Sequences, Systems Tracts, and Eustatic Cycles

Henry W. Posamentier and Peter R. Vail

Depositional sequences are composed of genetically related sediments bounded by unconformities or their correlative conformities and are related to cycles of eustatic change. The bounding unconformities are inferred to be related to eustatic fall inflection points. They will be either Type 1 or Type 2 unconformities depending on whether sea level fall was rapid (i.e., rate of eustatic fall exceeded subsidence rate at the depositional shoreline break) or slow (i.e., rate of eustatic fall was less than subsidence rate at the depositional shoreline break). Each sequence is comprised of a succession of systems tracts. Four systems tracts are recognized: lowstand, transgressive, highstand, and shelf margin systems tracts. The lowstand systems tract is subdivided into two parts lowstand fan and lowstand wedge where the basin margin is characterized by a discrete physiographic shelf edge, or lower and upper wedge where the basin margin is characterized by a ramp physiography. Each systems tract is comprised of a linkage of contemporaneous depositional systems.

Type 1 and Type 2 unconformities are each characterized by a basin-ward shift of coastal onlap concomitant with a cessation of fluvial deposition. The style of subaerial erosion characterizing each unconformity is different. Type 1 unconformities are characterized by stream rejuvenation and incision, whereas Type 2 unconformities typically are characterized by widespread erosion accompanying gradual denudation or degradation of the landscape. Stream rejuvenation and incision are not associated with this type of unconformity.

On the slope and in the basin, Type 1 unconformities typically are overlain by lowstand fan or lowstand wedge deposits, whereas Type 2 unconformities are overlain by shelf-margin systems tract deposits. Within incised valleys on the shelf, Type 1 unconformities are overlain by either fluvial (lowstand wedge) or estuarine (transgressive) deposits. Type 2 unconformities typically are characterized by a change in parasequence stacking pattern from progradational to aggradational.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.