Composite ichnofabric of a mid-Tertiary unconformity on a pelagic limestone

Ichnology and sedimentology of an omission surface on the pelagic Amuri Limestone (middle Oligocene) reveal a complex and variable history of unconformity development in Canterbury Province, South Island, New Zealand. Ichnofabric and trace fossil assemblages indicate the nature of the omission substrate: at some localities it was a softground, at others a firmground, at others a hardground, and at yet others a rockground. The irregular distribution of these substrate types among more than 50 localities over an area of several hundred square kilometers substantiates sedimentologic evidence for an irregular surface relief, which developed during an episode of gentle folding and uplift related to the onset of compressional tectonics accompanying establishment of a new plate margin in the New Zealand region. Softground conditions are represented by a wide range of burrow types. Firmground conditions are reflected by Thalassinoides and Zoophycos burrows. Hardground conditions are indicated by Trypanites borings, and by Thalassinoides borings which follow low-angle joints. Rockground conditions are represented locally by paleokarst features, commonly in the form of microkarstic solution-enhancement of joints and of omission-suite Thalassinoides burrows and borings, which reach to depths of 2 meters in the limestone. The rockground and clasts derived from it have been penetrated by another (postomission) set of Thalassinoides borings, which began as burrows in the sediment that overlies the unconformity. The original chalky consistency of the weakly indurated coccolithophorid biomicrite of the Amuri Limestone apparently offered no more resistance to burrowing than the unindurated sandy glauconitic fine biomicrite of the overlying sediments. Omission suite Thalassinoides burrows show evidence of reoccupation by postomission Thalassinoides progenitor organisms.