Coring at Site 1276 started at ~800 mbsf. Recovery was excellent throughout the entire cored interval, averaging 85% and approaching or exceeding 100% for many cores. Five lithologic units are recognized (Fig. F13), ranging in age from Albianuppermost Aptian(?) to Eocene. The units are defined primarily by the proportions of sediment types, sedimentary facies, and mineralogy of detrital and biogenic components. The sedimentary succession consists mainly of background hemipelagic mudrocks (bioturbated claystone and mudstone) with interbedded gravity flow deposits that vary from minor in some intervals to dominant in others. For example, Unit 2 and Subunits 5A and 5C are largely gravity flow deposits, whereas Subunit 5B is primarily hemipelagic mudstones and claystones. An exception to this general organization is Unit 4, in which the reddish brown, intensely burrowed muddy sandstones that occur (Fig. F14) are inferred to have been reworked by bottom currents on a well-oxygenated seafloor.
Gravity flow deposits show remarkable variability. They include the deposits of debris flows (Figs. F15, F16), low-density turbidity currents (Fig. F17), and mud-laden, viscous gravity flows that formed spectacular contorted structures in many beds (Figs. F18, F19). Texturally, the gravity-flow deposits range from siliciclastic mudrocks, siltstones, and sandstones to carbonate grainstones and marlstones. The sand fraction in these sediments is mainly (a) quartz, feldspar, mica, and rock fragments derived from metamorphic and plutonic source rocks; (b) recycled carbonate components derived from unlithified to loosely consolidated outer-shelf to slope sediments; (c) contemporaneous biogenic components; and (d) minor ash (e.g., in the mid-Paleocene). Bioclasts include benthic and planktonic foraminifers, red algae, bryozoans, mollusk fragments, and echinoderms. Other locally common particles include carbonate intraclasts and glauconite pellets. With rare exceptions, only the gravity flow deposits contain significant biogenic carbonate. This consists of bioclasts in grainstones and nannofossils in mudstones and marlstones. Most of the interbedded hemipelagic sediments are noncalcareous, reflecting deposition mainly below the calcite compensation depth (CCD).
Diagenesis is moderate throughout the succession, with minor mobilization and precipitation of silica in Units 1 through 3 and common carbonate cementation of grainstones, sandstones, and siltstones in Units 2 through 5. Authigenic siderite and dolomite form many concretionary bands and nodules in the hemipelagic mudrocks of Subunit 5B. Downhole changes in clay mineral assemblages, from mostly illite-smectite in Unit 1 through Subunit 5A to kaolinite-chlorite below Subunit 5A, are attributed to paleoenvironmental changes rather than to burial diagenesis.
Subunit 5C is intruded by two major diabase sills that locally altered and contact-metamorphosed their host sediments (Fig. F20). The remainder of the AlbianTuronian Unit 5 is very thick (>700 m). In addition to gravity flow deposits, it is characterized by ~5% finely laminated, organic-rich calcareous claystones to marlstones ("black shales") (Fig. F21) with total organic carbon (TOC) reaching ~10 wt% in some beds. The carbonate is mainly in the form of nannofossils. These laminites record times of enhanced input of mostly terrestrial organic matter under low oxygen conditions, with the notable exception of Ocean Anoxic Event (OAE) 2 (Cenomanian/Turonian), which contains significant amounts of marine organic matter.
Cored sediments at Site 1276 record postrift burial of irregular basement topography. Accumulation rate was rapid during the Albian to Turonian (maximum = ~100 m/m.y.) but then dropped steeply to <15 m/m.y. thereafter (Fig. F24). The Albian sediment influx is attributed to enhanced continental weathering in a warm humid climate, possibly accentuated by clastic input from active rifts farther north. After initial accumulation in a restricted ocean basin (i.e., black share deposition), Site 1276 experienced a pulse of bottom-current reworking during the TuronianSantonian that is inferred to originate from the establishment of a deep-ocean connection between the North and South Atlantic; the effect may have extended into the nascent Labrador Sea.
A hiatus in the middle Eocene at the boundary of Units 1 and 2 correlates with a discontinuity in the seismic reflection record that separates parallel reflections below from large sedimentary waveforms above. This may mark the initiation of strong abyssal circulation along the foot of the Newfoundland margin, correlative with Horizon Au in the western North Atlantic.
The AlbianEocene succession at Site 1276 has stratigraphic and facies similarities with DSDP and ODP sites in the western North Atlantic (Fig. F22) and on the conjugate Iberia margin. For example, dark-colored, locally carbon-rich mudrocks in the AlbianTuronian interval at Site 1276 are like those of the BarremianCenomanian Hatteras Formation. Also, the younger parts of the Site 1276 succession are comparable to the Plantagenet, Bermuda Rise, and Blake Ridge formations. Site 398 on the conjugate continental margin off Iberia is similar but contains significant nannofossil-rich sediment, indicating that it was above the CCD from the Campanian to early Oligocene.
One of the major discoveries at Site 1276 was two diabase sills emplaced within lower Albian sediments, 100200 m above basement as estimated from seismic reflection data. The upper sill is ~100 m thick, and the lower sill, which was not fully penetrated, is thicker than 10 m. Indirect constraints suggest emplacement soon after sediment deposition and at a very shallow level beneath the seafloor. These constraints are (1) the occurrence of vesicles in the sill, (2) the growth of porphyroblastic calcite occurring in contact metamorphosed sediments and predating compaction of these sediments, and (3) compaction folding of a calcite vein that was emplaced vertically in the sediments probably at the time of intrusion. The sills preserve chilled margins, and toward their centers they show an increase in average crystal size and a change of magmatic textures from predominantly interstitial to subophitic or ophitic (Fig. F23). Sill/sediment contacts preserve a thermal overprint marked by color changes, recrystallization processes, and very high reflectance of organic matter.
The sills are predominantly aphyric diabase, composed of primary plagioclase (40%60%), pyroxene (10%30%), magnetite (<5%), olivine (<5%), and glass (<20%). More differentiated rocks occur in segregation bands (e.g., sample 4i in Fig. F23) that form <5% of the rocks in the sills. Hydrothermal alteration in the sills ranges from high to complete at the margins to moderate toward the centers (see the alteration column in Fig. F23). The diabases are silica-poor basanites, with SiO2 values that range from 40 to 46 wt% and follow an alkaline differentiation trend. Samples taken from less altered parts of the sill show surprisingly coherent patterns for elements like K2O, NaO2, and CaO, assuming that the sills preserve their initial magmatic signature. Future petrological, geochemical, and age-dating studies of these rocks are expected to provide further information on the nature and evolution of the mantle source underlying the Newfoundland margin, as well as on the age of emplacement.
Paleogene and Cretaceous sediments cored at Site 1276 were deposited in abyssal depths (>2000 m) below the CCD. Hence, calcareous microfossil assemblages, particularly the planktonic foraminifers, were severely affected by dissolution in most intervals of Site 1276. High-resolution biostratigraphic analysis will ultimately depend on full integration of organic walled microfossils (palynomorphs: dinocysts, spores, and pollen), calcareous plankton (calcareous nannofossils and planktonic foraminifers), and siliceous microfossils (radiolarians). In situ agglutinated and redeposited calcareous benthic foraminifers also provide both a biostratigraphic and paleoenvironmental assessment of the deep western North Atlantic and the adjacent Newfoundland continental margin. Shipboard analysis of these varied microfossil groups provided a robust model of age vs. depth in the hole, as well as important insights into the changing paleoceanographic and depositional conditions during latest Aptian(?)early Albian to early Oligocene time.
Calcareous nannofossils provided excellent biostratigraphic control for most of the section, and planktonic foraminifers preserved in turbiditic sandstones proved helpful in refining that biostratigraphy. Palynomorphs provided a critical component for biostratigraphic age control in carbonate-free intervals. Calcareous benthic foraminifers indicated changes in the source areas for the turbidites cored at Site 1276. Reworking is a constant component in assemblages of all fossil groups, rendering the use of last occurrence datums problematic, particularly in the Paleogene section where reworking of older material is pervasive. On the other hand, a uniform age progression of the samples and minimal reworking of older material in the Cretaceous section indicates that redepositional processes appear to have been largely penecontemporaneous.
The age-depth model reveals marked changes in sedimentation rate, including at least one unconformity and two condensed intervals (see Fig. F24 for calculated sedimentation rates). The changes in slope correspond closely to lithologic unit boundaries. An unconformity marks the boundary between lithologic Units 1 and 2 based on palynomorph and nannoplankton data. The hiatus at this level indicates similarities in rift-wide sedimentation patterns when one considers results from studies on Sites 398, 900, 1067, 1068, and 1069 on the conjugate Iberian margin (McGonigal and Wise, 2001). Over the full stratigraphic section, comparison of the sedimentation rate at Site 1276 with that at Site 398 shows striking similarity not only in the positions of hiatuses or condensed intervals, but also in overall sedimentation rate (Fig. F24).
The paleoceanographic history of the opening of the North Atlantic is documented in the faunal composition of benthic foraminifers recovered at Site 1276. Exceptionally well preserved autochthonous deepwater agglutinated foraminifers (e.g., Kuhnt and Urquhart, 2001) throughout most of the sedimentary sequence imply that Site 1276 has remained at abyssal depths (>2000 m) since at least latest Aptian(?)early Albian time. Times of possible depression of the CCD are recognized in the late Albian, late Campanian, latest Maastrichtian, and late Paleocene.
All microfossil groups examined indicate that Site 1276 was influenced by transitional surface water masses during much of the Cretaceous and early Paleogene. This is suggested by the presence of select Boreal taxa and the absence or paucity of key Tethyan taxa. Hence, these taxa will be invaluable in determining oceanographic communication routes between the northwestern Atlantic, Tethys, and eastern Atlantic.
Although many of the sediments recovered are barren of calcareous fossils, debris flows and turbidites brought in more well preserved assemblages. Specifically, in the Paleocene and Eocene sections many debris flows contain robust assemblages of shallow-water origin, including calcareous benthic foraminifers indicating a neritic facies and abundant holococcoliths not generally preserved in deepwater sections. These taxa can provide insights into the paleoceanographic history of the nearby shallow-water areas. These allochthonous faunas indicate original depositional environments, together with the provenance of turbiditic packages and the timing of climatic change and/or tectonic disturbance.
For the extended AlbianTuronian section of Site 1276, palynological data indicate a strong terrestrial influence, which is documented by very high amounts of terrigenous organic matter. There is an overall trend toward a downhole increase of terrestrial influence. Samples from Albian black mudstones seem to be characterized by especially high amounts of terrestrial palynoclasts and sporomorphs. This is tentatively interpreted to indicate an accelerated hydrological cycle during the time of black mudstone deposition.
A series of "critical events" in Earth's history were cored at Site 1276. The recovered records are affected by abyssal burial depths below the CCD and by frequent turbidites, which make lithologic and microfossil information discontinuous. Nevertheless, the integrated results from the different microfossil groups, together with chemical and sedimentologic data, provide a preliminary documentation of the stratigraphy and environmental conditions during these intervals.
Worldwide, uppermost Paleocene marine sections are characterized by a sudden shift from carbonate-marl deposition to clayey, calcite-free deposition. The Paleocene/Eocene boundary interval is also characterized by an abrupt worldwide warming event referred to as the Paleocene/Eocene Thermal Maximum (PETM). In addition to the dissolution interval, the PaleoceneEocene transition in the oceanic sedimentary record is also distinguished by a sharp negative 13C excursion and by a benthic foraminiferal extinction event (Zachos et al., 1993; Thomas and Shackleton, 1996). The widespread pattern of decreased carbonate content is consistent with the hypothesis of a massive methane flux in the ocean-atmosphere inorganic carbon reservoir, which caused a short-term "super greenhouse" event (Dickens et al., 1997).
A nearly complete uppermost Paleocene to lower Eocene section is present at Site 1276. Even though the specific clay boundary layer is missing, probably due to incomplete core recovery, a complete succession of the calcareous nannofossil events occurring immediately above the boundary clay layer is recognized in Cores 210-1276A-13R and 14R. At the PaleoceneEocene transition, the calcareous nannoplankton community shows a great turnover characterized by peculiar biotic changes (Bralower et al., 1995; Angori and Monechi, 1996), which are recognized at Site 1276. This material is sufficiently well preserved to document a detailed biostratigraphic and evolutionary history of this fossil group, which can then be compared with other sections of the Atlantic domain in the framework of biochronological and paleoenvironmental changes associated with the PETM.
The Cretaceous/Tertiary (K/T) boundary records one of the most catastrophic perturbations to the Earth's biosphere and coupled ocean-climate system. A wealth of ODP research has provided data on the link between the K/T boundary mass extinction and a large-body impact on the Yucatan Peninsula. The Caribbean and North Atlantic sections are crucial to understand not only the marine biotic response to this event, but also to document disruption of the stratigraphic record as a consequence of the impact.
One of the few nearly complete upper Maastrichtian to lower Danian abyssal sections was recovered at Site 1276. Extensive reworking and frequent carbonate-free sediments prevent this section from being suitable for analyzing extinction processes, but the succession of biotic changes is obvious. The sequence includes a sharp increase of calcareous dinoflagellates ("Thoracosphaera bloom?") and an increase in Cretaceous survivor species, followed by blooms of dwarf earliest Danian species ("dwarf Biscutum bloom;" Gardin and Monechi, 1998). Penecontemporaneous reworking of planktonic foraminifers preserved in turbiditic sandstones provide an additional record of biotic extinction and recovery including an interval characterized by Cretaceous survivors (Guembelitria), together with earliest Danian species (Parvulorugoglobigerina eugubina, Woodringina sp.) and Thoracosphaera. The high sedimentation rates in the early Paleocene allow good resolution to study biotic recovery following the impact.
Oceanic Anoxic Events were short-lived episodes of widespread organic carbon burial, typically coupled with rapid changes in the ocean-climate system and marine biosphere. These perturbations in the global carbon cycle occurred during times of elevated tectonic activity, high global sea level, and generally warm climates of the mid- to Late Cretaceous (Leckie et al., 2002). A thick interval of gray to olive-black mudrocks was cored at Site 1276 (lithologic Unit 5). Much of the organic matter preserved in these hemipelagic and turbiditic sediments is likely of terrigenous origin, based on preliminary geochemical analyses and the preponderance of terrestrial organic matter observed in foraminiferal and palynomorph preparations. However, within Subunits 5A and 5C, there are several thin intervals characterized by laminated black shale, high TOC contents (37 wt%), and hydrogen index (HI) values (231452 mg HC/g TOC) characteristic of marine algal organic matter. The black shales in Core 210-1276A-31R correlate with the latest Cenomanianearliest Turonian OAE 2 ("Bonarelli" event). OAE 2 is one of the most widespread black shale events recognized in the marine record. It represents a time of enhanced productivity, perhaps triggered by tectonic activity or changes in ocean circulation. Black shale in Core 210-1276A-94R is potentially correlated with OAE 1b ("Paquier" event). A black shale in Core 210-1276A-98R, bounded by bioturbated sediments and located 8 cm above the lower sill cored at Site 1276, has yielded only strongly thermally altered palynoclasts.
Paleomagnetic studies consisted of routine measurements of natural remanent magnetization (NRM) and magnetic susceptibility of archive-half split cores of recovered sediments and rocks. Paleomagnetic data obtained from Site 1276 samples exhibit considerable variations in demagnetization behavior among various lithologies, and only one magnetic reversal was detected in the entire hole.
In the lower Oligocenemiddle Eocene cores in lithologic Unit 1, varicolored mudstone and claystone have low NRM intensity and low magnetic susceptibility. A few discrete peaks of higher NRM and susceptibility values could in some cases be tied directly to visible presence of pyrite. Although the NRM intensity of the middle Eoceneupper Paleocene sediments in lithologic Unit 2 was also low, we were able to define the characteristic remanent magnetization (ChRM) direction from a few intact cores. Both magnetic susceptibility and NRM intensity records show an anomalous peak at approximately the lower/upper Paleocene boundary. Lower Paleoceneupper Campanian lithologic Unit 3 has relatively high NRM intensity and magnetic susceptibility; it is caused by the presence of numerous dark burrowed beds that have relatively high concentrations of magnetic minerals. NRM intensity and magnetic susceptibility for lithologic Unit 4 are relatively high, perhaps because of the presence of fine-grained iron oxides. A strong drilling-induced overprint is present throughout this unit, which severely limits paleomagnetic work. Sedimentary cores from lithologic Unit 5 consist of lower Turonian to Albianuppermost Aptian(?) claystones and mudstones that have low NRM intensity and low magnetic susceptibility. There are more significant variations in susceptibility within lithologic Unit 5 than in lithologic Unit 4. Characteristic susceptibility peaks reflect carbonate and sandstone layers, and troughs correspond to green and gray claystones and mudstones. The susceptibility peaks correlating with the carbonates may reflect an iron component in these rocks, most likely in siderite.
Drilling in Subunit 5C recovered two diabase sills. The inclinations of the ChRM direction for these sills are all positive. The simplest explanation of the positive inclinations is that they represent normal polarity magnetization, probably acquired within the Cretaceous Normal Superchron. Significant changes in inclination and intensity values within the upper sill suggest that it may contain two intrusive units.
In order to test the archive-half data and to identify magnetic carriers in sediments of this hole, selected discrete samples from the five different lithologic units were demagnetized with stepwise alternating fields or were thermally demagnetized. Most samples show unblocking temperatures between 350°550°C, indicating that titanomagnetites are the likely magnetic carriers in these samples. Results from discrete sample measurements confirm the pass-through observation that no reversed magnetization polarity is present in these cores.
Time-series analysis was conducted on magnetic susceptibility and natural gamma ray data for several pelagic marlstone cores in lithologic Unit 5, with the goal of identifying paleoclimatic cycles driven by changes in the Earth's orbit (Milankovitch cycles). Preliminary results are encouraging, and they demonstrate that certain Leg 210 cores seem to be suitable for magnetic detection and extraction of climatic cycles and sedimentary changes in the Cretaceous.
At Site 1276 an extensive AlbianCenomanian black shale/mudstone sequence was cored. This lithologic unit (Unit 5) extends for >700 m (1064.241719.40 mbsf). It is characterized by moderately enriched TOC contents (mostly <2 wt%) (Fig. F25). This turbidite-dominated sequence shows low hydrogen index values (HI < 100 mg HC/g TOC) and C/N ratios averaging ~20 (Fig. F25), suggesting a strong influence of terrestrially derived organic matter. A positive correlation between C/N ratios and TOC contents supports the importance of a terrigenous contribution to the sedimentary organic matter. Tmax values ranging between 435° and 470°C could indicate that kerogen present in the sediments may be derived mainly from reworked, preheated terrestrial components (Wagner and Pletsch, 2001). Higher values of Ni/Al ratios in Unit 5 confirm the importance of anoxia, and therefore preservation, in the accumulation of organic matter throughout the sequence.
Headspace methane (C1) concentrations start to increase above background levels downhole only at a depth of 1140 mbsf in lithologic Subunit 5B (Fig. F25). In Unit 5, a possible weak correlation exists between higher organic carbon percentages and higher methane concentrations, suggesting that the concentration of organic matter may play a role in the generation of methane. C1 concentrations increase dramatically immediately below the upper diabase sill (Subunit 5C1; 1624.01 mbsf) and particularly in Cores 210-1276A-96R and 97R (1692.331703.5 mbsf) above the lower sill. These latter increases correspond to layers of anomalously underconsolidated high-porosity clays. Here, the methane concentrations are directly linked to the lithology of the formation; the sills probably formed seals and prevented gas and interstitial water from escaping. Low C1/C2 ratios and the presence, but small concentrations, of longer-chain volatile hydrocarbons in the deeper part of the hole, indicate that some thermogenically derived gas is present; it could have either migrated to this location or been generated in situ.
Within lithologic Unit 5, six sedimentary intervals have high TOC contents, in several instances coupled with high HI (characteristic of marine derived organic matter) and high S2, and they may record oceanic anoxic events.
The top of lithologic Subunit 5A (Cores 210-1276A-30R and 31R) is upper Cenomanianlowermost Turonian and it likely corresponds to the "Bonarelli" event, OAE 2. Core 210-1276A-33R (middle of Subunit 5A) probably represents the "mid-Cenomanian event" (Leckie et al., 2002). In lithologic Subunit 5B, Core 210-1276A-55R (characterized by terrestrially derived kerogen) may correspond to OAE 1c. The bottom of Subunit 5B (Core 210-1276A-73R) does not correspond to any of the known OAEs; however, it has the potential to represent an OAE-type layer. The top of Subunit 5B (Cores 210-1276A-42R to 44R) is late Albian and may correspond to OAE 1d, dominated by terrigenous organic matter (Leckie et al., 2002). Core 210-1276A-94R, in the middle of lithologic Subunit 5C is lower Albian and it might represent the "Paquier" event, OAE 1b.
Leg 210 physical property data for rocks and sediments include measurements of seismic velocity, density, porosity, natural gamma radiation (NGR), magnetic susceptibility, and thermal conductivity. Such data are a critical component for seismic stratigraphic studies and for understanding the complex physical and chemical processes responsible for the development of rift systems and passive margin sedimentation. At Site 1276, an excellent physical property data set was facilitated by very high core recovery through ~930 m of section. NGR, vertical velocity, bulk density, and porosity profiles are presented for Site 1276 in Figure F26. The data are color-coded to indicate lithology as defined from the barrel sheets, allowing assessment of sedimentary facies control on physical properties behavior of sediments and rocks. We are hoping that this type of display becomes standard procedure on future Integrated Ocean Drilling Program (IODP) legs.
Bulk density and porosity mirror one another downhole, with bulk density increasing from ~1.9 to 2.3 g/cm3 and porosity decreasing from ~50% to 20%. These trends result from the progressive mechanical reduction of void space with increasing burial depth. Abrupt variations in the general density and porosity trend correlate well with lithologic unit boundaries. High-density and low-porosity outliers tend to be associated with siderite-enriched carbonate concretions and grainstone and sandstone cementation. In marked contrast, a relatively thick zone of low density and high porosity at ~1690 mbsf defines a zone of undercompacted and low-density plastic mudstones.
Vertical (z-) velocity generally increases downhole from 1900 to 2600 m/s, and it is apparently insensitive to gross lithologic variations (Fig. F26). Rather, the velocity trend appears to be primarily a function of bulk density and porosity. Numerous high-velocity outliers exist, however, and they invariably relate to zones of increased cementation (both siliceous and carbonate cements) and zones of siderite precipitation. Low-velocity outliers (16501800 m/s) correlate with a zone of undercompacted, low-density mudstones between 1690 and 1710 mbsf.
The NGR profile shows a general increase downhole, with several large spikes that tie closely to lithologic unit boundaries. In general, NGR reflects changes in the relative amounts of sand (low NGR) and terrigenous clays (high NGR). The observed general decrease of NGR uphole may reflect the progressive increase of paleowater depth following rifting and breakup of the region, and thus the slow transition from terrigenous to pelagic sedimentation. This regional trend should be punctuated by eustatically enhanced sediment delivery to the deep basins.
From 1690 to 1710 mbsf, a zone of mudstones and calcareous mudstones have unusually high porosities (27%39%) and low velocities (16901960 m/s) for their depth of burial (Fig. F27). Furthermore, these sediments are very soft, with consistencies comparable to modeling clay. The porosity, velocity, density, and apparent plasticity of these mudstones are comparable to those of normally compacted sediments recovered higher in the section from 840 to 1020 mbsf (Fig. F27), and they clearly demonstrate that the sediments are significantly undercompacted with respect to their depth.
We surmise that the mechanical compaction of these mudstones was halted at a relatively shallow burial depth, perhaps related to the emplacement of two diabase sills, one above and one below the mudstones (Fig. F27). It is possible, but not necessary, that this interval was overpressured prior to Leg 210 drilling. Evidence of past fluid flow exists in the hydrothermally altered sediment immediately above the lower sill and underlying the undercompacted mudstones (Fig. F27). Furthermore, the highest measured concentration of hydrocarbons exists in this 20-m interval. Geochemical analysis measured C1 (methane) levels of nearly 19,000 ppm (Fig. F27), implying that fluids in this interval were trapped. This interval of section clearly was made incapable of normally compacting and expelling pore fluids, and perhaps it was sealed off due to the emplacement and effects of the bounding igneous intrusions.
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