Gunnar Sælen

Diagenesis of carbonate density-flow deposits controlled by differential uplift of platform segments: Examples from the Cretaceous of the Gargano Promontory (Italy)

Journal of Sedimentary Research, Jul 18, 2023

Diagenesis and construction of the belemnite rostrum

Palaeontology, 1989

Sequential growth of deformation bands in carbonate grainstones in the hangingwall of an active growth fault: Implications for deformation mechanisms in different tectonic regimes

Journal of Structural Geology, 2016

Deformation bands in porous sandstones have been extensively studied for four decades, whereas co... more Deformation bands in porous sandstones have been extensively studied for four decades, whereas comparatively less is known about deformation bands in porous carbonate rocks, particularly in extensional settings. Here, we investigate porous grainstones of the Globigerina Limestone Formation in Malta, which contain several types of deformation bands in the hangingwall of the Maghlaq Fault: (i) bed-parallel pure compaction bands (PCB); (ii) pressure solution-dominated compactive shear bands (SCSB) and iii) cataclasis-dominated compactive shear bands (CCSB). Geometric and kinematic analyses show that the bands formed sequentially in the hangingwall of the evolving Maghlaq growth fault. PCBs formed first due to fault-controlled subsidence and vertical loading; a (semi-)tectonic control on PCB formation is thus documented for the first time in an extensional setting. Pressure solution (dominating SCSBs) and cataclasis (dominating CCSBs) appear to have operated separately, and not in concert. Our findings therefore suggest that, in some carbonate rocks, cataclasis within deformation bands may develop irrespective of whether pressure solution processes are involved. We suggest this may be related to stress state, and that whereas pressure solution is a significant facilitator of grain size reduction in contractional settings, grain size reduction within deformation bands in extensional settings is less dependent on pressure solution processes.

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$Research paper thumbnail of Influence of key sedimentary interfaces on fracture pattern, linkage and cluster formation inperitidal carbonate rocks$

Influence of key sedimentary interfaces on fracture pattern, linkage and cluster formation inperitidal carbonate rocks

Marine and Petroleum Geology

Rock Physics Interpolation Used for Velocity Modeling of Chalks: Ontong Java Plateau Example

The Open Geology Journal, 2010

Velocity behaviour in chalks is determined by a combination of their depositional conditions and ... more Velocity behaviour in chalks is determined by a combination of their depositional conditions and subsequent diagenesis. Based on rock physics data from three drill sites on the Ontong Java Plateau, a strategy for velocity modelling is proposed where geological information is imposed via pore structure models. The Reuss and time-average relations define upper and lower bounds of the velocity data and the self-consistent approximation was applied to find direct links between velocity and changes in microstructure. The latter approach provides an estimate of the dynamic changes of the pore structure as a result of diagenesis in chalks. Depositional pore-models of chalks were determined from environmental conditions and can be defined from textural information. Mechanical compaction and cementation decrease porosity as a function of depth, but may increase the velocity by different rates as the pore structure changes differently. Discrimination between sediment stiffness and pore structure stiffness enables us to justify low velocity for indurated sediments and high velocity for soft sediments. Our results indicate the relevance of the pore structure to velocity interpretation because some variations in velocity data may result from pore structure differences rather than changes in fluid or porosity.

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Middle-Miocene (Serravallian) temperate carbonates in a seaway connecting the Atlantic Ocean and the Mediterranean Sea (North Betic Strait, S Spain)

Sedimentary Geology, 2010

Miocene deposits in the contact area between the Prebetic and Subbetic domains (Betic Cordillera,... more Miocene deposits in the contact area between the Prebetic and Subbetic domains (Betic Cordillera, S Spain) record the last stages of marine deposition on the southern Iberian margin and the final closure of the North Betic Strait in the early Tortonian (Late Miocene). Serravallian (Middle Miocene) shallow-water carbonates and mixed carbonate-siliciclastics formed on a platform at the southern margin of the North Betic Strait. These shallow-water deposits, informally termed the La Escalera Formation, onlap an erosion surface carved into underlying Miocene and older rocks. Carbonate components are mainly bryozoans, coralline red algae, molluscs, and benthic and planktonic foraminifers, typical of heterozoan lithofacies. Several facies types can be distinguished based on different grain sizes and sedimentary structures. Large-scale geometries and stratigraphic facies relationships indicate that the La Escalera Formation was deposited on a homoclinal ramp attached to a deeply eroded, emergent relief to the south. A beach system developed at the southern margin, with backshore lagoons and local incursions of coarse terrigenous sediments. Seawards of the beach system was a submarine-dune belt that extended over several square kilometres at the northeastern end of the La Escalera Formation. Longshore currents shifted submarine dunes to the northeast. Maximum production of carbonate skeletons took place seawards of the dune field. From this factory area, skeletal debris was moved landwards and incorporated into the dunes and beaches, whereas other skeletal particles were transported downslope by storms and fed a facies belt with hummocky and swaley cross-stratified sediments. The finestgrained particles accumulated below a storm-wave base, along with significant planktic debris. Carbonate deposits change laterally westwards to terrigenous sediments formed in similar facies belts. Terrigenous influx limited carbonate production on the western part of the ramp by disturbing the growth of coralline algae and bryozoans. Vertical variations of facies within the formation depict a major transgressiveregressive cycle, modulated by up to 10 higher-frequency cycles. Despite its location in an Atlantic-linked basin, the sedimentary model of the La Escalera Formation shows similarities to the models proposed for Neogene Mediterranean temperate carbonates, which imply a relatively low-energy regime on the ramp in comparison with the open-ocean platforms. This relatively low-energy regime can be explained by the position of the North Betic Strait at the eastern end of the Guadalquivir Basin. However, hummocky and swaley cross-stratified deposits are significantly more developed in La Escalera deposits than in known examples of Neogene Mediterranean carbonates.

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Characterization of dense zones within the Danian chalks of the Ekofisk Field, Norwegian North Sea

Petroleum Geoscience, 2013

The Ekofisk Field is a giant field which has been producing at a high level for more than forty y... more The Ekofisk Field is a giant field which has been producing at a high level for more than forty years and, since 1987, this production has taken place with the support of sea-water injection. The Danian-aged chalk deposits of the Ekofisk Formation and the Maastrichtian Tor Formation form the main reservoir units in the Ekofisk Field. The Ekofisk Formation principally consists of porous resedimented chalks intercalated with relatively thin and lower porosity beds, called dense zones. A multi-scale study of dense zones, from scanning electron microscopy to wells and seismic impedance data, has allowed the characterization and mapping of these deposits. Five main dense zone lithotypes have been identified: (1) argillaceous chalk; (2) chalk with abundant flint nodules; (3) chalk beds cemented with silica/nano-quartz; (4) calcite-cemented chalk; and (5) stylolitized chalk. The different types of dense zones tend to cluster in certain stratigraphic intervals, such as the EE and EM reservoir units at the base and in the middle part of the Ekofisk Formation. Dense zones have different mechanical properties compared to porous chalks and, depending on the connectivity of their fracture networks, they can act as preferential conduits or baffles for the reservoir fluids. An increased understanding of the distribution, characteristics and geological factors at the origin of the dense zones is fundamental to better define the reservoir architecture and ultimately identify unswept zones for future infill drilling targets. Supplementary material Descriptions of analytical procedures, composition, poro-perm and well log values of dense zone samples is available at: www.geolsoc.org.uk/SUP18573.

$Research paper thumbnail of Effects of sedimentary interfaces on fracture pattern, linkage, and cluster formation in peritidal carbonate rocks$

Effects of sedimentary interfaces on fracture pattern, linkage, and cluster formation in peritidal carbonate rocks

Marine and Petroleum Geology, 2010

The permeability of a reservoir is particularly dependent upon the proportion of its fractures th... more The permeability of a reservoir is particularly dependent upon the proportion of its fractures that penetrate or are arrested at interfaces such as contacts and discontinuities. Here we report on fracture penetration and fracture arrest in Lower Cretaceous peritidal deposits exposed in the Pizzicoli Quarry, Gargano Promontory, southern Italy. We measured more than 2000 fractures, in the field and using LIDAR data, of which 564 fractures from the field and 518 from LIDAR studies are the focus of this paper. Fracture arrest/deflection and penetration depend much on the effects of peritidal cycle interfaces such as paleosol horizons, laminated carbonate mudstones, and stylonodular horizons. The laminated mudstones have the greatest effect; 63e99% of the fractures are deflected or arrested at such interfaces, whereas 63e90% are deflected/arrested at paleosols, and 20e35% at stylonodular horizons. In the mudstones, many fractures are arrested at thin, internal laminae, such that few penetrate the entire laminated layer, and fewer still the boundaries between the layers. Paleosol interfaces deflect/arrest more than 60% of all fractures. However, when small-offset fractures above and below paleosols are regarded as penetrating, they are evenly spaced (non-clustered), so that fracture-related fluid transport may occur across the entire paleosol. Stylonodular horizons deflect/arrest and split some fractures, but generally have little effect compared with the other types of interfaces. We present three main mechanisms for fracture deflection and/or arrest: (1) the fracture-induced tensile stress ahead of its tip, referred to as the Cook-Gordon debonding mechanism; (2) rotation of the principal stresses at and across the interface, resulting in the formation of stress barriers; and (3) large elastic mismatch (particularly as regards Young's moduli) between layers across an interface. All these mechanisms are likely to have operated during fracture propagation and arrest in the carbonate rocks of the Pizzicoli Quarry.

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Heavy sedimentary sulfur isotopes as indicators of super-anoxic bottom-water conditions

Geology, 1993

Iron monosulfides and pyrite (total Cr-reducible S—TRS) in the bottom sediments of the exceptiona... more Iron monosulfides and pyrite (total Cr-reducible S—TRS) in the bottom sediments of the exceptionally sulfide-rich euxinic fjord, Framvaren, south Norway, are unusually enriched in 34 S. Most of these TRSs probably form in bottom waters, where reactive detrital iron phases encounter isotopically heavy dissolved sulfide. Enrichment in 34 S in Framvaren is a result of intense bacterial reduction of a sulfate reservoir whose exchange with the open sea is limited by shallow sills and a low-salinity surface-water layer. 34 S-enriched TRS in the deposits of ancient euxinic environments is commonly interpreted as evidence for early diagenetic sulfide formation where residual, less reactive iron phases were in contact with isotopically heavy dissolved sulfide (sulfate-depleted pore waters) for prolonged periods during burial. However, the Framvaren data indicate that isotopically heavy pyrite may form in relatively shallow (80-180 m), super-anoxic bottom-water pools. In contrast, the presence of isotopically light pyrite does not indicate an absence of restricted conditions, but only that most iron sulfide formation was completed before 34 S-enriched sulfide could be encountered in the water column.

Evidence of recycling of isotopically light CO2 (aq) in stratified black shale basins: Contrasts between the Whitby Mudstone and Kimmeridge Clay formations, United Kingdom

Geology, 1998

Calculation of [CO 2 (aq)] and pCO 2-atm based on proxy temperature and salinity data (δ 18 O and... more Calculation of [CO 2 (aq)] and pCO 2-atm based on proxy temperature and salinity data (δ 18 O and δ 13 C from belemnite rostra), and δ 13 C TOC (TOC-total organic carbon) from Toarcian marine organic-rich shales, yields unreasonably large intraformational pCO 2 (atm) variations. We postulate that the low δ 13 C TOC values in the shales can instead be explained by episodic release of isotopically light CO 2 that had built up below a relatively deep pycnocline (halocline?). The lowest δ 13 C TOC values would thus be associated with mixing and erosion of the pycnocline following

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Oyster shells, bulk carbonate sediment, and meteoric calcite cement as recorders of oceanic and radiogenic 87Sr/86Sr in mixed heterozoan carbonates and terrigenous sediment

Journal of Sedimentary Research, 2022

Well-preserved oyster shells from mixed heterozoan carbonates and terrigenous sediments of Miocen... more Well-preserved oyster shells from mixed heterozoan carbonates and terrigenous sediments of Miocene age in the La Chanata area (southern Spain) show varying luminescence and covarying δ18O and δ13C compositions > –2‰ VPDB. Temperature and salinity modeling based on the δ18O give temperatures ranging from 13 to 33 °C (average 22.4 °C) and normal-marine salinities (34–37 PSU). The isotopic compositions likely reflect environmental conditions during shell secretion; moreover, their identical intra-shell 87Sr/86Sr compositions (equal to or less than the analytical precision of 20 × 10–6) support a primary origin. Additional information from δ18O and 87Sr/86Sr compositions of Heterostegina foraminifera and sediment and cement samples show that: a) bulk sediment and cement samples that have significantly lower δ18O and δ13C values but 87Sr/86Sr values similar to the oyster shells likely derived their isotopic compositions from intraformational dissolution of high-Mg calcite and aragonit...

Contradictory coeval vertical facies changes in upper Miocene heterozoan carbonate–terrigenous deposits (Sierra de Gádor, Almería, SE Spain)

Journal of Sedimentary Research

While spatial facies patterns can be observed in modern systems, only vertical facies successions... more While spatial facies patterns can be observed in modern systems, only vertical facies successions can usually be examined in ancient deposits. Lateral facies relationships (depositional models) and relative sea-level changes throughout time are traditionally deduced from correlation of vertical facies successions along transects perpendicular to inferred paleo-shorelines. Establishing vertical facies successions and their corresponding time-equivalent proximal to distal facies patterns are, therefore, paramount in reconstructing ancient carbonate depositional systems and their response to sea-level change. In the present study of well-exposed panoramics of late Tortonian (8.1 Ma) mixed heterozoan carbonate and terrigenous deposits in La Chanata area in Sierra de Gádor, Almería, SE Spain, we show that frequent changes in facies width make it difficult to predict how variation in sea level impacts the facies distribution. The following facies are recognized: shoreline conglomerates di...

Oyster Shells As Recorders of Short-Term Oscillations of Salinity and Temperature During Deposition of Coral Bioherms and Reefs In the Miocene Lorca Basin, SE Spain

Journal of Sedimentary Research, 2016

We use d 18 O and d 13 C of oyster shells, inferred seawater d 18 O (d w), and temperature tolera... more We use d 18 O and d 13 C of oyster shells, inferred seawater d 18 O (d w), and temperature tolerance ranges of modern zooxanthellate coral communities to constrain and model paleo-salinities of marginal-marine waters of the Miocene Lorca Basin, Spain. Salinities ranged from 22 to 32 PSU during growth of coral bioherms and carpets of the Soriana and Parrilla formations, and from 35 to 39 PSU during deposition of the Hondo Formation reef complex. The periods with lower modeled salinities correspond with deltaic and alluvial-fan sedimentation in the area. The modeling is supported by 87 Sr/ 86 Sr data and provides new insights into the evolution of the local depositional settings. Departures of Sr-isotope values from the global oceanic curve occur within the intervals with the lowest modeled salinities, and the non-oceanic 87 Sr/ 86 Sr preserved in individual oyster shells was probably sourced from uplifted Aquitanian-Burdigalian limestones. 87 Sr/ 86 Sr values that plot along the global ocean Sr-isotope curve for the late Miocene indicate a Tortonian age for the upper part of the Soriana Formation and the Parrilla Formation, and a Messinian age for the Hondo Formation. More generally, we demonstrate how the chemical analyses of oyster shells provide additional information regarding short-term oscillations of salinity and temperature of seawater beyond what can be obtained by traditional paleoecological methods.

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Seismic stratigraphy of the Chalk Group in the Norwegian Central Graben, North Sea

Marine and Petroleum Geology

The Late Cretaceous to Early Palaeogene Chalk Group in the Norwegian Central Graben contains prol... more The Late Cretaceous to Early Palaeogene Chalk Group in the Norwegian Central Graben contains prolific hydrocarbon reservoirs that have been producing at high rate for more than forty years. Based on integration of regionally extensive 3D seismic data and numerous wells, this paper describes the seismic development of the Chalk Group with particular focus to the syndepositional geomorphological features produced by the activity of bottom currents and large-scale gravity flows. Using standard seismic stratigraphic interpretation techniques, eight seismic sequence boundaries are identified, which in turn define seven seismic stratigraphic sequences characterized by different seismic facies and well log signatures. The seismic sequences can be grouped into three sequence groups that reflect the general tectono-stratigraphic evolution of the chalk depositional system in the Norwegian Central Graben. During the Late Cretaceous, uplift of the Lindesnes Ridge and the Albuskjell Anticline due to inversion tectonic and halokinesis modified the physiography of the Norwegian Central Graben basin and affected the style of chalk sedimentation by influencing the location and importance of gravity flows. In addition, bottom currents, sea-level fluctuations and environment changes significantly influenced the chalk depositional system creating regional unconformities and influencing the input of terrigenous material in the chalk epeiric sea.

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Middle-Miocene (Serravallian) temperate carbonates in a seaway connecting the Atlantic Ocean and the Mediterranean Sea (North Betic Strait, S Spain

Sedimentary Geology, 2010

Miocene deposits in the contact area between the Prebetic and Subbetic domains (Betic Cordillera,... more Miocene deposits in the contact area between the Prebetic and Subbetic domains (Betic Cordillera, S Spain) record the last stages of marine deposition on the southern Iberian margin and the final closure of the North Betic Strait in the early Tortonian (Late Miocene). Serravallian (Middle Miocene) shallow-water carbonates and mixed carbonate-siliciclastics formed on a platform at the southern margin of the North Betic Strait. These shallow-water deposits, informally termed the La Escalera Formation, onlap an erosion surface carved into underlying Miocene and older rocks. Carbonate components are mainly bryozoans, coralline red algae, molluscs, and benthic and planktonic foraminifers, typical of heterozoan lithofacies. Several facies types can be distinguished based on different grain sizes and sedimentary structures. Large-scale geometries and stratigraphic facies relationships indicate that the La Escalera Formation was deposited on a homoclinal ramp attached to a deeply eroded, emergent relief to the south. A beach system developed at the southern margin, with backshore lagoons and local incursions of coarse terrigenous sediments. Seawards of the beach system was a submarine-dune belt that extended over several square kilometres at the northeastern end of the La Escalera Formation. Longshore currents shifted submarine dunes to the northeast. Maximum production of carbonate skeletons took place seawards of the dune field. From this factory area, skeletal debris was moved landwards and incorporated into the dunes and beaches, whereas other skeletal particles were transported downslope by storms and fed a facies belt with hummocky and swaley cross-stratified sediments. The finestgrained particles accumulated below a storm-wave base, along with significant planktic debris. Carbonate deposits change laterally westwards to terrigenous sediments formed in similar facies belts. Terrigenous influx limited carbonate production on the western part of the ramp by disturbing the growth of coralline algae and bryozoans. Vertical variations of facies within the formation depict a major transgressiveregressive cycle, modulated by up to 10 higher-frequency cycles. Despite its location in an Atlantic-linked basin, the sedimentary model of the La Escalera Formation shows similarities to the models proposed for Neogene Mediterranean temperate carbonates, which imply a relatively low-energy regime on the ramp in comparison with the open-ocean platforms. This relatively low-energy regime can be explained by the position of the North Betic Strait at the eastern end of the Guadalquivir Basin. However, hummocky and swaley cross-stratified deposits are significantly more developed in La Escalera deposits than in known examples of Neogene Mediterranean carbonates.

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Stable-Isotope Analyses of Belemnite Rostra From the Whitby Mudstone Formation, England; Surface Water Conditions During Deposition of a Marine Black …

Palaios, 1996

Page 1. RESEARCH REPORTS 97 Stable-Isotope Analyses of Belemnite Rostra from the Whitby Mudstone ... more

Sr/Ca in Calcitic Tests of Benthic Foraminifera-A Potential Water-Depth Proxy?

Well-preserved low-Mg calcite tests of modern benthic foraminifera from a depth transect off SW N... more Well-preserved low-Mg calcite tests of modern benthic foraminifera from a depth transect off SW Norway show decreasing Sr/Ca with increasing water depth (r =-0.84). The Mg/Ca ratios also decrease with increasing water depth, although in a curvilinear fashion. Moreover, the inter-and intra-species scatter is apparently larger than for Sr/Ca. A majority of the analysed specimens have Mn/Ca < 200 μmol/mol, and show Sr/Ca, Mg/Ca that fall within published ranges for benthic foraminiferal calcite, indicating that pristine biogenic calcite has been analysed. The uniform temperature and salinity of bottom waters below ~800 m moreover indicate that neither temperature nor salinity can explain the observed Me/Ca trends. The Sr/Ca and Mg/Ca depth trends cannot be explained by dissolution since bottom waters are generally supersaturated with respect to calcite, and dissolution is only observed in a few specimens. Moreover, a regular increase in sedimentary organic content with depth needed to invoke a decrease in porewater pH and concomittant calcite dissolution is not observed. We tentatively ascribe the decreasing Sr/Ca and Mg/Ca trends to depthrelated physiological phenomena. The relatively small intra-and interspecies scatter at a given water depth, as well as the linear decrease in Sr/Ca ratios with increasing water depth observed in modern oceans, may render the Sr/Ca in calcitic benthic foraminifera a potential water-depth proxy for fossil deposits.

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Contrasting watermass conditions during deposition of the Whitby Mudstone (Lower Jurassic) and Kimmeridge Clay (Upper Jurassic) formations, UK

The Whitby Mudstone Formation (WMF) and Kimmeridge Clay Formation (KCF) are both Jurassic organic... more The Whitby Mudstone Formation (WMF) and Kimmeridge Clay Formation (KCF) are both Jurassic organicrich 'black shale' units containing well-preserved marine organic matter. Despite such gross similarities they exhibit opposite trends in d13C TOC versus wt% decarbonated total organic carbon (TOC; r=−0.76 for WMF, and 0.71 for KCF); the d13C TOC versus TOC trend also varies geographically within both units. Only the KCF data exhibit a positive correlation between d13C TOC and d15N org (r=0.76) and the presence of aryl isoprenoids (probable indicators of photic zone anoxia). Maturity differences are insufficient to explain such contrasts. Bulk and compound-specific isotopic variations in the KCF are interpreted largely as a reflection of primary productivity during deposition, rather than a dominantly diagenetic signal due to incorporation of sulphurised carbohydrate. The contrasts between the formations, and the geographic variation in coeval shelf sediments, are explained by a model stressing the relative positions and stabilities of the chemocline, pycnocline, nutricline and euphotic zone, and the effect these have on productivity, CO 2 recycling, and bacterial floras.

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