Reconocimiento de ingresos de actividades ordinarias

Of the study sites for this project, four have overlapping time lines, with the Tangihau, Monck Road and Kopaatuaki sites experiencing approximately 2000 years of shared history. The sites of Arai Matawai and Otara Station also may have overlapped but the lack of age control at the termination of sedimentation of the sites excludes them. The linking of the four cores with known overlap was established through the stratigraphies and chronologies provided by the radiocarbon ages and the

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tephra layers discussed in chapter four. The geographic spread of the sub-catchments means they likely experienced differing environmental conditions and underwent differing response rates to shared events. Where a period of increased sediment influx is evident at only one site, it may be that the site experienced a disturbance event that was only locally felt. However, there are periods of both high and low sedimentation at the Kopaatuaki (WK2), Monck Road (WM1) and Tangihau (WT1) sites that can be correlated (Fig. 6.41).

Figure 6.41. Core log comparisons for the main sites.

The sediment columns for the sites of WK2, WM1 and WT1 shown in figure 6.41 all exhibited layers of sediments representing erosion from within the catchment caused by disturbance events. From the age and volume calculations for these sites it is evident that there are similarities in the timing, the volumes delivered and the relative core depths of these events for each of the sites. The timing of these events is estimated at approximately 2500-2200 cal. years BP. The disturbance events occurred during periods when the cores had settled into regular sedimentation patterns following the dam formation and the phase of rapid infill that usually follows. The disturbance events are most strongly evident in the coastal sites of WK2 and WM1

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where there are three clear individual layers of sediment where the texture and bulk density is different to the sediments above and below. The changes in texture and bulk density represent event delivery were rapid accumulation leads to less well packed coarser grain sediments that have a lower bulk density value. These event layers are less well defined for the more inland site of WT1 but sedimentation was similarly rapid during this time period.

In the sedimentary sequence at the further inland WT1 site, these three disturbance events are evident as a sequence of sediments of high organic content. As these cores all indicate disturbances of similar timing and across a wide region, it seems that each individual event was large enough to have a regionwide impact. Using the evidence from modern earthquakes compiled by Litchfield et al. (2009b) and discussed in the chapter three section on the geological and tectonic setting of the region, these events could be caused by faults such as the Repongaere fault of the central forearc region. Individual faults in this group are thought to have movement return intervals of 5500 years but for ground shaking events of MM7 plus from the combined faults return time is estimated to be 26 years (Litchfield et al., 2009b). The timing of these disturbance events does not line up with any of the known storm periods in the Tutira record, however the return time of approximately 1 in 100 years is consistent with return intervals of the Bola size cyclonic events. The diminished catchment response further inland would also suggest that while these events were large in themselves, they were more coastal in their impact and focused on the Gisborne region.

Figure 6.42. Comparisons for sediment delivery rates for Waipaoa core sites. 0.00 0.50 1.00 1.50 2.00 2.50 3.00 113 403 636 1147 1717 2332 2540 2587 Cal Years BP E ros ion r a te s ( m 3 /ha /y r) Kopaatuaki Monck Rd Tangihau Maungahuari

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Figure 6.42 shows that the Tangihau, Monck Road and Kopaatuaki field sites all record a period of increased sedimentation which peaks at 1717 cal. years BP. This correlates with a period of increased storm activity centring on 2000 cal. years BP in the Lake Tutira record. Page et al. (2010) described this period as being one of the most intense in terms of both storm magnitude and frequency. The curves in figure 6.42 show that this period of intense storm activity is also evident in the Waipaoa cores with the impact of the storm activity being felt region wide. High rates of infill also continue in the Waipaoa cores of Kopaatuaki, Monk Road and Tangihau during the next Tutira storm phase of 1250-1100 cal. years BP, although this period is not as intense as that of the 2100-1850 cal. years BP period of storm activity. Due to a lack of dating points in the cores it is not possible to narrow down further these high frequency periods and those of the intervening low activity periods. For those periods however, catchment erosion rates are above the average and background rates for infill. The most recent of the storm activity periods and the second highest in terms of activity and intensity from the Tutira record, occurs from 700-500 cal. years BP and is seen in the Waipaoa cores as shown in figure 6.42. This period of storm intensity was also of region wide impact and is recorded in the Kopaatuaki, Monck Road, Tangihau and the Maungahuri cores.

6.5 The spatial and temporal influence on sediment delivery