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doi:10.2204/iodp.proc.330.104.2012 SedimentologyThe following stratigraphic units were defined at Site U1373 on the basis of distinct sedimentary facies, sediment compositions, and cementation patterns observed at macroscopic and microscopic scales (Fig. F4):
Unit I
Stratigraphic Unit I is composed of basalt conglomerate or breccia that extends downhole from the seafloor to a volcanic interval with basalt lava flows in underlying Unit II at 3.05 mbsf (Fig. F4). Unit I was divided into three subunits on the basis of (1) distinct sedimentary textures and abundances of micrite, cement, and bioclasts and (2) the occurrence of an erosional contact between basalt conglomerate (above) and bioclast basalt conglomerate (below) at 2.66 mbsf (Section 330-U1373A-1R-3, 37 cm). The exact thicknesses of Subunits IA, IB, and IC are unknown because of poor recovery in Core 330-U1373A-1R (~30% recovery). Also, the relationship between Units I and II and Subunits IA and IB is ambiguous because interunit contacts were not recovered. As a consequence, the interval between 3.05 and 9.60 mbsf remains unconstrained in our lithostratigraphic interpretation. At Site U1373, paleontological data did not allow definition of a precise age of sediment deposition (see “Paleontology”). A Late Cretaceous or younger age was inferred for Unit I from paleontological data in Unit III (see “Paleontology”) and stratigraphic relationships with underlying units. The largest basalt clasts (as measured over 10 cm intervals) are similar in size throughout Unit I and range between pebble and cobble size (Fig. F4). Macroscopic and microscopic observations allowed recognition of 12 types of basalt clasts in Units I and III (see U1373A.DOC in CHAR in SEDIMENT in “Supplementary material”). The nature and occurrence of these clasts are summarized in Table T2. Unit I has a clast population compositionally similar to that of Subunits IIIA–IIIC. Subunit IA
Stratigraphic Subunit IA extends from the seafloor to 0.15 mbsf (Fig. F4). The lower boundary of Subunit IA was not recovered, and its lower depth was defined on the basis of an underlying lithofacies that contains distinctly fewer fossils (Subunit IB). Subunit IA is composed of grain-supported poorly sorted multicolor bioclast basalt conglomerate (Fig. F5A). The interpebble and intercobble spaces are composed of sandy–pebbly bioclast basalt sediment with a matrix composed of calcite cement and very pale brown foraminifer-bearing micrite. The bioclasts are rounded to well rounded and include shallow-marine biota (e.g., annelids, algae, and shell fragments). Thin Section 72 (Sample 330-U1373A-1R-1, 8–12 cm) indicates that the matrix of the conglomerate includes benthic and planktonic foraminifers and calcispheres, as well as fragments of sponge(?) spicules, echinoderms, annelids, algae, bryozoans, and bivalves. Rare altered fragments of volcanic glass were observed. Subunit IB
Stratigraphic Subunit IB is at least 2.51 m thick and extends between 0.15 and 2.66 mbsf (Fig. F4). The lower boundary of Subunit IB was defined by the occurrence of an erosional contact with underlying Subunit IC. Subunit IB is composed of a matrix-supported poorly sorted multicolor basalt conglomerate with few bioclasts (Fig. F5B). The interpebble and intercobble spaces consist of silty–pebbly basalt sediment with a matrix composed of pale brown azoic micrite. The sparse bioclasts have a similar composition but are smaller than those in overlying Subunit IA and underlying Subunit IC. Minor bioturbation of the finer grained sediment and biogenic encrustation of basalt clasts was observed throughout Subunit IB. Two thin sections (Samples 330-U1373A-1R-2, 7–9 cm [Thin Section 74], and 1R-2, 45–48 cm [Thin Section 75]) show that the matrix of the basalt sediment (i.e., pale brown micrite) contains a minor clay component not observed elsewhere in the sediment at Site U1373. Volcanic glass fragments and clinopyroxene grains are rare and generally altered. Some of the glass fragments are well-rounded, which attests to relatively significant reworking prior to final deposition in the sediment. Subunit IC
Stratigraphic Subunit IC is at least 0.39 m thick and extends between 2.66 and 3.05 mbsf (Fig. F4). Its lower boundary corresponds to the top of the underlying volcanic interval of lava flows (Unit II). The lithology of Subunit IC is very similar to that of Subunit IA, consisting of grain-supported poorly sorted multicolor bioclast basalt conglomerate (Fig. F5C). The interpebble and intercobble spaces are composed of sandy–pebbly bioclast basalt sediment with a matrix composed of calcite cement and very pale brown micrite. Macroscopic and microscopic observations (Sample 330-U1373A-1R-3, 66–70 cm [Thin Section 77]) indicate that the biogenic content of Subunit IC is also similar to that of Subunit IA. Biogenic encrustation of basalt clasts is common. Rare volcanic glass fragments are commonly completely altered. Unit II
Stratigraphic Unit II is 6.10 m thick and extends between 9.60 and 15.70 mbsf (Fig. F4). Its lower boundary is defined on the basis of bioclast basalt conglomerate that appears in the uppermost part of underlying Unit III. Unit II consists of a volcanic interval composed of three brecciated basalt flows interbedded with basalt breccia. The basalt flows are described in “Igneous petrology and volcanology.” Possible mingling textures of the basalt breccia (sediment) with the basalt (lava) occur throughout Unit II. Two thin (<0.50 m thick) interbeds of cemented multicolor basalt breccia occur between 10.98 and 11.42 mbsf (interval 330-U1373A-2R-2, 5–49 cm) and between 14.17 and 14.34 mbsf (interval 3R-1, 24 cm, to 4R-4, 45 cm). The breccia is grain supported, poorly sorted, and devoid of biogenic fragments (Fig. F5D). Its maximum clast size is similar to that of Unit I, but the average roundness of the largest clasts is lower than that of Unit I. Unit II is devoid of fossils, and its age was constrained on the basis of paleontological data from underlying Unit III (see “Paleontology”) and stratigraphic relationships. Unit III
Stratigraphic Unit III is composed of an 18.20 m thick interval of basalt conglomerate and breccia that extends between 15.70 and 33.90 mbsf (Fig. F4). The unit was divided into four subunits on the basis of distinct sedimentary textures and the abundances of residual pore space, cement, micrite, and bioclasts. The relationship between Subunits IIIB and IIIC and the exact thickness of Subunit IIIB are unknown because interunit contacts were lost as a result of the poor recovery of Core 330-U1373A-4R (22% recovery). A Late Cretaceous or younger age of deposition was defined on the basis of macrofossil content in Subunit IIIB (see “Paleontology”). Distinct cement textures were recognized in Unit III (Fig. F6), which helped constrain the deposition environments of the sediment (see below). It is important to note, however, that our cement analysis is preliminary. Only the most characteristic textures were described, and additional observations are needed to more accurately characterize the deposition environment of the sediment. Subunit IIIA
Stratigraphic Subunit IIIA is 1.37 m thick and extends from 15.70 to 17.07 mbsf (Fig. F4). A transitional contact was observed between Subunits IIIA and IIIB from ~17.02 to 17.12 mbsf. We defined the lower boundary of Subunit IIIA on the basis of the first occurrence of residual porosity and the layered structure in underlying Subunit IIIB. The composition and texture of Subunit IIIA are similar to those of Subunits IA and IC. Subunit IIIA consists of grain-supported poorly sorted multicolor bioclast basalt conglomerate (Fig. F5E). The interpebble and intercobble spaces include sandy–pebbly bioclast basalt sediment with a matrix composed of calcite cement and a very pale brown micrite. Similar to Subunits IA and IC, Subunit IIIA also includes rounded to well-rounded bioclasts of shallow-marine origin (e.g., annelids, algae, and shell fragments). Bioclasts in Subunit IIIA are, however, larger than those in Subunits IA and IC, with sizes up to ~5 cm. The maximum clast size and clast roundness in Subunit IIIA are similar to those in Subunits IA and IC (pebble to cobble size and subangular to very angular, respectively). Thin section observations (Samples 330-U1373A-3R-3, 10–13 cm [Thin Section 79]; 3R-3, 18–22 cm [Thin Section 80]; and 3R-3, 86–88 cm [Thin Section 81]) indicate that the finer grained sediment is composed of micrite-bearing grainstone with fragments of algae, bivalves, bryozoans, annelids, echinoderms, and gastropods, as well as rare benthic foraminifers. The occurrence of granular sparite and micritized cement in pore spaces (showing meniscus and dogtooth textures) suggests a possible intermittent subaerial exposure (Flügel, 1982). Together with observed dissolution features in thin section (Fig. F6A), these occurrences support our conclusion that sediment deposition likely occurred in a marine phreatic environment. Subunit IIIB
Stratigraphic Subunit IIIB is 6.73 m thick and extends from 17.07 to 23.80 mbsf (Fig. F4). Reduced cementation with depth in Subunit IIIB probably led to poor recovery of Core 330-U1373A-4R (22% recovery). The lower boundary of Subunit IIIB is believed to correspond to the disappearance with depth of poorly cemented sediment (correlated to an increased recovery rate) and was defined on the basis of the first occurrence in underlying Subunit IIIC of basalt conglomerate poorer in bioclasts. The composition of Subunit IIIB resembles that of overlying Subunit IIIA and consists of grain-supported multicolor bioclast basalt conglomerate (Fig. F5F). However, Subunit IIIB has several distinctive features compared with other sediments at Site U1373: (1) it is very well sorted, (2) grains are rounder, (3) cross-beds and layering structures defined by distinct amounts of volcanic and biogenic grains occur, (4) residual porosity widely occurs, (5) larger bioclasts are more abundant, and (6) biogenic encrustation on both sides of basalt clasts is more abundant. Maximum clast size in Subunit IIIB (granule to boulder size) decreases with depth and is generally lower than that of other units and subunits at Site U1373 (Fig. F4). Similarly, average grain roundness in Subunit IIIB increases with depth from very angular to subrounded. Identification of an extinct oyster (Flemingostrea sp.) suggests the age of Subunit IIIB is between Late Cretaceous and Miocene (see “Paleontology”). Thin section observations (Samples 330-U1373A-3R-3, 126–129 cm [Thin Section 82]; 3R-4, 63–68 cm [Thin Section 83]; 4R-1, 72–74 cm [Thin Section 84]; and 4R-1, 86–89 cm [Thin Section 85]) show that Subunit IIIB is composed of grainstone with fragments of algae, bryozoans, annelids, bivalves, and echinoderms. The occurrence of fibrous meniscus cement and abundant residual porosity (Fig. F6B) indicates that sediment was deposited in an intertidal or possibly subtidal environment (Flügel, 1982). Subunit IIIC
Stratigraphic Subunit IIIC is 5.37 m thick and extends from 23.80 to 29.17 mbsf (Fig. F4). Its lower boundary was defined on the basis of the appearance of matrix-supported dark multicolor basalt breccia in underlying Subunit IIID. The composition of Subunit IIIC ranges from multicolor basalt conglomerate with bioclasts to bluish-gray basalt conglomerate essentially devoid of bioclasts (Fig. F5G). A gradual transition between these two lithologies occurs from 24.19 to 24.29 mbsf (interval 330-U1373A-5R-1, 38.5–48.5 cm). The two conglomerate types of Subunit IIIC are poorly sorted and grain supported, and both have a bimodal grain size distribution and contain well-rounded basalt clasts. The multicolor basalt conglomerate has a matrix composed of cement, very pale brown micrite, and bioclast basalt sandstone conglomerate. This conglomerate also has a higher content of echinoderm fragments than other sediments at Site U1373. On the other hand, the bluish-gray basalt conglomerate has cemented pores and is devoid of micrite. Bioclast content in Subunit IIIC decreases sharply at the transition between the multicolor and bluish-gray basalt conglomerates. Bioclasts were not found below 24.49 mbsf at Site U1373. In Subunit IIIC, maximum clast size ranges from pebble to cobble size, with subangular to well-rounded average grain roundness. Thin section observations (Samples 330-U1373A-5R-1, 33–36 cm [Thin Section 86], and 5R-1, 101–104 cm [Thin Section 87]) show that the bioclast content in Subunit IIIC is similar to that of other bioclast-bearing sediments at Site U1373. The occurrence of dogtooth, meniscus, and granular cements, as well as vadose silt, dissolved shell fragments, and residual porosity (Fig. F6C, F6D, F6F), supports deposition of the upper part of Subunit IIIC (i.e., multicolor basalt conglomerate with bioclasts) in an intertidal or possibly subtidal environment (Flügel, 1982). Pore spaces in the lower part of the subunit (i.e., bluish-gray basalt conglomerate) are composed of micrite with possible meniscus texture, rim cement, and granular cement (Fig. F6E), which suggests that the sediment was deposited similarly in an intertidal and possibly subtidal to marine phreatic environment (Flügel, 1982). Subunit IIID
Stratigraphic Subunit IIID is 4.73 m thick and extends from 29.17 to 33.90 mbsf (Fig. F4). Its lower boundary corresponds to the uppermost part of the underlying volcanic basement sequence (see “Igneous petrology and volcanology”). Subunit IIID includes a poorly sorted matrix-supported dark multicolor basalt breccia devoid of bioclasts and carbonate, with a matrix composition ranging between dark gray siltstone and finer grained basalt breccia. The maximum clast size in Subunit IIID ranges between pebble and boulder size, with angular to subangular average grain roundness. The basalt clast composition is dissimilar to that of Unit I and Subunits IIIA–IIIC, with occurrences of clast Types 10–12 and the absence of Types 1–6 (Table T2). Thin section observation (Sample 330-U1373A-6R-2, 39–43 cm [Thin Section 88]) indicates that the sediment matrix is predominantly composed of altered glass fragments possibly replaced by clay minerals (Fig. F7). Sediment in underlying volcanic sequenceThe volcanic sequence starting with Unit IV includes several enclaves and interbeds of sediment at 37.91–37.94 mbsf (interval 330-U1373A-8R-1, 51–54 cm), 38.36–38.39 mbsf (interval 330-U1373A-8R-1, 96–99 cm), 38.82–40.27 mbsf (interval 330-U1373A-8R-2, 0–145 cm), and 43.96–44.21 mbsf (interval 330-U1373A-9R-2, 65–90 cm). The sediment consists of dark multicolor basalt siltstone-sandstone devoid of bioclasts and carbonate. Primary sedimentary structures within the sediment interbeds survived synvolcanic deformation and include moderately deformed laminae and fining-upward grain structures. Thin section observation (Sample 330-U1373A-9R-2, 76–79 cm [Thin Section 100]) shows that the composition of the fine-grained sediment is similar to that of the matrix of the dark multicolor basalt breccia encountered in Subunit IIID, predominantly altered glass fragments possibly replaced by clay minerals (Fig. F7). Interpretation of lithologies and lithofacies at Site U1373Seven lithofacies were recognized in the basalt conglomerate and breccia at Site U1373, permitting overall characterization of the environment of deposition (Figs. F4, F5, F6). Lithofacies 1 was found in Subunit IIIB and is characterized by (1) current structure at macroscopic and microscopic scales, (2) very well sorted grains, (3) generally well rounded grains, (4) increased occurrence of shallow-marine bioclasts relative to other sediments at Site U1373, and (5) cement textures (Fig. F6E) and high residual porosity indicative of an intertidal environment of deposition (Flügel, 1982). The conjunction of these characteristics supports sediment deposition in a beach environment. Lithofacies 2 corresponds to Subunits IA, IC, and IIIA. This lithofacies was interpreted to reflect sediment deposition in a shallow-marine and subtidal or intertidal environment on the basis of highly abundant shallow-marine bioclasts and the dissolution of some bioclasts and cement textures (Fig. F6A). Lithofacies 3 (upper part of Subunit IIIC only) is defined by the occurrence of shallow-marine bioclasts and meniscus, dogtooth, and dripstone cement textures (Fig. F6C, F6D, F6F). It is interpreted to represent an intertidal and subtidal environment of sediment deposition similar to that suggested by Lithofacies 2. However, clearer indications for cementation in an intertidal or possibly vadose environment were found in Lithofacies 3 (Point 1 in Fig. F6C, F6D). Lithofacies 4 (lower part of Subunit IIIC) corresponds to a cemented basalt conglomerate mostly devoid of micrite and bioclasts. Although clear discriminative criteria of the environment of deposition were not found, we tentatively propose that Lithofacies 4 reflects sediment deposition in a shallow-marine environment. The absence of bioclasts in the lower part of Subunit IIIC, deposition of overlying sediment in an intertidal–subtidal environment, and emplacement of underlying lava flows in subaerial conditions (see “Igneous petrology and volcanology”) suggest that Lithofacies 4 possibly records a subaerial to submarine transition. Lithofacies 5 corresponds to the basalt conglomerate of Subunit IB, which is interpreted as a mudflow deposited in a shallow-marine environment on the basis of (1) clay content in the conglomerate matrix, (2) matrix-supported texture of the conglomerate, and (3) bioclasts of shallow-marine origin and micrite embedded in the matrix of the conglomerate. Lithofacies 6 corresponds to the dark basalt conglomerate found in Subunit IIID and interbeds/enclaves of sediment in the underlying volcanic sequence. The sediment was interpreted as a matrix-supported debris flow deposit. Its textural and compositional characteristics include high angularity of basalt clasts, matrix-supported sedimentary texture, a lack of bioclasts, and predominant occurrence of altered ash and fine-grained vitric fragments without carbonate in the sediment matrix. These observations suggest that the sediment could represent a lahar deposit. Lithofacies 7 corresponds to interbeds of basalt breccia in Unit II (in an intermediate volcanic interval). It is characterized by a lack of bioclasts and high textural variability of the basalt clasts. These observations and the stratigraphic relationship of the volcanic interval with Units I and III suggest that the breccia represents proximal debris flows of Unit II emplaced in a shallow-marine environment. In summary, the sediment of Site U1373 defines a shallow-marine to beach environment of deposition on a previously volcanic island. We propose that the background sedimentation of shallow-marine conglomerate (Lithofacies 1–4) was punctuated by catastrophic emplacement of a mudflow deposit, a volcanic interval, and possible lahar deposit(s) (Lithofacies 5–7). The recurrence of such catastrophic deposits suggests that the environment of deposition was perhaps located close to the bottom of a valley, most likely close to a fluvial fan. Unlike Site U1372 on Canopus Guyot, no evidence for subsidence or significant eustatic change was found. |
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