Site Results

The location of Site 1051 was chosen to recover a thick Paleogene and Upper Cretaceous sequence. We anticipated that the Paleogene sequence in particular would be much more expanded than those at the other sites along the Blake Nose transect, as indicated by the seismic profile across the Leg 171B sites (MCS Line TD-5). We also expected to recover the Cretaceous/Paleogene boundary at intermediate water depths along Blake Nose and a more expanded sequence of Upper Cretaceous sediments than at Site 1049. The drilling results demonstrate that the Paleogene section at ODP Site 1051 is substantially thicker and includes younger Eocene sediments that are not found in the deeper water sites on the Leg 171B transect. The detailed shipboard biostratigraphy indicates that an almost complete sequence was recovered from the lowermost part of the upper Eocene to the upper part of the lower Paleocene. Unfortunately we had to stop drilling in the lower Paleocene section as our drilling rate became too slow to justify continued XCB coring.

We recovered a 630-m-thick sequence from two holes drilled at Site 1051. The lowermost upper Eocene to lower Paleocene sequence contains mainly oozes and chalks predominantly composed of nannofossils, siliceous microfossils, and clay. The siliceous component consists of generally well preserved radiolarians, sponge spicules, and diatoms. The clay content increases downhole in the lower Eocene and Paleocene. Over 25 ash layers were identified, spanning the majority of the Eocene sequence.

The sequence at Site 1051 is divided into four lithologic units, based on color, microfossil contents and lithology. The top of Unit I consists of several meters of manganese nodules and phosphatic, foraminifer sand, representing the present seafloor (Subunit 1A). The 63.95-m-thick Subunit IB is characterized by yellow middle Eocene siliceous nannofossil ooze with foraminifers and clay. A sharp transition from yellow to green is used to divide Subunits IB and IC. This transition is not marked by any change in microfossil or lithologic components and is clearly diachronous relative to a similar color change observed at Sites 1050 and 1052. We interpret the color change as a downhole diagenetic change in oxidation state. Subunit IC consists of a 66-m-thick section of predominantly siliceous greenish-gray nannofossil oozes. The transition between Subunits IC and ID is at the ooze/chalk transition. Unit ID is a 257-m-thick sequence of siliceous nannofossil chalk and nannofossil chalk with siliceous microfossils.

Unit II is 6.6-m-thick (376.1-382.7 mbsf) and was only partially recovered. It consists of strongly altered dark green porcellanitic smectite clay and several interbeds of silicified, white, foraminiferal packstone. Several distinctive firmgrounds are present in the recovered material and display white foraminifer sand that infills burrows in the green clay. The entire interval of clay and foraminifer packstone appears to coincide with a hiatus of ~2 m.y. in which bottom currents were episodically sufficient to thoroughly winnow the silt and clay fraction.

Lithologic Unit III is a 144.2-m-thick dark siliceous nannofossil chalk with clay. An apparently complete Paleocene/Eocene transition was recovered at Site 1051 and is partially laminated in the lowermost Eocene and IN parts of the upper Paleocene, indicating decreased bioturbation. There is also a distinctive soft-sediment breccia ~10 cm thick about 10 m below the Paleocene/Eocene boundary. The breccia occurs just above Chron C25n (55.9 Ma) and may represent part of a small slump. The slump appears to be within or just below the stratigraphic interval at which many Paleocene benthic foraminifer became extinct. Benthic foraminifer are very rare for more than 10 m within the extinction horizon and the fauna is reduced from about 40 taxa to only seven. An impoverished benthic fauna is present for a least 50 m above the onset of the extinction interval.

Occasional cross-bedded foraminifer sands occur at 450-470 mbsf. Unit IV is the oldest unit (lower Paleocene) and was recovered only in Hole 1051A. It is a 76.41-m-thick sequence of dark green siliceous nannofossil chalk to siliceous claystone or clayey spiculite.

Excellent age control was provided by biostratigraphy. Planktonic foraminifers and nannofossils are well preserved in the upper and middle Eocene. Preservation of both groups is variable in the lower Eocene, and becomes poor near the Paleocene/Eocene boundary, where foraminifers are infilled with calcite and recrystallized. Nannofossil preservation improves in the upper Paleocene and is moderate throughout the lower Paleocene. Planktonic foraminifer are overgrown in the lower Paleocene but are still useful for biostratigraphy. Almost all lower upper Eocene to lower Paleocene nannofossil and planktonic foraminiferal zones were recognized indicating that the sequence is complete except for two 1- to 2- m.y.-long hiatuses. The first hiatus coincides with the Unit II claystone and foraminfer packstone in the lowermost middle Eocene. A second hiatus occurs in the upper Paleocene where calcareous nannofossil Zone CP5 is missing.

Shipboard magnetostratigraphy was noisy, but a useful polarity pattern was obtained in both holes. Polarity interpretations were straightforward for the middle Eocene and upper Paleocene portions of the section and corroborate the biostratigraphic information. A well-defined magnetostratigraphy was obtained for the lower Eocene, but the sequence of polarity zones does not match the nannofossil biostratigraphy. A sequence of normal-polarity zones appears to have the right pattern and spacing to correspond to Chrons C23n through the base of C24n. However, nannofossil biostratigraphy suggests that the lower sequence of apparent normal-polarity intervals should correspond with the middle of Chron C24r (calcareous nannofossil Zone CP9a).

Color cycles are visible in nearly the entire sequence, with the exception of lithologic Subunits IA and IB and Unit II. The lower half of Subunit ID between 300 m to 380 m is badly biscuited by XCB coring and the record of color cycles is incomplete. The cycles in the middle Eocene may represent the 41-k.y. obliquity periodicity as judged from sedimentation rates determined by the biostratigraphy. In contrast, the Paleocene and early Eocene color records correspond more closely to a 23-k.y. precessional periodicity. The combination of lithologic cycles in the core and downhole log data should provide a high quality cyclostratigraphy that could enhance both the magnetochronology and biochronologies, as well as improve correlation between sites in the depth transect.

Hole 1051A was logged with three tool strings: the Triple Combo (natural gamma-ray, resistivity and formation density), the Formation MicroScanner (FMS) and the Geological High-Sensitivity Magnetometer (GHMT). The Sonic Digital Tool (SDT) was not used because of an apparent electrical incompatibility between the sonic and FMS tools that we were not able to fix during the time available for logging. Hole conditions were excellent with average diameter of 11 in and occasional washouts to about 15 cm. The hole was logged between 120 to 643 mbsf. Most of the logs clearly define the structure and depth of the lower-to-middle Eocene unconformity, as well as a prominent interval of soft-sediment deformation in the lower Eocene between 455 and 475 mbsf. Likewise, there are pronounced increases in gamma-ray, thorium, and magnetic susceptibility at ~510 mbsf that correspond to the depth of the benthic foraminifer extinction horizon, the lithologic evidence for soft-sediment deformation, and an increase in clay content. The transition from the upper Paleocene siliceous nannofossil chalk to diatomaceous nannofossil claystones (lithologic Unit IV) is associated with a gradual drop in magnetic susceptibility, as well as increases in gamma-ray attenuation and uranium content. The FMS produced high quality logs in two separate runs. The resistivity and magnetic susceptibility data from the FMS should help to produce a complete cyclostratigraphy for the Paleocene to lower-middle Eocene that will complement and check the cyclostratigraphies compiled from core measurements.

Sediments at Site 1051 are very low in organic matter, and gas samples consist largely of small quantities of methane and ethane. Both the inorganic chemistry of pore waters and analysis of gas samples detected marked changes in composition above and below the claystone and foraminifer packstone at about 380 mbsf. For example, strontium, lithium, calcium, and magnesium all show a clear shift in concentration across the hiatus. Apparently, the claystone acts as a seal that prevents upward flow of gas and pore waters. The same level also corresponds to an abrupt drop in carbonate content from about 75% to about 50%.

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