Gorda Ridge Cruise
August 5 - 21, 2002

August 18, 2000: Day #14

Dave Clague writes: Dive 199 explored a large sill complex at the southern Escanaba Trough. Previous work in this area had located several large inactive hydrothermal sulfide deposits and mapped a young lava flow. We started the dive in an area where a gravity core had recovered sulfide debris. The region, located on top of a 50-m thick sill, had numerous small to moderate sized sulfide deposits, including some chimneys that had not collapsed. We collected a number of samples and headed towards a basin between two lobes of the sills in the region. The edge of the sill has slumped and exposed the underlying lavas along slope-parallel faults. The same region had yet more sulfide deposits.

As we drove up the northern slope of the basin, we encountered more basalt outcrops and still more sulfide deposits, some quite large. Across the top of the nearly flat sill, sediment covers most of the lavas, but occasional outcrops of sheet flows and pillows protrude above the sediment. At the eastern edge of the sill, fractures similar to those around the basin expose fractured sediments, but no basalt or sulfide deposits were found.

At the base of the eastern edge of the sill, we encountered a flow that is exposed as fractured and broken pillow lavas. We continued to the northeast up this flow and crossed a fissure about 10 m wide that exposed basalt as far down as we could see. Continuing upslope to the northeast, the flow morphology changes from pillows to lobate and sheet flows. A brief exploration of the base of an uplifted sediment hill did not locate any additional sulfide deposits. We finally traced the flow up to its eruptive fissure near the base of one of the large uplifted sediment hills and then returned to a large sulfide deposit discovered on a SeaCliff dive in 1988.

We recovered additional sulfide samples and ended the dive. Along the way we recovered 29 samples of lava and sulfide and one unknown biological sample. We also filmed a remarkable animal swimming about 5 m off the bottom.

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