Enigmatic chambered structures in Cryogenian reefs: The oldest sponge-grade organisms?

doi:10.1016/j.precamres.2014.09.020

Precambrian Research

Volume 255, Part 1, December 2014, Pages 109-123

https://doi.org/10.1016/j.precamres.2014.09.020 Get rights and content

Highlights

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Complex chambered structures are widespread in Cryogenian carbonates.
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The structures probably represent the remains of a globally distributed organism.
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They most closely resemble some chambered reef-dwelling sponges.
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The structures may represent sponges-grade organisms, or their microbial precursors.

Abstract

Previously undescribed chambered structures are common and widespread in the Cryogenian (post-Sturtian glacial) carbonates of the Oodnaminta Reef Complex (Adelaide Geosyncline, South Australia), the Rasthof-Berg Aukas Formation (Namibia) and the Gauss Formation (Namibia). These carbonate structures have millimetre to centimetre-scale chambers separated by well-defined and generally thin micritic walls. Chamber walls now consist of dolomite, but were probably originally aragonitic. The chambers may have a lobate, polygonal or dendritic morphology and are often further divided into smaller chambers. Chambered structures occur as reefal growth frameworks; as cavity-fillings in neptunean dykes and growth cavities; and as intercolumnar material within stromatolite frameworks. In the Oodnaminta Reef Complex, these structures are only present in the sub-photic deep water framework.

These structures probably represent the calcified remains of an organism or community of organisms that was globally distributed and widespread for a significant time period following the Sturtian glaciation. No precisely analogous structures have been previously described from modern or ancient settings, but the complexity and degree of organization suggests a significant evolutionary advance over older Proterozoic fossils. The closest morphological analogues for the structures are: (a) some types of reef-dwelling sponges; and (b) some complex microbialites from Archaean and Paleoproterozoic carbonates. The structures lack spicules and ostia found in sponges, ruling out a true Poriferan origin. However, it is plausible that they are proto-sponges, sponge-grade organisms, or complex microbial precursors to sponge-grade organisms. Whatever their affinity, we suggest these structures record a significant evolutionary event on the path towards organic complexity.

Graphical abstract

Introduction

The sudden and widespread appearance of soft-bodied metazoan organisms during the Ediacaran and the later development of organisms with hard mineralized skeletons in the latest Ediacaran – Early Cambrian are events which challenge our understanding of early life (Conway Morris, 1993, Grotzinger et al., 2000, Maloof et al., 2010a, Kouchinsky et al., 2012). The relatively sudden appearance of these advanced organisms in the geological record and the lack of ancestral precursors has focussed attention on the preceeding early Ediacaran and Cryogenian periods. The discovery of biomarkers and spicules derived from demosponges (Du and Wang, 2012, Love et al., 2009), possible foraminiferans (Bosak et al., 2012), and possible sponge-grade calcified fossils from the Cryogenian (Maloof et al., 2010b) suggests that the Cryogenian is highly prospective for the discovery of complex fossils.

However, there has been considerable debate about the origin of many Precambrian structures that are purported to be fossils and the literature abounds with such controversies (e.g. Neuweiler et al., 2009, Planavsky, 2009, Brain et al., 2012). Even if a biological origin is confirmed, the affinity of many fossils from the Precambrian has also proven to be problematic, this being the case even with the well documented Ediacaran fossils themselves (e.g. Seilacher et al., 2003, Narbonne, 2005).

Here, we describe chambered structures consisting of carbonate from Cryogenian interglacial successions in Australia and Namibia. The structures are widely distributed and common, but have not previously been described. We compare these to previously described organic and inorganic structures and can find no precisely analogous structures documented from any time period. We attempt to deduce the possible affinity of these chambered structures by an analysis of similarities and differences from previously described structures.

Section snippets

Geological setting

The chambered structures described here occur within Cryogenian interglacial successions of South Australia (Northern Adelaide Geosyncline) and Northern Namibia (Otavi Mountainland and the Kaokoveld) (Fig. 1, Fig. 2). Cryogenian successions from the Adelaide Geosyncline (Australia) and Northern Namibia are closely comparable, with the Sturtian and Marinoan glacials being recognized in both successions, interglacial successions being dominantly carbonates, and similar age constraints (Fig. 3).

Morphology of chambered fabrics

The chambered structures from Namibia and Australia consist of multiple macroscopic and microscopic cavities (ranging from 1 to 30 mm diam.) with walls consisting of micritic carbonate (almost invariably now dolomite). The chamber walls are commonly thin (20–100 microns) but may be up to 1 mm thick (Fig. 7, Fig. 8, Fig. 9). Where the walls are thin, they are generally homogeneous. When the walls are thicker, they often display a laminated microstructure (Fig. 7B). The chambers are filled by a

Macroscopic occurrence of chambered structures

The chambered structures described here from both Australia and Namibia have a variety of stratigraphic associations, although virtually all occurrences of the structures are in some way associated with reefal frameworks (Fig. 12). Chambered structures occur as:

1.
Reefal growth frameworks
2.
Intercolumnar material within stromatolitic frameworks
3.
Within cavity systems in neptunean dykes and breccias

Chambered structures of various types are also reworked as allochthonous blocks in reefal debris and

Possible affinities for chambered structures

The structures described here have a relatively complex internal morphology consisting of multiple chambers with well-defined walls. The chamber size is relatively large, with a maximum size of up to several centimetres in diameter. The three major morphological variants (lobate, dendritic and polygonal) all have similar chamber walls that consist of micritic carbonate. We therefore compare these chambered structures with similar previously described void structures from the geological record.

Discussion

While it is tempting to ascribe a relatively simple origin, such as bubble-calcification (e.g. Bosak et al., 2010, Knoll et al., 2013) to these Cryogenian chambered structures, the analogy breaks down on closer inspection. Spherical fenestrae produced by bubbles, and calcified bubbles in microbialites do not have the well-defined wall structure of the Cryogenian examples (i.e., fenestrae lack thin chamber walls, having chambers that simply consist of a cavity surrounded by host carbonate).

Significance

Despite their abundance in Cryogenian carbonates from Australia and Namibia, these chambered structures appear not to have been previously described. Their widespread geographic distribution as here documented (Namibia and Australia) is consistent with a global occurrence. Since no other Cryogenian or early Ediacaran reefs have yet been found, the lack of recognized similar structures elsewhere is likely due to the dearth of known reefs during this time interval. It is also probable that they

Conclusions

1.
Enigmatic chambered structures consisting of carbonate are abundant in Cryogenian sediments from Namibia and Australia. The structures are hosted by the Oodnaminta Reef Complex of the Adelaide Geosyncline, Australia, the Rasthof-Berg Aukas Formation of Namibia, and the Gauss Formation of Namibia. They are invariably associated with growth frameworks.
2.
The structures consist of lobate, dendritic or polygonal chambers (millimetre to centimetres in scale) that have thin micritic walls. Chambered

Acknowledgements

We are most grateful to Marg and Doug Sprigg of Arkaroola Wilderness Sanctuary for their generous support of fieldwork over many years. We appreciate the help given by Gammon Ranges National Park staff for access to the Oodnaminta Reef. We also thank Sabre Resources (Matt Painter and Eddie van Dyke), Teck Namibia (particularly Andrea Reed and Eckhart Fryer) and Weatherly International PLC (Mike Stuart) for logistic support and access to drill core during fieldwork in Namibia. This research is

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Snowballs in Africa: sectioning a long-lived Neoproterozoic carbonate platform and its bathyal foreslope (NW Namibia)
2021, Earth-Science Reviews
Citation Excerpt :
Three explanations for this abrupt thinning have been offered. The first is a primary facies change involving northward deepening at a platform margin (Wallace et al., 2014), analogous to the Rasthof Fm facies change in eastern Tr6 (Fig. 76). A second is a lateral ramp in Ombepera detachment (Hoffman et al., 2016b), which hugs the top of the Rasthof Fm in S2 (Fig. 139).
Otavi Group is a 1.5–3.5-km-thick epicontinental marine carbonate succession of Neoproterozoic age, exposed in an 800-km-long Ediacaran−Cambrian fold belt that rims the SW cape of Congo craton in northern Namibia. Along its southern margin, a contiguous distally tapered foreslope carbonate wedge of the same age is called Swakop Group. Swakop Group also occurs on the western cratonic margin, where a crustal-scale thrust cuts out the facies transition to the platformal Otavi Group.
Subsidence accommodating Otavi Group resulted from S−N crustal stretching (770–655 Ma), followed by post-rift thermal subsidence (655–600 Ma). Rifting under southern Swakop Group continued until 650–635 Ma, culminating with breakup and a S-facing continental margin. No hint of a western margin is evident in Otavi Group, suggesting a transform margin to the west, kinematically consistent with S−N plate divergence. Rift-related peralkaline igneous activity in southern Swakop Group occurred around 760 and 746 Ma, with several rift-related igneous centres undated. By comparison, western Swakop Group is impoverished in rift-related igneous rocks.
Despite low paleoelevation and paleolatitude, Otavi and Swakop groups are everywhere imprinted by early and late Cryogenian glaciations, enabling unequivocal stratigraphic division into five epochs (period divisions): (1) non-glacial late Tonian, 770–717 Ma; (2) glacial early Cryogenian/Sturtian, 717–661 Ma; (3) non-glacial middle Cryogenian, 661–646 ± 5 Ma; (4) glacial late Cryogenian/Marinoan, 646 ± 5–635 Ma; and (5) non-glacial early Ediacaran, 635–600 ± 5 Ma. Odd numbered epochs lack evident glacioeustatic fluctuation; even numbered ones were the Sturtian and Marinoan snowball Earths. This study aimed to deconstruct the carbonate succession for insights on the nature of Cryogenian glaciations. It focuses on the well-exposed southwestern apex of the arcuate fold belt, incorporating 585 measured sections (totaling >190 km of strata) and > 8764 pairs of δ¹³C/δ¹⁸O_carb analyses (tabulated in Supplementary On-line Information).
Each glaciation began and ended abruptly, and each was followed by anomalously thick ‘catch-up’ depositional sequences that filled accommodation space created by synglacial tectonic subsidence accompanied by very low average rates of sediment accumulation. Net subsidence was 38% larger on average for the younger glaciation, despite its 3.5–9.3-times shorter duration. Average accumulation rates were subequal, 4.0 vs 3.3–8.8 m Myr⁻¹, despite syn-rift tectonics and topography during Sturtian glaciation, versus passive-margin subsidence during Marinoan. Sturtian deposits everywhere overlie an erosional disconformity or unconformity, with depocenters ≤1.6 km thick localized in subglacial rift basins, glacially carved bedrock troughs and moraine-like buildups. Sturtian deposits are dominated by massive diamictite, and the associated fine-grained laminated sediments appear to be local subglacial meltwater deposits, including a deep subglacial rift basin. No marine ice-grounding line is required in the 110 Sturtian measured sections in our survey.
In contrast, the newly-opened southern foreslope was occupied by a Marinoan marine ice grounding zone, which became the dominant repository for glacial debris eroded from the upper foreslope and broad shallow troughs on the Otavi Group platform, which was glaciated but left nearly devoid of glacial deposits. On the distal foreslope, a distinct glacioeustatic falling-stand carbonate wedge is truncated upslope by a glacial disconformity that underlies the main lowstand grounding-zone wedge, which includes a proximal 0.60-km-high grounding-line moraine. Marinoan deposits are recessional overall, since all but the most distal overlie a glacial disconformity. The Marinoan glacial record is that of an early ice maximum and subsequent slow recession and aggradation, due to tectonic subsidence. Terminal deglaciation is recorded by a ferruginous drape of stratified diamictite, choked with ice-rafted debris, abruptly followed by a syndeglacial-postglacial cap-carbonate depositional sequence. Unlike its Sturtian counterpart, the post-Marinoan sequence has a well-developed basal transgressive (i.e., deepening-upward) cap dolomite (16.9 m regional average thickness, n = 140) with idiosyncratic sedimentary features including sheet-crack marine cements, tubestone stromatolites and giant wave ripples. The overlying deeper-water calci-rhythmite includes crystal-fans of former aragonite benthic cement ≤90 m thick, localized in areas of steep sea-floor topography. Marinoan sequence stratigraphy is laid out over ≥0.6 km of paleobathymetric relief.
Late Tonian shallow-neritic δ¹³C_carb records were obtained from the 0.4-km-thick Devede Fm (~770–760 Ma) in Otavi Group and the 0.7-km-thick Ugab Subgroup (~737–717 Ma) in Swakop Group. Devede Fm is isotopically heavy, +4–8‰ VPDB, and could be correlative with Backlundtoppen Fm (NE Svalbard). Ugab Subgroup post-dates 746 Ma volcanics and shows two negative excursions bridged by heavy δ¹³C values. The negative excursions could be correlative with Russøya and Garvellach CIEs (carbon isotope excursions) in NE Laurentia.
Middle Cryogenian neritic δ¹³C records from Otavi Group inner platform feature two heavy plateaus bracketed by three negative excursions, correlated with Twitya (NW Canada), Taishir (Mongolia) and Trezona (South Australia) CIEs. The same pattern is observed in carbonate turbidites in distal Swakop Group, with the sub-Marinoan falling-stand wedge hosting the Trezona CIE recovery. Proximal Swakop Group strata equivalent to Taishir CIE and its subsequent heavy plateau are shifted bidirectionally to uniform values of +3.0–3.5‰.
Early Ediacaran neritic δ¹³C records from Otavi Group inner platform display a deep negative excursion associated with the post-Marinoan depositional sequence and heavy values (≤ + 11‰) with extreme point-to-point variability (≤10‰) in the youngest Otavi Group formation. Distal Swakop Group mimics older parts of the early Ediacaran inner platform δ¹³C records, but after the post-Marinoan negative excursion, proximal Swakop Group values are shifted bidirectionally to +0.9 ± 1.5‰. Destruction of positive and negative CIEs in proximal Swakop Group is tentatively attributed to early seawater-buffered diagenesis (dolomitization), driven by geothermal porewater convection that sucks seawater into the proximal foreslope of the platform. This hypothesis provocatively implies that CIEs originating in epi-platform waters and shed far downslope as turbidites are decoupled from open-ocean DIC (dissolved inorganic carbon), which is recorded by the altered proximal Swakop Group values closer to DIC of modern seawater.
Carbonate sedimentation ended when the cratonic margins collided with and were overridden by the Atlantic coast-normal Northern Damara and coast-parallel Kaoko orogens at 0.60–0.58 Ga. A forebulge disconformity separates Otavi/Swakop Group from overlying foredeep clastics. In the cratonic cusp, where the orogens meet at a right angle, the forebulge disconformity has an astounding ≥1.85 km of megakarstic relief, and km-thick mass slides were displaced gravitationally toward both trenches, prior to orogenic shortening responsible for the craton-rimming fold belt.
Characteristics of a Tonian reef rimmed shelf before the onset of Cryogenian: Insights from Neoproterozoic Kunihar Formation, Simla Group, Lesser Himalaya
2020, Marine and Petroleum Geology
Citation Excerpt :
Rimmed shelves have also been reported from the Cryogenian Period in Katakturuk Dolomite, Dabie River Formation (Hilda Subgroup) and Balcanoona Formation (Umberatana Group) (Table 3). It is interesting to note that the earliest chambered proto-sponge organisms formed the reefal framework of Balcanoona Formation (Umberatana Group) which was deposited between the Cryogenian glacial deposits (Wallace et al., 2004, 2015). Though algal stromatolites clearly persisted beyond the Cryogenian glaciations, complex Ediacaran reef building metazoans took over, initiating advanced forms of complex reefal framework systems (Penny et al., 2014; Wood et al., 2015; Bowyer et al., 2017).
The Kunihar Formation records evolution of a unique rimmed shelf during the Tonian, before the Cryogenian/Marinoan glaciation of Krol Formation, Lesser Himalaya. The mixed siliciclastic-carbonate system of Neoproterozoic Kunihar Formation provokes interest for outcrop based facies analysis and sequence stratigraphy in order to frame a better understanding on the mechanism and control of its deposition. The formation records a 450 m thick mixed siliciclastic-carbonate rimmed shelf. Five facies associations have been identified- Peritidal-FA1, Lagoonal-FA2, Barrier island/Ooidal shoal-FA3, Reef complex-FA4 and Fore reef-FA5 facies associations. Two systems tracts were delineated - transgressive (TST) and highstand (HST) systems tracts. TST is marked by dominant aggradation of stromatolitic carbonates, while siliciclastic incursion surpassed carbonate deposition during highstand. It conforms to the “lower carbonate-upper clastic” model of mixed siliciclastic-carbonates. During the Precambrian, reef rimmed shelves were considered more abundant in the Archaean and Palaeo-Mesoproterozoic. This study determines that development of rimmed shelves was more common during the Neoproterozoic than previously speculated. Neoproterozoic reef rimmed shelves are concentrated more within the Cryogenian and Ediacaran Period. Stromatolitic reef rimmed shelf of Kunihar Formation is the only rimmed shelf recorded in the Tonian Period till date. Increased microbial stromatolitic accretion was propelled by surge in nutrients due to chemical weathering of underlying tholeiitic basalt (Darla volcanics) associated with the rifting of supercontinent Rodinia. With increasing interest of Tonian-Cryogenian transition as a key period in both planetary and organic evolution, Kunihar Formation represents a terminal reef rimmed shelf system that evolved due to microbial stromatolites in the absence of complex organisms. Subsequent to this, Cryogenian and Ediacaran biota took over as the dominant reef builders.
Iron-rich carbonate tidal deposits, Angepena Formation, South Australia: A redox-stratified Cryogenian basin
2020, Precambrian Research
The Cryogenian Angepena Formation (ca. 650 Ma) of South Australia records deposition in a peritidal environment equivalent to, and landward of the Balcanoona reef complex, and offers valuable insights into shallow marine chemistry during the nonglacial interlude between two global ice ages. The sedimentary facies comprise an iron-rich marine red bed succession and include red mudcracked dolomitic shales, tepee dolostone, crossbedded intraclastic ooid grainstone, and tepee-related sheet cavities with marine cements. These facies are interpreted to have been deposited in a Cryogenian tidal flat to subtidal setting that is laterally equivalent to the reefal and back reef environments of the Balcanoona Formation. Marine cements from this environment are characterized by well-preserved, non- and bright-cathodoluminescent zoning, and strong negative to slightly positive cerium (Ce) anomalies (Ce/Ce*). We suggest that the Angepena Formation is indicative of intermittent synsedimentary iron oxide precipitation in an oxidized peritidal environment in an otherwise strongly ferruginous and anoxic oceanic setting. Large negative Ce anomalies develop over a narrower depth range when compared to modern oxic marine settings, highlighting the importance of dissolved Mn and Fe concentrations in controlling the magnitude of the negative Ce anomalies. Overall, coupled sedimentological and geochemical evidence suggest a Neoproterozoic shallow marine system at the interface of an oxidizing atmosphere and an anoxic ferruginous marine system.
Carbonates before skeletons: A database approach
2020, Earth-Science Reviews
Carbonate minerals have precipitated from seawater for at least the last 3.8 billion years, but changes in the physical and chemical properties of carbonate rocks demonstrate that the nature, loci, and causes of this precipitation have varied markedly through time. Biomineralization is perhaps the most obvious time-bounded driver. However, other changes in the sedimentological character of Archean and Proterozoic carbonates offer an under-considered record of fluctuations in Earth’s chemical, biological, and physical systems before and during the rise of animal life. Geobiologists and geochemists have successfully used large geochemical datasets to track changes in Earth’s surface chemistry through time, but inconsistencies between proxies make the recognition of clear spatial-temporal patterns challenging. In order to place new constraints on the environmental history of Archean and Proterozoic oceans, especially with regard to redox state, we present for the first time a high-resolution database of global Precambrian and Cambrian carbonate sedimentation.
Our initial datasbase, named Microstrat, comprises carbonate-bearing successions dating from c. 3.4 billion to ~500 million years old; these include more than 130 carbonate-bearing formations, digitized at the highest stratigraphic resolution possible and classified by environment of deposition. Lithofacies details are recorded at the finest scale available, including a range of microbial and other depositional fabrics, mineralogy, environment of deposition, age and location. We interrogate the Archean, Proterozoic, and Cambrian carbonate records, testing both long-standing and novel hypotheses about changes in Earth’s carbonate system through time. Changes in carbonate sedimentology illuminate global changes in bioturbation and microbial community structure; the depth-dependent timing of marine oxygenation and the transition from anaerobic to aerobic respiration at and within the seafloor; and temporal changes in patterns of dolomitization. Shifts in both the loci of carbonate precipitation and in markers of animal activity are consistent with time- and depth-dependent trends in ocean oxygenation. We also note changes through time in the morphologies of stromatolites and thrombolites, which we suggest reflect secular shifts in carbonate precipitation, clastic sediment transport, and eukaryotic evolution. Observations from the database and petrographic studies confirm that the nature and extent of Proterozoic dolomite contrasts with that of dolomite in the Phanerozoic, consistent with dolomite being an early or even, in some cases, primary precipitate prior to the Cambrian Period. Because this database preserves environmental context and the stratigraphic relationships of carbonate features rather than tracking the occurrence of single features through time, it permits new and detailed analyses of the physical sedimentology of Precambrian carbonates and correlations in time and space among different features.
We also consider how to build upon this initial dataset. Capturing and analyzing sedimentological data is difficult. Analysis becomes more challenging when data have been collected by multiple researchers working across Earth’s entire history and surface. Field observations have a great deal of power to describe and explain changes in Earth’s surface environments, yet they are often reported in idiosyncratic ways that make it difficult to compare them, obscuring their true explanatory power. Microstrat is an effort to pool idiosyncratic sedimentological data in hopes of illuminating Archaean, Proterozoic, and Cambrian carbonate depositional environments, yet it is unavoidably limited. We describe how other researchers can help to build Microstrat into a community-curated database by reporting and sharing data.
The Tonian and Cryogenian Periods
2020, Geologic Time Scale 2020
The Tonian and Cryogenian periods together span from 1000 to c. 635.5 Ma and are currently chronometrically divided at 720 Ma. The early Tonian followed the amalgamation of the Rodinia supercontinent and is a time for which the stratigraphic, chemostratigraphic, and fossil record is relatively sparse and poorly dated. The initiation of intracratonic basins on many cratons c. 850 Ma, while Rodinia was still intact, is responsible for a much richer late Tonian record. This record preserves evidence for eukaryotic diversification and the first documented pronounced negative carbon isotope anomaly in the Neoproterozoic—the Bitter Springs Anomaly. Much of the second half of the Tonian Period is characterized by high carbon isotope values (δ¹³C of carbonate >5‰), but recent studies indicate that at least one and probably two deep negative δ¹³C excursions occurred after c. 740 Ma, the latter immediately preceding the onset of Cryogenian glaciation. This glaciation appears to have initiated globally at c. 717 Ma, based on consistent, high-precision U–Pb zircon ages from multiple sedimentary successions. These ages will support formal definition of the Global Stratotype Section and Point for the base of the Cryogenian System. This first Cryogenian glaciation, commonly referred to as the Sturtian glaciation, was long-lived, ending c. 660 Ma. Because the second and briefer late Cryogenian (i.e., Marinoan) glaciation is known to have initiated prior to 639 Ma and ended c. 635.5 Ma, the Cryogenian nonglacial interval must have been relatively short-lived (c. 20 Myr). Nevertheless, this interval is well represented on many cratons, due in part to the formation of widespread rift basins and passive margins as Rodinia began to break up. Although molecular clock and biomarker data suggest that the earliest animals had appeared by this time, no unambiguous metazoan fossils have been recovered from Cryogenian strata, which show low overall fossil diversity.
Neoproterozoic marine dolomite hardgrounds and their relationship to cap dolomites
2019, Precambrian Research
Neoproterozoic cap dolomites are unusual and distinctive marker units that occur after large Cryogenian glaciations. Several thin dolomite units that have features resembling cap dolomites occur within the Cryogenian-Ediacaran succession of the Adelaide Geosyncline. While each of these dolomite units has distinctive features, they also share many similarities with cap dolomites including: stratigraphic thickness ranging from several centimetres to several metres; impure dolomicrite composition; graded beds with a sharp top that become more dolomite-rich upwards; and evidence of volume expansion in the form of sheet cracks, tepees or other soft- sediment deformation structures. The carbon isotopic compositions for these thin dolomite horizons have values ranging from ∼−5 to +2 per mil VPDB, which overlap those commonly found in both Sturtian and Marinoan cap dolomites. The generally negative carbon isotope compositions of these dolomites and cap dolomites is most easily explained by mixing of marine and authigenic carbonate.
Sedimentological evidence indicates the graded beds within cap dolomites and other dolomite units are a form of submarine hardground or firmground, where dolomicrite is precipitated authigenically at or near the seafloor during periods of little or no sedimentation. Dolomite replacement of clay and other silicates is an important process in the formation of these dolomicrite firmgrounds. Volume expansion structures are a product of this near-surface dolomicrite precipitation, brought about by the high degrees of dolomite supersaturation with a consequent large force of crystallization and unlimited (marine) carbonate source for the precipitating dolomite.
We suggest that this deepwater authigenic dolomicrite precipitation is a result of the dominantly anoxic nature of Cryogenian- early Ediacaran oceans and caused by alkalinity-producing anoxic bacterial reactions. When sedimentation rates slow sufficiently (e.g. during transgressions), thin dolomicrite horizons are produced. In this hypothesis, cap dolomites may largely be the result of sediment starvation, combined with a possible increase in carbonate saturation during and following the post-glacial transgressions associated with the Sturtian and Marinoan glacial events. This scenario explains most of the sedimentological features of these unusual carbonates. The widespread development of such authigenic carbonate would have profound implications for the interpretation of Precambrian marine carbon isotopic compositions.

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Enigmatic chambered structures in Cryogenian reefs: The oldest sponge-grade organisms?

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Geological setting

Morphology of chambered fabrics

Macroscopic occurrence of chambered structures

Possible affinities for chambered structures

Discussion

Significance

Conclusions

Acknowledgements

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Gondwana Res.

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