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doi:10.2204/iodp.proc.340.206.2016

Results

Of the 1818 core sections recovered during Expedition 340 (Expedition 340 Scientists, 2013), 36% contain fall-in or flow-in coring disturbances and 23%–32% of the total recovered core length is made of disturbed sediment (Tables T1, T2). The present data set shows that a large number of coring disturbances occur because of partial stroke (Jutzeler et al., 2014). Without considering failed holes (U1393A and U1396B), the cores from Montserrat (minimum 11.4% and maximum 13.2% of core length) are much less disturbed than those from Martinique (minimum 25.3% and maximum 30.6% of core length). This high percentage is probably due to the high abundance of volcaniclastic intervals offshore Martinique. Moreover, the relatively high shear stress of the hemipelagic mud at Martinique sites (Expedition 340 Scientists, 2013) likely increased friction of the host formation on the core barrel. High friction rates on the core barrel (1) reduce the overall velocity of the APC during penetration, favoring partial strokes, and (2) increase the tension on the core barrel when pulled out from the host formation, favoring flow-in.

Fall-in

Fall-in is commonly identified with coarse, ungraded to normally graded intervals at the uppermost part of a core section (Jutzeler et al., 2014). Where cores are entirely disturbed (e.g., Core 340-U1400B-5H), it is more difficult to assess whether and where there is a contact between fall-in and flow-in disturbances, but given such complete disturbance the distinction becomes irrelevant.

Flow-in

Basal flow-in is commonly easily identified from the presence of loose, soupy, commonly normally graded sand at the base of the core (Jutzeler et al., 2014). In some places, preserved halos of alteration with gradients of intensity (e.g., glaucony or dark alteration) coronae around clasts allow identification of nondisturbed domains.

Midcore flow-in, which can occur with or without basal flow-in, is identified where floating disaggregated pieces of hemipelagic mud occur in a loose volcaniclastic sand (Jutzeler et al., 2014) but should not be misidentified with primary textures of floating mud intraclasts in volcaniclastic sand. In other places, midcore flow-in is identified where sand of the same apparent componentry was injected into cracks in intervals of more cohesive material (commonly hemipelagic mud) or as the occurrence of a thin rim along the core liner (Jutzeler et al., 2014). Cracks in the mud are likely to be formed during initial recovery (pull) of the core from the formation when the core may be stretched along its entire length. In some cores, it becomes very difficult to distinguish midcore flow-in from local core extension (affecting a single interval) or from genuine mixing by creep or slump (e.g., Sections 340-U1398B-18H-4, 20–145 cm, and 340-U1399A-23H-3, 62 cm, to 23H-5, 140 cm).

Analyses on disturbed cores

Depending on the intensity and type of coring disturbances, some parts of the disturbed intervals may still be available for specific analyses. We give three examples:

1. The stratigraphy within an interval of flowed-in volcaniclastic sand may be destroyed, but bulk componentry and dating are still possible if the sample is considered as matching the entire length of the disturbance.
2. Similarly, if fall-in clasts are dated to the rough age of the preceding core, they may be available for analyses.
3. A piece of hemipelagic mud floating within volcaniclastic sand may still be used for oxygen isotope dating; however, its exact position in the stratigraphy of the core includes a large error, as loose sand may have been injected above it.

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