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The Origin of Third-Order Depositional Sequences: Eustacy, Tectonics, or Bolides: Abstract
Third-order depositional sequences characterize many stratigraphic successions, especially those consisting of paralic and shallow water lithologies. Each sequence is usually tens to hundreds of metres thick and spans a time interval of three to eight million years. Subaerial unconformities bound the sequences on the basin margins with submarine unconformities forming the boundaries in the central portion of the basin. In intermediate areas, sequence boundaries are commonly conformities which are difficult to pinpoint. Many sequences consist of a thin basal transgressive unit overlain by a thick regressive wedge of strata.
One hypothesis for the origin of third-order sequences is that eustatic sea-level change is the main forcing factor and that subsidence and sediment supply have background roles. This interpretation is favoured because many sequence boundaries appear to be synchronous over large areas of the globe. However, it must be mentioned that no physical mechanism that would produce eustatic cycles of three to eight million years duration is presently known.
In regards to tectonics there are a number of reasons for interpreting tectonism and associated changes in subsidence rate as the main factors in the origin of third-order sequences. These reasons include:
1. Many sequence boundaries are coeval with episodes of faulting and folding
2. Sediment source areas sometimes change markedly from one sequence to the next
3. Patterns of uplift and subsidence within a basin change dramatically across sequence boundaries
4. Sedimentary regimes in a basin are commonly very different from one sequence to another
5. Relative sea-level falls in rapidly subsiding basins are often greater than such changes in slowly subsiding areas
6. Some sequence boundaries have limited extent.
The one major barrier to accepting tectonics as the major cause of third-order cyclicity has been the great areal extent of many sequence boundaries. This hurdle has recently been overcome by the work of Cloetingh and others (1985, 1986, 1988) which has demonstrated that episodic variations in horizontal lithospheric stresses associated with major plate reorganizations are capable of producing relative sea-level changes of similar magnitude and extent to those recognized.
Finally, it is noted that the timing of most proposed bolide impacts, such as the Permian-Triassic boundary, the Triassic-Jurassic boundary, the Callovian-Oxfordian boundary and the Maestrichtian-Paleocene boundary, coincide with third-order sequence boundaries. Thus if one accepts the bolide hypothesis it would seem that plate reorganizations and associated tectonics, which result in sequence boundaries, are triggered by earth-shattering impacts which occur every 3 to 8 million years. At this time it would seem to be more reasonable to reject the bolide hypothesis and to look for more down-to-earth explanations for such phenomenon as the iridium anomalies which are sometimes present at sequence boundaries.
Acknowledgments and Associated Footnotes
1 Geological Survey of Canada, 3303 - 33rd St. N.W., Calgary, Alberta T2L 2A7
Copyright © 2009 by the Canadian Society of Petroleum Geologists