摘要:

We identify four sets of factors governing oil occurrence—climate (especially temperature), mineral nutrients, tectonic factors controlling initial basin formation, and tectonic factors controlling preservation of the oil. We argue that all factors are themselves subject to the framework imposed by plate tectonics. If we are to consider all Phanerozoic oil deposits, the only factor capable of quantitative comparison for all the periods is the first one, in that it is partly a function of latitude.A paleolatitude analysis has been made for both reservoir rocks and preferred source rocks for all petroliferous basins, with results weighted according to total reserves. No statistically satisfactory relationship was found between oil and paleolatitude that would embrace all Phanerozoic deposits. Most Paleozoic oil was formed in rocks deposited in low latitudes, but this may be an accident of preservation. The much larger Mesozoic deposits were similarly related to low paleolatitudes, but this result is heavily biased by the huge reserves of the Persian Gulf. If these are excluded, Mesozoic oil occurs with equal probability in high and in low paleolatitudes. Cenozoic oil is uniformly distributed with respect to paleolatitude.The distribution of oil with time reveals that 71% of all known oil was probably formed in the late Mesozoic, most of it (60%) in the mid-Cretaceous. The first requirement in any general theory of oil occurrence, therefore, is to understand why so much oil was formed near the present Persian Gulf, and to a lesser extent in Middle America, during such a short interval of geological time. We attempt to show that all four controlling factors were optimized in these two places for this brief time-span. In the timetable of plate tectonics, two large marine embayments opened astride the equator in the late Mesozoic, and these may or may not have been connected through the western Mediterranean. One embayment contained the Persian Gulf, and the other, Middle America. The renewal of mantle convection at about −100 m.y. activated these embayments, abruptly increased the rate of sea-floor spreading, and enlarged the oceanic ridges, causing maximum development of warm, shallow seas and releasing, through igneous activity, greatly increased quantities of mineral nutrients.The geometry of subsequent plate activity was such that the Persian Gulf was tectonically protected by the rapid northward movement of the Indian plate (which absorbed most of the impact with the Eurasian plate), and the Gulf of Mexico was protected by the northeastward movement of the Antillean arc.

ISSN:0008-4077    

DOI:10.1139/e74-001

出版商:NRC Research Press    

年代:1974

数据来源: NRC

摘要:

Two stable, ancient, N.R.M. components were isolated by a.f. demagnetization of samples from 29 sites in the Helikian (Middle Proterozoic) anorthosite and adamellite suites of the Mealy Mountains. The directions of the two components are chiefly to the northwest, 20° upward (northwest component) and to the east, 65 °downward (east component). The northwest direction is thought to be the primary igneous T.R.M. and no reversely magnetized sites with this direction were encountered in the Mealy suite. The 11 sites from which the northwesterly component was isolated yielded a mean pole at 179 °W, 8 °N, α95 = 12°. This is some distance away from poles derived from rocks of comparable age from elsewhere in the Canadian Shield and may reflect a 50° clockwise rotation of this section of the Grenville Orogen relative to the Superior after the initial magnetization of the Mealy rocks. Evidence of a second stable component was found in 14 sites (east component); 11 of these are reversely magnetized and three are normally magnetized. The 14 sites with easterly (and reversed easterly) magnetization yielded a combined pole for this component at 182 °W, 38 °S, α95 = 9°. This pole position suggests that the east component was acquired after the rotation of this part of the Grenville Orogen. The cores from 3 sites exhibited two clear end-points during a.f. demagnetization and thus contain both the northwest and the east component.The Helikian Shabogamo Gabbro at the south end of the Labrador Trough was sampled at 13 sites on a line extending 80 km north from the Grenville Front. After a.f. cleaning these sites yielded a pole at 171 °W, 10 °N, α95 = 12°. This pole lies about 20° northwest of poles for rocks of similar age in the Superior Orogen so it is suggested that the rocks of the sample area, though lying a bit north of the line generally accepted as the 'Grenville Front', have been rotated clockwise (about 20°) since intrusion of the gabbro.

ISSN:0008-4077    

DOI:10.1139/e74-002

出版商:NRC Research Press    

年代:1974

数据来源: NRC

摘要:

The Nonacho Group is a folded sequence of sedimentary rocks resting on a crystalline basement of probable Archean age. Both sedimentary and basement rocks are intruded by the Sparrow dikes. Fifty-four samples from 10 dikes have a stable magnetization with mean direction after a.f. cleaning of 131°, +51 °(N = 10,k = 50, α95 = 7°) and a corresponding pole at 12 °N, 069 °W. The presence of reversals and the approximate agreement between dikes and their baked contracts are taken to indicate that the dikes were probably magnetized at the time of intrusion.40Ar–39Ar studies suggest that the probable age of intrusion wasabout1700 m.y. The magnetization of the Nonacho sedimentary rocks collected at 12 sites (40 samples) have a mean direction after thermal cleaning of 148°, + 57 °(N = 12,k = 11, α95 = 14°) without correction for geological dip. The corresponding pole is at 13 °N, 086 °W. The precision after correction for geological dip is low (k = 3), so the magnetization is predominantly post-folding. The thermal decay characteristics and the geometry of the directions indicate that the magnetization of the sedimentary rocks was acquired during cooling following a period of mild heating presumably owing to deep burial. This process of secondary magnetization apparently reached its culmination betweenabout1700 and 1600 m.y., and continued with diminishing intensity untilabout1400 m.y. These results are used to modify the apparent polar wandering path for the Canadian Shield, so that the path now has a sharp bend (Hairpin 40) at about 1800 m.y. rather than at 1950 m.y. as previously postulated.

ISSN:0008-4077    

DOI:10.1139/e74-003

出版商:NRC Research Press    

年代:1974

数据来源: NRC

摘要:

Side-scan sonar imagery indicates the presence of numerous large-scale furrows on the continental shelf northeast of Belle Isle, Newfoundland. The furrows are attributed to the ploughing action of bottom-dragging icebergs. They are typically linear troughs bordered by raised shoulders with an average width of about 30 m, a maximum observed depth of 6.5 m, and a maximum length of at least 3 km. Considered collectively, the furrows have a pronounced north-south trend, which probably reflects the general southerly drift of icebergs in the Labrador Current.

ISSN:0008-4077    

DOI:10.1139/e74-004

出版商:NRC Research Press    

年代:1974

数据来源: NRC

摘要:

Small lenses of massive copper- and zinc-rich sulfide occur in ophiolitic volcanic rocks capping the Bay of Islands Igneous Complex near York Harbour, Newfoundland. Ore was discovered in 1897 and about 50 000 tons were produced between 1898 and 1912. Recent exploratory work has outlined several ore bodies totaling 282 000 tons that contain 1.92% Cu and 4.67% Zn, assuming 20% dilution.The deposits are stratabound, occurring along an intensely altered zone that separates two members of the layered pillow lava succession. Two types of mineralization are present. Lenticular massive bodies ranging in size up to 37 000 tons are composed of brecciated pyrite, sphalerite and chalcopyrite. These are stratigraphically underlain by chalcopyrite-rich stringer mineralization in altered lava.Microscopic and petrochemical investigations indicate that the host lavas have undergone pervasive spilitic alteration and intensive propylitic alteration near ore. Geochemical studies of Co:Ni ratios in pyrite show that Ni content is highest in disseminated pyrite from the propylitized lavas and Co is highest in pyrite from stringer ore. The Co:Ni ratio for all analysed pyrite is greater than unity, suggesting a magmatic-volcanic derivation for the deposits.The geological evidence suggests that the sulfides are volcanogenic and were formed from hydrothermal fluids rich in Fe, Zn, Cu and S. These moved upward along zones of primary brecciation in the lavas, leached Fe, Ni, and Ti from the ferromagnesian minerals and deposited the massive lenses by chemical sedimentation or precipitation from the metal-rich exhalations on or near the sea floor.Comparison of the York Harbour deposits to the recent heavy metal bearing sediments of the Red Sea 'deeps', and to the Cretaceous lavas and orebodies of Cyprus, supports the above genetic interpretation, and indicates that these ores are clearly an integral part of ophiolite complexes. The comparison provides new information about sulfide mineral paragenesis during initial sedimentation, diagenesis, burial, and later metamorphism.

ISSN:0008-4077    

DOI:10.1139/e74-005

出版商:NRC Research Press    

年代:1974

数据来源: NRC

摘要:

The Vendom Fiord Formation near the head of Cañon Fiord, central Ellesmere Island, hitherto thought to be unfossiliferous, contains mid to late Emsian trilete spores. Strata below the Vendom Fiord Formation, separated from it by an unconformity, contain a less diverse assemblage of spores that suggests a mid to late Gedinnian age. The spores are the only fossils known from these beds, and the first palynomorphs to be reported from the Early Devonian rocks of Ellesmere Island.

ISSN:0008-4077    

DOI:10.1139/e74-006

出版商:NRC Research Press    

年代:1974

数据来源: NRC

摘要:

A seasonal cycle in δ13C, varying between −22‰ in early spring and −28‰ in late fall, has been observed in maple leaves growing under natural conditions. A similar δ13C cycle, varying between −25‰ and −30‰ is indicated for grass growing in the same location as the maple leaves. The seasonal cycle appears to be analogous to the diurnal δ13C cycle.14C/12C ratios of various plants from different parts of Canada show that the14C produced by thermonuclear bombs has approximately a half-life of eleven years in the atmosphere over Canada. When due allowance is made for the isotope fractionation effect produced by plants, no significant variations in the14C content in plants from different localities across Canada can be detected. The reported14C peak observed in atmospheric CO2in late summer also shows up in maple leaves but with reduced intensity.14C levels show clearly that the first leaves in spring are composed primarily of carbon assimilated in the previous fall.

ISSN:0008-4077    

DOI:10.1139/e74-007

出版商:NRC Research Press    

年代:1974

数据来源: NRC

摘要:

Four erosional unconformities have been recognized within the Mesozoic-Cenozoic succession on the Scotian Shelf, on the basis of data from high resolution seismic reflection profiles. Older unconformities are known from well data and others may be revealed by detailed biostratigraphic studies.The oldest of the four unconformities discussed in this paper is of Early Cretaceous age and appears to mark, with discordance, the boundary between Jurassic and Cretaceous strata on the western part of the shelf. A second angular unconformity, of Late Cretaceous age, has been recognized on the central part of the shelf where the basal part of the Banquereau Formation (Tertiary and uppermost Cretaceous) oversteps the zero-edge of the Wyandot Formation (Upper Cretaceous) and lies upon truncated beds of the Dawson Canyon Formation (Upper Cretaceous). Cut-and-fill relationships characterize a third unconformity developed during Early Tertiary time. A fourth unconformity was developed in Late Tertiary – Pleistocene time by fluvial processes and later by glacial processes. Although in many areas the latest unconformity appears to be the most conspicuous one on the shelf, its configuration closely follows the geomorphic expression developed during the previous period of erosion. The regional extent of the Cretaceous unconformities is not known, and they might only occur near basin margins and on structural and basement highs.

ISSN:0008-4077    

DOI:10.1139/e74-008

出版商:NRC Research Press    

年代:1974

数据来源: NRC

摘要:

Reflection profiles shot about 10 km north of the Edmonton seismic observatory (EDM) indicate dips within the crust of 15 to 20° southeast, with a prominent reflecting horizon at 20 km apparently being offset 4 km by a fault. The average velocity in the crust to this horizon is 6.3–6.4 km/s. Deeper reflections tentatively correlated with the "Riel" discontinuity mapped in southern Alberta indicate a velocity in a second layer of 6.5 km/s to a depth of 32 km, with the base of the crust being essentially horizontal at 35.5 km. The lowermost portion of the crust appears to be significantly thinner here than in southern Alberta, although the general features of the seismic records appear similar in both locations.Velocities are determined by a modified version of the velocity spectrum technique, which does not require common depth point data.

ISSN:0008-4077    

DOI:10.1139/e74-009

出版商:NRC Research Press    

年代:1974

数据来源: NRC

摘要:

Gravity profiling at five different sites in an area of ice-cored topography indicates the usefulness of the technique as a method for the detection of ground ice and for estimating the thickness of the excess ice. Bouguer anomaly profiles using a density of 2.0 Mgm−3provide a quick method of assessing the relative amount of ice along a profile, the thickness of ice and the elevation being inversely proportional to the Bouguer anomaly value along each profile. The average amount of excess ice in the topography along the profiles is obtained by removing linear trends, obtaining the Bouguer density of the topography, and calculating the proportion of frozen saturated sediment and ice required to produce this density. Variations in the thickness of the body are obtained from significant gravity residuals. Finally, models are given to show the distribution of the excess ice with the aid of supplementary information. The above technique is unable to detect changes in the amount of excess ice that have a linear trend over the complete profile or a uniform slab of ice underlying the complete profile. A model for a pingo is constructed using stacked concentric cylinders. Geologic data from the region indicate that all models give reliable estimates of the amount of excess ice and probable thermokarst subsidence if the area were thawed.

ISSN:0008-4077    

DOI:10.1139/e74-010

出版商:NRC Research Press    

年代:1974

数据来源: NRC