15 research outputs found

    Sedimentological and ichnological analyses of the continental to marginal-marine Centenario Formation (Cretaceous), Neuquén Basin, Argentina: Reservoir implications

    Get PDF
    The upper Valanginian – lower Aptian Centenario Formation is a significant producer of oil and gas in the Neuquén Basin, western Argentina. This formation is located exclusively in the subsurface of the eastern and northeastern Neuquén Basin, and is 450–1000 m thick. The Centenario Formation laterally interfingers with the Agrio Formation. Previous studies addressing the paleogeographic history of the Centenario Formation are scarce, and a comprehensive geological model has yet to be put forward. The current study scrutinizes the Centenario Formation, especially its lower member, within the northeastern Neuquén Basin. The study area includes the Cerro Hamaca Oeste, Señal Cerro Bayo, and Volcán Auca Mahuida oilfields operated by Yacimientos Petrolíferos Fiscales (YPF). Sedimentological and ichnological core data, geophysical well logs, and petrographic thin sections have been utilized to construct a geological model. Eleven sedimentary facies and three facies associations have been identified from the core dataset, providing insights into the paleoenvironmental settings and their stresses on infaunal colonization. Basin-margin deposits from the northeastern part of the study region were formed in continental environments, comprising ephemeral fluvial channel complexes and floodplains, and are ichnologically represented by rare Skolithos and common rhizoliths. The central part of the study area is interpreted as recording deposition in ephemeral lakes, river-dominated lake deltas, and coastal lagoons and sabkhas, and is represented by a combination of stressed expressions of both the Skolithos and Scoyenia Ichnofacies. River-dominated, storm-influenced delta deposits are located towards the southwestern limit of the study area, and are ichnologically represented by the Skolithos and depauperate Cruziana Ichnofacies. Deltaic deposits gradually transition into the basinal facies of the Agrio Formation to the west. Overall sedimentologic characteristics suggest semi-arid to arid climatic conditions during deposition.Fil: Shchepetkina, Alina. Universidad Nacional de Río Negro; Argentina. University of Saskatchewan; Canadá. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Ponce, Juan Jose. Universidad Nacional de Río Negro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Carmona, Noelia Beatriz. Universidad Nacional de Río Negro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Mángano, M. Gabriela. University of Saskatchewan; CanadáFil: Buatois, Luis Alberto. University of Saskatchewan; Canadá. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Ribas, Soledad. YPF - Tecnología; ArgentinaFil: Villar Benvenuto, Marcela Celeste. YPF - Tecnología; Argentin

    A SEDIMENTOLOGIC, SEQUENCE STRATIGRAPHIC, AND ICHNOLOGIC CHARACTERIZATION OF THE CRETACEOUS U AND M2 SANDSTONE MEMBERS, NAPO FORMATION, ORIENTE BASIN OF ECUADOR

    Get PDF
    The Upper Cretaceous M2 and U Sandstone members of the Napo Formation are prolific hydrocarbon producers in the Oriente Basin, Ecuador. To understand the depositional origin of these reservoirs, a detailed sedimentologic, sequence-stratigraphic, and ichnologic study was performed, using 490 ft (ca. 149 m) of conventional core from six wells. Sedimentary facies, stratal stacking pattern, discontinuity surfaces, and trace fossils were documented. Nine lithofacies, two depositional sequences in each member, and depauperate and fully marine ichnofacies were identified. Both members present evidence of tidal (e.g. mudstone drapes on bedforms, double mudstone layers, flaser, wavy, and lenticular bedding, and thick-and- thin alternations of siltstone and claystone layers) and river (e.g. hyperpycnal flow deposits) influence. The shoreline was trending northeast-southwest within the study area and the predominant sediment source came from cratonic areas located to the east. In the study area, the U Sandstone Member represents three main broad environments: fluvial, estuarine, and deltaic. The base of the U Sandstone Member marks the base of depositional sequence 1 (DSU1), representing a subaerial unconformity formed as a result of valley incision during a relative fall of sea level. DSU1 comprises the lower and middle intervals and the lower part of the upper interval. DSU1 consists of lowstand moderate-sinuosity fluvial deposits, followed by transgressive estuarine deposits and highstand mixed tide- and river-influenced deltaic deposits. A subaerial unconformity marks the base of depositional sequence 2 (DSU2), which was followed by renewed lowstand fluvial deposition within an incised valley in the more proximal areas and transgressive estuarine sedimentation. DSU2 is recorded in the upper part of the upper interval. Trace fossils in the U Sandstone Member are recorded in the estuarine and deltaic deposits; fluvial deposits present sparse bioturbation. The depauperate Skolithos and Cruziana ichnofacies are commonly present in the estuarine and deltaic deposits, recording brackish-water conditions. The M2 Sandstone Member records sedimentation in a mixed tide- and river-influenced deltaic environment, encompassing delta front and prodeltaic subenvironments, as well as transgressive deposits signaling deltaic abandonment. Two depositional sequences have been recognized (DSM1 and DSM2). The underlying A Limestone Member, which pinches out towards the east, most likely represents the transgressive systems tract (TST) of DSM1. Deposition may have been controlled by an interplay of eustatic changes, tectonism, and active volcanism. This member consists of discrete thickening- and coarsening-upward packages that may represent either parasequences or intervals recording delta lobe switching. The parasequence sets exhibit progradational-stacking (seaward) patterns and have clinoformal geometry that exhibit both vertical and lateral facies changes. Various degrees of biogenic reworking are recorded (BI 1-6), commonly in the sandstone-dominated facies, generally representing the depauperate Cruziana Ichnofacies, indicative of brackish-water conditions. Integration of ichnology, sequence stratigraphy, and sedimentology was fundamental in order to provide detailed paleoenvironmental models for the U and M2 Sandstone members. This study represents the first detailed ichnologic study in Ecuador. It is expected that this research will encourage geoscientists in the country to adopt these conceptual and methodological tools in reservoir characterization

    ICHNOTAXONOMY AND PALEOENVIRONMENTAL ANALYSIS OF TRACE FOSSILS IN THE LATE DEVONIAN CATSKILL FORMATION, NORTH-CENTRAL PENNSYLVANIA, USA

    Get PDF
    The purpose of this thesis is to interpret the ichnotaxonomy, paleoenvironmental distribution, and paleoecological ramifications of trace fossils from the Frasnian to Famennian Catskill Formation (CF), north-central Pennsylvania, USA. The CF contains a suite of approximately 14 traces, 11 of which represent animal behavior, and 3 of which represent preservational styles and morphologies of plant roots. CF traces occur in paleosols and strata exhibiting no evidence of pedogenesis. Paleosol traces represent terraphilic to hydrophilic soil biota. Traces in strata with no evidence of pedogenesis represent aquatic organism behavior. Backfilled burrows--Beaconites antarcticus and B. barretti--represent dwelling and feeding by soil-dwelling arthropods. Rhizoliths represent shallow to deep rooting by plants with terraphilic to hydrophilic affinities. Backfilled burrows and rhizoliths form a characteristic ichnofabric that is superimposed on all pedogenically modified deposits. Lungfish estivation burrows--Hyperoeuthys teichonomos--are commonly superimposed on and subsequently overprinted by the dominant ichnofabric. Diplichnites gouldi is present in weakly developed paleosols and represents locomotion of an arthropod of unknown taxonomic affinity. In situ stump casts occur in paleosols of differing maturity and likely represent the life position of an arborescent plant. Camborygma eumekonomos and C. litonomos represent dwelling burrows of terraphilic to hydrophilic arthropods and are also overprinted by the dominant ichnopedofabric. Bivalve resting (Lockeia siliquaria), locomotion (Lockeia ornata), and escape traces, as well as fish swimming traces (Undichna multiloba), and Sagittichnus lincki--the resting trace of an unknown organism--represent aquatic organism behavior. The presence of terraphilic to hygrophilic and hydrophilic traces in CF paleosols indicates that Late Devonian soil organisms exhibited nearly as much behavioral complexity as Mesozoic to recent soil organisms. The abundance and degree of trace crosscutting increases in increasingly mature paleosols, indicating that CF paleosol ichnoassemblages, despite being controlled by paleohydrology, also represent ecological succession. Continental organisms are known exhibited behaviors that beneficially modify their environment (ecosystem engineering) by modulating resource flow paths (allogenic engineering) or modifying their bodies in ways that create new habitats for themselves (autogenic engineering). The idea that middle Paleozoic continental organisms were ecosystem engineers has not been examined. Our data suggest that the inception of allogenic ecosystem engineering in continental environments had occurred by the Late Devonian

    Ichnotaxonomy of the Cambrian Spence Shale Member of the Langston Formation, Wellsville Mountains, Northern Utah, USA

    Get PDF
    The Spence Shale of northern Utah is the oldest North American middle Cambrian (~506–505 Ma) Burgess Shale-type (BST) deposit and, unlike previously thought for BST deposits, has a very diverse ichnofauna. Twenty-four ichnogenera and 35 ichnospecies were identified: Archaeonassa (A. fossulata and A. jamisoni isp. nov.), Arenicolites carbonaria, Aulichnites, Bergaueria (B. hemispherica and B.aff. perata), Conichnus conicus, Cruziana (C. barbata and C. problematica), Dimorphichnus, Diplichnites (D. cf. binatus, D. gouldi, andD.cf. govenderi), Gordia marnia, Gyrophyllites kwassizensis, Halopoa aff. imbricata, Lockeia siliquaria, Monomorphichnus (M. bilinearis,M.lineatus, and M. cf. multilineatus), Nereites cf. macleayi, Phycodes curvipalmatum, Phycosiphon incertum, Planolites (P. annularius, P.beverleyensis, and P. montanus), Protovirgularia (P. dichotoma and P. cf. pennatus), Rusophycus (R. carbonarius, R. cf. pudicus, and R. cf.cerecedensis), Sagittichnus lincki, Scolicia, Taenidium cf. satanassi, Teichichnus cf. nodosus, and Treptichnus (T. bifurcus, T. pedum, and T.vagans). The ichnofossils comprise three ichnocoenoses—Rusophycus-Cruziana, Sagittichnus, and Arenicolites-Conichnus—representingdwelling, deposit- and filter-feeding, grazing, locomotion, and predation behaviors of organisms (e.g., annelid worms and trilobites).Two ichnofossil associations are suggestive of predation: (1) Planolites terminating at a Rusophycus; and (2) Archaeonassa crosscuttinga Taenidium. The Spence Shale ichnofauna represent a distal Cruziana Ichnofacies and depauperate, distal Skolithos Ichnofacies. Anew ichnospecies of Archaeonassa is proposed, A. jamisoni isp. nov., and Ptychoplasma (Protovirgularia) vagans is herein transferred toTreptichnus. This study is the first ichnotaxonomic study of the Spence Shale and North American BST deposits and shows highlydiverse ichnofaunas can be present in BST deposits

    ICHNOLOGICAL ASSESSMENT OF DEPOSITIONAL ENVIRONMENTS FROM ICEHOUSE AND GREENHOUSE GLOBAL CLIMATES IN POLAR AND NEAREQUATORIAL POSITIONS

    Get PDF
    This dissertation uses ichnological assessment to reevaluate depositional environments and the physicochemical controls on benthic organisms. I am assessing environments from icehouse and greenhouse global climates, and from polar and near-equatorial positions. This comparison originates from recent trends in the literature which suggested that the physicochemical controls on bioturbation are significantly different between climatic regimes and latitudinal positions. We identified these major controls from the high-latitude Permian Mackellar Fm, the mid-latitude Cretaceous Dakota Group, and the mid-latitude Cretaceous Greenhorn and Niobrara formations. In addition to interpretation of physicochemical controls, each chapter explores an application of ichnology: reevaluation of depositional environments (Mackellar Fm), identification of biogenic modification of porosity and permeability (Dakota Group), description of bioturbation trends through Oceanic Anoxic Event (OAE) strata (Greenhorn and Niobrara Fms), and the use of bioturbation intensity and ichnocoenoses to identify Milankovitch cyclicity (Greenhorn and Niobrara Fms). The Mackellar Fm was previously interpreted to had been deposited in a lacustrine system, but this study refutes this interpretation based on the identification of ichnogenera that only known from marine systems. Mackellar Fm ichnogenera have diminutive morphotypes, shallow penetration, low individual bed and high overall ichnodiversity and are interpreted as a short-lived benthic community that was stressed by freshwater input and high sedimentation rates in a river-dominated deltaic to fully marine system. The study on the Dakota Group assessed physicochemical parameters at the time of deposition using ichnocoenoses and ichnofacies to reinterpret depositional histories, reevaluate sequence stratigraphic models, and evaluate the influence of bioturbation porosity and permeability in these formations. Dakota Group depositional environments are interpreted as: fluvial to deltaic (Lytle Fm), paralic to nearshore (Plainview Fm), distal to proximal shoreface (Glencairn Fm), and continental to offshore marine depositional environments (Muddy Fm). Lastly, analysis of Greenhorn and Niobrara fm ichnology identified a high diversity with abundant bioturbation throughout oxic and OAE intervals. I interpreted that the anoxic events were not long-term events but a period of dysoxic conditions with punctuated periods of anoxia. Additionally, based on observed cyclic patterns in ichnocoenoses and bioturbation, identified harmonic cycles that match Milankovitch cyclicity (eccentricity, obliquity, and precession)

    Caracterización de la transición marino-continental Maastrichtiense-Daniense en el noroeste de la cuenca de Tremp-Graus. Integración de datos sedimentológicos, bioestratigráficos e icnológicos

    Get PDF
    Esta Tesis Doctoral comprende el estudio estratigráfico, sedimentológico, micropaleontológico e icnológico de la Formación Tremp (Cretácico terminal-Paleógeno) al noreste de la Cuenca de Graus-Tremp (región sur-pirenaica central) y su principales objetivos son: Establecer la edad de los depósitos de la Formación Tremp y precisar la posición del límite K/Pg. Reconstruir los paleoambientes del Cretácico Superior y Paleoceno (Formación Tremp) en el área de estudio. Realizar la caracterización icnológica de la Unidad Roja Inferior de la Formación Tremp para identificar las áreas afectadas por diferentes condiciones ambientales. Establecer los criterios sedimentológicos e icnológicos de reconocimiento del área de transición entre estos ambientes sedimentarios, en particular de la zona conocida como transición fluvio-mareal. Analizar diferentes casos de estudio de la transición fluvial-mareal para evaluar la naturaleza global o local del registro icnológico y sedimentológico en este ambiente sedimentario. Llevar a cabo una correlación general entre los depósitos estudiados y los localizados en el área oeste de la Cuenca de Tremp-Graus. Este trabajo doctoral está compuesto por tres artículos en los que se exponen la mayor parte de los resultados y constituyen la parte principal de la tesis, que finaliza con una discusión y unas conclusiones generales. A continuación se enumeran algunos de los resultados principales. Caracterización sedimentológica y bioestratigráfica de la Formación Tremp (Maastrichtiense pp.-Daniense pp.)..

    Sedimentology, Ichnology, Sequence Stratigraphy and Vertebrate Paleontology of the Belly River Group, southwestern Saskatchewan, Canada

    Get PDF
    The Belly River Group (BRG) in southwestern Saskatchewan records the interplay of accommodation space and base level in an epicontinental seaway. The Belly River Group has been the focus of numerous studies concerned with sedimentology, stratigraphy, and paleontology in Alberta and Montana, due to extensive badlands exposure, exceptional three-dimensional preservation of many sedimentary features, and the vast paleontologic data available from the deposits. The BRG has been studied with far less detail in Saskatchewan, due to the poor and sporadic nature of available outcrop, and comparatively little subsurface data. This study synthesizes all available data for the BRG in southwestern Saskatchewan, effectively characterizing the sedimentology, stratigraphy, ichnology, and paleontology and provides a comprehensive framework for these deposits in the province. This study provides critical insights into the BRG, particularly in regards to sequence stratigraphy. During the Campanian, the Western Interior Seaway bisected North America, and its western shore was located in southern Saskatchewan. This study provides critical insights into shoreline dynamics, both from a geologic and paleontologic perspective, and how these dynamics influence deposits further inland not directly in contact with the seaway. This study highlights the utility of integrating multiple, distinct datasets to understand depositional evolution along coastal plains, and its effects on biodiversity. McLean (1971) characterized the uppermost deposits of the BRG (then called the Judith River Group) in the Cypress Hills as being deposited in an extensive delta plain. In Chapter 3, detailed facies analysis indicates the upper DPF does not record sedimentation in a delta system. A reinterpretation determines the DPF was deposited in a low-relief coastal plain with a wave-dominated, tidally influenced, fluvially modified shoreline. Marginal-marine facies, interpreted as lagoons, tidal flats and estuaries, display a typical brackish-water trace-fossil assemblage, including Asterosoma isp., Chondrites isp., Cylindrichnus concentricus, Teichichnus rectus, and Skolithos isp. Fine-grained sandstone deposited in an estuarine mouth-bar and barrier-island complex protected the coast from wave reworking. As the seaway transgressed across the coast, fully marine wave-dominated parasequences replaced those of the coastal plain. Typical trace fossils include Asterosoma isp., Chondrites isp., Diplocraterion isp., Nereites missouriensis, Phycosiphon incertum, Planolites isp., Rhizocorallium isp., and Zoophycos isp., reflecting open, fully marine conditions. In Chapter 4, the Belly River Group and its associated formations are formally recognized for the first time in Saskatchewan with facies, depositional environments, and sequence stratigraphic framework interpreted to provide a concise treatment of the deposits in southwestern Saskatchewan. A new lithostratigraphic unit within the uppermost Dinosaur Park Formation is recognized based on laterally extensive barrier island, lagoon, and estuary basin deposits. The Manâtakâw Member is established as a means to aide in discussing the transition from nonmarine clastics of the BRG to marine shales of the overlying Bearpaw Formation. Unincised and incised valley systems are explored in detail, with lowstand systems tract deposits compared and contrasted between the Oldman and Dinosaur Park formations. Chapters 5 and 6 discuss microvertebrate localities from the Dinosaur Park Formation, one at Lake Diefenbaker (Ch. 5), and the other at Woodpile Coulee (Ch. 6). Palynology, ichnology, sedimentology, and vertebrate paleontology are integrated to determine paleoenvironmental and paleoecological conditions in the two regions and their stratigraphic positions. Both sites are interpreted as having been deposited at different times during transgression of the encroaching Bearpaw Sea. Though well studied and sampled in Alberta, the Dinosaur Park Formation is poorly exposed with little known associated vertebrate assemblages in Saskatchewan. These discoveries from the microvertebrate sites offer new insights into Late Cretaceous ecosystems near paleocoastlines, allowing for future studies of spatial diversity patterns relative to Albertan faunas

    Quantification of Ichnological, Paleoecological, Paleohydrological, and Paleoclimatological Information from the Upper Jurassic Morrison Formation

    Get PDF
    This dissertation takes a multifaceted approach to interpreting paleoenvironments and paleoclimate represented by the strata of the Morrison Formation (MF). The MF has been the subject of geological and paleontological investigations for well over a century, but a number of confounding factors have limited interpretations. One problem with interpreting MF ichnology is uncertainty about the sedimentary conditions under which deep dinosaur tracks were made. To solve this problem, I developed new methods to measure trace fossils, including footprints. I used multistripe laser triangulation scanning to create three-dimensional digital models of traces, from which I improved precision of traditional ichnological techniques. I also performed neoichnological experiments with elephants to collect empirical trackmaking data, to which I applied multiple regression to derive a quantitative relationship between physical trackmaking variables. Results showed that many deep sauropod tracks were created in near saturated conditions. Megafaunal track preservation was one factor taken into consideration when interpreting paleohydrology from moisture regimes represented by trace-fossil assemblages. I demonstrated the usefulness of ichnological moisture regimes by interpreting ichnocoenoses in MF avulsion deposits in the Bighorn Basin. I found a regular pattern of moisture profiles associated with crevassing that can be used to identify avulsion deposits in future ichnological studies. Ichnological moisture regimes were incorporated with other pedogenic features to develop a soil moisture index that was combined with measures of carbonate content, carbonate mineralogy, total organic carbon, and stable isotopes of carbonates and organic carbon to construct a detailed vertical profile through the MF in the Henry Mountains, Utah. This profile is useful for paleoclimatic and paleoenvironmental interpretations as well as for correlation to marine isotope records. I compared the vertical profile to paleoecological patterns determined from food-web network analyses and found a possible correlation between a global shift in organic carbon isotopes and an episode of biotic turnover. I also found that MF food webs were extremely stable, a factor that may have contributed to the success of dinosaur-dominated ecosystems during the Mesozoic

    The Chapel Island Formation of Newfoundland (Canada) revisited: integrating ichnologic and sedimentologic datasets to unravel early metazoan evolution

    Get PDF
    The Chapel Island Formation (CIF) is a 1000+ m-thick, mainly siliciclastic succession that is well exposed along the coastline of Burin Peninsula, southeastern Newfoundland, eastern Canada. The CIF contains an outstanding record of latest Ediacaran-early Cambrian trace fossils with some intervals rich in small shelly fossils, and in 1992 the Fortune Head section was ratified by the International Commission on Stratigraphy (ICS) and the International Union of Geological Sciences (IUGS) as the Global Stratotype Section and Point (GSSP) for the Cambrian System. This was the first system-level GSSP defined primarily on the basis of trace fossils, a decision that evoked considerable discussion among the geological community. This thesis represents the first modern study of the sedimentology and the first taxonomic appraisal of the trace fossils since the original studies that proposed the GSSP in the 1980’s. More than 1700 m of CIF strata were logged in the sea cliffs of Burin Peninsula at Fortune Head, Fortune North, Grand Bank Head, Lewin’s Cove, Little Dantzic Cove, and Point May. A revision of the sedimentology permitted the description and interpretation of fourteen sedimentary facies composing five facies association (FA), which are interpreted to be deposited in: (1) mud-flat, mixed-flat, sand-flat, and tide-dominated or -influenced embayments (FA-A); (2) middle and lower shoreface (FA-B); (3) offshore transition, upper offshore, and lower offshore (FA-C); (4) shelf (stricto sensu) (FA-D); and (5) carbonate subtidal and intertidal environments (FA-E). An extensive trace-fossil dataset was built from careful field observations and provided a comprehensive record of bioturbation intensity (1596 data points on vertical bioturbation, 1481 data points on bedding plane bioturbation) and of trace-fossil metrics (3162 data points on burrow width, 1473 data points on burrow depth). In addition, a comprehensive revision of the trace-fossil composition (3508 trace fossils identified) allowed the description of twenty-eight ichnogenera and fifty-one ichnospecies, which correspond to cf. Allocotichnus dyeri, Archaeonassa fossulata, Arenicolites aff. carbonaria, Arenicolites isp., Bergaueria perata, B. cf. radiata, Circulichnis ligusticus, C. montanus, Cochlichnus anguineus, C. luguanensis, Conichnus conicus, Cruziana problematica, Curvolithus multiplex, C. simplex, Curvolithus isp., Dendroidichnites aff. irregulare, Didymaulichnus miettensis, Dimorphichnus isp. A, Dimorphichnus isp. B, cf. Dimorphichnus isp., ?Diplocraterion isp., Gordia marina, Gyrolithes gyratus, G. scintillus, Halopoa imbricata, Helminthoidichnites tenuis, Helminthopsis abeli, H. hieroglyphica, H. tenuis, Monomorphichnus bilinearis, M. lineatus, M. needleiunm, Monomorphichnus isp., Palaeophycus annulatus, P. tubularis, Palaeophycus isp., Psammichnites gigas circularis, P. cf. saltensis, Rosselia socialis, Rusophycus avalonensis, Rusophycus isp. A, Rusophycus isp. B, Saerichnites kutscheri comb. nov., Teichichnus rectus, Torrowangea rosei, Treptichnus bifurcus, T. coronatum, T. pedum, T. pollardi, Trichichnus linearis, and Trichichnus isp. Sectioning and polishing of 47 lithic samples from throughout the CIF showed that the sediment mixed layer that characterizes modern oceans developed through a series of steps that took place in the early Cambrian rather than in the Silurian as previously advocated. The main evolutionary innovations took place in the offshore environment with three paleoecologic stages that comprised: (1) an Ediacaran matground ecology, with surficial and very shallow infaunal grazing organisms living on and within microbially bound seafloors; (2) a Fortunian matground/firmground ecology, with a burst in behavioural and anatomical innovations and the first evidence of colonization of the shallow-tier; and (3) a late Fortunian/Cambrian Age 2 mixground ecology, with the development of a shallow mixed layer and deeper discrete burrows of the transition layer. Evaluation of outcrop quality based on accessibility, lateral and vertical continuity of beds, stratigraphic completeness, and type of exposure, demonstrated that Fortune Head, Fortune North, Grand Bank Head, and Little Dantzic Cove represented the best suited section to perform trace-fossil analyses, whereas Lewin’s Cove and Point May suffered from exposure biases affecting their trace-fossil records. The Ediacaran-Cambrian boundary interval was also studied in detail at Fortune Head, Grand Bank Head, Lewin’s Cove, and Point May, and the base of the Cambrian was placed confidently at the first appearance of trace fossils of the Treptichnus pedum Ichno-Assemblage Zone. This study demonstrates that only through detailed, comprehensive, and integrative approaches, can research provide new empirical evidence that further unfold our understanding of the history of animal life on Earth

    Sedimentology, Ichnology and Sequence Stratigraphy of Black Shales from the Upper Jurassic - Lower Cretaceous Vaca Muerta Formation, Neuquén Basin, Argentina

    Get PDF
    During the past decades, the study of fine-grained successions has received considerable attention, mainly due to the groundbreaking work of various mudstone sedimentologists who suggested bottom transport processes as plausible mechanisms for mud sedimentation. Moreover, an increased interest has been growing due to the extensive development of unconventional shale reservoirs in USA. In this context, the Upper Jurassic-Lower Cretaceous Vaca Muerta Formation from the Neuquén Basin, Argentina, stands as a valuable shale reservoir for both industry and academic research that can be used to expand our knowledge on ancient fine-grained, organic-rich depositional environments. This Formation is the most important unconventional shale reservoir in South America, with exceptional characteristics for oil and gas extraction. It is well exposed in central western Argentina and contains a fair amount of core data that can be used to evaluate sedimentary processes and trace fossils. The present study analyzed sedimentologic, ichnologic and sequence stratigraphic datasets of the Vaca Muerta Formation to construct a robust depositional model and compare the results with other studies in order to test previous models and propose new ideas. For this purpose, outcrops from a basin margin location (Picún Leufú area), as well as outcrops (Yesera del Tromen) and cores (wells 1 to 9) from the basin centre (where most of the unconventional development is currently active), were analyzed in detail. In the basin margin location (Picún Leufú area), ten sections were logged in the Kimmeridgian to Tithonian succession. The interval encompasses two third-order sequences formed in continental environments and a mixed carbonate-siliciclastic, shelf-margin clinoform system. At the base, the Quebrada del Sapo Formation represents sandstone, conglomerate and mudstone of eolian and fluvial origin. On top, marine carbonaceous, mixed to calcareous mudstone and sandstone of the Vaca Muerta (bottomset and foreset of the clinoform) and Picún Leufú (topset) formations occur. At the base, the clinoform system consists of a thin, retrograding, open bay facies association comprising beach, bay margin, proximal bay and distal bay facies. Above, a siliciclastic basin facies association is constituted by basin, hyperpycnal lobe, lobe fringe, channel-fill complex and overbank facies, developed at the bottomset of the clinoform system. In the foreset and topset, slope mud belt, mixed slope, slope sand bodies, sandy shoal, sand bar complex and lagoonal facies are part of the foreset facies association. This analysis challenged the previous paradigm of a catastrophic transgression at the onset of deposition of the Vaca Muerta Formation. Catastrophic rates of sea-level rise (metres per day) are incompatible with the existence of a basal, retrograding, marginal marine succession suggesting a transition between continental and marine facies. A comparison to modern analogues of marine flooding over dunes indicates that rates of sea-level rise similar to Holocene ones (mm to cm per year) are plausible and may have generated a rapid, yet not catastrophic transgression. Hence, rapid transgressions represent an alternative to “catastrophic” and “normal” scenarios of marine flooding over eolian deposits. The Late Jurassic represents a non-glacial time, and the global sea-level maximum highstand pre-dated the Vaca Muerta transgressive event, indicating that part of the sea-level rise has to be attributed to tectonic/thermal subsidence and compaction. In addition, the combined analysis provides insights into sedimentary processes affecting bottomset and foreset of a fine-grained clinoform system, highlighting the role of wave-influenced hyperpycnal flows associated with higher fluvial input in Picún Leufú and their effect on diluting organic matter content. In the basin centre (Yesera del Tromen and cores from wells 1 to 9), the succession was represented by the Tithonian to Valanginian, mixed carbonate-siliciclastic, shelf-margin clinoform system of the Vaca Muerta (bottomset and foreset) and Quintuco (topset) formations. The succession mostly comprises mixed to calcareous mudstone and bioclastic to intraclastic wackestone developed in four third-order sequences (Sequence 1 to 4). Sequence 1 and 2 start with a transgressive, marginal marine facies association comprising beach and open bay facies. Above, the bottomset and lower foreset of the clinoform shows starved, anoxic, dysoxic and oxic basin facies grouped into the basin facies association, and crinoid-rich, bioclastic- and silt-rich drift, and mixed drift and fluid mud facies, included in the drift facies associations. In the foreset, distal, mid, proximal, and fluid mud-rich slope facies occur as part of the slope facies associations. The slope association evolves from dominantly hemipelagic facies in the older, low-angle clinoforms, to mostly fluid mud-rich slope facies due to higher foreset angles in younger clinoforms. Sequence 3 and 4 are poorly cored, yet low- and high-energy outer ramp facies of the outer ramp facies association with subordinate basin and slope facies associations can be observed. This analysis provides several insights into the understanding of the Vaca Muerta Formation and other ancient fine-grained depositional environments. First, climate and consequent basin circulation were found to control sediment partitioning along the clinoform. Warmer climates triggered equatorward migration of the arid belt, humid conditions and estuarine circulation, producing high TOC and siliciclastic content due to anoxia and enhanced riverine input, respectively. In contrast, cooler climates associated with a poleward migration of the arid belt generated arid conditions and anti-estuarine or weakened estuarine circulation, triggering intense contour currents at deep waters. Low TOC and high carbonate content is recorded, due to high burndown of organic matter under upper dysoxic to oxic conditions, and enhanced shelf export of carbonate sediments by cascading. Organic matter content was sourced from the water column (pelagic), indicating that bottom currents and sediment-gravity flows dilute the TOC content of these deposits. Second, the trace-fossil analysis shows an ichnofauna differing from the classic Chondrites and Zoophycos ichnoguild recorded in organic-rich mudstone successions. In contrast, cryptobioturbation, pellet-rich ichnocoenoses, and biodeformational structures are abundant in this formation. This characteristic can be explained by bioturbation in a food-rich benthic environment, which precluded specialized feeding and the development of tiered communities. Limited oxygenation is deemed responsible for reduced bioturbation index, penetration depth, burrow size, and ichnodiversity. Moreover, biodeformational structures in mudstone are associated with soupy to very soft substrates, whereas a change to soft and stiffgrounds during ash deposition enhanced preservation of discrete trace fossils. This last relationship indicates that tuff deposits and rock fabric can be used to evaluate bioturbation in homogeneous, fine-grained successions in order to claim biogenicity of structures or delineate bottom water oxygenation. Finally, the characteristics of this Formation (low bioturbation intensity due to oxygen deficiency and low foreset angles hampering gravity flow transport) allowed the delineation of contourite deposits. Contourite deposits (drifts) can be differentiated from sediment-gravity flow deposits by their evidence of long-term transport, low sediment concentration, and long-term oxygen introduction at the sea-floor. Furthermore, contourites are host to a particular trace-fossil assemblage that was controlled by food (deposited at the surface or in suspension), oxygenation (increasing oxygen levels), hydrodynamic energy (precluding biogenic reworking during higher energy events) and water turbidity (allowing suspension feeding). The basin-wide increase in oxygenation generated by contourites indicates that these were produced by a wind- and thermohaline-driven circulation system of deep waters that was intensified during arid and cooler climates, which were times of enhanced cascading of dense, shelf waters. The trace fossil data supports the existence of high bioturbation intensity in contourite deposits, yet indicate that sedimentary structures can be preserved in muddy contourites when different stress factors are combined
    corecore