Adjustment of Bank Depths to Present Ocean Level

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Adjustment of Bank Depths to Present Ocean Level

An important problem is opened up by the frequent recurrence of submarine banks, associated with marginal belt islands, which slope gradually seaward to depths of about 40 fathoms around their outer border; for that depth indicates that the detritus to be moved and the marine agencies for moving it are in adjustment with respect to present sea level. Daly has found from a study of many charts that the depth at which a marked increase of slope occurs on the outer part of continental shelves is not 100 fathoms, as it is often said to be, but is, as in the case of marginal-island banks, about 40 fathoms.

My own studies lead to the same conclusion with respect to the depth at which a change is made from a gentle slope to a steep pitch in the exterior submarine profile of Pacific coral reefs. This appears to mean that the activity of waves and currents is in the long run such that the fine detritus which prevails on the outer part of a bank or shelf will not be allowed to accumulate there at less depths than 40 fathoms but will be gradually shifted to the bank border where it will settle down upon the steeper slope and build it outward up to the standard 40-fathom depth. The longer a land mass remains stable, the broader may the continental shelf or bank in front of it become; but its border depth will remain of about the same measure.

The same relation should have obtained between the level of the lowered ocean in the Glacial epochs and the depth at the outer border of the detrital embankments that were then built of the material abraded in the production of low-level platforms. Hence, if the ocean was lowered 30 fathoms, the border of the embankment then formed should lie at a depth of about 70 fathoms below present sea level. But, as a matter of fact, bank-border depths of 70 fathoms are exceptional. The depth at which a change of declivity takes place between the gentle slope and the steeper pitch on continental shelves, on marginal-belt banks, and on the exterior profile of the veteran coral reefs in the tropical Pacific is generally 40 or 50 fathoms, not 70 fathoms.

Consequently, if we accept the fact of low-level abrasion during the Glacial epochs, whatever embankments were then formed appear now to have been so well aggraded in Postglacial time that they are no longer recognizable: the built-up banks are now as a rule in good adjustment with normal ocean level. It is perhaps surprising that detritus should have been provided (luring the relatively short Postglacial epoch in sufficient quantity to obliterate the effects of the lowlevel abrasion that was accomplished during a longer time; but no other satisfactory interpretation of the facts has been reached.

Evidently the smaller the measure of ocean lowering and the less the amount of abrasion then accomplished, the more readily will banks be brought to normal depths in Postglacial time. Two corollaries follow from these considerations. First, the depth of existing banks around marginal belt islands gives no safe indication of the depth of the abraded platforms that are supposed to be beneath them, because the thickness of Postglacial aggrading deposits is unknown. Second, the depth of the larger reef-enclosed lagoon floors in the coral seas of the Pacific should not be regarded as giving a rough measure of an abraded platform beneath them, not only because abrasion appears not to have taken place there but also because, even if it did, the uncertainty as to the thickness of Postglacial deposits prevents a safe inference as to platform depth from lagoon-floor depth.