Modeling the paleogeography of north-western Palaeotethys across the Permian-Triassic boundary: constraints and possible solutions

Abstract

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.