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Thrusting, folding and stratigraphy of the Ghaap Group along the southwestern margin of the Kaapvaal Craton
No full textThe lower sedimentary sequence of the Transvaal Supergroup underwent multiple folding and thrusting along the southwestern margin of the Kaapvaal Craton, between Prieska and Boegoeberg Dam. This tectogenesis is expressed in three folding phases with successively diminishing intensities. Thrusting occurred during the first and second phase. The stratigraphy of these rocks needs to be reviewed, taking into account the complex structure. -from Author
A structural sequence in low strain sedimentary rocks in the Beaufort Group of the south-western Karoo and its bearing on syngenetic and epigenetic mineralization
No full textSeveral sets of planar and linear structures are described from the very gently folded Karoo strata of the northern-most tectonic zone of the Permo-Triassic Cape Fold Belt. The superposition of these structures reveals a time sequence. Horizontal compression occurred before compaction was complete. After compaction further horizontal compression developed a sequence of structures that are readily interpreted in terms of visco-elastic strain in multilayers. This analysis reveals that a set of mainly antithetic kinks in the form of monoclines developed out of earlier incipient sinusoidal macrofolds during the final stages of horizontal N-S compression. The model is in agreement with evidence for synsedimentary folding that controlled the sites for erosion channels as well as the direction of sedimentary transport. Exploration efforts for syngenetic and/or fluid controlled epigenetic mineralization in permeable strata of this fluviatile sedimentary sequence should be directed along certain structural lines. © 1994.Articl
Structural history of the southwestern corner of the Kaapvaal Craton and the adjacent Namaqua realm: new observations and a reappraisal
No full textThe rocks along the southwestern margin of the Kaapvaal Craton were deformed up to 7 times during the Early to Middle Proterozoic. The oldest deformation D1 is recorded in the N-S-trending Uitkoms cataclasite of pre-Makganyene age (>2.24 Ga) on the craton, and interpreted as a bedding-parallel thrust. It is assumed to be a branch rising towards the surface from a blind sole thrust that initiated early N-S-trending F,-folds above it. D2 is represented by mainly N-S but also NE-SW and NW-SE-trending imbricates and recumbent fold zones ranging in size from small gravity slumps to large tectonic decollements in Asbesheuwel BIF and the Koegas Subgroup, and is younger than D1, or equals D1 in age. These age. These structures pre-date the Westerberg dyke-sheet intrusion. D3 south-verging folds and thrusts are the oldest post-Matsap deformations, just less than 2.07-1.88 Ga. D4 are upright to east vergent and N-S-trending folds deforming all previous structures. D4 post-dates the Westerberg dyke-sheet and probably reactivates N-S folds above the earlier sole thrust during renewed E-W compression. D5, producing the main NW-trending Namaqua structures, is only very feebly developed in the Kheis terrain and absent from the cratonic areas overlain by Olifantshoek and older strata, i.e. NE, E and SE of the Marydale High. Very gentle D6 E-W to ENE-WSW folds produce culminations and depressions in all NW-trending older structures. During D7 the NW-SE-trending Doornberg Lineament, an oblique left-lateral wrench, and smaller N-trending faults such as the Westerberg Fault developed. These and similar, but right-lateral faults are the last movements along the rim of the craton and occurred around 1.0 Ga. Multiple folding and thrusting with riebeckite mobilization happened prior to Namaqua events and resulted inter alia in discernable duplication and thickening of the Transvaal Supergroup along the southwestern margin of the Kaapvaal Craton and at least some 130 km into the craton interior. This complicates stratigraphic correlation as well as true thickness estimates of BIF units in Griqualand West, and affects the model for the environmental evolution of the Ghaap Group. A structural model of thin-skin decoupling at the base of the Transvaal Supergroup and starting in the Middle-Early Proterozoic is proposed. © 1991.Articl
Chemical composition of banded iron-formations of the Griqualand West Sequence, Northern Cape Province, South Africa, in comparison with other Precambrian iron formations
No full textMesobanded lithotypes (band rhythmites) of banded iron-formation (BIF) from the Griquatown and Kuruman Iron Formations of the Asbestos Hills Subgroup (Transvaal Supergroup, South Africa) have been sampled. 142 major and trace element analyses from diamond drill cores, an underground mine and an open pit mine were carried out. Out of these 19 open pit mine samples were excluded from the interpretation of the analytical results because of a different behaviour of the alkali elements. It is shown that other published data seem to suffer from the same effect. The results show that the chemical composition of the iron-formations is virtually independent of their stratigraphic and geographic localities over hundreds of kilometres across Griqualand West, if averaged element distribution patterns are compared. The major element composition of mesobanded iron-formation samples (magnetite chert and magnetite-carbonate chert, riebeckite-carbonate chert and ironsilicate chert) varies between 30 and 51 wt.% SiO2, 23 and 66 wt.% Fe2O3T, < 0.02 and 0.14 wt.% TiO2, < 0.04 and 1.9 wt.% Al2O3, < 0.02 and 1.05 wt.% MnO, 1.48 and 7.5 wt.% MgO, 0.14 and 12.09 wt.% CaO, 0.11 and 4.26 wt.% Na2O, < 0.007 and 2.39 wt.% K2O and < 0.01 and 1.57 wt.% P2O5. The trace element contents are generally low and range from < 1 to 2 ppm Nb (detected in only one sample), < 2 to 23 ppm Zr, < 3 to 31 ppm Y, < 2 to 152 ppm Sr, < 4 to 5 ppm U (detected in only one sample), < 2 to 240 ppm Rb, < 4 to 4 ppm Th, < 4 to 19 ppm Pb, < 3 to 4 ppm Ga, < 4 to 33 ppm Zn, < 6 to 69 ppm Cu, < 7 to 17 ppm Ni (detected in only one sample), < 5 to 59 ppm Cr, < 4 to 16 ppm V and < 10 to 177 ppm Ba. The intercalated stilpnomelane lutites have a very similar gross composition but regularly display higher concentrations of Al2O3, TiO2, K2O, and Zr. They have a different origin but certainly bear the imprint of the BIF environment and must be considered a clastic contaminant of the otherwise chemically or biochemically precipitated iron-formations. Clastic contamination and subsequent hydrothermal alteration are the most plausible agents which effected the element distribution pattern of the BIF, because they are best to reconcile with a model which assumes hydrothermal fluids as major sources of BIF. It can be concluded, from the general geochemical uniformity of BIF throughout the depository and from the absence of clear-cut relations of elemental distributions and ratios to crustal components (documented by low K/Rb, high Rb/Sr ratios, no relevant correlation between TiO2 and Al2O3), that a basinward hydrothermal system acted as a source for the Fe and Si in the BIF. The geochemical similarity between the microbanded Kuruman Iron Formation and the interbedded granular and microbanded Griquatown Iron Formation suggests deposition of the iron-formations in the same chemical environment over the entire basin from the inception of these conditions until their termination by clastic input. The somewhat lower trace element concentrations in the Griquatown Iron Formation as compared to the Kuruman Iron Formation probably do not mean an environmental change, but may reflect the diagenetic and tectonic evolution of the iron-formations. Regular but short-lived interruptions by distal volcanic ash had no influence on the bulk BIF composition. On the contrary, the volcanics reveal a strong overprint by the regional major element BIF chemistry. Therefore, the relationship between volcanogenic rocks and banded iron-formation seems to be coincidental and related to basin development. The general geochemical similarity of the Griquatown and Kuruman Iron Formations with other Proterozoic and Archaean iron-formations in the world and especially with those of the Hamersley Basin, leads us to the conclusion that the BIF of the Griqualand West Sequence are representatives of typical, large-scale iron-formations of the Precambrian, which all formed in chemically very similar environments. © 1995.Articl
The Gariep belt: stratigraphic-structural evidence for obliquely transformed grabens and back-folded thrust stacks in a combined thick-skin thin-skin structural setting
No full textThis paper deals with the tectonics of the Port Nolloth Zone (PNZ) of the Gariep belt, one of the Pan-African tectono-metamorphic belts along the southwest coast of Africa. Stratigraphical and sedimentological evidence indicates that deposition in the sub-basins was controlled by step and/or graben faults parallel to the craton edge. Some of these structures were inverted during the compressional phase to form oblique wrenches in a subzone proximal to the craton edge, whereas others developed into stacked thrusts, blind ramps and a refolded duplex in a distal subzone. A near vertical ZX plane of deformation strikes north-northwest to south-southeast for the entire PNZ and the shear planes are near horizontal in the distal subzone and near vertical in the proximal area. Because metamorphic grades are very similar everywhere, the difference in style is probably related to the original shape of the basement and the varying lithology of the Gariep rocks. Two-dimensional modelling shows that oblique obduction of an oceanic crustal slab to the southeast, followed by relaxation and renewed compression in three pulses, probably produced a combination of thick-skin and thin-skin elements in a zoned tectonite. © 1995.Articl
A stratigraphic-structural profile through parts of the Beaufort Group: fold genesis and relationship to U-distribution
No full textA technique for constructing a composite profile of a very gently folded sedimentary multilayer sequence, some 1200 m thick, is introduced and applied. Accuracy checks and balances are discussed. Real changes in layer thickness are found to be related to the position, attitude and intensity of folds. Upright as well as verging, open sinusoidal megafolds formed during diagenesis and compaction. Monoclines (macrokinks) formed late during the deformation history, and are antithetic with respect to the regional couple force. Most macrokinks eventually became locked. In some cases thrusting of the steep limb seems to have accompanied shortening inside the kinkband immediately before the locking occurred. Other monoclines have developed sets of short listric normal faults on the steep limb that merge with the beds on their downthrow side. The association of all structures is readily explained by a theory of visco-elastic folding of multilayers with varying competency. Some 50% of the stratiform uranium mineralization that is associated with the thicker, more permeable sandstone units occurs in monoclines. The positioning of monoclines relative to the axial planes of the early megafolds seems to be controlled by thickness variations in sequences of macrocycles. These variations can be linked to changes in transport direction across syndepositional early folds. Macrokinks can therefore be considered a valuable additional indicator for exploration targets. © 1994.Articl
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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