Modeling the paleogeography of north-western Palaeotethys across the Permian-Triassic boundary: constraints and possible solutions
We simulated for the first time the paleogeographic evolution of three thin depositional sequences of the shallowmarine western Palaeotethys, deposited in the Southern Alps (SA, Italy) during the devastating Permian-Triassic extinction. The simulation is calibrated by a rich set of published field data, measured in the uppermost Bellerophon Formation - lowermost Werfen Fm. Data and paleogeographic maps are located in the SA area palinspastically restored. The employed software is a preliminary version of SIMSAFADIM-CLASTIC, that simulates: a) the spatial distribution of terrigenous and clastic carbonate, b) the fossil content, c) the microbial content. The models (maps) were realized as a back-analysis, by calibration on the 3D architecture of the real sedimentary sequences, particularly on the spatial distribution of terrigenous-clastic carbonate ratio. The simulation covers a period of about 70 kyr, whereas each sedimentary sequence corresponds to a time span of 15-20 kyr; the low stand tract spans 5-6 kyr. The models that best match the reality was achieved by using a curve of sea level changes obtained empirically, by subsequent attempts. The maximum sea level change is about a dozen meters, and the study area underwent locally short periods of emersion, with soil or intertidal carbonates, followed by shallow marine, foreshore facies. This sea level changes curve is likely to represent the global reference. We interpret the model results in light of the hypothesis that the curve of sea level change presented here could were produced by alternating global warming and cooling of the oceans. This curve, obtained by an independent method, would be utilized as an important constraint of the global atmospheric, coupled with oceanic circulation, numerical models. On the contrary, the disappearence of Permian-type taxa (fusulinids, forams, bivalves and algae) pre-dating the P-T boundary does not match the field data, because the software is lacking a few specific functions; these biologic carbonate components seems to have beeen substituted by, for a still unknown environmental cause, the production of oolites and of carbonate of microbialitic origin.