Abstract

By comparison of three large deltas — Niger, Mississippi and Mackenzie — it is possible to analyze the overall architecture of these huge regressive accumulations of sediments. A number of theoretical models are proposed which analyze the most important variables: progradation, subsidence and sediment composition. A composite model which combines progradation, subsidence and flexing of the lithosphere approximates the conditions actually observed in nature.

For this composite model, the depression of the lithosphere caused by sediment loading extends beyond the point to which sediments can spread. A peripheral moat surrounds the depocenter, attracting from time to time the river and creating marked regression in one moat and marked transgression in the opposite one. In the Niger Delta, the repetition of this effect produces successive trans-gressive shales in the moat axis of the delta. The distribution of the majority of the producing oil fields and the vertical sequence of sediments can be explained by the distribution of the transgressive shales in the moat axis.

One consequence of this mechanism is that regression introduces its own transgressions. Ambiguities in the use of the terms transgression and regression in a deltaic environment are described. For example, the same sedimentary facies can be simultaneously transgressive on the alluvial plain and regressive towards the open sea.

The final conclusion is that the study of deltaic sediments is an excellent and subtle school for applying the classical concepts of geology.