Stacking patterns of the depositional sequences

In the Motuokura basin, the stacking pattern of the eleven sequences stacking pattern exhibits an overall remarkable retrogradational geometry (or architecture) that develops on a concave up erosion unconformity at the base of the Pleistocene basin fill (Fig. 5). In detail however,

shallow marine and coastal plain sediments. The surface is therefore strongly diachronous, and formed partly as a result of wave abrasion and subaerial erosion (Fig. 5, 6 and 7).

Two to Three regional megasequences have been identified on seismic data and are described below.

The uppermost megasequence (M1) reaches a maximum thickness of c. 1.3 s TWT (c. 1150 m

@ 1750 m/s). It is bounded below and above by S5 and S1 respectively and comprises four sequences (Seq1 to Seq4, eg. Fig. 5). The sequences exhibit typical lens-shape, shelf-edge clinoforms up to c. 250 meter-height relief and c. 0.3s to 0.4s-thick. Each sequence of M1 exhibits mostly seaward stepping lens-shaped sediment packages that define internal progradational clinoforms of a prominant shelf edge (Fig. 5). The seismic facies and their distribution within both the sequences and internal sediment packages are similar to Seq1 (Fig. 5). This implies that the ranges of sedimentary facies, depositional environments are equivalent between sequences and evolve from shallow marine sandy-silty deposits against the basal unconformity to deep marine silty clays toward the Motuokura ridge with sediment waves-like features at the shelf edge. This is illustrated by high amplitude reflections that grade basinward into weak reflectivity (Fig 5). This is consistent with lateral variation from shoreface and inner shelf deposits to shelf and slope muddy sequences. This also implies that M1 sequences developed under the same bathymetric conditions within the Motuokura basin during the overall retrogradation. The landward migration during M1 reaches c. 25% of the total migration of the megasequences.

The growth of tectonic structures beneath the shelf and upper slope has influenced the overall geometry of the megasequence. M1 is folded and faulted across the Waimarama thrust fault system and thins out on the western flank of the Motuokura ridge. The latter represents a tectonic growth sequence reflecting sedimentation contemporaneous with thrust faulting and uplift of the Motuokura Ridge. Sedimentary sequences exhibit gentle, concave up, bending (synform) migrating progressively eastward. The maximum of synform-bending of each sequence broadly corresponds to the location(s) of the following sequence(s) depocenter(s) (Fig. 5 and 10). The relative thickening and decrease of the slope at the base of the clinoforms build up the present day flat morphology of the Motuokura trough.

The middle megasequence (M2) reaches a maximum thickness of c. 0.7 s TWT (c. 630 m @ 1800 m/s). It is bounded below and above by S 9 and S 5 respectively, and comprises four sequences (Seq5 to Seq8, Fig. 5). The sequences exhibit pronounced lens-shape clinoforms up to c. 250 meter-height relief (c. 0.1s to c. 0.3s-thick) that thin out rapidly on the western flank

of the Motuokura ridge. M2 is thinner than M1, with thinner sequences but build up prominent shelf edge morphology (Fig. 5) and trough-like morphology at the base of clinoforms. Despite the loss of resolution in data, it remains possible to identify seismic facies that are equivalent to those seen in M1 with similar relative distribution within each sequence (Fig. 5). From this we infer that the sequences and their internal sedimentary facies formed in a similar environment to those in megasequence 1.

M2 sequences are tilted 3° to the WNW on the western flank (hangingwall sequence) of the Motuokura ridge. Compared to M1, the rate of overall retrogradation of the sequences decreases, with little landward shifts of the shelf edge margin and extension of the transgressive surfaces over the Cenozoic substratum. The landward migration of the LST shelf edge during M2 reaches less than 10% of the total lateral migration.

The lower megasequence (M3), reaches a maximum thickness c. 0.6 s TWT (c. 570 m@ 1900 m/s). It is bounded below and above by S12 and S9 respectively and comprises up to three sequences (Seq9 to Seq11, Fig.5). Sequences exhibit lens-shape clinoforms that are less pronounced than those from M1 and M2 sequences, with less than c. 100 meter-height (c. 0.1 to c. 0.2 s TWT). They show little thickness variation and poorly developed shelf edge morphology. The presence of shelf edge morphology is suspected downslope, west of the ridge in the northern part of the basin (05CM28 & 27– Fig. 8), and east of the ridge in the southernmost part (05CM30 – Fig. 8) but remains faint along the profile 05CM29 (Fig. 5).

The facies identification and distribution in the sequences is more speculative due to the loss of resolution in data. Nevertheless the distribution appears broadly similar to those of M1 and M2. The sequence is inferred to include shallow marine facies against the basal unconformity and deeper marine strata basinward within each sequence.. The landward migration of the LST shelf edge during M3 reaches less than 10% of the total migration.

M3 sequences are tilted to the WNW, on the western flank of the Motuokura ridge, with dip values up to c. 4°. Normal bending faulting resulting from flexural extension dissects the sequences along the Motuokura ridge anticline axis (eg. lines 05CM 29 and 30) but the diminished signature of growth faulting in the sequence geometry implies that most of deformation occurred largely after the deposition of M3.

and M3 megasequences merge together and wedge out laterally along the unconformity at mid-shelf position. M1 is largely distributed laterally over S5 unconformity and thicken in local sub-basins (Lachlan, Mahia and Kidnappers basins, Fig. 6, 7, 8 & 9) individualized between the main thrust fault ridges (Fig. 8). Evidence of channels between Fs3 and Fs4 on seismic data, in most of sequences over the present day shelf is in accordance with fluvial channel deposits interbedded with shallow marine and shelf deposits. This similarity between sequences M1, M2 and M3 implies that the bathymetric range and depositional environments were similar as sequences developed during the general retrogradation.