249 NUMMEDAL, D., and SWIFT, D. J. P. 1987. "Transgressive Stratigraphy 

 at Sequence-Bounding Unconformities: Some Principles Derived From Holocene 

 and Cretaceous Examples," Nummedal , D., Pilkey, 0. H., and Howard, J. D., 

 eds . , Sea-Level Fluctuations and Coastal Evolution : Special Publication No. 

 41, Society of Economic Paleontologists and Mineralogists, Tulsa, OK, 

 pp 241-260. 



Sequence stratigraphic concepts are powerful tools in the analysis of 

 the evolutionary history of sedimentary basins. The criteria used in the 

 identification of depositional sequences differ for outcrop and subsurface 

 data sets because of scale differences in stratal continuity and spatial 

 resolution. This paper documents sedimentary and stratigraphic 

 characteristics of sequence boundaries and the overlying transgressive 

 succession of depositional systems. By emphasizing the sedimentology and the 

 different patterns of systems stacking above the sequence boundaries, this 

 paper aids in identifying these key stratigraphic boundaries in outcrop. 



The key stratigraphic surfaces encountered in shallow marine sedimentary 

 sequences are: (1) subaerial unconformities, cut by episodes of sea- level 

 fall, and representing a significant hiatal break; and (2) diastems, surfaces 

 representing relatively short interruptions in sedimentation. Most diastems 

 in shallow marine and coastal settings are associated with transgressions and 

 may be loosely grouped as "transgressive surfaces." Diastems include the 

 ravinement diastem, which is formed by transgressive shoreface retreat, 

 channel-base diastems, and various marine erosion diastems. 



Subaerial unconformities of regional or interregional extent and their 

 correlative marine unconformities and conformities serve as boundaries of 

 depositional sequences. The depositional architecture between the sequence 

 boundary and the overlying transgressive surface, e.g., the ravinement 

 diastem, is controlled by the relief of the coastal plain and the rate of 

 transgression. 



Transgression of a coastal plain previously dissected by an episode of 

 significant sea- level fall will produce shore-normal estuaries and shore- 

 parallel lagoons. The resultant stratigraphic succession will be 

 characterized by an inner-shelf sand sheet above back-barrier deposits and 

 "ribs" of fluvial and estuarine sediments filling former subaerial valleys, 

 this architecture characterizes the Holocene stratigraphy of the United States 

 Atlantic and Gulf shelves, and the basal Cretaceous strata (Oak Canyon Mbr of 

 the Dakota Sandstone) of the San Juan basin in New Mexico. 



In rapidly subsiding basins, such as the Holocene Mississippi Delta 

 region of the Gulf Coast basin and the western foredeep area of the Cretaceous 

 Interior Seaway, relative sea- level fall may be insignificant or nonexistent. 

 Consequently, the sequence boundary may be a rather subtle unconformity, or 

 even a conformity. Nevertheless, the transgressions will cause erosional 

 shoreface retreat, the cutting of ravinement diastems ad formation of offshore 

 erosion surfaces. These surfaces generally are of limited spatial extent and 

 are discontinuous in the stratigraphic record, yet, they form distinct 

 lithostratigraphic breaks and may easily be mistaken for sequence boundaries. 

 Time lines cross these transgressive surfaces, but they do not cross sequence 

 boundaries . 



The Coniacian strata of the San Juan basin provide an example of a 

 sequence boundary and associated transgressive depositional systems formed in 

 response to a relatively small sea- level fall and subsequent rise in a rapidly 

 subsiding basin. A mid-Coniacian sea-level fall, detectable across the entire 



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