Cretaceous and Cenozoic Basin Evolution Based on Decompacted Sediment Thickness from Petroleum Wells around New Zealand

Decompacted sedimentary data from 33 New Zealand exploration wells is used to investigate basin evolution and tectonics from around New Zealand. This analysis is directed to both a comparison of basin behaviour and a search for common subsidence signatures. Common to almost all New Zealand basin subsidence curves is a sedimentary signature associated with rifting of the Gondwana super-continent (80-65 Ma). In the Great South Basin a second rifting event is inferred at 51 [plus or minus] 2 Ma, illustrated by a rapid increase in subsidence rates (with a maximum rate of 190 m.Myr-1 at Pakaha-1). Coinciding with the cessation of Tasman Sea rifting ([approximately] 53 Ma), and with the onset of rifting in the Emerald Basin ([approximately] 50 Ma), it is assumed that the event is related to the tectonic plate reorganization. An increase in sedimentation is noted at [approximately] 20 Ma in most South Island wells. Convergence on the Alpine Fault, leading to increased erosion is cited as a mechanism for this period of basin growth, consistent with the Cande and Stock (2004) model of plate motions. A second increase in sedimentation occurs at [approximately] 6 Ma in almost all wells around New Zealand. Climate-driven erosion resulting in isostatic uplift is thought to contribute to this event. Hiatuses in the sedimentary record for the Canterbury, Great South and Western Southland Basins during the late Oligocene are interpreted as the Marshall Paraconformity. It appears that the break in sedimentation located within a regional transgressional mega-sequence was caused by mid Oligocene glacio-eustatic fall and related oceanic current processes. Loading by the Northland Allochthon, in conjunction with paleobathymetry and subsidence data, is used to demonstrate the mechanical properties of the lithosphere. A lithospheric rigidity of 1.5 x [10 to the power of 22] Nm is estimated, with an elastic thickness of 12 km. Considerably lower than elastic thickness values previously calculated for the Plio-Pleistocene loading of the Taranaki Platform. It is noted that the Northland value represents a younger, hotter crust at the time of load emplacment. With the exception of the central Taranaki and Great South Basins, stretching factors ([Beta]) for the sedimentary basins surrounding New Zealand are below 2. This suggests crustal thickness prior to rifting was between 35 and 50 km, consistent with data from conjugate margins of Australia and Antarctica. An increase in water depth in the Taranaki Basin at 25 [plus or minus] 3 Ma is confirmed by this study. This coincides with a similar signature on the West Coast of the South Island at 26 [plus or minus] 2 Ma. It is suggested that a mantle flow caused by the initiation of the subduction zone at [approximately] 25 Ma extends over a broader region (>750 km) than previously thought

Constraints on the evolution of Taranaki Fault from thermochronology and basin analysis: Implications for the Taranaki Fault play

Taranaki Fault is the major structure defining the eastern margin of Taranaki Basin and marks the juxtaposition of basement with the Late Cretaceous-Paleogene succession in the basin. Although the timing of the basement over-thrusting on Taranaki Fault and subsequent marine onlap on to the basement block are well constrained as having occurred during the Early Miocene, the age of formation of this major structure, its character, displacement history and associated regional vertical movement during the Late Cretaceous- Recent are otherwise poorly known. Here we have applied (i) apatite fission track thermochronology to Mesozoic basement encountered in exploration holes and in outcrop to constrain the amount and timing of Late Cretaceous-Eocene exhumation of the eastern side of the fault, (ii) basin analysis of the Oligocene and Miocene succession east of the fault to establish the late-Early Miocene - Early Pliocene subsidence history, and (iii), regional porosity-bulk density trends in Neogene mudstone to establish the late uplift and tilting of eastern Taranaki Basin margin, which may have been associated with the main period of charge of the underlying Taranaki Fault play. We make the following conclusions that may be useful in assessing the viability of the Taranaki Fault play. (1) Mid-Cretaceous Taniwha Formation, intersected in Te Ranga-1 was formerly extensive across the western half of the Kawhia Syncline between Port Waikato and Awakino. (2) Taranaki Fault first formed as a normalfault during the Late Cretaceous around 85±10 Ma, and formed the eastern boundary of the Taranaki Rift-Transform basin. (3) Manganui Fault, located onshore north of Awakino, formed as a steeply east dipping reverse fault and accommodated about four km of displacement during the mid-Cretaceous. (4) Uplift and erosion, involving inversion of Early Oligocene deposits, occurred along the Herangi High during the Late Oligocene. This may have been associated with initial reverse movement on Taranaki Fault. (5) During the Early Miocene (Otaian Stage) the Taranaki and Manganui Faults accommodated the westward transport of Murihiku basement into the eastern margin of Taranaki Basin, but the amount of topography generated over the Herangi High can only have been a few hundred metres in elevation. (6) The Altonian (19-16 Ma) marked the start of the collapse of the eastern margin of Taranaki Basin that lead during the Middle Miocene to the eastward retrogradation of the continental margin wedge into the King Country region. During the Late Miocene, from about 11 Ma, a thick shelf-slope continental margin wedge prograded northward into the King Country region and infilled it (Mt Messenger, Urenui, Kiore and Matemateaonga Formations). (7) During the Pliocene and Pleistocene the whole of central New Zealand, including the eastern margin of Taranaki Basin, became involved in long wavelength up-doming with 1-2 km erosion of much of the Neogene succession in the King Country region. This regionally elevated the Taranaki Fault play into which hydrocarbons may have migrated from the Northern Graben region

Crustal evolution of the submarine plateaux of New Zealand and their tectonic reconstruction based on crustal balancing

Tectonics, marine geophysics, plate-tectonic reconstruction, new zealand, antarctica, seismic refraction/wide-angle reflection, Gondwana break-up. - The last supercontinent fell into pieces with the break-up of Gondwana. In this context, the separation of the microcontinent of New Zealand from Antarctica is a jigsaw puzzle of many pieces. Its parts lay at the convergent margin of East Gondwana, which changed into a divergent margin within a geologically short time. That is why the microcontinent of New Zealand experienced different tectonic regimes and phases of the Wilson cycle. Although it is a good object of investigation due to its changing history, remarkably little is known about the submerged parts of the microcontinent. Knowledge of the magmatic-tectonic development of the submarine plateaux such as Campbell Plateau and Chatham Rise will improve the understanding of the processes that led to the late Gondwana break-up, and, in turn, lead to better reconstructions of East Gondwana, as Zealandia is a key piece in plate-kinematic reconstructions of this part of Gondwana. The central part of this thesis deals with the separation process of Zealandia from Antarctica leading to an improved reconstruction of New Zealand with emphasis on the submarine plateaux. Bounty Trough separating Chatham Rise from Campbell Plateau, and the Great South Basin separating Campbell Plateau from the South Island are investigated with seismic refraction and reflection methods. They are interpreted jointly with magnetic and gravity data. The results of crustal thickness modelling based on satellite gravity data are combined with existing information about crustal thickness of Zealandia. With these data ...thesi

Analysis of the tectonic and basin evolution of the seychelles microcontinent during the mesozoic to cenozoic, based on seismic and well data

The Seychelles Microcontinent (SMc) is a fragment of continental lithosphere that experienced multiple phases of rifting and thermal subsidence during its isolation and submergence within the Indian Ocean. Originally part of central Gondwana, along with India and Madagascar, the SMc first emerged during Mesozoic fragmentation of Gondwana (ca. 220 – 180 Ma) along a complex rifted margin. Fragmentation involved three major rift phases, viz.: 1) Middle Triassic – Middle Jurassic (Rift I), associated with the “Karoo rifts” and break-up between [India-Madagascar-Seychelles] and East Africa; 2) Middle Jurassic – Early Cretaceous (Rift II), associated with the rifting and break-up of Madagascar from [India-Seychelles]; 3) Late Cretaceous (Rift III), associated with the rifting and final break-away of the SMc from India. In this study, the tectonic and sedimentary history of the SMc is analysed using 2D seismic reflection datasets and three exploration wells. Seismic to well-log correlations provide a chrono-stratigraphic framework that identifies seven sequences from the Middle Triassic to the Paleogene. This also identified horst and graben structures related to the extensional tectonics and thermal subsidence of this continental fragment. The latter is reflected also in changes of its litho-facies preserved on the SMc, from terrestrial to marine. The oldest sedimentary rocks identified on the SMc are Middle Triassic organic rich claystones (Sequence 7, Rift I), which grade upwards into alternating Upper Triassic sandstones and mudstones (Sequence 6, Rift I) followed by upward coarsening Lower Jurassic mudstones to sandstone units (Sequence 5, Rift I). These sequences are interpreted as lacustrine facies that evolved into fluvial channel migration facies and finally into progradational delta front facies. Sequence 5 is overlain by Middle Jurassic oolitic limestones that grade upwards into organic rich mudstones (Sequence 4, thermal subsidence after Rift I); the latter are interpreted as restricted-marginal marine deposits. Following Sequence 4, separated by a major break-up unconformity (BU), are the Upper Cretaceous open marine deposits comprising limestones, claystones and sandstones, and terminated with basaltic volcanics (ca. 66 Ma) prior to the separation of the SMc from India (Sequence 3, Rift III). This is overlain by the post-rift – thermal subsidence sequences comprising open marine claystones and shelf limestones (Sequence 2) followed by a sequence of shelf limestones (Sequence 1) that form the present carbonate platform, the Seychelles Plateau that lies approximately 200 m below the present sea-level. Backstripping and subsidence analysis quantifies 3 stages of subsidence; Phase A: Slow subsidence (ca. 5-20 m/Ma), from the Middle Jurassic to the Upper Cretaceous that terminated during a major marine transgression during ingression of the Tethys Sea between East Africa and [Madagascar-Seychelles-India]. This created marine conditions and the subsequent deposition of Sequences 4 and 3; Phase B: Accelerated subsidence (ca. 35-60 m/Ma) recorded throughout the Paleocene to the middle Eocene leading to deeper marine conditions and the subsequent deposition of Sequence 2; and Phase C: Reduced subsidence (ca. 10-30 m/Ma) following the interaction between the Carlsberg Ridge and the Reunion hotspot (ca. 55 Ma) that possibly introduced a reduction in subsidence and the subsequent deposition of Sequence 1 as the SMc drifted and thermally subsided to its submerged present location, and is now dominated mainly by marine carbonates. The effects of the Madagascar and Seychelles/India separation (ca. 84 Ma) are not observed in the subsidence analysis, possibly because it involved transcurrent-rotational movement between the two plates over a short period of time

Evolutionary and Biogeographical History of Penguins (Sphenisciformes): Review of the Dispersal Patterns and Adaptations in a Geologic and Paleoecological Context

Despite its current low diversity, the penguin clade (Sphenisciformes) is one of the groups of birds with the most complete fossil record. Likewise, from the evolutionary point of view, it is an interesting group given the adaptations developed for marine life and the extreme climatic occupation capacity that some species have shown. In the present contribution, we reviewed and integrated all of the geographical and phylogenetic information available, together with an exhaustive and updated review of the fossil record, to establish and propose a biogeographic scenario that allows the spatialtemporal reconstruction of the evolutionary history of the Sphenisciformes, discussing our results and those obtained by other authors. This allowed us to understand how some abiotic processes are responsible for the patterns of diversity evidenced both in modern and past lineages. Thus, using the BioGeoBEARS methodology for biogeographic estimation, we were able to reconstruct the biogeographical patterns for the entire group based on the most complete Bayesian phylogeny of the total evidence. As a result, a New Zealand origin for the Sphenisciformes during the late Cretaceous and early Paleocene is indicated, with subsequent dispersal and expansion across Antarctica and southern South America. During the Eocene, there was a remarkable diversification of species and ecological niches in Antarctica, probably associated with the more temperate climatic conditions in the Southern Hemisphere. A wide morphological variability might have developed at the beginning of the Paleogene diversification. During the Oligocene, with the trends towards the freezing of Antarctica and the generalized cooling of the Neogene, there was a turnover that led to the survival (in New Zealand) of the ancestors of the crown Sphenisciform lineages. Later these expanded and diversified across the Southern Hemisphere, strongly linked to the climatic and oceanographic processes of the Miocene. Finally, it should be noted that the Antarctic recolonization and its hostile climatic conditions occurred in some modern lineages during the Pleistocene, possibly due to exaptations that made possible the repeated dispersion through cold waters during the Cenozoic, also allowing the necessary adaptations to live in the tundra during the glaciations.Fil: Pelegrín, Jonathan S.. Universidad Santiago de Cali; ColombiaFil: Acosta Hospitaleche, Carolina Ileana Alicia. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. División Paleontología Vertebrados; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata; Argentin

Sedimentary facies and unconformity analysis of some Paleocene-Eocene sections, Marlborough and Campbell Island, New Zealand

Throughout the Late Cretaceous to Eocene, sedimentation in gradually subsiding basins on the passive eastern margin of the micro-continent of Zealandia recorded climatic and paleoceanographic changes in a greenhouse world. One such fundamental change in Southern Ocean circulation is hypothesised to be recorded in a regionally extensive unconformity surface and short-lived lithofacies changes contained within Late Paleocene to Early Eocene sedimentary successions at key sections throughout New Zealand, and investigated here on Campbell Island and in southeastern Marlborough. On Campbell Island, this oceanographic event is represented by an unconformity between the Late Cretaceous to Late Paleocene Garden Cove Formation and the Early Eocene to Oligocene Tucker Cove Limestone. This unconformity signifies a major lithofacies change from Garden Cove Formation which consists of siliceous mudstone containing fine sand to coarse silt sized siliciclastic grains, pelletal glaucony grains and rare quartz pebbles, to a nannofossil and foraminiferal limestone containing little to no siliciclastic grains comprising the Tucker Cove Limestone. Geochemically this lithofacies change is characterised by a dramatic decrease in terrigenous supply and a shift from siliceous to calcareous productivity, along with a significant concentration of Zr and rare earth elements. Lithofacies at this site are inferred to record possible episodes of ice rafting and eventual unconformity formation by invigorated intermediate depth ocean currents which resulted in winnowing of seafloor sediments and concentration of heavy minerals. At the distal Mead Stream site in southeastern Marlborough, deposition of bio-siliceous sediments of the Mead Hill Formation and Amuri Limestone was locally disrupted by deposition of the Waipawa Formation, the lateral equivalent of an important hydrocarbon source rock identified in several of New Zealand‟s sedimentary basins. In outcrop, the Waipawa Formation at Mead Stream is characterised by a very distinctive rusty‟ brown fissile appearance, while in thin section, though radiolarians and sponge spicules are common, the overall fine grained nature of the unit makes identification of other components difficult. Geochemical proxies show a significant increase in terrigenous supply in the Waipawa Formation, along with an increase in siliceous productivity concomitant with a decrease in oxygenation at the site. Lithofacies changes through the Late Paleocene at Mead Stream suggest the site lay under a zone of upwelling which resulted in an increase in siliceous productivity during the Late Paleocene. At the more proximal sites of Muzzle Stream and Kaikoura wharf in southeastern Marlborough, Mead Hill Formation and Amuri Limestone are separated by an unconformity, overlain by Teredo Limestone. The Teredo Limestone is considered to be a lateral equivalent of the Waipawa Formation, but both the base and top of the Teredo Limestone are timetransgressive. This means that at Muzzle Stream the unit is contemporaneous with the Waipawa Formation (Late Paleocene), while at Kaikoura wharf the unit is entirely Early Eocene in age. At these sites, the Teredo Limestone Member of the Amuri Limestone is a calcareous greensand sometimes containing phosphatised limestone clasts and sharks teeth. In thin sections, the unit consists of well sorted, fine to very fine sand sized siliciclastic grains and fine sand sized pelletal and vermicular glaucony set in a calcareous matrix that shows evidence of secondary silicification. Unconformity formation and the subsequent deposition of the overlying Teredo Limestone record a period of invigorated intermediate depth ocean currents that resulted in the transport of siliciclastic grains and glaucony to these bathyal sites. This interpretation is supported by a palinspastic map of the Teredo Limestone that suggests the unit was deposited under different conditions than those responsible for the deposition of the bounding Mead Hill Formation and Amuri Limestone. This map also suggests the Teredo Limestone was deposited as a skin drift‟, here named the Clarence Drift, possibly under the influence of contour currents. Based on similarities between unconformities and lithofacies changes in Late Paleocene to Early Eocene sedimentary sections and an earlier, well documented event at the Cretaceous/Tertiary boundary in southeastern Marlborough, evidence for a period of enhanced siliceous productivity, invigorated ocean currents and possible episodes of ice rafting is suggested to be consistent with a brief period of Antarctic ice sheet growth during a phase of global cooling in the Late Paleocene. The possible identification of Antarctic ice sheets, ephemeral though they may have been, not only challenges long held beliefs that the Antarctic continent remained ice free during the early Paleogene greenhouse world but also questions the suggested mechanisms responsible for Antarctic ice sheet growth. The lack of ocean gateways in the Southern Ocean during this time effectively rules out thermal isolation of the Antarctic continent as a driver. Given that this period of ice sheet growth is contemporaneous with a documented period of enhanced global ocean productivity and terrestrial carbon accumulation and related draw down in atmospheric CO2, it is suggested this may represent the driver responsible for brief Antarctic glaciation during this period, though the postulated link requires further investigation

Lithology and provenance of late Eocene - Oligocene sediments in eastern Taranaki Basin margin and implications for paleogeography

The latest Eocene and Oligocene was a time of marked paleoenvironmental change in Taranaki Basin, involving a transition from the accumulation of coal measures and inner shelf deposits to the development of upper bathyal environments. Up until the end of the Early Oligocene (Lower Whaingaroan Stage) Taranaki Basin had an extensional tectonic setting. Marine transgression culminated in the accumulation of condensed facies of the Matapo Sandstone Member of the lower part of the Ngatoro Group. During the Late Oligocene (Upper Whaingaroan Stage) Taranaki Basin's tectonic setting changed to one of crustal shortening with basement overthrusting westward into the basin on Taranaki Fault. The major part of the Ngatoro Group in thickness, including the Tariki Sandstone Member, Otaraoa Formation, Tikorangi Formation and Taimana Formation, accumulated in response to this change in tectonic setting. Various methods of stratigraphic and sedimentological characterisation have been undertaken to evaluate the stratigraphy of the Ngatoro Group. Wireline log records have been calibrated through particle sizing and carbonate digestion of well cuttings. A suite of wireline motifs have been defined for formations and members of the Ngatoro Group. The integration with other lithological and paleoenvironmental data sources has helped to better define the Late Eocene - Oligocene stratigraphy and sedimentary facies for eastern Taranaki Basin margin. U-Pb geochronology by laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) has been used to determine detrital ages for over 350 zircons from 13 samples of Late Eocene - Oligocene sandstone samples in eastern Taranaki Basin and correlative onshore North Island units. The spread of ages (1554 - 102 Ma) and the proportion of ages in particular age bands integrated with modal petrography data have aided provenance evaluation. A range of source rocks contributed to the Late Eocene - Oligocene sedimentary units analysed, mainly the Waipapa Terrane (Early Permian to Late Jurassic) as shown by 206Pb/238U zircon ages and the abundance of fine-grained sedimentary rock fragments observed in samples. The Median Batholith (i.e. Darran/Median Suite and Separation Point Suite) is also identified as a significant source, indicated by Early Triassic to Early Jurassic and Early Cretaceous 206Pb/238U zircon ages and an abundance of quartz in samples. Other minor sources identified include Murihiku and Caples Terranes, Rakaia Sub-terrane and possibly the Karamea Batholith. The Tariki Sandstone and the Hauturu Sandstone have the same source, with the main 206Pb/238U zircon ages of aggregated samples (124 - 116 Ma and 121 Ma, respectively) consistent with a Separation Point Suite/Median Batholith (124 - 116 Ma) source. Derivation of sediments from a landmass that existed to the east and southeast of the Wellington area has been inferred for the Late Eocene - Oligocene units, with subsequent migration of sediments northward into Taranaki Basin and the Waikato Region (i.e. Te Kuiti Group depocentre) via longshore drift. New provenance data have been used to revise understanding about the development of eastern Taranaki Basin margin through the Late Eocene to earliest Miocene. Three new paleogeography maps are presented for the Runangan (Late Eocene), Lower Whaingaroan (Early Oligocene) and Upper Whaingaroan (early-mid-Oligocene). New paleogeography interpretations illustrate a dramatic change in the basin development between Matapo Sandstone (Lower Whaingaroan) and Tariki Sandstone (Upper Whaingaroan) deposition, consistent with an Upper Whaingaroan age for the start of reverse movement on Taranaki Fault

Tectonostratigraphic controls on pore fluid pressure distribution across the Taranaki Basin, New Zealand

Significant variations in pore pressure across the Taranaki Basin, New Zealand, are attributed to changes in lithofacies and structure, usefully illustrated in terms of ten areas that we term geopressure provinces, each displaying individual pore pressure trends. Cretaceous to Early Miocene formations in different parts of the basin can be either normally pressured (near or at hydrostatic) or significantly overpressured (up to 28 MPa) at the same depth. Variations in Eocene–Oligocene facies types and thicknesses both within and between geopressure provinces provide first-order control on the magnitude, distribution and maintenance of overpressure across the basin. Examples of hydraulic compartmentalisation due to sealing faults and stratigraphic architecture are identified within the basin. Deep pore pressure transitions are sealed by diagenetic, structural or stratigraphic mechanisms in different places and are associated with an increase in mudrock volume (reduced permeability) or gas generation. Thus, pore pressure distribution in the Taranaki Basin is controlled by a combination of sediment loading, lithofacies variations, fault zone permeability and structural architecture. This work represents an appraisal of the pore pressure distribution across the whole of a multiphase structurally complex basin, and the approach taken provides a framework for better understanding the distribution of pore fluid pressures and pore fluid migration in other sedimentary basins

STRUCTURE AND STRATIGRAPHY OF THE AJDABIYA TROUGH AREA, EAST SIRT BASIN - LIBYA

The structural style within the deepest parts of the Ajdabiya Trough is defined by a system of Early - Late Cretaceous syn-depositional fault blocks bound by normal faults and basement highs devoid of syn-rift sediments, which are buried under a thick succession of Cenozoic post-rift deposits. The range of fault orientations likely reflects the conflicting influences of the ~NE-SW regional extension direction and the dominant ~N-S trending basement fabric. Mainly NW-trending normal faults dissecting Cretaceous and older rocks have been inferred from 2D seismic reflection and potential field data. Other faults trend NE-SW and E-W, and mainly cut Miocene and older strata. Some of these faults have both sinistral and dextral strike slip components and are possibly linked to on-going seismicity in the Sirt Basin and the Cyrenaica Platform. Vertical displacements on these faults are several hundred meters and are defined by large throws on Cretaceous and underlying horizons. Structural mapping confirms the presence of relay ramps associated with overlapping faults developed in the hangingwalls adjacent to west downthrowing normal faults along the eastern margin of the Ajdabiya Trough. The seismic stratigraphic framework is organised into six mega-sequences that correlate to variations in relative sea-level and/or sediment supply during Late Mesozoic and Cenozoic times. The stratigraphic architecture of the trough is largely influenced by relative sea level changes and minimal tectonic effects during the Cenozoic; observed progradation of the Paleocene, Early and Middle Eocene sequences along the trough margin is attributed to relatively rapid sedimentation rates and relatively slow rates of increase in accommodation space. Depositional environments are interpreted using the resultant facies analysis and the characterisation of the seismic reflections indicated that the geological units were deposited in marginal marine, shallow shelf and moderately deep marine environments. Special consideration is given to the principle of seismic sequence stratigraphy analysis of carbonate depositional systems where the facies group took initially place on a homoclinal ramp which later developed into a rimmed platform. This analysis additionally reveals that similar depositional architectures can be divided into systems tracts. The earliest systems tract of the Lower Eocene sequence is interpreted as lowstand prograding wedge distinguished on the basis of the component facies that indicate the dominant depositional regime. Localized debris flow or mass transport complex formed during early highstand systems tract deposition began during the Middle Eocene. The tectono‐stratigraphic analysis of the Ajdabiya Trough reveals that two major extensional pulses controlled the architecture of the trough during continental rifting with crustal stretching (β) factor ranging from 1.3 to 1.5 consistent with subsidence in the Ajdabiya Trough having been controlled by thermal cooling and isostatic adjustments of the crust beneath the trough. Growth strata within grabens and half-grabens denote persistent tectonic subsidence and demonstrate the progressive depocenter locus migration towards the north. In such a context, the current geometry of the Ajdabiya Trough is interpreted to have resulted mainly from rifting cycles and possible renewed continental extension. The investigations of the tectono‐stratigraphic controls reveal that after a period of relative tectonic quiescence, post‐rift tectonic reactivation affected the Ajdabiya Trough almost continuously since the latest Cretaceous to the Miocene. Burial history curves correlated with one-dimensional back-stripping assuming Airy isostasy shows that Cenozoic subsidence in the Ajdabiya Trough can be divided into three episodes of post-rift subsidence characterized by short and long-lived subsidence pulses and rapid sedimentation rates that may lead to development of overpressure by disequilibrium compaction

Trigger mechanisms for sand intrusions. Inferences from published data and investigation of 3D seismic data from the northern North Sea

Postponed access: the file will be accessible after 2020-02-02Numerous publications have addressed subsurface sediment remobilization, in terms of geometries of the intrusions, where the intrusions occur, the parent unit of the intrusions and trigger mechanisms. Several authors have suggested different trigger mechanisms. The most frequently suggested trigger mechanism is earthquake induced liquefaction, but the suggestion is rarely well supported by evidence. This thesis has investigated the possibility of a link between trigger mechanisms and the tectonic setting of the basin where sand intrusions occur. Several basins worldwide were investigated, and revealed an overrepresentation of subsurface sediment remobilization at convergent margins (including transform margins) and inverted passive margins. Convergent margins tend to show a relatively steep slope, and hence lateral pressure transfer was proposed as an important trigger mechanism at this type of tectonic setting. The subduction of the oceanic plate causes a step-wise compression of the deep sediments, and consequently the fluids are forced to escape rapidly, causing lateral fluid transfer to shallower strata. This process can result in rapid build-up of fluid pressures exceeding the lithostatic stress in the shallowest positions of the dipping (and permeable) strata, and trigger sand injections here. Investigations of 3D seismic data from the northern North Sea was carried out to examine trigger mechanisms of sand intrusions at inverted passive margins. One phase of subsurface sediment remobilization was recognized within the Early Oligocene to Mid Miocene succession. Several evidence point towards one alternative to a trigger mechanism causing subsurface sediment remobilization in the northern North sea: 1) remobilization took place along the basin-flank transition, but not in the basin center, 2) a detachment surface was interpreted along the base of the subsurface sediment remobilization, and pose a good candidate as the slide plane, 3) mounds are arranged in N-S trending ridges along the basin-flank transition, and are hence parallel to the eastern margin and a potential headwall scarp, 4) liquefaction of mud and sand are triggered by shearing, 5) the interpreted slide plane is parallel to the bedding. Accordingly, shearing along the slope caused by a submarine slab slide was suggested as the main trigger mechanism of subsurface sediment remobilization in the northern North Sea. Submarine slab slides represent sedimentary processes that are common on inverted passive margins. Consequently, the interpretation is considered at least partly applicable to other inverted passive margins worldwide.Masteroppgave i geovitenskapGEOV39