Aragonite-cemented sandstone from outer continental shelf of Delaware Bay; submarine lithification mechanism yields product resembling beachrock

Abstract

ABSTRACT A shelly sandstone containing a modern microfauna, recovered from the outer shelf off Delaware Bay at a depth of 43 fathoms (79 meters), had been cemented by cryptocrystalline aragonite and clear fibrous aragonite. Numerous organic borings, some algal, penetrate both cement and shells. The size-distribution and excellent sorting of the insoluble residue, and the textural relationships of the cement and algal borings, are identical with those of some kinds of beachrock. Beachrock forms today only in the intertidal zones of subtropical and tropical climates. However, faunal and geochemical data indicate that the sandstone is not a beachrock. All species of pelecypods, benthonic Foraminifera, and bryozoa are those whose modern representatives live variable distances north of New Jersey, in waters cooler than those found off the present coast of New Jersey. No tropical or subtropical species were found as would be expected if the sandstone were a submerged beachrock. The cement is greatly enriched in the light stable carbon isotope (12)C, suggesting that the carbon in t e aragonite comes from methane. A radiocarbon date of 4,390 = 120 years on molluscan shells contrasts with a date of 15,600 ± 250 years for the cement. Assuming that both dates are accurate, then the cementation of young shells by old cement can be explained only by postulating that the carbon of the cement originated earlier than the date of cementation, but was not introduced immediately into the sandstone. Methane originating in a submerged marsh is the most probable mechanism for creating a cement having a greater radiocarbon age than the shells cemented. On the basis of the radiocarbon-controlled sea level curve and on the supposition that the age of the shells indicates time of cementation, we conclude that cementation occurred on the sea floor under a cover of 75 m of seawater. Aragonite-cemented sandstone is associated with morphological evidence of a drowned shoreline which is a regional feature that extends all along the outer edge of the continental terrace of the eastern United States. Underwater television of cemented rock off Florida at the same depth as in the Delaware Bay sandstone, and other reports of rock near the shelf edge, suggest that the submerged littoral sand may have been cemented on a large scale. This cementation may have been controlled by methane originating in adjacent tidal marshes, now submerged. If this is true, a mechanism of lithification has been discovered by which quartz and carbonate sands can be cemented on the sea floor on a large scale without requiring unusual salinities of the sea water.