Page 74 - Carbonate Sedimentology and Sequence Stratigraphy

CHAPTER 4: CARBONATE FACIES MODELS 65

this case a reasonable proxy for restriction - is rather only subject to slow epeirogenic movements (see Har-
straightforwardly related to water depth and distance rison, 1990, for overview). Epeiric sediment formations
from the open ocean (Purser (1973). The same holds for are, therefore, on average thinner than coeval deposits
the Gulf of Carpentaria (Wolanski, 1993), albeit in the of continental margins and their hiatusses are longer.
opposite sense – salinity decreases shoreward because This characteristic hampers physical stratigraphic cor-
of freshwater run-off. relation, particularly in combination with far shifts of
➤ Topographic gradients are very low and more irregular facies belts during changes of sea level.
than on shelves of continental margins. An important
implication of the low slopes is that facies belts shift far The pecularities of tides, restriction and slope gradients
and rapidly as sea level changes. The Gulf of Carpen- in epeiric seas should be kept in mind when interpreting the
taria in northern Australia is a case in point. The gradi- facies belts of epeiric carbonates. Particularly critical is the
◦
ent from shore to the 60 m isobath is about 0.01 ;thus, a correct interpretation of facies belts 5 and 6 – reefs and sand
50 m drop in sea level will shift the shoreline and related shoals at the platform margin. On ocean-facing platforms,
facies by 230 km. Under these circumstances, the mi- these belts lie between a high, steep slope on the seaward
gration of facies belts with time is difficult to track and side and a flat lagoon on the landward side. The lack of high
sequences and systems tracts are difficult to assemble. slopes in epeiric seas complicates matters as water depth on
The low topographic gradients also imply that rates of the seaward side may differ only insignificantly from that in
progradation during stable sea level are higher than on the lagoon. In this situation, it becomes difficult or impos-
continental margins because less accommodation space sible to distinguish between sand shoals rimming a lagoon
needs to be filled. and sands of a tidal-bar belt that formed on the epeiric shelf
➤ Sedimentation rates are typically low in epeiric forma- in zones of amplified tides (Fig. 4.12). Similar ambiguities
tions because the continental interiors are stable and may develop with regard to reef belts (facies 5). As the depth

Fig. 4.12.— Paths of sand transport on
the NW European shelf - a recent epeiric
sea with dominantly siliciclastic sedimen-
tation. Transport is almost entirely con-
trolled by tides, even in the southern
North Sea where the shelf break is over
1000 km away. Distribution of sand bod-
ies is largely controlled by tidal currents
and poorly correlated with distance from
shore or from the shelf break. Note
frequent occurrence of bed-load part-
ings - sediment-starved areas that rep-
resent “watersheds” in the transport sys-
tem where sand is carried off in opposite
directions. After Johnson (1982).

bed-load parting
net sand transport directions
dominantly tidal
dominantly non-tidal
local direction on shelf
edge banks
net direction unknown
200 km