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doi:10.2204/iodp.proc.307.104.2006 LithostratigraphySite U1317 is located on the upper flank of Challenger Mound and consists of five holes (U1317A, U1317B, U1317C, U1317D, and U1317E) in water depths ranging from 790 to 840 m. A Pleistocene to lower Miocene succession of sediments was recovered to 268 mbsf. Based on the very distinct change in lithologies and depositional realms, we identified two major sedimentary units. The upper lithostratigraphic Unit 1 (0–130.1 mbsf in Hole U1317A) comprises the upper Pliocene to Pleistocene carbonate mound succession (Fig. F3). The underlying Unit 2 (146.2–270 mbsf in Hole U1317D) consists of the Miocene drift sediments (Fig. F4). The boundary between the two units is a sharp firmground representing a major hiatus as identified by the nannoplankton and planktonic foraminifer biostratigraphy (see “Biostratigraphy”) and indicated in the seismic data (see “Physical properties”). Sedimentological highlights include the recovery of the mound base unconformity and the underlying Miocene strata. The mound base from this site is strikingly similar to that of Site U1316. Also noteworthy is the presence of deepwater corals throughout the entire mound succession (lithostratigraphic Unit 1). The sedimentological descriptions are mainly based on cores from Holes U1317A and U1317D, which were split on board according to IODP standard procedure. Core sections from the other holes (Holes U1317B and U1317E) were split frozen at Gulf Coast Core Repository and Bremen Core Repository to avoid disturbance of the coral-bearing sediments. During drilling operations, only the uppermost five cores of Hole U1317C were split frozen (see “Lithostratigraphy” in the “Methods” chapter). Lithostratigraphic unitsUnit 1
Facies of the mound succession Unit 1 are divided into floatstone, rudstone, wackestone, packstone, grainstone, sandy clay, and clayey sand. Among these, floatstone and rudstone are the most common (Fig. F5). Grainstone, clayey sand, and sandy clay were rarely found in Unit 1. The facies are, in general, soft and unlithified but in some intervals partly to well lithified. The carbonate content ranges from 15 to 75 wt%. Cemented nodules are present in Section 307-U1317E-4H-4, 105–125 cm. Drilling breccias commonly occur in the top part of each core. Floatstone faciesThe floatstone (Fig. F6A) consists of coral fragments embedded in an unlithified greenish gray to dark greenish gray (5GY 5/1–5GY 4/1) wackestone to packstone matrix. Coral fragments >2 mm in size were estimated to represent >10% of the sediment volume. Corals fragments were randomly oriented and branch lengths of up to 8 cm are observed at the split surface. The largest coral diameters were recovered from Hole U1317E, closest to the mound summit. We have identified most coral fragments as L. pertusa, with minor to rare occurrences of M. oculata and Desmophyllum cristagalli. In addition to the coral fragments, the >2 mm fraction contains bivalve and gastropod shells, echinoid spines, and other unidentified biogenic components. The preservation of corals and other large biogenic components in the floatstone facies is dominantly moderate to good. In well-preserved intervals, the theca walls are solid and still show internal structures such as lamination of different aragonite layers. Only the centers of calcification of septa and outer theca show signs of dissolution. But in some intervals, the entire coral skeleton is dissolved and crumbles apart when touched. In addition to dissolution, several coral fragments show signs of bioerosion from clionid sponge Entobia spp. The matrix of the floatstone consists of unlithified wackestone to packstone dominated by reworked coral material and calcareous ooze of mainly coccolithophorids. Quartz grains are present to abundant throughout the facies. Smear slide analyses show that sponge spicules, glauconite grains, and benthic and planktonic foraminifers are present in the matrix. Dropstones are randomly distributed throughout the facies. The term “floatstone” was used based on the two-dimensional visual description, but a part of this facies is likely to be renamed to “bafflestone” if an autochtonous appearance of the coral branches is identified by postcruise three-dimensional computed tomography scan analyses. Throughout the mound succession, the floatstone facies correlates well with maxima in the red/green ratio and magnetic susceptibility (see “Physical properties”) as well as minima in sediment lightness (Fig. F3). This can be linked to the carbonate/siliciclastic component ratio. Siliciclastics are generally darker with a higher component in the red spectrum than the dominantly white to gray biogenic carbonates. Therefore, the red/green ratio can be a valid proxy for the terrigenous versus biogenic component content of the sediments. However, a clear correlation was not obtained between the carbonate content of discrete samples and the floatstone facies. This is most likely due to the bimodal nature (corals versus matrix) of the sediments. The thicknesses of the floatstone packages range from several decimeters to a maximum of ~10 m (104–114 mbsf) in Hole U1317A. As observed in sediments from Hole U1317A, the unlithified floatstone represents the dominant facies, comprising ~40% of Unit 1. Floatstone comprises 70% of Unit 1 in Hole U1317C (Fig. F5). Rudstone faciesThe recovered rudstone facies (Fig. F6B) is dominated by coral fragments and minor amounts of shells, echinoid spines, gastropods, and unidentified biogenic components. Colors range from greenish gray to dark greenish gray (5GY 6/1–5GY 4/1). Bioclasts >2 mm occur densely packed and form a clast-supported texture with an unlithified packstone, wackestone, or mudstone matrix. Long axes of the bioclasts commonly show a horizontal orientation. Based on visual estimates, coral fragments represent >20% of the sediment volume. The carbonate component of the matrix is dominantly nannofossil ooze (mainly coccolithophorids) subordinated by micrite and other skeletal fragments including benthic foraminifers. As noncarbonate components in the matrix, quartz grains are present and glauconite grains are rare. Coral rudstone composes up to ~50% of the mound succession. The rudstone packages vary in thickness from several centimeters up to 2 m. Wackestone faciesThe unlithified to partly lithified coral-bearing wackestone facies (Fig. F6C) consists dominantly of coral fragments <2 mm and nannofossils (mainly coccolithophorids). It ranges in color from light greenish gray to greenish gray (5G 7/1–5G 5/1). Smear slide analyses have shown that sponge spicules and quartz grains are present to common, whereas planktonic and benthic foraminifers are present to rare. The embedded coral fragments that rarely occur in the facies (<10% of the sediment) are strongly dissolved and disintegrate easily. As opposed to the floatstones, the wackestones correlate to the maxima in the lightness data and minima in the magnetic susceptibility and red/green ratio curves. Wackestones change gradually into floatstones accompanied by increasing content of large coral fragments. These gradual changes can also be identified in the color reflectance data (Fig. F3). The thicknesses of wackestone intervals vary from several decimeters to a maximum of 4 m between 97 and 101 mbsf in Hole U1317A. Approximately 15% of Unit 1 is classified as wackestone facies in Hole U1317A. Packstone faciesThe packstone facies (Fig. F6D) consists dominantly of sand-sized bioclasts and occasionally includes larger lithoclasts. The spaces between the bioclasts are filled by a micritic matrix. Bioclasts are dominated by coral fragments with minor amounts of shell fragments, echinoid spines, and small gastropods. Smear slide analyses show that nannofossils are the dominant component of the matrix. Silt-sized quartz is present to common, while foraminifers, sponge spicules, and glauconite grains are present to rare. The unlithified packstone facies is characterized by light gray to greenish gray beds of centimeter to decimeter thicknesses in the uppermost 10 m of the mound succession and in the 2 m above the mound base (128–130 mbsf) in Hole U1317A. Packstone intervals commonly have sharp erosional bases, fine upward, and change gradually into the overlying facies. The packstone facies accounts for <2% of Unit 1. Grainstone faciesThe grainstone facies rarely occurs and consists of highly fragmented coral skeletons in beds of few centimeter thicknesses. Sandy clay and clayey sand faciesBrown sandy clay and clayey sand are rare in the mound succession. They consist of a mixture of clayey sandy silt with grain-size biogenic components. Occasionally, larger lithoclasts such as dropstones occur (Fig. F7A). Lithologic cyclesRecurring lithologic cycles comprise repetition of lighter- and darker-colored strata (e.g., light gray–light bluish gray nodular wackestone–packstone gradually changes into the overlain greenish gray coral floatstone; dark greenish gray floatstone and greenish gray rudstone then change again gradually into nodular wackestone–packstone of light gray–light bluish gray color) (Fig. F3). Erosional surfaces generally occur on top of the lighter-colored wackestone–packstone facies, with burrowing into the partly lithified layer. Boundary between upper Pliocene–Pleistocene and Miocene strataThe boundary between upper Pliocene to Pleistocene strata (Unit 1) and Miocene strata (Unit 2) is represented by a firmground; a consolidated sediment that can be scratched with a knife. It shows a gradual color change in the upper 3 cm of Unit 2 from grayish green to light greenish gray at the boundary. This erosional horizon at the top of the Miocene silts is overlain by a 2 cm thick unlithified packstone with abundant benthic foraminifers and small coral fragments in Hole U1317A (Fig. F7B). The packstone is overlain by coral floatstone. This boundary transition occurs in all holes: in Hole U1317A at 130.1 mbsf, Hole U1317B at 143.5 mbsf, Hole U1317C at 148.0 mbsf, Hole U1317D at 148 mbsf, and Hole U1317E at 155.2 mbsf. Unit 2
Siltstone faciesThe upper part of Unit 2 (146.4–197.0 mbsf in Hole U1317D) shown in Figure F7C consists of greenish gray to dark greenish gray (5GY 5/1–5GY 4/1) sandy and clayey silt. This siltstone facies is moderately bioturbated yet contains well-preserved burrows and shows a gradual fining-upward trend to silty clay. Within the facies, the carbonate content varies between 26 and 52 wt%. Glauconite is present throughout the facies. Sandstone faciesThe underlying sandstone facies (Fig. F7D) consists of moderately bioturbated silty sand with colors from greenish gray to dark greenish gray (5GY 5/1–5GY 4/1). The sediments contain between 25 and 49 wt% carbonate. Only the samples from an interval significantly enriched in bivalve shells (231.5–232.4 mbsf, Hole U1317D) contain up to 75 wt% carbonate. X-ray diffraction matrix classificationAs many as 30 X-ray diffraction analyses were made on 26 matrix samples from Unit 1 of Hole U1317A and nine analyses of samples from Unit 2 of Hole U1317D. The mineralogical contents of the matrix sediments in Unit 1 consist primarily of low-magnesium calcite and quartz. The calcite/quartz ratio in the sediments, based on the peak height of the calcite (104) cps line to the quartz (101) cps line, ranges from a high of 3.32 to a low of 0.31. Eighteen of the samples appear to record calcite/quartz ratios of >1 (six of these having ratios >2). No clear relationship has been identified between the described lithology (floatstone, wackestone, etc.) and the calcite/quartz ratio. Aragonite is present in all of the samples and is likely to be derived from corals and to a lesser extent from mollusks. Other mineralogical components, in order of abundance, include illite (detected in all Unit 1 samples), dolomite (detected in all but one sample with a significant amount), and feldspar (detected in 23 samples). Mineralogic contents of sediments of Unit 2 include quartz, calcite, and illite. Quartz dominates all but two of the nine samples. Dolomite detected in one sample is of either diagenetic or detrital origin. Smear slides indicate that the calcite component of the matrix sediment is primarily made up of nannofossils (mainly coccolithophorids). Quartz, feldspar, and illite are identified as detrital fine sand, silt, and clay. DiscussionThe succession recovered in Holes U1317A–U1317E is 270 m thick and spans from the early–middle Miocene to Pleistocene (see “Biostratigraphy”). The oldest cored sediments, drift deposits of early–middle Miocene age, change gradually toward more clay rich intervals that are interpreted to represent relatively low energy environments in the younger Miocene succession. The strata end abruptly in the firmground shown in Figure F7B (Hole U1317A) and are erosively overlain by the upper Pliocene to Pleistocene mound succession. The hiatus between the two successions spans at least 1.65 m.y. by calcareous nannofossil or 1.3 m.y. by planktonic foraminifer biostratigraphic results from erosion or nondeposition. The mound initiation phase just above the firmground is represented by interbedded grainstone, floatstone, rudstone, packstone, and wackstone in decimeter thicknesses, all reflecting relatively rapidly changing depositional realms. These observations are contrary to previous reports based on seismic data, which predicted mound nucleation on a distinct and lithified hardground. This basal interval is followed by several meters thick, facies-specific sediment packages, which represent periods of rapid vertical and probably lateral mound expansion when correlated to the basal coral-bearing unit at Site U1316. This growth phase is dominated by floatstones (Fig. F5) which are tentatively interpreted to represent steady-state conditions between coral growth and background sedimentation. Following this phase, a distinct coral floatstone facies not only thins but also alternates to rudstone, packstone, and wackestone facies in the higher mound succession. The mound succession shows pronounced recurring cycles on a meter to several meter scale. The cycles generally start from floatstone and change into floatstone interbedded with rudstone to wackestone, in which the corals occur mostly in smaller fragments abraded or transported within the mound system. With regard to mound development, the floatstones represent optimal mound growth phases, in which the corals baffle and stabilize the fine-grained sediments. In contrast, the rudstone may represent periods of enhanced erosion and winnowing. With only visual core description and shipboard analyses, we cannot distinguish if the wackestone facies, in which the coral fragments are only poorly preserved, is a primary sedimentary feature or the product of early diagenesis. The poor coral preservation indicates that diagenetic processes are at least overprinting the sedimentary signal. The rhythmic recurring cycles can be clearly identified in the red/green ratio of the color reflectance data. Further investigation is needed to understand to what extent these cycles can be linked to climate changes. |
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