Seismic Geomorphological Analysis And Stratigraphic Architecture Of Submarine Channel Systems, Taranaki Basin, New Zealand

Submarine channels are important conduits for sediment transfer from the continent to oceans. Seismic geomorphological analysis provides insights into the evolution of ancient submarine channel systems and their preservation in the stratigraphic record. However, many seismic geomorphological studies lack borehole data to constrain the lithology and petrophysical expression of various deep-water depositional elements& such information can be derived from field-based studies, but most of these are 2D or quasi-3D at best. Here, we undertake borehole-constrained seismic geomorphological analysis of Middle Miocene submarine channel systems using a high-resolution, post-stack, time-migrated, 3D seismic reflection cube (c. 1,500 km² ) that is calibrated with well-logs and cuttings data from seven boreholes. Seismic attribute analysis utilising seven seismic horizons (H1 H7) mapped within two seismic units (SU1 and SU2), allow identification of three main seismically defined, depositional elements: (i) channel and intra-channel, (ii) levee/overbank, and (iii) slope/basin-floor.

SU1 contains two types of N-S trending channel system: (1) moderately sinuous (sinuosity index (SI) = 1.15), levee-confined channels (average width and thickness of c. 342 m and 35 m, respectively), and (2) highly sinuous (SI=2.25), partly coalesced channel complexes (average width and thickness of c. 346 m and 34 m, respectively). Attribute maps of channel-fills and overbank/levee deposits within SU1 show high amplitudes. SU2 contains NW-SE trending, low sinuosity (SU2 = 1.01 1.40), erosionally-to-levee-confined channel-fills that migrate laterally and stack vertically. These form channel complexes confined within slope valleys, with individual channel complexes being, c. 324 1080 m wide and c. 32 104 m deep. Attribute maps of SU2 channel-fills are only weakly reflective with high amplitudes, mostly confined to the overbank or levee environment. Well data show that the channel-fill seismic facies are variable, comprising sandstone, muddy-sandstone, siltstone and mudstone, with sandstone content decreasing upward. The levee/overbank seismic facies are overall finer grained, and characterised by muddy, very fine-to-fine-grained sandstone interbedded with siltstone and mudstone. The slope/basin floor seismic facies are mudstone-dominated.

SU1 and SU2 channel complexes also differ in that: (i) palaeoflow direction was northwards in SU1 times and northwestwards in SU2 times& (ii) SU1 channel complex-fills are sandier (20-45% sand content), compared to those in SU2 (10-18% sand content)& and (iii) SU1 channel complexes are relatively unconfined and characterised by lateral migration, whereas SU2 channel complexes are more commonly confined within valley systems. The mapped basal erosion surfaces of the channel complexes are highly time-transgressive, implying that the processes controlling channel evolution were similarly diachronous. Hence, we speculate that tectonically driven re-adjustment of the basin and hinterland during the Middle Miocene (between SU1 and SU2 times) resulted in a gradual change in slope orientation from N-to-NW-dipping, and an increase in slope gradient and a relative sea level fall, which potentially drove slope incision and increased the amounts of bypass of sand-grade material to the deeper basin.