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ABSTRACT: The Significance of Carbonate Megabreccias in Sequence Stratigraphy: Examples from the Triassic of the Dolomites, Northern Italy

Lyndon A. Yose, Lawrence A. Hardie

Megabreccia deposits in off-platform environments may accumulate in response to two highly contrasting processes: (1) platform-margin and slope-front erosion associated with sea level falls, or (2) platform-margin overproduction and slope instabilities generated during sea level rises or highstands. A series of sea level rises and falls could result in complex stacking of genetically different megabreccia systems in slope-to-basin carbonate environments. It obviously becomes crucial in sequence stratigraphy to correctly distinguish between lowstand and highstand megabreccia deposition, especially if these relationships are used to construct eustatic curves. Thus, additional criteria such as evidence of subaerial exposure (on the adjacent platform, and/or in the blocks), p esences or absence of platform-derived sands and muds, and/or influx of siliciclastics (where applicable) are required to establish the proper connection between sea level and megabreccias.

Spectacular exposures of megabreccia deposits (up to 120-m thick) in the Dolomites have been used as evidence for a global eustatic drop near the Ladinian-Carnian boundary. However, many aspects of these deposits are strikingly similar to other Ladinian and Carnian foreslope deposits of the Dolomites acknowledged to record platform growth during periods of sea level highstands. Similarities include (1) breccia and megabreccia clasts composed almost exclusively of margin-derived boundstones and corals, (2) cavities within clasts that are primarily constructional and lack evidence of extensive subaerial dissolution, and (3) proximal to distal relations across the base of slope that record a transition from amalgamated lower slope breccias to toe-of-slope megabreccias that interfinger wi h platform-derived carbonate sands and muds and basinal volcaniclastics. It is entirely possible, therefore, that the proposed lowstand megabreccias are actually highstand talus accumulations of active carbonate platforms. If this alternative hypothesis is correct, platforms of the Dolomites record a relative rise or highstand of sea level during the late Ladinian and the Triassic eustatic curve, if basin on these relationships, should be reevaluated.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990