Characterization of a heterogeneous sequence boundary surface perpendicular to the Patagonian fold-and-thrust belt: the role of fossil forest.

SEBASTIÁN RICHIANO; MARIANA BREA; AUGUSTO N. VARELA; ARI IGLESIAS; DANIEL G. POIRÉ

Toulouse

Congreso; 33rd INTERNATIONAL MEETING OF SEDIMENTOLOGY and 16ème CONGRÈS FRANÇAIS DE SÉDIMENTOLOGIE; 2017

International Association of Sedimentologists

The mid-Cretaceous Mata Amarilla Formation marks the onset of the foreland stage of t he Austral/ Magallanes Basin (southern Patagonia Argentina), which is related to the first stage of growth of the Andes.It is composed of t hr ee i nfor mal s ect i ons which were deposited in littoral and continental environments. The boundary between the lower and middle sections of the Mata Amarilla Formation shows a drastic reduction in accommodation/sediment supply (A/S) ratio, interpreted as a sequence boundary surface. This surface is characterized by a wel l-devel oped palaeosol, associated wit h t he extensive preservation of a podocarpdominatedfossil forest over a vast area (more than 5400 km). Sedimentological and palaeopedologicalanalyses, i n conj uncti on wit h for est str uct ur e, tree densit y and growt h ri ng anal yses, i ndicat e that t he midCenomanian sequence boundary can be distinguished as a non-unifor m r egional sur face devel oped over a short period of ti me. In a west-east transect perpendicular to the Patagonian fold-and-trust belt, this heterogeneous surface is deli mited in the western part by an erosional surface generated by a large lateral channel migration recorded by sheetli ke channel deposi ts wit h transport ed logs. By comparison, towards t heeaster n part of t he st udy area, it appears as a paraconfor mity bounded by a very mature vertic Alfisol, which may ha ve taken 40?100 ky to develop, and the preservation of a fossil forest in life position with a minimum age of 337 years. On the other hand, in a north-south transect parallel to the Patagonian fold-and-trust belt, this surface is interpreted as a forced regression surface because t owards t he basi n depocenter  overlies shallow marine deposit of the lower section of the Mata Amarilla For mati on. However, t hi s i nter pretati on isnot so clear in the west-east transect towards the basin margins where this surface overlies distal fluvial and littoral deposits of the lower section of the Mata Amarilla Formation. It is concluded that the extensive presence of these fossil forests at the same stratigraphic level in a vast region respond to changes in A/S ratio in a forced regressive surface at basin scale.