Late Cretaceous-early Tertiary Research Articles

A Review of the Bentong‐Raub Suture vis‐à‐vis New Insight of the Tectonic Evolution of Malay Peninsula, South East Asia

AbstractGenesis of the so‐called Bentong‐Raub Suture of Malay Peninsula does not fit to the model of subduction‐related collision. It has evolved from transpression tectonics resulting closure and exhumation of the inland basin which underwent extensive back‐arc extension during Triassic. Crust having similar thickness (average ∼35 km) below entire Malay Peninsula nagate collision of two separate continental blocks rather supports single continental block that collided with South China continental block during Permo‐Triassic. Westward subduction of intervening sea (Proto South China Sea) below Malay Peninsula resulted in widespread I‐ and S‐Type granitization and volcanism in the back‐arc basins during Triassic. Extensive occurrence of Permo‐Triassic Pahang volcanics of predominantly rhyolitic tuff suggest its derivation from back‐arc extension. Back‐arc extension, basin development and sedimentation of the central belt of the peninsula continued until Cretaceous. A‐Type granite of metaluminous to peraluminous character indicates their emplacement in an intraplate tectonic setting. Malay Peninsula suffered an anticlockwise rotation due to the rifting of Luconia–Dangerous Grounds from the east Asia in the Late Cretaceous–Early Tertiary. Extensive ductile and brittle deformation including crustal segmentation, pull‐apart fracturing and faulting occurred during the closure and exhumation of the basins developed in the peninsula during Late Cretaceous–Early Tertiary. Crustal shortening in the central belt of the peninsula has been accomodated through strike‐slip displacement, shearing and uplift.

The petroleum generation modeling of prospective affinities of Jurassic–Cretaceous source rocks in Tut oilfield, north Western Desert, Egypt: an integrated bulk pyrolysis and 1D-basin modeling

Total organic carbon (TOC) and Rock-Eval pyrolysis for 27 rock samples and geochemical model of Alam El Bueib, Masajid, Khatatba, and Ras Qattara Formations from Tut-1x well in the Tut Oilfield, North Western Desert, Egypt, were used to determine the source rock characteristics and petroleum generative potentials of prospective source rocks, including quantity, type of organic matter, and their thermal maturity level. The results were then incorporated into basin modeling in order to improve our understanding of burial/thermal histories and hydrocarbon generation and extraction from Jurassic–Cretaceous source rocks. The bulk geochemical results showed that Alam El Bueib and Ras Qattara formations contain type-III kerogen, while the Masajid and Khatatba formations displaying generally contain mixed kerogen types II–III, which have the ability to generate mixed oil and gas accumulations under thermal maturation level. Vitrinite reflectance values of the Jurassic–Cretacouse source rocks range from 0.42 to 0.86 % R, indicating sufficient thermal maturity for oil generation. Meanwhile, the burial/thermal history models indicate that the Alam El Bueib and Masajid formations initiated the early-mature stage of oil generation during the Late Cretaceous till the present day and the peak oil generation has not been reached yet. The source rock of the Khatatba and Ras Qattara formations reached to the peak oil generation at vitrinite reflectance values of 0.76 Ro% between 110 and 77 million years before present (mybp), and maximum rates of oil have been generated during late Cretaceous–early Tertiary (100–58 mybp). The transformation ratio of the main phase of oil generation varied from 25 to 65 %. The modeled hydrocarbon generation evolution suggests that the timing of hydrocarbon extraction from the Khatatba and Ras Qattara source rocks began in the early Tertiary (58 mybp) and persisted to the present day. This indicates that Khatatba and Ras Qattara formations can be consider as generative potentials of prospective source rock horizons in Tut oilfield.

Origins and Relationships of the Mixed Mesophytic Forest of Oregon–Idaho, China, and Kentucky: Review and Synthesis1

Abstract The Arcto-Tertiary Geoflora concept of Ralph Chaney, that the Mixed Mesophytic Forest of eastern Asia and eastern North America are relicts of a Northern Hemisphere high-latitude circumglobal deciduous forest of the Late Cretaceous–Early Tertiary that migrated south to the temperate zone as an intact unit, was shown by Wolfe and others to be invalid. To explain the origin and development of these disjunct forests, Wolfe and Tiffney developed the boreotropical hypothesis. Accordingly, a paratropical (near-tropical) rainforest flora containing a mixture of tropical, paratropical, and temperate genera developed at several places in the middle latitudes of the Northern Hemisphere in the Eocene and spread around the globe via the Bering and North Atlantic land bridges and shores of the Tethys Seaway. Further, the Mixed Mesophytic Forest of eastern Asia and eastern North America developed independently after disruption of the boreotropical flora by subsequent changes in climate and geography, thus acco...

Integrated petrographic, mineralogical, and geochemical study of the Late Cretaceous–Early Tertiary Dakhla Shales, Quseir–Nile Valley Province, central Egypt: implications for source area weathering, provenance, and tectonic setting

Late Cretaceous–Early Tertiary shales of the Dakhla Formation of the Quseir–Qena Province, central Egypt, were analyzed for major and selected trace elements to infer their provenance, source rock paleoweathering intensity, and tectonic setting. The studied formation consists of a series of marls and shales and is subdivided into two members, namely Beida Shale Member at the top and Hamama Marl Member at the base. The Dakhla Shales are texturally classified as mudstones. Mineralogically, these shales consist mainly of smectite and kaolinite. Chemical analysis of the major and trace elements generally exhibits a uniform distribution throughout the Upper Maastrichtian–Lower Paleocene sediments. SiO2, Al2O3, and Fe2O3 contents are higher than the values reported for the post-Archaean Australian shale (PAAS), while TiO2 and Na2O contents are found to be lower. Bivariate plot of Zr versus TiO2 diagram indicates that intermediate to felsic igneous rocks constitute the main supplying source rock. Average chemical index of alteration (CIA), plagioclase index of alteration (PIA), and chemical index of weathering (CIW) values (81, 92, and 93 %, respectively) imply intermediate to intense weathering of the source material in a semiarid climate. The bivariate discriminant function diagram reveals an active continental to passive margin setting for the Dakhla Shales. The developed soils were transported by rivers to the depositional basin. The inferred tectonic setting for the Late Cretaceous–Early Tertiary Dakhla Shales in Quseir–Qena Province is in agreement with the tectonic evolutionary history of central Egypt during the Late Cretaceous–Early Tertiary.

The Impact of Climate Change on Mangrove Forests

Mangrove forests have survived a number of catastrophic climate events since first appearing along the shores of the Tethys Sea during the late Cretaceous-Early Tertiary. The existence of mangrove peat deposits worldwide attests to past episodes of local and regional extinction, primarily in response to abrupt, rapid rises in sea level. Occupying a harsh margin between land and sea, most mangrove plants and associated organisms are predisposed to be either resilient or resistant to most environmental change. Based on the most recent Intergovernmental Panel on Climate Change (IPCC) forecasts, mangrove forests along arid coasts, in subsiding river deltas, and on many islands are predicted to decline in area, structural complexity, and/or in functionality, but mangroves will continue to expand polewards. It is highly likely that they will survive into the foreseeable future as sea level, global temperatures, and atmospheric CO2 concentrations continue to rise.

Kinematic analysis of mélange fabrics in the Franciscan Complex near San Simeon, California: Evidence for sinistral slip on the Nacimiento fault zone?

Asymmetric fabrics in Franciscan Complex shale matrix melange near San Simeon, central California, record synsubduction shear that has implications for the Late Cretaceous to early Tertiary tectonic evolution of the region. Outcrop-scale structural data including S-, C-, and C′-planes, slickenlines, block long axes, and offset blocks indicate that most asymmetric fabrics developed in a NE-dipping sinistral shear zone following Late Cretaceous chaotic mixing and accretion of the melange. Approximately 70% of C-planes strike within ∼45° of the major NW-striking structures in the region—the Neogene San Gregorio–Hosgri fault and the Late Cretaceous–early Tertiary Nacimiento fault zone. Slip vectors on these NW-striking C-planes consistently record sinistral or oblique sinistral shear. This dominant shear regime accommodated E-W to NE-SW shortening and N-S to NW-SE extension. Angles between S- and C-planes are typically ∼25°–35° and range up to ∼45°, suggesting dominantly simple shear. Sinistral shear in the melange is kinematically incompatible with Neogene to recent dextral faulting, indicating that these fabrics predate the early Miocene development of the Pacific–North America transform. We propose that sinistral shear in the melange is related to latest Cretaceous to early Tertiary slip on the adjacent Nacimiento fault zone. These kinematic data are compatible with previous studies that have suggested that the juxtaposition of the Salinian block against the Nacimiento block was accommodated by hundreds of kilometers of sinistral slip on the Nacimiento fault zone.

Modelling cover deformation and decoupling during inversion, using the Mid-Polish Trough as a case study

Seismic sections across the NW part of the Polish Basin show that thrust faults developed in the sedimentary units above the Zechstein evaporite layer during basin inversion. These cover thrust faults have formed above the basement footwall. Based on the evolution of the basin, a series of scaled analogue models was carried out to study interaction between a basement fault and cover sediments during basin extension and inversion. During model extension, a set of normal faults originated in the sand cover above the basement fault area. The distribution and geometry of these faults were dependent on the thickness of a ductile layer and pre-extension sand layer, synkinematic deposition, the amount of model extension, as well as on the presence of a ductile layer between the cover and basement. Footwall cover was faulted away from the basement only in cases where a large amount of model extension and hanging-wall subsidence were not balanced by synkinematic deposition. Model inversion reactivated major cover faults located above the basement fault tip as reverse faults, whereas other extensional faults were either rotated or activated only in their upper segments, evolving into sub-horizontal thrusts. New normal or reverse faults originated in the footwall cover in models which contained a very thin pre-extension sand layer above the ductile layer. This was also the case in the highly extended and shortened model in which synkinematic hanging-wall subsidence was not balanced by sand deposition during model extension. Model results show that inversion along the basement fault results in shortening of the cover units and formation of thrust faults. This scenario happens only when the cover units are decoupled from the basement by a ductile layer. Given this, we argue that the thrusts in the sedimentary infill of the Polish Basin, which are decoupled from the basement tectonics by Zechstein evaporites, developed due to the inversion of the basement faults during the Late Cretaceous-Early Tertiary.

Fluid flow systems of the Malta Plateau, Central Mediterranean Sea

The Malta Plateau is a shallow, asymmetric, north–south striking ridge located between Sicily and the Maltese Islands. New 2D seismic and side scan sonar data sets, sub-bottom profiles and seabed samples are investigated to characterize fluid flow systems on the Malta Plateau, determine their origin, and improve our general understanding of fluid flow focusing in terms of structural and stratigraphic controls. We demonstrate that fluid flow systems across the Malta Plateau are numerous, widespread and active. Two types of fluid flow systems are identified. The first type can be observed in the shallower parts of the western Malta Plateau. It consists of a shallow system where fluids ascend from gas-charged Plio-Pleistocene sediments and actively seep at the seafloor in the form of gas flares. The fluid migrating in this kind of system is likely autochthonous, biogenic gas (probably methane) forming at shallow depth. The second type comprises deep systems that can be observed in the central and eastern parts of the Malta Plateau. In these deep systems, fluids generated in Late Mesozoic sediments ascend through Late Cretaceous, Tertiary and Plio-Pleistocene units, and are expelled at the seafloor in the form of pockmarks. Late Mesozoic faults, Early Miocene to recent faults, and pipe structures constitute the preferred migration pathways. The migrating fluids are likely of thermogenic origin, possibly leaking from Mesozoic hydrocarbon reservoirs. Particularly in the north of Malta there is evidence that fluid migration is driven by overpressure at depth resulting from compressive events during the Late Cretaceous–Early Tertiary. Since the tectonic regime across the Malta Plateau is currently extensional, we propose that recent fluid migration and expulsion are at least partly driven by old overpressures and sustained by more recent normal faults. Our results show that fluid migration must be taken into account when assessing seabed stability on the Malta Plateau. Our results also indicate where chemosynthetic ecosystems may be located, and they improve our understanding of the petroleum geology of the Malta Plateau.

Fluid inclusions as constraints in a three‐dimensional hydro‐thermo‐mechanical model of the Paris basin, France

ABSTRACTFluid inclusion homogenization temperatures and three‐dimensional hydro‐thermo‐mechanical modelling were combined to constrain fluid flow, solute and heat transport in the Paris basin, France, focusing on the two main petroleum reservoirs i.e. the Dogger and the Triassic (Keuper) formations. The average homogenization temperatures of two‐phase aqueous inclusions in different samples range from 66 °C to 88 °C in the Dogger calcite cement, from 106 °C to 118 °C in the Keuper dolomite cement and from 89 °C to 126 °C in the Keuper quartz and K‐feldspar cements. The maximum homogenization temperatures for inclusions in the Keuper quartz and K‐feldspar cements were 17–47 °C higher than present‐day temperatures in the boreholes at similar depths. Processes that might explain higher temperatures in the past were examined through numerical simulations and sensitivity tests. A warmer climate in the Late Cretaceous–Early Tertiary resulted in a temperature rise of only 8 °C. Late Cretaceous chalk had a thermal blanketing effect that resulted in simulated temperatures as high as 15–20 °C above the present day ones. An additional 300 m deposition and subsequent erosion of chalk, not taken into account so far, has to be considered to simulate the high palaeo‐temperatures recorded by fluid inclusions in both reservoirs. In view of the simulated thermal history of the basin, in the Keuper, an age of about 85 Ma is consistent with quartz/K‐feldspar temperatures and an age of about 65 Ma is in agreement with the precipitation temperature of the dolomite cement. Our models suggest an age of about 50 Ma for the Dogger calcite cementation.

Petrology of chloritoid–ilmenite-rich rocks in the Indus Suture mélange of Pakistan: Implications for the Cretaceous paleolatitude of Kohistan

The Indian Plate and Kohistan Island Arc are juxtaposed to the north of Peshawar along the Indus Suture, which is characterized by a tectonic mélange consisting of ultramafic, mafic, and a variety of sedimentary rocks. Within metavolcanic greenstones, there are small, lensoid bodies comprising quartz, white mica (up to 4.5 wt% FeO), chlorite (pseudothuringite to ripidolite), pyrite, and abundant chloritoid (up to 60%) and ilmenite. These lenses consist essentially of SiO 2, TiO 2, Al 2O 3 and iron oxide. There is a clear petrographic and geochemical (major, trace and RE element) transition from these rocks to their host greenstones, suggesting a common parentage. The greenstones contain high TiO 2, Fe 2O 3 and P 2O 5. Their chondrite-normalized patterns show enrichment in light REE, depletion in heavy REE, negative Eu and positive Yb anomalies. Because the greenstones have probably been altered, the present set of data is not capable of deciphering the tectonic setting of these rocks, but previous geochemical studies have suggested an island arc setting for volcanic rocks in the mélange. By chemical analogy with lateritized basalts, the chloritoid-rich rocks are considered to be the product of weathering of basalts in tropical, probably equatorial, conditions before the northward drift of Kohistan. Mélange formation and greenschist facies metamorphism occurred during the collision of the Indian Plate and the Kohistan Magmatic Arc, to its north, in the Late Cretaceous-Early Tertiary.

Mesozoic episodic magmatism in South Korea and its tectonic implication

NE trending granitoid batholiths in South Korea are part of the huge Phanerozoic granitoid belt in east Asia. The crystallization ages of granitoid plutons show three episodes of Mesozoic magmatism in South Korea: Triassic (248–210 Ma), Jurassic (197–158 Ma), and Late Cretaceous–early Tertiary (110–50 Ma), with a Late Jurassic–Early Cretaceous (∼50 m.y.) hiatus. The granitoid ages and kinematic analyses of the Mesozoic structures in South Korea suggest that the Middle Jurassic (180–170 Ma) dextral strike‐slip ductile shear zones formed by the northwest (orthogonal) subduction of the Izanagi plate, whereas the Cretaceous sinistral faults formed by the northward (oblique) subduction of the same plate, which generally agree with proposed reconstruction of plate motions, albeit uncertain, around east Asia. Among the models proposed to explain magmatic lull in a continental arc, shallow angle subduction associated with high convergence rate and subduction of oceanic plateaus might be the probable cause for the magmatic hiatus in South Korea and Japan. The huge width (∼500 km) of the east Asian granitoid belt may be explained by oceanward retreat of subduction zone in the Cretaceous in association with change in the movement direction of the subducting plate.

Trace elemental and PGE geochemical constraints of Mesozoic and Cenozoic peridotitic xenoliths on lithospheric evolution of the North China Craton

Whole-rock major, trace, and platinum-group elemental (PGE) data, and major and trace element compositions of diopsides are reported for peridotite xenoliths from (1) early Mesozoic volcanic breccias in Xinyang, located at the southern margin of the North China Craton, and (2) Cenozoic basalts in Hebi and Shanwang, both of which are situated within the craton and lie on the North-South Gravity Line and the Tanlu fault zone, respectively. The early Mesozoic Xinyang xenoliths are harzburgites containing <2% Cpx with high Cr # and enriched in LREE but depleted in HFSE. These xenoliths have chondritic Pd/Ir (1.9–6.6) and Ru/Ir (3.5–4.0) ratios and high Ni and low CaO, Al 2O 3, and S contents, indicating derivation from a highly refractory mantle that experienced carbonatitic metasomatism. Negative Ce (mean δCe = 0.50) and low Mg/Si ratios of the Xinyang peridotites record the addition of crustal components likely produced from subducted continental material of the Yangtze Craton in the early Mesozoic. The subduction-related modification of the lithospheric mantle was limited to the area close to the collision zone rather than being pervasive throughout the craton. The Cenozoic Hebi peridotite xenoliths are harzburgites with ≤4.5% Cpx and have low CaO and Al 2O 3 but high Ni contents, chondritic Ru/Ir ratios (2.5–5.4), and a wide range of CaO/Al 2O 3, Na 2O/TiO 2, Pt/Ir (0.4–2.3), and Pd/Pt (1.1–8.5) ratios. These peridotites are interpreted as the shallow relics of the cratonic mantle. In contrast, the Cenozoic Shanwang xenoliths are lherzolites (5.6%–19.5% Cpx), which have low Ni contents and low Ni/Cu and Mg/Si ratios, but high CaO, Al 2O 3, S, and HREE contents, and relatively high Ru/Ir and Pd/Ir ratios. The Shanwang peridotites show pronounced positive Ti and Sr, negative Th, and slightly negative Y, Zr, and Hf anomalies. They are believed to represent newly accreted fertile lithospheric mantle derived from cooling of upwelling asthenosphere. The documented temporal and spatial variations in the Mesozoic-Cenozoic mantle support the previous suggestion that the buoyant refractory continental keel in the eastern part of the North China Craton was heterogeneously replaced by younger fertile lithospheric mantle in the late Cretaceous-early Tertiary.

Rediscovery of Choroterpes atramentum in Costa Rica, type species of Tikuna new genus (Ephemeroptera: Leptophlebiidae: Atalophlebiinae), and its role in the "Great American Inter-change"

A new genus, Tikuna, is described based on recent collections of adults and nymphs of Choroterpes atramentum Traver from western Costa Rica. All recent collections are from streams on or near the Nicoya Complex, the oldest geological formation in Lower Central America. Tikuna belongs to a lineage of South American Atalophlebiinae (Leptophlebiidae: Ephemeroptera) whose origin is hypothesized to have been in the late Cretaceous–early Tertiary. Some implications of the distribution of Tikuna for theories on the origin of Costa Rica’s biota are discussed.

Crustal structure of the ancestral northwestern California forearc region from seismic reflection imaging: implications for convergent margin tectonics

Analysis of a suite of 2-D seismic reflection profiles reveals that the northwestern Sacramento Valley and eastern Coast Range foothills, northern California, are underlain by a system of blind, west-dipping thrust faults. Homoclinally east-dipping and folded Mesozoic marine forearc strata exposed along the western valley margin define the forelimbs of northeast-vergent fault-propagation folds developed in the hanging walls of the thrusts. Exhumed coherent blueschists of the accretionary complex and attenuated remnants of the ophiolitic forearc basement presently exposed in the eastern Coast Ranges are in the hanging wall of the blind thrust system, and have been displaced from their roots in the footwall. Deep, east-dipping magnetic reflectors in the footwall of the thrust system may be fragments of sheared, serpentinized and attenuated ophiolitic basement. Restoration of slip on the thrusts suggests that the Coast Range fault, which is the exposed structural contact between the coherent blueschists and attenuated ophiolite, originally dipped east and is associated with the east-dipping magnetic reflectors in the footwall. This interpretation of the reflection data is consistent with previous inferences about the deep structure in this region, and supports a two-stage model for blueschist exposure in the eastern Coast Ranges: (1) blueschist exhumation relative to the forearc basin by attenuation of the ophiolitic basement along the east-dipping Coast Range fault system in late Cretaceous; (2) blueschists, attenuated ophiolite, and forearc strata all were subsequently uplifted and folded in the hanging wall of the blind thrust system beginning in latest Cretaceous–early Tertiary. The blind thrust system probably rooted in, and was antithetic to, the east-dipping subduction zone beneath the forearc region. Active transpressional plate motion in western California is locally accommodated, in part, by reactivation of blind thrust faults that originally developed during the convergent regime.

Timing of Precambrian melt depletion and Phanerozoic refertilization events in the lithospheric mantle of the Wyoming Craton and adjacent Central Plains Orogen

Garnet peridotite xenoliths from the Sloan kimberlite (Colorado) are variably depleted in their major magmaphile (Ca, Al) element compositions with whole rock Re-depletion model ages generally consistent with this depletion occurring in the mid-Proterozoic. Unlike many lithospheric peridotites, the Sloan samples are also depleted in incompatible trace elements, as shown by the composition of separated garnet and clinopyroxene. Most of the Sloan peridotites have intermineral Sm–Nd and Lu–Hf isotope systematics consistent with this depletion occurring in the mid-Proterozoic, though the precise age of this event is poorly defined. Thus, when sampled by the Devonian Sloan kimberlite, the compositional characteristics of the lithospheric mantle in this area primarily reflected the initial melt extraction event that presumably is associated with crust formation in the Proterozoic—a relatively simple history that may also explain the cold geotherm measured for the Sloan xenoliths. The Williams and Homestead kimberlites erupted through the Wyoming Craton in the Eocene, near the end of the Laramide Orogeny, the major tectonomagmatic event responsible for the formation of the Rocky Mountains in the late Cretaceous–early Tertiary. Rhenium-depletion model ages for the Homestead peridotites are mostly Archean, consistent with their origin in the Archean lithospheric mantle of the Wyoming Craton. Both the Williams and Homestead peridotites, however, clearly show the consequences of metasomatism by incompatible-element-rich melts. Intermineral isotope systematics in both the Homestead and Williams peridotites are highly disturbed with the Sr and Nd isotopic compositions of the minerals being dominated by the metasomatic component. Some Homestead samples preserve an incompatible element depleted signature in their radiogenic Hf isotopic compositions. Sm–Nd tie lines for garnet and clinopyroxene separates from most Homestead samples provide Mesozoic or younger “ages” suggesting that the metasomatism occurred during the Laramide. Highly variable Rb–Sr and Lu–Hf mineral “ages” for these same samples suggest that the Homestead peridotites did not achieve intermineral equilibrium during this metasomatism. This indicates that the metasomatic overprint likely was introduced shortly before kimberlite eruption through interaction of the peridotites with the host kimberlite, or petrogenetically similar magmas, in the Wyoming Craton lithosphere.

STRATIGRAPHIC CONTROLS ON PALAEOZOIC PETROLEUM SYSTEMS, GHADAMES BASIN, LIBYA

The distribution of oil‐ and gasfields in the Libyan Ghadames Basin is strongly influenced by the stratigraphic architecture of the Silurian‐Devonian succession. This interval encompasses part of two (second‐order) super‐sequences separated by the Caledonian unconformity: (i) a late Ashgillian‐Silurian super‐sequence, comprising the Melez Chograne‐Memouniat periglacial lowstand, the Tanezzuft transgressive to early highstand shales and the Acacus Formation late highstand systems tracts; and (ii) the lower part of a composite Devonian‐Carboniferous super‐sequence terminated by the Hercynian unconformity, which includes the lowstand Tadrart fluvial‐shallow marine sands, the Ouan Kasa (transgressive) and the Aouinet Ouinene transgressive to early highstand systems tracts of Devonian age.Hydrocarbon accumulations in Acacus, Tadrart and later Devonian reservoirs have been charged from a basal Silurian (Tanezzuft Formation) “hot” radioactive shale source rock. The Acacus and Tadrart sandstones provided regional migration conduits sealed by intra‐Acacus, intra‐Devonian (Emghaet Formation) and Permian (Bir Al Jaja Formation) shales. Regional stratigraphic continuity of these migration conduits permitted the development of low to moderate impedance petroleum systems dominated by lateral migration.Basin reconstructions and burial history modelling suggest three expulsion peaks from the basal Tanezzuft Formation source rock, preceding major periods of Hercynian, Austrian and Alpine (mid‐late Tertiary) uplift and exhumation. Hydrocarbons trapped prior to the Hercynian event were dispersed during later periods of structural deformation. Some pre‐Austrian accumulations may have survived locally in more robust traps. However, most if not all of the oil‐ and gasfields discovered in the Late Silurian ‐ Devonian succession were probably charged during the Late Cretaceous—Early Tertiary, only to suffer partial dispersal during later uplift, tilting and flushing. Three petroleum systems can be distinguished within the basin: (i) a Tanezzuft ‐Acacus system to the north; (ii) a Tanezzuft ‐ Tadrart system to the south; and (iii) a Tanezzuft ‐Acacus/Tadrart(+) system in the centre, with some leakage into overlying Devonian and Carboniferous sandstones locally.

Phylogeny and Systematics of Holasteroid Echinoids and Their Migration Into the Deep‐sea

Holasteroids are a major clade of irregular echinoids that today is confined to the deep‐sea, but which has a rich fossil record extending back to the start of the Cretaceous Period. A cladistic analysis of this clade, encompassing the great majority of living and fossil genera, is undertaken based on new data on test architecture. The classification of the group is reviewed and a new hierarchical scheme is proposed. The highly atypical apical disc structure of pourtalesiid holasteroids is discussed and critical data for understanding its plate homologies are provided by the most primitive pourtalesiid, the Late Cretaceous Galeaster. This supports the idea that plates previously identified as posterior genital plates in pourtalesiids are in fact ocular plates II and IV. An evolutionary tree for holasteroids is constructed by calibrating the cladogram against the stratigraphic record. This is then used to examine how holasteroid diversity and ecology have changed through time. Holasteroids have migrated into the deep‐sea at least four times independently. Three of these lineages can be traced back to sister taxa living in deep‐water continental shelf environments in the Late Cretaceous–early Tertiary, while a fourth represents a very much younger migration event, possibly Late Miocene in age.

Development of a compositional kinetic model for hydrocarbon generation and phase equilibria modelling: a case study from Snorre Field, Norwegian North Sea

AbstractThe possibility to model petroleum composition during hydrocarbon generation as well as the PVT behaviour of the fluids during migration has only recently become available in modern basin modelling software packages. While various compositional kinetic models of petroleum generation have been published in the past few years, none of the studies presented have attempted to match the composition, physical properties and phase state of known petroleum accumulations. Using compositional data from closed-system non-isothermal pyrolysis experiments, we developed a compositional kinetic model of hydrocarbon generation for a marine Type II source rock, which uses 13 components to describe the generated fluid. The data format selected is compatible with the compositional resolution used in reservoir engineering, thus allowing a direct comparison of predicted compositions and phase behaviour with PVT data of natural fluids. Compositional predictions of the model were tuned to a well-documented maturity sequence from the Tampen Spur, Norway, and the calibrated model implemented in a 2D basin modelling study of the Snorre Field, Norway. The results of the modelling led to an excellent correlation between predicted and reported reservoir fluid properties (formation volume factor, GOR and saturation pressure) for the present-day situation. The results indicate that the Snorre reservoir has received a continuous charge since the late Cretaceous-early Tertiary and that it most likely contained a two-phase system prior to the latest Plio-Pleistocene burial and overpressuring event.

Safah field, Oman: retrospective of a new-concept exploration play, 1980 to 2000

ABSTRACT The Safah oil field was discovered in 1983 on the north-plunging Lekhwair Arch of northwest Oman. The arch lacks any significant structural closure and the accumulation is stratigraphically trapped within chalky high porosity-low permeability Upper Shu’aiba carbonates of mid-late Aptian age. The complexity of its trapping geometry, internal reservoir architecture, reservoir quality and hydrocarbon charge history precluded easy explanation and geological models used to describe the field evolved quite significantly over time to accommodate new data and changing regional perspectives. These had a profound influence, first upon the decision to test what was a speculative new concept exploration prospect and later during appraisal and development, in defining an optimum static reservoir model, history matching and efficient field management strategies. The original play concept developed out of a loosely constrained regional structural and stratigraphic synthesis. Early isopach mapping had identified an enormous paleohigh on the North Lekhwair Arch, which appeared well placed to receive charge in the later Cretaceous and early Tertiary. This was tilted northward during the late Miocene, when any structurally trapped oil or gas must have been spilled to the south. However, nearby analogs suggested that the northeastern margin of the Upper Shu’aiba intraplatform Bab Basin crossed the arch in the vicinity of the paleohigh and it seemed possible that remigrating hydrocarbons might have been stratigraphically trapped against the impermeable basinal facies equivalents of Shu’aiba platform carbonates. Safah-1x was drilled to test this hypothesis, just to the north of the weakly defined Upper Shu’aiba shelf break. It encountered a thin pay zone at the northern end of what proved to be a more than 1 billion barrels STIIOP accumulation. The complexity of the field became increasingly apparent during appraisal drilling. Both differentiated shelf-to-basin and layered mid-shelf ramp depositional models were proposed to describe its unexpectedly heterogeneous internal reservoir architecture. Independent petrographic, fluid property and oil isotope analyses seemingly contradicted more likely stratigraphic correlations and consensus on a static reservoir model proved difficult to reach. As a result, geologically simple layered reservoir descriptions were favored during the early development of the field. However, as the regional perspective improved with better local analogs and increasing amounts of well and seismic data, attention eventually refocused back toward a more sophisticated stratigraphic explanation. The reservoir is now interpreted to be a low-energy mid-Shu’aiba highstand composite sequence with younger lowstand shales and offlapping carbonate shoals to the south. The updip trapping mechanism is far more complex than originally anticipated, formed by discontinuities between the porous lowstand shoals. The enigmatic relationship between stratigraphic architecture and in-reservoir PVT fluid properties and d13C isotope gradients appear to reflect dual charging by a high GOR Jurassic-sourced oil during the late Cretaceous-early Tertiary and low GOR Silurian oils in the Miocene. Internal stratigraphic baffles prevented complete homogenization and the PVT and isotope gradients remain as geochemical palimpsests. This resolution of initially rather contradictory observations was achieved by synthesizing data into a coherent narrative logic, most consistent with the available geological information at all scales, from the regional and general to the local and specific. Although more advanced seismic, petrographic and geochemical technologies certainly encouraged increasingly precise interpretations, the issues they raised were still geological and so still most effectively utilized within the context of such narratives. Ultimately, it was only by assessing these against broader geological perspectives that it proved possible to judge the validity of in-field interpretations with any confidence.

Crustal-Scale Cross-Section of the U.S. Cordillera, California and Beyond, Its Tectonic Significance, and Speculations on the Andean Orogeny

A cross-section across northern California from the San Andreas fault to central Nevada exhibits both major east- and west-vergent structures. East-vergent structures include crustal wedging and fault-propagation folds in the Coast Ranges, emplacement of the Great Valley ophiolitic basement over Sierran basement rocks, early east-vergent structures in the latter, displacement along the eastern margin of the Sierra Nevada batholith, and thrust faults in western Nevada. West-vergent structures include faults within the Franciscan complex and "retrocharriage" structures in the Sierra Nevada A model of evolution of the U.S. Pacific margin emphasizes the role of ophiolites, island arc-continental margin collisions, and subduction of a large oceanic plateau. Early Mesozoic subduction along the Pacific margin of North America was modified by a 165-176 Ma collision of a major intra-oceanic arc/ophiolite complex. A complex SW Pacific-like set of small plates and their boundaries at various times may have been present in southern California between 115 and 40 Ma. Subduction of an oceanic plateau about 85-65 Ma (remnants in the Franciscan) produced east-vergent tectonic wedging in the Coast Ranges, possible thrusting along the eastern Sierra Nevada batholith margin, and development of Rocky Mountain Laramide structures. The "Laramide orogeny" is herein redefined to include all late Cretaceous-Early Tertiary (75-45 Ma) fold-thrust structures from the Pacific Coast to the Rocky Mountains. A speculative model for collisional involvement in the Andean orogeny is also presented, based upon timing of the onset of the Andean orogeny, the presence of oceanic terranes along the western margin of the Andes, and the presence along part of the length of the chain of a remnant marginal basin.