Preliminary scientific assessment

Expedition 340 involved drilling in marine sediment and volcaniclastic material at nine sites located off the islands of Montserrat and Martinique, The overarching aim of the Expedition is to reach a better understanding of the constructive and destructive processes occurring along the Lesser Antilles volcanic arc. Two holes were planned for each of the nine sites, and logging was planned for eight sites. The primary objectives of this expedition were

  1. To drill through the chaotic units (as identified in the seismic data) interpreted as mass wasting deposits and to better understand their composition, origin, and relationship to on-land volcanic flank-collapse events;

  2. To core as many tephra layers as possible for tephrochronology studies designed to reconstruct the history of the volcanoes of both Montserrat and Martinique and the long-term magmatic evolution of the arc; and

  3. To retrieve a complete sediment record from each of the sites to study the sedimentation processes occurring along the entire volcanic arc.

Sites U1393–U1396 were located in the northern part of the arc, around Montserrat. Sites U1397–U1401 were located in the southern part of the arc close to Martinique.

Sites U1396 and U1397 were chosen to study the magmatic evolution and eruptive history based on tephrochronology of Montserrat and Martinique, respectively. We achieved our depth and coring objectives at Site U1396 (total depth of 139 mbsf, 296 m of material, and 104% recovery) and partly at Site U1397 (total depth of 260 mbsf, 276 m of material, and 54% recovery). Sites U1393, U1394, and U1399–U1401 were dedicated to the study of mass wasting deposits and associated erosional processes. Drilling proved to be extremely difficult in these chaotic, heterogenous formations. However, although core recovery was highly variable, ranging from 11% to 98% (Table T1), we were able to recover enough material to study the processes occurring during the emplacement of such deposits. Sites U1395 and U1398 were dedicated to the study of the distal sedimentation processes associated with the deposition of mass wasting deposits. In total we cored 284 m (65% recovery) of material at Site U1395 and 302 m (57% recovery) at Site U1398. Despite reaching our depth objectives at both sites in at least one of the holes, we did not meet our coring objectives below ~120 mbsf because of the poor recovery associated with the XCB. Nonetheless, the cored material should be sufficient to conduct the intended scientific studies. Logging was planned for the Hole B at each site, except for Site U1396, which, at 135 mbsf, was too shallow for downhole logging. Logging was successfully conducted at Sites U1394, U1395, U1397, and U1399. Because of unfavorable hole conditions, logging was not possible at Sites U1398 and U1400.

Despite the significant difficulties in drilling through the highly heterogeneous sediment, the expedition can be considered a success and the initial objectives were achieved. A total of 2384 m of core was recovered. We thank the drillers for their unrelenting efforts to drill through such difficult volcaniclastic material and all R/V JOIDES Resolution staff for their collaboration and efficient work on board.

The cores recovered during IODP Expedition 340 are very promising. Postcruise research will provide crucial information to better constrain the evolution of the Lesser Antilles volcanoes. Correlation of the chaotic deposits identified on seismic reflection profiles with sequences of turbidites or deformed marine sediment will lead to a better understanding of processes related to the instabilities of the Lesser Antilles volcanoes, as well as provide new perspectives for studies of similar volcanic settings.

1. Mass transport deposits: identify the mechanisms controlling processes and timing of potentially tsunamigenic large mass transport deposit emplacement.

Sites U1393 and U1394 were dedicated to the study of debris avalanche deposits offshore Montserrat. Coring conditions at Site U1393 were very difficult with poor recovery caused by the unconsolidated nature of the material being cored. The chaotic deposit was successfully cored at Site U1394 with some unexpected results. Based on seismic reflection profiles and previously published criteria (Moore et al., 1989; Urgeles et al., 1997; Deplus et al., 2001; Le Friant et al., 2004; Lebas et al., 2011; Watt et al., 2012), the chaotic unit has been interpreted as a debris avalanche deposit. However, the chaotic unit is dominated by a stacked sequence of predominantly thick, massive, relatively coarse grained turbidites that range in composition from bioclastic to volcaniclastic. The absence of planar or ripple cross-laminations in the upper part of the deposit suggests rapid deposition. Randomly distributed clasts in the lower part of the chaotic unit may suggest rapid emplacement by a debris flow. Overlying the chaotic unit are hemipelagic sediment and a series of basaltic tephra layers associated with the South Soufrière Hills Volcano (Harford et al., 2002). This gives a minimum age for the unit of ~138 k.y. However, to fully understand the emplacement processes of the turbidites and their association with the hummocky debris avalanche deposits observed on the upper submarine slope further investigations are required. Among these is the need for correlation between existing seismic data and the cored material. The physical properties data obtained correlate well with the different lithologies recovered. These measurements show that the hemipelagic mud is generally characterized by low values of magnetic susceptibility and P-wave velocity but relatively high values of NGR, with a generally low degree of variability of those properties. In contrast, the physical properties of the turbidites are relatively heterogeneous, with overall high values of magnetic susceptibility and P-wave velocity but low values of NGR and bulk density.

Three sites (U13999, U1400, and U1401) were dedicated to investigating the chaotic deposits offshore Martinique. Coring conditions at Site U1399 were very good. The first hole was terminated at a total depth of 275 mbsf, whereas the second hole had to be abandoned at 183 mbsf because of unstable hole conditions. At Site U1400 we cored three holes (U1400A, U1400B, and U1400C). Holes U1400A and U1400B were terminated at 51 and 213 mbsf, respectively, because of technical problems, whereas U1400C was terminated at 436 mbsf, reaching the depth objective for this site. Coring conditions at Site U1401 also proved to be difficult. Consequently, a series of short holes were cored instead of one longer hole. The objective at this site was to trace the evolution of the upper 15 m of material covering the chaotic unit along a ~1 km transect. Sediment cored at all three sites is dominated by hemipelagic mud with interbedded tephra and volcaniclastic turbidite layers of variable thickness. The sediment recovered at Sites U1399 and U1400 shows signs of severe deformation. Inclined and convoluted banding of hemipelagic sediment combined with deformed tephra or turbiditic layers were observed throughout almost the entire cored interval. Identifying the origin of these deformed units and their relationship with aerial flank-collapse events observed on land (retrogressive sediment failures, submarine slides, etc.) forms a major part of planned postcruise research. Timing of the event that deformed the sediment will be done by 18O dating of the undisturbed sedimentary sequences above and below the chaotic deposit. The study of the processes resulting in the observed fundamental differences between the chaotic deposits sampled offshore Montserrat and offshore Martinique (broadly turbidites versus deformed sediment) will also be part of postcruise research. Most likely these differences are related to the emplacement mechanism and the size of the different debris avalanche events, as well as the structure of the sedimentary basin surrounding the islands.

One of the main objectives of this expedition was to better characterize the seismic velocities of the different materials deposited around the Lesser Antilles (e.g., marine sediment versus volcaniclastic material). P-wave measurements from both whole-round sections and half sections show velocities ranging from 1500 to 1900 m/s. Generally, higher velocities (1650 to >1800 m/s) are obtained from the volcanic layers, whereas lower velocities are obtained from the hemipelagic background sediment (1550 to 1650 m/s). Velocities obtained during the vertical seismic profile (VSP) experiment conducted in Hole U1399C are generally higher, ranging from 1997 to 2072 m/s. These results will help improve estimates of the thickness of the chaotic units, which had previously been made using velocities between 1800 and 2200 m/s. Data will be processed on shore and will help provide constraints on the seismic stratigraphy of the area. Values could also be applied to seismic profiles obtained at other volcanic areas where similar chaotic deposits have been observed.

2. Tephrochronology studies: characterize the eruptive history, magmatic cycles, and long-term evolution of the arc.

Site U1396 was dedicated to the tephrochronology study of Montserrat and was successfully cored to 135 mbsf (Hole U1396A) and 139 mbsf (Hole U1396C). Material cored at this site is dominated by hemipelagic sediment with interbedded tephra layers and volcaniclastic sands. More than 180 visible tephra layers were identified, ranging from <1 to >10 cm in thickness. However, there may be many more crypto-tephra layers embedded within the hemipelagic mud. The magnetostratigraphic record obtained from the cored material is in excellent accordance with biostratigraphic observations. According to the magnetostratigraphy, the basal age of Hole U1396C (139 mbsf) is ~4.62 Ma; biostratigraphy places the basal age >4.48 Ma. The highest CaCO3 concentration observed in hemipelagic sediment is 72 wt% with most samples being much lower than this, indicating that a considerable amount of volcanic material is dispersed throughout the cores, despite the fact that it is not necessarily visible to the naked eye. The high recovery (104%) and good correlation between both holes mean that this site represents one of the most important time records for this expedition. The work planned for the material recovered at this site includes petrological, geochemical, magnetostratigraphic, and biostratigraphic studies to characterize the entire eruptive history of Montserrat. In addition, tephra layers obtained from Site U1395 will be correlated, by geochemical fingerprinting, with tephra layers from Site U1396.

Site U1397 was dedicated to the tephrochronology study of the volcanoes of Martinique and Dominica. Although both holes were drilled >250 mbsf, a continuous stratigraphic record was recovered for only the uppermost 120 m. The cored material consisted of combinations of hemipelagic mud, volcaniclastic or mixed (volcaniclastic-bioclastic) turbidites, and abundant tephra layers. At least 200 tephra layers (coming mainly from the Montagne Pelée Volcano) were recorded in the uppermost 28 m at this site. The turbidites contain variable amounts of fresh pumice in a crystal-rich matrix containing very low proportions of both carbonate material and lava clasts. Below 167 mbsf a larger block composed of andesitic lava containing large phenocrysts of amphibole and quartz was recovered. This lava originates from the Pitons du Carbet Volcano on Martinique and has been dated on-land at ~330–350 k.y. This age provides a maximum age for the base of the site. This corresponds well with biostratigraphic datums that assign the base of this site to ~400 ka or younger. Numerous tephra layers were also recognized at Sites U1398, U1399, U1400 (especially in the upper part), and U1401, representing material from the more recent history of the volcano. Postcruise research will include a full petrological, geochemical, and granulometric characterization of all identified tephra layers as well as an interhole correlation, based on geochemistry, of the tephra layers. In a final step, these tephra layers will be traced back to possible eruptions recorded on land both in Martinique and Dominica. With the cored material we should be able to reconstruct the entire history of the Montagne Pelée Volcano, which will be important to assess major volcanic hazards as well as volcano evolution.

Volcanoes from the northern and southern parts of the arc have marked differences in chemical composition. Postcruise analyses of the tephra and volcaniclastic turbidites obtained from the north to the south part of the arc will shed light on the evolution of the Lesser Antilles arc.

3. Sedimentation processes: characterize sedimentation processes along the deep backarc Grenada Basin.

Three sites were dedicated to studying basin sedimentation processes: Sites U1395 and U1396 in the Bouillante–Montserrat half Graben between Montserrat and Guadeloupe and Site U1398 in the backarc Grenada Basin. Cores recovered at Site U1395 consisted of hemipelagic mud with intercalated turbiditic sand and tephra. The majority of turbidites probably come from Montserrat. However, in the lower part (~90–100 mbsf), interbedded in the background sediment, are numerous layers of sand-sized volcanic material consisting of lava fragments, scoria, plagioclase, pyroxene, and, rarely, amphibole. This composition is characteristic for tephra deposits originating from volcanoes in the central part of Guadeloupe. The magnetostratigraphy may indicate a polarity change indicative of the beginning of the normal Jaramillo Chron (1.070 Ma) around 101 mbsf (Hole U1395A) and 116 mbsf (Hole U1395B). This polarity change not only suggests sedimentation rates on the order of 8 cm/k.y. above ~110 mbsf and slightly higher rates below this depth but also indicates that we are able to study a continuous sedimentation record over 1 m.y. The sedimentation record for Site U1396 reaches as far back as ~4.62 Ma, as indicated by the magnetostratigraphic and biostratigraphic studies done at this site. Using the GPTS of Cande and Kent (1995), nine periods of normal polarity and nine periods of reversed polarity could be identified for this site. The earliest polarity reversal we see is the beginning of Chron C3n.1n (4.29 Ma) at 129.4 mbsf in Hole U1396A, giving this hole a basal age of 4.29–4.48 Ma. The longer Hole U1396C record contains the end of Chron C3n.2n (4.48 Ma) at 4.48 Ma, giving this hole a basal age of 4.48–4.62 Ma. Average sedimentation rate for this site is ~3.1 cm/k.y. However, sedimentation rates vary over the observed depth interval cored at this site. Pliocene sedimentation rates are on the order of 4 cm/k.y., whereas Pleistocene rates are ~1.7 cm/k.y. and rates from the base of the core to the beginning of the Gauss Chron (3.58 Ma) are ~5.3 cm/k.y.

Site U1398 is located in the backarc Grenada Basin. The upper part of the site is dominated by volcaniclastic turbidites, whereas the lower portions are composed of various combinations of hemipelagic sediment with intercalated volcaniclastic turbidites and tephra. Abundant pumice clasts are common throughout the upper parts. XRD data obtained from discrete samples throughout the cores show that quartz and plagioclase dominate the volcanic material, whereas the marine sediment is dominated by calcite and lesser amounts of aragonite. Postcruise research will help to distinguish deposits from Dominica and Martinique by comparison with on-land products and geochemical fingerprinting. Based on detailed biostratigraphic studies, the cored material could be assigned to the late Pleistocene, indicating extremely high sedimentation rates in comparison with Site U1395. However, reworking of much older material is evident in several studied samples.

With the long-reaching sediment record obtained at these sites we will be able to conduct the desired research. Postcruise studies aim at comparing the sedimentation processes in the north and the south parts of the arc, correlating turbidites with volcanic activities (composition, petrology, geochemistry, and granulometry), refining the magnetostratigraphic-based chronology, and undertaking a complete provenance analysis of the hemipelagic sediment recovered.