Structure of the Blake-Bahama Outer Ridge

Abstract

Many of the recent high-resolution seismic reflection profile data substantiate the deposittonal nature of the Blake-Bahama outer ridge and show that its sedimentary structure is quite complex. The structure is revealed principally by the configuration of two prominent acoustic reflectors observed above horizon A, which are referred to as reflectors X and Y. Layer A-X is shown to have been deposited preferentially along the ridge axis; thus, the over-all shape of the ridge was established. Two major sedimentary regimes are distinguished above reflector X: a 'northern' regime, which mantles the crest of the northern half of the ridge, and a younger 'southern' regime, which laps onto the northern and creates sharp unconformable contacts in some localities. The most important of these contacts divides the regimes at latitude 30ø15'N and suggests that the post reflector X deposittonal history of the ridge north and south of this latitude is different.The Blake-Bahama outer ridge is a major topographic feature, over 1100 km in length, which trends seaward and southward from the northern part of the Blake plateau. In conjunction with the steep Blake escarpment the Blake-Bahama outer ridge constitutes a major departure from the usual slope-rise configuration of the continental margin off the east coast of the United States; it is, in essence, a detached continental rise. The regional topography of the ridge area has been discussed by Pratt and Heezen [1964]. Early seismic reflection work here was performed by Ewing and Ewing [1964].The advent of high-resolution continuous seismic reflection profiling has made possible a much more detailed investigation of the sedimentary structure of the ridge; many such data have been collected from the Blake-Bahama area in the last few years. This study is an attempt to synthesize the existing data into a coherent picture of the over-all structure of the ridge and to interpret the structure in terms of possible mechanisms of formation. The study is based on continuous seismic reflection data collected along 28,000 km of track. Both explox Lamont-Doherty Geological Observatory Contribution 1547. sive and pneumatic sound sources were used. Typical penetration amounted to 1.5 sec (twoway travel time); the maximum penetration achieved in the area was 3.5 sec. Some ambiguity exists in the term 'outer ridge.' According to Heezen et al. [1959], the term has no genetic implications, but is merely a morphologic term applied to fairly large offshore ridge features oriented parallel to the continental margin. Within this usage an outer ridge could be deposittonal, as are the Blake-Bahama outer ridge [Ewing and Ewing, 1964], Zapiola ridge [Le Pichon et al., 1970], and Feni ridge [Jones et al., 1970]; it could be predominantly erostonal, like the small ridge southeast of the Hatteras transverse canyon [Rona and Clay, 1967], or even tectonic in origin.Ambiguity also has existed in the nomenclature applied to the Blake-Bahama outer ridge. Two separate topographic ridges exist' a large one (over 550 ...

Regional Aspects of Deep Sea Drilling in the Western North Atlantic

“…have suggested that the Blake Plateau, which has suffered serious erosion near the head of the ridge, is the principal source, while Heezen et al (1966) have contended that the main source is the continental margins to the north. Bryan (1970) and Markl et al (1970) proposed a scheme in which both sources were called upon to initiate the development of the ridge, but agreed with Heezen and Hollister (1964) that the southward-flowing boundary current had the principal effect on its later development. The principal objective of the three ridge holes was to provide data concerning the provenance, composition and rate of accumulation of this massive feature.…”

“…Others have proposed that it is a sedimentary drift constructed of sediment transported along the sea floor by bottom currents (Heezen and Hollister, 1964;Ewing and Thorndike, 1965;Markl et al, 1970). The source of the sediment making up the ridge has also been a controversial subject.…”

“…This reflector was called Reflector Y by Markl et al (1970) who noted that it cut minor reflectors that appeared to be associated with bedding planes and, therefore, was probably due to diagenesis. The drilling results offer some clues as to the nature of the diagenesis.…”

Sedimentary Processes in the North Atlantic

“…Where the currents were slowed down by contact with an adjacent water mass moving with a different velocity they lost their competence and deposited their sediment load. This has perhaps been most elegantly demonstrated in the case of the Blake-Bahama Outer Ridge, which Bryan (1970) and Markl et al (1970) have suggested is the consequence of the interaction of the Gulf Stream and the Western Boundary Undercurrent. Sediment deposition would also be expected, in the less complex case of the interaction of a bottom current and a relatively static water mass ( Figure 4) and we might expect the development of long, narrow sediment ridges parallel to the currents.…”

The Future Supply of Nature-Made Petroleum and Gas

“…Ocean (Markl et al, 1970;Ewing and Hollister, 1972), and one has since been reported from the Bering Sea and elsewhere in the North Pacific and the Beaufort Sea in the Arctic Ocean (Grantz, et al, 1976).…”