ALBERT

Description: The drilling vesselSubmarex of Global Marine was used to drill and core sediments to a depth of 56.4 m on the Nicaragua Rise, between Walton Bank and Jamaica, in 610 m of water. Seismic reflection profiles revealed thick accumulations of layered sediments with some fossil reefs. The sediments consist of undisturbed layers rich in planktonic microfossils alternating with turbidite layers. Absence of older coccoliths indicates that the redeposited material was not appreciably older than the time of redeposition, and oxygen isotopic analysis of benthonic elements shows that this material was derived from a depth not much shallower. The lower portion of the cored section correlates with the Manchioneal Formation of Jamaica. Taxonornic analysis of the calcareous nannoplankton indicates that the level at 2354 cm correlates with the midportions of the eastern equatorial Pacific cores 58 and 62; with the “Nebraskan-Aftonian” boundary of the Gulf Coast; and with the appearance of Hyalinea baltica at Le Castella, southern Italy. This level, therefore, represents the Plio-Pleistocene boundary as officially designated, and an age of about 700,000 years is estimated for the bou ndary. Oxygen isotopic analysis shows important oscillations, with a full glacial-interglacial amplitude, occurring both above and below the Plio-Pleistocene boundary.

Description: The submarine Mascarene Plateau, lying east and northeast of Madagascar in the west central Indian Ocean, extends as a faulted composite arc for 2300 km, from the Precambrian granitic “micro-continent” of Seychelles Bank southward through the coral–reef–capped volcanic structures (?) of Saya de Malha, Nazareth, and Cargados Carajos banks to the faulted Tertiary–Quaternary oceanic volcanic island Mauritius. Locally the aseismic plateau is buttressed by spurs, is steep-sided and angular in plan, or is extended, disrupted, or offset by cross–faulting. Other major structural elements in a sector centered on the Mascarene Plateau are: (1) the seismically active Mid-Indian Ocean Ridge and rift zone (including that portion known as the Carlsberg Ridge) between the equator and its bifurcation near 23°S.; (2) the angular and faulted Amirante Trench lying between Seychelles Bank and the Madagascar-Farquhar complex; (3) the east–west late Tertiary–Pleistocene Rodriguez Ridge and its possible extension eastward as a fracture zone offsetting the Mid-Indian Ocean Ridge; (4) the Mauritius Trench, a 1400-km border deep-trending southwest from east of Mauritius to 28°S., 51°E. Irregular topography, attributed to volcanism and block faulting, is strikingly developed southeast of the intersection of Rodriguez Ridge with the Mascarene Plateau. Here linear, slightly sedimented deeps and narrow ridges lie subparallel to Mauritius Trench and to the southwest branch of the Mid-Indian Ocean Ridge. Except in the northwest sector where northwest and north trends lie within the Agalega–Seychelles–Amirante–Owen Fracture Zone complex, the dominant topographic lineations are northeast to east–northeast. These are displayed by large features such as fractures offsetting the mid-oceanic ridge (for example, Vema Trench), the Mauritius Trench and the linear topography southeast of Mauritius, the apparent offsets of the southern half of the Mascarene Plateau, spurs on the west flank of the mid-oceanic ridge, and the smaller ridges protruding; from the sedimented rise and abyssal plain between Tromelin and the Mascarene Plateau. Offsets of the Mascarene Plateau along northeast-trending faults-occurred in Tertiary time as adjustments accompanying formation of the Mascarene Basin. Activity on several of these faults has continued where they intersect the crestal portions of the mid-oceanic ridge system, but growth of the exceedingly rough, ridge in this region is primarily by addition of igneous material along its axis.

Description: The microtopography of the continental shelf in five areas between Nova Scotia and New York was investigated with side-scanning sonar. The results were checked against those previously obtained by conventional methods in all areas and against later visual observations of the bottom from a research submersible vessel in two areas. Side-scanning sonar proved to be an ideal device for learning the distribution and relationships of rock, boulder, and sand bottoms and for measuring the patterns and trends of several sizes of sand waves and of large ripple marks.