Trilobite biostratigraphy of the Tremadoc Bjørkåsholmen Formation on Öland, Sweden
The Lower Ordovician of Baltoscandia is characterized by the initiation of extensive carbonate deposits of the Ceratopyge Limestone. The limestone succession has a broad regional distribution and its associated sediments were deposited in a shallow water epicontinental sea across the Baltic platform during the late Tremadoc (Jaanusson 1976, 1982; Dronov & Holmer 1999). Those particular depositional factors have no present day equivalents. The unit is definitely remarkable in its apparent homogenous facies, lithological and faunal composition. The significance of the Ceratopyge Limestone succession was early referred by numerous authors and recognized sedimentologically and stratigraphically. Tjernvik (1956) reinvestigated the Lower Ordovician beds in Sweden in detail and completed an account of the rich Ceratopyge fauna.
In older studies a combined bio-litho stratigraphical concept of the unit has mostly been used, however a modern lithostratigraphical definition was given by Owen et al. (1990). The definition of the formation is based on the hypostratotype at Bjørkåsholmen in Slemmestad, Norway, and should be referred to instead of the historical synonym. The transition from the dark underlying Alum Shale Formation to the grey limestone beds marks the base of the Bjørkåsholmen Formation (BHF), this is evident in the change in not only lithology but also the typical associated fauna e.g. the Ceratopyge fauna (Tjernvik 1956; Ebbestad 1999). In Sweden the BHF is followed by the Hunneberg-Billingen Latorp Limestone and Volkhov Lanna Limestone. Shales of the Tøyen Formation follow the partially glauconitic limestone in Norway (Owen et al. 1990). The most recent revision of the trilobite fauna recognized 36 species assigned to 28 genera (Ebbestad 1999).
The present study investigates trilobite distribution of the BHF in southern Öland, the easternmost outcrop of the formation. The underlying crystalline bedrock on Öland dips weakly to the east resulting in the exposition of the oldest overlying sedimentary rocks in the west and the youngest beds in the east (Jaanusson & Mutvei 1982). The upper Cambrian Alum Shale Formation is continued in the Lower Ordovician successions and is subsequently overlain by the upper Tremadoc limestone deposits represented by the BHF. Outcrops of this unit are fairly rare, confined to a few localities in the southern and south-central parts of the island.
The material presented in this study was collected at the coastal section at Ottenby and at the Cementa quarry in Degerhamn. Sequences at Ottenby and Degerhamn were logged and material collected for a biostratigraphical study. To obtain trilobite abundances the sample frequency method (Jaanusson 1979; Nielsen 1995; Ebbestad 1999) was applied. The lower boundary of the BHF in Öland is marked by the occurrence of glauconiferous limestone nodules, represented in both localities. The main limestone beds in Öland, e.g. constituting continuous beds having the main trilobite abundance, are grey and micritic, and with some intercalations of glauconitic shale. In addition scattered grains of glauconite and small accumulations of pyrite are evident in the main limestone. The uppermost bed in the two successions, devoid in trilobites, marks the upper boundary of the formation, it is also exceptionally glauconitic suggesting slow deposition and starvation of sedimentation. The BHF at both Degerhamn and the Ottenby section has an approximately thickness of 0.6 meter and several discontinuity surfaces are evident.
The trilobite abundance logs give a proposal for the distribution of the trilobite fauna during the upper Tremadoc in Öland. The material collected from the section in Degerhamn and Ottenby belongs to the Ceratopyge fauna and biostratigraphically the unit is connected to the Apatokephalus serratus Zone. The trilobite abundance distribution from each of the localities is very consistent. Additionally, the fossil assemblages in Öland and the Oslo Region (Ebbestad 1999) are built up by the same typical Ceratopyge fauna, indicating a correlation. Faunal signals show several similarities in the trilobite abundance data and both areas have an upwards declination of trilobite quantities in the sequences. Faunal distribution of trilobites of the Ceratopyge fauna is thus very coherent throughout the platform. This suggests widespread stable conditions of the fauna throughout the Baltic Platform during sedimentation of the BHF. The Central Baltoscandian Confacies Belt (Jaanusson 1976), to which Öland belongs, and the Oslo Region belonging to the Oslo Confacies Belt, demonstrate no facies differentiation of the platform until post-Tremadoc (Jaanusson & Mutvei 1982).
The basal limestone nodules at the two investigated sections are devoid of trilobite remains. Periods of oxygenated lime mud sedimentation, reflected by the basal limestone nodules incorporated in the shale, may indicate times of fluctuations in the sea level. The main limestone units have little or no intercalations of shale, thus representing episodes of more stable sedimentary facies. The unstable settings may suggest non-favourable environments for the establishment of nileid communities (Fortey 1975), here represented by the Ceratopyge fauna, described by the absence of trilobites in the lowermost limestone nodules. Fluctuations of the sea level was probably confined to local settings and are suggested by extended periods with large inputs of oxygen during lime mud sedimentation. Constrained settings are most likely a result of diverse bottom topography and differences in current and wave distribution throughout the area (Ebbestad 1999). The faunal logs clearly demonstrate this idea. Ebbestad (1999) stated similar conditions for the depositions of the basal limestone beds in the BHF of the Oslo Region. The Ceratopyge fauna display coherent distribution in Öland and certainly across the platform, moreover, the distribution does not correlate with specific beds, presumably demonstrating dissimilar sedimentological developments of the two localities, suggesting that the deposition was diachronous.
The upper Tremadoc and early Arenig represents a time of large global sea-level changes by the lowering of the seas (Fortey 1984), this event the Ceratopyge Regressive Event (CRE) (Erdtmann & Paalits 1995) could have resulted in the depletion of sediments. In Baltica this is clearly shown by the end of the Apatokephalus serratus zone, the top of the Bjørkåsholmen Formation, and in addition the disappearance of its associated Ceratopyge fauna.
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2004. 141- p.
Bjørkåsholmen Formation, Tremadoc, trilobite, Öland, biostratigraphy, Baltoscandia