Site Results

Site 1049 is located on the eastern margin of the Blake Nose and represents the deepest site of the Blake Nose transect. The location was chosen to recover a sedimentary section of Eocene through Aptian-Albian deep-water strata to compare with age-equivalent, but more expanded, sections at shallower water depths. The site is at a present depth of 2656 m below sea level (mbsl) and is shallow enough to preclude significant dissolution in the lysocline, yet deep enough to record changes in bottom water chemistry produced by changes in the vertical structure of deep and intermediate waters during the Paleogene and Cretaceous.

The sediments were recovered with three adjacent APC/XCB holes in order to obtain complete recovery of the section. Although the middle Eocene, lower Paleocene-Maastrichtian and Aptian Albian sequences are condensed, they will be useful for paleoceanographic studies. The microfossil faunas are well preserved and in combination with magnetostratigraphy provide a good chronostratigraphic framework. In the upper 50 m of the section (middle Eocene), the GRAPE density and magnetic susceptibility records are remarkably featureless and do not provide an unambiguous composite record. On the other hand, we observed cyclic records in all three holes from the K/T boundary and below, which allowed us to develop a well-constrained composite Cretaceous section.

At Site 1049, the sediment/water interface is covered by a manganese sand and nodule layer that contains Pleistocene-aged planktonic foraminifers and nannofossils. Below the manganiferous sands we recovered middle Eocene to Aptian sediments that we divided into four lithologic units:

1. Middle to lower Eocene nannofossil ooze, with several ash layers.

2. Upper to lower Paleocene nannofossil ooze with several intervals of limestone and chert.

3. Paleocene to upper Campanian clayey nannofossil ooze and chalk. The K/T boundary was recovered at all three holes within this unit and seems well preserved and complete. The biostratigraphically constrained boundary consists of a 0.3-cm limonitic layer containing brown yellowish spherules (and presumably microkrystites and the Ir anomaly) overlying a 9 to 17 cm, normally graded bed of green clay spherules that is interpreted as microtektite ejecta (Fig. 11). The limonitic layer is overlain by 4 cm of gray mottled clay, representing the earliest Cenozoic. Below the K/T boundary, the 20-m-thick interval of Campanian to Maastrichtian oozes shows signs of slumping below the K/T boundary.

4. Lower Albian to Aptian bioturbated clayey nannofossil chalk and claystone with high frequency variations in color and magnetic susceptibility among light gray, dusky red, and greenish gray beds and including a 46-cm interval of organic-rich (maximum 11.5% total organic carbon [TOC]) black shale laminated on a millimeter scale.

Middle Eocene through upper Aptian sediments are characterized by a low average sedimentation rate of approximately 6 m/m.y. Sedimentation rates during the lower Eocene (approximately 11 m/m.y.) appear to have been slightly higher than those of the middle Eocene. The upper lower Eocene is separated from the middle upper Paleocene by a hiatus of at least 2 m.y., encompassing the Paleocene/Eocene boundary. Planktonic foraminifers and calcareous nannoplankton within the Paleocene are abundant and well preserved, radiolarians are absent, and benthic foraminifers are common and well preserved. Most noteworthy is the exquisite foraminifer preservation and complete recovery of all lower Danian planktonic foraminifer and calcareous nannofossil zones, including the basal Danian P0 Zone and the Pa Zone (P. eugubina Zone), which are either poorly preserved or unrecovered at other deep-sea sites containing the K/T boundary. Studies of this early Danian interval will provide a highly detailed record of paleoceanographic and evolutionary changes associated with the earliest radiation of oceanic plankton and benthos following the terminal Cretaceous extinction event.

An excellent composite record of middle Campanian through latest Maastrichtian time was also recovered. Sedimentation rates for this interval are very low, averaging about 2 m/m.y. Rhythmically bedded red and green calcareous claystones and white chalks at the base of the cored interval contain calcareous nannofossils and planktonic foraminifers of the early Albian and late Aptian. Paleodepth estimates based on benthic foraminifers revealed a deepening trend through time at Site 1049: from middle bathyal depths (~800-1000 m) during Albian times to lower bathyal depths (1000-2000 m) throughout the latest Cretaceous (Maastrichtian) and Paleogene (Paleocene to middle Eocene).

Portions from nearly all cores yielded magnetostratigraphic data, and most of these polarity intervals were reproduced at similar depth intervals in all three holes. Early to middle Eocene Chrons C19n through C22n appear to be complete, although these preliminary chron assignments may change as the shipboard Middle Eocene biostratigraphy is further refined. The K/T interval is clearly in C29r, but it was not possible to delineate the exact positions of the chron boundaries. Upper Maastrichtian Chrons C30n-C30r-C31n are present. A brief lower Albian reversed-polarity zone could be the elusive M"-2" reported on Leg 40. Within the Albian, an interesting "reversed" polarity interval is present adjacent to a black shale that appears to be caused by redox-induced precipitation of iron during a post-Santonian reversed-polarity period.

Sediment porosity decreases from 75% near the top of the hole to 40% at approximately 160 m. Over the same depth interval compressional-wave velocity increases from 1500 m/s to 1740 m/s. The compaction state of the upper sediment column suggests that about 75 m of sediment was removed by erosion at Site 1049. The K/T boundary was well resolved with P-wave, magnetic susceptibility, and GRAPE data. Magnetic susceptibility increases in magnitude, and GRAPE bulk density and P-wave velocity decrease in magnitude. These changes may be due to the occurrence of more iron-rich noncarbonate material, which has a coarser grain size and is less consolidated within the boundary layer. The Albian organic-rich black shale was well defined by MST data, which show a decrease in magnetic susceptibility and an increase in natural gamma radiation. An organic carbon-rich interval occurs in Albian variegated claystones and ranges from 1.7% to 11.5% TOC with hydrogen indices of 451 to 699 mg HC/gTOC. The organic matter has a marine origin in contrast to the terrestrial-sourced organic matter from the Aptian-Albian of the Blake Bahama Basin.

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