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Sequence
Boundaries on Carbonate Platforms
SCHLAGER, W. and JEROEN A.M. KENTER
oth exposure and drowning may interrupt shoalwater carbonate production on a
platform and thus generate bounding surfaces for sequence stratigraphy. The most
common scenarios are (1) exposure followed by shallow flooding; (2) exposure
followed by drowning, and (3) drowning without prior exposure. Scenarios (1) and
(2) qualify as sequence boundaries' in the standard systems-tract model of
sequence stratigraphy; scenario (3) does not, because it implies that a
highstand systems tract is overlain by a transgressive tract without intervening
exposure. This notwithstanding, the drowning of a platform without prior
exposure is likely to produce a distinct unconformity because sediment input
profoundly changes at this juncture and because the sharp topography of the
drowned platform amplifies ocean currents causing submarine erosion.
In seismic data, drowning events tend to be well recognizable because of the
impedance contrast between shoalwater carbonates and overlying pelagic
sediments. Exposure events within shoalwater carbonates show a comparable
impedance change only if there is a thick soil intercalation or if the rocks are
young and diagenetically immature (hard exposure crust between friable marine
intervals); as the rocks become tight and mineralogically uniform during burial,
this impedance contrast may completely disappear. Consequently, the seismic
expression of drowning events in carbonates tends to be more prominent than that
of exposure events and seismic
unconformities in carbonates are poor indicators
of exposure.