Virtually all these goals were met at Site 1052. The middle and upper Eocene section proved to contain well-preserved calcareous and siliceous microfossils, a very clean magnetostratigraphic signature, clearly defined cyclostratigraphy, and lacked unconformities between the top of Chron 19r (~41.6 Ma) and the top of the recovered section (~35 Ma). We recovered Chron C16n.1n in which the upper Eocene tektite strewn field is believed to occur, although we were not able to identify the tektite horizons. The completeness of the middle and upper Eocene section as defined by detailed chronostratigraphy will enable us to document the oceanic changes associated with the transition from the warm early Eocene to a cool late Eocene world in the low-latitude Atlantic Ocean. Cores from this site include a thick sequence of Danian and Maastrichtian strata that include a mostly complete K/T boundary sequence. We also penetrated a thick section of lower Cenomanian and upper Albian rocks at this site. These include laminated claystones that appear to represent low-oxygen environments near the shelf-slope break. The middle Cretaceous section contains a diverse assortment of calcareous microfossils, as well as ammonites that retain the iridescent luster of their aragonitic shells. The calcareous microfossils are extremely well preserved and ideal for meaningful stable isotope studies. Recovering the beautifully preserved middle Cretaceous assemblages was one of the main objectives of Leg 171B. Shore-based studies will document sea-surface and deep-water conditions during the middle Cretaceous warm period. Our one disappointment was not recovering the updip correlative section of the upper Aptian black shale sequence found at Site 1049.
Six holes were drilled at Site 1052. We divided the section into five lithologic units based on variation in microfossil and siliciclastic content, sedimentary structures, and color. Lithologic Unit I consists mainly of nannofossil or calcareous ooze with varying amounts of foraminifers and siliceous microfossils, and is subdivided into three subunits. The uppermost is a <5-m-thick layer of foraminifer sands and manganese nodules that is present across the entire Blake Nose transect. Foraminifers in this surficial layer range in age from Oligocene to Holocene and include substantial numbers of lower and middle Miocene taxa. The manganiferous foraminifer sands rest on pale yellow middle Eocene siliceous nannofossil ooze. As at Sites 1050 and 1051, a dramatic color change from pale yellow to light greenish gray occurs within the upper part of the Eocene ooze sequence. The Eocene section is generally well magnetized and contains an excellent record of cyclic variations in color and magnetic susceptibility, as well as a clear magnetic polarity stratigraphy. The youngest Eocene sediments belong to magnetochron C15r, planktonic foraminifer Zone P16 and the upper part of calcareous nannofossil Zone CP15b. There do not appear to be any substantial hiatuses within the Eocene section above Chron 19r, suggesting a long-term average sedimentation rate of about 2.8 cm/k.y.
Lithologic Unit II is divided into a ~33-m-thick interval with very pale upper lower to middle Eocene green nannofossil chalk and foraminifer chalk with chert layers and chert nodules and a ~38.4-m-thick interval composed of upper-lower to middle Eocene dark greenish gray to grayish green porcellanitic calcareous claystone. Recovery in the calcareous claystones was very poor, and drilling induced fragmentation is severe throughout due to the presence of chert layers. An extremely condensed section occurs at the base of this unit, where upper Paleocene to middle Eocene microfossils are mixed in a 5-m-thick interval of foraminiferal packstones, claystones, and chert. The unconformity partially represents the updip continuation of the hiatuses present in the upper Paleocene and middle Eocene at Site 1051, with the addition of the updip pinchout of all of the lower Eocene and a large part of the middle Eocene. We did not recover most of the upper Paleocene at Site 1052 because we suspect that the lithified layers tended to jam in the bit and cause the softer interbeds to wash away. Still, it is apparent that highly condensed sections of upper Paleocene and lower Eocene rocks on Blake Nose, like those present at Sites 1049 and 1052, tend to contain much more chert than the relatively expanded sections cored at Sites 1050 and 1051.
Lithologic Unit III is characterized by an alternation of dark greenish gray lower-to-upper Paleocene calcareous claystones and lighter greenish gray nannofossils with clay, and the colors grade downhole into light greenish gray. The top of this unit is defined at the lowest occurrence of chert, but the nature of the contact is not known due to poor recovery. The base of the unit is the K/T boundary where color changes from more uniform light gray to variable, mostly olive, tones. All told, the Danian and early part of the late Paleocene are represented by nearly 100 m of section. Microfossils tend to be moderately preserved through most of the section, although preservation improves markedly in the early Danian. The lowest part of the Paleocene appears to include a small portion of the lowermost Danian planktonic foraminifer Zone Pa, but the boundary ejecta bed was not recovered. However, some green spherules and quartz grains occur within burrows in lowermost Paleocene sediment.
Lithologic Unit IV includes an 87-m-thick interval that contains mostly greenish gray to light greenish gray Maastrichtian clayey nannofossil chalk and an 89-m-thick light greenish gray Maastrichtian nannofossil chalk to nannofossil chalk with clay. The uppermost Cretaceous is present (calcareous nannofossil Zone CC26b). We suspect that only a short section of the K/T boundary was not recovered. The entire Maastrichtian section exhibits meter-scale color cycles and slump deposits. Parts of the unit are faulted, but the faults appear to be contained within this lithologic unit, because major displacements that cut across older and younger strata are not evident on the MCS seismic Line TD-5. The middle part of the Maastrichtian appears to be relatively complete and to be almost completely recovered. Therefore, this section should be useful for studies of the climate history and biotic turnovers associated with the middle Maastrichtian. The base of the Maastrichtian is associated with a series of slump beds that contain reworked Coniacian nannofossils and rest directly on Cenomanian limestones and interbedded siltstones.
Lithologic Unit V includes hemipelagic Cenomanian to upper Albian sediments with a greater amount of terrigenous components than overlying sediment. Seismic profile MCS TD-5 shows that the Cenomanian-Albian sequence consists of two sets of clinoforms built over and to the northeast of a buried reef complex. The lower Cenomanian is a thin drape over a much thicker package of late Albian clinoforms, and the two appear to be partly unconformable. However, neither planktonic foraminifer nor nannofossil stratigraphy identify a substantial unconformity in Albian and Cenomanian rocks.
The Cenomanian includes dominant dark olive gray calcareous silty claystone to clay-rich siltstone. The sediment color varies from an olive gray to black, and darker intervals have a greater abundance of terrigenous components. An interval of small slumps and glauconite beds separates the Cenomanian from upper Albian deposits. The Albian sequence consists of green massive claystone alternating with dark laminated greenish-black claystones. The laminated claystones are rich in pyrite and contain clay with varying amounts of calcareous nannofossils, fine silt-sized quartz, fish remains, well-preserved ammonites, and organic debris. Total organic carbon content is always less than about 1 wt% and even the darkest laminated claystones are poor source rocks, because they are dominated by humic material. Indeed, gas content is uniformly extremely low throughout the rocks at Site 1052 and consists mostly of methane. Interbedded with the laminated claystones are lithified, coarser grained intervals that contain foraminifers, shallow-water limestone fragments and quartz. Color varies from light olive gray in the limestones to very dark olive gray in the more laminated rocks. Laminated claystones are more abundant and thicker toward the top of the Albian section. Near the bottom of Hole 1052E, the Albian sequence becomes dominated by slight to moderately bioturbated dark (dark olive gray) sandy siltstones clearly deposited in middle or outer shelf environments. These sandstones were probably deposited near storm wavebase, as suggested by the occurrence of well-sorted grainstones that are often features associated with sand waves. Apparently, the entire upper Albian clinoform stack represents a deepening upward package.
Albian and Cenomanian rocks all have normal polarity consistent with the Cretaceous Long Normal Chron C34n (Fig. 12). Rocks of this age were generally recovered in long coherent sections of core, which allowed us to collect high quality data on the inclinations associated with the natural remnant magnetizations of the rocks. Calculation of paleolatitude from these data suggests that Blake Nose was located at 23°N during the late Albian and early Cenomanian. These results are based on >700 data points and are statistically well constrained. The calculated paleolatitude is much less than the previously published estimates of North American paleolatitude of 30°N for Blake Nose during the Hauterivian to Santonian. Apparently, in previous reconstructions the middle Cretaceous North American pole has been placed approximately 1000 km too far southward with respect to the southeastern United States.
To 171B Site Results - Site 1053
To 171B Table of Contents