Site Results

Site 1050 was drilled in 2311 m of water on the Blake Nose 10 km upslope from Site 1049. Two APC/XCB holes were drilled, which produced extremely high (up to 95%) recovery throughout the section except for two minor intervals. We returned to Site 1050 at the end of the cruise and rotary drilled Hole 1050C to recover Albian to lower Danian strata. Although Site 1050 is a short distance from Site 1049, the middle Eocene and upper Paleocene intervals are much more expanded at Site 1050 than at Site 1049. The sediments that contain well-preserved calcareous microfossils will be used to reconstruct the deep-water mass characteristics in the Eocene through correlation of stable isotope data, lithostratigraphy, and benthic fauna between sites at different water depths on the Blake Nose. The record of Eocene sediments recovered on Leg 171B will document how depth shifts of the boundary between deep waters and intermediate waters occurred as climate switched from the early Eocene warm period to Antarctic glaciation in the late middle to late Eocene.

The lithostratigraphy is remarkably uniform through most of the Eocene and Paleocene and consists of green to pale-yellow siliceous nannofossil ooze and chalk with minor amounts of chert and diatomaceous nannofossil chalk. We divided the section into three lithologic units based on color and microfossil content. Lithologic Unit I contains an uppermost layer that comprises manganese nodules that probably represents the present seafloor (lithologic Subunit IA), and a lower section (~38 m) of yellow middle Eocene siliceous nannofossil ooze with varying amounts of foraminifers and clay (lithologic Subunit IB). A distinctive 5-cm-thick vitric ash occurs in the upper portion of lithologic Subunit IB. A sharp color change from yellow to green at ~40 meters below seafloor (mbsf) was used to divide lithologic Unit I from Unit II, but it should be noted that there is no apparent change in sediment composition across the contact. The color change from yellow to green does not appear to represent a primary depositional feature and may represent differences in redox conditions within the sediment cover of Blake Nose. Lithologic Unit II (~276 m thick; middle Eocene to late Paleocene in age) is composed of light grayish green nannofossil ooze to siliceous nannofossil ooze that grades downhole to a light grayish green siliceous nannofossil chalk and nannofossil chalk with varying contents of radiolarians, sponge spicules, and diatoms. The transition from ooze to chalk occurs at a depth of ~90 mbsf. The contact between Subunits IIA and IIB is placed at the top of a manganese hardground at ~154 mbsf that occurs within an interval with common thin chert layers and corresponds to a decrease in carbonate. At least 10 vitric ashes occur in Unit II. The ashes, together with color reflectance, magnetic susceptibility, and GRAPE bulk density data, make it possible to produce a detailed correlation between the two holes at this site. The ash beds are composed largely of clear glass shards. Several ash layers also contain euhedral biotite and feldspar grains that may provide an absolute chronology for the section. Most ash beds are about 1 cm thick, but many thinner beds have probably been destroyed by bioturbation or drilling disturbance, because we found that many of the gray wisps in the chalk contain volcanic glass shards. Peaks in abundance of siliceous microfossils in both holes were observed at 140 to 155 mbsf and 180 to 220 mbsf. Sediments containing the latest Paleocene benthic foraminifer extinction were not recovered in either hole because this interval consists of hard chalk or chert that jammed in the core barrel. However, lowermost Eocene and the upper Paleocene sediments adjacent to this interval of extremely low recovery are biostratigraphically and magnetostratigraphically complete which suggests that the interval of poor recovery does not contain a major hiatus. Lithologic Unit III (14.8 m thick; late early Paleocene to early late Paleocene in age) is a diatomaceous nannofossil chalk to nannofossil diatomite.

Hole 1050C contains a record of the upper Albian to lower Danian. The Danian section consists largely of calcareous claystone and clay-rick chalk. Microfossil preservation improves in the lowermost Danian and faunas just above the K/T boundary are typically very well preserved. The K/T boundary is represented by an unusually thick lowermost Danian sequence. Planktonic foraminifer Zone P0 is more than 1.5 m thick and may be even thicker, because the K/T boundary cores were not sampled in detail on the ship. It is possible that the great thickness of the lowermost Danian section is due to slumping, but if this is so, the slumps did not pick up older sediments, because the Danian rocks contain only trace amounts of Cretaceous microfossils. The boundary itself is a slightly burrowed contact that does not contain a spherule bed like that found at Site 1049. The uppermost Maastrichtian appears to be present, as indicated by the co-occurrence of both the nannofossil, Micula prinzi, and the planktonic foraminifer, Abathomphalus mayaroensis.

The Cretaceous section below the K/T boundary contains many slumped beds. Most of the Maastrichtian consists of slumped nannofossil chalks between apparently coherent packages of sediment that still retain burrows and primary sedimentary structures. The lower Maastrichtian appears to be missing in part and rests on upper Campanian white foraminiferal chalks. In turn, the Campanian strata rest unconformably on a highly condensed sequence of Coniancian and Turonian red nannofossil chalk, manganiferous hardgrounds, and chalk containing abundant clay ripup clasts. The Turonian chalk is disconformable over Cenomanian claystones.

The Cenomanian and upper Albian deposits are primarily black, laminated claystones and black shale with thin interbeds of more calcareous claystones and hard chalk. Much of the section is strongly slumped. Most of the slumps were soft-sediment features, but some are associated with conjugate shear surfaces, slickensides, and microfractured fold hinges that suggest at least partial lithification occurred before downslope transport. The almost complete absence of coarse-grained rocks and green-laminated claystones at this site is in sharp contrast to the dominance of these lithologies at the updip Site 1052. It is very likely that we have actually recovered the deep water equivalents of Site 1052 rocks. Unfortunately, the severe slumping renders much of the Cenomanian-Albian section useless for high-resolution paleoceanography. This is a pity, because microfossils are typically very well preserved.

A combination of magnetostratigraphy and biostratigraphy provides a preliminary chronostratigraphic framework for the sedimentary sequences at Holes 1050A and 1050B. Shipboard magnetostratigraphy within the middle Eocene interval is not well defined, although there are distinctive changes from intervals of predominately reversed polarity to intervals of normal polarity. Notably, both holes display the same patterns of stratigraphic change in apparent polarity. These results suggest that discrete samples may produce a well defined polarity scale during post-cruise research. The upper Paleocene-lower Eocene section displays a complete, well defined, magnetobiostratigraphy from Chron C27n through C22r. The deepest part of Hole 1050A was dated within the very top of the Danian (upper Chron C27n). Shore-based refinement of the magnetostratigraphy will make Site 1050 a reference site for calibration of the Paleogene biostratigraphy to magnetic polarity chrons.

The preservation of the calcareous nannofossils and planktonic foraminifers is good in the middle Eocene section and moderate to good in the lower Eocene and Paleocene. The exception is the interval straddling the Paleocene/Eocene boundary, where all calcareous microfossils show poor preservation due to extensive overgrowth and calcite cementation. The radiolarian faunas are generally well preserved. The youngest Eocene deposits correspond to Calcareous nannofossil Zone CP14a and Planktonic Foraminifer Zone P12, whereas the oldest sediments recovered belong to calcareous nannofossil Zone CP3 and the earliest part of Planktonic Foraminifer Zone P3a. The apparent absence of calcareous nannofossil Zones CP12a and CP10 suggests the presence of two hiatuses in the Eocene that each have a duration between 1.5 and 2 m.y.

Bulk density increases gradually in the upper 140 m of both holes and shows an abrupt increase followed by a equally sharp decrease at 150 mbsf, where the first chert stringers were encountered. Below this, bulk density increases steadily to a high value near 200 mbsf, and bulk density remains high through the upper Paleocene section. The transition to diatomaceous nannofossil chalk from nannofossil chalk at 300 mbsf corresponds to an abrupt drop in bulk density and P wave velocity. P-wave velocity increases dramatically about 90 mbsf corresponding to the transition from ooze to chalk and to the depth at which we were forced to switch from APC to XCB. Pelagic carbonates in Neogene sequences typically show a transition from ooze to chalk at ~160-200 mbsf. Hence, the relatively shallow depth of this lithologic and physical property transition at Site 1050 suggests that at least part of the sedimentary sequence has been WAS eroded. Estimates of the amount of sediment removed, based on measured shear strength and bulk density, suggest that as much as 147 m of section may have been eroded from the top of the Blake Nose at this site.

Interstitial water chemistry shows significant changes with depth. Strontium, lithium, potassium, magnesium, calcium, and alkalinity all display marked gradients, particularly between about 80 to 220 mbsf. Major cation changes are consistent with sea water interactions with the volcaniclastic sediments (such as the volcanic ashes found throughout the Eocene section) and/or the underlying Jurassic-Cretaceous carbonate platform. Strontium concentrations and strontium/calcium ratios are both consistent with recrystallization of biogenic carbonates and with pervasive calcite overgrowth and cements in the lower Eocene and upper Paleocene sequence. Carbonate content is about 70% 75% in the upper 150 m. Marked decreases in carbonate content occur between 150 and 160 mbsf and again at 300 mbsf, where carbonate drops first to 50% and then to 30%. Both decreases in carbonate content are associated with increases in biogenic silica.

Organic carbon content is extremely low throughout the section and averages about 0.05 wt% in the upper 200 m. Other than a few spikes of less than 0.3 wt%, total organic carbon is essentially zero below 225 mbsf. Methane decreases upsection, probably as a result of increases in methane consumption by aerobic bacteria in the shallow sediments.

To 171B Site Results - Site 1051

To 171B Table of Contents

Publications Homepage

ODP Homepage