Abstract

Stratal patterns within shelf depositional sequences are dependent on tectonically controlled subsidence rates and their regional patterns. In active tectonic basin settings, regional subsidence patterns can be very different from passive continental margin settings, resulting in substantial modifications of the basic sequence stratigraphic model. In ramp-type foreland basins, on the tectonically active side, subsidence rates decrease seaward in contrast with passive continental margins, where the opposite subsidence pattern exists. Analysis of the interplay between eustasy and subsidence suggests the existence of two tectono-stratigraphic zones, occurring proximally and distally with respect to the basin margin. Zone A is defined as the region within which the rate of subsidence always exceeds the rate of eustatic fall. Consequently, relative sea level rises continuously during a eustatic cycle, albeit at varying rates. Zone B is defined as the region within which the rate of eustatic fall periodically exceeds the rate of subsidence, resulting in an interval of relative sea-level fall during a eustatic cycle. On the tectonically active sides of foreland basins, zone A lies on the landward side of the basin margin, proximal to the orogenic belt, and zone B lies seaward off zone A, away from the orogenic belt where subsidence rates are lower. On passive continental margins, because of the opposite subsidence patterns, zone A lies seaward of zone B, where subsidence rates are greater. The location of the shoreline relative to these zones determines the stacking patterns and stratal discontinuities within a depositional sequence. If the shoreline remains in zone A, then only type 2 sequence boundaries will occur; if sufficient sediment flux is available, allowing the shoreline to prograde into zone B, then type 1 sequence boundaries can occur. In foreland basins, these type 1 sequence boundaries would become type 2 sequence boundaries updip in zone A. The pattern of seaward-decreasing subsidence on the tectonically active sides of foreland basins results in characteristic longitudinal facies and stratal patterns. When the two zones occur, the updip region of the basin proximal to the orogenic belt is characterized by nearly continuous nonmarine deposition, albeit at varying rates. The downdip region is characterized by forced regressions and deposition primarily of lowstand and transgressive systems tracts. A transitional region containing deposits of all three systems tracts commonly occurs between the updip and downdip areas.

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