Depositional Architecture and Stacking Patterns in Active Extensional Settings
Lesley S. McMurray and Rob L. Gawthorpe
Uplift and subsidence in extensional settings give rise to fundamental differences in stratal architecture and stacking patterns. Using both outcrop and sub-surface data, Quaternary age depositional systems in two areas undergoing active extension are compared. The outcrop study area is located in central Greece in a region undergoing uplift, whereas, the seismic study area is located in the north-west Aegean Sea, in an area undergoing subsidence.
The Gulf of Corinth basin is actively extending, but despite this, the area is undergoing uplift which has both regional and local components. Uplift rates vary across the area, from 0.3 m/ka to a maximum of 1.6 m/ka, and superimposed on high amplitude sea level variations, have resulted in a pulsed relative fall in sea level. A series of fan delta and marine terrace deposits display progradation during highstands of sea level. Successive highstand systems tracts show a downstepping and offlapping relationship characteristic of forced regressions.
The Thermaikos Gulf area, in the north-west Aegean, is a broad subsiding shelf. Seismic profiles from the gulf show a series of stacked deltas which successively backstep in response to a pulsed rise in sea level. The internal geometry of the delta packages is broadly aggradational to progradational. Stratal patterns are however, significantly modified by syn-depositional movement on a large number of small normal faults, which produce rotations and tectonically enhanced downlap surfaces.
Although the stratigraphy from the two areas are contemporaneous, the different tectonic signatures result in fundamentally different stratal architecture.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California