Niklas Axheimer

A core drilling through Cambrian strata at Almbacken, Scania, S. Sweden: trilobites and stratigraphical assessment

Abstract A core drilling at Almbacken in the village Södra Sandby, Scania, southern Sweden, penetrated a c. 30 m thick succession of upper lower Cambrian and middle Cambrian strata. Only the uppermost part of the lower Cambrian (Gislöv Formation) was recovered, comprising c. 1.50 m of unfossiliferous siltstones and a thin limestone bed at the base of the core. The core contains the most complete middle Cambrian succession so far documented in Scania. It contains a stratigraphical sequence from the Lejopyge laevigata Zone (upper Paradoxides forchhammeri Superzone) to the Holmia kjerulfi-group Zone (upper lower Cambrian). There is no faunal evidence for the presence of the Acadoparadoxides oelandicus Superzone. The middle Cambrian is 28.30 m thick and consists of dark grey to black mudstones and shales (Alum Shale) with stinkstones and limestone beds. Three major limestone beds were identified: the ‘Fragment Limestone’ (0.35 m), the Exsulans Limestone (0.40 m), and the Andrarum Limestone (1.55 m). The middle Cambrian is richly fossiliferous, except in the lower six metres, and agnostoid trilobites generally dominate the faunas. Polymeroid trilobites and indeterminate lingulate brachiopods are abundant at some levels. The succession of trilobite species was used to erect a biostratigraphical subdivision into seven biozones. The preservation is generally good and 32 trilobites were identified to species level. The excellent preservation in the limestones allowed a description of growth stages in pygidia of Ptychagnostus punctuosus (Angelin, 1851).

The middle Cambrian cosmopolitan key species Lejopyge laevigata and its biozone: new data from Sweden

The middle Cambrian Lejopyge laevigata Zone is poorly exposed in Scandinavia. Both this zone, however, and the succeeding Agnostus pisiformis Zone are well exposed at a classic locality at Gudhem, Vastergotland, south-central Sweden. The sequences consist of finely laminated alum shale with scattered stinkstone (orsten) lenses. Three measured and sampled sections yielded a diverse fossil fauna, dominated by trilobites, in particular agnostoids, and the bradoriid Anabarochilina primordialis. Fossils are excellently preserved but restricted to the stinkstones. The L. laevigata Zone at Gudhem includes several geographically widespread key agnostoid species, notably Tomagnostella sulcifera, Clavagnostus spinosus, Glaberagnostus altaicus, Lejopyge laevigata and L. armata. The L. laevigata Zone in Scandinavia is here extended to include the traditional Solenopleura? brachymetopa Zone, and its lower boundary is defined by the FAD of L. laevigata. Trilobite evidence shows that the upper part of the Scandinavian L. laevigata Zone approximately correlates with the Proagnostus bulbus Zone of China and elsewhere. (Less)

Skeletal carbonate productivity and phosphogenesis at the lower–middle Cambrian transition of Scania, southern Sweden

The lower-middle Cambrian transitional interval of Scania is largely represented by condensed limestone beds, lithostratigraphically grouped in the Gislov Formation (1-5.7 m thick), and the Forsemolla and Exsulans Limestone beds (lower part of the Alum Shale Formation, up to 4 in thick). The strata display a combination of skeletal carbonate productivity, episodic nucleation of phosphate hardground nodules, and polyphase reworking recorded on a platform bordering the NW corner of Baltica. The shell accumulations can be subdivided into three deepening-upward parasequences, separated by distinct erosive unconformities. The parasequences correspond biostratigraphically to the Holmia kjerulfi, Ornamentaspis? linnarssoni and Ptlychagnostus gibbus zones, the latter two generally being separated by a stratigraphic gap that includes the middle Cambrian Acadoparadoxides oelandicus Superzone. Except for the Exsulans Limestone, the carbonates reflect development of a prolific epibenthic biota, dominated by filter-feeding nonreefal chancelloriid-echinoderm-sponge meadows, rich in trilobites and brachiopods, and which were subjected to high-energy conditions. The absence of microbial mats or veneers encrusting the erosive surfaces of these event-concentration low-relief shoal complexes may be related to long hiatal episodes resulting in microboring proliferation. High levels of nutrient supply resulted in high primary productivity, eutrophic conditions, glauconite precipitation, phosphogenesis (in some case microbially mediated) and rnicroendolithic infestation. An early-diagenetic mildly reducing environment is suggested by the presence of authigenic (subsequently reworked) pyrite, which contrasts with the syndepositional normal oxygenated conditions reflected by macroburrowing and the abundance of benthic fossils.

A new lower Cambrian eodiscoid trilobite fauna from Swedish Lapland and its implications for intercontinental correlation

A lower Cambrian eodiscoid trilobite fauna and an associated holmiid trilobite, Holmia sp., are described from a bioclastic limestone at the top of the Tornetrask Formation in the Luobakti section, south of Lake Tornetrask, northern Sweden. Other associated polymerid trilobites include Orodes ? lapponica and Strenuaeva inflata . The precise age of the trilobite fauna cannot be determined, but its generic composition and stratigraphical position at the top of the lower Cambrian suggest that it was recovered from the Ornamentaspis ? linnarssoni Assemblage Zone. Two species of eodiscoids are present: Neocobboldia aff. dentata and Chelediscus acifer . The latter species is known previously from England and southeastern Newfoundland, and provides a novel link between upper lower Cambrian successions in Baltica and Avalonia.

The first record of Odontopleura ovata (Trilobita) from Scandinavia: part of a middle Silurian intercontinental shelly benthos mass occurrence

Abstract We report here, for the first time, the occurrence of the trilobite Odontopleura (Odontopleura) ovata Emmrich, 1839 from Scandinavia. The specimens came in light during a study of middle Silurian calcareous mudstone formed in offshore settings of the Baltic Basin (Grötlingbo-1 core, south-eastern Gotland). Intrabasinal correlation shows that the taxon had a brief mass occurrence during the latest dubius, parvus-nassa, and dubius-nassa graptolite chrons across the Baltic Shield and East European Platform without inhabiting the carbonate platforms that fringed the basin. The sudden appearance of O. ovata and an abundant brachiopod fauna in the core corresponds in time with a general shelly benthos mass occurrence in the earliest recovery phase of the middle Silurian global conodont and graptolite crisis (the Mulde Event) and correlates with a transgression and geographical expansion of the Baltic Basin.

THE MIDDLE CAMBRIAN EODISCOID TRILOBITE DAWSONIA OELANDICA (WESTERGÅRD, 1936)

The eodiscina comprises small, isopygous trilobites with either two or three thoracic segments. Small eyes may be present, but several genera and species lack visual organs. Eodiscoids are characteristic elements of many Cambrian faunas and are important for correlations of upper Lower Cambrian and lower Middle Cambrian strata (e.g., Robison et al., 1977; Ahlberg and Bergstrom, 1993; Geyer and Shergold, 2000; Fletcher, 2003). Only five eodiscoid species are known from the Middle Cambrian of Sweden; Eodiscus scanicus (Linnarsson, 1883), E. borealis Westergard, 1946, E. punctatus (Salter, 1864), Dawsonia oelandica (Westergard, 1936), and Opsidiscus bilobatus (Westergard, 1946). The last-mentioned species is from the upper Middle Cambrian Lejopyge laevigata Zone (sensu Westergard, 1946) in Sweden, and is one of the latest known occurrences of an eodiscoid hitherto known (Rasetti, 1952). The purpose of this paper is to refine and augment the concept of the Middle Cambrian eodiscoid trilobite D. oelandica . Insufficient work has been published on the species and the most recent thorough descriptions of the genus date back to the nineteenth and early to middle twentieth century (e.g., Matthew, 1896; Westergard, 1936, 1946; Rasetti, 1952). The specimens in this study were collected on the island of Oland, southeastern Sweden, and briefly discussed by Westergard (1936, 1946), supplemented with new, well-preserved material from the province of Jamtland, central Sweden. The material discussed in this paper was collected from three localities in Sweden (Fig. 1). Five cephala (Fig. 2.1–2.12) and two pygidia (Fig. 3.1–3.6) were collected by T. Weidner from a block of dark gray limestone found at a road section near the small community of Mon (Fig. 1.1, 1.2), located c. 30 km south of Ostersund, Jamtland, central Sweden. The road section, c. 100 m long and at most c. 8 …