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doi:10.2204/iodp.proc.308.214.2009

Results

The minerals identified (Table T2) are quartz, phyllosilicates, K-feldspars, plagioclases (sodic feldspar), calcite, dolomite, halite, and ankerite (Fe-rich dolomite, present as trace in a single sample [308-U1320A-2H-5, 44–46 cm]). Phyllosilicates typically dominate the <2 µm fraction and are thus often called clay minerals, even though they can occur in a wide range of size fractions. Quartz, phyllosilicates, plagioclases, and feldspars are typical detrital minerals. Dolomite can either result from in situ diagenesis or (more likely in the levee deposits considered here) can be eroded from ancient carbonate rocks and subsequently transported by fluvial processes. Calcite typically comes from benthic and pelagic foraminifers such as the ones identified in Holes U1320A and U1324B (see the “Expedition 308 summary” chapter). Halite is most likely precipitated from pore water within the sediment, and so its concentration is a function of porosity and not of the primary sedimentologic composition of the rock.

Stratigraphic evolution of mineralogy in Hole U1320A

Lithostratigraphic Unit V (174.4–299.6 meters below seafloor [mbsf]) is characterized by an average of 23% quartz, 33%–40% phyllosilicates, 3% K-feldspar, 6% plagioclases, 12% calcite, 14% dolomite, and 1% halite (Fig. F1A, F1B). The composition of this interval is very stable, although calcite and dolomite tend to decrease uphole of 200 mbsf in favor mostly of phyllosilicates.

Lithostratigraphic Unit IV (145.3–174.4 mbsf) is characterized by an average of 30% quartz, 44%–54% phyllosilicates, 3% K-feldspar, 5% plagioclases, 3% calcite, 4% dolomite, and 1% halite (Fig. F1A, F1B). Compared to the previous intervals, carbonates are much less important and the lithology is dominated by thick sand units and turbidites (see the “Expedition 308 summary” chapter).

Lithostratigraphic Unit III (137.50–145.3 mbsf) is characterized by an average of 20% quartz, 42%–43% phyllosilicates, 15% K-feldspar, 4% plagioclases, 12% calcite, 4% dolomite, and 1% halite (Fig. F1A, F1B). This interval contains the highest amount of K-feldspar found in this study and calcite values similar to Unit V. The lithology in this interval was interpreted as hemipelagic mudstones (see the “Expedition 308 summary” chapter).

Lithostratigraphic Unit II (2.65–137.50 mbsf) is characterized by an average of 47% quartz, 30%–34% phyllosilicates, 4% K-feldspar, 4% plagioclases, 6% calcite, 4% dolomite, and 1% halite (Fig. F1A, F1B). The mineralogical composition of this interval is variable, with quartz content ranging from 16% to 83%. This variability reflects the interbedding of sandstone and mudstone identified during Expedition 308 (see the “Expedition 308 summary” chapter). No samples were analyzed from lithostratigraphic Unit I (upper 2.65 m of the hole).

Stratigraphic evolution of mineralogy in Hole U1324B

Lithostratigraphic Unit II (364.70–600.80 mbsf) is characterized by an average of 33% quartz, 27%–30% phyllosilicates, 7% K-feldspar, 9% plagioclases, 6% calcite, 14% dolomite, and 1% halite (Fig. F2A, F2B). The composition of this interval is very stable, although quartz content starts to decrease uphole of 400 mbsf in favor mostly of dolomite and, to a lesser extent, calcite. This interval was interpreted as overbank deposits (see the “Expedition 308 summary” chapter).

Lithostratigraphic Unit I (0–364.70 mbsf) is characterized by an average of 25% quartz, 35%–39% phyllosilicates, 4% K-feldspar, 6% plagioclases, 20% calcite, 15% dolomite, and 1% halite (Fig. F2A, F2B). Dolomite content is initially very high in this interval (average = 17%; as much as 23%), but trends show a steady decline in the content of this mineral starting at the base of Subunit IID (151.0 mbsf). From Subunit IC (107 mbsf) to the top of the hole, dolomite content averages 10% with a minimum of 5%. This decrease in dolomite content is paralleled by an increase in calcite. The whole interval of Unit I was interpreted as a succession of rapidly deposited distal turbidite and intervals of discrete mass transport deposits (see the “Expedition 308 summary” chapter).

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