Abstract: The Role of Tectonics and Eustasy in the Development of Stratal Architecture in Foreland Basins
Henry
W. Posamentier, George P. Allen
Stratal patterns within shelf depositional sequences are dependent on tectonically-controlled subsidence rates and their regional patterns. Analysis of the interplay between eustasy and subsidence suggests the existence of two tectono-stratigraphic zones, occurring proximally and distally with respect to the basin margin. Zone A is defined as the region within which the rate of subsidence always exceeds the rate of eustatic fall. Consequently, relative sea level rises continuously during a eustatic cycle, albeit at varying rates. Zone B is defined as the region within which the rate of eustatic fall periodically exceeds the rate of subsidence resulting in an interval of relative sea-level fall during a eustatic cycle.
On the tectonically active sides of foreland basins, zone A lies on the landward side of the basin margin, proximal to the orogenic belt, and zone B lies seaward of zone A, away from the orogenic belt where subsidence rates are lower. On passive, continental margins, because of the opposite subsidence patterns, zone A lies seaward of zone B where subsidence rates are greater. The location of the shoreline relative to these zones determines the stacking patterns and stratal discontinuities within a depositional sequence. If the shoreline remains in zone A, then only type 2 sequence boundaries will occur; if sufficient sediment flux is available, allowing the shoreline to prograde into zone B, then Type 1 sequence boundaries can occur. In foreland basins, these Type 1 sequence boundarie would become Type 2 sequence boundaries updip in zone A. When the two zones occur, the region proximal to the orogenic belt, is characterized by nearly continuous non-marine deposition, albeit at varying rates. The downdip region is characterized by forced regressions and deposition primarily of lowstand and transgressive systems tracts.
AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994