Early Cretaceous Rifting Research Articles

Late Jurassic sedimentation and deformation in the west Iberia continental margin: insights from FMS data, ODP Leg 173

In the Iberia Abyssal Plain, Ocean Drilling Program (ODP) Site 1065 drilled the top of a continental tilted block, penetrating a Middle to Late Jurassic probable pre-rift sedimentary unit. Borehole images of this unit were obtained by the Formation MicroScanner (FMS) logging tool, thus allowing the identification and 3D measurement of two block tilting phases. The last one is an eastward 15° tilt, probably related to the main Early Cretaceous rifting phase, and consistent with the tilted block image on seismic lines. The first tilt, previously unknown, is Late Jurassic in age. The block tilting is 15° to the southeast, and is not associated with any noticeable change in the depositional environment. Both tilting episodes induced sedimentary instabilities, leading to listric faults and slumps. FMS images also showed that the sediments originated from the north (Galicia Bank) during Late Jurassic times.

Cenozoic crustal thickening, wrenching and rifting in the foothills of the southernmost Andes

The southernmost Andes form an orocline, between the Patagonian cordillera, trending N–S, and the Fueguian cordilleras, trending E–W. On the foreland side is the Magellan Basin. The area has a history of Paleozoic compression, Triassic to Early Cretaceous rifting and Late Cretaceous to Quaternary compression, in response to changing plate tectonics. Major structures of Late Cretaceous and Cenozoic age vary along the strike. In the Patagonian cordillera and foothills, folds and thrusts trend NNW, slightly oblique to the orogen, whereas strike–slip faults are parallel to the orogen and right-lateral. In the Fueguian cordilleras and foothills, folds and thrusts trend ESE, slightly oblique to the orogen, whereas strike–slip faults are parallel to the orogen and left-lateral. In the axial zone of the Magellan Basin, folds and thrusts are parallel to the orogen and rifts are sub-perpendicular to it. To a first approximation, the pattern of structures has mirror symmetry about the axis of the Magellan Basin. In detail, however, wrenching appears to be more prevalent in the Fueguian cordillera and foothills, than it is in the Patagonian cordillera and foothills. Minor faults of Cenozoic age are common in the foothills. From a kinematic analysis of fault–slip data: (1) shortening and stretching directions are mostly sub-horizontal; (2) shortening directions vary in trend, from ENE in the Patagonian foothills, to NE in the Fueguian foothills; and (3) stretching directions are sub-parallel to traces of major thrusts. In the Fueguian cordillera and foothills, strike–slip faulting is prevalent; in the Patagonian foothills, crustal thickening is prevalent over strike–slip faulting. The kinematics reflect a combination of thrusting and wrenching and they are consistent with the major structures. To investigate the origin of the Cenozoic structures, we used analogue models on a fully lithospheric scale, where an oceanic plate subducted beneath a continental corner. The corner was an area of transition, from frontal subduction, to transcurrent motion. The boundary conditions may not have been fully realistic, but the experiments did account for the major elements of the structural pattern in southernmost South America, including rifts that are perpendicular to the orogen and counterclockwise block rotations.

Role of extensional structures on the location of folds and thrusts during tectonic inversion (northern Iberian Chain, Spain)

The Aguilón Subbasin (NE Spain) was originated during the Late Jurassic–Early Cretaceous rifting due to the action of large normal faults, probably inherited from Late Variscan fracturing. WNW–ESE normal faults limit two major troughs filled by continental deposits (Valanginian to Early Barremian). NE–SW faults control the location of subsidiary depocenters within these troughs. These basins were weakly inverted during the Tertiary with folds and thrusts striking E–W to WNW–ESE involving the Mesozoic–Tertiary cover with a maximum estimated shortening of about 12 %. Tertiary compression did not produce the total inversion of the Mesozoic basin but extensional structures are responsible for the location of major Tertiary folds. Shortening of the cover during the Tertiary involved both reactivation of some normal faults and development of folds and thrusts nucleated on basement extensional steps. The inversion style depends mainly on the occurrence and geometry of normal faults limiting the basin. Steep normal faults were not reactivated but acted as buttresses to the cover translation. Around these faults, affecting both basement and cover, folds and thrusts were nucleated due to the stress rise in front of major faults. Within the cover, the buttressing against normal faults consists of folding and faulting implying little shortening without development of cleavage or other evidence of internal deformation. © Elsevier, Paris.

Comparative Jurassic and Cretaceous tectono-stratigraphy and reservoir development in the Jeanne d’Arc and Porcupine basins

The Jeanne d’Arc Basin offshore Newfoundland and the Porcupine Basin to the west of Ireland, developed under similar tectono-sedimentary conditions in pre-Late Cretaceous times. Rifting events in Permo-Triassic, Late Jurassic–Early Cretaceous and mid Cretaceous times were each followed by a phase of thermal subsidence. While a broadly common lithofacies pattern occurs within and between the basins, the detailed response of the basins differs in terms of style of structuring, reservoir architecture and quality of source rock development. The Late Jurassic–Early Cretaceous (Tithonian to Berriasian) rift episode is the most significant, with good reservoir sandstones and rich source rocks forming part of this rift sequence. A Late Oxfordian to Kimmeridgian onset warp phase resulted in the development of shallow marine limestones and deeper water organic-rich shales and marls in the Jeanne d’Arc Basin, with non-marine, shoreface and restricted marine sandstones and shales in the Porcupine Basin. The Late Jurassic–Early Cretaceous syn-rift episode resulted in the development of sandstone-prone successions which interfinger with good oil-prone source rocks in both basins. Reservoir architecture in both basins was influenced by this episode. Within the Porcupine Basin a range of lithofacies developed, ranging from fluvial channels and bars, to submarine fan turbidites. Within the Jeanne d’Arc Basin, periodic rejuvenation of fault footwalls produced conglomeratic pulses resulting in transverse alluvial fans which interfinger with axially-flowing braid channels and bars within the incised valley fills. Periods of increased subsidence resulted in the development of lacustrine facies and flood plain mudstones.

Hydrocarbon migration history, West of Shetland: integrated fluid inclusion and fission track studies

In sandstone reservoir rocks in the vicinity of the Rona Ridge, West of Shetland, there is evidence for former episodes of hydrocarbon migration in addition to the episodes responsible for current accumulations. An integrated study of fluid inclusion microthermometry and apatite fission track analysis (AFTA) has helped to clarify the history of fluid migration in the region. The Upper Jurassic Rona Sandstone contains both hydrocarbon and aqueous fluid inclusions. Hydrocarbons occur in trails of secondary inclusions through detrital grains, and as primary inclusions in quartz overgrowths and authigenic feldspars. This indicates at least 2 phases of migration before the presently trapped oil entered the rock: fluorescence colours indicate that they are of distinct compositions. Associated aqueous inclusions yield minimum trapping temperatures which indicate that the earliest hydrocarbons were associated with a phase of hot fluid migration, and that cooler conditions pertained during later migration. AFTA data provide independent evidence for a passage of hot fluid early in the history of the rock, which may be related to Late Jurassic–Early Cretaceous rifting. AFTA data also suggest a second episode of heating and cooling in the Tertiary following uplift, which may have been responsible for a later stage of hydrocarbon generation and migration. The results reflect repeated expulsion of fluids from deep basinal areas onto adjacent topographic highs, throughout the late Mesozoic to Recent.

The Hibernia Oilfield – effects of episodic tectonism on structural character and reservoir compartmentalization

The giant Hibernia Oilfield in the Jeanne d’Arc Basin, the first hydrocarbon development on the Grand Banks of Newfoundland, began production from an ice-resistant, concrete gravity base platform in November 1997. An estimated 615 × 10 6 BBL recoverable oil reserves are trapped in a highly faulted and south-plunging anticline. The tectonic stresses experienced on the western North Atlantic margin have largely controlled the density and size distribution of faults which limit and partition the Hibernia structure. More than 30 fault blocks have been identified at one major oil and gas reservoir alone, the fluvial–deltaic Hibernia Formation. Interpretations of relationships between the times of tectonism and changing stress directions with fault ages, orientations, and styles are based on comparison of a 1991 vintage 3D seismic survey along with data from one discovery, nine delineation and two development wells. There appear to have been three main episodes of extension that affected the oilfield. The first extensional episode spanned the Late Triassic into the Early Jurassic. Adjacent to the Hibernia Oilfield, initial rifting resulted in growth of NE-trending, en echelon faults that were subsequently amalgamated into the major basin-bounding Murre Fault. Deposition of the Hibernia Formation sandstones occurred during the second tectonic episode that spanned Late Jurassic into Early Cretaceous time. Faults of this vintage in the Hibernia Oilfield are normal and mostly formed synthetic to the re-activated Murre Fault in response to E–W oriented extension. The most marked fault growth occurred during the third tectonic episode. This late Early Cretaceous rift episode was co-eval with deposition of another major oil reservoir (the marginal to shallow marine ‘Avalon’ formation) which is targeted for development. Numerous normal faults formed orthogonal to the basin margin, thereby transecting and rotating the Hibernia structure. Fault development during the latest rift episode also responded to basin margin effects as a result of reactivation of the Murre Fault as a zone of oblique-slip. For example, the character of the large Rankin and Nautilus faults changes from relatively planar to distinctly listric moving from east to west. Additionally, sinistral deflections in the trace of the Murre Fault corresponding to linkages of the en echelon offsets formed during the first rift episode are interpreted to have controlled the location of the largest northeast dipping faults formed during the third rift episode. The largest faults formed during the third rift episode clearly compartmentalize the Hibernia reservoir while the sealing capacity of faults formed during deposition of the Hibernia sandstones remains unquantified.

Cretaceous to Cenozoic thermal evolution of the southwestern South Carpathians: evidence from fission-track thermochronology

The southwestern South Carpathian orogen is composed of various nappe complexes which were assembled during the Cretaceous–Cenozoic orogeny. These are from footwall to hangingwall: (1) the Danubian nappe complex including a Cadomian/Variscan basement; (2) the Arjana and Severin units with Jurassic to Early Cretaceous rift and oceanic sequences; and (3) the Getic nappe complex with Variscan continental basement. Fission track (FT) thermochronology on apatite, zircon and sphene from samples collected from various units of the South Carpathians, in conjunction with field constraints and previous geochronology is used to characterise the Alpine tectonic events and to restore the pattern and amount of exhumation since the Cretaceous. Zircon from the flysch unit and the Danubian Liassic cover sequence yields FT ages around 200 Ma suggesting cooling of the rift flanks prior to the opening of the Severin rift. Zircon and sphene from the Getic and Danubian basement units yield FT ages averaging 110 Ma and indicating cooling under 240°C of the basement contemporaneous with, or postdating thrusting. Apatite FT ages display a decreasing age trend from the hangingwall (65 Ma) to the footwall units (30 Ma). The age data and corresponding horizontal confined track length distributions suggest that exhumation of the nappe pile occurred in two stages: the first is related to the Late Cretaceous nappe stacking and the second one to the final thrusting of the South Carpathians onto the top of the Moesian platform. Apatite FT ages along major brittle wrench faults indicate reheating above ca. 120°C during fluid flow associated with fault (re)activation during Oligocene and Neogene times. Thus, shear zone rocks experienced a higher temperature overprint during Cenozoic time than rocks of the unaffected nappe pile. Temperatures of hydrothermal flow along these zones decreased below 100°C progressively starting with the Late Oligocene–Early Miocene when the area began to override the Moesian platform.

Continental breakup by detachment faulting: field evidence and geochronological constraints (Tasna nappe, Switzerland)

The Lower Tasna Detachment (LTD) is a low‐angle fault contact between serpentinized peridotite below and continental basement above. It was formed during Jurassic to Early Cretaceous rifting of a Tethyan continental margin and later captured in a thrust nappe during Tertiary plate convergence. Foliated gabbro, gabbro mylonite, and granitoid mylonite occurring along the LTD record shearing under decreasing temperatures. U–Pb dating of zircon from the gabbro mylonite yielded a Permian age, interpreted as the age of gabbro intrusion, whereas the breakup of the passive margin occurred as late as Early Cretaceous. This suggests that the gabbro belongs to a prerift, lower to middle crustal intrusion ‘smeared out’ along the detachment by extensional faulting. The juxtaposition of mantle and upper crust along the Lower Tasna detachment may serve as a model for several seismic reflectors observed in distal passive continental margins (e.g. S reflector of the Galicia margin).

Abstract: A Three-phase Early Cretaceous Rift History of the South Atlantic Salt Basins and its Influence on Lacustrine Source Facies Distribution

Abstract: A Three-phase Early Cretaceous Rift History of the South Atlantic Salt Basins and its Influence on Lacustrine Source Facies Distribution

Abstract: Early Cretaceous Rift Source Rocks from Eastern Brazilian Margin: Isotopic and Molecular Correlation

Abstract: Early Cretaceous Rift Source Rocks from Eastern Brazilian Margin: Isotopic and Molecular Correlation

A tectonic model for the Norwegian passive margin with implications for the NE Atlantic: Early Cretaceous to break-up

A sub-regional tectonic study of the mid-Norwegian margin has been assimilated into a broader, regional study of the NE Atlantic margin. The NE Atlantic margin experienced several extensional events between Devonian post-orogenic collapse and break-up in Early Tertiary. This paper focuses on the extensional events between Early Cretaceous and Early Tertiary. It is suggested that NE-trending Early Cretaceous rifting followed the entire margin from the southern Rockall Trough to the Bjørnøya Basin of the SW Barents Sea. This rift had a markedly different trend than the preceding N-trending Late Jurassic rift of, e.g., the North Sea. Seamounts may have been emplaced in the highly thinned basin axes during Early Cretaceous rifting. Subsequent rift events followed the Early Cretaceous rift trend. An enigmatic 'mid'-Cretaceous event, probably extensional, is observed in the northern part of the mid-Norwegian margin. The event is expressed by faulting in the inner Vøring Basin, and by uplift of the outer Vøring and Lofoten margins. Uplift is associated with a widespread unconformity. Uplift of the Gjallar Ridge in the Vøring Basin culminated during the latest Cretaceous to Early Tertiary rifting, with structural geometries broadly similar to those seen in core complexes. A new tectonic model for NE Atlantic margin is proposed based on these observations, and builds on analogues from the modern rift system in the Red Sea and Woodlark Basin of Papua New Guinea.

South Atlantic volcanic margins

The Early Cretaceous South Atlantic continental break-up and initial sea-floor spreading were accompanied by large-scale, transient volcanism emplacing the Paraná-Etendeka continental flood basalts and voluminous extrusive constructions on the conjugate margins south of the Torres Arch–Abutment Plateau. On the North Namibia margin we interpret four main tectono-magmatic crustal units: (1) oceanic crust; (2) thickened oceanic crust covered by huge seaward-dipping wedges; (3) a c. 150 km wide break-up related rift zone partly covered by the dipping wedges; and (4) thicker continental crust, partly deformed by Palaeozoic extension, east of the Early Cretaceous rift. Similar settings also characterize other South Atlantic margin segments. We infer an up to 300 km wide and 2400 km long rift zone representing lithospheric extension leading to breakup and formation of the South Atlantic volcanic margins. Comparison with other volcanic margins demonstrates, in spite of local and regional differences, gross similarities in tectono-magmatic style, crustal units and dimensions.

Exhumed Hydrocarbon Traps in East Greenland: Analogs for the Lower-Middle Jurassic Play of Northwest Europe

Four exhumed hydrocarbon traps crop out in the Traill O region of East Greenland, each at the footwall crest of a fault-block formed during Early Cretaceous rifting. Former oil accumulations are indicated by a pore fill or pore lining of solid bitumen within the Jurassic sandstone-dominated Vardekloft and Olympen formations. The Vardekloft Formation is divided into an undated fluvial-dominated lower unit (0-520 m) and a Bajocian-Callovian upper unit (65-1020 m) deposited in a shallow-marine environment. The Oxfordian Olympen Formation (0-250 m) contains shallow-marine and fluviodeltaic deposits. The sandstones are dominantly quartzarenites, and petrographic fabrics, such as dissolved feldspar, late quartz cement, and stylolites, are consistent with burial depths in excess of 2.5 km. Porosities ranged from 7 to 27% (generally about 20%, about one-half of which was primary), and permeabilities ranged from 1 to 622 md, prior to the formation of solid bitumen. The distribution of solid bitumen in each trap can be mapped out, allowing sealing elements and original oil-water contacts to be defined. Three of the four exhumed traps (Mols Bjerge, Laplace Bjerg, and Bjornedal) were simple one-seal structural traps. Conformable Upper Jurassic mudstone, unconformable Albian-Cenomanian mudstone, and normal faults are the three top-sealing elements. The fourth (Svinhufvuds Bjerge) was a poly-seal trap with a combined top-seal and a low-side fault closure. Preliminary estimates of the volume of original oil in place within these structures range from 0.2-1.1 billion bbl for the Mols Bjerge trap to 5.3-11.9 billion bbl for the Bjornedal trap. These estimates are prone to large errors, due to uncertainties in estimating original trap geometry, hydrocarbon saturation, and net/gross ratio, and in the understanding of volume changes of hydrocarbon in each trap during thermal degradation of the oil. The Upper Jurassic Bernbjerg Formation is the only known potential source rock in the region, which would require a drainage distance of less than the fault-block spacing to fill the largest of the traps. Secondary hydrocarbon migration into these traps occurred between the Cenomanian (age of the youngest sealing element) and early Eocene to late Oligocene (when widespread volcanism and sill intrusion resulted in thermal degradation of the oil). Each of these structures is relatively well exposed and accessible; we believe that they will provide excellent analogs for studies of enhanced recovery from the mature Lower-Middle Jurassic oil fields of Northwest Europe.

North Sea hydrocarbon plays and their resources

Exploration in the North Sea since 1964 has involved the drilling of 2617 new field wildcat wells and resulted in 722 discoveries. These discoveries, plus the 77 accompanying onshore finds, have originally recoverable hydrocarbon resources totalling 100 billion barrels oil equivalent. There are two main families of hydrocarbon plays when grouped according to their source rocks. In the centre and north of the North Sea, oil and gas finds are associated with a late Jurassic to early Cretaceous rift system buried beneath a Cretaceous and Tertiary cover.

The Neogene structural evolution of the western margin of the Pelagian Platform, central Tunisia

The deformation front of the Atlas Mountains in central Tunisia is a structure termed the North—South Axis. To the west of Kairouan, the North—South Axis consists of NE—SW to NNE—SSW-trending folds and thrusts. A main decollement thrust in Triassic evaporites has been mapped, with secondary detachment of out-of-sequence thrusts in Tertiary rocks and backthrusts at the base of the Tertiary and in Upper Cretaceous sediments. The Upper Cretaceous and Tertiary sediments are backfolded in the hangingwalls of the backthrusts, which are in turn folded in the SE-verging anticlines. The backthrusts are roof thrusts to ‘passive roof’ duplexes and imply the extensive development of thin-skinned ‘blind’ SE-verging thrusts. A similar structural style is seen further west in the Tunisian Atlas, but major NW-verging backthrusts are not exposed. Adjacent to the Pelagian Platform, the thrust structures are affected by later strike-slip faults. The thin-skinned thrusting at the deformation front is Middle Miocene in age and was synchronous with Africa—Europe collision. The North—South Axis is therefore interpreted as the frontal structure of a collision-related thrust belt. The thrust belt is underlain by a NW—SE- to E—W-trending Middle Jurassic to Early Cretaceous rift basin with probable ‘mid’ Cretaceous to Oligocene post-rift fill. The NE—SW-trending thrust belt is detached from NW—SE- to E—W-trending syn-rift basement faults, but changes in thrust belt geometry across the syn-rift faults indicate that basement structures have compartmentalized the thrust belt. The later strike-slip faulting resulted from the reactivation of the syn-rift basement faults in the post-collisional framework of the thrust belt.

State of intraplate stress and tectonism of northeastern America since Cretaceous times, with particular emphasis on the New England-Quebec igneous province

A paleostress analysis based on inversion of fault-slip data has been conducted in the Québec Appalachians and the St. Lawrence Lowlands in order to characterize the direction and state of stress during and after the emplacement of Cretaceous Monteregian plutons. Two major events with contrasting directions of extension are recognized: (1) an early, widely distributed NE-SW-directed extension (∼ 140 Ma), and (2) a N-S-oriented extension associated with the main phase of magmatism (∼ 125 Ma). Both these extensional paleostress regimes are consistent in direction with the result of statistical analysis of Cretaceous pluton and dyke trends in southern Québec and New England. The NE-SW-directed far-field tensional stress of the first event favours the reactivation of the earlier E-W-trending Proterozoic Ottawa-Bonnechère Graben and the reorientation of the local stress field. As a result, a restricted zone of N-S-directed extension was developed synchroneously with the emplacement of the Monteregians plutons. Cretaceous intrusions of the studied area are thus interpreted as the result of magmatism along reactivated Proterozoic basement faults, rather than in terms of the North American plate moving over a hot spot. In Late Cretaceous-early Tertiary times, the extensional stress regime changed to an ENE-WSW-directed compressional stress field. This compression is characterized by strike-slip faults and represents the youngest tectonic event in the area. Directions of compressional paleostress axes compare well to the present-day maximum compressive stress in northeastern America. The stress regimes inferred in the Québec-New England igneous province can be attributed to the Early Cretaceous rifting between Labrador and Greenland. Variations of spreading rate and plate boundary conditions of North America in the Late Cretaceous-early Tertiary led to stress inversion in eastern North America and to the establishment of a durable compressional stress field that is still present today.

Inner-Shelf to Lower-Shoreface Hummocky Sandstone Bodies with Evidence for Geostrophic Influenced Combined Flow, Lower Cretaceous, West Greenland

ABSTRACT Hummocky sandstone bodies were deposited on the margin of an Early Cretaceous rift basin in an overall progradational inner-shelf to lower-shoreface unit. Excellent outcrops of this unit permitted a detailed study of sedimentary structures and paleocurrents in relation to sandstone geometries. The sandstone bodies are lensoid and 0.5-4 m thick with lateral extensions from a few meters to more than 30 m. Laterally they pinch into thin sandstone sheets that continue for at least 130 m along the entire outcrop or swell within a distance of 20-30 m to form another sandstone lens. The lower boundaries are sharp with local erosional relief up to 1 m. Hummocky cross-stratification, wave-ripple cross-lamination, and low-angle cross-stratification are closely interbedded within the sandstone. aminated mudstone beds (10-60 cm thick) separate the sandstone bodies and are generally continuous over the entire length of the outcrop. Sedimentary structures in the sandstone bodies were produced by high-energy wave-generated oscillatory currents and unidirectional geostrophic currents. Vertical stratification successions from individual storm events reflect deposition during both increasing storm-current velocity succeeded by waning-storm deposits and, more typically, waning-storm facies successions. Deposition of the unusual vertical successions was probably controlled by an episodically high sediment supply. The local presence of the hummocky sandstone bodies on a regional scale was controlled by a paleogeographic setting where the bathymetric contours locally caused geostrophic cu rents to decelerate.

Tectono-stratigraphy of the North Celtic Sea Basin

Abstract The Celtic Sea, offshore southern Ireland, is underlain by several discrete rift basins, which are part of a larger set of such basins that developed marginal to the North Atlantic within the tectonic framework of Pangaean break-up and episodic opening of the North Atlantic Ocean. This paper documents the development of depositional systems within a rift basin setting by analysis of the sequence stratigraphic evolution of the North Celtic Sea Basin, the largest basin in the area, with particular emphasis on the influence of pre-existing structural fabric on that evolution. The Caledonian and Variscan orogenies each imprinted distinct structural grains on this area, and the intersection pattern of their respective tectonic trends provided the structural framework which influenced subsequent Triassic, Late Jurassic and Early Cretaceous phases of rifting. Detailed analysis of an extensive well and seismic database indicates that the mode of reactivation of pre-existing lines of structural weakness depended on the orientation of the principal extensional stress direction relative to these lineaments. Triassic and Late Jurassic northwest-southeast orientated extensional stress reactivated Caledonian lineaments as half-graben boundary faults and Variscan lineaments as accommodation zones and lateral transfer faults which offset the basin margin faults. In contrast, during the Early Cretaceous, extensional stress was orientated approximately north-south, oblique to the Caledonian trend. This resulted in transtensional pull-apart basin geometries dominated by reactivation of Variscan structures. The intrinsic relationship between plate tectonic setting, pre-existing structural fabric and the resulting structural style provides insight into the influence of tectonism on depositional system development in an evolving rift basin. During Late Jurassic and Early Cretaceous rifting, depositional system development was predominantly controlled by tectonism and partly conforms to empirically derived tectono-stratigraphic models of sequence development in non-marine rifts. Depositional environments changed from fluvial-dominant to lacustrine-dominant as each rifting episode evolved through its early to active rift stages. During post-rift subsidence, marine transgressions dominated depositional system development, overriding the influence of local tectonism. Spatially, pre-existing structural fabric and the style of structural deformation are the dominant influences on sediment input and drainage systems within the evolving rift basin. Recognition of the intrinsic structural fabric of a rift basin is one of the key factors in understanding and predicting stratigraphic distributions within this type of basin.

MESOZOIC TECTONICS OF THE OTWAY BASIN REGION: THE LEGACY OF GONDWANA AND THE ACTVE PACIFIC MARGIN—A REVIEW AND ONGOING RESEARCH

Mesozoic extension along Australia's southern margin and the evolution and architecture of the Otway Basin were probably controlled by three factors: 1) changes in global plate movements driven by mantle processes; 2) the structural grain of Palaeozoic basement; and, 3) changes in subduction along Gondwana's Pacific margin. Major plate realignments controlled the Jurassic onset of rifting, the mid-Cretaceous break-up and the Eocene onset of rapid spreading in the Southern Ocean.The initial southern margin rift site was influenced by the northern limit of Pacific margin (extensional) Jurassic dolerites and the rifting may have terminated dolerite emplacement. Changed conditions of Pacific margin subduction (e.g. ridge subduction) in the Aptian may have placed the Australia-Antarctic plates into minor compression, abating Neocomian southern margin rifting. It also produced vast amounts of volcanolithic sediment from the Pacific margin arc that was funnelled down the rift graben, causing additional regional subsidence due to loading. Albian orogenic collapse of the Pacific margin, related to collision with the Phoenix Plate, influenced mid-Cretaceous breakup propagating south of Tasmania and into the Tasman Sea.Major offsets of the spreading axis during breakup, at the Tasman and Spencer Fracture zones, were most likely controlled by the location of Palaeozoic terrane boundaries. The Tasman Fracture System was reactivated during break-up, with considerable uplift and denudation of the Bass failed rift to the east, which controlled Otway Basin facies distribution. Palaeozoic structures also had a significant effect in determining the half graben orientations within a general N-S extensional regime during early Cretaceous rifting. The late Cretaceous second stage of rifting, seaward of the Tartwaup, Timboon and Sorell fault zones, left a stable failed rift margin to the north, but the attenuated lithosphere of the Otway-Sorell microplate to the south records repeated extension that led to continental separation and may be part of an Antarctic upper plate.

Reservoir architecture of syn-rift lacustrine turbidite systems, early Cretaceous, offshore South Gabon

Abstract The Lucina Formation of offshore South Gabon comprises deep lacustrine facies deposited in an early Cretaceous rift lake. Both channellized and non-channellized turbidite sandbodies occur enclosed in deep-lacustrine mudstones. These sandbodies provide reservoirs in the Lucina and Lucina West Marine fields. Dramatic changes in sandstone body thickness occur from SW to NE across the strike of the rift lake. This was a consequence of the confinement of high-density sandy turbidity currents in fault-related lake-floor depressions. Limited palaeoccurrent data from orientated core indicate sediment transport from the SW towards the NE. This represents supply from the ramp margin of the lake towards the fault system bounding the lake along its northeastern margin. The modern Lake Malawi provides an analogue in terms of genetic elements and sediment distribution patterns. In addition, ancient marine turbidite systems such as those of the Oligo-Miocene fill of the Tertiary Piedmont Basin, Northwest Italy, provide insights into the relationships between topography in the receiving basin and sandbody geometries.