Tectono-stratigraphic Sequences Research Articles

A revision of the stratigraphy and geology of the south-western part of the Crotone Basin (South Italy)

The Crotone Basin, located on a stack of nappes piled up during the late Paleogene-Neogene, formed in the late Neogene to Quaternary as a forearc basin of the Ionian arc-trench system. The process of slab rollback caused rapid trench migration, resulting in an extensionaltranstensional regime persisting most of the time in the forearc area.The late Neogene tectonic evolution was strongly influenced by a NW-directed fault system, interpreted as basement wrench faults leading to partitioning of the basin into separate sub-basins subject to differential subsidence and mutual displacements. Major sequences identified in the area are regarded as tectono-stratigraphic sequences (TSS). The first of them was laid down in the late Serravallian (?) - Tortonian - early Messinian, during the basin opening stage and is bounded at the top by an erosional unconformity, which may be correlated with the well-known intra-Messinian event of the Mediterranean Salinity Crisis. The second TSS, of middle to late Messinian age, is characterized by strongly syntectonic deposits mostly derived from cannibalization of the lower Messiniansuccession, first infilling extensional troughs, then involved in an episode of sinistral transpression along the NW-trending fault system, which generated local overthrusts, sealed by a late Messinian erosional unconformity. The Messinian tectonics probably reflects the interplay between the processes linked to the kinematics of the Calabria block and those triggered by the Salinity Crisis. The erosional unconformity is overlain by widespread coarse fluvial conglomerates, which are the first onlapping term of the third TSS, represented by uppermost Messinian to lower Zanclean deposits, laid down in an extensional-transtensional regime. This TSS was closed by an important late Zanclean episode of dextral transpression along the NW-trending fault system, leading to inversion of the formerbasins, and limited SW-verging thrusts on fault-restraining bends.The unconformity sealing the structures has a clear expression in the northern, marginal part of the Crotone Basin and correlates downbasin with a conformable surface. The fourth TSS is characterized by a long-lasting phase dominated by extension-transtension, leading to high subsidence rate during the latest Zanclean to Early Pleistocene, and accommodating a thick succession of slope mudstones including clusters of diatomaceous bands mostly in the D. tamalis and D. brouweri Zones. In the northern part of the Crotone Basin two phases of drowning separated by an uplift pulse at ca. 2.55 Ma can be recognized, the second of which was a dramatic collapse, between 2.3 and 2.1 Ma. The fifth TSS is bounded at the base by an unconformity at the transition between «large» and «small» GephyrocapsaZones, i.e. at around 1.1-1.2 Ma, correlating basinwards with a conformable surface. The unconformity is erosional and locally angular in the marginal part of the basin, where it seals structures generated by a contractional event documented also elsewhere in the Calabria block. This event, which is accompanied by a strikeslip component, is inferred to be coeval to the Lower Pleistocene important transpressional episode along the Pollino shear system,which led to release of the Calabria block from the southern Apennines.The fifth TSS is characterized by resumed dextral transtension in the Middle Pleistocene along right-stepping NW-trending faults. This episode generated minor pull-apart sub-basins, showing spectacular growth structures in their infilling successions, which developed with shoaling trend up to inferred Marine Isotope Stages 9-8. The onset of shoaling trend was diachronous, being remarkably younger in the southern sub-basin. In the late Middle Pleistocene to Recent times extensional tectonics was dominating, accompanied by local gravity gliding towards the Ionian Sea, arguably triggered by increase in topographic gradient following hinterland uplift, and implying the activation of a linked, thin-skinned extensional and contractional NE- to NNE-directed fault system, with detachment surface possibly soling into Messinian evaporitic-mudstone deposits. It is concluded that the geologic evolution of the investigated forearc area was characterized by an alternation of long-lived stages of extension-transtension expressed by prolonged subsidence preceded by uplift pulses, and short-lived episodes of contraction-transpression.Major drowning episodes in the forearc area are thought to be coeval to the main phases of spreading in the Tyrrhenian basin.

Compaction trend and its implication in the overpressures estimate for the formations of the Colombian Foothills of the Eastern Plains

The main objective of this article is to raise a hypothesis to explain the main causes of overpressure in the formations of the Tertiary sequence for the stratigraphic column of the Colombian Foothill. For this purpose it was conducted an analysis of compaction trends from the Guayabo Formation until the unit C8 of the Carbonera Formation; in the area of foreland, where the tectonic affectation has been minimal. This analysis integrates the most representative basins modeling and tectono-stratigraphic events for such sedimentary sequence. It mapped overpressure areas, comparing them with geological parameters such as the subsidence, uplift, heat flow and speeds of sedimentation, to identify relations of these parameters with the overpressure of the area. As the main result, it stresses the identification of different sedimentation and compacting rates for each tectono-stratigraphic sequence and its relationship with the overpressure of the formations. These differences are represented in specific equations presented in this work. One of the main conclusions relates the rapid uplift of the basin, occurred in mid Miocene and the lack of liberation of stresses; as one of the causes of the pressures that are currently observed.

Structure of the Kasbah fold zone (Agadir bay, Morocco). Implications on the chronology of the recent tectonics of the western High Atlas and on the seismic hazard of the Agadir area

Detailed re-interpretation of the north-eastern segment of a profile realized across the Agadir bay along a NE-SW trend and crosscutting the main structures, together with analysis of available isochron maps, allowed us to retrace the geological history of the offshore western High Atlas. Two tectonostratigraphic sequences were distinguished: Unit II, which displays a simple structure, laying unconformably on Unit I, with a more complex structure dominated by a reverse fault (F1) striking E-W with a dip to the north. Correlation to boreholes Souss-1 and AGM-1 allowed us to assign Unit I to the Triassic – Palaeogene and Unit II to the Miocene – Present. The NE fault block shows a ramp-flat fault plane (F2) with an overlying SW-vergent fold that can be interpreted as a fault-bend fold. Three main stages were distinguished: (1) during the Cretaceous, F1 could have been a syndepositional normal fault with the NE block moving downwards; (2) towards the beginning of the Tertiary, the displacement of plane F2 induced the development of a fault-bend fold and erosion of the forelimb and hinge of the fold; displacement along F2 was transferred to fault F1; (3) afterwards, during the Miocene, reverse motion of F1 deformed and tilted the plane F2 and accentuated the folded structure. This evolution is typical for a frontal basin above a fault-related fold. Evaluation of the thickness and bed depth differences shows that the largest growth rate was recorded in Late Miocene times. Seismic activity recorded in the Agadir bay appears to be clearly related to this fault zone, as inferred from focal mechanisms. Seismic moment evaluation suggests that earthquakes of magnitude Mw≥6 are likely to occur, but could not be much larger because of the fault segmentation geometry of the High Atlas Front.

The Tertiary evolution of the prolific Nanpu Sag of Bohai Bay Basin, China: Constraints from volcanic records and tectono-stratigraphic sequences

The Bohai Bay Basin, located on the eastern Asian margin, is the second largest oil-production basin in China. It contains numerous depressions and sags, among which Nanpu Sag has become particularly important because of significant oil discoveries in recent years. Geologically and tectonically, however, the rifting mechanism and geodynamic evolution of the basin remain uncertain. This paper uses detailed volcanic and stratigraphic records obtained through extensive drilling and sampling in Nanpu Sag to interpret tectono-stratigraphic sequences and discuss basin dynamic evolution and crust-mantle interactions of the Bohai Bay Basin. Nanpu Sag contains voluminous volcanic rocks of dominantly alkaline basalts. Drill core samples, well logs, and seismic profiles reveal three volcanic cycles between the Eocene and Miocene Epochs. These basalts were created during a transition from garnet- to spinel-peridotite. Five tectono-stratigraphic sequences produced by episodic continental rifting have been identified, each composed of a basal coarse clastic sequence, a lower volcanic sequence, a middle deep-water clastic sequence, and an upper infilling sequence. Tectonically, Nanpu Sag experienced four evolutionary phases. The first three corresponded to three rifting events revealed by cyclic volcanism and sedimentary depositional features. The fourth phase was marked by extreme thermal subsidence and prograding fluvial deposition followed by tectonic inversion. A diapiric upper-mantle upwelling model is proposed to explain the dynamics that controlled the multiple rifting processes, the cyclic volcanism, and the periodic tectonic evolution of Nanpu Sag and the greater Bohai Bay Basin.

Tectonostratigraphic evolution of the Morichito piggyback basin, Eastern Venezuelan Basin

The Morichito piggyback basin (MPB) is a SW–NE-oriented depocenter in the Eastern Venezuelan Foreland Basin (EVFB). This piggyback basin formed by overlying the Pirital thrust during the middle to late Miocene as a result of oblique collision between the Caribbean and South-American Plates. The MPB covers an area encompassing approximately 1000 km 2 between the Serrania del Interior range and the Pirital high, which is a hanging wall uplift along the Pirital thrust that acts as a confining barrier on the southern boundary of the MPB. Previous studies have tried to address the tectonostratigraphic significance of the MPB, but new biostratigraphic information and recently acquired 3D seismic data have allowed us to expand the understanding of this basin. The MPB occupies a relatively small area of the EVFB; however, the MPB contains a valuable stratigraphic record that can be used to unveil the timing of the main deformational events that took place in the EVFB. This work presents the tectonostratigraphic evolution of the MPB by defining four tectonostratigraphic sequences (T1–T4). Each sequence was defined on the basis of integration of well logs, biostratigraphy, and seismic geomorphological interpretations. T1 (24–16 Ma) (late Oligocene to middle Miocene), which was deposited in shallow-marine environments, extends to the south of the Pirital high beyond the boundaries of the MPB. T1 is equivalent to the early foredeep stage of the EVFB, having been formed when structural deformation and uplifting were already occurring to the north on the proto-Serrania del Interior range (∼24–16 Ma) and the Pirital thrust was active (∼22 Ma). T2 (16–11 Ma) (middle to late Miocene) is composed of alluvial-fan deposits derived from the proto-Serrania del Interior range. The geometry and internal configuration of T2 indicate that during this time the basin was transitioning from an open-foreland basin to a confined piggyback basin. During deposition of T2, the Pirital fault was active as an out-of-sequence thrusting event (16–∼11 ma). T3 (late Miocene) and T4 (early Pliocene to Recent), composed of shallow-marine and fluvial deposits, were deposited in an already restricted piggyback basin. The Pirital high was already in place during deposition of T3 (∼11–9.3 ma). T3 and T4 represent the final phases of MPB infilling, when tectonic activity and subsidence were at their lowest rates. MPB sedimentary infilling dates the activity of thrusting events in the proto-Serrania del Interior (∼24–16 Ma), timing of maximum deformation associated with the Pirital out-of-sequence thrusting event (16–∼11 Ma), timing of final emplacement of the Pirital high (∼11–9.3 Ma), and the beginning of tectonic quiescence (<5.2 Ma).

Miocene to recent Cariaco basin, offshore Venezuela: Structure, tectonosequences, and basin-forming mechanisms

The Cariaco basin, located ∼40 km off the central part of the coast of Venezuela, is the largest (∼4000 km 2) and bathymetrically deepest (1400 m BSL) Neogene fault-bounded basin within the right-lateral strike-slip plate boundary zone that separates the Caribbean and South American plates. Using subsurface geophysical data, we test two previously proposed tectonic models for the age, distribution and nature of east–west-striking, strike-slip faults, and basin-forming mechanism for the two main depocenters of the Cariaco basin. The earliest interpretation for the opening of the twin Cariaco depocenters by Schubert (1982) proposes that both depocenters formed synchronously by extension along transverse (north-south) normal faults at a ∼30-km-wide rhomboidally-shaped pull-apart basin between the right-lateral, east–west-striking, and parallel San Sebastian and El Pilar fault zones. A later model by Ben-Avraham and Zoback (1992) proposes that both depocenters formed synchronously by a process of ”transform-normal parallel extension”, or rifting in a north–south direction orthogonal to the east-west-striking and parallel strike-slip faults. We use more than 4000 km of 2D single- and multi-channel seismic data tied to 11 wells to map 5 tectono-stratigraphic sequences and to produce a series of structural and isopach maps showing how the faults that controlled both Cariaco depocenters evolved from Paleogene to the present. Comparison of fault and isopach maps for dated horizons from Paleogene to late Neogene in age show three main phases in basin development: 1) from middle Miocene to Pliocene, the West Cariaco basin formed as a rhomboidally-shaped pull-apart at a 30-km-wide stepover between the northern branch of the San Sebastian fault and the El Pilar fault zone; 2) during the early Pliocene, a new strike-slip fault transected the West Cariaco basin (southern branch of the San Sebastian fault) and caused extension to cease; and 3) during the early Pliocene to recent, a “lazy-Z” shaped pull-apart formed along the curving connection between the southern branch of the San Sebastian and El Pilar fault zones.

Late Variscan (Carboniferous to Permian) environments in the Circum Pannonian Region

Late Variscan (Carboniferous to Permian) environments in the Circum Pannonian RegionThe Pennsylvanian-Cisuralian late-orogenic and post-orogenic paleoenvironments of the Circum Pannonian Region (CPR) include tectono-stratigraphic sequences developed from the Upper Bashkirian-Moscovian marine early molasse stage up to the Guadalupian-Lopingian post-orogenic stage, with gradual connection to the beginning of the Alpine (Neotethyan) sedimentary cycle. Shallow marine siliciclastic or carbonate siliciclastic overstep sequences started in the internal part of the Variscan orogenic belt during the latest Serpukhovian and Bashkirian-Moscovian. They overlapped unconformably the variably metamorphosed Variscan basement, or weakly deformed and metamorphosed foreland and syn-orogenic flysch sediments of Mississippian to Early Pennsylvanian age. The post-Variscan rifting largely affected the Variscan orogenic belt by reactivation of the Variscan lithosphere. The late- to post-orogenic terrestrial sequences started within the internal part of the Variscan orogenic belt during the Middle/Late Pennsylvanian. It continued gradually to terrestrial-shallow water carbonate-siliciclastic sequences in its external part through the Permian. According to the present configuration, the Alpine (Neotethyan) northward shifting transgression started during the Guadalupian/Lopingian in the South and during the Early Triassic in the North.

INTEGRATED GRAVITY AND SEISMIC INVESTIGATION OVER THE JABAL HAFIT STRUCTURE: IMPLICATIONS FOR BASEMENT CONFIGURATION OF THE FRONTAL FOLD‐AND‐THRUST BELT OF THE NORTHERN OMAN MOUNTAINS

A gravity survey was conducted over and around Jabal Hafit, located on the border between the United Arab Emirates (UAE) and Oman on the western edge of the northern Oman Mountains, as part of a study to investigate the subsurface structures and sedimentary sequences of the area. This new data, together with outcrop geology, well data and measurements of physical properties of rock samples, was integrated with a new interpretation of reprocessed commercial seismic reflection profiles recorded across the Jabal Hafit anticline.We recognize five major tectonostratigraphic sequences in the seismic profiles: Mesozoic shelf carbonates; Upper Cretaceous foreland (primarily Fiqa Formation); the Hawasina allochthon; and Upper Cretaceous to Lower Tertiary, and Upper Tertiary sequences. The seismic sections suggest that Jabal Hafit represents a backfolded anticline associated with a steep east‐vergent thrust that probably lies above a blind thrust fault along the base of the foreland basin sequence at depth. The lack of onlapping and thinning in the Lower Tertiary sequence suggests that the initiation of folding may have started after the Oligocene‐Miocene and corresponds to the beginning of the collision of Arabia and central Iran along the Zagros suture in Iran. In addition, the seismic sections suggest a pop‐up structure beneath the Jabal Hafit structure at the base of the foreland basin sequence. The structure is bounded by high‐angle listric reverse faults that cut downward through the Mesozoic shelf carbonates and are interpreted to be related to the inversion of deep structures.NNW to SSE trending positive residual gravity anomalies of up to +8 mGal occur on the Jabal Hafit and Al‐Ain anticlines. These positive gravity highs are attributed to uplifted basement structures which probably resulted from the reactivation of deep‐seated fault blocks. The residual Bouguer anomaly values decline gradually to the east and west of Jabal Hafit probably due to the deepening of the basement. A large negative gravity anomaly of &lt;−5 mGal is observed east of Jabal Hafit and coincides with a syncline which caused thickening of the Upper Cretaceous and Tertiary sequences.

Punctuated Neogene tectonics and stratigraphy of the African-Iberian plate-boundary zone: concurrent development of Betic-Rif basins (southern Spain, northern Morocco)

AbstractThis paper integrates the sequence stratigraphic and tectonic data related to the Neogene geodynamic and palaeogeographic development of the African-Iberian plate boundary zone between Spain and Morocco. Though the dating of individual tectonostratigraphic sequences and their delimiting sequence boundaries varies in accuracy and precision, their apparent correlation strongly suggests contemporaneous development of the Betic and Rif basins. This may likely be attributed to regional changes of the overall compressional intra-plate stress field. This, in turn, was governed by coeval plate-kinematic changes related to the ongoing collisional convergence of Africa and Iberia. The Neogene succession is characterised by brief tectonic pulses that governed the sequence stratigraphic development of the Betic-Rif basins (NBR phases). It broadly correlates with the coeval sequences in the compressional foreland basins and extensional rift basins in front of the collisional Alpine and Pyrenean orogens (CRF phases). These regionally correlatable basinal deformation events indicate that stress-related episodic changes at the African-Iberian plate-boundary zone resulted in an essentially direct cause-and-effect relationship between the tectonostratigraphic evolution of Neogene basins in northern African (Morocco), southern Europe (Spain) and western Europe (France, Switzerland, Germany).

An integrated analysis of an orogen-sedimentary basin pair: Latest Cretaceous-Cenozoic evolution of the linked Eastern Cordillera orogen and the Llanos foreland basin of Colombia

The integration of restored basin geometry and internal features of syntectonic units (e.g., stratal architecture, thickness, sandstone composition) with fl exural modeling of the lithosphere constrains the evolution of a basin and its fl exural history related to orogenic growth (spatial/temporal loading confi guration). Using this approach, we determined the Maastrichtian-Cenozoic polyphase growth of the Eastern Cordillera of Colombia, an inverted Mesozoic extensional basin. The record of this growth occurs in an Andean (post‐middle Miocene) thrust belt (the Eastern Cordillera) and in adjacent foreland basins, such as the Llanos Basin to the east. This approach permitted the identifi cation of fitectono-stratigraphic sequences in the foreland basin and fi ve phases of shortening for the Eastern Cordillera. Thermochronological and geochronological data support the spatial and temporal evolution of the orogen‐foreland basin pair. Tectono-stratigraphic sequences were identifi ed in two restored cross sections, one located at a salient and the other in a recess on the eastern fl ank of the Eastern Cordillera. The lower two sequences correspond to late Maastrichtian‐Paleocene fl exural events and record the eastward migration of both tectonic loading and depositional zero in the Llanos Basin. These sequences consist of amalgamated quartzarenites that abruptly grade upward to organic-rich fi ne-grained beds and, to the top, light-colored mudstones interbedded with litharenites in isolated channels. Amalgamated conglomeratic quartzose sandstones of the third sequence record ~15 m.y. of slow subsidence in the Llanos Basin and Llanos foothills during early to middle Eocene time, while shortening was taking place farther west in the Magdalena Valley. The fourth sequence, of late Eocene‐ middle Miocene age, records a new episode of eastward migration of tectonic loads and depositional zero in the Llanos Basin. This sequence begins with deposition of thick fi ne-grained strata to the west, whereas to the east, in the Llanos basin, amalgamated quartzarenites unconformably overlie Cretaceous and older rocks (former forebulge). Apatite fi ssion tracks in the axial zone of the Eastern Cordillera, growth strata in the

The Neoproterozoic Ottfjallet dike swarm of the Middle Allochthon, traced geochemically into the Scandian Hinterland, Western Gneiss Region, Norway

The Ottfjallet swarm of Late Neoproterozoic mafic dikes, cutting Neoproterozoic sandstones, became a key, 30 years ago, to the tectonics of the Scandian Caledonides. The sandstones were deposited in basins related to opening of lapetus, and intruded by dikes in distal parts of the Baltoscandian margin close to the developing spreading axis. The sandstones and underlying basement rocks were transported, from west of the present Norwegian coast, to as far east as western Sweden during the Silurian-Devonian Scandian Orogeny. The sandstones with dikes make up the Sarv Nappe, up to 2 km thick in Sweden, and the quartzites and amphibolites of the Saetra and equivalent nappes in Norway. These form the upper part of the Middle Allochthon. The lower part of the Middle Allochthon includes Middle Proterozoic basement gneisses and rapakivi granites, containing mafic rocks in some places. The dike-bearing quartzite is a key unit due to contrast with similar rocks lacking dikes at lower tectonic levels derived from inboard parts of Baltica. The Saetra Nappe and equivalents are well constrained on lithotectonic grounds in Norway at Oppdal, Leksdal, and Orkanger. It was also suspected to occur in deep, narrow synclines in the Western Gneiss Region where interlayered feldspathic quartzite and amphibolite are in correct tectonostratigraphic sequence, locally with a total thickness of only 1 to 3 m, and locally with dikes converted to eclogite. To test correlations, 127 samples of mafic rocks were collected from 14 areas west and southwest of Trondheimsfjord into the Western Gneiss region toward Alesund. Samples include mafic rocks in quartzites and others, some clearly dikes, from the underlying 1190 Ma rapakivi granite/augen gneiss of the Risberget Nappe and adjacent basement gneisses. Typical Saetra dikes in the Oppdal quarries, and mafic rocks from other quartzites and related rocks, have La n /Sm n ratios of 1.0 to 1.8, and Nb/La ratios of 0.8 to 1.4 (Oppdal group). Most REE patterns are moderately LREE-enriched with no or very small, mostly negative, Eu anomalies. All have similar multi-element patterns typically with small positive P anomalies, negative Zr-Hf anomalies and an absence of Nb-Ta anomalies, showing that the dikes are unrelated to arcs and have no notable continental crust component. A subset of Saetra, Risberget, and basement dikes is distinguished by higher La n /Sm n ratios of 1.8 to 2.5, but otherwise has very similar characteristics (Ystland Group). These data support correlation of the Saetra Nappe quartzite and dikes into highly deformed parts of the Western Gneiss Region and correlation of nearby dike-rich parts of basement gneiss with the Middle Allochthon. One sample in quartzite at Ura, at an unusual tectonostratigraphic position, has La n /Sm n = 0.7, Nb/La = 0.6, and a multi-element pattern different from Saetra dikes, suggesting it is unrelated to the Ottfjallet dikes. Non-Saetra-like amphibolites also occur in the Risberget Nappe, and have La n /Sm n ratios of 1.4 to 3, all Nb/La ratios <0.6, and multi-element patterns with sharp negative anomalies for Nb-Ta, P, Zr-Hf and Ti. These are probably Mesoproterozoic magmas and cumulates emplaced into the rapakivi granite protolith of the augen gneiss.

Kinematic sequence stratigraphy of the European Cenozoic Rift System and Alpine Foreland Basin: correlation with Mediterranean and Atlantic plate-boundary events

AbstractA review of the sequence stratigraphic development of the Tertiary basins of the North and West Alpine Foreland domains shows that their structural and depositional history was episodically affected by brief tectonic phases. These were associated with intermittent deformation events induced by the collisional convergence and compressional coupling of the Apulian and Iberian microplates with the European Plate. The plate kinematics-related episodicity was essentially isochronously recorded in the basin fills of the Alpine Foreland region. These are generally correlative with changes in eustatic sea level. The ensuing correlative successions of so-called Cenozoic Rift and Foredeep (CRF) sequences and phases can be traced throughout the European Cenozoic Rift System and Alpine Foreland Basin. Their temporal correlation indicates that, apparently, the changes in the plate collision-related stress regime of the Alpine Foreland were repeatedly accompanied by coeval changes in eustatic sea level. To test and substantiate the validity of this inferred causal relationship between intraplate deposition, plate kinematics and eustacy, the tectono-sedimentary evolution of the basins of the Mediterranean plate-boundary zone has been analysed in conjunction with a review of the plate-boundary events in the North Atlantic. Within the uncertainty range of available datings, synchroneity could thus be demonstrated for the punctuated tectonostratigraphic development of basins of the western Mediterranean (comprising the Liguro-Provençal Basin, Valencia Trough, Sardinia Rift and Tyrrhenian Basin), the Apenninic-Calabrian Arc, the Betic domain (including the Alboran Basin) and the North and West Alpine Foreland regions. Similar temporal correlations of plate tectonicsrelated events near the Mid-Atlantic Ridge in the North Atlantic and tectonostratigraphic sequences and phases of the Alpino-Pyrenean Foreland basins are further evidence of a common causal mechanism. The driving mechanisms appear to have been the northward drift of Africa and the resulting mechanical coupling of Apulia and Iberia with the southern passive margin of Europe, as well as the stepwise opening of the North Atlantic and accompanying episodic plate re-organisations of the Mid-Atlantic Ridge.

Evidence for a major Neoproterozoic orogenic unconformity within the Dalradian Supergroup of NW Ireland

An intriguing and contradictory scenario has recently developed in the Caledonides of the British Isles concerning the ages of deposition and deformation of the Neoproterozoic Dalradian Supergroup. Isotopic evidence, although limited, suggests that the lower parts of the sequence may have been deposited and undergone Precambrian deformation (i.e. pre-600 Ma) prior to deposition of the upper parts of the sequence (i.e. post-600 Ma). Given these existing constraints, it is clear that a major break (or breaks) would be required in the intervening sequence to maintain a coherent tectonostratigraphy. We present evidence for such an unconformity preserved at the base of the Easdale Subgroup in NW Ireland. Reworked clasts that contain a pre-existing tectonometamophic history are identified within a conglomerate that lies along this regionally recognized boundary. The underlying sequence also exhibits pre-existing deformational fabrics that display erosional truncation at the base of the conglomerates. These relationships, together with significant erosion and excision of the footwall sequence, and extensive thickness and facies variations in the hanging-wall units, imply that a major tectonic unconformity exists within this succession. As the conglomerate lies stratigraphically below Precambrian ( c . 600 Ma) lavas, the structural fabrics contained within the clasts and the underlying sequence must also be of Precambrian age and totally unrelated to the well-established Early Palaeozoic (Caledonian) orogenic deformation observed in the younger parts of the sequence. The Dalradian Supergroup, as defined, may actually comprise (at least) two distinct tectonostratigraphic sequences.

The Jurassic of East Greenland: a sedimentary record of thermal subsidence, onset and culmination of rifting

The Late Palaeozoic – Mesozoic extensional basin complex of East Greenland contains a record of deposition during a period of Rhaetian – Early Bajocian thermal subsidence, the onset of rifting in the Late Bajocian, its growth during the Bathonian–Kimmeridgian, culmination of rifting in the Volgian – Early Ryazanian, and waning in the Late Ryazanian – Hauterivian. The area was centred over a palaeolatitude of about 45°N in the Rhaetian and drifted northwards to about 50°N in the Hauterivian. A major climate change from arid to humid subtropical conditions took place at the Norian–Rhaetian transition. Deposition was in addition governed by a long-term sea-level rise with highstands in the Toarcian–Aalenian, latest Callovian and Kimmeridgian, and lowstands in the latest Bajocian – earliest Bathonian, Middle Oxfordian and Volgian. The Rhaetian – Lower Bajocian succession is considered the upper part of a megasequence, termed J1, with its base in the upper Lower Triassic, whereas the Upper Bajocian – Hauterivian succession forms a complete, syn-rift megasequence, termed J2. The southern part of the basin complex in Jameson Land contains a relatively complete Rhaetian–Ryazanian succession and underwent only minor tilting during Middle Jurassic – earliest Cretaceous rifting. Rhaetian – Lower Jurassic deposits are absent north of Jameson Land and this region was fragmented into strongly tilted fault blocks during the protracted rift event. The syn-rift successions of the two areas accordingly show different long-term trends in sedimentary facies. In the southern area, the J2 syn-rift megasequence forms a symmetrical regressive–transgressive–regressive cycle, whereas the J2 megasequence in the northern area shows an asymmetrical, stepwise deepening trend. A total of eight tectonostratigraphic sequences are recognised in the Rhaetian–Hauterivian interval. They reflect major changes in basin configuration, drainage systems, sediment transport and distribution patterns, and in facies and depositional environments. The sequences are bounded by regional unconformities or flooding surfaces and have average durations in the order of 10 Ma. They are subdivided into conventional unconformity-bounded depositional sequences with durations ranging from tens of thousands of years, in the Milankovitch frequency band, up to several million years. Deposition was alluvial and lacustrine in the Rhaetian–Sinemurian, but almost exclusively marine during the Pliensbachian–Hauterivian time interval when a marine strait, up to 500 km wide and more than 2000 km long, developed between Greenland and Norway, connecting the Arctic Sea and the North Sea. Coal-bearing fluvial and paralic deposits occur, however, at the base of the onlapping Middle Jurassic succession in the central and northern part of the basin complex. The sedimentary development is similar to that in the Northern North Sea and on the Norwegian shelf, and East Greenland offers important onshore analogues for virtually all of the types of deeply buried Jurassic depositional systems of these areas and especially their hydrocarbon reservoirs.

Chapter 3 Carboniferous Basin Development

Abstract The Carboniferous basin development of northern England is illustrated in a series of regional seismic lines presented in this chapter. Each of the major syn-rift basins will be described in turn, using representative seismic lines (Fig. 10) that have been tied to well and outcrop control. In addition to the seismic data, depth converted geological interpretations for each of the seismic lines are presented to illustrate the development of these tectono-stratigraphic sequences across the province. Particular attention is paid to the Widmerpool Gulf because the combination of seismic quality, well penetrations and the presence of exposure of the syn-rift along-strike in Derbyshire allows us to discuss the tectono-stratigraphic sequences in detail. The depth converted geological interpretations are based on corrected sonic logs taken from nearby boreholes, where available. Elsewhere, and for the deeper Dinantian section on most lines, seismic stacking velocities have been applied. The errors inherent in this latter method could result in errors in the depth section of as much as ±10%. Typical velocities used in the depth conversion of the Eakring and Welton sections in the East Midlands and the Northumberland Trough (Kimbell et al. 1989) are shown in Table 1. The Widmerpool Gulf is typical of the many fault-bounded Carboniferous basins of northern England forming the southern of two main depocentres that comprise the East Midlands province (Figs 1 and 11). It is bounded to the south by the Hoton Fault, the major basin-bounding fault, and to the north by the Cinderhill Fault, a major antithetic

Chapter 4 Palaeogeography and Facies Evolution

Abstract Having discussed the broad-scale tectono-stratigraphic subdivision of the north of England Carboniferous in the previous section, we now use the megasequences and tectono-stratigraphic sequences to determine the spatial and temporal evolution of depositional systems using sequence palaeogeographies. By late Devonian times, rifting had begun in northern England with sedimentation occurring in incipient half graben under an arid climate. The remnant Caledonian mountain belt to the north acted as a major sediment source (e.g. Gilligan 1920; Leeder 1988; Gawthorpe et al. 1989) and, in the study area, Caledonian structures were reactivated and also acted as local sediment sources. The northward drift of European Pangaea during the Dinantian led to a change to humid climatic conditions by the late Dinantian (Duff, 1980). This, together with regional transgression, caused a change from red-bed style deposition to fluvio-deltaic deposition in the north of the area, close to the major sediment source, and predominantly carbonate depositional systems in the south of the area, particularly on footwall highs starved of clastic sediment. The development of highfrequency cyclicity in late Dinantian times (e.g. Walkden 1987; Leeder &amp; Strudwick 1987) signifies the growing importance of glacio-eustasy as a control on stratigraphic development; a control which became dominant in the Silesian. There is general agreement that northern Britain occupied an equatorial position during the Namurian (Scotese et al. 1979; Smith et al. 1981), and the occurrence of coal and bauxitic soil horizons in Scotland indicates a humid, tropical climate (Cope et al. 1992). The increasing rainfall, and associated increase

Geology and Geotectonic Classification of Pan-African Gold Mineralizations in the Red Sea Hills, Sudan

Hundreds of ancient gold mines and prospects are known in the Red Sea Hills of the Sudan, and many are still of economic importance. Most of the mines are part of the Pan-African tectonostratigraphic sequence. The genesis of the gold expresses three geotectonic events that have left their marks on the style of the mineralization. The first type (I) is syngenetic with volcanogenic massive sulfide deposits that are associated with calc-alkaline, island-arc, acidic volcanism, where gold was deposited some 715 Ma ago. The second type (II) is related to a major tectonic-metamorphic episode during collisional orogeny, which caused the remobilization of gold and its precipitation in silica-barite and massive barite formations, possibly around 650 Ma. The third type (III) takes the form of auriferous quartz veins along first- and second-order shear zones related to postcollisional movement and retromorphism, at ∼550 Ma. Gold was remobilized from volcano-magmatic sequences to be precipitated along structurally more favorable sites.

Reservoir biostratigraphy of the Pedernales Field, Venezuela

Abstract The geographic and geological settings of the Pedernales Field, and its history from discovery to development, are briefly outlined. Biostratigraphy and palaeoenvironmental interpretation, and their impact on integrated reservoir description (IRD) and production geology, are discussed in detail. Historical and modern vintage micropalaeontological data enable the recognition of a number of ‘zones’ over the reservoir interval. These ‘zones’ are diachronous, although they do have correlative value when integrated into the framework of isochronous tectonostratigraphic sequence boundaries and flooding surfaces identified on seismic sections and well logs. The ‘zones’ and their containing sediments can be tentatively calibrated against global standard biozones and hence sequence boundaries and flooding surfaces on the eustatic sea-level cycle chart. Micropalaeontological and palynological data also enable the recognition of a number of biofacies interpreted on the basis of modern analogy as indicating a range of delta-top, delta-front and prodelta environments. The principal reservoir is interpreted as having been deposited in a range of delta-top to delta-front subenvironments, the seal in a range of distal prodelta subenvironments. The ongoing high-resolution biostratigraphic work programme has revealed the existence of a large number of correlatable events over the reservoir interval. Integrated high-resolution biostratigraphic, well log and seismic correlation can be achieved both within and between the compartments of the field. High-resolution biostratigraphic/biofacies data demonstrate the existence of a bathymetric gradient at the time the reservoir were being deposited. Gravity gliding along the observed gradient provides a possible mechanism for the locally observed section repetition and enhanced reservoir development, and also for the up-dip trapping mechanism.

A tectonostratigraphic framework for the Mid-Norway region

The tectonostratigraphic analysis presented here aims to establish a broad stratigraphic framework within which the likely distribution of both reservoir and source rock intervals within the deep-water Vøering and More basins can be evaluated. In terms of hydrocarbon potential, the Devonian to Triassic interval can be effectively discounted by its extreme depth of burial across most of the study area. The Middle Jurassic is interpreted to provide a potential reservoir target in the Gjallar Ridge area. The Cretaceous is seismically mappable across the region and has been divided into five tectonostratigraphic sequences. Late Cretaceous reservoirs are considered to offer targets in the Nyk High area. Six tectonostratigraphic sequences are described within the Cenozoic succession. Paleocene reservoirs are interpreted to offer targets over both the Ytterskallen and Ormen Lange areas of the Møre Basin. Source rock maturity is critical for exploration success. Mapping shows the Late Jurassic/earliest Cretaceous Spekk Formation to be overmature over most of the area, though the presence of possible DHIs indicates the probable presence of major gas accumulations in the region. Only on the western flank of the Vigrid Syncline does the Spekk shallow sufficiently to lie within the potential oil window. It is concluded that the region has the potential to become a significant new hydrocarbon province for the 21st century.

STRATIGRAPHIC RECORD OF THE EARLY MESOZOIC BREAKUP OF PANGEA IN THE LAURASIA-GONDWANA RIFT SYSTEM

▪ Abstract Rift basins of the Central Atlantic Margins (CAM) of North America and Morocco preserve largely continental sequences of sedimentary strata and less important minor basalt flows spanning much of the early Mesozoic. The best known is the Newark basin of New Jersey, New York, and Pennsylvania where an astronomically calibrated magnetic polarity time scale is developed. Lacustrine cycles of Milankovitch origin are commonly present in CAM basins, with the period changing from 10 ky (paleoequator with coals), to 20 ky (4°–10°N), to perhaps 40 ky northward with evaporites. Cycles of ∼100 ky, 413 ky, and ∼2 my are also important. Four mostly unconformity-bounded tectonostratigraphic sequences are present. The Anisian TS I is fluvial and eolian. TS II–TS IV (Late Triassic to Early Jurassic) consist of “tripartite” lacustrine sequences caused by extension pulses. The Newark basin accumulation rate history allows comparison with quantitative rift basin models. The North American plate's slow northward drift resulted in a relative shift of climate, although the rapid humidification during the latest Triassic and Early Jurassic is associated with a sea-level rise. The Triassic-Jurassic mass extinction is of independent origin, plausibly impact related.