Page 101 - Geology of Carbonate Reservoirs

82 STRATIGRAPHIC PRINCIPLES

4.1.2 Homoclinal and Distally Steepened Ramps

Homoclinal ramps are submerged platforms that extend from shore to basin without
a sharp increase in slope. Ramp slope angles may vary from one platform to another,
but the unchanging characteristic is the absence of a slope break. Monotony in

seaward slope and absence of a topographic “ filter ” to intercept incoming ocean
waves, tides, and currents cause shorelines on ramps to be high - energy zones where
grainstones and even carbonate gravels accumulate. Seaward transects of homocli-
nal ramps pass across increasingly muddy facies until basinal depths are reached.
In other words, the usual facies progression across a ramp is from grainstones on
the inner ramp (near shore) to mudstones on the outer ramp (distal portion of the
ramp). If the shoreline is attached directly to the mainland, the strandplain facies
on a homoclinal ramp will be beaches and coastal dunes. If longshore drift transports
carbonate sands and gravels parallel to the shoreline forming a barrier island just
offshore, lagoons will be ponded behind the barriers and muddy or evaporitic tidal

flats may line the protected lagoonal shores.
The absence of a slope break in shallow water is a deterrent to the formation of
buildups that might otherwise develop a topographic rim. Reefs develop on ramps,
but they occur typically as patch reefs rather than laterally continuous reef trends.
Reefs built by photozoans develop on hard seabeds in shallow, warm water where
warm temperatures favor the formation of aragonitic skeletons and where nutrients
and oxygen are abundant. Hard bottoms that are elevated above the surrounding
seafloor are sometimes associated with antecedent highs such as those over salt

domes, fault blocks, or erosional remnants. In short, some knowledge is required
about paleobathymetric highs and lows on ramps in order to understand and predict
locations of reef growth. Some buildups are constructed by heterozoans, by cements,
or by chemogenic processes associated with seafloor vents and seeps. These features

may develop in deep - water settings that are not related to topographic highs and
by organisms that do not depend on sunlight, warm water, or abundant oxygen in
near - surface water agitated by breaking waves. Because they are unrelated to
otherwise easily detectable bathymetric or depositional features, deeper water
buildups are somewhat more difficult to find in the subsurface.

Distally steepened ramps, like homoclinal ramps, exhibit high - energy facies near
shore that pass to lower - energy, muddy facies with increasing depth. In other words,
facies differences from shore to basin represent changes from mobile to stable sub-
strates — from grainy to muddy seabeds. Distally steepened ramps differ from homo-
clinal ramps by exhibiting slope breaks at some distance from shore. They differ
from open shelves in that slope breaks on distally steepened ramps do not have
facies changes that coincide with the steepening (deep - water slope breaks), as is
true of open shelves. For this situation to exist, slope changes on the ramp must
occur below the depth normally swept by the fair - weather hydrologic regime. If
carbonate buildups nucleate on slope changes along distally steepened ramps, they
will not be laterally persistent and will not be accompanied by continuous facies
differences that coincide with distal steepening. Such a situation is unusual and

would be difficult to distinguish from an open shelf break. Steepening is a topo-
graphic feature but it is not accompanied by facies changes. Homoclinal ramps and
rimmed shelves are end members of a range in platform shapes. Distally steepened
ramps and open shelves are variations in form and they may have similar geological