PALEOECOLOGIC SIGNATURE OF MACROFAUNAL ASSEMBLAGES IN CORES: IMPLICATIONS FOR SEQUENCE STRATIGRAPHIC INTERPRETATION

Paleoecology has a close relationship with sequence stratigraphy.  Associations of fossil taxa may provide important indicators of successively changing environmental conditions, which are critical in recognizing and interpreting systems tracts and key surfaces within sequences.  The sequence stratigraphic model can in turn provide a predictive framework with which to examine biotic changes and understand their causes.

       The Manutahi-1 core of Wanganui Basin, New Zealand, has provided a unique opportunity to calibrate the interpretation of depositional environments based on geophysical wireline log character to that constrained by high-resolution paleoecologic and sedimentologic information.  Thirty cyclothems of the Matemateaonga Formation (Miocene-Pliocene) have been identified in the core.  Temporal patterns in both lithofacies distribution and macrofaunal paleoecology indicate a close link with environmental shifts that took place over the duration of these sixth-order sea-level cycles (approximately 41 ka).  Because foraminiferal assemblages have proven to be depauperate in Manutahi-1 core, macrofossils, especially bivalves and gastropods, are used as the primary source of paleoecologic data.

     The paleoenvironmental analyses we present here have relied extensively on the transfer of ecologic data from the modern fauna to extant or closely-related species observed in the core.  Most notably a relative sea-level curve has been constructed for the Matemateaonga Formation from analysis of bathymetric data for appropriate modern taxa.  A range of exploratory statistical approaches, including cluster and ordination techniques, allow rapid, objective and reproducible classification of macrofaunal associations and the elucidation of large-scale paleoenvironmental patterns.

     Several implications arise from this study.  Firstly, sequence boundaries and other key surfaces are confidently identified.  Secondly, changes in depositional environment are revealed at a range of temporal scales.  These paleoenvironmental and sequence stratigraphic patterns matched to the wireline log character of Manutahi-1 has allowed increased confidence in the interpretation of wireline logs elsewhere in Wanganui Basin, where core material is not available.