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- PublicationRegional variations of Tournaisian and Viséan lithostratigraphy and their links to basin and structural evolution in Ireland(University College Dublin. School of Earth Sciences, 2023)This work details the lithological and unit thickness variability across a large portion of the Tournaisian and Viséan of Ireland, using 476 stratigraphically representative boreholes. A particular focus was put on how lithological changes link with their depositional environment and with the evolving rift structural framework. The deposition of carbonates during the Tournaisian in Ireland is characterised by a south to north diachroneity, reflecting a northward transgression, initially on a generally south-dipping overall ramp geometry. The transgression was accompanied by a northward stepping of facies belts in several sub- Waulsortian units across the study area. Compiled micropalaeontological information suggests that early extensional faulting initiated in the early Ivorian, represented by clear syn-sedimentary thickening of the Moathill Fm across faults in the Central Midlands province. Conversely, such relationships are not shown for the underlying units such as the Meath Fm. The character of the Waulsortian Limestone Fm changes dramatically in terms of facies and thickness across the study area. In south-eastern areas, the Waulsortian forms laterally continuous “sheet-reef” growth forms which substantially thicken towards the central Iapetus suture zone. However, towards the north its development becomes less consistent and is evidently more often localized to the immediate proximity of faults and in most northerly areas, the Waulsortian is commonly absent. Some of the reasons for this dramatic change in character are discussed. The Waulsortian Limestone Fm records the transition from a general ramp geometry to an increasingly fault-controlled basin architecture. The Lower Viséan to Arundian is marked by a major change in facies deposition. Continued syn-rift extension localized on fewer but faster moving faults, temporally associated with a complex facies mosaic comprising areas of interfingering coeval deep basinal and shallower water shelf deposition. This was interpreted to coincide with eustatic sea level changes which strongly influenced the sedimentary environment. Using newly compiled biostratigraphical data, and lithological correlations, regional shallowing and (sudden) deepening trends can be recognized which correlate with well documented examples in laterally equivalent basins in Belgium, Wales and Germany.
- PublicationTectnostratigraphic Interactions in Rift Basins - Constraints from Forward Stratigraphic Modelling(University College Dublin. School of Earth Sciences, 2022)Rift basin stratigraphy reflects a complex interplay between first-order tectonic, climate and base-level controls. Forward stratigraphic modelling (FSM) provides a means of simulating and better understanding aspects of this interaction. The study reported in this thesis used Sedsim from Stratamod to investigate: (1) how drainage is steered into fault-controlled depocentres and the extent to which autogenic cycles can develop despite constant external forcing; (2) the impact of contrasting extensional fault growth mechanisms on how and where sediment is delivered from the footwall to hanging wall basins by antecedent drainage, and (3) drainage patterns and stratigraphy occurring during salt-influenced rifting when the geometry of fault-controlled accommodation can be extensively modified by subsurface salt migration. The workflow incorporated a first set of simulations replicating physical experiments that included a relay ramp and a fault-controlled depocentre. Sedsim was configured to match the inputs and it faithfully reproduced drainage and depositional patterns down to episodic sediment storage and release that drove autogenic stratigraphic cycles in the hanging wall basin. Additional models were able to show the autogenic cycles persisted when the lateral tank walls were removed and when the simulation continued beyond the duration of the original physical experiments. FSM allowed upscaling of the physical model to natural rift basin dimensions and the inclusion of more realistic surface deformation around the faults; this confirmed the physical model reasonably represents reality, although varying the inputs showed autogenic cycles developed for only a subset of the possible combinations of input variables. Sedsim models were then produced for antecedent drainage interacting with a fault array developing according to either the Isolated or Constant Length fault growth models during rift initiation. Differences in the footprint and rate of hanging wall accommodation creation, the pattern of footwall uplift, and the relay ramp evolution explain significant contrasts in tectonstratigraphy between the two growth mechanisms. The drainage was less likely to be captured by the smaller initial depocentres in the Isolated fault growth examples, and when it was, sediment supply was able to balance space creation more effectively, with sediment delivered mainly via the developing relay ramps right through to breaching, and beyond relay breaching in cases where the discharge was high enough. Relay ramps were only utilised in the earliest stages of rifting associated with faults growing by the Constant Length model before drainage was diverted around the fault array tips to fill the interconnected hanging wall depocentres axially. Focussing of drainage around relays in the isolated growth model drove extensive footwall erosion close to the fault tips, whereas in the Constant Length case, where flow was sufficiently erosive, drainage could excavate valleys through the footwall at the point of maximum surface displacement. FSM was also used to address rift basin filling where subsurface salt migration initially compensates for sub-salt fault displacements before coupling of a fault across the salt layer creates reverse drag at the surface, but with accommodation still modified by upwelling salt. A conceptual model for salt-influenced rifting, honouring the dimensions of the Irish offshore rifts and drawing on a range of basins and physical experiments, was implemented in Sedsim and various sensitivities explored. Rapid changes in accommodation creation reflecting linkage of subsurface fault segments, migration of the early depocentre towards the coupled fault, and salt migration impacted facies distribution and stratigraphic development and underpin a new model for salt-influenced tectonostratigraphy that compliments the well-established half-graben models for rifting in the absence of subsurface salt-layers.
- PublicationApplied Diffraction Imaging: Conventional vs. Machine Learning Approach(University College Dublin. School of Earth Sciences, 2021)Diffractions are oft overlooked in favour of specular reflections for seismic imaging. Diffractions, however, are formed by objects and discontinuities which are comparable or smaller than the wavelength. Therefore, if the diffractions can be imaged, these objects and discontinuities can be directly imaged. Said features are geologically noteworthy as structural and stratigraphic features of interest which may directly affect hydrocarbon migration, flow, and trapping. While straightforward in theory, in application separating the diffractions from the wavefield is a complex and intricate task. This thesis aims to discuss what makes separation so problematic and attempts to address various methods for allaying some of these issues. Additionally, this thesis intends to tackle the real benefits diffraction imaging can add to a conventional seismic image. To achieve these goals, existing separation techniques are analysed in pre-migration and post-migration domains. These analytical methods leave a volume which contains both diffractions and noise and require additional inputs such as a dip field. Ergo, any errors present in the dip field are incorporated into the diffraction image, diminishing its quality. In this thesis, adaptations to existing methods have been proposed as well as a novel method in the pre-migration domain. These new methods incorporate common geophysical techniques to obtain a cleaner separation. The novel method, histogram-based separation, is an analytical curve-fitting technique to identify diffractions without the presence of noise. Deep learning has also been integrated with the methods. Firstly, with pre-migration separation deep learning has been used to automatically identify and separate diffractions, obtaining a cleaner separation in a faster time. Another neural network is also trained which can apply a post-migration-based diffraction imaging scheme directly on stacked, migrated, seismic data, something hitherto impossible. These methods have been applied to seismic and GPR data highlighting the benefits of diffraction imaging.
- PublicationThe kinematics and geometry of segment boundaries on normal faults(University College Dublin. School of Geological Sciences., 2014-09)Normal faults, when observed in detail, are commonly seen to consist of arrays of segments. The geometry of the boundaries between these segments has been extensively studied in outcrop and in subsurface geophysical datasets. Existing models generally consider that at low displacements the segments interact by deformation of the intervening rock volume, and become progressively linked as the displacement increases. The interaction and linkage of these segments is important in the formation and development of fault zones. The recent increase in availability of high quality 3-D seismic reflection data has allowed the detailed geometry of faults to be better imaged. While there are some studies focused on the geometry of segment boundaries, few have focused on the detailed small scale features of these structures and very few studies have focused on the kinematics of these structures. In this project a highly detailed analysis of the geometry and kinematics of segmented normal faults imaged in high quality 3-D seismic reflection data has been carried out. This analysis has revealed substantial departures from the relatively simple geometries and evolutionary models which are generally inferred from 2-D or lower resolution 3-D mapping. These departures include:- the formation of an array of parallel faults which become an array of interacting segments by the localisation of displacement onto shorter lengths of the established fault surfaces; the formation of a segment boundary by the propagation of a splay from a preexisting through going fault; the removal of large scale asperities and also stepping during lateral propagation of a fault giving rise to a segmented geometry. Adjustments to the existing models and a classification scheme for segment boundaries based on their 3-D geometry are proposed. Despite the observed variability in the nature of segment boundaries, all of the structures examined in this thesis formed in a way which was geometrically and kinematically coherent throughout their development and evidence for the development of segment boundaries by the coalescence of initially isolated faults has not been found. In each case, combining displacements accommodated by discontinuous (faulting) and continuous (folding) deformation yielded simple overall displacement distributions associated with a single structure. Kinematic analysis demonstrates that faulting and folding show, not only spatially complementary variations, but also, that these components are complementary in time. During the development of a segmented fault array segment boundaries are the areas where continuous deformation is most prevalent.
- Publication3D fault zone architecture in Kardia Mine, Ptolemais Basin, Greece(University College Dublin. School of Earth Sciences, 2016-12-06)Research on normal faults derives mainly from outcrop- and seismic-based studies. Outcrops are characterized by their high resolution but often lack a 3D context, in contrast with seismic-based data that can be fully 3D but with limited resolution compared to outcrop data. The novelty of this study is the examination of a truly 3D dataset of seismic scale fault zones at outcrop resolution. The dataset has been acquired in the active, opencast Kardia lignite mine in the Ptolemais Basin, NW Greece. The basin is affected by two fault systems related to two extensional episodes. The first, Late Miocene episode resulted in the formation of the basin in response to NE-SW extension. The normal faults that occur in the Kardia mine formed during the second, Quaternary episode in response to NW-SE extension. Repeated visits at 3-monthly intervals over a 5-year period have allowed serial sections through the faults to be examined. These sections have been analysed in three dimensions, providing a unique insight into the structure of normal faults. The faults in the Ptolemais mines are unusual in that they are associated with little or no fault rock generation but instead detailed internal fault zone structure, which would normally be comminuted to fault rock, is preserved at very high strains. This feature of the faults allows detailed study of the geometric evolution of the faults and, in the case of the Kardia mine, interaction with synchronous bed-parallel slip surfaces. The total throw on a fault can be considered to be partitioned onto three components 1) throw on the main fault surface, 2) throw on subsidiary fault surfaces and 3) throw accommodated by continuous deformation. Measurement and analyses of these components for the fault zones in Kardia mine demonstrates that the first of these becomes more important with increasing throw consistent with progressive strain localisation during fault growth. Rapid lateral variations in the degree of throw partitioning over a fault zone reflect the range of scales of segmentation of the initial fault. Continuous deformation constitutes an integral element of fault structure at all stages of fault growth and its contribution to total throw decreases with increased throw suggesting that continuous deformation, such as normal drag, develops during the early stages of a fault zone development. Detailed three-dimensional mapping of a seismic scale normal fault shows various degrees of fault linkage with a gradual transition from hard- to soft-linkage with increasing scale of segmentation. Average shear strains measured across the normal fault zone at 81 locations vary by four orders of magnitude. Fault zone geometrical features indicative of linkage between fault segments occur over the full range of shear strains encountered supporting a model in which fault segment linkage is the primary control on the internal structure of the fault zone. By analogy this conclusion suggests that fault segment linkage may be the main control on the thickness and distribution of fault rock in areas where the details of fault zone structure are not preserved but are comminuted to fault rock. Bed-parallel slip within the multilayer sequence in Kardia mine occurred towards the beginning of the second phase of extension and overlapped in time with Quaternary normal faulting. Bed parallel slip, which is attributed to flexural-slip caused by reverse-drag folding in the hanging wall of a major fault, has a persistent top to the north slip direction. Bed-parallel slip surfaces occur throughout the excavated section and individual slip surfaces have slip up to 4.5 metres. 3D mapping of bed-parallel slip surfaces demonstrate that they display many of the features of dip-slip faults, for example, bed-parallel slip surfaces can be segmented both parallel and normal to the slip direction and on a wide range of scales. The displacement to length ratios derived for bed-parallel slip surfaces are typical of those for normal faults but are significantly lower than for the normal faults in Kardia mine. Backstripping of fault zone evolution at the locations of mutually offsetting bed-parallel slip surfaces and normal faults demonstrate how complex fault zone structure arises in the presence of synchronous bed-parallel slip. Bed-parallel slip surfaces formed during fault growth effectively introduce new displacement markers that can be used to examine the growth history of blind faults.