Abstract

Important paleoenvironmental evidence about sandstones can come from the study of their associated faunas. Analysis of the sediments and faunas of the offshore tidal sand sheet and sand bank facies on the tidal current dominated part of the northwest European continental shelf has revealed several criteria that should be widely applicable to paleoenvironmental studies of sandstone successions. In order of decreasing tidal current strength, the bed form zones that are associated with the sand sheet facies are: furrows, sand ribbons, large and small sand waves, small sand waves alone, rippled sands and sand patches. Each bed form supports a characteristic fauna of potential body fossils consisting mostly of molluscs, echinoderms and crustaceans. Potential trace fossils are provided by polychaetes, echinoderms and other groups. The stable gravel floor adjacent to the furrows supports a diverse fauna. Sand ribbons are unlikely to be preserved. The adjacent gravel floor supports a moderately diverse fauna. Large sand waves (with small sand waves on them) support a fauna of low diversity consisting of few individuals with low preservation potential. Diversity is greatest in the zone of rippled sand and the preservation potential (here is much higher. Available evidence suggests that differentiation between active sand banks with sand waves on them and active sand wave fields is difficult if only faunal criteria are used. Bed forms on wave-dominated shelves are generally much more uniform and are broadly analogous to the rippled sand zone. By analogy, these wave-dominated sheets of sand support a diverse fauna of body and trace fossils with high preservation potential. Detailed studies are required to identify any geologically significant differences between faunas of wave-dominated sheets of sand and those of tidal current dominated rippled sands. The faunal composition of shell gravels reflects the composition of the local living fauna. The convex-up orientation of disarticulated bivalves in shelly layers in sandstones provides a means of recognition of storm deposits. Rapid deposition during the waning phase of the storm will preserve the convex-up orientation. If the shell layer remains uncovered at the sediment surface, the activities of the mobile benthic fauna turn over some valves into the concave-up orientation. The effects of major storms can be detected in shell pavements to depths of at least 100 m. Imbrication of valves occurs at depths of 100 m, thus indicating that it is not restricted to shallow locations. Imbrication may be also partially the result of biological activity.