Page 93 - Carbonate Sedimentology and Sequence Stratigraphy

84 WOLFGANG SCHLAGER

posure is extremely difficult to verify in seismic data. Even It simply means that at this boundary the pattern of sed-
in outcrop studies, demonstration of subaerial exposure in iment input or sediment dispersal changed abruptly. Sea-
carbonates often requires extensive laboratory analyses (e.g. level change commonly causes these shifts in input and dis-
Esteban and Klappa, 1983; Goldstein et al., 1990; Immen- persal because it reshuffles the sediment pathways on the
hauser et al., 1999). In siliciclastics, the situation is further shelf and slope. However, there is a significant number
complicated by the fact that much of the record of subaerial of examples where sea level interpretation is inadequate or
exposure is washed away by the subsequent transgression. outright inappropriate. An example from the Gulf of Mexico
The original definition of sequence boundary as an un- may serve to illustrate this point.
conformity between two units of conformable, genetically Figs 6.1 6.2 shows the mid-Cretaceous sequence boundary
related strata does not automatically imply sea-level control. (MCSB, Buffler, 1991). It is a basin-wide seismic marker that

2.0
NW Gulf of Mexico
UTIG Line GT2-3b
MCSB

4.0

10 km

6.0
sec
twt

Fig. 6.1.— Change of sediment input at mid-Cretaceous sequence boundary (MCSB, marked by red arrows) on Campeche Bank,
Gulf of Mexico. The carbonate platform on the right was drowned in the mid-Cretaceous and later covered with pelagics. As carbonate
input ceased, the debris apron of the platform was gradually buried by clastic sediments transported at right angle to the profile.
Seismic data courtesy of R.T. Buffler.

0.1 0.2 0.3 0.4
CONTOURS Fig. 6.2.— Sediment isopachs above and below the mid-
post-MCSB siliciclastics
Cenom-L. Paleocene Cretaceous sequence boundary of Fig. 6.1 show the change in
0.0
pre-MCSB carbonates
Albian-early Cenomanian sediment input at the unconformity. Pre-unconformity isopachs
reflect the influx of carbonate debris from the platforms, post-
unconformity isopachs reflects the influx of siliciclastic material
Florida Platform from the advancing Cuban island arc. After Schlager (1989),
24° N
modified.
0 60
km
J.K.
Campeche
Platform
0.0
23° N
Cuba
0.3 0.2 0.1 0.0 0.5 1.0