Seafloor structure and uppermost sedimentation in the
Pigafetta Basin, Magellan Seamount Chain, and East Mariana Basin
of the Central-Western Pacific
Mathews, N. J.; Oakley, A. J.; Lizarralde, D;
Tominaga, M.; Tivey, M.; Sager, W., 2013, Seafloor structure and
uppermost sedimentation in the Pigafetta Basin, Magellan Seamount
Chain, and East Mariana Basin of the Central-Western Pacific, Eos
Trans. AGU, Fall Meet. Suppl., San Francisco, CA
We conducted a marine geophysical survey of the Central-Western
Pacific seafloor in 2011 aboard the R/V Thomas G. Thompson (TN272).
Our survey imaged some of the oldest seafloor on the planet in a
region of sparse data coverage. We present new (3.5 kHz) and
bathymetry data from the Mesozoic Hawaiian magnetic lineations
(Jurassic Quiet Zone) and a transect from the south end of the
Pigafetta Basin (PB), west across the Magellan Seamount Chain (MSC)
and the East Mariana Basin (EMB) to the Mariana Trench. The Chirp
system penetrates the overlying sediment cover to a depth of ~50
meters below seafloor (mbsf). The deepest part of the Chirp record
is marked by a strongly reflecting horizon, which occasionally crops
out at the seafloor near volcanic peaks or bathymetric highs.
Correlation of these data to DSDP/ODP drill sites (801C, 802, 199,
585) enables us to compare seafloor structure and uppermost
sedimentation in the Jurassic Quiet Zone (JQZ), Pigafetta Basin,
Magellan Seamounts, and the East Mariana Basin.
Within the JQZ, and at the southern end of the PB, a 30- to
50-m-thick, acoustically-transparent sediment layer uniformly
overlies the strongly reflecting horizon observed at water depths
ranging from 4400m-5900m. We interpret this unit to be composed of
pelagic abyssal clay and radiolarian oozes ubiquitous in the
central-western Pacific. The basal horizon in this region is most
likely formed by a layer of chert-porcelanite. In the southeastern
PB (161.3 E, 17 N), this horizon reaches the seafloor at a depth of
5650 m and the transparent sediment package is truncated.
In the region of the MSC, ~115 km north of drill sites 199 and 585,
Chirp data show numerous volcanic peaks and a 50-m-thick sequence of
stratified reflections. Volcaniclastics likely contribute to the
layering. We first observe the stratified sediment package near
156.7 E, 15 N after a gap in data coverage. The sediment layer thins
to the west and onlaps the basal horizon near the base of a
seamount. Based on its depth, we interpret the basal horizon in this
region of the survey to be formed by interlayered nannochalk,
limestone, tuff, and chert.
In the EMB, the Chirp data image several small, sediment-filled
basins bounded by bathymetric highs formed by both faulting and
volcanism. There are also volcanic intrusions visible. Along a
broad, sloping bathymetric high centered on 151.9 E, 14.5 N, a
~15-m-thick transparent sediment layer of abyssal clay is observed
above a ~35-m-thick stratified section, possibly volcaniclastic
turbidites. Based on data from site 802 (~350 km to the south) we
interpret the basal horizon in this region to be formed by Miocene
tuff. At the western edge of the EMB, <250 km from the base of
the Mariana Trench, we observe several normal faults with offsets
from 5-45 m.
In both the PB and EMB, pelagic clay composes the youngest sediment
layers, whereas in the MSC, there is evidence of more recent
volcanic activity. Overall, abyssal clay thickness decreases toward
the trench, as crustal age decreases. The shipboard Chirp data
provides valuable information about seafloor deformation and shallow
sedimentation in a region of sparse data.
