Fossil forests in the Austral Basin (Argentina) marking a Cenomanian heterogeneous forced regressive surface

Abstract

The mid-Cretaceous greenhouse period in the southern Patagonia Argentina (Austral Basin) is represented by the Mata Amarilla Formation. 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 forced regressive surface. This surface is characterized by a well-developed palaeosol, associated with the extensive preservation of a podocarp-dominated fossil forest over a vast area (more than 5400 km2). Sedimentological and palaeopedological analyses, in conjunction with forest structure, tree density and growth ring analyses, indicate that the mid-Cenomanian forced regression can be distinguished as a non-uniform surface developed over a short period of time. This sequence boundary is recognized through a heterogeneous regional surface, delimited in the western part of the study area by an erosional surface generated by a large lateral channel migration recorded by sheet-like 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. 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.