Paleoclimate Considerations of the Palo Pintado Formation (Upper Miocene), Salta Province, Argentina

GALLI, C.I.; ANZOTEGUI, L.; REYNOLDS, J.; MORTON, S.; HORN. Y.

Mendoza

Congreso; 18th International Sedimentological Congress; 2010

International Associations of Sedimentologist

Fossil and sedimentological investigations were undertaken at the Palo Pintado Formation(Payogastilla Group, Upper Miocene), southern Calchaquí Valley. Strata comprising 1,387 m of clastic deposits interspersed with some pyroclastic beds are well exposed in the Quebrada El Estanque. Based on stratigraphic and sedimentological features, we introduce new paleoenvironmental interpretations: the strata consist of an transitional type between low andhigh sinuosity rivers forming a wandering fluvial system sand-gravel with small lakes.Channel and overbank deposits can be distinguished within this river system. Accumulationswithin channels are composed of gravel bars (GB) and sandy bedforms (SB). Overbank deposits are represented by: a) crevasse channels, b) small crevasse channels (CS) and c) floodplain (FP) deposits. The floodplains were drained during the dry season, or during longer time intervals, as suggested by desiccation cracks and gypsum in thin beds. The marshy subenvironment is developed by the shore of small overfilled lakes. It also constitutes the final filling stage of the lagoon trays, inside the flooding plains.Different levels of pelitic green and brown beds of the Palo Pintado Formation were analyzed(using X-ray diffraction), to be used as a base for provenance and paleoclimatic data. The basal strata are predominantly illite associated with quartz and albite.Intermediate levels are composed of saponite-illite, quartz and muscovite, while the top is composed of illite-montmorillonite, quartz, albite and clinochlore.Generation of incompletely altered clay minerals, such as illite and smectite (saponite and montmorillonite), are produced by hydrolysis in a temperate-humid climate.Description of the Palo Pintado Formation sedimentary paleoenvironment suggests a scenario thatmay have hosted known paleocommunities. The aqueous subenvironments (lagoons, swamps and rivers with slow to moderate current flow), gave shelter to the aquatic and marsh flora and fauna. The riverbanks and floodplains developed along a forested hydrophilic riverbank. In the other side of the floodplain the structures associated with abandoned channels are transitional to savannas and grasslands.The high frequency and good preservation of aquatic pteridophytes and pelecypods, developed inaqueous lenticular bodies, together with the good stratification of the riparian forests along withepiphytes, suggest stable communities that persisted for a long time.Some of the recorded plant taxa are presently living in tropical regions and some types of molluscsdeveloped under a humid and hot or warm temperate climate (18ºC ? 28ºC). Considering that both the spatial distribution and paleobotany (Barreda et al. 2007) of the analyzed fossils correspond to a Neotropical latitude, it is possible to establish that the paleocommunities developed under a tropical climate with warm and humid subtropical low seasonality, inthe Late Miocene (between 10.29 ± 0.11 Ma and 5.27 ±0.28 Ma). This study suggests that climate of the Palo Pintado Formation was more humid than previously proposed, and xeric vegetation was less represented than it is today.