--> ABSTRACT: Hydrodynamic and Rheologic Transitions from Traction to Sandy Sediment-Gravity Flows, by D. S. Anderson; #91021 (2010)

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Hydrodynamic and Rheologic Transitions from Traction to Sandy Sediment-Gravity Flows

ANDERSON, DONNA S.

Two outcrop examples from lake-delta settings display rarely documented changes from unidirectional traction to sediment-gravity flow conditions within beds over short distances. Each example shows progressive soft-sediment deformation of dunes or ripples culminating in massive beds of sandstone. The transformation probably involves disorganization of avalanche foreset laminae by vertical movements of pore water incorporated from the lake.

One example occurs in alternating lake and braidplain strata in the Hornelen Basin (Middle Devonian) of western Norway. Lake strata consist of largely ungraded Bouma A divisions composed of fine to very-fine sandstone. Axial braidplain macroforms up to 5 m thick consist dominantly of medium to fine, trough cross-stratified sandstone. Tracing macroforms laterally over hundreds of meters shows that trough cross-stratified sandstone becomes progressively fluidized lakeward: soft-sediment deformation structures range from isolated cusps affecting less than 10% of a macroform to assemblages of interpenetrative cusps, droplets, and pseudonodules which gradually destroy all traces of primary trough cross-stratification. The final transition to massive beds of Bouma A divisions of the lake facies tract occurs over <2 m laterally.

A second example occurs at the toes of Lake Lahontan (Late Pleistocene) delta clinoforms in Truckee River Canyon, Nevada. Individual sand beds in clinoforms display lakeward changes in sedimentary structures from climbing ripples to droplets and pseudonodules to massive bedding over <3 m laterally. 

AAPG Search and Discovery Article #91021©1997 AAPG Annual Convention, Dallas, Texas.