166 Preliminary Report


Site 1006 is located in 650 m of water approximately 30 km from the western platform edge of Great Bahama Bank. It is the most distal of the five transect sites positioned at the crossing of seismic lines 105 and 106. Drilling at Site 1006 was designed to core and log a thick sequence of drift deposits that were thought to extend through the Neogene with relatively few hiatuses. There were two principal objectives at this site. First, the sediments at Site 1006 were believed to have a greater pelagic component than the more proximal transect Sites 1003, 1004, 1005, and 1007. Therefore, they were more likely to be well suited for dating and establishing an oxygen and strontium isotope stratigraphy for the Neogene, thereby providing an independent indicator of sea level. Using the sequence stratigraphic framework, these datums could then be traced on the seismic reflections to the distal lobes of the prograding margins at the proximal sites. This approach will aid in the dating of the proximal sites where the chronostratigraphy is occasionally unclear as a result of missing sequences, poor recovery, diagenesis, and/or dilution by neritic sediments. The second objective was to determine if changes in the composition of the sediments varied in conjunction with variations in the strength of the Florida Current. If so, we will be able to correlate these variations with changes in sea level as recorded by the prograding and regressive sequences at the platform sites and the oxygen isotopic signature of the foraminifers.

A final objective was to examine pore-water chemistry in a more basinward site where pore water and geothermal gradients, in contrast to the slope sites, were believed to be predominantly diffusive in nature and not influenced by fluid flow.

A 717.3-m-thick middle Miocene to Holocene section was recovered at Site 1006. The sedimentary section consists of mixed pelagic and bank-derived carbonates with varying amounts of clay material believed to have been derived from Cuba and Hispaniola. The biostratigraphic control throughout Site 1006 is very good and almost all planktonic foraminiferal and nannofossil zones from the Pleistocene to upper middle Miocene are found. The abundant pelagic biogenic components are less diluted by platform-derived material, and microfossil preservation is less affected by diagenesis than in the upper slope sites (Sites 1003, 1004, and 1007). Benthic foraminifers indicate an upper middle bathyal paleodepth. An excellent record of magnetic susceptibility exists, showing fluctuations between negative and positive values that reflect the input of clay material. A magnetic reversal was found between 4.5 and 6.0 mbsf which was tentatively correlated with the Blake event (0.13 Ma). Good agreement was found between the integrated sonic and VSP logs and the calculated depth of the sequence boundaries. Chemical profiles reveal a shallow flushed zone extending to 30 mbsf. Below this depth, profiles are mainly diffusionally controlled with the exception of certain elements which are locally influenced by diagenetic reactions.

Sediments were divided into five lithostratigraphic units on the basis of compositional and textural changes. Unit I (0-125.95 mbsf, Hole 1006A; 0-127.9 mbsf, Hole 1006B; Pleistocene to late Pliocene) consists of largely unlithified, bioturbated nannofossil ooze (sand to silt-sized foraminifers) with a small component of aragonite needles in Subunit IA. Particle abundance and grain size increase downhole to the base of the subunit. In Subunit IB this ooze is interbedded with gray and olive clays reflecting erosion of siliciclastics from Cuba and/or Hispaniola.

Unit II (125.95-360 mbsf, Hole 1006A; 127.9-176.5 mbsf, Hole 1006B; Pliocene to early Pliocene) consists of nannofossil ooze and chalk in which some grains were infilled as a result of pyritization. In Subunit IIA there is an alternation between nannofossil ooze and a more aragonite-rich ooze containing some peloids. Subunit IIB is defined by the occurrence of chalk, but is generally very uniform as a result of the high rates of sedimentation. Minor to moderate bioturbation is pervasive.

Unit III (360-528.7 mbsf; early Pliocene to late Miocene) is composed primarily of light gray and light greenish-gray chalk. The unit consists of a series of fining-upward intervals. Firmgrounds characterized by sharp burrowed contacts are frequent in the upper part.

Unit IV (528.7-594.25 mbsf; late Miocene to latest middle Miocene) is composed of light gray and greenish-gray nannofossil chalk. The uppermost portion contains a series of thick intervals with sharp basal contacts. Within each interval, nannofossil chalk with bioclasts grade upward into nannofossil chalk and clay. The lower part of the sequence is punctuated by a series of firmgrounds.

Unit V (594.25-717.3 mbsf; middle Miocene) is composed of alternating intervals of olive nannofossil chalk and light gray nannofossil chalk with foraminifers. Throughout the entire unit, no primary sedimentary structures are visible. The degree of lithification increases downhole with the result that the lower portion consists of alternating intervals of chalk and limestone. The greenish gray to olive intervals are characterized by well-defined, flattened burrows that occur in association with Chondrites-type burrows. The light-gray intervals contain larger, well-defined burrows.

The facies succession at Site 1006 is interpreted as being governed by an interplay of current activity and sea-level fluctuations. We suggest that the bottom of cycles, which contain clay intervals, reflect the erosion of siliciclastics. These are overlain by nannofossil ooze with platform-derived bioclasts corresponding to neritic production. The uppermost stage consists of nannofossil ooze and planktonic foraminifers. These changes are reflected in the bulk mineralogy, with high aragonite recording high sea level. Aragonite content also appears to be highly correlated with the color reflectance at the 700-nm wavelength.

All nannofossil and planktonic foraminiferal zones were present throughout the middle Miocene to Pleistocene section. The lower Pliocene section of the sequence is expanded with foraminifers showing little evidence of diagenetic alteration, and it is ideal for paleoceanographic studies. An excellent record of magnetic susceptibility, which is suspected to be tied to changes in the clay content of the sediments, is present throughout the upper Neogene. A magnetic reversal was found between 4.5 and 6.0 mbsf, which was correlated with the Blake event (0.13 Ma).

Sedimentation rates vary considerably at Site 1006. The rates can be divided into four distinct periods. The rate for the Pliocene-Pleistocene section is 5 cm/k.y., which is considerably lower than the same interval at Sites 1003-1005. The rate for the lower Pliocene and uppermost Miocene sections is 12 cm/k.y., similar to the proximal sites, suggesting that similar depositional processes were influencing the sites during this period. The upper Miocene interval has a low sedimentation rate (3 cm/k.y.) that is within the range of normal pelagic sedimentation. The middle Miocene section has slightly higher rates, perhaps because of contourite deposition and the deposition of fine-grained, platform-derived material.

The predicted positions of the sequence boundaries using velocities calculated from the integrated sonic log and vertical seismic profile showed excellent agreement with the multi channel seismic data. These boundaries fell in the expected positions based on correlations determined at Site 1003. Measured discrete velocities, however, predicted significantly lower interval velocities, a phenomenon resulting from the soft nature of the rocks and the fact that shipboard velocities are not measured under in situ pressure.

A full suite of logs, with the exception of the Geochemical Logging Tool (GLT), was run from 103 to 716.8 mbsf. Throughout the entire sequence, the logs were characterized by small-scale cyclicity observed as alternations between thin, resistive, low natural gamma-ray layers and conductive layers with higher gamma-ray counts. The cyclicity is also well defined by porosity, density, and velocity. Firmgrounds are recognized by increased gamma ray, resistivity, and sonic velocity. A pattern of larger-scale cyclicity reflects changes in the abundance and spatial frequency of the cycles. These patterns may be related to depositional changes associated with current intensity and can be tentatively correlated with sequence boundaries. As a result of the high-percentage core recovery, these logs offer an excellent opportunity for core-log correlation.

The mineralogy at this site is dominated by low-Mg calcite. Aragonite is abundant in the Pleistocene age sediments decreasing to less than 5% in the upper Pliocene sequence. The concentration of aragonite increases to 40% in upper Miocene sediments. High-Mg calcite is present in the uppermost 20-30 mbsf, but quickly disappears coincident with the presence of dolomite. Dolomite is a minor but ubiquitous component throughout the core. With the exception of the upper 30 mbsf, the sediment profiles are dominated by diffusion from an underlying brine with local reactions involving Ca, Mg, Sr, K, Li, F, and Si providing deviations from the ideal diffusive profiles. In the upper 30 mbsf, there are no gradients in the major elements and only very small increases in alkalinity and Sr. Below this depth Cl- increases steadily to the base of Hole 1006A reaching a concentration of 717 mM. The Sr concentration increases steadily over this interval as a result of the dissolution of aragonite and the precipitation of calcite and dolomite. At 452.95 mbsf the Sr concentration reaches 7 mM, the highest values recorded in ODP or Deep Sea Drilling Project (DSDP) history. The Sr can attain this high concentration primarily because sulfate has been completely utilized in the oxidation of organic material. Concentrations of H2S are low at this site as a result of the abundance of iron which allows the formation of pyrite.

In summary, Site 1006 has met and exceeded all its initial expectations. Based on shipboard biostratigraphy, the sequence boundaries were dated and traced to the proximal sites where the ages agree well with those determined at Sites 1003 and 1005. The position of the boundaries was verified using a combination of VSP and integrated sonic logs. The excellent, continuous sedimentary sequence contains abundant well-preserved foraminifers, which will be dated using O- and Sr-isotope stratigraphy and tied to the shallow sites to date sea-level changes. In addition, the O-isotope stratigraphy provides its own independent record of sea-level changes. This correlation allows another primary goal of Leg 166 to be achieved; the isotopic and sedimentary record can be compared. The expanded Pliocene and upper Miocene sequence combined with the excellent microfossil preservation will allow this site to become a classic site for upper Neogene paleoceanography in the low-latitude Atlantic.

Site 1007 Results

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