Gamma-ray logging on peritidal carbonates: a case history from the Dolomia Principale/Hauptdolomit (Sella Group, Dolomites, Northern Italy)

Despite the Dolomia Principale/Hauptdolomit (DPR) is one of the most known lithostratigraphic units covering large parts of the Alpine chain, difficulties remain in correlating the inner platform succession from different sec- tors. This is due both to the absence of significant biomarkers and to the apparent uniformity of cyclic arrangement, making the stacking-pattern analysis hard, but also to the sin-sedimentary differential subsidence, resulting in quite lateral thickness changes (250 to over-1000 m). Integrating field observations with gamma-ray logging could help to interpret the sequence- and event-stratigraphic architecture of these successions, improving middle-to-large scale correlations. A case history is reported from the Sella Group (Dolomites, Northern Italy), where the DPR is entirely well-preserved and exceptionally exposed: it consists of dolostones arranged in m-scale, peritidal asymmetric cy- cles, for a total thickness of 285 m. The sedimentological patterns described by previous authors were furtherly integrated by new field observations, and a detailed stratigraphic column was prepared. On this section, an outcrop spectral gamma-ray survey was performed using a portable GR spectrometer. By means of a large NaI crystal detector, the total bulk-GR, and the K, U, Th components were detected. A 120 second count time was used with 1 m sampling step. Raw data were processed in order to obtain GR, SGR and SGR2 indexes. Results were plotted on the stratigraphic column, showing interesting features: at least 4 major shifts of the GR signal occur, each one characterized by high values of Th and U.
The basal part of DPR is characterized by a constant decrease of K, likely related to the retreat of the terrigenous coastline to which the platform was attached. A first GR-shift is recorded in the lower portion of DPR, where inter-supratidal facies prevail and high-energy facies are not rare in subtidal part of cycles, matching a peculiar pelitic horizon: relative low values of K and the residual character of Th support a diagenetic origin of pelites, in an organic-matter-rich environment (high values of U), like ponds in an emerged tidal flat. The second GR-shift roughly corresponds to a change in the cyclic arrangement of DPR, with prevailing subtidal facies characterized by intra-bioclastic or crystalline dolostones. A third GR-shift slightly precedes the transition to the upper part of DPR, dominated by inter-supratidal dolostones where stromatolites and pisoidal layers are common and paleokarsts are very frequent. The fourth GR-shift falls within the transition-interval to the overlying Rhaetian Dachstein Lime- stone, where reddish and well-developed paleokarsts are common: similarly, to the first one, the younger shifts are characterized by low values of K, contrasting relative high concentrations of Th and U, and the same sedi- mentological causes can be hypothesized. Considering the minimum time-span covered here by the DPR (15 Ma ca.) and assuming steady sedimentary control-factors, the Th-U shifts recur every 3,5/4 Ma and could be related to 3rd-order depositional sequences. Notably, the U trend shows minor shifts at high frequency, but cyclostratigraphic analysis are still underway to establish whether if they could be related to orbitally-driven cyclicity or not.