Carbonate platform evolution and conodont stratigraphy during the middle Silurian Mulde Event, Gotland, Sweden

Evidence from sedimentology and conodont biostratigraphy is used to reinterpret the mid-Homerian (Late Wenlock) succession on Gotland, Sweden. A new conodont zonation includes from below: the Ozarkodina bohemica longa Zone (including five subzones), the Kockelella ortus absidata Zone and the Ctenognathodus murchisoni Zone (two taxa are named, Ozarkodina bohemica longa and Pseudooneotodus linguicornis). These new zones are integrated with facies in order to correlate strata and infer the major depositional environments and the controls on deposition during the mid-Homerian Mulde Event. Reef-associated and skeletal carbonate deposition predominated before and after the event, i.e. during the uppermost O. s. sagitta Zone and, again, in the C. murchisoni Zone. These periods are characterized by the expansion of reefs and shoal facies across marls in the topmost Slite Group on eastern Gotland and in the lower parts of the Klinteberg Formation on western Gotland, respectively. The intervening O. b. longa and K. o. absidata zones are initially characterized by rapid facies changes, including siliciclastic deposition, and later stabilisation of a carbonate depositional system. The composition of sediments and depositional rates are closely related to the creation and destruction of accommodation space and reflects a classical case of depositional bias of the carbonate and siliciclastic depositional systems. Based on coastline migration, stratal boundaries, and the stratigraphic position of major reef belts, several facies associations can be fitted into a sequence stratigraphic model for platform evolution. A highstand systems tract (HST) situation prevailed prior to, and during the early part of the event; the upper Slite Group including the lower Frjel Formation. This HST was characterized by prolific skeletal production and regional reef development except for during the latest stage when carbonate production declined at the onset of the Mulde Event. Platform growth was inhibited during a following regressive systems tract (RST) when regional siliciclastic deposition predominated; the Gannarve Member. The subsequent lowstand resulted in regional emersion and karstification, i.e. a complete termination of the platform. The post-extinction transgressive systems tract (TST) is exclusively composed of non-skeletal carbonates; the Bara Member of the Halla Formation. Re-occurrence of reefs and a prolific skeletal production marks platform recovery during a second HST; the remaining Halla and the lower Klinteberg formations. Integration of high-resolution biostratigraphy and sequence stratigraphy reveals that the major physical control on platform evolution was a 5th order eustatic sea-level change during an early part of the Mulde Event, and that the bulk of the strata accumulated when the platform aggraded and prograded during the highstand systems tracts. Thus, Silurian oceanic events and associated sea-level changes had profound impact on the neritic carbonate system. The Gotland-based middle and late Homerian sea-level curve shows two rapid regressions, both leading to truncation of highstand systems tracts. The first lowstand occurred at the very end of the C. lundgreni Chron, and the second at the end of the Co.? ludensis Chron. The intervening interval was characterized by stillstand or possibly slow transgression

High-resolution carbon isotope stratigraphy of the Lower and Middle Ordovician succession of the Yangtze Platform, China

Variation in the relative abundance of the stable carbon isotopes has been widely used to correlate Ordovician marine successions over the past two decades. To date, only a few of studies of Ordovician carbon chemostratigraphy have been conducted in South China. Most of the previous studies in this field have focused on specific time intervals and/or events in the Middle and Upper Ordovician. The Lower and Middle Ordovician of the Yangtze Platform is typically represented by a sedimentary succession dominated by carbonate rocks, which is ideal for studying the carbon chemostratigraphy. Three sections spanning the Nantsinkuan/Lunshan, Fenhsiang, Hunghuayuan, and Dawan/Zitai formations, corresponding to the TremadocianâDapingian in age, have been sampled for high-resolution δ13C chemostratigraphy. Our new δ13C data reveal five tie-points with the potential for global correlation: (1) a positive δ13C excursion in the lower Nantsinkuan Formation within the Tremadocian Rossodus manitouensis Zone; (2) an excursion with two peaks roughly within the late Tremadocian Paltodus âdeltiferâ Zone; (3) a positive δ13C shift in the lower Hunghuayuan Formation, within the early Floian Serratognathus diversus Zone; (4) a gradual positive δ13C shift in the late Floian, ranging from the uppermost S. diversus Zone to the basal Oepikodus evae Zone; (5) a minor negative shift in the lower Dawan/Zitai Formation, within the early Dapingian Baltoniodus triangularis Zone. These excursions are herein used for correlation of the Yangtze Platform strata with successions from South China, North China, the Argentine Precordillera, North America and Baltica. From a palaeogeographical perspective, the Gudongkou, Xiangshuidong and Daling sections represent depositional environments along an inner to outer ramp profile. The δ13C data from these sections show successively heavier (higher) δ13C values with increasing depositional depth, which can be interpreted as due to remineralization of organic carbon within the carbonate rocks formed in the shallow-water environment

Osmium and lithium isotope evidence for weathering feedbacks linked to orbitally paced organic carbon burial and Silurian glaciations

The Ordovician (∼487 to 443 Ma) ended with the formation of extensive Southern Hemisphere ice sheets, known as the Hirnantian glaciation, and the second largest mass extinction in Earth History. It was followed by the Silurian (∼443 to 419 Ma), one of the most climatically unstable periods of the Phanerozoic as evidenced by several large scale (>5‰) carbon isotope (δ13C) perturbations associated with further extinction events. Despite several decades of research, the cause of these environmental instabilities remains enigmatic. Here, we provide osmium (187Os/188Os) and lithium (δ7Li) isotope measurements of marine sedimentary rocks that cover four Silurian δ13C excursions. Osmium and Li isotope records resemble those previously recorded for the Hirnantian glaciation suggesting a similar causal mechanism. When combined with a new dynamic carbon-osmium-lithium biogeochemical model we suggest that astronomical forcing of the marine organic carbon cycle, as opposed to a decline in volcanic arc degassing or the rise of early land plants, resulted in drawdown of atmospheric CO2, triggering continental scale glaciation, intense global cooling and eustatic sea-level lows recognised in the geological record. Lower atmospheric pCO2 and temperatures during the Hirnantian and Silurian glaciations suppressed CO2 removal by silicate weathering, driving 187Os/188Os and δ7Li variability, supporting the existence of climate-regulating feedbacks

Stratigraphy, facies development, and depositional dynamics of the Late Wenlock Fröjel Formation, Gotland, Sweden.

A lithostratigraphic subdivision of the topmost Slite Beds on western Gotland is proposed, viz. the Fröjel Formation (9-11 m) including the Svarvare Mudstone Member (2-3 m) and the Gannarve Siltstone Member (7-8 m). The Fröjel Formation resulted from increased siliciclastic deposition within an intracratonic carbonate platform setting during the mid-Homerian (Wenlock, Silurian). The temporal and spatial development of sedimentary facies and the characters of formation boundaries can be explained with a sequence stratigraphic approach to depositional dynamics. The accelerating influx of siliciclastic material was connected to a sea-level fall which therefore initiated the formation of a gradationally based parasequence (the Gannarve Member). The depositional trend is supported by both the facies (upward decrease of accommodation space) and faunal development. The Svarvare Mudstone Member reflects a late highstand period, when hemipelagic deposition and weak, small-scale density currents contributed to sedimentation. The subsequent short period of shallowing resulted in deposition of the overlying Gannarve Siltstone Member, consisting of siltstone tempestites and associated fair-weather mudstones deposited in successively decreasing palaeodepth. The shallowing culminated with the formation of an unconformity, which marks the top of the Cyrtograptus lundgreni Biozone, and which is overlain by oolites. This unconformity correlates to the transgressive surface in offshore settings. The siliciclastic deposition is regarded as regional for this part of the Baltic Basin, influencing an area from offshore east Öland to Saaremaa west of Estonia. The depositional history of the Fröjel Formation is discussed in terms of relative and eustatic sea-level changes and with respect to regional correlation of the mid-Homerian eustatic regression