Magazine article Oceanus

DSDP/ODP Downhole Measurements in Hole 504B

Magazine article Oceanus

DSDP/ODP Downhole Measurements in Hole 504B

Article excerpt

The Ocean Drilling Program's continuous downhole measurements have become essential to seafloor exploration. (See "Borehole Measurements Beneath the Seafloor," page 129 for a description of downhole measurements.) They supplement and verify information obtained from core studies and often provide data in sections where cores are not recovered. Over the past 10 years, these methods have steadily become more important to research on the structure and dynamics of Earth's upper crust.

DSDP/ODP Hole 504B, by far the deepest hole yet drilled into the oceanic basement, illustrates the importance of downhole measurements to earth science. The work done in 504B provides the scientific community with an excellent means for verifying models of the upper oceanic crust's structure and evolution.

Hole 504B is located beneath 3,475 meters of eastern equatorial Pacific water and penetrates ocean crust assembled on the southern flank of the Costa Rica rift. Beginning with DSDP Leg 69 in 1978, eight ocean drilling expeditions have been dedicated to drilling Hole 504B, to reach the present depth of 2,111 meters below the seafloor. Beneath 275 meters of sediments, including pelagic oozes and chert, 1,836 meters of basaltic basement has been cored, with recovery often less than 20 percent. The basaltic section comprises about 600 meters of pillow lavas and massive lava flows extruded 5.9 million years ago on the seafloor at the rift spreading center. Below the basalt layer lies a transition zone that leads to a 1,200-meter-thick section of sheeted dikes, solidified conduits from the magma chamber to the seafloor. As the gabbros (the prime objective of the last two drilling legs) were unfortunately not reached, downhole measurements covering the entire basement section were recorded.

The data collected from downhole measurements can generally be classified into two main categories associated, respectively, with the structure and the dynamics of the penetrated section. That related to structure reveals, either in terms of physical properties or lithostratigraphy, continuous data around the drillhole, generally at meter scale. From within the borehole, dynamic parameters reveal information at kilometer scale for mapping present or past fluxes, as well as force fields such as those associated with tectonic stresses.

Probably the most important finding in Hole 504B downhole measurements is a strong downflow of sea-bottom water into the upper basement. This vigorous flow was first discovered in 1979 as the hole was being re-entered during DSDP Leg 70. Scientists were surprised to observe a temperature profile showing 2 |degrees~ to 3 |degrees~ C water down to 300 meters, a few tens of meters into basement, where they expected to see water at 60 |degrees~ C. This was the clue for the downflow. Flow experiments with downhole packers, combined with geophysical measurements of electrical resistivity, revealed the presence of a 30-meter-thick, porous, permeable and underpressured aquifer located under a 14-meter-thick massive sheet flow of basalt at 300 meters. Since that time, temperature data are routinely recorded first whenever the hole is being re-entered for deepening and downhole experiments.

The temperature profile taken at the beginning of Leg 111 in 1986 showed that ocean-bottom water was still flowing into the aquifer, proving the large extent of this underpressured reservoir and, at the same time, the similarly large extent of its basalt seal.

The seal must have originated in a massive outpouring of basalt onto the seafloor near the ridge axis, an eruption different in many ways from somewhat more classic modes of volcanism that lead to emplacement of pillow lava. …

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