Page 147 - Geology of Carbonate Reservoirs

128 DEPOSITIONAL CARBONATE RESERVOIRS

between depositional and bypass slopes were illustrated in simplified form by James
and Mountjoy (1983) . Steep slopes commonly fail and coarse debris is shed as
“ periplatform talus ” or, farther away from the slope, as “ debrites ” and turbidites.

5.2.10 Depositional Rock Properties in the Slope and
Slope-Toe Environments

Slope facies characteristics reflect environmental processes, slope characteristics,
and proximity to the slope break above. Depending on the steepness of the slope,
most sediment accumulation takes place near the base of the slope and just beyond.
In other words, there is comparatively little deposition directly on steeply sloping
surfaces. Depositional facies basinward from the toe of slope typically include
slumps, slope talus, debris fans or debrites, grain flows, and turbidites. Gravity is the

primary engine that drives sediment transport and the sediment source is material
dislodged from the slope and slope break, material shed from the platform top, and
admixed pelagic sediment. In situ sediment production is usually negligible because
the slope environment is below the depth of production by the carbonate factory.

Slumps and debris flows consist of poorly sorted coarse particles in a fi ner matrix.
In some cases slope failure yields blocks as large as automobiles and even entire

segments of shelf margins can slump downslope (C. Kerans, 2006 , personal com-

munication). Debris flows, or debrites, commonly form as fan - shaped deposits at the
toe of slope and beyond, depending on the competence and capacity of the transport
mechanism. Examples from the Permian Basin of Texas are described in Saller

et al. (1989) and Mazzullo (1994) . Because debris flows at the base of slope and
beyond usually have a high percentage of fine matrix, interparticle porosity is low.

Intraparticle porosity formed before the debris was dislodged from the slope margin
may be present, commonly as separate vugs. However, it may also be reduced by

cement and matrix infill. In most slumps or debris flows reservoir porosity has either
been created or enhanced by burial diagenesis (Mazzullo and Harris, 1992 ).
Grain flows and turbidites are finer grained than debrites. Grain flows are gravity -

driven “ rivers of sand ” that pour down steep slopes after unstable accumulations of
grainy sediment have been dislodged. The fundamental difference between grain

flows and turbidites is that grain flows move in traction flow rather than in suspen-

sion and the flows are not driven by density contrasts induced by turbidity. Turbidites
are deposits produced by a special form of density currents — turbidity currents —
that result when fine sediment is put in suspension to form a water mass with greater

density than its surroundings. Typically, turbidity is increased dramatically when
storm surges pound sediments at shelf margins, during seismic tremors, and gravi-
tational failure of unstable slope sediment. Turbidity currents can reach speeds of
tens of kilometers per hour and may flow basinward for many kilometers. As with

some grain flows and debris flows, proximal turbidites (those closest to the base of

slope) may accumulate in fan or lobe - shaped deposits. Entrained grains may scour
soft bottom sediments so that some turbidity currents excavate channels in the

seabed. These channels are subsequently filled by channelized turbidites. Essentially
all base - of - slope deposits are set in motion by an “ event. ” Whether the event is a
storm, a tremor, or a slope failure, the initial response is usually vigorous and intense
sediment transport followed by declining transport velocity and subsequent deposi-
tion. Some deposits, particularly thermohaline current deposits and turbidites, refl ect