Page 117 - Geology of Carbonate Reservoirs

98 STRATIGRAPHIC PRINCIPLES

accompanied by facies changes. That is, the sea may deepen rather abruptly but the
fundamental properties of the sediments landward and seaward of the slope change
are identical. In general, this requires that the distal steepening has to occur at
depths great enough so that there are no differences in the hydrological regime or
the sediment supply system on either side of the slope change. In other words, there
is no difference in wave or current activity or in sediment sources on either side of
the slope change. There must be a difference in the hydrological regime coincident
with the slope break on shelves and there may be differences in sediment sources
or mode of supply on the inboard and outboard sides of the shelf - slope break. These
differences in the hydrologic environment and sediment input produce different
facies inboard and outboard of the slope break. Likewise, the slope environment is
characteristic of shelves only, not of ramps. Slopes extend from the slope break to
the slope toe at the inboard margin of the basin. No slope angle is included in the

definition; it may vary from steep to gentle, depending on substrate stability, the
hydrological environment, and tectonic stability. Sloping surfaces are not places
where gravity lets sediment accumulate; therefore one can think of slopes either as

the source of sediment supplied to the basin floor, as the “ slide ” down which the
sediment was transported, or both. The site of ultimate deposition is the base, or
toe, of the slope. Deposits on the toe of the slope, still included with slope deposits
in this text, consist of the mass wasted material from the slope break and the slope

such as slumps, rock slides, debrites, grainflows, proximal turbidites, and rock falls.
Essentially everything is out of its original place, it is probably out of its original
up – down orientation (if it had one), and it is out of its original, shallower - water
environmental setting, probably interbedded with or encased by pelagic mud.

There is no specific depth range for basinal environments because they occur in
such a wide range of depths. Oceanographers define the bathyal environment as the

zone that extends from 200 to 1000 m in the modern oceans. They define the envi-

ronment deeper than 1000 m as the abyssal zone. Rarely, if ever, do carbonate sedi-
ments accumulate — especially in reservoir facies — in bathyal or abyssal depths at
least partly because carbonates dissolve below the CCD, which is in the abyssal
depth range. As for the range of depths that can be considered basinal, the deepest
parts (basinal depths) of the modern Persian Gulf are only 200 m, but basinal depths
off the Great Bahama Bank are over 1 km. We can defi ne the basinal environment
therefore as the environment that exists at the greatest depth where carbonate sedi-
ments accumulate normally and on a regular basis within any particular basin.
Environments in which carbonates do not accumulate but where siliciclastic oozes,
clays, and other noncarbonate sediments accumulate are not “ carbonate deposi-
tional environments. ” Basinal environments of all types are not affected by surface
waves, tides, or shallow - water currents. They are characterized by the absence of
light, by dysoxia to anoxia, and by low taxonomic diversity. True, deep - water depos-
its consist mainly of a fine rain of pelagic detritus including pelagic skeletal detritus,

clays, extraterrestrial “ dust, ” and organic matter. Organic matter may be well pre-
served in deep - water settings because they are usually characterized by reducing
and aphotic conditions where water motion is limited in most cases, except for
density currents including contour and turbidity currents. Basinal successions include
lamintes (millimeter - scale microlaminated beds), distal turbidites, density fl ow

deposits, and rhythmites (cyclically repeated coarse - to - fine beds of millimeter to
centimeter scale).