ERSC 3P12 Lecture 20: 20. The parasequence II

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5 Aug 2016
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The parasequence II
Example from ramp margin (rather than margin with shelf break) Falling Stage System Tract
(FSST) sediments are more continuously deposited.
The Falling Stage Systems Tract (FSST)
Represents low (t8) to maximum(t11) and to low (t16) rates of rsl fall.
Comprises large volumes of fluvially-derived sediments deposited basinward (i.e. distally)
due to falling sea levels. Coastline both prograding and lowering (forced regression).
On ramp margin, downstepping parasequences, on shelf break, first two parasequences
downstep, then submarine fans develp at base of continental slope.
FSSTs can be excellent hydrocarbon reservoirs!
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The Lowstand Systems Tract (LST)
Relative sea level (rsl) has begun to rise again. LST =minimum rsl to maximum rate in rsl
rise.
Parasequence stacking patterns progradational (with sediment supply outpacing creation of
accommodation space) to aggradational. Prograding shoreline, but downstepping ceases, as
may sediment bypass.
Rivers cease to incise.
The transgressive surface (ts)
During the LST, the rate of increase in rsl increases markedly and causes a marine
transgression.
Because sediment supply is outpaced, the shoreline moves landwards.
Retrogradational parasequence sets develop. The base of these is the TS. They overstep the
downstepping parasequences of the FSST. On shelf, minor unconformity due to landward-
moving wave base. Hence, marine sediments may overlie non-marine. Water table rises,
infilling of incised valleys.
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