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doi:10.2204/iodp.proc.340.107.2013 LithostratigraphyTwo holes were drilled at Site U1397: Hole U1397A was drilled to 265 mbsf, and Hole U1397B was drilled to 255 mbsf. In both holes a good stratigraphy can be established to 120 mbsf. Below this depth, large parts of the section were not recovered, and it was not possible to establish a detailed stratigraphy. Sediment recovered at Site U1397 was divided into eight lithostratigraphic units (A–H). Each unit consists of various combinations of hemipelagic muds, volcaniclastic or mixed (volcaniclastic-bioclastic) turbidites, and tephra fallout. Each lithology is described in detail “Lithostratigraphy” in the “Site U1394” chapter (Expedition 340 Scientists, 2013b). Good correlations can be established between the two holes, and the depths of the correlated units are similar. We observed a larger proportion of turbidites and a smaller proportion of tephra in some of the units in Hole U1397B than were observed in Hole U1397A. It is likely that most of the missing tephras were eroded during the emplacement of the turbidites. As with previous sites, boundaries for each of the lithostratigraphic units are defined by abrupt or gradational changes in the abundance of lithologies, distinctive marker horizons, or different mineralogical compositions of tephra and unusually thick turbidites. Unit A
Unit A extends from 0 to 28 mbsf in Hole U1397A and from 0 to 26 mbsf in Hole U1397B. The top of Unit A is marked by a 13 cm thick, oxidized brown hemipelagic mud that contains very fine sand layers and has high water content. Below this, the main part of Unit A is composed of a thick sequence of gray, muddy hemipelagic sediment with abundant interbedded volcaniclastic sand layers. Most of these sand layers, massive or normally graded and always sorted, are interpreted as tephra fallout deposits; only three thicker unsorted layers are recognized as turbidites. The number of visually identified tephra layers is ~200. There are probably more tephra layers, as some of the layers, which are ≤50 cm thick, are the result of successive eruptions. These successive eruptions form stacked tephra layers that are not separated by hemipelagic mud. We observed good correlation between Holes U1397A and U1397B, and the base of Unit A is at 28 and 26 mbsf, respectively. The only significant difference in Unit A between the two holes is the presence of slightly thicker turbidite sequences in Hole U1397B. Based on the mineralogical composition of the tephra (presence or absence of amphiboles), we can identify their origin. Most of the tephras are from Montagne Pelée (Martinique), whereas the others are from the volcanoes of Dominica. Dominica is located north of Site U1397. The stratigraphy recognized in the uppermost part of Unit A is identical to the stratigraphy established on the basis of previous studies done on a 7 m long piston core (Boudon et al., submitted). Unit B
Unit B extends from 28 to 53 mbsf in Hole U1397A and from 26 to 53 mbsf in Hole U1397B and is composed of a thick sequence of interbedded hemipelagic mud, tephra, and volcaniclastic turbidites. The proportion of turbidites is higher than that found in Unit A. Most of the turbidites are 1–3 m thick, are normally graded, and contain white, nonaltered pumices in proportions from a few percent to >40%. The turbidite matrix is composed of a high proportion of crystals, a very low proportion of carbonates, and lava clasts (massive to poorly vesiculated). The low proportion of carbonate in these turbidites indicates that they are associated with explosive volcanic events, either by direct emplacement of pyroclastic flows or by remobilization of pyroclastic flow deposits. Most of the tephra layers are dark and fine-grained and are frequently normally graded. They resemble the tephra layers in Unit A, except for two or three thick layers that are several tens of centimeters thick and contain abundant pumice granules. These thick and coarse tephra originate from large plinian eruptions of the neighboring islands. Unit C
Unit C extends from 53 to 77 mbsf in Hole U1397A and from 53 to 78 mbsf in Hole U1397B and is separated into two subunits (C-1 and C-2). Subunit C-1Subunit C-1 (53– 74 mbsf in Hole U1397A and 53–72 mbsf in Hole U1397B) is dominantly composed of thick mixed (bioclastic-volcaniclastic) turbidites. The proportion of biogenic and detrital carbonate grains can vary from 40% to 80%. Volcanic components are dominated by crystals, massive lava, and, in some cases, pumice in low proportions. One of the turbidites is 5 m thick and massive, but it shows an evolution from the base to the top from dark gray to light gray corresponding to a decreasing proportion of volcanic material. At the base of Subunit C-1, two volcaniclastic turbidites are recognized, one rich in pumice. The proportion of hemipelagic mud is very low between turbidites, and intervening hemipelagic mud is frequently absent. Subunit C-2Subunit C-2 (74–77 mbsf in Hole U1397A and 72–78 mbsf in Hole U1397B) comprises a debris flow deposit and a section of deformed sediment (340 cm and 520 cm thick, respectively, in Holes U1397A and U1397B). The deformed sediment contains abundant, highly contorted mud and lava clasts. The debris flow deposit contains very abundant deformed mud clasts. This debris flow facies comprises the main part of this subunit in Hole U1397A (320 cm) and is 160 cm thick in Hole U1397B. Subunit C-2 can be interpreted as a slump of submarine material, including sediment and a debris flow. Unit D
Unit D extends from 77 to 91 mbsf in Hole U1397A and from 78 to 95 mbsf in Hole U1397B and is composed of a series of volcaniclastic turbidites and few tephra layers interbedded with hemipelagic mud. Tephra layers are better represented in Hole U1397A than in Hole U1397B. These layers are located both at the top and base of the unit where two successions of ~20 tephra layers are observed. Turbidites are the dominant component of Hole U1397B. Hemipelagic sediment is in very low proportion, attesting to the important erosional character of the successive turbidites. In Hole U1397A, a 1 m thick zone of deformed sediment is present. This zone is not observed in Hole U1397B. Unit E
Unit E extends from 91 to 150 mbsf in Hole U1397A and from 95 to 150 mbsf in Hole U1397B and is composed of a series of thick massive to normally graded volcaniclastic turbidites. These turbidites contain a large proportion of massive to poorly vesiculated lava. They represent 95% of the unit, with hemipelagic mud only representing 5%. No tephra layers are observed. Unit E is observed in Holes U1397A and U1397B approximately at the same depth. The base of Unit E is not observed. Below the depth of 122 mbsf in Hole U1397A and 115 mbsf in Hole U1397B, the stratigraphy is more uncertain because core recovery was very low (<10%). No sediment was recovered between 122 and 150 mbsf in Hole U1397A or between 115 and 150 mbsf in Hole U1397B. Unit F
Unit F is observed between 150 and 168 mbsf in Hole U1397A and between 150 and 160 mbsf in Hole U1397B. Unit F can be divided into two subunits (F-1 and F-2) on the basis of sediment compaction. Subunit F-1 (150–163 mbsf) is composed of a package of hemipelagic muds with a few interbedded tephra layers. This subunit is thicker in Hole U1397A than in Hole U1397B. Subunit F-2 (163–168 mbsf) is composed of the same sequence of sediment types as seen in Subunit F-1; sediment is weakly compacted at the top and progressively lithified into mudstones. Only two tephra layers are identified in Hole U1397A. Abundant bioturbation characterizes the mudstones. Unit G
Unit G extends from 168 to 230 mbsf in Hole U1397A and from 160 to 247.5 mbsf in Hole U1397B (bottom of the hole) and is composed of mud-rich sandstone layers and semiconsolidated, highly fractured, and contorted mudstones. This deformed unit comprises beds with different colors, which often exhibit strongly inclined contacts. Unit G contains abundant lava clasts and few pebbles. In Hole U1397A, at 227 mbsf, a clast consists of green to violet clay, probably resulting from hydrothermal alteration on land. The presence of pebbles may also indicate an origin from the emerged island. Poor recovery from the two holes at this depth makes establishing a precise stratigraphy difficult; however, highly deformed sediment seems to be present, having a thickness of ~60 m. This deformed sediment is most likely indicative of mass movement derived from the subaerial or submarine parts of the neighboring islands. At the base of Hole U1397B, at 229 mbsf, only a few clasts and a larger block were recovered. The block is composed of andesitic lava containing large phenocrysts of amphibole and quartz. Similar lava is generated by the Piton du Carbet Volcano, south of Montagne Pelée, and is unique in the area of Martinique and Dominica. The period of activity that generated this type of magma is dated ~330–350 ka on the basis of K-Ar dates obtained on the lava domes from the Piton du Carbet Volcano (Germa et al., 2011; Samper et al., 2008). This composition gives a maximum age for the base of the core. Unit H
Unit H is only present in Hole U1397A and extends from 230 to 259.6 mbsf at the base of the hole. It is composed of heavily bioturbated hemipelagic mud. A few layers of bioclastic sandstone are interbedded within the mudstone. No volcanic layers are observed. |
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