161 Preliminary Report


Figure 1. Leg 161 drill sites in the western Mediterranean. SG: Strait of Gibraltar; AB: Alboran Basin; SB: South Balearic Basin.

Figure 2. The Alpine chains surrounding the Alboran Sea, general tectonic subdivision of crustal domains (from Balanyá and Garcia-Dueñas, 1987). Onshore distribution of these domains indicates that the continental basement beneath the Alboran Sea belongs to the Alboran Crustal Domain.

Figure 3. Bathymetric map of the Alboran Sea showing position of Leg 161 sites and DSDP Site 121. Contours in meters.

Map onshore. Stippled: Miocene marine sediments; Shaded: Alboran Crustal Domain (Metamorphic Complexes in Betics and Rif).

ACH: Alboran Channel; Al: Alboran Island in the Alboran Ridge; CHB: Chella Bank; EAB: Eastern Alboran Basin; SAB: South Alboran Basin; SBB: South Balearic Basin; WAB: West Alboran Basin; XB: Xauen Bank; YB: Yusuf Basin; YR: Yusuf Ridge.

Figure 4. Structural sketch of the Alboran Sea, based on interpretation of MCS profiles and the surrounding Betic and Rif chains (from Comas et al., 1992; Comas et al., 1993). Positions of Leg 161 drill sites and DSDP Site 121 within the structural setting are shown.

Figure 5a. Composite ("spliced") section construction at multiple offset holes seeks to provide a complete, continuous, and undisturbed stratigraphic record, utilizing overlapping intervals of cores from the different holes. In this example, Site 974 high-resolution (2 cm) color reflectance data (550 nm) for four offset holes (Holes 974A, 974B, 974C, and 974D) is plotted against meters composite depth (mcd), with tie lines used to a construct composite depth section. Distinctive features were correlated between cores from each hole and depth-shifted to mcd to produce the single composite section. Shifts in core depths from mbsf to mcd were accomplished using color as well as magnetic susceptibility and GRAPE data.

Figure 5b. ODP Leg 161 drill sites and Pleistocene occurrences of sapropels and ORL's in the western Mediterranean. Preliminary time scale is based on shipboard biostratigraphic data. In the Tyrrhenian and Balearic seas, sapropels are discrete layers up to 30 cm thick. In the Alboran Sea, ORL's contain 0.8% to 2.5% organic carbon and are up to 3 m thick. These layers are more dispersed and thus do not resemble type sapropels. Yet, ODP Leg 161 drilling results suggest that organic-rich sedimentation occurred basinwide in the western Mediterranean.

Figure 6. Lithostratigraphic summary of Leg 161 drill sites in the western Mediterranean.

Figure 7. Summary sediment-accumulation-rate diagram for Leg 161 sites. Data were obtained from calcareous nannofossils and planktonic foraminifers. Water depths for each site are given in meters.

Figure 8. Close-up of seismic line ST01, showing the relative positions of Sites 652 and 974. Dipping reflectors at Site 974 are due to fault-block tilting of the sedimentary sequence.

Figure 9. Single-channel (80-in.3 air-gun source) seismic profile shot during site approach across the southern flank of the Menorca Rise over Site 975. The acoustic "M-reflector" was penetrated at a sub-bottom depth of 305 mbsf, and an evaporitic Messinian sequence (gypsum) was recovered.

Figure 10. Portion of JOIDES Resolution single-channel seismic profile across Site 976. Below a Pleistocene to middle Miocene sedimentary sequence, 258.97 m of metamorphic basement was recovered at this site, consisting primarily of high-grade schist and gneiss, and migmatitic gneiss, with lesser amounts of granite, marble, and calc-silicate rocks.

Figure 11. P-T diagram showing estimated conditions for the deformation in high-grade schist (light gray areas) and gneiss (dark gray areas). Uncertainties in boundary P-T conditions of the different deformations are shown by question marks. Approximate field of migmatite formation is also shown. P-T grid for pelites is in the KFMASH system, composed from Spear (1993) and references therein. Aluminum silicate triple point after Holdaway (1971). The minimum melting conditions for pelitic rocks and the H2O contents (as XH2O in melt) necessary to saturate the granitic liquid are from Le Breton and Thompson (1988). The garnet-plagioclase-rutile-ilmenite-quartz reaction is from Bohlen and Liotta (1986). The corundum-in and muscovite-out reactions are from Helgeson et al. (1978).

Mineral abbreviations: aluminosilicate (As); andalusite (And); biotite (Bt); chlorite (Chl); cordierite (Crd); corundum (Crn); garnet (Grt); ilmenite (Ilm); kyanite (Ky); muscovite (Ms); orthopyroxene (Opx); phlogopite (Phl); plagioclase (Pl); potassium feldspar (Kfs); quartz (Qtz); rutile (Rt); sillimanite (Sil); and staurolite (St).

Liquid (L) and vapor (V).

Figure 12. Location of Sites 977 and 978 on migrated multichannel seismic profile CONRAD 823. Al-Mansour Seamount divides the graben into two sub-basins in which Sites 977 and 978 were cored. The "M" Messinian reflector was penetrated at Site 978 and was just reached at Site 977. Submersible basement sampling of the northern flank of Al-Mansour Seamount recovered dacitic and rhyodacitic volcanic rocks.

Figure 13. Location of Site 979 on MCS profile RAY-36. Submersible basement sampling of the northern side of Alboran Ridge recovered rhyodacitic volcanic and volcaniclastic rocks.

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