Sedimentary Facies Model Research Articles

A sedimentary facies model for stepped, fluvial tufa systems in the Iberian Range (Spain): the Quaternary Piedra and Mesa valleys

AbstractStratigraphic and sedimentological analyses of the Quaternary tufa and associated deposits in the Piedra and Mesa river valleys allowed a number of stages of their sedimentary evolution to be characterized, and a depositional sedimentary model for this north‐central sector of the Iberian Range (Spain) to be established. The proposed sedimentary facies model may explain tufa arrangements in other medium to high gradient, stepped, fluvial tufa systems with narrow transverse profiles occurring in temperate, semi‐arid areas, in both recent and past scenarios. There are several tufa deposits within the Piedra and Mesa river valleys that, over a maximum thickness of about 90 m, record one or more stages of tufa deposition produced following the fluvial incision of the bedrock or previous tufa deposits. Each depositional stage begins with coarse detrital sedimentation. Six fundamental, vertical sequences of tufa facies with small amounts of detrital material reveal the sedimentary processes that occurred in different fluvial environments: channel areas with: (i) free‐flowing water; (ii) barrages and/or cascades; and (iii) dammed water and palustrine floodplains. The proposed sedimentary model involves narrow, stepped, fluvial valleys in which tufa cascades were common. Alternating intervals of bryophyte and stromatolite facies commonly formed at some cascades. Many of these represented barrage‐cascade structures that consisted of phytoclast rudstones, thick phytoherms of mosses and associated stromatolites, and curtain‐shaped phytoherms of stems. Upstream of these structures, dammed areas with bioclastic sands and silts developed and palustrine vegetation grew. The channel stretches between barrages and/or cascades were loci for extensive stromatolite growth in fast flowing water. The palustrine floodplain was home to pools and drainage channels. The model also explains the growth of some barrages in the River Piedra that surpassed the height of the divide, with the diffluence of the main channel into a secondary course forming other tufa deposits in the area. The distribution and abundance of certain types of tufa facies in fluvial basins may be an indicator of differences in their gradients. The facies studied in this work suggest that the gradient of the ancient River Piedra was steeper than that of the ancient River Mesa. Assuming similar scenarios for climate and hydrology, the depositional settings mentioned above and their dimensions would have been determined mainly by the gradient and width of the associated river valleys. This sedimentary model may also be useful for inferring variations in other river basin slopes, as well as accounting for the presence of tufas in areas that normally have no permanent water input.

3D Geological Modeling in Chang109 Block of Changchunling Oilfield

With the improving of reservoir development level, reservoir geologic research urgently need some new and practical technical methods to describe reservoir more accurately and meticulous. The three-dimensional geological modeling exactly is one of the main aspects to resolve the problem. Take the Chang109 block of Changchunling oilfield for an example. Using Petrel, which is multi-disciplinary and synthetical software for researching reservoir and to establish a 3D geological model as the outstanding characteristic, to build the reservoir model displaying geological information system. That, including the structure model, the sedimentary facies model and the property model, will provide reliable basis potential finding and well placement.

Dominance of lateral over axial sedimentary fill in dryland rift basins

ABSTRACT Facies models for continental rift basins suggest longitudinal (axis-parallel) fluvial activity dominates sediment transport and deposition. In contrast, modern basins in the arid to semi-arid northern Basin and Range Province, USA, show axial drainage development to be characterized by short endorheic systems that contribute little to the basin fill. Mapping and calculation of the proportionate distribution of surficial facies of three representative basins at different stages of rift evolution show that basin fill is dominated by the deposits of transverse catchments, and axial fluvial deposits are restricted to a narrow corridor by the progradation of lateral systems. Drainage integration in these dryland rifts is limited by the moisture-stressed climate, with a resultant reduction in stream power, and the complex, tectonically-induced physiography, which limits potential drainage pathways. River systems that flow through multiple structural basins are rare, restricted to those systems with catchment headwaters lying outside the dryland climatic regime. These data imply that long-range, axial fluvial deposits should not automatically be included as a significant part of dryland rift sedimentary facies models. Sediment routing pathways in ancient dryland rift systems may be much shorter than commonly predicted, so affecting the spatial distribution of lithofacies. Climate, therefore, has a much stronger control on drainage and lithofacies at all stages of rift development than is generally stated.

Characteristics and Evolution of Sedimentary Microfacies of Chang 6–4+5 Layer in the Northern Area of Western Mahuang Mountain

Although there are widespread indications of oil and gas in the Chang 6–4+5 layer of western Mahuang Mountian, well test proves that there is more water and less oil with the distribution and continuity of sand being not yet clear. To understand the distribution and evolution of sedimentary microfacies in the area, six kinds of sedimentary microfacies and ten kinds of sedimentary structures were identified, and ten patterns of contact between microfacies and five kinds of sedimentary sequence model were summarized through detailed observation and description of the core of rock and logging data. Each model represents different sedimentary facies. Reversed cycle is dominant and normal short-term cycle also exists in the vertical direction; Mouth bar and subaqueous distributary channel developed alternately in the lateral direction. The lower channel sand is “large in quantity but small in size with the layer narrow and thin”, whereas the upper sand is “few in quantity but large in size with a wide and thick layer”. The “single-factor analysis and multifactor comprehensive” research method was used to divide this area into three phase belts—delta inner front, delta outer front, and predelta. Subaqueous distributary channel, distributary mouth bar, and interdistributary deposits are prevailing in the delta inner front. Distal bar and sand sheet are dominant in the delta outer front, and predelta mud is dominant in the predelta. According to modern sedimentary facies model, the area is a constructive braided river-lake delta. Three stages appeared in its evolution: the initial development period (Chang 6 3), in which the predelta and the delta outer front are the main facies belts; the early-middle development period (Chang 6 2–Chang 6 1), in which the inner delta front and the delta outer front dominates and each micro-facies is a single phase and is deposited alternately; the middle-late development period (Chang 4 +5 2–Chang 4 +5 1), in which the delta inner front dominates and subaqueous distributary channel shows multiphase stacking pattern.

A sedimentary facies model for glacial-age sediments in Baldwin Lake, Southern California

A combined sedimentological and high-resolution petrographic analysis was conducted on a glacial-age (20,000–65,000 cal yr BP) sediment core from Baldwin Lake, Southern California. The results of this research represent the most complete glacial-age, terrestrial climate record from Southern California to date. These results are used to characterize the different sediment types and to investigate the difference in depositional processes and environments between the core's three predominant sediment types: massive, semi-laminated, and laminated sediments. Massive sediments are commonly associated with a blocky texture and/or desiccation cracks, are organic-poor, have high magnetic susceptibility values, and are coarser-grained. Thin-sections from massive sediments reveal a homogenous sediment fabric. Sub-centimeter-scale laminated and centimeter-scale semi-laminated sediments are generally organic-rich, have low magnetic susceptibility values, and are finer grained. Thin-sections from laminated and semi-laminated sediments reveal diffuse sub-millimeter- to millimeter-scale laminae. This combination of sedimentological and high-resolution petrographic data enabled us to characterize four sediment facies, each related to specific depositional processes and environments: (1) a playa lake; (2) a perennial shallow lake; (3) an intermediate lake with variable lake level; and, (4) a perennial deep lake. At centennial-to millennial-timescales, lower lake levels are represented by deposition of massive to semi-laminated sediments in a playa to a perennial shallow lake environment. At similar timescales, higher lake levels are recorded by semi-laminated to laminated sediments deposited in an intermediate lake to a perennial deep lake environment. These results provide an additional sedimentological study for comparison to similar arid environment basins, and for comparison to existing regional paleoclimatic reconstructions.

Model building for Chang-8 low permeability sandstone reservoir in the Yanchang formation of the Xifeng oil field

In order to build a model for the Chang-8 low permeability sandstone reservoir in the Yanchang formation of the Xifeng oil field, we studied sedimentation and diagenesis of sandstone and analyzed major factors controlling this low permeability reservoir. By doing so, we have made clear that the spatial distribution of reservoir attribute parameters is controlled by the spatial distribution of various kinds of sandstone bodies. By taking advantage of many coring wells and high quality logging data, we used regression analysis for a single well with geological conditions as constraints, to build the interpretation model for logging data and to calculate attribute parameters for a single well, which ensured accuracy of the 1-D vertical model. On this basis, we built a lithofacies model to replace the sedimentary facies model. In addition, we also built a porosity model by using a sequential Gaussian simulation with the lithofacies model as the constraint. In the end, we built a permeability model by using Markov-Bayes simulation, with the porosity attribute as the covariate. The results show that the permeability model reflects very well the relative differences between low permeability values, which is of great importance for locating high permeability zones and forecasting zones favorable for exploration and exploitation.

3D geological modeling of dual porosity carbonate reservoirs: A case from the Kenkiyak pre-salt oilfield, Kazakhstan

Abstract The 3D geological model of dual porosity reservoirs in the Kenkiyak pre-salt oilfield is built step by step based on the detailed core observation, employing core data, log data, geological data, and seismic data. Firstly, the structural model is built. Then the sedimentary facies model is built on the basis of analyzing seismic facies and electro-facies, and the models of reservoir matrix properties controlled by the sedimentary facies are built based on structure analysis and methods optimization. Secondly, the fracture distribution model is established by applying the results of the seismic property analysis and the core fracture description. Thirdly, the three-dimensional integrated geological model of the dual porosity reservoirs is set up by combining the matrix property models with the fracture distribution model and applying the filtration theory between matrix and fractures. The set of methods finally provides a fracture distribution model, a fracture equivalent permeability model, and a dual porosity equivalent permeability model, which can be directly provided to reservoir engineers for numerical simulation.

Sediment–landform assemblages and digital elevation data: Testing an improved methodology for the assessment of sand and gravel aggregate resources in north-western Britain

This paper uses the results of a series of sand and gravel resource assessment surveys to refine and improve the methods used for identifying aggregate resources in north-western Britain. Geomorphological surveys of the Cors Geirch and Rhosesmor areas of north Wales and the lower Ribble valley in Lancashire, carried out by 1:10,000 scale field mapping, are compared to resource assessments made using high resolution Digital Elevation Models (DEMs) of the same ground. The DEMs proved useful tools for remote geomorphological survey, but precision was significantly better for the Intermap NEXTMAP and, particularly, the Environment Agency LiDAR dataset. Only the LiDAR data revealed with any clarity small-scale and low relief features, for example scroll-bar palaeochannels on river terraces. The identification of sediment–landform assemblage zones from geomorphological survey permits the identification of the primary sedimentary process responsible for each assemblage and the attachment of an ideal sedimentary facies model to it. Each model can then be used to predict sedimentary composition, the optimal location of boreholes and the location of resource blocks. Combining the use of DEM data and the Spatial Analyst toolset within ArcGIS™ (ESRI) allows rapid accurate delineation of landforms and the use of cut and fill functions allows more accurate estimation of the volume of resources than the traditional calculations based on area and average thickness. These new methods improve the accuracy and cost-effectiveness of geomorphology-based aggregate assessments, notably where the mineral-bodies are concealed beneath minimal overburden and produce characteristic landforms.

Sedimentary Facies Models on Carbonatite in the Upper Shuaiba Member of Lower Cretaceous in Daleel Field, Oman

ABSTRACT The Upper Shuaiba Member (USH) is the main force pay bed in the Daleel field in northern Oman; 5 layers including A, B, C, D, and E were divided in profile, and layer D and layer E are the main beds. With the development of exploration in the Daleel oil field, studying the sedimentary systems about their inner composition and the collocation in dimension, and setting up the sedimentary models in the USH are becoming more and more necessary and important to meet the further exploration requirement. Based on the data of geology, seism, and paleo-biology, according to the analysis method on carbonatite depositional system, the litho-facies assemblage and sedimentary environment in the USH were studied. Intershoal low-lying sub-facies (where the water depth is 10–50 m) and shallow shoal sub-facies (where the water depth is not more than 10 m) were extinguished in the layer D, and storm deposit was found in layer E1, in which intershoal low-lying sub-facies also developed. The feature of the sedimentary sub-facies and the sedimentary condition were analyzed, and the sedimentary model was set up in the article: the carbonatite intershoal low-lying developed under the background of open land in shallow sea, where storm events usually occurred in the Lower Cretaceous in the area.

Drowned coralline algal dominated deposits off Lanai, Hawaii; carbonate accretion and vertical tectonics over the last 30 ka

We present detailed bathymetry, remotely operated vehicle (ROV) and submersible observations, and sedimentary and radiocarbon age data from carbonate deposits recovered from two submerged terraces at − 150 m (T1) and − 230 m (T2) off Lanai, Hawaii. The tops of the terraces are veneered by relatively thin (< 5 m) in situ accumulations of coralline algal nodule, coralgal nodule, Halimeda and a derived oolitic facies deposited in intermediate (30–60 m) to deep fore-reef slope settings (60–120 m). The data are used to develop a sedimentary facies model that is consistent with eustatic sea-level variations over the last 30 ka. Both nodule facies on T1 and T2 initiated growth 30–29 ka following a fall in sea level of ∼50 m and increase in bottom currents during the transition from Marine Isotope Stage 3 to 2. The nodules accreted slowly throughout the Last Glacial Maximum when sea-level was relatively stable. Drowning occurred during the early deglaciation (17–16 ka) and was marked by the complete drowning of coralline algal nodules facies on T2 and incipient drowning of coralgal facies on T1. Abrupt sea-level rise during the middle deglaciation, perhaps associated with global meltwater pulse 1A (14–15 ka), finally drowned the coralgal facies on T1, which in turn was overlain by a deep-water Halimeda facies or an oolitic facies derived from upslope. Our data indicates that Lanai has experienced relatively little vertical tectonic movement over the last 30 ka. Using paleobathymetric data derived from the sedimentary facies, age vs. depth relationships, and published sea-level curves, we estimate that Lanai could be either slowly uplifting or subsiding, but at rates < 0.1 m/kyr (uplift) or < 0.4 m/kyr (subsidence) over this 30 kyr period.

Ephemeral deposition, seabed mixing and fine-scale strata formation in the York River estuary, Chesapeake Bay

A process-oriented sedimentary facies model is developed for the York River estuary, a sub-estuary of the lower Chesapeake Bay. This facies model was based on 210Pb and grain-size profiles, as well as X-radiographs taken from kasten cores and box cores collected in a series of across-river transects. Throughout most of the energetic microtidal York River, the seabed is characterized by physical mixing to depths of 25–200 cm. A strong cross-estuary gradient in processes is observed with one side, including channel, flank and shoal, dominated by frequent deep erosion and redeposition (physical mixing), while physical mixing is reduced on the other side, resulting in a greater preservation of biological mixing signatures. Within the physically dominated side of the river, the mixed layer is characterized by ‘stair-stepped’ 210Pb profiles with one or more segments (∼25–200 cm thick) of nearly uniform excess activity. X-radiographs reveal that, although a record of limited biogenic sediment modification is preserved, sedimentary structures within the mixed layer are dominated by centimeter to decimeter scale units of finely to coarsely laminated strata bounded by hiatal surfaces. This demonstrates that mixing results primarily from erosion, resuspension and deposition. Reduced salinity limits the number of benthic species in the York River. Physical disturbance leads to an impoverishment of this community, which is composed primarily of small, opportunistic species with a paucity of larger macrofauna. As a result, mixing in the biologically dominated side of the river is generally on the order of a few centimeters, but may be as deep as 40 cm, and 210Pb geochronology yields low biodiffusion rates (0.43–3.35 cm 2 yr −1). X-radiographs reveal the presence of some laminations which suggest that although the mixing is controlled by biological processes the mixing intensity is relatively low. Based on 210Pb geochronologies, residence time estimates for particles within the mixed layer are on the order of centuries. Residence time calculations based on the sediment mass in the physically mixed layer is equivalent to 70 yr of river sediment yield, consistent with century-scale residence times from core data. The frequency and intensity of seabed mixing appears to differ between the lower and upper river. The lower York River is wider and deeper, and is more susceptible to large storms and sea surges, which we suspect drives much of the recorded seabed mixing. Within the upper river, longer-term events (storms) may cause the deepest mixing, but much of this record is destroyed by shorter-term, high-frequency events which produce shallow to mid-depth (<50 cm) mixing, probably driven by spring/neap tides, co-phased tidal constituents, and river flooding.

Sedimentology of rocky shorelines: 1. A review of the problem, with analytical methods, and insights gained from the Hulopoe Gravel and the modern rocky shoreline of Lanai, Hawaii

Hypotheses advanced concerning the origin of the Pleistocene Hulopoe Gravel on Lanai include mega-tsunami, abandoned beach, ‘multiple event,’ rocky shoreline, and for parts of the deposit, Native Hawaiian constructions and degraded lava flow fronts. Uplift of Lanai shorelines has been suggested for deposits occurring up to at least 190 m. These conflicting hypotheses highlight problems with the interpretation of coarse gravel deposits containing marine biotic remains. The geological records of the processes implied by these hypotheses should look very different. Discrimination among these or any other hypotheses for the origins of the Hulopoe Gravel will require careful study of vertical and lateral variations in litho- and biofacies, facies architecture, contact relationships and stratal geometries of this deposit. Observations of modern rocky shorelines, particularly on Lanai adjacent to Hulopoe Gravel outcrops, have shown that distinctive coarse gravel facies are present, several of which occur in specific geomorphic settings. Tectonic, isostatic and eustatic changes which cause rapid shoreline translations on steep slopes favour preservation of former rocky shorelines and associated sedimentary deposits both above and below sea level. The sedimentary record of those shorelines is likely to be complex. The modern rocky shoreline sedimentary environment is a hostile one, largely neglected by sedimentologists. A range of high-energy processes characterize these shorelines. Long-period swell, tsunami and storm waves can erode hard bedrock and generate coarse gravel. They also erode older deposits, depositing fresh ones containing mixtures of materials of different ages. Additional gravelly material may be contributed by rivers draining steep hinterlands. To fully evaluate rocky shoreline deposition in the broadest sense, for both the Hulopoe Gravel and other deposits, sedimentary facies models are needed for rocky shorelines occurring in a range of settings. Recognition and description of rocky shoreline deposits are crucial for correctly interpreting the geological history of oceanic and volcanic arc islands, for distinguishing between ancient tsunami and storm deposits, and for interpreting coarse-grained deposits preserved on high energy coasts of continents. Problems include not only the absence of appropriate sedimentary facies models linking rocky shoreline deposits and environments but also, until recently, lack of a systematic descriptive scheme applicable to coarse gravel deposits generally. Two complementary methods serve to integrate the wide range of bed and clast attributes and parameters which characterize complex coarse gravel deposits. The composition and fabric (CAF) method has a materials focus, providing detailed description of attributes of the constituent clasts, petrology, the proportions of gravel, sand and mud, and the ways in which these materials are organized. The sedimentary facies model building (FMB) method emphasizes the organization of a deposit on a bed-by-bed basis to identify facies and infer depositional processes. The systematic use of a comprehensive gravel fabric and petrography log (GFPL), in conjunction with detailed vertical profiles, provides visual representations of a range of deposit characteristics. Criteria useful for distinguishing sedimentary facies in the Hulopoe Gravel are: grain-size modes, amount of matrix, bed geometry, sedimentary structures, bed fabric and clast roundness.

STONECIPHER, S. A. 2000. Applied Sandstone Diagensis–Practical Petrographic Solutions for a Variety of Common Exploration, Development, and Production Problems. SEPM Short Course Notes no. 50. v+143 pp. Tulsa: SEPM (Society for Sedimentary Geology). Price US $63.00 (member's price US $45.00), plus shipping and handling; paperback. ISBN 1 56576 074 3.

This fine little book does exactly what the title states. It contains six chapters, five of which are dedicated to applying basic petrographic data to applied hydrocarbon exploration and production. The final chapter is a single page cautionary note. Each of the first three chapters is a case study. Chapter 1 describes relationships between diagenetic mineralization and sedimentary facies models using data from the Wilcox Formation (Paleocene–Eocene), Texas Gulf Coast. Chapter 2 examines the sequence stratigraphic context of diagenesis and introduces the reader to the issue of …

Latest Oligocene–Late Miocene lacustrine systems of the north-central part of the Ebro Basin (Spain): sedimentary facies model and palaeogeographic synthesis

In the central part of the Ebro Basin (Spain), north of the Ebro River, three allostratigraphic or tectosedimentary units differentiated within the latest Oligocene–Late Miocene (upper Agenian–Vallesian) interval are sedimentologically characterized. These deposits represent carbonate and sulphate lacustrine environments fed by Pyrenean alluvial systems (Luna system and Huesca system). In each unit, several facies associations or lithofacies were mapped. These interfinger through time: gypsum lithofacies occupy the southernmost part of the study area and are surrounded by carbonate and/or detrital lithofacies to the north. Each lithofacies consists of different carbonate, sulphate and/or detrital facies, arranged in simple sequences, decimetres to metres thick. The four types of sequences characterized reflect a complex shallowing evolution involving carbonate or sulphate contexts. Thus, each lithofacies represents the sedimentation in a particular lacustrine or alluvial subenvironment. A single lacustrine system is proposed for the central part of the basin. Its evolution followed cycles of water level variations, which caused kilometre-scale migrations of the lake shoreline. In this model, two distinct situations or lakes alternated through time. (a) One represents high levels associated with a single body of dilute carbonate water, where massive carbonate facies formed influenced by organism proliferation. These lakes had wide palustrine vegetated margins, in which bioturbated facies developed. (b) The other represents low levels that correspond to a playa-lake model. Laminated sulphate deposits formed in the lake waters, while nodular gypsum originated in the surrounding saline mud flats and within the exposed, previously deposited carbonate sediments. Transitions from one situation to the other, including minor water level fluctuations within carbonate sedimentation conditions, were marked by the development of laminated and stromatolitic facies. Lake level variations were caused by climatic cycles and gave rise to the simple sequences and to the lithofacies lateral relationships recognized in this study. Correlation among tectosedimentary units in the studied Pyrenean domain and in the Iberian domain of the Ebro Basin shows that for units N1 and N2 the facies that resulted from dilute carbonate waters are rare in the lacustrine areas of the Iberian margin. This indicates that a strong hydrological contrast existed between both regions. On this basis, a theoretical asymmetrical facies model is proposed for the single lacustrine basin that extended over these regions.

Estimating Hydraulic Conductivities in an Alluvial Basin from Sediment Facies Models

AbstractIt is not uncommon that hydraulic conductivity data are sparse in an area, and that information about subsurface sediment distributions are even more sparse or of poor quality. It is therefore desirable to have rational ways to estimate conductivity distributions for ground‐water flow modeling. Interpretations of the geologic history of a fault‐bounded sedimentary basin in south Santa Clara County, California, were used to estimate variations in sediment distribution and resulting relative hydraulic conductivities. Spatial sediment distributions were interpreted from the dominant infilling processes, using standard sedimentary facies models for alluvial fans and fluvial deposition. These sedimentary facies were categorized into interpreted hydrogeologic facies, which were then compared with measured hydraulic conductivity data obtained from scattered aquifer pumping tests in the basin. The comparison suggests that sedimentary facies models can provide a good basis for estimating the distribution of relative hydraulic conductivity. Further, the comparison provides a procedure for estimating hydraulic conductivities in areas where no measured values are available. This procedure provides a better basis for estimating hydraulic conductivities than methods which do not consider sedimentary processes.

A sedimentary facies model for perennial and meromictic saline lakes: Holocene Medicine Lake Basin, South Dakota, USA

Medicine lake is a small (about 1 km2), shallow (up to 10 m deep), saline (50–170 g l−1) and meromictic lake formed after the retreat of the Wisconsin ice in the north American Great Plains. Based on a detailed sedimentological analysis of cores, we describe and interpret 13 sedimentary subfacies grouped in 9 associations which characterize the following lacustrine subenvironments: clastic littoral (freshwater and saline), springs, microbial mats, bench slope, and pelagial (oxic, alternating oxic-anoxic, anoxic and hypersaline, and organic-dominated). Lateral distribution and vertical evolution of subfacies in our model are controlled by climate fluctuations, climate-related limnological parameters (lake level, TDS and brine composition, and redox conditions), and autocyclic processes (progressive infilling of the basin and higher sedimentation rate in the pclagial realm). Microbial and chemical processes govern deposition in this system, and meromixis plays a decisive role in lake dynamics. Phototropic bacterial plate communites at the chemocline dominated as pelagial organic producers during stable meromictic periods, whereas benthic microbial communities developed during mixed water periods. Water stratification during the Holocene was mainly controlled by three parameters: 1) basin morphometry, 2) lake level, and 3) differences in TDS values between mixolimnion and monimolimnion waters. Sedimentary facies analyses is a powerful descriptive and interpretative tool that greatly contributes to deciphering the high resolution paleoenvironmental information archived in lake sequences. Depositional and paleoenvironmental models provide a dynamic framework for integrating paleolimnological data and other proxy paleorecords. Medicine lake serves as a facies model for shallow, perennial hypersaline, meromictic lakes in modern and ancient lacustrine basins. The sediment sequence from Medicine lake cores is consistent with the general paleoclimatic evolution of the northern Great Plains since the retreat of ice sheets. Our study reveals a plethora of rapid fluctuations in the water cycle both during the middle and the late Holocene. These augment prior paleoclimate reconstructions based on diatom studies of the lower Holocene freshwater to saline transition and on pollen profiles which show little variability during the subsequent long prairie grass episode.

Application of the organic facies method in the thrace basin

Thrace is an intermontane basin and the sedimentary sequence encompasses a typical transgressive-regressive depositional cycle. We apply sedimentary organic facies as an accessory method to develop a distinctive sedimentary facies model of the Thrace Basin. This model suggests comparing the relative abundance of different types of kerogen with the associated sediment. We recognize four types of kerogen: (i) amorphous/algae; (ii) herbaceous; (iii) woody;, and (iv) coaly. The relative abundance and physical properties of the kerogens show close consistency with the sedimentary facies. Well preserved amorphous/algae material is associated mainly with shale, while degraded/oxidized woody material is dominant in siltstone and sandstone. Considering these phenomena we have composed organic-matter abundance charts along the transgressive-regressive sections in the wells. A good correlation exists between organic-matter type and sedimentation mode; the transgressive sections are represented by upward increasing amorphous/algae kerogens and, reversely, the regressive sections by increasing woody material. Vitrinite reflectance and the Spore Colour Index are used to define the thermal history of the basin. As a result, the close relation between organic matter features and sedimentary mode is considered as a useful parameter to clarify the sedimentary facies of the Thrace Basin.

Interpretation of lithologies and depositional environments of Cretaceous and Lower Permian rocks by using a diverse suite of logs from a borehole in central Kansas

As part of an extensive research program, a borehole was drilled to extend sedimentary facies models of the Cretaceous into the subsurface on the eastern side of the western interior Cretaceous basin in Kansas. Lithodensity-neutron and spectral gamma-ray logging runs were completed to facilitate interpretation of rocks penetrated by the borehole. Th/K, Th/U, and composition profile logs based on apparent grain density and photoelectric absorption index were prepared and used to show vertical changes in geochemical facies and clay mineralogy. These logs were compared with the gamma-ray log and drill cuttings to interpret depositional environments. The Th/K log sharply defines the Cretaceous/Permian boundary and, together with the Th/U log, emphasizes the contrast between marine Upper Cretaceous rocks of the Greenhorn depositional cycle and nonmarine to transitional rocks of the Lower Cretaceous. The long-term cyclic pattern of the Th/U log is an excellent indicator of a broad transgression/regression during the Greenhorn cycle on an open marine shelf, whereas extreme fluctuations of Th/U in the Lower Cretaceous rocks suggest a high degree of short-term environmental variability. Interpretation of the RHOMAA-UMAA compositional profile in the Dakota Formation indicates several pulses of marine transgression and regression prior to the initiation of the Greenhorn cycle in central Kansas.

Facies Analysis and Seismic Stratigraphic Interpretation of Cored Paleocene Carbonate Sequence in Block III, Lake Maracaibo, Venezuela: ABSTRACT

The cored Paleocene carbonate sequence found in wells VLC-812 and VLC-950, together with ditch samples from wells VLC-750 and VLC-693 in Block III, Lake Maracaibo, present a recurrent lithology which can be represented in seven sedimentary facies groups. The recognized facies range from a mollusk-rudstone to a sandy oolitic grainstone. The most important facies is a highly porous, oil-bearing grainstone. The above-mentioned facies were grouped into a facies assemblage which was then interpreted in terms of its depositional environment. A genetic facies model was constructed in order to explain lateral and vertical facies changes and for regional prediction purposes. This facies model predicts a commercial porosity development in a preferred orientation in the Block III area. A seismic-stratigraphic interpretation, which took into account the sedimentary facies model, documents a paleoshoreline at the time of the oolitic grainstone sedimentation. This finding confirmed the favorable prospective zones proposed by the sedimentary facies model for better porosity development in the carbonate sequence. This sedimentary facies/seismic-stratigraphic model can form the basis for hydrocarbon exploitation and exploration for carbonate rocks with similar geological characteristics and most importantly can be used where there are widely spaced wells.

A Facies Model for Temperate Continental Glaciers

The presence and dynamics of continental glaciers dominated the physical processes of erosion and deposition in the mid-latitudes during the Pleistocene. The stratigraphy of these glacigenic deposits can be synthesized into a sedimentary facies model and used as a guide to recognize ancient temperate continental glacier deposits and to predict lithofacies variation on a local scale (of a few km) in ancient glacigenic sediments. Deposits in epicontinental areas may extend for several hundred km and be less than 0.1 km thick as the basins created by isostatic depression are temporally and spatially limited. Thicker deposits form along continental margins intercalated with glaciomarine sediments. Deposition from temperate continental glaciers occurs near the peripheries of the glaciers (the outer 10–100 km) and predominantly during the waning phase. Each successive glacial advance tends to erode previous deposits, leaving isolated patches of the older deposits on top of striated bedrock. A generalized sedime...