End-member Facies Research Articles

Geology and geochronology of the El Peñón granitic pluton: A contribution to the famatinian magmatic evolution in the Sierra de San Luis, Argentina

The El Peñón pluton is a peraluminous granite located in the northeastern sector of the Sierra de San Luis. According to its textural and mineral variations two endmember facies have been distinguished along it: a coarse-grained muscovite-bearing facies, and a fine-grained biotite-bearing facies. Field relationships aligned with mineralogical and geochemical compositions, allow us to establish an anatectic origin for this pluton. Regarding the inherited zircon ages, the provenance area of the metasedimentary source could be linked to the orogenic systems of Western Gondwana. The analysis of the internal structure of the intrusive body related with the structural arrangement observed in the country rocks indicates a syn-kinematic emplacement of the igneous body with respect to the main deformational event described in the Sierra de San Luis, which has been assigned to the closure of the Famatinian arc. New U-Pb (LA-ICP-MS) zircon ages constrain the emplacement/crystallization age of the El Peñón granite within the time lapse between ∼470 and 460 Ma. The geochronological data presented here, together with geochemical and structural analysis, enables us to correlate the origin of the El Peñón pluton and the deformation that occurred during its emplacement with the Famatinian Orogeny. Thus, these results allow us to rule out the presence of Pampean magmatism in the Sierra de San Luis but also establish a new absolute age for the main deformational event of the range.

Evaluating the depositional environment, lithofacies variation, and diagenetic processes of the Wolfcamp B and lower Spraberry intervals in the Midland Basin: Implications for reservoir quality and distribution

Detailed facies characterization in the Wolfcamp B and lower Spraberry intervals of two drill cores in the Midland Basin has yielded five main lithofacies with distinctive physical, chemical, and biologic attributes. The main attributes for facies identification include lithology and/or mineralogy, texture and/or fabric, porosity, hydrogen index, and total organic carbon content. The methodologies are focused on detailed core description, thin-section petrography, quantitative x-ray diffraction, and field emission scanning electron microscopy (SEM). Porosity data are primarily based on Gas Research Institute (Core Laboratories) measurements and field emission SEM assessment. Moreover, the depositional and diagenetic controls on facies development are addressed to assess the dominant geological processes that govern reservoir quality and distribution. A model of five end-member facies is proposed to characterize source and reservoir elements through delineating facies tracts. The five end-member facies are as follows: facies 1: silty mudstone, optimal source, and optimal to fair reservoir; facies 2: muddy siltstone (optimal source and optimal reservoir); facies 3: silty calcareous mudstone (good source and good reservoir); facies 4: bioclastic wackestone–floatstone (fair source and fair reservoir); and facies 5: packstone–grainstone (poor source and poor to excellent reservoir). The proposed facies scheme aims to provide a more comprehensive approach to capture the high vertical and lateral variability in this mixed carbonate and fine-grained clastic succession. Through this detailed textural, compositional, sedimentologic, and diagenetic approach, this facies model can be used to better understand reservoir quality and distribution throughout the Midland Basin.

Distinguishing between Deep-Water Sediment Facies: Turbidites, Contourites and Hemipelagites

The distinction between turbidites, contourites and hemipelagites in modern and ancient deep-water systems has long been a matter of controversy. This is partly because the processes themselves show a degree of overlap as part of a continuum, so that the deposit characteristics also overlap. In addition, the three facies types commonly occur within interbedded sequences of continental margin deposits. The nature of these end-member processes and their physical parameters are becoming much better known and are summarised here briefly. Good progress has also been made over the past decade in recognising differences between end-member facies in terms of their sedimentary structures, facies sequences, ichnofacies, sediment textures, composition and microfabric. These characteristics are summarised here in terms of standard facies models and the variations from these models that are typically encountered in natural systems. Nevertheless, it must be acknowledged that clear distinction is not always possible on the basis of sedimentary characteristics alone, and that uncertainties should be highlighted in any interpretation. A three-scale approach to distinction for all deep-water facies types should be attempted wherever possible, including large-scale (oceanographic and tectonic setting), regional-scale (architecture and association) and small-scale (sediment facies) observations.

Fine-Grained Debris Flows In Coarse-Grained Alluvial Systems: Paleoenvironmental Implications For the Late Paleozoic Fountain and Cutler Formations, Colorado, U.S.A.

Abstract: The Fountain and Cutler formations are coarse-clastic alluvial wedges that mantled ancestral Rocky Mountain uplifts. Muddy granule sandstone (MGSF) is a volumetrically important facies to both systems. The facies is massive and unsorted with grain-size distributions that range from clay to granule. These deposits are best characterized as cohesive fine-grained debris flows. Yet, the MGSF rarely contains clasts > 10 mm, although other intercalated alluvial facies commonly contain cobbles and small boulders. Competence modeling was undertaken to assess the amount of water needed to account for the observed coarsest fraction of the MGSF. These results indicate that flows would need inflation with water by one-third to two-thirds, depending on clay mineralogy and the range of clay in the MGSF. This paper proposes that flows began as debris flows but underwent flow transformation through incorporation of water during flow in the paleohighlands. Dilation of the flow reduced competence, and each flow lost the coarser than 10 mm fraction. Flows would rheologically stiffen upon reaching the unconfined alluvial surface, ultimately behaving as cohesive fine-grained debris flows. The MGSF at both study areas does not fit into end-member facies models of alluvial fans. Depositional systems elsewhere that exhibit similar facies may want to consider flow transformation and the associated environmental implications when inferring the depositional setting.

Hydroclimatic variability in the Levant during the early last glacial (∼ 117–75 ka) derived from micro-facies analyses of deep Dead Sea sediments

Abstract. The new sediment record from the deep Dead Sea basin (ICDP core 5017-1) provides a unique archive for hydroclimatic variability in the Levant. Here, we present high-resolution sediment facies analysis and elemental composition by micro-X-ray fluorescence (µXRF) scanning of core 5017-1 to trace lake levels and responses of the regional hydroclimatology during the time interval from ca. 117 to 75 ka, i.e. the transition between the last interglacial and the onset of the last glaciation. We distinguished six major micro-facies types and interpreted these and their alterations in the core in terms of relative lake level changes. The two end-member facies for highest and lowest lake levels are (a) up to several metres thick, greenish sediments of alternating aragonite and detrital marl laminae (aad) and (b) thick halite facies, respectively. Intermediate lake levels are characterised by detrital marls with varying amounts of aragonite, gypsum or halite, reflecting lower-amplitude, shorter-term variability. Two intervals of pronounced lake level drops occurred at ∼ 110–108 ± 5 and ∼ 93–87 ± 7 ka. They likely coincide with stadial conditions in the central Mediterranean (Melisey I and II pollen zones in Monticchio) and low global sea levels during Marine Isotope Stage (MIS) 5d and 5b. However, our data do not support the current hypothesis of an almost complete desiccation of the Dead Sea during the earlier of these lake level low stands based on a recovered gravel layer. Based on new petrographic analyses, we propose that, although it was a low stand, this well-sorted gravel layer may be a vestige of a thick turbidite that has been washed out during drilling rather than an in situ beach deposit. Two intervals of higher lake stands at ∼ 108–93 ± 6 and ∼ 87–75 ± 7 ka correspond to interstadial conditions in the central Mediterranean, i.e. pollen zones St. Germain I and II in Monticchio, and Greenland interstadials (GI) 24+23 and 21 in Greenland, as well as to sapropels S4 and S3 in the Mediterranean Sea. These apparent correlations suggest a close link of the climate in the Levant to North Atlantic and Mediterranean climates during the time of the build-up of Northern Hemisphere ice shields in the early last glacial period.

Macrobenthic community structure in a high-productivity region: Upper Campanian Mishash Formation (Israel)

A paleoecological study of molluscan fossils from the Upper Campanian Mishash Formation of southern Israel was undertaken to provide new insights into the inference of bottom-oxygen levels, food availability and bottom water energy on high-productivity seafloors. Quantitative data for molluscan analysis were collected from each of the main lithologies that comprises the high-productivity facies-mosaic of the Mishash sea, i.e., organic-rich carbonates, thin-porcelanite and -chert beds, and loosely- to densely-packed shell beds. Diagenesis has obliterated faunal remains in carbonate layers. Pollution studies in modern environments have demonstrated the close connection between stressful conditions – such as organic load, toxicity and water and sediment aeration – and benthic community structure, as expressed through species richness, evenness, taxonomic composition, body sizes, and trace fossil assemblages. These connections and behavior patterns were investigated in the varied benthic fossil assemblages of the Phosphate Member of the Mishash Formation. Bivalves and gastropods dominated the macrofauna in all rock types but in varying abundance and taxonomic compositions, changing from diverse assemblages (15–22 species per sample) to less rich ones (5–7 species) and even barren intervals. This range of abundance neatly describes the lateral mosaic and heterogeneous nature of the upwelling environment of the Upper Campanian and is comparable to the mosaic of sub-facies that characterize modern upwelling facies-belts today (e.g., in the Benguela upwelling system). The variations shown for the macrobenthic assemblages within the Mishash Formation high-productivity regime are associated with proximity to the high-productivity center or to periods of intensified upwelling. The inferred, variable oxygen levels and organic-load changes point up how dynamic these regions really are. This is expressed in community structure and composition, which vary between two extremes: The low-oxygen biofacies 1 is characterized by a smaller range of shell sizes, infaunal deposit feeders and low species richness, while the high-oxygen biofacies 3 is characterized by greater ranges in shell size, infaunal but also epifaunal suspension feeders and higher species richness. The chert and porcelanite beds exhibit a variety of sub-environments and seem to represent an intermediate facies that bridges the two extremes of biofacies 1 and 3. The above ecological criteria may provide evidence of the original carbonate environment for the thin-chert and -porcelanite layers. The chert or porcelanite samples that contain a moderately even and rich assemblage of species intermediate to the values of the two end-member facies are taken to represent a shallow-water carbonate marine environment marginal to the upwelling center, whereas layers of chert and porcelanite containing very low species richness and evenness are taken to characterize primary depositional environments proximal to the paleo-upwelling cells.

A log correlation of the Rotliegend of the northern Cleaver Bank High: the search for controls on reservoir sand distribution

Gamma ray log correlations of some 50 Open File wells were used to investigate the distribution and relative age of aeolian Leman Sandstone developments across the northern Cleaver Bank High. The correlations led to a sequence stratigraphic interpretation based on a depositional record of climatically forced fluctuations in the level of the Silverpit Formation playa lake system. This confirmed the widely reported diachroneity of the Leman Sandstone and identified the onlapped Base Permian palaeotopography and climatically forced, generally positive, changes in the water budget of the Silverpit Lake as the chief controls on aeolian sand distribution. However, a ‘1/f n ’ power law distribution of changes in water budget is preferred to the idea of strictly cyclic climatic control. On this basis, the transgression of the northern Cleaver Bank High and the occurrence of aeolian and lacustrine end-member facies within depositional sequences of uniform thickness can be understood. Interval isochores were used to derive palaeotopographic maps of the Base Permian. Taken in conjunction with the observed facies of the onlapping depositional sequences, the maps locate under-explored basal aeolian Leman Sandstone play fairways.

Classification of clastic coastal depositional environments

This paper proposes a new classification for clastic coastal environments which includes the full range of major depositional settings including deltas, strand plains, tidal flats, estuaries and lagoons. This classification includes both morphologic and evolutionary components and is based on dominant coastal processes. It has the potential to predict responses in geomorphology, facies and stratigraphy. The significance of this classification is its evolutionary capability, and its inclusion of all major clastic coastal depositional environments, making it more comprehensive than previous classifications. We employ a ternary process classification with two axes. The first (horizontal axis) is defined as the relative power of wave versus tidal processes. The second (vertical) axis represents relative fluvial power (increasing upward). A ternary diagram defined by these axes can be used to illustrate the genetic process-response relationships between major coastal environments. The evolutionary classification combines the concept of two sediment sources (river and marine) with a relative sea-level parameter to classify embayed as well as linear and elongate/lobate shorelines. This approach identifies the evolutionary relationships between coastal sedimentary environments. The new ternary approach to process classification can be applied to estuaries and lagoons to define wave and tide end-member facies models, each consisting of a tripartite facies zonation. The evolutionary classification is compatible with sequence stratigraphy because sediment supply and relative sea level are included, and serves as a starting point for more refined coastal stratigraphic analyses.

Geology and Seismic Modeling of an Aneth-Type Desert Creek Mound Trend, Paradox Basin, Southwest Colorado: ABSTRACT

In the Paradox basin of southwest Colorado, a major facies change from thick, porous algal dolomite to thin, tight anhydrite occurs within the Desert Creek zone of the Pennsylvanian Paradox Formation. This northwest-trending algal mound belt is subparallel to and separated from the ancestral Uncompaghre mountain front by lagoonal anhydrite and nearshore arkosic fanglomerates. In the belt, carbonate rocks attain a gross thickness of 200 ft, which can include 100 ft of reservoir quality dolomite. These rocks have an updip and abrupt eastward facies change to thin evaporites that form a regional master trap for potentially significant hydrocarbon accumulations. Reservoir parameters and trapping mechanisms are similar to Utah's Aneth field, where oil production exceeds 340 million bbl. The subsurface stratigraphic changes have been seismically modeled with a microcomputer for the purpose of defining the extent of each facies. The modeling is based on sonic logs from two wells that exhibit the end-member facies of thick, porous dolomite versus thin, tight evaporite. An interpolation routine creates intermediate logs between these end-members, thereby defining the geometry of the model. The depth model is convolved with a zero-phase wavelet of 10/20-40-60 hz, and the result is an extra seismic cycle where reservoir thicknessmore » exceeds 40 ft. Updip loss of the cycle defines the transition from reservoir dolomite to trap anhydrite. The modeling demonstrates that this major facies change can be detected at frequencies reasonably attainable by modern seismic methods. Regional subsurface stratigraphic analysis and modern seismic data provide an integrated approach to hydrocarbon prospecting in the Pennsylvanian Desert Creek zone.« less