Cascades and Plunge Pools in the Gulf of Alaska

TitleCascades and Plunge Pools in the Gulf of Alaska
Publication TypeBook Section
YearSubmitted
AuthorsMayer, LA, Gardner, James V.
Book TitleAtlas of Submarine Glacial Landforms: Modern, Quaternary and Ancient
VolumeMemoir

In the summer of 2005, the University of New Hampshire’s Center for Coastal and Ocean Mapping/Joint Hydrographic Center, mapped more than 321,400 sq. km. of seafloor in the Gulf of Alaska in support of efforts to establish the limits of the juridical continental shelf of the United States as defined under the Law of the Sea Treaty. Both bathymetry and high-resolution backscatter data were collected with a 12-kHz EM120 multibeam echoshounder aboard the University of Hawaii’s R/V Kilo Moana. The resulting digital terrain models and backscatter mosaics (presented as average backscatter normalized to 45o) provide a remarkable view of the processes on a tectonically active, glaciated margin including the formation of numerous submarine channels (Gardner and Mayer, this volume) and large deep submarine fans (Gardner and Mayer, this volume). In the course of these surveys, several morphological features similar to cascades and plunge pools that are found beneath terrestrial waterfalls were identified on the continental slope.

Full Text

In the summer of 2005, the University of New Hampshire’s Center for Coastal and Ocean Mapping/Joint Hydrographic Center, mapped more than 321,400 sq. km. of seafloor in the Gulf of Alaska in support of efforts to establish the limits of the juridical continental shelf of the United States as defined under the Law of the Sea Treaty. Both bathymetry and high-resolution backscatter data were collected with a 12-kHz EM120 multibeam echoshounder aboard the University of Hawaii’s R/V Kilo Moana. The resulting digital terrain models and backscatter mosaics (presented as average backscatter normalized to 45o) provide a remarkable view of the processes on a tectonically active, glaciated margin including the formation of numerous submarine channels (Gardner and Mayer, this volume) and large deep submarine fans (Gardner and Mayer, this volume). In the course of these surveys, several morphological features similar to cascades and plunge pools that are found beneath terrestrial waterfalls were identified on the continental slope.

Description

Cascades:

Although several cascade-like features are seen along the continental slope of the south-eastern Alaskan margin, the most prominent is found southwest of Yakutak Bay, and south of the Malispina Glacier and the St. Elias Mountain Range. The St. Elias Mountain Range are some of the highest mountains in North America with remarkable vertical relief (e.g., Mount St. Elias rises to 5489 m above sea-level in less than 16 km horizontal distance, Figs. 1a, b, and e). Offshore of Yakutak Bay is the Yakutak Valley, a large, glacially excavated, submarine valley that cuts NE-SW across the continental shelf. Yakutak Valley originates in Yakutak Bay with a width of approximately 50 km and a depth of 120 m as it reaches the shelf break 90 km from the mouth of the bay (Fig. 1a). Within the multibeam sonar data set, the cascade starts at a depth of about 1800 m with eight steps down to a depth of about 2800 m over a horizontal distance of about 14 km.  The regional slope in this region is relatively smooth with an average of about 5o over the depth range of 1800 m to 2800 m. The headwalls of the cascade steps typically have slopes of 10o to 13o with vertical drops of 150 m to 250 m over horizontal distances of 1 to 2 km (Fig. 1e).  The cascade steps stop at 2800 m water depth; deeper than this depth the seafloor slope abruptly changes to an average slope of less than 1.5o in its descent to abyssal depths of approximately 3500 to 3600 m. Interestingly, the headwalls of the cascades are not orthogonal to the strike of the margin but are asymmetric and concave with an angle of about 45o from the fall line of the slope (oriented NE-SW; Fig. 1a).

Below 2800 m, where the major change in gradient occurs, there appears to be no bathymetric continuation of the cascade structure. However, the backscatter mosaic simultaneously collected with the bathymetry (Fig. 1c) reveals a narrow 300 to 400 m wide channel that continues downslope where it eventually merges into a series of tributary channels that eventually join one of the major deep-sea channels of the Alaskan margin (see Gardner and Mayer, this volume).

 

Plunge Pools:

Also evident in the multibeam sonar data collected along the southeast Alaskan margin are several features that resemble plunge pools typically found at the bases of terrestrial waterfalls. The most prominent of these is located approximately 70 km WNW of the cascade described above and approximately 85 km ESE from a major change in the orientation of the margin that represents the intersection of the Transition Fault along the southeast Alaskan margin and subduction zone along the Aleutian Trench (Gulik et al., 2007) (Figs. 1a and d). The plunge pool is located at 2800 m water depth along the trace of the Transition Fault). The gradient of the upper slope in this region (2o to 4o) is less than that in the vicinity of the cascade (>5o) but there is a steep (up to 38o) topographic step with an 800 m vertical extent (Fig. 1d) immediately above the plunge pool.  Unlike the relatively smooth upper slope in the vicinity of the cascade, the region of the upper slope in the vicinity of the plunge pool shows a series of ridges and valleys with 100 to 200 m of relief that trend 30˚ to 45˚ from the regional dip of the slope. The regional gradient is less than 0.4o below the trace of the transition (Figs. 1a and d).

The plunge pool is nearly circular in plan with a diameter of approximately 2.3 km.  At its deepest point, it is approximately 250 m deeper than the seafloor on the downslope side of the plunge pool and approximately 600 to 900 m deeper than the seafloor on the upslope side (Fig. 1d).  A 250 m wide, 25 m high, bathymetric rampart rims the plunge pool on the downslope side; a series of small bathymetric steps mark the transition from the upslope side into the plunge pool (Fig.1d).

Interpretation

Taking advantage of the availability of high-resolution multibeam sonar data, Lee et al. (2002) catalogued the occurrence of a series of plunge pools and associated bathymetric ramparts on the New Jersey and California margins and offered two possible mechanisms for their creation: 1) reduced shear stress downstream of hydraulic jumps in submarine density flows that causes deposition of a downstream bar and 2) the impact of high-momentum density flows that can excavate a depression.  The plunge pool described here is much deeper than those described by Lee et al. (2002);  250 m deep as compared to a maximum depth of 75 m in the 150 plunge pools described by Lee et al. (2002). Given the relationship of the plunge pool to the surrounding seafloor (it is clearly excavated rather than deposited), impact of density flows is the most likely mechanism, with the rim being deposited as material is eroded from the pool (Fig. 1d).  Downslope flow of either sediment laden density flows or hyperpycnal flows of cold dense water derived from Icy Bay are likely to be constrained by the large bathymetric ridge to the west, that would allow the flows to accelerate over the 800 m, 38o slope to create the plunge pool (Fig. 1a and d).

In contrast to the plunge pool where flow appears to be bathymetrically constrained over a steep slope, the cascade occur on a relatively smooth slope (Figs. 1a and e). Here, it appears that hyperpycnal flows out of Yakutak Valley, have moved down the relatively smooth upper slope and encountered bedrock outcrops that created the hydraulic jumps to generate the cascade structure. The orientation of the headwalls of the cascades are similar to those of the ridge-like structures seen to the east and west of the region of smooth upper slope.  Evidence that the flow continued down slope is found in the backscatter but not the bathymetry, indicating a non-erosive flow below the depth of the cascade (Fig. 1c).

References

GARDNER, J.V., and MAYER, L.A., 2005, U.S. Law of the Sea cruise to map the foot of the slope and 2500-m isobath of the Gulf of Alaska continental, http://ccom.unh.edu/lawofthesea

GULICK, S.P.S., LOWE, L.A., PAVLIS, T., MAYER, L.A., and GARDNER, J.V., 2007, New insights into the Transition Fault debate:  Propagating strike-slip in response to stalled subduction in the Gulf of Alaska, Geology, v. 35, no. 8, pp 763-766.

LEE, S.E., TALLING, P.J., ERNST, G.G.J., and HOGG, A.J., 2002, Occurrence and origin of submarine plunge pools at the base of the US continental shelf, Marine Geology, v. 85, pp. 363-377.

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