Stratigraphic Packaging and Syndepositional Extensional Tectonism: a Preliminary Study of the Dolomia Principale, the Dolomites, N. Italy

Robert M. Forkner
The University of Texas at Austin, Department of Geological Sciences
Austin, TX
rforkner@sbcglobal.net

The Norian (late Triassic) Dolomia Principale (DP) is a broad peritidal carbonate complex that nucleated atop several mid-Triassic fault-bounded platforms preserved in the Dolomites of northern Italy. Early studies by Bosellini and Hardie (1985) recognized depositional cyclicity throughout the DP, with laminate-capped peritidal facies composing the basal two-thirds of the package and massive subtidal dolostones with diagenetic exposure caps dominating the top one-third. Doglioni, (1992); Jadoul et al., (1992); and Cozzi, (2000) recognized the effects of syndepositional tectonism associated with the opening of the Tethys that led to the formation local basinal, marginal, and slope facies within the DP complex. Preliminary results of this study indicate that syndepositional extension almost certainly modified the stratigraphic packaging within the DP, and may have served as a long-term accommodation-generating mechanism upon which an allocyclic record is superimposed.

Evidence for synsedimentary extension in the DP can be observed at several scales, and includes centimeter scale open-mode fractures with fill from overlying beds, depositionally overlapped meter-scale normal faults, and hundred-meter variability in complete vertical section thickness over a 30- kilometer transect. Additionally, a comparison of the thickness of complete DP sections at the Sella group, Puez group, Tofane de Roces, and Monte Cristallo indicate that the DP more than doubles in thickness from west to east over a few tens of kilometers. Measured sections from the bases of the Sella, Tofane and Monte Cristallo show subtle variability in facies as well as measurable differences in cycle thickness. Fischer plots generated from measured sections indicate that cycles thicken from west to east rather than increase in number, likely a result of sedimentation keeping up with differential subsidence.

Synsedimentary extension and subsidence driven by rifting and gravitational sliding of sedimentary blocks on mobile media can be readily observed in the subsurface in seismic lines from most passive margins, including the Gulf of Mexico, offshore Brazil and offshore Angola. It is likely that the multi-scaled evidence for synsedimentary extension in the Dolomia Princiaple is also the direct result of extensional normal faulting driven by rifting and mobilization on underlying evaporite.