RECOGNITION OF CLINOFORMS IN OUTCROPS; ILLUSTRATED FROM JURASSIC LAS LAJAS – LOS MOLLES FORMATIONS, SOTHERN NEUQUEN BASIN, ARGENTINA

The Lower to Middle Jurrasic deposits of Challaco – Las Lajas – Los Molles formations in Neuquen Basin in Argentina fill the basin by the means of building shelf-slope-basin floor clinoforms. Exceptional outcrops expose 1000s m of stratigraphy along 20 km dip and 10 km strike direction in the southern part of the basin. The deposits show large scale (1000 m) tripartite lithology with sandy basin floor, muddy slope and sandy shelf deposits that is characteristic for a basin fill by clinoformal geometry. The clinoforms have been mapped in the outcrops by following the “flooding surfaces” (continuous muds). The peculiarity of the clinoforms in this area is the coarseness of the deposits which are conglomeratic at times from the shelf to the basin floor. The depositional elements indicate a shelf dominated by fluvial and tidal processes, a shelf edge penetrated at times by high energy discharge (water and sediment), basin slope is dominantly muddy punctuated by coarse grained channel deposits and extremely thick (hundreds of meters) basin floor fan deposits composed of channelized to unchannelized lobes. The recognition of the Lajas-Los Molles clinoforms show how the coarse deposits extends from shelf through the shelf-edge into the upper slope and then deposits (coarse grained) are discontinuous until the basin floor where form continuous coarse sandstone units (fans).

Comparison of Neuquen Basin clinoforms with other basins where 100s to 1000s m thick tripartite sand-mud-sand succession suggests the way to map and understand clinoforms filling the basins. Clinoforms in outcrops can be recognized when the exposures are large and continuous with sandstones punctuating the slope clinoforms (100s to 1000s m thick) like in Spitsbergen, Norway and Neuque Basin. When outcrops are combined with subsurface data like in Washakie Basin, Wyoming or Dacian Basin, Romania the clinoformal interpretation of outcrops become easier being driven the subsurface geometries despite limited (not clinoform conducive interpretation) outcrops. However, in the case of limited subsurface data despite large or multiple outcrops such as Karoo Basin, South Africa, Tyee Formation, Oregon, Fish Creek – Valecito Basin, California the interpretation of clinoforms is more difficult and requires the use of a model like it is proposed in study.