Dictionary:Fig S-32

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FIG. S-32. System tracts and the seismic patterns that indicate sea-level changes. Coastal onlap is evidence of a rise of sea level, a seaward and downward shift in onlap is evidence of a fall of sea level, and a landward movement of sediment packages is evidence of transgression. (a) Types of reflection terminations. (b) System tracts; a stratigraphic sequence begins with a sea-level fall at the end of a highstand tract (HST) and ends with the next highstand tract. A lowstand tract (LST) is the first unit after a large sea-level fall, lying on top of a type 1 sequence boundary (SB1). It is subdivided into basin-floor fan (bf), slope fan (sf), and lowstand wedge (lsw). A rapid rise of sea level produces a transgressive tract (TST), at the top of which there lies the maximum flooding surface (mfs) and a thin, fossil-rich condensed section. The first unit after a sea-level fall that does not fall below the shelf edge (type-2 sequence boundary, SB2) is a shelf-margin tract (SMST). (From Vail, 1987, 2, 4.)