The lower and middle Cambrian of Sweden: trilobites, biostratigraphy and intercontinental correlation

University dissertation from Department of Geology, Lund University

Abstract: This thesis is based on studies of Cambrian successions in Sweden, with particular focus on the middle Cambrian biostratigraphy and its correlative relationship to the proposed global agnostoid zonation. The investigated material, mainly trilobites, was collected from both outcrops and drill cores from five provinces in Sweden: Skåne (Scania), Öland, Västergötland, Jämtland and Lapland.

The Almbacken drill core penetrated c. 30 m of Cambrian strata, constituting one of the stratigraphically most complete successions of this age in Scania. Thirty-two trilobites were identified to species level and used to subdivide the core into seven biozones; from the Ptychagnostus gibbus Zone of the lower middle Cambrian to the Lejopyge laevigata Zone of the upper middle Cambrian. Another drill core (Andrarum-3) was taken at Andrarum, south-eastern Scania. It covers c. 29 m of middle Cambrian to Furongian (upper Cambrian) strata. Based on the fossil content, the core was subdivided into eight biozones; from the middle Cambrian P. atavus Zone to the Furongian Parabolina spinulosa Zone. A series of alum shale samples yielded a positive ?13C excursion corresponding to the globally recognisable Steptoean Positive Carbon Isotope Excursion (SPICE). This is the first time SPICE is documented in Baltica, and based on organic matter from an alum shale setting.

New material collected from Västergötland showed that the P. punctuosus and Goniagnostus nathorsti zones are considerably more extensively developed in this area than previously thought. Trilobites collected from eight localities show that the two zones are represented in a 15 cm thick and impersistent conglomeratic limestone at both Mount Kinnekulle and in the larger area of Falbygden-Billingen. Moreover, the classical locality of Gudhem yielded a trilobite fauna including several widespread key agnostoid species. In particular, L. laevigata (Dalman, 1828) was studied as its first appearance datum (FAD) currently is proposed to define the base of the uppermost stage in the Cambrian Series 3. We suggested that the base of the L. laevigata Zone of Scandinavia should be defined by the FAD of the eponymous species. Of similar global stratigraphical importance is P. atavus (Tullberg, 1880). Its FAD is proposed to define the base of the middle stage of the Cambrian Series 3. A syntype series collected from the Forsemölla-Andrarum area of Scania of both this species and the closely similar P. intermedius (Tullberg, 1880) were studied. It was concluded that they are conspecific, and that P. intermedius is the junior synonym of P. atavus.

A revision of the conspicuous eodiscoid Dawsonia oelandica (Westergård, 1936) was made, based on well preserved material from Mon, Jämtland. Reconstructions of this species were presented, and its functional morphology and relationship to closely related taxa were discussed. Associated trilobites placed the material stratigraphically within the lower middle Cambrian P. praecurrens Zone. Another eodiscoid fauna, including the first reported occurrence from Scandinavia of Neocobboldia aff. dentata (Lermontova, 1940) and Chelediscus acifer Rushton, 1966, was also studied. These eodiscoids, recovered from the Luobákti section, Lapland, offered a tentative correlation between the uppermost lower Cambrian strata of Baltica and eastern and western Avalonia.

From the studies included in this thesis it has been shown that the proposed global zonation can be applied to Swedish middle Cambrian successions, substituting the traditional zonation in our overall strive for a common global zonation. Accordingly, eight biozones can be recognised (in ascending order): the Eccaparadoxides insularis, P. praecurrens, P. gibbus, P. atavus, P. punctuosus, G. nathorsti, L. laevigata and Agnostus pisiformis zones.