An upper Pliocene-Holocene section is present at each of the four Alboran Sea sites drilled during Leg 161 (Fig. 1) and, at the resolution of micropaleontological sampling, each appears to have been deposited without significant nondepositional or erosional breaks. The dominant sediment type present in the studied interval of all sites is gray to green nannofossil clay and silty clay (see Shipboard Scientific Party, 1996a, for lithologic classification), which is variously structureless, mottled, or burrowed. Several subordinate lithotypes are present, and these impart a variable stratigraphy, discussed below, across the Alboran Basin. The variations are at least in part caused by local influences at the widely spaced sites.
A continuous section from the seafloor to below the upper/lower Pliocene boundary was cored at Sites 976 and 977. At Site 978 the uppermost 213 m below seafloor (mbsf) was drilled without coring except for two 9-m spot cores, but the hole did penetrate the upper/lower Pliocene boundary. At Site 979, the section was cored from the seafloor and drilling was terminated in sediments of late Pliocene age. The stratigraphy of the studied intervals is given in Figure 2. Organic-rich layers (ORLs) are present in the Pleistocene-Holocene interval at Sites 976, 977, and 979, and these can be correlated across the Alboran, Balearic, and Tyrrhenian Basins, and across the eastern Mediterranean (see Murat, Chap. 41, this volume). The host sediment for these enriched layers is the nannofossil clay and silty clay that is the subject of this study, but the description, interpretation, and significance of the ORLs themselves are discussed in Murat (Chap. 41, this volume).
Five holes were drilled at Site 976. Of these, only Hole 976B continuously cored from the seafloor to beneath the base of the upper Pliocene. Hole 976C, offset 50 m to the east of Hole 976B, cored most of the section from the seafloor, but was terminated above the base of the upper Pliocene. The stratigraphic section intersected by the two holes correlates well, there being no significant difference between the studied intervals (Fig. 2). Only samples collected from Hole 976B were available for this study.
At Site 976, the upper Pliocene-Holocene section is divided into two parts (Fig. 2), an upper nannofossil clay- and silty clay-dominated unit (lithostratigraphic Unit I, 0-362.1 mbsf) and a lower unit (lithostratigraphic Unit II), which is present between 362.1 and 518.3 mbsf. Core recovery from Unit II was low (17 m of the 142 m drilled, ~12%; Shipboard Scientific Party, 1996b). However, the described sediment types include sand- and silt-dominated beds present as discrete layers interstratified with nannofossil clay and silty clay. The sub-sand-grade sediment is indistinguishable from that in Unit I in texture and mineral composition. At least 22% of Unit II comprises sand or silt beds of this type (Shipboard Scientific Party, 1996b, p. 189). Hole 976B continued to a depth of 928.7 mbsf, where it was terminated in metamorphic basement. Hole 976C penetrated all of Unit I (362.8 m) and ~17 m of Unit II.
In Hole 976B, the Pliocene/Pleistocene boundary is located near the base of lithostratigraphic Unit I in the interval 357.9-361 mbsf. Unit II is entirely late Pliocene in age, lying unconformably over lower Pliocene marine clay. The oldest late Pliocene fauna present in Unit II comprises nannofossil zone NN18 (Shipboard Scientific Party, 1996b, p.201), four zones above the upper/lower Pliocene boundary.
Layering of two types is present within the dominant nannofossil clay and silty clay of lithostratigraphic Unit I. It is defined either by subtle color alternations or by horizontal to slightly inclined burrows (Fig. 3). At some locations (see Shipboard Scientific Party, 1996b, table 3), color banding can be attributed to a slight enrichment in organic matter that formed a recognized ORL, but this is not the case everywhere. The origin of many color bands remains unknown. Rare subordinate lithologies present in Unit I include silt laminae enriched in pyrite and shell fragments.
One fully cored hole, Hole 977A, was drilled at this site. The upper Pliocene-Holocene section is ~475 m thick, and sedimentation was continuous from the early Pliocene (NN14) until the Holocene (Shipboard Scientific Party, 1996c, p. 309). The lower/upper Pliocene boundary is located at 475-476.7 mbsf, and the Pleistocene/Pliocene boundary is between 262 and 264 mbsf.
The interval included in this study lies entirely within lithostratigraphic Unit I, which is dominated by structureless to moderately bioturbated nannofossil and calcareous clay and silty clay. Unit I was divided into three subunits by shipboard scientists (Shipboard Scientific Party, 1996c, fig. 5), and the upper two of these encompass the section included in this study. Prominent millimeter- to centimeter-scale color bands and intense bioturbation characterize the lower Subunit IB, with the color banding indicating alternations of calcareous and nannofossil clay (Shipboard Scientific Party, 1996c, p. 305). Common folded and dismembered intervals of color-banded units that are contemporaneous with confining strata indicate intraformational slumping and mean that discontinuities within the sequence are possible. In the overlying Subunit IA, the dominant clay and silty clay is characterized by commonly interstratified sand laminae and thin beds of both bioclastic and mixed bioclastic-siliciclastic composition, and by ORLs. Visible bioturbation in the clay and silty clay is moderate to slight when compared with Subunit IB, and soft-sediment deformation is rare. The gradational boundary between the two subunits is placed at 417.4 mbsf.
Three stratigraphic divisions are recognized within the sedimentary interval cored at Site 978. The single hole, Hole 978A, was drilled without continuous coring to a depth of 213 mbsf and was then fully cored to total depth at 698 mbsf. Two 9-m spot cores were taken from the upper part of the hole; from 110.3 to 119.9 mbsf and from 168.4 to 178.0 mbsf. The section is late Pliocene-Pleistocene in age and is contained wholly within lithostratigraphic Unit I. Sedimentation was continuous across both the upper/lower Pliocene boundary, which is located between 457.7 and 459.8 mbsf, and the upper Pliocene/Pleistocene boundary, which is present in the interval between 222.8 and 223.4 mbsf.
Lithostratigraphic Unit I was divided into three subunits designated IA, IB, and IC from the seafloor downwards, with the late/early Pliocene boundary located in the lowermost Subunit IC. Variably bioturbated nannofossil clay and silty clay are the principal sediment types present in Unit I. Stratification, where present, is defined by decimeter-scale "dark" and "light" color bands, in which some examples of the former have a silty lower interval up to 10 cm thick, overlying a sharp basal contact (Shipboard Scientific Party, 1996d, p. 357). Color banding was determined shipboard to be a function of carbonate content, with the darker bands having less than the lighter bands in any one sequence. The division into subunits is based mainly on details of sedimentary features observed within the color bands, and also on Subunit IC being considered sufficiently lithified to be called a claystone (see Shipboard Scientific Party, 1996a, for lithologic distinction). Minor calcareous (shell debris and foraminifer tests) and mixed calc-siliciclastic sand and silty sand layers are present throughout the interval studied. ORLs are assumed to have been present in the noncored upper section of the hole.
A single fully cored hole, Hole 979A, was drilled at this site and reached a total depth of 580.9 mbsf in upper Pliocene (NN16A) marine clayey sediment. All nannofossil zones younger than this are present in the hole, which supports continuous sedimentation. The late Pliocene/Pleistocene boundary is located in the interval 341-345 mbsf (Shipboard Scientific Party, 1996e, p. 397). The stratigraphic section was remarkably uniform in lithologic character at this locality, and only one stratigraphic unit, designated lithostratigraphic Unit I, was established. The predominant lithology is nannofossil clay, which is commonly burrowed. ORLs, in which nannofossil clay is the host sediment, are confined to the Pleistocene section (Shipboard Scientific Party, 1996e, table 2), as are most of the diatom-bearing intervals. Discrete sand and silt layers (Shipboard Scientific Party, 1996e, table 3) are regularly present throughout the entire section.