Preservation of Trace Fossils in Flint in European Chalk: ABSTRACT

Abstract

It has long been known that many flint concretions in the European Upper Cretaceous chalk represent burrows of organisms. In particular, the ichnogenus Thalassinoides is readily recognizable in branched networks of flint developed more or less parallel to bedding, while other ichnogenera such as Zoophycos and Chondrites are commonly preserved on the surfaces of such nodules. However, until recently, the value of this qualitative information has been restricted because the trace fossils are more or less invisible beyond the boundaries of the flint. It has not been clear, therefore, to what extent trace fossils preserved in flint zones are present at flintless zones. For example, in flint-bearing sequences, was the formation of beds of flints restricted in an degree to horizons especially bioturbated with Thalassinoides suevicus? Conversely, is heavy Thalassinoides suevicus bioturbation restricted to zones bearing thalassinoid flints? The development of a technique involving painting oil onto polished surfaces of chalk is now greatly refining our knowledge of the distribution of trace fossils in pure white chalk, where they are otherwise barely visible, and we are now in a better position to evaluate the ichnological evidence provided by flint preservation. Patterns of distribution are emerging. It is becoming clear, for example, that the many different morphologies of thalassinoid flints are characteristically associated with different sedimentological settings. Thus, fully developed, sizable thalassinoid flint boxworks occur in autochthonous chalks containing correspondingly large, idiomorphic T. suevicus boxworks; straggly, malformed thalassinoid or finger flints have been found in slightly allochthonous chalk where the burrows have been dislocated or streaked out; and in unbioturbated allochthonous chalk, thalassinoid concretions are absent and flint may occur as rounded or irregular forms incorporating siliceous sponge body fossils. Such flint fabrics are easily recognizable and visible in the field and are showing themselves to be a useful aid in detailed sedimentological interpretation of chalk sequences. End_of_Article - Last_Page 430------------