An evaporitic facies in Neoproterozoic post-glacial carbonates: The Gifberg Group, South Africa
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Diamictite
Chemostratigraphy
Snowball Earth
Diamictite
Siliciclastic
Terrigenous sediment
Snowball Earth
Paleoclimatology
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Abstract The long-conceived idea of the glacial origin of Blaini diamictite of Lesser Himalayan Neoproterozoic succession reached its maxima when the diamictites and capping pink limestone were attributed to the Neoproterozoic Snowball Earth event and its aftermath, respectively. Occurrences of diamictite-limestone association in two different levels have also been correlated with the Sturtian and Marinoan glaciations. Critical review, however, reveals that the interpretations of the glacial origin of diamictites are not well founded. The diamictite-limestone association, which occurs at the lower part of a thick, light brown shale unit and laterally grades into light brown shale, primarily indicates episodic surge events in an otherwise tranquil condition favorable for hemipelagic sedimentation. The lithology, bed geometry, internal organization, and disposition of the diamictite bodies suggest deposition of debris flow fan lobes along fault scarps in a rift setting. Emplacement of subaqueous debris flows is indicated by the associated deposits of entrained turbidity currents. The limestone also bears the signature of claciturbidites. The appearance of diamictite bodies and associated limestone in two distinct levels is not a stratigraphic disposition; on the contrary, the deposits were dislocated and repeated by two successive regional thrust faults. The Chemical Index of Alteration (CIA) values of the light brown shale and the matrix of the diamictites indicate that these sediments formed through prolonged subaerial weathering. The events leading up to development of the rift system and evidence of prolonged weathering within the basin-fill sediments are consistent with supercontinental break up, the prologue of Snowball Earth.
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Snowball Earth
Dalradian
Subaerial
Mudflow
Debris flow
Chemostratigraphy
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ABSTRACT There is much debate regarding the intensity and geographic extent of glaciation during the Neoproterozoic, particularly in response to recent geochemical work suggesting that the Neoproterozoic earth was at times ice covered from equator to poles (the ‘Snowball Earth’ hypothesis). A detailed sedimentological analysis of the Neoproterozoic Smalfjord Formation of northern Norway was conducted in order to determine the extent and intensity of glacial influence on sedimentation. In the Tarmfjorden area, the Smalfjord Formation consists of a stacked succession of diamictites interbedded with fine‐grained laminated mudstones containing rare outsized clasts. Diamictites and interbedded mudstones are interpreted as the product of subaqueous mass flows generated along the basin margin. In the Varangerfjorden area, chaotically interbedded diamictites, conglomerates and sandstones are overlain by a thick succession of stacked sandstone beds; onediamictite unit at Bigganjargga overlies a striated pavement. The Varangerfjorden outcrops appear to record deposition on a subaqueous debris apron. Although diamictites contain rare striated and faceted clasts, suggesting a glacial sediment source, their origin as subaqueous mass flows prevents the interpretation of ice mass form or distribution. Rare lonestones may be associated with floating ice in the basin, which may be of glacial or seasonal origin. Glacial ice may have contributed poorly sorted glacial debris to the basin margin, either directly or through fluvioglacial systems, but there is no evidence of direct deposition by ice at Varangerfjorden or Tarmfjorden. The overall fining‐upward trend identified in the Smalfjord Formation and overlying Nyborg Formation is consistent with depositional models of rift basin settings. This fining‐upward trend, the predominance of mass flow facies including breccias associated with scarps and the evidence for extensional tectonic activity in the region suggest that tectonic activity may have played an important role in the development of this Neoproterozoic succession. The Smalfjord Formation at Tarmfjorden and Varangerfjorden does not exhibit sedimentological characteristics consistent with severe glacial conditions suggested by the snowball Earth hypothesis.
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Abstract Palaeoenvironmental reconstruction of Neoproterozoic successions has been the subject of long‐standing debate, particularly concerning the interpretation of diamictites. The Wilsonbreen Formation of north‐east Svalbard is a 130 to 180 m thick diamictite‐dominated glacigenic succession deposited during a late Cryogenian (Marinoan) glaciation. Previous research has highlighted a complex sedimentary architecture with evidence of subaqueous, subglacial and non‐glacial conditions. This study combines well‐established sedimentological techniques with the first sedimentological application of the anisotropy of magnetic susceptibility technique in Neoproterozoic glacial sediments, to investigate the origin and palaeoenvironmental significance of glacigenic sediments within the Wilsonbreen Formation. A range of lithofacies occurs within the succession, dominated by massive diamictites, sandstones and conglomerates. Some of these facies display evidence of primary deformation and can be grouped into a Deformed Facies Association; these are interpreted to have been formed through glacitectonic deformation in a subglacial environment. Fabric investigation reveals that this deformation was associated with glacier flow towards the north. In addition, an Undeformed Facies Association records deposition in ice‐proximal and ice‐distal subaqueous environments. Taken together with intervening non‐glacial facies, the glacigenic sediments record a series of advance–retreat cycles, with ice flow involving sliding and sediment shearing below wet‐based ice.
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Cryogenian synglacial deposits are regionally thin but locally thick, considering glacial duration, but the reasons for local thickening are poorly known. We studied three local thickenings of the Sturtian Chuos Formation in northern Namibia by measuring closely spaced columnar sections, not only of the synglacial deposits but also of the bounding pre- and post-glacial strata. This enabled incised paleovalleys filled by glacial debris to be distinguished from morainal buildups. In case 1, a U-shaped paleovalley, ∼450 m deep by ∼3.0 km wide, is incised into pre-glacial strata and 10% overfilled by ice-contact and subglacial meltwater deposits. In case 2, a wedge of glacial diamictite, ∼220 m thick by 2.0 km wide, overlies a disconformity that is demonstrably not incised into underlying pre-glacial strata. The wedge, draped by a post-glacial cap carbonate and argillaceous strata, is erosionally truncated at its apex by Marinoan glacial deposits and their basal Ediacaran cap dolomite. The wedge was a positive topographic feature, either a terminal moraine or an erosional outlier of formerly more extensive glacial deposits. In case 3, a wedge of conglomerate, glacial diamictite, and subglacial lake deposits thickens to >2000 m where it abuts against granitoid basement rock uplifted along a border fault. Fault movement ceased before the Sturtian cap carbonate was deposited. The locus of maximum deposition shifted over time from proximal to distal with respect to the border fault, similar to Mesozoic half grabens developed above listric detachments imaged seismically on offshore North Atlantic margins.
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In the northern peripheral part of the Carpathian Foredeep, the Middle Miocene (Badenian) gypsum deposits comprise two major, laterally extensive members: the lower is mostly autochthonous, of selenitic facies and the upper is allochthonous, of clastic facies and cumulate deposits. Towards the south, gypsum is replaced in the subsurface by anhydrite which displays relict textures of the primary gypsum. The facies variation and succession throughout the gypsum section, as well as geochemical indicators, reflect varied sedimentary conditions on the basin margin. Deposition took place on the periphery of a platform made up of a system of widespread shallow-water lagoons (sub-basins) separated by fault-controlled, NW-SE elongated islands or shoals. In these physiographically differentiated palaeoenvironments, facies relationships were largely diachronous. The water depth varied from a few metres to some tens of metres, and subaerial exposure episodically affected the gypsum deposition, as suggested from the sedimentary record and comparison of the facies with modem evaporitic environments. Variations in brine depth, salinity and water dynamics are expressed in the cyclic succession of the progressively changing facies associations. Sedimentary conditions changed drastically at the boundary of the lower (selenitic) and upper (clastic) members, and at the end of sulphate deposition, following major sea-level changes.
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Siliciclastic
Chemostratigraphy
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