Oligocene sediments near Chain Ridge, northwest Indian Ocean: structural implications

Marine Geology Elsevier Publishing Company, Amsterdam - Printed in The Netherlands

Letter Section Oligocene sediments near Chain Ridge, northwest Indian Ocean: structural implications*

ANTHONY C. PIMM1, R.H. BURROUGHS2 and E.T. BUNCE2

1Scripps Institution o f Oceanography, La Jolla, Calif. (U.S.A.) 2 Woods Hole Oceanographic Institution, Woods Hole, Mass. (U.S.A.) (Received March 6, 1972) (Accepted after revision May 8, 1972)

ABSTRACT Pimm, A.C., I~urroughs, R.H. and Bunce, E.T., 1972. Oligocene sediments near Chain Ridge, northwest Indian Ocean: structuralimplications. Mar. GeoL, 13 :MI4-MI8. A hiatus of 26 m.y. was detected in a 431 cm-long piston core taken from the east flank of Chain Ridge. This hiatus at 356 cm separates nannoplankton chalk of Early Oligocene age from overlying nannoplanktonsilty clay of Late Pliocene age. An unconformity between Late Pliocene and Pleistocene is marked by a manganese nodule at 278 cm. Mass movement down a slope of 11° initiated by tectonic activity in the area is the best explanation for the 26 m.y. hiatus.

GEOLOGIC SETTING

Oligocene material, the oldest sampled to date in the northern Somali Basin, lies in a structurally complex area between Chain and Carisberg ridges (Fig. 1). There the topography consists of a series of basement highs and intermountain sediment-filled basins (Fig. 2). In the Somali Basin to the west the basement is deep, 2.5 km subbottom, and covered by a section of uniformly stratified and flat-lying sediments (Bunce et al., 1967; Ewing et al., 1969). Dredged basalts from the ridge have been dated at 89 m,y. which is considered to be a minimal age for the feature (Bunce et al., 1967). The total thickness of sediments over basement at Core 28 cannot clearly be established. The 3.5-kHz and 12-kHz records are varied; they show a series of side echoes, some clear indications of 20 m of sediment in conformable beds, and some evidence of greater thickness up to 50 m.

*Contribution no. 2842 of the Woods Hole Oceanographic Institution.

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Fig. 1. Location of cores (dots) and the 89-m.y. dredge sample (cross) near Chain Ridge. Line indicates bathymetric section (Fig. 2).

CHAIN RIDGE

10 KM

Fig. 2. Bathymetric section from Somali Abyssal Plain (5,100 m) to core 28 (4,150 m). CORE DESCRIPTION

An examination of Core 28 shows that from the surface to a depth of 278 cm the sediment is a white to light-gray calcareous nannoplankton chalk which is slightly to moderately mottled (Fig. 3). Siliceous fossils (Radiolaria, sponge spicules, diatoms, silicoflagellates) and Foraminifera are present in small amounts ( < 15%). The remaining nonbiogenic fraction is mostly clay-size quartz with much smaller amounts of kaolinite, iUite, montmorillonite, and plagioclase. Calcium carbonate content is 71.6% at the surface and 81.7% at 200 cm. All age determinations were made on the basis of calcareous nannoplankton. The pilot core, 100 cm in length, is all Pleistocene;it contains an abundance of Geophyrocapsa

M 16

LETTER SECTION LITHOLOGY

NANNOPLANKTON

AGE

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NANNOPLANKTON CHALK

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200

300

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Discooster brouweri D.perdol'odiotu~ D. voriobHrs D. SUrCUlUS

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Sphenolithus 400

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Fig. 3. Core log,

oceanica throughout. Samples from 135 cm and 200 cm in the piston core yielded an abundant flora ofPseudoemiliania lacunosa indicating an Early Pleistocene age. Also present are Helicopontosphaera selli, H. kamptneri, Cyclococcolithina leptopora, and Pontosphaera discopora; the presence of the last-named species - a delicate form susceptible to solution - indicates an extremely well.preserved flora. A reworked Oligocene species, Reticulofenestra abisecta, was also noted. In summary the top unit of the piston core and all of the pilot core is a Pleistocene nannoplankton ooze. From 278 cm, where a manganese nodule marks the boundary, to 355 cm the sediment is olive-gray nannoptankton silty clay (CaCO3 = 48.8%). This second unit contrasts with the sediment above, as it contains both a noticeable terrigenous component and redeposited carbonate in addition to the pelagic calcareous nannoplankton material. The unit is half clay-size and half sand-coarse silt-size material. Of the clay.size material about 30% is carbonate and the remainder contains quartz, plagioclase, K-feldspar, montmorillonite, kaolinite, and illite. The fine sand-coarse silt-size material is about 70%

LETTER SECTION

M 17

fragmented calcite (mostly Foraminifera test fragments), and 20% subangular quartz grains of the same size grade. The remaining 10% consists of plagioclase feldspar, iron oxide, radiolarian fragments, sponge spicules, pyrite, echinoid spines, light-colored glass (R.I. < 1.55), brown phosphatic fragments, chloritic material, and heavy minerals. A Late Pliocene flora occurs in this second unit a.t 300 cm and 355 cm. This flora is characterized by abundant Discoaster brouweri, D. pentaradiatus, D. variabilis, and D. surculus and rare Ceratolithus rugosus, and the absence of Reticulofenestra pseudoumbilicus. All species are strongly etched. Spheolithids are lacking at 300 cm and this suggests the interval is in the uppermost part of the Late Pliocene. The sample from 355 cm appears a little lower in the Late Pliocene as it contains numerous Sphenolithus abies. In addition to the Late Pliocene flora there are a few reworked species from the Middle Tertiary - Discoaster deflandrei, D. divaricatus and large Sphenolithus

moriformis. At 356 cm there is a very sharp contact with uniform compact lithified white chalk of Early Oligocene age, which continues to the base of the core at 431 cm. This third unit (CaCO3 = 71..2%) is mostly nannoplankton with about 3% small whole planktonic Foraminifera tests, and small amounts of clay size material - mostly plagioclase and K-feldspar with Smaller amounts of quartz, kaolinite and illite. The contact at the top of this white chalk unit coincides with a hiatus of approximately 26 million years, since a sample at 357 cm yielded an Early Oligocene flora characterized by an abundance of Sphenolithus predistentus and Reticulofenestra umbilica. CONCLUSIONS From the surface to a depth of 355 cm the age of the sediment ranges from Pleistocene to Late Pliocene, about 3 m.y. The presence of a manganese nodule coinciding with a lithologic change at 278 cm indicates an unconformity coincident with the Pliocene-Pleistocene boundary. The marked improvement in preservation of calcareous nannopiankton and Foraminifera from the Pliocene to Pleistocene may suggest some uplift in the depositional area. The major hiatus occurs at 355 cm within the core. Several other piston cores (Fig. 1) taken in this area showed more continuous sedimentary records, so it seems unlikely that bottom currents are responsible for this 26 m.y. hiatus. Dissolution takes place to varying degrees throughout the core, but is not a reasonable explanation for the major hiatus.. Mass movement is the most likely explanation for the major hiatus. The slope of the bottom at the core location is 11 ° to the south (Fig. 4) which can be sufficient to cause mass movement in unconsolidated material (Ross, 1971). Side echoes are common because the ship's track lies obliquely to the maximum slope, making the bathymetric record difficult to interpret in detail. However, minor relief may indicate mass movement. Uplift and/or seismic activity along Chain Ridge could have initiated the gravity slides that removed the Early Oligocene to Late Pliocene sediment sequence.

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Fig. 4. Bathymetric contours in area of Core 28 (meters). The relationship o f Owen Fracture Zone-Chain Ridge to the evolution of the Indian Ocean is not clearly known. Laughton (1966) and Matthews (1966) conclude that the major tectonic readjustment of the area, the offset of the mid-ocean ridge, occurred during the Late Tertiary. If our interpretation o f the causes of the hiatus is correct, the evidence from Core 28 which is near the juncture o f the Carlsberg and Chain ridges suggests that the major tectonic activity occurred after Early Oligocene and terminated before Late Pliocene. ACKNOWLEDGEMENTS

We thank Captain D. Casiles, the officers, the crew, and scientists of the R/V "Chain", Cruise 100, Leg 4. C.D. Hollister critically reviewed the manuscript and provided many helpful comments. This work was supported by a National Science Foundation Grant, GA-27516 to the Woods Hole Oceanographic Institution. REFERENCES Bunce, E.T., Langseth, M,G., Chase, R,L. and Ewing, M., 1967, Structure of the Western Somali Basin. J. Geophys. Res., 72:2547-2555. Ewing, M., Ettreim, S., Tmchan, M. and Ewing, J.I., 1969. Sediment distribution in the Indian Ocean. Deep-Sea Res., 16:231-248. Laughton, A.S., 1966. The Gulf of Aden. Philos. Trans. R. Soc., Set. A, 259:150-171. Matthews, D.H., 1966. The Owen Fracture Zone and the northern end of Cadsberg Ridge. Philos. Trans. R. Soc., Ser. A, 259:172-197. Ross, D.A., 1971. Mass physical properties and slope stability of sediments of the northern Middle America Trench. J. Geophys. Res., 76:704-712.