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

Abstract

-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 Fröjel 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 postextinction 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...

Upper Homerian (Silurian) high-resolution correlation using cyclostratigraphy: an example from western Lithuania

“…Based on the conodont data, this ecological perturbation is named the Mulde Bioevent (Jeppsson et al 1995). The Mulde Bioevent was distinguished following detailed investigations of Gotland conodonts, which showed the conodont turnover occurring in three event levels or datum points (Calner and Jeppsson 2003;Jeppsson and Calner 2002). Acritarchs and prasinophytes experienced reorganisation in their communities (Masiak 1998(Masiak , 1999Porębska et al 2004;Calner et al 2006).…”

Silurian carbonate platforms and extinction events—ecosystem changes exemplified from Gotland, Sweden

“…CIE -Carbon Isotope Excursion; data from Samtleben et al (2000). Biostratigraphic data from various sources, e.g., Calner and Jeppsson (2003), Jeppsson and Aldridge (2000), and L. Jeppsson (pers. comm.…”

“…Points 3, 4, 5, and 6 have previously been largely overlooked in the study of Silurian extinction events but they are important since they imply that major ecological changes took place in carbonate platforms (i.e., on shallow-water tropical shelves) during the separate events (see Calner and Jeppsson 2003;Munnecke et al 2003;Calner 2005). Importantly, these factors also suggest that extinctions among, e.g., graptolites and the very substantial changes in carbonate platform deposition occur simultaneously.…”

Converting scanned images of seismic reflection data into SEG-Y format

“…The Silurian strata on Gotland consist of a sequence of about 10 barrier reef complexes, which prograde to the SSE (Manten 1971;Calner et al 2004). As the strata are very well preserved on Gotland, it has become a globally important locality for the study of Silurian geology and has been extensively documented (Calner and Säll 1999;Calner and Jeppsson 2003;Eriksson and Calner 2005;Calner 2005 etc.). However, despite this, a stratigraphic framework to describe the reef complexes on Gotland is only loosely defined and in general, the correlation between the surface geology and the deeper Silurian succession is poorly understood (Calner et al 2004).…”