EUSTATIC CONTROLS ON CLASTIC DEPOSITION II—SEQUENCE AND SYSTEMS TRACT MODELS

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Abstract
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 margin is characterized by a ramp physiography.Two sequence types are recognized: a type 1 sequence composed of lowstand, transgressive-, and highstand systems tracts, and a type 2 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. Type 2 unconformities typically are characterized by a change in parasequence stacking 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 type 1 sequences, fluvial deposits occur as linear, incised-valley fill during the time of lowstand wedge and transgressive deposition. Fluvial deposits also may occur during highstand deposition as more widespread floodplain deposits within the late highstand systems tract. Fluvial deposits in type 2 sequences are usually limited to widespread floodplain deposits occurring within the late highstand systems tract.

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