161 Preliminary Report

Site 979

Site 979 is located in the southern Alboran Basin, a narrow depression between Alboran Island and the Moroccan coast, about 45 km north of Cabo Tres Forcas, Morocco. The site is situated south of the northeast-southwest-trending Alboran Ridge. The ridge, which is >30 km wide and ~150 km long, rises ~1000 m above the surrounding basin floor and above sea level at Alboran Island. This site was located where seismic data indicated a zone of syn- and post-sedimentary deformation, including Pleistocene-Holocene deformation and tilting (Fig. 13). The deformation is expressed as a series of folds and faults that extend from the southern flank of the Alboran Ridge to the adjacent basin floor. Our main objectives were to determine the age and stratigraphy of the uppermost part of the basin fill and to constrain the age of the deformation, thought to represent later stages of contractive reorganization of the Alboran Basin.

Site 979 was drilled to a total depth of 580.9 mbsf, where drilling was terminated to allow time for logging operations before the end of the leg. After excellent recovery and core quality in the upper part of the section, APC coring was terminated at 134.5 mbsf. Recovery in the XCB-cored interval was high, but the cores were extensively biscuited. Unfortunately, the biscuiting destroyed a significant portion of the original sediment texture, severely limiting sedimentological and structural observations to short (<10 cm) pieces of cores. In addition, recovered cores were highly gassy, which also caused considerable disturbance.

The recovered stratigraphic interval ranges from upper Pliocene (Subzone NN16a, MPL5a) to uppermost Pleistocene/Holocene (NN21, G. truncatulinoides excelsa zone). The Pliocene/ Pleistocene boundary is approximated by the NN19B/NN19A Subzonal boundary (between 340.77 and 345.33 mbsf). Calcareous nannofossils and planktonic foraminifers are abundant and well preserved in most of the sequence. Zone MPL5a (upper Pliocene) is consistently present down to 569.78 mbsf, and Zone MPL4b was not reached. Benthic foraminifers suggest lower epibathyal (500-1300 m) to upper mesobathyal (1000-1800 m) depths for these sediments. Average sedimentation rates at Site 979 were calculated at 196 m/m.y. for the Pleistocene and
176 m/m.y. for the late Pliocene. A short hiatus within the upper Pliocene was recognized, below which the biostratigraphic age data suggest an increase in sedimentation rate to 696 m/m.y.

The sediments recovered at Site 979 were quite uniform. Only one lithological unit was recognized.

Unit I (0-580.9 mbsf) is composed of Pleistocene-to-Pliocene, open-marine hemipelagic deposits with minor siliciclastic detrital layers. The dominant lithology is gray-to-green, nannofossil clay, which accounts for about 40% of the stratigraphic section. A typical composition of these hemipelagic facies is about 53% clay, 40% calcareous nannofossils, 6% micrite, 1% foraminifers, and trace amounts of detrital mica, opaque minerals, sponge spicules, and fecal pellets. Shell fragments and dispersed, silt-sized foraminifers are variably present. Disseminated grains, clusters, and rare nodules of pyrite are common throughout the unit, as well as trace amounts of glauconite. Bioturbation is common but varies in intensity throughout the entire sequence and includes Chondrites, Planolites, and Zoophycos. Minor lithologies include diatom-rich, silty-clay, ORL's, discrete sandy and silty layers, and intraformational breccia. Silty and sandy turbidites occur throughout the sequence. The unconformity identified by biostratigraphic data between 475.35 and 477.46 mbsf is marked by a weakly bioturbated to structureless nannofossil-rich clay with no obvious breaks in the sedimentary record.

The Pliocene-to-Pleistocene sediments recovered at Site 979 are for the most part horizontal, but there are local intervals of dipping beds that may have resulted from slump folding. Some intervals show a variably developed and variably oriented fissility oblique to the bedding and, in addition, they show fractures parallel to the core. The fissility appears to be an incipient disjunctive cleavage, and may be a response to deformation associated with the nearby Alboran Ridge.

The interstitial water profiles at Site 979 appear to be influenced by two main processes, early diagenesis of organic matter and the presence of a saline brine at depth. There are downhole linear increases in calcium, salinity, chlorine, sodium, and lithium which suggest a deep supply for these elements. Alkalinity peaks at 16 mM at 24.96 mbsf, and decreases to below seawater values toward total depth. Strontium concentrations increase from seawater concentrations at the top of the cored sequence to 3 mM at total depth, which is >30 times higher than seawater concentrations. Sulfate decreases from seawater concentrations to zero at 24.95 mbsf, which reflects bacterial degradation of organic matter through sulfate reduction.

Heat flow at Site 979 is 79 mW/m^2, which is significantly less than that reported at other sites in this area. Calculating heat flow using the shallowest measurement at 20.5 mbsf and its delta temperature (in situ minus mud line) of 2.04°C, the result is 111 mW/m^2, close to the 118 8 mW/m^2 reported from nearby sites. This value suggests that there may have been a recent change in bottom-water temperature.

Concentrations of carbonate carbon vary up to 8.3% and are equivalent to 2% to 69% sedimentary CaCO3. TOC fluctuates between 0% and 2.0%. Average TOC concentration is 0.7%, which is more than twice the deep-sea average of 0.3%. The elevated TOC concentrations of these sediments probably are a consequence of high sedimentation rates, which improve organic-matter preservation. Organic-matter C/N ratios reach as high as 20, but most are between 4 and 8, which is representative of algal organic matter. Rock-Eval analyses indicate that some marine organic matter has been oxidized, probably by microbial reworking.

Elevated amounts of headspace gas, including higher molecular-weight thermogenic gases, were encountered. High C1/C2 ratios indicate that the methane is biogenic in origin. Concentrations of propane exceed those of ethane in sediments below about 300 mbsf and exceed 100 ppm at the bottom of Hole 979A, suggesting that propane was produced by thermal degradation of sedimentary organic matter during some former period of elevated heat flow in the southern Alboran Basin.

Remanent magnetic directions seem to be extensively overprinted at Site 979. The overprint, which is almost antiparallel to the radial remagnetization observed at the other Leg 161 sites (Sites 974, 975, 976, and 978), points to an acquisition of anhysteretic remanent magnetization. Due to this overprinting, magnetostratigraphic polarity zones could not be identified at Site 979.

Physical property data indicate smooth variation in properties downhole. GRAPE data show several distinct changes in slope, which may be related to subtle changes in the sediments' strength. Velocity measurements were hampered by the gassy nature of the cores.

Quad-combo log data were acquired from 60.0 to 277 mbsf. The entire logged section appears to be rather homogeneous, as is reflected in the recovered cores. Between 157 and 168 mbsf, however, intervals with abrupt decreases in resistivity and acoustic velocity are found.

Site 979 results will allow us to constrain the age of the deformation in the southern Alboran Basin and around the Alboran Ridge. A comparison between seismic data and drilling results suggests that tectonic activity, including uplifting by folding and/or faulting of the Alboran Ridge, occurred from the late Pliocene to the Holocene. The high sedimentation rates suggest that active subsidence and sedimentation were coeval with the recent contractive reorganization of the Alboran Basin. No volcanic or volcaniclastic material was found at Site 979, indicating that the upper Pliocene-to-Pleistocene contractive reorganization was not accompanied by volcanic activity and implying that the volcanic Alboran Ridge was not active during these times. Although we terminated coring
~40 m before reaching a major angular unconformity visible on seismic profiles, extrapolation of our data indicates that the unconformity may correspond to the same erosional unconformity that was cored at Sites 977 and 978, and likely corresponds to the "M-reflector" in the southern Alboran Basin.

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