halts, and as a result of microbial degradation of organic matter. Barium was supplied to porewaters within the methanogenic zone, precipitating as barite when, migrating upwards, reached the boundary with the overlying sulfidic zone.

Taking the heat out of British Jurassic septarian concretions

AbstractSeptarian carbonate concretions in marine mudrocks contain calcite cements that should represent evolving conditions from ambient temperature, shallow subsurface environments to warmer, burial diagenetic conditions. Clumped isotope results from British Middle and Upper Jurassic concretions indicate that most concretion body calcites formed at temperatures between 9 ± 5°C and 18 ± 5°C from marine pore waters with δ18O values between 0.2 ± 1.1‰ and −2.2 ± 1.1‰VSMOW. Early diagenetic, brown, fibrous calcite fracture cements mostly formed at temperatures between 15 ± 5°C and 19 ± 5°C, again from marine‐derived pore fluids with δ18O compositions between −0.5 ± 1.1‰ and 0.3 ± 1.2‰VSMOW. Two of these cements showed evolution to warmer temperatures and more evolved pore fluids with growth, indicating transition to deeper burial conditions. Later diagenetic, sparry calcite cements gave more variable temperatures but all indicated involvement of meteoric pore fluids. The highest clumped isotope temperature (43 ± 6°C) is within error of the 50°C regional maximum burial temperature estimate. These results are consistent with published geological and stable isotope constraints on the formation of Jurassic septarian concretions and highlight their potential as robust archives of marine benthic palaeotemperatures. Some of these results differ from clumped isotope data in an earlier study that reported higher temperatures for concretion body and early diagenetic fibrous cement fringes probably due to methodological differences.

Lectostratotype of maikopian group – Belaya River section upstream of Maikop town (Western Ciscaucasia) in the Oligocene part

Belaya River Section of Maikop deposits is the closest to the Maikop town, typical for Western and Central Ciscaucasus, well outcropped and studied and should be considered as a type section for the Maikop Group. Here it is possible to observe a continuous transition from the underlying light marls of Belaya Glina Formation to the dark clays of Pshekha Formation at the base of the Maikop succession. Pshekha Fm is characterized by nannoplankton NP21 and NP22 zones, dinocysts Phthanoperidinium amoenum (D13) and Wetzeliella symmetrica zones, mollusk association with Pterolucina batalpashinica at the bottom and rich fish fauna. The succession shows a normal polarity chron C13n for the base of the Pshekha and revers polarity C12r above.
 Marl of Polbian Bed with brackish Lower Solenovian mollusks, ostracod association with Disopontocipris oligocaenica, nannoplankton of NP23 Zone and dinocysts of Wetzeliella gohtii Zone lies above with erosion contact. Overlying Lower Morozkina Balka Subformation contains from non-calcareous clays and concludes the remains of fish and dinocysts of Wetzeliella gohtii Zone. The Upper Morozkina Balka Subformation contains calcareous sublayers with nannoplankton NP23 Zone in lower part and NP24 one above, dinocysts Chiropteridium partispinatum Zone, Rhombodinium draco Subzone. Polarity pattern we interpret to correspond to C10n, C9r and C9n.
 Batalpashinsk Formation corresponds to the time of maximum hydrogen sulfide contamination of the Late Oligocene basin and is dated by nannoplankton NP25 Zone, and dinocysts Chiropteridium patrispinatum Zone with Rhombodinium draco Subzone in lower part and Deflandrea spinulosa (non typica) Subzone in the middle and upper parts. Septarian+Zelenchuk Fm is composed of clays with septarian concretions, and interbedded sands with the same dinocysts and foraminifera with Spiroplectammina aff. terekensis. Calcareous interlayers in the top of the formation conclude nannoplankton of NP25-NN1 Zone. Karadzhalga Formation is composed mainly “fish lithofacies” and includes dinocysts zone Labyrinthodinium truncatum (Lower Miocene) in basal part.

Silica-rich septarian concretions in biogenic silica-poor sediments: A marker of hydrothermal activity at fossil hyper-extended rifted margins (Err nappe, Switzerland)

Silica-rich septarian concretions occur within Pliensbachian-Oxfordian syn-rift to early post-rift sediments of the fossil distal Adriatic rifted margin today exposed in the Err nappe (Central Alps). The concretions, ranging in size from few centimeters to about 2 m, consist of partially to completely siliceous spheroidal bodies developed within fine-grained sedimentary rock (mud to fine sand) with no or very scarce content in biogenic silica such as radiolarian tests. These concretions are characterized by a network of septarian cracks filled with chalcedony, quartz and locally, saddle dolomite. Some concretions are in their original position, whereas others are fragmentary and preserved in coarse-grained turbiditic beds, implying that concretionary growth had occurred prior to significant burial. Fluid inclusion temperatures (~150 °C), the positive δ18O values for original fluids (up to +7‰ SMOW), and the high 87Sr/86Sr values of dolomite point to a deep circulation of seawater into fault damage zones developed in granitic basement rocks. In their ascent, these hot, silica-rich fluids crossed compact sediments giving rise to quartz-dolomite veins. Lastly, when reaching the uncompacted soft sediments close to the seafloor, silica (quartz and chalcedony) locally precipitated in sediment pores and/or fissures leading to the formation of the siliceous septarian concretions. The anomalous quartz-rich mineralogy of the concretion body and of the authigenic crack-filling minerals, the occurrence in clastic sediments from deep-water distal rifted margins as well as high formation temperatures represent a new discovery enlarging the spectrum of septarian concretion types and the environment where they may grow. They provide a possible record of the subbottom products of hydrothermal venting typifying hyper-extended rifted margins where extensional detachment faults exhume crustal rocks to the seafloor and lead to an intense fluid-rock interaction and silica enrichment in the fluids.

Clumped-Isotope Constraints On Cement Paragenesis In Septarian Concretions

Septarian concretions exhibit multiple generations of cements that include body, fringe, and spar phases. Classic paragenetic interpretations include initial precipitation of the body followed by fringe(s) and then by spar in more or less discrete events. Traditional approaches (e.g., carbon and oxygen isotope analyses) are generally unable to distinguish paragenetic trends as they relate to specific formation environments (e.g., precipitation during burial or with meteoric influx). Here we present carbonate clumped-isotope, δ13C (δ13Ccarb), and δ18O (δ18Ocarb) values for septarian concretions taken from four host units in order to assess cement paragenesis and overcome traditional shortcomings. Clumped-isotope and δ18Ofluid data exhibit a wide range of values, with carbonate precipitation temperatures between ∼ 20 and 50°C and δ18Ofluid compositions of ∼ −14 to +4‰ (VSMOW). In stable-isotope cross-plots, specific cement phases group together and confirm the paragenesis indicated by superposition. In some cases, samples analyzed from concretion bodies yield temperature and δ18Ofluid values that indicate formation at shallow depths, consistent with independent data (e.g., high minus-cement porosity, external laminae deflection). In contrast, other concretion-body analyses indicate relatively high body temperatures that conflict with shallow-formation indices. Petrographic and backscatter scanning electron microscopy (SEM) reveal that concretion bodies partially consist of a secondary, replacement phase, which could explain the higher temperatures expressed in bulk body samples. Based on data for different phases in these septarian concretions, we suggest that initial body-cement precipitation occurred at relatively shallow depths from unmodified seawater, followed by fringe formation at elevated temperatures that likely coincided with the emplacement of the secondary body phase. When considered together, late-stage spar phases exhibit temperatures and δ18Ofluid values supportive of spar precipitation from fluids with a significant meteoric component, possibly during uplift.

Fatally bitten ammonites from septarian concretions of the ‘Marston Marble’ (Lower Jurassic, Sinemurian), Somerset, UK, and their taphonomy

A small polished slab of ‘Marston Marble’, Lower Jurassic, from Marston Magna, Somerset, UK, contains several ammonite specimens with pieces of shell missing from the body chamber immediately in front of the last suture. The damage is visible in sagittal sections, yet its position and extent are almost identical to damage reported from several Lower Jurassic ammonite genera from the Dorset coast and elsewhere, and attributed to the trace fossil Bicrescomanducator rolli. A second cut, but unpolished slab of ‘Marston Marble’ shows two more ventrally damaged ammonites, one in sagittal section. The damage is interpreted as the result of predation, by a teuthoid cephalopod and affected two prey species. This is the first time ventral bite marks have been reported from sectioned ammonites.Some ammonites, but not bitten examples, have their body chambers largely filled with diagenetic calcite and were buried with the body inside. Often the calcite does not fill the full width of the body chamber, creating a false geopetal structure. As these are variously orientated with respect to bedding, the ammonites were possibly reworked before final burial. The polished slab derived from a septarian concretion. As the septarian cracks opened some ammonites were pulled apart. During shrinkage the sediment maintained a tight grip on fossils.

Diagenetic alterations in a silt- and clay-rich mudstone succession: an example from the Upper Cretaceous Mancos Shale of Utah, USA

An understanding of the nature and scales of diagenetic variability within organic-rich mudstones is critical to the accurate assessment of shale-gas reservoir properties, as well as for elucidating chemical evolution pathways within mudstones. Here we integrate field observations with thin section descriptions (optical and electron optical techniques) and mineralogical data for the Blackhawk Member time-equivalent Mancos Shale in Book Cliffs, Utah, to determine the impacts of early and burial diagenesis on this mudstone succession.The detrital assemblage in the Mancos Shale comprises quartz-silt, feldspar, clay minerals, dolomite and organic matter (TOC of 1 to 2.5%). Biogenic silica is negligible. Field mapping reveals laterally continuous (km scale), ferroan dolomite cemented units up to 0.3 m thick, are present. These cemented units cap both coarsening-upward units (1 to 3 m thick), and stacked successions of coarsening-upward units (5 to 15 m thick). These upward-coarsening sediment packages, capped by dolomite cemented strata, correlate to bedsets and parasequences in updip settings. Pervasive cementation in these dolomite-cemented units is likely to have occurred prior to compaction as a result of bacterially mediated respiratory processes. Cementation at these levels is particularly evident because cement precipitation occurred during breaks in sediment accumulation below marine flooding surfaces. The abundance of dolomite cements highlights the importance of macroscopic-scale diagenetic carbonate mobility in these mudstones.In addition to carbonate-cements, diagenetic alteration and precipitation of quartz and alumina-silicate minerals are also important in these mudstones. Kaolinite is present both in uncompacted test of organisms and as vein fills in septarian concretions. Kaolinite precipitation is interpreted to have occurred prior to significant compaction and indicates that both silicon and aluminium were mobile during early diagenesis. We interpret the abundance of early diagenetic kaolinite cement to be the result of Al-mobilization by organic acids generated during organic matter oxidation reactions, with the Al sourced from poorly crystalline detrital aluminium oxides and clay minerals. There is also indirect evidence for burial diagenetic kaolinitization of feldspar grains. Quartz cement takes the form of quartz overgrowths and microcrystalline quartz crystals. Textures and CL spectra for the quartz microcrystalline cement suggests that recrystallization of biogenic silica (opal-A) was likely to have been an important source for quartz cements, although smectite-to-illite transformation may have contributed some. These mineral phases highlight that microscopic-scale diagenetic mobility of silica is important, even within mudstones lacking obvious sources of biogenic silica and is likely to be an important processes in a wide range of mudstones.

Petrography and Geochemical Signatures in Cracks Filling Calcite Sequences in Septarian Concretions, Sanganeh Formation, Kopet-Dagh Basin, NE Iran

Petrography and Geochemical Signatures in Cracks Filling Calcite Sequences in Septarian Concretions, Sanganeh Formation, Kopet-Dagh Basin, NE Iran

Spherical and columnar, septarian, 18O‐depleted, calcite concretions from Middle–Upper Permian lacustrine siltstones in northern Mozambique: evidence for very early diagenesis and multiple fluids

AbstractCalcite septarian concretions from the Permian Beaufort Group in the Maniamba Graben (NW Mozambique) allow controls on the composition and nature of diagenetic fluids to be investigated. The concretions formed in lacustrine siltstones, where they occur in spherical (1 to 70 cm in diameter) and columnar (up to 50 cm long) forms within three closely spaced, discrete beds totalling 2·5 m in thickness. Cementation began at an early stage of diagenesis and entrapped non‐compacted burrows and calcified plant roots. The cylindrical concretions overgrew calcified vertical plant roots, which experienced shrinkage cracking after entrapment. Two generations of concretionary body cement and two generations of septarian crack infill are distinguished. The early generation in both cases is a low‐Mn, Mg‐rich calcite, whereas the later generation is a low‐Mg, Mn‐rich calcite. The change in chemistry is broadly consistent with a time (burial)‐related transition from oxic to sub‐oxic/anoxic conditions close to the sediment–water interface. Geochemical features of all types of cement were controlled by the sulphate‐poor environment and by the absence of bacterial sulphate reduction. All types of cement present have δ13C ranging between 0‰ and −15‰(Vienna Peedee Belemnite, V‐PDB), and highly variable and highly depleted δ18O (down to 14‰ Vienna Standard Mean Ocean Water, V‐SMOW). The late generation of cement is most depleted in both 13C and 18O. The geochemical and isotopic patterns are best explained by interaction between surface oxic waters, pore waters and underground, 18O‐depleted, reducing, ice‐meltwaters accumulated in the underlying coal‐bearing sediments during the Permian deglaciation. The invariant δ13C distribution across core‐to‐rim transects for each individual concretion is consistent with rapid lithification and involvement of a limited range of carbon sources derived via oxidation of buried plant material and from dissolved clastic carbonates. Syneresis of the cement during an advanced stage of lithification at early diagenesis is considered to be the cause of development of the septarian cracks. After cracking, the concretions retained a small volume of porosity, allowing infiltration of anoxic, Ba‐bearing fluids, resulting in the formation of barite. The results obtained contribute to a better understanding of diagenetic processes at the shallow burial depths occurring in rift‐bound, lacustrine depositional systems.

Stable isotope geochemistry of calcrete nodules and septarian concretions in a Quaternary ‘red clay’ paleovertisol from Hungary†

Calcrete nodules and concretions in unusually large amounts are embedded in the Quaternary clay-rich (Vertisol-type) ‘red clay’ soil-sedimentary complex at the pediment of the Mátra Mountains (Hungary). Stable isotope signatures were studied in nodules and septarian concretions, uncommon due to their several millimeter sized calcite crystals filling voids and fractures, to reveal their origin. The isotope composition of calcrete covers a wide range: δ18O=−5.9 to−10.4 ‰ and δ13C=−8.9 to−12.3 ‰ (vs. V-PDB). Isotope compositions support pedogenic (sensu stricto) and/or shallow groundwater origin for the calcrete nodules and concretions, the role of ‘evolved’ (isotopically modified) groundwaters in the formation of secondary carbonate was possibly subordinate. Late-stage, large, Mn-rich euhedral calcite crystals in concretions have the lowest δ13C values, which are interpreted as a result of larger contribution of isotopically light organic carbon due to decomposition of organic matter under reducing conditions. Precipitation of late calcite crystals in concretions occurred in early diagenetic environment after shallow burial of the ‘red clay’ paleovertisol. †Revised version of a paper presented at the VIII Isotope Workshop of the European Society for Isotope Research (ESIR), June 25 to 30, 2005, Leipzig-Halle, Germany

Jurassic septarian concretions from NW Scotland record interdependent bacterial, physical and chemical processes of marine mudrock diagenesis

AbstractSeptarian concretions in the Staffin Shales Formation (Kimmeridgian, Isle of Skye) allow controls on concretion rheology and septarian cracking to be investigated. Stratabound concretions consist of anhedral ferroan calcite microspar enclosing clay and minor pyrite. Intergranular volumes range from 77% to 88%, and calcite δ13C and δ18O values in most concretion bodies range from −10·0‰ to −17·3‰ and +0·3‰ to −0·6‰ respectively, consistent with rapid and pervasive cementation in marine pore fluids. Septarian rupture occurred during incipient cementation, with a sediment volume reduction of up to 43%. Crack‐lining brown fibrous calcite records pore fluid re‐oxygenation during a depositional hiatus, followed by increasing Fe content and δ13C related to bacterial methanogenesis. Brown colouration results from an included gel‐like polar organic fraction that probably represents bacterially degraded biomass. A new hypothesis for concretion growth and septarian cracking argues that quasi‐rigid ‘proto‐concretions’ formed via binding of flocculated clays by bacterial extracellular polysaccharide substances (EPS). This provided rheological and chemical conditions for tensional failure, subcritical crack growth, volume contraction, calcite nucleation, and incorporation of degraded products into crack‐lining cements. Bacterial decay of EPS and syneresis of host muds provided internal stresses to initiate rupture at shallow burial. Development of septarian (shrinkage) cracks in muds is envisaged to require pervasive in situ bacterial colonization, and to depend on rates of carbonate precipitation versus EPS degradation and syneresis. Subsequent modification of septarian concretions included envelopment by siderite and calcite microspar, hydraulic fracturing associated with Cretaceous shallow burial or Palaeogene uplift; and cementation by strongly ferroan, yellow sparry calcite that records meteoric water invasion of the host mudrocks. An abundance of fatty acids in these spars indicates aqueous transport of organic breakdown products, and δ13C data suggest a predominantly methanogenic bicarbonate source. However, the wide δ18O range for petrographically identical cement (−1·3‰ to −15·6‰) is difficult to explain.

Organic geochemistry and stable isotope composition of New Zealand carbonate concretions and calcite fracture fills

Carbonate concretion bodies, representing a number of morphological types, and associated calcite fracture fills, mainly from New Zealand, have been studied both organically and inorganically. Extracted organic material is dominated by a complex polymeric dark brown highly polar fraction with a subordinate less polar and lighter coloured lipid fraction. The relative proportion of the two fractions is the principal control on the colour of fracture fill calcites. Concretions are classified mainly by reference to their carbonate stable carbon and oxygen isotope and cation composition. Typical subspherical calcitic septarian concretions, such as those in the Paleocene Moeraki and the Eocene Rotowaro Siltstones, contain carbon derived mainly by bacterial sulfate reduction in marine strata during early diagenesis. Other concretions, including a septarian calcitic type from the Northland Allochthon, have a later diagenetic origin. Siderite concretions, abundant in the non‐marine Waikato Coal Measures, are typically dominated by methanogenic carbon, whereas paramoudra‐like structures from the Taranaki Miocene have the most extreme carbon isotope compositions, probably resulting from methane formation or oxidation in fluid escape conduits. Lipids from concretion bodies and most fracture fill calcites contain significant concentrations of fatty acids. Concretion bodies dominated by bimodally distributed n‐fatty acids with strong even‐over‐odd preference, in which long chain n‐acids are of terrestrial origin, have very low hydrocarbon biomarker maturities. Concretion bodies that lack long chain n‐acids often have higher apparent biomarker maturity and prominent α‐ω diacids. Such diacids are abundant in fracture fill calcites at Rotowaro, especially where calcite infills the septaria of a siderite concretion in the non‐marine Waikato Coal Measures, and support the view that fluid transport resulted in carbonate entrapment of the fracture‐hosted acids. Diacids also occur in Northland calcite concretion bodies, but not in their septarian fracture fill. Release from kerogen into migrating pore fluid during an early organic maturation stage is suggested as a plausible origin of the diacids. Their site of entrapment may have been serendipitous, depending on the timing of concretion body and fracture fill carbonate precipitation.

Eogenetic siderite as an indicator for fluctuations in sedimentation rate in the Oligocene Boom Clay Formation (Belgium)

Horizons with septarian concretions are a salient feature of the marine Boom Clay Formation. At most horizons, the concretions consist of ferroan calcite with variable amounts of pyrite, but at stratigraphic level S60 they also contain siderite. S60 is situated at the centre of an intensely bioturbated zone that is underlain by a pyrite-rich layer. Furthermore, the enclosing clay is strongly enriched in iron, manganese and phosphorous. The sedimentological and chemical zoning is indicative for low sedimentation rates, which allowed the concentration of iron in the aerobic zone of the sediment. Concentration of iron was the prerequisite for the formation of the siderite-containing concretions. The co-precipitation with pyrite is an argument for a formation in the sulphate reduction zone, and is indicative for a high rate of iron-reduction. The latter was due to the rapid burial of the iron-enriched layer below the redox boundary. The abrupt fluctuations in sedimentation rate were a response to the maximum flooding event of the second Rupelian third-order relative sea-level cycle, which caused a brief pushback of the detrital sediment wedge to its source areas. As this response is logically explained by the general sequence stratigraphic model [Spec. Publ.-Soc. Econ. Paleontol. Mineral. 42 (1988) 109], early diagenetic siderite may be widespread at maximum flooding surfaces in rapidly prograding marine mudstones.

The first finding of the crab Ctenocheles (Decapoda) in Oligocene beds in Slovenia

In the paper remains of crabs of genus Ctenocheles are discussed. They were found in the Oligocene grey marly mudstone at Čeplje at Vransko. This is the first evidence of this crab genus in Slovenia. In the Oligocene mudstone at Čeplje also numerous other fossil remains were registred, and also abundant septarian concretions.

Septarian concretions from the Oxford Clay (Jurassic, England, UK): involvement of original marine and multiple external pore fluids

Septarian concretions occur at several horizons within the Oxford Clay Formation, a marine mudstone containing pristine aragonite and immature biomarker molecules. They record the passage of at least four generations of pore fluids, the first of marine origin and the last still present in cavities. Concretion bodies formed, cracked, and calcite and pyrite precipitated in and around the cracks within the sulphate reduction zone, as demonstrated by C, O, S and Sr isotopic composition (Pore fluid 1). Before major compaction, sandstone dykes were intruded locally, and baryte precipitated, followed by coarse calcite cements with isotopically light oxygen and radiogenic strontium, indicating the introduction of meteoric‐derived water (Pore fluid 2). Later, coarse celestine within concretions has distinct sulphur‐isotopic composition and requires a further, geographically restricted, water source (Pore fluid 3). Celestine‐bearing concretions contain water in tight cavities whose isotopic composition is close to that of modern precipitation. Its chemistry shows that it is equilibrating with pre‐existing minerals implying a relatively recent origin (Pore fluid 4). The mineralogy of the Oxford Clay concretions shows that complex results can follow from a simple burial and uplift history, and that multiple generations of pore fluids can pass through a low‐permeability clay.

Septarian concretions: internal cracking caused by synsedimentary earthquakes

Septarian concretions are abundant in many Phanerozoic marine and marginal‐marine shales and mudstones. They range from a few centimetres to several metres in size and are spherical or ellipsoidal in morphology. In general, formation by localized calcite or siderite cementation in argillaceous sediments began under less than a few metres of burial. Septarian cracks vary widely in shape and configuration: included are networks of wide, vertically, radially and sometimes concentrically oriented, lenticular shrinkage cracks; and narrower, parallel‐sided, straight to irregular tension cracks locally accompanied by brecciation, and plumose and en échelon sigmoidal cracks indicative of shear stresses. Crack types are intergradational; many concretions exhibit multiple cracking events. Enclosed macrofossils and isopachous fibrous calcite cement that lines earlier formed cracks are commonly broken and displaced. In some cases, cracks contain injected lime mud and silt. These features, taken together, testify that cracking involved a spectrum of responses in concretion interiors, from loss of shear strength, dewatering and shrinkage to brittle failure, demonstrating variations in, and contrasts between, the rheological properties of the matrix and enclosed objects. Localization to interiors and outward tapering of lenticular cracks make a case for fracture partitioning and indicate that interiors were softer than exteriors at the time of shrinkage. Parallel‐sided cracks point to greater stiffness, and evolving crack shape in multiply cracked concretions shows that rigidity increased with time. Crack orientations indicate highly variable tensile and shear stress directions within individual concretions. Rupture, brecciation, displacement of fragments, loss of shear strength, liquefaction and injection of unconsolidated granular sediment suggest that deformation events were rapid, if not virtually instantaneous. Previous explanations for the internal cracking, such as gas generation, spontaneous chemical dehydration or localized overpressuring due to compaction, seem either untenable or fail to account for the spectrum of observed features. However, syndepositional earthquake‐induced shaking of cementing bodies of varying rigidity at shallow burial depths seems to be a plausible source for the requisite short‐lived, variable to anisotropic, high‐stress regime inside the concretions. Septarian concretions may thus preserve a signature of basin seismicity as it relates to their cementation history.

An assessment of calcite crystal growth mechanisms based on crystal size distributions

Calcite crystal growth experiments were undertaken to test a recently proposed model that relates crystal growth mechanisms to the shapes of crystal size distributions (CSDs). According to this approach, CSDs for minerals have three basic shapes: (1) asymptotic, which is related to a crystal growth mechanism having constant-rate nucleation accompanied by surface-controlled growth; (2) lognormal, which results from decaying-rate nucleation accompanied by surface-controlled growth; and (3) a theoretical, universal, steady-state curve attributed to Ostwald ripening. In addition, there is a fourth crystal growth mechanism that does not have a specific CSD shape, but which preserves the relative shapes of previously formed CSDs. This mechanism is attributed to supply-controlled growth. All three shapes were produced experimentally in the calcite growth experiments by modifying nucleation conditions and solution concentrations. The asymptotic CSD formed when additional reactants were added stepwise to the surface of solutions that were supersaturated with respect to calcite (initial Ω = 20, where Ω = 1 represents saturation), thereby leading to the continuous nucleation and growth of calcite crystals. Lognormal CSDs resulted when reactants were added continuously below the solution surface, via a submerged tube, to similarly supersaturated solutions (initial Ω = 22 to 41), thereby leading to a single nucleation event followed by surface-controlled growth. The Ostwald CSD resulted when concentrated reactants were rapidly mixed, leading initially to high levels of supersaturation (Ω >100), and to the formation and subsequent dissolution of very small nuclei, thereby yielding CSDs having small crystal size variances. The three CSD shapes likely were produced early in the crystallization process, in the nanometer crystal size range, and preserved during subsequent growth. Preservation of the relative shapes of the CSDs indicates that a supply-controlled growth mechanism was established and maintained during the constant-composition experiments. CSDs having shapes intermediate between lognormal and Ostwald also were generated by varying the initial levels of supersaturation (initial Ω = 28.2 to 69.2) in rapidly mixed solutions. Lognormal CSDs were observed for natural calcite crystals that are found in septarian concretions occurring in southeastern Colorado. Based on the model described above, these CSDs indicate initial growth by surface control, followed by supply-controlled growth. Thus, CSDs may be used to deduce crystal growth mechanisms from which geologic conditions early in the growth history of a mineral can be inferred. Conversely, CSD shape can be predicted during industrial crystallization by applying the appropriate conditions for a particular growth mechanism.

Stable Carbon and Oxygen Isotope Evidence of Cretaceous Sea-Level Fluctuations Recorded in Septarian Concretions from Pueblo, Colorado, U.S.A.

ABSTRACT Limestone concretions in the Blue Hill Member of the Carlile Formation (middle Middle Turonian) exposed in the Lake Pueblo State Park, Colorado, U.S.A., range in size from 20-cm-diameter, approximately spherical forms to flattened spheroids >2 m in width and >1 m in height. The earliest matrix and septarian calcite cement, with 13C and 18O values ranging mostly from -18o/oo to -8o/oo (PDB) and -3o/oo to -8o/oo (PDB), respectively, precipitated from diluted seawater during shallow burial where sulfate reduction largely controlled pore-water bicarbonate composition. In contrast, the latest septarian cements exhibit very depleted 18O values (-16o/oo) with 13C values approaching values of -1o/oo to -3o/oo, reflecting a strong meteoric-water component to pore fluids. Overall, 13C and 18O values are inversely covariant. Concretion matrix calcite and septarian cements precipitated from mixtures of modified marine and meteoric fluids. Pore-water reversals resulted from sea-level fluctuations leading up to and associated with the formation of a sequence boundary, or possibly the development of parasequences near the top of the Blue Hill Member. Relative sea-level falls caused pore-water freshening as the coastal mixing zone migrated seaward. Similarly, modified marine-derived pore waters replaced fresher pore fluids accompanying subsequent marine flooding. Carbonate cements precipitated during this active hydrologic history recorded the isotopic characteristics of their parent fluids. Negative 18O values of the meteoric end-member calcite compositions suggest derivation from high-altitude precipitation in the Sevier highlands to the west. Septarian cements in concretions, despite their enclosure in relatively impermeable host sediment, can act as sensitive indicators of relative sea-level fluctuations.

Carbonates and oxalates in sediments and landfill: monitors of death and decay in natural and artificial systems

Subaqueous sediments and landfill waste disposal sites are both natural locations for anaerobic microbiological metabolism, where sulphates are reduced to sulphide, and organic matter (particularly acetate) is oxidized and degraded to produce ultimately CO 2 and CH 4 . The microbiologically mediated geochemical processes in each case are similar. However, natural sedimentary porewaters can be difficult to sample and monitor, whereas landfill sites are relatively easily accessible, yielding leachate samples that are routinely monitored over periods of time. In both environments, carbonate precipitation is predicted to be a normal consequence of anaerobic microbiological activity, consistent with changes in leachate composition and the predictions of geochemical modelling. Initial investigation of scale from a landfill drainage system confirms the presence of calcite, together with gypsum and iron minerals. Calcium oxalate minerals (whewellite and weddelite) are widespread in their occurrence within septarian concretions from a number of geological formations; whewellite is reportedly stable up to 160°C. The solubility of calcium oxalate is very low, and precipitation readily takes place when oxalate (oxalic acid) is present (derived from fungal processes in sediments and soils). Calcium oxalate minerals should be expected wherever vegetable matter is decomposing; this link can be demonstrated for natural sediments, but has yet to be proven for landfill.

Stable Carbon and Oxygen Isotope Evidence of Cretaceous Sea-Level Fluctuations Recorded in Septarian Concretions from Pueblo, Colorado, U.S.A.

Stable Carbon and Oxygen Isotope Evidence of Cretaceous Sea-Level Fluctuations Recorded in Septarian Concretions from Pueblo, Colorado, U.S.A.