Trace fossils at the basal Upper Greensand (Albian, Cretaceous) unconformity surface in east Devon (southwest England) and the nature of the unconformity surface

摘要

Along the east Devon coast the early Cretaceous Upper Greensand Formation rests unconformably on bioturbated firmground and hardground surfaces on mid-Triassic to early Jurassic rocks (Mercia Mudstone Group to Lias Group). The classification and interpretation of the burrows and borings preserved on and beneath these surfaces are discussed, and compared with those from similar bioturbated surfaces elsewhere in Europe. In east Devon, the nature of the preservation of these trace fossils is dependent not only on the nature of the substrate but also on that of the infilling materials. These range from poorly defined, irregular infillings composed of pebbly mudstone to well-defined casts of cemented fine-grained sandstone that preserve detailed external ornaments. The most prominent trace fossils recorded are regularly spaced, flask-shaped Gastrochaenolites ornatus Kelly & Bromley produced by an as yet unidentified bivalve that rotated during penetration. At Branscombe, where the Upper Greensand rests on Triassic mudstones, many of the crypts are ellipsoidal to subhemispheroidal in cross-section. Their producer(s) are also enigmatic. Some infillings contain fragments of Myopholas or Girardotia, bivalves that rotate during penetration of soft to firm substrates. These burrows were probably initiated above the unconformity surface and extended down into an already perforated and softened mudstone surface. A few burrows may be due to a burrowing coelenterate. Bioturbation at the sub-Albian unconformity is ubiquitous in southern and eastern England, and indicates that the erosion surface was available for colonization for a considerable period of time.