Gray Sandstone Research Articles

Reduced Productivity Impairment For Fracture Stimulated Gas Condensate Wells

Abstract Development of a retrograde condensate reservoir requires accurate well productivity predictions for a capital commitment to gas processing facilities. Historically, Fussell(1) identified that liquids condensing in the reservoir will result in a substantial productivity impairment. A single well model, which included a hydraulic fracture as part of the grid system, was developed to perform sensitivities for well test interpretation and to predict long term performance. Interesting results were obtained. The productivity of fractured wells was not impaired to the degree expected. Radial modelling confirmed the results obtained by Fussell. Current simulation technique allows for direct modelling of a hydraulic fracture instead of using an equivalent well bore radius. The distribution of pressure drawdown and condensate dropout around a hydraulic fracture results in limited productivity impairment. The methodology used and the results obtained are described. Introduction This work was originally completed to forecast production from wells in a new field, which was being developed in the Deep Basin area of Alberta, Canada. This study was comprised of geological review, PVT characterization, numerous well test sensitivities, as well as simulating the effects of condensate dropout on well productivity. The ultimate objective of this work was to make a nomination for a sour gas plant. This paper concentrates on the most important technical point which is that the productivity of wells, which are hydraulically fractured, may not be as adversely affected by condensate precipitation as was previously reported. However, in any actual study, there are a significant number of factors which are derived from different disciplines within petroleum engineering, which must be integrated. We have therefore placed our main results within an abbreviated outline of the process used in solving an actual field problem. In the authors' opinion, this is more valuable to practicing engineers. As much as possible, the material is presented chronologically so as to follow the development of the technical work. Material is presented under the following major headings: Geological Review, PVT Characterization, Model Construction, Well Test Modelling, Effects of Condensate Dropout, and Conclusions. Originally this work was completed for a single well, which was later expanded to include other wells in other pools. Only the work done on the first well analysed is presented and does not apply universally to the area. Geological Review The main objective of this phase of the study was to develop a quantitative reservoir description. This included porosity, permeability, water saturation and layering (heterogeneity). Since this was a new play, relatively little information was available on this zone. The lithology of the sandstone in this well was derived from a core description. Overall, the sand comprises interbedded dark grey-black shaley siltstones and light grey dolomitic sandstones. The shaley siltstones occur in beds that range in thickness from centimetres (inch) to metres (3 to 6 ft.). The sandstone occurs in beds 1/10 of a metre (4 in.) to metres (3 to 6 ft.) thick. Layering can be a major consideration in the design of a simulation, particularly for secondary recovery.

Nagthat Formation: An example of a progradational, tide-dominated Proterozoic succession in Kumaun Lesser Himalaya, India

The Proterozoic Nagthat Formation of the Krol-belt succession, in the Nainital area, is composed mainly of fine- to coarse-grained quartzarenite with a subordinate amount of purple to grey sandstone, siltstone-shale and conglomerate horizons. The association with spilitic lava flows, variable palaeocurrent trends and the restricted lateral extent of the Nagthat Formation within the Krol-belt succession imply an active role for tectonism in the basin of deposition. In the upward coarsening succession of the Nagthat Formation, six major lithofacies have been identified: medium- to coarse-grained gravelly quartzarenite (Lithofacies A), planar cross-bedded, medium-grained quartzarenite (Lithofacies B), horizontally laminated, fine-grained quartzarenite (Lithofacies D), interbedded sandstone-shale (Lithofacies E) and matrix-supported conglomerate (Lithofacies F). The constituent lithofacies are repetitive in nature, forming upward fining unit cycles and interpreted to reflect deposition as upper shore-face, shoals and bars, barrier-beachface, tidal channels (inlets), intertidal–sandflat–mixedflat environments and, occasionally, in the form of gravity flows in subtidal channels. The general upward coarsening succession of the Nagthat Formation represents deposition in a progradational (regressive) barrier island system. The palaeocurrent pattern in the Nagthat Formation is distinctly polymodal and indicates sediment distribution across the roughly NW–SE trending shoreline, in response to a dominating flood tidal current system. The palaeocurrent pattern shows higher variability in the upper shore-face deposits than in the tidalflat domain. A recycled metasedimentary terrain served as the source for the Nagthat Formation, probably supplying the sediments from E, NE and S directions.

Paleomagnetism of the Distal Member of the New Glasgow Formation, Nova Scotia, Canada: New Constraints on Magnetic Polarity Stratigraphy near the Base of the Kiaman Superchron

The ca. 315–260‐Ma Kiaman Reversed (Permo‐Carboniferous) Magnetic Superchron is an important time marker that can be used for correlation of stratigraphic sequences from a local to a global scale. Earlier workers, from a paleomagnetic study of the Joggins stratigraphic section of the western Cumberland Basin of Nova Scotia, demonstrated that the base of the superchron lies above redbeds of the lowermost portion of the late Namurian–Westphalian A Boss Point Formation. Although a detailed sequence of reversals was observed below this point, no normal magnetic polarity zones were recognized above it. In our study, samples were collected from redbeds of the distal member of the Westphalian A New Glasgow formation of the eastern Cumberland Basin, which directly overlie the gray sandstones of the Boss Point Formation. Most sampling sites carry a normal polarity magnetization, probably acquired at about the time of deposition. Therefore, the base of the Kiaman Reversed Superchron lies within or above the New Glasgow formation. Either the top of the Boss Point Formation is diachronous between the two areas, or, more likely, the base of the superchron is significantly higher in the stratigraphic section than previously recognized, a conclusion that is important for both intrabasin and intercontinental stratigraphic correlations. It implies that the pattern of pre‐Kiaman magnetic reversals reported from below the Boss Point Formation in the Joggins section is significantly older than previously thought and, therefore, cannot be used for comparison with magnetic reversal patterns that closely underlie the base of the superchron at other localities in the Cumberland Basin or on other continents.

The Queenston Formation: shale-dominated, mixed terrigenous-carbonate deposits of Upper Ordovician, semiarid, muddy shores in Ontario, Canada

The Upper Ordovician Queenston Formation constitutes the westernmost part of the Taconic clastic wedge of eastern North America. In Ontario, the Queenston Formation consists of a mixed terrigenous-carbonate succession composed primarily of red silty shales, which are locally gypsiferous, alternating, in the middle and lower parts of the formation, with thin grey shales, calcareous siltstone and sandstone, and bioclastic limestone. Brachiopods, some ostracodes, and, in the lower beds, bryozoans constitute the body fossil assemblage. Diplocraterion, Skolithos, Chondrites, and occasional large vertical burrows of unknown origin form the ichnofauna. Plane beds, ripple marks, and occasional swaley and hummocky cross-stratifications are present in the calcareous units. Mudcracks, gypsum nodules, occasional halite hopper casts, and gilgai-like structures typically occur in horizons where shales alternate with siltstone and limestone. These features are indicative of prograding muddy and storm- and tide-affected shores. In southernmost Ontario, the upper part of the formation is mostly composed of a red shale, with local recognizable shale-filled channels. This shale is of paralic origin, and it is truncated by the unconformity separating the Ordovician from the Silurian units in this region. This last event may, in part, be related to a eustatic sea-level drop due to the contemporaneous Saharan glaciation, if not exclusively due to tectonic movements.

Provenance of coarse-grained detritus in fan-delta systems, Miocene Pohang Basin, SE Korea: implications for boundary fault movements

In fan-delta systems, contemporary activities of boundary faults can be effectively constrained by tracing coarse-grained detritus. In order to reveal provenance and tectonic activities in the western margin of the Pohang Basin (Miocene), a total of 6000 clasts were examined in four fandelta systems for composition and compared with probable source rocks in the drainage area. Clasts in the Doumsan, Maesan, and Duksung fan-delta systems are dominated by granite, granodiorite, gray mudstone, and sandstone with subordinate amounts of rhyolite, dacite, and tuff. The Malgol fandelta system comprises silicic volcanics (mainly dacite), red or light gray sandstone, and mudstone. These clasts are similar in composition to the rocks exposed in the drainage area (Cretaceous Banyawol and Kisadong formations and Eocene granite and dacite bodies). The results indicate that strike-slip movements along the basin margin were inactive during the deposition of these fan deltas and each system was self-sustained.

A playa deposit of pre-Yellow Sands age (upper Rotliegend/Weissliegend) in the Permian of northeast England

A complexly interbedded thin succession of sandstone, siltstone and mudstone beneath the Permian Yellow Sands Formation (upper Rotliegend/Weissliegend) was cored in a borehole 11 km off the coast of northeast England. The cored strata rest unconformably on fissured Westphalian B mudstone and mainly comprise low-dipping fine- to coarse-grained grey sandstone with unevenly spaced thinner dark-red to grey mudstone beds and laminae. Most of the sandstone beds are moderately to poorly sorted, fine- to very fine-grained subarkoses, with some sublitharenites, and are weakly to strongly cemented by dolomite (predominant), kaolinite and illite; sedimentary structures include subhorizontal plane to wavy lamination, ripple lamination, salt ridges and soft-sediment deformation. The siltstones and mudstones are more uniform in lithology and primary thickness; they contain sandstone-filled desiccation cracks, mudstone saucers and narrow sandstone dykes. Traces of possible former evaporite minerals are concentrated in the sandstones. By comparison with modem desert depositional environments, these strata are interpreted as the deposits of a playa occupying a depression on a stony, deflationary desert surface on a rock pediment or peneplain flanking the ancestral Pennines. The floor of the depression, which probably lay in the capillary fringe just above the contemporary water table, was periodically flooded so as to form a shallow playa lake. Comparable successions beneath the Yellow Sands have not been recorded from surface exposures in northeast England but mudstone beds at this stratigraphical level have been recorded in four other cored offshore boreholes and are presumed to be of similar origin to those described in this study. The stratigraphical position of these deposits suggests that they may be of pre-Yellow Sands age and coeval with or younger than the Basal Permian Breccia. Palynological analysis of mudstone samples from the core reveal the presence of plant material and indeterminate, pyrite-invested, possibly indigenous bisaccate pollen indicative of a vegetated hinterland with deposition occurring within a reducing, possibly sulphide-rich environment.

Biostratigraphy and paleoenvironment of foraminiferal assemblages from neogene and quarternary deep-sea sediments collected during the nautimate cruise (Pacific Ocean, off Mexico, 18°N-21°N). Geodynamic implications

Deep-sea dives with the submersible Nautile allow to reconstruct the lithostratigraphic series, and to collect samples of plutonic rocks and sediments along the inner wall of the Acapulco trench, off Mexico (Manzanillo area, 18°N-19°N, Pacific Ocean). Plutonic rocks are unconformably overlain by brown and grey sandstones and siltstones, constituting a continuous sequence of sediments, several hundreds meters thick. Assemblages of planktonic foraminifera give a late Miocene-early Pliocene age for the lowest part of the sedimentary sequence. Benthic foraminifera indicate that the minimum depth of deposition was located into the middle bathyal zone. Characteristic species are the following: Bolivina foraminata, Bulimina mexicana, Galliherina avigerinaformis, Gyroidina healdi, Hansenisca rotundimargo, Nodogenerina lepidula, Parafrondicularia miocenica, Fontbotia wuellerstorfi, Plectofrondicularia californica, Protoglobobulimina pseudotorta, Pyrgo murrhina, Siphonodosaria advena. These assemblages are dominated by endofaunic species (bolivinids, buliminids, uvigerinids and cassidulinids) and oxygen-minimum zone indicators (Buliminella subfusiformis, Epistominella smithi, Uvigerina subperegrina). These late Miocene-early Pliocene sediments were probably deposited into the oxygen-minimum zone, at the boundary between the lower and upper parts of the middle bathyal zone, corresponding approximately to a depth of 1000 m. Their present position (between 2500 and 4000 m) reveals a strong and rapid subsidence of this area (between 1500 and 3000 m) since the early Pliocene. These stratigraphic and paleoenvironmental results indicate the importance of these stepwise downward movements of this zone since Eocene. An additional study of late Miocene-Quarternary foraminiferal sediments collected during deep-sea dives conducted to the North (South of Tres Marias Islands, 20°N-21°N) gives complementary data on the stratigraphy and paleoenvironment of these deposits. It completes the understanding of the complex geodynamic history affecting the western boundary of the North America plate.

The role of climate in stratigraphic patterns exhibited by late Palaeozoic rocks exposed in Kansas

Abstract Late Palaeozoic “cyclothems” of the midcontinental U.S. cannot be represented by a single ideal facies sequence. Rather, they encompass a wide range of cycle types depending on their stratigraphic position. The exposed Late Palaeozoic (Pennsylvanian and Permian) rocks of Kansas, as compiled by Zeller (1968), can be divided into nine major lithofacies. These lithofacies were ordered based on their times of greatest abundance through the section. From oldest to youngest, they attain their peak abundance in the following order: (1) coal; (2) gray mudrocks and sandstones; (3) black mudrocks; (4) limestones; (5) cherty limestones; (6) fossiliferous mudrocks; (7) variegated mudrocks; (8) evaporites; and (9) variegated dolomitic siliciclastics. Climatically, these nine lithofacies suggest a change from generally wetter, at the base, to generally drier, at the top of the section. This climatic trend was probably generated by a complex interaction of factors primarily driven by global tectonics. Climatic changes can be used to explain the five generally recognized types of lithofacies “cyclothems” in Kansas, and thus are a potential explanation for the changing stratigraphic patterns of the entire Late Palaeozoic time interval of Kansas, and perhaps elsewhere.

Abstract: Depositional Environments and Sandstone Diagenesis in the Tyler Formation (Pennsylvanian), Western North Dakota 

Thirty-three oil well cores were examined and approximately 3,100 oil well logs were used to determine the depositional environments of the Tyler Formation (Pennsylvanian) in northwestern North Dakota. Ninety sandstone thin-sections were described and twenty-nine sandstone samples were analyzed by scanning electron microscope/X-ray microanalysis techniques to characterize sandstone diagenesis. The Tyler Formation in northwestern North Dakota is correlative with the lower unit of the Tyler Formation in southwestern North Dakota. The upper unit of the Tyler Formation in southwestern North Dakota is not present in northwestern North Dakota, contrary to previous workers suggestions. The Tyler Formation in northwestern North Dakota consists of varicolored mudstone, medium-gray claystone, thin bituminous coal beds, dark-gray shale and limestone, and gray sandstone. The upper unit of the Tyler Formation in southwestern North Dakota consists of gray sandstone, dark-gray limestone and shale, and varicolored mudstone. Lenticular sandstones present in the middle and rarely at the base of the formation, which occur in northeast-southwest linear trends, indicate that the Tyler Formation in northwestern North Dakota was deposited as a river channel system on a low-lying, prograding coastal plain. Depositional environments associated with the coastal plain include river channels, flood plains, lakes, estuaries, caliche paleosols, and backswamps. Throughout deposition of the Tyler Formation, several major river channels flowed in a predominantly southwest direction across a low-lying coastal plain and transported quartz sand toward a delta or shoreline, not identified in this study, at the margin of a shallow epeiric sea. Initially, the presence of detrital clay and infiltration clay and early hematite and chlorite coatings on quartz grains was the most important factor influencing porosity reduction in Tyler sandstones. The first major diagenetic event responsible for porosity reduction was the development of authigenic quartz overgrowths. Other cements that have contributed to porosity reduction in Tyler sandstones include amorphous silica (opal), calcite, ankerite, siderite, anhydrite, barite, hematite, and pyrite, In addition, authigenic kaolinite, illite, and chlorite have reduced porosity in Tyler sandstones. Dissolution of carbonate cement resulted in the formation of secondary porosity. Sandstone diagenesis took place shortly after sand was deposited in a marine environment, while sediments were being flushed with meteoric ground water, and while sands were being flushed with chemically evolved pore water released after diagenesis and compaction of interbedded mud-rich sequences. River-channel sandstones in northwestern and southwestern North Dakota and barrier-island sandstones present only in southwestern North Dakota contain considerable amounts of intergranular porosity. Petroliferous sandstones in southwestern North Dakota are fine- to medium-grained, well-sorted, texturally mature…

Kinderscoutian and Marsdenian successions in the Bradup and Hag Farm boreholes, near Ilkley, west Yorkshire

SUMMARY The Bradup and Hag Farm boreholes were drilled by the British Geological Survey (BGS) into strata of the Upper Carboniferous Millstone Grit Group near Ilkley. They prove (beneath glacial till) a rock succession of Namurian (late Kinderscoutian to earliest Marsdenian) age including five faunal marker bands, two of which are the widespread Reticuloceras coreticulatum (R 1c 4) and Bilinguites gracilis (R 2a 1) marine bands. The other three yielded mainly benthonic assemblages and lacked diagnostic ammonoid taxa, but are tentatively assigned to the R 1c 2, R 1c 3, and Butterly marine bands. Four late Kinderscoutian cycles are identified, and a basal fifth cycle is incomplete. Each comprises a coarsening-upwards sequence from marine mudstones to prodelta-slope siltstones and sandstones, then to fluvial sandstones and other delta top deposits. Three of the fluvial sandstones have conglomeratic bases and overlie erosion surfaces which, in the third and fifth cycles, are immediately underlain by laminites comprising alternating grey sandstone and dark grey clay-rich sandstone laminae, the former varying rhythmically in lamina thickness, and the latter invariably having a thickness of 1–2 mm. Preliminary measurements suggest a possible tidal influence on the deposition of the laminites, an influence previously unrecorded in the Namurian of this part of the Central Pennine Basin.

Petrographic and Reservoir Features of Hauterivian (Lower Cretaceous) Shatlyk Horizon in the Malay Gas Field, Amu-Darya basin, East Turkmenia: ABSTRACT

Malay gas field in Amu-Darya basin, eastern Turkmenia, is located on the structural high that is on the Malay-Bagadzha arch north of the Repetek-Kelif structure zone. With 500 km{sup 2} areal coverage, 16 producing wells and 200 billion m{sup 3} estimated reserves, the field was discovered in 1978 and production began in 1987 from 2400-m-deep Hauterivian-age (Early Cretaceous) Shatlyk horizon. The Shatlyk elastic sequence shows various thickness up to 100 m in the Malay structural closure and is studied through E-log, core, petrographic data and reservoir characteristics. The Shatlyk consists of poorly indurated, reddish-brown and gray sandstones, and sandy gray shales. The overall sand-shale ratio increases up and the shales interleave between the sand packages. The reservoir sandstones are very fine to medium grained, moderately sorted, compositionally immature, subarkosic arenites. The framework grains include quartz, feldspar and volcanic lithic fragments. Quartz grains are monocrystalline in type and most are volcanic in origin. Feldspars consist of K- Feldspar and plagioclase. The orthoclases are affected by preferential alteration. The sandstones show high primary intergranular porosity and variations in permeability. Patch-like evaporate cement and the iron-rich grain coatings are reducing effects in permeability. The coats are pervasive in reddish-brown sandstones but are notmore » observed in the gray sandstones. The evaporate cement is present in all the sandstone samples examined and, in places, follows the oxidation coats. The petrographic evidences and the regional facies studies suggest the deposition in intersection area from continental to marine nearshore deltaic environment.« less

A paleomagnetic study of Mesozoic sediments from the Junggar and Turfan basins, northwestern China

Mesozoic sedimentary rocks have been collected from the Karamay area (northwestern Junggar block) and in the Turfan basin (northeastern Tarim block) for a paleomagnetic study with the aim of testing the rigid nature of these blocks and to better constrain their paleogeographic relationships within central Asia prior to the collision with India. The results from the Karamay area are disappointing, Lower Cretaceous grey sandstones having apparently been recently remagnetized. This remagnetization is carried by magnetite, and yields highly clustered normal polarity directions. Both a medium (MTC) and a high (HTC) unblocking temperature component are isolated from flat-lying or weakly tilted Jurassic red sandstones. The MTC could be carried by maghemite and the HTC is carried by hematite. The two components are statistically identical, and seem to correspond to an ancient magnetization. However, there is no fold test and comparison with previous data from Junggar and the reference APWP of Siberia does not allow us to decide between a Middle Jurassic primary magnetization and an Upper Jurassic or Cretaceous remagnetization. Stronger constraints are obtained from Upper Jurassic to lower Eocene fine-grained red sandstones from anticlines that have been thrust southwards, in the Turfan area of South Tien Shan. After the elimination of a recent overprint that is probably related to weathering, a high coercivity, high unblocking temperature component carried essentially by hematite is isolated. All directions from the Jurassic to the lower Eocene, which yield positive fold and reversal tests, are statistically indistinguishable. This is compatible with a primary origin for these magnetizations, similar to what has previously been found in Junggar, Tarim and western Tibet. The corresponding Turfan pole is not statistically distinct from that of Tarim for the same period, in accordance with the notion of a large, quasi-rigid Tarim block remaining attached to Eurasia in a rather constant position and at a constant latitude throughout the second half of the Mesozoic. This new datum can therefore be included to derive a new mean pole for the Tarim that is valid for the period from the Middle Jurassic to the end of the Cretaceous: 68.4° N, 224.5° E ( A 95 = 3.1°).

Magnetic mineralogy of sandstones from the Lunde Formation (late Triassic), northern North Sea, UK: origin of the palaeomagnetic signal

Abstract The minerals responsible for the magnetic properties of the Late Triassic Lunde Formation have been examined using a combination of magnetic mineral extraction, microscopy and rock magnetic measurements. The magnetic mineralogy is controlled by the depositional environment and the effects of diagenesis, by a process of differential preservation and modification of the original detrital minerals. The magnetic susceptibility and remanence properties of the sediments are strongly influenced by the proportion of silt and clay sized clastic material, which contains most of the magnetic minerals. Paramagnetic minerals, both of depositional origin and produced during diagenesis, are largely responsible for the magnetic susceptibility. A suite of residual ferrimagnetic oxides (chromite and Mn-substituted magnetite) is responsible for the remanence properties in grey sandstones, because most of the originally deposited magnetic Fe-Ti oxides have been removed by reducing conditions during diagenesis. However, a small number of reddened horizons preserve a higher proportion of original detrital Fe-Ti oxides (ferrimagnetic ilmenite, ferrimagnetic chromite and magnetite), although authigenic hematite has replaced most of the original mineralogy in the reddened horizons. Magnetic oxide inclusions in quartz and feldspars are important contributors to the remanence signal, particularly in horizons which have a low content of discrete detrital ferrimagnetic grains, either through diagenetic dissolution, or paucity of the silt and clay fraction. Within the grey sandstones, the Triassic palaeomagnetic signal is carried by magnetite as inclusions in detrital silicates, with a contribution from detrital ferrimagnetic chromite. A model of the effects of diagenesis on the magnetic mineralogy in deeply buried sequences is proposed.

Palaxius caucaensis n. sp., a Crustacean Microcoprolite from the Basal Nogales Formation (Campanian to Maastrichtian) of Colombia

From a sandstone layer of the basal Nogales Formation (Campanian to Maastrichtian) of Colombia (South America) the new crustacean microcoprolite Palaxius caucaensis is described. It represents the first finding of these microfossils in Cretaceous beds in South America. INTRODUCTION Until now crustacean microcoprolites from South America have been known only from Triassic and Jurassic sediments (Forster and Hillebrandt 1984, Senowbari-Daryan and Stanley 1986, Blau et al. 1993, Blau and Senff 1993). Whereever these coprolites occur, they are often the only fossil group preserved and therefore of great stratigraphical interest. The value of crustacean microcoprolites for biostratigraphy has been demonstrated by Molinari Paganelli et al. (1980, 1986). Additionally, crustacean microcoprolites are a good tool for paleobiogeography and the reconstruction of faunal migration paths (e.g., Blau et al. 1993). In this paper, we describe the first Cretaceous crustacean microcoprolite from South America. ORIGIN OF THE SAMPLE The investigated outcrop is situated SE of Monteloro (Departamento Valle del Cauca) at the western flank of the Central Cordillera of Colombia on sheet 261 Tulua. Close to the locality known as El Paraiso, about 200m of shallow marine conglomerates, sandstones and claystones crop out in a nameless eastern confluent of the Quebrada (creek) La Mina (text-fig. 1). The sample which yields the coprolites originates from a sandstone layer near the base of the profile. The Nogales Formation The Nogales Formation crops out in a narrow north-south directed strip along the western flank of the Central Cordillera of Colombia, southeast of the city of Tulua. It is limited to the west by a fault against rocks of the Amaime Formation and the batolith of Buga, while to the east the Nogales Formation disappears under the tertiary cover. It was described first by Nelson (1957) as a unit consisting of black cherts and grey sandstones. Pardo et al. (1993) subdivided the Nogales Formation into three units. The lower part contains (graded) sandstones and matrix-supported conglomerates which sometimes have an important content of carbonate. The conglomerates are composed of metamorphic, volcanic and sedimentary rock fragments and show diameters up to 10cm. The middle part of the Nogales Formation shows an alternation of sandstones and claystones with some intercalated chert layers. Slumping structures are frequent. The upper part is dominated by claystones and cross-bedded, fine-grained sandstones. Especially in the lower part, the Nogales Formation yields plant remains, internal molds of bivalves, gastropods and ammonites. F. Etayo-Serna (Bogota) kindly determined Trochoceramus sp. and Nostoceras sp., which indicate a Late Cretaceous (Campanian to Maastrichtian) age for the lower part of the Nogales Formation which also contains the finding horizon of the coprolite-bearing sandstone layer. In the upper part of the Nogales Formation van der Hammen (1960) found a sporal assemblage of Paleocene age. According to these data, the Nogales Formation is the youngest marine sedimentary unit at the western flank of the Colombian Central Cordillera. SYSTEMATIC PALEONTOLOGY Phylum ARTHROPODA Subclass MALACOSTRACA Superorder EUCARIDA Order DECAPODA Family THALASSINIDAE Genus Palaxius Bronnimann and Norton 1960 The genus Palaxius was defined by Bronnimann and Norton (1960) as having crescent shaped canals in cross-sections. Recently, Blau et al. (1994) have demonstrated that crescent-shaped canals also oc ur together with triangle-shaped canals in Parafavreina thor netensis Bronnimann et al. 1972a. Palaxi s caucaensis Blau, Moreno and Senff n. sp. Text-figures 2a-e, 3a-d Etymology: Named after the Valle del Cauca, where the coprolites were found. Holotype: The specimen illustrated in text-fig. 2c, thin section no. M-12, tored at the Institut fur Angewandte Geowissenschaften, Fachgebiet Palaontologie, Justus-Liebig-Universitat, Diezstrasse 15, D-35390 Giessen. Type locality: Nameless eastern confluent of the Quebrada La Mina (text-fig. 1). micropaleontology, vol. 41, no. 1, pp. 85-88, text-figures 1-5, 1995 85 This content downloaded from 157.55.39.159 on Fri, 09 Dec 2016 05:24:19 UTC All use subject to http://about.jstor.org/terms Joachim Blau et al.: Palaxius caucaensis n. sp., a crustacean microcoprolite from the basal Nogales Formation of Colombia

Tectonostratigraphy and kinematics of the eastern end of the Sivas Basin (central eastern Turkey): Implications for the so‐called ‘Anatolian block’

AbstractThe eastern end of the Sivas Basin, just west of Erzincan, is known as the Kemah Basin. The lithostratigraphy of the Kemah Basin thus corresponds with the general characteristics described for the Sivas Basin: the upper stratigraphic sequence begins with the Kemah Limestones of Burdigalian age. Above these, the sediments (grey limy marls, sandstones and conglomerates) are not yet dated, but probably span Late Miocene to Plio‐Quaternary times. The lower sequence is made of (Late Oligocene?) non‐fossiliferous, mainly continental deposits (variegated clays, silts and gypsum at the base; conglomerates, sandstones and some volcanics in the footwall of the limestones).Both the basin deposits and substratum (arc volcanics, volcanoclastics and ophiolites) are involved in thrust systems with displacements consistently directed to the SSE. The sequence of thrusting deduced from the geometrical pattern of the imbricate zone and from the stratigraphic record includes at least two stages of thrust propagation. The sequence shows the rise of successive waves of active new thrusts and folds from the north, cutting through earlier thrusts in a general ‘break‐back’ sequence. The southern edge of the Neogene basin against the ophiolites covering the Munzur Limestones (Taurides) shows a moderate northward tilting.A first attempt at a (incomplete) balanced cross‐section and restoration along the Burdigalian limestones as datum gives a minimum displacement of 11 km along a section length of 12 km. The kinematics and the contraction ratio question the concept of an ‘Anatolian block’ and related displacement models during the Late Neogene.

Early stages of foreland basin evolution in the Lesser Himalaya, N India

Abstract In this paper we report preliminary results of an integrated sedimentological and structural study of the Late Cretaceous-Early Tertiary facies exposed in the Lesser Himalaya. The Precambrian-Palaeozoic basement of the Indian plate was initially transgressed in Late Cretaceous times (after c. 100 Ma), with the deposition of shallow shelf carbonates, (50 m thick Singtali Formation). These are depositionally overlain by green ferruginous mudstones and thin limestones of the 150 m thick Palaeocene-Eocene ( c. 65–40 Ma) Subathu Formation. During Late Eocene-Oligocene times ( c. 40–30 Ma), clastic sediment input increased and red lenticular sandstones, thinner planar-bedded sandstones, mudstones and caliche of the 350 m thick Dagshai Formation were deposited. A semi-arid, meandering fluvial/floodplain setting is envisaged, with thick channelised and overbank sands. Palaeocurrent evidence indicates a general SE progradation. Our detailed field mapping shows that where the entire Subathu and Dagshai Formations are intercalated, this is the result of post-depositional tectonics, rather than primary intertonguing as described in some previous reports. The relative abundance of sandstones greatly increased in upper Dagshai times, continuing into the Early-Mid-Miocene ( c. 30–10 Ma) when the 250 m thick Kasauli Formation was laid down. This formation is characterized by lenticular and planar-bedded grey sandstones, rich in plant material including occasional logs (but without caliche) interbedded with minor grey mudstones. A rapidly prograding, braided fluvial environment is proposed, with the petrography (e.g. presence of garnets) suggesting erosion and derivation from deeper levels of the Himalayan mountain belt to the north. The climate had by then changed from semi-arid to humid, possibly in response to the onset of the monsoon, initiated when the mountain belt had reached sufficient height to interfere with the jet-stream. During Late Miocene-Quaternary times (younger than c. 15 Ma), the Tertiary foreland basin was deformed and incorporated into three structural levels of a southward migrating thrust stack. At the lowest level, thick successions of the Tertiary Subathu, Dagshai and Kasauli Formations are structurally underlain by fluvial sediments (Siwalik Gp.) and overlain by the Krol Precambrian-Palaeozoic sedimentary nappe. At the intermediate structural level, Early Tertiary sediments unconformably overlie the stratigraphy within this nappe and comprise, often thin (<50 m), successions of mainly Subathu and Dagshai Formation sediments. The highest structural level sediments form small imbricates, highly dismembered units and melange (including evaporites) beneath the major metamorphic Jutogh Nappe. We interpret the Late Cretaceous-Palaeocene Singtali Formation as pre-collisional transgressive sediments related either to flexural downwarping of the Indian plate due to the obduction of the Spontang ophiolite onto the north Indian plate margin, or to extensional tectonics related to the detachment and drift of India from Gondwana and Neo-Tethyan subduction beneath Asia. This would have been coupled with the Late Cretaceous global eustatic sea-level high stand. The marine Subathu Formation is interpreted as having been deposited between initial and terminal India-Eurasia continental collision, with the overlying fluvial/floodplain Late Eocene-Oligocene Dagshai and Miocene Kasauli Formations as southward prograding foreland basin successions related to progressive stages of India-Eurasia continental collision.

Lithostratigraphic comparison of three Diamictite successions of Nepal Lesser Himalaya

ABSTRACT
 The Proterozoic Blaini-type of diamictite sequences have been identified from the Lesser Himalaya (Brookfield, 1987). A tillitic sequence called the Sisne Formation of Gondwana age was reported by Sakai (1983) from Tansen, the Western Nepal Lesser Himalaya. The newly reported Kokaha Diamictite and Sallyan Diamictite situated to the east and west of Tansen, respectively, exhibit some lithostratigraphic similarity with the Sisne Formation, though the underlying and overlying rocks are quite different
 The Kokaha Diamictite from the Barahakshetra area of Eastern Nepal is underlain by grey quartzite with coal seams and conglomerate lenses. Sometimes tuffs and agglomerates are also found together with the diamictite in some places. It is followed upsection by grey calcareous sandstone, siltstone and shale. The Sisne Formation from the Tansen area of Western Nepal overlies grey dolomite with a disconformity and is followed by the Taltung Formation separated by another disconformity. The Taltung Formation itself is composed of basic volcanics, conglomerate, sandstone and siltstone. A non-Blaini type of diamictite horizon called the Sallyan Diamictite from the Sallyan area of West Nepal occurs in low -grade metamorphic rocks. It disconformably overlies quartzite, meta-conglomerates and phyllites, and is transitionally followed by black carbonaceou s slates. Lack of reliable fossils, metamorphism and strong deformation pose difficulties in correlating it with other diamictites.

The Permian in Northern Bulgaria. I. Formal lithostratigraphy related to the Lower Permian

Two lithostratigraphic groups, corresponding to Lower Permian and Upper Permian, have been distinguished after stating the principles of division of the Permian sediments. In the first group, called North-Bulgaria Group, are distinguished: a variegated clayey-terrigenous formation, Nanevo and Komunari Formations (Lower Rotliegendes?), Severci Formation (Upper Rotliegendes?) and Bdin and Dolna Zlatica Formations (undivided Rotlegendes), as well as Kamen Briag, Bajacevo and Balgarevo Members in Nanevo Formation and Bononia, Delejnska and Rasovo Members in Bdin Formation. The officially distinguished lithostratigraphic units are documented by eight type sections and two reference sections, which are described according to drill-holes, taking into consideration the requirements of the Stratigraphic Code. The variegated clayey-terrigenous formati on comprises red and grey-green sandstones, silt stones and argillites. In the Nanevo Formation, from the bottom upwards, dominate various middle-acid volcanites and tuffs (Kamen Brjag Member), conglomerates, sandstones and a small amount of silt stones with tuffs and redeposited volcanic material (Bajačevo Member) and hypoargillitcs, hyposiltstones, a small amount of impure gravel-sandy rocks (Bălgarevo Member). Polygenetic brecc iaconglumcrates. conglomerates, red and black grey sandstones and siltstones alternate in the Komunari Formation (by a lateral disappearance of the psephilic rocks). The Belin Formation mainly contains conglomerates with carbonate fragments (with a small amount of admixtures of volcanic fragments, mainly in the Bononia Member), the composit ion of the Delcjnska Member being hypoa lcuropclitic. The Dolna Zlatica Formation is built of brown-reddish, mainly massive argillites and clayey siltstones with rare bedded interbeds consisting of siltstones and grained sandstones. Breccia conglomerates, sandstones and siltstones (a small amount of argillites), the psephites bearing fragments of sediments (carbone ones inclusive), low-metamorphic and volcanic rocks, are in an alternation in the Severci Formation. The Severci Formation overlies an eroded surface, while the other formations interfinger laterally with a correlation of the separate triads of members. The Komunari Formation is spread over the southern margin of the estern part of North Bulgaria, the Dolna Zlatica Formation - in the Tiirnovo Depression and the areas around, the Nanevo Formation - in the Bulgarian Coastal Dobrudza and around the North-Bulgaria Swell, the Severci Formation - in NE Bulgaria. Lower Permian, in the Bulgarian pari of the Moesian area, has a more detailed lithostratigraphic division than on the territory of Romania. The majority of the formations and members described may have analogues in the drill-holes north of the Danube.

Paleomagnetic evidence for rotation during Stephano-Permian extension in southern Massif Central (France)

We present results of a preliminary paleomagnetic study devised as a test of a tectonic model proposed by Van Den Driessche and Brun [1] to interpret the main structural features of the Montagne Noire (South Massif Central, France). This model suggests that crustal extension during Stephanian to Permian times is responsible for a major flat-lying detachment. The inferred geometrical relationships between fault azimuth and extension direction is believed to induce a counter-clockwise rotation of the Paleozoic cover during its northward motion. In order to test this model, we have sampled Autunian (Lower Permian) grey and red sandstone beds within two basins opened during the extension, the Lodève and the St-Affrique basins. The paleomagnetic study of six sites (three in each basin) using thermal demagnetization procedures, reveals the presence of 3 magnetization components. A first one (A), which is unblocked below 400°C, is interpreted as a recent overprint due to weathering of the sandstones. The B component is unblocked between 400° and 575°C. This B component is assumed to be primary, although the gentle dip of the beds does not allow a fold test. It fits the Lower Permian field direction in the Lodève basin ( in situ: D m = 194° , I m = 6° , k = 50, tilt corrected: D m = 194° , I m = −10° , k = 53). In contrast, it shows a systematic difference in declination in the St-Affrique basin ( in situ: D m = 173° , I m = 2.5° , k = 101, tilt corrected: D m = 172° , I m = 6° , k = 37). This is interpreted as a post-Autunian counter-clockwise rotation of at least 20° of the St-Affrique basin, thus supporting one of the main implications of the tectonic model. Finally, red sediments from two sites of the Lodève basin yield a third component (C) with unblocking temperatures up to 670°C and carried by hematite. Its direction is also consistent with the Permian field direction, showing that hematite crystallisation occurred early in the diagenesis history of the sediments.

Neogene stratigraphy and paleoenvironments of China

Neogene deposits are widely distributed in north and west China, whereas in southeast China they are confined to isolated basins. The Neogene in China is predominated by non-marine facies and marine deposits are limited to the Leizhou Peninsula, some islands and continental shelves, although Miocene brackish-water deposits were encountered in Xinjiang, northwest China, and a very brief marine transgression took place in north China near the end of the Pliocene. Non-marine Neogene deposits in China are largely fluvial and lacustrine in origin and composed of mudstones, sandstones and conglomerates with local occurrences of lignites and diatomites. The maximum thickness may exceed 2000 m (e.g. the Qaidam Basin). A rich fauna and flora have been found in many localities and the most famous are the Middle Miocene diatomites at Shanwang, Shandong Province, with unusually well preserved vertebrate and plant fossils. A total of 7 mammal stages have been distinguished in China (5 in the Miocene, 2 in the Pliocene) and tentatively correlated with the European stages. A major realignment of climatic zones occurred before the Paleogene/Neogene boundary, when the Paleogene climatic zones, with a NW-trending broad arid zone across the whole of China, was eliminated and the arid zone moved to northwest China, presumably indicating the onset of the monsoon circulation system. During the last few years, the marine Neogene has been intensively studied in the Leizhou Peninsula and the three major sedimentary basins off the south China coast: the Beibu Gulf (Gulf of Tonkin) Basin, the Zhujiangkou (Pearl River mouth) Basin and the Yinggehai Basin. This sequence is over 3000 m thick, ranges from the Oligocene to the Pleistocene and consists of grey mudstones, sandstones and conglomerates, interbedded with biogenic deposits such as reef limestones and sandy chalk. At least 6 foraminiferal assemblages can be recognized in the three basins: the Psudohastigerina micra assemblage from the Oligocene, the Globigerina ciperoensis-Cassigerinella chipolensis assemblage from the Lower Miocene, the Globoratalia siakensis-Orbulina suturalis assemblage from the Middle Miocene, the Globigerina nepenthes-Globorotalia acostaensis assemblage from the Upper Miocene and/or Lower Plicoene, the Globorotalia tosaensis- Globigerinoides extremus assemblage from the Upper Pliocene and the Globorotalia truncatulinoides assemblage from the Quaternary. The nannofossil succession and larger foraminiferal faunas confirm the ages. The tropical nature of the Neogene faunas is revealed by the occurrences of large foraminifers and warm-water species of other groups. The transgression cycles are controlled by tectonic processes during the Oligocene and Early Miocene, but later by eustatic movements.