Ice-rafted dropstones in “postglacial” Cryogenian cap carbonates

Heron, Daniel Paul Le; Busfield, Marie; Kettler, Christoph

doi:10.1130/g48208.1

Cited by 14 publications

(5 citation statements)

References 33 publications

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“…This interpretation is reinforced by similar textures in delicately laminated diamictites at the microscopic scale (Figure 8), and could have two possible origins. The first is introduction of dropstones via floating icebergs, a model that is widely appealed to in interpretations of other ancient records (e.g., Le Heron, 2015;Le Heron et al, 2021). However, given the dominance of diamictite in the succession, in conjunction with the recognition of coarsening-upward cycles over tens of metres (Figure 3), we propose that dropstones were probably introduced beneath a floating ice shelf near the grounding zone which delivered an abundance of material by rain-out.…”

Section: Discussionmentioning

confidence: 87%

A grounding zone wedge origin for the Palaeoproterozoic Makganyene Formation of South Africa

Heron

Busfield²,

Smith³

et al. 2022

Front. Earth Sci.

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The Makganyene Formation is a Siderian (2.45–2.22 Ga) diamictite-dominated succession, with both outcrop and subcrop in the Griqualand West Basin of the Transvaal Group of South Africa. We provide new outcrop and core descriptions from this succession, supplemented by microscopic analyses, to present an updated depositional model for a classic Palaeoproterozoic diamictite. Although internal correlation of core and outcrop successions is not possible, a recurring pattern is observed where diamictites are organised into coarsening-upward motifs at the tens of metres scale. With additional finds of striated clasts, and evidence for dropstones both at the core scale and at the microscopic scale, earlier interpretations of glacial control on sedimentation can be substantiated, with modification of glacial diamictites by mass flow processes also recognised. Overall, given the characteristic progradational stratigraphic architecture, we propose a new model for the Makganyene Formation which is considered to represent deposition of a grounding zone wedge at an ancient, oscillating ice margin.

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Section: Discussionmentioning

confidence: 87%

A grounding zone wedge origin for the Palaeoproterozoic Makganyene Formation of South Africa

Heron

Busfield²,

Smith³

et al. 2022

Front. Earth Sci.

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“…The brownish‐yellow massive pebbly dolostone of the basal‐lower cap dolostone contains many subangular gravels that originated from the erosion of the underlying basal granite or quartzite. The gravels float irregularly in the host rock; preferred orientations along the long‐axis or dropstone textures, as found in other cap carbonates (Le Heron et al ., 2020), are not seen. Therefore, these textures could represent contemporaneous reworking during the initial formation of cap dolostone in, for example, a nearshore environment.…”

Section: Discussionmentioning

confidence: 92%

Sedimentology of the Ediacaran barite‐bearing cap dolostone from Gaolan, northern Three Gorges, South China

et al. 2022

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While Ediacaran cap dolostones research is a hot topic, detailed sedimentological analysis of barite‐bearing cap dolostones remains scarce. Here, new data are presented from Xiangerwan in northern Three Gorges, South China, to illustrate the sedimentary characteristics, depositional environment and formation mechanism of the barite‐bearing cap dolostone. In ascending order, the succession is divided into: (i) pebbly dolostone; (ii) dolostone with unusual sedimentary structures (for example, sheet cracks and tepee structures); and (iii) barite‐bearing facies. The gravels of the pebbly dolostone record the slow relative sea‐level rise during post‐glacial transgression, with a local terrestrial influence. Sheet cracks and tepee structures represent water escape structures in the supratidal and upper intertidal zone, and gently wavy stromatolites in barite‐bearing facies are deposits typical of the upper intertidal zone. In addition, primary radial length‐slow dolomite cements were originally precipitated at the seafloor. In general, the cap dolostone was formed in a shallow‐water ramp environment. The form and sedimentary characteristics of barites indicate a primary (syngenetic) origin and help to divide the barites into bladed (Type I) and needle‐like (Type II) variants. With the ongoing melting of the Marinoan glacier and water density stratification, the cap dolostone was gradually precipitated, accompanied by exotic gravels input. The stratification persisted, and special sedimentary structures were widely developed. Phosphorus and barium ions were brought up from the deep basin by upwelling, indicated by the flourishing stromatolites and barite fan deposition. Above the sediment–water interface, the Type I barite first grew upward from substrates, then the Type II barite could grow either in the reverse ‘V‐shape’ cavities between and within the Type I barite clusters, forming rhythm layers with bedded dolomite, or scattered in the laminae of stromatolites or surrounding dolomite. Finally, the formation of barite fans indicates the end of cap dolostone deposition and deeper water sedimentation.

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“…2. Outsized clasts interpreted to be dropstones, where the patterns of deposition and sedimentary structures are seldom recognised or even reported in detail or in great numbers, e.g., the often small size of these clasts (one or a few centimetres), or if large size the appearance and position may indicate SGFs, for example, in the Cryogenian deposits of Namibia (Domack & Hoffman, 2011;Hoffman et al, 2021;Le Heron et al, 2021a).…”

Section: Discussionmentioning

confidence: 99%

Patterns, processes and models - an analytical review of current ambiguous interpretations of the evidence for pre-Pleistocene glaciations

Molén

2023

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Models (paradigms) and former interpretations have often been presupposed when conducting field research. In the 19th century diamictites were for the first time interpreted to have originated from ancient glaciations. These interpretations have to a large part prevailed in the geological community, although there has been much progress in the areas of sedimentology, glaciology and physical geography. The present work is an effort to find criteria which most clearly discriminate between geological features produced by different processes, mainly glaciation and mass flow, the latter predominantly sediment gravity flows. Geological features which have been interpreted to have formed by glaciation throughout pre-Pleistocene Earth history are compared to similar-appearing geological features formed by mass flow and tectonics, so as to uncover variations in the appearance between features resulting from these different processes. The starting point for this comparison is documentation of the appearance of Quaternary products of erosion and deposition, in order to discern the origin of older formations. It is shown that the appearance and origin of pavements, dropstones, valleys, small-scale landforms, surface microtextures and most other geological features may in some cases be equivocal, but in others the details are indicative of the process which generated the feature. Detailed geological field data which have been compiled by geologists from outcrops of pre-Pleistocene strata, more often than is considered in most papers, commonly point to a mass flow origin, mainly a sediment gravity flow origin, rather than a glaciogenic origin. A process of multiple working hypotheses or interpretations is therefore advocated, based mainly on a comparison of the appearance of features formed by different geological processes documented from different research disciplines. Instead of starting with current interpretations or models, this multiple working hypothesis or methodology helps to avoid confirmation bias and jumping to conclusions.

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Ice-rafted dropstones in “postglacial” Cryogenian cap carbonates

Cited by 14 publications

References 33 publications

A grounding zone wedge origin for the Palaeoproterozoic Makganyene Formation of South Africa

A grounding zone wedge origin for the Palaeoproterozoic Makganyene Formation of South Africa

Sedimentology of the Ediacaran barite‐bearing cap dolostone from Gaolan, northern Three Gorges, South China

Patterns, processes and models - an analytical review of current ambiguous interpretations of the evidence for pre-Pleistocene glaciations

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