Page 146 - Geology of Carbonate Reservoirs

DEPOSITIONAL ENVIRONMENTS AND PROCESSES 127

environment. Slope environments characterize both rimmed and open shelves but
not ramps. With the possible exception of rare cases, slopes on distally steepened
ramps are nothing more than changes in slope without accompanying differences
in environmental characteristics or sedimentary rock properties. The absence of
steep gradients in environmental processes above and below distal steepening on
ramps indicates how monotonous sedimentation had to be for the distally steepened
area to have rock properties no different from the surrounding seabed above and
below it.
Carbonate slopes differ from siliciclastic slopes in that carbonate slopes are
steeper (5 ° – 15 ° as compared to 3 ° – 6 ° ), they tend to have a concave profi le com-
pared to the slightly convex or straight profile on siliciclastic slopes, they are prone

to oversteepening as reef growth or cementation enhances slope steepness, and their
slope angle increases with slope height (Coniglio and Dix, 1992 ). Physical processes
that characterize slope environments vary depending on water depth, on the nature
of the hydrologic regime, on slope characteristics, and on proximity to the slope.
Environmental processes on slopes are dominated by gravitational forces and
pounding from waves. Upper slope zones in relatively shallow water may be subject
to wind or storm waves, oceanic currents, and tides much the same as the slope -
break environment. Steep slopes with abrupt and great changes in water depth are
constantly subject to slope failure owing to steady gravitational pull on wave and
storm - ridden upper slopes. Slopes are commonly sites for upwelling, initiation of
density or turbidity currents, and initiation of slumps, rock falls, and debris fl ows
triggered by slope failures. Middle slope and base of slope zones are typically below
fair - weather wave base, below the influence of surface currents, and relatively less

influenced by tidal currents than the upper slope zone, where the shorter water
column is more vigorously moved during tidal exchange.
Deeper parts of slope zones are sites where rocks and sediments swept off the
slope by shallow - water processes come to rest. Currents on the middle and outer
slope are mainly geostrophic and density currents. Geostrophic currents may be
active to any depth because they depend only on fl ow down a density gradient bal-
anced by Coriolis force. These currents commonly flow parallel to the bathymetric

contours along the toe - of - slope and are referred to as “ contour currents. ” Density

currents result when more dense water flows in the direction of less dense water.
Typically, higher density results when salinity is high, or temperature is low, or when
both factors operate simultaneously. Density currents driven by differences in the
amount of suspended particulate material, or turbidity, in the water masses are
known as turbidity currents and their deposits are called turbidites. Turbidites are
a type of density current deposit but they differ from thermohaline density current
deposits by having high matrix content.
Slope characteristics and proximity to the slope break probably have the greatest
influence on what type of deposits will occur at the base of slope and farther beyond

on the basin floor. Steep, unstable slopes are ideal places for rock falls, debrites, and

coarse grain flows to be deposited. Gentle slopes are probably more likely to be
associated with turbidites, rhythmites, and soft sediment slumps. Accretionary slopes
that develop from reef growth and extensive sediment production and retention at
the slope break are known as depositional margins . Slopes with inherited (anteced-
ent) or eroded escarpment margins are known as bypass margins because their
steepness facilitates slope failure and off - slope sediment shedding. The differences