Grain counts and XRD mineralogy of turbidite sand and hemipelagic mud from ODP Leg 168 sites

Sequences of late Pliocene to Holocene sediment lap onto juvenile igneous crust within 20 km of the Juan de Fuca Ridge in northwestern Cascadia Basin, Pacific Ocean. The detrital modes of turbidite sands do not vary significantly within or among sites drilled during Leg 168 of the Ocean Drilling Program. Average values of total quartz, total feldspar, and unstable lithic fragments are Q = 35, F = 35, and L = 30. Average values of monocrystalline quartz, plagioclase, and K-feldspar are Qm = 46, P = 49, and K = 5, and the average detrital modes of polycrystalline quartz, volcanic-rock fragments, and sedimentary-rock plus metamorphic-rock fragments are Qp = 16, Lv = 43, and Lsm = 41. Likely source areas include the Olympic Peninsula and Vancouver Island; sediment transport was focused primarily through the Strait of Juan de Fuca, Juan de Fuca Channel, Vancouver Valley, and Nitinat Valley. Relative abundance of clay minerals (<2-µm-size fraction) fluctuate erratically with depth, stratigraphic age, and sediment type (mud vs. turbidite matrix). Mineral abundance in mud samples are 0%-35% smectite (mean = 8%), 18%-59% illite (mean = 40%), and 29%-78% chlorite + kaolinite (mean = 52%). We attribute the relatively low content of smectite to rapid mechanical weathering of polymictic source terrains, with little or no input of volcanic detritus from the Columbia River. The scatter in clay mineralogy probably was caused by converging of surface currents, turbidity currents, and near-bottom nepheloid clouds from several directions, as well as subtle changes in glacial vs. interglacial weathering products.

Foraminifers from DSDP Leg 87 holes

Leg 87 investigated two sites in the Nankai Trough, off southeastern Japan, and one in the Japan Trench, off northeastern Japan. Several holes at the Nankai Trough sites penetrated mostly Quaternary interbedded sandy turbidites and hemipelagic mud. Foraminifers are common only in certain turbidite sands because both sites are at or just below the carbonate compensation depth. The planktonic assemblages from these sandy layers consist of mixed cool-temperate and warm-water species, and include both solution-resistant and solution-prone species. The benthic assemblages from these same layers are composed of mixtures of shelf to abyssal species. The northward-flowing Kuroshio is important in producing the mixed planktonic faunas, whereas turbidity currents are the primary agents in mixing benthic faunas and in the rapid burial of both planktonic and benthic foraminifers, which protects them from solution. Interbedded hemipelagic muds are barren or contain sparse faunas. Hole 582B penetrated through the trench-fill deposits into hemipelagic sediments that originated in the Shikoku Basin. These muds contain a dissolution facies of solution-resistant planktonic species, partially dissolved tests, and deep bathyal benthic species. Drilling at Site 584, on the landward midslope of the Japan Trench, penetrated a section of dominantly diatomaceous mudstone. This section contains a meager Pliocene calcareous fauna in its upper third and a nearly monospecific assemblage of Martinottiella communis in the lower two-thirds. Diatom biostratigraphy indicates that this change in assemblages occurs near the Miocene/Pliocene boundary. Similar biofacies changes are observed in neighboring sections drilled during Legs 56 and 57. The change from agglutinated to calcareous faunas is probably related to a relative drop in the carbonate compensation depth at the end of the Miocene.

Cenozoic sediments facies and geochemistry of ODP Hole 103-637A

The late Cenozoic deposits recovered at ODP Site 637 from the Iberian Abyssal Plain near the continental margin off northwestern Spain include three main facies groups. Turbidites are the dominant facies association (two-thirds of the total thickness), followed by pelagites (one-fourth), and subordinate amounts of contourites (one-tenth). Slump deposits occur locally in the upper Miocene and middle Pliocene. Turbidity currents and pelagic settling were the significant sediment depositional processes from the Pliocene to the Pleistocene, whereas bottom currents predominated during the late Miocene. Fine-grained, base-cut-out turbidites, normally starting with the Td division, are the most abundant sequence type. The pelagites include both carbonate-rich pelagic and hemipelagic facies. The two types of contourites, sandy and calcareous-rich or fine-grained terrigenous, record two types of bottom-current processes. The Cenozoic deposits at Site 637 show a general upward transition from contourites in the upper Miocene to turbidites in the Pliocene-Quaternary. The entire section is rhythmically bedded and has a poorly developed cyclic pattern defined by variations in the total carbonate content. The low sedimentation rates also show the same cyclicity, with lower values for the late Miocene and late Pliocene. This evolution reflects the predominant depositional processes and the dissolution of carbonates by a lower CCD during the late Miocene.

Grain size composition and geochemistry of volcaniclastic sediments in the Lau Basin

Early Pliocene to Pleistocene volcaniclastic sediments recovered during Ocean Drilling Program Leg 135 from Sites 834 to 839 in the Lau Basin show a wide range of chemical and mineralogical compositions extending the spectrum previously known from the Lau Basin, Lau Ridge and Tofua Arc. The following major types of volcaniclastics have been distinguished: (1) primary fallout ashes originating from eruptions on land, (2) epiclastic deposits that resulted from subaerial and submarine eruptions, (3) subaqueous fallout and pyroclastic flow deposits resulting from explosive submarine eruptions, and (4) hyaloclastites resulting from mechanical fragmentation and spalling of chilled margins of submarine pillow tubes and sheet-lava flows. Vitric shards are mostly basaltic andesitic to rhyolitic and broadly follow two major trends in terms of K2O enrichment: a low-K series (LKS) with about 1 wt% K2O at 70 wt% SiO2, and a very low-K series (VLKS) with only about 0.5 wt% K2O at 70 wt% SiO2. Sites 834 and 835 on "old" backarc basin crust, >4.2 and 3.4 m.y. old, comprise LKS rhyolites >3.3 m.y. old. Calc-alkaline basaltic turbidites originating from the Lau Ridge flowed in at 3.3 Ma. In the period from 3.3 to 2.4 Ma basaltic andesitic to rhyolitic, fine-grained LKS and VLKS volcaniclastics were deposited by turbidity currents and subaerial fallout. Three thin, discrete fallout layers (2.4-3.2 m.y. old) with high-K calc-alkaline compositions probably erupted in New Zealand. Volcaniclastics from Site 836, all <0.6 m.y. old, make up 24% of the sediments and comprise local basaltic andesitic to andesitic hyaloclastites with low Ba/Zr ratios of 0.9 to 1.4 and polymict andesitic sediments with Ba/Zr ratios of up to 5.5, containing clasts altered to lower greenschist facies. In Sites 837-839, drilled on young crust (1.8-2.1 m.y. old), volcaniclastics make up 45%-64% of the total sediment. Glass compositions are often bimodal with a mafic and a rhyolitic population. Large-volume rhyolitic, silt- to lapilli-sized volcaniclastics are interpreted as pyroclastic flows from explosive eruptions on a seamount 25-50 km away from the sites. Ba/Zr ratios are 2 to 4, partially overlapping with some Lau Basin basement lavas that show an "arc" signature, and they can reach values >5 in thin volcaniclastic layers <0.6 m.y. old.

Sulfur-isotope analyses of gypsium samples from the Southeast Greenland margin, southern Alaska and Gulf of Alaska (Table 1)

Gypsum grains were identified in Miocene-Pleistocene sediment cores from two deep-water ODP sites, Site 918 off the SE Greenland margin and Site 887 in the Gulf of Alaska, and in Holocene sediment cores from shallow-water localities in Disenchantment Bay and Muir Inlet in southern Alaska. Although initial morphologic and textural observations suggested a complex system in which the gypsum may have had more than one origin, quantitative sulfur isotope analyses of the gypsum provide evidence of its detrital nature. d34S values in gypsum from southern Alaska range between +0.0 and +7.1 per mil. Gypsum has d34S values between -27.1 and -27.5 per mil in the Gulf of Alaska and values between -28.5 and +0.2 per mil off the SE Greenland margin. All of these isotopic signatures are too highly depleted in d34S to have precipitated from seawater, present or past. In addition there is no significant change in d34S values for gypsum crystals with differing physical characteristics (abraded vs. unabraded) from the same stratigraphic horizon, suggesting all the gypsum is detrital regardless of the degree of abrasion. The isotopic and physical evidence, in combination with the onshore geology the environmental setting, and site characteristics of the gypsum-bearing marine localities, lead us to propose that the ultimate source of the gypsum is precipitation from freeze-induced terrestrial sediment or soil brines. Furthermore the combined evidence suggests that the subsequent occurrence of gypsum in glacimarine sediments results from ice-rafting (by icebergs or sea ice) of the frozen regolith and/or, in the proximal glacimarine setting of southern Alaska, very rapid burial via turbidity currents.

(Table 3) Textural and bulk mineralogy of Pliocene and Pleistocene sediments from ODP Hole 161-976B

The Pliocene and Pleistocene deposits recovered at Site 976 from the northwestern Alboran Sea at the Málaga base-of-slope include five main sedimentary facies: hemipelagic, turbidite, homogeneous gravity-flow, contourite, and debris-flow facies. The thickness and vertical distribution of these facies into lithostratigraphic Units I, II, and III show that the turbidites and hemipelagic facies are the dominant associations. The Pliocene and Pleistocene depositional history has been divided into three sedimentary stages: Stage I of early Pliocene age, in which hemipelagic and low-energy turbidites were the dominant processes; Stage II of early Pleistocene/late Pliocene age, in which the dominant processes were the turbidity currents interrupted by short episodes of other gravity flows (debris-flows and homogeneous gravity-flow facies) and bottom currents; and Stage III of Pleistocene age, in which both hemipelagic and low-energy gravity-flow processes occurred. The sedimentation during these three stages was controlled mainly by sea-level changes and also by the sediment supply that caused rapid terrigenous sedimentation variations from a proximal source represented by the Fuengirola Canyon.

(Table 1) Age determination of sediment cores MD925-2002 and MD03-2690

The compilation of results obtained on three giant piston cores from the Whittard, Shamrock and Guilcher turbidite levees reveals a high-resolution stratigraphic record for the Bay of Biscay. Due to the abundance of reworked sediments in these sedimentary environments, a specific methodological approach, based on an X-ray-assisted subsampling phase associated with sedimentological, geochemical and micropalaeontological analyses, was implemented. With an accurate chronological framework, this multi-proxy investigation provides observations on the 'Fleuve Manche' palaeoriver and the British-Irish Ice Sheet (BIS) histories over the last 20,000 years. The results obtained highlight the direct influence of the decay of the BIS on the Bay of Biscay deep-sea clastic sedimentation during the last European deglacial phase. During this period, the annual BIS cycle of meltwater seems enough to generate seasonal turbidity currents associated with exceptional sedimentation rates in all the Celtic and Armorican turbidite systems. With very high sedimentation rates, the turbidite levees represent the main deep-sea clastic depositional area. Long coring combined with a very careful subsampling method can provide continuous high-resolution palaeoenvironmental signals.

Ripple migration directions and grain fabric measurements on ODP Hole 210-1276A sediments

Albian turbidites and intercalated shales were cored from ~1145 to 1700 meters below seafloor at Site 1276 in the Newfoundland Basin. Strata at this level dip ~2.5° seaward (toward an azimuth of ~130°) based on seismic profiles. In contrast, beds dip an average of ~10° in the cores. This higher apparent dip is the sum of the ~2.5° seaward dip and a measured hole deviation of 7.43°, which must be essentially in the same seaward direction. Using the maximum dip direction in the cores as a reference direction, paleocurrents were measured from 11 current-ripple foresets and 11 vector means of grain fabric in planar-laminated sandstones. Five of the planar-laminated sandstone samples have a grain imbrication 8°, permitting specification of a unique flow direction rather than just the line-of-motion of the current. Both ripples and grain fabric point to unconfined flow toward the north-northeast. There is considerable spread in the data so that some paleoflow indicators point toward the northwest, whereas others point southeast. Nevertheless, the overall pattern of paleoflow suggests a source for the turbidity currents on the southeastern Grand Banks, likely from the long-emergent Avalon Uplift in that area. On average, turbidity currents apparently flowed axially in the young Albian rift, toward the north. This is opposite to what might be expected for a northward-propagating rift and a young ocean opening in a zipperlike fashion from south to north.

Sedimentology of surface samples from the Weddell Sea, Antarctica

Mineralogical and granulometric properties of glacial-marine surface sediments of the Weddell Sea and adjoining areas were studied in order to decipher spatial variations of provenance and transport paths of terrigenous detritus from Antarctic sources. The silt fraction shows marked spatial differences in quartz contents. In the sand fractions heavy-mineral assemblages display low mineralogical maturity and are dominated by garnet, green hornblende, and various types of clinopyroxene. Cluster analysis yields distinct heavy-mineral assemblages, which can be attributed to specific source rocks of the Antarctic hinterland. The configuration of modern mineralogical provinces in the near-shore regions reflects the geological variety of the adjacent hinterland. In the distal parts of the study area, sand-sized heavy minerals are good tracers of ice-rafting. Granulometric characteristics and the distribution of heavy-mineral provinces reflect maxima of relative and absolute accumulation of ice-rafted detritus in accordance with major iceberg drift tracks in the course of the Weddell Gyre. Fine-grained and coarse-grained sediment fractions may have different origins. In the central Weddell Sea, coarse ice-rafted detritus basically derives from East Antarctic sources, while the fine-fraction is discharged from weak permanent bottom currents and/or episodic turbidity currents and shows affinities to southern Weddell Sea sources. Winnowing of quartz-rich sediments through intense bottom water formation in the southern Weddell Sea provides muddy suspensions enriched in quartz. The influence of quartz-rich suspensions moving within the Weddell Gyre contour current can be traced as far as the continental slope in the northwestern Weddell Sea. In general, the focusing of mud by currents significantly exceeds the relative and absolute contribution of ice-rafted detritus beyond the shelves of the study area.

(Table T1) Correlations between marker beds and drill holes for ODP Leg 171B sites

More than 50 discrete volcanic ash layers were recovered at the five drill sites of the Blake Nose depth transect (Leg 171B, western central Atlantic). The majority of these ash layers are intercalated with Eocene hemipelagic sediments with a pronounced frequency maximum in the upper Eocene. Several ash layers appear to be deposited from volcanic fallout with little or no indication of secondary remobilization. They provide excellent stratigraphic markers for a correlation of the Leg 171B drill sites. Other ash layers were probably redeposited from volcaniclastic-rich turbidity currents, but they still represent geologically instantaneous events that can be used in stratigraphic correlation between adjacent drill holes. Additional nonvolcanic marker beds, like the suspect late Eocene impact event layer, were included in our hole-to-hole correlations. Stratigraphic and downcore positions of marker beds were compiled and plotted against existing composite depth records that were constructed to guide high-resolution sampling. Comparison of our correlation with the spliced composite sections of each drill site reveals several minor and some major discrepancies. These may result from drilling distortion or missing sections, from the lack of unambiguous criteria for the synchronism of ash layers, or from the systematic exclusion of marker-bed data in the construction of the spliced record. Integration of both correlation approaches will help eliminate most of the observed discrepancies.