Glauconitic Sediments Research Articles

Stratigraphic and Geochemical Evidence for the Alteration of Calcareous Glauconitic Marine Sediments to Calcium Bentonite

ABSTRACT Calcium bentonite mined in Smith County, Mississippi, has been reported in numerous publications to be derived from the weathering of volcanic ash. These interpretations were based on the bentonite having similar properties to bentonites actually formed from volcanic precursors. No recent detailed stratigraphic mapping in combination with modern laboratory analyses had been conducted for this area. Calcium bentonite, found at Olmstead, Illinois, in contrast, formed in situ from the weathering of biotite, glauconite, quartz, and other materials found in shallow calcareous marine deposits. Detailed stratigraphic mapping in Smith County in 2011 was carried out by Mississippi State University as part of a proposed surface reservoir study. The mapping, utilizing GPS and continuous sonic core samples, indicated that calcium bentonite was present and similarly formed by the in situ weathering of calcareous, glauconitic, marine marl in several formations in the Oligocene Vicksburg Group. The bentonite was not restricted to one stratigraphic interval as would be the case for a true ashfall deposit. Additional research conducted at Mississippi State University and more recently by others confirmed that the bentonite was formed by weathering and contemporaneous microbial action of the calcareous glauconitic marls. No volcanic ash was detected in any of the samples tested. The preponderance of other material present in the Smith County bentonite renders the presence of minute amounts of volcanic detritus volumetrically insignificant in the formation of the clay mass. A conceptual model is presented showing how the bentonite was formed and why it is restricted to this small area of the Oligocene outcrop.

New paleoenvironmental insights on the Miocene condensed phosphatic layer of Salento (southern Italy) unlocked by the coral-mollusc fossil archive

From the Late Oligocene to the Late Miocene, the central Mediterranean area was characterized by the extensive deposition of phosphate-rich sediments. They are usually represented by 10 to 20-cm-thick hardgrounds made of phosphatic and glauconitic sediments containing a rich macrofossil association. This study represents the first thorough investigation of the biotic assemblage of Mediterranean phosphorites aimed at collecting new information on the environmental factors controlling their deposition. The Serravallian/Tortonian phosphatic deposits of the Salento Peninsula (“Aturia level”) have been selected for the abundance of fossil remains and special attention is given to the coral–mollusc association. Two different facies have been recognized: a basal coral rudstone that includes most of the macrofossils, superimposed by a detrital rudstone made of thin layers mainly composed of phosphatic fragments. These two facies are separated by a phosphatic crust several millimeters in thickness. The coral assemblage contains at least 17 azooxanthellate taxa belonging to four families, while the molluscs are represented by a rich gastropod fauna (26 species), associated with bivalves (18 species) and cephalopods (two species). Four distinct depositional phases have been recognized, with the coral rudstone representing the key-facies to reconstruct the onset of the “Aturia level” and the original environment of its fossil content. The composition of the coral–mollusc association has been reliably compared with present-day analog taxa, suggesting the occurrence of a heterogeneous seafloor formed by rocky substrates and accumulations of soft sediment, at around 100–350-m water depth, and under the influence of moderate-to-strong bottom currents rich in nutrients and resuspended organic matter.

The palaeomagnetism of glauconitic sediments

The palaeoenvironmental significance of glaucony has long been appreciated, but accurate palaeomagnetic dating of events recorded by glauconitic horizons requires an understanding of how glauconitic sediments acquire a remanent magnetization. Pure glauconitic minerals are paramagnetic, but glauconite grains are large and slow-forming (over periods that can exceed 100kyr), with complex and variable morphologies. It is, thus, possible that small magnetic grains within glaucony particles may carry a significant fraction of the remanence in weakly magnetized sediments. Any remanence carried by glauconitic grains may therefore represent the geomagnetic field at a time significantly later than the time of deposition, or a time-averaged signal over some or all of the formation period. We investigated this problem using weakly magnetic Palaeocene glauconitic siltstones from southern New Zealand. We disaggregated the rock and separated it magnetically into glauconitic and non-glauconitic fractions. Results from stepwise isothermal remanent magnetization (IRM) acquisition, alternating-field demagnetization, temperature dependence of magnetic susceptibility, and stepwise thermal demagnetization of a triaxial IRM were used to demonstrate that the remanent magnetization is carried by single-domain or pseudo-single-domain magnetite in the non-glauconitic sediment fraction, and that the glauconite grains themselves make no contribution to the remanent magnetization. However, accurate measurement of the primary remanence is complicated by a strong viscous overprint and mineral alteration during thermal demagnetization studies. Identification of magnetite as the remanence carrier in sediments within a reducing diagenetic environment gives confidence that the remanence has a depositional origin. Glauconite does not carry a remanence; therefore, its effect is to dilute and weaken the overall magnetization. Furthermore, the use of rock magnetic parameters may be problematic when glauconite concentrations are (as in the studied sediments) orders of magnitude greater than remanence carrier concentrations, because in such cases the glauconite susceptibility can dominate that of the remanence carriers.

Microbial transformations of arsenic: Mobilization from glauconitic sediments to water

In the Inner Coastal Plain of New Jersey, arsenic (As) is released from glauconitic sediment to carbon- and nutrient-rich shallow groundwater. This As-rich groundwater discharges to a major area stream. We hypothesize that microbes play an active role in the mobilization of As from glauconitic subsurface sediments into groundwater in the Inner Coastal Plain of New Jersey. We have examined the potential impact of microbial activity on the mobilization of arsenic from subsurface sediments into the groundwater at a site on Crosswicks Creek in southern New Jersey. The As contents of sediments 33–90 cm below the streambed were found to range from 15 to 26.4 mg/kg, with siderite forming at depth. Groundwater beneath the streambed contains As at concentrations up to 89 μg/L. Microcosms developed from site sediments released 23 μg/L of As, and active microbial reduction of As(V) was observed in microcosms developed from site groundwater. DNA extracted from site sediments was amplified with primers for the 16S rRNA gene and the arsenate respiratory reductase gene, arrA, and indicated the presence of a diverse anaerobic microbial community, as well as the presence of potential arsenic-reducing bacteria. In addition, high iron (Fe) concentrations in groundwater and the presence of iron-reducing microbial genera suggests that Fe reduction in minerals may provide an additional mechanism for release of associated As, while arsenic-reducing microorganisms may serve to enhance the mobility of As in groundwater at this site.

Discussion of “The bioeroded megasurface of Oura (Algarve, south Portugal): implications for the Neogene stratigraphy and tectonic evolution of southwest Iberia” by Cachão et al. (Facies 55(2): 213–225, DOI 10.1007/s10347-008-0172-2)

With reference to the above cited paper, we were surprised to see that the authors present four isotopic ages obtained in diVerent localities from that of the Wgured section (Oura Neogene section, Fig. 2) and use them to support very speculative (and most likely erroneous) conclusions. None of the four numerical dates presented were obtained from the Oura section. Instead, they derive from diVerent outcrops and diVerent sections, with no direct correlations. In addition, no reference to isotopic dates from the Oura section published in Pais et al. (2000) was made despite the paper was cited by Cachao et al. (2009). Also, some paleontological and biostratigraphic data (Antunes et al. 1981) concerning the Oura section are not discussed. The Oura Neogene section coordinates (Google) are 37°05 02 /8°13 50 . In fact, the numerical dates of 8.15 Ma (§0.29) and 7.15 Ma (§0.27) presented in the lower part of bed 3 (Fig. 2; Cachao et al. 2009) were obtained by Boski et al. (1995) in glauconitic sediments from the Gale/Castelo region, at a location with coordinates 37°04 25 /8°17 50 (Fig. 1). We obtained another date of 10.1 Ma (§0.25 Ma), in the same Gale/Castelo region, from a locality with coordinates 37°04 51 /8°18 58 , which is not referred to by Cachao et al. The isotopic age 3.0 (+2.5/i1.0) Ma presented from the upper part (bed 8) of the Wgured succession (Oura Neogene section, Fig. 2) was obtained by us in a diVerent section named “Olhos de Agua”, with coordinates 37°05 27 / 8°10 40 , in a mollusc-rich bed, intercalated with whitish, and thin Xuvial sandstones (Fig. 2; Legoinha (2001), p. 178, Wg. 64). Although the description presented by Cachao et al. for bed 8 (Oura Neogene section, Fig. 2) may correspond with that of the original level dated in the Olhos de Agua section, it does not bear much similarity to the observed lithology of bed 8, in the Oura section, which is a conglomerate containing quartz and highly abraded oyster clasts. We also studied and logged the “Oura Neogene Section”, but under the name Auramar (“Corte de Auramar”) (Antunes et al. 1981; Pais 1982; Legoinha 2001). A correspondence between our description and measured section (see Legoinha 2001, p. 177, Wg. 63) and the graphic columnar section of Cachao et al. (2009) can easily be established. Notice that in bed 3 (where they place the age from Boski et al. 1995) we have obtained an isotopic age of 9.5 (+1.0/i0.5) Ma (which was also referred to in Pais et al. 2000, in the table of isotopic age data). Furthermore, we have obtained an isotopic age of 8.3 (+2.2/i3.3) Ma, for bed 6 of Cachao et al. (2009), which they do not make reference to. Notice again that in bed 12 of our section (coarse conglomerate with large oysters), which corresponds to bed 8 of Cachao et al. (2009), they erroneously put the age that we obtained in the Olhos de Agua section. On the other hand, in the “Oura Neogene section”, we have not obtained any numerical dates for the Lagos Biocalcarenite (Lagos Portimao Formation). However, Cachao et al. indicate 11.3 Ma (+0.9/i1.3), which was obtained by us at an outcrop (some 7 km westward) at Gale Beach. We should note that in the Olhos de Agua section, which is nearer to the Oura section (some 3 km eastward), we obtained an isotopic age of 14.4 Ma (+0.5/i0.6) for the top of the Lagos Biocalcarenite (Lagos Portimao Formation) J. Pais (&) · P. Legoinha CICEGe, Faculdade de Ciencias e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Lisbon, Portugal e-mail: jjp@fct.unl.pt

Arsenic in sediments, groundwater, and streamwater of a glauconitic Coastal Plain terrain, New Jersey, USA—Chemical “fingerprints” for geogenic and anthropogenic sources

Abstract Glauconite-bearing deposits are found worldwide, but As levels have been determined for relatively few. The As content of glauconites in sediments of the Inner Coastal Plain of New Jersey can exceed 100 mg/kg, and total As concentrations (up to 5.95 μg/L) found historically and recently in streamwaters exceed the State standard. In a major watershed of the Inner Coastal Plain, chemical “fingerprints” were developed for streambed sediments and groundwater to identify contributions of As to the watershed from geologic and anthropogenic sources. The fingerprint for streambed sediments, which included Be, Cr, Fe and V, indicated that As was predominantly of geologic origin. High concentrations of dissolved organic C, nutrients (and Cl−) in shallow groundwater indicated anthropogenic inputs that provided an environment where microbial activity released As from minerals to groundwater discharging to the stream. Particulates in streamwater during high flow constituted most of the As load; the chemical patterns for these particulates resembled the geologic fingerprint of the streambed sediments. The As/Cr ratio of these suspended particles likely indicates they derived not only from runoff, but from groundwater inputs, because As contributed by groundwater is sequestered on streambed sediments. Agricultural inputs of As were not clearly identified, although chemical characteristics of some sediments indicated vehicle-related inputs of metals. Sediment sampling during dry and wet years showed that, under differing hydrologic conditions, local anthropogenic fingerprints could be obscured but the geologic fingerprint, indicating glauconitic sediments as an As source, was robust.

Integrated Late Santonian-Early Campanian sequence stratigraphy, New Jersey Coastal Plain: Implications for global sea-level studies

Recent studies of the Santonian-lower Campanian section (Merchantville Formation) in two coreholes (Ancora and Bass River) from the New Jersey coastal plain postulated the existence of concatenated sequences in glauconitic facies (Miller et al. 2004). The lithologic expression of unconformities within the Merchantville glauconitic sediments is obscure and their resolution requires an integrated stratigraphic approach. Here, we present detailed litho-and biostratigraphic analyses of the Merchantville Formation sequences in four New Jersey coastal plain coreholes (Sea Girt, Ancora, Millville, and Bass River). Biofacies analysis reveals four benthic foraminiferal assemblages from inner to middle neritic environments. Correlation of benthic foraminiferal biofacies, percentage of planktonic foraminifera, and lithologic data indicates that paleowater depths varied between 30 and 75m. The correlation of biostratigraphy, benthic foraminiferal biofacies, lithology, and gamma logs delineates unconformities, and identifies three sequences within the Merchantville Formation (MeI, MeII, MeIII). The ages of these sequences have been constrained by calcareous nannoplankton biostratigraphy (MeI: CC16; MeII: CC17-18; MeIII: CC18-19) from 78.8 to 84.5 Ma. The Merchantville I sequence is observed in the Sea Girt and Bass River coreholes. The Merchantville II and III sequences are present throughout the New Jersey coastal plain, and the ages of the Merchantville II and III sequences boundaries are similar to those of North Carolina, and North-Western Europe suggesting an interregional cause of sequences boundaries. Comparison of the New Jersey shallow marine sequences with the deep-sea record from DSDP Site 511 (Falkland Plateau) show that the episodes of sea-level fall in New Jersey are synchronous with 18O increases. Our data imply that the Merchantville sequence boundaries may be attributed to ice volume increases and support the contention that global sea-level changes were controlled by ice-volume variations even during the peak warmth of the Cretaceous.

A particular temper: mineralogical and petrographic characterisation of ceramic fabrics with glauconitic inclusions

This paper focuses on the mineralogical and petrographic characterisation of ceramic fabrics rich in glauconitic pellets, found out in a few local productions of European and Mediterranean sites dated from Prehistory to the Middle Ages. The main distinguishing features of glauconitic inclusions are their rounded shape, dark colour and peculiar chemical composition. During the firing – as also evidenced by a few preliminary experimental tests on glauconitic sediments – the pellets change in colour (from green to red or black in thin section) because of the oxidation of divalent iron and their texture becomes more homogeneous, up to the partial or complete vitrification at moderately high temperatures.

A new hexactinellid sponge from the Cretaceous of Devon Island, Canadian High Arctic

Over 20 specimens of the new genus and species Nunavutospongia irregulara, an irregular stump-like to columnar, or fan- to blade-shaped, hexactinosid hexactinellid sponge, have been recovered from exposures of the Upper Cretaceous upper Kanguk Formation on Devon Island, Nunavut Territory, in the Canadian High Arctic. The species and genus are characterized by dermal surfaces with prominent radial, irregularly vertical, rib-like flanges that have a single linear series of separated oscula perforating their rounded crests. The thin, but persistent dermal layer of fused hexactine-based spicules has rays thicker than those of the similarly fused endosomal, quadrangularly arranged, hexactines of the principal skeleton. Ostia of coarse exhalant canals, and finer inhalant canals, are extensively and uniformly developed, but irregularly placed, in the dermal layer. A thin, less prominent, gastral layer of thickened spicules lines the short, internally tapered spongocoel openings. The skeleton exposed in the commonly upward-arcuate base of each sponge has a radiate structure that does not have significant ostia. The new genus is tentatively included in the Family Cribrospongiidae Roemer, 1864, within the Order Hexactinosa Schrammen, 1903. The specimens were found in unconsolidated fine-grained glauconitic sediments along with other fossils such as fish teeth, bird bones, lingulids, coprolites, and wood. The sedimentology, stratigraphic context, and co-occurrence with fossil wood suggest that these sponges inhabited the neritic zone. As such, they would have been subject to polar light regimes, because paleogeographic reconstructions indicate that Devon Island was above the Arctic Circle during the Late Cretaceous. The distribution of the specimens suggests that they were solitary sponges that used their arcuate bases to colonize sandy substrates or biotic debris.

Ground Water Stratification and Delivery of Nitrate to an Incised Stream under Varying Flow Conditions

Ground water processes affecting seasonal variations of surface water nitrate concentrations were investigated in an incised first-order stream in an agricultural watershed with a riparian forest in the coastal plain of Maryland. Aquifer characteristics including sediment stratigraphy, geochemistry, and hydraulic properties were examined in combination with chemical and isotopic analyses of ground water, macropore discharge, and stream water. The ground water flow system exhibits vertical stratification of hydraulic properties and redox conditions, with sub-horizontal boundaries that extend beneath the field and adjacent riparian forest. Below the minimum water table position, ground water age gradients indicate low recharge rates (2-5 cm yr(-1)) and long residence times (years to decades), whereas the transient ground water wedge between the maximum and minimum water table positions has a relatively short residence time (months to years), partly because of an upward increase in hydraulic conductivity. Oxygen reduction and denitrification in recharging ground waters are coupled with pyrite oxidation near the minimum water table elevation in a mottled weathering zone in Tertiary marine glauconitic sediments. The incised stream had high nitrate concentrations during high flow conditions when much of the ground water was transmitted rapidly across the riparian zone in a shallow oxic aquifer wedge with abundant outflow macropores, and low nitrate concentrations during low flow conditions when the oxic wedge was smaller and stream discharge was dominated by upwelling from the deeper denitrified parts of the aquifer. Results from this and similar studies illustrate the importance of near-stream geomorphology and subsurface geology as controls of riparian zone function and delivery of nitrate to streams in agricultural watersheds.

MORPHOLOGY AND CHARACTERIZATION OF SOILS FORMED IN MARLBORO CLAY REGOLITH

The Marlboro Clay is a late Paleocene/early Eocene geologic formation that outcrops in significant portions of Prince George's County, Maryland. The formation exhibits slope instability and is unsuitable for most building structures. In this study, we describe and characterize the mineralogical and shrink-swell properties of soils formed in Marlboro Clay regolith. Samples were collected from 2 pits, 7 deep cores, and 38 1-m surface auger borings, five of which were collected at the same site as the deep cores. Potential volume change, particle size, and pH value were analyzed on all samples. X-ray diffraction scans were performed on selected samples. Mineralogical analysis indicated that the clay fractions of surficial samples were from approximately 10 to 50% smectite. However, the swell index values resulting from the potential volume change analysis were generally moderate for soil samples (most samples were less than 18,000 kg m -2 ). Deep cores generally had greater swell index values (mean, 21,205 kg m -2 ) than did surficial samples, indicating that pedogenesis reduces the shrink-swell capacity of these soils. Air-dried pH values were less than 4.0 in many samples, with a minimum of 2.7, indicating sufficient infilling of sulfide-bearing glauconitic sediments from underlying and overlying formations to cause sulfuricization upon oxidation. The pedogenesis of Marlboro Clay materials reduces their shrink-swell potential, although these soils may still be problematic for slope instability and construction because of low permeability.

Glauconite grains in continental shelf sediments around the Korean Peninsula and their depositional implications

Glauconite grains are an important component of the surficial sediments on the Korean continental shelf. Relatively high glauconite contents (>20%) occur near Huksan Island in the Yellow Sea and on the outer shelf of the southern East Sea where they are associated with relict, sandy sediments. By contrast, glauconite grains are rare (<1%) in clay-dominated sediments. The grains can be classified into four major categories: (1) very abundant, green to dark green, spheroidal and lobate grains displaying bulbous and honeycomb exteriors with well-developed surface cracks; (2) dark green, discoidal grains with poorly defined cracks; (3) scarce, moderately dark green, accordion-shaped grains characterized by series of closely spaced, parallel surface ridges; (4) scarce, green to dark green, foraminifer-shaped grains. Microprobe analyses as well as optical microscopy and SEM observations show that most grains are composed of mineral mixtures (e.g., quartz, mica, feldspar) rather than a single mineral species. The mineralogy, morphology, and textural properties suggest that the grains may have formed mainly by replacement of fecal pellets, and the alteration of mica and clays which have filled foraminifera tests. The high potassium contents, rosette-shaped clay structures, and bulbous shapes reflect an “evolved” (mature) stage of glauconitization. Glauconite grains in Korean shelf sediments are presumably relict, and have been produced by the reworking of older glauconitic sediments during the Holocene sea-level transgression.

Strontium isotope geochemistry of groundwaters and streams affected by agriculture, Locust Grove, MD

The effects of agriculture on the isotope geochemistry of Sr were investigated in two small watersheds in the Atlantic coastal plain of Maryland. Stratified shallow oxic groundwaters in both watersheds contained a retrievable record of increasing recharge rates of chemicals including NO 3 −, Cl, Mg, Ca and Sr that were correlated with increasing fertilizer use between about 1940 and 1990. The component of Sr associated with recent agricultural recharge was relatively radiogenic ( 87Sr/ 86Sr=0.715) and it was overwhelming with respect to Sr acquired naturally by water–rock interactions in the oxidized, non-calcareous portion of the saturated zone. Agricultural groundwaters that penetrated relatively unoxidized calcareous glauconitic sediments at depth acquired an additional component of Sr from dissolution of early Tertiary marine CaCO 3 ( 87Sr/ 86Sr=0.708) while undergoing O 2 reduction and denitrification. Ground-water discharge contained mixtures of waters of various ages and redox states. Two streams draining the area are considered to have higher 87Sr/ 86Sr ratios and NO 3 − concentrations than they would in the absence of agriculture; however, the streams have consistently different 87Sr/ 86Sr ratios and NO 3 − concentrations because the average depth to calcareous reducing (denitrifying) sediments in the local groundwater flow system was different in the two watersheds. The results of this study indicate that agriculture can alter significantly the isotope geochemistry of Sr in aquifers and streams and that the effects could vary depending on the types, sources and amounts of fertilizers added, the history of fertilizer use and groundwater residence times.

The geometry of the Rupelian and Chattian depositional bodies in the Lower Rhine district and its border area: implications for Oligocene lithostratigraphy

Three profiles are constructed through the Oligocene succession of the borderland between Belgium, Germany and the Netherlands. The profiles illustrate the geometric relationships between Rupelian and Chattian successions and the lateral and vertical variations within the Rupelian and Chattian deposits. The end-Rupelian profile between Central Belgium and the Achterhoek area in the Netherlands shows a rather uniform thickness distribution. This is in sharp contrast to the fault controlled sedimentation during the Chattian as represented on an end-Chattian profile between the same areas. Within the Chattian succession several lithostratigraphic units can be identified and correlated using a dense net of geophysical well logs. Remarkably similar subdivisions are found in the different tectonic blocks, suggesting eustatic sea-level fluctuations. In the central part of the subsiding area the Chattian deposits are largely continental to deltaic. Laterally,these deposits developed into marine glauconitic sediments. The end-Chattian profile between Bonn and Asten in the Netherlands runs approximately parallel with the subsidence axis. The cyclicity expressed by facies alternations due to a periodically shifting coastline can be correlated over the different blocks again suggesting the involvement of eustatic controls. On this last profile, an anomalous geometry is due to SE-NW stress-induced movements along the Variscan Aachen overthrust.The three lithological profiles are meant to represent a frame for formal stratigraphic subdivisions and nomenclature.

Clastic limestone dikes from the late Proterozoic Bhima Group, South India

Clastic dikes of the late Proterozoic Bhima Group, South India, intruding the Kasturpalli Glauconitic Member are reported from two localities. These glauconitic sediments of the Rabanpalli Formation form the oldest transgressive sequence of the Bhima Group, which starts with basal conglomerates and grades upward into supermature quartz arenites. These are followed successively by glauconitic fine-grained sediments and shales. This sequence is capped by thick deposits of micritic flaggy limestones of the Adki Formation. The dikes are generally emplaced vertically, but also have small lateral apophyses along the bedding planes of the host rocks. The clastic dikes are chiefly composed of a matrix of micritic calcite along with quartz, clay minerals, etc. In places, crystals of calcite are observed in the form of veins. In one locality the dikes are emplaced in the vicinity of a post-depositional fault. This faulting seems to have triggered the intrusion of the clastic material, which shows a thixotropic behaviour, into the glauconitic sandstones and siltstones.

MINERALOGY AND CHEMICAL PROPERTIES OF TWO ULTISOLS FORMED IN GLAUCONITIC SEDIMENTS1

We examined two Ultisols formed from glauconitic sediments of Eocene age for chemical, physical, and mineralogical characteristics. The parent glauconitic sediments have been subjected to intense weathering that has resulted in the release of large quantities of iron and the alteration of glauconite sequentially to vermiculite → smectite → hydroxy-interlayered vermiculite. These soils exhibited high capacities to adsorb K and P. Most of the naturally occurring K was adsorbed on high-energy-specific adsorption sites. Fertilizer K would be expected to be strongly retained by the clay minerals in these soils. Phosphorus adsorption was proportional to the clay and iron oxide content, exchangeable Al3+, and surface area. Adsorbed P was not easily removed by extraction with dilute CaCl2, indicating that it was not readily utilized by plants.

Glauconitic Sediments on the Continental Shelf off Vancouver Island, British Columbia, Canada

ABSTRACT The tectonically active Vancouver Island continental margin is characterized by a very narrow (< 80 km) continental shelf and an extremely steep continental slope. The shelf is marked by high wave and current energies on the inner, nondepositional shelf and very slow rates of sediment accumulation on the outer shelf. Within the muddy sands of the outer shelf and slope, glauconitic grains are frequently an important constituent of the sand fraction, occurring in four principal forms: (a) locally abundant, pale green infillings of benthic foraminifera; (b) very abundant, pale to moderately dark green, dull grains displaying varying degrees of fracturing of the external surfaces; (c) scarce, moderately dark green, altered and expanded mica grains; and (d) scarce, very dark green, small (< 200 µm) grains with polished, highly fractured surfaces. Mineralogically, the grains mainly reflect their origins as detrital sediments. They consist most commonly of quartz, plagioclase minerals, chlorite, smectite, and illite. Also, a mixed-layer mineral, smectite-illite, occurs in significant amounts in glauconitic grams but is absent in unaltered mud fragments and in the clay fraction of sediments from the outer shelf. The appearance of this mineral reflects the first stages of glauconite authigenesis. Bulk analyses of glauconitic grains yielded K2O values of less than 2%, reflecting their very early stage of evolution, but 19-28% Fe2O3, several times the level in the surrounding matrix sediments. Microprobe analyses and scanning electron microscopy show that glauconitic smectite forms in the pores and along fractures in the host grains, commonly as a boxwork arrangement or rosette clusters of blades. Fe2O3 and K2O values at these sites can reach 32.5% and 7.5%, respectively. Several lines of evidence, including radiocarbon age determinations on glauconitized foraminifera tests, indicate a Recent age for the Vancouver Island shelf glauconitic grains.

Alteration Products of Glauconite in Burgan Oil Field, Kuwait

ABSTRACT Core samples of the Cenomanian glauconitic sediments from the Great Burgan oil field, Kuwait, display important mineralogical changes both under surface and subsurface conditions. Unconformities can be demarcated with confidence by studying the chemical weathering of unarmored glauconite which results in the precipitation of hydrated iron oxides and alunite. In the subsurface, close to oil reservoirs, the reduction of the hydrated iran oxide dust in glauconite pellets results in the development of black magnetic siderite. The presence of this mineral may explain the currently observed magnetic anomalies in clastic sediments close to oil reservoirs.

Phosphatic and glauconitic sediments

A meeting on ‘Phosphatic and Glauconitic Sediments‘; is particularly appropriate in view of the considerable but often widely scattered literature, both published and unpublished, that has become available on this subject in recent years. Phosphatic and glauconitic sediments are those in which the calcium phosphate mineral, francolite, a variety of apatite and the hydrated potassium-iron silicate, glauconite, are prominent constituents. As such they have intrigued geologists for many years, particularly in connection with the nature of the chemical reactions responsible for their formation and the relative importance of individual parameters controlling these reactions. These sediments are widely distributed in the geological column, particularly in strata of Cambrian, Cretaceous–Lower Tertiary, and, in the case of phosphatic sediments, Permian and Miocene–Pliocene age. The association of apatite and glauconite in sediments has frequently been reported but integrated studies of the two minerals have, until recently, been comparatively few. These have shown that the minerals form by direct precipitation from sea water at the sediment/water interface, or diagenetically by interstitial replacement or replacement of pre-existing sediment. Phosphatic sediments are of great economic importance since they provide most of the raw material, known commercially by the imprecise term ‘phosphate rock’, for the manufacture of phosphate fertilizers and some phosphorus-based chemicals. For example, sedimentary formations accounted for c. 16 million tonnes ( c. 89%) of the total world output of c. 18 million tones of P in 1978. Although production is dominated by USA, the USSR and Morocco, there are other important producers of phosphate rock. In marked

Red bed genesis by lateritic weathering of glauconitic sediments

Studies of some Mesozoic-Cenozoic 'red beds' in France and in W African basins (Senegal and Ivory Coast) show that they are the result of in situ lateritic weathering of glauconitic sediments. The alterations took place without destruction of pre-existing sedimentary structures. Glauconitic grains are weathered to ferruginous öoids consisting of Al-goethite, or are weathered to a kaolinite+goethite matrix. Original clay fractions are transformed into kaolinite and ferruginous oxi-hydroxides. Apatite nodules or beds gave rise to crandallite, millisite and variscite