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doi:10.2204/iodp.proc.307.204.2009

Results

Biological and lithological composition

Facies

The main common facies is unlithified floatstone in which coral fragments are the dominant bioclasts. The fragments are embedded in a matrix showing packstone or wackestone to mudstone fabrics (Fig. F3). In addition to the coral remains, other biologic components (bivalve and gastropod shells, echinoid spines, etc.) are present. The fine-grained fraction (from 10 µm to 2 mm) is composed of a mixture of carbonate and siliciclastic material (Figs. F3, F4). The mound sediments are often “unlithified” and appear dark and greenish gray (Fig. F3A). However, some “semi-lithified” horizons <1 m thick occur and appear lighter in color (Fig. F3B). Typical burrow features are present in the semilithified horizons (Fig. F4A). Coral-free intervals ~10 cm thick consisting of brown sandy to clayey silt are rarely observed. The mound base is a “firmground” corresponding to an erosional surface overlain by a basal mound succession of packstone containing coral debris. This basal facies is in contrast with the mound body where occurrence of large coldwater coral fragments appears within floatstone fabric (see the “Expedition 307 summary” chapter).

Four main facies types were coded during Expedition 307: floatstone, wackestone, rudstone, and packstone (Fig. F3). However, detailed microscopic investigation in this study reveals that the previous facies coding was related simply to the presence or absence of corals and the relative proportion of bioclasts and siliciclastic minerals (Fig. F3).

The floatstone facies consists of coral fragments and a greenish matrix of wackestone fabric (Fig. F3A). Macrofauna comprises essentially coldwater corals (12.7%) and echinoid spines. The matrix is made of micrite (or microsparite) and microbioclasts (<100 µm). Micrite forms 65.5% of sediment. The identified bioclasts are siliceous sponge spicules, echinoderms (Fig. F4D), bivalves, and planktonic (7.0%) and benthic foraminifers and radiolarians. Glauconite is common (Figs. F3A, F4F) as well as framboidal pyrite (3.3%) and silty quartz (10.3%).

The wackestone facies consists of coral debris and a grayish matrix of wackestone to packstone fabrics (Fig. F3B). The matrix is composed of micrite (or microsparite) and silty quartz (15.6%). Micrite forms 47.5% of sediment. Bioclasts are dominated by coral debris (16.3%) and planktonic foraminifers (15.6%) and include siliceous sponge spicules and radiolarians. Glauconite is present, and phosphate is frequent in the foraminifer chambers (Fig. F3B).

The rudstone facies consists of coral debris and a greenish to grayish matrix of wackestone fabric (Fig. F3C). The matrix consists of micrite or microsparite (50.5%) and silty quartz (21.8%). Bioclasts are dominated by coral debris (5.0%) and planktonic foraminifers (18.8%). Glauconite and framboidal pyrite are common (Fig. F4B, F4C) but their contents are <4% of sediment (Fig. F3C).

The packstone facies consists of planktonic foraminifers and a grayish to greenish matrix of packstone to wackestone fabrics (Fig. F3D). The matrix is composed of micrite or microsparite (67.0%) and silty quartz (20.2%). A distinct feature of this facies is that it is devoid of coral debris. Bioclasts are dominated by planktonic foraminifers (16.5%) and contain echinoderms and benthic foraminifers (Fig. F4E). Glauconite, framboidal pyrite, and phosphate (in the foraminifer loges) are common, but these authigenic minerals constitute <6% of the sediment volume (Fig. F3D).

Detailed matrix composition

Results of our SEM and XRD investigations indicate that the matrix consists of two main fractions: biogenic and terrigenous (siliciclastic). The ratio between these two fractions likely reflects on the sediment color that records the cyclic change throughout the mound sequence (Ferdelman et al., 2006; see the “Expedition 307 summary” chapter).

Biogenic fraction

Calcareous nannofossils (Fig. F5) form the major part of the carbonate content of the mound sediment matrix. Besides the nannofossils, a large amount of fine-grained bioclasts is present, such as small coldwater coral debris, fragments of echinoderms, siliceous sponge spicules, radiolarians, planktonic and benthic foraminifers, gastropods, bivalves, and other unidentified biogenic fragments (Fig. F5). XRD analyses show the presence of calcite and aragonite in the matrix (Fig. F6). Aragonite likely derives from corals and to a lesser extent from mollusks. Pure calcite is mainly from nannofossils (coccolithophorids), whereas a limited amount of euhedral calcite is present (Fig. F7C).

Terrigenous (siliciclastic) fraction

Data from thin sections, SEM, and XRD analyses show that fine sands and silts are dominantly composed of quartz (from 10% to 20% of the whole sediment) (Figs. F4, F5, F7A). Clay minerals are abundant (from 10% to 50% of the mound sediment) and consist of the same association throughout the entire mound succession, comprising illite (37%–45.6%), kaolinite and chlorite (24%–28.7%), and smectite (1%–10%) (Fig. F6). Silt-sized grains of glauconite are also commonly observed (Figs. F3, F4B). The siliciclastic fraction has a very immature characteristic indicating detrital origin from nearby sources.

Diagenetic fraction

Framboidal and euhedral pyrite crystals are frequent and especially located within foraminifer chambers and perforations of corals (Fig. F7D). This pyrite probably has a diagenetic origin linked to bacterial sulfate reduction of organic matter and subsequent sulfide precipitation.

Phosphate precipitates are also common in some foraminifer chambers and intergranular spaces of the matrix.

Coral bioerosion

As mentioned above, the main producers of skeletal carbonate were coldwater corals. The coral fragments are ubiquitous throughout the mound. These fragments range from microscopic debris to branches 8 cm long, and all of them are randomly orientated in the matrix (Figs. F3, F4, F5, F7, F8). Most of the fragments, for which taxonomy was identified belong to the species L. pertusa. Many minor occurrences of Madrepora oculata and Desmophyllum cristagalli were noted (see the “Site U1317” chapter). The preservation of coral fragments and other large biogenic components is various throughout the mound succession. In some parts, coral fragments are partly pyritized or are associated with framboidal pyrite (Fig. F8B). Several corals show evidence of intense bioerosion (Fig. F8). Traces of fungi and sponges are quite common on the observed coral fragments. Large borings are attributed to clionid sponges (Fig. F8D).

Large tunnel-like borings from 5 to 10 mm in diameter (Fig. F8C) are identified as Orthogonum lineare (Glaub, 1994), whereas the round chambers are identified as Saccomorpha clava (Radtke, 1991). These traces are typical constituents and index ichnofossils for aphotic ichnocoenoses (Wisshak et al., 2005a, 2005b). In conclusion, bioerosion is an important factor in weakening the coral skeletons and contributing to the production of carbonate grains. However, chemical aragonite dissolution occurred as well (dissolution cupules on aragonite crystals are recognizable) and enhanced the collapse of the coral frames.

Organic matter content

The results (Table T1) show that all 17 analyzed samples are low in total organic carbon (TOC) content (from 0% to 0.4%; average = 0.2%) (Fig. F9). The average value of the T_max parameter around 407°C (Fig. F9) indicates that the organic matter has not experienced strong thermal maturation. However, four samples (Samples 307-U1317A-5H-3, 85–90 cm; 10H-3, 85–90 cm; 12H-3, 75–80 cm; and 14H-1, 135–140 cm) have a bimodal S2 pattern (Fig. F9), indicating that organic matter contains two fractions with different stability (maturity). One sample (307-U1317A-7H-1, 135–140 cm) yields a higher T_max value of 455°C, suggesting that this sample is abundant in mature organic particles (reworked from older rocks?).

Hydrogen index (HI) values are usually low (<200 mg hydrocarbon [HC]/g TOC) indicating that the organic matter was highly oxidized and can be classified as Type IV (altered organic matter). Nevertheless, Sample 307-U1317A-10H-3, 85–90 cm, shows a high HI value (589 mg HC/g TOC) suggesting that the organic matter has a predominantly marine origin and can be classified as Type II (algae) or I (altered bacterial organic matter) (Espitalié et al., 1986).

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