Lithofacies Distribution Research Articles

Abstract: Geologic and Computer Modeling of Upper Jurassic Smackover Reef and Carbonate Shoal Lithofacies, Eastern Gulf Coastal Plain

ABSTRACT Reefs and carbonate shoals have long been known from the Upper Jurassic Smackover Formation in the Gulf Coastal Plain; however, these carbonate lithofacies have unique acoustic properties that make them difficult to define using 3-D seismic reflection technology. In the eastern Gulf Coastal Plain, microbial reef and shoal buildups occur on pre-Jurassic paleotopographic basement features on a carbonate ramp margin. Development of these buildups is a result of the interplay among paleotopography, sea-level changes, and carbonate productivity. Geological and computer modeling indicates that Smackover reef and shoal development is restricted to the flanks of high (emergent) relief structures, while reef and shoal development occurred on the crest and flanks of low-relief structures (submergent). For these submergent features, modeling indicates that carbonate productivity during reef growth was significantly greater than during shoal development. In addition to the initial paleorelief, the rate of sea-level rise, the level of carbonate productivity, and duration of reef growth appear to be critical factors in determining thickness and distribution of the reef lithofacies. Subsidence and compaction are minor factors. Shoal development is greatly influenced by reef distribution, sea-level changes, level of carbonate productivity, and duration of shoal deposition. Subsidence, compaction and sediment redistribution are also factors. Modeling of parameters affecting reef and shoal development associated with pre-Jurassic paleohighs in combination with 3-D seismic reflection studies increases the chances of drilling a successful exploration well.

Evolution of Pliocene to Pleistocene sedimentary environments in an arc-continent collision zone: evidence from the analyses of lithofacies and ichnofacies in the southwestern foothills of Taiwan

Lithofacies and ichnofacies analyses reveal an excellent example of gradually subsiding and rapid infilling and uplifting of the sedimentary basin in the southwestern foothills of Taiwan, along the strike of the active Plio-Pleistocene orogeny. Ten well-exposed sections, roughly evenly distributed along a southwestern transect, were studied to construct the three-dimensional distribution of lithofacies and ichnofacies. Basin evolution is discussed in terms of temporal and spatial facies variations. In early Pliocene time, the shallow marine facies of the northern part of the study area were deposited on the passive Eurasian continental shelf. The sedimentary basin was deeper in the south, with a lower slope and/or basin floor setting. The ongoing arc-continent collision caused flexural bending of the crust, inducing gradual subsidence of the basin during Pliocene and early Pleistocene time. Around 0.98 Ma, the onset of more rapid orogenic movements accelerated the basin subsidence, as indicated by rapid deepening of the facies northward along the basin axis, as well as by the widespread development of mass-gravity transportation deposits and channel cutting structures. However, the basin was soon infilled and uplifted as part of the orogen within 0.8 my. Basin subsidence is much greater than previously indicated.

Evolution of Pliocene to Pleistocene sedimentary environments in an arc-continent collision zone: Evidence from the analyses of lithofacies and ichnofacies in the southwestern foothills of Taiwan

Abstract Lithofacies and ichnofacies analyses reveal an excellent example of gradually subsiding and rapid infilling and uplifting of the sedimentary basin in the southwestern foothills of Taiwan, along the strike of the active Plio-Pleistocene orogeny. Ten well-exposed sections, roughly evenly distributed along a southwestern transect, were studied to construct the three-dimensional distribution of lithofacies and ichnofacies. Basin evolution is discussed in terms of temporal and spatial facies variations. In early Pliocene time, the shallow marine facies of the northern part of the study area were deposited on the passive Eurasian continental shelf. The sedimentary basin was deeper in the south, with a lower slope and/or basin floor setting. The ongoing arc-continent collision caused flexural bending of the crust, inducing gradual subsidence of the basin during Pliocene and early Pleistocene time. Around 0.98 Ma, the onset of more rapid orogenic movements accelerated the basin subsidence, as indicated by rapid deepening of the facies northward along the basin axis, as well as by the widespread development of mass-gravity transportation deposits and channel cutting structures. However, the basin was soon infilled and uplifted as part of the orogen within 0.8 my. Basin subsidence is much greater than previously indicated.

North and South China collision along the eastern and southern North China margins

Lithofacies distribution and petrography and intracontinental deformation in the North China block (NCB) and the South China block (SCB) show an active continental margin flanked by the eastern NCB, and a passive continental margin was present along the west side of the SCB, both margins initially trending N-NE. The Tanlu fault possibly initiated as part of the subduction zone during Paleozoic time along the eastern margin on the NCB. The NCB and the SCB were divided along the eastern–southern North China margin by the Qinling–Dabie–Tanlu–Sulu–Imjingang–Yanji zone from southwest to northeast. These two continental blocks made contact first in the northeast during late Early Permian time, finally in the southeast at Late Triassic time, and the clockwise collision probably lasted to Middle Jurassic time. The eastern suture, the Yanji–Imjingang–Sulu–Tanlu zone, was dominated by contractional deformation. The southern suture, the Qinling–Dabie zone, was a transpression belt dominated by right-lateral strike-slip.

The Paradox of Nonmarine Ichnofaunas in Tidal Rhythmites: Integrating Sedimentologic and Ichnologic Data from the Late Carboniferous of Eastern Kansas, USA

The occurrence of trace fossil assemblages dominated by arthropod trackways and surface grazing trails within Carboniferous tidal rhythmites has puzzled sedimentologists and ichnologists, who interpreted them either as marine or nonmarine. The Virgilian (Stephanian) Tonganoxie Sandstone Member (Stranger Formation) at Buildex Quarry (eastern Kansas) consists, for the most part, of planar-laminated coarse grained siltstones deposited on an upper tidal flat, close to or at the fluvial-estuarine transition of a macrotidal estuarine paleovalley. Recurrent thickness fluctuations demonstrate the strong influence of tidal process and provide evidence that these deposits are tidal rhythmites, with thicker strata representing spring tides and thinner ones recording neap tides. The Buildex sequence hosts a moderately diverse ichnofauna composed of arthropod trackways (Dendroidichnites irregulare, Diplichnites gouldi, Diplopodichnus bifurcus, Kouphichnium isp., Mirandaichnium famatinense, Stiallia pilosa, Stiaria intermedia), grazing traces (Gordia indianaensis, Helminthoidichnites tenuis, Helminthopsis hieroglyphica), subsurface feeding traces (Treptichnus bifurcus, T. pollardi, irregular networks), apterygote insect resting and feeding traces (Tonganoxichnus buildexensis, T. ottawensis), fish traces (Undichna britannica, U. simplicitas), and tetrapod trackways. In contrast to trace fossil assemblages from brackish-water estuarine settings, the Buildex ichnofauna is characterized by moderate to relatively high ichnodiversity, ichnotaxa commonly present in terrestrial/freshwater environments, dominance of surface trails and absence of burrows, dominance of temporary structures produced by a mobile deposit-feeder fauna, a mixture of traces belonging to the Scoyenia and Mermia ichnofacies, moderate density of individual ichnotaxa, and absence of monospecific suites. This ichnofauna is thought to record the activity of a typical freshwater/terrestrial benthos. The presence of this mixed freshwater/terrestrial ichnofauna in tidal rhythmites is regarded as indicative of tidal flats that were developed in the most proximal zone of the inner estuary under freshwater conditions, more precisely in a zone between the maximum limit of landward tidal currents and the salinity limit further towards the sea. Although lithofacies distribution in estuarine valleys is mainly salinity-independent, the distribution of benthos is not. Accordingly, ichnologic studies have the potential to provide a high-resolution delineation of fluvio-estuarine transitions.

Structural kinematics and depositional history of a Laramide uplift-basin pair in southern New Mexico: Implications for development of intraforeland basins

The kinematic and erosional history of the Rio Grande uplift, a large northwest-trending, basement-involved, thrust-bounded, block uplift of Laramide age (latest Cretaceous–early Tertiary) in south-central New Mexico, is documented by clastic rocks that accumulated in the complementary Love Ranch basin. The synorogenic to postorogenic McRae and Love Ranch Formations are as much as 1460 m thick; they filled the Love Ranch basin and onlapped the Rio Grande uplift. Present outcrops of the two formations cover an area of 100 km2 and reveal the stratigraphic architecture of the basin fill in three dimensions. Lithofacies distribution, clast size and composition, paleoflow data, syndepositional structures, nature of the basal unconformities, and ages of basin fill provide essential data for constraining uplift history. Laramide shortening began between Campanian and latest Maastrichtian time, and initially created open, symmetrical, northwest-trending folds, as well as a broad, approximately symmetrical uplift. This incipient Rio Grande uplift was capped by Upper Cretaceous volcanic rocks of intermediate and silicic composition, which were the primary source of volcanic detritus in the latest Cretaceous–Paleocene(?) McRae Formation. At this stage of uplift, the northeastern flank dipped gently northeastward away from the crest and served largely as a sediment transport surface; McRae strata accumulated only on distal parts of the surface in an embryonic Love Ranch basin to the northeast. Following an interruption in tectonism lasting as much as 10 m.y., renewed shortening in Paleocene(?) time elevated the Rio Grande uplift and formed the Love Ranch basin. At least 900 m of upward-fining, clastic Love Ranch strata of Paleocene-Eocene age accumulated in the basin. The Love Ranch lithofacies record a gradual southwestward shift of alluvial-fan depocenters, which resulted from growth of basin-margin structures and increasing basin asymmetry. Syndepositional synclines and angular unconformities record the growth of both intrabasinal folds and basin-margin thrusts. Clast compositions document progressive erosional unroofing of the Rio Grande uplift from Upper Cretaceous volcanic rocks into Precambrian granite and metamorphic rocks. Canyons 0.4 km deep locally drained the uplift, and maximum topographic relief may have approached 2 km. By late Eocene time, Love Ranch piedmont-slope deposits onlapped the uplift, burying all but the higher granite peaks. At this stage, the Love Ranch basin broadened and was the site of broad alluvial plains crossed by braided rivers draining to saline lakes. Our analysis of syntectonic sedimentary rocks in the Love Ranch basin supports recent models of evolution of Laramide basement-involved block uplifts in which early stages produce approximately symmetrical structures, and sediment derived from the uplift is transported across most of the uplift flank to be deposited in a distal setting. At this stage the future footwall of uplift-boundary faults dips basinward in a ramp-like fashion, providing a sediment transport surface. As boundary thrust faults and fault-propagation folds evolve and grow, basin asymmetry rapidly develops, causing depocenters to shift toward footwall positions near the overthrust margins. This evolution from symmetrical to asymmetrical structures is reflected in an overall upward-fining sequence in the basin fill.

Eastern Mississippi delta: late Wisconsin unconformity, overlying transgressive facies, sea level and subsidence

Analysis of 92 engineering core logs located in the Balize sector of the eastern Mississippi delta focuses on the late Wisconsin unconformity and lithofacies of strata lying immediately below and above this stratigraphic horizon. This major sequence boundary is a key feature used to correlate strata across shelf and slope to the basin. Observations emphasize lithofacies distributions of the latest Pleistocene sediments underlying the unconformity, the late Pleistocene to early Holocene transgressive facies, and the immediately overlying deltaic deposits. Maps and a cross-section compiled with this information highlight the critical relation between lithofacies distributions and late Wisconsin sea-level oscillations. Core analysis reveals that the transgressive facies comprises distinct environments of deposition, offshore to onshore. Findings identify criteria to determine the approximate position of the shoreline at the late Wisconsin maximum sea-level lowstand and at the extent of early Holocene maximum marine inundation in the Balize complex. Mapping of lithologies along the late Wisconsin unconformity serves to improve correlation of sediment facies with changes in acoustic response along high-resolution seismic profiles. An estimate of long-term averaged land subsidence suggests lowering of at least 1 mm/year near the shelfedge during the past ∼ 18 000 years. This mapping also refines sea-level response models for the Mississippi delta, including criteria to locate key paleogeographic features such as shelfedges and maximum flooding surfaces in other Recent marine deltas and in older, stacked deltaic sequences.

Lacustrine fan-deltaic sedimentation in a Proterozoic rift basin: the Sopa-Brumadinho Tectonosequence, southeastern Brazil

The Sopa-Brumadinho Tectonosequence (100 to 200 m thick) is the fourth unit in ascending order of the up-to-2000 m thick Mesoproterozoic Espinhaço Supergroup, representing the depositional products of the third and main phase of the rift stage of the Espinhaço basin. The Sopa-Brumadinho Tectonosequence in the study area consists of diamond-bearing lacustrine fan-delta deposits, bounded by prominent angular and erosional unconformities. Detailed lithofacies studies were carried out in the central part of the southern Serra do Espinhatço, southeastern Brazil, to characterize the paleogeography and basin-fill architecture of the Sopa-Brumadinho Tectonosequence. Thirteen lithofacies are present: massive clast-supported conglomerates (lithofacies UGCS), inversely graded clastsupported conglomerates (lithofacies IGCS), normally graded clast-supported conglomerates (lithofacies NGCS), massive to crudely stratified clast-supported conglomerates (lithofacies CSCS), sandy matrix-supported conglomerates (lithofacies S-MS), muddy matrix-supported conglomerates (lithofacies M-MS), talus-breccia clast-supported conglomerates (lithofacies TB-CS), massive sandstones (lithofacies Sm), horizontal stratified sandstones (lithofacies Sh), trough cross-stratified sandstones (lithofacies St), planar cross-stratified sandstones (lithofacies Sp), graded-stratified sandstones (lithofacies Sg), and pelites (lithofacies P). They represent the products of several types of subaerial and subaqueous sediment gravity flows (ranging from turbidity currents to cohesive debris flows) and ephemeral braided streams. The deposits are usually arranged in coarsening- to fining-upward tectono-depositional intervals. Each tectono-depositional interval represents a progradational lobe, which starts at the base with lacustrine pelites and evolves with time from distal (subaqueous) to proximal (subaerial) characteristics. The fining-upward part represents the abandonment phase of the depositional lobes. The origin, characteristics, distribution and spatial arrangement of the various lithofacies, the interbedding of gravityflow and stream-flow deposits, the predominance of debris-flow conglomerates over aqueous current-formed ones, the semi-radial, fan-like dispersion pattern of the paleocurrents, and the recognition of associated subaerial and subaqueous depositional settings are all indicative of a lacustrine alluvial-fan/fan-delta environment, or a fan-delta system. An interplay between several intrabasinal and extrabasinal controls probably determined the fan evolution. The sedimentation of the lacustrine fan-delta systems of the Sopa-Brumadinho Tectonosequence probably took place in a N-S elongated continental rift basin, compartmentalized by N-S trending normal faults and W-E trending transfer faults, under a tropical semi-arid climate. The basin-fill architecture was probably determined by episodic subsidence, clockwise block tilting and asymmetric-graben evolution, as well as by inherited characteristics of the source area. Additionally, a new descriptive scheme for classification of conglomerate lithofacies is here proposed.

Depositional framework of Permian coal-bearing strata, southern Victoria Land, Antarctica

Abstract The Permian Weller Coal Measures of Antarctica are exposed in a narrow belt that extends along the edge of the Polar Plateau from the Mawson Glacier to the Mulock Glacier in southern Victoria Land. These rocks record a rapid change from glacial to postglacial conditions, with the establishment of polar peat forming conditions, during the late Paleozoic in Antarctica. The 222 m thick Weller consists of interstratified fine- to coarse-grained quartzo-feldspathic sandstone, laminated siltstone, mudstone and coal. Regional lithofacies patterns and paleocurrent orientations define a narrow elongate basin. In the lower Weller, coarse-grained sandstone sheets occur along both eastern and western basin margins. These deposits are similar to those of modern sandy braided streams. Toward the axis of the basin, this lithofacies grades into coarse-grained tabular sandstone. Lateral-accretion beds suggest that deposition of this lithofacies occurred within meandering streams. Coarse-grained basin-margin deposits interfinger with fine-grained basin-axis lithofacies of interstratified laminated siltstone and graded sandstone beds deposited from suspension and underflow currents within a lacustrine environment. In the upper Weller, braided stream deposits dominate much of the basin, grading downstream into meandering streams. Establishment of vegetation in southern Victoria Land occurred immediately following late Paleozoic deglaciation. Upward in the Weller, the distribution of plant fossils and coal seams is lithofacies dependent. Thick coals and abundant plant fossils occur in meandering stream lithofacies, while they are scarce within the lacustrine and braided stream deposits. Abrasion during transport probably contributed to the destruction of vegetable material in the braided stream environments, and to material introduced into the axial lake. Depositional patterns within the Weller Coal Measures are similar to those in the Takrouna Formation in northern Victoria Land. Similarities suggest deposition in either the same or a similar basin. Comparison of the Weller Coal Measures with Permian strata in the central Transantarctic Mountains suggest deposition in two separate basins, which are now juxtaposed across the Byrd Glacier, the sight of a possible Cenozoic strike-slip fault. Causes for the formation of the depositional basin containing the Weller Coal Measures are unknown; however, the geometry of the basin and the regional distribution of lithofacies are similar to those of rift basins and to basins excavated by glacial erosion. The ultimate cause of basin subsidence however, may be related to long-distance stress produced during late Paleozoic orogenic activity along the paleo-Pacific margin of Antarctica.

Sequential Stratigraphy of Outcropping Strata Equivalent to Arab-D Reservoir, Wadi Nisah, Saudi Arabia

ABSTRACT Two outcrops in Wadi Nisah of Central Saudi Arabia expose sedimentary successions of the Jubaila and Arab formations which are stratigraphic equivalents of the Arab-D reservoir in Ghawar field. These carbonate exposures consist of lithofacies and stacking patterns similar to the succession found in Arab-D reservoir. Nineteen vertical sedimentary cycles were identified by collectively evaluating disconformable relationships, textural characteristics, and lithofacies assemblages. These, in turn, define 3 depositional cycle types: (1) stromatoporoid, (2) skeletal bank, and (3) thinning-upward. The lateral distribution of lithofacies within the stromatoporoid cycle shows that stromatoporoid grainstones form organic buildups flanked by burrowed mud-dominated carbonates. Stromatoporoid banks locally exhibit thickness increases of about 3 meters (10 feet) over a horizontal distance of 500 meters (1, 700 feet). Conversely, the five foot-thick skeletal grainstone capping the skeletal bank cycle forms a continuous sheet exceeding 2 kilometers (1.2 miles) in width. Thinly bedded carbonate sand units within the thinning-upward cycle exhibit pronounced lateral discontinuity. Beds pinch out against topographic highs formed by stromatoporoid buildups over distances of less than a hundred meters (328 feet). Results of the outcrop stratigraphic analysis enable visualization of lateral and vertical stratigraphic relationships and potential fluid-flow pathways. This study demonstrates that outcropping cycles and their lithofacies components have several important implications for ongoing subsurface reservoir characterization and modeling of the Arab-D. Firstly, cycle definition identifies stratigraphic units whose bounding surfaces describe envelopes that constrain the interwell distribution of lithofacies. Secondly, the lithofacies define geometries for the interwell distribution of petrophysical characteristics. Thirdly, the understanding of lateral relationships of these carbonates aids in reservoir simulation modeling.

Lithofacies Distribution and Reservoir Heterogeneity Within Pennsylvanian Phylloid Algal Mounds, Western Orogrande Basin, New Mexico: ABSTRACT

Pennsylvanian strata within the San Andres Mountains (western Orogrande basin) contain very well-developed phylloid algal bioherms, but these bioherms remain understudied owing to their location within the bounds of the U.S. Army White Sands Missile Range. The exposed Upper Pennsylvanian section within Hembrillo Canyon affords a three-dimensional view of mound structure, and thus an excellent opportunity for characterizing lithofacies distribution and reservoir heterogeneity that may prove useful for exploration/exploitation efforts in analogous petroliferous systems. The mounds are developed within a mixed carbonate-clastic shallow marine section punctuated by shoaling-upward cycles. Each mound site consists of a slack of individual biohermal growth events characterized by a subtidal wackestone initiation phase, core boundstone phase, and peritidal to subaerially exposed packstone/grainstone terminal phase. Individual biohermal growth events range up to 30 m in thickness; vertical stacking of these bioherms has produced aggregate mounds reaching up to 100 m in stratigraphic thickness and 300 m in diameter. Individual blohermal thicknesses decrease abruptly and markedly away from mound sites, and calcareous mudstones dominate in intermound regions. The controlling influences of paleogeography and glacioeustasy, respectively, produced the pronounced lateral and vertical heterogeneity characterizing these and analogous phylloid algal mound systems. Reservoirs within these systems are highly compartmentalized:more » wackestone initiation phases and peritidal to subaerial termination phases that envelope core facies may serve as porosity and permeability barriers that effectively partition the reservoir. Recognition of the scale, character, and probable controls on these lateral and vertical changes is important for effective exploration and exploitation in phylloid algal mound systems.« less

Stochastic Modelling of Turbidite Reservoirs Constrained by Seismic Informations (Campos Basin, Offshore Brazil): ABSTRACT

A new methodology of reservoir probabilistic modelling was applied at an oil field scale in the brazilian continental margin. The Namorado field, located in the Campos basin in the passive margin of eastern Brazil, is a mature oil field with 60 wells and 1326 km of 3D seismic lines. The reservoir (Albian/Cenomanian), deposited in an overall-transgressive context, produces from turbidite sandstones and is composed by massive sandstones, classical Bouma turbidities, debris-flows, marls and shales. Three depositional sequences were correlated across the field, each of them corresponding to cycles of relative sea-level falls. The stochastic simulation of the spatial lithofacies distribution as a non stationary truncated gaussian function was carried out to generate realistic reservoir images. The important lateral variation in the facies and thickness of the sedimentary bodies is characteristic of non stationarity. In this case, the seismic data integration is crucial for a successful modelling of the phenomenon. A reservoir oriented processing of the seismic data was carried out, including a post-stack stratigraphic inversion. Seismic attributes on inversion results were calibrated at wells in terms of lithofacies. In the non stationary approach, the images of the reservoir lithofacies distribution are conditioned to wells and deduced from the Vertical Proportionmore » Matrix, which represents the percentage of each lithofacies at a given spatial coordinate and in each depositional sequence. This matrix, defined over a regular grid, is computed from wells and seismic derived constraints. The incorporation of the seismic information in the non stationary gaussian function model has ensured a successful reproduction of the reservoir architecture.« less

Impact of Lithofacies Architecture and Distribution on Fluid Flow: Examples from the Cretaceous Ferron Sandstone, East-Central Utah: ABSTRACT

Detailed outcrop studies of the Cretaceous Ferron Sandstone reveal the spatial relationships between facies in a fluvial-deltaic system. Outcrop data include detailed and regional geological mapping, gamma ray transacts, and core plug transacts. These data are supplemented by laboratory measurement of plug porosity and mini-permeater measurement of plug permeability, petrographic analysis of plugs, {open_quotes}behind-the-outcrop{close_quotes} boreholes, detailed mini-permeameter measurements on these cores, and wireline logs. Prograding delta front clinoforms within -the seaward-stepping Kf-1 parasequence show a systematic decrease in permeability from the proximal delta front (k{sub avg} {approx} 20 md) to the distal delta front (k{sub avg} 1 00 md). Individual parasequences are up to a few 10s of meters thick, are amore » really extensive, and are often isolated from one another by nearly continuous mudstone and siltstone up to a meter thick. Multiphase flow simulations illustrate how lithofacies architecture and the juxtaposition of related lithofacies impact reservoir efficiency.« less

A summary of Paleogene sequence stratigraphy in northwest Europe and the North Sea

Abstract Sequence stratigraphic analysis of Paleogene central North Sea well-log, seismic and biostratigraphic data recognizes patterns of cyclic sedimentation seen in the physical stratigraphy and biostratigraphy. Numerous authors have documented cyclic sedimentation resulting from relative changes in sea level in northwest Europe, but interregional integration of these observations with North Sea subsurface data is lacking in the literature. Presented here is a chronostratigraphic correlation framework for the Paleogene of northwest Europe, built by integrating subsurface and outcrop data using sequence stratigraphic first principles. Biostratigraphic data from many sources is ordered with the composite standard method. Graphic correlation of this data documents certain correlations and helps suggest previously unrecognized ties. Paleogene North Sea sediments record five major regressions and their intervening major transgressions. Overprinting this low frequency signal are 19 higher frequency sequence cycles that control lithofacies distribution. In northwest Europe, western basins (London-Hampshire, Paris and Belgian) have shallow marine to non-marine settings which reveal basinward and landward facies shifts that indicate sea level changes. The biostratigraphy of these shallow water deposits is linked to deep water central North Sea biostratigraphy by correlating through deeper water deposits outcropping in Denmark that have been tied to western basin stratigraphy. Using this biostratigraphic framework, key bounding surfaces are correlated between basins using sequence stratigraphic principles. Depositional sequences are recognized onshore that are completely sediment starved in the North Sea. The mixing of low and high frequency sea level signals requires that all of northwest Europe be studied to recognize the ‘true’ signal. Final correlations resolve 30 depositional sequences with five long-term sea-level changes that can vary from one sub-basin to another.

An integrated stratigraphy for the Paleocene and Eocene of the North Sea

Abstract A high-resolution sequence stratigraphy has been developed for the Paleocene and Eocene rocks of the North Sea Basin, using electric log data and biostratigraphy. A total of nine stratigraphic sequences has been defined, together with a further 11 subsequences. The sequences are bounded by high-gamma log markers representing maximum condensation surfaces that can be readily correlated over the whole basin. Biostratigraphic analysis has allowed the recognition of 37 last occurrence or top abundance dinocyst bioevents that have also been correlated with the microfaunal succession. Integration of the sequence stratigraphy and biostratigraphy demonstrates that a consistent relationship exists between individual sequence boundaries and bioevents, suggesting that these surfaces are time lines. The usefulness of developing an accurate stratigraphic framework is illustrated by a series of maps showing the lithofacies distribution for individual sequences on a regional scale. At the prospect or field level, it can be used in conjunction with seismic data to map the more subtle Tertiary hydrocarbon plays in the North Sea Basin.

The evolution of basement gneiss domes of the Sergipano fold belt (NE Brazil) and its importance for the analysis of Proterozoic basins

Four ductilely reworked mantled basement gneiss domes are prominent structures inside the Neoproterozoic Sergipano fold belt. The belt is an ESE-WNW trending wedge comprising sediments, metasediments, metavolcanics, and meta-intrusive rocks, and is divided into a cratonic domain to the south, passing into highly deformed miogeoclinal and eugeoclinal domains to the north. A detailed stratigraphic and structural analysis around two of these domes, in a key area spreading over the three domains, revealed that the basement domes most likely began as extensional structures controlling the distribution of sedimentary lithofacies and the variations of their thicknesses in the precursor basin of the Sergipano fold belt. Closure of the basin led to the evolution of these extensional basement structures in the core of basement-cored fold nappes. The largest of the domes is capped by a thick quartzite sequence and controlled the partition of the contractional deformation of the belt. Combined with new geophysical data, such analysis indicates that the three lithotectonic domains are continuous parts of the basin. Therefore, detailed studies of the basement-cover interface allow us to characterize the tectonic evolution of Proterozoic basins in the same way as their Phanerozoic counterparts.

Sedimentary formations of the Cretaceous Sakhalin Basin (Far East Asia)

The marine sedimentary formations of the Middle Albian to Maastrichtian in the Cretaceous Sakhalin Basin (CSB) were investigated. These successions of strata consist of interbedded sandy, clayey and calcareous rocks which are underlain by heterogeneous metamorphosed (up to greenschist facies) Paleozoic to Mesozoic (pre-Aptian) rocks. The studied sections display several different facies reflecting geological settings ranging from an inner shelf to a continental slope. Three depositional complexes bound by regional subaerial unconformities are recognized within the marine successions. Since the Albian, the CSB has been a rapidly subsiding marginal part of the Okhotsk Sea plate. The Naiba Valley succession, corresponding to a sublittoral zone, shows extremely high sedimentation rates up to 190 m/Ma. The stratigraphic distribution of lithofacies indicates that the CSB became shallower from the Middle Albian to the Maastrichtian.

Fragmentation versus transportation mechanisms in the pyroclastic sequence of Monte Pilato-Rocche Rosse (Lipari, Italy)

Monte Pilato is a rhyolitic pumice cone formed about 1400 years ago during the last volcanic activity which occurred at Lipari. Its activity consisted of numerous explosive episodes, ending with the extrusion of the Rocche Rosse obsidian lava flow. We have identified nine different pyroclastic lithofacies, of which four main lithofacies represent 90% of the erupted products; they include: (1) deposits consisting of decimeters- to meters-thick well-sorted, graded beds of pumice lapilli, which are of fall origin; (2) massive deposits consisting of decimeters-thick beds of pumice lapilli in an ash matrix, interpreted as being of pyroclastic flow origin; (3) deposits of centimeters-thick ash beds of surge origin; (4) decimeter-thick fine ash layers with scattered subcentimetric accretionary lapilli of turbulent “flow” origin. The complex grain size distributions of the main lithofacies were distinguished using a multivariate statistic analysis ( R-mode factor analysis). Variations of the grain size characteristics in relation to the distance from the vent area have been investigated in order to discriminate the roles played by fragmentation and transportation processes. In the lithofacies 1 fall deposits, the grain size distribution is always unimodal and varies from a Rosin-type distribution near the vent to a Gaussian one in the distal outcrops. This is caused by the transport process and the different particle trajectories. In lithofacies 2, 3 and 4, the grain size distributions are always polymodal. The polymodality is not caused by preferential processes occurring during transportation, but seems, rather, to be connected with the original fragmentation of the pyroclastic material. The pumice lapilli present both in lithofacies 1 and 2 have mean vesicularity index values always in excess of 65%, and the value is homogeneous throughout the sample. These data suggest that for these two lithofacies, the pumice lapilli originated from exsolution of magmatic gases. In contrast, the fine ash particles in lithofacies 2, and also the main mode of lithofacies 3 and 4 deposits, show typical features of hydromagmatic fragmentation processes. The grain size population is always centered at 4–5 ϕ (0.032–0.064 mm) and, interestingly, the same grain size has been obtained recently (Zimanowski et al., 1991) for fragments produced by experimental magma/water interaction. Field and laboratory data suggest that during Monte Pilato-Rocche Rosse eruptions magmatic and hydromagmatic mechanisms were frequently operative at the same time.

Three-Dimensional Distribution of Lithofacies, Bounding Surfaces, Porosity, and Permeability in a Fluvial Sandstone--Gypsy Sandstone of Northern Oklahoma

Where it was studied at two sites in north-central Oklahoma, the Gypsy sandstone of the Pennsylvanian Vamoosa Formation provides significant insights into the controls on reservoir quality in a meandering river deposit. At an outcrop site west of Tulsa, a three-dimensional architectural framework, based on erosional surfaces, allowed the recognition of a channel-fill sequence consisting of six channel sand bodies and representing at least three channel belts. Belts are vertically stacked and are incompletely separated by low-permeability flood-plain deposits, which are locally eroded. Porosity and permeability are primarily related to depositional facies. Within channel belts, mudclast lags associated with erosion surfaces at the base of channels have the potential to act as vertical permeability baffles between channel sand bodies, as do mud-draped lateral accretion surfaces within channel sand bodies. At a subsurface site 31 km (19 mi) to the west of the outcrop site, nine wells were drilled and cored through the Gypsy interval. At this site, it was possible to identify and correlate three channel belts comprising amalgamated channel sand bodies. Although lower level architectural elements could be recognized in cores, they could not be correlated with confidence given a 100-m (330-ft) well spacing.

Geologic Framework of a Transect of the Central Brooks Range: Regional Relations and an Alternative to the Endicott Mountains Allochthon

Geologic Framework of a Transect of the Central Brooks Range: Regional Relations and an Alternative to the Endicott Mountains Allochthon