Page:The Journal of geology (IA journalofgeology11893univers).pdf/53

This page has been proofread, but needs to be validated.
CONDITIONS OF SEDIMENTARY DEPOSITIONS.
479

the land determine the deposition of calcareous formations they will do so most efficiently during this topographic phase, and in the absence of mechanical sediments the corresponding deposits will be limestones or dolomites. As the topographic phase passes to its close and the sloping plains sink to base-level, the power of streams to transport mechanical sediment fails, and rivers finally carry only silt in lessening proportion; hence the upper portions of a great limestone deposit may be less clayey than the lower. Furthermore, the mantle of residual clays, accumulating upon the extended base-level, will check solution, and thus, in so far as the deposition of limestone is influenced by contributions from the land, will limit the growth of the formation; and with the cessation of both mechanical and chemical supply, terrigenous deposits will cease to form beneath the sea. Then, while these conditions endure geologic ages may pass without record in sediments unless there is a marine source of supply.

Thus far this statement has tacitly assumed a constant relation of elevation between coast and ocean. Assume that the long quiet, which has been necessary for the reduction of a mountain range to base-level and the deposition of the corresponding sediments, is interrupted by sinking or heaving of the land area. The surface is low, flat and covered by a mantle of residual sand and clay intimately mingled. Moderate subsidence must lead to extensive transgression and the invading sea, margined by tide flats, will spread arenaceous, clayey deposits, bearing the marks of shallow water formations and resting unconformably upon the ancient rocks. If the residual soil be red, the sediments will be of similar color, since the process of deposition on tide flats does not involve much attrition and the ferruginous coating of the grains will remain.[1] The base of the deposit may be a zone of transition, composed of cores of undecomposed rocks, imbedded in more or less re-arranged products of partial decomposition.[2]

  1. ↑ Bull. U. S. G. S.No. 52.I. C. Russell,Subaërial Decay of Rocks and Origin of Red Color of Certain Formations.
  2. ↑ R. Pumpelly.The Relation of Secular Rock Disintegration to Certain Transitional Schists.Bull. Geol. Soc. of America.Vol. II., p. 209.
This article is issued from Wikisource. The text is available under Creative Commons Attribution-Share Alike 4.0 unless otherwise noted. Additional terms may apply for the media files.