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Andrea Cozzi1
(1) Trinity College Dublin, Dublin 2, Ireland

Abstract: The effects of extensional tectonics on shallow water carbonate platform facies and geometry: the case of the Upper Triassic Dolomia Principale, Carnian Prealps, northern Italy

During the Late Triassic (Norian) the plate-scale reorganization that led in the Jurassic to the opening of the North Atlantic was at its beginning. The initiation of the rifting phase was recorded in the shallow water carbonate platform deposits of the Dolomia Principale Fm. (northern Italy). The platform interior was intensely dissected by cm-wide fractures, shatter breccias, neptunian dikes, normal faults and m-scale laterally discordant breccias, all having in common a vertical development and a penecontemporaneous infill by marine cements and/or sediment. The platform margin deposits record the onset of the extensional tectonic phase with a switch from lateral progradation to vertical aggradation, indicative of an overall increase in accomodation space dictated by tectonic subsidence. This coincides with a time when the global sealevel curve for the Late Triassic predicts the exact opposite. During the margin aggradation, an anoxic fault-bounded relatively deep water depression formed within the platform interior facies. The strikes of the tensional disruption features follow two principal orientations, one perpendicular to the other, and a third at 45 degrees from them. A coupled N-S and E-W extension is suggested to fully explain this complex array. The directions of the main tensional stresses are in good agreement with those postulated for the rifting of the NW Tethys Gulf during the Late Triassic-Early Jurassic. It is concluded that small and large scale tensional disruption features are reliable paleostress indicators and can be used to better understand the accomodation history of shallow water carbonate platforms and their sequence stratigraphic patterns in extensional tectonic settings.

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