Trace fossils and paleoenvironmental control of ichnofacies in a late Quaternary gravel and loess fan delta complex, New Zealand

Remarkably well-preserved trace fossil assemblages of a small, Late Quaternary (> 3000 yr B.P.) fan delta complex are exposed in sea cliffs south of Conway Flat, North Canterbury, New Zealand. Marine trace fossils are common and well-preserved in the bar, embayment and prodelta facies, but deltaic distributary facies contain no trace fossils at all. The preceding paper in this journal (Lewis and Ekdale, 1991) describes the lithofacies and their paleoenvironmental setting in this fan delta complex. Common trace fossils include the ichnogenera Anconichnus, Arenicolites, Asterosoma, Cylindrichnus, Diplocraterion, Gordia, Helminthoida, Ophiomorpha, Piscichnus, Planolites and Skolithos. One new ichnogenus (Arborichnus), two new ichnospecies (Arborichnus sparsus and Diplocraterion asymmetrium), and three informal varieties of Diplocraterion parallelum (var. lingum, arcum and quadrum) are introduced. Local ichnofacies include (a) low-divsity Anconichnus horizontalis Ichnofacies of densely packed burrows of deposit-feeding animals in shallow-marine, fine-grained sediment of bar and distal embayment facies, (b) high-diversity Diplocraterion parallelum var. lingum Ichnofacies of burrows of suspension-feeding animals in interbedded mud, clean sand and gravelly sand of the bar facies, (c) moderate-diversity Diplocraterion parallelum var. quadrum Ichnofacies in interbedded mud and sand of distal embayment facies, and (d) low-diversity Planolites montanus Ichnofacies of deformed burrows in bioturbated mud of the prodelta facies. In addition, sparse animal escape structures (fugichnia) occur locally in interbedded sand and gravel of the bar facies. The trace fossil associations of the Conway Flat fan delta complex are unusual in their excellent state of preservation, and many trace fossils are exposed in full-three-dimensional relief. Because of their very young age, they must reflect similar associations of biogenic sedimentary structures in modern deltaic settings that are as yet undescribed. This report appears to be the first documentation of a fully marine trace fossil assemblage in loessial sediments.