Print this pageSequence 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.