Trace Fossil Daedalus in Lower Ordovician Storm Deposits of Armorican Massif, West France

Jacques Durand

The lower Paleozoic Series in the Armorican massif consists of a major transgressive terrigenous sequence including basal red beds (Pont-Rean Formation) unconformably overlying Precambrian rocks, pale sandstone, and shale-sandstone alternations (Gres Armorican Formation) and monotonous shales (Traveusot Formation).

Two species of Daedalus exist in the Gres Armorican Formation (Arenig): Daedalus halli and Daedalus labechei. They belong to distinct ichnofacies. In the sand-flat environment (lower quartzarenite lithofacies), Daedalus halli is associated with Skolithos and Diplocraterion. These vertical cone-shaped burrows are well-developed, and the morphological features suit a three-dimensional dwelling structure. The interbedded lithologies consist of tidal-dominated deposits, and the trace fossil associations contain Cruziana, Monocraterion, Phycodes, and Daedalus labechei (food-searching burrow).

In the storm deposits, the ichnofacies, which do not appear as a characteristic association, is composed of Cruziana, Daedalus halli, Teichichnus, Lingulichnites, and Planolites. Daedalus halli is restricted to the top of the storm sequences in the hummocky amalgamated sandstones and completely crosses the strata in the interbedded lithologies. The burrows may be entirely truncated and set off the disparities in the storms intensity. The burrow sizes are less developed than in the sand-flat environment, and the features comparatively show a partly different behavior. The animal growth has not been recorded in the burrow. Unsuccessful escape structures of the brachiopod Dinobolus are fossilized in the lower part of the sequences.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.