Sequence stratigraphy, palynology and biostratigraphy across the Ordovician-Silurian boundary in the Röstånga-1 core, southern Sweden

Abstract: A multiproxy study on sedimentary rocks spanning the Ordovician-Silurian boundary has been performed on material from the Röstånga-1 core, with the aims to improve the biostratigraphical and sedimentological framework and to interpret the depositional environment during this interval in Earth’s history. The sedimentary succession in the Röstånga-1 core is developed in the Scanian Lithofacies Belt and is considered an excellent upper Middle Ordovician-Lower Silurian reference section for southernmost Sweden. The studied succession includes the Sularp Shale, the Skagen Limestone (early middle Katian), the Mossen-Fjäcka formations, the Lindegård Mudstone and the Kallholn Formation. The results from this study, which includes sedimentological, palynological and stratigraphical investigations, add to the knowledge on the climatic changes across the Ordovician-Silurian boundary. During the time interval covered in the drill core, an intense Ordovician volcanism was climaxed by a short-lived glacial pulse which in turn led to a eustatic regression. The intense volcanism is evidenced by several K-bentonite beds in the Röstånga-1 core. This event left its signature as a development of a sequence boundary and solution structures at the top of the Skagen Limestone. The regressive phase was rapidly succeeded by a return to deeper-water conditions evident within the Mossen-Fjäcka succession, implying that the rate of glacio-eustatic regression was greater than the subsidence rate at that time in Scania. During the deglaciation phase, anoxic black shale coincides with a flooding surface at the base of the Fjäcka Shale. This possibly signifies a high palaeo-productivity and preservation. Following the Fjäcka Shale, the Lindegård Mudstone marks a sea-level highstand and a stable environment. Based on the palynological results, the succession was subdivided into three informal local palynozones. Palynozone I is of late Katian (pre-Hirnantian Ashgill) age and is characterized by the occurrence of the following acritach taxa: Baltisphaeridium, Buedingiisphaeridium balticum and Orthosphaeridium inflatum. The occurrence of cryptospores within this palynozone provides the oldest evidence of early land plants in Baltica. Through the upper part of this palynozone, the occurrence of diagnostic Early-Middle Ordovician reworked acritarchs such as Striatotheca, Coryphidium and Acanthodiacrodium spp. indicates a detrital Avalonian sedimentary input in front of the Caledonian Deformation and provides an evidence for foreland-type sedimentation. Palynozone II ranges from early Hirnantian-latest Hirnantian and is characterized by the occurrence of long-ranging and tolerant acritarch taxa such as Veryhachium and Micrhystridium and a major micro-phytoplankton turnover recognized by the appearance of several taxa with a Silurian affinity, including Ammonidium spp., Diexallophasis denticulate and the first Baltic record of Tylotopalla caelamenicutis. The cryptospore assemblage, through Palynozone II, comprises taxa that have been reported in coeval assemblages globally, possibly indicating homogeneous land plant assemblages. The palynofacies results reveal a relatively high abundance and diversity of scolecodonts coupled with a decrease in the relative abundance of chitinozoans at the base of this palynozone, reflecting a regional similarity with previous results. Palynozone III ranges from latest Hirnantian-early Llandovery (Rhuddanian) and is characterized by high abundance of graptolites and the occurrence of sphaeromorph acritarchs. The Kallholn Formation, comprising this palynozone, rests on a maximum flooding surface and coincides with a eustatic sea level rise related to the Hirnantian deglaciation, marking new anoxic black shale and a high palaeo-productivity and preservation. Through this palynozone, the acritarchs show the relatively lowest abundance, corresponding to the general feature of the acritarch assemblages through the start of the Silurian, implying that anoxic conditions might be related to this early Silurian “crisis”. The acritarchs from the Lindegård Mudstone are brownish black, indicating a Thermal Alteration Index (TAI) of 4 which is considered to be post-mature with regard to oil generation. There are many controls such as the sedimentary burial and magmatic heating that might have had an effect on the thermal maturity of the organic matter in this basin.