Conference Agenda

The late Carboniferous–Early Triassic Krkonoše Piedmont Basin in the northern Bohemian Massif was initiated as a fault-controlled basin during the waning stages of the Variscan orogeny and its transition to an intra-plate setting. The basin was filled with 9 formations of mostly red beds with a total thickness more than 2000 m, deposition was accompanied by extensive volcanic activity dominated by andesitic lava flows and ash-flow and ash-fall tuffs. The U–Pb detrital zircon geochronology on 17 samples taken up-section across the basin stratigraphy points to local as well as distant sources of detritus derived predominantly from the nearby Saxothuringian and Teplá–Barrandian units, with Archean–Paleoproterozoic, Ediacaran, Cambro-Ordovician, Late Devonian and middle–late Carboniferous age peaks. Furthermore, magnetic fabric of the red beds was analysed at 29 sampling sites using the anisotropy of magnetic susceptibility (AMS) method in order to understand both the syn- and post-depositional history of the basin. Five different fabric types were detected across the basin and interpreted in terms of random orientation of magnetic grains due to turbulent flow (Type 1), strong alignment of magnetic minerals representing depositional fabric and recording paleocurrent directions (Type 2), superposition of compaction strain onto pre-existing depositional fabric (Type 3), tectonic fabric formed due to small increments of regional strain (Type 4), and an anomalous, mineralogy controlled inverse fabric (Type 5). Altogether the detrital zircon ages and multiple magnetic fabrics reveal a long term and complex sedimentary and post-sedimentary evolution and inversion of the basin in the Pangea interior.

Results of a systematic study are presented combining U-Pb ages, Hf isotope data and shape parameters (length, width, aspect ratios, roundness, roughness, typology) of detrital zircon populations from low- to medium-grade greywackes of the Badenweiler-Lenzkirch Zone (BLZ), which is squeezed between high-grade gneisses of the Central and Southern Black Forest Gneiss complexes. Nine sample are investigated from three formations: Sengalenkopfschist, Schleifenbachschist, and Protocanites Greywacke unit, assumed to be deposited from the Early Ordovician to Early Carboniferous based on biostratigraphic record. This interpretation, however, is at odds with detrital zircon U-Pb ages, revealing robust maximum depositional ages between 368 and 378 Ma for rocks of all three units. Age spectra show peaks at 380-400 Ma, 480-500 Ma, 600-620 Ma, 700-750 Ma, 0.9-1.1 Ga, 1.8-2.2 Ga, and 2.6 Ga, and Hf isotopes a juvenile input at 380-400 Ma (εHf_t up to +5). The combined age-Hf data point to a similar provenance like the metamorphic rocks exposed in Southern Black Forest gneiss complex, hosting relics of different Gondwana-derived terranes, in addition to a Late Devonian arc-back arc system. Similar proximal sources are also indicated by the finding of abundant euhedral zircon grains with ages <550 Ma in all samples, and a significant overlap in zircon typology. The low degree of zircon roundness and roughness reflect sediment transport in water-saturated media, but in some samples has been significantly modified by post-depositional structural-metamorphic overprint, causing mechanical zircon peeling and chemical dissolution in contact with sheet silicates, in particular during garnet-forming dehydration-reactions.

There is no firm evidence for glaciation for over 1.5 billion years, i.e. from the Great Oxidation Event (or Episode) until the onset of the Cryogenian Period. By contrast, global glaciation became the predominant climate state for the next 85 million years, followed by a series of regional ice ages that culminated in the Ediacaran-Cambrian biological radiations. There is increasing evidence that each of these climatic events was preceded by a negative carbon isotope anomaly, potentially caused by imbalance within the global sulphur cycle and related redox and productivity feedbacks. In this presentation, I aim to outline the evidence for primary carbon isotope anomalies before transitions into glaciation at c. 720 Ma, c. 660 Ma, c. 580 Ma and c. 560 Ma, as well as similar isotopic events that have identical geochemical context, but for which no glacial deposits have yet been identified. Cryogenian-to-Cambrian carbon cycle elasticity reflects a distinct earth system state, which was likely related to a dynamically changing organic carbon reservoir, oxidation of which was coupled to sulphate and ferric iron reduction. Organic carbon likely became the dominant redox (and climate) capacitor in the exogenic earth system only once atmospheric oxygen ceased to be a limiting factor for the weathering of iron sulphide minerals. The extent to which sulphur cycle imbalance forced climatic and environmental change after the Cambrian explosion remains to be determined.

The terminal Ediacaran Urusis Formation of the Nama Group (southern Namibia and northwestern South Africa) is a fossiliferous, mixed carbonate-siliciclastic succession with numerous silicified volcanic tuff interbeds. Studies of the Urusis Formation have historically focused on the Swartpunt area of southern Namibia, where post-depositional thrusting associated with the Gariep orogeny transported and emplaced the Formation as a series of thrust plates, which abut autochthonous Nama Group deposits to the east. Over thirty years of geochronological, chemostratigraphic and paleontological investigations have made the Swartpunt area a key reference section for terminal Ediacaran chronostratigraphy. Recent geochronological data from an expanded succession, exposed on the border between Namibia and South Africa, have cast doubt on the interpretation that ash beds in the vicinity of Swartpunt record depositional ages, possibly due to zircon reworking. The resulting implications for the temporal calibration of global terminal Ediacaran chemostratigraphy, as well as confidence in the maximum reported uncertainty of radioisotopic dates across geological time, are numerous. However, alternative regional structural complications that may resolve these issues have not yet been fully considered. Here, we build upon foundational observations of structural tectonics in the Swartpunt area, combining high resolution geological mapping, outcrop and drill core stratigraphy obtained through the ICDP GRIND-ECT project, carbonate carbon isotope (δ¹³C_carb) chemostratigraphy, and new high precision radioisotope geochronology (zircon U-Pb chemical abrasion-isotope dilution-thermal ionization mass spectrometry, CA-ID-TIMS). We use these data to explore structural alternatives that may resolve the chronostratigraphy of the Urusis Formation without invoking insidious zircon reworking.

Marine carbonates are potential archives of geochemical proxies, such as U-Pb and Sr isotopes, which can be utilized in the reconstruction of past climate conditions and ancient seawater composition. The ability to confidently reconstruct environmental conditions in the past times is of great importance since they are linked with changes in the biosphere. For example, the Ediacaran-Cambrian transition was a period of time where significant evolutionary change modified the biosphere. An intact, continuous record of environmental conditions will help to understand better the timing, nature and sequence of events that preceded or accompanied such changes in biodiversity. However, carbonate rocks are susceptible to numerous post-depositional processes (such as: oxidative weathering, diagenesis, burial, lithification, deformation, dissolution and reprecipitation), which may alter the geochemical record. Additionally, detrital components may increase the complexity of the geochemical signature and the carbonate composition.

Thus, we have to understand and identify the presence or absence of such processes, before extracting meaningful geological information from these archives. Laser Ablation – Inductively Coupled Plasma – Mass Spectrometry (LA-ICP-MS), is a tool that offers spatial resolution when performing geochemical analyses, which may help to interpret the geochemical data. Here, we combine observations from U-Pb and Sr isotopic systematics supported by trace element abundances to identify domains that are indicative of post-depositional processes, over protracted time and variable in their extent.

Neoproterozoic Nama Group sediments in SW Namibia provide an example of the interplay between the deposition of clastics and carbonates that do not obviously follow the “rules” of sequence stratigraphy. The unconformity surface beneath the sediments is planar and extensive. Basal interbedded carbonates and coarse clastics are overlain by a massive influx of fluvial clastics (Kliphoek Member). These fine upwards and are overlain by fine grained clastics with interbedded carbonates and minor turbidite sands (Aar Member) that contain most of the fossils found in this part of the Nama Group. Carbonate beds become more common towards the top of this member, which is overlain by a thick carbonate unit of region extent (Mooifontein Member).

Southwards the basal clastics (Kanies Member) become thicker, while carbonate beds in the overlying Mara Member are more dominant. The Kliphoek Member becomes finer grained but includes some coarse grained clastics, and the Aar Member contains more and thicker carbonate beds. Paleocurrents show a dominant sediment transport towards the west and southwest.

Questions posed by the stratigraphy include whether the basin subsided at a constant rate, where was the shoreline at any given time, where were the input points of the clastic sediments and how and why did the influx rate vary? Did changes in sea level, and therefore shoreline position, influence the deposition of carbonates, or was this mostly influenced by a lack of clastic influx? We do not have accurate ages for the sediments or the rate at which they were deposited.