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

Abstract

The mid-Cretaceous Mata Amarilla Formation marks the onset of the foreland stage of the Austral/ Magallanes Basin (southern Patagonia Argentina), which is related to the first stage of growth of the Andes. It is composed of three informal sections 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 well-developed palaeosol, associated with the extensive preservation of a podocarpdominated fossil forest over a vast area (more than 5400 km). Sedimentological and palaeopedological analyses, in conjunction with forest structure, tree density and growth ring analyses, indicate that the midCenomanian sequence boundary can be distinguished as a non-uniform regional surface developed over a short period of time. In a west-east transect perpendicular to the Patagonian fold-and-trust belt, this heterogeneous surface is delimited in the western part by an erosional surface generated by a large lateral channel migration recorded by sheetlike channel deposits with transported logs. By comparison, towards the eastern part of the study area, it appears as a paraconformity bounded by a very mature vertic Alfisol, which may have 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 towards the basin depocenter overlies shallow marine deposit of the lower section of the Mata Amarilla Formation. However, this interpretation is not 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.