Siliciclastic Sandstones Research Articles

Tectonic, sedimentary and metallogenic re-evaluation of basal strata in the Mesoproterozoic Bylot basins, Nunavut, Canada: Are unconformity-type uranium concentrations a realistic expectation?

The Mesoproterozoic Borden, Hunting-Aston, Fury and Hecla, and Thule basins, collectively known as the Bylot basins, are a series of sub-parallel extensional depressions associated with the Mackenzie igneous event at ∼1270Ma. In the Borden basin, basal strata of the Eqalulik Group are dominated by tholeiitic basaltic rocks and siliciclastic sandstones previously interpreted as predominantly terrestrial flows and fluvial deposits respectively. New field data indicate that thick basalt flows in the Nauyat Formation contain cryptic pillow structures and typically have fine-grained, fragmental tops. A subaqueous origin for these flows is supported by the presence of stromatolitic carbonate on the top of several flows, and the presence of peperite and sand-injectite structures at the base of others. Associated quartz arenite units are all of marine origin. Although much of the overlying Adams Sound Formation was previously interpreted as fluvial, there is little evidence for this, except locally at the base of the succession near Paquet Bay. Paleocurrents do not exhibit a strong unimodal pattern to the northwest, as indicated in earlier work, but reflect a less restricted basin, with paleoflow oblique to later graben margins. Most of the Adams Sound Formation was deposited on a tide- and storm-dominated shelf, with local development of large sand-wave complexes. A similar, marine-dominated setting can be demonstrated for basal terrigenous clastic units in both the Hunting-Aston, and Fury and Hecla basins. The latter contains thin sheets of fluvial strata only in the Nyeboe Formation on the south side of the basin, and in the basal 3m of the Whyte Inlet Formation on the northeast margin of the basin. Paleocurrent patterns in all three basins do not reflect confinement between the walls of restricted grabens. This suggests that the early development of the basins may have been related to transtensional extension rather than aulacogen development.Although uranium, present in basement granites north of the Fury and Hecla Strait, may have been scavenged by fluids well after lithification of the Mesoproterozoic basin fill to produce local vein-type uranium deposits, the potential for extensive unconformity-type deposits, analogous those in the Athabasca basin, is exceptionally low. Most of the sandstone units have early siliceous cement, and low initial feldspar content, both of which would have inhibited development of extensive secondary porosity.

RESERVOIR QUALITY ASSESSMENT AND PETROFACIES OF THE LOWER CRETACEOUS SILICICLASTIC, CARBONATE AND HYBRID ARENITES FROM THE JEQUITINHONHA BASIN, EASTERN BRAZIL

A study of the rift to early drift phase, fluvial, deltaic and shallow‐marine Lower Cretaceous (Upper Aptian to Lower Albian) siliciclastic sandstones, hybrid arenites and calcarenites from the Jequitinhonha Basin, eastern Brazilian margin, has shown that the distribution of diagenetic alterations and of related reservoir quality evolution can be constrained within a sequence stratigraphic framework. Description of cores, wireline logs, thin sections and petrophysical (porosity and permeability) analyses were integrated in order to investigate the controls on patterns of diagenesis in these rocks, and hence of reservoir quality distribution. The results of this study will be relevant to the exploration of rift and early drift successions in Atlantic‐type passive margin settings. The study suggests that depositional facies and detrital composition impact the diagenetic evolution and the characterization and prediction of reservoir quality.Reservoir quality assessment was based on the concept of reservoir petrofacies, which is defined in terms of the main attributes affecting reservoir quality including depositional structures, textures, primary composition, diagenetic processes and products, and pore types. This paper demonstrates the practical use of the reservoir petrofacies concept in the evaluation of the controls on porosity and permeability, as well as on seismic and log signatures.

Diagenetic Evolution and Porosity Destruction of Turbiditic Hybrid Arenites and Siliciclastic Sandstones of Foreland Basins: Evidence from the Eocene Hecho Group, Pyrenees, Spain

Abstract This study aims to unravel the impact of diagenetic alterations on porosity loss of foreland-basin turbiditic hybrid arenites and associated siliciclastic sandstones of the Eocene Hecho Group (south-central Pyrenees, Spain). In this succession, hybrid arenites and calclithites are extensively cemented by mesogenetic calcite cement (δ18OVPDB = −10.0‰ to −5.8‰; Th, mode = 80° C; salinity mode = 18.8 wt% eq. NaCl), Fe-dolomite (δ18OVPDB = −8.5‰ to −6.3‰) and trace amounts of siderite. The extent of carbonate cementation is interpreted to be related to the amounts of extrabasinal and intrabasinal carbonate grains, which provided nuclei and sources for the precipitation and growth of carbonate cements. Other diagenetic alterations, such as pyrite and albitization, had no impact on reservoir quality. Scarce early diagenetic cements, coupled with abundant ductile carbonate and siliciclastic framework grains, have led to rapid porosity loss owing to compaction. Conversely, abundant quartz in the sandstones prevented rapid loss of porosity by mechanical compaction. Reservoir quality was affected by mesogenetic cementation by quartz overgrowths, calcite and dolomite intergranular pressure dissolution of quartz grains, and formation of fracture-filling calcite cement (δ18OV-PDB values from −10.4‰ to −7.8‰; Th temperatures of 150° C), which are attributed to deep circulation of hot meteoric waters during extensional stages of tectonism. The results of this study illustrate that diagenetic evolution pathways of the arenites and sandstones are closely linked to the variation in detrital composition, particularly the proportion and types of extrabasinal noncarbonates, extrabasinal carbonates, and intrabasinal carbonate grains. These insights suggest that marine turbiditic hybrid arenites and calclithites of foreland basins are subjected to more rapid and extensive porosity loss owing to compaction and cementation than associated siliciclastic sandstones. Degradation of reservoir quality makes these hybrid arenites, calclithites, and sandstones suitable as tight gas reservoirs, but only if fracture porosity and permeability develop during tectonic deformation.

Climatic and tectonic control on the distributionof the Oligocene reefs of the Tertiary Piedmont Basin (NW Italy)

The Tertiary Piedmont Basin (TPB) stretches along the Piedmont-Liguria border and is a late- to post-orogenic basin that evolved in a piggy-back position on the Monferrato thrust belt. Its depositional story is strongly controlled by tectonic and eustatic events. The basin was filled with mainly marine sediments (upper Eocene-upper Miocene), which unconformably overlie the Ligurian Alps and the Northern Apennines. The early stage of sedimentation of TPB records a transgressive phase (time-transgressive from the eastern to the western sectors, upper Eocene-upper Oligocene), characterized by the deposition of alluvial fan and fan delta siliciclastic conglomerates and sandstones, marine shallow-water coarse to fine grained siliciclastic sediments, and reef limestones.The TPB reefal buildups, which developed close to the paleoshoreline, have a lower Rupelian-middle Chattian stratigraphic distribution and are restricted to the central part of the southern margin of the TPB. The Valzemola-San Bernardino reef is the younger and the westernmost settlement, while the Val Lemme reef represents the older and easternmost one.The depicted stratigraphic and geographic distribution may be the result of a combined climatic and tectonic control. The lack of reef settlements west of Valzemola-Bric S. Bernardino may be due to climatic condition unfavourable for the reef development (temperature cooler than those of the early Rupelian-middle Chattian time interval). The absence of reefs in the area to the east of Val Lemme may be the consequence of bottom instability and high sedimentation rates (unfavourable conditions for the reef development) caused by the syn-sedimentary tectonic activity along the Scrivia fault, during the time interval spanning from the late Eocene to the early Oligocene (notwithstanding the favourable climatic conditions).Finally, the age of the older reefal complex, i.e. early Rupelian (Val Lemme), gives an obvious constraint to the age of the end of the N-vergent thrusting phase of the Voltri Unit onto the flysch units and its sedimentary cover.

Nuovi dati litostratigrafici e biostratigrafici sulla Scaglia Toscana nella Toscana meridionale (area di Rapolano Terme)

Nuovi dati litostratigrafici e biostratigrafici sulla Scaglia Toscana nella Toscana meridionale (area di Rapolano Terme)

Lithostratigraphic controls on bedding-plane fractures and the potential for discrete groundwater flow through a siliciclastic sandstone aquifer, southern Wisconsin

Outcrop-analog studies of the Upper Cambrian Tunnel City Group sandstones in southern Wisconsin show the utility of lithostratigraphic information in hydrostratigraphic studies of siliciclastic sandstone aquifers. Recent work supports the lateral continuity of discrete groundwater flow through these sandstones. Lithologic description of the Reno Member of the Lone Rock Formation (Tunnel City Group) in outcrop and core reveals repeating sequences of three dominant lithofacies, including flat-pebble intraclast conglomerate with a glauconite-rich matrix; glauconitic and feldspathic subquartzose sandstone with horizontal-planar, low-angle, and hummocky lamination; and feldspathic subquartzose sandstone with dolomite-filled burrows. The vertically stacked Reno Member sequences have been interpreted as having a storm-related origin, and they are laterally continuous on the scale of an outcrop. Horizontal fracture locations correlate with bedding planes at contacts between lithofacies. They are most commonly associated with the base of the flat-pebble intraclast conglomerate or with partings along laminae and erosional surfaces in the laminated subquartzose sandstone lithofacies. Sequences show upward increases in natural gamma radiation due to increasing potassium feldspar content. The incorporation of the detailed lithostratigraphic information allows a more accurate interpretation of borehole natural gamma logs where the rocks are buried and saturated and clarifies the role of sedimentary structures in the distribution of features that might promote discrete flow through these rocks.

Albacora Leste Field Development: Reservoir Aspects and Development Strategy

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper OTC 18056, "Albacora Leste Field Development: Reservoir Aspects and Development Strategy," by W.P. Lemos, M.R. Baiao de Castro, C.M. Soares, J.F. Rosalba, and A.A.G. Meira, Petrobras, prepared for the 2006 Offshore Technology Conference, Houston, 1–4 May. Albacora Leste, one of the largest Campos basin deepwater oil fields, comprises Miocene sandstones with high porosity and permeability. Net thickness ranges from 5 to 35 m, suggesting horizontal wells. After the deposition stage, erosive channels introduced flow barriers that generated different reservoir compartments. The full-length paper presents the key aspects of the reservoirs as well as the strategy adopted during project implementation. Introduction The giant Albacora Leste deepwater oil field in the northern part of the Campos basin has reservoir depths ranging from 2300 to 2600 m. The reservoirs are high-quality siliciclastic Miocene sandstones with 30% average porosity, 3000-md average absolute permeability, and 16.5 to 21.5°API oil. Total recovery for Albacora Leste is estimated to be 90×106 m3 of oil and 10×109 m3 of gas. Discovered in 1986, the challenging field environment slowed field development because it could be supported only by technologies emerging at that time such as long horizontal gravel-packed wells and thermally insulated pipelines. Development-well drilling began in 2002. A 28 600-m3-oil-capacity floating production, storage, and offloading vessel (FPSO), the P-50, will be used for field production. Peak oil production will be reached at the end of 2006 when all production wells will be producing. Seismic The first 3D-seismic data were acquired in 1987, but their acquisition parameters and processing routine were not sufficiently good for reservoir characterization. During 2001, a new 3D seismic was purchased that supported reservoir characterization for the field development. Fig. 1 shows the difference between the 1987 and 2001 surveys for the main reservoir zone. The new seismic data brought significant improvements to interpretation of the Albacora Leste geological model. Because of improved seismic resolution, reservoir boundaries, sandstone channels, and erosive channels were defined better. Usually, reservoirs show low impedance in the seismic cubes and are clearly distinct from the surrounding hemipelagic shales that show higher impedance. In Albacora Leste, presence of low-impedance shale and interlaminated rocks (very thin shale/sandstone stacked sequences) causes negative-amplitude seismic responses, similar to the best reservoir responses. Usually the low-impedance shale and the interlaminated rocks can be identified by amplitude-variation-with-offset (AVO) analysis and elastic inversion, but in the case of Albacora Leste, this does not solve the problem because of the low seismic resolution. For the same reason, identification of small gas caps is difficult. Models indicate that the small gas thicknesses observed in the drilled wells would not have been detected previously even using prestacking techniques because of seismic pulse interference effects. In 2005, a new 3D seismic with a larger number of traces on a smaller bin was purchased to obtain larger frequency content. This seismic will provide better support for future reservoir management and possibly for 4D studies to identify poorly drained reservoir volumes.

Quartz cementation mechanisms and porosity variation in Baltic Cambrian sandstones

Quartz is an import cementing material in siliciclastic sandstones that can reduce porosity and permeability severely. For efficiently predicting and extrapolating petrophysical properties such as porosity and permeability, the controls on the occurrence and the degree of quartz cementation need to be better understood. Toward this end, a succession of Cambrian marine quartz arenites in Lithuania, at a present burial depth ranging from about 1 to 2 km, has been studied. In the central part of the Baltic Basin these quartz arenites are heavily cemented by quartz and the degree of quartz cementation is the main control on reservoir properties. On a regional Baltic Basin scale, the amount of quartz cement, and inversely the porosity and permeability, are correlated to burial depth and palaeotemperature. Porosity of the sandstones in the Baltic Basin flanks is around 23–28% and decreases toward the west to 15–18% in the central part and to 2–4% in west Lithuania. However, superposed on general trends between petrophysical properties and depth, large variations exist even between closely spaced wells making forward prediction of reservoir quality difficult. Evidently, factors other than general physical conditions and overall chemical conditions related to burial depth must have locally influenced the processes of quartz cementation and controlled the location and amounts of quartz cement. Such local factors are detrital composition, sedimentary structures and reservoir architecture, inherent to depositional facies. Clay-induced chemical compaction and pressure dissolution of detrital quartz at shale–sandstone contacts and within thin shale lamina is probably the main process yielding silica for local quartz cement. The supply of silica for quartz cement in the sandstones is thus dependent on the number of thin clay lamina and clay intercalations within the reservoir sandstones. The sandstone/shale ratio and thickness of sandstone bodies control the location and the degree of quartz cementation and thus reservoir quality.

BORINGS IN LARGER BENTHIC FORAMINIFERS FROM TURKEY AND THEIR PALEOENVIRONMENTAL SIGNIFICANCE

Cretaceous/Paleogene outcrops of Turkey provide important areas for the study of trace fossils on larger benthic foraminiferal tests such as Orbitoides and Loftusia from the Maastrichtian, and Nummulites, Assilina and Discocyclina from the Eocene. The current investigation synthesizes previous and new records of borings on Upper Cretaceous and Paleogene foraminifers with information on faunal associations, stratigraphical correlation and paleoenvironmental conditions. The types of borings were distributed according to their shallow-water paleoenvironments. Orbitoides tests in siliciclastic sandstones and calcarenites from northwestern and eastern Turkey contain four forms of borings: semicircular tubes, circular cavities, subspherical cavities and undulating tubes. Benthic foraminiferal associations confirm the Maastrichtian age of the borings. Towards the end of the Cretaceous, a regression took place during the Maastrichtian in the Osmaneli area, resulting in a shallower, bivalve-dominated facies. During initial phases of the regression, semicircular tubes and circular cavities were formed, while later phases of the regression led to the formation of undulating tubes. The Loftusia tests from southeastern and eastern Turkey contain cylindrical holes, paraboloid pits and subspherical cavities formed as predational and dwelling borings. Borings occur on the Eocene larger foraminifera Nummulites , Assilina and Discocyclina from the Ypresian to Bartonian carbonates and siliciclastic sediments in southwestern Turkey. These Eocene sediments comprise back-reef, nummulite-bank, fore-reef and open-sea facies. The benthic foraminifers may contain borings such as cylindrical holes, paraboloid pits, subspherical cavities and spiral tubes. The Eocene foraminiferal tests less commonly contain borings and endobiontic foraminifers than those from the Maastrichtian. The Eocene borings were formed in a shallower carbonate setting.

COMMENT ON MELLUSO ET AL. (2003): The Ricetto and Colle Fabbri wollastonite and melilite-bearing rocks of the central Apennines, Italy

The bimodal rock of Ricetto consists of a felsic glassy component, which prevails volumetrically at the top of the rock, and mafic spherules of acicular diopside + anorthite + wollastonite pseudomorph after melilite, which are concentrated at the bottom of the rock. This texture clearly indicates liquid immiscibility, which can be modeled in the compositional space SiO 2 - Al 2 O 3 -MgO-CaO and Al 2 O 3 -CaO-(FeO T + MgO). The spinifex and dendritic forms are due to quenching from a melt at temperature of 1000-1100 °C: melilite was the first phase to crystallize and the subsequent rapid cooling caused its breakdown to a more stable assemblage. The strict chemical similarity between the rock under study and the surrounding siliciclastic sandstones, together with the results of 14 C radiometric analysis on coal fragments found at bottom of the convex central part of the body, demonstrate that the rock of Ricetto is a man-made pyrometamorphic product. The genetic attribution of the Colle Fabbri occurrence to pyrometamorphic phenomena somewhat similar to Ricetto is difficult to be asserted, as the crystallization history at Colle Fabbri has been much more complicated than at Ricetto. The common presence of melilite and wollastonite cannot account for a direct comparison between the Ricetto and Colle Fabbri rocks, as conspicuous mineralogic, petrologic, and geologic differences exist between these two occurrences.

The early Pliocene Titiokura Formation: Stratigraphy of a thick, mixed carbonate‐siliciclastic shelf succession in Hawke's Bay Basin, New Zealand

This paper presents a systematic stratigraphic description of the architecture of the early Pliocene Titiokura Formation (emended) in the Te Waka and Maungaharuru Ranges of western Hawke's Bay, and presents a facies, sequence stratigraphic, and paleoenvironmental analysis of the sedimentary succession. The Titiokura Formation is of early Pliocene (Opoitian‐Waipipian) age, and unconformably overlies Mokonui Formation, which is a regressive late Miocene and early Pliocene (Kapitean to early Opoitian) succession. In the Te Waka Range and the southern parts of the Maungaharuru Range, the Titiokura Formation comprises a single limestone sheet 20–50 m thick, with calcareous sandstone parts. In the vicinity of Taraponui Trig, and to the northeast, the results of 1:50 000 mapping show that the limestone gradually partitions into five members, which thicken markedly to the northeast to total thicknesses of c. 730 m, and concomitantly become dominated by siliciclastic sandstone. The members (all new) from lower to upper are: Naumai Member, Te Rangi Member, Taraponui Member, Bellbird Bush Member, and Opouahi Member. The lower four members are inferred to each comprise an obliquity‐controlled 41 000 yr 6th order sequence, and the Opouahi Member at least two such sequences. The sequences typically have the following architectural elements from bottom to top: disconformable sequence boundary that formed as a transgressive surface of erosion; thin transgressive systems tracts (TSTs) with onlap and backlap shellbeds, or alternatively, a single compound shellbed; downlap surface; and very thick (70–200 m) highstand (HST) and regressive systems tracts (RST) composed of fine sandstone. The sequences in the Opouahi Member have cryptic TSTs, sandy siltstone to silty sandstone HSTs, and cross‐bedded, differentially cemented, fine sandstone RSTs; a separate variant is an 11 m thick bioclastic limestone (grainstone and packstone) at the top of the member that crops out in the vicinity of Lake Opouahi. Lithostratigraphic correlations along the crest of the ranges suggest that the Titiokura Formation, and its correlatives to the south around Puketitiri, represent a shoreline‐to‐shelf linked depositional system. Carbonate production was focused around a rocky seascape as the system onlapped basement in the south, with dispersal and deposition of the comminuted carbonate on an inner shelf to the north, which was devoid of siliciclastic sediment input. The rates of both subsidence and siliciclastic sediment flux increased rapidly to the northeast of the carbonate “platform”, with active progradation and offlap of the depositional system into more axial parts of Hawke's Bay Basin.

Late Miocene to early Pliocene biofacies of Wanganui and Taranaki Basins, New Zealand: Applications to paleoenvironmental and sequence stratigraphic analysis

The Matemateaonga Formation is late Miocene to early Pliocene (upper Tongaporutuan to lower Opoitian New Zealand Stages) in age. The formation comprises chiefly shellbeds, siliciclastic sandstone, and siltstone units and to a lesser extent non‐marine and shallow marine conglomerate and rare paralic facies. The Matemateaonga Formation accumulated chiefly in shelf paleoenvironments during basement onlap and progradation of a late Miocene to early Pliocene continental margin wedge in the Wanganui and Taranaki Basins. The formation is strongly cyclothemic, being characterised by recurrent vertically stacked facies successions, bounded by sequence boundaries. These facies accumulated in a range of shoreface to mid‐outer shelf paleoenvironments during conditions of successively oscillating sea level. This sequential repetition of facies and the biofacies they enclose are the result of sixth‐order glacio‐eustatic cyclicity. Macrofaunal associations have been identified from statistical analysis of macrofossil occurrences collected from multiple sequences. Each association is restricted to particular lithofacies and stratal positions and shows a consistent order and/or position within the sequences. This pattern of temporal paleoecologic change appears to be the result of lateral, facies‐related shifting of broad biofacies belts, or habitat‐tracking, in response to fluctuations of relative sea level, sediment flux, and other associated paleoenvironmental variables. The associations also show strong similarity in terms of their generic composition to biofacies identified in younger sedimentary strata and the modern marine benthic environment in New Zealand.

Paleogeographic Orientation of the Sino‐Korean Plate Based on Evidence for a Prevailing Silurian Wind Field

Abstract The regional geology of southern Inner Mongolia records relationships between an upper Silurian coastline and a small island on what was formerly the continental shelf of the Sino‐Korean Plate (North China Block). In both cases, rocky shorelines are defined by unconformities between strata of Ludlow age in the Xibiehe Formation and underlying igneous rocks of Ordovician age. The paleoisland occurs near Bater Obo about 50 km south of the Chinese‐Mongolian border. The diorite core of this geological inlier is oriented on an axis N 55° E. A simple dichotomy of exposed windward versus sheltered leeward environments accounts for contrasting carbonate facies on the flanks of the paleoisland. Wider corroboration of a prevailing Silurian wind field is supported by evidence from other unconformities about 30 km to the southwest, where a linear paleoshore extends for 15 km on an average bearing of N 60° E. Siliciclastic sandstone derived from a granodiorite source in the Gushan area now sits in close proxi...

Raised Pleistocene marine terraces of the Crotone peninsula (Calabria, southern Italy): facies analysis and organization of their deposits

Wide marine terraces well known in the Italian literature are present in the Crotone peninsula, Ionian Calabria. They are related to the interplay between Pleistocene sea-level changes and the progressive uplift of the Calabrian Arc from the middle Pleistocene onwards. We present a description of facies organization and stratigraphical characteristics of the terrace deposits, along with a new geological map of the area. All the marine terraces unconformably overlie a Plio-Pleistocene slope succession known as Cutro Clay. The oldest and highest terrace, named Cutro terrace, is placed between 90 and 200 m of elevation and is gently inclined south-eastwards. Four younger terraces placed at progressively lower elevations and dipping seawards at low angles have been recognized. The sedimentary deposits of the terraces are represented by mixed carbonate and siliciclastic bodies, up to 10 m thick. Algal reefs and grainstone to rudstone deposits are common in the south-eastern part of the study area, whereas mostly siliciclastic sandstone units of shoreface environments are more abundant northwards and westwards. The deposits of some terraces are made up of minor cycles, bounded by subaerial, transgressive, and flooding surfaces referred to minor sea-level fluctuations. NNE-, ENE-, E-, and WNW-trending normal fault systems are present within the study area and dissect the marine terraces. Specifically, the activation of ENE-trending faults took place after the formation of the oldest terrace but prior to the formation of the younger terraces.

High-temperature rapid pyrometamorphism induced by a charcoal pit burning: The case of Ricetto, central Italy

Bulk chemistry and mineralogy of the peculiar rock of Ricetto (Carseolani Mts., Central Apennines, Italy) was studied to resolve its controversial origin: igneous dyke or anthropic product. This hybrid rock consists of a colorless, felsic component made up of glass plus quartz, and a brown, femic component made up of fans and spherulites of diopside, calcic plagioclase, wollastonite, and melilite. Textural relationships indicate very rapid cooling and immiscibility phenomena. The bulk chemistry of the rock is the same as that of the surrounding siliciclastic sandstone. The 14C analysis of a coal fragment from bottom of the body yields the conventional age of 227(±50) years. The Ricetto occurrence is an example of pyrometamorphism of a siliceous limestone induced by a charcoal pit burning. The small size of the heat source at Ricetto caused an intense but short-lived melting of the country rock. Prograde metamorphism caused a temperature increase up to 1,000–1,100 °C when melilite crystallization conditions were reached at appreciable P(CO2) and high f(O2). Melting occurred in a close system represented by the simplified equation: 3Cal+16.5Qtz+Ms+Bt→Mel+Melt+2H2O+3CO2+0.5O2. Diopside+calcic plagioclase+wollastonite formed by melilite breakdown during rapid cooling, through the reaction: 6Mel+6Qtz+0.5O2→3Di+2An+7Wo. Liquid immiscibility caused the separation between the felsic melt component and the femic melilite-bearing component. Immiscibility was characterized by different fractionation of alumina and alkalies between these two phases. Differences in bulk, glass, and mineral chemistry between the Ricetto and other melilite-bearing pyrometamorphic rocks can be attributed mainly to different protoliths.

空知‐エゾ帯の“未分離日高累層群”から高圧変成鉱物の発見

High P/T metamorphic minerals are found from a Late Cretaceous accretionary complex (Ganpi-yama Complex) separately occurring within a serpentinite mass in the Kamuikotan Zone of the Sorachi-Yezo Belt, Hokkaido. The complex was previously referred to as the "undivided Hidaka Supergroup". The Yon-no-sawa Unit, a major member of the complex, is composed of deformed alternation of siliciclastic sandstone and mudstone, greenstones and chert. Sodic pyroxenes coexisting with quartz are found from greenstone, and metamorphic aragonite from sandstone and greenstone. The finding of these high P/T metamorphic minerals suggests that the Ganpi-yama Complex has, at least, a metamorphic affinity to the Kamuikotan Zone. An exact tectonostratigraphic correlation of the complex, however, is still open to the argument.

ABSTRACT: Tectonic Setting and Provenance, Petrography and Reservoir Characteristics of Nine Siliciclastic Sandstone Formations, Mississippian to Tertiary age, Sagavanirktok Quadrangle, Eastcentral Brooks Range Foothills and Arctic Slope, Alaska

ABSTRACT: Tectonic Setting and Provenance, Petrography and Reservoir Characteristics of Nine Siliciclastic Sandstone Formations, Mississippian to Tertiary age, Sagavanirktok Quadrangle, Eastcentral Brooks Range Foothills and Arctic Slope, Alaska

Recognition and distinction of normal and forced regression in cyclothemic strata: a Plio-Pleistocene case study from eastern North Island, New Zealand

Abstract The Tangoio block of eastern North Island, New Zealand contains an exceptionally well exposed, 550 m thick sequence of Plio-Pleistocene cyclothemic sedimentary rocks, the Petane Group, that were deposited along the western margin of a shallow (<200 m) pericontinental seaway. The cyclicity that characterizes the Petane Group in this area was generated through recurring 40 ka (sixth-order) sea-level changes of c. 75–150 m. Data from more than 100 sections indicates that the strata consist of five distinct cyclothems, each consisting of a lower formation of mudstone deposited in midshelf environments (highstand system tract, HST), and an upper formation dominated by coarse-grained facies deposited in shallow marine and occasionally non-marine environments. Western exposures of HST mudstones in the lower portion of the Petane Group shoal up into shallow marine siliciclastic sandstones, fluvial gravels and siltstones. The transition is gradational and is consistent with progradation of a siliciclastic shoreline during late sea-level highstand. The shoaling upward siliciclastic sandstone interval represents a regressive system tract (RST) whereas the fluvial beds represent a lowstand system tract (LST). LST gravel beds are sharply overlain by a transgressive surface of erosion and fining-upward siliciclastic sandstone facies of a transgressive systems tract (TST). Carbonate sediments replaced siliciclastic sandstones as the dominant coarse-grained lithology in the upper Petane Group, probably due to bypass of siliciclastic sediment during sea-level fall. Transitions from HST mudstone into carbonate sand and bioclastic limestone (coquina) are generally sharp everywhere except the downdip eastern portions of the Tangoio block. The contacts represent regressive surfaces of erosion produced during falling stages of sea-level and/or ravinement surfaces formed during rising phases of sealevel (transgressive surfaces of erosion). Carbonate facies atop these erosional surfaces are generally interpreted as TST deposits, except for the uppermost limestones in the Petane Group that occur below a sequence boundary and therefore represent a forced regression systems tract (FRST). Stratal position is the only distinguishing characteristic of FRST limestones in the Petane Group, otherwise they are identical to TST deposits. In eastern portions of the Tangoio block, HST mudstones pass gradationally into RST calcareous sandstones and LST limestones. Systems tracts here are bounded by correlative conformities rather than erosive surfaces indicating continuous sedimentation during sea-level fall and lowstand. Differences in sedimentation and subsidence rates between carbonate and siliciclastic sedimentary systems, are responsible for the different stratal architecture of systems tracts in the Petane Group.

Abstract: Environmental Geology of Urban Growth Areas within the Edwards Aquifer and Balcones Fault Zone, South-Central Texas

Abstract: Environmental Geology of Urban Growth Areas within the Edwards Aquifer and Balcones Fault Zone, South-Central Texas

Effects of Differential Cementation on the Sonic Velocities of Upper Cretaceous Skeletal Grainstones (Southeastern Netherlands)

ABSTRACT P-wave velocities measured in Cretaceous skeletal grainstones from Limburg, southeastern Netherlands, are significantly higher in moderately cemented samples than in poorly cemented samples from similar depositional facies. Petrography suggests that this relationship is due to differences in the extent of development of intergranular cementation as influenced by original grain mineralogy, rather than a significant difference in primary porosity. The time-average velocity equation for a calcite matrix fails to predict the cementation effect on sonic velocity at high porosities. Linear and nonlinear time-average regressions show strong correlation between velocity and porosity. Whether the underlying trend is linear or nonlinear is a fundamental question that cannot be determined with this data set. Comparison between carbonate grainstones and siliciclastic sandstones reveals that: (1) the grainstones have gradients in velocity-porosity space similar to those in siliciclastic arenites but have velocities that are up to 0.3 km/s higher than those in clean arenites and up to 0.8 km/s higher than in argillaceous arenites; and (2) grainstones develop a rigid framework that transports elastic waves at significantly higher porosities than arenites, 40-50% compared to 30-40%. Higher sonic velocity presumably results from the higher matrix velocity of calcite versus quartz, and the higher elastic moduli created by the effective cement bridges connecting grains.