The Utsira High is a prominent intrabasinal basement structure in which the eroded remnants of a widespread, Late Permian, Zechstein carbonate shelf are preserved locally. The western margin of the central Utsira High is mainly characterized by weathered basement rocks with a thin Mesozoic cover. However, the recently discovered Symra Field forms an isolated sedimentary inlier basin of the area, where deeply eroded Zechstein shelf carbonates are preserved within a half-graben. The Zechstein carbonates consist of mainly ZS2 marginal marine carbonate facies and are similar to those described elsewhere from the Zechstein Basin. They are unconformably overlain by Paleogene chalks. Based on detailed facies analysis of two recently drilled cores combined with detailed petrographic and stable isotopic analysis and supplemented by age dating of selected carbonate phases using U-Pb geochronology, we show that the Zechstein carbonates have been subjected to several phases of near-surface diagenetic alterations. Volumetrically the most dominant diagenetic product comprises nonplanar dolomites interpreted to have formed by recrystallization of a precursor reflux-type dolomite phase. The recrystallized dolomites retain enhanced reservoir quality in comparison to stratigraphic equivalent ZS2 reflux-type dolomites found elsewhere on the Utsira High. Based on U-Pb-derived age constraints, the recrystallization took place during a long-lived Late Triassic exposure event of the Utsira High, and a near-surface origin for the recrystallized dolomites is proposed. The porosity enhancement occurred contemporaneously with the dolomite recrystallization process and was facies controlled. Reservoir modifications associated with the exposure led to the dissolution of CaSO4 cement, and metastable dolomite phases, in addition to the enlargement of existing pores which had created zones of weakness prone to further dissolution. Overall, the Zechstein carbonates preserved on the Utsira High illustrate the complexity of the diagenetic alterations resulting from the uplift of a carbonate shelf, and its importance for reservoir quality.

We used high-resolution petrographic and dense-mineral data on modern sand to investigate erosion patterns of the El Rosario, San Fernando, and San Vicente river basins of Baja California (Mexico) to better understand the interrelationships between a complex magmatic arc terrane and surface processes. Modern sand composition of these three rivers reflects the nature of the source region, which lies in the central part of the Alisitos arc (Peninsular Ranges, Baja California, Mexico). The sand detrital modes correspond well with the main structural units drained by the El Rosario, San Fernando, and San Vicente rivers: 1) the Early Cretaceous oceanic arc of the Alisitos Group, 2) the Paleozoic to Mesozoic continental-margin metasedimentary rocks, 3) the Cretaceous plutons, 4) the Upper Cretaceous to Tertiary sedimentary rocks, and 5) the Tertiary volcanics. The modern sand of the San Vicente, San Fernando, and El Rosario rivers is fed chiefly from erosion of a magmatic arc and consists mostly of minor feldspatho-lithic (Fl) to quartzo-litho-feldspathic (qFL) sand and dominant quartzo-feldspatho-lithic (qLF) and litho-feldspatho-quartzose (lQF) sand. Framework petrography also suggests a progressive increase in quartz, K-feldspar, sedimentary and metamorphic lithic fragments, and a decrease in volcanic lithic fragments. Sand, in the Lv field, microlitic (Lvmi), felsitic (Lvf) and lathwork (Lvl) types, and trace amounts of vitric grains (Lvv), such as pumice particles. The andesitic volcanic province of the Alisitos arc sheds quartz-poor sand containing mainly microlitic lithic fragments and plagioclase, whereas sand derived from more felsic rhyolites and rhyodacitic and trachyandesitic products contains largely felsitic volcanic lithics and minor lathwork lithics are mainly derived from subordinate basalts. The abundance of intrusive rock fragments and volcanic and sedimentary lithics of the sampled river sands faithfully represents the relative abundance of a heterogeneous bedrock exposure consisting of sedimentary and metasedimentary rocks, as well as volcanic, plutonic, and medium-to high-grade metamorphic rocks in each drainage basin. Transparent heavy-mineral assemblages including major amounts of amphibole, pyroxene, epidote, titanite, zircon, and minor amounts of staurolite, rutile, actinolite, tourmaline, garnet, kyanite, andalusite, sillimanite, and apatite are in good agreement with a mixed provenance characterized mainly by magmatic, primarily volcanic (andesite, rhyolite, and basalt) and secondarily plutonic (granitoid rocks) and metamorphic source rocks. Some labile species such as hornblende and pyroxene grains show mainly corroded to etched morphologies due to dissolution processes and by chemical weathering processes occurring in a paleo and current semiarid climate. The Zircon+Tourmaline+Rutile index of the heavy-mineral modes, coupled with their subrounded to rounded grain surface texture, indicates recycling from the sedimentary source rocks. Heavy-mineral abundance and weathering textures in the San Fernando and San Vincente river sands match predominantly volcanic bedrock lithologies, while the El Rosario river sands match sedimentary and metasedimentary source rocks.

The Utsira High (North Sea) records rift faulting that culminates with Jurassic-Cretaceous crustal-scale rollover towards the Viking Graben. This deformation is superimposed on Paleozoic fault-bounded basins on a substrate of Caledonian nappes. The latter contains multilevel crustal-scale shear systems that domed under the Utsira High during the Devonian, as indicated by mapping and interpretation of a large long-offset 2D seismic reflection dataset. Restorations show that isostatically driven doming from excision of overthickened crust caused uplift and erosion of basins and the underlying nappe stack. Doming took place above a crustal rollback system as a symmetrical metamorphic core complex formed. The dome discloses vertical flattening kinematics below bidirectional shear systems, evident by opposite shear-fabric kinematics on opposite dome flanks. A major detachment marks the upper boundary of a transition between upper and lower plate strain regimes, coinciding with low-reflective granitic units above strongly reflective Caledonian nappes. This detachment hosts transportparallel corrugations that strike E-W to NE-SW. A series of deeply eroded half-grabens on top of the upper plate are bounded by pre-Permian faults exhibiting top-NE extensional kinematics. Faults sole out in the fundamental detachment level, locating the brittle-ductile transition, with all structures subsequently rotated during doming. A new localized detachment formed at a shallower level above the region of maximum crustal uplift, seen as upwards detachment migration driven by heating. Later faulting locates to the dome flanks, either rejuvenating rotated shear zones or cutting to deeper crustal levels while recording predominantly down-eastward transport. During Jurassic-Cretaceous rifting, deformation localized to the Viking Graben Boundary Fault, giving room for thick growth wedges in the Viking Graben and establishing the Utsira High as a crustal-scale rollover structure.

Porous and permeable crystalline rocks have the potential of being hydrocarbon reservoirs. In many cases, reservoir properties are created by rock-altering physico-chemical processes occurring in the near-surface environment. Hydrocarbon reservoirs in altered granitoids may comprise both arenaceous (clay-poor) and argillaceous (clay-rich) palaeoregoliths, and hence outcrop analogues from various environments are required. In the Sila Massif, southern Italy, the arenaceous weathering of medium to coarse grained granodioritic rocks has been studied. The pore system, dominated by microfractures, was described and quantified at each stage in the weathering process (altered coherent rock facies -> saprock facies -> lower and upper saprolite facies -> soil) in relation to changes in chemical and physical properties. The transformation of rock to saprolite was close to an isochemical process, as demonstrated by the low variations in the bulk mass transfer coefficient (tau(bulk)). At the same time, trans-, intra- and intermineral microfractures increased in frequency and aperture upwards through the regolith profile. It was the main reason for the reduction of bulk densities (rho(b)) from 2.66 g/cm(3) in the altered coherent rock to 1.88 g/cm(3) in the saprolite, corresponding to the formation of up to 30% porosity (phi(c)). The results suggest that a volume increase of > 50%, represented by the volumetric strain factor (epsilon), was the main cause for the transformation of solid crystalline rock to porous saprolite. Anisovolumetric arenaceous regoliths with minimal mass loss, are the result of one of the major weathering processes, notwithstanding that they are much less understood than isovolumetric, anisochemical, argillaceous regoliths. Arenaceous saprolites potentially can have excellent porosity and permeability properties, with open and well-connected microfractures. Reservoir quality of arenaceous saprolites is in stark contrast to the poor permeability observed in some argillaceous saprolites where porosity is dominated by isolated dissolution voids within a clay-rich matrix.

The Paleocene–Eocene Thermal Maximum (PETM; ∼ 55.9 Ma) was a period of rapid and sustained global warming associated with significant carbon emissions. It coincided with the North Atlantic opening and emplacement of the North Atlantic Igneous Province (NAIP), suggesting a possible causal relationship. Only a very limited number of PETM studies exist from the North Sea, despite its ideal position for tracking the impact of both changing climate and NAIP activity. Here we present sedimentological, mineralogical, and geochemical proxy data from Denmark in the eastern North Sea, exploring the environmental response to the PETM. An increase in the chemical index of alteration and a kaolinite content up to 50 % of the clay fraction indicate an influx of terrestrial input shortly after the PETM onset and during the recovery, likely due to an intensified hydrological cycle. The volcanically derived zeolite and smectite minerals comprise up to 36 % and 90 % of the bulk and clay mineralogy respectively, highlighting the NAIP's importance as a sediment source for the North Sea and in increasing the rate of silicate weathering during the PETM. X-Ray fluorescence element core scans also reveal possible hitherto unknown NAIP ash deposition both prior to and during the PETM. Geochemical proxies show that an anoxic to sulfidic environment persisted during the PETM, particularly in the upper half of the PETM body with high concentrations of molybdenum (MoEF > 30), uranium (UEF up to 5), sulfur (∼ 4 wt %), and pyrite (∼ 7 % of bulk). At the same time, export productivity and organic-matter burial reached its maximum intensity. These new records reveal that negative feedback mechanisms including silicate weathering and organic carbon sequestration rapidly began to counteract the carbon cycle perturbations and temperature increase and remained active throughout the PETM. This study highlights the importance of shelf sections in tracking the environmental response to the PETM climatic changes and as carbon sinks driving the PETM recovery.

The paleoregolith in this study was unconformably overlain by lower Cretaceous (Valanginian) marine deposits of the Asgard Formation and buried below two kilometers of sediments, on the Utsira High, Norwegian North Sea. The upwards intensification of chemical alteration, manifested in the replacement of feldspars by clays, leaching of base cations and reduced mechanical rock strength, in addition to the presence of terrestrial biotitic features, suggested weathering during subaerial exposure of the Utsira High during the Mesozoic. The paleoregolith can be divided into three weathering facies: altered coherent rock, saprock and saprolite, whereas the absence of true soil horizons suggested erosional truncation of the regolith before burial. Possible remnants of the eroded soil were identified as pedogenic mud aggregates in the overlying Asgard Formation, bearing similar compositional characteristics as the saprolite. Clay mineralogy, bulk geochemical composition, micromorphology and depth of biotic features were used to reconstruct the paleoenvironmental conditions that prevailed during the formation of the regolith in the Mesozoic North Sea. Our results point to a regolith formation under periodically water-restricted conditions, promoting the co-existence of interstratified illite-smectite (beidellite) and kaolinite, deep presence of endomycorrhizae (the fungal extension of the root structure) and precipitation of iron oxide nodules. Application of recently developed geochemical climofunctions indicated that the regolith formed in a mesic (10-12 degrees C) and subhumid (1157-1406 mm/yr) environment, and under strongly acidic soil conditions (pH 5.2-5.6).

Large-scale magmatic events like the emplacement of the North Atlantic Igneous Province (NAIP) are often coincident with periods of extreme climate change such as the Palaeocene–Eocene Thermal Maximum (PETM). One proxy for volcanism in the geological record that is receiving increased attention is the use of mercury (Hg) anomalies. Volcanic eruptions are among the dominant natural sources of Hg to the environment; thus, elevated Hg∕TOC values in the sedimentary rock record may reflect an increase in volcanic activity at the time of deposition. Here we focus on five continental shelf sections located around the NAIP in the Palaeogene. We measured Hg concentrations, total organic carbon (TOC) contents, and δ13C values to assess how Hg deposition fluctuated across the PETM carbon isotope excursion (CIE). We find a huge variation in Hg anomalies between sites. The Grane field in the North Sea, the most proximal locality to the NAIP analysed, shows Hg concentrations up to 90 100 ppb (Hg∕TOC = 95 700 ppb wt %−1) in the early Eocene. Significant Hg∕TOC anomalies are also present in Danish (up to 324 ppb wt %−1) and Svalbard (up to 257 ppb wt %−1) sections prior to the onset of the PETM and during the recovery period, while the Svalbard section also shows a continuous Hg∕TOC anomaly during the body of the CIE. The combination with other tracers of volcanism, such as tephra layers and unradiogenic Os isotopes, at these localities suggests that the Hg∕TOC anomalies reflect pulses of magmatic activity. In contrast, we do not observe clear Hg anomalies on the New Jersey shelf (Bass River) or the Arctic Ocean (Lomonosov Ridge). This large spatial variance could be due to more regional Hg deposition. One possibility is that phreatomagmatic eruptions and hydrothermal vent complexes formed during the emplacement of sills led to submarine Hg release, which is observed to result in limited distribution in the modern era. The Hg∕TOC anomalies in strata deposited prior to the CIE may suggest that magmatism linked to the emplacement of the NAIP contributed to the initiation of the PETM. However, evidence for considerable volcanism in the form of numerous tephra layers and Hg∕TOC anomalies post-PETM indicates a complicated relationship between LIP volcanism and climate. Factors such as climate system feedbacks, changes to the NAIP emplacement style, and/or varying magma production rates may be key to both the onset and cessation of hyperthermal conditions during the PETM. However, processes such as diagenesis and organic matter sourcing can have a marked impact on Hg∕TOC ratios and need to be better constrained before the relationship between Hg anomalies and volcanic activity can be considered irrefutable.

Saprock-saprolite associations were studied by field and laboratory methods (optical microscopy, X-ray powder diffraction, scanning electron microscopy, electron microprobe) in order to describe regolith development in the crystalline rocks of the Nygard kaolin pit (Bornholm, Denmark). The clay sequences and stages of porosity development are similar to those observed for reservoir rocks from the Utsira High (Riber et al., 2016). The weathering of the parent granite began before the end of the Mesozoic. Two stages of syn-/pre-burial alteration, followed by diagenesis during burial, and then post-uplift weathering have been recognized. In stage I, plagioclase and some biotite (biotite-vermiculite-kaolinite) reacted to form elongate booklets of highly-ordered kaolinite or smaller, blocky pseudohexagonal crystals. Stage II represented more extreme weathering developed along local fracture systems. The higher potential for fluid flow in the fractures caused highly-ordered kaolinite to alter to halloysitic, poorly-ordered kaolinite. Plagioclase, biotite, and K-feldspar continued to interact with formation water and formed additional quantities of secondary clay minerals. As a consequence of inter-, intra-, and trans granular cracks, the original rock fabric deteriorated and porosity increased. During later burial, diagenetic siderite crystals filled voids and chlorite formed at the expense of kaolinite. After re-exposure, calcite precipitated in localized zones. Smectite formed on a broader scale wherever high ionic strength groundwaters were present. A mineralogical key to predicting where high porosity and permeability zones occur is the recognition of halloysitic, poorly-ordered kaolinite. Highly-ordered kaolinite is abundant, but it is ubiquitous. Siderite, chlorite and smectite adversely affect permeability.Riber, L., Dypvik, H., Sorlie, R. and Ferrell, R. (2016) Clay minerals in deeply buried paleoregolith profiles, Norwegian North Sea. Clays Clay Miner. DOI: 10.1346/CCMN.2016.064036. (C) 2016 Elsevier Ltd. All rights reserved.

For the first time on the Norwegian Continental Shelf, deeply buried paleoregolith profiles have been identified as part of the petroleum reservoirs in recently discovered oil fields on the Utsira High, Norwegian North Sea. Reservoir properties (porosity and permeability) in the granitic basement on the Utsira High are mainly the result of physical and chemical alteration of the rock occurring in the near-surface environment during sub-aerial exposure of the high in the Mesozoic. Evaluating the reservoir potential of altered basement rocks requires a different approach than in conventional petroleum exploration. In this paper, macroscopic, mineralogical and micromorphological alteration features observed in two deeply buried paleoregolith profiles are compared with surface paleoregoliths from No Klack, Sweden and Georgia, USA. The paleoregolith profiles are subdivided into specific weathering fades (altered coherent rock fades, saprock facies and saprolite facies) based on the rock fabric and mechanical strength. The reservoir potential of each weathering facies is controlled by the type and degree of alteration. In the altered coherent rock fades, porosity and permeability is mainly controlled by joints and microfractures that developed prior to subaerial exposure of the granitic pluton. In the saprock fades, intensified chemical dissolution of plagioclase enhanced porosity and the development of mesofractures improved the connectivity between pores. In the saprolite facies, progressive dissolution of plagioclase creates porosity, but the precipitation of clays within voids and mesofractures has a destructive effect on the overall reservoir properties. The deeply buried paleoregolith profiles from the Utsira High display comparable macroscopic, mineralogical and micromorphological alteration features to what was observed in surface paleoregoliths from No Klack and Georgia. Outcrop studies may therefore be an important tool when evaluating the reservoir potential in subsurface paleoregoliths. (C) 2017 Elsevier B.V. All rights reserved.

Recent discoveries of oil in deeply buried paleoregolith profiles on the Utsira High, Norwegian North Sea, was the first time basement rocks had been demonstrated to be petroleum reservoirs on the Norwegian continental shelf. The present study aimed to establish the processes responsible for the primary weathering sequence, distinguish them from other phases of alteration, and create a model for the development of reservoir properties in crystalline basement rocks. Hand-specimen and laboratory tests revealed a link between reservoir properties in weathered granitic rocks and alteration facies. Samples were obtained from two distinct paleoregolith profiles on the Utsira High. The core samples were studied in detail by optical microscopy, X-ray powder diffraction, scanning electron microscopy, and X-ray fluorescence. In the altered coherent rock facies, porosity and permeability were mainly created by joints and fractures prior to subaerial exposure. In the altered compact rock and altered incoherent rock facies, the development of reservoir properties was increasingly affected by physicochemical interactions between the rock and percolating fluids during subaerial exposure and early diagenesis. In well 16/3-4, the altered coherent rock facies contained R0 illite-smectite (I-S), well ordered kaolinite, and a mixture of fine-grained mica and illite, produced in semi-open and closed microsystems. In the altered compact rock and altered incoherent rock facies, disordered kaolinite became more abundant at the expense of R0 I-S, well ordered kaolinite, plagioclase, and biotite, suggesting alteration in semi-open microsystems. The collapse of the rock structure and clogging of mesofractures by clays contributed to reduced permeability in the clay-rich upper part of the altered incoherent rock. In contrast, well 16/1-15 represented a more deeply truncated weathering profile compared to 16/3-4, characterized by open and interconnected mesofractures and moderate formation of clay. R0 I-S was present and kaolinite was rare throughout the profile, suggesting stagnant conditions. During burial, a porosity-reducing serpentine-chlorite Ib β = 90° polytype formed in the overlying sandstone and the regolith. Application of these results should improve the success of exploration and production efforts related to hydrocarbon reservoirs in the altered crystalline basement.

As part of the recent discoveries on the Utsira High (Edvard Grieg and Johan Sverdrup fields), altered and fractured basement rocks were for the first time shown to act as a reservoir and possible migration paths for commercial hydrocarbon deposits on the Norwegian Continental Shelf Altered basement rocks are underlying the main Upper Jurassic reservoir rocks in Johan Sverdrup and the main Cretaceous, Jurassic and Triassic reservoir rocks in Edvard Grieg. In the present study, eighteen basement cores from the Utsira High have been classified and investigated for signs of alteration, including subaerial weathering. The results show highly variable basement composition, including metasandstones, phyllites, granites, granodiorites and gahhroic rocks. In core view most of the basement rocks show signs of a medium to high degree of fracturing. Alteration has taken place in most of the cores, ranging from slight discoloration to disintegration along fractures, and to total fragmentation of the rock. The fragmentation of the rock is commonly associated with the dissolution of primary minerals and precipitation of secondary clays in the newly formed pore space. The upwards increasing disintegration and increasing amount of clay observed in the basement rocks from two of the wells (16/3-4 and 16/1-15) indicate that subaerial weathering was the main alteration agent.

The study deals with environmental changes during the Paleocene-Eocene Thermal Maximum (PETM) and its background conditions in Spitsbergen through analysis of benthic foraminiferal assemblages (FA) in a section drilled in the Paleogene Central Basin. The impact of this extreme global warming occurs here in prodelta shelf mudstones composing the lower part of the Gilsonryggen Member (Frysjaodden Formation). The start of the PETM perturbation is marked by a faunal turnover, in which the medium-diversity circumpolar Reticulophragmium assemblage was replaced by a low-diversity Trochammina fauna. During the hyperthermal period, benthic foraminiferal diversity decreased severely, while the dominance of small-sized taxa with epifaunal morphology strongly increased. This low-diversity fauna occurs in sediments with a reduced thorium/uranium ratio (proxy for oxygenation) and kaolinite enrichment (proxy for high humidity). The faunal changes were thus caused by the combined effects of hypoxic and hyposaline conditions in a stratified water column, due to extreme warming with its accompanying intensified hydrologic cycle. The PETM acme coincides with the maximum flooding surface (MFS) of the Gilsonryggen depositional sequence, composed of the Gilsonryggen Member and the overlying Battfjellet and Aspelintoppen formations. The transgressive phase of the sequence was initiated by local tectonics, while the eustatic sea-level rise of the PETM was superimposed on this transgression.

For the first time the Paleocene–Eocene Thermal Maximum (PETM) interval has been identified in drillcores and field outcrops from the Paleogene Central Basin in Svalbard, based on mineralogical and geochemical information. Grumantbyen and Frysjaodden formations, a more than 300m thick Paleogene interval, representing deltaic, delta-influenced marine shelf and deep water slope environments. The PETM has been recognized in the deepest and most distal deposits in the lower parts of the Frysjaodden Formation. Here, this global event is displayed in clay mineralogical distributions as significantly increased kaolinite abundances reflecting a period with intensified chemical weathering in the land areas surrounding this Paleogene basin. The period was characterized by dominantly reducing sea floor conditions, as seen in the Th/U distribution and the well developed parallel lamination in the pyrite-rich shales; all to be expected during an interval with increased temperature and precipitation.

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