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Sierra Leone
The 2017 Regent Landslide, Freetown Peninsula, Sierra Leone
Inclusions in an isoferroplatinum nugget from the Freetown Layered Complex, Sierra Leone
The mineralogy, geochemistry and genesis of the alluvial platinum-group minerals of the Freetown Layered Complex, Sierra Leone
West Africa: The World’s Premier Paleoproterozoic Gold Province
Re-Os Geochronological Evidence for Multiple Paleoproterozoic Gold Events at the Scale of the West African Craton
Magnesium isotope evidence for a recycled origin of cratonic eclogites
THE FIRST REPORT OF PLATINUM-GROUP MINERALS IN MAGNETITE-BEARING GABBRO, FREETOWN LAYERED COMPLEX, SIERRA LEONE: OCCURRENCES AND GENESIS
Abstract In recent years deep-water petroleum exploration has been booming. The Gulf of Mexico, some West African regions, and Brazil are leading this growing activity. Current deep-water plays focus on presalt, subsalt, stratigraphic pinch-out and deep-water folded belt targets. Comparisons of conjugated margins across the Atlantic are commonly used by explorationists to extend the prospectivity of known plays. However, what are deep-water plays? Are they only plays presently located in water depths more than 2000 m water depths or do they also include plays developed initially in relatively deep oceanic tectonic settings but are now underneath shallow water? For that matter, what about plays which developed in relatively shallow water but are now located in water depths greater than the continental slope. Present deep-water tectonic settings are mostly compressional toe-thrust regions of larger massive gravitational collapses related to (A) major deltas (B) allochthonous salt provinces, or (C) subduction-related accretionary wedges. Back-arc extensional basins in deep-water settings are underexplored, except for the Black Sea. Other plays presently in deep waters but from a geologic perspective initially formed in relatively shallow marine water and subsequently subsided are the pre-evaporitic plays of the Campos-Santos basins in Brazil and Angola offshore basins. Some elements of deep-water petroleum systems are still poorly understood. Although the presence of widespread deep-water source beds on oceanic crust is mostly known through DSDP/ODP wells, the role of tectonics and volcanic activity in the generation and maturation has yet to be adequately evaluated. Classical upwelling models intended to explain marine source beds may need to be refined. Note that most current deep-water-plays involve siliciclastics. There is no reason to preclude deep-water carbonate plays, the reservoirs of which are analogous to the outer platform, relatively deep-water pelagic carbonates producing in the Bay of Campeche in Mexico. Traditional seismic stratigraphy views unconformities as being caused by eustatic sea level changes. However, the origin of widespread deep-water unconformities needs to be further elucidated. An increasing number of deep crustal seismic surveys along West Africa show low-angle extensional detachments similar to those found on the Galicia margin of Spain and Portugal and comparable with tectonic styles of the Basin and Range Province of the western United States or else in the western Mediterranean ( e.g. , Western Alps or Betic Cordillera). Classical rifting models always need to be updated to be “on target” with new and often surprising seismic observations. In addition to the four principal types of deep-water plays currently being explored, additional plays should be found in deep-water carbonates, volcanic margins including hot spots or volcanic-lineaments, and subsided rifted systems; these may account for significant yet to be explored future plays in many parts of the “deep-water” world.
New insights into prospectivity of Liberia-Sierra Leone Basin because of improvements in seismic acquisition and processing
Developments in the Understanding and Application of Lithium Isotopes in the Earth and Planetary Sciences
Regional 2D seismic interpretation and exploration potential of offshore deepwater Sierra Leone and Liberia, West Africa
The Origin of Platinum Group Minerals from the Freetown Intrusion, Sierra Leone, Inferred from Osmium Isotope Systematics
Mid- to late Quaternary environments and stratigraphy of the southern Sierra Leone shelf, West Africa
Open-system magma chamber process in the Freetown Complex of Sierra Leone; evidence from Zone 3
The mobility of the platinum-group elements in the soils of the Freetown Peninsula, Sierra Leone
Spin glass behavior in a single crystal of chromite
Tropical Residual Soils Geological Society Engineering Group Working Party Report
Tectonic fabric of Sierra Leone, West Africa: implications for Mesozoic continental breakup
Contrasting accreted terranes in the southern Appalachian orogen and Atlantic–Gulf Coastal Plains and their correlations with West African sequences
A variety of lithostratigraphic units may be outlined within the pre-Cretaceous crystalline basement of the Atlantic and Gulf Coastal Plains. An expanse of undeformed granite occurs within central peninsular Florida; biotite within the pluton yields 40 Ar/ 39 Ar plateau dates of ca. 525 to 530 Ma. The high-level character of the granite and its emplacement age are similar to post-kinematic granitic plutons within northern portions of the Rokelide orogen in Guinea, West Africa. A sequence of calc-alkaline, felsic volcanic-volcaniclastic rocks, together with associated hypabyssal granite, occurs throughout the basement terrane. These are variably altered and display markedly discordant 40 Ar/ 39 Ar age spectra. Crystallization ages are likely best represented by ca. 675 to 690 Ma Rb-Sr whole-rock and 40 Ar/ 39 Ar hornblende plateau ages previously reported for correlative calc-alkaline granites exposed in southeastern Senegal. A Lower Ordovician–Devonian sedimentary sequence with non-Laurentian paleontological characteristics occurs within the Suwannee basin of the northern Florida subsurface. Detrital muscovite from Lower Ordovician sandstones in the succession records 40 Ar/ 39 Ar plateau ages of ca. 500 to 510 Ma. The sequence is similar to that within the Bové basin in Senegal and Guinea. A small area of high-grade metamorphic rocks occurs in the east-central Florida subsurface. Hornblende from this terrane records 40 Ar/ 39 Ar plateau ages of ca. 485 to 495 Ma. The character and post-metamorphic cooling history of these rocks are similar to that of units within the Rokelide orogen in Sierra Leone and Liberia, West Africa. In the subsurface Wiggins Arch of southwestern Mississippi, hornblende and biotite from interlayered gneiss and amphibolite record similar 40 Ar/ 39 Ar plateau ages of ca. 300 to 310 Ma. Phyllite from a lower-grade metasedimentary sequence penetrated in the Wiggins Arch of southwestern Alabama records a 40 Ar/ 39 Ar whole-rock plateau age of ca. 315 Ma. Characteristics of the pre-Cretaceous basement units suggest that they represent an extension of the Mauritanide, Bassaride, and Rokelide orogens of West Africa. There is no apparent record of Paleozoic tectonothermal activity in central and southern portions of the Atlantic and Gulf Coastal Plain subsurface basement terrane. This is in marked contrast to complete late Paleozoic reworking of basement units penetrated in the area of the Wiggins Arch. This suggests that a major dextral transcurrent fault system was likely active during the late Paleozoic, and that proximal basement units were directly involved in collisional aspects of Pangea assembly. The various units of the Atlantic and Gulf Coastal Plain basement are not correlative with any of the northern non-Laurentian terranes exposed in the southern Appalachian orogen (e.g., Carolina terrane of the eastern Piedmont) which had earlier accreted to exterior positions along the eastern margin of the North American craton. These were transported into their present structural positions along a basal décollement during late Paleozoic collision of Gondwana and Laurentia, and are separated from the stranded Gondwana units of the Coastal Plain basement by a suture approximately marked by the Brunswick-Altamaha magnetic anomaly.