The role of extensional tectonism in the sequence stratigraphic interpretation of shallow water carbonates: A case study from the Upper Triassic Dolomia Principale, Carnian Prealps, northern Italy.

摘要

Upper Triassic (Norian) shallow water carbonates of the Dolomia Principale Formation (DP) and its deep water counterparts (Forni Dolomite Formation, DF) have been studied in the Carnian Prealps. A well preserved platform-to-basin transition in the M. Pramaggiore area has made it possible to differentiate between inner platform, outer platform and margin sedimentary facies of the DP and upper slope, lower slope and basinal sedimentary facies of the DF. After an initial platform lateral progradation phase during the Early Middle Alaunian, a switch to vertical aggradation took place in the late Alaunian-Sevatian. This is reflected in an increase in platform relief, steeper foreslopes angles and coarser-grained slope facies. Coeval with the change from progradation to aggradation is a record in the platform deposits of the onset of synsedimentary tectonic activity during the Late Alaunian-Sevatian, expressed as fractures, normal faults, shatter breccias, neptunian dikes and laterally discordant meter-wide breccias. When interpreted in sequence stratigraphic terms the entire succession fails to reveal any sequence boundary, shelf margin systems tract, onlap or downlap surfaces, which are predicted to occur in the Norian by the global sealevel curve of Haq et al. (1987, 1988) that was derived from the Dolomites region, only 50 km to the west of the study area. Further investigation revealed a discordant series of depositional sequences in other parts of the Southern Alps, a result of different subsidence rates controlled by block faulting. A further comparison with the Northern Calcareous Alps, where the Rhaetian part of the Exxon global chart was derived, and the Transdanubian Central Range revealed a similar pattern if compared with the Carnian Prealps, with absence of unconformities on the platform tops and tectonically controlled subsidence during the Norian and the Rhaetian. Remarkable is the time-coincidence of intense extensional tectonic activity and the two depositional sequences suggested by Haq et al. for the Late Triassic. It is suggested that the global sealevel curve for the Late Triassic, based on outcrops located at that time in the same tectonic setting, were determined by regional tectonics and not global sealevel changes. Moreover, changes in tectonic subsidence rates can produce 3rd order depositional sequences on a regional scale, identical to the one predicted for eustatic sealevel oscillations by sequence stratigraphy.