Late Ordovician palaeoceanographic changes as reflected in the Hirnantian-early Llandovery succession of Jamtland, Sweden
Summary, in English
A study of the Upper Ordovician-Lower Silurian strata in Jamtland, central Sweden, shows that large-scale changes in shelf deposition took place close to the systems boundary. These changes include unconformity development and the replacement of a siliciclastic shelf with a carbonate-dominated shelf, suggesting the interaction of allocyclic controls such as changing eustatic sea-level and climate. The 6-m-thick Ede Formation is a key lithosome for interpretation of this transition. Its sediments were deposited in the Caledonian foreland basin, situated east of the closing Iapetus Ocean on the western margin of the Baltic craton. A major part of the late. Caradoc to late Ashgill (into the Hirnantian) was characterised by continuous and uniform deposition over wide areas (Kogsta Formation), whereas erosional surfaces and complex lateral facies relationships characterise the Ordovician-Silurian boundary strata (Ede Formation and lateral equivalents). The Ede Formation represents the end of terrigenous deposition, which in the middle Aeronian was followed by regional expansion of carbonate deposition (Berge Formation). A syn-sedimentary erosional surface, with at least I in of relief locally, forms the lower boundary of the Ede Formation. This surface is overlain by two types of conglomerate. Lower parts of the Ede Formation consist of medium to thick-bedded quartzites. A second erosional surface with only minor (few centimetres) relief occurs on top of these quartzites. The upper parts of the Ede Formation consist of a thin, basal favositid biostrome overlain by thin bedded, calcareous sandstones, limestones and intensely bioturbated shales. Analysis of stratigraphic boundaries and the facies succession suggests that the lower Ede Formation represents a major downward shift in coastal onlap and by-pass sedimentation that created the lower erosional surface. The erosional surface in the middle of the Ede Formation is inferred to have formed during the subsequent maximum lowstand or as a ravinement surface, and is interpreted as an unconformity. The succession is subdivided into four facies associations, each corresponding to a specific systems tract: (a) a Shale-Siltstone Association (uppermost Kogsta Formation), deposited during a highstand situation in mid-outer shelf areas; (b) a Quartzite Association (the lower Ede Formation), deposited during forced regression in a shoreface environment; (c) a Mixed Carbonate-Siliciclastic Association (the upper Ede Formation), deposited during transgression in a wave-dominated, proximal shelf environment when elastic supply was reduced; and (d) a Micritic Limestone Association (lowermost Berge Formation), deposited during a second highstand situation in a low-energy, offshore environment. Conodont data, together with a previously reported Hirnantia fauna, constrain the position of the Ordovician-Silurian boundary to the lower 1.65 m of the Ede Formation, or less likely, to the uppermost metre of the underlying Kogsta Formation, i.e., within a 2.65-m-thick uncertainty interval. The base of the Berge Formation is about 4 m above the top of the uncertainty interval, and is dated as being mid-Aeronian in age, suggesting condensation and/or a hiatus close to, or at, the Ordovician-Silurian boundary. These data tie the unconformity and the regional facies change from a siliciclastic to a carbonate-dominated shelf to Late Ordovician-Early Silurian eustatic and climatic changes. (C) 2004 Elsevier B.V. All rights reserved.