Stratal Units Research Articles

Nucleus Reuniens Thalami Modulates Activity in Hippocampal Field CA1 through Excitatory and Inhibitory Mechanisms

The nucleus reuniens thalami (RE) originates dense projections to CA1, forming asymmetrical synapses on spines (50%) and dendrites (50%). The hypothesis that RE input modulates transmission in CA1 through excitation of both pyramidal cells and interneurons was tested using electrophysiological methods in the anesthetized rat. The RE-CA1 afferents were selectively stimulated at their origin; evoked field potentials and unit activity were recorded in CA1. RE-evoked depth profiles showed a prominent negative deflection in the stratum lacunosum-moleculare and a positive one in the stratum radiatum. The lacunosum-moleculare sink-radiatum source configuration is compatible with RE-elicited depolarization of apical dendrites of pyramidal cells. Despite a consistent and robust paired pulse facilitation of RE-evoked field potentials, population spikes in the stratum pyramidale were not detected at any tested condition. This indicates the inability of RE-CA1 input to discharge pyramidal cells. However, stimulation of RE-elicited spiking of extracellularly recorded units in strata oriens/alveus and distal radiatum, indicative of the activation of local interneurons. Thus, RE seems to modulate transmission in CA1 through a (subthreshold) depolarization of pyramidal cells and a suprathreshold excitation of putative inhibitory oriens/alveus and radiatum interneurons. RE-evoked monosynaptic or disynaptic field potentials were associated with stimulation of rostral or caudal RE, respectively. Anatomically, a projection from caudal to rostral RE was demonstrated that can account for the disynaptic RE-CA1 input. Because caudal RE receives input from the hippocampus via the subiculum, we propose the existence of a closed RE-hippocampal circuit that allows RE to modulate the activity in CA1, depending on hippocampal output.

Tectonic control on sea-floor relief and the localization of Lower Mississippian Waulsortian mounds, New Mexico

Lower Mississippian carbonate mud-rich bioherms, generally referred to as Waulsortian mounds, are commonly associated with low-paleolatitude carbonate ramp settings and have recently been recognized as important hydrocarbon reservoirs. The factors controlling localization of Waulsortian mounds have heretofore been poorly understood. Stratal relations exposed in the Alamogordo Member of the Lake Valley Formation in the Sacramento Mountains of New Mexico illustrate the effects of tectonism on carbonate sedimentation prior to, during, and after mound growth. They indicate that mound initiation and localization were strongly controlled by tectonically generated, intraramp, sea-floor topography. These observations bear strongly on understanding the controls on localization and growth of mud mounds in general. Stratal geometries observed in the underlying Andrecito Member indicate that this topography was modified by erosional and depositional processes prior to mound initiation. Mounds formed on the surfaces and margins of the intraramp topography as the result of aggradational, in situ accumulation of biogenic sediment. Differences in growth geometry of stratal units within individual mounds and differences between mounds are correlated with position of the mound on the ramp and the deformation occurring immediately prior to mound growth. It is probable that local tectonism continued during mound growth, and that local differences in the amount of relative uplift resulted in different amounts of space for growth of individual mounds, and thus determined differences in mound size and geometry.

Periprocedural myocardial ischemia in patients with severe symptomatic coronary artery disease undergoing endoscopy: Prevalence and risk factors: J.G. Lee, M.W. Krucoff, S.R. Brazer. Gastroenterology Section, Portland VAMC, Oregon Health Sciences University, Portland, OR. Am J Med 1995;99:270–5

This paper refers to thermal energy storage units based on fully mixed and stratified liquid storage tanks. The stratified unit was modeled by using three partial differential equations governing the time and space dependence of hot gas, liquid storage medium and pipes wall material, respectively. For the study of fully mixed units a standard approach based on the number of thermal units concept was slightly generalized to take into account the time dependence of the heat transfer coefficients. The performance indicator adopted here is the minimum exergy destruction. All relevant irreversibility sources were considered. Every operation regime has an optimum charging time, which increases by decreasing the hot gas inlet temperature. Also, there is an optimum gas mass flow rate that makes the exergy destruction number a minimum minimorum. The stratified units are normally more effective than the fully mixed units from the point of view of exergy conservation. The standard approach based on the number of thermal units concept tends to overestimate the performance of thermal energy storage systems. Its accuracy is better in case of fully mixed units.

Pyroclastic flows and surges over water: an example from the 1883 Krakatau eruption

Pyroclastic deposits from the 1883 eruption of Krakatau are described from areas northeast of the volcano on the islands of Sebesi, Sebuku, and Lagoendi, and the southeast coast of Sumatra. Massive and poorly stratified units formed predominantly from pyroclastic flows and surges that traveled over the sea for distances up to 80 km. Granulometric and lithologic characteristics of the deposits indicate that they represent the complement of proximal subaerial and submarine pyroclastic flow deposits laid down on and close to the Krakatau islands. The distal deposits exhibit a decrease in sorting coefficient, median grain size, and thickness with increasing distance from Krakatau. Crystal fractionation is consistent with the distal facies being derived from the upper part of gravitationally segregated pyroclastic flows in which the relative amount of crystal enrichment and abundance of dense lithic clasts diminished upwards. The deposits are correlated to a major pyroclastic flow phase that occurred on the morning of 27 August at approximately 10 a.m. Energetic flows spread out away from the volcano at speeds in excess of 100 km/h and traveled up to 80 km from source. The flows retained temperatures high enough to burn victims on the SW coast of Sumatra. Historical accounts from ships in the Sunda Straits constrain the area affected by the flows to a minimum of 4x103 km2. At the distal edge of this area the flows were relatively dilute and turbulent, yet carried enough material to deposit several tens of centimeters of tephra. The great mobility of the Krakatau flows from the 10 a.m. activity may be the result of enhanced runout over the sea. It is proposed that the generation of steam at the flow/seawater interface may have led to a reduction in the sedimentation of particles and consequently a delay in the time before the flows ceased lateral motion and became buoyantly convective. The buoyant distal edge of these ash-and steam-laden clouds lifted off into the atmosphere, leading to cooling, condensation, and mud rain.

Quaternary Sedimentary Processes and Budgets in Orphan Basin, Southwestern Labrador Sea

The continental slope in Orphan Basin, northeast of Newfoundland, is underlain by several seaward-thinning debris-flow wedges alternating with acoustically stratified, regionally extensive, mainly hemipelagic sediments. δ18O stratigraphy and volcanic ash layers in a 11.67-m core indicate that the uppermost debris-flow wedge formed during the last of several sea-level lowstands in isotopic stages 2–4. Similarly, seismic reflection correlation of dated levels at DSDP Site 111 with the Orphan Basin succession suggests that two deeper debris-flow wedges were deposited during oxygen isotopic stages 6 and 8. The oldest of the debris-flow deposits in at least three of the wedges formed well into the corresponding glacial cycle, after ice sheets had reached the edge of the continental shelf. Slower deposition by hemipelagic processes and ice rafting formed the acoustically stratified units, including Heinrich layers. The youngest three debris-flow wedges each have volumes of 1300–1650 km3. Approximately two-thirds of this material is attributed to glacial erosion of Mesozoic and Tertiary strata beneath the Northeast Newfoundland Shelf. The remainder is believed to have been derived by glacial erosion of older bedrock that now forms the island of Newfoundland. The observed sediment volumes and the inferred basal and upper ages of the debris-flow wedges imply an average glacial denudation rate of about 0.13 mm/yr for this older bedrock, and an average of about 60 m of glacial bedrock erosion since oxygen isotope stage 22. This denudation rate is similar to estimates from the Barents Sea region off Norway.

Episodic growth of a Waulsortian buildup: the Lower Carboniferous Muleshoe Mound, Sacramento Mountains, New Mexico, USA

Abstract Muleshoe Mound is a composite Waulsortian buildup that crops out along the western escarpment of the Sacramento Mountains, New Mexico. Exceptional exposures of Muleshoe Mound have enabled a detailed stratigraphic model to be developed, subdividing the buildup into five separate stratal units. Each stratal unit differs in facies, geometry and symmetry, reflecting changes in the character and intensity of different environmental conditions (energy, carbonate production, oxygenation and accommodation space). Hiatal surfaces mark periods of mound crisis and/or sea-level fluctuations. These hiatal surfaces highlight the episodic nature of the mound’s growth. Episodic growth acted as an important control on this buildup’s development, facies and early diagenesis. Features and patterns of the Muleshoe Mound conflict with the traditional interpretation of the Waulsortian as quiet, stable, deep-water buildups. Muleshoe Mound appears to have grown in appreciable currents where accommodation space was a crucial control on growth.

Stratal Thickness Frequencies and the Prevalence of Orderedness in Stratigraphic Sequences

Assuming the existence of discernable hierarchies in the thickness and/or temporal recurrence of stratal units within sedimentary sequences has become an increasingly important axiom of sequence and cyclostratigraphic studies, and multiple orders of stratigraphic cyclicity are now commonly associated with inferred durations and magnitudes of rhythmic variation in global sea level. Beyond the desire to establish an informal nomenclature relating stratigraphic thickness to periodicity of eustatic change, an assumption of stratigraphic orderedness also supports the inference that groups of sedimentary units with distinct modal dimensions in fact do occur with some generally distinct frequency in stratigraphic sequences. In contrast to such perceived patterns of stratigraphic order, many groupings of sedimentary units exhibit lognormal frequency distributions wherein most of the population has an exponentially increasing frequency of occurrence with linearly decreasing class size. Such thickness distributions typify a wide range of sedimentary entities, including individual lithofacies, formally named stratigraphic units, epoch-interval sedimentary sections, and cyclic peritidal lithofacies associations, as well as durations of unconformity-bounded stratigraphic sequences and the magnitudes and durations of presumed change in global sea level. These distributions indicate that most natural populations of sedimentary units comprise a non-modal series of increasing frequency with decreasing size. The importance of this statement lies in the fact that discrimination of stratigraphic hierarchies and their designation as nth-order cycles may constitute little more than the arbitrary subdivision of an uninterrupted stratigraphic continuum.

The Utility of Continual Reservoir Description: An Example from Bindley Field, Western Kansas

Continual revision of geologic reservoir description is an important component of reservoir management. New data should be incorporated into existing reservoir models in light of evolving geologic concepts. Revisions may have significant impacts on the approach and success of reservoir management strategies. A reevaluation of Bindley field (Mississippian), Hodgeman County, Kansas, serves as an illustration of this process. Prior study of this field suggested that the reservoir interval is comprised of a single, relatively uniform facies (bryozoan dolomite) having no apparent internal structure. A waterflood attempt based on this concept of reservoir architecture resulted in minimal response. A revised model of reservoir architecture and petrophysics resulted from integration of new core data, a new stratigraphic correlation scheme, updated well production histories, and capillary pressure data. The principal reservoir interval is vertically segregated into six stratal units, each consisting of storm-winnowed, normal marine carbonates at the base and capped by low permeability, restricted dolomudstones. The reservoir is confined to grain-supported dolostones in the normal marine portion of four of the stratal units (flow-unit type I; k>=10 md). Each grain-supported dolostone grades laterally into low permeability mud-supported fabrics and is vertically isolated by the intervening restricted dolomudstones. Reservoir quality is primarily determined by matrix permeability, which is not covariant with porosity. Permeability variation within flow-unit type I rocks is caused by variation in pore-throat size distribution. Cumulative primary fluid production is proportional to the transmissivity (kh) of flow-unit type I rocks that are in communication with the well bore. The revised geologic model reveals specific methods to improve primary recovery and rectify the poor waterflood performance. These methods include selective perforation of all oil-saturated type I flow units to optimize primary recovery, and remedial waterflood design to assure continuity of fluid flow between injection and production wells. The revised model demonstrates the benefit of revising geologic reservoir models regardless of field size or maturity.

Cyclic alteration of proximal and distal storm facies; Kope and Fairview formations (Upper Ordovician), Ohio and Kentucky

The Upper Ordovician Kope and Fairview Formations and the Bellevue Tongue of the Grant Lake Limestone exposed in the Cincinnati, Ohio region, represent storm-dominated sedimentation on a prograding, intracratonic ramp. The upper Kopelower Fairview transition zone (25 m thick) is an outer to intermediate ramp lithofacies and is composed of numerous (1-3 m thick) shallowing-upward subtidal cycles. Bedding cycles are composed of two depth-controlled facies: 1) a shale-dominated distal facies and 2) a fossiliferous proximal facies . The distal facies records low-energy, largely single-storm events and consists of thin graded shale beds, nearly in situ fragile-fossil accumulations, thin parautochthonous packstones, and quartzose siltstone beds. Siltstones contain abundant storm-depositional indicators: tool-marked soles, gutter casts, mud-filled scours, hummocky lamination, and hummocky bedforms. The proximal facies represents deposition in an energetic, storm-wave-swept environment and consists of amalgamated sets of thick-bedded, fine- to coarse-grained skeletal grainstones and packstones with low-angle cross-lamination, megaripples, and shale lithoclasts. Sedimentary structures and paleocurrent patterns from both facies record deposition under a combined flow regime with the unidirectional current vector oriented in the offshore direction. Cycles are laterally continuous across the 35 km by 32 km study area. Cycle boundaries (flooding surfaces) are parallel to a unique storm-generated marker bed, implying that these surfaces are effective time lines. Their widespread character and relatively deep subtidal-facies assemblage argues against autocyclic processes in cycle formation. We propose that the alternation of proximal and distal facies was tied to fluctuations in storm-wave base driven by short-term glacio-eustasy. Three scales or orders of sedimentary cyclicity are recognized in the study area. Meter-scale bedding cycles are considered fifth-order parasequences (20-100 Ka duration) and stack into genetically related parasequence sets. These fourth-order units have been previously described as member-scale lithostratigraphic tongues. Analysis of Fischer plots suggest that cycle thickness and subfacies composition was controlled by both accommodation and siliciclastic sediment supply. These higher-order stratal units are super-imposed onto the highstand portion of the Kope to Bellevue depositional sequence and give insight into the high-frequency controls on the progradation of the ramp complex.

Stratigraphic relationships of some Tertiary lowstand depositional systems in the Central North Sea

The application of sequence stratigraphic techniques, in conjunction with detailed palynofacies and sedimentological analyses, provides a new methodology for the prediction of stratigraphic relationships in selected lowstand depositional systems from the Tertiary of the Central North Sea. Traditional lithostratigraphy invokes facies changes to explain stratal variations within a depositional system. A sequence stratigraphic approach emphasizes the ‘temporal’ dimension to explain and predict the distribution of stratal units and their respective stacking patterns. Sequence definition requires an in-depth knowledge of chronostratigraphically equivalent stratal units in a regional basin context. A biostratigraphic zonal scheme for the Upper Paleocene and Lower Eocene is presented, and provides the chronostratigraphic constraints to the sequence interpretation. Each sequence is composed of a series of systems tracts and parasequences whose stratal expression evolved in response to accommodation space; the primary control on accommodation space being the inter-relationship between eustatic sea-level, subsidence/uplift and sediment supply. Palaeoenvironments responded to changes in relative sea-level which are reflected in the qualitative and numerical aspects of the faunal and floral assemblages at a variety of scales. These basinal lowstand depositional systems are represented predominantly by fans and channelized complexes. Alternating high and low (less turbulent) energy conditions possess characteristic palynomorph signatures. The stratigraphic relationships of several contrasting Tertiary lowstand depositional systems are examined. In each case, a series of stratigraphic methodologies are presented that integrate detailed palynofacies, sedimentologic, well and seismic data at both field and regional scales, using a database of more than 200 wells and 15 000 km of 2D and 3D seismic data. The first case study comprises the Late Paleocene (zones ET4–ET7), in places Lista Formation equivalent; type well, Gannet D, 22/21-5. The composition of the microflora and kerogen strongly infers a distal basinal setting. The lowstand system provides reservoir quality sands, whereas the overlying distal prograding highstand affords an effective intraformational seal. The second case study is in the Late Paleocene (zone ET8), in places Forties Formation equivalent; type well, Gannet B, 21/25-1. Certain features of this palynofacies assemblage contrast with the previous example, and suggest a more proximal palaeoenvironment strongly influenced by an adjacent coastal deltaic complex. A further case study, from the Eocene of the Gannet area, investigates fan and channel morphologies in areas where palaeotopographic relief is influenced by halokinesis. The theme of stratal relationships continues with discussion of an interdigitating leveed channel complex of Eocene age from the Kittiwake area. The application of these sequence stratigraphic techniques to hydrocarbon prospectivity, how they open new avenues for exploration and, at a production scale, new insights into reservoir geometry and continuity, are discussed.

Tectonic episodes of cratons: conflicting North American concepts

ABSTRACTOver the past one hundred years North American geologists have been influenced by concepts of Earth history based on periodic events involving marked (‘Neocatastrophic’) changes and competing concepts involving steady‐state (‘Uniformitanan’) behaviour. T.C. Chamberlin, a late 19th‐/early 20th‐Century exponent of a nonuniformitarian scenario of earth history, proclaimed ‘diastrophism as the ultimate basis of correlation’ ‐ meaning that geological history is punctuated by orogenic paroxysms causing retreat of epicontinental waters to oceanic basins, creating segmentation of the stratigraphic and chronological record. At mid‐Century James Gilluly gained wide support for the proposition that orogeny has followed a Uniformitarian, near‐steady‐state course. Accordingly, episodic diastrophism is a myth and cannot be the basis for periodic eustatic sea‐level change, interregional unconformities, etc.In more recent decades two powerful new influences on geological thought have emerged. The global tectonic paradigm has forced upon us the realization that rearrangements of lithospheric plates are capable of changing the freeboard of continents. At the same time technological advances in the acquisition and processing of seismic‐reflection data have been seized by receptive minds to organize the newly available stratigraphic imagery for application to the discipline of seismic stratigraphy which emphasizes the identification and correlation of unconformity‐bounded successions of strata. Expansion of the methodology to the outcrop, beyond the limitations of seismic resolution, has brought about the flowering of the New Sequence Stratigraphy, doctrine that has deservedlyachieved wide application and acclaim. Practitioners of sequence stratigraphy, impressed by the distances over which unconformity‐bounded stratal units can be correlated, hew to the belief that eustatic (commonly glacially engendered) sea‐level change is the control.This paper examines evidence of tectonic episodes affecting cratons and their margins. These offer promise of a more satisfactory understanding of stratigraphic sequences and of the Earth history they represent. Consideration of these matters makes it inescapable that the elevation and subsidence of cratons (and their component basins and arches) reflect tectonic phenomena of the continental lithosphere quite independent of eustatic events.

Inventory of Indigenous Forest and Woodland in Southern Africa

SYNOPSIS An inventory is an important component of a series of activities to ensure the sustained use of natural forest and woodland resources. Three levels of inventories are considered: national level to determine general forest policy: regional level to provide a basis for integrated multiple-use management: and local level for detailed working plans. Different approaches to mapping resource areas are discussed, with particular emphasis of the use of satellite imagery. Sampling design requirements are discussed with particular emphasis on accuracy and precision, stratification of the resource area. shape and size of the sampling units, random and systematic distribution of sampling units, sampling intensity and the allocation of sampling units to stratified vegetation units. Requirements for sampling variables, and analyses and presentation of inventory results are briefly discussed. Several examples are used from inventories conducted in forest and woodland in southern Africa.

The tectonic factor in sea level change: A countervailing view

It is now demonstrable that many unconformity‐bounded assemblages of strata, the sequences and groupings of sequences of seismic stratigraphy, can be correlated subglobally. The facies tracts that are identifiable within sequences clearly represent depositional environments that bear a systematic relationship to the strandline and to water depths, and thus to sea levels. Lacking a more suitable explanation for the intercontinental correlation of sequences it has become widely accepted that eustatic changes of sea level are responsible for the interruptions in sedimentation that separate sequences and for the shifts of coastal onlap and of facies tracts implicit in sequence analysis. Students of cratonic basins are convinced that continental interiors are subject to a tectonic evolution of positive and negative vertical movements and that these control relative sea level and the distribution of sedimentary environments. Proof of this control is buried hi the sedimentary fills of cratonic basins, but it has been difficult to produce such proof in a form digestible by Neo‐Neptunist doubters. Digitization of isopach maps of successive stratal units produces thickness data amenable to numerical study. For example, the logarithms of thicknesses of a stratal unit commonly exhibit a uniform areal distribution, the hallmark of a bivariate‐normal distribution. This revelation makes available a number of Gaussian statistical measures as descriptors of time‐variable basin geometry. Extraction and analysis of geometric parameters lead to the realization that basin evolution, necessarily including relative sea levels and the transgression and progradation of strandlines and sedimentary environments, has a strong tectonic component that gives evidence of synchrony of widely separated basins on separate cratons. Secular variation in radial heat flow, leading to episodic inflation and deflation of continents is suggested as a possible globally effective mechanism.

Sedimentology of a Proterozoic erg: the Venkatpur Sandstone, Pranhita‐Godavari Valley, south India

ABSTRACTReappraisal of the Late Proterozoic Venkatpur Sandstone indicates that the bulk of the sandstone is aeolian in origin. Aeolian stratification types, namely (i) inverse graded translatent strata, (ii) adhesion laminae, (iii) grainflow strata and (iv) grainfall strata, are present throughout the outcrop belt. Nine facies have been identified that represent both aeolian and related aqueous environments within a well‐developed erg. Cosets of large cross‐beds at the Bellampalli section in the NW of the study area record dune fields in the interior of the sand sea. To the SE, at the Godavari River and Ramgundam sections, a progressive increase in the relative proportion of the flat‐bedded to cross‐bedded facies and intercalated non‐aeolian facies delineates the transition from the dune‐field to sand‐sheet environment. An alternating sequence of aeolian and marine sediments at Laknavaram, in the extreme SE, marks the termination of the sand sea. Palaeocurrent data suggest that the NW‐SE trend of the sections represents a transect across the sand sea in a direction normal to the resultant primary palaeowind direction.Abundant horizontally stratified units in the Vankatpur Sandstone do not always represent the interdune sediments. On the basis of the thickness and geometry of the units, nature of bounding surfaces and associated facies sequence, the facies is variously interpreted to represent interdune, inland sabkha, sand sheet and coastal sand flat deposits.

Seismic reflection investigation of Kalamalka Lake: a "fiord lake" on the Interior Plateau of southern British Columbia

A uniboom seismic reflection profile survey has revealed the nature of bedrock relief and the acoustic character of Pleistocene glacial sediment fill beneath Kalamalka Lake in southern British Columbia. Despite its continental interior setting, Kalamalka Lake basin has many attributes of coastal fiords, such as being overdeepened below sea level and having closed bedrock depressions and a thick sediment fill.The bedrock surface beneath Kalamalka Lake has been eroded as much as 417 m below lake level (26 m below sea level) and is characterized by a series of closed, glacially overdeepened depressions. We suggest that the location of the lake basin is structurally controlled but was overdeepened by rapidly flowing ice that drained the interior portions of the Cordilleran ice sheet during repeated Pleistocene glaciations.Up to 272 m of sediment has been deposited beneath Kalamalka Lake. The greatest thickness of the sediment fill (up to 237 m) is a seismically transparent unit that overlies a thin (up to 20 m), discontinuous lower stratified unit and is overlain by a thin (up to 15 m), continuous upper unit that is well stratified. The sedimentological nature of the lower stratified unit is not known but could represent a discontinuous coarse lag. The thick, middle transparent unit is interpreted as a massive silt deposited rapidly in a proglacial lake from suspended-sediment plumes during deglaciation. The thin overlying stratified unit may be correlative with laminated glaciolacustrine "white silt" deposits that outcrop extensively across central and southern British Columbia, suggesting a common history of deglaciation and sedimentation.An ambitious research program focused on seismic stratigraphic definition, coupled with direct drill-core sampling, is needed to take full advantage of the extensive sediment record that exists beneath the large, glacially overdeepened lakes of southern British Columbia.

Depositional processes of the Suwolbong tuff ring, Cheju Island (Korea)

ABSTRACTThe Suwolbong pyroclastic sequence in the western part of Cheju Island, Korea, comprises partly preserved rim beds of a Quaternary basaltic tuff ring whose vent lies about 1 km seaward of the present shoreline. The sequence consists of breccia, lapillistone, lapilli tuff and tuff. Eighteen sedimentary facies are established and organized into six lateral facies sequences (LFS) and seven vertical facies sequences (VFS).The LFS 1, 4 and 5 begin with massive lapilli tuff which transforms downcurrent into either planar‐bedded (LFS 1), undulatory‐bedded (LFS 4) or climbing dune‐bedded (LFS 5) (lapilli) tuff units. They are representative of relatively ‘dry’ base surge whose particle concentration decreases downcurrent with a progressive increase in both tractional processes and sorting. The LFS 2 begins with disorganized and massive lapilli tuff and transforms into crudely stratified units downcurrent. It results from relatively ‘wet’ base surge in which sorting is poor due to the cohesion of damp ash. The LFS 3 comprises well‐sorted lapilli tuff and stratified tuff further downcurrent, suggestive of deposition from combined fall and surge of relatively ‘dry’ hydroclastic eruption. All seven vertical facies sequences generally comprise two facies units of coarse‐grained fines‐depleted lapilli tuff and an overlying fine‐grained tuff. These sequences are suggestive of deposition from base surge that consists of a turbulent head and a low‐concentration tail.Depositional processes in the Suwolbong tuff ring were dominated by a relatively ‘dry’ base surge. The base surge comprises turbulent and high‐concentration suspension near the vent whose deposits are generally unstratified due to the lack of tractional transport. As the base surge becomes diluted downcurrent through fallout of clasts and mixing of ambient air, it develops large‐scale turbulent eddies and is segregated into coarse‐grained bedload and overlying fine‐grained suspension forming thinly stratified units. Further downcurrent, the base surge may be either cooled and deflated or pushed up into the air, depending on its temperature. The Suwolbong tuff ring comprises an overall wet‐to‐dry cycle with several dry‐to‐wet cycles in it, suggestive of overall decrease in abundance of external water and fluctuation in the rate of magma rise.

A stratified. Two-stage sampling design for estimation of the biomass ofMytilus edulisL. in Lindåspollene, a land-locked fjord in western Norway

Abstract A two-stage, stratified random sampling design for estimating the total biomass of mussels in a land-Io~ked fjor.d !s described. Stratification based on a visual survey of the population resulted In four distInct strata. As a bonus, a detailed map of the distribution of mussels along the shores of the fjord was obtained. Segments of shoreline, rather than the traditional areal units, were used as sampling units, mainly as an attempt to reduce the variance component associated with variation in the width of the mussel belt along the shores. The ~nit.s adopted were 100 m for the primary units, and 2 m and 0.25 m for the secondary units In different strata. ESllmates of the biomass In May and October of 1978 showed that in three of the strata the change in biomass was not significant, while in the fourth roughly 30 % of the biomass were lost over the period. The standing crop of mussels was estimated to 343 and 282 tons wet weight (shells and water included) respectively, with 95 % confidence interval lengths of 22 % to 25 % of the point estimates. This biomass was distributed on about 86 km o~ s~oreline, ~nd the .mean biomass of mussels was thus slightly less than 4 kg per m of shore. Similar samphng deSIgns may be used whenever estimates of the standing crop of dominant rocky-shore organisms, like mussels, barnacles or seaweeds, are wanted.

Application of Sequence Stratigraphic Analysis to Thin Cratonic Carbonate-Dominated Shelf Cycles (Upper Pennsylvanian) in Mid-Continent: ABSTRACT

Seismic sequence analysis has proven extremely effective and practical in identifying potential petroleum reservoir plays. This analysis requires defining stratal units, 10s of meters in thickness, bounded by unconformities and translated on seismic traces. A similar approach, using different techniques, is applied to subsurface data to improve the resolution of the sedimentary architecture in thin Upper Pennsylvanian cratonic cycles. These cycles reflect the relative rise and fall of sea level, aspects of which vary between cycles in terms of rate, magnitude, and duration. Understanding aspects of the fall of relative sea level and accordant sediment aggradation and progradation is important in order to recognize the character of the basinward migration of favorable shallow-water carbonate facies tracts. Because of their short duration, individual aggradational and progradational events cannot be resolved via biostratigraphy. Correlatable events, bounded by omission surfaces, can be recognized through detailed lithofacies analysis of cores. Wireline logs provide a basis from which to extrapolate thicker (>0.7 m) core-derived lithofacies. Very high resolution CMP seismic profiling (less than or equal to300-Hz) has been used successfully at penetration depths to about 450 m to distinguish and characterize reflections from beds (1.5 m minimum) within cycles and internal depositional surfaces within thickermore » deposits. Potential exists for using detailed seismic sequence analysis to assist in defining geometry of these component beds at reservoir scale.« less

Carbonate Sequence Stratigraphy and Controls on Carbonate Platform Development--Case Study from Permian of West Texas-New Mexico: ABSTRACT

Integration of seismic stratigraphic concepts with detailed field studies and geohistory analysis provides powerful interpretation leverage for deciphering the complex geologic history of carbonate platform-basin areas. Changes in carbonate productivity as well as platform growth and the resultant facies distribution are controlled most importantly by changes in relative sea level. The structural history of the Permian basin during the Permian shows two subsidence cycles of 10-20 m.y. duration. These subsidence cycles were major factors in the long-term (106 - 107 m.y.) development of the Permian carbonate platforms. During periods of relatively rapid subsidence, aggradation was dominant; during periods of slow subsidence, major platform progradation occurred. Superimposed on the long-term tectonic cycles is a series of third-order eustatic cycles (0.5-3 m.y.), which controlled development of 27 depositional sequences. Each sequence is composed of three depositional systems tracts: (1) a lower basin-restricted wedge interpreted to have been deposited during a relative fall and lowstand of sea level, (2) a transgressive systems tract of variable thickness, and (3) an upper, relatively thick, aggradational-to-progradational carbonate platform system, which includes significant allochthonous deposition in the basin and is interpreted to have been deposited during a relative highstand in sea level. The lowstand systems tracts are composed dominantly of quartz sandstone, commonly intercalated with carbonate debris beds at the toe of the slope. S quence boundaries display erosional truncation (subaerial on platform or at platform margin, subaqueous on slope) and/or subaerial exposure. Erosion and debris deposition occurs both within and outside submarine-canyon feeder systems. Two highstand depositional styles are differentiated here: (1) a keep-up system, which represents a relatively rapid rate of accumulation able to keep pace with periodic rises in sea level and displays a mounded-oblique stratal geometry at the platform margin, and (2) a catch-up system, which represents a relatively slow rate of accumulation and displays a sigmoid profile at the platform margin. Individual strata units of the platform margin and slope area of the catch-up carbonate system have a much longer sea-floor residence time and display significantly greater amounts of early submarine cement. The underlying transgressive systems tract tends to have a keep-up or give-up (i.e., thin, drowned) depositional style. End_of_Article - Last_Page 1522------------

SUPERPOSITION AND THE LAW OF REGULARITY IN STRATAL ORDER—KEYS TO THE PRACTICE AND THEORY OF GLOBAL STRATIGMPHY

In petroleum geology, the widespread practice of mapping lateral variations of lithology within the confines of general formations instead of time‐zones inevitably provokes differences of opinion over the scope and definition of stratigraphy. These differences can be traced back to a blending together of the separate concepts of two founders of geology, William Smith and Abraham Gottlob Werner. Werner originated a lithologic definition of rock formations; also, he taught the concept that the relative ages of formations could be determined by fossils. Smith contributed to the development of stratigraphy with his invention of the term ‘stratigraphical’, which referred to the occurrence of specific fossils and rock types within units of strata.The outcome of reconciling these different concepts has been that geologists have used Smith's basic terminology, the principle of superposition, and the formational units of stratigraphy, but have redefined them to accord with Werner's theory of Earth history. These changes in definition have confused the concept of stratigraphy, because they misrepresent its spatial form evident from ‘regularity in stratal order’. This general law was accepted by William Smith as the basis for his stratigraphical arrangement of rocks and fossils, which in the practice of correlating rocks by superposition led to a stratigraphic system of strata and the economy of a single stratigraphic scale.