Offshore Seismic Data Research Articles

Sedimentation and volcanism linked to multiphase rifting in an Oligo-Miocene intra-arc basin, Anglona, Sardinia

Three extensional phases can be recognized in the northern, Anglona area of the Oligo-Miocene Sardinian Rift during a fifteen million year period which spanned Corsica–Sardinia continental microplate separation and Western Mediterranean back-arc basin opening. In response to this multiphase rifting, a complex facies architecture involving clastic, carbonate and volcanic rocks developed. Integrated onshore facies and structural analysis, dating and offshore seismic data are here used to reconstruct the tectono-stratigraphic history of the Anglona area. Initial late Oligocene extension created a half-graben geometry with syn-rift clastic deposits shed locally from fault-bounded highs, passing laterally to lacustrine marlstones. Calc-alkaline volcanic activity subsequently predominated as volcanic centres developed along one half-graben bounding fault. Voluminous pyroclastic and epiclastic material was supplied to the adjacent half-graben accommodation space and was deposited in marginal to marine conditions. Second-phase mid-Aquitanian–early Burdigalian extensional faulting, recognized from localized clastic syn-rift stratal wedges, truncated and subdivided the half-graben. The syn-rift sediments were sealed by a regionally correlated ignimbrite that in turn was offset by late second-phase faulting. Third-phase extensional fault movement which reactivated the original fault trend then occurred. A perched lake developed in the resultant topography coeval with the progressive marine transgression of lower areas. As sea-level rose during mid-Burdigalian times, reefal carbonates and grainstones developed on fault-block highs whilst calcarenites and marlstones were deposited in hangingwall locations. Initial extension was coeval with the formation of the Sardinian proto-rift and the initiation of the Western Mediterranean basin. Second-phase faulting occurred as the Corsica–Sardinia microplate rotated to its present position during Western Mediterranean back-arc basin spreading. Final extension can be correlated to a second major extension phase along the Oligo-Miocene Sardinian Rift following back-arc basin opening, as extension was transferred towards the fore-arc. In Anglona, the main influence of multiphase tectonism was on rift topography, providing accommodation space and localized uplifted source areas. Varying relative sea-level mainly controlled the broad types of facies belts that developed. Contemporaneous calc-alkaline volcanism played a major role in the supply of basin filling material and in changing the topography locally.

PALYNOLOGICAL AND GEOCHEMICAL ANALYSIS OF CARBONIFEROUS BOREHOLE AND OUTCROP SAMPLES FROM THE ISLE OF MAN

Palynological and geochemical analyses were performed on Namurian (mid‐Carboniferous) samples from the Ballavaarkish and Shellag Point boreholes from the north of the Isle of Man, and on Dinantian/Namurian outcrop samples from Black Marble Quarry in the south of the island.The boreholes yielded a rich and diverse (over 60 species) Namurian B‐C (Kinderscoutian‐Yeadonian) palynomorph assemblage equivalent to the KV/FR Zones of Clayton et al. (1977). Reworked Brigantian‐Namurian A palynomorphs of the NC/TK Zones were recorded in the Shellag Point borehole. Offshore seismic data indicates a period of major fault activity in the early Namurian which may support this concept of early Namurian reworking. Amorphous organic matter indicating anoxic/dysoxic depositional conditions together with prasinophyte algae and goniatites suggesting a marine environment were recorded in the Shellag Point borehole.Vitrinite reflectance and spore colouration measurements coupled with Tmax data from Rock‐Eval pyrolysis indicate that the Namurian shales from the Shellag Point borehole are currently in the oil window. Rock‐Eval data and kerogen analyses indicate that these shales represent moderate to good gas‐prone source rocks which have yet to attain full maturity. The Namurian samples from Ballavaarkish are mainly in the oil window or late mature zone and have poor gas potential. Extract chromatography indicates a relatively mature product and the possible onset of hydrocarbon biodegradation, particularly in the Ballavaarkish borehole samples.Outcrop samples from near the top of the succession at Black Marble Quarry underwent thermal alteration as a result of dyke intrusion and subsequent volcanism. Although previously believed to be Brigantian (end‐Dinantian) in age based on goniatites, corals and conodonts, the samples have produced a sparse palynological assemblage indicating an apparent age no older than Namurian. The samples are late‐ to post‐mature in terms of hydrocarbon generation and have no remaining source potential.The results indicate that where Namurian sediments are preserved beneath the base‐Permian unconformity, for example in the Solway Basin, hydrocarbons are likely to have been generated.

Maximum extent of the Eurasian ice sheets in the Barents and Kara Sea region during the Weichselian

Based on field investigations in northern Russia and interpretation of offshore seismic data, we have made a preliminary reconstruction of the maximum ice‐sheet extent in the Barents and Kara Sea region during the Early/Middle Weichselian and the Late Weichselian. Our investigations indicate that the Barents and Kara ice sheets attained their maximum Weichselian positions in northern Russia prior to 50 000 yr BP, whereas the northeastern flank of the Scandinavian Ice Sheet advanced to a maximum position shortly after 17000 calendar years ago. During the Late Weichselian (25 000–10000 yr BP), much of the Russian Arctic remained ice‐free. According to our reconstruction, the extent of the ice sheets in the Barents and Kara Sea region during the Late Weichselian glacial maximum was less than half that of the maximum model which, up to now, has been widely used as a boundary condition for testing and refining General Circulation Models (GCMs). Preliminary numerical‐modelling experiments predict Late Weichselian ice sheets which are larger than the ice extent implied for the Kara Sea region from dated geological evidence, suggesting very low precipitation.

Size and morphology of the Chicxulub impact crater

The Chicxulub impact in Mexico has been linked to the mass extinction of species at the end of the Cretaceous period. From seismic data collected across the offshore portion of the impact crater, the diameter of the transient cavity is determined to be about 100 km. This parameter is critical for constraining impact-related effects on the Cretaceous environment, with previous estimates of the cavity diameter spanning an order of magnitude in impact energy. The offshore seismic data indicate that the Chicxulub crater has a multi-ring basin morphology, similar to large impact structures observed on other planets, such as Venus.

Neogene deformation of the back-arc shelf of Southwest Japan and its impact on the palaeoenvironments of the Japan Sea

The evolutionary process of the back-arc basins of Southwest Japan is presented on the basis of geological and geophysical data-sets. Surface geology and borehole information demonstrate rapid subsidence throughout the back-arc margin in the Early Miocene as a result of continental rifting. Uplift in the Middle Miocene and extensive truncation in the Late Miocene on the back-arc margin tend to become more intense eastward and westward, respectively. Palaeomagnetic studies show that the Middle Miocene uplift is linked to collision of the Izu-Bonin Arc with the eastern part of Southwest Japan and confined deformation of the island arc. Offshore seismic data illustrate that the Late Miocene unconformity event was caused by intra-arc folding under N-S compression. Coeval resumption of subduction of the Philippine Sea Plate is the probable cause of this episode. Seismic profiles also indicate that the folds converge into the Tsushima Strait. The crustal contraction brought about closure of the Tsushima Strait at the western end of Southwest Japan, blocking the warm Tsushima current, and so impacting strongly on the Neogene palaeoenvironments of the Japan Sea.

Absolute dating of brittle fault movements: Late Permian and late Jurassic extensional fault breccias in western Norway

40Ar/39Ar geochronological and palaeomagnetic dating methods applied to fault breccias in western Norway have isolated two brittle reactivation episodes of the syn‐post‐Caledonian, extensional Nordfjord‐Sogn Detachment. These events, of latest Permian and latest Jurassic–Early Cretaceous ages, demonstrate temporal relationships between development of chemical remanent magnetism and partial resetting of Ar isotopic systems during distinct breccia‐forming episodes. A third event of Carboniferous age was also identified in the breccias with the 40Ar/39Ar technique and is a relict unroofing signature inherited from the fault wall‐rocks. These brittle faults are significant time markers and become relevant to interpretations of offshore seismic data which attempt to place ages on faults that have undergone multiple reactivation episodes.

Interaction of Basement-Involved and Thin-Skinned Tectonism in the Tertiary Fold-Thrust Belt of Central Spitsbergen, Svalbard

The Tertiary fold-thrust belt in Oscar II Land, central Spitsbergen, consists of three major zones of distinct structural style: (1) a western basement-involved fold-thrust complex, (2) a central zone of thin-skinned fold-thrust units above a decollement in Permian evaporites, and (3) an eastern zone characterized by a frontal duplex system in the fold-thrust belt, bounded eastward by steep, basement-rooted reverse faults (Billefjorden and Lomfjorden fault zones) beneath subhorizontal platform strata. Offshore seismic data from Isfjorden (Statoil) confirm the threefold zonation and document thick-skinned and thin-skinned structural interactions in both the fold-thrust belt and the foreland section. An admissible cross section yields about 45%, or 20 km, of shortening in Oscar II Land. Deeper parts of the seismic profiles show fault-bounded Devonian (central and east) and Carboniferous (west) basins. The structural grain of the Tertiary fold-thrust belt partly coincides with the margin-bounding normal faults of these basins, suggesting that preexisting structures and stratigraphy controlled the Tertiary fold-thrust belt development. A kinematic evolution of the fold-thrust belt is invoked: (1) north-northeast-directed, bedding-parallel shortening, (2) major west-southwest-east-northeast shortening, with in-sequence foreland fold-thrust propagation, (3) basement-involved, west-southwest-east-northeast uplift in the eastern foreland zone, (4) eastward out-of-sequence propagation of thrusts, and (5) west-east extension in the hinterland. Our regional structural compilation map and synthesis of the central Spitsbergen transect advocates structural variation and linked basement-involved thrusting in the hinterland and thin-skinned/thick-skinned reactivation and out-of-sequence thrusting in the east (foreland), and is new compared with previous work of the region. The synthesis also raises several important new structural play concepts for investigating hydrocarbon prospects in Spitsbergen and adjacent regions; for example, inverted Carboniferous basins, and traps produced by Tertiary thin- and thick-skinned contraction and reactivation structures.

Tectonic structure and petroleum potential of TayabasBay southeast Luzon, Philippines

Tayabas Bay is one of four offshore Philippine areas where the Australian GeologicalSurvey Organization and the Philippine Department of Energy conducted a cooperative marine seismic, gravity, magnetic, bathymetry and geochemical survey. The project acquired new seismic data and reprocessed the 1983 World Bank seismic sections which were all integrated with previous oil company data. the absence of wells drilled offshore, interpretation of offshore seismic data was complemented by onshore well log information and stratigraphy of the Bondoc Peninsula. Geochemistry data, both offshore and onshore, were analyzed to confirm the presence of mature source rocks and hydrocarbon migration. A new seismic interpretation has revealed the structure of this tectonically active geologically complex area. A major structural feature interpreted in offshore Tayabas Bay was a N-NW-trending strike-slip fault which is believed to be a northern splay of the Sibuyan Sea Fault. The authors named this fault the Tayabas Bay Fault and due to its association with the Philippine Fault System the movement is assumed to be left-lateral. The present study suggested the presence of a prolific source rock in the Middle Miocene Vigo Formation and/or the Late Oligocene to Early Miocene Panaon Limestone. Oil and gas generation have been and are occurring in the Bondoc Sub-basin. Two groups of reservoirs were identified, the shelf carbonates beneath the Middle Miocene shales on the Marinduque Platform and the early Middle Miocene carbonates and basin-floor clastics near the base of the Vigo Formation. Carbonate reservoirs are believed to be present in traps formed when the Late Oligocene to Early Miocene carbonate reefs and shelf deposits of the Panaon Limestone were buried by the Middle Miocene shales. A later set of traps was formed and possibly superimposed by the intense deformation associated with the Philippine Fault System which has continued from the Late Pliocene up to the present. Evaluation for hydrocarbon reserves of several possible traps identified three significant leads, namely the Yuni Lead in the south, the Mulanay in the central area and the Mabio in the North.

The geological evolution of the Falkland Islands continental shelf

The Falkland Islands are surrounded by four major sedimentary basins: the Falkland Plateau Basin to the east, the South Falkland Basin to the south, the Malvinas Basin to the west, and the North Falkland Basin to the north. The four main basins appear to have formed initially as Triassic through earliest Cretaceous extensional rifts associated with the break-up of Gondwana. A ?Valanginian end to rifting was followed by thermal sag. There is evidence of Cenozoic uplift in at least the North Falkland Basin, possibly coincident with Andean compression and the development of overthrusting along the plate boundary to the south of the islands resulting from opening of the Scotia Sea. There is no evidence from offshore seismic and gravity-magnetic data to support interpretations that the Falkland Islands have rotated clockwise through up to 180° during Gondwana separation. With the exception of the South Falkland Basin all the major basins probably underwent initially, more or less east-west extension, and had a similar orientation to adjacent South American and western southern African basins. The Falkland basins probably shared a similar geological history with the offshore southern African and South American basins.

Actively evolving microplate formation by oblique collision and sideways motion along strike-slip faults: An example from the northeastern Caribbean plate margin

The pattern of folding, faulting, and late Quaternary coral-reef uplift rates in western and central Hispaniola (Haiti and Dominican Republic) suggest that the elongate Gonave microplate, a 190,000-km 2 area of the northeastern Caribbean plate, is in the process of shearing off the Caribbean plate and accreting to the North American plate. Late Cenozoic transpression between the southeastern Bahama Platform and the Caribbean plate in Hispaniola has inhibited the eastward motion of the northeastern corner of the plate. Transpression is manifested in western and central Hispaniola by the formation of regional scale folds that correspond to present-day, anticlinal topographic mountain chains continuous with offshore anticlinal ridges. Areas of most rapid Quaternary uplift determined from onland coral reefs 125 ka and younger, coincide with the axial traces of these folds. Offshore data suggest recent folding and faulting of the seafloor. Onshore reef data do not conclusively require late Quaternary folding, but demonstrate that tectonic uplift rates of the axial areas of the anticlines decrease from the Northwest Peninsula of Haiti (0.37 mm/yr) to to the central part of the coast of western Haiti (0.19 mm/yr) to the south-central part of western Haiti (0 mm/yr). Formation of the 1200-km-long Enriquillo-Plantain Garden-Walton fault zone as a ‘bypass’ strike-slip fault has isolated the southern edge of the Gonave microplate and is allowing continued, unimpeded eastward motion of a smaller Caribbean plate past the zone of late Neogene convergence and Quaternary uplift of coral reefs in Hispaniola. Offshore seismic reflection data from the Jamaica Passage, the marine strait separating Jamaica and Haiti, show that the Enriquillo-Plantain Garden fault zone forms a narrow but deep, active fault-bounded trough beneath the passage. The active fault is continuous with active faults mapped onshore in western Haiti and eastern Jamaica; the bathymetric deep is present because the Jamaica Passage fault segment represents a 50-km-wide, transtensional left-step of the fault trace between Haiti and Jamaica. Onshore satellite imagery and field observations suggest that the Enriquillo-Plantain Garden fault forms a continuous trace extending from central Hispaniola east of Lake Enriquillo, Dominican Republic, to the westernmost end of the southern peninsula of Haiti. The regional lineament corresponds to a recent fault scarp in Quaternary alluvium of the Clonard pull-apart basin in the central part of the southern peninsula of Haiti and suggests that at least this part of the lineament has undergone recent slip. Calmus (1983) has suggested a total offset of 30–50 km of the Enriquillo-Plantain Garden fault zone in the southern peninsula of Haiti using the apparent offset of lithologic units. Seismic reflection data from Lake Enriquillo document recent deformation of Quaternary lake sediments where the lineament crosses the lake. Leveling of the crest of a late Holocene coral reef and associated algal tufa around Lake Enriquillo demonstrate late Holocene vertical movement and tilting in a 1500-m-wide zone parallel to the fault trend. Lateral offset is difficult to show in the Enriquillo Valley area because of rapid recent sedimentation into the valley. The pattern of inactive strike-slip faults and fold belts of Cenozoic age in Cuba and the Yucatan basin suggest that two elongate microplates were sheared off the proto-Caribbean plate and accreted to the North American plate by a similar process in Paleocene and Eocene times. Age of terminal deformation in western, central and eastern Cuba is consistent with southeastward younging and migration of arc collision. The similarity of the size and sequence of events in Cuba suggests that the process of oblique collision and sideways motion of the plate along a new strike-slip fault towards a free face may be an important process of microplate formation and interplate transfer in other areas.

Syn-rift thermal structure and post-rift evolution of the Oslo Rift (southeast Norway): New constraints from fission track thermochronology

The Permo-Carboniferous (240–305 Ma) Oslo Graben in southeast Norway is a classical magmatic continental rift. We present fission track (FT) data on apatites, zircons and sphenes from the rift and surrounding areas in order to clarify its syn- and post-rift thermal evolution. Zircons and sphenes within the rift record FT ages of between ∼ 270 and 180 Ma (i.e., syn-post-rifting). In contrast, the Precambrian basement surrounding the graben yields zircon FT/sphene FT ages of ∼ 500–650 Ma, indicating that heating associated with rifting was focused inside the rift as a result of advective heat flow. Syn-rift to early post-rift temperatures reached > 240°C at the present erosion level in the graben. Syn-rift advective heating ( ∼ 260–270 Ma) was probably a result of large-scale batholith intrusion; post-rift ( ∼ 180–220 Ma) heating seems to have occurred by hydrothermal circulation of high-temperature (100–300°C) fluids. The FT data, organic geochemical indicators and fluid inclusions suggest that hydrothermal circulation followed a highly complex pattern controlled by the tectonic and volcanic structure of the rift. Apatite FT ages and confined track length distributions reveal the post-rift exhumation history of the area. Apatite FT ages are continuous within and outside the graben and decrease from Triassic (200–240 Ma) in the southeast to Jurassic ( ∼ 160 Ma) in the northwest, indicating that post-rift exhumation took place on a much larger scale than Permian rifting. Modelling of apatite FT ages and track lengths suggests cooling/denudation events in the Triassic, Jurassic and Neogene, accounting for a total post-Permian erosion of 3–4 km. Triassic-Jurassic denudation coincides with the migration of rifting from the Oslo-Skagerrak area to the North Sea/Danish Basin, as revealed by offshore seismic data, and caused destruction of the hydrothermal regime. Neogene uplift and erosion of the area is also supported by seismic evidence. The inferred timing and amount of regional post-rift erosion suggests that post-rift subsidence and sedimentation within the Oslo Graben was minimal.

Profile of a piggyback basin: Early Miocene Otaua Group and Waipoua Subgroup, western Northland, New Zealand

Abstract The Otaua Group of west Northland is interpreted as comprising three phases of sedimentation: (1) a locally preserved, bathyal, upward‐fining sequence of grit, sandstone, and mudstone of late Waitakian age (Harnetts Formation, new); (2) a deltaic and regressive sequence of sandstone and mudstone (Waitiiti Formation), sandstone and grit (Otueka Formation, new), and conglomerate (Waiwhatawhata Conglomerate, including the new Smoothy Member), deposited at upper bathyal to shelf depth in the late Waitakian ‐ Otaian (first cycle); and (3) a second regressive sequence of sandstone and mudstone (Pokas Formation, new) and conglomerate (Omapere Conglomerate) deposited in upper bathyal to terrestrial conditions in the Otaian (second cycle). The Otaua Group is overlain conformably and unconformably by the late Otaian‐Altonian Waipoua Subgroup of the Waitakere Group, comprising basaltic volcanics (Waipoua Basalt), which contain near‐basal lacustrine sediment in the east (Rhine Member, new) and which are overlain by conglomerate (Pukorukoru Formation) in the northwest. Assessment of stratigraphic relationships suggests that the boundary between the Otaua Group and the Waipoua Subgroup should be redefined as the base of the Waipoua Basalt. Following its deposition upon the Northland Allochthon, Harnetts Formation was tectonically incorporated as a large block into the substrate during late‐stage movement. During deposition of the first sedimentary cycle, deformation about northeast‐striking trends was accompanied by southward‐directed thrusting: these thrusts cut the early sediments (Waitiiti Formation) and are truncated by the Waiwhatawhata Conglomerate. After deposition of the first sedimentary cycle, regional folding about both northeast‐trending and NW‐WNW‐trending axes took place, and the sequence was stacked along observed and inferred southwest‐dipping thrusts. Sediments of the second cycle were then deposited to the west of the first, overstepping the earlier sediments and structures. Subsequent local folding of the second‐cycle sediments (about northwest‐trending axes only) was accompanied by emergence and followed by eruption of the Waipoua Basalt. The Waipoua Basalt and second‐cycle sediments were in turn monoclinally folded about NW‐WNW‐trending axes during the final phase of uplift. Deformation is inferred to have ceased during Waipoua volcanism in the latest Otaian and early Altonian. The sediments and structures are interpreted as recording a progressive change, from extensional deformation within the Northland Allochthon during its southward emplacement at bathyal depth in the Waitakian, to north‐south and then northeast‐southwest compression and regional uplift during the Otaian. These structures are inferred to be related to the formation of an early Miocene convergent margin in the vicinity of Northland. Westward regression of the loci of deposition and of deformation, and the southwestward dip of major thrust structures, suggests northeastward‐directed compression (from the southwest); however, interpretation of offshore seismic data shows no evidence of a convergent margin off Northland's west coast. The structures may instead be related to regional shortening in the basement of east Northland but the evidence to date is inconclusive.

Hydrocarbon Prospect Mapping Using Balanced Cross Sections and Gravity Modeling, Onin and Kumawa Peninsulas, Irian Jaya, Indonesia: ABSTRACT

Exploration field geology mapping and acquisition of gravity data has been conducted on approximately 650 line kilometres of26 surveyed traverse lines across the Onin and Kumawa Peninsulas of western Irian Jaya. Karstified New Guinea Limestone with a maximum thickness of 2,150 m is the predominant surface outcrop, and precludes use of seismic. However, integrated use offield geology data, balanced cross-sections, and gravity modelling has enabled us to identify two giant hydrocarbon prospects. The Onin and Kumawa Peninsulas lie at the margin of Jurassic age faulting associated with the Australian Northwest Shelf. Jurassic rift sands of the Lower Kembelangan Formation are the primary reservoir target. During the Plio-Pleistocene, collision of the Australian plate. and the Banda Arc inverted sections of the rift system, including the Onin and Kumawa Peninsulas. A better understanding of the regional structure was gained by integrating the Mobil Oil gravity data (of 1992) and that collected by Shell Oil (in the 1950's) in the structurally less deformed Bomberai region east ofOnin and Kumawa. Bouguer reduction was carried out using 2.4 glcm3 density, GRS 1967 and IGSN 1971. A sequence of gravity maps were generated, including Bouguer, regional, residual, downward continued, and second derivative. Spectral analyses indicate that basement is about 3 km depth at Onin and about 6 km in the Bomberai area. The Bomberai and Onin-Kumawa regions are separated from one another by a steep gravity gradient which has a SE-NW strike direction. This gravity gradient may represent a change of lithology. Prospect definition was obtained by evaluating traverse profile data. Balanced cross-sections were constructed, using detailed biostratigraphy to determine formation thicknesses and amount of fault offsets. Where possible, onshore balanced section profiles were tied to offshore seismic profiles and wells. Two dimensional forward gravity models were calculated, using formation densities from well data. The calculated profiles were then compared to profile observed values. Differences between calculated and observed profiles were resolved by adhering to the exploration model, which required that the main thickness changes would be in the Lower Kembelangan Formation rift section. Density values for the seven formations were held constant. Constraining the variables to one (Lower Kembelangan thickness) in the gravity profile modelling maintains credibility of the technique. Errors inherent in the structural maps derived from the cross­ sections are likely to be vertical shift which would be approximately constant across the prospects. The interactive work between geologists and geophysicists was effective in producing a logical representation of subsurface geology, which in turn allowed selection of drill sites with some degree of confidence. Subsequent to completing the modelling, offshore seismic data across the western plunge of the Onin Peninsula was obtained. The structural style demonstrated in the modelling and the seismic data are comparable. This enhances our confidence in the modelling results.

PETROLEUM POTENTIAL OF RAGAY GULF, SOUTHEAST LUZON, PHILIPPINES

During March-May 1992, the Australian Geological Survey Organisation and the Philippine Department of Energy conducted a cooperative marine seismic and underway geochemical survey in four offshore Philippine basins which included Ragay Gulf. The project was funded and supported by the Australian International Development Assistance Bureau.The newly acquired and reprocessed seismic data from Ragay Gulf show a significant improvement in penetration and stratigraphic resolution. Seismic interpretation has revealed the existence of five sedimentary sub-basins with 2.5 – 6 seconds of Eocene to Recent sediments. Several potential traps have been evaluated for hydrocarbon reserves and new prospects are identified.No well has been drilled offshore in the Ragay Gulf. Onshore well log information and stratigraphy have assisted in the correlation and interpretation of offshore seismic data and allowed potential reservoirs to be recognised. The primary reservoir targets are the carbonate sequences of both Early and Late Miocene age. The widespread volcanoclastic sand may be an important secondary target, especially for gas reservoirs.Onshore and offshore geochemical data have confirmed the presence of mature source rocks from which generated hydrocarbons are currently migrating to the surface and, by inference, to reservoirs.A wide diversity of play types is recognised which could have been sourced from three separate source kitchens in the Bondoc, Ragay and Burlas Sub-basins. Specific entrapment possibilities are:Compressional fault-dependent traps (e.g. Anima Sola).Compressional anticlinal fault independent traps (e.g. Alibijaban and Palad).Late Miocene carbonate reefal buildups (e.g. Apud and Gorda).Early Miocene carbonate reservoirs in drape over highs (e.g. San Narciso and Bagulaya).

A preliminary result of the Ragay Gulf survey in the Philippines

During March-May 1992, the Australian Geological Survey Organisation and the Philippine Office of Energy Affairs conducted a cooperative marine seismic, gravity, magnetic, bathymetry and geochemical sniffer survey in four offshore Philippine basins: NE Palawan, Cuyo Platform, Tayabas Bay and Ragay Gulf. The project was funded by the Australian International Development Assistance Bureau. The survey work was conducted using the Australian Government's geoscientific vessel, Rig Seismic. The cruise achieved all its objectives and was a major success. Of the four survey areas, the Ragay Gulf is of particular interest, with complex structures, multi-phase tectonism and massive sediment infill. It was in this region that many geochemical sniffer seeps were located. The geochemical sniffer data and its C-13 and C-14 isotope analyses have indicated the presence of significant source rocks. These seeps are thermogenic in origin and have reached sufficient maturity to generate significant quantities of hydrocarbons. In the near future, the geochemical data will be integrated with the seismic data to provide clues for hydrocarbon migration. The preliminary seismic interpretation have revealed the existence of five sedimentary sub-basins with 2.5 - 5 seconds of Eocene to Recent sediments. Several types of potential traps were documented. No well has been drilled in the offshore Ragay Gulf. Onshore well log information and stratigraphy have assisted in the correlation and interpretation of offshore seismic data. In addition, the importance of the carbonate and clastic reservoirs was recognised. Overall, the oil and gas potential in the Ragay Gulf area looks most encouraging.

Seismostratigraphy and Neogene-Recent depositional history of the south central continental margin of Cyprus

The Neogene-Recent depositional history and tectonic setting of offshore and onshore central southern Cyprus can be correlated using offshore seismic data and onshore geology. Seismic data allow the south Cyprus continental margin to be divided into a narrow inner shelf area and a wider, irregular outer slope area. The well known top Messinian reflector is recognized over the slope and western part of the shelf. To the east, an inferred Quaternary, ?eustatically-induced erosion surface, possibly a drowned marine terrace, has been cut into lithified Miocene sediments. Zones of folding in the offshore Miocene sequence can be traced onshore, where they correlate with several WNW-ESE trending structural lineaments of Miocene age. The formation of the lineaments is attributed to compression resulting from northwards subduction to the south of Cyprus. Plio-Quaternary sediments are relatively thin to absent along the shelf of south central Cyprus, but thicken across the slope where they infill end Miocene topography. Their distribution is consistent with evidence from onshore which indicates an end to deformation in southern Cyprus around the close of the Miocene, relative inactivity in the Pliocene and relative uplift in the Quaternary. The end of deformation in the Pliocene may relate to southwards ‘roll-back’ of the inferred subduction zone trench.

Structural and tectonic evolution of the Humber Zone, western Newfoundland, 1. Implications of balanced cross sections through the Appalachian structural front, Port Au Port Peninsula

The Appalachian structural front in western Newfoundland, which is principally a submarine feature that comes ashore on Port au Port Peninsula, is interpreted to be a structural triangle zone, or tectonic wedge. Rocks within the triangle zone, which are therefore transported, include the Taconian (Middle Ordovician) Humber Arm Allochthon, Taconian foreland clastic sediments, and the Cambro‐Ordovician platform succession. The transported clastic sediments and platformal rocks, as well as structurally involved Grenville crystalline basement exposed east of the peninsula, compose the Acadian (Siluro‐Devonian) Port au Port Allochthon, which carried the Humber Arm Allochthon as a high structural slice; Acadian transport of tens of kilometers westward is suggested. This interpretation contrasts markedly with the traditional and widely accepted interpretation of the Taconian and Acadian orogenies in western Newfoundland. Here we reinforce our earlier arguments for an allochthonous interpretation by presenting a series of six closely spaced, balanced cross sections across the Port au Port Peninsula. These sections illustrate our interpretation of complex and noncylindrical structures within the triangle zone, as well as the envisioned structural linkage between exposures on Port au Port Peninsula and the geometry of the triangle zone interpreted from offshore seismic data 23 km along strike to the northeast. The southeast vergent Tea Cove thrust, which formed an early upper detachment to the triangle zone, is offset and overturned by the north‐northwest vergent Round Head thrust. The Round Head thrust is interpreted to flatten into a second southeast vergent thrust, the Red Brook detachment, which formed a late upper detachment to the triangle zone. Some map‐scale structures are interpreted as fault‐bend folds above oblique ramps on the Red Brook detachment. Allochthonous crystalline basement within the triangle zone, beneath its attached platformal cover, is interpreted as a result of thrust reactivation of a preexisting basement‐cutting normal fault, which may have formed during rifting of the early Paleozoic Iapetus Ocean. This interpretation is suggested by the stratigraphy and geometry of spatially restricted coarse conglomeratic units on Port au Port Peninsula, and is supported by the restored cross sections.

Potential Cretaceous Play in the Rharb Basin of Northern Morocco

The autochthonous Cretaceous in the Rharb basin of northern Morocco is located underneath a cover of neogene sediments and of the Prerif nappe olistostrome, which was emplaced during the Tortonian 7 m.y. The presence of infranappe Cretaceous sediments is documented in a few onshore wells in the Rharb basin and in the adjacent Prerif Rides area, as well as in the Rif Mountains. Their presence in the deeper portion of the Rharb basin is difficult to detail because of poor seismic resolution data beneath dispersive prerif nappe. A recent study of offshore seismic data acquired by PCIAC in 1987 indicates that the infranappe interval can be more than 1500 m thick in some of the offshore Kenitra area. These sediments have seismic signatures that would correspond to Middle Cretaceous transgressions, culminating with a Turonian highstand. Their deposition systems were located on the northern and western flanks of the Meseta and were followed by a hiatus lasting until the Miocene. Regional studies of gravity and magnetic data provide and additional understanding of the Rif province, its evolution, and the possible presence of autochthonous Cretaceous sediments below the prerif nappe cover. The infranappe of Rharb basin has a good potential to developmore » into a major hydrocarbon play with the presence of middle Cretaceous reservoir rocks, Turonian-Cenomanian black shale source rocks, as well as the timely combination of trap formation, source rock maturation, and hydrocarbon migration.« less

Potential Cretaceous Play in the Rharb Basin of Northern Morocco

The autochthonous Cretaceous in the Rharb basin of northern Morocco is located underneath a cover of neogene sediments and of the Prerif nappe olistostrome, which was emplaced during the Tortonian 7 m.y. The presence of infranappe Cretaceous sediments is documented in a few onshore wells in the Rharb basin and in the adjacent Prerif Rides area, as well as in the Rif Mountains. Their presence in the deeper portion of the Rharb basin is difficult to detail because of poor seismic resolution data beneath dispersive prerif nappe. A recent study of offshore seismic data acquired by PCIAC in 1987 indicates that the infranappe interval can be more than 1500 m thick in some of the offshore Kenitra area. These sediments have seismic signatures that would correspond to Middle Cretaceous transgressions, culminating with a Turonian highstand. Their deposition systems were located on the northern and western flanks of the Meseta and were followed by a hiatus lasting until the Miocene. Regional studies of gravity and magnetic data provide and additional understanding of the Rif province, its evolution, and the possible presence of autochthonous Cretaceous sediments below the prerif nappe cover. The infranappe of Rharb basin has a good potential to developmore » into a major hydrocarbon play with the presence of middle Cretaceous reservoir rocks, Turonian-Cenomanian black shale source rocks, as well as the timely combination of trap formation, source rock maturation, and hydrocarbon migration.« less

Structure, Stratigraphy, and Climate of the Mesozoic Chignecto Subbasin, Bay of Fundy, Canada

The Chignecto subbasin is one of three Triassic/Jurassic subbasins in the Bay of Fundy rift system of southeastern Canada. Although no wells have been drilled in the subbasin, offshore seismic data and onshore outcrop data show that the basin contains typical rift structures and stratigraphy. The Chignecto subbasin is an asymmetric rift basin bounded on the west by a series of gently to moderately dipping normal faults. The seismic data show that the total heave on these boundary faults locally exceeds 10 km. Most strata within the basin dip gently to the west toward the boundary faults. Field data show that a divergent wrench fault zone composed of numerous strike-slip, oblique-slip, and normal faults bounds the Chignecto subbasin on the south. The normal faults bounding the basin on the west and the divergent wrench fault zone on the south are reactivated Paleozoic compressional structures. Major stratigraphic features in the Chignecto subbasin include an unconformity that subdivides the basin strata into pre- and syn-rift sequences and an asymmetric arrangement of sedimentary and seismic facies and corresponding lithologies. The Triassic/Jurassic syn-rift strata within the basin thicken toward the western boundary faults. Coarser-grained strata of alluvial-fan and fluvial origin interfinger with finer-grained stratamore » of fluvial and lacustrine origin near the boundary faults. Eolian deposits and evaporites in adjacent Mesozoic rift basins indicate an arid climate for the Late Triassic/Early Jurassic. Large-amplitude reflections observed within the western half of the basin probably represent high-velocity evaporites or carbonates deposited within the deepest part of the Chignecto subbasin.« less