Synsedimentary Extension and Iits Effect on Platform Carbonate Cycle Stacking: The Norian Dolomia Principale, the Dolomites, North Italy
Robert Forkner1 and Linda A. Hinnov2
1 The University of Texas at Austin, Austin, TX
2 Johns Hopkins University, Baltimore, MD
The Norian (late Triassic) Dolomia Principale (DP) is a broad, basin-fringing peritidal carbonate complex that is preserved along the Eurasian Tethyan suture, and is particularly well exposed in the Dolomites of northern Italy. Frequency analyses of exposure-ranked series from vertical measured sections along a 40 km EW transect indicates bundling of cyclic stratigraphy differs systematically from locality to locality. Extreme up-dip sections in the western Dolomites (near Ala) contain packages of condensed megacycles grouped into bundles of 3. Sections taken from the central Dolomites show bundling at variable frequencies, including between 4 and 6 cycles per megacycle. Finally, sections and grayscale scans taken from localities furthest east suggest a dominant bundling frequency of 5. The variable bundling frequencies discovered within the DP can be explained in two ways. One, that the great expanse of the platform allowed for appreciable lateral sediment transport and associated autocyclicity, leaving few localities with predictive allocyclic records. And two, that synsedimentary extension and differential subsidence broke down the allocyclic record in some portions of the platform. This is evidenced by differential formational thickness within the DP as well as depositionally overlapped fractures, faults, and grabens with slickensides indicative of normal motion. Facies within the DP point to a dominance of peritidal environments through the measured successions, suggesting that the carbonates were able to keep pace with differential subsidence. As a result, cycle thickness may not be a reliable record of eustatic cyclicity in this circumstance. As an alternative to correlation via analysis of cycle stacking, plots of exposure rank series derived from measured sections provide a promising method by which to correlate stratigraphy in regions with variable local subsidence.