Small Seamounts Research Articles

Studies of oceanic tectonics utilizing detailed surveying and stratigraphy

The sea floor off the coasts of Washington and southern British Columbia consists of two major features: the Cascadia Basin and the chain of seamounts to the west of it, called the Cobb Rise. The Basin is magnetically featureless and appears to be a syncline deeply filled by sediment. Clues to the tectonic history of the region are therefore more likely to be found on and beyond the Cobb Rise, which forms the western boundary of the Basin and interrupts the westward flow of terrigenous sediment. A program of detailed topographic and magnetic surveving, complemented by heat-flow coring and seismic profiling, is underway to attempt to determine the structure and history of the abyssal hills on the seaward side of Cobb Rise. Knowledge of the stratigraphy and the topography of the basement-sediment interface is expected to reveal the approximate age of the sea floor in this area and possibly the nature of the basement itself. The magnetic and geothermal data may provide direct evidence of the subcrustal structure associated with Cobb Rise, particularly if the transition zone between the high heat flow on the rise and the low heat flow west of it can be studied in detail. A small seamount has been discovered at 47°N, 132°W and has been intensively surveyed as the nucleus of the future survey area. It rises 630 meters above the surrounding bumpy plain and has extremely steep faces to the south and east. Heat flow near the seamount is somewhat below the oceanic average and the several long gravity cores obtained in the area show some correlable layering in the pelagic ooze.

Association of a seismic reflector and upper cretaceous sediment in the Bay of Biscay

Geological and geophysical investigations have been carried out over a small seamount near 45°N, 8°W in the central part of the Bay of Biscay. Recordings made with an airgun seismic profiler have suggested that the feature has been formed as a result of folding of a sequence of stratified sediments, which may be composed largely of turbidites, and by later deposition of accoustically more uniform pelagic material. The fold movements apparently took place before a series of turbidites nearly 1 kilometre thick was laid down to form the Biscay Abyssal Plain. A strong seismic reflector marking the top of the distorted stratified sequence is generally covered by thick sediments, but in two places it apparently outcrops at the sea floor. Samples of cocolith ooze of Maestrichtian age were obtained by gravity coring and dredging in one of the outcrop areas. It is suggested that the strong seismic reflector is approximately Maestrichtian in age and may record an important change in the sedimentary regime in the deep parts of the Bay of Biscay near the Mesozoic-Tertiary boundary. This occurrence of relatively old sediment does not conflict with current ideas on the formation of the Bay of Biscay by an anticlockwise rotation of Spain in late Jurassic-early Cretaceous times.

Altered Lavas from the Floor of the Eastern North Atlantic

RITTMANN and others have long argued that basic lavas erupted on the ocean floor under great depths of water may suffer immediate alteration and solidify as rocks of the serpentine–spilite association, and recent experimental work by Clark and Fyfe1 has directed attention to this possibility. The altered basalts, the physical properties and bulk chemistry of which were outlined in a previous communication2, were almost certainly extruded under deep water and may provide an example of mild alteration under these conditions. The specimens were dredged from a small abyssal hill, named Swallow Bank, standing 4–5 km. above the Mohorovicic discontinuity under 4,960 m. of water at 41° 21′ N., 14° 28′ W. These specimens, and a second collection from a nearby small seamount (the Western Seamount, summit depth 2,600 m.), have been examined petrographically. In thin section the altered lavas are 50 per cent opaque; none retains unaltered groundmass material, and only three specimens out of four hundred preserve unaltered phenocrysts or glass. The present communication outlines the mineralogy of the alteration process which is believed to have affected them.

NORTH ATLANTIC HYDROGRAPHY AND THE MID-ATLANTIC RIDGE

Research Article| October 01, 1949 NORTH ATLANTIC HYDROGRAPHY AND THE MID-ATLANTIC RIDGE IVAN TOLSTOY; IVAN TOLSTOY COLUMBIA UNIVERSITY, NEW YORK, N. Y. Search for other works by this author on: GSW Google Scholar MAURICE EWING MAURICE EWING COLUMBIA UNIVERSITY, NEW YORK, N. Y. Search for other works by this author on: GSW Google Scholar GSA Bulletin (1949) 60 (10): 1527–1540. https://doi.org/10.1130/0016-7606(1949)60[1527:NAHATM]2.0.CO;2 Article history received: 07 Sep 1948 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation IVAN TOLSTOY, MAURICE EWING; NORTH ATLANTIC HYDROGRAPHY AND THE MID-ATLANTIC RIDGE. GSA Bulletin 1949;; 60 (10): 1527–1540. doi: https://doi.org/10.1130/0016-7606(1949)60[1527:NAHATM]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract This paper presents primarily the soundings obtained by the Research Vessel ATLANTIS during the summer field season of 1947. It also utilizes some of the data obtained by later expeditions of the ATLANTIS and by several other vessels. The ATLANTIS is equipped with a continuously recording fathometer with a range up to 4000 fathoms.The outstanding features of the area investigated in 1947 (ATLANTIS cruise #150) are: (1) A conspicuously flat plain at a depth of 2900 fathoms occupies the deeper sections of the North Atlantic basin between Bermuda and the Azores. More recent data indicate that this horizontal stretch of sea floor extends at least as far south as Lat. 29°N. and possibly as far north as Lat. 40°N., between Long. 50°W. and 56°W., and also that at the same depth a similar plain is found east of the Ridge in the Northern Canary basin. They are dotted with small sea mounts of variable height whose exact shapes have not yet been determined. The size and number of these appear to increase toward the south, and the southern extremities of the plains consist of smooth horizontal stretches interrupted by pronounced elevations and depressions.(2) The Mid-Atlantic Ridge is characterized by two strongly contrasting types of topography: (a) The central backbone of the Ridge, or Main Range, which is shoaler than 1600 fathoms, consists of a series of parallel ranges trending NE.–SW. Several of these rise to less than 800 fathoms. Their trend follows roughly that of the Main Range. The portion of the Main Range to which the most attention was devoted is between Lat. 30° and 34°N. In this area its width is of the order of 150 miles, and it is of the order of several hundred miles in the area of the Azores.(b) The flanks, between the 1600- and 2500-fathom isobaths, consist of a succession of smooth shelves, each from 1 to 50 nautical miles or more in width. This zone is 200–300 miles in width.(c) The region between the limit of the 2900-fathom plain and the foot of the first of the series of shelves characterizing the flanks of the Ridge, at 2500 fathoms, in some localities stands out as a distinct physiographic province.(3) The following local features are noteworthy:(a) Close to 31°N.Lat. a deep east-west trench extends from about 41° to 43°W.Long. and cuts deep into the Main Range. Its deepest point is at 2800 fathoms. Crushed and metamorphosed ultra-basics were brought to the surface by dredging its flanks.(b) At 30°15'W., 34°N., a flat-topped sea mount rises to 180 fathoms, about 100 miles southeast of the Main Range. The dredge brought up calcareous discs of probably Cenozoic age off its top. They were about 15 cm in diameter and about 4 cm thick.(c) Another sea mount was found at 33°43'N., 62°30'W., 150 miles NE. of Bermuda, shoaling to 780 fathoms, having a NW.-SE. elongation, an unknown total length, and a width of 4 miles across its flat top.(d) Another flat suboceanic plain occupies the bottom of the basin separating Bermuda from the Northeastern United States, at a depth of 2650 fathoms. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.