Depositional age and provenance of the Cobar Supergroup

Abstract

The turbidite-filled, Lower Devonian Cobar Basin is characterised through a detrital zircon study. Uranium–Pb age data for six samples were combined with published data to show the basin has a unique age spectrum characterised by a subordinate Middle Ordovician (ca 470?Ma) peak superimposed on a dominant ca 500?Ma peak. Maximum depositional ages for 3 samples were ca 425?Ma, close to the published Lower Devonian (Lochkovian 419–411?Ma) biostratigraphic ages. A minor ca 1000?Ma zircon population was also identified. The major source of the 500?Ma zircons was probably the local Ordovician metasedimentary basement, which was folded, thickened and presumably exposed during the ca 440?Ma Benambran Orogeny. The ca 470?Ma age peak reflects derivation from Middle Ordovician (Phase 2) rocks of the Macquarie Arc to the east. The I-type Florida Volcanics, located ～50?km eastward from the Cobar Basin, contains distinctive Middle and Late Ordovician zircon populations, considered to be derived from deeply underthrust Macquarie Arc crust. Protracted silicic magmatism occurred before, during and after Cobar Basin deposition, indicating that the basin formed by subduction-related processes in a back-arc setting, rather than as a continental rift.     

Geophysical and hydrogeologic investigation of groundwater in the Karoo stratigraphic sequence at Sawmills in northern Matabeleland,Zimbabwe: a case history

Geophysical and hydrogeological investigations have been carried out around Sawmills in Zimbabwe, Africa. The investigations are components of a larger investigation to assess the groundwater potential of the Karoo sedimentary basin with regards to supplying water to Bulawayo City. The Sawmills area was selected due to the availability of borehole logs indicating favourable stratigraphy for groundwater availability and due to the high yields from the aquifers measured from these boreholes. Data collected using two geophysical methods are presented here: transient electromagnetic (TEM) and continuous vertical electrical sounding (CVES) data. The data have also been processed using laterally constrained inversion (LCI). Because the CVES provides greater detail in the shallow subsurface, whereas TEM is more effective at depth, a more accurate image of the entire subsurface profile is provided based on using both methods. The results suggest that LCI of CVES and TEM data, in the subsurface at the required depths at Sawmills, is able to provide a substantially more accurate image of the subsurface than either method alone. The hydrogeological interpretation of the geophysical data is valuable for determining the depth to and thickness of the potential aquifer horizon(s) and for identifying the position of potential recharge zones.     

Proterozoic Rifting in the Pranhita-Godavari Valley: Implication on India-Antarctica Linkage

The Pranhita-Godavari (PG) Valley, a major lineament within the South Indian cratonic province, that preserves sediment dominated deposits spanning from Mesoproterozoic to Mesozoic, appears to be a key element in supercontinent reconstruction. The sedimentary basins of the Valley include a thick succession of Early Mesoproterozoic to Late Neoproterozoic rocks, the Godavari Supergroup, which is unconformably overlain by the Late Palaeozoic-Mesozoic Gondwana sequence. The Godavari Supergroup is internally punctuated by several regional and interregional unconformities into a number of unconformity-bound sequences having group level and subgroup level status. The lithostratigraphic attributes of the succession indicate multiple events of fault controlled sedimentation marked by transgression and regression, as well as uneven rates of uplift and subsidence of the basin floor in an extensional tectonic regime. The amplitude of translation of the unconformity surfaces across the base level attests to collective role of tectonic movement and sea level changes in building the stratigraphic framework of the Valley. The stratigraphic framework and depositional systems, such as fan and fan-deltas, together with local outburst of felsic volcanism further indicate repeated rifting of the craton.Geochronologic data indicate that the rift basin started to open in Early Mesoproterozoic, concomitantly with the breakup of the Mesoproterozoic supercontinent during which the India and East Gondwana fragments were separated. The spatial variation in the declivity of the unconformity surfaces, and the trend of thickness variation of the unconformity-bound sequences point that the basin deepened and opened towards southeast to join an ocean that developed between the South Indian craton and East Antarctica. The contractional deformation structures preserved in several lithounits were produced under NE-SW directed regional compression during Late Neoproterozoic basin inversion.     

Tectonic and lithological constraints on the evolution of the Karoo graben of northern Malawi (East Africa)

The results of a lithostratigraphic, tectonic and kinematic study of the Karoo deposits of northern Malawi are reported. The objective of the lithostratigraphic study is to correlate the deposits of the Karoo basins of northern Malawi with the well-known deposits of southern Tanzania, thus establishing a stratigraphic framework through which the timing of faulting can be constrained. The kinematic analysis of faulting constrains the opening direction for the Karoo graben in this area and provides basic data to discuss the Karoo graben development within the regional tectonic framework of south-eastern Africa. The studied adults are defined by moderately to steeply dipping cataclastic zones with a width of up to 15 m and are characterized by an array of slickensided fault surfaces with different orientations and slip directions. In this study, small faults (offset < 10 m) and meso-scale faults (offset > 10 m, but generally not exceeding 30–40 m) have been distinguished. Methods used to analyse the kinematic data include the ‘pressure tension’ (PT) method, which estimates the principal axes for the bulk brittle strain, and the internal rotation axis (IRA) method, which estimates the axis of bulk internal rotation and the overall sense of slip at the faults. A mass balance calculation reveals a volume increase of up to 16% during cataclastic deformation in the fault zones. The PT method shows an approximately east trending extension direction for faults that occur only in the latest Carboniferous (?) and Early Permian strata, whereas the fault kinematics from faults that cut middle Permian to Early Triassic rocks is characterized by a ESE to SE trending extension direction. The small faults yield essentially the same kinematic results as the meso-scale faults. In a transport-parallel cross-sectional view, the principal extension axes are at an acute angle of approximately 60° to the major fault planes. Given the moderate fault density, the relatively high angle between the orientation of the principal extension axis and the fault planes suggest only a moderate amount of horizontal extension across the Karoo graben of northern Malawi. Riedel structures in the fault zones formed within two conjugate sets of localized shear zones; slip on one set was top to the W/NW and, on the other, top to the E/SE. The two conjugate sets of Riedel structures have an acute angle about the regional shortening axes, implying that no pronounced rotation of the strain axes occurred. The internal rotation axes for the Riedel structures reveal a largely bimodal distribution and inferred weakly monoclinic to orthorhombic symmetry. Therefore the overall deformation during Karoo rifting in northern Malawi is interpreted to be close to a coaxial deformation with a limited amount of horizontal extension.[/p]     

Silicification in the Belt Supergroup (Mesoproterozoic), Glacier National Park, Montana, USA

Nodular cherts can provide a window on the original sediment composition, diagenetic history and biota of their host rock because of their low susceptibility to further diagenetic alteration. The majority of Phanerozoic cherts formed by the intraformational redistribution of biogenic silica, particularly siliceous sponge spicules, radiolarian tests and diatom frustules. In the absence of a biogenic silica source, Precambrian cherts necessarily had to have had a different origin than Phanerozoic cherts. The Mesoproterozoic Belt Supergroup in Glacier National Park contains a variety of chert types, including silicified oolites and stromatolites, which have similar microtextures and paragenesis to Phanerozoic cherts, despite their different origins. Much of the silicification in the Belt Supergroup occurred after the onset of intergranular compaction, but before the main episode of dolomitization. The Belt Supergroup cherts probably had an opal-CT precursor, in the same manner as many Phanerozoic cherts. Although it is likely that Precambrian seas had higher silica concentrations than at present because of the absence of silica-secreting organisms, no evidence was observed that would suggest that high dissolved silica concentrations in the Belt sea had a significant widespread effect on silicification. The rarity of microfossils in Belt Supergroup cherts indicates that early silicification, if it occurred, was exceptional and restricted to localized environments. The similarity of microtextures in cherts of different ages is evidence that the silicification process is largely controlled by host carbonate composition and dissolved silica concentration during diagenesis, regardless of the source of silica.     

Aseismic controls on in situ soft-sediment deformation processes and products in submarine slope deposits of the Karoo Basin, South Africa

Intervals of soft‐sediment deformation features, including vertical fluid escape and load structures, are common and well‐exposed in Permian lower slope deposits of the Tanqua Depocentre, Karoo Basin. The structures mainly comprise elongated flames and load structures associated with ruptured sandstones and structureless siltstones, observed over a range of scales. The presence of an upper structureless siltstone layer linked to the flames, interpreted as a product of the debouching of fine‐grained material transported through the flame onto the palaeo‐seabed, together with the drag and upward folding of lower sandstone layers is evidence that the flames were formed in situ by upward movement of sediment‐rich fluids. Flames are oriented parallel to the deep‐water palaeoslope in lateral splay deposits between two major slope channel complexes. Statistical correlation and regression analyses of 180 flame structures from seven stratigraphic intervals suggest a common mechanism for the deformation and indicate the importance of fluidization as a deformation mechanism. Importantly, deformation occurred in an instantaneous and synchronous manner. Liquefaction and fluidization were triggered by incremental movement of sediment over steeper local gradients that were generated by deposition of a lateral splay on an inherited local north‐west‐facing slope. Seismic activity is not invoked as a trigger mechanism because of the restricted spatial occurrence of these features and the lack of indications of earthquakes during the time of deposition of the deep‐water succession. The driving mechanisms that resulted in the final configuration of the soft‐sediment deformation structures involved a combination of vertical shear stress caused by fluidization, development of an inverse density gradient and a downslope component of force associated with the local slope. Ground‐penetrating radar profiles confirm the overall north‐east orientation of the flame structures and provide a basis for recognition of potential larger‐scale examples of flames in seismic reflection data sets.     

Taxonomy and affinity of Early Mesoproterozoic megascopic helically coiled and related fossils from the Rohtas Formation, the Vindhyan Supergroup, India

A well preserved assemblage of compressed, straight, circular to sinuously coiled megascopic and helical carbonaceous fossils and other varied megascopic morphoforms are known from the Early Mesoproterozoic Rohtas Formation, Semri Group within Vindhyan Supergroup exposed in Katni district of central India. These megascopic remains are preserved as impressions, compressions, partially mineralized remains, and/or epi-relief. Some of the forms are typical filamentous empty sheaths and others are trichomes, with cell like entities under various stages of degradation. This study, based on fresh collections and also of the topotype material of the helically coiled megascopic fossils, straight forms and related fossilized remains occurring as epi-relief from Katni indicate that the two morphotaxa are distinct entities and possibly appear to be prokaryotes. Grypania spiralis and Katnia singhii are most likely of cyanobacterial origin. Spirally coiled and circular fossils, with epi-relief, and which probably represents a tissue grade organism, are considered as Spiroichnus beerii Mathur, 1983. Linear sheet-like carbonaceous solitary form has been placed in the morphotaxon Proterotainia and described as P. katniensis n. sp. Certain rare circular, carbonaceous forms are considered as Chuaria sp. A few circular disc-like forms found in the assemblage are treated as dubiofossils.     

The Horto-Baratinha itabirite-hosted iron ore: A basal fragment of the Espinhaço basin in the eastern São Francisco Craton

The Horto-Baratinha (HBD) iron ore deposit is located at the eastern border of São Francisco Craton, comprising BIF-hosted high-grade bodies (>60 wt.% Fe) associated with polydeformed quartz-mica-schists, amphibole-schist of Statherian maximum deposition age, enclosed by Statherian granitoids of the Borrachudos Suite and Neoarchean gneiss. All the sequence is crosscut by undeformed dikes and sills of pegmatitic bodies probably formed during Late Ediacaran-Cambrian. The metasedimentary sequence is stratigraphically correlatable with the Orosirian-Statherian Serra da Serpentina and Serra de São José Groups that comprise the basal units of the Espinhaço Supergroup and was intensively segmented into distinct tectonic blocks. The sedimentary/diagenetic bedding of the metamorphosed BIF (itabirite) is generally transposed by an axial planar schistosity. The lamellar hematite from itabirite is the oldest iron oxide generation, which was formed during the syn-deformational stage, parallel-oriented to the rock foliation. The (keno)magnetite grains from itabirite, iron ore and pegmatite bodies developed as idioblasts that grew over the foliation formed during late and post-deformational stages. Magnetite oxidizes subsequently to martite and granular hematite. Coarse lamellar hematite crystals randomly oriented in the border of the pegmatitic bodies also formed during the post-deformational stage due to hydrothermal reaction with itabirite. The country rocks have undergone at least three stages of deformation developed during the syn-collisional and late-collisional (Ediacaran to early-Cambrian) phases of the Brasiliano Orogeny: stage 1 with the development of a pervasive foliation (S₁), parallel to axial plane to tight folds and transposition of all sedimentary structures; stage 2 with folding of S₁; stage 3 with refolding of S₁. Both fold systems interfere with each other making up a dome and basin refolding shape. During the late-collisional (Ediacaran to early-Cambrian) and post-collisional/gravitational collapse (Cambrian) the sequence was intruded by anatectic pegmatitic bodies, which are part of the Eastern Brazilian Pegmatite Province, one of the most significant pegmatitic regions worldwide. The fluid related with these intrusions could be related with the Si leaching, crystallization of magnetite and granular hematite, and consequent formation of high-grade iron bodies.     

Early-Middle Jurassic Dolerite Dykes from Western Dronning Maud Land (Antarctica): Identifying Mantle Sources in the Karoo Large Igneous Province

A suite of dolerite dykes from the Ahlmannryggen region of westernDronning Maud Land (Antarctica) forms part of the much moreextensive Karoo igneous province of southern Africa. The dykecompositions include both low- and high-Ti magma types, includingpicrites and ferropicrites. New ⁴⁰Ar/³⁹Ar age determinationsfor the Ahlmannryggen intrusions indicate two ages of emplacementat 178 and 190 Ma. Four geochemical groups of dykes have beenidentified in the Ahlmannryggen region based on analyses of60 dykes. The groups are defined on the basis of whole-rockTiO₂ and Zr contents, and reinforced by rare earth element (REE),⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd isotope data. Group 1 were intrudedat 190 Ma and have low TiO₂ and Zr contents and a significantArchaean crustal component, but also evidence of hydrothermalalteration. Group 2 dykes were intruded at 178 Ma; they havelow to moderate TiO₂ and Zr contents and are interpreted tobe the result of mixing of melts derived from an isotopicallydepleted source with small melt fractions of an enriched lithosphericmantle source. Group 3 dyke were intruded at 190 Ma and formthe most distinct magma group; these are largely picritic withsuperficially mid-ocean ridge basalt (MORB)-like chemistry (flatREE patterns, ⁸⁷Sr/⁸⁶Sr_i 0·7035, Nd_i 9). However, theyhave very high TiO₂ (4 wt %) and Zr (500 ppm) contents, whichis not consistent with melting of MORB-source mantle. The Group3 magmas are inferred to be derived by partial melting of astrongly depleted mantle source in the garnet stability field.This group includes several high Mg–Fe dykes (ferropicrites),which are interpreted as high-temperature melts. Some Group3 dykes also show evidence of contamination by continental crust.Group 4 dykes are low-K picrites intruded at 178 Ma; they havevery high TiO₂–Zr contents and are the most enriched magmagroup of the Karoo–Antarctic province, with ocean-islandbasalt (OIB)-like chemistry. Dykes of Group 1 and Group 3 aresub-parallel (ENE–WSW) and both groups were emplaced at190 Ma in response to the same regional stress field, whichhad changed by 178 Ma, when Group 2 and Group 4 dykes were intrudedalong a dominantly NNE–SSW strike. KEY WORDS: flood basalt; depleted mantle; enriched mantle; Ahlmannryggen; Karoo dyke     

Storm Event Beds in a Paleoproterozoic Rift Basin, Aravalli Supergroup, Rajasthan, India

Storm event beds in the Paleoproterozoic riftogenic sedimentary succession of Aravalli Supergroup are described from a 12.8 m-thick sandstone-mudstone interbedded unit in Zawar area, Rajasthan, India. The storm event beds include different primary structural assemblages indicating deposition from waning storm current. Sequential arrangement of beds with characteristic primary structural assemblages suggests deposition under a transgressive phase, and overall retrogradational evolution of the storm-succession provides evidence in favour of faster downsagging of the basin floor. The Pb-Zn sulphide ore bearing sedimentary succession of Zawar records repeated downsagging and exhumation of the basin floor in the frame of continental rift tectonics.