Landslides - Extreme Events: A Physical Reconstruction and Risk Assessment - page 196

2380

2370

13.7 ka
18 ka
14.8 ka

2360

Major mudslide
episodes

2350

27.7 ka
24.7 ka
38 ka

33.3 ka
>42.5 ka

2340

2330

0 25 m

Figure 7.11. Mudslide layers in the Yakatabari Valley, Papua New Guinea (after Blong
and Goldsmith, 1993 ). Note multiple event layers and radiocarbon dates showing
initiation of slides around 40 ka. Dates from other boreholes show that the slide
complex has remained active until at least 4 ka.

thelast glacial maximum as sea levels fell and later by isostatic uplift of the
surrounding terrain.
Blong and Goldsmith (1993)investigated the stratigraphy of the Yakatabari
mudslide complex in Papua New Guinea. Boreholes in colluvial deposits at the
study area allowed the construction of an isopach map of colluvial thickness and
theidentification of numerous phases of infilling of the valley due to landslides
(Fig. 7. 11 ). A number of landslide layers are evident in the 8--14 m deep bore-
holes drilled through the complex. Wood remains found in the sample cores
and in near-surface exposures were radiocarbon dated. The wood would have
been incorporated into the slides as trees were uprooted and destroyed during
individual events. The complex, predominantly composed of mudslides, appears
to have been active over the last 40 000 years. This is one of the few studies to
ever identify such a long-lived landslide complex, but as Blong and Goldsmith
(1993)note it is unlikely to be unique. It was not possible from this study to iden-
tify periods of more than a few thousand years when the mudslide was stable.
These periods of stability are marked by the presence of buried soils. Buried soils
represent periods of ground surface stability when the uppermost level of sedi-
ments undergoes weathering and is covered with vegetation. The soils will stop
forming when another slide buries the existing ground surface. Studies such as
this, where slides can be recognised and dated over considerable periods of time,
highlight the persistent risks from landslides in the area. Blong and Goldsmith
(1993)found no correlation between late Quaternary climatic phases and slide