Page 125 - Carbonate Sedimentology and Sequence Stratigraphy

116 WOLFGANG SCHLAGER

central reef domain backreef apron platform interior

Fig. 7.13.— Reef-lagoon transition in Triassic Dachstein Fm. of the Northern Calcareous Alps. Massive limestones on the left gradually
pass into well-bedded limestones of the platform interior on the right (bedding tilted by Alpine tectonics). Red line in massive limestones
indicates the approximate boundary between in-situ reef belt on far left and rubble and sand of the backreef apron in the center; red
arrows indicate boundary between reef apron and bedded lagoon facies (based on map by Zankl, 1969). Seismic data would almost
certainly show reef and apron deposits as one zone of incoherent reflections and pick up the transition to the rhythmically bedded
limestones as a facies change.

try of sediment bodies, which tends to be dominated by Highstand shedding
differences in slope angle.
The rules of thumb are based on first principles of carbon- It is a well-established fact that in the Pleistocene, sili-
ate sedimentation as well as observations on recent and an- ciclastic sediment supply to the deep sea was at its max-
cient carbonate systems tracts. However, the link between imum during glacial lowstands of sea level. The insight
geometrically defined systems tracts and carbonate facies re- that rimmed carbonate platforms were in antiphase to this
mains an indirect and tenuous one that is easily perturbed rhythm developed first in the Bahamas: Kier and Pilkey
by other effects on facies. Predicting facies from systems (1971) and Lynts et al. (1973) showed that sedimentation
tract geometry will remain a blend of art and science for rates in the interplatform basins peaked during the inter-
some time to come. glacials when large volumes of aragonite mud were swept
off the platforms. Schlager and Chermak (1979) observed
T SEQUENCES IN DEEPER-WATER
that turbidite input, too, was high during the Holocene
Periplatform environment - part of the platform system
and low in the last glacial. Mullins (1983) first empha-
Platform, slope and debris aprons on the basin floor are sized this “carbonate way” of responding to sea level for
one connected system in the T factory. In this respect the T which Droxler and Schlager (1985) coined the term “high-
factory resembles siliciclastic systems where sediment sup- stand shedding”. It indicates that carbonate platforms pro-
ply from land and downslope transport in the marine do- duced and exported most sediment during interglacial high-
main create one interconnected system of depositional en- stands when the platform tops were flooded. The pattern is
vironments and facies. In the T factory, the sediment feed- best documented for the Bahama Banks (Fig. 7.14; Droxler et
ing the systems is produced in the photic zone at the plat- al., 1983; Mullins, 1983; Reijmer et al., 1988; Spezzaferri et al.
form top. Because of this shallow location, production is 2002; Rendle and Reijmer, 2002). However, the same trend
very sensitive to sea-level changes that expose and flood the has been observed on the platforms of the Caribbean, the In-
platform top. The effects of changing production propagate dian Ocean and the Great Barrier Reef (Fig. 7.15; Droxler et
downslope through the system and make themselves felt al., 1990; Davies et al., 1989; Schlager et al., 1994; Andresen
as changes of facies and sedimentation rates on the slopes et al., 2003).
and basin floors surrounding the platforms. Two important Highstand shedding is pronounced on tropical carbonate
topics related to sea-level effects on deeper-water environ- platforms because of the combined effect of sediment pro-
ments are highstand shedding and the origin of megabrec- duction and diagenesis. Sediment production of a platform
cias. They are discussed below. increases with its size, and the production area of a plat-