Recognition Of Facies Research Articles

The impact of rapid early- to mid-Holocene palaeoenvironmental changes on Neolithic settlement at Nea Nikomideia, Thessaloniki Plain, Greece

The site of Nea Nikomideia is one of the oldest and most important Neolithic settlements in Northern Greece and the wider Balkan Peninsula, having been first occupied by early farmers at around 6500 cal. BC. Important archaeological excavations conducted in the 1960s suggested that the settlement was located close to an ancient coastline during the Neolithic. However, palaeoenvironmental change and landscape evolution in the vicinity of the site have seldom been considered in detail. Six cores from the western and central parts of the Thessaloniki Plain were therefore drilled in 2008 and subjected to palaeoenvironmental analyses, including sedimentology (LASER grain size and magnetic susceptibility measurements), chemical analysis (loss on ignition and carbonate content), stable isotopes analysis coupled with X-Ray diffraction measurements, molluscan faunal analysis and radiocarbon dating. The recognition of several important facies representing freshwater (lacustrine and fluvial) and brackish (lagoonal and marine-influenced) conditions have shed light on the environmental and landscape evolution of the western part of the Thessaloniki Plain and associated impacts on human occupation during the last 10,000 years. The general sequence proved in the cores indicates the predominance of lacustrine conditions during the early Holocene, with the occurrence of a marine transgression at c. 6000/5800 cal. B.C. This major palaeoenvironmental change corresponds with the 8.2 Ka event and is a likely cause for the desertion of Nea Nikomideia at that time. Subsequent regression of the shoreline to the east saw that the area around Nea Nikomideia returns to predominantly terrestrial conditions and the deposition of lacustrine and fluvial deposits.

Recognition and characterisation of the sedimentary facies deposited by the 1960 tsunami in the Maullín estuary, Chile.

The tsunami triggered by the earthquake of May 22, 1960, flowed violently into the Maullin River estuary. According to eyewitnesses, the tsunami deposited a widespread sand layer throughout the estuarine marshes. The aim of this study was to determine whether the sand sheet was incorporated into the estuarine stratigraphy. A sedimentological analysis of the sand, located on a saltmarsh opposite Maullin, was carried out. The sand layer was found between the 1960 saltmarsh and the present soil. Considering the sand sheet grain size characteristics and tilt of the buried vegetation, it is probable that the sand was transported from the neighboring dunes and/or sandbars located in the estuary mouth. The spatial distribution of granulometric parameters shows that the marsh vegetation reduced the wave transport capacity during seawater run-up. This illustrates the importance of estuarine marshes in diminishing the impact of tsunamis.

Petrographic modelling of Egyptian limestones for quicklime manufacture

The main objective of this article was to construct a mathematical model for the optimum petrographic variables that affect quicklime production along with others (physico-mechanical and chemical) during Egyptian limestone calcination at different firing conditions. Limestone microstructure is a key player during its calcination. Eighteen limestone technological samples representing the limestones in ten occurrences were petrographically analysed through the microscopic examination of a large number of thin sections in detail. The description and the semi-quantitative measures of the petrographic variables were determined by the aid of the descriptive and comparison charts. The data obtained permitted the delineation of 14 petrographic compositional and textural variables which, in turn, led to the recognition of different petrographic facies. The limestone technological samples were fired at eight firing conditions (900°C and 1,000°C for 15, 30, 60 and 120 min) in an electric muffle. After firing, the obtained quicklime was ground and chemically analysed for free lime determination. The graphical presentations of the relationships between the independent (petrographic) and the dependent (free lime measures) variables declared different behaviours with different strength (correlation) of each petrographic variable on the rate of lime liberation. To recognise the petrographic compositions for the optimum lime yield at each firing condition, statistical (mathematical) models were constructed. The obtained petrographic and free lime data sets were statistically analysed by applying the stepwise regression method. This method examines the relationships between each dependent variable (free lime at eight firing conditions) and the independent variables (14 petrographic measures). This treatment of data led finally to eight mathematical models which involve the more important compositional and textural variables which affect the rate of lime liberation at each firing condition (temperature and soaking time). The relations between the observed and expected cumulative probabilities at all firing conditions proved that the acquainted models require further enhancement to be more applicable. This is quite acceptable especially when it is known that the chemical and physico-mechanical properties are also key players in the quicklime manufacture. Microstructure studies are also recommended for the fired limestone (lime) for testing these models where the type and amount of each petrographic variable could be determined pre- and post-firing.

Seismic analysis of clinoform depositional sequences and shelf‐margin trajectories in Lower Cretaceous (Albian) strata, Alaska North Slope

ABSTRACTLower Cretaceous strata beneath the Alaska North Slope include clinoform depositional sequences that filled the western Colville foreland basin and overstepped the Beaufort rift shoulder. Analysis of Albian clinoform sequences with two‐dimensional (2D) seismic data resulted in the recognition of seismic facies inferred to represent lowstand, transgressive and highstand systems tracts. These are stacked to produce shelf‐margin trajectories that appear in low‐resolution seismic data to alternate between aggradational and progradational. Higher‐resolution seismic data reveal shelf‐margin trajectories that are more complex, particularly in net‐aggradational areas, where three patterns commonly are observed: (1) a negative (downward) step across the sequence boundary followed by mostly aggradation in the lowstand systems tract (LST), (2) a positive (upward) step across the sequence boundary followed by mostly progradation in the LST and (3) an upward backstep across a mass‐failure décollement. These different shelf‐margin trajectories are interpreted as (1) fall of relative sea level below the shelf edge, (2) fall of relative sea level to above the shelf edge and (3) mass‐failure removal of shelf‐margin sediment. Lowstand shelf margins mapped using these criteria are oriented north–south in the foreland basin, indicating longitudinal filling from west to east. The shelf margins turn westward in the north, where the clinoform depositional system overstepped the rift shoulder, and turn eastward in the south, suggesting progradation of depositional systems from the ancestral Brooks Range into the foredeep. Lowstand shelf‐margin orientations are consistently perpendicular to clinoform‐foreset‐dip directions. Although the Albian clinoform sequences of the Alaska North Slope are generally similar in stratal geometry to clinoform sequences elsewhere, they are significantly thicker. Clinoform‐sequence thickness ranges from 600–1000 m in the north to 1700–2000 m in the south, reflecting increased accommodation from the rift shoulder into the foredeep. The unusually thick clinoform sequences suggest significant subsidence followed by rapid sediment influx.

Reconhecimento e mapeamento de fácies sísmicas nos pontais arenosos da lagoa dos Patos, RS, Brasil

O objetivo deste trabalho é reconhecer e mapear os elementos sísmicos arquiteturais dos pontais arenosos que ocorrem na lagoa dos Patos, os quais se encontram localizados na planície costeira do Estado do Rio Grande do Sul, Brasil. O trabalho é baseado em dados de alta frequência (kHz 3,5) e resolução, oriundos de levantamentos sísmicos realizados na lagoa dos Patos, a bordo da lancha Larus, da Universidade Federal do Rio Grande. Cerca de 400 km de perfis sísmicos foram coletados em várias seções. Consideraram-se, para o estudo, os perfis sísmicos obtidos próximo aos pontais arenosos da margem lagunar. A análise dos registros sísmicos permitiu o reconhecimento e mapeamento de fácies sísmicas que construíram os pontais arenosos da lagoa dos Patos. A progradação dos pontais arenosos é bem marcada por refletores oblíquos tangenciais que mergulham em direção ao interior da laguna. O pacote formado por estes refletores progradantes recobre um pacote de refletores subparalelos ou, às vezes, uma discordância de erosão. Ao longo dos registros, fácies sísmicas compostos por refletores paralelos, regulares e mergulhantes ocorrem no topo da sequência. Estas estão relacionadas com a sedimentação lagunar

Carbonate platforms in the Dolomites area of the Southern Alps – historic perspectives on progress in sedimentology

AbstractThe area of the Dolomites in the Southern Alps exhibits some of the best outcrops of carbonate platforms and platform‐to‐basin transitions in the world. The region has attracted geologists since the early 19th Century and has been a centre of stratigraphic and sedimentological research ever since. The interpretation of the platforms in the Dolomites as coral reefs dates back to the 1850s and was inspired by the work on modern reefs in the Pacific and Indian Oceans at that time. Very soon, however, studies in the Dolomites triggered developments of their own. Fieldwork on these Triassic atolls led to a first understanding of the dramatic facies changes at the periphery of reefs and atolls, including the recognition of slope facies and basinward transport of platform material by gravity. In addition, the platforms were used to prove long‐term syndepositional subsidence on the scale of kilometres as early as 1860. After World War II, the study of modern reefs and platforms led to process‐based facies models and this, in turn, guided the sedimentological analysis of the Alpine–Mediterranean belt. Repeatedly, however, the roles became reversed and the Alpine–Mediterranean platforms contributed to the fundamental underpinnings of sedimentology. In the 1980s, mountain‐size outcrops in the Alps and the Apennines were recognized as models for the interpretation of shoal‐water carbonates in seismic data. Seismic models of outcrops in the Dolomites, for instance, gave rise to the concept of pseudo‐unconformities, i.e. rapid lateral facies changes that appear as unconformities in seismic images. In the 1990s, the Alpine–Mediterranean Mesozoic along with the North American and European Palaeozoic revealed the significance of microbially induced carbonate precipitation in constructing large limestone bodies. Studies of ancient rocks led this development because this particular carbonate factory is far less prominent now than it was at certain times in the past.

Sedimentology and sequence stratigraphy of Aquitanian and Burdigalian stratotypes in the Bordeaux area (southwestern France)

The historical stratotypes of the Aquitanian and Burdigalian in the Aquitaine Basin are studied here by using the tools of facies sedimentology and the concept of sequence stratigraphy. This analytical method combines recognition and sequential organization of facies, and several types of stratigraphic markers. This method allows identification of at least six depositional sequences within the Miocene of the Saucats area: the four lower ones belong to the Aquitanian, the fifth one to the whole Burdigalian, while the sixth sequence corresponds to the Serravallian. In addition, this method provides evidence of a period of emersion before each transgression, suggesting potential fluvial erosion. Nevertheless, these phases are of lower amplitude than those observed in the Rhodano-Provençal Basin.

Introduction to this special section—Seismic Attributes

Seismic attributes are a great aid in both the qualitative and quantitative mapping of subsurface geologic features. Even before classical interpretation of picking horizons and generating maps begins, animating through appropriate seismic attribute volumes can quickly define the structural fabric and delineate major stratigraphic features. Seismic attributes have proven useful in almost every geologic environment, from clastics through carbonates to volcanic, and from normal faulting through wrench faulting to reverse faulting. Thus, seismic attributes facilitate recognition of depositional environments and enhance the recognition of seismic facies. Once calibrated at well locations, attributes can be used to identify seismic facies and provide information about both lithology and fluids, with or without the use of reference horizons. For this reason, seismic attributes form an integral part of most interpretation projects completed today. A complete book on seismic attributes, Seismic Attributes for Pro...

Facies recognition using wavelet based fractal analysis and waveform classifier at the Oritupano-A Field, Venezuela

Abstract. We have used a Wavelet Based Fractal Analysis (WBFA) and a Waveform Classifier (WC) to recognize lithofacies at the Oritupano A field (Oritupano-Leona Block, Venezuela). The WBFA was applied first to Sonic, Density, Gamma Ray and Porosity well logs in the area. The logs that give the best response to the WBFA are the Gamma Ray and NPHI (porosity) logs. In the case of the logs, the lithological content could be associated to the fractal parameters: slope, intercept and fractal dimension. The map obtained using the fractal dimension shows tendencies that generally agree with the depositional patterns previously observed in conventional geological maps. According to the results obtained in this study, zones with fractal dimension values lower than 0.9 correspond to sandstone channels. Values between 0.9 and 1.2 coincide with the interdistributary deltaic shelf and values greater than 1.2 might be associated with zones of greater shale content. The WBFA and WC results obtained for the seismic data show no relation with the lithofacies. The lost of low and high frequencies in these seismic data, as well as phase problems, could be the reasons for this behavior.

Fluvial and aeolian deposition in the Siluro-Devonian Swanshaw Sandstone Formation, SW Scotland

Synopsis The recognition of fluvial and aeolian facies within the Siluro-Devonian Swanshaw Sandstone Formation of SW Scotland allows a more detailed depositional history of the Lanark basin in the southern part of the Midland Valley of Scotland to be inferred. The formation is well exposed in Ayrshire and has four main sedimentary facies associations: Channel, Aeolian, Floodplain and Mudflat. Each combines several sedimentary facies: eight facies are recognized in the channel association, three in the aeolian, four in the floodplain and two in the mudflat. These facies associations co-existed in seven different depositional settings in the western Lanark basin. Deposition occurred on a semi-arid proximal alluvial plain with an aeolian component described for the first time in the Lanark basin. Sedimentation was pulsed, largely driven by periods of source area uplift that resulted in the deposition of coarse-grained conglomeratic and sandstone-dominated facies. The principal transport was via river channels, commonly in shallow braided systems. Periods of low sediment flux are marked by fewer channels and by the preservation of floodplain successions including aeolian deposits. Abandonment of the fluvial system towards the top of the formation in Ayrshire is marked by a progression through sandflats and mudflats to the overlying volcanic formation. The relationship of the fluvial and aeolian deposits is analysed. The palaeo-wind direction is compatible with that recorded in sandstones of similar age in the Orcadian basin.

DEVELOPMENTS IN THE CENTRAL AND NORTHEASTERN EAST COAST BASIN, NORTH ISLAND, NEW ZEALAND

Oil production in the East Coast Basin began in the late 19th century from wildcat wells near oil seeps. By the mid-20th century, geology was being applied to oil exploration, but with little success. In the late 20th century, seismic techniques were added to the exploration arsenal and several gas discoveries were made. At each stage it was recognised that exploration in this difficult but tantalising basin required more information than was available. Continuing work by exploration companies, as well as by the Institute of Geological & Nuclear Sciences (GNS), has begun to reduce the risk of exploration. Source rocks have been identified and sophisticated thermal models show that petroleum is being generated and expelled from them as shown by numerous oil and gas seeps onshore. Many potential reservoir sequences have been recognised from outcrop studies and depositional models are being refined. All components of petroleum systems have been demonstrated to be present. The most important deficiency to date is the general lack of high-quality seismic data which would allow recognition of reservoir facies in the subsurface.During early 2005, Crown Minerals, the New Zealand government group charged with promoting and regulating oil and gas exploration, commissioned a high specification regional 2D survey intended to address some of the main data gaps in the offshore East Coast Basin. A broad grid was planned with several regional lines to be acquired with a 12,000 m streamer and infill lines to be acquired with a streamer 8,000 m long. It was expected that the long streamer would increase resolution of Paleogene and Cretaceous units. Several of the lines were actually acquired with a 4,000 m streamer due to unexpectedly high rates of unserviceability. The resulting 2,800 km data set consists of a series of northwest–southeast lines approximately orthogonal to the coast at a spacing of about 10 km as well as several long strike lines.GNS was contracted to produce a series of reports covering source rock distribution, a catalogue of reservoir rocks, a regional seismic interpretation, thermal models and structural reconstruction. The data package and reports are available free of charge to any interested exploration company to accompany the licensing round that was announced on 1 September 2005. The new data set has confirmed the existence of a large, little-deformed basin to the north of North Island and the Bay of Plenty; it has elucidated the complex structure of a large part of the East Coast Basin and has enabled generation of a general sequence stratigraphic model which assists in delineating reservoir targets. On 1 September 2005, the New Zealand government launched a licensing round covering about 43,000 km2 of the East Coast Basin, from the far offshore East Cape Ridge in the north to the northern Wairarapa coast in the south. Four blocks (I, J, K and L) were on offer for a competitive staged work programme bid, closing on 17 February 2006.

Reply to comment by C. A. Boulter on “A new geodynamic interpretation for the South Portuguese Zone (SW Iberia) and the Iberian Pyrite Belt genesis”

[1] Our paper [Onezime et al., 2003] was basically aimed at providing a new, and thus debatable, geodynamic interpretation at a regional scale of the South Portuguese Zone, of which the Iberian Pyrite Belt (IPB) is part. Our study relied mainly on new structural data and facies analysis, with some emphasis on the volcanic and related facies of the Volcano-Sedimentary Complex (VSC) of the IPB, on which Boulter’s [2005] comments concentrate. Boulter complains that his ideas about the IPB and more specifically his interpretations of the VSC volcanic and volcaniclastic facies [Boulter, 1993a, 1993b, 1996] were not correctly rendered and were partly misinterpreted in our paper. A huge literature exists about the IPB geology, and various authors have dealt in more or less detail with the volcanological aspects of this belt. There was reasonably no room in our paper for an in-depth discussion of all relevant previous interpretations on this topic, including Boulter’s sill-sediment model. Some other workers in the IPB have already stressed the poor consistency of this model with the field observations at the regional scale [e.g., Carvalho et al., 1999], and recent reviews of the IPB geology and metallogeny do not rely much on this model [e.g., Leistel et al., 1998; Saez et al., 1996, 1999]. We restrict our reply to the issues raised by Boulter in his comment. [2] We acknowledge the fact that Boulter [2005] reported extrusive volcaniclastics and resedimented volcanic deposits in the VSC. We thus acknowledge that our quoting of his model as ‘‘a complete intrusive model’’ was a bit loose in this regard. However, the claim by Boulter that ‘‘he did recognize the importance of stratified volcaniclastic rocks in the Pyrite Belt’’ does not give any support to the sillsediment model he in fine continues to defend. It rather tends to weaken his case since such volcaniclastic facies are not normal components of intrusive magma-sediment systems. Boulter interprets them as mostly ‘‘extrusive hydroclastic breccias’’ derived from disruption, eventually explosive, of the sediment cover above the high-level intrusive system. In this interpretation, there is implicit assumption that such volcaniclastic deposits should be volumetrically trivial in the VSC, if this is still to be described as a sill-sediment system. We thus envisioned the extrusive component as being subsidiary in Boulter’s model and not worth to be quoted in our paper. It is concerning, and rather contradictory to his claim, that Boulter himself states that ‘‘the majority of the volcanic facies in the Rio Tinto district were peperitic intrusions that did not supply detritus to the sedimentary basin.’’ [3] Boulter [2005] pretends to understand that we are unaware of the possible formation of volcaniclastic rocks in intrusive conditions. This is rather unfair misreading of our writing. The exact wording of his comment is ‘‘Despite important amounts of volcaniclastic ‘‘deposits’’ (quotation marks added), a complete intrusive model has been proposed. . .’’. ‘‘Volcaniclastic deposits’’ in our writing referred to material that has been transported and deposited (either pyroclastic or resedimented), according to the common meaning. It was not to refer to the intrusive autoclastic facies reported and emphasized by Boulter in his papers. Intrusive (and extrusive as well) autoclastic facies in submarine environment of relevance here are mostly formed by quench fragmentation (granulation in our paper). We have made due recognition of such facies in the VSC (e.g., Figure 8 of our paper, our ‘‘autoclastic brecciated facies’’). Although locally conspicuous at the outcrop scale, they do not represent a prominent volumetric part of the volcanic province as a whole. [4] The issues raised by Boulter [2005] and discussed above should not obscure some more significant differences between his description of the VSC and ours. A major point is the evidence gained from our fieldwork that volcaniclastic deposits, both primary and resedimented, are, along with volcanogenic sedimentary deposits, a major component of the VSC in terms of volume. The volcaniclastic deposits are dominated by sandstones and lapillistones. In the perspective of the issues raised by Boulter in his comment, it is clear that this evidence does not match with the sillsediment hypothesis. There is obvious contradiction between a model where volcanics are dominated by intrusions, TECTONICS, VOL. 24, TC1010, doi:10.1029/2004TC001775, 2005

Kamafugitic diatremes: facies characterisation and genesis—examples from the Goiás Alkaline Province, Brazil

This paper describes the internal organisation of two diatremes (Águas Emendadas and Neuzinha) and one small breccia-filled conduit (Tigre) in the central portion of the Late Cretaceous Goiás Alkaline Province (GAP), central Brazil, and explores the criteria for facies recognition. The GAP kamafugitic diatremes are emplaced into Carboniferous sandstones of the Aquidauana Formation, at the northern margin of the Paraná Basin. They are usually elliptical structures, not longer than 900 m, filled with breccia and partially covered by thin kamafugitic to basanitic lava flows. The breccias are dominated by juvenile pyroclasts, with subordinate amounts of cognate fragments and xenoliths. In addition to variations in ash and lapilli proportions, juvenile fragment types may be used to discriminate genetic processes and the corresponding pyroclastic deposits.An extensive field, textural and compositional dataset was analysed by multivariate statistical techniques. Combined with field observations, this allowed us to define a set of facies for kamafugitic diatremes, and, more importantly, to understand the internal structure of the studied bodies and to cross-correlate them. Seven distinct facies were recognised. The Fluidised Conduit Facies (FCF) represents high-energy, strongly fluidised but only moderately fragmented systems. It occurs in a confined environment, and is typical of deeper parts of the conduit, before the actual diatreme level is reached by the ascending fluidised magma. Large amounts of spinning droplets are formed within this region. The Fluidised Conduit–Diatreme Facies (FCDF) is characteristic of intermediate depths in the conduit, where highly fluidised and highly fragmented systems produce large amounts of ash. Spinning droplets decrease in abundance, ordinary juvenile fragments become very common, and xenoliths from the country rock in the immediate vicinity of the diatreme are present. The Fluidised Fragmented Facies (FFF) and the Magmatic Fluidised Facies (MFF) produce very heterogeneous deposits that dominate the shallower part of the system, making up most of the diatreme-filling materials. The Fluidised Fragmented Facies can be distinguished by much higher degrees of fluidisation, fragmentation and system energy. It occupies the internal part of the diatreme and is characterised by the common presence of tuff pockets, tuff fragments, and accretionary and armoured lapilli. The Magmatic Fluidised Facies typically occupies the outer portion of the diatreme and can be distinguished from the Fluidised Fragmented Facies by the dominance of lapilli over ash and by the presence of abundant wrapped fragments. The Magmatic Facies (MF) and the Coherent Magmatic Facies (CMF) are volumetrically subordinate and represent late stages, when less fluidised and less fragmented material, or even coherent magma erupts relatively passively, in the aftermath of the main explosive stage that generated the diatreme. The Border Facies is defined by the increased abundance of material from the immediate country rock. At Águas Emendadas and Neuzinha this facies is marked by the presence of fragments of peperite-like rock, formed by the interaction of the fluidised magma with friable sandstone.

Kamafugitic diatremes: facies characterisation and genesis?examples from the Goi�s Alkaline Province, Brazil

This paper describes the internal organisation of two diatremes (Águas Emendadas and Neuzinha) and one small breccia-filled conduit (Tigre) in the central portion of the Late Cretaceous Goiás Alkaline Province (GAP), central Brazil, and explores the criteria for facies recognition. The GAP kamafugitic diatremes are emplaced into Carboniferous sandstones of the Aquidauana Formation, at the northern margin of the Paraná Basin. They are usually elliptical structures, not longer than 900 m, filled with breccia and partially covered by thin kamafugitic to basanitic lava flows. The breccias are dominated by juvenile pyroclasts, with subordinate amounts of cognate fragments and xenoliths. In addition to variations in ash and lapilli proportions, juvenile fragment types may be used to discriminate genetic processes and the corresponding pyroclastic deposits. An extensive field, textural and compositional dataset was analysed by multivariate statistical techniques. Combined with field observations, this allowed us to define a set of facies for kamafugitic diatremes, and, more importantly, to understand the internal structure of the studied bodies and to cross-correlate them. Seven distinct facies were recognised. The Fluidised Conduit Facies (FCF) represents high-energy, strongly fluidised but only moderately fragmented systems. It occurs in a confined environment, and is typical of deeper parts of the conduit, before the actual diatreme level is reached by the ascending fluidised magma. Large amounts of spinning droplets are formed within this region. The Fluidised Conduit–Diatreme Facies (FCDF) is characteristic of intermediate depths in the conduit, where highly fluidised and highly fragmented systems produce large amounts of ash. Spinning droplets decrease in abundance, ordinary juvenile fragments become very common, and xenoliths from the country rock in the immediate vicinity of the diatreme are present. The Fluidised Fragmented Facies (FFF) and the Magmatic Fluidised Facies (MFF) produce very heterogeneous deposits that dominate the shallower part of the system, making up most of the diatreme-filling materials. The Fluidised Fragmented Facies can be distinguished by much higher degrees of fluidisation, fragmentation and system energy. It occupies the internal part of the diatreme and is characterised by the common presence of tuff pockets, tuff fragments, and accretionary and armoured lapilli. The Magmatic Fluidised Facies typically occupies the outer portion of the diatreme and can be distinguished from the Fluidised Fragmented Facies by the dominance of lapilli over ash and by the presence of abundant wrapped fragments. The Magmatic Facies (MF) and the Coherent Magmatic Facies (CMF) are volumetrically subordinate and represent late stages, when less fluidised and less fragmented material, or even coherent magma erupts relatively passively, in the aftermath of the main explosive stage that generated the diatreme. The Border Facies is defined by the increased abundance of material from the immediate country rock. At Águas Emendadas and Neuzinha this facies is marked by the presence of fragments of peperite-like rock, formed by the interaction of the fluidised magma with friable sandstone.

Recognition, Characterization and Implications of High-Grade Silicic Ignimbrite Facies from the Paleoproterozoic Bijli Rhyolites, Dongargarh Supergroup, Central India

A controversy regarding the distinction between the highly welded lava-like ignimbrites sometimes showing strongly rheomorphic characters, and the extensive silicic lava flow has been overwhelming in the recent literature. However, a rethinking, after Walker (1983), has brought into light the concept of ‘grade’ referring to the degree and extent of welding between the pyroclasts. Various parameters and characteristics were suggested for strengthening the idea of densely welded ignimbrites, which differentiate them from lava. Here, a comprehensive study on early Proterozoic acid magmatic rocks forming lower part of the Dongargarh Supergroup, central India, has been made to suggest extensive occurrence of high-grade welded rheomorphic tuffs. The possibility of their being welded ignimbrite rather than lava flow has been explored in the light of facies analysis as well as detailed microscopic evidences. Despite having overall monolithologic look various units bear distinction on account of their nature of welding, enrichment of phenocrysts and degree of stretching. The presence of vitroclastic texture, melt inclusions and radial fracturing of phenocrysts suggests pyroclastic nature of these deposits. Based on these characters four facies — A, B, C and D from bottom to the top respectively, have been identified from field studies around Salekasa. Facies-A and B represent clast-supported/matrix-supported welded pyroclastic flow deposits. Facies-C represents extremely welded thinly laminated rheomorphic tuffs while lava-like tuffs with an autobreccia carapace is represented by facies D. A complete gradation of facies A/B to D through C exists. High to extremely high-grade nature of welding in these deposits suggests a low column-height subaerial plinian to fissure eruption of a very high temperature silicic magma in a continental setting.

Geochemical Stability of Natural Zeolites

Zeolites are used in numerous agricultural, commercial, and environmental applications (e.g. as soil conditioners and fertilizers, and as adsorbents for ammonia, heavy metals, nuclear and organic wastes). It is important to understand their stability to insure their persistence and effectiveness in these applications. An analysis of the occurrence of natural zeolites provides basic data on conditions favorable for zeolite stability and formation. All such environments have neutral to alkaline waters and, with few exceptions, are associated with low-temperature (<300°C) alteration of highly reactive volcanic rocks containing natural glasses. The principal exceptions are certain deep sedimentary sequences, sometimes associated with petroleum maturation where volcanic ash falls are absent, and deep-sea sediments where biogenic opal may be the highly reactive phase. Since initial recognition of the zeolite facies in diagenetic alteration (Coombs et al. 1959), investigators have struggled to develop a quantitative basis for defining zeolite stability fields by development of both the thermodynamic relations and kinetic factors governing zeolite formation. An additional motivation for study in the United States and elsewhere is the occurrence (and use) of zeolites at potential sites for disposing of high-level radioactive waste in underground repositories. Regulatory requirements for a high-level radioactive waste repository, reflecting public concerns over the integrity of subsurface disposal facilities, require unprecedented quantitative predictions of the physical-chemical response of zeolites to heat generated during radioactive decay and to waste-rock interactions over a time period exceeding tens of thousands of years. In light of such requirements, investigators are taking a new look at the thermodynamic and kinetic factors affecting zeolitization. The geochemical conditions that result in zeolite formation have been outlined in numerous studies (e.g. Hay 1966, 1978; Iijima 1978, Surdam and Sheppard 1978, Sheppard and Hay, this volume; Hay and Sheppard, this volume). Aqueous silica activity, cation concentrations, and …

The interpretation of seismic facies in the molassic deposition of Preadriatic Foredeep

The purpose of this article is to illustrate the principles of seismic facie analysis used in the interpretation of sedimentary rocks, in siliciclastic deposits, especially in molassic one. The recognition and definition of a seismic facies and the analysis of its vertical evolution (facies associations) lead to an environmental interpretation, which can give useful information on both sedimentary facies and reservoir characteristics. With this aim, the major depositional systems, from continental to deep marine, and the depositional elements in which they can be subdivided, will be briefly overviewed in terms of extension, geometry, continuity and lateral variations. For each of these systems, it is pointed out, the major physical active processes during the deposition, the resulting sedimentary structures and their vertical and lateral evolution. The comparison between the environmental interpretation derived from bottom cores, well - logs and that derived from the current depositional models, is used to predict the nature and distribution of reservoir and sealing rocks.

Recognition and significance of streamflow-dominated piedmont facies in extensional basins

Recognition and significance of streamflow-dominated piedmont facies in extensional basins

Recognition of Facies, Bounding Surfaces, and Stratigraphic Patterns in Foreland-Ramp Successions: An Example from the Upper Devonian, Appalachian Basin, U.S.A.

ABSTRACT A detailed stratigraphic study of the Upper Devonian Lock Haven Formation in the Appalachian basin provides new interpretations applicable to understanding sedimentation and stratigraphic architecture in the foreland-ramp setting. The common occurrence of graded beds, hummocky cross stratification, and shell lags indicates that storm processes played an important role in deposition. Gradational-based, coarsening-upward shoreface sequences of interbedded sandstone and mudstone formed during gradual fall in relative sea level. In contrast, sharp-based regressive sequences consist of laminated sandstone facies overlying a submarine erosion surface that developed in response to more rapid fall in relative sea level. Transgressive sequences become finer grained upward from conglomerate lag, which was deposited on a basal transgressive surface, to heterolithic facies and mudstone facies. A combination of eustasy and tectonic subsidence of the foreland ramp produced changes of relative sea level. These fluctuations provided a means for transporting sand onto the ramp and for producing repetition of stratigraphic patterns in the Lock Haven Formation. The Upper Devonian clastic succession, which includes both the Lock Haven Formation and the Catskill Formation, represents northwestward progradation of shoreline to nonmarine environments as sediment supply to the foreland ramp exceeded the rate of formation of accommodation. Compared to passive-margin shelves, thick fine-grained successions such as the Lock Haven Formation are more likely to form in foreland basins, where tectonic subsidence provides a mechanism for creating accommodation. Sediment influx from uplifted areas and absence of a slope break on the foreland ramp results in a higher rate of fine-grained clastic sedimentation than occurs on shelves of passive margins.

Recognition of Facies, Bounding Surfaces, and Stratigraphic Patterns in Foreland-Ramp Successions: An Example from the Upper Devonian, Appalachian Basin, U.S.A.

Recognition of Facies, Bounding Surfaces, and Stratigraphic Patterns in Foreland-Ramp Successions: An Example from the Upper Devonian, Appalachian Basin, U.S.A.