Basinal setting and origin of thick (1·8 km) mass-flow dominated Grand Conglomérat diamictites, Kamoa, Democratic Republic of Congo: Resolving climate and tectonic controls during Neoproterozoic glaciations (original) (raw)
Related papers
2017
Thick successions of poorly-sorted admixtures of clasts and matrix (diamictite) are widely regarded as evidence for synchronous ‘panglacials’ of global extent during the Cryogenian period (c. 720-635 Ma). In this thesis, diamictite facies are examined in detail using an impressive collection of newly available sub-surface data consisting of some 300 km of drill core through the Grand Conglomérat, Democratic Republic of Congo, and by field investigations of the exceptionally well-exposed Kingston Peak Formation in eastern California, USA. Diamictites are shown to be of mass flow origin (debrites) present within thick (<3 km) successions of genetically-related sediment gravity flow facies (olistostromes, slumps, and turbidites) deposited in deep-water within rapidly subsiding rift basins. Thick slope aprons likely formed at the base of fault scarps and tectonically oversteepened and unstable gravelly slopes that periodically underwent collapse, forming diamictite as a consequence o...
Geologica Belgica, 2017
The Upper Diamictite Formation of the West Congo Supergroup is a diamictite-dominated succession variously interpreted as a continental tillite, glaciomarinite, and glacially-influenced or non-glacial debrite. This paper presents a detailed macroand microscale analysis of soft-sediment deformation structures in order (1) to resolve the long-standing debate on the genetic origin of the Upper Diamictite Formation, and (2) to constrain the paleoenvironmental conditions during the Marinoan global event. The predominance of ductile and brittle deformations and grain-to-grain compression, considered as evidence of high strain rates and local high stress conditions, indicate that the diamictites were deposited as mass flows. The presence of probable pelagic clays, lonestones, and the absence of direct ice-contact deposits point to a subaqueous gravity flow origin. These diamictites were deposited along the margin or at the foot of the basin slope. They were probably triggered by oversteepening and/or tectonic shocks in the Araçuaí-West Congo Orogen between 630 and 660 Ma. The Upper Diamictite Formation provides no support for the postulated global Marinoan glaciation at this time and underscores the importance of a local tectonic control on the sedimentation.
Neoproterozoic diamictites around the Congo river basin : a critical reappraisal of their origin
Glossary of Geology. American Geological Institute. Falls Church, Virginia, USA, 751 p.) is a comprehensive, nongenetic term for a nonsorted or poorly sorted, terrigenous sedimentary rock that contains a wide range of particle sizes, such as a rock with sand and/or larger particles in a muddy matrix. Typically, it is a matrix-supported conglomerate, whose matrix is generally of black, grey, green to violet colour and composed of floating angular fragments of clay-to silt-size particles. The term matrix is here used in a purely descriptive, non-genetic and non-compositional sense. Neoproterozoic diamictites are well known all over the world, and also frequently occur in the Precambrian basement surrounding the Congo River Basin of Central-Eastern Africa. There, three large domains of Neoproterozoic sequences are exposed around the Congo Basin: 1) to the W, the West Congo Supergroup, 2) to the SE, the Katanga Supergroup and 3) to the N and NE, the Lindi Supergroup. In the West Congo and Katanga Supergroups, two separate diamictite horizons are distinguished in the lithostratigraphic columns, i.e. respectively the Formations of the Lower and Upper Mixtites (West Congo) and the Formations of the (lower) "Grand Conglomérat" and (upper) "Petit Conglomérat" (Katanga). In the Lindi Supergroup only one diamictite sequence has been recognized, i.e. the Akwokwo Tillite. The absolute age of deposition of these diamictites as well as stratigraphic intercorrelation attempts on a Central-Eastern Africa scale are still a matter of debate. In the Democratic Republic of the Congo (DRC), other Neoproterozoic diamictite horizons occur in more restricted and/or scattered regions. The following list is not restrictive and same localities may have been
Journal of African Earth Sciences, 2019
During the Cryogenian-Ediacaran, tectonically-and climatically-driven perturbations created sea level changes. Snowball Earth-type glaciations suggest~500-800 m sea level fall or at least 1,000-1,500 m in eustatic sea-level change as a result of severe climate changes. However, in Central Africa, geologic evidence of such processes is lacking. In the Lower Congo region (Democratic Republic of the Congo), detailed facies analysis and sequence stratigraphy of the Neoproterozoic uppermost Haut Shiloango-Lukala carbonate ramp system allowed to reconstruct the multiple order relative sea-level changes. Two hundred twenty-three fifth-order elementary parasequences, grouped into eleven fourth-order depositional sequences or parasequence sets, recorded a severe sea-level fall up to 15 m for slope-outer-to-outer ramp facies of the uppermost Haut Shiloango Subgroup-Upper Diamictite Formation followed by two distinct sea-level rises for the slope-outer-to-inner ramp facies of the Lukala Subgroup. First marine transgression shows a cumulative~60 m of sea-level rise throughout C1 to C3 (Lukala Subgroup) sediment accumulations. Second marine transgression shows a cumulative~50 m sea-level rise throughout C4 to C5 sediment accumulations. The transition from uppermost Haut Shiloango to Upper Diamictite ramp system points to an isostatic rebound and uplift of the rift flanks of the Congo Craton, creating paleoreliefs, potentially allowing local glaciation and periglacial sedimentation. This rebound can be ascribed to a diachronous far-field effect of~660 Ma Macaúbas Basin opening in Brazil. First marine transgression is interpreted as a consequence of syn-to post-rifting related to sea-floor spreading, which initiated the subsidence of the basin. A~30 m of sea-level rise drowned the C1 formation carbonate ramp, which turned on the oversupply and progradation of offshore to nearshore marine deposits on the wedges of the basin. An abrupt change from both marine transgressions points to~25 m of sea-level rise that is interpreted as the consequence of the development of~585-560 Ma Araçuaì-West Congo Orogen, which significantly increased the siliciclastic supply in the basin. Our results show that no anomalous climatic or eustatic events such as those proposed in the Snowball Earth model are recorded in DRC. On the contrary, relative sea-level changes result from long-term overriding regional tectonic processes controlling diachronous sedimentation along the western passive margin of the Congo Craton. Hoffman and Schrag, 2002). However, the link between tectonicallyand climatically-driven perturbations has not sufficiently been highlighted throughout Neoproterozoic times, particularly during the Ediacaran period, which conditioned carbonate factories. Other alternative hypotheses (Slushball-Waterbelt Earth, Zipper-rift Earth, High-Obliquity Earth) for the Snowball Earth model, which have been
Palaeogeography, Palaeoclimatology, Palaeoecology, 2016
The upper carbonate-rich parts of the West Congo Supergroup (~1000-560 Ma) from the Democratic Republic of the Congo have hitherto been considered as a record of abrupt eustatic and climatic events accompanying glaciation and deglaciation of a Snowball Earth-type Marinoan ice age that was of global extent. These strata have however never been investigated in detail. Results of new sedimentological work at key outcrops over a 1300 km outcrop belt show that pre-and post-Marinoan carbonates are respectively, storm-influenced sediments deposited principally in a mid/outer-ramp setting, and deep-water slope carbonates (calicturbidites) representing a lobe-fringe or levee-overbank setting. The Upper Diamictite Formation held previously by some to be a subglacial tillite, comprises gravity flows (debrites) deposited in deep water below wave base along the unstable margins of a carbonate ramp. A direct glacial influence on sedimentation for diamictites or any accompanying facies cannot be readily identified. Sedimentary facies reported here primarily record the presence of deep-water submarine to alluvial fan systems related to extensional tectonic processes of the central-southern Macaúbas Basin (now located in Brazil) between 700 Ma and 660 Ma followed by the 630-Ma onset of the pre-collisional magmatic arc in the Araçuaí-West Congo Orogen. No extreme short-lived climatic or eustatic events of a Snowball Earth-type ice age are recorded in the studied succession, which primarily reflects longterm overriding regional tectonic controls resulting in diachronous sedimentation along the western margin of the Congo Craton.
New ion microprobe U-Pb zircon ages, as well as some geochemical and isotopic analyses, for key igneous units within the central part of the West Congo belt are integrated with geological information to provide an updated geological map (1:1000000 scale) and a synthetic type cross-section of the belt, as well as an updated lithostratigraphic chart of the 'West Congo Supergroup'. Three Neoproterozoic units are recognised, from oldest to youngest, the Zadinian, Mayumbian and West Congolian 'Groups'. Emplacement of early Zadinian peralkaline granites (Noqui massif, 999 97 Ma) and rhyolites (Palabala) was accompanied by incipient rift sedimentation, corresponding to the onset of transtensional rifting, preferentially in a transverse mega-shear setting along the margin of the Congo craton. Subsequent upper Zadinian magmatism produced a thick (1600-2400 m) basaltic sequence (Gangila), which has geochemical characteristics typical of continental flood basalts (CFBs). The Gangila basalts, associated with major pull-apart rifting, were followed rapidly by the 3000-4000 m thick Mayumbian rhyolitic lavas, dated at 920 9 8 Ma at the base and 912 97 Ma at the top. The felsic lavas are intruded by coeval high-level (micro)granites, whose emplacement is dated at 924 925 Ma (Mativa body) and at 917 914 Ma (Bata Kimenga body) in the Lufu massif. This voluminous bimodal magmatic province is similar to the Paraná and Deccan provinces, and shares similar lithospheric sources. It corresponds to the initial, transtensional rifting stage along the western edge of the Congo craton before Rodinia breakup. The early Neoproterozoic rocks of the West Congo Supergroup rest unconformably on a ca. 2.1 Ga Palaeoproterozoic polycyclic basement (Kimezian Supergroup). No Mesoproterozoic events are recorded in the area. Following the initial, transtensional early Neoproterozoic (ca. 1000-910 Ma) rifting stage, Bas-Congo behaved as a passive margin of the Congo craton, as indicated by deposition of ca. 4000 m of Neoproterozoic (pre-Pan-African) platform sediments (lower part of West Congolian Group) preceding ca. 2000 m of Pan-African molasse-type sediments (upper part of West Congolian Group). In the late Neoproterozoic, during Pan-African assembly of Gondwanaland, the Bas-Congo passive margin, which was largely protected by thick lithosphere of the Congo craton, collided with a western active margin to form the Brasiliano-Araçuaí belt, now : S 0 3 0 1 -9 2 6 8 ( 0 1 ) 0 0 1 9 2 -9 L. Tack et al. / Precambrian Research 110 (2001) 277-306 278 preserved adjacent to the São Francisco craton of Brazil. This collision, which ended in Bas-Congo at ca. 566 Ma, induced relatively limited effects in the West Congo belt, which experienced no late Neoproterozoic magmatic activity.
Reconsidering the glaciogenic origin of Gondwana diamictites, Dwyka Group, South Africa
2022
The Gondwana Late Palaeozoic Ice Age is probably best represented by the Dwyka Group in South Africa. Striated and grooved surfaces or pavements are commonly considered to have formed subglacially, as are diamictites which have been interpreted as in-situ or reworked tillites. These interpretations were tested by investigation of outcrops in formerly well-studied areas, throughout South Africa. Detailed analyses have focused on striated surfaces/pavements and surface microtextures on quartz sand grains in diamictites. The sedimentological context of four pavements, interpreted to be glaciogenic, display features commonly associated with sediment gravity flows, rather than glaciation. A total of 4,271 quartz sand grains were subsampled from outcrops that are considered mainly to be tillites formed by continental glaciation. These grains, analysed by SEM, do not demonstrate the characteristic surface microtexture combinations of fracturing and irregular abrasion associated with Quaternary glacial deposits, but mainly a mix of surface microtextures associated with multicyclical grains. The Dwyka Group diamictites warrant reinterpretation as non-glacial sediment gravity flow deposits.
A Proterozoic-rift origin for the structure and the evolution of the cratonic Congo basin
Earth and Planetary Science Letters, 2011
Keywords: intracratonic basins "Cuvette Centrale" of Congo continental rifting sediment backstripping We interpret the cratonic Congo basin, a large circular "Cuvette Centrale" filled with up to 9 km of Proterozoic to Neogene sediments, as the consequence of a Neo-Proterozoic rift. Firstly, the magnitude and the long-term subsidence are consistent with the thermal time-constant of a 200-250 km thick lithosphere inferred from several tomographic studies. Secondly, the surface accumulation of sediments is compensated at depth by crustal thinning, whose magnitude can be estimated from the analysis of the surface gravity: after backstripping the effect of the sediments, a residual NW-SE positive and narrow gravity anomaly is observed across the "Cuvette Centrale" and is interpreted as the remaining crustal thinning associated with this rift. Assuming that isostasy is governed by a necking level and a flexural response to sediment loads, we have estimated the combination of the depth of necking and the equivalent elastic thickness of the lithosphere that provide the best fit with the residual gravity, i.e. 10 km and 100 km respectively. The corresponding uplift of the upper mantle is in the continuity of the Mbuji-Mayi Supergroup to the SE and the Liki-Bembian Group to the NW. These two groups represent older stages of rifting in the Congo craton, which shows that rifting has periodically affected and weakened the "Cuvette Centrale" during a long period of time.
Reconsidering the glaciogenic origin of Gondwana diamictites of the Dwyka Group, South Africa
Geologos, 2022
The Gondwana Late Palaeozoic Ice Age is probably best represented by the Dwyka Group in South Africa. Striated and grooved surfaces or pavements are commonly considered to have formed subglacially, as are diamictites which have been interpreted as in-situ or reworked tillites. These interpretations were tested by investigation of outcrops in formerly well-studied areas, throughout South Africa. Detailed analyses have focused on striated surfaces/pavements and surface microtextures on quartz sand grains in diamictites. The sedimentological context of four pavements, interpreted to be glaciogenic, display features commonly associated with sediment gravity flows, rather than glaciation. A total of 4,271 quartz sand grains were subsampled from outcrops that are considered mainly to be tillites formed by continental glaciation. These grains, analysed by SEM, do not demonstrate the characteristic surface microtexture combinations of fracturing and irregular abrasion associated with Quaternary glacial deposits, but mainly a mix of surface microtextures associated with multicyclical grains. The Dwyka Group diamictites warrant reinterpretation as non-glacial sediment gravity flow deposits.
American Journal of Science, 2018
Neoproterozoic glacial diamictites and rift-related volcanics are preserved throughout the North American Cordillera, yet the nature and timing of both glaciation and rifting are poorly constrained. New geochronological, geochemical, and stratigraphic data from the Cryogenian Gataga volcanics and bounding units at Gataga Mountain, in the Kechika Trough of northern British Columbia, better constrain the age of these rift-related volcanics and suggest that they erupted during glaciation. At Gataga Mountain, three informal sequences are exposed; a basal quartzite, the Gataga volcanics, and an overlying mixed carbonate-siliciclastic succession. The basal quartzite is dominated by cross-bedded sandstone with an intertidal facies assemblage including bidirectional cross-stratification and mud-cracks, indicative of non-glacial deposition. The overlying Gataga volcanics are over one kilometer thick, comprising both mafic and felsic units, with volcaniclastic breccia and interbedded sedimentary units including iron formation and matrix-supported diamictite with exotic clasts. Magmatic ages in the upper Gataga volcanics span 696.2 ؎ 0.2 to 690.1 ؎ 0.2 Ma, and detrital zircon from the underlying non-glacial quartzite provide a maximum age constraint on the onset of glaciation <735.8 ؎ 0.6 Ma. We interpret interfingering beds of matrix-supported diamictite with exotic clasts within the Gataga volcanics to record sub-ice shelf sedimentation and volcanism during the Sturtian Glaciation. Although volcanic facies are consistent with eruption in a sub-ice to sub-aqueous (below ice shelf) environment, we acknowledge the difficulty of distinguishing sub-glacial from sub-aqueous explosive volcanic facies. Overlying the Gataga volcanics, a mixed carbonate-siliciclastic succession contains minor basalt flows that are geochemically distinct from the underlying volcanic rocks. Based on chemostratigraphic and lithostratigraphic similarities, we suggest that this sequence is correlative with Ediacaran strata to the north. Together, we suggest that the stratigraphy and geochemical signature of volcanic rocks at Gataga Mountain records two episodes of Neoproterozoic extensionrelated sedimentation and volcanism, the first indicated by the Cryogenian Gataga volcanics and interbedded sedimentary strata and the second by the overlying Ediacaran carbonate-siliciclastic succession with interfingering basalt.