Volcaniclastic lacustrine sedimentation in the Pleistocene Guayllabamba intermontane basin in the Ecuadorian Andes

In this work, we present the description of the sedimentary fill of a well-exposed lacustrine succession in the Ecuadorian Andes. The Guayllabamba basin is an intermontane basin located in the Andean range of Ecuador, and part of its sedimentary history is represented by a volcanically-influenced ∼100 m thick lacustrine unit of the Pleistocene age. We create a stratigraphic cross-section from the eastern to western lake margins and identify nineteen facies that were used to carry out a paleoenvironmental reconstruction. The Guayllabamba paleolake was developed in a tectonic depression surrounded by volcanoes and it was filled by sediments derived from the erosion of the volcanic edifices, the reworking of unconsolidated pyroclastic deposits, and deposition of pyroclastic currents into the lake. The lake shows a deepening trend, passing from shallow deltaic sedimentation to varved diatomites with turbidites. Abundant ash-fall beds, monolithological pumiceous deltaic sequences, and pumice-dominated thick ignimbrites show the impacts of volcanism on lacustrine sedimentation within this basin. Soft-sediment deformation and gravity flow deposits are common due to the intrabasinal tectonic activity and to the intrusion of a lava body. Aulacoseira-rich diatomites dominated the background lake sedimentation. The outcrops of the Guayllabamba basin are outstanding examples of the interaction between volcaniclastic and lacustrine sedimentation.

The geology of Pegasus Basin based on outcrop correlatives in southern Wairarapa and northeastern Marlborough, New Zealand

<p>Acquisition of high quality 2D seismic data by the New Zealand Government in 2009-10 (the PEG09 Survey) sparked new interest in Pegasus Basin, an offshore frontier basin situated east of central New Zealand. Although no wells have been drilled in Pegasus Basin, strata exposed onshore in southern Wairarapa and northeastern Marlborough provide useful analogues for the sedimentary fill of the basin. Using field observations in combination with petrographic analysis and seismic interpretation, this study provides a more complete understanding of the geology of Pegasus Basin. 13 outcrop localities are described from the surrounding southern Wairarapa and northern Marlborough regions, which are inferred to have been deposited in a range of depositional environments including fluvial, terrestrial and shallow marine deposits, through to inner – mid shelf, and deep marine channel-levee and submarine fans, with fine-grained sedimentation at bathyal depths. These outcrops provide representative and well-exposed examples of facies and lithologies typical of the depositional environments that are likely to exist in Pegasus Basin. Petrographic analysis of six Cretaceous and six Neogene sandstones from Marlborough and Wairarapa regions has revealed that they are compositionally classified as litharenites and feldspathic litharenites, derived from the Torlesse Supergroup. Primary porosity is best preserved in Neogene sandstones, whilst Cretaceous sandstones only tend to preserve secondary porosity, in the form of fractures or dissolution of framework grains. Carbonate cementation, compaction and authigenic clay formation are the biggest contributing factors that degrade reservoir quality. Seismic interpretation of the PEG09 survey has revealed that Pegasus Basin contains a sedimentary succession over 10,000 m thick, that mantles Early Cretaceous syn-tectonic strata in various states of deformation attained during mid-Cretaceous subduction at the eastern Gondwana margin. Key horizons mapped extensively over the basin highlight seismic reflection packages, which are linked to described outcrop localities onshore, based on reflection characteristics and geometries. The Miocene succession contains up to 4,000 m of sediments that are likely to include promising reservoir lithologies akin to the Great Marlborough Conglomerate of Marlborough, or the Whakataki Formation of Wairarapa.</p>

The geology of Pegasus Basin based on outcrop correlatives in southern Wairarapa and northeastern Marlborough, New Zealand

<p>Acquisition of high quality 2D seismic data by the New Zealand Government in 2009-10 (the PEG09 Survey) sparked new interest in Pegasus Basin, an offshore frontier basin situated east of central New Zealand. Although no wells have been drilled in Pegasus Basin, strata exposed onshore in southern Wairarapa and northeastern Marlborough provide useful analogues for the sedimentary fill of the basin. Using field observations in combination with petrographic analysis and seismic interpretation, this study provides a more complete understanding of the geology of Pegasus Basin. 13 outcrop localities are described from the surrounding southern Wairarapa and northern Marlborough regions, which are inferred to have been deposited in a range of depositional environments including fluvial, terrestrial and shallow marine deposits, through to inner – mid shelf, and deep marine channel-levee and submarine fans, with fine-grained sedimentation at bathyal depths. These outcrops provide representative and well-exposed examples of facies and lithologies typical of the depositional environments that are likely to exist in Pegasus Basin. Petrographic analysis of six Cretaceous and six Neogene sandstones from Marlborough and Wairarapa regions has revealed that they are compositionally classified as litharenites and feldspathic litharenites, derived from the Torlesse Supergroup. Primary porosity is best preserved in Neogene sandstones, whilst Cretaceous sandstones only tend to preserve secondary porosity, in the form of fractures or dissolution of framework grains. Carbonate cementation, compaction and authigenic clay formation are the biggest contributing factors that degrade reservoir quality. Seismic interpretation of the PEG09 survey has revealed that Pegasus Basin contains a sedimentary succession over 10,000 m thick, that mantles Early Cretaceous syn-tectonic strata in various states of deformation attained during mid-Cretaceous subduction at the eastern Gondwana margin. Key horizons mapped extensively over the basin highlight seismic reflection packages, which are linked to described outcrop localities onshore, based on reflection characteristics and geometries. The Miocene succession contains up to 4,000 m of sediments that are likely to include promising reservoir lithologies akin to the Great Marlborough Conglomerate of Marlborough, or the Whakataki Formation of Wairarapa.</p>

Cretaceous–Quaternary seismic stratigraphy of the Tanga offshore Basin in Tanzania and its petroleum potential

In this study, the available 2D seismic lines have been interpreted to understand the basin development and petroleum potential of the Late Cretaceous–Quaternary stratigraphy of the Tanga offshore Basin in Tanzania. Conventional seismic interpretation has delineated eight sedimentary fill geometries, fault properties, stratal termination patterns and unconformities characterizing the studied stratigraphy. The Late Cretaceous was found to be characterized by tectonic quiescence and uniform subsidence where slope induced gravity flows that resulted during the Miocene block movements was the major mechanism of sediment supply into the basin. The Quaternary was dominated by extensional regime that created deep N-S to NNE-SSW trending graben. The graben accommodated thick Pleistocene and Holocene successions deposited when the rate of tectonic uplift surpasses the rate of sea level rise. Thus, the deposition of lowstand system tracts characterized by debris flow deposits, slope fan turbidites, channel fill turbidites and overbank wedge deposits, known for their excellent petroleum reservoir qualities, especially where charged by Karoo Black Shales. Subsequent tectonic quiescence and transgression lead to the emplacement of deep marine deposits with characteristic seismic reflection patterns that indicate the occurrence of Quaternary shale sealing rocks in the study area. The occurrence of all the necessary petroleum play systems confirms the hydrocarbon generation, accumulations and preservation potential in the Tanga Basin.

Fjord network in Namibia: A snapshot into the dynamics of the late Paleozoic glaciation

Fjords are glacially carved estuaries that profoundly influence ice-sheet stability by draining and ablating ice. Although abundant on modern high-latitude continental shelves, fjord-network morphologies have never been identified in Earth’s pre-Cenozoic glacial epochs, hindering our ability to constrain ancient ice-sheet dynamics. We show that U-shaped valleys in northwestern Namibia cut during the late Paleozoic ice age (LPIA, ca. 300 Ma), Earth’s penultimate icehouse, represent intact fjord-network morphologies. This preserved glacial morphology and its sedimentary fill permit a reconstruction of paleo-ice thicknesses, glacial dynamics, and resulting glacio-isostatic adjustment. Glaciation in this region was initially characterized by an acme phase, which saw an extensive ice sheet (1.7 km thick) covering the region, followed by a waning phase characterized by 100-m-thick, topographically constrained outlet glaciers that shrank, leading to glacial demise. Our findings demonstrate that both a large ice sheet and highland glaciers existed over northwestern Namibia at different times during the LPIA. The fjords likely played a pivotal role in glacier dynamics and climate regulation, serving as hotspots for organic carbon sequestration. Aside from the present-day arid climate, northwestern Namibia exhibits a geomorphology virtually unchanged since the LPIA, permitting unique insight into this icehouse.

Late Cretaceous – Early Palaeogene inversion-related tectonic structures at the NE margin of the Bohemian Massif (SW Poland and northern Czechia)

A brief, regional-scale review of the Late Cretaceous – Early Palaeogene inversion-related tectonic structures affecting the Sudetes and their foreland at the NE margin of the Bohemian Massif is presented and complemented with results of new seismic studies. The Sudetes expose Variscan-deformed basement, partly overlain by post-orogenic Permo-Mesozoic cover, containing a wide spectrum of tectonic structures, both brittle and ductile, in the past in this area referred to as young Saxonian or Laramide. We have used newly reprocessed legacy seismics to study these structures at the two main post-Variscan structural units of the area, the North-Sudetic and Intra-Sudetic synclinoria, and discuss the results together with regionally-distributed examples coming from quarries and underground mines as well as those from the literature. The Late Cretaceous – Early Palaeogene tectonic structures in consecutively reviewed Sudetic tectonic units, from the north to south, typically include gentle to moderate buckle folds of detachment type or fault-related, high-angle reverse and normal faults, as well as low-angle thrusts – often rooted in the crystalline basement. The structures hitherto described as grabens, are frequently believed to be bounded by reverse faults (hence we use the term ‘reverse grabens’) and typically reveal strongly synclinal pattern of their sedimentary fill. The crystalline basement top, as imaged by seismic sections in the North Sudetic Synclinorium below the detachment-folded cover, is synformally down-warped with a wavelength of up to 30 km, whereas on the elevated areas, where the basement top is exposed at the surface, it is up-warped (i.e. tectonically buckled). The reviewed compressional structures typically show an orientation fitting the regionally-known Late Cretaceous – Early Palaeogene tectonic shortening direction of NE-SW to NNE-SSW The same applies to the regional joint pattern, typically comprising an orthogonal system of steep joints of c. NW-SE and NE-SW strikes. All the reviewed structures are considered as due to the Late Cretaceous – Early Palaeogene tectonic shortening episode, although some of the discussed faults with a strike-slip component of motion may have been modified, or even produced, by later, Late Cenozoic, tectonism.

Jurassic Arc: Reconstructing the Lost World of eastern Gondwana

The tectonic setting of the Australian sector of the eastern Gondwanan margin during the Jurassic and Cretaceous is enigmatic. Whether this involved convergent tectonism and a long-lived continental magmatic arc or rift-related extension unrelated to subduction is debated. The paucity of Australian Jurassic–Cretaceous igneous outcrops makes resolving these competing models difficult. We used the detrital zircon record of the Jurassic–Cretaceous Great Australian Superbasin (GAS) as a proxy for igneous activity. We attribute the persistent magmatism recorded in GAS sedimentary fill throughout the Mesozoic to ca. 95 Ma to continuation of the established Paleozoic continental arc system. The detrital zircon record signals short (~10 m.y.) pulses of elevated Jurassic and Cretaceous magmatic activity and strongly positive εHf values, indicating juvenile crust or mantle-derived magmatism. Margin reconstruction indicates sustained continental growth at rates of at least ~55 km3 km–1 m.y.–1, mainly to the tract now represented by submerged northern Zealandia, due to the retreat of this arc system. We posit that arc retreat was a key factor in rapid crust generation and preservation, and that continental sedimentary systems globally may host cryptic records of juvenile crustal addition that must be considered in estimating crustal growth rates along convergent plate margins.

GPR signature of Quaternary faulting: a study from the Mt. Pollino region, southern Apennines, Italy

With the aim of unveiling evidence of Late Quaternary faulting, a series of Ground Penetrating Radar (GPR) profiles were acquired across the Campotenese continental basin (Mt. Pollino region) in the southern Apennines active extensional belt (Italy). A set of forty-nine 300 MHz and 500 MHz GPR profiles, traced nearly perpendicular to a buried normal fault, were acquired and carefully processed through a customized workflow. The data interpretation allowed us to reconstruct a pseudo-3D model depicting the boundary between the Mesozoic bedrock and the sedimentary fill of the basin, which were in close proximity to the fault. Once reviewing and defining the GPR signature of faulting, we highlight in our data how near surface alluvial and colluvial sediments appear to be dislocated by a set of conjugate (west and east-dipping) discontinuities that penetrate inside the underlying Triassic dolostones. Close to the contact between the continental deposits and the bedrock, some buried scarps which offset wedge-shaped deposits are interpreted as coseismic ruptures, subsequently sealed by later deposits. Although the use of pseudo-3D GPR data implies more complexity linking the geophysical features among the radar images, we have reconstructed a reliable subsurface fault pattern, discriminating master faults and a series of secondary splays. We believe our contribution provides an improvement in the characterization of active faults in the study area which falls within the Pollino seismic gap and is considered potentially prone to severe surface faulting. Our aim is for our approach and workflow to be of inspiration for further studies in the region as well as for similar high seismic hazard areas characterized by scarcity of near-surface data.

Origin and evolution of unconformities in a continental rift basin: a case study from the Lishu Depression in Songliao Basin, China

A variety of unconformity types are commonly developed during the rifting of a continental basin. Identifying their origin and evolution is prerequisite to understanding the complex tectono-sedimentary evolution of the basin and to evaluate the basin’s oil and gas resources. Based on the 2D/3D seismic, geophysical logging, and subsurface lithological data, the unconformities within the rift layer of the Lishu Depression in the Songliao Basin are identified as disconformities, nonconformities and four ‘types’ of angular unconformities. Their evolution and origin are analyzed by associating the unconformities with the basin’s tectonic evolution. The results show that the rifting of the Lishu Depression was not a continuous process. Instead, several regional tectonic episodes influenced the basin’s evolution. Episodic compressional events and a phase of extension along the main faults determined the basin configuration and sedimentary fill characteristics of different sub-rifting stages. The tectonic history and related paleogeomorphology changes are the dominant factors in the formation of these different unconformity types. During the rifting process, the type and distribution of unconformity varied. In the initial rifting stage, the basin developed a nonconformity and four types of angular unconformities as the result of both regional uplift and fault block tilt. Rotation and tilting of the fault blocks during the intensive rifting resulted in two types of angular unconformities. Finally, in the recession rifting stage, a different form of angular unconformity and disconformity developed, mainly as a result of the regional uplift and subsequent subsidence.