The geometry of Units 1 and 2 is related to large-amplitude asymmetric folds described by Masson et al. (1994); the location of these structures is shown in Figure 2. The characteristic features of these units are that Unit 2 onlaps the steeper western side of these folds, whereas Unit 1 occurs as a continuous sheet above them (Fig. 3A-C, Fig. 4, Fig. 5). The wedge-shaped geometry of Unit 2 is caused by a combination of onlap at its base and erosional truncation at the base of Unit 1, resulting in the absence of Unit 2 over the crests of the folds. The two units cannot be distinguished from one another to the east of Site 898.
The base of Unit 1 is defined in the western part of the study area, where erosional truncation of Units 2 and 3 occurs. Erosional truncation is well displayed in the vicinity of Site 897 on lines SO16 and LG04 (Fig. 3A-B, Fig. 4) and to the west of Site 898 on line LG12 at about shotpoint (SP) 2700 at 7200 ms TWT (Fig. 3C, Fig. 5). Unit 1 shows a sheetlike geometry, ranging in thickness from less than 100 ms in the east to 270 ms west of Site 897. It is characterized by high-amplitude reflections with moderate to good continuity. East-northeast of Site 897, there is a marked change in reflection amplitude into the underlying Unit 3, in which amplitudes are low (Fig. 4).
The base of Unit 2 is defined by onlap onto the broad asymmetrical anticlinal fold structures affecting Unit 3 and deeper units. Onlap is well displayed to the west of Site 897 on lines SO16 (Fig. 3A, Fig. 4) and LG04 (Fig. 3C), between Site 898 and SP 2500 on LG12 (Fig. 3C, Fig. 5), and at about SP 3000 on SO22. Reflection terminations on the gently dipping east limb of the fold structure enable the base of Unit 2 to be identified on LG04; a minor fault and associated minor folding are truncated by the base of the unit at SP 1595 on this line (Fig. 3B).
Unit 2 ranges in thickness from 0 to 460 ms, thickening westward away from the folds shown on Figure 2, back pocket. It shows parallel low- to moderate-amplitude reflections to the west of the fold on the Sonne lines, but moderate to high amplitude reflections on the Lusigal lines. This difference is probably related to different processing parameters.
As previously noted, Units 1 and 2 cannot be mapped separately to the east of Site 898. Unit 1 probably truncates Unit 2 just to the west of Site 898 (Fig. 3C). At Site 900, sediments of the same age as those equivalent to Unit 2 at Site 897 were drilled, but it is not possible to distinguish the two units on seismic sections.
To the east of Site 898 on line LG12, low-amplitude (10-20 ms) sediment waves are evident on the seafloor. These, and the wavy reflections within Units 1 and 2, have a wavelength of 1.5 to 2.0 km (Fig. 5). This wavy reflection pattern is not seen on the larger scale Sonne lines (SO18, SO20 [Fig. 8], and SO22), which show high-amplitude reflections with low continuity (usually about 0.5-1.5 km long) and a very slightly undulatory pattern beneath mostly parallel reflections near the seafloor. It is difficult to trace the base of Unit 1 through this zone. To the east of Site 900 on LG12 (Fig. 5), the wavy reflection pattern is replaced by parallel to very slightly undulatory, moderate- to low-amplitude reflections. Here, the base of the combined Units 1 and 2 is recognized by reflection terminations (between Site 900 and SP 4100 on LG12; Fig. 3C, Fig. 5).
Table 1 summarizes information concerning the boundaries of the seismostratigraphic and lithostratigraphic units recognized during Leg 149 (Sawyer, Whitmarsh, Klaus, et al., 1994) and biostratigraphic boundaries and hiatuses identified by de Kaenel and Villa (this volume) at Sites 897-900. Three key points emerge from this information:
1. The boundary between seismic Units 2 and 3 coincides with the early/middle Miocene boundary at Sites 897, 898, and 900.
2. The base of Unit 1 cannot be mapped to the east of Site 898. Between Site 900 and SP 4100 on LG12 (Fig. 3C, Fig. 5), a few reflection terminations are recognizable and define the base of the combined Units 1 and 2, as shown on Fig. 3C. Biostratigraphic information from Site 900 shows that this boundary coincides with the late/middle Miocene boundary. Therefore, we conclude that adjacent to Site 898 (where middle Miocene sediments are only 19 m thick; de Kaenel and Villa, this volume) the base of Unit 1 must truncate Unit 2 toward the crest of the fold. This is consistent with the conclusion of Milkert et al. (this volume) that the later onset of turbidite sedimentation at Site 398 (2.0 Ma compared to 2.6 Ma at Sites 897 and 900) indicates that earlier turbidites may have been eroded.
3. The base of Unit 1 at Sites 897-899 coincides with the base of the upper Pliocene, but at Site 899 it appears to tie with the base of the upper Miocene. This discrepancy is explained by the difficulty of tracing the seismic units on single-channel JOIDES Resolution data adjacent to Site 899.
In the light of the above, we conclude that the ages of Units 1 and 2 are
Unit 1: late Pliocene to Holocene;
Unit 2: middle Miocene to late Pliocene.
Unit 1 and the bulk of Unit 2 are equivalent to the turbidite/pelagite sequence described by Milkert et al. (this volume); they suggest that the onset of turbidite sedimentation was caused by a glacioeustatic sea-level fall at 2.6 Ma.
The onlap of reflections within Unit 2 onto the top of Unit 3 constrains the timing of the main pulse of folding to the middle/early Miocene boundary, which is identical to the timing of inversion in the Lusitanian Basin in Portugal (Wilson et al., 1989). This event coincides with, and possibly caused, a major change in sedimentation. Sediments equivalent to Unit 3 are characterized by upward-darkening rhythms (5-15 cm thick) of carbonate-dominated turbidites and contourites with thin siliciclastic sand or silt bases and siliciclastic hemipelagic tops with sporadic nannofossil chalks. Silicoflagellates occur in these sediments but are absent in the lithologies equivalent to seismic Unit 2. Sediments equivalent to the latter unit consist of heavily bioturbated nannofossil claystones and oozes. The absence of silicoflagellates in sediments equivalent to seismic Unit 2 suggests that the upwelling of cooler bottom waters during seismic Unit 3 times was "shut down" at the same time as folding occurred; we do not have an explanation for this coincidence of events.
The most characteristic feature of Unit 3 is the occurrence of westward-inclined reflections forming a sequence up to 230 ms thick between SP 3000 and 4000 on LG12 (Fig. 3C, Fig. 5) and along almost the entire length of SO18 and SO20 (Fig. 3D). The base of the inclined reflections defines the base of the unit. The only exception is a short length on SO20 at SP 200, where a separate narrow interval of inclined reflections occurs just below the top of Unit 4 (the separation is too small to be shown on Fig. 3D, but is visible on the full-scale seismic line). Recognition of the base of Unit 3 in areas where there are no inclined reflections was achieved by tracing the reflection at the base of the inclined interval around the seismic grid. Unit 3 has a sheetlike geometry and ranges in thickness from 520 ms to the east of Site 900, diminishing to 200 ms west of Site 898, (LG12, SP 2600-2200, Fig. 3C), and then thickening to 270 ms west of Site 897.
The interval of inclined reflections is laterally extensive on east-west lines (LG12, SO18, SO20, and SO22), thins west of Site 898, and disappears completely at about SP 2600 on LG12 (Fig. 3C, Fig. 5). On lines LG06, SO16, and SO17 and to the east of Site 900 on LG12 the lateral equivalent to the inclined zone shows low- to moderate-amplitude reflections or is seismically almost transparent (Fig. 5). Figure 6 shows the inclined reflector sequence occupies an elongate zone extending some 60 km westward from Site 900, with a north-south width of about 15-20 km.
On north-south lines SO17, SO19, and SO21, Unit 3 shows moderate- to high-amplitude continuous reflections. Short lengths of oblique reflections occur on lines LG04, SO17, and SO19. Where the west-northwest–east-southeast–oriented line GP20 illustrated by Groupe Galice (1979) and shown here in Figure 7 intersects LG12 to the east of Site 900, it shows the oblique reflection pattern.
On LG12 between Sites 898 and 900 a zone of undulatory reflections occurs above the inclined interval with wavelengths of 1.5 to 2.0 km and amplitudes of 10-20 ms (Fig. 5). This pattern is just discernible on the larger scale Sonne lines, where slightly undulatory discontinuous reflections occur.
At Sites 897, 898, and 900, drilling results show that Unit 3 is middle Eocene to early Miocene in age. Thinning (referred to above) is caused by erosion of the top of the unit over the fold structures at Site 897, but the decrease in thickness west of Site 898 on LG12 (Fig. 3D) may be due both to distance from sediment sources on the continental slope and shelf and to an increase in interval velocity resulting from deeper burial.
The undulatory and inclined reflections described above are interpreted as related to structures formed by the migration of large-scale bed forms produced by contour currents. The height and wavelength of the undulatory reflections is comparable to bed forms observed along the western Iberia Abyssal Plain margin by Gardner and Kidd (1987) at depths greater than 4000 m. The waves show slight migration to the east, which is the opposite direction to flow directions observed along the foot of the margin by Gardner and Kidd, but this is not surprising, as both upcurrent and downcurrent migration of wave forms is common on sediment drifts (Faugéres and Stow, 1993).
The westward-inclined reflections characteristic of the base of Unit 3 are associated with short reflections dipping in the opposite direction (Fig. 5). The reflection pattern is superficially analogous to the arrangement of laminae in cross stratification produced when the migration of bed forms is accompanied by very low depositional rates or slight erosion on their upcurrent side. However, in this case the bed forms were probably migrating in an upcurrent, rather than downcurrent, direction. Thus, the westward-inclined reflections are analogous to cross stratification set boundaries and the short eastward-dipping reflections between them to cross strata. It must be stressed that the inclined reflections are not clinoforms indicating sediment progradation. The absence of the inclined reflections in other areas may be due to changes in rates of sedimentation and/or contour current strength.
The true dip of the inclined reflections is to the west, or within 10°-20° of this direction. This is consistent with Gardner and Kidd's (1987) observation that the orientation of sediment waves along the western Iberia Abyssal Plain Margin ranges between about 15° and 18° to the regional contours. In the Leg 149 area, the present-day regional contours trend approximately northwest-southeast or west northwest-east southeast (Fig. 6).
The base of Unit 4 is defined as the downward change from strong continuous reflections to the very discontinuous reflections of Unit 5, which appears almost seismically transparent in comparison to the overlying unit. Unit 4 shows thinning and onlap toward basement highs, and its overall thickness steadily diminishes westward, from 600 ms between Sites 900 and 901 to 240-300 ms to the west of Sites 897 and 898. This unit consists largely of an interval dominated by moderate- to high-amplitude continuous reflections (Fig. 3, Fig. 5, Fig. 8). The lower part of the unit onlaps onto highs in the acoustic basement and so it is impossible to trace its base continuously along most seismic lines. Reflection convergence occurs toward the highs.
The deepest borehole penetration into Unit 4 was achieved at Site 900, where upper Paleocene sediments rest on the acoustic basement, which consists of metagabbros. The change from strong continuous to very discontinuous reflections across the boundary between Units 4 and 5 is very similar to that observed around Site 398 across the boundary between Units 2 and 3 of Groupe Galice (1979). This correlation is illustrated at the crossover of LG12 and GP 20 (Fig. 1, Fig. 7), where the base of Unit 4 of this study ties with the base of Unit 2 of Groupe Galice (1979). Drilling at Site 398 showed the boundary to be marked by a late Cenomanian-Turonian (and possibly Coniacian) age hiatus. Therefore, it is probable that Unit 4 in the Leg 149 area ranges in age from middle Eocene to Coniacian or late Santonian (Fig. 9).
Drilling at the Leg 149 sites showed that the top part of seismic Unit 4 consists of numerous upward-darkening units (10-30 cm thick) consisting of dominantly carbonate turbidites and siliciclastic hemipelagites, possibly reworked by contour currents. At Site 900, the siliciclastic sand content of the sequence is significantly higher, leading the Shipboard Scientific Party (1994c, p. 223) to suggest a possible lobe fringe setting on a submarine fan. On either side of Site 901, several mound features, between 150 and 400 ms high, are present within Unit 4 (Fig. 3C). These are interpreted as sediment drifts formed between basement highs.
During the deposition of sediments equivalent to Unit 4, the basement highs west of Site 901 were completely buried. The lower part of Unit 4 onlaps the basement highs. The equivalent Unit 2 in the Group Galice (1979) study area shows some discordance and overstepping onto the underlying Unit 3, indicating tectonism related to Pyrenean movements; such features were not observed on the Iberia Abyssal Plain seismic profiles.
The base of Unit 5 is taken at a continuous high-amplitude reflection (in some areas by a pair of strong reflections on the Lusigal lines) or at the top of acoustic basement. Unit 5 is characterized by parallel, low- to moderate-amplitude, low- to moderate-continuity reflections on the Lusigal lines, with slightly more continuity on the Sonne lines (Fig. 5, Fig. 8). However, on both groups of lines, the unit appears almost seismically transparent compared with the overlying high-amplitude continuous reflections of Unit 4. In the area west of Site 898, Unit 5 rests directly on acoustic basement or on a very discontinuous or rather chaotic reflection interval that may be equivalent to Unit 6. The unit onlaps basement highs. If thickness changes caused by draping over the highs are ignored, the unit shows an overall gradual thickening westward from ~ 60 ms adjacent to Site 900 to ~ 200 ms adjacent to Site 897 and in areas to the south of this site.
Unit 6 drapes the irregular surface of the acoustic basement. It is recognized with confidence only east of a line through Sites 897 and 898. On the Sonne lines it shows moderate-amplitude semicontinuous to discontinuous reflections, but on the Lusigal lines higher amplitudes and continuity are usually displayed (compare Fig. 5, Fig. 8).
In the area between Sites 898 and 900, three Subunits can be recognized within Unit 6 on SO18, SO19, and SO20 and on a reprocessed version of LG12 (Krawczyk et al., this volume, their fig. 3). They are shown on Figure 3D and Figure 8. The descriptions below refer to appearance of the Subunits on SO18 and SO20 (Fig. 3D, Fig. 5, Fig. 8).
Subunit 6A shows slightly undulatory, discontinuous, moderate-to high-amplitude reflections, capped by a moderate- to high-amplitude continuous reflector marking the boundary with Unit 5. This Subunit has a sheetlike geometry and is 200 ms thick; it dies out to the east at SP 180 on SO20, and to the west on SO18 it onlaps the basement ridge to the south of Site 898 (Fig. 3D).
Subunit 6B shows parallel, low- to moderate-amplitude reflections that onlap onto the irregular topography of the acoustic basement. The Subunit onlaps Subunit 6C and is overstepped by Subunit 6A (Fig. 3D, Fig. 8). To the east the boundary between Subunits 6A and B dies out at about SP 150 on SO20 (Fig. 3D) where, in terms of seismic facies characteristics, it appears that Subunit 6A merges into Subunit 6B. Its thickness ranges from 0 to 600 ms.
Subunit 6C is a wedge-shaped reflection package, 0-250 ms thick, banked, to the west, against a steep slope in the acoustic basement between SP 200 and 400 on SO20 (Fig. 3D, Fig. 8). It is about 10 km wide and almost transparent, but faint reflections downlap onto basement at the foot of slope and onlap onto the slope (Fig. 8).
The tie between lines GP20 (Groupe Galice, 1979) and LG12 (Fig. 1, Fig. 7) shows that seismic Units 5 and 6 in the Leg 149 area are equivalent to Unit 3 of the earlier study. However, we question whether Groupe Galice correctly identified the base of Unit 3 on their figure 9A for the following reasons:
1. Apart from figure 9A of Group Galice (1979), their Unit 3 is shown in other extracts of seismic lines (e.g., their figures 8A, B, C; 9B, C; 15A) as being virtually transparent and not exhibiting significant reflection continuity. This seismic facies characteristic matches our Unit. 5.
2. The base of Group Galice's Unit 3 is characterized by a thin, strong reflection package on their figure 8.
3. It is not clear how Groupe Galice tied line GP20 (their figure 9A) to Site 398, as this line crosses the Vasco da Gama Seamount, against which all their units are shown to terminate at a fault boundary in their figure 9C, with their Unit 3 developing a sedimentary prism against the fault (but not showing reflection divergence toward the fault). The location of the extracts from seismic lines in the Groupe Galice paper is not clearly defined (neither is the horizontal scale given) but it appears that their figures 9C and 9A are contiguous as there is a perfect match between the depths of all the reflections shown at the northeast and southwest ends of the two extracts. Beneath the seismically virtually transparent prism a package of strong reflections occurs that is not identified as the base of their Unit 3, in contrast to the interpretation of another part of the line shown in their figure 9B, where the base of the unit is taken at the change from weak to strong reflections.
In light of the above, we believe that the strong double reflection 130 ms below the base of Groupe Galice Unit 2 shown on line GP20 (on the right side of our Fig. 7) is the base of their Unit 3. This means that our Unit 5 equates to their Unit 3 as identified elsewhere in their study area, including the area of DSDP Site 398. Likewise, we equate our Unit 6 to Unit 4 of Groupe Galice. This interpretation is consistent with the strong similarities in reflection characteristics between the units defined in this study and those of Groupe Galice illustrated in Figure 10.
The correlation of our units with those of Groupe Galice as summarized in Figures 9 and 10 enables the former to be equated with the sequence drilled at Site 398. This indicates that Unit 5 is Albian to Cenomanian in age and that Unit 6 is Hauterivian to Aptian in age. If our Unit 6 is partly equivalent to Unit 5A of Mauffret and Montadert (1988), as suggested by comparing reflection characteristics as illustrated in Figure 10, then it may extend into the Valanginian.
At Site 398, the mid-middle Albian to Cenomanian interval consists of 236 m of marly nannofossil chalk to calcareous mudstone interbedded with dark gray to black claystone with minor siliciclastic sands. The remaining part of the Albian beneath this sequence comprises 218 m of dark gray to black laminated to homogeneous claystone. The low-amplitude/transparent nature of Unit 5 in the Leg 149 area suggests that similar sediments may occur here. The relatively stronger reflection characteristics of Unit 6 are consistent with the variety of lithologies found over the equivalent interval at Site 398. Here, the Aptian-Barremian sediments include black to dark dray and olive green mudstones and claystones with interbedded mud and limestone pebble debris flows and siliciclastic turbidites. The Hauterivian consists of radiolarian-rich nannofossil limestones interbedded with dark mudstones with some thin siliciclastic turbidites (Sibuet, Ryan, et al., 1979).
The wedge shape of Subunit 6C and the faint internal reflection pattern (Fig. 3D, Fig. 8) suggest eastward progradation of a submarine fan or talus deposit.
The principal features of the seismic units beneath the Iberia Abyssal Plain are summarized in Table 2, and the relationships betweeen them illustrated in Figure 11.