In a study of the dependence of the velocity of compressional waves in marine sediments upon the thickness of overburden, the velocity‐depth relationship in shelf sediments is shown to be distinctly different from that in deep basin sediments. The difference between the two cases may be illustrated by comparing the straight lines that best represent the data. These are [Formula: see text] shallow water, [Formula: see text] deep water where V is in km/sec and Z is in kilometers. Shallow and deep water are defined arbitrarily to be under 100 fathoms and over 1,500 fathoms respectively. The observed variation of average compressional velocity in the shallow and deep water sediments, taken together with the known limited range of variation of velocity for a given porosity, yields limits in turn upon the porosity‐depth dependence in the two environments. It is shown that at the same depth of overburden porosity is much greater in deep water sediments than in shallow. A physical argument is presented to show that there is implicit in the observed narrow range of variation of velocity with porosity a simple relation between porosity and rigidity. Thus quantitative estimates of shear velocity may be made from compressional velocity alone. In this way the original data are used to place rather narrow limits on the depth variation of shear velocity, porosity, and density. A number of comparisons with observation are employed to test the conclusions at each stage of the discussion.
A description is given of the bottom topography, seismic refraction profiles, magnetic and gravity observations in the Red Sea. The deep trough along the centre is associated with positive Bouguer gravity anomalies, large magnetic anomalies and seismic velocities of 7.1 km/s. It appears that this represents a fissure in the continental crust, partly filled with basic, igneous material. A structural map, based on all the geophysical evidence, has been prepared and it is suggested that the complex Red Sea rift was formed as a result of crustal tension. Finally, a discussion is given of the Red Sea as part of the world rift system. H.M.S. Dalrymple on a cruise to the Persian Gulf and obtained total intensity magnetic field records along more than 3 ooo km of track in the Red Sea. The track was planned to supplement the data obtained aboard Vema the previous year and is also shown in Figure I. The data obtained by Dalrymple are being published separately.
The geologic record of terminal Cretaceous environmental events indicates that iridium and other associated elements were not deposited instantaneously but during a time interval spanning some 10,000 to 100,000 years. The available geologic evidence favors a mantle rather than meteoritic origin for these elements. These results are in accord with the scenario of a series of intense eruptive volcanic events occurring during a relatively short geologic time interval and not with the scenario of a single large asteroid impact event.
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