Deep Sea Drilling Project Initial Reports Volume 23
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3. SITE 219 The Shipboard Scientific Party1 With Additional Reports From Norman Hamilton, University of Southampton, United Kingdom Johannes Hunziker, Mineralogisch-petrographisches Institut der Universitàt, Bern, Switzerland and Paleontological Laboratories of the Geological Survey of India SITE SUMMARY Early Early 70 75C Early Figure 1. Position of Site 219 and adjacent Leg 23 sites (shown by +). Contours at 200, 1000, and 4000 meters, from Laugh ton et al. (1971). SITE DATA Dates: 1315 10 Mar-1945 13 Mar 72 Time: 78 hours Holes Drilled: 2 ; Position (Figure 1): 9°01.75 N, 72°52.Ó7'E Water Depth by Echo Sounder: 1764 corr. meters Total Penetration: 411 meters Total Core Recovered: 223.1 meters from 42 cores Early Age of Oldest Sediment: Late Paleocene Basement: Unknown ABSTRACT Shallow-water Upper Paleocene limestones, sandstones, and siltstones are succeeded by Eocene chalk and ooze. The sea bed began to sink about 2000 meters in Early Eocene times. Biogenic silica, sometimes as chert, is common in the Eocene. Sedimentation was at a minimum in the Oligocene. Mainly detrital silty clay nanno ooze was laid down in post-Early Miocene times. Upwelling began in the Middle Miocene and continues today. Volcanic glass is found in the Pleistocene. Robert B. Whitmarsh, National Institute of Oceanography, Wormley, Godalming, Surrey, United Kingdom; Oscar E. Weser, Scripps Institution of Oceanography, La Jolla, California: Syed Ali, Godalming, Surrey, United Kingdom); Tapas K. Mallik, Geological State University of New York, Stony Brook, New York; Joseph E. Survey of India, Calcutta, India; Albert Matter, Geologisches Boudreaux, Texaco, Inc., New Orleans, Louisiana; Robert L. Institut, Universitàt Bern, Bern, Switzerland; Catherine Nigrini, Fleisher, University of Southern California, Los Angeles, California; Lexington, Massachusetts; Hassan N. Siddiquie, Geological Survey Dan Jipa, M.I.M.G. Institutul Geologic, Bucharest, Romania; Robert of India, Calcutta India (Present address: National Institute of B. Kidd, Southampton University, Southampton, United Kingdom Oceanography, Goa, India); Peter Stoffers, Laboratorium fur (Present address: National Institute of Oceanography, Wormley, Sedimentforschung, Universitàt Heidelberg, Heidelberg, Germany. 35 SITE 219 BACKGROUND AND OBJECTIVES Lambeck, 1971). This anomaly is about -60 mgal in the Maldive area, suggesting that the isostatic anomaly, if any, Site 219 is located on the crest of the Laccadive-Chagos attributable to the sinking of the Maldive Islands is far less Ridge (Figure 1). This ridge is a major linear feature of the than Glennie supposed. floor of the Indian Ocean. Its origin has been said, even Magnetic observations by ships during the IIOE have recently, to be both oceanic and continental by different shown that substantial magnetic anomalies (Figure 2), authors. The almost complete lack of samples from the several hundred gammas in amplitude and with wavelengths ridge, other than surficial sediments, has prevented detailed of tens of kilometers, exist along tracks passing through the study of its history and nature. Thus a drill hole was clearly Laccadive and Maldive islands (D. H. Matthews, unpub- desirable but, although the ridge is a feature probably lished IIOE Atlas charts; Admiralty Hydrographic Office, deserving half a dozen holes, time was only allotted for one 1963 and 1966). These observations led to wide acceptance reconnaissance hole on the northern part of the ridge. by oceanographers that the Laccadive-Chagos Ridge has a During Leg 24, a second hole was drilled near the Chagos basaltic and, therefore, probably oceanic foundation. Islands. The Laccadive-Chagos Ridge extends a distance of This conclusion has been borne out by the crustal 2200 km in a north-south direction, from the Chagos velocities of 4.8 to 6.1 km/sec and 6.7 to 7.1 km/sec found Islands at 6°S to the Laccadive Islands at 14°N. The ridge is by seismic refraction studies over the Ridge at latitudes slightly arcuate, being concave to the west. A considerable 9°N, 2°S, and 5%°S (Francis and Shor, 1966). However, length of the crest of the ridge is composed of shoals, only at the northernmost station, situated very close to Site banks, and coral reefs at depths less than 1500 meters, and, 219, was an upper mantle velocity detected, and this broadly, these shoal areas are grouped as the Laccadive, unreversed line indicated a crustal thickness of 17.5 km. At Maldive, and Chagos islands. Between the shoals, the water 2°S, the Moho is probably at least 20 km deep. Thus, depth is commonly around 2000 meters and, exceptionally, Francis and Shor indicate that the depth at which a normal reaches 4000 meters between the Maldive and Chagos upper mantle velocity is reached beneath the Laccadive- islands. To the west of the ridge, the sea floor everywhere is Chagos Ridge is comparable with that close to Eniwetok relatively steep at first and then flattens out at around 4000 Atoll, the inference being that the Maldive Islands rest on a meters. The east flank is modified by the proximity of the subsided volcanic foundation. Before Leg 23, only five Indian continental slope and abutting abyssal plain seismic reflection profiles which crossed the northern part sediments at 2500 meters depth north of 3°N, but south of of the ridge were available (Ewing et al., 1969; Harbison the equator a scarp is developed, which drops down to at least 4500 meters, with a parallel ridge and trough (the Chagos Trench) flanking the Chagos mass (information from unpublished charts prepared for the International Indian Ocean Expedition [IIOE] Atlas by R. L. Fisher and A. S. Laugh ton). Before the drilling by Glomar Challenger, no sedi- mentary rock, other than coral rock and associated detritus, was known to have been obtained from anywhere along the whole length of the Laccadive-Chagos Ridge (Wilson, 1963). A single sample of a glassy basalt from the west flank of the Laccadive Islands (11°18'N, 71°00'E) has been reported by Vinogradov et al. (1969). Probably the earliest geophysical measurements made on the ridge were the pendulum gravity observations of Glennie (1936) in the Maldive Islands. He found large negative isostatic anomalies (about -75 mgal) which he interpreted in two ways. First, if the anomalies are due to local superficial anomalies of density, such as a considerable thickness of coral rock, then the Maldive Islands "mark the WESTSOUTHWEST original site of an oceanic ridge, not a continental block, which has since sunk down under isostatic adjustment." A second hypothesis proposed that the Maldives could lie over "an area where a block of sial has subsided as a result of the downwarping of the lower crustal layers." Glennie - 1000 calculated coral thicknesses on the basis of these ^^____^-v/^^ NO DATA hypotheses, but his figures must be tempered by the fact - 2000 that he used the Helmert gravity formula of 1901, not the 1930 formula which yields larger negative anomalies. On the other hand, the former gives a gravity value at this Figure 2. Bathymetric and magnetic profiles across the latitude which differs little from the newly adopted 1967 northern part of the Laccadive-Chagos Ridge obtained gravity formula (Woollard, 1969). In addition, however, by H.M.S. Owen (Admiralty Hydrographic Office 1963). account should be taken of the large regional negative The profiles are arranged in order of decreasing latitude free-air anomaly in this part of the Indian Ocean, recently from top to bottom. Site 219 is situated between the discovered from satellite observations (Gaposchkin and two topmost profiles. 36 WEST Figure 3. Two seismic reflection profiles (CD and EF) across the northern end of the Laccadive-Chagos Ridge (courtesy ofN.O.A.A. Atlantic Oceanographic and LO Meteorlogical Laboratories, Miami). The arrow on profile EF marks the proposed site. For tracks of these and other available profiles see inset profiles AB -J CD, EF from Oceanographer, 1967; profile GH from Vema-19, 1963; profile IJ from Conrad-9, 1965. SITE 219 and Bassinger, 1973), and three of these were close together The soft pelagic sediments on the sea floor were easily within the channel between the Laccadive and Maldive penetrated and the first 13 cores, spanning the interval 8 to islands (Figure 3 inset). Due to the absence of any presite 127 meters, were continuously cored. A punch core was survey, these profiles, therefore, were the sole data from taken at the surface to minimize sediment deformation. which a proposed site could be chosen. In general, all the The somewhat more indurated chalks occurring between profiles, except the northernmost one (profile AB, Figure 3 127 and 210 meters were not as easily penetrated, and inset), show a sea bed over the ridge which varies from circulation was broken several times. smooth to hummocky. It has a layer of transparent At Hole 219, some difficulties were encountered sediment usually at least 0.4 sec thick, overlying an between 210 and 273 meters. Hard layers consisting mostly irregular and probably faulted acoustic basement. In places, of chert alternated with softer material, including sands. the basement crops out. Profile AB apparently has even Recovery of the softer material was poor as circulation had thicker sediments which include turbidites, and the basement to be broken frequently. The inner barrel of the final core does not crop out but is only clearly seen at one point. The (Core 28) became stuck and the drillers were unable to JOIDES Indian Ocean Panel had made a preliminary effect circulation suggesting that the water jets in the bit suggestion for a site in the general area of profile AB. In were plugged. The drill string then had to be tripped back view of the above comments, however, and the possible to the surface, terminating operations at this hole.