8. SITE 359: WALVIS RIDGE (SEAMOUNT)
The Shipboard Scientific Party1
Total Length of Cored Section: Hole 359: 50.0 meters Hole 359A: 19.0 meters Total Core Recovered: Hole 359: 27.5 meters Hole 359A: 14.9 meters Principal Results: Site 359 was a contingency site selected near the crest of a positive feature (seamount) on the trend of the Walvis Ridge. Fifty-seven meters of upper Eocene and younger foraminifer-nannofossil ooze overlie 29 meters of upper Eocene calcareous volcanic mud. An important hiatus spans the upper Eocene to lower to middle Miocene. A middle to upper Miocene hiatus is also probably present. An upper Eocene sanidine-rich volcanic tuff was cored between a depth of 86 meters and the bot- tom of the hole at 107 meters.
BACKGROUND AND OBJECTIVES After leaving Site 358 24 hr early because of bad weather, we held open the alternative of occupying a shallow-water site, to utilize remaining operational time on Leg 39. Because we had not sufficient time to deviate from the direct course to Cape Town, our op- tions were to locate a site either on the shallow portion of the Walvis Ridge or on the shallow portion of the continental slope off South Africa (Figure 1). In the late morning of 10 December, we reached the western flank of a positive feature which is part of the structural lineament of the Walvis Ridge. The reflection 45°W 15°E profile (Figure 2) showed what appeared to be igneous basement covered by 0.1-0.3 sec of semiconformable SITE DATA reflectors. We elected to drill on the Walvis Ridge if we could find a place with suitable sediment thickness in Date Occupied: 10 Dec 74 (1455Z) water shallow enough to allow meaningful work in the time left, about 30-36 hr. Just west of the crest of the Date Departed: 11 Dec 74 (2340Z) feature we did find such a place; about 0.3 sec of sedi- Time on Site: 1 day, 8 hours, 45 minutes ment appeared to overlie basement and the section had Position: 34°59.10'S, 04°29.83'W intermediate reflectors at 0.11 and 0.20 sec. The objectives at this site were to core the sedi- Water Depth: 1658 meters (drill pipe measurement) mentary section, paying particular attention to Penetration: 107 meters recovery of upper Paleogene (Oligocene, Eocene) sedi- Number of Holes: 2 ments if these were present, and to sample igneous basement if time and drilling rates permitted. The site is in the region of anomaly 25 (Ladd, 1974), which 'K. Perch-Nielsen, Eidg. Technische Hochschule, Zurich, Switzer- probably represents a crustal age of about 53 m.y.B.P. land (Co-chief scientist); P.R. Supko, Scripps Institution of (Tarling and Mitchell, 1976). Oceanography, La Jolla, California (Co-chief scientist); A. Boersma, Lamont-Doherty Geological Observatory, Palisades, New York; R.L. Carlson, University of Washington, Seattle, Washington; M.G. Cored Dinkelman, The Florida State University, Tallahassee, Florida; R.V. Int.Units Lithology Age Fodor, University of New Mexico, Albuquerque, New Mexico; N. 0 Kumar, Lamont-Doherty Geological Observatory, Palisades, New -+- -*-->- APlio-Pleist. H i Miocene York; F. McCoy, Lamont-Doherty Geological Observatory, Λ 2 Palisades, New York; J. Thiede, Oregon State University, Corvallis, LJ -»-|£.»_* Eocene Oregon; H.B. Zimmerman, Union College, Schenectady, New York. 100 -Pi 3 o\\ UlltfW
373 SITE 359: WALVIS RIDGE (SEAMOUNT)
6° Since Site 359 was a contingency site selected at sea, we were working without the advice of the JOIDES Atlantic Advisory Panel or the JOIDES Advisory Panel on Pollution Prevention and Safety. On the basis of the presumed pelagic nature of the sediments and the shallow water depth, we were fully confident that no potential hazard existed.
OPERATIONS Glomar Challenger approached the vicinity of Site 359 on a course of 086°T at a speed of about 10 knots. The reflection proflle obtained over the western flank of the Walvis Ridge province indicated (locally) at least two reflectors lying semiconformably over presumed basaltic basement 0.1 to 0.3 sec below (Figure 2). We crossed the crest of the topographic feature at 1325Z. After traversing the east flank until it lay at a depth that would have compromised operations in the short time available, we decided that the most favorable drill site Figure 1. Base map showing the location of Site 359 in the was on a low sediment rise about 4-5 miles west of the western (seamount) province of the Walvis Ridge. From ridge crest. We executed a Williamson turn onto bathymetric chart by Simpson (1974); heavy contour is 267°PGC at 1345Z, slowed the ship to 6.5 knots, and 4500 meters, other contour lines at 500 meter intervals. recrossed the crest at 1410Z (Figure 2). We dropped a
— 5
— 6
Figure 2. Glomar Challenger seismic reflection profile obtained on approach to Site 359.
374 SITE 359: WALVIS RIDGE (SEAMOUNT)
13.5-kHz beacon while underway at 1455Z, retrieved 0005Z 12 December, and passed over the site with the geophysical gear, and returned to position the ship over beacon 70 meters to starboard at 0030Z (Figure 3). We the beacon. held this course for 15 min (~2.5 nautical miles), then We had considerable difficulty in attempting to lock executed a starboard turn of one mile radius and came onto the beacon: the computer display showed the ship onto a course of 087°PGC for the steam to Cape Town. to be moving relative to the beacon at an unlikely rate. Various means of troubleshooting and computer repair were of no avail. During these repair attempts, the bottom-hole assembly units were magnafluxed. This 0000 12 Dec, was required preparatory to running in the hole at Site start turn 359 anyway, so the time spent on trying to positon over the beacon was not totally lost. u/w 23 to 11 Dec. The beacon signal began to fail after about 6 hr, and SITE 359 at 2130 hr (Z time zone same as LCT) we dropped a 16- o/c 087c kHz beacon at the same location. Problems continued with the new beacon. We started running pipe slowly at 2200 hr, in hope of finally having the computer accept start turn the beacon. At about 0500 hr we satisfactorily locked 0045 12 Dec. T onto the beacon. The site location (average of 10 1 mile satellite fixes) is 34°59.10'S, 04°29.83'W. Water depth ± by PDR is 1655 meters. Drill pipe measurement at bot- Figure 3. Glomar Challenger track on departure from Site tom contact was 1668 meters to the rig floor; a water depth of 1658 meters is accepted. 359. Site 359 was spudded at 0630 hr 11 December. The top 107 meters of section was spot-cored; three cores LITHOLOGIC SUMMARY were unconsolidated sediment, one was unconsolidated The sediments and volcanic rocks of this site are sediment and a hard black tuff, and one was tuff (Table divided into three distinct lithologic units (Figure 4, 1). During the cutting of Core 5, the drill bit apparently Table 2) became plugged, which accounts for the low recovery of tuff in Core 5 and the zero recovery in Core 6. We had decided earlier that if the tuff appeared to be Unit 1 a thick deposit, our time would be better spent attemp- Unit 1 is Pleistocene to late Eocene in age, and divid- ting to core the interval 9.5-28.5 meters, which would ed into Subunits A and B on the basis of a slight change contain either missing section or evidence of a hiatus. in composition and color. A thin pinkish light gray Before we could attempt this, we had to free the bit. A horizon marks the upper Eocene/middle Miocene chisel-faced center bit, pumped down the pipe at high hiatus and separates the two subunits. pump pressure, succeeded in freeing the lodged Subunit A (Core 1 to Sample 2-6, 60 cm, 0.0-36.1 m) material from the bit. Hole 359A was spudded at 1800 consists of a white soupy foraminifer-nannofossil ooze. hr and the required two cores were taken.We began The very homogeneous sediments contain occasional running out of the hole at 1915 hr. small pale yellowish specks. The dominant color grades Glomar Challenger got underway at 2340Z 11 downsection (Core 1 of Hole 359 to Core 2 of Hole December 74. We steamed north to stream gear, ex- 359A) into very pale orange; this color first occurs in ecuted a slow turn to starboard, came up to 10 knots at several thin horizons in Core 1A. The major sediment
TABLE 1 Coring Summary, Site 359
Depth From Depth Below Drill Floor Sea Floor Cored Recovered Recovery Core (m) (m) (m) (m) Hole 359 1 1668.0-1677.0 0.0-9.0 9.0 6.1 68 2 1696.0-1705.5 28.0-37.5 9.5 9.5 100 3 1724.5-1734.0 56.5-66.0 9.5 9.5 80 4 1753.0-1762.5 85.0-94.5 9.5 3.5 37 5 1762.5-1772.0 94.5-104.0 9.5 0.8 8 6 1772.0-1775.0 104.0-107.0 3.0 0.0 0 Total 50.0 27.5 55 Hole 359A 1 1677.0-1686.5 9.0-18.5 9.5 8.4 88 2 1686.5-1696.0 18.5-28.0 9.5 6.5 68 Total 19.0 14.9 78
375 SITE 359: WALVIS RIDGE (SEAMOUNT)
359 359A 1 I I I I 0 10 20 30 40 50%
Z z Foraminifers Silty clay z z Zeolite
Foraminiferal Nannofossils ooze ,\\ Volcanic breccia
DO Micarb Nanno-Foram ooze Glauconite no Sili ceous 000 Dolomite p p Pyrite,other opaque minerals microfossils P P Figure 4. Lithologic summary, Site 359. Sediment composition data based on shipboard smear- slide examination. components of Subunit 1A are calcareous nannofossils mixture of biogenic and volcanogenic materials whose (50%-70%), planktonic foraminifers (10%-30%), and proportions change erratically. The term volcanic mud, authigenic carbonate. Many calcareous fossils are therefore, includes sediments of quite different com- heavily encrusted. Opaline skeletons are almost com- positions. The dominant color is light olive-gray; pletely absent. Except for the faint color changes men- irregular areas are slightly greener. Except for the tioned above, no sedimentary structures have been pumice layers (see below) and a thin volcanic ash, no preserved in the soupy sediments. sediment structures are evident. Dominant components The sediments in Subunit IB below the upper are foraminifers, calcareous nannofossils, sponge Eocene/middle Miocene hiatus (from Sample 2-6, 60 spicules, volcanic glass, clay minerals, and glauconite; cm to Sample 3-1, 45 cm, 36.1-56.9 m) consist of 20.8 authigenic carbonate, hornblende, pyroxene, feldspars, meters of foraminifer-nannofossil oozes and occasional zeolites, diatom remains, radiolarians, dolomite nannofossil-foraminifer oozes. The yellowish-gray rhombs, opaque minerals, and Fe oxides are of minor color of these sediments begins below a sharp contact importance. Bivalves, brachiopods, and a solitary coral (the Eocene/Miocene hiatus), which is marked by a are macroscopic representatives of shallow water thin, slightly darker horizon. The sediments are similar faunas. to but slightly coarser than those of Subunit 1A. In ad- Three pumice layers, 5 to 10 cm thick, with generally dition to the above-mentioned main components, they lapilli-sized fragments and with distinct contacts, are contain glauconite, mainly as fillings within foraminifer intercalated with the sediments described above. The tests and shallow (?) water fossils, such as small oyster- volcanic glasses differ from the surrounding volcanic like bivalves, bryozoans, and barnacles. Small, dark mud by their olive-gray to grayish-olive color and by brown sand-sized fragments of maganese crust occur their distinct appearance as coarse (average grain frequently in the sediments of this unit. diameter 0.5 ±2 mm) layers. A few centimeters above the tuff of Unit 3, a marly Unit 2 limestone, 10-20 cm thick (Sample 4-2, 5 cm to 15 cm), The calcareous, mostly sandy volcanic muds which occurs intercalated with volcanic mud. The sediments consistute this upper Eocene unit consist mainly of a directly in contact with Unit 3 are laminated and show
376 SITE 359: WALVIS RIDGE (SEAMOUNT)
TABLE 2 Lithologic Summary, Site 359 (Walvis Ridge)
Depth Below Thickness Unit Cores Sea Floor (m) (m) Age Description Hole Hole 359 359A 1 1-3 1A 0-56.9 56.9 Pleistocene to Subunit 1A: Foram nanno ooze (white to pale and Miocene and yellow orange) 2A L. Eocene Subunit IB: Foram nanno to nanno foram ooze (yellowish-gray) 2 34 56.9-86.0 29.1 L. Eocene Calcareous volcanic mud with pumice layers (light olive-gray) 3 4-5 86.0-95.5 9.5 L. Eocene Volcanic ashflow tuff (black, medium to dark gray) lensy bedding (sand in clayey sediments). They exhibit BIOSTRATIGRAPHIC SUMMARY settling cracks with up to 1 cm displacement. The eight cores taken at Site 359 range in age from Unit 3 Pliocene to late Eocene. The biozones represented are Below the Eocene volcanic muds, 3.3 meters of shown in Figure 5. medium gray ash-flow tuff were recovered in Cores 4 and 5. At least 10 or 20 meters more probably escaped Pliocene/Pleistocene recovery because of a plugged drill bit. The ash-flow Only a veneer of mixed Pliocene and Pleistocene sedi- tuff contains abundant feldspar crystals (up to 5 mm ment occurs at the top of the first core; below this level long), minor amounts of green pyroxene crystals and the sediment is early Pliocene in age. Thus, a hiatus ex- biotite, and medium light gray pumice fragments up to ists between the Pleistocene and the lower Pliocene. 1 cm, in a fine-grained matrix. It is moderately in- Pliocene foraminifer faunas in the core contain sub- durated, although it crumbles slightly when rubbed. In tropical species; nannofossil assemblages consist of hand specimens the tuff appears to be free of alteration high- and mid-lattitude elements. It appears that small and weathering. nannofossil species have been winnowed out of the Thin-section examination shows alkali feldspar as sediment. Benthic foraminifer faunas indicate bathyal rectangular crystals averaging 0.5 to 1 mm in size, with water depths. No radiolarians are present. distinct cleavage planes and Carlsbad twinning. Overall, the feldspar is unaltered. Infrequent clino- Miocene pyroxene grains appear to be fresh, whereas the biotite Miocene sediments occur in Core 2, Hole 359, and in is highly oxidized. Oxides are also present. The rock both cores from Hole 359A. According to the type is probably one of intermediate composition. foraminifer evidence, the section spans the upper Pumice fragments are abundant; compaction ratios Miocene into upper middle Miocene. The nannofossils, vary from 1:1 to about 10:1, and indicate moderate however, indicate a section down to the lower middle welding. Cellular and fibrous structures are no longer Miocene. The foraminifer assemblages contain sub- visible, probably because of devitrification and tropical open marine species, and benthic species in- seawater alteration to clays. Similarly, the groundmass dicate bathyal depths. The nannofossils are poorly looks rather dusty and glass shards are not visible. preserved and no radiolarians occur. Eocene GEOCHEMISTRY A hiatus between the middle Miocene and the upper We analyzed three interstitial water samples for pH, ++ ++ Eocene occurs at the bottom of Section 359-2. The alkalinity, salinity, Ca and Mg content. The results foraminifers from Cores 2 through 4 all belong to one are presented in Table 3. zone, the lower upper Eocene Zone PI5, and the nan- nofossils represent two lower upper Eocene zones PHYSICAL PROPERTIES (NP19 and 18). The characteristics of the upper Eocene fauna and No report for this site. flora are entirely different from those of the Neogene.
TABLE 3 Summary of Shipboard Geochemical Data Sample Subdepth Alkalinity Salinity CA++ Mg++ (Interval in cm) (m) pH (meq/1) (°/oo) (mmoles/1) (mmole/1) 3-3, 144-150 61 7.63 2.57 35.2 13.63 48.82 4-1,75-80 85.5 7.64 2.54 35.3 13.60 48.27 1A4, 144-150 15 7.57 2.97 35.2 11.58 51.48
377 SITE 359: WALVIS RIDGE (SEAMOUNT)
Core Planktonic Epoch ^^^ and Foraminifer Nannofossil Zones ^^^^ Series Depth (m) Zones 5. dehisaens- PLIOCENE E 1 ° NN15-NN13 • N19 G. altispira L G. tumi•da N17 plesiotumida NN11 MIOCENE 1 D. suturaizs- N9 NN5 M 2 G. peripheroronda
50
P15 G. mexiaana NP18 EOCENE L 3
4 unzoned unzoned I5~ 100 Figure 5. Biostratigraphic summary, Site 359. For example, some poorly preserved radiolarians occur Benthic species are fairly scarce, but are represented by in Core 3. The presence of Lychnocanium bellum, com- bathyal species of the genera Laticarinina, Angulo- monly found in the southern Indian Ocean, indicates gerina, Cassidulina, and Stilostomella. Echinoid that cool-temperate waters flow into the eastern Atlan- fragments and smooth ostracodes also occur. tic up to at least the area of this site. Many invertebrate fossils, some of shallow shelf Miocene origin, such as pelecypods and barnacles, occur mixed Miocene sediments occur in Core 359-2 and in two in with the foraminifer faunas in Cores 2 and 3. Only cores from Hole 359A. These Hole 359A cores appear the more resistant planktonic species are present in to represent the section below Core ^59-1 but above most samples; the benthic foraminifers are diverse and Section 359-2-6. Core 159A-1 contains sTmoderately suggest outer shelf depths. The nannofossil flora con- well preserved fauna that includes the index species tains members of the Braarudosphaeraceae, which ap- Globoquadrina dehiscens, Globorotalia conomiozea, parently prefer restricted conditions. In Core 4 the Globigerina nepenthes, and Globigerinoides sacculifer. Braarudosphaerids decrease in abundance, as do the The specimens in Core 359A-2 are poorly preserved shallower invertebrate fossils. The shallower water in- and the fauna is depleted, but the same index species, dicators may have reached the depth of the deposition except G. sacculifer, are present. site from higher parts of this positive feature. Since G. conomiozea evolves in Zone N17 and G. dehiscens becomes extinct above Zone NI7, we assign Foraminifers these cores to the upper Miocene (Zone NI7). Sample 359-2-6, 61 cm, is apparently older. Globo- Pliocene/Pleistocene rotalia conomiozea is no longer present, but Globo- Core 1 contains both Pliocene and Pleistocene rotalia miozea is. Globigerina nepenthes, G. woodi, planktonic foraminifers. In Sample 1-1, 46-48 cm (the Globorotalia conoidea, and Globigerina bulloides are top), a thin tan layer contains a residue of Pleistocene abundant. This part of Section 2-6 apparently belongs (Globorotalia truncatulinoides Zone), mixed with in the middle to upper Miocene (Zones N14-N16). Pliocene (G. margaritae Zone). Sediments below that Below Sample 359-2-6, 61 cm, a marked lithologic layer are white, like the core-catcher sample, which change occurs, from the Miocene creamy marl above to contains foraminifers of the Pliocene G. margaritae gray clayey marl below. Sample 359-2-6, 95 cm, from Zone, such as the nominate taxon, G. crassula, G. pun- the marl sequence, contains a mixed fauna of Miocene ticulata, G. conomiozea, and Sphaeroidinellopsis sub- and upper Eocene. The Miocene fauna, however, dehiscens. appears to be older than that in Sample 359-2-6, 61 cm. If Pleistocene overlies the lower Pliocene, as The co-occurrence of Globorotalia praemenardii and G. suggested by the top layer of this core, then the upper miozea in this sample suggests a middle Miocene age. Pliocene and the rest of the Pleistocene are missing at Despite differences in preservation of the faunas, this site. these subtropical Miocene planktonic foraminifer The planktonic forms in this core are subtropical and faunas are markedly similar; few other fossils or com- typical of open marine conditions. They are partially ponents occur in the residues, and the benthic species overgrown with calcite crusts, and many are corroded. characterize bathyal depths.
378 SITE 359: WALVIS RIDGE (SEAMOUNT)
Eocene colith zonations. Thus the presence of C. macintyrei in Fossils of Eocene age occur in Samples 2-6, 95 cm, all samples must be taken to indicate an age of middle through 4-1, 120 cm. These fossils include not only to late Miocene. Its co-occurrence with Cyclicargo- planktonic foraminifers, but abundant shelf benthic lithus floridanusindicate s an early middle Miocene age foraminifers, echinoid remains, fish teeth, pelecypod for Sections 1 through the top of 6 of Core 359-2 and remains, a few siliceous fossils, and structures resembl- Core 359A-2; Core 359A-1 may be of late middle to late ing "grouped" barnacles. From the diversity of fossils it Miocene age. is difficult to assess how much of the material is in place Late Eocene (Cores 2, 3 and 4) and how much transported. Glauconite, also common in many samples, suggests microreducing conditions. Common and fairly well preserved coccolith assem- blages of late Eocene age occur below Sample 359-2-6, Samples 2-6, 95 cm and 2, CC contain upper Eocene 60 cm, and in Core 3 and the top of Core 4. Besides faunas mixed with considerable numbers of Miocene planktonic forms. However, the Eocene foraminifer Isthmolithus recurvus and Chiasmolithus oamaruensis, faunas in this and Sections 3-1 through 4-1 are very which indicate an early late Eocene age, the assem- similar, and contain the species Globigerinatheka mex- blage includes common Zygrhablithus bijugatus and few icana, Globigerapsis index, Globigerapsis kugleri, Braarudosphaera bigelowi in Sample 2, CC. In Sample Globorotalia (Turborotalia) cerroazulensis, and in better 3, CC, /. recurvus is missing, Z. bijugatus, B. bigelowi, preserved samples, "GlobigerinitcT howei, Hantkenina and Micrantholithus sp. are few, and C. oamaruensis in- alabamensis, Pseudohastigerina micra, and dicates the zone of this name. Both assemblages reflect Chilogumbelina wilcoxensis. Sample 4-1, 120 cm, con- a depth of deposition shallower than the present depth tained in addition several species of round-edged of this site (1658 m). Common Z. bijugatus suggests acarininids. depths of less than 1000 meters, and members of the All these samples indicate subtropical conditions and family Braarudosphaeraceae preferred very shallow belong to the upper Eocene G. mexicana Zone (PI5). and/or restricted seas (bays, fjords, etc.). The coccolith Each sample contains differing amounts and types of assemblage in the upper part of Core 359-4 is still of benthic fossils, and evidence for environmental type late Eocene age, but contains a more diverse flora, with from these fossils often conflicts. For example, Sample fewer representatives of Braarudosphaeraceae. Small 4-1, 120 cm, contains a slope fauna of benthic pieces of clay attached to the volcanic tuff of the core- foraminifers, but also macroscopic pelecypods. Sample catcher sample of 359-4 contain only rare cocco- 3, CC contains typically slope-dwelling benthic species liths—probably contaminants. of Stilostomella, Uvigerina, Gavelinella, and SEDIMENT ACCUMULATION RATES Anomalinoides, along with structures that seem to be barnacles and pelecypod remains. If the deeper- Figure 6 summarizes the sediment accumulation dwelling fossils are in place, then Core 4 sediments may history at Site 359 in the southwestern part of the have come from a slightly deeper environment than Walvis Ridge. Age was determined from planktonic sediments in Cores 2 or 3. foraminifers in the Neogene and from foraminifers and calcareous nannofossils in the Eocene. Calcareous Nannofossils No fossils occur in the hard volcanic tuff recovered in Core 5 and the bottom of Core 4; we obtained a K/Ar Calcareous nannofossils of early Pliocene to late date of 40.7 ±1 m.y. from the tuff. The oldest fossils Miocene-middle Miocene age and of late Eocene age (foraminifers and coccoliths) indicate at least a late occur in Cores 1 to 4 of Hole 359 and Cores 1 and 2 of Eocene age for the lowermost fossiliferous volcanic Hole 359A. Figure 5 summarizes the distribution of mud in Core 4. The bottom of Section 359-2-6 is of only coccolith zones. Tables in Perch-Nielsen (this volume) slightly younger age than Core 4, so the accumulation show the distribution of coccolith species. rate was thus at least about 2 to 3 cm/1000 yr. A hiatus spanning the uppermost Eocene, the entire Pliocene (Core 1) Oligocene, and the lower to middle Miocene was cored Core 1 contains common but poorly preserved coc- in the foraminifer-nannofossil oozes in Section 359-2-6. coliths, including only a few barely definable dis- Another hiatus may occur between Cores 2 and 2A and coasters. The assemblage consists almost exclusively of include middle to upper Miocene. This "hiatus" may, large forms. This suggests extensive winnowing of this however, also be an artifact caused by the use of data high- to mid-latitude assemblage. An early Pliocene age from two holes. The accumulation rate during the late is suggested by Cyclococcolithina macintyrei, Miocene to early Pliocene was probably about 1 Sphenolithus abies, Reticulofenestra pseudoumbilica; cm/1000 yr. Pseudoemiliania lacunosa is absent. Sample 359-1-1, 46 Sediments of late Pliocene and early Pleistocene age cm contains some species (Syracosphaera histrica and were not recovered at this site. Since only 68% of the Rhabdosphaera claviger) which indicate a warmer sediment was recovered in Core 359-1, taken at the climate (or less dissolution) than that inferred from sediment surface, the Quaternary may have been pre- Sample 359-1, CC. sent but missed in coring. Miocene (Cores 2, 1A, and 2A) CORRELATION OF REFLECTION PROFILE The Miocene assemblages also are poorly preserved WITH DRILLING RESULTS and include none of the discoasters, sphenoliths, or Figure 7 contains a line drawing interpretaion of the ceratoliths used in the tropical and subtropical coc- Glomar Challenger seismic reflection profile obtained
379 SITE 359: WALVIS RIDGE (SEAMOUNT)
water depth 1658 m Wal vis Ridge 359 MIOCENE OLIGOCENE EOCENE PALEOCENE -M Q- 1.0 20 30 40 50 60 Lith. Cores I I I I I I years
2-3 cm/1000 yrs IFORAMS 100 empty 1 NANNOS
Figure 6. Sedimentation history, Site 359.
PLIO-PLEISTOCENE middle-late MIOCENE
Figure 7. Correlation of the seismic reflection profile with the stratigraphic sequence.
on approach to Site 359 (Figure 2). Reflectors are pre- The tuff horizon seems to crop out slightly upslope sent at 0.11 and 0.20 sec. A reflector appears at 0.32 sec from the drill site (Figure 7). The outcropping tuff im- on the original strip chart record, but its validity is parts a very dark reflective appearance to the seismic doubtful. profile record. No subbottom reflectors are evident in The reflector at 0.11 sec is correlated with the top of the region where tuff is exposed. the black tuff encountered in Core 4 at a depth of 87 meters. This would result in an average sonic velocity of SUMMARY AND CONCLUSIONS 1.58 km/sec for the upper ooze sequence. The mode(s) of origin and structural history of the The tuff layer, of which we penetrated 20 meters, Walvis Ridge are critical to understanding the develop- probably does not extend from 0.11 to 0.20, since a tuff ment of the South Atlantic ocean basin, particularly over 100 meters thick would absorb all the non- with regard to the ridge's role as a barrier to surface reflected energy within itself and its top at 0.11 sec water exchange and deep flow in the meridional direc- would appear as acoustic basement on the profile tion at various stages of development of the South record. The reflector at 0.20 sec indicates that there is Atlantic. The most comprehensive compilations of data probably nontuffaceous sediment in the interval are those of Connary (1972), Barnaby (1974), Goslin et between 0.11 and 0.20 sec. It is unclear whether al. (1974), and Goslin and Sibuet (1975); Dingle and acoustic basement is the 0.20 sec reflector or whether a Simpson (manuscript) review these and other reflector actually occurs at 0.32 sec. references.
380 SITE 359: WALVIS RIDGE (SEAMOUNT)
Wilson (1965), Dietz and Holden (1970), and about 50-55 m.y.B.P., and drifted east as the sea-floor Morgan (1971, 1972) have proposed that the Wal vis spread and underwent geochemical differentiation until Ridge was generated by the southeast Atlantic crustal the late Eocene, when volcanic activity ceased. At a plate having moved away from a "hot spot" on the ac- spreading rate of 2 cm/year, the seamount would have creting plate boundary of the Mid-Atlantic Ridge. Le moved laterally away from the spreading center a dis- Pichon and Hayes (1971) and Francheteau and Le tance of 200-300 km before volcanic activity ceased; Pichon (1972) favor a transform-fault origin. Goslin this implies that an active magma chamber was en- and Sibuet (1975) explain the different strikes of the trained beneath the seamount for at least 10 m.y. three recognized subprovinces (Connary, 1972) of the An attempt to reconcile the supposedly subaerial Walvis Ridge as resulting from extrusion of mantle- nature of the tuff with the environmental indicators derived volcanics along structurally controlled lines of (fossils) in the overlying calcareous volcanic mud of weakness. roughly the same age must confront some enigmas. The Site 359 lies within the western Walvis Ridge sediments contain abundant open marine planktonic province where between it and the Mid-Atlantic Ridge nannofossils and foraminifers; but they also contain the Walvis Ridge consists of individual seamounts, benthic foraminifers of continental slope depth ranges guyots, and the islands of Tristan da Cunha and and shallow-water indicators such as large mollusc Gough. Between Site 359 and about 3°E, the western, fragments and brachiopods. Boersma (this volume) or seamount, province consists of discontinuous and states that environmental data are difficult to determine narrow elongate elements with steep sides, presumbaly from fossils at this site, but that if the more obvious chains of closely spaced seamounts (Dingle and Simp- shallow-water indicators were introduced from up- son, in preparation). Although other factors may be of slope, the site could have had a depth of about 900 controlling importance for the central and eastern meters in the late Eocene. Working with this as a con- Walvis Ridge, we assume that the seamounts of the straint, Fodor et al. (this volume) have suggested that western province were generated at a point on the ac- the subaerial tuff subsided catastrophically about 1000 creting plate boundary and carried to the east by the meters by summit collapse of a shield volcano. sea-floor spreading process. We do not know whether A problem with the hypothesis that the depositional the source of igneous material was a mantle plume or site was at a depth close to 1000 meters is that if its sub- merely a locus of increased igneous activity nor do we sidence rate is presumed to be along the curve defined know whether such a feature has migrated over time. by Sclater and Detrick (1973), subsidence between 40 The ash flow tuff which comprises the basal rocks m.y.B.P. and the present should have resulted in a pre- recovered at Site 359 has a trachytic bulk composition, sent site depth of about 2400 meters rather than 1650 similar to the trachytes on the nearby islands of Tristan meters. An alternative explanation, whereby the da Cunha and Gough. Fodor et al. (this volume) benthic slope fauna represents a somewhat shallower describe the mineralogy and chemical composition of depth (say, 300 m), increases the chances of introduc- the tuff, and note that ash flow tuffs are reported from tion of very shallow water components, is more easily several Atlantic oceanic islands. They suggest, on the reconcilable with the tuff being subaerial, and allows bases of lack of graded bedding, lack of evidence or submergence along the model favored for normal reworking, and lack of entrained shell material and oceanic crust. The abundance of open marine pelagic clasts of marine sediment, that the Site 359 ash flow organisms is explained by the proximity of deep water may have been subaerially emplaced. This, if so, would all around. imply either that any sediments underlying the tuff (see The major upper Eocene-middle Eocene hiatus Correlation section, above) are nonmarine or that the within the pelagic ooze sequences may be a conse- seamount was tectonically raised above sea level quence of nondeposition, erosion by currents, or slum- between deposition of such sediments and deposition of ping. It is of local importance only, since sediments the tuff. representing these ages were recovered at Sites 17 and Site 359 is at the approximate location of anomaly 18, on the eastern flank of the Mid-Atlantic Ridge close 23-25 (Ladd, 1974), which has an age range of 47-53 to the Walvis Ridge (Maxwell, Von Herzen et al., m.y.B.P., according to Tarling and Mitchell (1976)' 1970). A Plio-Pleistocene hiatus, if present, could also The average South Atlantic spreading rate of 2 cm/year be a result of any one of the causes mentioned above. for this period (Larson and Pitman, 1972) would ac- The uppermost nannofossil and foraminifer oozes count for its present position about 1000 km from the have been intensively winnowed, resulting in a rela- crest of the Mid-Atlantic Ridge. The tuff is 40.7 ± 1 tively coarse grain size and a large portion of foramin- m.y. old according to a K/Ar date obtained on feldspar ifers. Heavy overgrowth on both foraminifers and nan- (Fodor et al., this volume) and is overlain by calcareous nofossils are qualitative indicators of a high CaCOj volcanic mud of roughly the same age (late Eocene, mobility in the interstitial waters of these sediments. Zone PI5). Overlying upper Eocene biogenic oozes contain no volcaniclastic components; this indicates REFERENCES Jhat volcanic activity ceased before the end of the Eocene. The seamount Ihus probably came into being Barnaby, A.M., 1974. The structure of the Eastern Walvis as a basaltic outpouring on the Mid-Atlantic Ridge Ridge and the adjacent continental margin: M.Sc. thesis, University of Cape Town. Connary, S..A., 1972. Investigations of the Walvis Ridge and 2Although more recent work indicates an age range of 54-59' environs: Ph.D. thesis, Columbia University, Palisades, m.y.B.P. for anomalies 23 to 25 (La Brecque et al., in press). New York.
381 SITE 359: WALVIS RIDGE (SEAMOUNT)
Dietz, R.S. and Holden, J.C., 1970. Reconstruction of Larson, R.L. and Pitmann, W.C. Ill, 1972. Worldwide Pangaea: breakup and dispersion of continents, Permian correlation of Mesozoic magnetic anomalies, and its to Present: J. Geophys. Res., v. 75, p. 4939-4956. implications: Geol. Soc. Am. Bull. v. 83, p. 3645-3662. Dingle, R.V. and Simpson, E.S.W., in preparation. The Le Pichon, X. and Hayes, D.E., 1971. Marginal offsets, Walvis Ridge: a review: Submitted to Tectonophysics. Francheteau, J. and Le Pichon, X., 1972. Marginal fracture fracture zones, and the early opening of the South zones as structural framework of continental margins in Atlantic: J. Geophys. Res., v. 76, p. 6283-6293. South Atlantic Ocean: Am. Assoc. Petrol. Geol. Bull., Maxwell, A.E., Von Herzen, R.P., et al., 1970. Initial Reports v. 56, p. 991-1007. of the Deep Sea Drilling Project, Volume 3: Washington Goslin, J. and Sibuet, J.C., 1975. Geophysical study of the (U.S. Government Printing Office). easternmost Walvis Ridge, South Atlantic: deep structure: Morgan, W.J., 1971. Convection plumes in the lower mantle: Geol. Soc. Am. Bull., v. 86, p. 1713-1724. Nature, v. 230, No. 42. Goslin, J., Mascle, J., Sibuet, J.C., and Hoskins, H., 1974. , 1972. Plate motions and deep mantle convection: Geophysical study of the easternmost Walvis Ridge, South Geol. Soc. Am. Mem. 132, p. 7. Atlantic: morphology and shallow structure: Geol. Soc. Sclater, J.G. and Detrick, R., 1973. Elevation of midocean Am. Bull., v. 85, p. 619-632. ridges and the basement age of JOIDES Deep Sea Drilling La Brecque, J., Kemp, D., and Cande, S., in press. Reversal magnetic polarity time scale for the Late Cretaceous and sites: Geol. Soc. Am. Bull., v. 84, p. 1547-1554. Cenozoic: Geology. Tarling, D.H. and Mitchell, J.G., 1976. Revised Cenozoic Ladd, J.W., 1974. South Atlantic seafloor spreading and polarity time scale: Geology, v. 4, p. 133-136. Caribbean tectonics; Unpublished Ph.D. thesis, Columbia Wilson, J.T., 1965. Submarine fracture zones, aseismic ridges University, New York. and the ICSU line: proposed western margin of the East Pacific Rise: Nature, v. 207, p. 907-911.
382 SITE 359: WALVIS RIDGE (SEAMOUNT)
APPENDIX A Smear-slide Summary