ABSTRACT

Within the Aberystwyth Grits (Lower Silurian, Wales) the trace fossils Paleodictyon and Squamodictyon occur with marked preferred orientation on the soles of turbidite sandstones. One hundred twenty-three orientation measurements have been made, and in parts of the succession this orientation is remarkably constant stratigraphically through hundreds of meters of sediment. The trace fossils were formed as complex open tunnel systems dug within the inter-turbidite mud and preserved on the base of the overlying turbidite sandstones. The preferred orientation cannot be convincingly ascribed to soft sediment or tectonic deformation but apparently reflects an animal response which pre-dates the turbidity current. The low slope angles normally present in the deep oceans preclud a slope induced animal response. Paleodictyon and Squamodictyon tunnel systems do, however, have numerous vertical shafts opening on to the ocean floor and orientation of the system parallel with bottom currents would facilitate water flow within the tunnels. On this hypothesis, the orientation of the systems can be used to determine bottom current trend during the period between turbidite deposition.

Over much of the area studied, trace fossil orientation and sole marks are both commonly parallel with the SSW-NNE trough axis. However in the south within thicker bedded, more proximal parts of the succession, interpreted as channel-fill deep-sea sand fan deposits, trace fossils are oriented axially but sole marks indicate downslope marginal supply from the west. This reflects the pattern in some modern oceans where turbidity currents transport sediment down slope and bottom currents contour axially. Locally, between Wallog and Borth, sole marks are oriented SW-NE but trace fossils are aligned E-W and are interpreted as reflecting bottom currents oblique to the trough axis.