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USGS1999

Chapter 4: Compositional zoning and petrology of the Thera pyroclastics

Compositional zoning is a common feature of pyroclastic deposits erupted from calderas worldwide (Smith 1979; Hildreth 1981; Bacon & Druitt 1988; De Silva 1991; Feeley & Davidson 1994). Compositionally zoned tuffs provide a geologically instantaneous snapshot of the vertical stratigraphy and density stratification in the chamber immediately before eruption. Magma bodies are commonly zoned in density, with light, relatively cool, silicic magma overlying hotter, more mafic magma. Vertical zonation can arise by a number of processes, including crystallization and convective fractionation on the chamber.

Santorini↗

USGS1999

Chapter 5: Cumulate nodules in the Thera pyroclastics

Gabbroic and dioritic nodules are common in some horizons of the Thera pyroclastics. These nodules contain pyroxene as the main ferromagnesian phase in contrast to the early Akrotiri centre, which contain nodules with abundant hornblende (Nicholls 1971a; Pichler & Kussmaul 1972). Calcsilicate nodules and gabbroic nodules have also been recognized in Santorini lavas (Fouqu6 1879; Nicholls 1971a). Previous studies of Santorini volcanic rocks indicate that crystal fractionation has played an important role in generating the compositional range basalt-andesite-dacite-rhyodacite (Nicholls 1971a; Mann 1983; Huijsmans 1985; Barton & Huijsmans 1986). This interpretation is also consistent with much of the petrological data presented in Chapter 4 and the geochemical data and models presented in Chapter 6. However, disequilibrium phenocryst assemblages in some lavas (Huijsmans 1985; Chapter 4), banded pumice clasts in many pyroclastic deposits (Druitt et al. 1989; Chapters 4), isotopic disequilibria in phenocrysts (Pyle et al. 1988; Chapters 6 and 7) and complex zoning patterns in plagioclase crystals (Stamatelopoulou-Seymour et al. 1990) indicate that magma mixing has also occurred. In addition, trace element and isotopic abundances can be interpreted in terms of assimilation of continental crust during fractionation (Barton et al. 1983; Stamatelopoulou-Seymour et al. 1990; Chapter 6). The nodules provide information on petrogenetic processes that have influenced the evolution of Santorini magmas. Their petrology is documented in this chapter and compared with that of associated juvenile ejecta. The nodules are identified as cumulates from andesitic and dacitic magmas, con- firming the importance of fractional crystallization in the petrogenesis of Santorini magmas. They also demonstrate that mafic plutonic rocks can be formed from intermediate to silicic magmas.

Santorini↗

USGS1999

Chapter 7: Studies of short-lived radionuclides in Santorini volcanics

Disequilibria between the short-lived radioactive nuclides 226 Ra, 230 Th and 238 U in young volcanic rocks can be used both for dating the time of crystallisation of a rock, and as isotopic tracers of the sources of magmas and the chemical evolution of volcanic systems. The principles of these methods have been reviewed extensively elsewhere (Condomines et al . 1988; Gill et al . 1992; Gill & Condomines 1992; Macdougall 1995). In this chapter, we present new 238 U- 230 Th- 226 Ra disequilibria data for the Thera pyroclastics and for selected lavas. These data complement the radiogenic isotopic data presented in Chapter 6 on the compositional evolution through time of the pyroclastic ejecta. In particular, we find that the ( 230 Th/ 232 Th) isotopic ratio corrected to the time of eruption varies in a systematic way with time through the second cycle of the Thera pyroclastics and variations in this ratio are correlated with the variations of 87 Sr/ 86 Sr, 143 Nd/ 144 Nd and Pb isotopes.

Santorini↗

USGS1999

Chapter 8: The evolution of Santorini

There now follows a synthesis of the main results of this study. We interpret the observations in terms of the interplay between magmatic, tectonic and volcanic processes and place these processes within a global context. Santorini has displayed a wide variety of igneous and volcanic phenomena over a period of at least 650 000 years of almost continuous and focused volcanism. Perhaps the most intriguing issues are the causes of volcanic and magmatic cyclicity, the secular variations in magma composition and the way in which magmatic processes have influenced the style of volcanism.

Santorini↗

USGS1999

Appendix 1: Analytical methods and errors

Thirty eight K-Ar and eight 40Ar/39Ar high-precision age determinations were made at the US Geological Survey, Menlo Park, on a total of 22 rocks from the entire volcanic field. Duplicate or triplicate determinations were carried out on 14 samples in order to improve analytical precision. All ages were measured on whole-rock samples selected after thin-section examination. Most of the samples meet the usual criteria for whole-rock dating (Mankinen © Dal-rymple 1972), but some contain minor amounts of glass and a few samples are very glassy. The samples selected for dating were crushed to 0.5-lmm (-18 to +35 mesh). For K-Ar dating aliquots weighing c. 25 g were used for the Ar measurements. A 10 g aliquot was ground to -200 mesh and splits of the powder were used for K20 measurements, which were made in duplicate on each of two separate splits of sample powder by flame photometry after lithium metaborate fusion and dissolution (Ingamells 1970). Ar analyses were by isotope-dilution mass spectrometry using a high-purity (>99.9%) 38Ar tracer and techniques and equipment described previously (Dalrymple & Lanphere 1969). All samples for Ar extraction were baked overnight at 280°C. Mass analyses were done on a 22.68 cm radius, multiple-collector mass spectrometer with a nominal 90° sector magnet, using automated data collection (Stacey etal 1981). Errors given for the calculated K-Ar ages of individual measurements are estimates of the standard deviation of analytical precision. The errors were calculated using formulae derived by Cox & Dalrymple (1967) and Dalrymple & Lanphere (1969).

Geological Society, London, Memoirs↗

USGS1999

Appendix 2: Tabulated analytical data

Details of analytical methods and errors are given in Appendix 1. bd means below detection and na not analysed. Samples numbers which begin with the letter S are from Druitt (1983) and those that begin with LS are from Edwards (1994). Sample numbers which begin with the number 8 are either from Mellors (1988) or Pyle (1990 b ). Subunits of each major tuff of the Thera pyroclastics are denoted by upper case letters (A to D). These are listed in Table 3.4. In the case of the minor sequences (Appendix 2.3), the sequence abbreviations (M6 to M8) are shown in Fig. 3.17 and the individual unit abbreviations (CD1 to CD10) are from Edwards (1994).

Geological Society, London, Memoirs↗

USGS2003

Current controlled deposition on the Wilkes Land continental rise, Antarctica

Turbidite, contourite and hemipelagic deposition are the main components of Wilkes Land continental rise sedimentation above the regional unconformity WL2. On the continental shelf, unconformity WL2 marks the start of shelf progradation, which is interpreted to correspond with the onset of glacial conditions in this segment of the east Antarctic margin. Unusually large (i.e. up to 900 m relief and 18 km between levee crests) channel-levee deposits, and high relief (up to 490 m) mounded contourite-style deposits develop above unconformity WLlb. Unconformity WLlb overlies unconformity WL2 and is interpreted to have formed under a fully continental glacial regime where ice streams reached the palaeo-continental shelf edge. Based on an analysis of multichannel seismic profiles and sediment cores, we differentiate three phases in the development of the sedimentary unit between WLlb and the present seafloor. From older to younger these are: Phase 1, dominated by turbidite deposition; Phase 2, dominated by turbidite and contourite deposition with significant mound building; and Phase 3, dominated by turbidite and contourite deposition without active mound building. We hypothesize that building of the mounds during Phase 2 corresponded with times of expansion of the Antarctic ice-sheet when vast amounts of sediment were eroded from the continent and continental shelf. The large amount of unsorted glacial sediment supplied to the outer shelf apparently travelled down the slope canyons and rise channels as turbidity current flows to feed the usually large continental rise channel-levee complexes. The suspended fines of the turbidity flows were then entrained in a palaeo-nepheloid layer and carried by the westward flowing palaeo-contour currents until their deposition in the mounds. During Phase 3, sediment supply to the continental rise, although important in volume and capable of turbidite and contour-current deposition, was insufficient to support further building of the mounds. We believe the decrease in sediment supply to the continental rise from Phase 2 to Phase 3 could be the result of a change on sediment depocentres, with most of the sediment supplied to the margin during Phase 3 being trapped on the continental shelf. We believe that ultimately these changes are related to the stage of glacial evolution of the continent.

Geological Society, London, Memoirs↗

USGS2011

Geology and petroleum potential of the Timan-Pechora Basin Province, Russia

The Timan–Pechora Basin Province represents the northeastern-most cratonic block of Eastern European Russia. More than 16 billion barrels of oil (BBO) and 40 trillion cubic feet of gas (TCFG) have been discovered in this basin. Three geological assessment units (AU) were defined for assessing the potential for undiscovered oil and gas resources. The NW Izhma Depression AU encompasses all potential structures and reservoirs in the northwestern part of the Izhma–Pechora Depression, but this part of the basin contains little source and reservoir rocks and so was not assessed quantitatively. The Main Basin Platform AU includes all structures and reservoirs that developed in the central part of the basin where the tectonic evolution and development of petroleum systems were complex. The Foredeep Basins AU includes all potential reservoirs within the thick sedimentary section of the foredeep basins developed during the Permo-Triassic Uralian Orogeny. For the Timan–Pechora Basin Province, the estimated means of undiscovered resources are 3.3 BBO, 17 TCFG and 0.3 billion barrels of natural gas liquids (BBNGL). For the areas of the AUs north of the Arctic Circle in the Timan–Pechora Basin Province, the estimated means of undiscovered resources are 1.7 BBO, 9.0 TCFG and 0.2 BBNGL. The Pechora Sea was assessed with the South Barents Sea Province and is not included in this assessment.

Pechora Basin Province↗

USGS2011

Geology and petroleum potential of the east Barents Sea Basins and Admiralty Arch

The US Geological Survey (USGS) recently assessed the potential for undiscovered oil and gas resources of the East Barents Basins and Novaya Zemlya Basins and Admiralty Arch Provinces as part of the USGS Circum-Arctic Resource Appraisal. These two provinces are located NE of Scandinavia and the northwestern Russian Federation, on the Barents Sea Shelf between Novaya Zemlya to the east and the Barents Platform to the west. Three assessment units (AUs) were defined in the East Barents Basins for this study – Kolguyev Terrace Assessment Unit (AU), South Barents Basin and Ludlov Saddle AU, and North Barents Basin AU. A fourth, defined as Novaya Zemlya Basins and Admiralty Arch AU, is coincident with the Novaya Zemlya basins and Admiralty Arch Province. These AUs, all lying north of the Arctic Circle, were assessed for undiscovered, technically recoverable resources resulting in total estimated mean volumes of approximately 7.4 billion barrels of crude oil, 318 trillion cubic feet of natural gas and 1.4 billion barrels of natural gas liquids.

Geological Society Memoir↗

USGS2011

Sedimentary successions of the Arctic Region (58–64° to 90°N) that may be prospective for hydrocarbons

A total of 143 sedimentary successions that contain, or may be prospective for, hydrocarbons were identified in the Arctic Region north of 58–64°N and mapped in four quadrants at a scale of 1:11 000 000. Eighteen of these successions (12.6%) occur in the Arctic Ocean Basin, 25 (17.5%) in the passive and sheared continental margins of the Arctic Basin and 100 (70.0%) on the Circum-Arctic continents of which one (<1%) lies in the active margin of the Pacific Rim. Each succession was assigned to one of 13 tectono-stratigraphic and morphologic classes and coloured accordingly on the map. The thickness of each succession and that of any underlying sedimentary section down to economic basement, where known, are shown on the map by isopachs. Major structural or tectonic features associated with the creation of the successions, or with the enhancement or degradation of their hydrocarbon potential, are also shown. Forty-four (30.8%) of the successions are known to contain hydrocarbon accumulations, 64 (44.8%) are sufficiently thick to have generated hydrocarbons and 35 (24.5%) may be too thin to be prospective.

Geological Society Memoir↗

USGS2011

Geology and petroleum potential of the Arctic Alaska petroleum province

The Arctic Alaska petroleum province encompasses all lands and adjacent continental shelf areas north of the Brooks Range–Herald Arch orogenic belt and south of the northern (outboard) margin of the Beaufort Rift shoulder. Even though only a small part is thoroughly explored, it is one of the most prolific petroleum provinces in North America with total known resources (cumulative production plus proved reserves) of c . 28 BBOE. The province constitutes a significant part of a displaced continental fragment, the Arctic Alaska microplate, that was probably rifted from the Canadian Arctic margin during formation of the Canada Basin. Petroleum prospective rocks in the province, mostly Mississippian and younger, record a sequential geological evolution through passive margin, rift and foreland basin tectonic stages. Significant petroleum source and reservoir rocks were formed during each tectonic stage but it was the foreland basin stage that provided the necessary burial heating to generate petroleum from the source rocks. The lion's share of known petroleum resources in the province occur in combination structural–stratigraphic traps formed as a consequence of rifting and located along the rift shoulder. Since the discovery of the super-giant Prudhoe Bay accumulation in one of these traps in the late 1960s, exploration activity preferentially focused on these types of traps. More recent activity, however, has emphasized the potential for stratigraphic traps and the prospect of a natural gas pipeline in this region has spurred renewed interest in structural traps. For assessment purposes, the province is divided into a Platform assessment unit (AU), comprising the Beaufort Rift shoulder and its relatively undeformed flanks, and a Fold-and-Thrust Belt AU, comprising the deformed area north of the Brooks Range and Herald Arch tectonic belt. Mean estimates of undiscovered, technically recoverable resources include nearly 28 billion barrels of oil (BBO) and 122 trillion cubic feet (TCF) of nonassociated gas in the Platform AU and 2 BBO and 59 TCF of nonassociated gas in the Fold-and-Thrust Belt AU.

Alaska↗

USGS2011

Regional magnetic domains of the Circum-Arctic: A framework for geodynamic interpretation

We identify and discuss 57 magnetic anomaly pattern domains spanning the Circum-Arctic. The domains are based on analysis of a new Circum-Arctic data compilation. The magnetic anomaly patterns can be broadly related to general geodynamic classification of the crust into stable, deformed (magnetic and nonmagnetic), deep magnetic high, oceanic and large igneous province domains. We compare the magnetic domains with topography/bathymetry, regional geology, regional free air gravity anomalies and estimates of the relative magnetic ‘thickness’ of the crust. Most of the domains and their geodynamic classification assignments are consistent with their topographic/bathymetric and geological expression. A few of the domains are potentially controversial. For example, the extent of the Iceland Faroe large igneous province as identified by magnetic anomalies may disagree with other definitions for this feature. Also the lack of definitive magnetic expression of oceanic crust in Baffin Bay, the Norwegian–Greenland Sea and the Amerasian Basin is at odds with some previous interpretations. The magnetic domains and their boundaries provide clues for tectonic models and boundaries within this poorly understood portion of the globe.

Geological Society Memoir↗

USGS2011

Geology and petroleum potential of the West Greenland-East Canada Province

The US Geological Survey (USGS) assessed the potential for undiscovered oil and gas resources of the West Greenland–East Canada Province as part of the USGS Circum-Arctic Resource Appraisal programme. The province lies in the offshore area between western Greenland and eastern Canada and includes Baffin Bay, Davis Strait, Lancaster Sound and Nares Strait west of and including part of Kane Basin. A series of major tectonic events led to the formation of several distinct structural domains that are the geological basis for defining five assessment units (AU) in the province, all of which are within the Mesozoic–Cenozoic Composite Petroleum System. Potential petroleum source rocks include strata of Ordovician, Lower and Upper Cretaceous, and Palaeogene ages. The five AUs defined for this study – the Eurekan Structures AU, NW Greenland Rifted Margin AU, NE Canada Rifted Margin AU, Baffin Bay Basin AU and the Greater Ungava Fault Zone AU – encompass the entire province and were assessed for undiscovered technically recoverable resources. The mean volumes of undiscovered resources for the West Greenland–East Canada Province are 10.7×10 9 barrels of oil, 75×10 12 cubic feet of gas, and 1.7×10 9 barrels of natural gas liquids. For the part of the province that is north of the Arctic Circle, the estimated mean volumes of these undiscovered resources are 7.3×10 9 barrels of oil, 52×10 12 cubic feet of natural gas, and 1.1×10 9 barrels of natural gas liquids.

Geological Society Memoir↗

USGS2011

Assessment of NE Greenland: Prototype for development of Circum-ArcticResource Appraisal methodology

Geological features of NE Greenland suggest large petroleum potential, as well as high uncertainty and risk. The area was the prototype for development of methodology used in the US Geological Survey (USGS) Circum-Arctic Resource Appraisal (CARA), and was the first area evaluated. In collaboration with the Geological Survey of Denmark and Greenland (GEUS), eight ‘assessment units’ (AU) were defined, six of which were probabilistically assessed. The most prospective areas are offshore in the Danmarkshavn Basin. This study supersedes a previous USGS assessment, from which it differs in several important respects: oil estimates are reduced and natural gas estimates are increased to reflect revised understanding of offshore geology. Despite the reduced estimates, the CARA indicates that NE Greenland may be an important future petroleum province.

Geological Society Memoir↗

USGS2011

A first look at the petroleum geology of the Lomonosov Ridge microcontinent, Arctic Ocean

The Lomonosov microcontinent is an elongated continental fragment that transects the Arctic Ocean between North America and Siberia via the North Pole. Although it lies beneath polar pack ice, the geological framework of the microcontinent is inferred from sparse seismic reflection data, a few cores, potential field data and the geology of its conjugate margin in the Barents–Kara Shelf. Petroleum systems inferred to be potentially active are comparable to those sourced by condensed Triassic and Jurassic marine shale of the Barents Platform and by condensed Jurassic and (or) Cretaceous shale probably present in the adjacent Amerasia Basin. Cenozoic deposits are known to contain rich petroleum source rocks but are too thermally immature to have generated petroleum. For the 2008 USGS Circum Arctic Resource Appraisal (CARA), the microcontinent was divided into shelf and slope assessment units (AUs) at the tectonic hinge line along the Amerasia Basin margin. A low to moderate probability of accumulation in the slope AU yielded fully risked mean estimates of 123 MMBO oil and 740 BCF gas. For the shelf AU, no quantitative assessment was made because the probability of petroleum accumulations of the 50 MMBOE minimum size was estimated to be less than 10% owing to rift-related uplift, erosion and faulting.

Geological Society Memoir↗

USGS2011

Chapter 50 Geology and tectonic development of the Amerasia and Canada Basins, Arctic Ocean

Amerasia Basin is the product of two phases of counterclockwise rotational opening about a pole in the lower Mackenzie Valley of NW Canada. Phase 1 opening brought ocean–continent transition crust (serpentinized peridotite?) to near the seafloor of the proto-Amerasia Basin, created detachment on the Eskimo Lakes Fault Zone of the Canadian Arctic margin and thinned the continental crust between the fault zone and the proto-Amerasia Basin to the west, beginning about 195 Ma and ending prior to perhaps about 160 Ma. The symmetry of the proto-Amerasia Basin was disrupted by clockwise rotation of the Chukchi Microcontinent into the basin from an original position along the Eurasia margin about a pole near 72°N, 165 W about 145.5–140 Ma. Phase 2 opening enlarged the proto-Amerasia Basin by intrusion of mid-ocean ridge basalt along its axis between about 131 and 127.5 Ma. Following intrusion of the Phase 2 crust an oceanic volcanic plateau, the Alpha–Mendeleev Ridge LIP (large igneous province), was extruded over the northern Amerasia Basin from about 127 to 89–75 Ma. Emplacement of the LIP halved the area of the Amerasia Basin, and the area lying south of the LIP became the Canada Basin.

Geological Society, London, Memoirs↗

USGS2011

US geological survey Circum-Arctic Resource Appraisal (CARA): Introduction and summary of organization and methods

The USGS has assessed undiscovered petroleum resources in the Arctic through geological mapping, basin analysis and quantitative assessment. The new map compilation provided the base from which geologists subdivided the Arctic for burial history modelling and quantitative assessment. The CARA was a probabilistic, geologically based study that used existing USGS methodology, modified somewhat for the circumstances of the Arctic. The assessment relied heavily on analogue modelling, with numerical input as lognormal distributions of sizes and numbers of undiscovered accumulations. Probabilistic results for individual assessment units were statistically aggregated taking geological dependencies into account. Fourteen papers in this Geological Society volume present summaries of various aspects of the CARA.

Geological Society Memoir↗

USGS2011

Oil and gas resource potential north of the Arctic Circle

The US Geological Survey recently assessed the potential for undiscovered conventional petroleum in the Arctic. Using a new map compilation of sedimentary elements, the area north of the Arctic Circle was subdivided into 70 assessment units, 48 of which were quantitatively assessed. The Circum-Arctic Resource Appraisal (CARA) was a geologically based, probabilistic study that relied mainly on burial history analysis and analogue modelling to estimate sizes and numbers of undiscovered oil and gas accumulations. The results of the CARA suggest the Arctic is gas-prone with an estimated 770–2990 trillion cubic feet of undiscovered conventional natural gas, most of which is in Russian territory. On an energy-equivalent basis, the quantity of natural gas is more than three times the quantity of oil and the largest undiscovered gas field is expected to be about 10 times the size of the largest undiscovered oil field. In addition to gas, the gas accumulations may contain an estimated 39 billion barrels of liquids. The South Kara Sea is the most prospective gas assessment unit, but giant gas fields containing more than 6 trillion cubic feet of recoverable gas are possible at a 50% chance in 10 assessment units. Sixty per cent of the estimated undiscovered oil resource is in just six assessment units, of which the Alaska Platform, with 31% of the resource, is the most prospective. Overall, the Arctic is estimated to contain between 44 and 157 billion barrels of recoverable oil. Billion barrel oil fields are possible at a 50% chance in seven assessment units. Undiscovered oil resources could be significant to the Arctic nations, but are probably not sufficient to shift the world oil balance away from the Middle East.

Geological Society Memoir↗