An occurrence of radially symmetric sedimentary structures in the basal Ediacaran cap dolostone (Keilberg Member) of the Otavi Group

Snowball Earth cap carbonate sequences provide an archive of what are likely the most dramatic climate transitions in all of Earth history. One approach to gain insight into these events is the detailed observation of sedimentary structures within these post-glacial units. Here, we report on newly discovered radially symmetric sedimentary structures within the Keilberg Member post-Marinoan ‘cap dolostone’ from the Otavi Group of northwest Namibia. We describe the local expression of over 60 decimeter-scale cymbal or disc structures from a single location. We interpret these features, which we name Zildjian structures, to be of likely abiotic origin. Through morphological comparisons, we suggest that Zildjian structures are most similar to Astropolithon, a pseudofossil that formed as a result of fluid or gas expulsion.


Introduction
Persuasive geological and geochemical evidence suggests that the Neoproterozoic Era was punctuated by a pair of 'Snowball Earth' glaciations -the Marinoan (646 +/-5 to 635 Ma; Kendall et al. 2006;Prave et al. 2016;Condon et al. 2005) and the Sturtian (717 to 661 Ma; Macdonald et al. 2010;MacLennan et al. 2017;Rooney et al. 2014) -during which time the oceans were covered from pole to pole by dynamic ice sheets (Kirshvink 1992;Hoffman & Schrag, 2002;Hoffman et al. 2017;Hoffman et al. in press). The transitions out of these extreme climate states are documented by so called 'cap dolostones', which (in the case of the Marinoan) are layers of organic-poor micro-clotted, pseudopeloidal (also described as micropeloidal or dolopelarenite) dolomite overlying glacial deposits and glacial erosion surfaces. These postglacial carbonates have been observed to range in thickness from 10s of centimetres to 100s of metres (Grotzinger & Knoll, 1995;Hoffman et al. 1998Hoffman et al. , 2011Hoffman & Li, 2009). Cap dolostones are found on virtually all palaeocontinents and palaeogeographic reconstructions place deposition typically at ≤ 50˚ palaeolatitude (Hoffman & Li, 2009). These units represent the transgressive systems tract (i.e. post-glacial flooding) of thick depositional sequences that may have formed because of prolonged subsidence in a slow sedimentation regime (Partin et al. 2016).
Cap dolostones contain many unusual (and, in certain cases, enigmatic) sedimentological features, including tubestone stromatolites (Corsetti & Grotzinger, 2005), digitate and fanning barites (Bao et al. 2008;Crockford et al. 2016Crockford et al. , 2018Crockford et al. , 2019, trochoidal bedforms interpreted as giant wave ripples (Allen & Hoffman, 2005;Lamb et al. 2012), and sheet cracks filled with fibrous isopachous dolomite cement (Hoffman & Macdonald, 2010;also cf. Hoffman, 2011 for an in-depth review of cap dolostone sedimentology). In fact, Marinoan cap dolostones are so distinctive in character and setting that they defined the base of the Ediacaran Period (Knoll et al. 2006) before their age and synchroneity were known radiometrically (Rooney et al. 2015;Zhou et al. 2019). In the top centre map panel of Figure 1, the area bearing the Zildjians in the foreland thrust-fold belt of the Ediacaran Kaoko orogen is outlined by the white rectangle. Bedrock includes pre-orogenic carbonate formations of the Otavi Group (770-600 Ma) and synorogenic clastics of the Sesfontein Formation (Mulden Group). The Glacigenic Chuos and Ghaub formations are too thin to show at this scale. The white rectangle is in the W-facing but E-dipping limb of an anticline in the hanging wall of a W-directed backthrust. Local topographic relief is 425 m relative to the Hoanib River, which has perennial flowing surface water in this area. A vehicle track (purple line) connects westward to Khowarib and eastward to Omukutu and Ombaatjie. The field campsite is indicated by a small yellow triangle. In the top right panel, a representative columnar section of Keilberg Member cap dolostone in the white rectangular area of the map panel and red rectangle of the cross section in the lower panel are presented. Numbered lithologic units: 1 -Ghaub Formation (Marinoan) carbonate diamictite inferred to be a lodgement tillite derived from underlying upper Ombaatjie Formation (left panel); 2 -low-angle crossstratified dolopelarenite (peloid grainstone); 3tubestone stromatolite (Corsetti & Grotzinger, 2005); 4 -low-angle cross-stratified dolopelarenite with peloidal sand volcanoes at horizons indicated; 5 -thin planar-laminated dolomicrite with argillaceous partings increasing upward; 6marly calcite rhythmite. Stratigraphic height is in metres above the base of the Keilberg Member (0.0 m). In the lower cross section panel, selected Ombaatjie Fm sections are plotted from the OPz and IPz which outline the Omarumba Trough. The insert map at the bottom of the panel shows relative section locations. Palaeotopography is reconstructed assuming as a datum when carbonate carbon isotope values from previous studies cross 0.0 per mil in cycle b7 (Hoffman et al. in press), which, elsewhere, is supported by correlation of Keilberg Member thickness with stratigraphic height above this datum (Hoffman et al., in press). The Omarumba trough has been inferred to be a subglacial bedrock trough formed via partial removal of the b8 and b7 cycles via Marinoan glacial erosion (Hoffman et al. in press).
One way to gain new perspectives on cap dolostone depositional processes is through the careful accounting and analysis of sedimentary features within them. Such features -from millimetre-scale wave ripples to metre-scale microbial buildups and kilometre-scale mud volcanoes -represent a record of physical forcings that can be used to understand past environmental conditions (Hoffman & Macdonald, 2010;Lamb et al. 2012). With such analyses in mind, we present a description of decimetre-scale radially symmetrical sedimentary cymbal-shaped structures that are located within the Keilberg Member cap dolostone of the Congo craton in modern day Namibia. Due to their size and shape, we call these cymbal-like structures "Zildjians", after an Armenian-Turkish-American family of cymbal manufacturers since 1618.

Geological Setting
The focus of this study is the Keilberg Member of the Maieberg Formation (Hedberg, 1979;SACS, 1980;Hoffman & Halverson, 2008), which is the basal Ediacaran formation within the Otavi Group. The study site is located near the village of Omukutu on the upper Hoanib River east of Khowarib, in the Kunene Region of northwest Namibia (S19˚17'20.04" E13˚54'5.4"; Fig. 1). At the study location, the Maieberg Fm is exposed as sub-vertically dipping, slightly overturned beds that face to the west. The Keilberg Member documents the initial post-glacial transgression from the Marinoan glaciation (Hoffman et al. 1998(Hoffman et al. , 2011Hoffman et al. in press) across northwest Namibia and correlates with other, globally distributed formations that record similar geological events (Hoffman et al. 2017). The Omukutu area is situated on the inner Otavi Group carbonate platform at the western sidewall of the Omarumba Trough (Fig. 1), a broad shallow depression cut by south-southwestward-flowing Marinoan ice. In this location, the Keilberg Member directly overlies the glacial erosion surface, marked by scraps of lodgement tillite, and passes gradationally upwards into marly limestone rhythmite of the middle Maieberg Formation postglacial maximum-flooding interval. The Keilberg Member at Omukutu is ≈23 m thick. Regionally, sections within the Omarumba Trough range between 10-20 m in thickness but outside the trough they can expand to between 30 and 100 m (Fig. 1).

Field observations
Sixty-one different Zildjian structures were recorded in the Omukutu area (Fig 2). Most structures were observed to be between 7.8 and 9.1 m above the base of the Keilberg Member, although several were also found at 11.2, 13.0 and 13.5 m above the base of the section ( Table 1). The stratigraphic interval containing the Zildjian structures was deposited above 'tubestone' stromatolites (Corsetti & Grotzinger, 2005) and is comprised of dolomitized micropeloidal grainstone (i.e. dolopelarenite) characterized by swaley low-angle cross-stratification ( Fig. 1). The Zildjians were identified as concentric circular ridges and depressions (Fig. 2). As previously mentioned, in the Omukutu area, the beds are slightly overturned. Therefore, field measurements of Zildjians were made on the undersides of the structures (Fig. 2). Although the preservation of Zildjian structures varied across the outcrop (due to differential weathering), we applied a consistent measurement scheme to document those instances that we were safely able to reach.
In total, we were able to study approximately half of the 61 observed structures (n = 35). In what follows, we present observations as if observing Zildjian structures from rightway-up in horizontal beds unless otherwise specified. For each Zildjian, we measured an outer rim diameter (D1), an inner trough diameter (D 2 ), a central axial pit diameter (D 3 ), and the distance to the next closest Zildjian (from center to center; S), all parallel to bedding ( Fig. 3C; Table 1).
We also measured an overall vertical relief where the undersides of structures protrude down from the bedding plane (H) and stratigraphic height (Z), both normal to bedding (  (Table 1). As observed, none of the structures exhibited any markings radiating away from the axial pit. Such observations have been documented in a number of interpreted Ediacaran and Cryogenian circular fossil imprints (MacGabhann, 2007;Inglez et al. 2019;Burzinski et al. 2020). We note that many of the Zildjian structures displayed a slight depression beyond the D1 perimeter, approaching, in some cases, one metre in diameter. This slight depression radiating away from the underside of the Zildjian structures implies a slight doming of the bedding plane ( Fig. 2A). Together, these measurements depict a regularity of Zildjian dimensions as well as somewhat regular spacings between them . Two weathered blocks of float provided cross-sectional views of the Zildjian structures ( Fig. 3A & 3B) thereby allowing a more detailed description of their sedimentological characteristics. In these two samples, we observed regular laminations in grainstone parallel to the bedding plane away from the structures. Moving towards the center of the structure, laminations deflected downward, reaching an angle of ≈ 45 degrees. Further inward, laminations curved back up toward the axial zone of the structures. In the axial zone, in the lower portion of the structures, the laminations appeared to stop and were replaced by infill (likely micritic) which in one sample displayed convex layering (Fig. 3B & 3D). Tracing the axial zone further up, however, laminations do bridge across the structures (Fig.  3D). This finding is consistent with continued sedimentation that draped over the resulting Zildjian bedform. In cross-section, we observed that, when vertically tracing the axial zone downwards (~ 10 cm), D 2 and D 3 varied. A possibility for the vertical heterogeneity of inter-Zildjian widths is differential exposure (i.e. differences in where the bedding plane intersects with different Zildjians) rather than true size dissimilarities between structures.

Interpretation
In what follows, we compare the Zildjian structures to reported discoidal sedimentary features of both abiotic and biotic interpreted sedimentary origins. Specifically, we focus on some of the key features highlighted above: the regular spacing between the Zildjians, the dimensions of the structures, the partial destruction of laminations in the axial pit, and the slight doming outside the D 1 diameter. We utilise these observations to consider a biological versus abiotic origin for these structures. We would like to note, however, that further research, utilizing analyses such as detailed petrography and/or microscopy,will be needed to conclusively rule out particular interpretations.
There are several instances of documented sedimentary features that are both morphologically similar to Zildjians and have inferred biological origins. Examples of such features include Ediacaran and Cryogenian discoidal fossils such as Aspidella (MacGabhann, 2007), rooting or frond structures (Luzhnaya & Ivantsov 2019), Cambrian medusae (Young & Hagadorn, 2010) or features formed via microbial mats. Importantly, slight outer doming analogous to that observed in the Zildjian structures beyond the D 1 diameter (see above), has not been described in any of these examples. Zildjian structures have a decimetre-scale range of outer rim diameters (i.e. from 0.12 to 0.7 m) which is considerably larger than the range of diameters reported for Ediacaran discoidal fauna imprints (maximum diameter of < 0.15 m, with many reported in the sub-cm range; MacGabhann, 2007;Inglez et al. 2019;Burzinski et al. 2020) or the frond-like Petalonamae Ediacaria flindersi Sprigg (outer diameter of < 0.02 m; see Luzhnaya & Ivantsov, 2019) or so called 'scratch circles' (Jensen et al. 2018). While Cambrian medusoids can reach similar sizes to the Keilberg Zildjians (Young & Hagadorn, 2010) other morphological characteristics disprove such an affinity. In particular, the destruction of bedding in the axial pit rules out an interpretation of Zildjians as surficial impressions resulting from a dead medusa-like organism. Additionally, the observed regular spacing of Zildjians is inconsistent with the expected spatial distribution of a death assemblage of medusae (i.e. maximum concentration in local troughs, Hagadorn & Miller, 2011). While such regular spacing may be induced via holdfasts of fronds, again, the magnitude of relief of the Zildjians is unlike reported scratch circles and the size of these structures does not match reported imprints from frond-bearing organisms. The final possibility is that Zildjians formed as a direct result of microbial construction such as a stromatolite. A challenge to this interpretation is that Zildjians are of different scale and morphology from documented stromatolite occurrences of this age (James et al. 2001;Bosak et al. 2013). While some instances of slightly crinkly laminations (Fig. 3) away from the axial zone may conform to expectations of microbial laminite morphology, the spacing (i.e. a lack of lateral contact) of Zildjians and relief is very different from documented domal microbial laminite occurrences (Romero et al. 2020). In sum, the outer doming, size, vertical disruption, and regular spacing of the Keilberg Zildjian structures do not match those of previously reported Neoproterozoic or early Cambrian fauna, flora or microbial structures. Therefore, the lack of overlap of these key observations motivates consideration of an abiotic origin.
If the Keilberg Zildjian structures are unlikely to be of biological origin, then what sort of processes led to their formation? The shape, size, axial pit and distribution of Zildjians are very different from discoidal features produced by diagenetic concretions (Schwid et al. 2021) but are similar to interpreted gas and fluid escape structures (Dionne, 1973;Lowe, 1975) such as sand volcanoes. In particular, the structures exhibit a striking morphological resemblance to the pseudofossil Astropolithon, which is characterised by positive convex relief, a central sediment plug, circular shape, and a diameter of several millimetres to tens of centimetres (Pickerill & Harris, 1979). Astropolithon has been documented elsewhere in time and space (Walter, 1972;Mount, 1993;Seilacher & Goldring, 1996;Seilacher et al. 2002;Hagadorn & Miller, 2011), but, in contrast to the Zildjian structures, they have typically been reported in siliciclastic-dominated units. Indeed, this difference in host-lithology may be responsible for the spectacular preservation (i.e. clearly visible deformation of laminations in cross section) of Zildjian structures in the Omukutu area. Initially Astropolithon was interpreted to be a trace fossil by Dawson (1878) but later investigations noted how the pseudofossil bears the same characteristics as sand or mud volcanoes (Seilacher et al. 2002). Thus, Astropolithon are now considered to be genetically similar to those sedimentary structures, forming as a result of the expulsion of over-pressurized gases or fluids (contained within pore spaces) out of a breach in the sediment-water interface (Lowe, 1975;Pickerill & Harris, 1979). The only suggested distinction between sand or mud volcanoes and Astropolithon is the presence of a less permeable surface layer in the latter, which results in slight doming beyond the central vent or aperture (Seilacher et al. 2002). In the case of a Silurian example from the Kufra Basin (Seilacher et al. 2002), this less-permeable surface layer was suggested to be a 'biomat'. A potential point of contrast between Astropolithon and the Zildjian structures reported here, are that no evidence for an organic-rich seal was found in our study location. That said, at this time we cannot rule out the possibility of a microbial mat acting as a seal or impermeable layer. Additionally, we note that rapid cementation of carbonate laminae may have had a similar sealing effect where deformation then occurred within partially lithified sediments. With these considerations in mind, we explore further the potential origins of Zildjian structures below.
If the Zildijian structures are indeed Astropolithon-like constructions, they formed because of either gas or fluid escape from sediments and, in turn, these physical events were likely triggered by either degradation of organic matter, seismic activity, or rapid sediment loading (Fig. 4).  Multiple exposed horizons make selecting a definitive set and order of genetic events challenging. We first consider gas escape. A possible formation mechanism is the degradation of organic matter, which may have produced pockets of gases that pooled in place until sudden expulsion through beds occurred. Previous work has suggested that 'balloon' structures in sands (Hilbert-Wolf et al. 2016) are a key feature of gas escape. However, such features are absent from our study area. Although a difference in host lithology may be responsible for the lack of balloon structures, their absence potentially supports fluid escape versus gas escape as the primary expulsion events resulting in Zildjian structures.
A second potential piece of evidence in support of fluid escape is the upward deflection of beds into the axial pit; similar features have been shown in fluid-escape experiments conducted in siliciclastic sand and silt (Nichols et al. 1994). While there are many documented examples of fluidization structures with inferred relationships to seismicity, we did not observe, nor can we correlate, episodes of faulting or other physical indicators that would pinpoint a seismic trigger. Moreover, the appearance of the Keilberg Zildjians in multiple beds, as well as the possibility of the variation of the D2 and D3 diameters at the outcrop being caused by multiple expulsion episodes, appears to require a mechanism for repeated triggering. The combination of post-glacial sea level rise and glacial unloading could potentially produce seismic activity in the study area. However, further work is needed to identify and link such observations. An alternative possibility is that rapid loading may have been the underlying cause of the Keilberg Zildjians. Indeed, rapid sedimentation events have been suggested as the most common cause of fluid-escape structures in the sedimentary record (Lowe, 1975). However, this hypothesis is negated if fine mm-scale laminations in cross section require slower sedimentation. In sum, through morphological comparison Zildjians appear to bear the most similarity to Astropolithon and are likely the result of fluid or gas escape from sediments, however further work is warranted in order to constrain their origins at this time more precisely.

Conclusions
Here we have described a new sedimentary feature within post-Marinoan cap carbonate in the Omukutu area of Namibia. The features most closely resemble the pseudofossil Astropolithon indicating fluid or gas expulsion and are therefore unlikely to be the result of a fossil imprint or direct microbial construction. At this time, further interpretation is challenging without detailed petrography, microscopy, and geochemical analyses, which are greatly encouraged in future work. Importantly, if such structures resulted from fluid escape, they may provide support for models of rapid cap dolostone sedimentation. The lack of prior reports of Zildjian structures, and their discovery in the Omukutu area within the most extensively studied Cryogenian field area that has been developed to date, is potentially due to their exposure in vertically dipping beds with wellexposed bedding planes. Indeed, there may be many other roughly time-equivalent occurrences of Zildjians or similar structures within post-Marinoan strata and therefore further exploration is warranted.