Stratigraphic and compositional study of mixed shallow-water carbonate-siliciclastic units of carnian age (late triassic) in Dolomites and Julian Alps (Italy)

The Julian to Tuvalian (Carnian, Late Triassic) units of the Dolomites and Julian Alps (Eastern Southern Alps, Northern Italy) have been studied under a sedimentological and petrographical point of view. This stratigraphic interval records an important turnover in the sedimentary system with at least two phases of coarse terrigenous input interfingered with carbonate deposits. This episode seems coeval with a wet climatic event named “Carnian Pluvial Event (CPE)”. The arenite petrography of the Carnian coarse siliciclastics has been studied in order to provide insights on the structure and climatic nature of the CPE, and the underlying carbonates of Dogna were examined, in order to provide an understanding of the “benchmark” sedimentation before the CPE in a region where the depositional profile did not change significantly throughout the Carnian. The Julian (Early Carnian) succession of the Dogna area (Julian Alps, Eastern Northern Italy) shows the geometrical character of a carbonate ramp (associable to a “cool-water factory”) with terrigenous influx, in contrast with the rimmed platforms of the adjacent Dolomites area (recognized as “tropical-water factory”). The contradictory carbonate succession of Dogna has thus been investigated under a sedimentological and petrographical profile in order to refine the sedimentological model of the ramp, identifying the depositional sub-environments and to test the fit of the cool-water carbonate factory model. Lithofacies recognized are micrite-supported (indicator for “tropical-water”), while petrographical associations of bioclasts are mainly heterotrophic (“cool-water” indicator) but with dominance of halimedacean algae in the inner ramp facies. Fragmented halimedaceans represent also the origin of micrite, unraveled by SEM analyses. Overall, five sedimentary sub-environments for the subtidal ramp have been recognized from integration of field observations and petrographical analyses. A new insight for carbonate production in the Upper Triassic is given: the coexistence of tropical- and cool-water features can be explained by the terrigenous influx, hampering the growth of autotrophic organisms (as presently occurring in stressed tropical platforms), and halimedacean algae appear to be already major carbonate producers. Arenites of the Heiligkreuz and Travenanzes Formations (Dolomites) and of the “Dogna” formation (Julian Alps) were analyzed with standard sedimentary petrology techniques in order to detect the influence of the climatic change in the sedimentary systems. Arenites composition displays a repetitive alternation between lithic and quartzose lithologies, with an overall increase in quartz content. Within each terrigenous phase, arenites composition displays repetitive alternations of lithic and quartzose arenites, with an overall quartz increase. Compositional variations have been related to climate with the detection of the modifications of known mechanisms affecting arenite composition. Short-term variations can be caused by the different weathering extent in the pedogenetic environment, while long-term variations from the change of the general depositional setting. The CPE can thus be described as a parossistic event with extreme seasonality, represented by the repeated alternation of humid (lithic arenites) and arid (quartzose arenites) periods, in strong contrast with the current understanding of climate influence on arenite compositions