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doi:10.2204/iodp.proc.311.208.2008

Results

A total of 186 samples of hemipelagic samples, turbiditic samples, and carbonate concretion samples were analyzed in this study. Figures F3, F4, F5, F6, and F7 display downhole variations of clay minerals for each site. The X-ray diffraction data are illustrated in Table T1.

As previously stated, we used the factors of Biscay (1965) to estimate semiquantitative clay mineral percentages in order to compare our results with already published data. However, we must note that using these factors and 1 as the factor for I/V "minimizes" the proportion of I/V compared to semiquantitative estimates based on other methods described in Holtzapffel (1985). I/V mixed layers were never observed offshore Oregon (e.g., Karlin, 1980; Underwood, 2002; Gràcia et al., 2006; Underwood and Torres, 2006), whereas they are present in Pleistocene northwestern Atlantic sediments and relate to interglacial periods (e.g., Vanderaveroet et al., 2000). Vermiculite and random mixed-layer minerals were also described in late Pleistocene sediments of Vancouver Island (Blaise, 1989). The expandability values in R0 I/S for each site are quite similar (mean = ~60%–65%; lower values are obviously observed in smectite-poor samples). Therefore, as with Underwood and Torres (2006), we suppose that the expandable clay is probably detrital in origin with small amounts of interlayered illite.

Site U1326

Site U1326 is located on top of the first uplifted ridge of accreted sediments at the far western downslope end of the transect. Thirty-three samples from lithostratigraphic Unit I (Hole U1326C), Unit II (Holes U1326C and U1326D), and Unit III (Hole U1326D) were analyzed (Table T1; Fig. F3). Three samples analyzed for Unit I show a similar clay assemblage composed of smectite (19%), illite (42%), chlorite (28%), and kaolinite (12%). There is an anticorrelation between smectite and illite in Unit II and between illite and chlorite (+ kaolinite) in Unit III. Moreover, as noted by Bahr et al., there is a trend toward higher illite and chlorite (+ kaolinite) contents in the lower part of Unit II and all of Unit III. The lower smectite content in Unit III is accompanied by a slight decrease in the amount of expandable layers (Percent expand in Table T1). The authigenic carbonate–rich sample at 235.90 meters below seafloor (mbsf) does not show any particular clay content.

Site U1325

Site U1325 is located within the first slope basin in the southwestern part of the transect. Thirty-five samples from lithostratigraphic Units IA, IB, II, and III (Hole U1325B) and Unit IV (Hole U1325C) were analyzed. As for Site U1326, clay mineral assemblages are composed of smectite, illite, chlorite, kaolinite, and I/V (Table T1; Fig. F4). No global trend was noted except in Units I–II, in which the observed moderate anticorrelation between smectite and illite could be linked to sediment grain size, turbiditic samples being more illitic, whereas hemipelagic samples are more smectitic. Unit IV (as Unit III for Site U1326) appears slightly richer in chlorite and I/V than Units I–III and poorer in smectite. The authigenic carbonate–rich sample analyzed in Unit III does not show any particular clay content.

Site U1327

Site U1327 is located on the midcontinental slope off Vancouver Island, several hundred meters from Site 889 (Leg 146). Forty-two samples from lithostratigraphic Unit I (Hole U1327C), Unit II (Holes U1327C and U1327D), and Unit III (Hole U1327C) were analyzed (Table T1; Fig. F5). Units I–III display an anticorrelation between smectite and illite related to sediment grain size. Unit III is slightly richer in illite and poorer in smectite than Units I and II, whereas Unit I displays a little more I/V. Six authigenic carbonate–rich samples were analyzed (one in Unit I, four in Unit II, and one in Unit III) with no particular clay content.

Site U1328

Site U1328 is located 3.7 km southeast of Site U1327 on the midcontinental slope off Vancouver Island. Forty-four samples from lithostratigraphic Unit I (Holes U1328B and U1328C), Unit II (Hole U1328C), and Unit III (Hole U1328C) were analyzed (Table T1; Fig. F6). As for the precedent sites, an anticorrelation trend between smectite and illite, related to sediment grain size, was observed. The most smectitic samples (and poorest in illite) are observed at the base of Unit I between 105 and 130 mbsf. Lithostratigraphic Unit II is slightly richer in illite and poorer in smectite than Units I and III. Four authigenic carbonate–rich samples were analyzed in Unit III with no particular clay content.

Site U1329

Site U1329 is the easternmost and shallowest site of the transect. Moreover, it is the only site to display Miocene sediments occurring in discordance below Pleistocene sediments. Twenty-seven samples from lithostratigraphic Units I–III (Hole U1328C) were analyzed (Table T1; Fig. F7). From 0.74 to 35.35 mbsf the clay content of samples is characterized by the lack of kaolinite (all of Unit I and the first sample of Unit II). Lithostratigraphic Unit I appears also slightly poorer in smectite than Units II and III. Globally, an anticorrelation trend between smectite and illite fractions is also observed in the three units. Compared to Pleistocene sediments, Miocene sediments are slightly richer in smectite and kaolinite and poorer in illite and chlorite.

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