Individual Facies Research Articles

Poisson processes of carbonate accumulation on Paleozoic and Holocene platforms

ABSTRACT Cambro-Ordovician carbonate lithofacies units in the Elbrook and Conococheague formations exposed at Wytheville, Virginia, as well as those in many other Phanerozoic peritidal sequences, exhibit exponential thickness frequency distributions. That is, occurrence frequency decreases exponentially with linear increase in unit thickness. Such distributions are characteristic of waiting times between independent Poisson events. This relative frequency of spaces of different size between horizons of lithologic change is what one would expect if the horizons were distributed randomly throughout carbonate successions. Abundances of different lithologies, both as net stratigraphic thickness and as number of occurrences, also decreases exponentially among successively rare sediment types, with each lithology being about 60% as plentiful as the next more abundant rock type. Relations between net thickness and number of occurrences for each facies define a linear trend coincident with a mean thickness for all Wytheville units of 0.48 m, a relation indicating that thickness distribution is independent of facies type. Similar relations are apparent for the horizontal extent of carbonate sediment bodies from the Holocene Florida-Bahama platform. Areal extents of individual facies units (lithotopes) are described by a frequency distribution in agreement with that anticipated for a population of equidimensional facies elements whose diameter distribution follows an exponential frequency distribution. Although regional gradients in sediment texture and composition are also apparent along most transects from platform margin to interior, such frequency distributions indicate that lateral extents of individual sediment reflect a largely stochastic distribution of facies boundaries across this Holocene surface. Lithotope abundances also yield trends of exponentially decreasing dominance among successively subordinate facies, with each being about 70% as extensive the next more abundant sediment type. Relations between areas and abundances for all lithotopes define a covariant trend corresponding to a mean area of 2.2 103 km2 for all Florida-Bahamas lithotopes. We consider several numerical models of stochastic carbonate accumulation; although not demonstrably unique, scenarios incorporating the sequential superposition of randomly placed coniform lithotopes result in thickness and area frequency distributions that are the same as those observed in ancient and modern platform deposits. Such simulations of Poisson processes of sediment accumulation are in general agreement with stochastic models of lithologic heterogeneity that have been more widely applied to petroleum reservoirs and groundwater aquifers. To the as yet unknown degree that peritidal lithofacies area and thickness are correlated, data from Paleozoic and Holocene platforms suggest that carbonate units should exhibit length/height ratios of approximately 105. Given the decimeter scale over which facies are designated in most Paleozoic peritidal successions, these relations predict mean lateral extents on the order of several tens of kilometers, a value in general agreement with the few data that exist on spatial continuities of peritidal lithotopes in Paleozoic carbonate sequences.

The Relationship Between Hydrogeologic Properties and Sedimentary Fades: An Example from Pennsylvanian Bedrock Aquifers, Southwestern Indiana

AbstractThe relationship between the hydrogeologic properties and sedimentary fades of shallow Pennsylvanian bedrock aquifers was examined using detailed sedimentologic descriptions, aquifer (slug) tests, and gamma ray logs. The main goal of the study was to determine if it was possible to reliably estimate near‐well hydraulic conductivities using core descriptions and logging data at a complex field site, based on assignment of consistent conductivity indicators to individual facies. Lithologic information was gathered from three sources: core descriptions, simplified lithologic columns derived from the core descriptions, and drillers’ logs. Gamma ray data were collected with a conventional logging instrument. Slug tests were conducted in all wells containing screened zones entirely within the Pennsylvanian facies of interest.Simplified subsets of sedimentologic facies were assembled for classification of subsurface geology, and all rocks within the screened intervals of test wells were assigned to individual facies based on visual descriptions. Slug tests were conducted to determine the bulk hydraulic conductivity (a spatial average within the screened interval) in the immediate vicinity of the wells, with measured values varying from 10−4 m/s to 10−8 m/s. Gamma ray logs from these wells revealed variations in raw counts above about 1.5 orders of magnitude.Data were combined using simple linear and nonlinear inverse techniques to derive relations between sedimentologic facies, gamma ray signals, and bulk hydraulic conductivities. The analyses suggest that facies data alone, even those derived from detailed core descriptions, are insufficient for estimating hydraulic conductivity in this setting to better than an order of magnitude. The addition of gamma ray data improved the estimates, as did selective filtering of several extreme values from the full data set. Better estimates might be obtained through more careful field measurements and reduction of associated errors, collection and incorporation of some form of continuous porosity log, or use of spatial statistics or more sophisticated inversion techniques. The utility of the approach remains to be tested through modeling and additional direct measurements.

PERMEABILITY AND POROSITY DISTRIBUTION PATTERNS IN A SHOREFACE RESERVOIR: LOWER CRETACEOUS FLACOURT FORMATION, BARROW SUB-BASIN, NORTH WEST SHELF, WESTERN AUSTRALIA

Porosity and permeability measurements on 926 core plugs were taken from 18 exploration, wildcat and producing wells from the Lower Cretaceous Flacourt Formation, Barrow Sub-basin and analysed for their spatial and temporal variability using both sedimentological and geostatistical methods.The porosity and permeability distribution shows a strong relationship to depositional facies and diagenetic modification. The samples are variably cemented with porosity ranging from 1.3 to 39.4 per cent and permeability from less than 0.01 to 24,400 md. Both porosity and permeability exhibit strong heterogeneity as a result of change of the depositional facies within the same well and the amount of detrital matrix within each facies.The degree of spatial and temporal variability observed in both porosity and permeability data is well reflected by the Levy-stable statistical analysis and variogram modelling. The heterogeneity is shown to be strongly controlled by texture, composition and sedimentary structures of the specific sedimentary facies. The heterogeneity of both permeability and porosity for each individual facies was characterised by the correlation length in the vertical direction from both the variogram and Levy C-gram analyses, and Levy index parameter.The sands of the Flacourt Formation, which form the principal reservoirs in the Barrow Sub-basin, were deposited in a paralic environment. Five major facies were recognised including a tidal channel, an upper shoreface, a middle shoreface, a lower shoreface and a background facies.Sedimentological and statistical analysis of the porosity and permeability of the Flacourt Formation shoreface sandstones enabled us to better understand the heterogenity of the petrophysical properties. The finding can be used to understand similar heterogeneous reservoirs elsewhere, and will provide a basis for reservoir simulations. We have shown that the approach of using genetically related sedimentary facies, can significantly improve the predicability of the porosity and permeability distribution.

石灰岩油層の岩質キャラクタリゼーション

This paper proposes a novel method for the grouping of rock types for the successful estimation of the in-situ absolute permeability accurately and for the smooth interpolation of relative permeability endpoints which are inevitably required for reservoir simulation models. The method consists of the grouping reservoir rocks based on the concept of electrical facies in accordance with their response to the wireline log attributes. The results showed excellent matching of the estimated absolute permeability and relative permeability endpoints to the core data prior to the application of the hydraulic unit method and multi-variate regression on each data group.The individual electrical facies showed the unique petrophysical characteristics each other, and their distribution frequency in each sedimentary zonation exhibited distinguishing feature possibly reflecting the sedimentary environment.The proposed approach lead to the great success on the description of the carbonate reservoir, of which petrophysical characteristic is so complicated that accurate enough estimation of the pointwise petrophysical properties could not be obtained by simple application of the ordinary hydraulic unit method nor multi-variate regression. The methodology is totally general and is considered to be applicable for a variety of reservoirs.

Sedimentology of a lacustrine fan‐delta system, Miocene Horse Camp Formation, Nevada, USA

ABSTRACTA 1600‐m‐thick succession of the Miocene Horse Camp Formation (Member 2) exposed in east‐central Nevada records predominantly terrigenous clastic deposition in subaerial and subaqueous fan‐delta environments and nearshore and offshore lacustrine environments. These four depositional environments are distinguished by particular associations of individual facies (14 defined facies). Subaerial and subaqueous fan‐delta facies associations include: ungraded, matrix‐and clast‐supported conglomerate; normally graded, matrix‐ and clast‐supported conglomerate; ungraded and normally graded sandstone; and massive to poorly laminated mudstone. Subaqueous fan‐delta deposits typically have dewatering structures, distorted bedding and interbedded mudstone. The subaerial fan‐delta environment was characterized by debris flows, hyperconcentrated flows and minor sheetfloods; the subaqueous fan‐delta environment by debris flows, high‐ and low‐density turbidity currents, and suspension fallout.The nearshore lacustrine facies association provides examples of deposits and processes rarely documented in lacustrine environments. High‐energy oscillatory wave currents, probably related to a large fetch, reworked grains as large as 2 cm into horizontally stratified sand and gravel. Offshore‐directed currents produced uncommonly large (typically 1–2 m thick) trough cross‐stratified sandstone. In addition, stromatolitic carbonate interbedded with stratified coarse sandstone and conglomerate suggests a dynamic environment characterized by episodic terrigenous clastic deposition under high‐energy conditions alternating with periods of carbonate precipitation under reduced energy conditions. Massive and normally graded sandstone and massive to poorly laminated mudstone characterize the offshore lacustrine facies association and record deposition by turbidity currents and suspension fallout.A depositional model constructed for the Horse Camp Formation (Member 2) precludes the existence of all four depositional environments at any particular time. Rather, phases characterized by deposition in subaerial fan, nearshore lacustrine and offshore lacustrine environments alternated with phases of subaerial fan‐delta, subaqueous fan‐delta and offshore lacustrine deposition. This model suggests that high‐energy nearshore currents due to deep water along the lake margin reworked sediment of the fan edge, thus preventing development of a subaqueous fan‐delta environment and promoting development of a well‐defined nearshore lacustrine environment. Low‐energy nearshore currents induced by shallow water along the

PROBE PERMEABILITY STUDIES FOR BETTER RESERVOIR UNIT EVALUATION WITH EXAMPLES FROM THE TIRRAWARRA SANDSTONE, COOPER BASIN AND THE MURTA AND MCKINLAY MEMBERS, EROMANGA BASIN

Probe permeametry studies have enabled a better understanding of permeability characteristics of three clastic reservoirs: The Tirrawarra Sandstone in the Tirrawarra and Gidgealpa fields of the Cooper Basin and the Murta and McKinlay Members in the Jena and Biala fields of the Eromanga Basin.An empirical relationship between Hassler-sleeve core plug measurements and probe permeametry is used for calibration. The probe permeameter takes sufficient non-destructive readings to define permeability better within an individual facies or reservoir flow unit.Frequency versus log (permeability) plots and semi-variograms allow visual comparison of individual reservoirs and facies. Histograms of plug and probe data show similar distributions, but the plug data generally have a lower variance than the probe data.For the Tirrawarra Sandstone there is a better match between probe permeability data and wireline predicted permeability than between the latter and core plug permeability.Azimuth directional permeability is evident for the braided fluvial sands of the Tirrawarra Sandstone. Permeability within the measured formations is primarily related to depositional facies. Secondary reduction in permeability within the Tirrawarra Sandstone is mainly due to ductile rock fragment compaction and authigenic clay precipitation with possible groundwater flow control in the Gidgealpa field.

Sequence Stratigraphic Analysis of Individual Depositional Successions: Effects of Marine/Nonmarine Sediment Partitioning and Longitudinal Sediment Transport, Mannville Group, Alberta Foreland Basin, Canada

In the Falher Member of the Mannville Group (Aptian-Albian) of western Canada, two shoreline successions contain the reservoir conglomerates for the giant Elmworth gas field. The Falher B succession has a basal sheetlike shoreface unit of hummocky cross-stratified sandstone that thins seaward and terminates about 30 km north (seaward) of the landward limit of the transgression. Another 25 km farther basinward, the succession shows a 20-30-m-thick sandstone, unattached to the prograding shoreface, and an overlying coarsening-upward shoreface succession with thin muds and coals, interpreted as back-barrier deposits. These basinward facies are the results of a relative sea level fall and the early stage of the subsequent rise. In the upper (Falher A) succession, immediately andward (south) of the barriers, fluvial valleys were incised into nonmarine mudstones and coals during the base-level fall. As relative sea level subsequently rose, in nonmarine areas the valleys were filled by estuarine and fluvial sands, then a widespread sheet of fine-grained nonmarine sediment was deposited. At the same time, the shoreline migrated back across the shelf. As it reached the original shorezone (structurally controlled), reworking of underlying deposits successively generated three gravelly barrier islands superimposed on the sandy shoreface succession. The conglomeratic reservoirs all rest above the unconformities, in the transgressive depositional system. Because this sequence is essentially a set of linked facies and not a composite of stacked individual facies successions, it is affected by sediment partitioning between facies. During relative sea level rise, little marine sedimentation occurred because sediment was trapped mainly in nonmarine areas. Conversely, during sea level fall, most deposition occurred in marine areas because of the absence of nonmarine accommodation. Westward (alongshore) toward the thrust belt, no falling or lowstand sea level succession developed. Instead, a wide regressive shoreface sandstone with a transgressive cap occurs. Subsidence rates were higher in this area, and relative sea level appears always to have risen, but at varying rates. The surface under the transgressive facies changes from a type 1 unconformity in eastern, lower subsidence areas to a type 2 unconformity in western, higher subsidence areas. Any two-dimensional sequence stratigraphic model, therefore, is inadequate to describe the lateral variation of the sequence and distribution of shoreface sandstones, because the subsidence gradient was not parallel to the direction of shoreface progradation.

Shannon Sandstone in Hartzog Draw-Heldt Draw Fields (Cretaceous, Wyoming, USA) Reinterpreted as Lowstand Shoreface Deposits

ABSTRACT The lower Campanian Shannon Sandstone at Hartzog Draw-Heldt Draw (Powder River Basin, Wyoming) is interpreted in the literature as a shelf-ridge complex deposited 160 km from the contemporaneous shoreline. This interpretation poses difficult problems, and does not discuss the significance of bounding discontinuities preserved in the sandbodies. Cross sections show that the Shannon Sandstone is present at several stratigraphic horizons, and is bounded by regionally extensive discontinuities. The presence of these bounding discontinuities suggests that fluctuations in relative sea level were important in controlling geometry of sandbodies on the Shannon shelf. In the Hartzog Draw-Heldt Draw area of Wyoming the base of the Shannon is a planar surface with respect to a lower bentonite marker and is characterized by the abrupt appearance of bioturbated sandstone on top of black basinal mudstone. Siderite pebbles < 1 cm in diameter are common at this contact. In Hartzog Draw the bioturbated sandstone interval has an average thickness of 10 m and is separated from an overlying unit of cross-bedded fine to medium sandstone (up to 20 m thick) by a ubiquitous (50 cm thick) horizon of mud clasts in a cross-bedded, highly glauconitic, medium sandstone. These three units are laterally continuous in Hartzog Draw except where removed by erosion. The top of the Shannon is marked by an abrupt change from the cross-bedded sandstone or bioturbated medium sandstone to laminated black mudstone. Mud clasts, siderite pebbles, chert grains, and glauconite mark this abrupt contact, suggesting erosional transgression. This surface can be traced westward, where in many areas it is a mud-on-mud contact marked by a lag of shell debris. This transgressive surface, which terminates Shannon deposition, is characterized by at least one regionally extensive onlapping marker in the Hartzog Draw-Heldt Draw area. The presence of depositional discontinuities in the Shannon Sandstone is suggested by (1) the sharp basal contact, (2) the lateral continuity of individual facies, (3) the abrupt vertical facies changes and the association of mud clasts, siderite pebbles, and glauconite accompanying these abrupt juxtapositions of facies, (4) the stratigraphic variability, and (5) the termination of Shannon deposition by erosive transgression, marked by onlapping markers. These depositional discontinuities are the bounding discontinuities. These bounding discontinuities have asymmetrical one-sided scour morphologies, with steeper landward margins that flatten basinward. These geometries and the linearity of the overlying sandstone and the troughs defined by these scoured surfaces suggest incised shoref ce profiles truncated by erosive transgression. Thus, the Shannon Sandstone in Hartzog Draw-Heldt Draw is reinterpreted as a lowstand shoreface deposit.

Genetic Facies Characterization of Unstructured Deltaic Reservoirs Using Detailed Stratigraphic Correlations and Computer Generated Maps

Ancient deltaic environments contain the predominant light oil producing reservoirs in the United States. Similar deltaic reservoirs in Venezuela, both in the Maracaibo Basin as well as in the Eastern Venezuelan Basin have produced significant amounts of hydrocarbons for the past 60 years. The geologic characteristics of these reservoirs influence the volume distribution and potential recovery of remaining mobil and ommibile oil. One of the primary geological factors that can affect recovery potential is the original condition under which the reservoir sediments were deposited. The depositional processes involved determine reservoir characteristics. In order to understand the major constraint to oil production, the reservoir heterogeneities, E-logs from 1007 wells in 4 townships (144 mi[sup 2]/373 km[sup 2]) in east central Louisiana were correlated in detail. An 800-ft Paleocene Middle Wilcox interval containing stacked deltaic reservoirs was investigated. The correlations include 16 tops and 16 net sand intervals. The resulting data set was loaded into the Terrasciences mapping program from which detailed structural as well as net sand isopach maps could be generated. The net sand geometries and the stratigraphic relationships between the sands, shales and lignites permitted the authors to isolate and identify individual genetic facies within each deltaic sequence. Themore » observed facies found to be oil prone include distributary channel deposits, overbank bay fill and transgressive barrier sands. Understanding the heterogeneities found in deltaic reservoirs are fundamental for planning advanced production methods such as infill and horizontal/slant well drilling and related enhanced oil recovery processes.« less

The complex nature of a Jurassic multistorey, alluvial sandstone body, Whitby, North Yorkshire

Abstract The complex alluvial sandstone body of the Saltwick Formation (Aalenian) exposed in the town of Whitby, North Yorkshire is 700–1000 m wide, up to c. 25 m thick and extensive in the palaeoslope direction (N-S). Exposed sections are approximately parallel and perpendicular to the palaeochannel orientation. The sandstone body is made up of a large number of channel storeys stacked in a complex vertical and lateral pattern and it is composed predominantly of fine- to medium-grained sandstone (&gt; 85% by volume) with subordinate amounts of mudstone and siltstone. The sedimentary facies architecture indicates that channels repeatedly reoccupied the same site and that periods of erosion and incision alternated with periods of rapid bed aggradation. The sandstone facies patterns are related to migration of channel bars, dunes and ripples. Fine-grained sediment facies which occur between some of the storeys are laterally discontinuous because of their position within abandoned channels and their partial erosion by later channels. Within the sandstone, permeability ranges from 0.8 to 682.1 mD with a geometric mean of 83.5 mD. Permeability heterogeneity is apparent from laminae to sandstone body scale. An attempt was made to characterize permeability on the basis of sedimentary facies. However, there is considerable scatter in the data and individual facies show a varying degree of heterogeneity of permeability. Two major sandstone storeys which appear very similar in origin and lithological character were found to have systematically different permeability distributions (geometric mean permeabilities of 36.0 and 108.5 mD). Facies variations can only partly explain this. There is a difference in the mean permeability of the same facies in the two sandstone storeys (e.g. geometric mean permeability of cross bedded sandstones in the two units are 63 mD and 100 mD respectively).

Subaqueous sediment gravity flow deposits: practical criteria for their field description and classification

ABSTRACTA new method for the description and classification of subaqueous sediment gravity flow deposits is proposed. The classification scheme employs a convenient letter code and divides deposits (individual beds) into descriptive categories of two hierarchical levels: facies and subfacies. Facies, as the higher rank categories, are distinguished chiefly on the basis of sediment type (i.e. bed grain size/texture). A total of 13 facies have been distinguished: G= gravel; GS = gravel‐sand couplet; GyS = gravelly sand; S = sand; SM = sand—mud couplet; MS = mud—sand couplet; TM = silt—mud couplet; MT = mud—silt couplet; M = mud; MyS = muddy sand; SyM = sandy mud; MyG = muddy gravel; GyM = gravelly mud. Subfacies, as the lower rank categories, are distinguished within the individual facies on the basis of the bed's internal structures. The number of subfacies is unlimited, and their labelling code includes particular facies symbols (see above) preceded by lower—case letters denoting specific sedimentary structures and their vertical arrangement. Subfacies thus refer to the bed's intervals, or divisions, which are labelled as follows: m = massive (unstratified and ungraded); g = graded (unstratified and graded); s = plane‐stratified; x=cross‐stratified; 1 = parallel– and/or cross‐laminated; q = liquefied. For example, subfacies gsG (graded to plane‐stratified gravel) are gravel beds that have a lower graded interval and an upper plane‐stratified interval; subfacies xG (cross‐stratified gravel) are gravel beds that are cross‐stratified throughout; subfacies slS (plane‐stratified to laminated sand) are sand beds that have a lower plane‐stratified interval and an upper laminated (parallel‐ and/or cross‐laminated) interval.

A new petrogenetic grid for low‐grade metabasites

Abstract We have used internally‐consistent thermodynamic data to present calculated phase equilibria for the system Na2O‐CaO‐MgO‐Al2O3‐SiO2‐H2O (NCMASH), in the range 0–500° C and 0.1–10 kbar, involving the phases anorthite, glaucophane, grossular, heulandite, jadeite, laumontite, lawsonite, paragonite, prehnite, pumpellyite, stilbite, tremolite, wairakite, zoisite with excess albite, clinochlore, quartz and pure water. Average activity terms derived from published mineral chemical data were included for clinochlore, glaucophane, prehnite, pumpellyite, tremolite, and zoisite. The new petrogenetic grid delineates stability fields and parageneses of common index minerals in zeolite, prehniteactinolite, prehnite‐pumpellyite, pumpellyite‐actinolite, blueschist and greenschist facies metabasites. The stability fields of mineral assemblages containing prehnite, pumpellyite, epidote, actinolite (+ albite + chlorite + quartz) were analysed in some detail, using activity data calculated from five specific samples. For example, the prehnite‐actinolite facies covers a P‐T field ranging from about 220 to 320° C at pressures below 4.5 kbar. The transition from the prehnite‐actinolite and pumpellyite‐actinolite to greenschist facies occurs at about 250–300° C at 1–3 kbar and at about 250–350° C at 3–8 kbar. P‐T fields of individual facies overlap considerably due to variations in chemical composition.

Sedimentology and petrology of the Cañete Dolomites and Limestones Formation (Muschelkalk Facies, Middle to Upper Triassic) Southern Iberian Ranges, Eastern Spain

The Canete Dolomites and Limestones Formation (Muschelkalk Facies, Middle to Upper Triassic) of the South Iberian Ranges (East Spain) displays a carbonate ramp evolution with characteristic features like shoal, lagoon and tidal flat subenvironments. Six individual facies and five main facies associations have been defined. The general facies association succession allows to recognize a main regressive disposition for the general sedimentological evolution of the Canete Dolomites and Limestones Formation as well as a short transgressive initial development. An oscillating subsidence, eustatic variations in sea-level and sediment supply are related to the general sedimentological evolution.

Hydrogeologic facies models to delineate large-scale spatial trends in glacial and glaciofluvial sediments

Recent interest in contaminant transport in ground water has led hydrogeologists to the conclusion that predicting the movement of solutes requires information on the distribution of spatial trends and heterogeneities in porous media. Description of spatial trends has long been of interest to sedimentologists who have produced a large body of geologic information on the subject. In this paper, facies models are used to construct conceptual models of hydrogeologic facies for glacialmeltwater-stream sediment and till. These hydrogeologic facies models, which delineate large-scale trends in heterogeneity, are appropriate for use in designing hydrogeologic field tests and for estimating input to regional ground-water flow and transport models. This paper treats each facies as a homogeneous, anisotropic hydrogeologic unit. The models presented herein conceptualize the hydrogeologic relationships among facies and illustrate one method of converting the apparent chaos in nature into an orderly system that can be tested scientifically and modeled mathematically. The principles used to create conceptual models of hydrogeologic facies for the types of sediment considered in this paper can be extended to other sedimentary environments. It should be recognized, however, that such models do not address the small-scale heterogeneity present within individual facies. Additional basic research is required to measure hydraulic conductivity variation within representative hydrogeologic facies and to develop statistical descriptions to represent the variations. Such detailed descriptions of hydraulic conductivity may be necessary to describe ground-water flow at a local scale for analysis of contaminant transport.

Is cratonic sedimentation consistent with available models? An example from the Upper Proterozoic of the West African craton

The Atar Group, part of the Upper Proterozoic sequence covering the West African craton, stable since 2000 Ma, is characterized by an alternation of extensive carbonate beds and mixed siliciclastic and carbonate facies. The carbonate beds comprise essentially columnar stromatolite biostromes and bioherms which reflect sublittoral environments. The mixed facies contain a variety of laterally discontinuous facies which imply more variable environmental conditions. The settings of the mixed facies are not always clear but they do not contain thick sequences of high-energy facies. Few obvious facies sequences are discernable; those that are present are considered to be punctuated aggradational cycles (PACs) and they always start with biostromes of columnar stromatolites with very few sediments. Composite sequences are interpreted as due to shallowing upward or increasing energy environments that may be laterally contiguous, despite the fact that the contacts are not gradational. However, much of the stratigraphic sequence cannot be subdivided into cycles and seems to consist of unrelated individual facies, bound by sharp boundaries. The basin analysis reveals that biostromes of columnar stromatolites start after an instantaneous geological event corresponding to a sea-level rise. Consequently, their appearance can be considered as a time-line. We describe, in the Atar Group and its equivalents, three sedimentation trends, all of which are interpreted to be of shallowing upward character. The Atar Group appears to have been deposited in an epeiric sea (i.e. an extremely flat ramp). There are two contrasting styles of sedimentation: (1) after the submergence of the whole area, columnar stromatolites built extensive biostromes; (2) during the stable phase, sediments are deposited in a mosaic of laterally-discontinuous facies. Tidal influence cannot be recognized in the sequence, neither can a salinity increase toward the land; both common features in published epeiric sea models. A cratonic sedimentation area such as this is characterized by its size and flatness. Only during the stable phase of the cycle does small-scale topographic relief lead to deposition of a mosaic of facies. The sedimentation is storm- and wave-dominated.

Lahar facies of the Mount Dutton Formation (Oligocene-Miocene) in the Marysvale Volcanic Field, southwestern Utah

Lahar facies of the Mount Dutton Formation (Oligocene-Miocene; Marysvale Volcanic Field, Utah) differ in clast-size gradation, presence or absence of a self- supporting clast framework, and, to a lesser extent, thickness. The two most common of these fades (matrix supported and either inversely graded [facies 1A] or nongraded [1B]) are absent from the closest modem analog, Mount St. Helens (MSH), Washington, as are three others (matrix supported normally graded [3], clast supported, inversely graded [5], and clast supported, nongraded [7A]). One common Mount Dutton fades (matrix supported, inversely to normally graded [4]) is the peak-flow overbank and transition lahar fades at MSH. One less common Mount Dutton lahar fades (clast supported, inversely to normally graded [6]) is the sole facies or channel facies at MSH. Mount Dutton facies 2 (thin, matrix supported, nongraded) is present at MSH as the lahar runout facies. Plots of maximum particle size (MPS) versus bed thickness (BTh) show significant positive correlations for seven fades, as expected of debris-flow deposits. Analogy with MSH lahars suggests that Mount Dutton lahars were cohesionless debris flows (grainflows) that formed when turbulent floods or pyroclastic surges bulked by eroding material from their bed. Dispersive pressure produced inverse grading where shearing occurred; inverse grading throughout beds is inherited from transient shearing episodes. Normal grading may be inherited from graded suspensions developed during bulking of turbulent flows or may reflect progressive deposition, rather than frictional or cohesive freezing, of a laterally graded flow. Slopes and intercepts of correlation lines for individual facies on an MPS-BTh plot form natural groups. Three groups, comprising facies 5 and 7A, 1A and 1B, and 3 and 4, differ chiefly in the intercept of the correlation line with the MPS axis. This intercept indicates thickness-independent strength, the result of cohesion and possibly other factors. All three groups are stronger in this respect, by a factor of 2 to 100, than a sampling of ancient nonvolcanic debris flows. These three groups correspond to diamictite subfacies of nonvolcanic debris flows from the Permian of Colorado. Facies 6 is unlike other fades of the Mount Dutton Formation but similar to ancient nonvolcanic debris flows in its strength properties. It resembles whaleback bars that formed during nonuniform flow in the North Fork lahars at MSH. Two Mount Dutton fades (2 and 7B [clast supported, normally graded]) do not have a significant positive correlation of MPS and BTh. They are probably deposits of hyper-concentrated fluids, not lahars in the strict sense.

Facies Control on Reservoir Properties within a Barrier Island Complex, Sparta Formation, Pointe Coupee Parish, Louisiana: ABSTRACT

Analysis of conventional core, well logs, and petrophysical data has yielded an interpretation of the Sparta B sandstone in south-central Louisiana as a prograding barrier island complex. Four sedimentary facies possess reservoir potential: lower shoreface, upper shoreface, storm washover, and tidal-inlet channel sandstone facies. Distinct porosity and permeability values are associated with each reservoir facies. Predictable porosity and permeability trends associated with the Sparta B sandstone barrier island complex allows for identification of distinct fluid flow units within the sequence. In addition, individual sedimentary facies possess characteristic well log signatures; therefore, the geometry, thickness variability, and lateral continuity of each fluid flow unit can be determined and heterogeneities within the reservoir interval can be resolved.

Projection of joints from surface to tunnel level — a Case Study

A joint-study of a granitic batholith recorded orientations and spacings of joint-sets at outcrop and at three separate sections of a water diversion tunnel. A second joint-survey in the tunnel determined the average joint spacing along the spring lines. Joint-set spacing studies show a 40–50% reduction of joint-set spacing underground, compared to those at the surface. Individual equal-area plots of surface joints for the two major facies of the granitic rock showed different maxima. But when the joints in the two facies were combined into one equal-area plot, the predominent facies, which represented 73% of the joints plotted, masked the prominent sets of the second facies. Hence, joint surveys for individual rock facies are warranted. Comparison of equal-area plots of surface and tunnel joints in the same rock facies reveals a striking similarity in joint-set maxima.

Early Proterozoic bimodal volcanic rocks in central Colorado, U.S.A., Part I: Petrography, stratigraphy and depositional history

Early Proterozoic metamorphic rocks of the Salida area include two exceptionally well-preserved sections of undeformed volcanogenic rocks: a northern section nearly 4000 m thick and a southern section 1700 m thick. The stratigraphic and lithologic character of the sections are the same, indicating that they were formed contemporaneously in the same subaqueous environment. The suite is strongly bimodal, consisting of felsic volcaniclastic rocks (50%), mafic volcanic and volcaniclastic rocks (22%), and gabbro/diabase sheets (23%). The remaining rocks are mostly pelitic metasediments. The felsic rocks, which dominate the upper portions of both sections, comprise four distinct facies: crystal-lithic tuff breccia, crystal-lithic lapilli-breccia tuff, bedded tuff, and massive tuff. Lithic fragments are typically foreign, dominated by pelitic material. Crystal clasts are feldspar, mainly plagioclase, and quartz. Sequences of these facies, representing cycles of eruptive activity, are uniform in thickness and extend along strike for the entire width of Precambrian exposure, up to 4 km, but individual facies units, especially the coarser ones, are limited in their lateral extent. Individual sequences average about 100 m thick, but continuous exposures of felsic rocks locally exceed 600 m. Primary sedimentary structures and vertical and lateral variations indicate deposition, typically following remobilization as debris flows and turbidites, in a shallow marine environment of low relief. Mafic volcanic and volcaniclastic rocks form accumulations up to 600 m thick and extend uniformly for several kilometers along strike, although individual units are more restricted laterally. The principal rock types are basalt flows, basaltic lithic breccia, pillow lava/pillow breccia, bedded basaltic lapillistone, and laminated basaltic tuff, all deposited in a submarine setting. Accumulations, typically including several types, are interbedded with felsic rocks, but mechanical mixing of mafic and felsic volcaniclastics is rare, suggesting separate eruptive centers for the mafic and felsic volcanics. The northern section may be more proximal than the southern section, as indicated by the greater abundance of flows, the thicker sheets of gabbro, and the greater abundance of mafic rocks relative to felsic rocks. Gabbro/diabase sheets, up to 350 m thick and dominantly concordant, intrude throughout both sections. They are characterized by lack of internal differentiation, simple mineralogy consisting of plagioclase and hornblende, pseudomorphous after pyroxene, and sub-ophitic texture. The minor pelitic material indicates that the area was accessible to continental sediment sources, although their location is not known. The Salida bimodal volcanic suite is part of a more extensive accumulation of similar 1700–1800 Ma supracrustal rocks that extends west to the Gunnison area and perhaps south into northern New Mexico.

Hydrocarbon Potential of a Hoh Turbidite Sequence (Miocene), Western Olympic Peninsula, Washington: ABSTRACT

The type section of S.L. Glover's Browns Point formation, a turbidite sequence within the Hoh rock assemblage, was examined for its hydrocarbon potential. In addition, the turbidite facies are matched to the organic facies throughout the entire vertical section (4000 ft) to correlate kerogen distribution, type, and maturation level with organic and turbidite facies. Outer and middle fan turbidite facies association were analyzed for organic facies lateral continuity. Distribution of organic concentrations and organic carbon types suggests terrestrial input are dominant. Thermal alteration indexes (TAI) and vitrinite reflectance data (R/sub 0/) demonstrate a marginally mature thermal alteration level (R/sub 0/ = 0.5-0.6). Visual inspection of kerogen displays a mixed to structured kerogen with Type III/IV to Type IV kerogen predominant. A gas source with marginal oil potential is inferred. However, individual turbidite facies display a significant relationship to the amount, type, and maturation level of organic matter present within each facies and facies association. Hydrocarbon exploration in the Browns Point sequence demonstrates how frontier basin analysis can be expedited through proper field identification of facies and subsequent geochemical examination of the amount, type, and maturation level of the organic matter present in each facies. These processes can help the explorationistmore » develop sedimentological, environmental, and structural reconstructions of the basin.« less