Site 976, located in the western Mediterranean (Alboran Sea), 60 km off the southern Spanish coast and about 110 km east of the Strait of Gibraltar, is situated on the lower part of a gentle slope that dips to the south from the Spanish margin toward the western Alboran Basin. The site is located in a water depth of 1108 m and is 8 km northeast of DSDP Site 121.
Site 976 (Fig. 10) is the first of three Leg 161 sites that were planned to address tectonic objectives in the western Mediterranean Sea. It is the westernmost site of the trans-Mediterranean drilling transect, which was designed to refine paleoceanographic models for sapropel formation. The main tectonic question in the Alboran Basin deals with the long-standing problem of understanding convergent plate boundaries, i.e., the development of extensional basins in collisional settings. Paleoceanographic objectives at Site 976 focused on monitoring late Cenozoic Atlantic-Mediterranean water exchange with special emphasis on periods of sapropel deposition.
Five holes were drilled at Site 976. Except for Hole 976A, which was used to determine the mud line, these holes were dedicated to distinct scientific objectives. Hole 976B continuously cored the entire 650 m-thick sedimentary sequence and penetrated 267 m into metamorphic basement, thus meeting our highest priority tectonic objective. Hole 976C was dedicated to paleoceanographic objectives and cored down to the Pleistocene/Pliocene boundary at 375 mbsf. Hole 976D was dedicated to high-resolution, interstitial-water geochemistry and was cored to a depth of 30 mbsf. Hole 976E was designed to fill gaps in recovery around major unconformities within the sedimentary sequence and to sample and log the sediment-basement transition.
The stratigraphic interval cored and sampled at Site 976 ranges from uppermost middle Miocene (Zone NN7, N. continuosa/G. Siakensis Zone) to uppermost Pleistocene/Holocene (Subzone NN21b, G. truncatulinoides excelsa Zone). The Pliocene/Pleistocene boundary is between 357.92 and 361.01 mbsf. Three major hiatuses were recorded: between the late and early Pliocene (Zanclean and Piacenzian), between the early Pliocene and latest Miocene (Zanclean and Messinian), and within the late Miocene (Tortonian). Sedimentation rates were calculated as 205 m/m.y for the Pleistocene/ Holocene, 341 m/m.y. for the late Pliocene, 167 m/m.y. for the early Pliocene, and 33 m/m.y. for the late Miocene.
Sediments at Site 976 were subdivided into four lithostratigraphic units.
Unit I (Hole 976A, 0-5.9 mbsf; Hole 976B, 0.0-362.1 mbsf; Hole 976C, 0.0-362.8 mbsf; Hole 976D, 0.0-30.0 mbsf) contains a Holocene-Pleistocene, open-marine, hemipelagic facies of nannofossil-rich clay, nannofossil clay, and nannofossil silty clay. Continuous and discontinuous, clayey, silt laminae occur irregularly throughout the unit. Carbonate content averages 28%. The carbonate fraction consists of nannofossils, foraminifers, bioclasts, micrite, inorganic calcite, and dolomite. Laminated beds of diatomaceous ooze, up to 5 cm thick, can be correlated between holes. Downhole variations in detrital siliciclastic grains suggest that Unit I contains three major cycles of upward-increasing terrigenous input.
Twenty eight ORL's occur in five discrete intervals within Unit I. These ORL's, which consist mainly of nannofossil clay to nannofossil-rich clay and generally contain 0.9 to 1.3% TOC (background is 0.5% TOC), are identified by low magnetic susceptibilities and subtle color changes. The ORL's range in thickness from <20 cm to >2 m. Examination of the smear slides reveals an amorphous organic component, as well as terrigenous plant fragments and spores.
Unit II (Hole 976B, 362.1-518.3 mbsf; Hole 976C, 362.8-379.7 mbsf) consists of Pliocene sand, silt, calcareous silty clay, and nannofossil clay. Core recovery was low (12%), probably because poorly consolidated sand intervals were washed during drilling. Average carbonate content is 33%. Where recovered, the sand consists mainly of quartz and shell fragments, with minor components of rock fragments (including schist and serpentinite), feldspar, micas, heavy minerals, plant fragments, and traces of glauconite.
Unit III (Hole 976B, 518.3-660.2 mbsf; Hole 976E, 543.8-652.0 mbsf) is Miocene/Pliocene in age and consists of grayish-olive, nannofossil and nannofossil-rich clay and claystone. Hiatuses/unconformities occur between the early and late Pliocene, at the Miocene/Pliocene boundary, and within the Tortonian. The average carbonate content is 37%. Bioturbation is extensive, and Chondrites and Zoophycos ichnofacies are found throughout the unit. Laminations are present in a few intervals and, in places, are delineated by aligned organic matter. At Hole 976 E, immediately above Unit IV, the clays of Unit III exhibit a well-defined fissility (shale).
Unit IV (Hole 976B, 660.2-669.73 mbsf; Hole 976E, 651.95-652.08 mbsf) immediately overlies basement in both holes. In Hole 976B, Unit IV consists of coarse-grained, poorly sorted, coarse pebbly sand. The pebbly sand, which is Serravallian in age, is of marine facies and composed of quartz, biotite, feldspar, and rock and shell fragments. Rounded, gravel-sized, metamorphic clasts are present as minor components throughout the sandy interval. In Hole 976E, Unit IV is composed of a 15 cm-thick interval of glauconite-rich, sandy-silty claystone, also Serravallian in age.
Beneath the sedimentary sequence, we cored 259 m (669.7-928.7 mbsf) and 50.53 m (652.08-702.5 mbsf) of high-grade metamorphic rocks in Holes 976B and 976E, respectively. The contact between basement and the middle Miocene sediments is sharp and has an irregular topography, possibly produced by faulting. Faulting of the basement is indicated by breccia throughout the basement in Holes 976B and 976E. In the upper 40 m of basement, some fault breccias are formed by highly angular metamorphic clasts in a matrix with a Miocene sedimentary component.
The high-grade metamorphic basement rocks of the Alboran Basin at Site 976 are formed of the following lithotypes.
(1) High-grade schist: dark-gray graphitic schist with biotite, sillimanite aggregates, with andalusite and garnet porphyroblasts in some places.
(2) Gneiss: medium-gray felsic gneiss, commonly with biotite, feldspar, plagioclase, sillimanite, andalusite porphyroblasts up to 1 cm, inky blue or blue-green cordierite porphyroblasts up to 1 cm, and locally some muscovite. In places, the gneiss grades, with increasing biotite content, into high-grade schist, and with increasing felsic component, into migmatitic gneiss.
(3) Migmatitic gneiss: medium-gray, felsic biotite-cordierite-sillimanite-andalusite gneiss with irregular veins and patches of light-gray, weakly foliated or unfoliated granite, with biotite and tourmaline. The granitic material forms veins parallel to, or cutting across, the foliation. Associated coarse-grained quartz veins with tourmaline are abundant. In places, the granitic material contains cordierite.
(4) Marble: very pale green, gray, or white crystalline dolomite marble and calcite marble with minor amounts of phlogopite and chlorite. The calcite marble near the top of the basement in Hole 976E is interlayered, on a small scale, with calc-silicate rock and biotite-sillimanite schist.
(5) Calc-silicate rock: banded rocks with thin layers of calcite or dolomite, garnet, plagioclase, green calc-silicate minerals, including diopside and calcic amphibole, and serpentine(?) after forsterite(?). These minerals commonly occur as reaction zones between marble and schist.
(6) Granite: discrete pieces of light gray-to-white, fine-grained, hypidiomorphic, granular leucogranite occurs throughout the sequence, probably in the form of dikes. The granite has small amounts of biotite and tourmaline.
With the exception of the leucogranite dikes and the granitic leucosomes, all basement rock types show a well-developed foliation. These rocks also show evidence of penetrative ductile deformation, which produced a suite of small-scale structures and fabrics, followed by extensive brittle fracturing. At least three sets of ductile fabrics and structures can be systematically distinguished. The metamorphic sequence is also cut by numerous zones of fault breccia and fault gouge, which mark zones of brittle faulting. Marble occurs as layers dispersed throughout the sequence, and the dolomitic marble, in particular, is commonly associated with zones of brecciation and faulting. At the bottom of Hole 976B, samples include a large amount of well-cemented fault breccia. Some left-lateral oblique slip along discrete faults is suggested by striae on the subvertical fault planes that cross-cut the basement rocks.
First estimates of P-T conditions of metamorphism, based on data published by Spear (1993, and references therein), suggest that the high-grade schist underwent a significant decrease in pressure accompanied by constant, or possibly increasing, temperature. Migmatite gneiss and gneiss also indicate a late superimposed high-T metamorphism under low-P conditions associated with granite formation. The metamorphic history of basement rocks at Site 976 is most easily explained by tectonic exhumation of middle crustal rocks accompanied by substantial heating (Fig. 11).
The basement rocks recovered from Holes 976B and 976E closely resemble high-grade metamorphic rocks belonging to the Alpujarride Complex of the western Betic Cordillera (Spain), which have early Miocene radiometric ages, particularly those neighboring the Ronda peridotite massif. In that region, the high-T/low-P metamorphism is dated at 18-22 Ma (Zeck et al., 1992; Monié et al., 1994), i.e., early Miocene.
Site 976 sediments average 0.5% TOC with maximum values up to 1.6%. C/N ratios of samples containing a minimum of 1% TOC average 15.5 and indicate that the organic carbon-rich sediments contain a mixture of partially degraded algal material and continental organic matter. TMax values are relatively low, showing that organic matter is thermally immature with respect to petroleum generation. Concentrations of headspace methane are high at Site 976 and are probably derived from in-situ microbial fermentation of the marine organic matter. Gas levels never became hazardous at Site 976.
Interstitial-water concentrations of salinity, chlorinity, sodium, and bromine increase downcore to approximately 2-3 times seawater concentrations. Calcium, magnesium, strontium, and lithium also increase linearly with depth. These profiles suggest the presence of a deep-seated brine, which is preliminarily interpreted to be either a Messinian-age paleo-fluid, or a brine originating from dissolution of salts in a deeper part of the basin. The circulation of this brine may be driven by compaction or by hydrothermal influx along the basement/sediment contact.
High-resolution (1.5 m-interval), interstitial-water sampling of the upper 30 m at Hole 976 revealed a classical sequence of diagenetic redox reactions driving organic-carbon degradation. The Mn- and Fe-reduction zones are located within the upper 1.5 m of the core. Sulfate decreases linearly and is depleted at 19.95 mbsf, indicating that organic-matter degradation above this depth is primarily sustained by sulfate reduction. Below 20 m, headspace methane concentration increases rapidly, marking the onset of bacterially mediated methanogenesis, which is responsible for organic-matter degradation in the absence of interstitial-water sulfate.
Downhole temperature measurements with the ADARA and WSTP temperature tools indicate a heat flow of 102 mW m-2 at this site, which is in excellent agreement with other values measured nearby (Polyak et al., in press). Remanent magnetization of the sediments at Site 976 is weak and exhibits a stepwise decrease, by about one order of magnitude, down to 50-60 mbsf. Declinations are scattered and inclinations dominantly positive with only a few negative inclinations above 360 mbsf for Holes 976B and 976C. Between 675 and 710 mbsf at Hole 976B, negative inclinations suggest a reversed interval. A strong magnetic overprinting makes magnetostratigraphy difficult. Reliable MST velocity and GRAPE measurements could not be made at Site 976, owing to high gas concentrations in the sediments. Velocities in basement samples range from 3.3 to
6.5 km/s and have 20%-30% anisotropy.
One of the most exciting results of Site 976 is the discovery that basement beneath the Alboran Basin is formed by rocks of continental origin that have undergone high-temperature meta-morphism during exhumation and isothermal decompression. We cored a >250 m-thick section of high-grade metamorphic basement and obtained a spectacular suite of basement logs (quad-combo, FMS, BHTV, and GLT). Tectonic models that propose an early-to-middle Miocene continental extensional origin for the Alboran Basin postulated the existence of such metamorphic rocks at depth. Drilling results from Site 976 will significantly contribute to our understanding of the tectonic evolution of the Alboran Sea, as well as other backarc basins in the Mediterranean Sea. Integration of tectonic and paleoceanographic results from Site 976 will help to establish links between the paleogeographic history of the Atlantic-Mediterranean gateway and the evolution of Atlantic-Mediterranean water exchange since the Miocene.
To Site 977
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