D. P. Le Heron

Sea ice−free conditions during the Sturtian glaciation (early Cryogenian), South Australia

In the Neoproterozoic snowball Earth hypothesis, shutdown of the planet9s hydrological system has been attributed to a global ice cover during one or more extreme glaciations. In the central Flinders Ranges, South Australia, the Yudnamutana Subgroup of Sturtian age includes diamictite, sandstone, and siltstone units of glaciomarine origin as much as 5000 m thick, and is overlain by postglacial transgressive siltstone and shale of the Tindelpina Shale Member, Tapley Hill Formation. In the central Flinders Ranges, the Yudnamutana Subgroup consists of (1) the Pualco Tillite (gravity resedimented glacial deposits), (2) the Holowilena Ironstone (glacioturbidites), (3) poorly stratified pebbly diamictite of the Warcowie Dolomite Member, lowermost Wilyerpa Formation (gravity resedimented glacial deposits), succeeded by (4) siltstones and sandstones with abundant hummocky cross-stratification (HCS: storm deposits), and (5) a lonestone-bearing succession with cobble-sized clasts in the upper Wilyerpa Formation (ice-rafted debris interpreted to record a glacial re-advance) in which HCS is absent. Because the action of oscillating waves is required to produce HCS on the seafloor, its presence indicates an interval of significant meltback prior to glacial re-advance. Given that the HCS occurs ∼2 km beneath the Tindelpina Shale Member, it signifies a major ice-free interval during the Sturtian glaciation.

Glaciogenic Reservoirs and Hydrocarbon Systems

Glaciogenic reservoirs and hydrocarbon systems occur intermittently throughout the stratigraphic record, with particular prominence in Neoproterozoic, Late Ordovician, Permo-Carboniferous and Late Cenozoic strata. Recent interest in glaciogenic successions has been fuelled by hydrocarbon discoveries in ancient glaciogenic reservoirs in North Africa, the Middle East, Australia and South America. Glaciogenic deposits of Pleistocene age are noteworthy for their content of groundwater onshore and potentially prospective and/or hazardous gas accumulations offshore. The abundant imprints of Pleistocene glaciations in both hemispheres can be used to reconstruct complex histories of repeated ice cover and retreat, and glacier-bed interactions, thus informing our view on the dynamics of older ice caps and predictions of future glaciations. This volume aims to provide a better understanding of glaciogenic processes, their stratigraphic record and reservoir characteristics of glaciogenic deposits. The book comprises 3 overview papers and 16 original case studies of Neoproterozoic to Pleistocene successions on 6 continents and will be of interest to sedimentologists, glaciologists, geophysicists, hydrologists and petroleum geologists alike.

Sequencing the Sturtian icehouse: dynamic ice behaviour in South Australia

The Cryogenian record of South Australia includes the type region of the Sturtian glaciation, the oldest of three pan-global icehouse intervals during the Neoproterozoic. Data are presented from previously little described sections at Holowilena Creek, Oladdie Creek and Hillpara Creek in the central and southern Flinders Ranges, where five facies associations are recognized. These are (1) diamictite and conglomerate, (2) interbedded heterolithic deposits, (3) hummocky cross-stratified sandstone, (4) lonestone-bearing siltstone, and (5) ferruginous siltstone and sandstone. The succession reveals significant lateral and vertical facies variation, which is linked to a complex inherited palaeotopography and distance from the sediment source. Repeated stratigraphic occurrences of striated clasts and abundant ice-rafted debris strongly support recurrent glacial influence on sedimentation. The intercalation of gravitationally reworked diamictites, dropstone-bearing siltstone and dropstone-free siltstone testifies to dynamic sedimentation within a periodically glacially influenced subaqueous environment. Sequence stratigraphic analysis identifies four glacial advance systems tracts, separated by three glacial retreat systems tracts, wherein hummocky cross-stratified sandstones attest to open water conditions. These findings support dynamic ice sheet behaviour in South Australia, and provide clear evidence for repeated intra-Sturtian ice sheet recession.

Glaciogenic reservoirs and hydrocarbon systems: an introduction

Abstract Glaciogenic reservoirs host important hydrocarbon and groundwater resources across the globe. Their complexity and importance for exploration and palaeoclimate reconstruction have made glaciogenic successions popular subjects for study. In this paper we provide an overview of the palaeoclimatic and tectonic setting for Earth glaciation and a chronological account of glaciogenic deposits since c. 750 Ma, with particular emphasis on their reservoir potential and associated hydrocarbon systems. Hydrocarbon accumulations within glaciogenic reservoirs occur principally in Palaeozoic (Late Ordovician and Permo-Carboniferous) sandstones in South America, Australia, North Africa and the Middle East, with relatively minor occurrences of shallow gas hosted in Pleistocene deposits in the North Sea and Canada. Groundwater reserves occur within glaciogenic sandstones across the northern European lowland and in North America. The main glaciogenic environments range from subglacial to glacier front to proglacial and deglacial. Rapidly changing environments, hydrodynamic regimes and glacier-front and subglacial deformation often result in very complex glaciogenic sequences with significant challenges for reconstruction of their origin and resource importance, which this volume seeks to address.

A eukaryote assemblage intercalated with Marinoan glacial deposits in South Australia

Composite hematite–silica structures recovered from a siltstone bed in the Elatina Formation of South Australia include (1) sub-circular to whorl-shaped forms, (2) elongate to half-moon-shaped forms and (3) and lozenge-shaped forms locally linked into chains. They range from 200 to 500 µm in diameter and are interpreted as eukaryote tests. Evidence for internal etching of a calcite core of some tests indicates that at least some of the hematite–silica fabrics were acquired through replacement. Carbon isotope values of −20‰ δ13C are suggestive of precipitation by microbial activity, and imply a change in ambient fluid chemistry associated with a pH reduction. The tests occur within sandstone beds that were deposited on a tidally modulated braidplain during the Marinoan glaciation at the end of the Cryogenian. The quartz grains in the sandstone sample lack the typical textures (surface striae, internal fractures or irregular grain boundaries) expected for glacially transported material. Thus, on textural grounds we argue that the eukaryote tests represent a proglacial ecosystem during a late Cryogenian snowball Earth event. Supplementary material: Video files of digital X-ray tomographs (μCT) in the longitudinal and transverse planes are available at: https://doi.org/10.6084/m9.figshare.c.2209723.

The Hirnantian glacial landsystem of the Sahara: a meltwater-dominated system

During the Hirnantian (443 Ma), ice sheets expanded towards the present-day north over North Africa and the Arabian Peninsula when the western part of the Gondwana supercontinent straddled the South Pole. A glacigenic succession is exposed around the flanks of several Saharan cratonic basins, including Murzuq and Al Kufrah basins in Libya. As a result of the present-day hyperarid context of the outcrops, a suite of palaeo-glacial landforms has been identified and, in some cases, mapped. These landforms occur along glacial erosion surfaces, defined as unconformities derived through direct ice sculpting, meltwater erosion or a combination of these processes. Unconformities, mostly resulting from ice sculpting and including glacially striated pavements and mega-scale glacial lineations (MSGLs), are identified at a number of locations along these erosion surfaces, in association with drumlinized features. The majority of reported examples are of ‘soft sediment character’; that is, generated through the deformation of unconsolidated sediment. Features mainly associated with meltwater erosion include tunnel valleys which occur in swarm-like belts, defining palaeo-grounding-line positions as the ice fronts retreated. A ‘smeared’ series of palaeo-ice-stream footprints, resulting from multiple advance–retreat cycles, is characteristic of the landsystem. Deformation of sediments into a series of push moraines, expressed as kilometre-scale fold–thrust belts, allows the locations of the former ice front to be identified.During the Hirnantian (443 Ma), ice sheets expanded towards the present-day north over North Africa and the Arabian Peninsula when the western part of the Gondwana supercontinent straddled the South Pole. A glacigenic succession is exposed around the flanks of several Saharan cratonic basins, including Murzuq and Al Kufrah basins in Libya. As a result of the present-day hyper-arid context of the outcrops, a suite of palaeo-glacial landforms has been identified and, in some cases, mapped. These landforms occur along glacial erosion surfaces, defined as unconformities derived through direct ice sculpting, meltwater erosion or a combination of these processes. Unconformities, mostly resulting from ice sculpting and including glacially striated pavements and mega-scale glacial lineations (MSGLs), are identified at a number of locations along these erosion surfaces, in association with drumlinized features. The majority of reported examples are of ‘soft sediment character’; that is, generated through the deformation of unconsolidated sediment. Features mainly associated with meltwater erosion include tunnel valleys which occur in swarm-like belts, defining palaeo-grounding-line positions as the ice fronts retreated. A ‘smeared’ series of palaeo-ice-stream footprints, resulting from multiple advance–retreat cycles, is characteristic of the landsystem. Deformation of sediments into a series of push moraines, expressed as kilometre-scale fold–thrust belts, allows the locations of the former ice front to be identified. The Hirnantian glacial record of North Africa is a classic example of a continent-scale archive of an ancient palaeo-ice sheet. Expeditions to the Sahara during the late 1960s, focusing on the Algerian record, revealed a full suite of glacial indicators including striated pavements, pingos, roches moutonnees and a variety of meltwater channels (Beuf et al. 1971). Subsequently, largely as a result of their significance as hydrocarbon reservoirs, more recent studies re-evaluated these outcrops from a sedimentological perspective. Much effort focused on unravelling the glacial stratigraphy in neighbouring …

Influence of microbial framework on Cryogenian microbial facies, Rasthof Formation, Namibia

Abstract The Rasthof Formation is a mid-Cryogenian cap carbonate succession deposited in Namibia following the Sturtian glaciation. It includes a microbial member, typically >100 m thick. This member exhibits contorted intervals, and is divisible into two informally defined units. The lower unit (microbial member 1: MM1) comprises thickly laminated microbialites (1–6 mm); the upper unit (MM2) is characterized by thinly laminated microbialites (sub-millimetre layering). Contortion of the microbialite deposits – a recurrent feature of this succession – is interpreted to result from soft-sediment deformation. Deformed intervals and styles range from metre- to decimetre-scale chaotic folds in MM1 to a few centimetre-scale, localized roll-up structures in MM2. Study of the microfacies of MM1 and MM2 reveals two essentially different architectures. In MM1 the microfacies is dominated by an alternation of thin micritic laminae with thicker cemented intervals; this probably gave less rigidity to the sediments than in MM2 where the laminated fabric is also present but connected vertically as well, forming a continuous framework. We suggest that the continuity of this framework limited the frequency and scale of soft-sediment deformation. In the Rasthof Formation, the microarchitecture is thus suggested to translate into different degrees of rigidity of the macrofacies.

Precambrian olistoliths masquerading as sills from Death Valley, California

Olistolith production and magmatism are processes commonly associated with extensional tectonic settings, such as rift basins. We present a cautionary exemplar from one such Precambrian basin, in which we reinterpret metabasite bodies, previously documented as sills, to be olistoliths. We nevertheless demonstrate that, on the basis of field observation alone, the previous but erroneous sill interpretation is parsimonious. Indeed, it is only by using isotopic age and compositional analysis that the true identities of these metabasite olistoliths are revealed. We present new data from metabasites and metasedimentary strata of the Kingston Peak Formation (Cryogenian) and Crystal Spring Formation (Mesoproterozoic) of Death Valley, USA. These include field observations, U–Pb apatite ages, U–Pb zircon ages (detrital and igneous) and whole-rock geochemistry. These data also provide a new maximum age for the base of the Pahrump Group and suggest that the Crystal Spring Diabase was more tholeiitic than previously thought. Similar sill/olistolith misinterpretations may have occurred elsewhere, potentially producing erroneous age and tectonic-setting interpretations of surrounding strata. This is particularly relevant in Precambrian rocks, where fossil age constraints are rare. This is illustrated herein using a potential example from the Neoproterozoic literature of the Lufilian belt, Africa. We caution others against Precambrian olistoliths masquerading as sills. Supplementary material: Details of a meta-igneous boulder from P12 of the Silurian Hills, LA-ICP-MS and whole-rock geochemistry methods and standards, and U–Pb apatite and zircon isotopic data, including standards and selected cathodoluminescence images, are available at https://doi.org/10.6084/m9.figshare.c.3990639

The Early Palaeozoic Glacial Deposits of Gondwana: Overview, Chronology, and Controversies

Abstract The Early Palaeozoic glacial record is confined to Gondwana, and is expressed in successions of late Ordovician through early Silurian age. Glacial or glacially related successions crop out in Africa, Arabia, Europe, and South America. The high-quality exposures of North African and Arabian Hirnantian age strata are of particular interest. The record of extensive ice sheets across the area is generally simpler further away from the centre of glaciation (e.g., Morocco, Turkey, Iran), and more complex within the inner glaciated shelf. Following a glacial maximum, where ice sheets were drained by a network of palaeo-ice streams, an extensive belt of tunnel valleys was cut across large swathes of the region. Some authors have proposed a porewater control on the location of these. The glacial deposits themselves include glacially striated pavements, dropstone-bearing diamictites and boulder pavements. Multiple lines of evidence suggest that the majority of striated pavements developed in largely unconsolidated sediments, but their relationship to warm versus cold-based ice remains controversial.

Styles, origins and implications of syndepositional deformation structures in Ediacaran microbial carbonates (Nama Basin, Namibia)

Abstract Outstanding exposures of Ediacaran-aged thrombolite–stromatolite bioherms and biostromes crop out in the Nama Basin, SW Namibia. Fieldwork, dovetailed with remote sensing and a terrestrial laser scanning (LiDAR) survey, allow the fracture network of this succession to be characterized, and the relative age of fracture sets and families to be determined. The results show that the microbial carbonates were affected by intense syndepositional brittle and ductile deformation. Early brittle fracturing was favoured where early lithification of microbialites took place upon deposition. Such deposits were prone to gravitational collapse due to internal weaknesses during early lithification. Timing of syndepositional fracturing of bioherms and biostromes is demonstrated by contemporaneous microbial overgrowth over brecciated material in open-mode fractures. Ductile deformation occurs preferentially around massive thrombolite domes and columns, represented by folding of mud-dominated sediments in inter-column fill. Secondary fractures developed during the long-lived structural history of the Nama Basin, resulting in a complex fracture network of syndepositional fractures overprinted by secondary fractures. These findings have important implications for carbonate reservoir characterization in microbial reservoirs and subsurface fluid-flow estimations. The observed syndepositional fractures form due to body forces that are intrinsic to the microbial system and thus do not require an external tectonic driver.