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doi:10.2204/iodp.proc.330.108.2012 SedimentologySediment at Site U1377 was encountered in the sedimentary cover of Hadar Guyot. Two stratigraphic units were defined on the basis of compositional and textural characteristics of the sediment at macroscopic and microscopic scales in Holes U1377A and U1377B (Fig. F3). Hole U1377AAlthough the exact nature and thickness of the units are poorly constrained because of very low (<1%) recovery, two stratigraphic units were defined in Hole U1377A, and these likely correspond to Units I and II defined in Hole U1377B (see below):
Unit I
Stratigraphic Unit I corresponds to the sediment retrieved in the core catcher of Core 330-U1377A-1R. Its exact extension is poorly constrained because of very low (<1%) core recovery in the uppermost part of Hole U1377A (Fig. F3). The lower boundary of this unit was not sampled. The sediment of Unit I is composed of light gray nannofossil foraminiferal ooze with volcanic glass fragments. Foraminiferal and nannofossil assemblages define an early Pliocene to Holocene age of deposition (see “Paleontology”). Smear slide observations (Sample 330-U1377A-1R-CC-PAL-SED, 0–2 cm) showed that the nannofossil foraminiferal ooze includes some fragments of fresh volcanic glass and plagioclase. Unit II
Stratigraphic Unit II corresponds to the sediment retrieved in the core catcher of Core 330-U1377A-2R, and its lower boundary was not recovered (Fig. F3). Unit II includes bioturbated multicolor foraminiferal limestone with ferromanganese encrustations and black dendrites (Fig. F4A). Foraminiferal assemblages found in the limestone provide an age of deposition between the middle and late Eocene (see “Paleontology”). X-ray fluorescence analyses using the shipboard Thermo Scientific Niton XL3 Analyzer (see XL3_EVAL.PDF in XRF in “Supplementary material”) show that the limestone has a high phosphorous content. These analyses also confirm the occurrence of ferromanganese material in most of the limestone. Some cuttings accumulated in the core catcher during retrieval of Core 330-U1377A-3R, including some sand-size grains of white and yellowish foraminiferal limestone and possible echinoderm fragments. These fragments originated from the interval above 24.50 mbsf (the bottom depth of the cored interval for this core) and likely indicate the presence of unrecovered shallow-water or hemipelagic limestone in Unit II above. Thin section observations (Sample 330-U1377A-2R-1, 15–17 cm [Thin Section 269]) showed that limestone recovered in Unit II includes very abundant planktonic foraminifers with rare echinoderm fragments and altered rounded glass fragments. The dendroid shapes of probable ferromanganese-oxide minerals were often observed in thin section. Hole U1377BAlthough recovery of the uppermost sequence in Hole U1377B was very poor, two stratigraphic units were defined, and these likely correlate with Units I and II in Hole U1377A:
Unit I
Because of poor recovery in Hole U1377B, Unit I is poorly constrained, extending from the seafloor to at least 0.79 mbsf (Fig. F3). Although it was possibly sampled in underlying Section 330-U1377B-1R-2 or ghost Core 330-U1377B-3G (see below), the lower boundary of Unit I was not identified. Macroscopic and smear slide (Sample 330-U1377B-1R-1-SED, 10 cm) observations revealed that Unit I is composed of pale yellow nannofossil foraminiferal ooze with rare volcanic glass fragments (Fig. F4B). Foraminiferal and nannofossil assemblages define a Pliocene to Holocene age of deposition (see “Paleontology”). Unit II
Stratigraphic Unit II corresponds to a 0.29 m thick sedimentary unit, the exact interval of which is poorly constrained because of low recovery in Hole U1377B (Fig. F3). Unit II includes poorly sorted multicolor basalt conglomerate with ferromanganese encrustations (Fig. F4C). The clasts in the conglomerate range in size from coarse sand to boulder. The largest clasts are rounded, whereas the smallest clasts are generally angular. The conglomerate is heterolithic and includes distinct types of basalt clasts (see “Igneous petrology and volcanology”), as well as few clasts of volcanic sandstone-breccia and shallow-marine bioclasts (echinoderms, larger foraminifers, bryozoans, red algae, and shell fragments). The matrix of the conglomerate is composed of bioturbated multicolor foraminiferal limestone with ferromanganese encrustations. A geopetal structure defined by infilling of foraminiferal limestone in a cavity or burrow occurs in Section 330-U1377B-1R-2, 5.5 cm (Point 2 in Fig. F4C). Foraminiferal assemblages found in the matrix of the conglomerate provide an age of deposition between the late Paleocene and early Eocene (see “Paleontology”). Partial collapse of the hole during drilling led to the retrieval of a ghost core (Core 330-U1377B-3G; see “Operations”) that contains rock fragments fallen from the previously penetrated interval between the seafloor and 18.10 mbsf. Five pieces of sediment were found in the ghost core (Fig. F4D). Four pieces are composed of foraminiferal limestone with ferromanganese encrustations that closely resembles the limestone of Unit II in Hole U1377A. Another piece of sediment (Piece 7) contains the following lithologies (from top to bottom):
The orientation of this piece is well constrained by a geopetal structure found in a gastropod and by the texture of the ferromanganese crust. This piece is therefore believed to provide reliable indications on the stratigraphic arrangement of the distinct types of sediment found at Site U1377 (see below). Thin section observations (Sample 330-U1377B-1R-2, 8–12 cm [Thin Section 276]) showed that the limestone recovered in Unit II is a poorly washed grainstone with very abundant planktonic foraminifers and rare larger benthic foraminifers, echinoderm fragments, altered rounded glass fragments, plagioclase, and fish debris. Interpretation of sediment at Site U1377Although characterization of the stratigraphy at Site U1377 was limited by very poor recovery, observations in Holes U1377A and U1377B allowed definition of a consistent sedimentary pattern on top of Hadar Guyot. The uppermost sediment (Unit I) recovered in both holes is composed of early Pliocene to Holocene nannofossil foraminiferal ooze (see “Paleontology”) that strongly resembles the soft sediment recovered in the uppermost part of Site U1372 on Canopus Guyot, Site U1374 on Rigil Guyot, and Site U1375 on Achernar Guyot. Similar to preceding sites, the soft sediment recovered at Site U1377 on Hadar Guyot is considered to reflect recent pelagic sedimentation on top of the drilled seamount. Cores and cuttings retrieved in Holes U1377A and U1377B support the existence of a lithologically consistent Unit II at Site U1377. Two distinct lithofacies and several ferromanganese encrustations were recognized in this unit, which indicates that the sediment represents one or more condensed sections on top of Hadar Guyot. Lithofacies 1 of Unit II corresponds to bioturbated foraminiferal limestone with ferromanganese encrustations and rare shallow-marine bioclasts (see “Paleontology”). This lithofacies occurs in Section 330-U1377A-2R-1 and ghost Core 330-U1377B-3G (Fig. F4) and is interpreted to reflect sedimentation in a pelagic to hemipelagic environment on the basis of highly abundant planktonic foraminifers and few shallow-water bioclasts. Lithofacies 2 of Unit II corresponds to heterolithic basalt conglomerate with a matrix composed of foraminiferal limestone with shallow-marine bioclasts (see “Paleontology”). This lithofacies is also associated with ferromanganese encrustations and occurs in Section 330-U1377B-1R-2, ghost Core 330-U1377B-3G (Fig. F4), and cuttings in the core catcher of Core 330-U1377A-3R. Lithofacies 2 is interpreted to reflect deposition in a possible shallow-marine to hemipelagic environment on the basis of its abundance of planktonic foraminifers with well-preserved shallow-marine macrofossils and rounded basalt pebbles. Very low recovery at Site U1377 did not allow a determination of the exact thickness and recurrence of these two lithofacies. However, an unoriented limestone piece in ghost Core 330-U1377A-3G (Piece 7) includes (from bottom to top) Lithofacies 2, a ferromanganese crust, and Lithofacies 1 (Fig. F4). The sedimentary texture of this piece and the paleontological ages of Lithofacies 1 and 2 (between middle and late Eocene and between late Paleocene and early Eocene, respectively; see “Paleontology”) suggest that deposition of Lithofacies 2 occurred prior to Lithofacies 1. Possibly, this may reflect transition from a shallow-marine to hemipelagic-pelagic environment of deposition on Hadar Guyot between the late Paleocene and late Eocene. |
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