Previous   |    Next

doi:10.2204/iodp.proc.340.104.2013

Lithostratigraphy

The primary lithologies at Site U1394 are described here, followed by associations of lithologies that form a series of lithostratigraphic units (A–F).

Hemipelagic mud

Site U1394 background sediment mainly consists of fine-grained calcareous biogenic fragments and siliciclastic sediment (Fig. F2A). The background sediment is often pale yellowish gray to dark gray in color and varies from fine silt to fine mud in grain size. The variety in colors could be caused by the compositional ratios between biogenic grains, volcaniclastic grains, and grains of other origins. The sediment mainly consists of a moderately to poorly sorted homogeneous carbonate-rich matrix, but it has diffusely laminated intervals or mottled coloration in places. It occasionally contains millimeter-scale dark greenish layers (possibly altered clay minerals) that can be sharp or diffuse, tiny black organic matter, small shell fragments, and heterogeneous sandy patches (possibly in-filled burrows). In case of interbedded sandy turbidities or ash layers, layer boundaries are distinct and sometimes erosive at the base of turbidites.

Turbidite sand and mud

Turbidites are characterized by normal grading and massive well-sorted mud (for thin deposits a few centimeters thick) to very coarse sand with maximum grain size (for thick deposits measured in meters) (Fig. F2B). Turbidite deposits generally consist of a mixture of volcaniclasts and bioclasts. Volcaniclasts include fragments of fresh andesitic lava and pumice, altered lava, and crystals (feldspar, amphibole, and so on). Bioclasts include fragments of carbonate materials such as corals and shells. Occasionally, a few (<10) percent of granule- to pebble-sized clasts (<5 cm) or mud clasts are included. The ratio of volcaniclastic to bioclastic components varies. In some turbidite units bioclasts occupy >50 vol%. Thin turbidites (a few centimeters thick) are generally interlayered in Units A and E, and thick turbidites (as thick as several meters) occur in the upper part of Unit B and most of Unit D.

Mafic volcaniclastic/tephra deposits

Well-sorted dark color layers that represent fallout from a series of eruptions of basaltic magmas from the South Soufrière Hills volcanic center (131–138 ka; Le Friant et al., 2008) are seen in Subunit A-7 in Holes U1394A and U1394B. Basaltic eruptions are absent during activity of the Soufrière Hills volcanic center (250 ka to recent); thus, basaltic fallout layers are good regional markers in cores around Montserrat.

Other tephra fall deposits

Other types of volcanic ash layers, probably of fallout origin, occur elsewhere in the cores. The interpretation of ash layers is based on visual core observations (such as color, a well-sorted nature, grading, and so on) and petrographic features seen in smear slide analysis. At the base of the cores from Hole U1394B, several dark tephra layers composed partly of scoria are observed. It is probable that some of these tephra layers originate from volcanoes on Guadeloupe, as several basaltic eruptions are already recognized there.

Lithostratigraphic units

Lithostratigraphic unit boundaries at Site U1394 are defined by abrupt or gradational changes in the abundance of lithologies and by distinctive marker horizons such as basaltic fall and turbidity current deposits. Uncertainties concerning the depths of these unit boundaries are sometimes significant.

Unit A

• Depths: Hole U1394A = 0–6.5 mbsf, Hole U1394B = 0–8 mbsf

Unit A extends from the seafloor to a series of distinctive basaltic fall and flow deposits from 6.5 mbsf (Hole U1394A) to 8 mbsf (Hole U1394B). Unit A is divided into a series of seven subunits (A-1 to A-7).

Subunit A-1

The youngest stratigraphic subunit, A-1, extends 21 cm (0–0.21 mbsf; Hole U1394A) or 48 cm (0–0.48 mbsf; Hole U1394B) below the seafloor and comprises two (Hole U1394A) or three (Hole U1394B) volcaniclastic turbidites (Fig. F2). Each turbidite fines upward from a sandy base into a muddy top and comprises mainly andesitic lava grains with very little bioclastic material. This subunit is poorly consolidated and often appears deformed, most probably as a result of the coring process. APC coring typically fails to recover the uppermost few centimeters of “soupy” sediment immediately below the seafloor, so this subunit may actually be as much as a few tens of centimeters thicker. The lower boundary of Subunit A-1 is a sharp contact with underlying hemipelagic sandy mud.

Subunit A-2

Subunit A-2 comprises the 20 cm of hemipelagic mud below Subunit A-1 and an underlying volcaniclastic turbidite, which is 2–6 cm thick and consists of fine sand.

Subunit A-3

Subunit A-3 comprises 33–41 cm of hemipelagic mud immediately below Subunit A-2 and an underlying, more bioclastic rich turbidite, which also has a significant volcaniclastic component. The turbidite comprises 2–3 cm of fine sand and probably as much as 4 cm of overlying turbidite mud.

Subunit A-4

Subunit A-4 comprises 20 cm of hemipelagic mud below Subunit A-3 and a thicker series of stacked multiple turbidites. The turbidites grade from sand to mud, although in many cases there is erosive sand-on-sand contact. The turbidites have a variety of more bioclastic and more volcaniclastic compositions, shown by lighter or darker gray hues. Indeed, a single turbidite can grade from more bioclastic rich to more volcaniclastic rich. Hole U1394A contains ~9 turbidites where Subunit A-4 is 140 cm thick, and Hole U1394B contains at least 10 or 11 turbidites where Subunit A-4 is ~200 cm thick. A series of three finer grained and thinner turbidites occurs within the subunit in both holes.

Subunit A-5

The upper part of Subunit A-5 comprises 23 cm (Hole U1394B) to 50 cm (Hole U1394A) of hemipelagic silty mud, underlain by two stacked volcaniclastic turbidites that total 14 cm (Hole U1394A) or 19 cm (Hole U1394B).

Subunit A-6

Subunit A-6 comprises 50 cm (Hole U1394A) or 55 cm (Hole U1394B) of hemipelagic mud beneath Subunit A-5; this hemipelagic mud is underlain by six stacked turbidites that are 55 cm (Hole U1394A) or 45 cm (Hole U1394B) thick. The turbidites are normally graded from sand to mud. In both holes the uppermost five turbidites are volcaniclastic and the basal turbidite is more bioclastic.

Subunit A-7

Subunit A-7 is better exposed in Hole U1394A, where it was not sampled in the core catcher (Fig. F2). Three distinctive basaltic fallout layers are recognized in intervals 340-U1394A-2H-2, 19–25 cm (Layer BF-a); 2H-2, 25–38 cm (Layer BF-b); and 2H-2, 58–83 cm (Layer BF-c). The depths at the top of the uppermost fallout layer (BF-a) are 6.5 mbsf (Hole U1394A) and 7.8 mbsf (Hole U1394B). Two basaltic turbidites are interlayered with the fallout units. The first turbidite overlies basaltic fallout Layer BF-a, from 0 to 19 cm, and the other basaltic turbidite separates Layers BF-b and BF-c between 38 and 58 cm. The three basaltic fallout layers are normally graded, fining upward from pebble to fine sand. The thicknesses of Layers BF-a, BF-b, and BF-c are 6, 13, and 25 cm, respectively.

Two of the same basaltic fallout layers (BF-a and BF-b) are recognized in intervals 340-U1394B-1H-6, 40.5–44.5 cm; and 1H-6, 44.5–60 cm, respectively. The thicknesses of the basaltic fallout layers in Hole U1394B are 4 cm (Layer BF-a) and 15.5 cm (Layer BF-b), similar to the thicknesses of those layers in Hole U1394A. Grain sizes are finer in the basaltic fallout deposits of Hole U1394B, both of which fine upward from granule to mud.

Unit B

• Depths: Hole U1394A = 6.5–50 mbsf, Hole U1394B = 8–50 mbsf

Unit B extends from 6.5 or 8 mbsf to ~50 mbsf (see Section 340-U1394B-7X-1).

Subunit B-1

Subunit B-1 immediately underlies the basaltic horizons of Subunit A-7, and its upper part comprises a relatively thick interval of stacked and amalgamated turbidites that are often relatively coarse grained. The variable composition of the turbidites, more bioclastic or more volcaniclastic rich, produces complex changes in their gray hues. Individual fining-upward turbidites show compositional (gray hue) zoning. The turbidites are always massive (Ta division of Bouma, 1962), such that planar or ripple cross-lamination is always absent, suggesting rapid deposition that prevented bed-load reworking into laminae. The turbidites contain andesitic clasts, bioclasts (including coral), and mud clasts, which can be several centimeters in length. The subunit is at least 18 m thick.

A 1 m thick interval of very contorted hemipelagic silty mud with fine laminae lies within Section 340-U1394A-2H-6. This interval is most likely a mud clast but could represent a deformed hemipelagic interbed. A second interval of hemipelagic mud and flat-lying thin turbidites in Section 340-U1394A-3H-1 is deformed, but only weakly. This second interval could either be in situ or a second clast. Hemipelagic intervals are also found in Core 340-U1394B-3X.

Subunit B-2

The lower part of Unit B was not recovered in Holes U1394A and U1394B, suggesting that it is even more difficult to recover with either piston or rotary coring and therefore may potentially be coarser grained. The core catcher samples comprise a few centimeters of mixed volcaniclastic-bioclastic sand and andesitic pebbles as large as 4 cm.

Unit C

• Depths: Hole U1394A = 50–74 mbsf, Hole U1394B = 50–74 mbsf

Unit C extends from 50 to 74 mbsf, but its boundaries are poorly constrained. Unit C contains a greater abundance of hemipelagic sediment than Units B and D. Individual continuous intervals of hemipelagic silty clay are up to 130 cm thick. This unit also contains volcaniclastic turbidites that are up to 40 cm thick.

Unit D

• Depths: Hole U1394A = 74–91.5 mbsf, Hole U1394B = 74–110 mbsf

Unit D extends from ~74 mbsf to either 91.5 or 110 mbsf. This unit was only recovered well in Hole U1394B, where it is dominated by massive coarse-grained turbidite sandstones that comprise variable amounts of volcaniclastic and bioclastic material. This variability causes changes in gray hue. Individual graded turbidite units are often >1 m thick, and there are numerous graded units. Planar lamination or cross-lamination (divisions Tb, c, and d of Bouma, 1962) are always absent. The chaotically distributed clasts in some ungraded sections may suggest en masse emplacement by debris flow, and clasts of sand and mud (as well as andesitic lava and bioclasts) can be several centimeters in length. Adjacent massive but graded sand intervals are most likely deposited in a layer-by-layer fashion from the base of high-density turbidity currents. There are no fine mud interbeds in this unit, and its duration of emplacement is uncertain. It could represent multiple events or multiple pulses in one event.

Unit E

• Depths: Hole U1394A = 91.5–181 mbsf, Hole U1394B = 110–181.4 mbsf (bottom of hole)

Unit E extends from either 91.5 or 110 mbsf to at least 181 mbsf. This unit comprises interbedded mud hemipelagite and turbidite intervals and was much better recovered in Hole U1394B. The position of its upper boundary is uncertain and is placed at the first interval of hemipelagic mud. If an interval in Sections 340-U1394B-12H-1 and 12H-2 is indeed hemipelagic mud, then the top of Unit E is at 91.5 mbsf. However, if this interval is turbidite silt and mud, then the top of Unit E is at 110 mbsf. The interval between 91.2 and 110 mbsf is coarse-grained material, sometimes with chaotic clasts suggestive of en masse emplacement by debris flow and sometimes with massive sand that may have flowed during coring. The first unequivocal interval of hemipelagite mud is at 110 mbsf in Section 340-U1394B-14H-1.

Unit E contains significant amounts of hemipelagic mud. The upper part of Unit E contains relatively thick and coarse-grained turbidites, and turbidite abundance and thickness is lower in the middle part of Unit E. Turbidite thickness could increase again near the base of the unit. The turbidite sands can be volcaniclastic, bioclastic, or mixed in character, and they range in thickness from a few centimeters to a few meters.

At several levels (e.g., Sections 340-U1394B-16H-1 and 16H-2; Section 340-U1394A-20X-1), there are massive silty brown tephra layers composed of ash, some of which are 20 cm thick. These brown silt layers tend to occur in clusters.

The lower–middle part of the interval is heavily bioturbated by distinctive large burrows (e.g., Core 340-U1394A-17X).

A prominent 8 m thick and coarse unit (154–162 mbsf; Core 340-U1394B-19H) contains ~5% pumice clasts, which are even more abundant near the top of the unit. The pumice clasts are ≤5 cm in diameter. The finer matrix comprises crystals. The unit does not contain obvious breaks in sedimentation and could be a single event bed. A >5 m thick volcaniclastic turbidite occurs at the base of Unit E in Hole U1394B.

Unit F

• Depth: Hole U1394A = 181–235 mbsf (bottom of hole)

Unit F extends deeper than 181 mbsf and certainly deeper than 215 mbsf, but the base of the unit was not reached. In Hole U1394A this unit was poorly recovered, suggesting that it was relatively coarse grained. Andesitic clasts were recovered in core catcher samples below ~215 mbsf. Hole U1394B terminated within Unit E and did not reach Unit F.

Igneous petrology and alteration

Hard rock petrology at Site U1394 reflects the dominant magmatic products of Montserrat. Thin sections were prepared of a gray, poorly vesicular andesite clast (Sample 340-U1394A-6X-CC, 9–13 cm), a white-gray andesite pumice clast (Sample 22X-1, 27–28 cm), and grain mounts of four volcaniclastic tephra layers (Samples 2H-2, 65–66 cm; 24X-1, 89–90 cm; 24X-2, 16–18 cm; and 24X-2, 42–43 cm).

The andesite lava and pumice fragment contained similar phase assemblages but differed in texture. The lava clast is porphyritic with an approximately 60:40 mix of groundmass and phenocrysts. The phenocryst assemblage consists of plagioclase (70%), amphibole (25%), Fe-Ti oxides (4%), and orthopyroxene (1%). The plagioclase phenocrysts are euhedral, lath-shaped grains as long as 3.5 mm, commonly exhibiting albitic twinning. Many of the grains contain sieved cores, evidence of previous resorption, although all cores are surrounded by clean overgrowth. Two populations of amphibole phenocrysts are observed: the larger crystals (≤4.6 mm long) are fully reacted to a fine-grained mix of oxides and pyroxenes (decompression reaction rims; Devine et al., 1998); the smaller crystals (≤0.25 mm long) are thin rimmed. Both populations are euhedral to subhedral and elongate. No significant oxidation of the smaller clasts is observed. Orthopyroxene and Fe-Ti oxides occur as euhedral tabular and subhedral microphenocrysts, respectively. The phenocrysts are contained in a microcrystalline matrix of plagioclase, oxide, and pyroxene microlites. No flow lineation was observed in the lava clast.

The pumice clast contained a similar phase assemblage with a distinctly different groundmass. Vesicle population made up ≥50% of the clast. The remaining clast was a 70:30 mix of groundmass and crystals. The groundmass was composed of glassy materials exhibiting flow lineation. The phenocryst proportions were similar to the andesite lava clast.

The grain mount of the uppermost volcaniclastic tephra layer (Sample 340-U1394A-2H-2, 65–66 cm) appears to be basaltic in composition. It contains fragments of plagioclase, olivine, clinopyroxene, and oxides in descending abundance. Some of the microcrystalline matrix is still present on the rims of some of the grains. This layer forms part of the A-7 marker subunit.

Grain mounts lower in the core (Samples 340-U1394A-24X-1, 89–90 cm; 24X-2, 16–18 cm; and 24X-2, 42–43 cm) appear to be of basaltic andesite or andesite composition. These grain mounts correspond to three distinct volcaniclastic sand units. The mineralogy of the mounts consists of fragments of plagioclase, amphibole, orthopyroxene, and Fe-Ti oxides. Small fragments of biogenic material and microcrystalline magmatic matrix are present as well. The average grain size within the grain mount is <0.2 mm. The mineral assemblage does not appear to vary significantly between units. No textural interpretation of the grain mounts is possible.

Top of page   |    Previous   |    Next