Palaeokarst evidence for widespread regression and subaerial exposure in the middle Katian (Upper Ordovician) of Baltoscandia: Significance for global climate Mikael Calner a, ⁎, Oliver Lehnert b , Jaak Nõlvak c a GeoBiosphere Science Centre, Lund University, Sölvegatan 12, SE-223 62 Lund, Sweden b University of Erlangen-Nürnberg, Institute of Geology, Schloßgarten 5, D-91054 Erlangen, Germany c Tallinn University of Technology, Institute of Geology, Ehitajate tee 5, 19086 Tallinn, Estonia abstract article info Article history: Received 12 June 2009 Received in revised form 18 November 2009 Accepted 19 November 2009 Available online 3 December 2009 Keywords: Palaeokarst Baltoscandia Fjäcka Shale Slandrom Limestone Saunja Formation Katian Ordovician We report on widespread and contemporary palaeokarst in the middle Katian (Upper Ordovician) of Sweden, Estonia and Latvia, the first major palaeokarst horizon to be reported from the Ordovician of Baltoscandia. The solution features occur within a few metres of thick zone of limestone immediately below the widespread Fjäcka Shale and are related to an unconformity with preserved palaeorelief in the Slandrom Limestone in Sweden and the time-equivalent Saunja Formation in Estonia and Latvia. Facies evidence for a karst origin comes from several outcrops and core sections and includes 1) frequent karren-like morphologies interpreted as ‘Swiss-cheese’ karst [sensu Baceta et al., 2001], 2) local occurrences of solution/collapse breccia, 3) presence of bladed pseudospar crystals in solution cavities, and 4) carbon isotope values indicating meteoric influence to the succession. These findings are herein put in context with previously reported, large-scale erosional channels that may cut down several tens of metres below the Fjäcka Shale in the subsurface Baltic Sea area, and with regional, anomalous thickness variations in the Slandrom Limestone and Saunja Formation, all together forming strong support for regional exposure of the Baltoscandian continent in the middle Katian. High-resolution stable isotopic data show that the regression and lowstand of sea-level overlap with the Waynesville carbon isotope excursion. It resulted in basin-wide cessation of carbonate production near the Amorphognathus superbus and A. ordovicicus conodont zonal boundary. The contemporary development of palaeokarst in different confacies belts of the basin suggests that this was an extraordinary sea-level lowstand, herein interpreted as reflecting a middle Katian glaciation. © 2009 Elsevier B.V. All rights reserved. 1. Introduction The identification of palaeokarst in ancient marine basins is of significant interest since it implies a regression of sea-level and a time period of subaerial exposure of already lithified strata. Many present-day karst terrains or caves have formed during the last Pleistocene glacial and have been preserved during the geologically swift Holocene transgres- sion, i.e. over a time period that is substantially shorter than any biozone of the Lower Palaeozoic. For this reason, palaeokarst is potentially an excellent tool for recording short-term (sub-zonal level) sea-level change. Palaeokarst surfaces are commonly associated with the up-dip portion of many sequence boundaries and are therefore of significant importance also in basin analysis. Occurrence of palaeokarst should be emphasized also in biostratigraphy and event stratigraphy because it may explain lack of a certain biozone or the temporal architecture of an extinction event in the marine environment. In many cases, however, the evidence for karst weathering is lost due to transgressive erosion. Except for a statement of possible palaeokarst at the top of the Darriwilian Komstad Limestone (Nielsen, 1995, p. 18) there are to our knowledge no previous reports on palaeokarst from the Ordovician of Sweden. This is surprising because the time interval is represented by a very thin veneer of mostly carbonate rocks, formed in the cratonic interior of Baltoscandia and therefore yields frequent and quite substantial stratigraphic gaps (e.g. Calner et al., 2010). By comparison, small-scale palaeokarst have been reported from younger strata in the same basin, e.g. at several stratigraphic levels in the Silurian carbonate platform strata of Gotland (Cherns, 1982; Calner, 2002; Eriksson and Calner, 2008). In this paper we document a thin but conspicuous middle Katian palaeokarst horizon immediately below the widespread Fjäcka Shale. In Sweden, the karstified limestone formation has variously been termed ‘knyckelkalk’ or ‘curly birch limestone’— names that well reflect the peculiar texture. The morphologies are herein interpreted as analogous to ‘Swiss-cheese’ karst, which forms through dissolution in the marine- meteoric mixing zone and which is an important process in cave development and in present-day carbonate platforms (Baceta et al., 2001). Our observations come from central and southern Sweden, Estonia, and Latvia (Fig. 1) and suggest contemporaneous exposure of Palaeogeography, Palaeoclimatology, Palaeoecology 296 (2010) 235–247 ⁎ Corresponding author. E-mail address: mikael.calner@geol.lu.se (M. Calner). 0031-0182/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2009.11.028 Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo