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San Lorenzo Vista Hermosa, Mexico

McG Miller R.,Consulting Geologist
South African Journal of Geology | Year: 2012

Mesoproterozoic crustal evolution of southwestern Africa involved a complex interplay over 600 million years of subduction, platemargin or intraplate sedimentation, volcanism and plutonism followed by terrane amalgamation, tectonism, collision, magmatism, metamorphism and final, late-stage isostatic rebound of high-grade terranes. Most terranes are underlain by ∼2.0 Ga continental crust but the Areachap and Garies Terranes were juvenile Mesoproterozoic features with respective model ages of ∼1.5 to ∼1.3 Ga and ∼1.25 Ga. Pre- and syntectonic metasediments in northwestern Namibia and in the Namaquan terranes are mid to late Mesoproterozoic in age. Volcanism or plutonism accompanied sedimentation in places. A southward younging of terrane accretion is apparent from about ∼1.37 Ga in northwestern Namibia, to -1.24 Ga(?) in the region of the central Damara Orogen, <1.33 Ga in the Konkiep Subprovince, the latter on the west-dipping Namaqua Front, ∼1.2 Ga (O'okiepian episode) in the Kaaien Terrane, to between ∼1.11 and ∼1.07 Ga in the Bushmanland and Garies Terranes. Widespread intrusion of granite into the base of the crust accompanied collision in northwestern Namibia. Syntectonic granite magmatism was extensive and voluminous during O'okiepian collision in the Gordonia Subprovince. Thereafter, local intermittent magmatism and sedimentation continued in the Rehoboth and Konkiep Subprovinces and the Kaaien, Bushmanland and Garies Terranes. In most regions from the Rehoboth Subprovince to the Garies Terrane there was an -1.1 Ga period of magmatism and/or metamorphism which Hanson et al. (2004, 2006) relate to plume activity. Redbed successions post-date the youngest magmatic rocks in the Rehoboth and Konkiep Subprovinces. The Bushmanland and Garies Terranes appear to have been subjected to intense deformation, recumbent folding and thrusting between ∼1.11 and ∼1.07 Ga. Open, upright folding thereafter in these two terranes was accompanied by intrusions of the Spektakel and Koperberg Suites and granulite-facies metamorphism between ∼1.06 and ∼1.02 Ga. Pegmatites mark the final magmatic event at about 1000 Ma. The high-grade rocks of the Gordonia Subprovince were juxtaposed against low-grade terranes to the north and south during isostatic rebound on major, late-stage, dextral shear zones at or near the margins of the subprovince. Cooling to 300 to 350°C had taken place by 850 to 800 Ma. © 2012 December Geological Society of South Africa.

Maxwell T.A.,Smithsonian Institution | Issawi B.,Consulting Geologist | Haynes Jr. C.V.,University of Arizona
Geology | Year: 2010

Space Shuttle Radar Topography Mission (SRTM) data have revealed new details on the extent and geomorphic relations of paleodrainage in southern Egypt. Following a period of late Tertiary drainage from the Red Sea Hills south through Wadi Qena and west across the Tushka region, the Nile River as we now know it established its connections with Central Africa and the Mediterranean in the middle Pleistocene (oxygen isotope stage, OIS 7 to OIS 5). SRTM topography reveals a lake level at ~247 m that is coincident with the elevation of middle Pleistocene fish fossils 400 km west of the Nile, and with the termination of shallow runoff channels in northern Sudan that were active during the middle Pleistocene and Holocene pluvial periods. An additional lake level at ~190 m is based on the current elevation at Wadi Tushka, and is consistent with Paleolithic sites at Bir Kiseiba followed by Neolithic sites at lower topographic levels. Overflow of the Nile through Wadi Tushka during the wetter north African climate of the middle Pleistocene, coupled with limited local rainfall, was the likely source of water for these lakes. © 2010 Geological Society of America.

Sallam E.,Benha University | Issawi B.,Consulting Geologist | Osman R.,Benha University
Arabian Journal of Geosciences | Year: 2015

The study of eight stratigraphic sections along Cairo–Suez district, between Gebel Ataqa and Gebel Mokattam, reveals a thick Lower Eocene Ypresian carbonate section, approximately 210 m thick, at Gebel Abu Treifiya just to the west of Gebel Ataqa. It is represented by the Minia Formation rich in Nummulites praecursor, Orbitolites pharaonum Schwager, Alveolina frumentiformis Schwager, and one of the precursors of Nummulites gizehensis group. The Minia Formation is a fairly clear, warm, and shallow marine facies. The Middle Eocene Lutetian sediments are totally missing in all studied sections probably reflecting instability in deposition echoed in the active block movements the area witnessed since the Paleozoic. The Bartonian sea transgressed over the area depositing Gebel Hof Formation at the base, Observatory Formation in the middle, and Qurn Formation at the top. The Gebel Hof Formation was deposited in an open-marine environment, passed upward into shallow marine, neritic to reefal facies; for the Observatory and Qurn formations, the former changed laterally into a sheltered lagoon facies, Sannor Formation, rich in Somalina stefaninii Silvestri, Dictyoconus aegyptiensis Chapman, and Idalina cuvillieri Bignot and Strougo. The Upper Eocene sediments, Maadi Formation, are mainly represented by a carbonate–clastic section showing shallowing-upward cycles resulted by the retreating of the sea shoreline northward during the Late Eocene, with high supply of terrigenous sediments. The lower carbonate cycle of the Maadi Formation was deposited in a restricted platform and tidal flats, whereas the upper siliciclastic cycle containing Carolia placunoides banks was probably deposited in a winnowed platform edge. These depositional environments were developed in grabens, ramps, and footslopes of the down-faulted Middle Eocene blocks, i.e., syn-tectonic deposition. The fluviatile Oligocene sediments were highly controlled by the structural and topographic lows, where a substantial thickness was deposited, Gebel Ahmer Formation, occupying several grabens and gently sloping areas between many synthetic faults. © 2014, Saudi Society for Geosciences.

Grantz A.,Consulting Geologist | Grantz A.,U.S. Geological Survey | Hart P.E.,U.S. Geological Survey
Marine and Petroleum Geology | Year: 2012

Reconnaissance seismic reflection data indicate that Canada Basin is a >700,000 sq. km. remnant of the Amerasia Basin of the Arctic Ocean that lies south of the Alpha-Mendeleev Large Igneous Province, which was constructed across the northern part of the Amerasia Basin between about 127 and 89-83.5 Ma. Canada Basin was filled by Early Jurassic to Holocene detritus from the Beaufort-Mackenzie Deltaic System, which drains the northern third of interior North America, with sizable contributions from Alaska and Northwest Canada. The basin contains roughly 5 or 6 million cubic km of sediment. Three fourths or more of this volume generates low amplitude seismic reflections, interpreted to represent hemipelagic deposits, which contain lenses to extensive interbeds of moderate amplitude reflections interpreted to represent unconfined turbidite and amalgamated channel deposits.Extrapolation from Arctic Alaska and Northwest Canada suggests that three fourths of the section in Canada Basin is correlative with stratigraphic sequences in these areas that contain intervals of hydrocarbon source rocks. In addition, worldwide heat flow averages suggest that about two thirds of Canada Basin lies in the oil or gas windows. Structural, stratigraphic and combined structural and stratigraphic features of local to regional occurrence offer exploration targets in Canada Basin, and at least one of these contains bright spots. However, deep water (to almost 4000 m), remoteness from harbors and markets, and thick accumulations of seasonal to permanent sea ice (until its possible removal by global warming later this century) will require the discovery of very large deposits for commercial success in most parts of Canada Basin. © 2011 Elsevier Ltd.

Nelson C.E.,Consulting Geologist | Proenza J.A.,University of Barcelona | Lewis J.F.,George Washington University | Lopez-Kramer J.,Institute Geofisica
Geologica Acta | Year: 2011

The Greater Antilles host some of the world's most important deposits of bauxite and lateritic nickel as well as significant resources of gold and silver, copper, zinc, manganese, cobalt and chromium. Beginning in Jurassic time, sedimentary exhalative base metal deposits accumulated in marine sedimentary rift basins as North and South America drifted apart. With the onset of intraoceanic subduction during the Early Cretaceous, a primitive (tholeiitic) island arc formed above a southwesterly-dipping subduction zone. Podiform chromite deposits formed in the mantle portion of the supra-subduction zone, directly above subducted Proto-Caribbean oceanic lithosphere. Within the nascent island arc, bimodal-mafic volcanogenic massive sulfide deposits formed in a fore-arc setting; mafic volcanogenic massive sulfide deposits formed later in mature back-arc basins. The Pueblo Viejo gold district, with five million ounces in production and twenty million ounces in mineable reserves, formed at 108-112Ma, in an apical rift or back-arc setting. By Late Cretaceous time, calc-alkaline volcanism was well established along the entire length of the Greater Antilles. Volcanogenic massive sulfide deposits including shallow submarine deposits characteristic of the primitive island arc gave way to porphyry copper and epithermal precious metal deposits typical of the mature island arc. Oblique collision of the Greater Antilles with North America began in the Late Cretaceous in Cuba and migrated eastward. Orogenic gold and tungsten deposits that formed during the collision event are preserved in ophiolites and in metamorphic core complexes. Since the Eocene, regional tectonism has been dominated by strike-slip motion as the North American continent moved westward relative to the Caribbean Plate. Large nickel-cobalt laterite deposits were formed when serpentinites were exposed to weathering and erosion during the mid-Tertiary. Bauxite deposits were derived from the weathering of volcanic ash within a carbonate platform of Eocene to Miocene age.

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