Argillaceous Facies Research Articles

The Leptodus Shales of central Peninsular Malaysia: distribution, age and palaeobiogeographical affinities

The terms ‘ Leptodus Shales’ and ‘ Leptodus Beds’ have been used to describe a rich brachiopod bearing unit within the Permian argillaceous facies of the Central Belt of Peninsular Malaysia. To date there has been no formal description of this unit regarding its age, spatial distribution or faunal composition. A review of previous literature, backed by recently collected data from our field surveys and biostratigraphical studies reveals that there is a sequence of fossiliferous assemblages within the Leptodus Shales, which range in age from Middle Permian to possibly early Late Permian and extend geographically from southern Kelantan to southern Pahang, Peninsular Malaysia. These assemblages are found in argillaceous sediments which are often highly tuffaceous, and in northern Pahang are associated with pyroclastic volcanics of probable island-arc origin. The faunas are of Palaeo-equatorial affinity and are taxonomically close to faunas in Indochina, such as the Sisophon fauna in Cambodia. Typical elements include Vediproductus cf. punctatiformis (Chao), Transennatia gratiosa (Waagen), T. termierorum Sone, Leman and Shi, Uncuninellina timorensis (Beyrich), Leptodus richthofeni Kayser, L. cf. tenuis (Waagen), Leptodus nobilis (Waagen), Gubleria aff. ninglangensis Fang and Jiang, and Spyridiophora gubleri Termier and Termier.

A geomechanical survey of some different facies in relation to stability at a mine in the Eastern Transvaal Coalfield, South Africa

The stability of roofs and floors in bord areas in coal mines is affected by a number of properties of the strata above and below the coal seam that is worked. These include the lithological character of the rocks, the sedimentary features contained within them, the geomechanical properties of the rocks, notably their strength and deformation moduli, the durability of the rocks on exposure, and the incidence and geometry of discontinuities. Accordingly, core material was obtained from a mine in the Eastern Transvaal Coalfield in order to investigate the influence of certain of these properties on the stability of the roof and floor of the mine. The investigation showed the existence of a number of sedimentary facies that had different geological characteristics and geomechanical properties. A large number of compressive and tensile strength tests were carried out on the core material, and the geodurability of the rocks of the argillaceous facies was also determined. The geodurability is assessed in terms of the free swell coefficient and the uniaxial compressive strength. It indicated that some of the argillaceous material was of very poor quality. The geodurability and tensile strength would appear to provide indications of the potential behaviour of the roof rocks as mining occurs and the uniaxial compressive strength helps in the understanding of failure of floors in coal mines. In particular, poor roof and floor conditions were found to be associated with rocks of the argillaceous facies. The most stable strata were the fine-grained arenaceous facies whilst those of the coarse-grained facies were somewhat less stable. The triaxial compressive strength and deformation moduli of some of the sandstones also were determined.

Sedimentation, stratigraphy, and petroleum potential of Krishna-Godavari basin, East Coast of India

The Krishna-Godavari basin is located in the central part of the eastern passive continental margin of India. The structural grain of the basin is northeast-southwest. Exposures of Upper Cretaceous sedimentary rocks demarcate the basin margin toward the north west, where the northwest-southeast-trending Pranahita-Godavari graben abuts the basin. The basin contains thick sequences of sediments with several cycles of deposition ranging in age from Late Carboniferous to Holocene. A major delta with a thick, argillaceous facies that has prograded seaward since the Late Cretaceous is a hydrocarbon exploration target. Magnetic and gravity data predicted the basin architecture, which was subsequently confirmed by a multichannel seismic sur vey. The basin is divided into subbasins by fault-controlled ridges. Sediments accumulated in subbasins more than 5 km thick. Above the basement ridges, thin sediments are found. Until the Jurassic period, sediments were deposited in the rift valley and in topo graphic lows. This sequence is completely overlain by a Lower Cretaceous, transgressive sedimentary wedge. Later, continued delta progradation characterized basin sedimentation. With an areal extent of approximately 45,000 km2, this proven petroliferous basin has potential reservoirs ranging in age from the Permian to the Pliocene. Exploratory drilling of more than 350 wells in more than 160 structures has resulted in the discovery of 42 oil and gas bearing structures. Good source rocks are known from sequences ranging in age from Permian-Carboniferous to early Miocene. Because the reservoir sand bodies have limited lateral variation, understanding the stratigraphy and depositional sub environments in different sequences is essential to decipher the favorable locales for reservoir sands. Tilted fault blocks, growth faults, and related rollover anticlines provide the structural traps. (Begin page 1624)

Chonetoidea (Brachiopoda) from the Lopingian (Late Permian) of South China

Ten species of the superfamily Chonetoidea from the Lopingian (Late Permian) of South China are described or revised. A review of all recorded Chonetoidea species from the Lopingian (Late Permian) of South China indicates that some 22 species of five genera can be recognised. Species of Tethyochonetes and Neochonetes are characteristic in the lithofacies dominated by mudstone, siltstone or siliceous rocks in the Lopingian and some argillaceous limestone and clay rock facies near the Permian-Triassic boundary. New taxa are Neochoneles (Zhongyingia) subgen. nov., Neochonetes (Huangichonetes) subgen. nov. and Tethyochonetes flatus sp. nov.

GEOLOGIA E GÊNESE DOS DEPÓSITOS DE CAULIM FLORESTA E CAMBUÍ (FORMAÇÃO CAMPO ALEGRE - SC) - 1. FACIOLOGIA E MINERALOGIA DAS ROCHAS E MINÉRIOS

Cambuí and Floresta kaolin deposits occur within the rocks of Campo Alegre Formation, a tardi-orogenic basin filled with volcanic acidic rocks, dated as 536±65 Ma, and volcano-sedimentary rocks. All mapped rocks in deposits are argillised, but mapping allowed to identify original volcanic rocks and to distinguish several argillaceous facies. The common features observed in both mapped deposits were: (a) Acidic intrusions are always present; (b) Intrusive rocks are hosted by tuffs and volcanic breccias; (c) Veins and fractures are inside and next to intrusions, forming stockworks filled with white argillaceous materials; (d) Locally argillaceous ores are overburden by massive beds of argillite, greywakes and conglomerates. These sedimentary beds contain buried vegetal organic debris at their basal portions, (e) All rocks of both deposits are covered by beige and black massive soils. The identified argillaceous facies were: (a) white or white-reddish ore (argillised laminated rhyolites and/or dacites); (b) green ore (argillised andesitic tuffs and breccias); (c) white massive sedimentary ore; (d) beige massive soil with vegetal organic residues; (e) micaceous, green fractures related rocks, inside the white ore; (f) white argillaceous material that fills veins in stockworks; and (g) black soils. Cambuí and Floresta kaolin deposits were originated in volcanogenic proximal environments, and they have typical conduct and volcanic cone lithofacies. The mineralogy of the argillised rocks allowed to recognize an evolutionary sequence that starts with the fracture, micaceous, hydrothermal high temperature facies (300°C to 500°C) changing upward and laterally to the stockwork-vein, kaolinic, facies, probably formed at low temperature (10CTC to 300°C). The stockwork facies grades, without apparent discontinuity, to the supergenic, low temperatures (20°C to 50°C) altered rocks. If supergenesis started only after hidrothermal rocks have been exhumed, a compositional but not a temporal gradding exists.

GEOLOGIA E GÊNESE DOS DEPÓSITOS DE CAULIM FLORESTA E CAMBUÍ (FORMAÇÃO CAMPO ALEGRE - SC) - 1. FACIOLOGIA E MINERALOGIA DAS ROCHAS E MINÉRIOS

Cambui and Floresta kaolin deposits occur within the rocks of Campo Alegre Formation, a tardi-orogenic basin filled with volcanic acidic rocks, dated as 536±65 Ma, and volcano-sedimentary rocks. All mapped rocks in deposits are argillised, but mapping allowed to identify original volcanic rocks and to distinguish several argillaceous facies. The common features observed in both mapped deposits were: (a) Acidic intrusions are always present; (b) Intrusive rocks are hosted by tuffs and volcanic breccias; (c) Veins and fractures are inside and next to intrusions, forming stockworks filled with white argillaceous materials; (d) Locally argillaceous ores are overburden by massive beds of argillite, greywakes and conglomerates. These sedimentary beds contain buried vegetal organic debris at their basal portions, (e) All rocks of both deposits are covered by beige and black massive soils. The identified argillaceous facies were: (a) white or white-reddish ore (argillised laminated rhyolites and/or dacites); (b) green ore (argillised andesitic tuffs and breccias); (c) white massive sedimentary ore; (d) beige massive soil with vegetal organic residues; (e) micaceous, green fractures related rocks, inside the white ore; (f) white argillaceous material that fills veins in stockworks; and (g) black soils. Cambui and Floresta kaolin deposits were originated in volcanogenic proximal environments, and they have typical conduct and volcanic cone lithofacies. The mineralogy of the argillised rocks allowed to recognize an evolutionary sequence that starts with the fracture, micaceous, hydrothermal high temperature facies (300°C to 500°C) changing upward and laterally to the stockwork-vein, kaolinic, facies, probably formed at low temperature (10CTC to 300°C). The stockwork facies grades, without apparent discontinuity, to the supergenic, low temperatures (20°C to 50°C) altered rocks. If supergenesis started only after hidrothermal rocks have been exhumed, a compositional but not a temporal gradding exists.

Chapter 18 Triassic history

The Triassic Period of about 40 million years dutation spanned about a third of that of the Carboniferous and Permian interval. The Triassic rocks of Svalbard are easily distinguished from the underlying Permian strata because of a distinct desconformity between them and a marked contrast in facies from the resistant. pale coloured, cherls and siliciclastics of the Kapp Starostin Formation to the softer, darker areno-argillaceous Vardebukta and equivalent formations. Figure 18.1 shows the distribution of Triassic strata in Svalbard. The minor angular unconformity represents a hiatus mainly in the Permian rather than the Triassic record. The dominantly argillaceous facies constitute the Early Triassic to Late Middle Triassic Sassendalen Group . The rocks can be well dated from ammonoids, typically within calcareous concretions in the shales. The succeding Kapp Toscana Group is distinguished by a dominatly sandy deltaic facies in which age determinations are difficult. It spans both Late Triassic and Early Jurassic spoehs (roughly mid-Ladinian to mid-Bathonian). The Triassic-Jurassic boundary is not easy to estimate. Nevertheless towards the end of Triassic time (e.g. Rhaetian) the overall scene changed. Thus of the three formations of the Kapp Toscana Group the lower two (Tschermakfjellet and De Geerdalen) belong to the Triassic story. The overlying Wilhelmøya Formation may possibly range from Latest Triassic through Liassic time, and due to its complexity it is also discussed in the Jurassic-Crataceous chapter (19). The facies of the two groups reflect two distinct environmental configurations. The Sassendalen Group was deposited on a distal marine muddy shelf with a

Reservoir Characterization of Lacustrine Sediments from the Late Triassic, Beryl Field, UK North Sea: ABSTRACT

Located on the western flank of the Viking Graben, the Beryl Field has been producing from the Late Triassic Lewis reservoir since first oil in 1976. The Beryl A Triassic contains an estimated STOIIP of 256 mmstb, with a cumulative production of 31 mmstb (8/95) (12% recovery). Low recovery to date, coupled with high remaining reserves potential, necessitated a new simulation model and optimized development program for the Lewis reservoir. This paper summarizes the revised reservoir description of the Beryl A Triassic, a complex lacustrine and fluvial system, and its integration with the reservoir simulation. The Triassic reservoir is subdivided into four zones: Lewis Units I, II, III and IV. Six lithofacies associations are identified in core: offshore lacustrine, lacustrine sandflat, marginal lacustrine, floodplain, sheetflood/overbank, and fluvial. Detailed petrological studies were conducted which confirmed that both depositional and diagenetic processes influenced reservoir properties and quality. Optimal reservoir quality is preserved in fluvial and lacustrine sandflat deposits. Argillaceous floodplain and lacustrine facies are non-reservoir and form barriers to vertical fluid migration. Calcrete lags (concentrated at the base of fluvial channels) and carbonate paleosols form baffles to flow. Fieldwide correlation of core and log facies resulted in the identification of 27 geneticmore » flow-units. This geologically-based layering scheme was integrated with production data to generate the framework for vertical zonation for the new reservoir simulation. The simulation studies produced as accelerated development program for the Beryl A Triassic. Reserves have increased as result of optimizing secondary recovery.« less

Depositional Environmental History of the Chhattisgarh Basin, M.P., Based On Stromatolites and Microbiota

Abstract The Proterozoic Chhattisgarh basin, comprising 2000m thick sediments mainly limestonel/dolomite, sandstone and shale, shows prolific growth of stromatolites and presence of micro biota dominantly cyanobacteria. Added to these there are many sedimentary features and structures found in the rocks throughoutthe sequence. Depositional environment is thus well doculnented by physicochemical factors in general and biological features in particular. Based on all these biogeological features, it has been deduced that the deposition is largely marine under tidal flat conditions, sources of sediments being both precipitation as well as terrigenous material. Sedimentation shows a distinct cyclic pattern commencing with arenaceous facies culminating into argillaceous facies with intervening calcareous facies. During the waning stage of sedimentary history of the Chhattisgarh basin, the cycles repeated rapidly, interpreted from the intercalatory sedimentation of the upper horizon, namely Raipur Limestone. Disposition of stromatolites ancLtheir morphologies reveal low as well as high-energy conditions of deposition, levels of deposition being both intertidal and subtidal ones. The sedimentological, mineralogical and petrological evidences point towards an alternating oxidising and reducing environments throughout the sequence irrespective of cycles or lithologies. Overwhelming dominance of procaryotic algae and absence (7 or near absence) of eucaryotic algae have possibly the following implications, namely the temperature during deposition was raised above 40°C at least for major part during deposition and also the pH of the environment. was 7 or more.

Sequence Stratigraphy and Depositional Facies of the Silurian-Devonian Interval of the Northern Permian Basin

The Silurian and Devonian intervals of the northern Central Basin platform area of west Texas and southeastern New Mexico include the Fusselman, Wristen, and Thirtyone formations and the Woodford Shale. The carbonate-rich Fusselman, Wristen, and Thirtyone formations record a transition from ramp to platform deposition. Oolite grainstones of the lower Fusselman Formation were deposited in a ramp setting during an Upper Ordovician/Lower Silurian transgression. The overlying crinoid packstones and grainstones represent shoals that developed along a break in slope separating the evolving platform from a southward-dipping starved basin. By the close of Fusselman deposition, the platform was well developed, with shallow peridtidal mudstones and wackestones, and high-energy grainstones deposited as near-parallel facies tracts over the platform area. The platform system became fully developed during the deposition of the Wristen Formation. Porous dolomitic peridtidal and platform margin facies grade downdip into nonporous, limy and argillaceous open-shelf facies. Platform facies are typified by numerous shallowing-upward parasequences that terminated at subaerial exposure surfaces. The rocks of the Lower Devonian Thirtyone Formation were deposited as a wedge that onlaps the exposed Silurian platform margin. This formation contains a porous, chert-rich, lowstand deposit; a transgressive disconformity; and variably porous, grain-rich highstand deposits representing an overallmore » sea level rise. A major unconformity marks the contact between the karsted upper surface of the Thirtyone Formation and the overlying organic-rich, anoxic Woodford Shale.« less

Sedimentologic and paleotectonic evolution of carbonate platforms on a segmented continental margin: example of the African Tethyan margin during Turonian and Early Senonian times

During Turonian and early Senonian times, there were extensive shallow-water carbonate platforms on the northern margin of the African craton (Eastern Algeria, Central Tunisia and adjacent offshore areas), at the edge of the Tethys ocean. The development of these carbonate platforms (mid-Turonian to late Coniacian) was especially controlled by the segmentation of the African (Saharan) margin by extensional fault systems, as evidenced by block-faulting, volcanic activity and halokinetic movements. From the mid-Turonian to early Coniacian times, the carbonate platforms underwent an incipient drowning, indicated both by their sedimentologic evolution (rimmed-shelves to carbonate ramps) and the landward shift of facies belts. A partial drowning of carbonate platforms during late Turonian times was probably induced by climatic and/or oceanographic changes. The late Coniacian is the period of continuation and further extension of deepening events with the widespread development of deep ramp facies. At the Coniacian-Santonian boundary, the sinking of the Saharan margin induced an abrupt increase of subsidence rates and the complete drowning of carbonate platforms, covered by deep-water argillaceous facies. This renewal of tectonic activity at the Coniacian-Santonian boundary is recorded in many areas in North Africa (“Intra-Senonian phase”).

Upper Carboniferous Fine-Grained Turbiditic Sandstones from Southwest England: A Model for Growth in an Ancient, Delta-Fed Subsea Fan

ABSTRACT The Upper Carboniferous Crackington and Bude Formations of southwest England comprise laminated black mudstones, gray siltstones, thin-, medium- and thick-bedded sandstones, and contorted, silty sandstones. The thick-bedded sandstones only occur in the Bude Formation, which overlies the Crackington. The sandstones were deposited predominantly by turbidity currents in a relatively shallow basin. They are unusual for two reasons: 1) sediment coarser than fine sand is entirely absent; 2) unlike many previously described ancient-subsea-fan deposits, organized bundles of sandstones displaying asymmetric upward-thickening or upward-thinning sequences are no more common than symmetric sequences, and both types of organized bundles are much less common than disorganized (nonsequential) bundle , particularly in the Bude Formation. Sediment was supplied directly from a delta to the north, by resedimentation of delta-front sands, and slumping of upper pro-delta muds and silts to form a lower pro-delta turbidite fan. Fan growth is not readily explained by current models for single-sourced, tripartite subsea fans displaying radial growth. It developed, instead, a bipartite growth pattern, which was strongly influenced in its upper part by external variables, including grain size, distributary channel switching and other deltaic processes, active tectonism within the basin, and possibly sea-level fluctuations. The fan grew by down-slope construction of independent levee/channel complexes, each of which headed in the upper pro-delta and even possibly the delta front. Slumping and mobilization of argillaceous facies resulted in the random occurrence of contorted beds in all parts of the fan. Regionally extensive mudstones, known as key shales, within the fan deposits probably reflect large-scal , upslope, delta-lobe abandonment. The Crati and Ebro Fans of the Mediterranean Sea would appear to be good modern analogs for the Crackington-Bude fan system.

Brackish Water in Unsaturated Confining Beds at a Texas Lignite Minea

ABSTRACTDeposits of low permeability are hydrologic barriers between reclaimed land and juxtaposed aquifers and should limit the impact of mining on ground‐water quality. Clay‐stone and mudstone in argillaceous facies of the Calvert Bluff Formation (lower Eocene, Wilcox Group) function as confining beds in the East Texas Basin. In the subsurface and at the outcrop, water in argillaceous deposits is brackish to saline. Samples of vadose water from the outcrop of confining beds at the Big Brown lignite mine in Freestone County, Texas, have a chloride concentration of up to 3,500 mg/l and total dissolved solids of up to 8,000 mg/l. Ground‐water composition evolved from Eocene sea water by seven‐ to nine‐fold dilution with rain water. Ion exchange, pyrite oxidation, and calcite dissolution further modified water composition. The amount of recharge through the vadose zone where confining layers crop out is probably negligible over an extremely long time. Meteoric flushing in reclaimed land at the surface mine is many times greater than that in unmined mudstone deposits, and the chemical composition of vadose water in reclaimed land is changed by further dilution and water‐rock reactions.

Status of Antler Orogeny in Central Idaho--Clarifications and Constraints from Pioneer Mountains: ABSTRACT

Structural telescoping of argillaceous lower Paleozoic sequences with coeval calcareous and quartzitic rocks in central Idaho has commonly been presumed to be analogous to the Roberts Mountains thrust in Nevada. However, all datable thrusts in the Pioneer Mountains are post-middle Permian to pre-Eocene in age; if Antler age thrust existed, they were reactivated or obscured by major post-Antler movement. Furthermore, differences in fabric or structural style between thrusted argillaceous pre-Mississippian sequences and calcareous-arenaceous Pennsylvanian-Permian sequences, which have been cited in support of Antler orogenesis, are equivocal. These differences more likely resulted from disharmonic response to entirely later thrusting in rocks of different competencies or at different structural levels. The Copper Basin Formation, a dominantly clastic deposit requiring an argillite-chert-quartzite western highland source in Mississippian time, remains the only evidence in central Idaho for an Antler highland. The Copper Basin now occurs in two superimposed allochthons which, when palinspastically restored, require that their original depositional basin extended at least 50 to 75 km west from the present Pioneer Mountains. Thus, in Mississippian time, the Antler highland reached no farther east than westernmost Idaho. Emplacement of the Pioneer Mountains allochthons during mainly Mesozoic time involved (1) tectonic slices of high-grade metamorphic rocks and Precambrian crystalline basement, (2) eastward movement and imbrication of Antler detritus, and (3) thrusting of argillaceous facies lower Paleozoic rocks from the Antler highland over the tectonic remnants of its own debris. At least 100 km of post-Antler eastward translation is estimated; a comparable amount of earlier facies telescoping could be accommodated within the Antler highland based on reasonable facies reconstructions, but thrusting has not yet been demonstrated to have accompanied Antler highland development in Idaho. End_of_Article - Last_Page 700------------

Shallow subtidal sediments of Soudleyan (Caradoc) age in the Berwyn Hills, North Wales, and their palaeogeographic context

Soudleyan rocks in the Berwyn Hills, North Wales, consist essentially of marine mudstones and thin siltstones and fine sandstones, but also include interbedded pyroclastic formations which are at least partly subaerial in origin. In the marine sediments the association of current and wave ripples, small steep sided channels, rare desiccation cracks plus a Planolites , Skolithos, Teichichnus and Cruziana -dominated ichnofauna associated with shallow marine benthic brachiopod-dominated shelly assemblages suggests a shallow subtidal origin with water depths of less than 25 m. Four lithofacies (regular layered, irregular layered, mottled and homogeneous) are differentiated on the basis of their geometry, lithology, sedimentary and biogenic structures and associated faunas, and are described in detail. Comparable subtidal facies extend northwestwards across North Wales into the Arenig-Bala and Snowdon-Tremadoc districts and southeastwards into South Salop, with slightly deeper water argillaceous facies in the intervening Shelve and Breidden Hills areas. Southwest of the Berwyn Hills offshore mudstones are present in the Corris-Cader Idris and Builth-Abbey Cwmhir areas and clastic turbidites in the Cardigan area. A regional palaeogeography for the Soudleyan of the Welsh Basin is proposed.

Geochimie des illites du Gres a Voltzia du (Buntsandstein superieur) des Vosges du Nord, France

The fine-particle fraction of the triassic formation of the Grès à Toltzia (sandstone) in the Vosges Mountains is often made up of pure illite. Chemical analyses, done on about one hundred samples, did not show any variation in the chemical composition of the major elements from one facies to the other. The amounts of B, Li, Rb, Cr and Sr however are different according to whether the illites come from the sandstony, dolomitic or argillaceous facies. The progressive increase of these amounts is a function of water concentration in the environment of deposition, and this is in good agreement with the paleontological and sedimentological results.

Stratigraphy and Potential Prospects of Devonian Reefs of New York: ABSTRACT

Reefs are found in the outcrop sections of several Lower and Middle Devonian units in New York State. The most prominent of these occurs in the Edgecliff Member of the Onondaga Limestone. The Onondaga Limestone was first described and named by James Hall of the New York Geological Survey in 1839. The present fourfold division of the Onondaga, in descending order, Seneca, Moorehouse, Nedrow, and Edgecliff, was proposed by Oliver in the early 1950s. The type section is located in Onondaga County, New York. In the subsurface, the uppermost Seneca Member is a massive limestone and can only by separated from the similar underlying Moorehouse Member by the presence of the Tioga Bentonite Bed, which gives a characteristic peak on the gamma ray log. The Seneca is absent in the central-southern part of New York, where a pronounced thinning of the Onondaga occurs. The Moorehouse is a massive cherty limestone and is also missing in the extreme central-southern part of New York in the previously mentioned area of thinning. The Nedrow is a shaly cherty limestone and is persistent throughout the state and in the area of thinning, except over known subsurface reefs in the underlying Edgecliff. The lowermost Onondaga member, the Edgecliff, is a coarse-grained light-gray to grayish-white biostromal limestone, present in an area from northeastern southwestward through central New York. In eastern and southeastern New York this unit is represented by an argillaceous facies, whereas in far western New York it is highly cherty. The Edgecliff shows a pronounced thinning in central and southern New York and in north-central Pennsylvania, where it is mostly 10 ft or less thick. In the southwestern part of this thin area, three subsurface Edgecliff reefs, all 150-200 ft thick and containing gas, have been discovered since 1967. At least 21 smaller reefs are known in the outcrop section of this member in eastern New York, one in central New York and two in the Buffalo area. The reefs were formed in a clear-water shallow subtidal environment on the Edgecliff platform. Biostromal facies and reefing are also present on the outcrop in several zones in the Middle Devonian Hamilton Formation, which overlies the Onondaga. Most important of these zones are in the Ludlowville Member of the Hamilton in the Syracuse area of central New York. Two of these zones, the Joshua and Staghorn Point, occur over areas of 40 and 120 sq mi, respectively, according to Oliver. No reef buildup in these zones has been encountered in drilling as yet, but no systematic search has been made for reefs in the subsurface. Several smaller reefs are known from outcrops of the Coeymans Formation of the Helderberg Group in central New York and northwestern New Jersey. End_of_Article - Last_Page 2110------------

Carbonate Porosity Related to Depositional Fabric--Zelten Field, Libya: ABSTRACT

Production from the Zelten field, Libya, is from the highly porous shelf carbonates of the Zelten Member (main pay) of the Paleocene and lower Eocene Ruaga Limestone. Fifteen facies are easily recognized, mapped, and predicted. In the Zelten field, primary and secondary porosities, recorded as high as 40%, are related to the original depositional fabric of the sediment and are, therefore, facies controlled. Porosity is best developed in the coralgal wackestone and packstone and Discocyclina-foraminiferal packstone and grainstone, which together form a northwest-southeast trend across the northern part of the field. Porosity is lowest in the miliolid-foraminiferal-wackestone and argillaceous bryozoan/echinoid-wackestone facies, both of which are blanketlike in distribution over the top of the field and form the cap for the reservoir. Porosity is also low in the argillaceous molluscan-wackestone facies south of and equivalent to the coralgal and Discocyclina-foraminiferal facies. It is concluded that early compaction of the soft carbonate sediments determined the amount of porosity preserved in the reservoir today. The grain-supported facies were not compacted and much of the original primary porosity is presumed to have been enlarged later by leaching. However, the mud-supported facies were compacted; the original porosity was lost early and consequently, later leaching was inhibited. End_of_Article - Last_Page 603------------

Submarine Channel and Fan Deposits, Silurian of Central Wales, United Kingdom: ABSTRACT

Mapping and detailed sedimentological study of a group of geosynclinal formations of Llandoverian age (Lower Silurian) around Rhayader, central Wales, have been undertaken jointly by the writer and M. A. Woollands (University College, London). These formations occur near the southeastern margin of the Welsh trough and consist essentially of bodies of sandstone and conglomerate, lenticular through distances of several kilometers, and enclosed within a framework of graptolite-bearing argillaceous sediments. Marked lateral and vertical variations in the geometry, internal features, and transport patterns of the coarser intercalations may be demonstrated and are attributed to differences in the mechanisms and sites of deposition. On the southeast, an argillaceous facies with a few thin, laterally derived tractional sandstone beds and calcareous bands, displaying slumps, is interpreted to represent a slope environment. This facies contains a few longitudinally derived distal turbidite units which appear in more profusion on the northwest in an argillaceous facies of inferred basin-plain origin. Mantling the pelagic sediments are broadly lenticular bodies of coarse sandstone interpreted as proximal turbidites. Sole marks in these sandstone beds present a complex but essentially longitudinal transport pattern but ripple marks and other depositional features indicate modification of the northeast-southwest longitudinal pattern by lateral tractional currents (from the southeast). Such proximal turbidites grade both laterally and vertically into more distal turbidites and into coarse granule-gritstone and boulder conglomerate. The rudites occupy successive, steep-sided channels excavated in the pelagic basin floor and slope sediments, and display features characteristic of fluxoturbidite associations. The orientation of channels, sole marks, ripples, and pebble imbrication consistently indicates current movement toward the north-northwest whereas the internal features and fabric of the coarse sediments suggest emplacement by a sand-avalanche End_Page 535------------------------------ mechanism in which shearing processes were dominant. The rudites are interpreted as a complex of submarine channel fills, excavated in base-of-slope sediments and possibly representing the lower ends of small submarine canyons. The more extensive proximal turbidite bodies may then be regarded as fan accumulations fronting the canyon mouths and feathering out into the distal representatives. One of the most interesting features of this sequence is the abruptness of the change from channel rudite through proximal facies to distal turbidites, which in several localities may be shown to occur within less than 8 km down-current. The short-lived and unusual character of the turbidity currents in this example may be linked to the clean, mud-deficient nature of the original supply of gravel and sand. End_of_Article - Last_Page 536------------

THE GEOLOGY OF WESTERN PAPUA

Two principal phases of sedimentary deposition are known in Western Papua, the Mesozoic and Tertiary. Knowledge of older sediments is limited to a small outcrop of Permian on the western end of the Kubor Range beneath which are granites and metamorphic rocks forming basement.The Mesozoic rocks outcrop in the central ranges of New Guinea and have been penetrated in 20 wells in the southern coastal area. Elucidation of the detailed Mesozoic structure and stratigraphy is incomplete but the outline of the history of sedimentation is emerging and appears to be similar in broad structural pattern to that of the overlying Tertiary sequence.Folding emergence and erosion occurred at the end of the Mesozoic in response to epeirogenic uplift of the northeastern margin of the Australian continent; with the subsequent Tertiary transgression being represented for the most part by Eocene and Oligocene limestones of limited thickness. On the basis of facies characteristics and sediment thickness it appears that during the Miocene western Papua was the site of an arcuate orthogeosynclinal complex. In the sector north of tilte Papuan Gulf the subdivision into a eugeosynclinal and miogeosynclinal trough is clearly represented by the development, in Kereruan times, of the Aure Trough and the Omati Basin. During the Lower and Middle Miocene huge thicknesses of greywackes and shales were deposited in the Aure Trough and westwards and southwestwards of the trough a limestone succession consisting mainly of basinal, fore -reef limestones and shoal reefs was developed over an area some 200 miles wide with its maximum thickness at Ornati. Definition of limestone facies within this huge limestone area is still incomplete. The Upper Miocene was marked by widespread development of argillaceous and arenaceous facies with only minor development of limestone. Movements during the Upper Miocene led to restriction of the Pliocene basin of sedimentation and the formation of shallow water arenaceous rocks with lesser mudstones and coals. The final major phase of folding and uplift probably began in late Pliocene times and has continued to the present day, accompanied by vulcanism and vigorous erosion.