ACS PRF | ACS
All e-Annual Reports
43893-AC8
Carbonate-Hosted Ediacaran Fossils from Siberia
During two seasons of summer fieldwork we measured fifteen at lamina-scale, covering the entire 360-m thick succession of the Khatyspyt and Turkut Formations. Detailed stratigraphy and sedimentology of the Khorbusuonka River section allowed precise correlation among widely separated localities, discovery of additional fossiliferous horizons, and the collection of 160 oriented fossils and 100 oriented lithological samples for subsequent laboratory analyses. In addition, we collected 212 samples for chemostratigraphy, 68 samples for organic-walled microfossils, 85 bulk samples for small shelly fossils, and 9 bulk samples for biomarker analysis. We also discovered and sampled multiple interstratified volcanic ashbeds to further resolve the age of these strata. The Khatyspyt+Turkut succession turned out to be 100 m thicker than previously thought (e.g. Knoll et al 1995). In addition, we discovered, that the Turkut dolostone is overlain by a unit of thick-bedded and wavy-bedded sandstone which is intruded by a dolerite sill. The top of this unit is a sequence boundary, marked by a regolith and a paleosol horizon which was earlier misinterpreted as a product of phreatomagmatic eruption. Zircons separated from the paleosol horizon have an age of 543.9±0.3 Ma (Bowring et al 1993), which is apparently the age of the dolerite sill. This date has always been regarded as the first reliable radiometric dating of the base of the Cambrian, but it is, in fact, simply the minimum age of the Turkut Formation.
Analysis of fossil distributions over a wide range of scales yielded major insights into the ecological structure of carbonate-hosted Ediacaran communities. We documented in situ fossil distributions, associations and orientations by systematic excavations of fossiliferous facies at each of the outcrops studied. Quadrate mapping of fossil distribution on a single bedding surface revealed 101 individuals of Inaria khatyspytia forming two monospecific clusters at a 0.01–1 m scale, within heterospecific patches on a 1–100 m scale. On a 1–10 km scale the fossil distribution is demonstrably controlled by facies – the essentially continuous exposure of individual strata over 1000's of meters allowed mapping of the distribution of Ediacaran fossils from carbonate ramp facies to the transition with stromatolitic biostromes.
Analyses of trace fossil distribution have yielded major insights into the ecological structure of late Neoproterozoic carbonate basins. In particular, we discovered horizons with the Phanerozoic-style ichnofabric interstratifying (and sedimentologically linked) with Ediacaran fossil assemblages in what are undoubtedly Precambrian strata. At least in the carbonate basin of Siberia, the Ediacaran biotope appears to have been engineered by burrowing organisms that prevented the formation of microbialites and mediated the substrate for subsequent colonization by Ediacaran organisms. In the course of our fieldwork, we recorded the levels and types of bioturbation on a millimeter scale utilizing the ichnofabric index method of Droser & Bottjer (1986). This bioturbation data allows for a detailed characterization of the substrate on which the carbonate-hosted Ediacaran biota benthic suspension feeders were living. It also raises challenges current views of bioturbation patterns across the Precambrian/Cambrian boundary.
Laboratory analysis of the Khatyspyt fossils has focussed primarily on the Charnia, Hemalora/Mawsonites, Inaria preserved in calcareous concretions. Petrographic analysis reveals a coarsely cemented sedimentary infill of silt-sized peloidal, occasionally oolitic grainstone. The sediment surrounding the concretions is conspicuously more clay-rich, but thin uneven layers of non-concretionary limestone can sometimes be traced into a fossil concretion. There are no internal organic structures preserved; however, the external surface exhibits signs of flexible deformation, suggesting that the concretionary growth did not exceed beyond the outer wall of the organisms. Thus the early cementation, and fossil preservation, appears to have been associated with internal microenvironments. The presence of clastic sediments within these soft-bodied fossils is enigmatic, but implies that it accumulated during organism growth (Grazhdankin and Seilacher 2002).
We also carried out an exhaustive analysis of the Khatyspyt collections in Novosibirsk (Vodanjuk 1989). The majority of the specimens in the collection can be attributed to three globally distributed species: Mawsonites pleiomorphus, Inaria khatyspytia and Inaria karli. From biostratinomic and diagenetic features in the type specimens of Mawsonites pleiomorphus we identified a much broader spectrum of morphotypes, modes of preservation and ontogenetic stages that has hitherto been appreciated. The revised taxonomy of Vodanjuk's Fossil Type Collection now includes 17 species.
The Khatyspyt assemblage is dominated by holdfast structures (Mawsonites pleiomorphus, Mawsonites new species, Inaria khatyspytia, Inaria karli, Protodipleurosoma wardi, Aspidella terranovica). It also includes a frondose erect organism Khatyspytia grandis; a rangeomorph Charnia masoni; serially and radially chambered structures (Palaeopascichnus delicatus and Eoporpita medusa); still problematic Hiemalora stellaris; as well as common discoidal structures (Cyclomedusa davidi, Ediacaria flindersi, Paliella patelliformis, Nimbia occlusa) that can be interpreted as fossilized microbial colonies. In addition, there are elongate structures bearing thin, curvilinear folds oriented sub-perpendicular to the main axis (Cucullus fraudulentus) and compressed thin-walled originally spherical bodies (Beltanelloides sorichevae).
