REGIONAL GEOLOGICAL SETTING OF PROPOSED SITES
Hole 801C
Although located almost 1000 km from the Mariana Trench, Hole
801C is the most promising site for penetrating Jurassic MORB in
the region. Throughout much of the Pigafetta and East Mariana
Basin (Fig. 2),
"basement" consists of Cretaceous flows and sills that
overlie the "normal" Jurassic crust. Because these
Cretaceous units have already been sampled by Leg 129 drilling,
the remaining goal is the MORB section. Hole 801C is the only
location where Jurassic-aged material (165 Ma) was reached in a
reasonable amount of drilling time, and that material should be
essentially the same as what is now being subducted beneath the
Marianas (also fast spreading and within 15 m.y. in age). It is
necessary to penetrate several hundred meters into the upper
oxidative layer of Jurassic basement to constrain that part of
the crustal input equation, and that section is now available
directly beneath the bottom of Hole 801C. Hole 801C was left
clean with a reentry cone that is cased and cemented into
basement, and is ready for more extensive basement drilling.
Above basement, the 460 m sedimentary section is dominated by pelagic clays and cherts, with notable intervals of volcaniclastic material from the mid-Cretaceous, derived from the nearby Magellan Seamounts. Specifically, the sedimentary stratigraphy at Site 801 consists of a thin pelagic brown clay unit (64 m, Cenozoic-Campanian /Maestrichtian?) overlying 63 m of brown chert and porcellanite (Campanian-Cenomanian) overlying 192 m of volcaniclastic turbidites (Cenomanian-Albian) overlying 125 m of brown radiolarite (Valangian-Oxfordian), with a basal unit of alternating red radiolarite and claystone (19 m, Callovian). Further background on Site 801 can be found in Lancelot, Larson, et al. (1990).
Site
BON-8A
Site BON-8A is ~60 km east of the Izu-Bonin Trench, where the
plate surface is broken by normal faults as it bends into the
subduction zone (Fig. 7). Avoiding some of
this complexity, Site BON-8A is located on the top of a fault
block in flat-lying sediments. Coring the complete sedimentary
section as well as the altered oceanic crust at Site BON-8A will
sample the main crustal components that enter the Bonin trench.
Based on our assessment of regional variations in subducting
sediments elsewhere (Plank and Langmuir, 1998), we feel confident
that a single reference site will provide adequate constraints on
the crustal inputs to the Izu-Bonin Trench. Even though the
sedimentary stratigraphy will vary regionally, changes in unit
thickness can be mapped more efficiently seismically than with
multiple drill holes. Site BON-8A should provide us with samples
of the largely pelagic sediments from the region. Of the ~470 m
of sediment, we anticipate 170 m of pelagic clay and volcanic arc
ash above 300 m of mid- to Early Cretaceous cherty porcellanites
and chalks (Fig. 7). The basement
should be Early Cretaceous MORB (135 Ma), with the upper 200-300
m of extrusives containing the oxidative alteration zone.