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Edmund R. Gustason1, Arthur S. Trevena2, Lawrence S. Jones3
(1) Schlumberger Holditch-Reservoir Technologies, Denver, CO
(2) Unocal Corporation
(3) Schlumberger Holditch-Reservoir Technologies

Abstract: Jurassic Highways and Byways: A new mechanism for avulsion in the Salt Wash Member of the Upper Jurassic Morrison Formation, east-central Utah

The Salt Wash Member of the Upper Jurassic Morrison Formation in east-central Utah is a low net-to-gross fluvial deposit characterized by two distinct populations (larger highways and smaller byways) of low sinuosity, ribbon sandbodies encased in variegated floodplain mudstone. Highways, greater than 1 meter thick, tens of meters wide, and containing planar and cross-stratified, fine- to medium-grained sandstone, formed from lateral and downstream accretion of mid-channel braid bars within a non-migrating, fixed-width channel. Smaller byways are less than 1 meter thick, a few meters wide, and are characterized by ripple cross-laminated and planar laminated, very fine to fine-grained sandstone. Mudstones range in thickness from less than 1 meter to a few meters and contain carbonate-rich horizons, calcareous nodules, agglutinated nodules, and distinct color banding indicative of widespread soil formation in moderately to well-drained floodplains. Dinosaur tracks and dinosaur bones are abundant.

The floodplain location of channels was controlled by avulsion. Avulsion generally results from a gradient differential between the existing channel slope and floodplain slope caused by high sinuosity and/or natural levee development, but Salt Wash channels are low sinuosity and natural levees have not been found. We propose that avulsion occurred during floods when sand bedload permanently clogged the main highway and smaller byway channels. Pre-existing dinosaur trackways aligned in the direction of most favorable slope provided a pathway for floodwaters and were quickly scoured into new channels. A single dominant channel developed (highway), while smaller channels (byways) may have provided floodplain drainage during small floods

This mechanism for avulsion is consistent with the spatial distribution and connectivity of ribbon sandbodies. A modern analogue is provided by the Okavango Fan of Africa, where floodplains are criss-crossed by Hippo trackways that focus floodwater, thereby controlling the location of avulsed channels.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana