Site 978 is located in the eastern Alboran Basin, to the south of Cabo de Gata and 24 km north of Site 977. The site lies in a small, east-west-trending basin within the same 35 km-wide graben as Site 977, but north of the Al-Mansour Seamount (Fig. 12). Site 978 was selected because, like Site 977, seismic data showed a lower sequence that may represent the earliest syn-rift sediments that filled the eastern Alboran Basin. This tilted sequence is overlain by relatively uniform, mostly flat-layered sediments. Site 978 was drilled to complete objectives that were not achieved at Site 977. Our main interest was to penetrate below the same unconformity that was penetrated at Site 977, with the objective of sampling the underlying syn-rift sequence of the eastern Alboran Basin, and of determining the age and nature of these deposits, as well as the relative proportions of syn-rift and post-rift subsidence. In addition, Site 978 results will permit accurate seismic correlation between the Alboran Basin and the South Balearic Basin.
A continuous sequence of 485 m (from 213 to 698.0 mbsf) of upper Miocene (Zone NN11) to Pleistocene (Subzone NN19) sediments was recovered at Site 978. The Pliocene/Pleistocene boundary, as approximated by the first occurrence of Gephyrocapsa oceanica, is between 222.77 and 223.35 mbsf; the Miocene/Pliocene boundary occurs between 607.51 and 611.39 mbsf. All Miocene cores are assigned to Zone NN11 (Messinian or uppermost Tortonian). In the Pliocene interval, foraminifers are abundant and preservation is generally good. Miocene foraminifers are moderately to poorly preserved. Using foraminiferal and nannofossil ages and geomagnetic polarity events, sedimentation rates were calculated at 127 m/m.y. for the Pleistocene, 110 m/m.y for the late Pliocene, 122 m/m.y. for the early Pliocene, and 100 m/m.y. for the late Miocene.
The sedimentary sequence sampled at Site 978 was divided into three lithologic units.
Unit I (213.0-620.9 mbsf) consists of early Pleistocene-to-Pliocene, grayish-olive nannofossil clay to claystone, which is variably bioturbated. Foraminifers and shell fragments are dispersed throughout the unit. The carbonate fraction consists predominantly of nannofossils, micrite, bioclasts, and foraminifers. Terrigenous components include quartz, feldspar, mica, sedimentary and low-grade metamorphic rock fragments, and accessory minerals such as garnet and zircon. Minor amounts of organic debris (up to 5%) and diagenetic opaque minerals (up to 3%) are present.
Unit II (620.9-630.67 mbsf) consists of an upper Miocene, gravel-bearing interval, containing pebbles of volcanic and sedimentary rocks. The contact between Units I and II was not recovered. Some pebbles have smooth, rounded, weathered(?) surfaces and some are partly covered by a thin coating of microcrystalline calcareous material, zeolites, and smectite, possibly representing matrix or cement. The pebbles are formed of andesitic basalt to andesite, chert, limestone, quartzite, and metamorphic rocks.
Unit III (630.67-694.3 mbsf) consists of Miocene sandy and silty layers, which exhibit parallel and cross lamination and inverse to normal grading. Sparse bioturbation and in- situ brecciation and clastic dikes are observed throughout this unit.
Sediment bedding is mostly horizontal, and sporadic slump folding and small, syn-sedimentary faults occur. Slump folds are well preserved near the base of the Pliocene. Late Miocene sediments are significantly more consolidated than the overlying Pliocene sequence. Bedding in the Miocene section is also mostly horizontal, but some units are cut by numerous dilational fractures with irregular orientations. The fractures possibly represent hydraulic fractures formed by overpressured and underconsolidated zones within the sequence.
Carbonate content varies between 10% and >60% in the Pliocene sediments. Miocene sediments are distinctly lower in carbonate content. TOC averages 0.3% and reaches values up to 0.9% in some ORL's. Organic C/N ratios are mostly between 4 and 8. From Rock-Eval analysis, it is inferred that the organic matter has been heavily oxidized, probably by microbial reworking.
Concentrations of headspace methane are high in sediments at Site 978. The source of the methane is probably in-situ microbial fermentation of marine organic matter. Concentrations of propane, iso-butane, and iso-pentane exceed, or equal, those of ethane in sediments from about 300 to 500 mbsf. These C3, C4, and C5 gases were probably produced by thermal degradation of sedimentary organic matter.
Interstitial water salinity, chloride, sodium, and calcium increase downhole, with a steepening of the concentration gradients below 450 mbsf. Sulfate concentrations are close to zero to 450 mbsf and increase steeply below 500 mbsf. Because no halite salts are known at depth in this area, the high concentrations of these elements are likely due to trapped Messinian-age paleo-seawater or to lateral migration of saline fluids that are produced by dissolution from Messinian halite deposits that are present in the South Balearic Basin, about 30 km east of Site 978. The downhole lithium increase may also be partly related to evaporitic fluids. Alkalinity shows a maximum of 3.5 mM at 269.5 mbsf, which corresponds to the highest ammonia concentrations and indicates that organic- matter decomposition is most extensive at that depth. Magnesium concentrations are consistently below seawater concentrations, indicating precipitation of high-magnesium calcite or dolomite in the sediments. Strontium shows a maximum at 492.20 mbsf, likely indicating carbonate recrystallization.
At least 11 magnetic polarity zones were identified between 390 and 610 mbsf at Site 978. Correlation with biostratigraphic data suggests that these represent polarity subchron C2An.2n through subchron C3n.4n (3.22 to 4.98 Ma). A sharp increase in intensity occurs at 440 mbsf, about 50 m below the last well-defined polarity change. A similar increase in intensity was observed at 420 mbsf at Site 977, indicating good correlation between the two neighboring sites. Remanent directions, however, are substantially different between the sites, probably due to different coring techniques. RCB drilling at Site 978 resulted in the only reasonable magneto-stratigraphy obtained during Leg 161, as well as better preservation of sediment structures and less intense biscuiting than at Site 977, which was drilled using XCB.
The post-Messinian stratigraphy established at Site 978 supports our results and interpretations from Site 977, and an accurate correlation between the post-Messinian sequence at both sites is feasible. The uppermost Miocene, gravel-bearing interval, containing pebbles of volcanic rocks in Unit II and encountered at 620.9 mbsf at Site 978, can be seismically correlated with the gravel interval sampled at 598.5 mbsf at Site 977. This correlation confirms the correspondence between this gravel-bearing sedimentary interval and the M-reflector, and the fact that this reflector represents a strong erosional event in the eastern Alboran Basin, possibly from the early Pliocene flooding.
Drilling at Site 978 was successful in sampling the post-rift and the upper part of the syn-rift sequence of the eastern Alboran Basin and in determining that late, syn-rift sediments correspond to the upper Miocene (Tortonian). Post-cruise studies will help to clarify the paleodepth of deposition for this upper Miocene facies and will allow us to evaluate the rate of syn- to post-rift subsidence for the eastern Alboran Basin.
To Site 979
161 Table of Contents