Timothy A. Cross1
(1) Colorado School of Mines, Golden, CO
Abstract: Facies successions, stratigraphic cycles and correlation of deep-water sandstones
Deep-water sandstones record
energy cycles of stratigraphic base-level oscillations. The cycles are recorded
by systematic vertical facies successions which reflect changes in topographic
gradient
that occur during progradation and aggradation of fan complexes.
Sediment gravity flows are
sustained by inertial and potential energy. Potential energy is gradient
.
Inertial energy is a function of the mass and concentration of the sediment
remobilized at the failure site. Resistance to flow is from friction. Reduction
of energy input causes flows to decelerate and freeze. A population of flows
feeding one part of a basin have a narrow range of inertial energy, therefore
the inertial term is approximately constant and
gradient
is the dependent
variable.
As submarine fan complexes
prograde and aggrade, topographic slopes increase and decrease. There is a
dynamic balance between gradient
and the viscosity of flows which freeze at
different geographic positions.
Deep-water sandstones record the
changes in energy required to sustain them, and therefore the changes in
gradient
through time. When the
gradient
is reduced below a critical angle for
a flow of particular viscosity, the flow freezes. Vertical facies successions
record
gradient
changes through time. A common facies succession recording
increasing
gradient
through time is: Bouma cde; Bouma a-e; Bouma a-c; dispersed
granule ss; thin-bedded structureless ss; convolute lamination (subdivisible);
chaotic fabrics; saucers; dishes; pillars; pipes; feathers; streaky lamination;
thick-bedded structureless ss. This facies succession records a half-cycle of
stratigraphic base-level fall (decreasing accommodation/sediment supply), and a
mirror image records a half-cycle of base-level rise.These cycles are
correlatable through sand-prone parts of submarine fans.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana