Aspects of a Middle Cambrian Thanatotope on Öland

The Paradoxissimus Siltstone, representing the Middle Cambrian zone with Tomagnostus fissus and Ptychagnostus atavus in the Öland area, was deposited in a bay under the influence of rhythmic or pendulating current action from NE. Longer periods of mud deposition and an abundant animal life alternated with short periods of silt influx causing a devastation of the mud-burrowing zoocoenoses over large areas. Occasionally the currents turned, transporting some silt in the opposite direction. Among the characteristic animals of the muddy bottoms was the large trilobite Paradoxides paradoxissimus which is found in abundance in the siltstone but only as fossil exuviae; in the shale, representing the environment in which it lived, shelly and trace fossils have been obliterated by diagenetic processes. The silt content in the Paradoxissimus Beds shows lateral variations within the Öland area, and the mainly shaly portions, mostly in the lower part of the sequence, contain better preserved dead trilobites and exuviae in situ. Primary sedimentary structures in connection with the current activity (ripples of different types, traction marks, and priels) are briefly discussed, and the trace fossils are dealt with in more detail. Practical problems in the treatment of trace fossils and ichnocoenoses are discussed as well as some ichnosystematic aspects of the Cambrian “genera” Halopoa and Scotolithus. The Paradoxissimus Siltstone contains hydrocarbons. There are some gas seepages, and surface samples of the siltstone have proved to contain about 0.3% oil.