The Palaeozoic rocks of the Dingle Peninsula provide a record of the evolution of the Caledonides, Acadides and Variscides. The succession ranges from Early Ordovician deep-water sediments, through Silurian shallow marine to non-marine sediments and volcanic rocks to an Old Red Sandstone (ORS) succession topped by Carboniferous marine shales. Comparison of structural styles in the unconformity-bounded groups, together with a detailed analysis of fault zones, allows the tectonic history to be deduced. The older rocks record Caledonian processes on the margin of Avalonia during Early Ordovician time and convergence then soft collision with Laurentia during Silurian time. The Dingle Basin was developed during the late Silurian -Early Devonian transtension in the Iapetus suture zone and was inverted in the latest Emsian Acadian orogenic episode. Post-Dingle Group ORS groups in the north of the peninsula were deposited in a syn-rift footwall block to the main Munster Basin. The Acadian transpressional and Munster Basin extensional structures were reactivated or overprinted in the Variscan deformation such that Acadian folds are transected by Variscan cleavage in both plan and vertical views. After Iapetus closure, changes in the tectonic regime are believed to be a result of adjustments in the geometry of subduction of the Rheic Ocean.

The phyllitic mudrocks, quartz wacke sandstones and mélange units of the Annascaul Formation in the SE Dingle Peninsula, SW Ireland have been previously assumed to be Silurian in age, similar to overlying fossiliferous Wenlock rocks of the Ballynane Member. The contact between the Ballynane Member and the underlying Annascaul beds is either faulted or unconformable , and the Annascaul mudrocks contain two or more cleavages not present in the Silurian Ballynane and younger strata. The Ballynane Member is therefore differentiated from the Annascaul Formation by upgrading the proven Wenlock Ballynane beds to formational status. Grey and black mudrocks in the Annascaul Formation have yielded diagnostic palynomorph assemblages indicative of an Early Ordovician age. The Annascaul Formation invites comparison with similar Early Ordovician rocks, south of the putative trace of the Iapetus Suture, in SE and eastern Ireland, in the English Lake District, and in central Newfoundland.

Abstract The Late Silurian to Mid-Devonian Dingle Basin occupies a central position within the Iapetus Suture Zone of SW Ireland. The basin is believed to have formed from late Silurian times onwards as a product of sinistral transpression along several major faults within this suture zone. Conglomeratic sediments were deposited by moderately large gravelly fans shed into the basin from the NW (Glashabeg Formation) and the SE (Trabeg Conglomerate Formation). The systems fed into a large apparently through-going sandy, axial river (Slea Head Formation) that flowed towards the NE. The lateral, basin-margin systems were sourced from two disparate source terranes. To the NW lay a basic volcanic hinterland with some intermediate volcanic rocks and limestones, mudstones, sandstones and chert. During the Early Devonian time the SE drainage basin was underlain by sandstones, quartzites, phyllites and limestones probably intruded by a granite. Some of these lithologies can be found in outcrop in the pre-Dingle Group of the peninsula. Others need to be correlated with rocks of the oceanic terranes in the northern part of the suture zone. The southerly derived clasts have corollaries in the rocks of the Avalonian Leinster Terrane south of the suture. Following partial inversion of the Dingle Basin, the southerly hinterland was apparently further unroofed during mid-Devonian time, when the Inch Conglomerate Formation was deposited by alluvial fans shed northwards from a source area formed along the Dingle Bay Lineament. The Inch conglomerates are characterized by distinctive clasts of schist, gneiss, mylonite, tourmalinite and granite. The general picture of the Early Devonian deformation, intrusion by granite and unroofing of terranes currently partially exposed in central and southern Ireland within the Iapetus Suture Zone is largely consistent with clast lithotypes. However, some exploratory isotopic data indicate at least two possible vagaries in the interpretation. First, model ND TDM ages of Trabeg sedimentary clasts yield several results older than typical southern Iapetean or Avalonian crustal material. This suggests a complex history of sedimentary mixing of material across the developing Iapetus Ocean. Second, two of three Rb-Sr muscovite-whole-rock dates of Inch metamorphic clasts indicate Silurian ages. These data similar to Rb-Sr dates derived from the Carnsore and Saltees granites in the Rosslare Terrane, perhaps extending the geographical spread of this Silurian deformation.

Abstract Most SE Asian Tertiary-aged petroleum has been derived from paralic (lower delta plain to prodelta, higher land plant dominated) source rocks, although the larger proportion of oil is from lacustrine (freshwater/brackish algal) sources. Because many SE Asian petroleum provinces have been largely explored without penetrating source rock, the source is inferred from the oil’s geochemistry. Prolific lacustrine sources develop mainly in rift lakes in the Palaeogene syn-rift megasequence common to many SE Asian basins, but floodplain lake sources are also important. Paralic source rocks include both coals and particularly coaly mudrocks developed within the Miocene post-rift megasequence with an oil-prone zone. Oil-prone source rocks are preferentially developed in the paralic realm between the lower coastal plain and lower estuary/delta front facies, perhaps involving a mangrove system. The stratigraphic position and kinetic expulsion behaviour of different source types, migration, seal timing and pressure-temperature-depth relationships converge to make younger plays, including carbonate buildups, more gas prone. The volumetric significance of biogenic gas is still poorly understood. Vertical migration is common in overpressured SE Asian basins; lateral migration is thus commonly restricted to distances of 20 km or less from the kitchen.

Abstract New well and seismic data acquired during recent exploration of the SE Nam Con Son Basin, offshore Vietnam, have been evaluated to assess the tectonostratigraphic evolution. The offshore Vietnamese region has evolved in response to the complex relative motions of Indochina, Peninsular Malaysia, Borneo and the East Vietnam/South China Sea during the Cenozoic. On a regional scale these motions have been accommodated by strike-slip fault development, crustal extension and contraction. Rift pulses occurred in the SE Nam Con Son Basin from the Palaeogene to the earliest Late Miocene in response to the interaction of East Vietnam/South China Sea rift propagation and regional transtensional shear to the west of the evolving ocean basin. The structural evolution was complicated by mild contractional deformation during the Middle Miocene which was broadly synchronous with development of the largest inversion structures in the nearby West Natuna Basin. The oldest dated Tertiary rocks in the SE Nam Con Son Basin are fluvio-deltaic sediments of Late Oligocene age which have been penetrated by several wells. Early to Mid-Miocene depositional environments ranged from non-marine to outer shelf, with a predominantly clastic basin-fill. The thickest Lower to Middle Miocene occurs in N-S- to NE-SW-trending half grabens. A regionally recognized truncational unconformity of late Mid-Miocene age has resulted partly from the combined effects of the mild inversion and by the erosion of uplifted footwalls. During the Late Miocene there was rapid and widespread deepening of depositional environments, synchronous with net-extensional fault reactivation on many previously developed rift structures. The stratigraphic response to this increased bathymetry was the growth of isolated carbonate build-ups on pre-existing structural highs, progradation of the palaeo-Mekong delta system and associated deep-marine submarine channel development and erosion. This pulsed structural and stratigraphic evolution has resulted in deposition of source, reservoir and seals, and produced a variety of potential trapping styles.

The Iapetus suture in Ireland and Britain is that line which separates Caledonian rocks of the Laurentian and Avalonian continents. The suture is cryptic: nowhere is there an exposed fault-zone containing ophiolite remnants, blue-schist melanges, or trench deposits. Instead a suture line may be traced with varying degrees of confidence through a series of faults that traverse the Iapetus suture zone, which contains two or more tectonostratigraphic terranes. Several data sets are utilized to constrain this trace. The distribution of faunal provinces caused by oceanic separation can be used most reliably to define early Ordovician terranes. Fauna1 intermingling in the mid-Ordovician reduces provinciality and the confidence of terrane identification using faunal data. The late Ordovician and particularly Silurian tectonostratigraphic histories of terranes either side of the suture suggest that amalgamation of the terranes, elimination of the Iapetus Ocean and development of the suture zone had begun by the Ashgill. Therefore the resolution of the suture trace becomes unreliable where Ordovician rocks do not crop out, because Silurian turbidite fans of Laurentian provenance may have dispersed across a significant width of Avalonian crust. Similarly, the precision of structural identification of the suture zone, by correlating the suture with a major fold-facing confrontation, may be weakened by late overthrusting. Tectonic interleaving of crustal flakes in the zone is supported by geophysical evidence which suggests that some major boundaries in the lower crust do not always coincide with their inferred projections in outcrop. Tectonic and sedimentary mixing of crustal blocks and sediments in the zone is further reflected by Nd and Pb isotopic patterns; the Rb-Sr patterns of granites record a stronger distinction between north and south of the suture. Despite these difficulties, we depict a possible suture trace in Ireland that departs significantly from the traditionally placed line by following a NE-SW-aligned trace through the Slane fault and the Navan-Tipperary lineament, and thence through a dog-leg to pass north of the Dingle Peninsula.

The study of sedimentary provenance interfaces several of the mainstream geological disciplines (mineralogy, geochemistry, geochronology, sedimentology, igneous and metamorphic petrology). Its remit includes the location and nature of sediment source areas, the pathways by which sediment is transferred from source to basin of deposition, and the factors that influence the composition of sedimentary rocks (e.g. relief, climate, tectonic setting). Materials subject to study are as diverse as recent muds in the Mississipi River basin (Potter et al. 1975), Archaean shales (McLennan et al. 1983), and soils on the Moon (Basu et al. 1988). A range of increasingly sophisticated techniques is now available to workers concerned with sediment provenance. Provenance data can play a critical role in assessing palaeogeographic reconstructions, in constraining lateral displacements in orogens, in characterizing crust which is no longer exposed, in testing tectonic models for uplift at fault block or orogen scale, in mapping depositional systems, in sub-surface correlation and in predicting reservoir quality. On a global scale, the provenance of fine-grained sediments have been used to monitor crustal evolution. We introduce below some of the novel techniques which are currently being used in provenance work, and some of the areas in which provenance studies are making, and promise to make, an important contribution to our understanding of earth processes. Many of the techniques and applications are covered by papers collected in this volume. These papers represent a selection of those contributed to a joint British Sedimentological Research Group/Petroleum Group meeting on ‘Developments in Sedimentary Provenance Studies’

The term ‘inversion’ to describe an inverted basin was first used by Glennie & Boegner (1981) although inverted basins had been recognized many years before e.g. Lamplugh (1920) and Stille (1924). During this meeting it became apparent that the application of the term had broadened to such an extent that the understanding of ‘inversion’ in the petroleum industry was incompatible with much of the current usage. The discussion that follows illustrates many of the points of disagreement, perhaps the most contentious of which is the use of the term ‘negative inversion’ although this was also introduced by Glennie & Boegner (1981). Most of the discussion was presented verbally at the meeting and was recorded, transcribed and returned to speakers for their corrections. In addition, a number of written contributions were received. All contributions to the discussion have been edited as gently as possible so as to retain the exact meaning intended by the contributor. All contributors are included as co-authors in this discussion article but clearly this does not mean that individuals necessarily accept all the points made by other contributors. The editors have identified individual contributions. The discussion commenced with some proposals by the editors which are briefly reproduced here. This discussion article concludes with a considered revision of the proposals on nomenclature which aims to satisfy some of the shortcomings identified in the discussions. Our initial definition of inversion relied on the concept of regional elevation. The regional elevation of a marker horizon is the structural elevation of the horizon

AbstractA sheet‐like appinite from Breaghy Head, North Donegal, is closely associated with a spessartite intrusion. Field, mineralogical, and geochemical similarities indicate that the parent magmas were very similar, and the appinite magma may have been derived from the spessartite by small degrees of crystal fractionation. The appinite is divided into an amphibole‐rich lower part and a felsic upper part; the mineralogical contrast between the two is best explained by two‐pulse intrusion of a batch of spessartitic magma within which amphibole and pyroxene had already segregated due to crystal settling or flow sorting. Unlike other Caledonian appinites, the Breaghy Head intrusion is not one of a cluster, and appears to be an isolated development. Possible reasons for its location are discussed.

The study of alluvial fan sequences, the most proximal components of the alluvial spectrum, can be one of the most profitable avenues of research in basin analysis because fans are particularly sensitive to external influences and have a relatively high preservation potential. Tectonism has long been invoked to explain many variations in fan sediments, and increasingly the presence and nature of fan sequences in the rock record is being used to denote proximity to active faults and to document movement on these faults. There are, however, many difficulties involved with the interpretation of alluvial fan sequences, not the least of which is deciding whether the sediments are truly components of an alluvial fan or the products of a different depositional system such as an alluvial braidplain. Many of the problems arise from a lack of knowledge about the relative roles in fan construction of allocyclic processes such as climatic change and tectonic activity, and of autocyclic processes such as channel avulsion and fanhead entrenchment. Whilst researching alluvial fan sediments in a variety of tectonic settings and ages, it has become increasingly clear to the authors that there are many inadequacies in our understanding of alluvial fans, and hence weaknesses in the conclusions drawn from these sediments. This prompted the one-day meeting organized to promote discussion of the problems of distinguishing tectonic from other influences on the evolution of modern and ancient fans. 110 industrial and academic geologists gathered at the University of Bristol on 17th October 1987 to hear 16

AbstractThe structurally attenuated greenschist facies metasedimentary succession at Cullenstown Strand in south Co. Wexford comprises three formations which in stratigraphic order are (1) the Quartzite Formation, (2) the Greywacke/Quartzite Formation, and (3) the Greywacke Formation. The entire sequence is inverted and lies within the lower limb of a large scale overturned D1 anticlinal structure that closes towards the southeast. The three formations are lithologically and sedimentologically similar to the Shelmaliere Quartzite, the Cullentra Greywacke, and Newtown Greywacke Formations of the Bray Group in the Forth Mountain ‐ Ferrycarrig area that probably lie on the normal limb of the D1 structure. The Cullenstown strata are therefore considered to be part of the Bray Group in southeast Leinster, and hence Cambrian in age.The rocks were affected by three major phases of deformation. D1 produced an overturned anticline containing an important tectonic slide. The D1 structures are modified by upright D2 and D3 structures, but from regional considerations it is argued that D1 is restricted to strata of Cambrian to Lower Ordovician age in southeast Leinster.A model for the tectonic evolution of the southeastern part of the Leinster Basin during Cambro‐Ordovician times is presented and discussed. In this model, the southeast margin of the Leinster Basin was delineated by a mylonite zone along the northwest margin of the Precambrian Rosslare Complex. Bray and Ribband Group sediments close to the margin were deformed during the Llanvirn as a consequence of basin inversion which caused shortening and thrusting of the basin fill across the Rosslare block. The D1 compressional structures were subsequently modified during a period of extension, marked by the resumption of sedimentation and outbreak of voluminous volcanicity near the basin margin during the Llandeilo and Caradoc. The D2 and D3 structures in the Bray and Ribband Groups are an expression of later Caledonian deformation.

AbstractThe lower part of the Old Red Sandstone in the Dingle Penisula has been previously assigned to one lithostratigraphic group (Dingle Group) despite marked variations in sedimentary facies. However the apparently oldest non‐marine sequence in the northwest of the peninsula has sedimentary and lithological attributes that contrast strongly with those of the late Silurian‐early Devonian Dingle Group to the south. This northern sequence, here renamed the Smerwick Group, evolved independently of the Dingle Group in a separate basin of deposition. Field relationships between the two groups in the north of the peninsula are interpreted as indicating that the Smerwick Group overlies, with angular unconformity, a normal Dingle Group succession. Similarly, it is argued that the Smerwick Group overlies, with angular unconformity, the Dingle Group in the northwest of the peninsula, but there the Dingle Group is attenuated, represented only by a conglomerate unit some 10 m thick. In the absence of biostratigraphic evidence the age of the Smerwick Group is poorly constrained. Nevertheless, we propose a tectonic model that suggests that the Smerwick Group evolved within a small extensional half‐graben on the northern margin of the Munster Basin. This model accounts for the stratigraphic and structural relationships observed, and implies that the Smerwick Group is of Late Devonian age.

A clear correlation exists in the southern Irish Variscides between extensional structures which controlled late Palaeozoic sedimentation in the Munster Basin, and contractional thrusts and folds of end-Carboniferous age. This correlation is interpreted to indicate that the extensional structural framework of the basin, including a basal low-angle detachment and an array of superjacent normal faults, was reused during transpressive inversion of the basin to produce the fold-thrust belt. The extensional framework, which in turn was to some extent derived by reactivation of Caledonian structures, therefore, exerted a fundamental control on the geometry and style of contractional deformation. It is not evident that the detachment had a discrete link to the fold belt in south Wales or the main Variscan orogen in Europe.

The Dingle Peninsula exposes the most complete Old Red Sandstone (ORS) succession in Ireland, ranging from Late Silurian to Early Carboniferous in age. It lies within 20 km of several ENE-WSW aligned lineaments, including the Dingle Bay Lineament immediately to the south, and the putative trace of the Iapetus Suture to the north. These were formed in a regime of sinistral transpression in the Late Silurian to Early Devonian. It also forms part of the northern margin of the Munster Basin formed by Late Devonian N-S extension. The sequence is divided into five groups: the Dingle Group, the Caherbla Group, the Smerwick Group, the Pointagare Group and the Glengarriff Harbour Group in ascending order. The Dingle Group records initially lacustrine dominated sedimentation followed by deposition in laterally and axially draining fluvial systems within the NE-SW aligned Dingle basin. Generation and inversion of the Dingle basin, and subsequent deposition of the Caherbla Group — composed of alluvial fan and aeolian dunefield deposits — may be related to sinistral strike-slip displacements along the Dingle Bay Lineament. The Smerwick and Pointagare Groups, also consisting of fluvial and aeolian facies, are interpreted to have been deposited in small half grabens restricted to the north of the peninsula. Further Late Devonian extension and subsidence allowed the more widespread deposition of the Glengarriff Harbour Group comprising braided river sediments. Thus the lower part of the ORS of the Dingle Peninsula is interpreted to reflect the Late Silurian to Early Devonian development of localised basins adjacent to sinistral strike-slip faults that were active during the final oblique convergence within the British and Irish Caledonides. In the later Devonian these lineaments were reactivated as normal faults during N-S extension to form the northern margin of the more extensive Munster basin. This change in tectonic regime is also reflected in the swing of major fluvial drainage from an early NE-SW along-basin alignment to a later southerly directed paleoflow.