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).