The five sites cored during Leg 207 recovered sedimentary sequences that record the sedimentary and paleoceanographic history of the tropical Atlantic. The sedimentary pile can be divided into three broad styles of depositionsynrift clastics, restricted marine "black shales," and open marine chalk and calcareous claystones (Fig. F8). Synrift deposits show a variety of lithologies, and important questions remain regarding depositional water depth, terrigenous sources, transport paths, and possible differences in age within and among the sites. Pelagic deposition included long intervals of relatively constant sedimentation recovered at multiple sites suitable for detailed chronostratigraphic and paleoceanographic studies. In addition, the changing proportions of terrigenous and biogenic components, the distribution of regional hiatuses, and benthic isotopic studies should help constrain the evolution of circulation within the tropical Atlantic. The most dramatic lithologic features recovered during Leg 207, however, were the sedimentological expression of the P/E and K/T boundary events and black shale deposition encompassing several OAEs.
The P/E boundary is placed at the level of a greenish clay bed ~30-cm thick that is unique at each site (Fig. F9A). It is provisionally interpreted as the record of the shoaling of the CCD following the P/E boundary event and is expected to coincide with both the benthic foraminiferal extinction event and the carbon isotopic excursion. The 15 cm of the Paleocene below the clay shows a progressive increase in clay content toward the boundary. Above the boundary, the clay is relatively thick and distinctly laminated at Site 1260, but this fabric is not seen at the other sites. Either the record at Site 1260 is exceptionally complete or there were paleobathymetric differences in expression of the event on Demerara Rise. A 12- to 20-cm-thick red interval begins 220 cm above the P/E boundary at Sites 1258, 1259, and 1261 but not at Sites 1257 or 1260, documenting additional site-to-site variability.
Magnetic susceptibility measurements suggest that the clay-rich part of the boundary sequence ranges from ~1 m in thickness at Site 1261 to ~2 m in thickness at Site 1258. The differential thickness of the magnetic susceptibility peak may reflect depth-related changes in carbonate preservation or variations in sedimentation rate. All the sites display pronounced cyclicity in physical property measurements and sediment color. Cyclicity is particularly pronounced at Site 1259, where a short period cycle with an ~25- to 30-cm wavelength is modulated by an ~1- to 1.5-m cycle that resembles the modulation of the orbital precession cycle with a period of ~21 k.y. by the 100-k.y. eccentricity cycle. The pervasive cyclicity in physical property records from the lower Eocene offers the possibility not only of refining the chronology of the P/E boundary event, but also cross-checking the results between sites.
Compared to the P/E boundary, the lithologic character of the K/T boundary is remarkably consistent at the three sites in which it was recovered (Fig. F9B). At each site, sediments ~0.5 m above and below the boundary have a yellowish brown tinge, but the causal relationship of these colors to the boundary is unclear. The boundary itself occurs within an interval of pelagic sediment with no visible change in depositional style until 0.3 cm below an apparent ejecta layer marking the boundary. This layer is 1.52 cm thick and is composed of normally graded, green spherules. Spherules decrease in diameter from ~1 mm at the base to ~0.25 mm at the top; the uniformity of this layer among sites suggests settling of ejecta material without subsequent reworking (i.e., an air fall deposit). The spherule bed sits on a ~3-mm-thick gray fine-grained homogeneous layer, probably composed of Maastrichtian nannofossils. Possible interpretations of this layer include (1) fine material suspended or elutriated by impact shaking, (2) a thin layer of reworking between the time of the impact and the arrival of the first spherules, and (3) a diagenetic artifact related to rapid burial and alteration of the spherules. The spherule bed is overlain by a clay-rich interval containing characteristic earliest Tertiary as well as bloom foraminifers and nannofossils. Carbonate content gradually increases over ~0.5 m, and the basal Danian seems to be expanded, suggesting that details of the paleoceanographic conditions in the earliest Tertiary may be preserved on Demerara Rise.
In contrast to these two event layers, tens of meters of organic-rich laminated sediments (black shale) were recovered at all sites during Leg 207 (Fig. F8). These rocks represent the local equivalent of widespread organic-rich sedimentation in the southern part of the mid-Cretaceous North Atlantic (Kuhnt et al., 1990). They include the globally recorded OAE 2 around the Cenomanian/Turonian boundary.
On Demerara Rise, black shale sedimentation began abruptly in the early Cenomanian and extended into the Campanian. The black shales unconformably overlie Albian shallow-water siliciclastic sediments, including a unit interpreted as a tidal flat deposit. Some of the Albian claystones have a high organic carbon content, but organic matter composition and a lack of lamination distinguishes them clearly from the younger black shales. During the Coniacian, and especially in the Santonian, oceanographic conditions in the region were apparently less stable, as indicated by intercalated glauconitic and bioturbated intervals. Bioturbation affected large intervals of the organic-rich claystones and destroyed the lamination. Furthermore, probable tectonic instability on Demerara Rise lead to widespread mass wasting, resulting in numerous debris flows affecting the ConiacianSantonian part of the black shale sequence. Sediment clasts of Campanian age in one debris flow indicate mass wasting lasted at least into the Campanian. The lithologic transition from the black shales into the overlying Campanian pelagic deposits at some sites occurs over several meters of glauconite-rich bioturbated claystone. At other sites, it is a sharp contact at the top of a debris flow deposit. The basal Campanian chalks typically contain some glauconite, quartz, and a strong accumulation of zeolite-replaced radiolarians.
The black shales are characterized by the following three facies:
Throughout the sediment and in particular the organic-rich claystone is characterized by a distinct and sometimes very high content of well-preserved fish debris and phosphatic nodules (~2 cm). These occurrences form either discrete layers/intervals or are scattered in the background sediment. Diagenetic calcite formation is common at the shallower sites. Calcite replaces organic-rich sediment in the form of "beef" or cone-in-cone structures up to several centimeters thick.
Black shale sediments seem to have been deposited in a shallow to moderately deepwater environment. Existing seismic lines do not support a silled basin model but rather an intensified oxygen-minimum layer impinging on Demerara Rise. This interpretation is further supported by occasional glauconite-rich bioturbated intervals at the shallower sites marking the weakening or even retreat of the oxygen minimum zone.
A principal objective of Leg 207 was to recover relatively expanded, shallowly buried Paleogene and Cretaceous sediments that could be used for paleoceanographic study of the tropical Atlantic. Of the five sites drilled, all recovered multiple sequences of Cenomanian and Turonian black shales, CampanianMaastrichtian chalk, as well as Paleocene, lower Eocene, and middle Eocene chalk. In aggregate, the recovered sections form a continuous record of tropical sedimentation from the late middle Eocene (~38 Ma) to the late Campanian (~76 Ma) and from the Santonian (~83.5 Ma) to the late early Cenomanian (~98 Ma). The oldest sedimentary rocks recovered during Leg 207 are lower and middle Albian claystones (Site 1258), and the youngest are PliocenePleistocene clay-rich nannofossil oozes (Site 1261). At all but one site (1261), the Paleogene was exposed at the seafloor or by <90 m of burial beneath Neogene sedimentary cover.
Neogene or Oligocene calcareous ooze is present at the seafloor at all the Leg 207 sites and consists mostly of winnowed foraminifer sands. The thickest Neogene sequence is present at Site 1261, where 370 m of Pleistocene, Pliocene, and upper Miocene strata is present above chalk of late middle Eocene age. The Neogene section at Site 1261 was spot cored through most of its thickness but appears to comprise an expanded sequence of lower Pliocene and upper Miocene deposits that form a drape over the shallower parts of Demerara Rise shoreward of the Leg 207 drill sites. Elsewhere, drilling encountered lower Miocene and lower Oligocene calcareous ooze below a thin veneer of Pleistocene foraminifer nannofossil ooze. At Site 1259, nearly 125 m of Oligocene and lower Miocene strata is present, but at least the upper 28 m represents a remobilized sequence that was slumped or eroded and redeposited in inverse stratigraphic order. Redeposited lower Oligocene calcareous ooze also is present at Site 1260. The E/O boundary is unconformable or highly condensed at all sites.
There are widespread Paleogene unconformities in the upper Eocene, the lower middle Eocene, and upper Danian (Figs. F10, F11). However, with the exception of the upper Eocene, the other hiatuses are mostly represented by sediment in at least one of the sites. Most notably, the widespread lower Eocenemiddle Eocene hiatus, which is found virtually everywhere in the North Atlantic, is represented by apparently continuous sedimentation at Site 1258. Indeed, recovery at Site 1258 appears to include the most complete lower Eocene section cored by DSDP and ODP in the tropical oceans (Figs. F10, F11). In the Cretaceous, there is a condensed surface or unconformity across the Santonianlower Campanian and another one between the upper Albian and lower Cenomanian (Figs. F12, F13). Further biostratigraphic work may well recover zonal markers for parts of the lower Campanian in the sequence of glauconitic sands that is present at the contact between the TuronianSantonian black shales and the upper Campanian chalk. The age of the AlbianCenomanian unconformity may be diachronous with the oldest Cenomanian section present at Sites 1258 and 1260, whereas Cenomanian sedimentation began slightly later in the late Cenomanian at the other sites.
Preservation of microfossils is highly variable at all Leg 207 sites. Calcareous microfossils are best preserved in parts of the Albian at Site 1258, the black shale sequence, and the Paleoceneall sequences rich in clay. In some cases, particularly in the black shales, foraminifers are preserved with translucent skeletons and primary microstructure. Much of the lower and middle Eocene suffers from extensive reprecipitation and recrystallization of calcite. Siliceous microfossils (mainly radiolarians) are present in parts of the middle Eocene, the lower Eocene, and Campanian and in spots within the black shales and underlying Albian claystone. Middle Eocene siliceous nannofossil chalks at Site 1260 are particularly notable for their expanded sequence of middle Eocene radiolarian zones with excellent preservation.
During Leg 207, we recovered the P/E boundary at all five sites, with a remarkable 10 cores spanning the boundary interval. At all sites, the P/E boundary is represented by dark green clay-rich beds that form a sharp contact with underlying chalk. The chalk below the boundary is frequently well lithified and shows a transition to slightly darker sediment that is often laminated ~12 cm below the first dark green clay. The green beds themselves are either massive (e.g., Sites 12571259) or display fine laminations (e.g., Sites 1260 and 1261) in the lower parts. Its preservation throughout the depth transect implies nearly equivalent paleowater depths at the time of deposition. Bioturbation gradually returns between 5 (Site 1261) and ~30 cm (Site 1259) above the base of the green bed.
Samples taken from the massive green beds at Sites 1258 and 1259 contain few calcareous foraminifers but common, poorly preserved radiolarians. Calcareous microfossils begin to appear at about the same level that the massive or laminated sequence begins to become bioturbated. The foraminiferal assemblages are characteristic of dissolved faunas, consisting mostly of small benthic foraminifers. However, as preservation improves, a distinct fauna of planktonic foraminifers appears. These species include M. allisonensis, A. sibaiyensis, and A. africana, all of which have been named the excursion fauna because they have been previously recognized only in the interval of the 13C anomaly. Leg 207 sites appear to contain a few additional, as yet unnamed, species of the excursion fauna (e.g., Kelly et al., 1996). In addition, the foraminiferal assemblage from Site 1259 includes a clavate species, P. paleocenica, which has thus far been described only from coastal Senegal and the equatorial Pacific (Site 1220). About 2 m above the base of the green beds at Site 1259, the excursion fauna appears to be rare or absent, but several other foraminifer species are present that may be undescribed. These include a nearly biconvex variant of Morozovella aequa.
Although Demerara Rise is located some 3500 km southeast of and therefore upwind from the Chicxulub impact site, ejecta layers ~1.02.0 cm thick were cored at the K/T boundary in six holes at three sites (Fig. F14). This is the first such evidence of the event reported from the South American continent proper.
The ejecta usually rests on a 1- to 2-mm-thick white calcareous layer (Fig. F14) composed of Cretaceous microfossils that were possibly suspended into the water column by the seismic shock of the impact. The ejecta itself is typically laminated, sometimes consisting of sublayers of darker and lighter color with different concentrations of spherules, black or dark to light green in color. These are often graded upward, and individual spherules may range in size from 1 to 3 mm in diameter.
Above the ejecta layer, the dark green boundary clay is relatively soft, low in carbonate, and sometimes drilling disturbed. In the most complete sections (at Sites 1258 and 1259) (Fig. F14), the greenish chalk beneath contains the latest Maastrichtian-age calcareous nannofossils Micula prinzii and Micula murus along with the corresponding planktonic foraminifers Abathomphalus mayaroensis and the rare Plummerita hantkenoids, which suggests that the Leg 207 sites preserve an unusually complete record of latest Cretaceous paleoclimate and biotic evolution.
Immediately above the boundary clay, the "disaster forms" Braarudosphaera and Thoracosphaera are present, but other nannofossils are difficult to extract from the more lithified Danian chalk farther uphole. Instead, planktonic foraminifers provide better age control for the biotic "recovery," beginning with rather small specimens of P. eugubina and associated forms that distinguish the basal Tertiary Zone P and hence progressing upsection through Subzone P1b or Zone P2 before a disconformity intervenes.
Each of the five sites (Sites 12571261) drilled in the course of Leg 207 recovered a sequence of claystones rich in organic matter, referred here to as black shales (Fig. F13). The black shale sequence has been attributed a Cenomanianearliest Campanian age based on biostratigraphic observations (Fig. F12). Throughout the depth transect covered by the five sites, the thickness of the black shales is relatively consistent (Site 1261: shallow; 89 m; Site 1259: shallow; 56 m; Site 1260: intermediate; 93 m; Site 1257: deep; 57 m; Site 1258: deep; 56 m).
The black shales rest on shallow marine to nonmarine sediments of Albianearliest Cenomanian age:Upper Albianlower Cenomanian: siltsandstones (Site 1261),
The black shales rest on shallow marine to non-marine sediments of Albian to earliest Cenomanian age: upper Albianlower Cenomanian siltsandstones (Site 1261), lower Cenomanian siltsandstones (Site 1259), lower middle Albian claystones (Site 1260), upper Albian siltstones (Site 1257) and upper middle Albian black shales and claystones (Site 1258). The onset of the black shale sedimentation occurred, except at Site 1258, in the Cenomanian over a period of ~3 m.y. (Fig. F13). Because of the low resolution, poor preservation, and absence of both calcareous nannofossils and planktonic foraminifers in the lower part of the black shale succession at most sites, an accurate age assignment of the basal part is impossible. The current biostratigraphic data cannot solve the question of whether the onset of the black shales at the various sites is synchronous or diachronous over a period of ~3 m.y. within the Cenomanian. A very dark black shale interval, intercalated in the lower part of the black shale succession of the shallow sites (1259 and 1261) is of late Cenomanianearly Turonian age and may equate the OAE 2.
The top of the black shale succession is diachronous, with ages ranging from latest Turonian (Site 1258) to mid-Coniacian (Site 1260) to late Santonian (Site 1261) to early Campanian (Sites 1257 and 1259). The different ages of the cessation of black shale deposition are partly caused by postdepositional erosion and/or condensation.
Shipboard paleomagnetic measurements of chalks generally indicate a polarity pattern consistent with chrons expected from the biostratigraphy. Chrons C30n through C32r were resolved in MaastrichtianCampanian clayey chalks at most sites; the upper Paleocene succession at Site 1257 displayed Chrons C24rC26n, and Chrons C18C19r were observed in mid-Eocene (upper Lutetian to lower Bartonian) siliceous chalk.
Shipboard assignment of polarity zones within some white to green-gray chalk intervals, especially the lowermiddle Eocene (lower Lutetian), was difficult. In this facies, the magnetic intensities after low alternating-field (AF) demagnetization steps approached the background noise level of the pass-through cryogenic magnetometer. In addition, when the color of the lithified clayey chalk was brownish to reddish, such as within the upperlower Eocene (upper Ypresian) and uppermost Maastrichtian, it was not possible to remove secondary overprints by AF demagnetization. Magnetostratigraphy within similar facies at ODP sites requires progressive thermal demagnetization of minicores in a magnetically shielded room and analysis with a cryogenic magnetometer, having an additional order-of-magnitude sensitivity. Therefore, we collected a large set of oriented minicores for shore-based analyses, which should enable resolution of the full EoceneCampanian polarity pattern.
An exciting discovery of Leg 207 was ubiquitous cyclic sediments in expanded sections of lower to middle Eocene (55035 Ma) siliceous chalks and in Maastrichtianupper Campanian (8065 Ma) clayey chalks at all five sites on Demerara Rise. Both of these time slices lack direct cyclostratigraphic calibration of the magnetic polarity pattern and associated geological time scale. Similar cycle-magnetic tuning of the Paleocene timescale (Chrons C29C24; ~6653 Ma) has been accomplished with Leg 171 and other DSDP/ODP sites. Therefore, the array of Leg 207 sites will complete a high-resolution astronomically tuned magnetic polarity timescale spanning the 50 m.y. of the CampanianEocene.
Linear sedimentation rates (LSRs) were derived from age-depth models at each of the Leg 207 sites. At all sites and with current stratigraphic resolution, the sediment sequences can be characterized by continuous but variable rates of sedimentation separated by distinctive hiatuses, condensed intervals, and/or mass flows. Each of the hiatuses comprises, based on biostratigraphic dating, at least 1 m.y.
The absence of biostratigraphic zonal markers, as well as the extensive normal polarity Superchron C34n, restricted the age assignment in most of the LowerUpper Cretaceous sequences recovered during Leg 207 (AlbianSantonian). Therefore, LSRs calculated for these intervals should be considered as imprecise estimates. These intervals include the black shale sequences (CenomanianSantonian) that were characterized by LSRs of 35 m/m.y. at Sites 1257, 1258, and 1259 and slightly higher values of ~8.5 m/m.y. at Sites 1260 and 1261 (corresponding to 0.30.9 cm/k.y. in Fig. F15). In addition, mass accumulation rate (MAR) calculations were limited by the lack of reliable dry bulk density (DBD) data at Sites 1258 and 1259.
Average LSRs increased markedly across the K/T boundary interval at all sites (Fig. F15). They were at least 1.5 and up to 2.8 times higher during the Paleogene (PaleoceneEocene) than rates recorded during the latest Cretaceous (CampanianMaastrichtian). Highest LSRs are consistently recorded for the Paleocene and Eocene, with no discernible change in LSRs across the P/E boundary interval at all Leg 207 sites (Fig. F15). Late Paleocene and early Eocene LSRs ranged from 7 to 15 m/m.y. (Fig. F15), characteristic for pelagic nannofossil chalk, which is the dominant lithology in this interval. The middle Eocene interval is characterized by a distinct decrease in LSRs from 11 m/m.y. (during the early Eocene) to 3 m/m.y. at Site 1257, whereas a relative increase in LSRs is recorded for the same time interval at Sites 1260 (from 12 to 20.5 m/m.y.) and 1261 (from 7 to 9 m/m.y.) (Fig. F15).
MAR calculations, using the LSRs and average DBD data, allow for a better assessment of sedimentation processes because the influence of compaction has been taken into account. This effect is clearly illustrated at Site 1261, where overburden by Neogene sediments on the Paleogene chalks is greatest and, hence, is reflected in lowest LSRs calculated for the Paleogene sequences on Demerara Rise. However, MARs are comparable to those found at the deeper Site 1257 (Fig. F15).
In contrast at Site 1258, where the change in LSR across the K/T boundary was least pronounced, MARs only increased by 10% during the Paleocene and Eocene, compared to 50% using LSR only (Fig. F15). Nevertheless, the conclusion that sedimentation rates, hiatuses notwithstanding, were higher overall during the Paleogene than during the Late Cretaceous still holds when MARs are considered.
Neogene oozes recovered during Leg 207 are too thin relative to sampling density at four of the five sites for a meaningful sedimentation rate to be calculated. In addition, slumping and hiatuses are present in these sequences. At Site 1261, however, the upper 368 m is unique among the five sites cored during Leg 207. The youngest interval is Pleistocene in age, which is separated by a gap of ~3 m.y. from an interval of Plioceneupper Miocene nannofossil clay and nannofossil ooze. Sedimentation rates in this section are the highest calculated for Demerara Rise (65 m/m.y.) (Fig. F15). This interval of high sedimentation rate sits above a thick (60 m) series of mass flow deposits that occupy a stratigraphic position corresponding to a >30-m.y. gap in pelagic sedimentation.
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