Submersible Observations in Escanaba Trough, Southern Gorda Ridge

Abstract

A series of eight dives using the U.S. Navy submersible Sea Cliff was conducted in July and August 1986 to map hydrothermal sulfide deposits and determine the distribution of benthic fauna at two study areas in Escanaba Trough at the southern end of Gorda Ridge. Five dives examined three seafloor hills that had been surveyed and sampled during previous surface ship programs, including one at latitude 41° N, longitude 127° 30′ W (the NESCA site), and two at latitude 40° 45′ N, longitude 127° 30′ W (the SESCA site). The hills, approximately 700 m in diameter and 100 m high, are complex polygenetic features produced by a combination of intrusive and extrusive volcanic processes. The flanks of the hills are characterized by steep, scarplike pitches separated by gently sloping terraces. The small hills are mostly sediment covered, although the steep flanks locally expose stratified mudstone. Extensive deposits of iron-rich massive sulfide were encountered on the mid and lower flanks of all three hills, in water depths ranging from 3,180 to 3,230 m. Some of these deposits were several tens of meters wide and more than 100 m long, suggesting sustained hydrothermal discharge.

Two dives investigated areas several hundred meters from the domes where surface ship measurements indicated a change in both bathymetry and bottom morphology. The seafloor at these two sites is characterized by closely spaced ridges up to 10 m high. Some of the ridge slopes expose mudstone slabs that have broken, contributing talus into the intervening swales. Hydrothermal sulfide deposits are located on the crests of some of these ridges. These deposits differ from those on the hills, occurring as discrete mounds, broken plates, and small chimneys spaced approximately 1 to 2 m apart. The nature of these deposits indicates diffuse hydrothermal circulation and sulfide deposition within the uppermost sediment layers.

The easternmost flank of the central hill at NESCA was studied during a single dive. This area, near one of the north-trending fault zones that bounds Escanaba Trough, is the site of extensive extrusive volcanism. Lightly sedimented basalt pillows up to 3 m in diameter cover the seafloor; glassy rinds were occasionally seen.

No signs of active hydrothermal discharge were seen during the 41 hours of bottom observations conducted during the dive program. The generally weathered nature of the sulfide deposits on the hills indicates that active venting slowed some time ago, at least in the areas visited by the submersible. The extensive but much less massive sulfide deposits observed away from the hills might be somewhat younger. The subsurface growth of sulfide appears to have lifted and deformed the most recently deposited sediment.