Site U1394 |
Site U1395 |
Site U1396 |
Site U1397 |
Site U1398 |
IODP Expedition 340:
Lesser Antilles Volcanism and Landslides
Site U1393 Summary
PDF file is available for download.
Background and objectives
Ocean Drilling Program (IODP) Site U1393 (CARI-02C; 16°43.13'; 62°5.06'W; 914 m
below sea level [mbsl]) is located close to the Soufrière Hills volcano at
Montserrat (13.7 km from point Shoerock, the SE tip of Montserrat).
The on-going eruption of the Soufrière Hills volcano at Montserrat started in 1995.
Activity has included lava dome growth, pyroclastic flows from dome collapse,
explosive activity with tephra fall and pumice flows, flank collapse with
debris avalanches and volcanic blasts. More than >70% of erupted material
from the on-going eruption has been transported to the sea (Le Friant et al.,
2009, 2010; Trofimovs et al., 2006). The rapid entrance of volcanic material
into the sea has caused smaller tsunamis.
The site survey data obtained for Site U1393
show that the distal parts of the pyroclastic flows as well as some underlying,
older debris avalanches have been deposited at this site. The English's crater event, which occurred ~2000 years
ago, produced deposit 1. The debris avalanche deposit 2 probably resulted from
a combined submarine and subaerial flank collapse of the eastern flank of the
volcano, as well as sediment failure. The site survey seismic data indicate that Site U1393 might penetrate through the erupted material from the on-going eruption and into the underlying debris avalanche deposits 1 and 2.
The objective of Site U1393 is to characterize the
processes occurring during debris avalanche emplacement, associated erosional
processes and tephra diagenesis. Analysis of 5 m piston cores taken in this
area show that the pyroclastic material from the 2003 Soufrière
Hills volcano dome collapse
mixed with seawater and immediately deposited the coarse components out of the
suspension (Trofimovs et al., 2006). The coarse debris avalanche deposit will
enhance our understanding of the emplacement processes.
Comparing the geochemical
signatures (pore waters and sediments) of cored material with surface sediments
(from the 2007 cruise) will allow us to characterize the alteration rates of
volcanic material in seawater. In addition, we will examine the dependency of
alteration rate and style on grain size, layer thickness, admixture of
Coring through both the smaller debris
avalanche deposit 1 and the larger volume deposit 2 will allow comparing the
emplacement processes of debris avalanches of different magnitudes. We will
undertake a detailed lithological, sedimentological, and textural fabric
analysis of the retrieved material at the macro- and microscopic scale to
investigate the transport and deposition processes, the nature and magnitude of
erosional processes and interaction with the substratum (e.g., bulking;
Komorowski et al., 1991; Glicken, 1991, 1996). These data will provide valuable
insights into the chronology (one or several pulses) and the debris avalanche
mobility, which have implications for tsunamigenesis.
was planned to core two holes at Site U1393 and recover as much of the young
debris avalanche deposit called deposit 1 of the Soufrière-Hills
volcano on Montserrat as possible. However, due to the unfavorable
drilling conditions encountered at this site only one hole was drilled. The
hole was cored to a depth of 47.55 mbsf with an 117/16"
diameter APC/XCB core bit and a 135.75 m long bottom hole assembly (BHA). A mud
line core with the APC system established a water depth of 926 mbsl. The second
APC core bounced off the formation and the APC system was changed over to the
XCB coring system. Coring conditions proved to be very difficult and recovery
was very poor. Coring was terminated when the XCB system failed, leaving part
of the XCB core barrel in the hole. The depth objective at the site was 250 m
and this appeared to be unreachable with the tools planned for the site.
Overall core recovery for Site U1393 was 11.4% of the 47.5 m cored (5.42 m of
Based on the lithological characteristics the sediments recovered at Site U1393 only one
lithostratigraphic unit was defined. This unit is termed Unit 1. Unit 1 extends
from the seafloor to a depth of 4.24 mbsf, however, the lower stratigraphic
boundary of this unit cannot be defined due to the poor core recovery. The
upper part of Unit 1 consists of mud clasts embedded in a sandy matrix. This is
followed by dark brownish gray-black sand consisting of volcaniclastic material
containing medium to very coarse sand-sized grains. The grains are mainly
composed of andesite and rarely of carbonate. Occasionally larger clasts of
amphibole-rich andesite are present. The unit is moderately well to well
sorted, massive and normally graded. The lowest part of Unit 1 consists of
andesite clasts up to 3 cm in size embedded in a coarse sand matrix. Below Unit
1 the material recovered consists mainly of andesite clasts. The clasts show
variable signs of hydrothermal alteration and subaerial oxidation. These clasts
probably are derived from the deposit 1 debris avalanche deposit. The upper
4.24 m of the material cored are a product of the most recent eruption of the Soufrière-Hills volcano.
Generally, the cores retrieved at this site contain only very few micro- and nannofossil remains, which is consistent with the lithostratigraphic information we have for this site. Due to the small number of fossil remains an age determination of the cored material was not possible. The majority of the observed foraminifers are typical for a reef environment shallower than 30 m water depth, suggesting a re-deposition caused by the debris flow.
The physical property data obtained on
the cored material, display the behavior expected coring moderately to
well-sorted sands with an andesitic bulk composition. The measured grain
density of 2.80 g/cm3 and the measured bulk porosity of 41% are
consistent with an andesitic composition and medium to well-sorted sand that
has undergone little consolidation, respectively. The magnetic susceptibility
(maximum value of 3.75 x 10-2 at a depth of 1.07 m) as well as the
natural gamma radiation (NGR, 13 counts/second) is much higher than for pelagic
sediment, consistent with sediments dominated by volcaniclastic particles. Meaningful
paleomagnetic directions for the interpretation of the geomagnetic field at Site
U1393 could not be obtained. This is mainly due to the grain size and the
consolidation stage of the material recovered. Sand sized grains are usually
comprised of multidomain size magnetic grains, which are inefficient at
recording paleomagnetic directions and unconsolidated sands are usually highly
sensitive to disturbances. The grains themselves are often too large to
orientate the Earth's magnetic field.
Glicken, H.X. (1991) Sedimentary
architecture of large volcanic-debris avalanches, in Fisher R.V., and Smith
G.A., eds, Sedimentation in volcanic settings: SEPM (Society for Sedimentary Geology) Special Publication, 45, 99-106.
Glicken, H.X. (1996) Rockslide-debris
avalanche of May 18, 1980, Mount St. Helens Volcano, Washington. US Geol. Surv.
Open File Rept. US Geol Surv., Washington, DC, 98pp.
Komorowski, J-C, Glicken, H, Sheridan,
M.F. (1991) Secondary electron imagery of microcracks and hackly fracture
surfaces in sand-size clasts from the 1980 Mount St. Helens debris-avalanche
deposit: Implications for particle-particle interactions. Geology 19, 261-264.
Le Friant, A., Deplus, C., Boudon, G.,
Feuillet, N., Trofimovs, J., Komorowski, J-C., Sparks, R.S.J., Talling, P.,
Loughlin, S., Palmer, M., Ryan, G. (2010) Eruption of Soufrière Hills
(1995-2009) from an offshore perspective: Insights from repeated swath
bathymetry surveys. Geophysical Research Letters 37, L11307, doi: 10.1029/2010GL043580.
Le Friant, A., Deplus, C., Boudon, G., Sparks,
R. S. J., Trofimovs, J., Talling, P., (2009) Submarine deposition of
volcaniclastic material from the 1995–2005 eruptions of Soufrière Hills volcano, Montserrat. Journal of the Geological Society 166, 171-182: doi: 10.1144/0016-76492008-047.
Trofimovs, J., Amy, L., G. Boudon, C.
Deplus, E. Doyle, N. Fournier, M.M.B. Hart, J-C Komorowski, A. Le Friant, E.
Lock, C. Pudsey, G. Ryan, R.S.J Sparks, P.J. Talling (2006) What happens when
pyroclastic flows enter the ocean? Geology 34, 549-552.