Subaqueous debrites of the Grand ConglomÉrat Formation, Democratic Republic of Congo: A model for anomalously thick Neoproterozoic: “Glacial” diamictites

Abstract

Very thick (> 1 km) successions of matrix-supported conglomerates (diamictites) are a very distinctive component of many Neoproterozoic basins. Classically interpreted as glacially deposited sedimentary rocks, their thickness has been seen as requiring exceptional depositional conditions such as world-wide “panglacial” climates. The Neoproterozoic Grand Conglomérat Formation (GC) of Katanga Province, southeastern Democratic Republic of Congo, is a 1.8-km-thick diamictite succession hosting one of the world's largest stratiform copper deposits. Examination of more than 300 km of recently acquired large diameter (up to 4 inches, 10.2 cm) core identifies the diamictites of the GC as debrites that accumulated as part of a deep-water “mass-transport complex” in a tectonically active and volcanically influenced anoxic rift basin. Detailed sedimentological descriptions of debrite facies and their lateral and vertical variability permits new insights into processes of debris-flow formation and transport, and their wider paleoenvironmental and paleotectonic significance. A genetic model is herein presented that highlights the importance of slumping and subaqueous downslope mixing of basin-margin fan-delta gravels, fault breccias, volcanic debris from contemporaneous basaltic fissure eruptions with basinal muds, and the downslope ponding of flows in narrow fault-bounded depocenters. Preservation of such an exceptionally thick subaqueously deposited debrite-dominated mass transport complex lacking any evidence of shallow-water deposition, requires rapid subsidence. Any direct sedimentary evidence of a glacial influence on sedimentation in the wider basin hinterland (if present) has been destroyed and homogenized by mass flow. Indirect evidence of a cold-climate setting is possibly expressed as lonestones in laminated turbidite facies dropped by either glacial or seasonal ice, and by exceptionally rare scratched clasts that may have been striated subglacially. Descriptions and interpretations presented here provide clues to the origin of other unusually thick debrite and turbidite successions elsewhere in other Neoproterozoic basins; their primary paleoenvironmental significance is that they appear to record ponding and focusing of mass flows in narrow, rapidly subsiding fault-bounded depocenters, rather than any unique glacial paleoclimate.

The Laurentian Neoproterozoic Glacial Interval: reappraising the extent and timing of glaciation

“…These workers emphasized the importance of applying a 'tectonosequence' approach in contrast to the simple bedby-bed climatostratigraphic models of earlier workers. Other investigations in Congo has similarly demonstrated the fundamental importance of a tectonic driver on facies types in Neoproterozoic rift basin fills, especially in promoting the accumulation of the 'diamictite/turbidite association' that is characteristic of this interval globally where diamictites commonly approach 1 km in thickness (Kennedy et al, 2018;Kennedy and Eyles, 2019). This work also demonstrates that comparison of very detailed sedimentary logs in terms of facies, clast composition and stratigraphic stacking patterns permit no correlation whatsoever even over a few hundred metres (Tofaif et al, 2019), with major implications for identifying "type sites".…”

Late Cryogenian and late Paleozoic ice ages on the São Francisco craton, east Brazil

“…The tectonic setting of continents can exert a strong influence on glacial dynamics and ultimately on the preservation and representativeness of the glacial record and on the basin stratigraphic arrangement Isbell et al, 2008;Ramstein et al, 2019). The late Neoproterozoic glacial record is well known as being influenced by plate tectonic processes Eyles and Januszczak, 2004;Arnaud and Etienne, 2011;Li et al, 2013;Le Heron et al, 2017;Kennedy and Eyles, 2019). The rifting of the Rodinia occurred concomitantly with major glaciations and generated many basins presenting glacial and slope-derived material.…”

Syn‐rift mass flow generated ‘tectonofacies’ and ‘tectonosequences’ of the Kingston Peak Formation, Death Valley, California, and their bearing on supposed Neoproterozoic panglacial climates

“…Furthermore, the thickness of units may primarily reflect ponding of debris flow in relatively small fault‐bounded depocentres (see also Kennedy et al. , 2019; Kennedy & Eyles, 2019).…”

“…, 2011; Naruse & Otsubo, 2011; Kennedy et al. , 2019; Kennedy & Eyles, 2019). Internal churning during flow acts essentially as a concrete mixer and homogenizes poorly‐sorted antecedent sediment (e.g.…”

“…The rationale of this study and others recently concluded by the authors (Kennedy et al. , 2019; Kennedy & Eyles, 2019) is to more closely examine the sedimentology of distinctive Neoproterozoic deposits that have been ascribed to one or more global panglacials commonly known as ‘Snowball Earth’ (e.g. Hoffman et al.…”