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AAPG Bulletin


Volume: 21 (1937)

Issue: 9. (September)

First Page: 1101

Last Page: 1157

Title: Sedimentation in Barred Basins, and Source Rocks of Oil

Author(s): W. G. Woolnough (2)


While the DOCTRINE OF UNIFORMITY must always form the basis of geological theory, it is necessary to admit that phenomena have occurred during geological time which have not been observed at all, or not in their entirety, during the very limited period of modern scientific investigation.

Reasons are given for the belief that major coal measures, major primary salt deposits, major fresh-water series of sediments, and source rocks of oil deposits are not now being deposited anywhere on the earth.

No continent-wide peneplanation of advanced maturity, comparable, for example, with the Middle Tertiary peneplanation of Australia, can be recognized at the present day. It is believed that the present is largely a period of orogeny rather than of peneplanation.

It seems probable that periods of greatly enhanced humidity and of greatly enhanced aridity, and others of wide temperature fluctuations, have marked many eras in the geological past, and have left their impress on sediments formed during such eras. The present condition of the earth can not be assumed to be the only "normal" one in earth history. In cases where the characteristics of geological formations require explanations other than those afforded by the strict application of the Doctrine of Uniformity it is legitimate and necessary to invoke causes not now observed in operation, provided always that objective evidence, not conformable with conventional explanations, can be adduced.

The close relationship between sediments and their environment has been emphasized by Twenhofel, and the fossil contents of such sediments generally but not always conform with this environment. The grade of this relationship under different circumstances is discussed in some detail.

It is insisted that under normal conditions of deposition in any environment there should be preserved a BALANCED ASSEMBLAGE of fossil forms, including benthos, nekton, and plankton in suitable proportions. Any unbalance of assemblage indicates something abnormal in conditions of deposition, and demands explanation.

The main thesis of this paper is an amplification of the "Bar Theory" of Ochsenius. It is suggested that, when all implications of this theory are considered, the possibility is indicated of existence in the geological past of "barred basins" of dimensions and characters entirely unrepresented at the present day.

Reasons are given for associating barred basins chiefly with maturity of peneplanation. Climatic factors are considered and it is concluded that humidity favors development of lacustrine conditions, of coal, boghead, and oil shale; while aridity

End_Page 1101------------------------------

controls formation of black shales, source rocks of oil, and ultimately salt deposit. Certain characteristics are common to all such deposits.

The occurrence of unbalanced planktonic assemblages of fossils, generally erroneously interpreted as indicating pelagic conditions, is shown to be compatible with deposition in barred basins. Fineness of texture and lamination, blackness of shale deposits, high organic content, pyritization, and composition of oil-field waters are believed to be due to barred-basin environment and not to excessive depth of water.

The suggestion is put forward, merely as a suggestion at this stage, that certain lithological characters and structural features may be genetically associated with such depositional environments. Amongst these may be mentioned concretionary structure cone-in-cone, barren argillaceous limestones, and organic siliceous cherts, particularly if dark in color.

The relations of barred-basin deposits with the "euxinic facies" of van der Gracht, and the "sargasso environment" of Ruedemann are discussed.

As possible examples of deposits of barred basins are cited the saline formations of New Mexico, Irwin River basin (Western Australia), "shoestring sands" of Kansas and Oklahoma, Cretaceous limestones of Venezuela, Yates pool (Texas), some of the formations of southern California, and black shale deposits of New York and elsewhere.

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