Continental sedimentation of the Las Flores Formation (Early Palaeogene) in Golfo San Jorge Basin (Patagonia, Argentina).

RAIGEMBORN, M.S.; KRAUSE, J.M.; MATHEOS, S.

Alghero, Italia

Otro; 27th Meeting of Sedimentology; 2009

IAS

The Late Palaeocene–Lower Eocene sedimentation at north of the Golfo San Jorge basin, Central Patagonia, Argentina, is recorded by the Las Flores Formation. It is a fluvial-lacustrine unit, which spans a time deposition contemporary with part of the Paleogene global warming. Their type section is defined as essentially composed by mudstones. However, the well-exposed outcrop in the southeast of the Chubut province reveals significant sedimentary differences in the unit. Two main areas, eastern and western ones, were defined. The former is composed by 33–52 m thick, white and pink sandstones (lithic feldsarenites), conglomerates and silstones. The western area is 24–44 m thick and mainly consists of grayish siltstones and minor epiclastic and volcaniclastic sandstones. In both areas, kaolinite dominates the clay mineral assemblage. By recognizing textures, sedimentary structures and geometry, two main facies associations (FA) were identified. The FA-I, exclusive of the eastern area, comprises normally graded bodies constituted by sandstones, pebbly sandstones and fine pebbles; showing trough-cross bedding, low-angle cross stratification and with inclined large-scale surfaces (epsilon cross-stratification). The geometry of the bodies ranges from lenticular to tabular, being the basal surface slightly erosional concave-up or sharp. Fossil content is characterized by oriented woods, mammals, coprolites and leaf compressions. Recognized paleosols show a poor development. The FA-II, particularly restricted to the western area, have a lateral relationship with the FA-I. This is composed by volcaniclastic silstones and very fine sandstones (fine member), and scarce fine to medium sandstones and reworked tuffs (coarse member). They are dominantly massives or laminated; with scarce biogenic structures and pedogenetic features such as nodules and mottles. Paleosols show relative higher development degree and frequency than those present in the eastern area. Bodies of the FA-II are dominantly laterally continuous tabular ones with planar base. However, scarce bodies with lenticular geometry and erosional base may also be recognized. Both, fine and coarse members yielded abundant siliceous microfossils (phytoliths and other siliceous remains derived from arboreal, palm and herbaceous components) and fossil leaves. In this FA the clay mineral composition shows a trend to increase kaolinite respect to smectite. The FA-I represent multistory channels and they are interpreted to be the deposit of a sinuous, essentially sandy, fluvial system. The FA-II deposition probably took place in distal floodplain and in shallow lacustrine environments. The overbank deposits were accumulated from suspension (fine member) and traction (coarse member) current. The fine volcaniclastic material indicates transportation from ash fall-out and deposition in low gradient, subaerial and subaqueous areas of the floodplains. Both, development degree and frequency of paleosols, suggest common subaerial exposure conditions. The presence of paleosols, bioturbated beds and fossil remains suggests that the floodplain of the system supported vegetal and animal communities. Fossil and clay-mineral assemblages indicate that the Las Flores Formation developed under subtropical-tropical climate conditions, intermittently affected by episodic input of volcaniclastic materials, and with subaerial exposition lapses. Main channel facies suggest higher depositional rates in the eastern area. The properties of paleosols in the western area suggest higher pedogenesis-lower sedimentation rates respect to the eastern area. The only cause for significant facies variation seems to be the position of the FA-I and FA-II in the basin. A further study will allow to define the climatic response of the Las Flores Formation and the interaction between depositional and pedogenesis rates, along the time.