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Recognition and Significance of Peri-Platform Ooze Facies and Micrite-Chip Breccias in Carbonate Gravity-Flow Systems: An Example from the Middle Jurassic of Northeastern Italy

ZEMPOLICH, WILLIAM G., and LAWRENCE A. HARDIE, Johns Hopkins University, Baltimore, MD

The Vajont Limestone is predominantly composed of resedimented oolite that was deposited by gravity-flow processes in a deep-water setting. The present study examines associated micrite ooze and micrite-chip breccias found in upper and lower slope settings. An open seaway gravity-flow model is proposed for Vajont deposition.

A peri-platform ooze facies was found to overlie a thick succession of oolitic gravity-flow deposits. This facies is composed of (1) bedded, platform-derived, radiolaride-rich micrite; and (2) channelized, fossil- and clast-rich, oolitic debris flows. Bedded micrite contains burrows and borings that are filled by micrite and/or oolite. These features suggest that bioturbation occurred before and after lithification. Channelized debris flows contain skeletal fragments, oolite, and pebble- to boulder-size clasts of platform lithologies.

Micrite-chip breccias are found in lower slope settings and are composed of micrite clasts and oolite. Breccias exhibit both clast- and matrix-supported fabric and are found at the base of fining- and thinning-upward depositional cycles. Micrite clasts contain burrows and borings similar to those found in bedded micrites. These data suggest that breakup and transportation of micrite clasts and oolite, by gravity flow processes, occurred after significant periods of hemipelagic sedimentation in upper slope settings.

Peri-platform micrite ooze and micrite-chip breccias exhibit a complex history of sediment deposition, bioturbation, and lithification in upper and lower slope environs. These and other field data suggest that an open seaway gravity-flow model is appropriate for Vajont deposition.

 

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)