slow or rapid rise or fall, and so on) will be globally effective. The 

 significance of eustatic fall-and-rise inflection points is considered with 

 regard to the occurrence of unconformities are related to rapid eustatic 

 falls, and type 2 unconformities are related to slow eustatic falls. 

 (Authors) . 



277 POSAMENTIER, H. W. , and VAIL. P. R. 1988. "Eustatic Controls on 

 Clastic Deposition II - Sequence and Systems Tract Models," Wilgus , C. K., 

 Hastings, B. S., Kendall, C. G., Posamentier, H. W. , Ross, C. A., and Van 

 Wagoner, J. C, eds . , Sea-Level Changes: An Integrated Approach . Special 

 Publication No. 42, Society of Economic Paleontologists and Mineralogists, 

 Tulsa, OK, pp 125-154. 



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 are 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 composed of a succession of 

 systems tracts. Each systems tract is composed of a linkage of 

 contemporaneous depositional systems. Four systems tracts are recognized: 

 lowstand, transgressive , highstand and shelf margin. The lowstand systems 

 tract is divided into two parts: lowstand fan followed by lowstand wedge, 

 where the basin margin is characterized by a discrete physiographic shelf 

 edge, or lower followed by upper wedge, where the basin is characterized by a 

 ramp physiography. Two sequence composed of shelf -margin, transgressive-, and 

 highstand systems tracts. 



Type 1 and type 2 unconformities are each characterized by a basinward 

 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. Within 

 incised valleys on the shelf, type 1 unconformities typically are 

 characterized by a changes in parasequence standing pattern from 

 progradational to aggradational . 



Timing of fluvial deposition is also a function of eustatic change 

 insofar as global sea-level is the ultimate base level to which streams will 

 adjust. The elevations of stream equilibrium profiles are affected by 

 eustatic change, and, assuming constant sediment supply, streams will aggrade 

 or degrade in response to eustatically induced shifts in these profiles. 

 Fluvial deposition occurs at different times in type 1 and type 2 sequences 

 and is characterized by different geometries within each type of sequence. In 



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