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By MARLAN DOWNEY

Recognizing Turbidites Was No Tall Tale

Perhaps a review of the past may make us better geologists in the future?

"Significant Features of Graded Bedding," by Ph. H. Kuenen; AAPG BULLETIN, Vol. 37, #5, 1953.

"Graded bedding caused by volcanic eruptions should be readily recognized by the nature of the materials. Deposits formed by a river while its competency is changing should be graded . . . The grading should ordinarily be from coarse at the bottom to fine at the top.

"Graded marine graywackes are characterized by regularity of bedding in thick and widespread series, absence of clear wash-out channels or wind ripple marks, and by uniformity in direction of supply."

Key arguments for attributing marine graded graywackes to turbidity currents are:

• "Absence of tidal actions and symmetrical wave ripple marks."
• "Absence of autochthonous benthonic life in the graded beds."
• "Graded beds show a gradual and perfectly smooth decrease in competency from a maximum at the start to a minimum at the close."

Those were glory years in sedimentology and the understanding of sedimentary structures, and Kuenen developed many of the important criteria for recognizing turbidites on which we still rely -- 50 years later.

I remember walking down an Arkansas road cut looking at Lower Atoka sandstones in 1958. On the east side of the exposure, "Boo" Bernard was detailing the reasons why the sandstones were shallow water, low energy delta-front sandstones; on the west side of the roadcut, Ken Hsu was describing Kuenen's work and identifying the sandstones as deposited by turbidity currents in deep water.

Ken Hsu was right, of course -- but "Boo," a wonderful Cajun contemporary of Rufus LeBlanc, easily won the story-telling contest.