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Refining palaeoenvironmental analysis using integrated quantitative granulometry and palynology

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Abstract

Accurate palaeoenvironmental analysis is at the heart of producing reliable interpretations and depositional models. This study demonstrates a multivariate statistical approach to facies analysis based on relationships between grain size and quantitative palynology. Our methodology has the advantage that it can be used on small amounts of sample, such as core or well cuttings, as the basis for facies analysis. Proof of concept studies involving the collection of grain-size and palynological datasets from well-exposed outcrops of the Middle Jurassic, Lajas Formation of the Neuquen Basin, Argentina, demonstrate that canonical correspondence analysis can be used to consistently recognize facies and aid in the determination of depositional environments. This study demonstrates the link between depositional facies, grain-size distribution, palynomorph hydrodynamics and assemblage taphonomy of palynomorphs. This knowledge can be transferred into a semi-automated statistical facies prediction technique for the subsurface in complex depositional settings, particularly when calibrated against conventional sedimentary facies analysis. Supplementary material: The full set of grain-size data and statistical scores are available at:

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... During investigation of the Middle Jurassic (Bajocian) Lajas Formation, Neuquén Basin, Argentina (Stukins et al. 2013(Stukins et al. , 2017, tetrads of Classopollis pollen were recorded in high numbers. This material is reexamined here, focusing on Classopollis and tetrads thereof. ...
... 147 before the separation of the tetrad, which is deemed unusual. The high number of tetrads recorded in this study does not represent environmental stress, but is rather a taphonomic signal for preferential deposition of buoyant tetrads out of suspension, as opposed to those separated into individual pollen grains from current derived sediments, within a pro-delta depositional environment (Stukins et al. 2017). This has also been documented in other marginal marine settings (Leckie and Rumpel 2003), and there are extensive published records of Classopollis tetrads from throughout the Mesozoic (Online Supplemental File) highlighting how common tetrad occurrence is. ...
... In other depositional settings of the Lajas Formation, there are fewer tetrads recovered (Stukins et al. 2013). This is due to the grains breaking apart because of the turbulent action of the current controlled processes in these depositional paleoenvironments (Carvalho et al. 2006;Stukins et al. 2013Stukins et al. , 2017. ...
Article
Aberrant forms of many different spore and pollen taxa (sporomorphs) are often used to assess timings and extent of environmental stress at major extinction/climate events. However, little is known about the normal level of malformations in these taxa. Malformation can manifest in several different ways, including significant size differences (± from the accepted range for the species), retention of pollen/spores within tetrads, and unusual and inconsistent morphological aberrations. This study analyses one commonly used pollen genus, Classopollis, and its aberrant forms, from the Bajocian of Argentina. Tetrads, including those incorporating malformed/aborted pollen grains, are found to be common, showing that tetrads of Classopollis are not a reliable signal of major environmental disturbance. The results and discussion presented here emphasize why palynological data must be interpreted in context of the depositional dynamics, facies changes and how they affect the assemblages in conjunction with understanding variations in the strategies of individual flora.
... Previous examinations of Prince Creek palynomorph assemblages and microbiota have mainly been used for chronostratigraphic purposes to identify significant age diagnostic taxa from presence-absence or semi-quantitative data [28][29][30][31][32]. Palynomorphs provide a basis for biochronology, as well as a wealth of paleoenvironmental information. Moreover, the three-dimensional stratigraphic distribution of palynomorphs in a sedimentary body is linked to the stratigraphic architecture of enclosing sediments and, by inference, to the occurrence of paleoenvironments [36][37][38][39][40][41][42][43][44][45][46][47][48][49]. The purpose of this study is to quantitatively analyze palynomorph and microbiotic assemblages contained within the PCF paleosol horizons described previously by Flaig et al. [22]. ...
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Late Cretaceous coastal plain deposits of the Prince Creek Formation (PCF) offer a rare glimpse into an ancient, high-latitude, arctic greenhouse ecosystem for which there is no modern analog. Here, we employ quantitative biofacies analysis to explore the spatio-temporal variability in PCF palynomorph and microbiota assemblages from nine paleosol horizons exposed along the Colville River, North Slope, Alaska. Biofacies results provide insight into paleoenvironmental controls on the coastal plain ecosystem. Cluster and ordination analyses recognize five biofacies and the following two assemblage types: (1) fern and moss dominated assemblages and (2) algae dominated assemblages. Ordination arrays biofacies along environmental gradients related to soil moisture and marine influence. Fern and moss dominated biofacies from regularly water-logged paleosols along lake and swamp margins on the lower delta plain clearly segregated from algae dominated assemblages of periodically drier levee-overbank paleosols. These results support previous interpretations from the sedimentology, paleopedology, and geochemistry of PCF paleosols that suggest that fluctuations in the water table, related to seasonal river discharge and variations in topography and drainage, controlled soil development and vegetation growth across the coastal plain. This quantitative biofacies-based approach provides an independent predictive tool and cross-check for interpreting environmental conditions along any ancient coastal ecosystem.
... The samples have high sand percentages (Table 2, Figs. 6 and 7) which is not favourable to pollen preservation (Stukins et al., 2017;Evaldt et al., 2012), since sandy sediments tend to be more porous, pollen sporopollenin is in greater contact with oxygen and is then oxidized (Evaldt et al., 2014). ...
Article
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The Coastal Plain of Rio Grande do Sul (CPRS) underwent several climatic and environmental changes during the Holocene. In order to characterize this evolution, a paleoclimatic and paleoenvironmental reconstruction was carried out in the Butiazal de Tapes region, on the west shore of the Patos Lagoon. In this region there is a remnant of butiazal, a native ecosystem characterized as an arboreal savanna dominated by palms of the genus Butia. This vegetation is very sensible to climatic changes, making the bioindicators of this ecosystem excellent proxies for a paleoclimate and paleoenvironment study. In order to perform these reconstructions, phytolith analysis and palynology, as well as X-ray diffraction, particle size analysis, Total Organic Carbon determinations, radio-carbon dating (C14) and determination of the carbon isotopic ratios. With this multiproxy approach it was possible to perceive the not-so-expressive climate changes of the Holocene in the region and to make relationships between sea level variations and paleoenvironmental changes. Relationships were also made between changes in bioindicators and particle size results, showing paleoenvironmental changes caused by the transition from a transgression to a regression.
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Spore and pollen (sporomorph) assemblages from Middle Jurassic marine deposits of the Brent Group in the northern North Sea are investigated to assess temporal and spatial variations in vegetation and depositional processes. Four wells were sampled for palynology from the Penguins Cluster and the Don North East fields through the Rannoch Formation shoreface succession. Hyperpycnite deposits occur throughout, but are concentrated within the lower part of the section. These are expressed by sand-prone beds displaying waxing and waning current motifs, normally graded muddy beds and structureless mudstones. Hyperpycnal/hypopycnal deposits resulting from episodic river flooding represent important sedimentary features as they may be preserved below fair weather wave base in more offshore settings and potentially be the only record of the former presence of a nearby river mouth. The hyperpycnites typically contain abundant Botryoccocus spp., Amorphous Organic Matter (AOM) and hinterland sporomorph taxa with relatively few marine components compared to associated marine shoreface facies. Variations in palynofacies assemblages and Botryococcus spp. abundances indicate frequent river mouth avulsion. Ordination of samples using non-metric multidimensional scaling (NMDS) indicates that shoreface samples of the sampled wells are relatively distinct, but hyperpycnite samples are highly similar regardless of their sampled well. This suggests that depositional processes and spore/pollen sources (i.e. catchment zones) were similar among hyperpycnite events across different wells. Abundant bisaccate pollen, Botryococcus spp. and AOM within interpreted hyperpycnites suggest sediment mixing along the fluvial drainage path during flooding events. The terrestrial signature of hyperpycnite sporomorph assemblages demonstrates that underflows remained coherent as they descended the shoreface profile with little turbulent mixing with ambient marine waters. Sporomorph assemblages display few large changes through time suggesting vegetation on the adjacent coastal plain was relatively static through the studied interval.
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The Middle Jurassic Cuyo Group in the~uthern Neuquen Basin comprises shallow marine to continental beds up to 1200 m of thickness. Twelve sedimentary sections were measured through the succession, in which facies and sequence stratigraphic analysis have been carried out. The study allowed us to recognize eight depositional sequences, related to both third and fourth-order scales (sensu Exxon). Because these outcrops extend from mainly fluvial deposits on the east to coeval shallow marine deposits on the west, they allow to analyze sequence stratigraphic relationships, facies, and paleoenvironmental changes during the evolution of a depositional sequence. (1) In shallow marine environments, third-order sequences start with a major erosive and non-depositional event followed by up to 20 meters of sandy-braided fluvial to high sinuosity estuarine channel deposits linked with a late lowstand systems tract - early transgressive systems tract stage. The transgressive systems tract deposits are characterized by 2-6 meters thick shallowing upward tidal bars with a retrogradational parasequence set. Highstand systems tract deposits starts with open-shelf mudstones followed by 4-6 meters of shallowing upward cycles of input to wave dominated stream-mouth bars, with a progradational parasequence set. (2) In mainly continental areas, third order sequences begin with up to 10 meters of coarse grained braided-river deposits resting over a regional discontinuity. These deposits are interpreted as developed in early transgressive systems tract stage. The Transgressive systems tract deposits are expressed by 12-14 meters of marsh levels, with tidal influence. The highstand systems tract deposits start with off-shore marine mudstones, followed by a strongly prograding input-dominated deltaic systems, and ending with thick high-sinuosity sandy fluvial deposits.
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A common element of sequence stratigraphic models for ancient shallow marine systems is the shift from wave-dominated shoreface deposition during highstand to tidal/ estuarine deposition within incised valleys during early base-level rise. In this widely used model, tide-dominated sedimentation is thus restricted to specific base-level positions, which has important implications for prediction of reservoir geometry and quality. However, this model neither adequately explains nor allows reservoir prediction in aggradational tide-dominated reservoir successions. The Lower-Middle Jurassic Lajas Formation of the Neuquén Basin, Argentina, comprises 500 m of well-exposed tidedominated facies deposited within low-frequency unconformity-bounded sequences, which are tide dominated throughout. The maintenance of macrotidal conditions over several complete base-level cycles during the 8 m.y. within which the Lajas was deposited is interpreted as being due to the structural topography inherited from rifting, in which the whole sub-basin behaved as a structurally controlled estuary. Facies associations include prograding tidal deltas, stacked sandy tidal channel fills, extensive lagoonal deposits, and tidal-flat successions, which are locally cut by heterolithic tidal-channel-fill facies. Despite the narrow bathymetric depositional range of the Lajas sediments and the complex facies variability, parasequences can be defined and correlated on a kilometer scale. Unlike simpler fluvial or wave-dominated shallow marine systems, there are two types of parasequence, each with different net:gross sandstone ratios and different vertical/lateral reservoir-quality trends. These parasequences form the building blocks for accurate reservoir modeling in complex tidal reservoirs.
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Crevasse subdeltas develop on modern river-dominated delta plains, and may be affected by the interaction of river currents and marine processes. However, their sedimentology and stratigraphic architecture is poorly constrained, leading to simplistic depositional models of delta-plain systems in the ancient record. Extensive exposures of the Middle Jurassic Lajas Formation permit the architecture, main stratigraphic surfaces, and lateral and vertical facies variations of crevasse subdelta deposits to be constrained. Lower-delta-plain successions studied in the Lajas Formation consist of up to 5-m-thick distributary channels and interdistributary-bay deposits, interpreted as crevasse subdeltas. Crevasse subdelta deposits consist of small-scale lenticular units (∼ 1–2 m thick) interpreted as crevasse channels and upward-coarsening and upward-thickening packages (∼ 2 m thick) with clinothems interpreted as crevasse mouth bars. These deposits preserve interbedding of coarser and finer sediments that are interpreted as river flood and interflood couplets associated with variations in river discharge. River flood beds are commonly structureless and erosionally based, and show little evidence of tidal action and brackish-water conditions. Interflood deposits show rhythmically distributed mudstone drapes, bimodality, and brackish trace fossils. This study highlights an important but largely undocumented component of interdistributary deposits consisting of tide-influenced, but strongly river-dominated, prograding depositional bodies. An implication is that some coarsening-upward, forward-accreting units previously interpreted from the rock record as interchannel “tidal bars” may instead represent minor mouth bars of tide-influenced crevasse subdeltas. Furthermore, present-day crevasse subdeltas are restricted to river-dominated delta systems that flow into semi-enclosed or enclosed seas and lakes with microtidal conditions and limited wave action, which is comparable to paleogeographic reconstructions for the Neuquén Basin during the Middle Jurassic.
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The Early Jurassic Tilje Formation on the Halten Terrace, offshore mid-Norway, was deposited in a relatively narrow but long seaway connecting the Boreal Ocean in the north and the Tethys Ocean in the south. Sediments of the Tilje Formation in the Smørbukk and Heidrun Fields have been classified into ten facies associations. Many of the lithofacies are mud rich and typified by strong grain-size contrasts. In addition, all but two of the facies associations are tidally influenced or dominated. As a result, 80 percent of the total rock volume consists of very heterolithic sediments characterised by rapidly alternating grain-size changes between mudstone or siltstone and fine- to medium-grained sandstone. In the Tilje Formation, the recognition and interpretation of heterolithic facies is crucial to understanding the depositional conditions and stratigraphic architecture. The classification scheme and the associated facies breakdown in cored wells served to define two successive conceptual depositional models that are placed in a sequence stratigraphic framework. The lower part of the Tilje Formation (T1 and most of T2) are envisaged to have formed in response to base-level fall, creating a series of low-relief valleys, and subsequent base-level rise resulting in the formation of a tide- and wave-dominated estuarine system. The upper part of the Tilje Formation (top of T2 to T6) is interpreted to have formed as a progradational and retrogradational, tide- and fluvial-dominated delta system. These two contrasting depositional styles resulted in different three-dimensional facies architectures, relative facies proportions, and facies stacking patterns, which have implications for reservoir model-building methods.
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NOTE: A HIGH RESOLUTION PDF OF THIS WORK CAN BE SENT ON REQUEST: j.h.vandenberg@uu.nl------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ Atlas of sedimentary structures in estuarine and tidally-influenced river deposits of the Rhine-Meuse-Scheldt system Their application to the interpretation of analogous outcrop and subsurface depositional systems This prolifically illustrated atlas is the outcome of an outstanding 'town and gown' cooperation between the Norwegian oil company, Statoil, and the Faculty of Geosciences at Utrecht University in the Netherlands. Recognizing the present focus on sedimentary sequences rather than the diagnostics of sedimentary structures, the authors help to redress the imbalance by providing a comprehensive overview of characteristic sedimentary structures formed in recent inshore tidal environments and in tidally-influenced river channels subject to microtidal to mesotidal conditions. The work also establishes the basis for seamlessly transferring information from modern depositional systems to aid the interpretation of analogous outcrops and, more especially, subsurface depositional systems revealed only in cores. Having dealt with the fundamentals of tides, tidal currents and sediment dynamics, the deposits of the Rhine-Meuse-Scheldt system are analyzed in extraordinary detail from temporary construction pits in the Netherlands and a large open cast mine in Germany. These exposures revealed a variety of exceptionally well-preserved fluvial, tidal and transitional fluvial-tidal sediments whose information content was captured in meticulous diagrams and sections, high quality lacquer peals and photographic images. Their interpretation was facilitated by the environmental background of the Holocene deposits, which has been reconstructed here with unprecedented accuracy from historical and recent hydrographic maps and measurements of river flow and tides (some of the records go back several centuries). The Rhine-Meuse-Scheldt system is then used as a standard of reference and a comparator for worldwide ancient outcrop analogues believed to cover a similar range of natural variability. The information obtained from the recent exposures (and, to some extent, the outcrop analogues) is finally applied to the interpretation of subsurface core data from the Norwegian Continental Shelf to illustrate a variety of analogous and comparable tidal facies and sedimentary structures encountered in prospective Late Triassic to Jurassic sedimentary systems. The uniqueness of the atlas relates not only to the unparalleled overview of sedimentary structures in the recent deposits but also to the incorporation of a vast amount of unpublished data and the structured collation of information dispersed throughout the sedimentological literature. Furthermore, it provides sedimentologists and reservoir geologists alike with criteria, models and a library of examples which can be used to understand better the spatial distribution and character of reservoir heterogeneities. Of particular value is a list of the limited diagnostic criteria that can be gleaned from cores and an appreciation of information loss caused by burial-related compaction and consolidation.
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The Tilje Formation (Early Jurassic; 120-300 m thick) consists predominantly of heterolithic deposits and is thought to have accumulated in tide-dominated estuarine and deltaic environments in an active rift setting. Anomalously thick (>0.5 cm) and internally structureless mudstone layers, which are interpreted to represent fluid-mud deposits, are widespread and occur in three different environmental settings: (1) in the basal part of upward-fining tidal-fluvial channels where they generate upward-sanding successions: (2) in the deposits of mouth bars and terminal distributary channels where they are associated with the coarsest sands and the least-bioturbated sediments, suggesting deposition during tidally modulated river floods; and (3) in delta-front successions where they immediately overlie thick, wave-generated storm beds, suggesting that these fluid-mud deposits result from wave resuspension of previously deposited mud. These observations provide criteria for the recognition of ancient fluid-muds and for interpreting their origin. The tectonic setting may be responsible for their abundance.
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Book
Paleopalynology, second edition, provides profusely illustrated treatment of fossil palynomorphs, including spores, pollen, dinoflagellate cysts, acritarchs, chitinozoans, scolecodonts, and various microscopic fungal and algal dispersal bodies. The book serves both as a student text and general reference work. Palynomorphs yield information about age, geological and biological environment, climate during deposition, and other significant factors about the enclosing rocks. Extant spores and pollen are treated first, preparing the student for more difficult work with fossil sporomorphs and other kinds of palynomorphs. Recognizing that palynomorphs occur together in rocks because of chemical robustness and stratigraphic distribution, not biological relationship, the central sections are organized stratigraphically. Among many other topics presented are the sedimentation and geothermal alteration of palynomorphs, and palynofacies analysis. An appendix describes laboratory methods. The glossary, bibliographies and index are useful tools for study of the literature.
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Facies models for regressive, tide-influenced deltaic systems are under-represented in the literature compared with their fluvial-dominated and wave-dominated counterparts. Here, a facies model is presented of the mixed, tide-influenced and wave-influenced deltaic strata of the Sego Sandstone, which was deposited in the Western Interior Seaway of North America during the Late Cretaceous. Previous work on the Sego Sandstone has focused on the medial to distal parts of the outcrop belt where tides and waves interact. This study focuses on the proximal outcrop belt, in which fluvial and tidal processes interact. Five facies associations are recognized. Bioturbated mudstones (Facies Association 1) were deposited in an offshore environment and are gradationally overlain by hummocky cross-stratified sandstones (Facies Association 2) deposited in a wave-dominated lower shoreface environment. These facies associations are erosionally overlain by tide-dominated cross-bedded sandstones (Facies Association 4) interbedded with ripple cross-laminated heterolithic sandstones (Facies Association 3) and channelized mudstones (Facies Association 5). Palaeocurrent directions derived from cross-bedding indicate bidirectional currents which are flood-dominated in the lower part of the studied interval and become increasingly ebb-directed/fluvial-directed upward. At the top of the succession, ebb-dominated/fluvial-dominated, high relief, narrow channel forms are present, which are interpreted as distributary channels. When distributary channels are abandoned they effectively become estuaries with landward sediment transport and fining trends. These estuaries have sandstones of Facies Association 4 at their mouth and fine landward through heterolithic sandstones of Facies Association 3 to channelized mudstones of Facies Association 5. Therefore, the complex distribution of relatively mud-rich and sand-rich deposits in the tide-dominated part of the lower Sego Sandstone is attributed to the avulsion history of active fluvial distributaries, in response to a subtly expressed allogenic change in sediment supply and relative sea-level controls and autocyclic delta lobe abandonment.
Article
The Middle Jurassic Lajas Formation of the Neuquén Basin, Argentina, is interpreted as a 400–600 meter thick aggradational, macrotidal tide-dominated deltaic system. Tide-dominated systems are most commonly found in estuarine settings, and thus the biota found in such settings are generally influenced by lowered salinities. This paper documents the ichnology of the tide-dominated Lajas Formation, which includes a diverse ichnofauna, much of which is subject to tidal conditions, though not necessarily to reduced salinity. The ichnogenera recorded include: Asteriacites, Arenicolites, Asterosoma, Dactyloidites, Didymaulichnus, Diplocraterion, Chondrites, Cruziana, Helminthorhaphe, Macaronichnus, Ophiomorpha, Parahaentzschelinia, Palaeophycus, Planolites, Polykladichnus, Protovirgularia, Rhizocorallium, Rosselia, Schaubcylindrichnus, Scolicia, Siphonichnus, Taenidium, Teichichnus, and Thalassinoides. The range of tidal environments from which the trace fossils were collected encompasses tidal flats, tidal channels, and tide-dominated delta fronts, passing offshore into shelf mudstones of the Los Molles Formation and landward into the fluvial deposits of the Challaco Formation. The paleoenvironmental controls on the distribution of ichnotaxa are discussed in relation to Lajas Formation ichnology and sedimentology.
Article
The coefficient of sorting in sedimentary petrography is commonly considered to be the statistical measure introduced by Trask. Several investigators have objected to the use of the Trask coefficient on theoretical grounds. Others have continued to uphold its usefulness. Two new sorting coefficients have been introduced within recent years (Inman's and Folk and Ward's sorting measures), based on the geometry of the normal curve. On theoretical grounds these measures should more adequately describe the sorting characteristics of sediments than does the coefficient of Trask. However, some practical considerations argue against the use of these new sorting parameters for many types of sediments. Empirical correlations have been made in this study between the three sorting measures (Trask's, Inman's, and Folk and Ward's) and the standard deviation. The standard deviation is a moment measure which represents the most suiTable statistic for describing the sorting characteristics of sedimentary rocks. The empirical correlations permit an appraisal of the advantages and disadvantages of each sorting parameter which has not been possible in the past. Correlation diagrams show that both the Trask coefficient and the Inman sorting measure approximate the standard deviation and that a high correlation exists between the standard deviation and the sorting measure of Folk and Ward. The Inman measure is more satisfactory for describing the sorting of moderate to poorly sorted sandstones. The Trask coefficient is more satisfactory for describing very well to well-sorted sandstones. The Folk and Ward sorting measure appears to be satisfactory for the entire range of sorting characteristics. A new genetic classification of sorting based on the standard deviation and on environmental significance is recommended. The standard deviation ranges for sands from different environments are tabulated. Three figures in this paper demonstrate how the standard deviation of the sediment distribution compares with the sorting coefficients of Trask, Inman, and Folk and Ward. Trask sorting values corresponding to the class intervals of the recommended standard deviation classification serve as a new sorting classification for studies where grain size is expressed in millimeters. A new genetic classification of sorting based on the standard deviation and on environmental significance is recommended. The standard deviation ranges for sands from different environments are tabulated. Three figures in this paper demonstrate how the standard deviation of the sediment distribution compares with the sorting coefficients of Trask, Inman, and Folk and Ward. Trask sorting values corresponding to the class intervals of the recommended standard deviation classification serve as a new sorting classification for studies where grain size is expressed in millimeters. To determine the extent of deviation from lognormality of the grain-size distribution of sandstones, a more sensitive method than the straight-line probability plot has been employed. A normal curve can be defined as a frequency distribution which has a skewness (third moment) of 0 and a kurtosis (fourth moment) of 3. A study of 612 ancient and modern sand samples indicates that the grain-size distribution of most sands and sandstones does not follow a lognormal function. Omitting the sign for skewness, the mean for skewness for the samples studied is 0.76 and the mean for kurtosis is 5.31. These data indicate that the grain-size distribution curve of the average sand is skewed and considerably more peaked than that of the normal curve. There is a tendency for deviation from lognormality as the sands are being deposited. River and dune sands are for the most part positively skewed, whereas beach sands are generally negatively skewed. Deviation from lognormality of the grain-size distribution of sands is an environment-sensitive parameter. The standard deviation (sorting index) of a sand is another significant environment-sensitive textural attribute. plot of deviation from lognormality as expressed by the skewness (third moment) against the sorting index (standard deviation) for medium-to fine-and very fine-grained sands serves to distinguish beach sands from river sands.
Article
Many modern deltas show complex morphologies and architectures related to the interplay of river, waves and tidal currents. However, methods for extracting the signature of the individual processes from the stratigraphic architecture are poorly developed. Through an analysis of facies, palaeocurrents and stratigraphic stacking-patterns in the Jurassic Lajas Formation, this paper: (i) separates the signals of wave, tide, and river currents; (ii) illustrates the result of strong tidal reworking in the distal reaches of deltaic systems; and (iii) discusses the implications of this reworking for the evolution of mixed-energy systems and their reservoir heterogeneities. The Lajas Formation, a sand-rich, shallow-marine, mixed-energy deltaic system in the Neuquén Basin of Argentina, previously defined as a tide-dominated system, presents an exceptional example of process variability at different scales. Tidal signals are predominantly located in the delta-front, the subaqueous platform and the distributary channel deposits. Tidal currents vigorously reworked the delta front during transgressions, producing intensely cross-stratified, sheet-like, sandstone units. In the subaqueous platform, described for the first time in an ancient outcrop example, the tidal reworking was confined within subtidal channels. The intensive tidal reworking in the distal reaches of the regressive delta front could not have been predicted from knowledge of the coeval proximal reaches of the regressive delta front. The wave signals occur mainly in the shelf or shoreface deposits. The fluvial signals increase in abundance proximally but are always mixed with the other processes. The Lajas system is an unusual clean-water (i.e. very little mud is present in the system), sand-rich deltaic system, very different from the majority of mud-rich, modern tide-influenced examples. The sand-rich character is a combination of source proximity, syndepositional tectonic activity and strong tidal current reworking, which produced amalgamated sandstone bodies in the delta front area, and a final stratigraphic record very different from the simple coarsening-upward trends of river-dominated and wave-dominated delta fronts.This article is protected by copyright. All rights reserved.
Chapter
The Middle Jurassic (Bajocian–Bathonian) Beryl Formation of the Beryl (Bravo) Field (Viking Graben), has been produced since 1979 and contains estimated remaining reserves of 180 × 10 ⁶ BBL oil. Previous studies of this deltaic/shallow marine succession have defined five reservoir units. Production characteristics indicate that this lithostratigraphically-based zonation scheme does not adequately define reservoir flow units and constrain sweep efficiency. Here, a more refined reservoir zonation is used to form the basis for an upgraded reservoir simulation model. High density well coverage (50 wells) in the Beryl Bravo area allows the construction of a high resolution reservoir zonation scheme within the main producing interval of the Beryl Formation: Unit 3. The scheme is based on integration of sedimentology, ichnology, biostratigraphy, dynamic data and well log character. Nine reservoir zones (consisting of nine genetic sequences) have been defined and confirmed and are supported by drilling and production data. The complex reservoir zonation scheme is based upon the recognition of regionally extensive marine/brackish mudstones which, in many cases, form traceable pressure barriers across tilted fault blocks. This framework has allowed subdivision into 3 to 25 m thick correlatable units. Individual units record the progradation and transgression of tide-dominated deltaic deposits. Variations in the thickness and development of these units result from sedimentation in actively subsiding half-graben that were subject to regionally extensive base-level changes. The integrated approach taken here has substantially improved the understanding of the architecture and production characteristics of the Beryl Formation.
Article
Numerous measures are used in the literature to describe the grain-size distribution of sediments. Consideration of these measures indicates that parameters computed from quartiles may not be as significant as those based on more rigorous statistical concepts. In addition, the lack of standardization of descriptive measures has resulted in limited application of the findings from one locality to another. The use of five parameters that serve as approximate graphic analogies to the moment measures commonly employed in statistics is recommended. The parameters are computed from five percentile diameters obtained from the cumulative size-frequency curve of a sediment. They include the mean (or median) diameter, standard deviation, kurtosis, and two measures of skewness, the second measure being sensitive to skew properties of the "tails" of the sediment distribution. If the five descriptive measures are listed for a sediment, it is possible to compute the five percentile diameters on which they are based (phi 5 , phi 16 , phi 50 , phi 84 , and phi 95 ), and hence five significant points on the cumulative carve of the sediment. This increases the value of the data listed for a sediment in a report, and in many cases eliminates the necessity of including the complete mechanical analysis of the sediment. The degree of correlation of the graphic parameters to the corresponding moment measures decreases as the distribution becomes more skew. However, for a fairly wide range of distributions, the first three moment measures can be ascertained from the graphic parameters with about the same degree of accuracy as is obtained by computing rough moment measures.
Article
Three-dimensional geological modelling and reservoir simulations of an outcrop analogue to reservoirs of the Halten Terrace, offshore mid-Norway, are presented. The model of the outcrop incorporates (a) a detailed sedimentological understanding, (b) a set of stochastic realizations highly-constrained to the geological models and (c) streamline waterflood flow simulations assuming typical subsurface petrophysical properties from the Halten Terrace. Statistical analysis of simulation results has been used to show the importance of both the facies architecture and the spatial petrophysical model. The outcrop model has significantly improved the estimation of facies dimensions and architecture and gives a valuable insight into understanding petroleum reservoirs of the Halten Terrace.
Article
Tidal depositional systems are often interpreted as lowstand/ transgressive estuarine deposits within sequences that are either wave or river dominated during highstand times. The Middle Jurassic Lajas Formation of the Neuquén Basin, Argentina, comprises 600 m of well-exposed tide-dominated facies deposited within four unconformity-bounded sequences, spanning approximately 4.5 Ma. Facies associations include tide-dominated deltas, sandy-heterolithic tidal channel fills and extensive progradational tidal-flat successions, which are locally cut by heterolithic tidal channel fills. Despite the narrow bathymetric depositional range and the complex facies variability, flooding surfaces can be defined and mapped along a 48 km-long outcrop belt. These flooding surfaces allow definition of three distinct types of parasequence that exhibit coarsening-upwards, fining-upwards and coarsening- to fining-upwards motifs. Sequence boundaries are marked by widespread, but shallow, incision, and the juxtaposition of stacked fluvial/tidal channel fills on a variety of subtidal and intertidal facies. Unconventional grain-size changes at sequence boundaries can occur where basinward facies shifts are marked by juxtaposition of heterolithic-argillaceous intertidal/supratidal mudflat deposits on subtidal sandflat facies. The maintenance of macrotidal conditions through complete base-level cycles is interpreted as being due to the structural topography inherited from rifting, causing the whole sub-basin to behave as a structurally controlled embayment.
Article
Silicified coniferous wood is commonly found in the Middle Jurassic (Bajocian) Lajas Formation of the Neuquén Basin, west-central Argentina. The wood is preserved in a succession of sandstones, siltstones, mudstones and minor conglomerates that represent deposition as part of tide-dominated deltas and fluvial plains across which large rivers meandered. Most of the wood occurs as dense accumulations in the tidal and fluvial channels. The wood fragments are worn, abraded, and lack both bark and branches, indicating that they were transported prior to deposition. The material is typically 20-30 cm long, with only infrequent examples of larger trunks (c. 80 cm in diameter, 5-6 m long). No trunks were found with root systems attached, and no stamps were found upright and in situ. The fossil wood genus Araucarioxylon dominates the assemblage. Growth rings are largely absent from the specimens, although one sample (from Rhea Gorge) displays highly diffuse and irregularly spaced rings, suggesting that it grew in different conditions from the others studied. Large-scale interpretations for southern Gondwana suggest a seasonally dry climate. However, these fossil wood specimens show no evidence of this, indicating that in this area at least the effects of any seasonal component to the climate may have been overridden by factors such as a locally plentiful supply of water and/or the possibility that growth was to some extent controlled by the taxonomic affinity of the trees.
Article
Tide-dominated deltas are poorly known from the stratigraphic record and are notoriously complex, owing to the wide spectrum of facies encountered and their spatial/temporal variability. The tide-dominated deltaic palaeoenvironment combines the ecological harshness of brackish-water settings with complex tidal channel/tidal-flat type facies architecture on the delta top, in association with more classic deltaic facies-stacking patterns. The Ile Formation is interpreted herein as a tide-dominated delta deposited in a microtidal setting. Its palaeoenvironments are interpreted based on a combination of ichnology, ichnofabric analysis and sedimentology. Ichnofabric stacking patterns are used to elucidate the internal architecture of the notoriously problematic aggradational multi-storey tidal channel units. The tide-dominated deltas of the Ile Formation have a distinctive ichnological signature that may be used to characterize tide-dominated deltas. In comparison to typical river-dominated deltas the Skolithos ichnofacies is less well developed and ichnodiversity is lower than expected in wave-dominated deltas. The ichnofabric model presented has potential to be used, with modification, in other tide-dominated deltaic settings.
Article
This work compares the approaches taken by the two schools of applied ichnology (ichnofacies and ichnofabric analysts) and proposes an integrated approach for future improvement. There are many similarities between the two approaches, which mainly differ in the resolution at which ichnological analysis is undertaken. Ichnofabric work is by necessity performed on a bed-by-bed basis and interpreted at a scale consistent with that of the sedimentologist's facies. Ichnofacies studies, in contrast, seldom report bed-by-bed changes in ichnological content, instead highlighting bedset to facies association-scale changes in ichnology through comparison with summary models of ichnological trends.
Article
Multivariate techniques summarize large sets of data and present their major trends in a graphically simplified manner. This paper discusses ways in which these techniques can be used to correlate biostratigraphical data. Stratigraphically constrained cluster analyses, which highlight distinct zones, may be performed on a number of sections. These can be plotted together to provide a concise summary that allows sequences to be compared. Ordinations, such as principal components and detrended correspondence analyses, can be performed on several sections simultaneously to extract a new set of axes that represent the combined trends of various taxa. The first few major axes can be plotted against depth as 'biostratigraphical logs'; the rest can be ignored as insignificant variation or noise. Sections can then be correlated based on these logs, either manually or using procedures such as sequence slotting. These techniques have been tested using artificial data from Edwards (1984), consisting of taxa of known spatial and temporal ranges. Correlations between sections were found to match closely the true ages. Inconsistencies in the data, such as time-transgressive taxa, were isolated on one axis, allowing them to be removed or studied further.
Palaeoenvironmental controls on the ichnology of tide-influenced facies with an example from a macrotidal tide-dominated deltaic depositional system
  • D Mcilroy
  • R G Bromley
  • L A Buatois
  • G Mángano
  • Genise
  • McIlroy D.