Abstract

The thickest part of asymmetric rift, pull-apart, and foreland basin fills commonly consists of large-scale (hundreds to thousands of metres thick), tectonically generated cyclothems of fine-grained marine, lacustrine, or longitudinal fluvial deposits and coarse-grained transverse braid-plain or alluvial-fan deposits. The appearance of coarse clastics in these basin fills is typically noted as the time of tectonically rejuvenated source-area uplift, based on the conceptual tie between relief and coarse grain size, and on the application of the Davis theory of landform development. We propose the opposite interpretation: that the commencement of fine-grained sedimentation above coarse-grained deposits in a tectonic cyclothem is the best indicator of renewed tectonic activity. This reinterpretation is more consistent with (1) modern examples, (2) the consideration of source-area and sedimentary-basin geomorphology, (3) the disparity between the reaction rates of the various environments to subsidence, (4) the disparity between the rates of tectonic uplift and erosion, and (5) the controls on elastic-wedge progradation. In our model, extensive coarse-grained clastic-wedge progradation is an indicator of tectonically quiescent phases.

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