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

Abstract

The Kamoa sub‐basin, in the south‐eastern part of the Democratic Republic of Congo, is a rift basin that hosts a world‐class stratiform copper deposit at the base of a very thick (1·8 km) succession of matrix‐supported conglomerates (diamictite) (Grand Conglomérat Formation) that has been interpreted by some as the product of deposition in the aftermath of a planet‐wide glaciation. Newly available subsurface data consisting of more than 300 km of drill core throws new light on the origin of diamictite and associated facies types, and their tectonic, basinal and palaeoclimatic setting. Initiation of rifting is recorded by a lowermost subaqueous succession of fault‐related mass flow conglomerates and breccias (the ‘Poudingue’) with interdigitating coeval and succeeding sandstone turbidites (Mwashya Subgroup). Overlying diamictites of the Grand Conglomérat were deposited as subaqueous debrites produced by mixing and homogenization of antecedent breccias and gravel from the Poudingue and Mwashya sediments with basinal muds. Failure of over‐steepened basin margins and debris flow was likely to be triggered by faulting and seismic activity, and was accompanied by syn‐depositional subaqueous basaltic magmatism recorded by peperites and pillow lavas within diamictites. The thickness of diamictites reflects recurring phases of faulting, volcanism and rapid subsidence allowing continued accommodation of rapidly deposited resedimented facies well below wave base. A distal or indirect, glacial influence in the form of rare dropstones and striated clasts is evident, but tectonically‐driven mass flow destroyed any primary record of glacial climate originally present in basin margin sediments. Such basin margin settings were common during Rodinia rifting and their stratigraphy and facies record a dominant tectonic, rather than climatic, control on sedimentation. Deposition occurred on tectonic timescales inconsistent with a Snowball Earth model for Neoproterozoic diamictites involving a direct glacial contribution to deposition.

The nonglacial diamictite of Toutswemogala Hill (lower Karoo Supergroup, central Botswana): Implications on the extent of the late Paleozoic ice age in the Kalahari–Karoo Basin

“…Interpretation.-The discrimination between glaciogenic and nonglaciogenic diamictite can be problematic (e.g., Visser 1994;Eyles and Januszack 2007;Le Heron et al 2017;Vesely et al 2018;Kennedy et al 2019). Most of the clasts found in this FA appear to be locally derived (pink to green granite and gneiss; Figs.…”

“…However, the absence of striations on clasts, combined with their local origin, suggests that the diamictite does not originate from glaciogenic processes. The chaotic aspect of the deposits and the normal grading, rather point toward a scenario whereby successive debris, mass, or hyperconcentrated flows deposited the sediments (e.g., Prior and Bornhold 1988;Mulder and Alexander 2001;Caron et al 2011;Moxness et al 2018;Kennedy et al 2019). Basal shear at the base of the flow is evidenced by rip-up clasts which show virtually no displacement (Fig.…”

“…While a glaciogenic origin is commonly invoked to interpret such facies (tillite, grounding-zone wedges, rain-out under an ice shelf), they may also constitute the product of a wide range of nonglaciogenic sedimentary processes such as subaqueous sediment gravity flows and subaerial alluvial and colluvial fans or debris flows, among others (e.g., Fedorchuk et al 2018). To further complicate the interpretation of these deposits, diamictite may also result from resedimentation through non-glaciogenic processes, of initially glaciogenic deposits (Caron et al 2011;Le Heron et al 2017;Kennedy et al 2019).…”

Geological observations in the Five-Klippes area, northwestern Katanga Copperbelt (Democratic Republic of the Congo)

“…Rocks of the Paleo-Mesoproterozoic basement, that are bordering the KCB to the west, are uncomformably overlain by a transgressive basal conglomerate. Formations overlying this conglomerate along the basement are the Mwale (Grand Conglomérat) to the southeast, and Mwashya or undistinguished Roan rocks to the northwest and to the south (François, 1973, 1995Kennedy et al, 2019. Recent observations questioning the autochthonous massif in that region showed that: (1) the Mamfwe anticline, eastern portion of the autochthonous massif ( Fig.…”

A slippery slope for Cryogenian diamictites?

“…Given this, distinguishing between diamictites of glacial versus slope origin in the rock record is a vital, yet often problematic, component of correct palaeo‐environmental and climate interpretations (Kennedy, Eyles, & Broughton, ; Letsch & Kiefer, ). This is especially true for Cryogenian diamictites of Death Valley, which often record coeval glaciation and rifting (Eyles & Januszczak, ; Miller, ; Prave, ; Vandyk et al., ).…”