It is instructive to compare seismic reflection profiles, lithologic units, and sediment accumulation patterns on the conjugate margins of the Newfoundland–Iberia rift. This is true particularly for the deepest parts of the sedimentary section, which were deposited at a time when the two margins were close to one another. For this comparison we consider Site 1276 and Site 398 (DSDP Leg 47B), drilled on continental crust of southern Galicia Bank on the Iberia margin (Fig. F28). This site has the most complete and expanded stratigraphic range of sediments among the Iberia margin sites. Other sites drilled during Legs 149 and 173 were on basement highs and did not penetrate significant sections of pre-Tertiary sediments. At the time of Anomaly M0 (Aptian; ~121 Ma), Sites 1276 and 398 were separated by only ~250 km (Fig. F3). The paleoceanographic environment in this young ocean basin should have been similar on both margins. Indeed, we observe comparable occurrences of mid-Cretaceous black shales and Upper Cretaceous red-brown and multicolored shales, although there are some significant differences in details of sedimentary processes. Below, we summarize and compare principal features of the sedimentary sequences on the two margins.
Seismic sequences in the Newfoundland Basin around Site 1276, outlined more fully in "Geological Setting," in "Introduction," are summarized in Figure F29. The deepest sequence, A, is seismically laminated, flat-lying, and contains intervals with sufficiently high impedance contrasts that signal penetration to the underlying basement and appears to be limited. Its top is the flat, strong, basin-wide U reflection. The sequence covers and smooths the underlying basement topography, with its greatest thicknesses between underlying basement blocks (Fig. F10). Correlation of the exact top of the seismic sequence to Site 1276 borehole results is not certain. In one interpretation the upper of two diabase sills (at 1612 mbsf) correlates with U, and in an alternate interpretation the lower sill at 1705 mbsf is correlative. Both sills intruded into dark mudstones, siltstones, and sandstones of early Albian age (nannofossil biozone NC8a–NC8b).
Seismic Sequence B is not strongly laminated compared to the underlying interval. It correlates with dominantly green-gray to black claystones and mudstones, with variable amounts of siltstones and sandstones. The sediments were deposited in the lowermost Albian to upper Turonian.
The overlying seismic Sequence C is flat-lying and strongly reflective. It correlates with brown sandy mudstones, overlain by multicolored (e.g., brown, red, and gray green to black) hemipelagic claystones interrupted by frequent turbidites. The sediments are latest Turonian to middle Eocene in age.
Seismic Sequence D was sampled at Site 1276 only in its lower part, where it consists of middle Eocene to lower Oligocene(?) gray-green hemipelagic claystones and mudstones. Although the sequence shows distinct sediment waves in the landward seismic section (Figs. F8, F28), there is little indication of current-controlled deposition at Site 1276 and the section is dominated by turbidites and debris flow deposits.
Seismic Sequences E and F were not sampled during Leg 210, although they can reasonably be interpreted from their seismic signature. Sequence E is an abyssal fan marked by irregular reflections, poorly developed sediment waves, and local debris-filled channels. Sequence F is an abyssal plain with widespread planar reflections that are very characteristic of turbidite deposits. Both sequences record extensive downslope sedimentation during Oligocene to recent time. The Sequence F turbidites most likely are latest Pliocene and Quaternary.
Site 398 was drilled at a depth of 3900 mbsf, ~20 km south of Vigo Seamount on the southern margin of Galicia Bank (Fig. F3A). The site is on thinned continental crust on the seaward side of Galicia Interior Basin and ~40 km north of the Leg 149/173 drilling transect. A representative MCS reflection profile extending northwest–southeast through the site (GP-19; Figs. F28, F29) displays a complete sedimentary sequence that shows little apparent interruption by unconformities or condensed sections.
Basement on profile GP-19 exhibits some coherent internal reflections and apparent fault blocks that appear to record tectonic extension of the continental crust (Figs. F28, F29). The overlying moderately smooth and seismically laminated seismic sequence is named acoustic Unit 4 (Groupe Galice, 1979), and it is capped by a moderately strong reflection termed the orange reflection. In the lowest part of the sequence, reflections are moderately conformable to basement and locally may diverge, but reflections in the upper part are unconformable and lap against the flanks of the fault blocks. This sequence was interpreted as being deposited during tilting of the fault blocks (Groupe Galice, 1979; Shipboard Scientific Party, 1979), but the onlap relations indicate that at least the upper part of the sequence is postrift. Site 398 drilling results show that the sequence consists of thin sand-silt-clay graded beds that are interbedded with thick slumps or debris flows (Shipboard Scientific Party, 1979). The sequence is interpreted as an upward-fining set of abyssal fan deposits. Its top (orange reflection) appears to match a small stratigraphic gap at the Aptian/Albian boundary (Sigal, 1979). At its base is an interval of white indurated limestone and marlstone/siltstone of Hauterivian–Barremian age (Shipboard Scientific Party, 1979). This has been interpreted as part of the basement complex at Site 398.
A large set of MCS profiles in this area allows us to correlate acoustic Unit 4 from the thinned continental crust of Galicia Bank southward onto the transitional crust beneath the Iberia Abyssal Plain (Fig. F30). Mapped thickness patterns show that on Galicia Bank the sequence is deposited between north-south–oriented basement blocks that plunge to the south under the abyssal plain. Just south of the Leg 149/173 drilling transect, the surface of the sequence reaches across the tops of all these blocks. The sequence also thins toward the west, indicating a source of the clastic sediments along the shallow Iberia margin. It pinches out on crust of late Barremian age ~30 km west of ODP Site 1070.
The overlying acoustic Unit 3 is a weakly reflective layer in which faint, semicontinuous reflections are observed. It correlates with a series of dark gray to black laminated to homogeneous claystone and interbedded mudstone that includes black shale and calcareous mudstone overlain by nannofossil chalk and claystone. The sediments are lower Albian–lower Cenomanian.
Acoustic Unit 2 is flat-lying and highly stratified, and it consists of siliceous mudstone, red marly chalk, and brown, dark reddish, and reddish gray mudstone and claystone, predominantly massive and unborrowed. Ages of the sequence at Site 398 are Coniacian to early Eocene.
The shallowest acoustic Unit 1 is stratified in its lower part but has only weak reflections in its upper part. The lower part is marly nannofossil chalk, nannofossil chalk, and siliceous marly chalk, and the upper part is rhythmically bedded marly nannofossil ooze. Although the sequence locally is eroded away, apparently by bottom currents (Fig. F28), its internal reflections are flat-lying and show few if any signs of current-controlled erosion or deposition. Ages of the sequence range from middle Eocene to Quaternary.
It is clear that there are significant similarities in the deeper parts of the seismic sections and in the correlative lithofacies of the conjugate Newfoundland and Iberia margins (Figs. F28, F29). These similarities persist up through seismic Sequence C on the Newfoundland margin and the correlative acoustic Unit 2 on the Iberia margin. They indicate that the two margins shared common depositional environments when the ocean basin was young and relatively narrow. At shallower levels in the sedimentary record the seismic signatures become significantly different, reflecting divergence in sedimentation patterns as the margins became more widely separated. We summarize this development below.
Basal seismic Sequence A off Newfoundland and acoustic Unit 4 on Galicia Bank show comparable seismic signatures. The age at the base of the sequence at Site 1276 is unknown, but it may be very similar to the Hauterivian age of basement limestones cored at Site 398. Magnetic anomalies M3 and M0 lie to the east of Site 1276. If the crust at the drill site is oceanic and we extrapolate a crustal age from these anomalies to the drill site (assuming constant spreading rate), the basement dates to ~126 Ma at the Hauterivian/Barremian boundary. Although the top of seismic Sequence A appears to be slightly younger than that of acoustic Unit 4 (lower Albian vs. Aptian/Albian boundary), depositional conditions were comparable on both margins. Dark-colored, low-carbonate hemipelagic sediments indicate that the basin was a low-oxygen environment below the CCD, but sedimentation was dominated by turbidity currents and debris flows. Widespread dispersal of these flows across the basin floor accounts for the seismically laminated signature of the interval.
Newfoundland seismic Sequence B matches the seismic signature of Iberia acoustic Unit 3. The correlative sedimentary section on both margins is Albian to lower Cenomanian–upper Turonian gray-green and black shales, similar to the Hatteras Formation in the main North Atlantic Basin (Jansa et al., 1979). At Site 1276 these shales are hemipelagic through most of the section, with gravity-flow deposits being important at the top and base of the section. At Site 398 gravity-flow deposits are important at the top, but sediments are hemipelagic in the middle and mostly calcareous at the base. During the Albian–Turonian, the basin width increased from 600 to 1000 km. Nonetheless, the two margins show remarkable correspondence in sediment facies, and they also have very similar sediment accumulation patterns. Notably, there are high rates (to 55 m/m.y.) in the lower part of the section and somewhat reduced rates (to 18 m/m.y.) at the top of the section on both margins (Fig. F31). These correlations indicate that both margins provided similar source areas for sediment input to the deep basin. The deep seafloor was at least intermittently anoxic and was below the CCD.
In the Late Cretaceous (approximately Cenomanian–Campanian), both margins developed either hiatuses or had severely reduced sedimentation rates (Fig. F31). This condensed or missing record corresponds to the top of seismic Sequence B off Newfoundland, where truncations of reflections indicate that an unconformity is present. Similar truncation in the Iberia GP-19 seismic profile is not apparent, although a slight unconformity was identified on other profiles (Groupe Galice, 1979). This correlation between margins indicates a rift-wide event, possibly the development or invigoration of deep circulation that marked the end of significant black shale deposition. This is consistent with the presence of red and brown sediments deposited on a more oxygenated seafloor in the overlying section on both margins.
Similar seismic signatures between margins persist upward into seismic Sequence C off Newfoundland and acoustic Unit 2 off Iberia. Basin width increased from 1000 to 2000 km during this period (Turonian–middle Eocene). The sequences are red-brown mudstones on the Newfoundland margin and marly chalks on the Iberia margin, deposited under mostly oxygenated seafloor conditions. Turbidity currents delivered carbonate sediments to the deep seafloor in large quantities on the western side, but they mostly bypassed Site 398, where pelagic carbonates are abundant (50–60 wt%). Even so, sedimentation rates were similar (5.7 m/m.y.) on both margins. The difference in sedimentation processes at the two drill sites is not apparent in the seismic signature.
Beginning with the middle–late Eocene record, there is significant divergence in the seismic signature of sediments on the two margins. Newfoundland seismic Sequence D shows clear evidence of sediment waves developed by strong abyssal circulation, but there is no similar seismic signature on the Iberia side (Figs. F8A, F8B, F29). The reflection at the base of Newfoundland seismic Sequence D appears to correlate with an unconformity between lithologic Units 1 and 2 at Site 1276, and biostratigraphic data indicate a middle Eocene hiatus at this level (Fig. F31; see "Biostratigraphy," in "Site 1276 Synthesis"). All these features indicate that the reflection may be equivalent to Horizon Au in the western North Atlantic, which marks the influx of cool bottom waters from the Norwegian-Greenland Sea and corresponds to initiation of strong abyssal circulation in the basin (e.g., Miller and Tucholke, 1983). Because of the Coriolis effect, the deep currents were primarily concentrated along the western boundaries of the ocean basins. Although the currents seem not to have affected the seismic architecture of the Iberia margin at Site 398, there is an indication of a short middle Eocene hiatus there (Fig. F31), so there may be some subtle erosion or attenuation of the sedimentary record.
The uppermost sedimentary sections on the opposing margins are markedly different in seismic character (Figs. F28, F29), and they clearly show a divergence in sedimentary processes during the late Cenozoic. As already noted, seismic Sequences E and F on the Newfoundland margin are dominated by downslope sedimentation. In contrast, acoustic Unit 1 around Site 398 is a less reflective sequence of largely pelagic sediments. It is clear that this southern part of Galicia Bank became progressively isolated from downslope sedimentation throughout the Cenozoic.