科研之友
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摘要
Debris ridges in New Zealand are routinely assumed to be moraines%26apos; and used as key Southern Hemisphere paleoclimatic sites without detailed evaluation of ridge origin. Here we assess the origin of a debris ridge adjacent to Dundas Ridge in the Tararua Range, North Island, New Zealand, through measurements of ridge morphology and sedimentary properties. The ridge has a steep c. 35 degrees distal slope (height 18 m), compared with the c. 19 degrees proximal slope (height 6 m), and on all transects the distal slopes contain the coarsest material (median b-axis clast widths of 0.18-0.25 m), compared to distal samples (0.34-0.37 m). Clast shape (C40 range 40-60%) and angularity (RA%26gt;65%) indicate typically angular and slabby%26apos; clasts, and along with the lack of fines, and the ca. 40-m-distance between the ridge crest and the foot of the backwall, lead us to reject a glacial (moraine) origin for the ridge. The single ridge morphology precludes a protalus rock glacier origin, while the lack of a broad hillslope scar and debris apron beyond the ridge excludes a landslide origin. Instead, we interpret the ridge as a pronival (protalus) rampart formed by supranival debris supplyfrom the ca. 200 m-high southeastern slopes of Dundas Ridgeacross a snowbed. Re-distribution of snow by prevailing westerlies from Mt Dundas Ridge into the basin would have nourished the snowbed, which is likely to have formed during the interval 24-18 ka BP, when a minor alpine-style glaciation affected sectors of the Tararua Range. This is the first pronival rampart detailed in New Zealand, raising the possibility that debris ridges of pronival origin may also be present elsewhere in New Zealand%26apos;s mountains.
- 出版日期2013-1
