ABSTRACT: Synrift Sequences: Their Recognition and Significance in Basin Analysis

PROSSER, SARAH D., University of Edinburgh, Edinburgh, Scotland

Synrift sequences are those that accumulated at the same time as displacement occurred on the normal faults that bound the extensional basin in which they are deposited. But how do these sequences typically appear? How do we distinguish them from sequences deposited during tectonic quiescence? Do we in fact distinguish them correctly? How does compaction affect the sequence geometries? And can we predict the facies and architecture that develop at each stage of synrift sequence accumulation?

These questions can be addressed through a combination of seismic interpretation, fieldwork, computer modeling, and the consideration of theoretical sedimentological concepts. Examples are illustrated from asymmetric extensional basins in various geological and geographical settings. They demonstrate how synrift sequences can be recognized or distinguished from the postrift infill sequences that commonly overlie them. The case studies presented here include true synrift sequences and megasequences as well as apparent synrift sequences that need to be carefully separated in terminology, and thus significance, once the true control on the development of these latter sequences has been developed. Thus, synrift sequences, synrift megasequences, growth sequences, and compaction-deformed po trift sequences can be recognized and defined.

The comparison of extensional basin fills and integration of the techniques mentioned above can help to suggest a common pattern to the three-dimensional linked depositional systems that will develop at each stage of rifting. Rift initiation, rift climax, immediate postrift and late postrift stages can be recognized in this manner, each with a characteristic appearance on seismic profiles and in spatial distribution. The conclusions regarding synrift sequences are that they are unlikely to be major constituents of a basin fill, and that estimating the duration of rifting without considering the true origin of apparent synrift sequences can lead to gross overestimation of the duration of rift events.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)