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