Inferred bivalve response to rapid burial in a Pleistocene shallow-marine deposit from New Zealand

Orientations of 16 species of conjoined bivalves were analysed in relation to the sedimentary history of the middle Pleistocene Upper Castlecliff Shellbed and the overlying Karaka Siltstone in Wanganui Basin, New Zealand. The shellbed represents mid-cycle condensation, and the Karaka Siltstone (highstand systems tract) was deposited by coastal progradation. Most of the shell orientations represent either life orientation or reworked reclining orientation. However, orientations inverted compared to the normal feeding position are not uncommon for infaunal shallow-burrowing bivalves, such as Notocallista multistriata, but only in the lower unit of the Karaka Siltstone. This orientation is interpreted as produced by upward migration of bivalves when they were buried rapidly. Such bivalve orientations are evidence for a rapid change in sedimentation rate from the starved condition represented by the midcycle condensed shellbed (Upper Castlecliff Shellbed), across a downlap surface, to the highstand systems tract (Karaka Siltstone) where episodic rapid sedimentation began.