Orientation of agnostid shields in Alum Shale (Upper Cambrian): Implications for the depositional environment

Abstract

Polar angles (azimuth orientations) and vertical positions (convex side up or down) of cephala and pygidia of Agnostus pisiformis were recorded from 18 shale surfaces within a 0.85 m sequence of Upper Cambrian Alum Shale in Västergötland, Sweden. The absence of articulated specimens, and of thoracic segments, on the surfaces suggest that the agnostids did not live in the depositional area, but that exuviae had been transported into this area, possibly following storms. Two of the surfaces, where most of the shields were deposited with the convex side down, indicated absence of bottom currents. On the other 16 surfaces shields were deposited with the convex side up, indicating that their position had been affected by currents. On 12 of these 16 surfaces the orientations were significantly bipolar, suggesting a wave regime. Hence the results indicate that the sediments forming the Alum Shale (of the A. pisiformis Zone) were deposited above storm wave base (<50 m deep).

Keywords:

• Agnostus pisiformis
• sedimentation
• trilobites
• Västergötland
• wave currents