| Summary |
Foraminiferal assemblages were used to investigate the nature of sedimentation on the tectonically active, high terrestrial-input Waipaoa River Margin (WRM) of New Zealand. Recent research has been focused on understanding the contribution of small mountainous rivers to the global sediment budget, and this study is part of a greater effort to define the processes that link sediment source and sink. Fifty-five surface samples from box-, gravity- and multi-cores from the shelf and slope of the WRM were used to document the distribution of modem benthic foraminifera. Twenty samples from two gravity cores retrieved from a canyon, considered to be a conduit of sediment to the deep ocean (Lachlan Canyon), were analyzed to evaluate the record of down-slope sediment transport. Six biofacies, recognized based on cluster analysis of census data of surface samples, are strongly related to water depth. The inner-shelf biofacies (26-43 m) is characterized by Elphidium charlottense, Zeaflorilus parri and Ammonia aoteana. The shelf biofacies (~30-130 m) is distinguished by Cassidulina laevigata, Eilohedra sandiegoensis, Saidovina karreriana, Bulimina marginata f. marginata (the four most abundant taxa in the foraminiferal dataset), Textularia earlandi, Bulimina elongata and Bolivina cf B. paula. The outer-shelf biofacies (60-250 m) is characterized by Saidovina karreriana, Bulimina marginata f marginata, Eilohedra sandiegoensis, Evolvocassidulina orientalis, Reophax subfusiformis, Sphaeroidina bulloides and Cihicides sp. A. The upper-slope biofacies (230-350 m) is distinguished by Dorothia scabra, Evolvocassidulina orientalis, Saidovina karreriana, Bolivina alata, Chilostomella oolina and Spiroloxostoma glabra. The lower-slope biofacies (870-1255 m) is characterized by Haplophragmoides sp. A, Dorothia scabra, Uvigerina peregrina, Bulimina marginata f. marginata, Cribrostomoides subglobosus, Bolivinita quadrilatera, Discammina compressa, Haplophragmoides sp. A and indeterminate agglutinated. A sixth biofacies is confined to Lachlan Anticline, a mid-Tertiary, emergent structure in the southern portion of the WRM shelf It is characterized by the largest foraminifera (inner and mid-shelf species), the most diverse assemblages and the highest percentage of reworked planktonic tests in the foraminiferal dataset. Many planktonic and benthic specimens have probably been derived from outcropping rock of the anticline. A secondary high in the percentage of reworked (fossil) planktonic foraminifera occurs at the mouth of Poverty Bay, and it likely represents sediments that have been shed from the rapidly uplifting Plio-Pleistocene strata within the Waipaoa River drainage basin. Analysis of three shore-normal surface sediment transects showed contrasting evidence for sediment transport; shallow water indicator taxa (e.g., Elphidium charlottense, Haynesina depressula and Notorotalia sp.) were present only in surface samples at slope depths in the northern WRM transect. Cluster analysis of down-core and surface samples together showed that assemblages throughout the head (360 m) and mid-canyon thalweg (890 m) cores from Lachlan Canyon contained higher percentages of shallower-water indicator taxa than other samples at similar depths outside the canyon, supporting the idea that Lachlan Canyon is channeling sediment from shelf and upper slope depths to the deep ocean. However, different patterns of down-core distributions between the canyon head and mid-slope thalweg sites suggest that the nature of sedimentation varies down-canyon. Abundances of shallower-water foraminifera indicate that shelf taxa are being supplied from a consistent source to the canyon head, while shelf and upper slope taxa are more variably supplied to the canyon thalweg. |
