SUMMARYAn early study of part of the Zagros mountains of south-western Iran put forward evidence for extensive overthrusting and nappe formation. A simplified interpretation of the stratigraphy now obviates the necessity for such tectonic complications.The north-eastern limit of Zagros folding is marked by a long narrow belt of structural deformity and stratigraphic discontinuity known as the Crush Zone, which includes basic intrusive rocks contemporaneous with the late Tertiary foldings.It is suggested that the Crush Zone is closely related to the Zagros orogeny, and geophysical evidence is used to speculate on a possible explanation. The general concept of the Crush Zone could prove to be of more widespread application.
The well-known Carboniferous cyclothems of North America and Europe are discussed as examples to illustrate the great extent and lateral variation of sedimentary cycles. Very many other instances of cyclic sedimentation, in rocks of most ages, in many parts of the world, are also characterized by a more or less regular lateral migration of simultaneously existing depositional environments, caused by repeated transgressions and regressions of the sea over an epicontinental platform. Theories of varying complexity which have sought to explain cyclic sedimentation are briefly summarized, and it is suggested that the two basic requirements are a slowly subsiding sedimentary basin and more or less regular eustatic changes of sea level. The former will decide the geographical location. The latter could be a natural, periodic and world-wide consequence of the combination of the continuous effects of sedimentation and sostatic or orogenic movements of the sea floor.
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