Large slides and rockfalls are a common, but intermittent, transport process in areas of active tectonic activity. On dry land, these events destroy infrastructure and cause the loss of life. In the case of a recent rockfall near Newhalem, WA, the large boulders shown in the photo below fell on SR 20 within 15 minutes of a passing commuter. When these events occur in the ocean, they can produce tsunamis, which result in comparable devastation along adjacent coasts. However, data are rare about these episodic, unpredictable events. This is particularly true in the deep sea, where instrumentation and detailed mapping is essentially nonexistent.
SR 20 near Newhalem, just after
the 9 Nov 2003 rockfall (photo courtesy of WSDOT)
Ancient events preserved in the rock record provide an important tool to provide constraints on the mechanisms responsible for their occurrence and the dynamics of material along the flow path. The Cow Head Group exposed on the west-central coast of Newfoundland has been extensively studied because of its continuous sections of deep-marine units. From these studies, the paleogeography has been reconstructed fairly accurately. At Lower Head, a single megabreccia is exposed that has a volume on the order of a cubic kilometer (wow, that's big!). The largest clast within this event bed is Big White One, which is shown in the picture below. My colleague, Giff Kessler (now deceased), and I measured the primary-clast-size distribution and examined the various sedimentological contacts (shown above: Bed 11 underlies the Lower Head deposit, Bed 14) associated with this monster deposit. We found that the clast-size distribution obeyed a power law. The magnitude of the power law (i.e., the exponent) was consistent with laboratory experiments of impulsive fragmentation. That is, the clast-size distribution was dominated by the initial failure event, not subsequent shear and clast-clast impacts along the flow path. Combined with the amount of erosion observed, this characteristic provides a tight constraint on the propagation rate and internal shear of the parent flow and hints at the importance of acoustic fluidization of the underlying beds and possibly the parent flow. These results will soon be submitted to Geology.
Giff is shown standing with
his right hand out on Big White One, outlined in black.
In mountainous regions, the threat to human life and property is more immediate. For instance, the Squire Creek slide (shown below) endangered the nearby town of Darrington, when it created a debris dam on Squire Creek. Fortunately, the dam failed incrementally and posed no threat to people living along it downstream. Detailed analysis is underway of the seismic signature of this event, which was recorded at two different seismographs in the North Cascades. Combined with a perilous survey of the headwall last year, these valuable direct observations should enlighten the mechanics responsible for slide runout. Unlike the Lower Head flow, evidence abounds that the material associated with the slide interacted and fragmented significantly during its quick trip down the hillside. It is common to see primary material such as serpentine imbedded several cm into wood, as seen in the photograph at the bottom of this page.
This work is closely related to work being performed by Chris Brummer, who is studying the recovery of Squire Creek and the past history of slides within this steep gorge.
The upper portion of the
Squire Creek slide as seen from SR 530 outside Darrington, WA
Gravel imbedded into cedar
at the terminus of the Squire Creek slide (photo courtesy of Tina Lomnicky)