Difficulties in Identifying Synrift Growth Beds

Martha Oliver Withjack and Roy W. Schlische
Rutgers University, Piscataway, NJ

Growth beds constrain the timing of rifting, providing critical temporal information about trap development, source-rock maturation, and hydrocarbon migration. Synrift growth beds commonly fan or thicken toward active normal faults. Field, seismic, and experimental studies, however, show that not all synrift growth beds share this characteristic. In symmetric rift basins bounded on both sides by fault zones (e.g., the Rhine graben), most growth beds do not thicken appreciably toward either boundary fault zone. In asymmetric rift basins bounded on one side by a low-angle fault zone (e.g., the Newark basin, the Fundy basin), most growth beds have a very subtle fanning geometry. Without regional seismic lines and ample well/outcrop data, it is easy to mistake these synrift growth beds for prerift or postrift rocks. The presence of salt also affects the characteristics of growth beds in rift basins. Basement-involved normal faults cannot propagate upward through thick salt. Instead, fault-propagation folds develop in the sedimentary cover in rift basins with salt (e.g., the Suez rift, Haltenbanken region of offshore Norway). Growth beds deposited during movement on basement-involved normal faults, thin toward the crests of the folds rather than thicken toward the active, underlying faults. Postrift salt movement, triggered by differential sediment loading and/or regional tilting, creates detached normal faults in rift basins with salt (e.g., the Jeanne d'Arc basin, Orpheus graben). Without high-quality seismic data, it is easy to mistake these postrift growth beds associated with detached normal faulting for synrift growth beds associated with basement-involved normal faulting.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005