Lowermost Zone Research Articles

Depositional Environment of Jurassic Beds of Bela Island, Rann of Kachchh, Gujarat, India Using Ostracodes

The present study focuses on understanding the depositional environment of Jurassic beds of Bela Island, Rann of Kachchh, Gujarat, India which is deduced mainly based on evidence furnished by ostracode fauna. Based on the distribution of 57 ostracode species in Jurassic beds of Bela Island, three biozones, in ascending order, Cytheropteron micropunctata Range Zone (early-middle Bathonian), Progonocythere laeviscula Range Zone (late Bathonian- early Callovian) and Habocythere mouwanaensis Range Zone (early-middle Callovian), have been established. The beds of lowermost Zone were deposited in shallow brackish water environment in the initial phase of marine transgression; the beds of middle zone were deposited at moderate rate of sedimentation in shallow sublittoral environment during the transgressive phase of sedimentary cycle; and the beds of uppermost zone were deposited in the shallow and brackish water condition in regressive phase of sedimentary cycle. Keywords: Ostracode, Depositional Environment, Jurassic Beds, Bela Island, Rann of Kachchh

Geomorphic response to historic and ongoing human impacts in a large lowland river

AbstractLowland coastal plain rivers are important for navigation and ecosystems, having unique concerns regarding water and sediment management. The Apalachicola River in Florida, the lower end of the ACF (Apalachicola–Chattahoochee–Flint) system, had historical modifications beginning in the 1800s. Most impacts are attributable to the Apalachicola Navigation Project in the latter half of the 20th century, which included dredging, disposal, rock and snag removal, artificial cutoffs, and wing‐dike building.As one of few studies using paired hydrographic surveys over a long reach and time (170 km, 50 years), we defined four zones of bed elevation change between 1960 and 2010 based on the cumulative thalweg elevation plots. The river has four bed change zones: (1) uppermost, marked degradation from river mile (RM) 106 to 85 because of dam construction; (2) middle, minimal degradation from RM 85 to 34; (3) artificial cutoff zone, highest degradation, from RM 34 to 27; and (4) lowermost zone, aggradation from RM 27 to Apalachicola Bay because of upstream bed and bank sediment disturbances. Artificial cutoffs and lithology influence lateral migration, with rates being highest in cutoff zones and least in areas with some bedrock upstream and finer sediments downstream. Length changes from 1941 to 2019 are stable, except for the middle reach, which lengthened nearly 10%, and the lower non‐tidal reach, where length shortened with cutoffs then regained in subsequent decades. Minimizing disturbance has led to 18% of the sand bar area revegetating since a decade of dredging ended. This framework helps in understanding historic and ongoing human activities, notably upstream water consumption, connectivity changes, sediment inputs, and climate change, that will affect the water and sediment management of the river, its floodplain, and bay, thus varying approaches for restoration.

Corynexochine trilobites of the Harkless Formation and Mule Spring Limestone (Cambrian Series 2, Stage 4), Clayton Ridge, Nevada

AbstractMost Cambrian Series 2 faunas of Laurentia are dominated by olenelline trilobites; however, non-olenelline trilobites occur with the olenellines and sometimes dominate the assemblages. Reported here are such non-olenelline trilobites from the Harkless Formation and Mule Spring Limestone at Clayton Ridge, Nevada. At the bottom of the Saline Valley Tongue, Harkless Formation, are two assemblages that are characterized by corynexochines and/or ptychoparioids, with olenellines occurring as only rare components. The corynexochines present in these assemblages include Bonnia cf. B. brennus (Walcott, 1916), Ovatoryctocara cf. O. yaxiensis Yuan et al., 2009, Protoryctocephalus? aff. P.? arcticus Geyer and Peel, 2011, Ogygopsis sp. indet., and Oryctocephalops frischenfeldi Lermontova, 1940. These assemblages are from the mid-Dyeran Stage, below the lowermost zone in the upper Dyeran (Arcuolenellus arcuatus Biozone), and can be correlated to Series 2 Stage 4 (Cambrian) assemblages in Greenland, Siberia, and South China based on the corynexochines.Also in the Saline Valley Tongue and the overlying Mule Spring Limestone and lowermost Emigrant Formation are olenelloid-dominated assemblages that contain the corynexochines Bonnia columbensis Resser, 1936, Zacanthopsis aff. Z. levis (Walcott, 1886), and Z. sp. indet.

A Cretaceous dinoflagellate cyst zonation for NE Greenland

AbstractA palynostratigraphic zonation is for the first time established for the entire Cretaceous succession in NE Greenland from Traill Ø in the south to Store Koldewey in the north (72–76.5° N). The zonation is based on samples from three cores and more than 100 outcrop sections. The zonation is calibrated to an updated ammonite zonation from the area and to palynozonations from the northern North Sea, Norwegian Sea and Barents Sea areas. The palynozonation is primarily based on dinoflagellate cyst and accessory pollen. The Cretaceous succession is divided into 15 palynozones: seven Lower Cretaceous zones and eight Upper Cretaceous zones. The two lowermost zones are new. The following five (Lower Cretaceous) zones have already been described. Two of the Upper Cretaceous zones are new. The zones have been subdivided into 20 subzones, 11 of which have been described previously and one of which has been revised/redefined. Nine subzones (Upper Cretaceous) are new. More than 100 stratigraphical events representing more than 70 stratigraphic levels have been recognized and presented in an event-stratigraphic scheme.

Tracer dispersion in trickle beds under tilts and roll motions – CFD study and experimental validation

A wire-mesh sensor was used to measure the liquid residence time distribution (RTD) and tracer dispersion in trickle beds subjected to stationary tilts and roll motions. The measured tracer characteristics were confronted to three-dimensional transient Euler-Euler CFD simulations to expose the role of inclined and oscillating porous media. The simulation results satisfactorily reproduced the widening tracer signals with increased bed inclination reflecting in asymmetric RTDs prompted by permanent gravity-driven stratified flow zone through the packing. Similarly, tracer dispersion was predicted to increase for rolling beds by increasing tilt angle and motion period. While an increase of tilt angle shortened RTD breakthrough time due to the faster tracer stream evolving in the liquid-rich lower-most zones, tracer elution tended to slow-down with increasing the rolling period. In addition, it was revealed that with altering the column angular position during roll motions the fluid flow structures inevitably varied inside the packing leading to different mixing behaviors and residence time distributions.

How much of the sediment in Gale crater's central mound was fluvially transported?

The origin of the sedimentary mound within Gale crater, the landing site for the Mars Science Laboratory rover Curiosity, remains enigmatic. Here we examine the total potential contribution of fluvial material by conducting a volume-based analysis. On the basis of these results, the mound can be divided into three zones: a lower, intermediate, and upper zone. The top boundary of the lowermost zone is defined by maximal contribution of water-lain sediments, which are ~13 to 20% of the total mound volume. The upper zone is defined by the elevation of the unbreached rim to the north (-2.46 km); sediments above this elevation cannot have been emplaced by flowing water. These volume balance calculations indicate that mechanisms other than flowing water are required to account for the overwhelming majority of the sediments transported into Gale crater. The most likely candidate process is settling from eolian suspension.

Water sources of plant uptake along a salt marsh flooding gradient.

Salt marsh plants are affected by regular tidal inundation exposing them to saline water as a potential water source. This study aimed at quantifying the water uptake of plants depending on their distance from the sea and exploring plant responses to changing inundation regimes. We used stable isotope ratios (δ18O) to determine the proportions of seawater and precipitation water used by three salt marsh species (Spartina anglica, Atriplex portulacoides and Elytrigia atherica) from a German North Sea coast salt marsh. Additionally, A. portulacoides was transplanted to experimental islands at three elevation levels to investigate its plasticity in water use in the course of future sea level rise. We found a marked gradient in plant seawater use from the lowermost pioneer zone (79-98% seawater uptake by S. anglica) to the lower marsh (61-95% by A. portulacoides) and the upper marsh (25-39% by E. atherica). Seasonal differences in water use were not pronounced, likely due to the absence of longer dry periods during summer in these temperate salt marshes. Contradicting our expectation, roots in deeper soil showed higher water uptake rates per fine root mass than topsoil roots suggesting effective root adaptation to the anoxic subsoil. Transplanted A. portulacoides plants significantly increased the uptake of seawater with increasing inundation indicating flexibility in the use of water sources by this species which may facilitate acclimation to rising sea levels. We conclude that the zonation of salt marsh vegetation reflects the availability of water sources along the inundation gradient.

Photosynthetic activity and productivity of intertidal macroalgae: In situ measurements, from thallus to community scale

The photosynthetic activity and productivity of four dominant canopy intertidal macroalgae were measured under emersion and immersion, at saturating light levels (PAR>300μmolphotonsm−2s−1), and compared at two sites (eastern and western English Channel) in spring and summer. The photosynthetic activity of thalli was measured by the electron transport rate (ETR) using pulse-amplitude modulated (PAM) fluorescence and the productivity of individuals and of communities was measured by carbon fluxes in closed chambers. Under emersion, when thalli were still hydrated, the uppermost species Pelvetia canaliculata had higher photosynthetic activity (mean ETR between 327 and 460μmole−m−2s−1) and individual gross productivity (between 60 and 212μmolCgDW−1h−1) than the lowermost species Laminaria digitata (mean ETR between 24 and 53μmole−m−2s−1 and gross productivity between 2 and 38μmolCgDW−1h−1), whatever the site and season. P. canaliculata had higher ETR in air than underwater (averaged 146μmole−m−2s−1) and L. digitata had lower ETR in air than underwater (averaged 112μmole−m−2s−1), while they exhibited, respectively, 3 and 5 times higher gross productivity underwater. At the community scale, the low mid-shore zone of Fucus serratus had the highest mean gross productivity under emersion (47mmolCm−2h−1) while rates were higher for the uppermost than lowermost zone at the eastern site (average 20 and 6mmolCm−2h−1, respectively) and of the same order of magnitude for both zones at the western site (about 30mmolCm−2h−1). Finally, the variability of under emersion primary productivity among sites and seasons was reduced when the measurements were performed on entire communities compared to isolated individuals of the dominant species.

Record of Juraphyllites ex gr. libertus (Gemmellaro 1884) from the Ururoan Stage (Early Jurassic, New Zealand) and observations on the paleoenvironmental dynamics of the Ururoan ammonite fauna

Juraphyllites libertus is a common member of Tethyan Juraphyllitid assemblages of Pliensbachian age in Central and Southern Europe and the eastern Mediterranean. Judging from these occurrences, the age of J. ex gr. libertus in New Zealand probably ranges from the Jamesoni Zone to the Margaritatus Zone, i.e. from the lowermost zone of the Lower Pliensbachian to the middle zone of the Upper Pliensbachian. The presence in New Zealand of J. ex gr. libertus, together with other ammonite taxa previously recorded from the Ururoan (Harpoceras, Zugodactylites and Catacoeloceras), all with strong Tethyan affinities, is discussed. The paleoenvironmental factors that enabled them to migrate to the southwest Pacific, on the southern margin of Gondwana are considered. Their migration was probably facilitated by a combination of the following factors: 1. dispersal ability of taxa; 2. global sea levels; 3. availability of migration routes; 4. local seawater temperatures; and 5. local facies conditions.

Early Campanian silicoflagellates from the Ural Federal District, Russia: a taxonomic and biostratigraphic reexamination of the A. P. Jousé sample suite

Nine samples of broad Late Cretaceous age, previously studied by the respected Russian diatomistA.P. Jousé, were reexamined in order to document the taxonomic composition of silicoflagellates at several localities within the Ural Federal District of Russia. New observations are provided on the range of morphological variability in several important taxa, and a new morphological term, “segment”, is presented for the basal skeletal components of Cornua. An attempt is also made to refine the age determinations for these samples using a recently established silicoflagellate biostratigraphic zonation for theCanadianmargin.Our general conclusion is that the zonation developed based on the Canadian sections can be successfully used to provide age control for sections exposed in the northern part of Eurasia. The samples studied here are probably correlative to the lower Campanian Cornua trifurcata Partial Range Zone and the lowermost Campanian Schulzyocha ruppelii Range Zone of northern Canada. However, the recognition of the Schulzyocha ruppelii Zone is based on the presence of a secondary zonalmarker (Gleserocha tapiae), due to the absence of Schulzyocha. Thismay indicate that only the Cornua trifurcata Zone has interregional applicability, while the Schulzyocha ruppelii Zone may be restricted to the Canadian successions due to the endemism of Schulzyocha. Further studies on Late Cretaceous silicoflagellate paleobiogeography are required to resolve these issues.

The genusPlumbago(Plumbaginaceae) in Ethiopia and Eritrea

Summary The genus Plumbago has a concentration of indigenous species in eastern tropical Africa and Madagascar: nine out of a total of between twelve and twenty-five species. In the Flora of Ethiopia and Eritrea, Vol. 5, published in 2006, only two indigenous species were accounted for: the widespread and common P. zeylanica and a new species, P. truncata, restricted to south-western Ethiopia. The name P. truncata was not formally validated. Since then more collections and field observations of Plumbago have been made in Ethiopia: after revision of the entire material it is concluded that P. truncata is conspecific with P. dawei, known from Uganda, Kenya, Tanzania and Madagascar, and that another tropical East African species, P. montis-elgonis, known from Kenya and Tanzania, also occurs in south-western Ethiopia. In Ethiopia the two species are restricted to areas originally covered by moist forest: P. dawei to Transitional Rain Forest and Riverine Forest, while P. montis-elgonis to the lowermost zone of...

Onshore–offshore gradient in reductive early diagenesis in coastal marine sediments of the Ria de Vigo, Northwest Iberian Peninsula

Early diagenetic modification of magnetic properties is an important process in marine sediments, but temporal and spatial variability of diagenetic processes have rarely been reported for recent coastal sediments. The magnetic properties of sediments from the Ria de Vigo (NW Spain) define a marked three-part zonation with depth. The uppermost zone is magnetically dominated by (titano-)magnetite. In the intermediate zone, rapid down-core dissolution of (titano-)magnetite increases the relative influence of high-coercivity magnetic minerals, which react more slowly during reductive dissolution than (titano-)magnetite. This zone is characterized by the ubiquitous occurrence of framboidal iron sulphides. Pyrite is the dominant iron sulphide, but framboidal ferrimagnetic greigite is also frequently observed in association with pyrite. The lowermost zone is characterized by an almost complete depletion of magnetic minerals associated with progressive reduction of detrital iron oxides with depth. This zonation is controlled by organic matter diagenesis, which varies with water depth and wave-induced sediment resuspension and organic matter reoxidation in the water column. This leads to a shallowing and thinning of each zone with more intense reductive diagenesis toward the interior of the ria. Such a zonation seems to be a common feature in shallow water marine environments. If preserved, the described zonation and its spatial variability provide a potential tool for detecting estuarine-like environments in the geological record. Magnetic detection of current or past reductive conditions also has important implications for assessing paleoenvironmental proxies that are sensitive to diagenetic redox state.

Foraminiferal Distribution in the Central and Western Parts of the Late Pleistocene Champlain Sea Basin, Eastern Canada

Marine sediments from the late-glacial Champlain Sea have been sampled at 20 localities representing the deeper part of the basin, between Ottawa and the Rivière St-François, Québec. Foraminiferal assemblages have been extracted and a sequence of three deep water and two shallow water ecozones recognized. The lowermost zone (A) is characterized by Cassidulina reniforme, Islandiella helenae and I. norcrossi and represents a paleosalinity of 25 to 30%o. The overlying zone B is dominated by Elphidium excavatum. It represents salinities decreasing from 25 to as low as 10%o. The uppermost zone (C) contains only a sparse assemblage of a morphotype of E. excavatum. If suggests a paleosalinity of no more than 10%o. A mostly unfossiliferous silt and clay layer of variable thickness (post-C) occurs above zone C. It is probably lacustrine. Below zone A and above the Late Wisconsinan till there is a pre-A interval whose variable assemblages represent hyposaline environments east of Montréal, predominantly lacustrine conditions west of Montréal and alternating hyposaline/ lacustrine environments near and south of Montréal. Bottom water temperatures were probably "Arctic" (within a few degrees of 0°) from the pre-A interval up to zone B inclusively. The data from zone C are too poor to estimate temperatures. The shallow water zones indicate environments with high (zone EH) or low (zone EA) salinities but of shallower depths than the deep water zones. The existence of two sequences is interpreted as the result of (probably seasonal) water stratification. The data does not allow to determine the depth of the limit between the shallow and deep waters.

New Opportunities for Time-Sensitive Gulf of Mexico Completions

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper OTC 18857, "New Opportunities for Time-Sensitive Gulf of Mexico Completions," by Allen W. Womble, SPE, Ed Van Sickle, SPE, and Mike Mckown, SPE, Baker Oil Tools, prepared for the 2007 Offshore Technology Conference, Houston, 30 April-3 May. Adaptation of single-trip multizone sand-control completion systems in the Gulf of Mexico (GOM) for frac packing has created new opportunities for time-sensitive completions. Frac packing involves relatively large amounts of complex fluids and deployment of capital-intensive equipment. Time becomes a significant variable in project economics. In 1990, after completing several hundred wells successfully in California, single-trip multi-zone technology was introduced to the GOM. Two key drivers provided the initial opportunity for GOM applications: minimizing rig time and reducing formation damage by reducing formation exposure time to completion fluids. Introduction Single-trip multizone gravel-pack systems are not new to the GOM, but the adaptation to frac packing has created new opportunities for this technology. Single-trip multizone gravel-pack completions were proved first in the early 1980s in the Beta field offshore California. The productive interval there was composed of as many as nine separate zones with a gross interval thickness of 1,200 ft. The system provided zone isolation in the form of packers without the normal packer slips; hence, the first zone-isolation packers were created. The gravel-pack-liner assembly consisted of a series of screen/isolation-packer/gravel-pack port combinations, one combination per zone. Sealbores were positioned above each isolation packer and below the gravel-pack port, to be used for positioning of a gravel-pack-seal assembly during gravel packing. All producing zones were first perforated simultaneously by use of tubing-conveyed-perforating guns. The appropriate number of gravel-pack assemblies was assembled as a single unit, and a conventional gravel-pack packer, with slips, was positioned at the top gravel-pack assembly. The entire single-trip multizone system then was run to bottom on drillpipe. Once the system is positioned across the producing interval, the upper gravel-pack packer is set, the setting is released, and the gravel-pack-seal assembly is used to set all zone-isolation packers hydraulically. A smaller pipe string was run concentrically through the drillpipe to bottom and stung into the gravel-pack-seal assembly. The lowermost zone then was gravel packed, with slurry transported down the inner work string and clean return flow moving up the miniannulus between the two strings. Repositioning the tool to pack all upper zones required pulling both the drillpipe and concentric pipe up the incremental distance between zones. This original system, given the name "Beta" system in reference to the field where the system was used, was used successfully in more than 250 wells involving more than 2,000 zones from 1981 to 1991, resulting in significant completion time savings.

Physical characteristics of the Karoo sediments and mode of emplacement of the dolerites

The mode of emplacement of the dolerites and their relation with the overlying basalts are revisited in the light of new information provided by a recent resistivity study of the structure of the Karoo basin and augmented by an analysis of age and palaeomagnetic data. The different styles of intrusion from the bottom to the top are explained in terms of anisotropy, lithology and increasing upward bending stresses accompanied by a decrease in overburden thickness. The lowermost zone 3 consisting of flat lying dolerite sills of large extent occurs in well-laminated, homogeneous shales with a high degree of anisotropy. The thick dolerite-rich middle zone 2 extending from the Upper Ecca Subgroup through the Beaufort Group and partially into higher Formations is characterised by the occurrence of large basin structures of dolerite. A model, which assumes a central dyke as source and based on the principle that the propagating magma front always follows the path of least resistance is developed to explain the formation of a basin structure including the steepening and flattening of the rim. The model also explains the variations in shape and size such as are encountered in the field by taking into consideration the structure of the nexus of lenticular sandstones intercalated with shales and mudstones occurring throughout this zone. The dominant style of intrusion of, at least, the upper part of zone 1 that extends from the surface to a depth of about 700 m is that of dykes and steeply dipping sheets. Another striking feature of this zone is its dolerite-poor nature. An analysis of the magnetic polarities of the dolerites and basalts leads to the conclusion that the reversely polarized dolerites, which only occur in zone 1 were the feeders to the outpouring of the basalts. The younger normally polarized dykes that cut through the entire basalt succession are associated with the period of cooling and contraction following the main phase of dolerite intrusion. The implications, aided by well resolved age determinations are that the duration of intrusion of the Karoo magmatic event was short. Finally, the significant increase in both the amount of dolerite and the thickness of the dolerite bearing layer towards the east supports the view of a magma source in that direction.

North polar cap of Mars: Polar layered deposit characterization and identification of a fundamental climate signal

The record of recent climate change on Mars is encoded in the polar layered deposits within the north polar cap. Individual Mars Orbiter Camera (MOC) images of exposed layer sequences in cliffs and troughs provide the equivalent of high resolution “cores” through many sections in the upper part of the north polar layered terrain. In order to decode this record it is necessary 1) to quantitatively characterize the layers in individual “cores” and 2) to assess possible correlations between “cores” in vertical layered deposit sequences across the cap. We use two techniques commonly employed in paleoceanography for the study of deep‐sea sediment cores on Earth to establish the characteristics of layers in individual cores (Fourier analysis) and to determine the correlation between cores (curve‐shape matching algorithms). Application to “cores” (vertical sections) of the north polar layered terrain on Mars reveals several fundamental properties of north polar cap stratigraphy: 1) Fourier analysis of the layer vertical sequences reveals a characteristic and repetitive wavelength of ∼30 m thickness throughout the upper part (Zone 1) of all sequences analyzed. 2) Application of curve‐shape matching algorithms demonstrates that layers correlate across at least three quarters of the cap (∼6 × 105 km2) in the 13 images analyzed to date. 3) Assessment of geometric relationships shows that layers are not horizontal, but rather have an apparent dip of approximately 0.5 degrees. We interpret these results as follows: 1) The fundamental ∼30 m signal is interpreted as a climate signal that may correspond to a 51 kyr insolation cycle. 2) The lateral correlation and broad distribution of these layer sequences strongly imply that layer accumulation processes are widespread across the cap, rather than confined within a single trough or region. 3) Local to regional variability in individual layer thicknesses (and thus accumulation and sublimation rates) is typically less than a factor of 2.5, providing the ability to study regional trends, but often making it difficult to correlate visually the vertical sequences in individual cores. Finally, initial examination of layers located deeper in the stratigraphic sequence within the north polar cap than the ∼300 m thick Zone 1 provides evidence for a unit less than 100 m thick (Zone 2) in which the fundamental ∼30 m sequence is not detected. We interpret this as a deposit having formed during a recent high‐obliquity phase of Mars, during which time polar volatiles underwent mobilization and were transport equatorward, leaving a polar lag of dust‐rich material. The most recent “ice age” (∼0.5–2 Ma) offers a plausible candidate for this period of ice cap removal and lag deposit formation. An underlying Zone 3 (∼200 m) contains a dominant 35 m signal, and a lowermost Zone 4 (∼200 m) contains multiple signals but no dominant one. Together these four zones represent ∼800 m of vertical stratigraphic section, about one‐fourth of the total thickness of the cap. These findings support earlier interpretations that orbital parameter variations could cause significant erosion and possibly complete removal of the polar caps. The interpreted crater retention ages of the layered terrain are consistent with the correlations and vertical sequences described here, suggesting that the polar caps wax and wane throughout geological history, depending on the evolution of orbital parameters. Definition of the ∼30 m unit signal holds promise for determining 1) the detailed origin of individual layer types, 2) the nature of deposition and sublimation processes and their relation to insolation geometry across the polar cap, and 3) correlation with and comparison to the south polar layered terrain record.

Hot-spring and supergene lanthanide mineralization at the Buru carbonatite centre, Western Kenya

AbstractThe Tertiary (22 Ma) carbonatite centre at Buru hill is located towards the eastern end of the Nyanza rift, western Kenya. As revealed by geochemistry and petrology of drill core samples, the Buru centre ranges in composition from surficial lateritized pyroclastic ferrocarbonatite to recrystallized ferruginous calciocarbonatite at depth.Mineralogically the lateritic cover of the Buru carbonatite centre is characterized by goethite, hematite, psilomelane, baryte, fluorite with minor calcite, bastnäsite and significant amounts of monazite. A transitional middle zone consists of ferrocarbonatite grading into ferruginous calciocarbonatite containing siderite, baryte and fluorite, a greater proportion of bastnäsite replacing calcite, but lesser proportions of monazite. The lowermost zone consists of ferruginous calciocarbonatite containing intimate mixtures of siderite and calcite, with parisite and synchysite existing as major replacive components of calcite down to depths of 200 m. Synchysite appears to be the more stable form of lanthanide fluorcarbonate at depths below 150 m.Oxygen isotope measurements on separated calcite concentrates from three of the Buru drill cores reveal significant deviations from primary igneous carbonatite values suggesting isotopic re-equilibration with meteoric water. Accepting that the African plate may have been stationery during the past 30 Ma, it is possible to use a modern sample of water from the Kenya rift to estimate the equilibration temperature at which Buru carbonatite in western Kenya has recrystallized. Temperatures of ∼60 to 90°C suggest that the hot-spring activity encouraged the formation of replacive lanthanide fluorcarbonate in ferruginous calciocarbonatite. Lanthanide enrichment also occurred closer to the surface within the pyroclastic ferrocarbonatite at temperatures of ∼40 to 60°C corresponding to waning stages of hot-spring activity. Further lanthanide enrichment redistribution and oxide formation occurred at surface supergene temperatures of ∼20 to 30°C forming the lateritized top of the carbonatite.The wider implications from these carbonatite studies are the recognition that lanthanide enrichment can be preserved in volcanic-subvolcanic centres, and that lanthanide fluorcarbonates can replace calcite in response to circulating hot-spring waters. From a classification point of view, ferrocarbonatites should be seen as products of meteoric hydrothermal systems unrelated to carbonatite magmatic processes

Late Quaternary sediments from Nal Sarovar, Gujarat, India: Distribution and provenance

A 54-m long core was raised from the bed of the Nal Sarovar, a large shallow lake located in the middle of the low-lying region linking the Gulfs of Kachchh and Khambhat, in western India. A three-layer sequence comprising: Zone-1 (top 3 m), predominantly silty-clay/clayey; Zone-2 (3–18 m), sandy; and Zone-3 (18–54 m), dominated by sticky silty-clay/clayey-silt with occasional thin sand layers and basalt fragments was identified. Smectite and illite are the dominant clay minerals with minor amounts of kaolinite and chlorite. Very high content of smectite (53–97%) in the clays of the lowermost zone (18–54 m) and the geomorphic features of the surrounding region suggested that the sediments were derived from the basaltic terrain of Saurashtra and/or via the Gulf of Khambhat. The clay content in the middle zone (3–18 m), dominantly sandy, is very low. Therefore, provenance for this zone was derived using heavy minerals in the sand fraction. The heavy mineral species in this zone suggested the mixed metamorphic and igneous terrain of Aravallis as the major source. The grain-size distribution of this zone closely matched with the sediments underlying the modern Sabarmati riverbed at Ahmedabad, suggesting fluvial depositional environment. Clays also dominate sediments of the topmost (0–3 m) zone with illite as the dominant (74–81%) specie followed by smectite suggesting derivation from the mixed metamorphic and igneous terrain of Aravallis.

THE GWYDION DISCOVERY: A NEW PLAY FAIRWAY IN THE BROWSE BASIN

The Gwydion-1 oil and gas discovery well is located in exploration permit WA-239-P on the sparsely explored Yampi Shelf area of the Browse Basin. The Gwydion feature was first recognised as a series of stacked seismic amplitude anomalies, which were interpreted to represent hydrocarbon-bearing Barremian to Albian age shallow marine sandstones draped over a prominent basement high. Amplitude versus offset analysis and modelling supported this interpretation.Gwydion-1 was spudded on 4th June, 1995, and discovered three gas bearing zones and one oil and gas bearing zone. The lowermost zone is Barremian to Hauterivian in age and consists of 12.6 m of net gas-filled glauconitic sand overlying a 9.5 m net oil-filled quartz sand. The three overlying hydrocarbon zones consist of glauconitic reservoirs of Barremian to Albian age.The play fairway for Gwydion-style traps has been named as the Echuca/Swan-Bathurst Island Group/Shelfal Play Fairway. It comprises mature Swan Group and Echuca Shoals Formation source rocks, and Bathurst Island Group reservoirs and seals. The limits of the play fairway on the shelf are controlled by the existence of topographic relief in the underlying basement metasediments. Migration pathway analysis suggests that the eastern margin of the Browse Basin is favourably situated to receive charge from the mature source rocks within the basin.The dominant northwesterly dip of the strata on the Yampi Shelf limits the potential for structural traps. Accordingly, a thorough understanding of the sequence stratigraphic architecture of the succession is necessary in order to generate the stratigraphic play concepts which hold the bulk of the prospectivity in the area.Gwydion-1 was plugged and abandoned as an uneconomic oil and gas discovery. It was, however, significant as it validated a new play type and generated renewed interest in the eastern margin of the Browse Basin for the first time since the mid 1970s; an area previously thought to be too shallow, too far from mature source and lacking reliable seal.

Detecting a sequence boundary across different tectonic domains: an example from the middle Miocene of the northern Apennines (Italy)

ABSTRACTSequence stratigraphic concepts can provide a powerful tool for understanding the tectono‐sedimentary evolution of areas extending across different tectonic domains. An example is provided by the upper Serravallian strata of the northern Apennines, where a sedimentological and biostratigraphic study allows a sequence boundary to be traced across the foredeep and piggy‐back basin successions. Turbidite sedimentation of predominantly alpine and subordinate apenninic provenance occurred in the apenninic foreland basin throughout the middle Miocene. Deep‐water sedimentation in the foredeep was laterally associated with deposition in shelf to slope environments in the piggy‐back basins.In the piggy‐back basin succession, the upper Serravallian sequence boundary is a laterally extensive unconformity within homogeneous marly deposits. This unconfonnity is laterally correlative with the base of lenticular turbidite bodies. A stratigraphic lacuna affecting Zone N14 characterizes the marginal areas, where glaucony‐rich deposits assigned to Zone N15 unconformably overlie marls displaying association of Zone N13. In the depocentres, where no significant stratigraphic gap has been detected, the sequence boundary is narrowly constrained to lowermost Zone N14.The upper Serravallian unconformity of the piggy‐back basins succession is correlative with time‐equivalent features in two distinct parts (inner basin and outer basin) of the foredeep. In the inner basin the sequence boundary separates basin margin turbidites from overlying slope hemipelagites. In a more external position (outer basin) the sequence boundary is the base of a characteristic mega turbidite of apenninic provenance (Turrito layer). In other sectors of the outer basin, where turbidite sedimentation was entirely of alpine provenance, the sequence boundary has no clear physical expression.The observed facies distribution in the study area suggests that an important thrusting event affected the northern Apennines in the late Serravallian, resulting in submarine channel incision and nondeposition in the piggy‐back basins. Compressional activity in the foredeep was responsible for the closure of the inner basin and subsequent shifting of turbidite sedimentation in the outer basin. Slope instability led to widespread remobilization of previously deposited turbidites, triggering turbidite events of huge volume. The different characteristics of the sequence boundary in the various parts of the foredeep constitute an example of differential response of a multisourced supply system to tectonic deformation.