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

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

Middle cambrian trilobites and biostratigraphy of the Almbacken drill core, Scania, Sweden

A core drilling at Almbacken in Södra Sandby, Scania, southern Sweden, penetrated a c. 30 m thick succession of Middle Cambrian-upper Lower Cambrian strata. The Middle Cambrian of this core is the most complete one so far documented in Scania. The core contains a stratigraphical sequence from the Lejopyge laevigata Zone (upper Paradoxides forchhammeri Stage) to the Holmia kjerulfi-group Zone (upper Lower Cambrian). The Acadoparadoxides oelandicus Stage is probably missing. The Middle Cambrian is c. 28.3 m thick and consists of dark grey to black mudstones and shales (alum shale) with stinkstone and limestone beds. Three major limestone beds can be identified: the "Fragment Limestone", the Exsulans Limestone, and the Andrarum Limestone. The Middle Cambrian is richly fossiliferous and the faunas are dominated by agnostid trilobites. Polymerid trilobites and indeterminate lingulate brachiopods are common at some levels. The succession of trilobite species are used for a biostratigraphical subdivision of the succession. The preservation is generally good and 31 trilobite species were identified. The excellent preservation in the stinkstones allowed a description of growth stages in pygidia of Ptychagnostus punctuosus (Angelin, 1851). Only the upper part of the Lower Cambrian (Gislöv Formation) was recovered, comprising c. 1.5 m of siltstones and a thin limestone bed at the base of the core.Populärvetenskaplig sammanfattning: För drygt 500 miljoner år sedan var Sverige beläget söder om ekvatorn och utgjorde tillsammans med resten av Skandinavien och östra Europa en kontinent kallad Baltica. Vid denna tid, under den period som kallas kambrium, var klimatet ganska kallt. I början av kambrium började havet stiga upp över Baltica så att dagens Skåne, samt övriga delar av Sverige, täcktes av vatten. I ett relativt grunt hav avsattes sand på den prekambriska berggrunden. Den underkambriska lagerföljden i Skandinavien utgörs därför till största delen av sandsten. I mellersta kambrium steg havsytan ytterligare. Med ett vattendjup på kanske 25-100 m avsattes nu finare partiklar, mestadels silt och lera. Vid tillfälliga perioder med lägre vattenstånd avsattes även konglomerat och kalksten. På grund av en mycket låg syretillgång vid bottnen var den mikrobiella nedbrytningen av organiskt material mycket långsam. De mellankambriska silt- och lerstenarna är därför mörka till färgen, på grund av ett högt organiskt innehåll. Dessa mörka och fint skiktade silt- och lerstenar kallas allmänt för alunskiffer på grund av dess användande för utvinning av alun förr i tiden. Det var under den kambriska perioden som djurlivet på jorden verkligen utvecklades. Man brukar kalla detta plötsliga uppträdande av en rad olika djurgrupper för den kambriska explosionen. Med hjälp av fossil kan man bestämma den relativa åldern av lagerföljder samt jämföra dessa med övriga delar av världen, en teknik som kallas för biostratigrafi. En djurgrupp som visat sig vara mycket lämplig för biostratigrafiska undersökningar är trilobiter, en utdöd grupp leddjur som hade ett yttre kalcifierat hårt skelett och en kropp bestående av ett huvud, en mellankropp och ett stjärtparti. Trilobiter med två segment i mellankroppen kallas för agnostider. De skandinaviska trilobiterna från kambrium är ofta mycket välbevarade, något som ofta kan förklaras av den syrefattiga miljön närmast bottnen. I Skåne finns de mest omfattande mellankambriska avlagringarna i hela Sverige. Dessa avlagringar är i de flesta fall täckta av yngre sediment, förutom vid ett par gamla dagbrott i sydöst där mellankambriska alunskiffrar och kalkstenar finns blottade. För att kunna beskriva mellankambrium i Skåne har man därför borrat ner i marken och på så sätt plockat upp borrkärnor, som sedan kan studeras. Ett antal borrningar genomfördes i början på 40-talet runt om i Skåne. Anledningen var att man ville veta mer om alunskifferns utbredning och tjocklek för eventuell utvinning av alun. En av dessa borrningar gjordes i Södra Sandby, strax öster om Lund. Denna borrning misslyckades tyvärr så en kompletterande borrning gjordes 1949 vid Almbackengården i Södra Sandby. Det visade sig att Almbackenkärnan omfattar den mest kompletta lagerföljden av mellankambrium som hittills har dokumenterats från Skåne. Mellankambrium i Almbackenkärnan har en mäktighet på ca 28,3 m. Även en del av övre underkambrium finns representerad i borrkärnan, totalt 1,5 m. Det mellankambriska avsnittet domineras av alunskiffer samt mindre kalkstenslager. Den underkambriska delen av Almbackenkärnan utgörs av siltsten samt ett tunt kalkstenslager. Små agnostider hittas rikligt i alunskiffern och kalkstenarna från Almbackenkärnan. Dessa trilobiter har använts för att dela in Almbackenkärnan i sju biozoner. Även en hel del större trilobiter, s.k. polymerida trilobiter, och skal av armfotingar (brachiopoder) påträffades i vissa nivåer. Totalt har 31 olika trilobitarter identifierats. Deras vertikala utbredning har studerats i detalj och särskilt agnostiderna visade sig vara användbara för åldersbestämning och indelning av lagerföljden. Många av de identifierade agnostidarterna har en kosmopolitisk utbredning och lagerföljden i Almbackenkärnan kan därför jämföras med likåldriga lagerföljder på andra kontinenter

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

A core drilling at Almbacken in the village Sodra Sandby, Scania, southern Sweden, penetrated a c. 30 in thick succession of upper lower Cambrian and middle Cambrian strata. Only the uppermost part of the lower Cambrian (Gislov 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)

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 Tornetr¨ask Formation in the Luob´akti section, south of Lake Tornetr¨ask, 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

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

The middle Cambrian Ptychagnostus punctuosus and Goniagnostus nathorsti zones in Vastergotland, Sweden

New trilobite discoveries from eight localities in Vastergotland, south-central Sweden, demonstrate that the middle Cambrian Ptychagnostus punctuosus and Goniagnostus nathorsti zones are present in most Lower Palaeozoic outcrop areas of Vastergotland, though as a thin and impersistent conglomeratic limestone, up to approximately 15 cm thick. These zones have not previously been recognised in Vastergotland, except for a thin bed of limestone on Hunneberg