The migration of hydrocarbons on formation and accumulation on commercially viable oil and gas deposits/reserves/reservoirs. Reflection 2D/3D seismic data can be used to identify the position of these fluids in the subsurface and analyse their habitat and pathways in the context of the basin architecture. A multichannel reflection 2D seismic dataset from a large area offshore Falkland Islands (South American Plate) was analysed to interpret direct hydrocarbon indicators (DHIs) and basin architecture with the aim to find a link between these two elements. The first results of this research work have led to the definition of a new class of hydrocarbon indicator, here called vertical anomaly cluster (VAC) and observed in the east Falkland Basin. A VAC represents a group of individual hydrocarbon occurrences linked by a vertical migration of hydrocarbons within fine-grained sequences. VACs are interpreted to be low-impact fluid flow phenomena where the migration process exploits pre-existing permeability structures. In the second part of this research a suite of DHIs, including bottom simulating reflections (BSR), enhanced reflections, pipes, gas chimneys and flat spots have been analysed in the South Falkland Basin. The work demonstrates a significant link between the style of the migration and basin features. In particular it is observed that vertical migration phenomena such as pipes and gas chimneys are clearly linked to the secondary structural features of the basin while the gas hydrate system, composed of BSR and enhanced reflections, is more controlled by primary depositional features. The presence of a mature source rocks suggests that the fluids involved in the formation of the observed DHIs are of thermogenic origin. The third part of this research was focussed on the analysis of depression structures located in the North east Falkland Basin. The basin hosts other DHIs, such vi as VACs, gas chimneys and pipes. The depression structures propagate in depth and are associated with material withdrawal. These structures have therefore been interpreted as mud volcano conduits. The lack of mud edifices has been tentatively interpreted to be related with seabed erosion operated by bottom currents active in the study area. The fluids involved in the withdrawal of the material are interpreted to be related to hydrocarbon generation and expulsion. The comparison between the results of this research with the wider literature has been used to propose a classification where the style of the migration process and the DHIs are linked to the permeability architecture of the basin. Two styles of DHIs have been defined: (1) permeability-controlled DHIs, derived essentially from low impact migration processes acting in a pre-existing permeability structure (e.g.: VACs, enhanced reflection and BSR), and (2) fluid-controlled DHIs, derived from migration processes able to generate the migration pathways (e.g. pipes, mud volcanoes). Two speculative and, at the time of this research, not tested hypotheses have been proposed to justify the style of the migration process. The first hypothesis suggests that the style of the migration process is directly linked to the geometry of the deep structures where the hydrocarbon fluids are focussed in relatively small areas. These areas are suggested to be located at structural highs. Once the fluid is focussed in this region then the vertical migration is believed to not be sensitive to barrier and seals within the overlying lithologies. This hypothesis is consistent with the position of fluid-controlled DHIs (e.g. pipes and MVs) at the margin of the hydrocarbon plumbing systems and where the basement structures are relatively shallower. DHIs located in the inner regions are instead of the type sensitive to the permeability structures. vii The second hypothesis suggests that the style of the DHIs is function of the time. Hence in this hypothesis, assuming a given region of active fluid migration, the first DHIs to be generated are simple amplitude anomalies which then evolve to VACs. If the migration of hydrocarbons is still active the probable condition of high saturation may trigger some failure within the hosting lithologies and the formation of additional pathways. The newly generated permeability structure may finally be consistent with the one of more dramatic DHI style, such as mud volcanoes.

Hydrocarbon migration is one of the key processes that takes place in the development of a successful petroleum system. Understanding when fluids migrated, how they migrated and which routes they took either into primary reservoirs or via transient seals into shallow reservoirs is paramount for successful extraction. Fluids in the sub-surface can be imaged in seismic reflection data as anomalously high reflection amplitudes owing to their contrasting acoustic properties (density and p-wave velocity) with sedimentary rocks. This thesis uses 3-Dimensional seismic reflection data from the Møre Basin, offshore mid-Norway and the Lower Congo Basin, West African margin to investigate the migration of fluids from primary reservoir intervals through overlying successions of fine-grained sediments. These shallow intervals are typically considered as regional seal layers and understanding how seals were breached and fluids migrate through them is vital to risking exploration targets. The Møre Basin case study investigates a gas-associated amplitude anomaly at the crest of a domal structure cored with alternating fine-grained biosiliceous and calcareous ooze sediments. The anomaly has a rather unique convex-upward basal contact which is explained by the superposition of lateral velocity variations through the gas-filled dome. The centre of the dome has more gas than the flanks resulting in a lower velocity which pushes the basal contact to deeper positions with respect to the flanks. The domal trap was charged from gas migrating from depth via capillary entry pressure and possibly via sub-vertical pathways created by compaction-derived polygonal faults which pervasively deform the host stratigraphy. The Lower Congo case study investigates a range of high-amplitude seismic amplitude anomalies in a thick sequence of hemipelagites (the waste zone) above a deep-seated turbidite reservoir. Anomalies take many forms and include; Linear anomalies, Sub-circular anomalies, Patchy anomalies at which finger-shaped anomalies emanate from their lateral edges, and Discrete filamental anomalies. The Sub-circular and Patchy anomalies were interpreted as being related to the presence of hydrocarbons. Detailed analysis of a sub-set of the hydrocarbon-bearing amplitude anomalies suggest leakage occurred through two means; 1) vertical leakage through feeders and 2) via deep-seated extensional faults formed during gravity-driven gliding of vi an underlying salt detachment. Vertical leakage is expressed in the form of Vertical Anomaly Clusters which comprise vertically stacked assemblages of high-amplitude anomalies. A common aspect of the two case studies are that high-amplitude anomalies within fine-grained sedimentary successions are linked to vertical or sub-vertical migration pathways provided either by faults or pipe-like structures formed during overpressure. These results have implications for our understanding of how seals are breached when reservoirs are overpressured.

The aim of this thesis is to assess the spatial and temporal recurrence of mass-transport deposits (MTDs) within salt withdrawal basins to unravel the complex interaction between mass-wasting processes and salt halokinesis. A high-quality 3D seismic dataset from the mid-continental slope of Espírito Santo Basin, SE Brazil, was used to assess the provenance of mass-transport deposits and their potentials as structural markers for seafloor perturbation and fault activity. A new proposition from this work includes scale-independent classification of mass- transport deposits into homogeneous and heterogeneous types. Heterogeneous MTDs are composed of seismic facies corresponding to slides, slumps and debrites. Homogeneous MTDs comprise consolidated debrites considered in this work as comprising barriers to fluid flow. In addition, MTD composed of rafted blocks displayed a disproportionate relationship between their shape, transporting distance and degree of remobilization. Drag zones denote sections of MTDs that are uplifted during salt diapir rise. These drag zones are extended and shortened along their long and short axis respectively. Ramps flanking salt diapirs are formed by either complete or partial erosion of paleo-seafloors and pre-existing fault scarps. In this thesis, it is shown that the risk of remobilized sediments is highest within drag zones. The erosive nature of mass-wasting processes is justified by the decoupling history of faults eroded by MTDs. An innovative method to assess fault decoupling history on continental margin is the use of cumulative throw character. MTD-decoupled faults are iv | P a g e characterised by shorter propagation rate and cumulative throw and are potentially sealing compared to their non-decoupled counterparts. The information from this study is crucial information for successful hydrocarbon exploration and risk assessment in deep-water environments. The methodologies and results from this thesis are applicable to continental margins worldwide.

The integration of several industry and scientific 2D seismic surveys with various well data allowed for the first time a detailed analysis of the sedimentological, stratigraphic and architectural changes recorded during syn- to post-rift transitions on passive margins. The Northwest Iberia margin and its conjugate margin of Newfoundland formed the basis for an interpretive model. Comparison with the South Australia�East Antarctica conjugate margins enabled hypothesis testing and premise refinement. The breakup unconformity concept is revised and a more comprehensive term is proposed for the stratigraphic surface recording the transition between syn- and post-rift: the lithospheric breakup surface. This new term: a) discriminates between continental crust breakup and complete lithospheric breakup as verified in several magma-poor margins, and b) takes into account the different character this surface can show according to its position on the margin. The concept of a breakup sequence is proposed as a sedimentary sequence showing a distinct architecture to strata deposited prior to the lithospheric breakup event. The breakup sequence records the depositional changes occurring across the lithospheric breakup surface due to lithospheric adjustments triggered by lithospheric breakup. Contourites were identified for the first time as being associated with lithospheric breakup, supposedly being triggered by the lithospheric plate in-plane stress release occurring at the time of lithospheric breakup. Consequently, it is proposed that contourites can be used as an indicator for established lithospheric breakup. On the East Antarctica margin, a surface usually dated as mid Eocene to early Oligocene by comparison with the conjugate South Australia margin, is dated as latest Maastrichtian�earliest Palaeocene using data from IODP Site 1356. This new date suggests that the surface is a lithospheric breakup surface, which can explain its generation and the overlying strong contouritic deposition.

This thesis is an investigation of the magmatic processes that operated in the plumbing system of the Franklin Large Igneous Province (LIP). The Franklin LIP (~723-716 Ma) extends >2500 km across northern Canada and is superbly exposed in the Minto Inlier of Victoria Island. The exposed plumbing system is dominated by sills, with subordinate fault-guided feeder dykes. Locally, sills are characterised by olivine-enriched bases, with overlying doleritic gabbros. One example is the Lower Pyramid Sill. This sill was investigated in detail using petrography, whole-rock chemistry and in-situ mineral chemistry. Petrological and geochemical observations, combined with inverse melt modelling show that the olivine-enriched layer formed by olivine slurry replenishment into a resident gabbroic mush. In contrast, the overlying gabbros formed by in-situ differentiation. Intervening clinopyroxene-rich cumulates formed by a metasomatic reaction between different pore melts. The study emphasises that both slurry emplacement and in-situ differentiation played a role in the formation of cumulate layering in this sill. Sections examined through the same sill along strike, collectively termed the Fort Collinson Sill Complex, reveal that the olivine slurry propagated >50 km downstream. Radiogenic and stable isotope data support this interpretation. The geochemical consequences of cumulate remobilisation and mixing in the sill complex are demonstrated, as well as quantified, using Fe=Mg modelling. Additionally, the effects of dolostone assimilation are considered, which can potentially affect phase equilibria. This thesis also investigates the Ni-Cu-PGE potential of the Franklin LIP by studying the Southern Feeder Dyke Complex. This dyke complex was fault-guided and brecciated dykes contain microdoleritic microxenoliths that are impregnated by sulphides. Immiscible sulphide liquids formed when the magma became sulphur saturated due to the ingestion of local sulphur-rich wallrock. This study demonstrates the importance of external sulphur addition to magma in triggering sulphide mineralisation and highlights the economic potential of the Franklin LIP.

A temporal and spatial relationship between plume magmatism, cratonic lithosphere and the occurrences of orthomagmatic Ni-Cu and platinum-group element (PGE) sulphide mineralisation has been documented in the literature. However the underlying causes for this correlation have yet to be resolved – is there an inherent feature of the cratonic lithosphere and its mantle ‘keel’ that controls mineralisation? Or is this correlation purely a preservational bias in the geological record? Scotland has experienced multiple tectono-magmatic events and provides an ideal testing ground, or ‘framework’, in which to assess the role of lithospheric mantle on chalcophile element (Ni and Cu) and precious metal (PGE and Au) abundances through time. Given the well-documented geological history of the region (including several suites of mantle xenoliths), coupled with exploration campaigns in Greenland (with which Scotland has comparable geology), this thesis aims to assess the contributions and influences of lithospheric mantle vs. asthenospheric mantle during melting and mineralisation. It also evaluates the Ni-Cu-PGE mineralisation potential for Scotland, particularly in a Noril’sk-type conduit-hosted setting within the British Palaeogene Igneous Province (BPIP). The earliest major tectono-magmatic event following cratonisation of the North Atlantic Craton (NAC) occurred c. 2.4 Ga during Palaeoproterozoic extension, forming the maficultramafic Scourie Dyke Swarm. Despite evidence for lithospheric mantle melting at this time, the subcontinental lithospheric mantle (SCLM) below the Scottish portion of the NAC did not become severely depleted in sulphides or PGE. Instead, spinel lherzolite mantle xenoliths from this region (e.g., Loch Roag) record an influx of carbonatite-associated sulphides at this time, enriched in PGE, and providing a deeper indication of continental extension that may be correlated to carbonatitic intrusions in Greenland. Subsequent collision and orogenesis of the NAC in the late Palaeoproterozoic (c. 1.9 to 1.7 Ga) represents a second significant tectonomagmatic event, recorded in the Scottish SCLM as sulphide (re-)melting and formation of discrete Pt-sulphide minerals (cooperite). Hence the lithospheric mantle here became appreciably enriched in precious metals during the Palaeoproterozoic, but crucially this preserved multiple co-existing populations of sulphides, distinct in their petrographic setting and geochemistry. Cratonic basement and associated mantle lithosphere are absent in the southern terranes of Scotland. This provides a direct comparison between lithospheric mantle geochemistry for Archaean-Palaeoproterozoic terranes north of the Great Glen Fault vs. Palaeozoic terranes south of the Great Glen Fault. Rifting of Rodinia and opening of the Iapetus Ocean in the late Neoproterozoic thus marks a significant change in geodynamic setting. This is especially apparent in the concentration of cobalt in lithospheric mantle sulphides, which appears to be inherently linked with the formation and/or later destruction (subduction) of oceanic crust during the Grampian event of the Caledonian orogeny. The impingement of the proto-Icelandic mantle plume initiated in the Palaeogene at the base of the NAC lithospheric mantle keel of Scotland and Greenland. The earliest Palaeogene magmas are enriched in Pt (i.e., have a high Pt/Pd ratio), whilst subsequent magmas associated with the opening of the Atlantic Ocean have successively lower Pt/Pd ratios. High Pt/Pd ratios are therefore coincident with magmas that have intruded through cratonic lithosphere. The SCLM at the margin of this region is known to be Pt-enriched (with cooperite) and therefore the changing Pt/Pd ratio of North Atlantic Igneous Province magmas suggests a fundamental interaction between the mantle plume and pre-enriched SCLM. Thus, whilst the concentration of metals, particularly Ni and Cu, is largely based on the high degree of asthenospheric mantle melting associated with the plume itself, the ratio of precious metals, such as Pt/Pd, can be strongly influenced by SCLM geochemistry. Overall, the intricate subtleties of metasomatic signatures recorded by mantle xenolith sulphides (or populations of sulphides) could allow for metallogenic ‘mapping’ of the upper mantle. This may identify areas of geochemical and mineralogical ‘preconditioning’, and together with geophysical constraints such as major lithospheric lineaments, it may be possible to establish the craton-specific fertility of a region. Finally, in order for orthomagmatic sulphide mineralisation to occur, magmas must achieve sulphur saturation in the upper crust, forming an immiscible sulphide liquid and thereby collecting PGE and chalcophile elements, possibly to economic grades. Thus a crucial part of assessing the mineralisation potential of a region must entail an investigation into the causes and locations of S-saturation. Given that crustal sulphur contamination is a common trigger for magmatic S-saturation, this thesis establishes the first S-isotopic (δ34S) framework for western Scotland in order to identify areas of sulphur contamination in the BPIP. In Scotland, the most readily available and S-rich rocks occur in the Mesozoic Hebrides Basin. Sulphur contamination of BPIP rocks is widespread and both S-saturation and S-undersaturation can be observed, suggesting that the region may be extremely fertile for orthomagmatic Ni-Cu-PGE mineralisation. By reconstructing the Hebrides Basin stratigraphy we can assess locations of contamination, even if these are above the current level of exposure (and since removed by erosion), and in some situations sulphide liquid sinking may be demonstrated, suggesting further possibilities for mineralisation present ‘up-stream’ in magmatic conduits. In conclusion, the Scottish BPIP represents a new exploration frontier not yet identified by industry for orthomagmatic Ni-Cu-PGE mineralisation. The conclusions are based on approximately 500 rock samples from across Scotland, which have been analysed for major elements and over thirty trace elements (including PGE) and S-isotopes. All data are available on an accompanying CD.

The PGE mineralisation in the Fazenda Mirabela ultramafic-mafic and Jinchuan ultramafic intrusions has been characterised by determining the platinum-group mineralogy of each complex and the concentration of PGE in solid solution in the base metal sulphides (BMS). The Mirabela intrusion is largely unaltered and hosts two PGE occurrences from which 128 Au- and Ag-bearing minerals and 716 PGM have been identified; predominantly Pt-Pd-Ni tellurides in the Santa Rita sulphide ore deposit and to a lesser extent in the underlying sulphur-poor dunite. A localised Pd-Cu±Pb alloy assemblage is identified in the dunite in the central zone of the intrusion. The PGM assemblages at the edges of the intrusion are relatively As-rich containing sperrylite (PtAs2); arsenic may have been introduced through crustal assimilation. Two orebodies (#1 and #24) have been studied from the Jinchuan intrusion which has undergone extensive greenschist facies alteration. A total of 64 Au- and Ag-bearing minerals and semimetal alloys, and 93 PGM have been identified including michenerite (PdTeBi), froodite (PdBi2), members of the hollingworthite-irarsite-platarsite solid solution series ([RhIrPt]AsS), sperrylite and maslovite (Pt[BiTe]2) in decreasing order of abundance. The PGM vary across these two orebodies as a result of sulphide fractionation, with the Ni/MSSrich orebody #1 hosting early crystallising sulpharsenides and As-bearing PGM whereas the relatively Cu/ISS-rich orebody #24 hosts more Pd-bearing PGM with Pd partitioning into the Cu-rich sulphide liquid during MSS crystallisation. These studies show that Bi- and Te-bearing PGM ([PtPdNi]Te2, PdBi2, PdBiTe) may exsolve from the BMS during sub-solidus cooling after these elements have partitioned into the BMS at high temperatures whereas As-bearing PGM (PtAs2, [RhIrPt]AsS) and sulpharsenides (gersdorffite-cobaltite [NiAsS-CoAsS]) may crystallise early directly from an As-bearing immiscible sulphide melt. In the Mirabela intrusion, the IPGE and Rh have partitioned into MSS, from which pentlandite, pyrrhotite and pyrite have exsolved. Osmium and Ir preferentially partitioned into pyrite (with Co) whereas Ru and Rh partitioned equally between pentlandite and pyrite. In the Jinchuan intrusion, Ir, Rh and Pt have preferentially partitioned into early crystallising sulpharsenides (from which [RhIrPt]AsS PGM exsolve) depleting MSS in these elements. Palladium is identified in pentlandite in both complexes studied and may have diffused from ISS/chalcopyrite or partitioned into MSS at an earlier magmatic stage. Platinum does not usually partition into BMS (excluding sulpharsenides) and instead forms PGM. However, Pt does partition into pentlandite in the Mirabela sulphur-poor dunite where semimetal concentrations are very low and Pt-bearing PGM form only in low quantities, if at all. Minor localised serpentinisation of the Mirabela intrusion does not remobilise the PGE with the primary magmatic distribution of PGE and PGM preserved; the latter are predominantly associated with interstitial BMS and occur in sulphide stringers shown to be of magmatic origin. Pervasive greenschist facies hydrothermal alteration at Jinchuan altered and oxidised the BMS during a process of sulphur loss, resulting in the formation of secondary magnetite and the liberation of Pd, Bi and Te which coalesce to form secondary froodite and michenerite in situ at the edges of these replacement oxides; however, the PGE are not extensively remobilised. Both complexes show that the semimetal content of the ore-forming magma is critical in controlling the distribution of PGE into BMS and/or PGM.

The Rodrigues Ridge is a linear east-west trending volcanic ridge, located between 18°S and 20°S in the western Indian Ocean. The trend of the Rodrigues Ridge is contrary to the ocean floor fabric of the underlying crust, which formed on the Central Indian Ridge (CIR) between ca. 48 My and 10 My. Dating of dredged basalts from the Rodrigues Ridge showed them to be 8-10 My, with no systematic variations with longitude. All samples recovered from the Rodrigues Ridge were olivine and plagioclase phyric with traces of chrome spinel. Only the most western site contained any phenocryst clinopyroxene. Phenocryst olivine was in the range Fo88-79 and plagioclase was in the range An79-60. Where present the clinopyroxene was titaniferous (2.0-3.7% TiO2) in the range Ca4852Mg30-19Fe 19.33. The Rodrigues Ridge lavas are transitional alkali basalts which display systematic geochemical variations with longitude. Most notably the trace elements Ba, Nb, Rb, Sr, Th, Y, Zr and the LREE increase from east to west, while Sc decreases. These variations may be described as ranging from depleted (in incompatible elements) MORB-like compositions in the east, to enriched OIB-like compositions in the west. This is manifested by at least a three-fold increase in the concentration of the incompatible elements. In accordance with the model presented by Ellam (1992) it is proposed that this behaviour is controlled by the depth to the base of the lithosphere, which acts as an upper limit to melting. In this model the upper limit on the melting is much deeper under the older thicker lithosphere than it is under the young lithosphere. Thus melt composition will increasingly be influenced by the presence of residual garnet under older ocean crust. Even in the absence of residual garnet clinopyroxene in the upper mantle may be capable of retaining the HREE and some trace elements such as Y and Zr. Furthermore shallow mantle is more likely to have been subjected to one or more previous melting episodes beneath spreading ridges, leading to melts depleted in incompatible elements being derived from beneath young ocean crust. Rayleigh-type modelling for most sites along the Rodrigues Ridge produced only a poor correlation with the observed data. At several sites, notably RR3, some incompatible elements (eg. Zr, Y, REE) show buffered or decreasing trends with decreasing MgO. Although the causes of this behaviour remain ambiguous, it is possible that these melts have not experienced significant high-level fractionation, and this unusual behaviour has its origin in melt-rock reactions within the upper mantle. At these depths the partition coefficients for Zr, Y and the mid-HREE in clinopyroxene have been shown to be greater than unity (Blundy et al., 1998 and Vannucci et al., 1998). Thus the reaction between the melt and coexisting clinopyroxene within the upper mantle, in conjunction with the crystallisation of olivine, may explain how these buffered and declining trends (with declining MgO) have developed. To characterise their isotopic signature a subset of samples were analysed for Sr, Nd and Pb. Like the trace elements, the isotopes display clear linear trends with longitude. On paired isotope plots the Rodrigues Ridge lavas form similar linear trends between samples from the CIR (the Marie Celeste Fracture Zone) and the Reunion hotspot (notably Mauritius), suggesting that they are products of mixing between the mantle sources of the CIR and Reunion. It is proposed that the upper mantle has been passively contaminated by the Reunion plume (with circa 87Sr/86Sr = 0.7042, 143 Nd/ 144Nd = 0.7042, 206Pb/204Pb = 18.788, 207Pb/204Pb = 15.585 and 208Pb/204Pb =38.849). At shallow levels, immediately below the lithosphere, the upper mantle is made up predominantly of a depleted MORB-like source, with circa 87Sr/86Sr = 0.7031, 143Nd/144Nd = 0.51305, 206Pb/204Pb = 18.354, 207 Pb/204Pb = 15.517 and 208Pb/204Pb =38.214, while at greater depths more Reunion plume material is available. The observed linear array, on paired isotope plots, reflects the differing contributions made by these deep and shallow sources to the Rodrigues Ridge lavas. It is proposed that the Rodrigues Ridge was formed due to a build up of stress, possibly resulting from its proximity to the Ridge-Ridge-Ridge triple junction (see Patriat &Ségoufin 1988), causing the rigid African plate to rupture parallel to the principal stress direction. This would have resulted in decompression melting in the upper mantle, so facilitating rapid but short-lived volcanism. Although volcanism has ceased, there is still elevated heat-flow within this area, suggesting that the upper mantle is still anomalously hot (von Herzen & Vacquier 1966). Isostatic readjustment could account for the magmatic reactivation at 1.5 My which formed Rodrigues island.

The formations were examined along the Cotswold scarp (160km), and subcrop data were also utilised. The spatial and temporal characteristics of the formations allow sedimentological patterns to be related to structures in the pre-Permian basement and in the overlying Middle Jurassic strata. Sedimentation was strongly controlled by an actively subsiding block faulted basement, which formed part of the North Atlantic Rift system. The generalised Pliensbachian-Bajocian model of Sellwood and Jenkyns (1975) is supported by evidence in the Cotswolds. Both formations show cyclic sedimentation characterised by upward changes in grain size, mineralogy, thickness, sedimentary structures and fauna. Spatial patterns reflect the N-S structures of the basement. Primary controls on the cyclicity are shown to be tectonic rather than eustatic. Five facies are recognised in the Marlstone Rock Bed Formation. The stratigraphic interpretation of the formations is refined. There was a break in sedimentation at the end of the Pliensbachian. The base of both formations is diachronous, and spread from the centre of the basin outwards to both E and W margins. Randomly-interstratified illite-smectite in these rocks is interpreted as a weathering product of illite, while smectite was produced by alteration of air-fall volcanic ash. Both were derived from adjacent land areas. Ferruginous ooids probably formed through mechanical and/ or. algal accretion in temporary reducing conditions on the sea bed. The iron-rich sediments were formed at the boundary between siliciclastic and carbonate regimes. True ironstones are virtually absent as a result of rapidly changing patterns of sedimentation within the rift. Widespread 'wavy' bedding is shown to be mostly diagenetic pseudo-bedding, although some appears to have been produced by wave rippling or by compaction alone.

Invasive tufa deposits on Foel Fawr, South Wales are forming from lime kiln waste deposited from the 18th century until the 1950s. The tufa deposits are unusual in their anthropogenic origin and form the largest site of this type in the UK. At emergence pH is extreme (>11) and this consequently generates significant pressure on organisms living in and around the system. Morphologically, the calcium carbonate tufa deposits are similar to those formed in other extreme environments (e.g. hot springs, caves), forming many of the same features (e.g. terraces, rimpools, pisoids, stalactites). By identifying and mapping tufa facies at Foel Fawr, the spatial distribution of these facies has been directly compared to these potentially analogous environments. Analysis of the fabrics associated with each facies highlights the importance of physico-chemical precipitation in the system; proximal facies are dominated by abiotic fabrics, while distal and marginal facies show an increasing degree of biological influence. This process is comparable to the partitioning of fabrics and facies observed in hot spring systems, however, the role of microbes in precipitation is apparently less important on Foel Fawr. The present day extreme chemistry of the site selectively excludes organisms and generates a partitioning of biology. Hydrochemical monitoring of the site reveals that the extreme pH of the system is in decline. The recession of the extreme hydrochemistry is confirmed by colonisation of previously excluded organisms and allowed the physical decay of the site. Fabrics preserved within the deposits support the suggestion that the extreme environments were previously much more widespread.