Lithostratigraphy and sedimentology of a Lower Ordovician limestone sequence at Hälludden, Öland, Sweden

In order to achieve a useful lithological zonation of the Lower Ordovician (Arenig-Llan-virn) limestone sequence at Hälludden, N. Öland, a method of presence/absence analysis was used in combination with correspondence analysis. Also conventional modal analysis was tried, but proved less effective for zonation purposes. The zonation achieved at Hälludden is valid at least locally, as indicated by the results from a preliminary correlation analysis with a corresponding sequence on northern Öland (Hagudden). Based on the presence/absence analysis, the section at Hälludden is subdivided into seven more or less distinct zones representing different depositional environments. The four lowermost zones (I–IV) represent shallowing conditions, reaching subaerial conditions in zone IV, while the sediments of the overlying zones (V–VII) were laid down in a shallow to very shallow, mainly low-energy environment. There is a scarcity of readily interpretable structures and constituents in the sequence. Presence of spar, grain-support and polished glauconite grains are some of the high-energy criteria used in the study, while shallow-water conditions are recognized mainly through the composition of the biota. The sediments of Zone IV (the “ooid horizon”) display a complexity in composition and texture indicating a mixing of sediments with different affinities. Evidence of selective leaching indicates early diagenesis under subaerial conditions. The distribution of different morphologies of glauconite grains in the sequence reflects their polygenetic origin and, in the case of internal molds, probably also faunal changes. Differences in the Fe2+/Fe3+ ratio cause the color of the sediment to shift from red to grey in the lower part of the sequence. This is mainly a synsedimentary feature, related to time of exposure at the sea-bottom. The environmental models discussed assume deposition in a wide, shallow sea with very limited tidal movements.