Page 97 - Geology of Carbonate Reservoirs

78 STRATIGRAPHIC PRINCIPLES

Wright, 1992 ). Platform geomorphology varies, especially in the amount and regu-
larity of slope on the depositional surface. It is the interaction of the oceanic hydro-
logical regime with platform geomorphology that determines the anatomy of the
depositional facies on the platforms, the location of the facies on the platforms, and
the rock types exhibited in the facies. Unbroken slopes from shore to open sea are
swept by open ocean waves and currents; those with slope breaks occurring within
fair - weather wave base will have “ high - energy ” facies at the slope break and
restricted circulation behind it. It is the interaction of oceanic hydrodynamics with
antecedent platform geomorphology that determines facies characteristics and evo-
lutionary trends in platform anatomy. Some platforms slope continuously from
beach to basin without a pronounced break in slope; they are called homoclinal

ramps (Figure 4.1 ). Note that the original definition of ramp does not specify any
angle of slope for ramps, although some later authors impose a slope of 1 ° or less
for ramps. Actually, the slope on ramps may vary depending on antecedent topog-

raphy. Ramps with well - defined slope changes on their distal margins are called
distally steepened ramps (Figure 4.2 ). There are no facies changes associated with
slope changes on distally steepened ramps because the distal steepening occurs at
depths below the depths at which oceanic waves and currents affect bottom sedi-
mentation. Other platform types may have steeper or fl atter slopes, but they differ
markedly from ramps in that they have a major break in slope accompanied by
distinct facies changes some distance from shore. At the change in slope, water depth
increases markedly over a relatively short lateral distance, forcing the hydrologic

Rimmed Shelf

Slope-break
Tidal Flat Shallow Subtidal (Shelf-edge)
High Tide Low Tide
Slope Toe
Basin
Homoclinal Ramp

Tidal Flat – Barrier Isle –
Sebkha Lagoon Beach Shallow Subtidal Outer Ramp Basin
High Tide Low Tide
Inner-
Middle Ramp

Figure 4.1 Examples of rimmed shelf and homoclinal ramp platform types. These models

represent the most commonly encountered, end - member platform geometries. Rimmed
shelves typically have reef rims but sand – wave complexes of carbonate grainstones can also
act as hydrologic baffles (rims). An example of such a sand – wave rim is shown in Figure 5.5 .

All shelves have pronounced slope breaks accompanied by facies changes that mark the shift
from shelf interior to slope or slope toe. Homoclinal ramps may be attached to the mainland
shores or more commonly, as in this illustration, a barrier island – beach complex is separated
from the mainland shore by a lagoon. The seabed on homoclinal ramps passes laterally from
the strandline to basinal depths without a pronounced break in slope. Facies changes are
gradational rather than abrupt.