Just back from four days in the Cascade River valley, one of the first initiations into the complexities of fieldwork along the Alpine Fault in South Westland. The target was to find dateable material in earthquake-deformed post-LGM glacial silts to constrain the glacial and earthquake records and then to map out the extent and timing of other Quaternary deposits. Geomorphically the central Cascade valley is
one of the most complicated areas of New Zealand- it has the greatest
diversity of rock types anywhere in NZ, it has the longest glacial
record of any single valley in NZ, it has two enormous 1 km^3 rock avalanche deposits, two former rock
avalanche produced lakes, three youthful gorges that were cut after
these rock avalanches, partial inundation during sea level
highstands, major recent stream piracies and reorganizations, young slips, active and
abandoned fans, pressure ridges, terminal moraines, multiple active
fault traces, and it is constantly being lengthened by dextral slip on the
plate boundary. In short it is a land of mystery and intrigue just waiting to be unraveled.
The helicopter flight in went without event, though I never tire of the views. We made quick work setting up camp and headed off for a solid half day of collecting samples from key outcrops, finding new ones, and mapping out the heavily bushed terra incognita between the two traces of the Alpine Fault. It rained much of the day.
After a frosty but bright morning, day 2 was spent mapping some lateral moraines and then climbing up the ultramafic Red Hills for a view of the valley below and of the Mt Raddle headscarp. We chose a less than ideal route on the way up but set things right on the way down. While Gen and I geologized, Tammo botanized the straggly plants doing what they can to live on the toxic soils. Once back on the valley floor we finished the day off with some newly found exposures of the glacial silts which exhibit large wavelength folds here.
Day 3 (heavier frost!) we crossed the Cascade and revisited the Saddle Creek Alpine Fault outcrop to collect samples for dating. We collected fault gouge formed as a mixture between fault gouge formed at depth and abrasion against young river gravels with the hopes of finding dateable material mixed in. We then headed up some previously unvisited fault parallel creeks which eerily lacked water. No outcrops of geomorphology of use though we were clearly on the trace of the fault for much of the time.
The last day I got to put my packraft to full use to document the last outcrops along the Cascade River I had yet to reach. These outcrops were either in uncrossable sections of the river or adjacent to a deep pool. I suited up and set off downstream, initially making excellent time as I paddled through minor riffles and passed outcrops I had already visited. That all changed when I reached the gateway to the main gorge and was presented with an absolutely stunning 20m tall by 200m long outcrop with some of the most incredible and intricate rock avalanche textures I have ever seen. Some of the textures resemble major shear zones in faults but the amazing thing is that all the textures I saw formed in one event seconds long, not thousands of years of earthquakes. This outcrop completely rekindled my interest in rock avalanches (big rocky landslides) and hopefully I'll get the chance to do some more work on it. Fingers crossed my collected samples contain dateable material! I could have easily spent hours here but time was ticking to get to my destination before the chopper arrived.
I passed many more rock avalanche deposit outcrops in the quick moving gorge, my attention divided between the rocks and not-inconsequential rapids. Due to time mental notes and a quick photo often had to suffice. A couple rapids I had to scout to be able to determine the safe passage through log and boulder gardens. I was being quite productive despite enjoying the wilderness and having lots of fun. The river would have been too low for a whitewater kayak but my Alpacka excelled where others would have scraped and beached.
Upon exiting the gorge I noticed an upcanyon wind had begun to whip at me. Bugger! All of a sudden I was behind schedule and paddled hard against the wind to recover some time. The open stretch near Woodhen Creek afforded excellent views of the surrounding mountains and some interesting observations on river genesis where the river had jumped its bed several years before and was now working to smooth out a new course through what was once bush.
I made great progress through the Cascade rock avalanche gorge, outcrops of which I had already documented at length. However one last surprise was in store at the last outcrop where the rock avalanche absolutely reamed the tree-ed swamp on the valley floor. Here I marveled and quickly documented the incredible deformation and grabbed some super-primo wood detritus for radiocarbon dating which should reliably date the age of the rock avalanche and potentially an Alpine Fault earthquake that may have triggered it. Once around the last bend the river gradient dropped dramatically through the braided river flats of the lower Cascade. I took out at Charlies Bump just in time to see the chopper deliver Gen and Tammo back at the car. I hurried best I could to meet them. In all a most productive few days and with any luck we will turn up with some useful dates to help us unravel the complex Quaternary history of the Cascade Valley. Thanks to Gen and Tammo.
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