Facies Trends Research Articles

Influence of the Atlantic inflow and Mediterranean outflow currents on Late Quaternary sedimentary facies of the Gulf of Cadiz continental margin

The late Quaternary pattern of sedimentary facies on the Spanish Gulf of Cadiz continental shelf results from an interaction between a number of controlling factors that are dominated by the Atlantic inflow currents flowing southeastward across the Cadiz shelf toward the Strait of Gibraltar. An inner shelf shoreface sand facies formed by shoaling waves is modified by the inflow currents to form a belt of sand dunes at 10–20 m that extends deeper and obliquely down paleo-valleys as a result of southward down-valley flow. A mid-shelf Holocene mud facies progrades offshore from river mouth sources, but Atlantic inflow currents cause extensive progradation along shelf toward the southeast. Increased inflow current speeds near the Strait of Gibraltar and the strong Mediterranean outflow currents there result in lack of mud deposition and development of a reworked transgressive sand dune facies across the entire southernmost shelf. At the outer shelf edge and underlying the mid-shelf mud and inner shelf sand facies is a late Pleistocene to Holocene transgressive sand sheet formed by the eustatic shoreline advance. The late Quaternary pattern of contourite deposits on the Spanish Gulf of Cadiz continental slope results from an interaction between linear diapiric ridges that are oblique to slope contours and the Mediterranean outflow current flowing northwestward parallel to the slope contours and down valleys between the ridges. Coincident with the northwestward decrease in outflow current speeds from the Strait there is the following northwestward gradation of contourite sediment facies: (1) upper slope sand to silt bed facies, (2) sand dune facies on the upstream mid-slope terrace, (3) large mud wave facies on the lower slope, (4) sediment drift facies banked against the diapiric ridges, and (5) valley facies between the ridges. The southeastern sediment drift facies closest to Gibraltar contains medium–fine sand beds interbedded with mud. The adjacent valley floor facies is composed of gravelly, shelly coarse to medium sand lags and large sand dunes on the valley margins. By comparison, the northwestern drift contains coarse silt interbeds and the adjacent valley floors exhibit small to medium sand dunes of fine sand. Because of the complex pattern of contour-parallel and valley-perpendicular flow paths of the Mediterranean outflow current, the larger-scale bedforms and coarser-grained sediment of valley facies trend perpendicular to the smaller-scale bedforms and finer-grained contourite deposits of adjacent sediment drift facies. Radiocarbon ages verify that the inner shelf shoreface sand facies (sedimentation rate 7.1 cm/kyr), mid-shelf mud facies (maximum rate 234 cm/kyr) and surface sandy contourite layer of 0.2–1.2 m thickness on the Cadiz slope (1–12 cm/kyr) have deposited during Holocene time when high sea level results in maximum water depth over the Gibraltar sill and full development of the Atlantic inflow and Mediterranean outflow currents. The transgressive sand sheet of the shelf, and the mud layer underlying the surface contourite sand sheet of the slope, correlate, respectively, with the late Pleistocene sea level lowstand and apparent weak Mediterranean outflow current.

High-resolution sequence stratigraphic correlation in the Upper Jurassic (Kimmeridgian)-Upper Cretaceous (Cenomanian) peritidal carbonate deposits (Western Taurides, Turkey)

Upper Jurassic (Kimmeridgian)±Upper Cretaceous (Cenomanian) inner platform carbonates in the Western Taurides are composed of metre-scale upward-shallowing cyclic deposits (parasequences) and important karstic surfaces capping some of the cycles. Peritidal cycles (shallow subtidal facies capped by tidal-¯at laminites or fenestrate limestones) are regressive- and transgressive-prone (upward-deepening followed by upward-shallowing facies trends). Subtidal cycles are of two types and indicate incomplete shallowing. Submerged subtidal cycles are composed of deeper subtidal facies overlain by shallow subtidal facies. Exposed subtidal cycles consist of deeper subtidal facies overlain by shallow subtidal facies that are capped by features indicative of prolonged subaerial exposure. Subtidal facies occur characteristically in the Jurassic, while peritidal cycles are typical for the Lower Cretaceous of the region. Within the foraminiferal and dasyclad algal biostratigraphic framework, four karst breccia levels are recognized as the boundaries of major second-order cycles, introduced for the ®rst time in this study. These levels correspond to the Kimmeridgian±Portlandian boundary, mid-Early Valanginian, mid-Early Aptian and mid-Cenomanian and represent important sea level falls which affected the distribution of foraminiferal fauna and dasyclad ¯ora of the Taurus carbonate platform. Within the Kimmeridgian±Cenomanian interval 26 third-order sequences (types 1 and 2) are recognized. These sequences are the records of eustatic sea level ¯uctuations rather than the records of local tectonic events because the boundaries of the sequences representing 1±4 Ma intervals are correlative with global sea level falls. Third-order sequences and metre-scale cyclic deposits are the major units used for long-distance, high-resolution sequence stratigraphic correlation in the Western Taurides. Metre-scale cyclic deposits (parasequences) in the Cretaceous show genetical stacking patterns within third-order sequences and correspond to fourth-order sequences representing 100±200 ka. These cycles are possibly the E2 signal (126 ka) of the orbital eccentricity cycles of the Milankovitch band. The slight deviation of values, calculated for parasequences, from the mean value of eccentricity cycles can be explained by the currently imprecise geochronology established in the Cretaceous and missed sea level oscillations when the platform lay above fluctuating sea level.

Chalk palaeoenvironments and depositional model, Valhall-Hod fields, southern Norwegian North Sea

Abstract Palaeoenvironmental analysis in the North Sea chalk is hampered by the prevalence of widespread allochthonous units, intervals of massive reworking of microfossils and an often monotonous lithology of massive chalks of uncertain depositional history. Most difficult of all is devising precise regional biostratigraphic correlations in a complex section with large, but very subtly expressed, disconformities and condensed zones. Facies change quickly both laterally and vertically. Without very precise regional correlations, it is impossible to formulate any detailed palaeoenvironmental model. Through a multidisciplinary palaeontological approach, employing graphic correlation methodology and a composite standard database, a highly accurate and detailed regional chronostratigraphy was generated. Biofacies recognized within the section were then scaled in absolute time, allowing for more reliable recognition of facies trends at any one time horizon. Biofacies were formulated based upon recovered benthic foraminifera, planktic foraminifera, calcispheres, siliceous microfossils and macrofossil debris. Biofacies mapping for several higher-order sequences recognized in the Upper Cretaceous chalks has allowed recognition of four major periods of deposition: (1) early Turonian-early Santonian high pelagic productivity and rapid deposition of chalks over a relatively continuous ridge from the Valhall to Hod fields; (2) early Campanian-earliest Maastrichtian deep-water autochthonous chalk deposition off-structure with structural crests mantled by submarine hardgrounds; (3) very heterogeneous autochthonous and allochthonous middle Maastrichtian chalk deposition controlled by local grabens formed from crestal collapse; and (4) widespread sediment flows and redeposition of sediments with crestal shoaling and winnowing in the latest Maastrichtian culminating in crestal erosion of Cretaceous chalks in the early Danian.

98/00916 Methods for studies of coal structure: Jasienko, S. et al. Chemical Fiz. Wegla, 1995, 90–145. (in Polish). Edited by Jasienko, S., Oficyna Wydawnicza Politechniki Wroclawskiej, Wroclaw, Poland

Fifteen samples of South African coals from the Waterberg, Soutpansberg (Northern Province) and Main Karoo (Mpulanga and KwaZulu-Natal) provinces were investigated by petrographic and geochemical methods. The aim of this work was to establish the relationship between these two methods for evaluation of coal maturity in the case of South African coals. The petrographic analyses included maceral and microlithotype evaluation by routine quantitative methods as well as random reflectance measurements. Maceral analysis included identification of vitrinite, liptinite and inertinite groups. The same samples were used for microlithotype analysis. Vitrite, liptite, inertite, clarite, durite,vitroinertite, duroclarite, clarodurite and carbominerite were identified. To assess the facies trends, liptinite-poor and liptinite-rich durite and clarodurite were counted separately. Sporoclarite and cutinodetrite were also treated independently. Both maceral and microlithotype analyses were performed to confirm the geochemical and petrographic compatibility in interpretation of the primary organic material which was deposited in the biochemical stage of coal formation. The reflectance measurements were carried out in incident light using a Zeiss Universal microscope. The 546 nm interference filter was applied. Distributions of aliphatic and aromatic hydrocarbons present in coal extracts were analysed by gas chromatography-mass spectrometry (GC-MS) for assessment of both source/environment of deposition and coal thermal maturity. Normal alkane distribution shows odd-over-even carbon number predominance in the case of coals in the range of 0.63–0.80% vitrinite reflectance. Several groups of compounds related to conifer resin were identified indicating predominant source of organic matter of the coals, for example tetracyclic diterpanes, retene and simonellite.The coal rank was assessed by 13 thermal maturity parameters based on isomerisation reactions of sterane and triterpane diastereomers together with ratios and indices of aromatic hydrocarbons and methyldibenzothiophenes based on different thermal resistance of their α- and β-alkyl isomers. The correlation of reflectance (Rrv%) with two sterane diasteromer parameters: C29αββ/(ααα+αββ), C29ααα20S/(20S+20R) and aromatic hydrocarbon parameters of thermal maturity such as: MDR (Methydibenzothiophene ratio), MPI-1 (MethylPhenanthrene Index-1) revealed good agreement. Lower correlation coefficients were obtained for TMR-2 (Trimethylnaphthalene ratio), TeNR (Tetramethylnaphthalene ratio) and DMPR (Dimethylphenenthrene ratio) with respect to Rrv%. Other correlations of biomarker and aromatic hydrocarbon parameters with Rrv% were poor.

Permian deposition in the north central Brooks Range, Alaska: Constraints for tectonic reconstructions

Two opposing tectonic models have been offered to explain the regional structural relations in the north central Brooks Range fold‐thrust belt of northern Alaska. The first suggests that rocks of the northern Endicott Mountains were thrust from south to north over the area of the present Mount Doonerak high and are therefore highly allochthonous. The second implies that the rocks of the northern Endicott Mountains were deposited in a basin that lay north of the Mount Doonerak high and later were thrust a short distance southward onto the northern flank of the high and are thus parautochthonous. To provide stratigraphic constraints for these models, this study examines Permian facies of the north central Brooks Range. Permian rocks in the north central Brooks Range comprise a thin (40 to 160 m thick), fining‐upward succession of clastic, storm‐influenced shelf deposits. When the rocks of the northern Endicott Mountains are restored south of the Mount Doonerak area, a minimum distance of 80 km, the Permian deposits grade systematically from distal facies (Siksikpuk Formation) in the southwest to proximal facies (Echooka Formation) in the northeast. Facies trends in the reconstructed Permian basin include, from southwest to northeast, (1) an increase in carbonate content and corresponding decrease in silica content, (2) a general darkening and thickening of shaley intervals, (3) an increase in proximal features of storm beds, including coarser, thicker, more abundant, and more closely spaced beds, and (4) an increase in abundance and diversity of the faunal assemblage with a corresponding decrease in age. These stratigraphic relations imply that rocks of the northern Endicott Mountains are allochthonous and structurally overlie a proximal stratigraphie succession similar to that exposed in the Mount Doonerak area and northeastern Brooks Range.

Axial–transverse fluvial interactions in half‐graben: Plio‐Pleistocene Palomas Basin, southern Rio Grande Rift, New Mexico, USA

Accurate magnetostratigraphic dating of Plio‐Pleistocene alluvium in the Palomas half‐graben permits correlation of transverse and axial deposits, thus enabling analysis of the movement of alluvial facies belts in time and space for the first time. Northern areas show evidence for basinward progradation of footwall‐sourced Matuyama‐age alluvial fan deposits over axial channel belt deposits of the ancestral Rio Grande, despite both deposits having similar deposition rates. This gradual ‘forced’ westward migration of the axial belt was in opposition to ongoing eastward growth of hangingwall‐sourced fans and tectonic tilt imposed by the bounding Caballo normal fault. Fan growth was coincident with a recently proposed gradual climatic shift that may have increased sediment flux out of transverse catchments. It is also possible that continuing tectonic footwall uplift and divided retreat caused catchment areas to increase, contributing to these trends.Southern areas of the Palomas half‐graben feature late Gilbert/early Gauss deposits indicative of rapid westwards progradation of large low‐gradient, footwall‐sourced fans over axial deposits. This ‘forced’ migration of the ancestral Rio Grande may have occurred due to footwall catchment and fan growth consequent upon initiation and growth of the Red Hills Fault. Subsequent eastward movement of the axial channel belt in late Gauss and Matuyama times overwhelmed these large fans. We attribute this to continued tilting on the Red Hills Fault and to development of the Jornada Fault to the south‐east, the axial river belt avulsing north and eastwards through a developing Red Hills/Jornada crossover transfer zone.We conclude generally that facies architecture of axial and transverse elements in half‐graben must reflect both climatic influences and the effects of fault development. Careful field mapping, accurate dating and palaeoclimatic studies are all necessary to determine the relative importance of these controls. Although adequate as broad guides, previous purely ‘fixist’ tectonosedimentary models allow for no fault growth, decay or climatic modulation of facies trends and are thus generally inadequate to explain important aspects rift basin stratigraphy.

Sedimentary model and high‐frequency cyclicity in a Mediterranean, shallow‐shelf, temperate‐carbonate environment (uppermost Miocene, Agua Amarga Basin, Southern Spain)

ABSTRACTUppermost Tortonian to lower Messinian temperate carbonates crop out in the Agua Amarga Basin (SE Spain). They consist of four units. The lower three units can be tentatively assigned to the lowstand systems tract of a fourth‐order sequence, constituting in turn the lowstand (‘megatrough unit’), transgressive (‘breccia unit’) and highstand (‘bedded unit’) stages of a higher‐order cycle. All these materials were deposited in a small pull‐apart basin related to the sinistral Carboneras strike‐slip fault system. The best represented is the bedded unit (up to 25 m thick), which consists of bioclastic, bryozoan/bivalve‐dominated calcarenites/calcirudites with abundant fragments of echinoids, barnacles, benthic foraminifers, coralline algae, brachiopods and solitary corals. Facies trends within this unit are roughly arranged in an E‐W direction, with the coastline to the north of the basin. The depositional model is that of a gentle ramp with prograding beaches and shoals in its higher parts. Seaward of the shoals was the ‘factory area’, where most organisms lived and maximum carbonate production took place. From this area some of the skeletons were washed landwards by waves and/or currents during storms and incorporated into the shoals and beaches, and others moved downslope along the ramp as mass‐flows, accumulating to form the ‘fan‐bedded zone’.The factory‐area and fan‐bedded sediments intercalate five well‐defined, thick beds of calcarenites/fine‐grained calcirudites. They show bar morphologies (single or amalgamated), or make up sand‐waves with very consistent tabular cross‐bedding pointing landwards. These beds formed in a very shallow, wave/current‐influenced, coastal environment. The bars and sand waves in the fan‐bedded zone developed during lowstands, while those located higher up in the ramp interbedded with the factory facies are related to transgressive stages. Prograding beaches, shoals, factory facies and fan‐bedded layers developed during the highstands. Net skeletal production occurred mainly during the highstands.Sediment‐accretion values of these sediments are similar to those of present and ancient shallow‐marine, temperate carbonates considering that the whole bedded unit was deposited in a 100 000‐year interval (equivalent to the short eccentricity cycle). The five cycles inside the bedded unit would therefore correspond to the c. 20 000‐year precession cycles of the Milankovitch band.

Modeling Heterogeneity in a Low-Permeability Gas Reservoir Using Geostatistical Techniques, Hyde Field, Southern North Sea

Hyde field is a small (133 Gcf (FOOTNOTE *) reserves) gas field in the United Kingdom southern North Sea. The reservoir, the Permian Rotliegende Group, contains eolian, fluvial, and sabkha facies. The eolian facies constitute the dominant flow units. Illite cementation results in low reservoir quality. In the eolian facies permeability averages 2.5 md and rarely exceeds 10 md. To maximize recovery from this field, with its thin gas column and low reservoir quality, Hyde has been developed with three long-reach horizontal wells. A reservoir model that could accurately predict future production had to include a full range of heterogeneities from the effects of thin muddy laminations at the core-plug scale to the spatial distribution of eolian, fluvial, and sabkha facies over the entire field at the largest scale. By carefully defining the model's layering scheme, we incorporated strongly deterministic elements where facies trends were influenced by laterally extensive stratigraphic surfaces. The distribution of facies within stratigraphic units was modeled using geostatistical techniques. Average permeability values were assigned to the poorest quality fluvial and muddy sabkha facies. Permeability values for the eolian and sandy sabkha facies were determined by upscaling models of their internal permeability structure. The resulting distribution of permeabilities across the field, present in a 2.2-million-cell model, was upscaled into a coarser grid suitable for simulating the full field. This new model produced a significantly better match to dynamic data than did an earlier simple layer model that was used to take the field to sanction. The results of our work suggest that stochastic modeling allowed us to represent a level of heterogeneity that was not captured by the earlier simple layer model. By capturing this level of heterogeneity, we were able to achieve a significantly better match of model predictions to production data.

Integrated Reservoir Characterization of a Tulare Steamflood Finds Bypassed Oil - South Belridge Field, Kern County, California: ABSTRACT

Reservoir quality and producibility are directly related to the characteristics of the depositional lithofacies. Electric log gamma ray/resistivity profiles were used to define facies trends within the Tulare steamflood at South Belridge. Channel and non-channel facies profiles are distinctive across the lease with the channel sands having the better quality reservoir and greater net pay values. Sidewall core permeabilities were averaged over the main producing Tulare intervals with the channels averaging 2000-3000 millidarcies and non-channels 200-500 millidarcies. This supports the lithofacies trend and net pay maps. Although the approach is qualitative, it illustrates the dramatic permeability contrast between the channel and non-channel lithofacies. Temperature maps using downhole temperature surveys and flowline temperatures indicate channel facies temperatures up to 300[degrees] with the non-channel facies having 90[degrees] to 100[degrees] temperatures (near ambient). Higher temperatures also relate to higher average daily production rates for channel associated wells. Channel wells averaged greater than 30 BOPD while non-channel wells averaged 10 BOPD or less. New and replacement well nations have been high graded resulting in favorable production responses. Integration of the lithofacies, permeability and temperature data plus ongoing preventive production optimization work has led to a more efficient Tulare steamflood and identification of bypassed oilmore » on the King-Ellis lease in the South Belridge Field.« less

Preservation of premetamorphic oxygen and carbon isotopic trends in granulite facies marbles from Rogaland, Southwest Norway

Preservation of premetamorphic oxygen and carbon isotopic trends in granulite facies marbles from Rogaland, Southwest Norway

Lacustrine bioherms, spring mounds, and marginal carbonates of the Ries-impact-crater (Miocene, Southern Germany)

The petrographic investigation of the Miocene Rieslake-carbonates gave rise to a modifiedDunham-scheme for classifying non-marine carbonates. If the fabric is not exclusively the result of hydromechanical or biogenic effects,Dunham-terms were extended by interpretative attributes describing processes responsible for the secondary fabric development (pedogenesis, early meteoric diagenesis). The lowermost investigated section reveals a distinct zonation of the lake shore during humid stages (carbonates of the infra-, eu-, and supralittoral), interrupted by playa-like interstages of arid conditions (desiccated mudflat). Dolomitic successions of sinter-veneered bioherms, built by green algae and cyanobacteria, form an incomplete ‘reef belt’ at the northern crater rim. Bioherm sequences were controlled by a seasonally oscillating lake level and fluctuations of higher order, which correspond to small-scale climatic fluctuations in the range of several hundreds to thousands of years. A superposed facies trend is recognized, which is the result of the decreasing eutrophy, alkalinity, and carbonate supersaturation. This reflects the climatic change to generally humid conditions at the end of Miocene. Originally thrombolitic sublacustrine spring mounds were caused by upwelling groundwater from permeable bedrock. Cement framestones of the mound core are considered to result from fabric alteration within the upwelling ground water and by emersions. Marginal carbonates of the Ries basin, inclusively bioherms, probably were originally Mg-calcitic. Aragonite was restricted to gastropod shells and sublacustrine spring mounds. Dolomitization is interpreted as essentially due to a fluctuating phreatic mixing-zone caused by meteoric groundwater, which underflowed saline water at the soda lake margin. The algal flora comprises cyanobacteria, possible cyanobacteria, green algae, few charophytes, endoliths, and problematic forms. A Recent contamination by endolithic fungi and lichens is evident. Occasionally preserved insect larval tubes, mass accumulations of pupal cases of flies, and arthropod eggs demonstrate that these groups were once a prominent part of the fossil soda lake ecosystem of the Nordlinger Ries.

Geochronologic and isotopic evidence for Triassic-Jurassic emplacement of the eugeoclinal allochthon in the Mojave Desert region, California

The geologic history of the outer continental margin (eugeoclinal) rocks in the El Paso Mountains and northern Mojave Desert has long been important in models for the development of the active continental margin in the western Cordillera. Current interpretations call for either strike-slip or thrust juxtaposition of eugeoclinal rocks against miogeoclinal/cratonal (platformal) rocks, or some combination of both strike-slip faulting and thrusting. Two broad and interrelated aspects of the history of the eugeoclinal rocks are at issue: (1) How much primary displacement is necessary to account for the present position of the eugeoclinal rocks? and (2) When were the eugeoclinal rocks thrust against platformal rocks? This study primarily addresses the second issue. Lithologic correlation indicates that the outcrop belt of eugeoclinal rocks is bounded by platformal rocks to the east and south. Platformal rocks are also present to the west, but many of these exposures restore to a position structurally beneath the eugeoclinal rocks when Tertiary extension is restored, implying stacking by a thrust. New U-Pb zircon geochronology and whole rock geochemistry and Sr, Nd, and Pb isotopic data for plutons in the El Paso Mountains and northern Mojave Desert lend insight into the timing of this thrusting. The outcrop belt of eugeoclinal rocks coincides with the only known Permian and Triassic plutons in the El Paso Mountains and northern Mojave Desert, which are dated at 260 to 240 Ma by U-Pb zircon in this report. Initial Sr and ϵ Nd(t) isotopic values from these plutons are distinct ≤0.704 and ≥+2, respectively) from continental lithosphere isotopic signatures ≥0.705 and ≤−2) of both Middle Jurassic plutons in the same area and Triassic plutons in the southern and eastern Mojave Desert. Feldspar common lead data for the Permian and Triassic plutons within the eugeoclinal outcrop belt also indicate limited crustal involvement and do not overlap previously reported values for common lead data from Mesozoic plutons in the eastern Mojave Desert region where Proterozoic basement is widespread. The observations and data reported here indicate that Late Permian–Early Triassic plutons in the northern Mojave Desert and El Paso Mountains were generated within or passed primarily through oceanic lithosphere, but later Jurassic plutons were derived from and/or interacted extensively with continental lithosphere. We hypothesize that the eugeoclinal rocks were deposited on oceanic crust that was thrust eastward over Precambrian cratonal basement and overlying strata between approximately 240 Ma and 175 Ma, a time of little documented tectonic activity in the Mojave Desert region. Postulated Permian or late Middle Jurassic east-directed thrusting is incompatible with our data and observations. The data also fill an important gap in palinspastic reconstructions of the early Mesozoic arc and indicate that this arc was northwest-trending and oblique to Paleozoic facies trends in the Mojave Desert.

Cyclostratigraphy of Middle Devonian Carbonates of the Eastern Great Basin

ABSTRACT Middle Devonian carbonates (250-430 m thick) of the eastern Great Basin were deposited along a low energy, westward-thickening, distally steepened ramp. Four third-order sequences can be correlated across the ramp-to-basin transition and are composed of meter-scale, upward-shallowing carbonate cycles (or parasequences). Peritidal cycles (shallow subtidal facies capped by tidal-flat laminites) constitute 90% of all measured cycles and are present across the entire ramp. The peritidal cycles are regressive- and transgressive-prone (upward-deepening followed by upward-shallowing facies trends). Approximately 80% of the peritidal cycle caps show evidence of prolonged subaerial exposure including sediment-filled dissolution cavities, horizontal to vertical desiccation cracks, rubble and ka st breccias, and pedogenic alteration; locally these features are present down to 2 m below the cycle caps. Subtidal cycles (capped by shallow subtidal facies) are present along the middle-outer ramp and ramp margin and indicate incomplete shallowing. Submerged subtidal cycles (64% of all subtidal cycles) are composed of deeper subtidal facies overlain by shallow subtidal facies. Exposed subtidal cycles are composed of deeper subtidal facies overlain by shallow subtidal facies that are capped by features indicative of prolonged subaerial exposure (dissolution cavities and brecciation). Average peritidal and subtidal cycle durations are between approximately 50 and 130 k.y. (fourth- to fifth-order). The combined evidence of abundant exposure-capped peritidal and subtidal cycles, transgressive-prone cycles, and subtidal cycles correlative with updip peritidal cycles indicates that the cycles formed in response to fourth-to fifth-order, glacio-eustatic sea-level oscillations. Sea-level oscillations of relatively low magnitude (< 10 m) are suggested by the abundance of peritidal cycles, the lack of widely varying, water-depth-dependent facies within individual cycles, and the presence of noncyclic stratigraphic intervals within intrashelf-basin, slope, and basin facies. Noncyclic intervals represent missed subtidal beats when the seafloor lay too deep to record the effects of the short-term sea-level oscillations. Exposure surfaces at the tops of peritidal and subtidal cycles represent one, or more likely several, missed sea-level oscillations when the platform lay above fluctuating sea level, but the amplitude of fourth- to fifth-order sea-level oscillation(s) were not high enough to flood the ramp. The large number of missed beats (exposure-capped cycles), specifically in Sequences 2 and 4, results in Fischer plots that show poorly developed rising and falling limbs (subdued wave-like patterns); consequently the Fischer plots are of limited use as a correlation tool for these particular depositional sequences. The abundance of missed beats also explains why Milankovitch-type cycle ratios ( 5:1 or 4:1) are not observed and why such ratios would not be expected along many peritidal-cycle-dominated carbonate platforms.

Rhythmic Sedimentation in a Mixed Tide and Wave Deposit, Hazel Patch Sandstone (Pennsylvanian), Eastern Kentucky Coal Field

ABSTRACT The Hazel Patch sandstone is an informal unit of the lower Middle Pennsylvanian Breathitt Formation, which is locally dominated by facies containing alternating sand and shale layers, or rhythmic bedding. Three types of rhythmites are defined. Cyclic rhythmites show a sinusoidal vertical stacking trend of thickening and thinning laminae, amalgamated rhythmites show partially or possibly cyclic trends that have been reworked or erosively truncated by overlying formsets, and noncyclic rhythmites show only random thickness distribution and no evidence of original cyclicity. Lower in the sandstone, noncyclic rhythmites are associated with swaly to quasi-planar bedding, even bedding, and ripples with rounded crests, and are inferred to have formed in response to storm waves and combined flows. Higher in the sandstone, noncyclic, amalgamated, and cyclic rhythmites are present in association with herringbone stratification and pinstripe laminae, and are inferred to have formed under the influence of tidal currents. Cyclic tidal rhythmites are present near the base of a shallow channel deposit. These rhythmites contain sandstone-shale couplets indicative of presumed diurnal tides, 11-12 laminae cycles indicative of neap-spring tidal sedimentation, and 25-27 laminae cycles indicative of lunar monthly sedimentation. Upward and laterally within the channel trunc tion of neap-cycle laminae by spring-cycle ripples, and a loss of continuous clay drapes in amalgamated rhythmites, led to preservation of only monthly or greater depositional cycles. Vertical facies trends in the sandstone are interpreted to indicate upward shallowing from a wave-influenced outer estuarine or shallow shelf environment to an intertidal sand flat to an exposed low-lying coast upon which peats accumulated. The fact that cyclic rhythmites were preserved only in a shallow channel, stratigraphically near the middle of the mail may indicate that accommodation space and a medial estuarine position with little wave reworking and bioturbation controlled the preservation of the various orders of cycles.

The late jurassic ‘Massenkalk fazies’ of Southern Germany: calcareous sand piles rather than organic reefs

Upper Jurassic (Malm δ to ζ1) massive limestones (‘algal-sponge-reefs, sponge-reefs, reef-complexes, reefs, algal-sponge-bioherms, biolithites, Massenkalk, bioherms, Stillwasser-Mudmounds’) were analyzed in the Southern Swabian Alb, the Southern Franconian Alb and in drilling wells in the Molasse basin (Southern Bavaria). This analysis was carried out within the frame of a multidisciplinary DFG-study with the objective of decifering the controls on the development of Upper Jurassic spongiolites, their three-dimensional distribution, their characteristic faunal composition, and the diagenetic trends of the different primary facies.

Stratigraphic responses to geologic processes: Late Pennsylvanian eustasy and tectonics in the Pedregosa and Orogrande basins, Ancestral Ricky Mountains

The primary factors that influence stratigraphy are tectonics, eustasy, and sediment supply. Because a change in any of these factors produces a concomitant change in relative base level or accommodation space, the link between a given process and its stratigraphic response is nonunique. Despite this ambiguity, distinguishing the origins of stratigraphic signals is theoretically possible by examining the temporal and especially the spatial scales of the signal. Correlation is a particularly critical tool for distinguishing global eustasy from regional tectonism. Upper Pennsylvanian strata of the Pedregosa and Orogrande basins (southern Ancestral Rocky Mountains) were deposited during a time of major continental collision coincident with extensive continental glaciation, and thus they contain a composite record of significant changes in tectonism, eustasy, and sediment supply. High-frequency stratigraphic cyclicity expressed as repetitive successions of lithofacies at the 10 m scale is prevalent in all sections and displays several features that collectively imply a primarily glacioeustatic origin: (1) abrupt juxtaposition of dissimilar lithofacies, signaling rates of base-level change best attributed to glacioeustasy; (2) apparent intrabasinal, interbasinal, and (provisional) interregional correlation of high-frequency cycles across and between contrasting tectonic environments; and (3) apparent cycle frequencies that bracket the 413 k.y. periodicity typical of orbital eccentricity, the probable forcing mechanism for Pennsylvanian glaciations. Aside from the prevalent high-frequency cyclostratigraphy, multiple-cycle trends in facies and/or thicknesses define low-frequency stratigraphic patterns at the 10 2 m scale. Isolating and identifying the controlling factors for these patterns is not straightforward; nevertheless, qualitative analysis of multicyclic trends implicates distinct eustatic and tectonic processes as contributing influences. The eustatic component may derive partially from low-frequency glacioeustasy. The tectonic component almost certainly reflects Marathon-Ouachita collisional orogenesis.

Pliocene Facies Trends and Controls on Deposition of Lower Gusher Shallow Gas Reservoirs, North Coles Levee Field, San Joaquin Basin, California: ABSTRACT

Net sand isochore maps of three Pliocene-age Lower Gusher sands in the Etchegoin Formation at North Coles Levee field, southern San Joaquin basin, California display geometries suggestive of deposition in delta front settings. The north-south depositional strike of these sands approximately parallels the orientation of the paleoshoreline. The sands thicken and display greater lateral continuity near distributary channel sands, which are oriented east-northeast approximately perpendicular to the shoreline. A comparison of the isochore maps of each of the three sand bodies show that they are stacked vertically above each other, indicating that the position of the shoreline remained stationary during deposition of the Gusher interval. This apparent stillstand of the shoreline is superimposed on an overall regression of the sea from the San Joaquin basin during the Pliocene. Therefore, we believe that the Lower Gusher sands were deposited during a period of relatively rapid basin subsidence, which negated the effects of the marine regression and caused vertical aggradation of shoreline facies in the North Coles Levee area. The Lower Gusher interval at North and South Coles Levee contains the most prolific shallow gas reservoirs on the Bakersfield Arch. A thorough knowledge of depositional trends in the Lower Gusher interval increases more » the probability of encountering reservoir-quality facies in exploration programs focusing on Pliocene gas. « less

Geology of the Niederlausitz Lignite district, Germany

One of the largest brown coal producing districts in the world is the Miocene of Niederlausitz Lignite area in the southeastern part of Germany. Production is in a range of 320 million t/yr. The resources of the first (shallowest) Miocene seam have nearly been exhausted and it is now mainly the second seam which is being mined. A fourth is being explored. The third Miocene and the Oligocene seam, Calau, are unminable. All brown coal is mined in open pits. The rank of brown coal of the second Miocene seam ranges from fuel coal to coking coal. Its heat value (dry) from 22.2 to 23.5 MJ, its ash content (dry) from 6% to 13%, its moisture from 57% to 59%, and its seam thickness from 10 to 12 m. Due to the close relationship between swamp facies and the main coal quality parameters, the coal quality can be directly determined from the drill log. The Oligocene and Miocene brown coal formation was synchronous with the alpine orogenesis and the seafloor spreading of the North Atlantic Ocean, which both caused north and east oriented migrations of the labile basement of central Europe. Periods of compression alternated with longer periods of isostatic subsidence and sedimentation. Additionally, the trends of thickness and facies of sediments were controlled by a block system in the basement of the brown coal district of Niederlausitz, uplifting, subsiding, collapsing, rotating or spreading. The second Lower Miocene seam is situated at sea level in the north of the Niederlausitz area and rises to the south to +150 to +180 m above sea level, due to considerable widespread subsidence and uplifting since the Lower Miocene. Horizontal tectonic movements were caused by the collapse of asymmetric grabens with slight tendency to rotation. Regional shear movements led to block faulting followed by volcanism. Counter-clockwise rotation of the basement blocks is assumed, a hypothesis supported by recent tension measurements and seismic observations. Ice cover in the Pleistocene caused wide destruction zones, narrow and deep channels and intensively folded or imbricated seam structures. Some gravity-induced plastic structures were also formed.

Architecture and sedimentary facies evolution of a marine, expanding outer‐arc half‐graben (Crete, late Miocene)

AbstractVertical trends in architecture and facies of delta systems are preserved in a clastic wedge of an expanding marine half‐graben in which tectonics, eustatic sea‐level change and climatic change are roughly known from independent evidence. The studied half‐graben is situated on Crete (Greece) and part of a larger, E‐W‐trending extensional domain situated north of the Hellenic subduction zone. The extension seems to be related to the southward migration of the trench (roll‐back) in early Late Miocene times. The infill pattern is discussed in the light of theoretical fault‐growth models for expanding half‐grabens.The geometry of the half‐graben fill is typically wedge shaped, with a thickness of nearly 1000 m near the fault scarp thinning to c. 50 m about 20 km away from the scarp. The lower part of the wedge (Stratified Prina Series) contains coarsening‐upward units representing progradational, shallow‐marine deltas. At the base of the wedge these units are thin and retrogradationally stacked. Upwards in the succession, the units become composite (coarsening‐upward subunits), thicker and finer grained. The composite structure, the thickening and the fining trend is related to progressive increase in accommodation space inherent in fault growth.Rapid deepening of the basin from the photic zone (evidenced by intercalated coral and stromatolite beds) up to a depth of 900 m started at the top of the Stratified Prina Series. The deepening continued over some tens of metres of marly sediments of the base of the Kalamavka Formation and may be related to structural collapse of the fault block. After the structural collapse, basin depth remained more or less constant and basin infilling occurred by progradation of deep‐water delta systems. These systems are characterized by a muddy delta slope with channelized conglomerates, and by mainly aggradation of prodelta turbidites deposited in small lobes at the base of slope.

Palaeogeography of the western Pelagonian continental margin in Beotia (Greece) during the Cretaceous—biostratigraphy and isotopic compositions ( δ13C, δ18O) of calcareous deposits

The palaeogeography of Beotia, Greece, during the Cretaceous is outlined on the basis of biostratigraphic evidence derived predominantly from the distribution of rudist bivalves and planktonic foraminifera. Observed facies trends are evaluated together with carbon and oxygen isotopic compositions of limestones and rudist shells. The interaction between global sea level fluctuations and gradual subsidence of the Pelagonian continental margin controlled facies distribution during most of the Cretaceous. The sedimentary record begins with Berriasian flysch-type deposits that accumulated during incipient Eohellenian orogenic movements. Barremian deposits with similar sedimentological characteristics probably belong to the same tectonostratigraphic unit. During Aptian-Cenomanian times, terrigenous clastics with a few intercalated marine limestones accumulated in South Beotia. The fossil content of patch reefs presents important biostratigraphic marks in this terrigenous sequence. Carbon and oxygen isotopic compositions recorded unstable marine environments and subsequent decrease of continental influx. Continuous subsidence and rising sea levels are evident in the Late Turonian onlap of rudist limestones on the Eohellenian basement of the Ptoon Mountains as well as in the interfingering of calcareous slope deposits and basinal Globotruncana-mudstones of South Beotia. During Late Coniacian-Early Santonian, a marked regression finally led to a regional emersion and the development of a karst relief. This regression is predated by “heavy” oxygen isotopic compositions in the Coniacian limestones, which probably resulted from isolation of a larger marine embayment from oceanic circulation. The subsequent Late Santonian-Campanian transgression flooded all of Beotia. Its sediments lie over allochthonous laterites. Extended rudist biostromes developed as soon as continental influx ceased, as indicated both by the stable isotope compositions of rudish shells and embedding limestones. During the Late Campanian, the rather uniform platform-type depositional environments differentiated and pelagic sedimentation recurred in South Beotia, whereas depositional environments to the north remained shallow. Incipient tectonics movements were responsible for a Late Maastrichtian emersion of North Beotia, while basinal conditions persisted in the south. The onset of flysch-type sedimentation occurred simultaneously during the Late Paleocene in North Beotia, as well as in the adjacent Parnassus and Pindos zones. However, this important facies transition has not yet been precisely dated in South Beotia.