FIRST EARLY OLIGOCENE PLANT MACROFOSSILS FROM NORTHERN PATAGONIAN ANDES

MAURO G. PASSALIA; JUAN I. FALCO; ARI IGLESIAS; VANESA D. LITVAK; WILF PETER; FLORENCIA BECHIS; GABRIELLA ROSSETTO-HARRIS

Puerto Madryn

Congreso; Congreso Geológico Argentino; 2022

Asociación Paleontológica Argentina

ThePaleogene is a period where major geological changes took place affecting SouthAmerican environments and paleocommunities. Major episodes are the earlyPaleogene global warming ending in a pick in the Early Eocene Climatic Optimum,the subsequent abrupt decrease in temperatures and later, during the latestPaleogene to Miocene, a more or less continuous aridization due to a higherrate of the Andean Cordillera uplift. Rain shadow is interpreted to be muchless affected than during late Oligocene (Palazzesi and Barreda 2012), andcontinuous wet forests are interpreted far east in southern Patagonia. Theevolution in paleocommunities cannot be isolated from tectonic evolution(Barreda et al. 2020). In Patagonia early Oligocene sediments are representedmostly by marine or estuarine environments (Malumián and Náñes 2011) andcontinental floras are only known for southern Patagonia. During the latest eocene-earlyoligocene transition occurs a global biotic change associated with a globaldecrease in temperature. In Patagonia it is related with a northward expansionof micro- to mesothermal plant taxa, and absence of further records ofmegathermal elements. Precise time when the microthermal taxa reach northernPatagonia remains unrecorded, being observed a rich subtropical humid florauntil the middle Eocene (Wilf et al. 2005) and microthermal stabilized florasin the late Oligocene (Brea et al. 2015). ThePaleogene in northern Patagonia constitutes a period of drastic change in thetectonic scenario. The Paleocene-Eocene is represented by the development ofthe Pilcaniyeu Volcanic Belt intraplate volcanism; meanwhile the middle Eoceneto early Oligocene is related to El Maitén Belt, and represents there-establishment and evolution of arc-type magmatism (37-33 Ma; Fernández Pazet al. 2020). Thenew locality is placed in the central sector of the Pilcaniyeu Volcanic Belt.The fossiliferous outcrops occur in a volcaniclastic sequence overlaying thePaleocene-Eocene Huitrera Formation, the area is located 1.6 km to the east ofthe well-known Río Pichileufu plant locality, where an early Eocene age of 47.8Ma was obtained (Wilf et al. 2005). The new fossiliferous outcrops are exposedin the northern slope of a small hill. The strata are sub-horizontal to gentlyfolded, and show lateral facies with thickness variations interpreted asinfilling of paleo-relief carved out in an Eocene ignimbritic unit. The fossilswere recognized in massive and laminated fine grained deposits, near the middlesection of the unit. The upper fossiliferous level represents a diatomite andpreserves exquisite details of leaf venation and plant morphology. Themacrofloras are dominated by Nothofagus leaves (Figure) with at least threedifferent species (also distinguished by reproductive structures); in a generallow leaf diversity (xx angiosperm morphotypes). Cupressaceae branches andbroad-leaved Araucariaceae were noted among conifers. Seeds, cuticles, andinsects were also recorded (Figure). Rock samples were dated at two differentlevels intercalated within the new fossiliferous section. The mid section datedsample corresponds to a massive vitreous lapillitic tuff. Well preserved andfresh pumice fragments and shards were recognized in thin sections, togetherwith subordinate lithic fragments that correspond to pyroclastic andintermediate volcanic rocks, within a volcanic ash-rich matrix. The upper datedsample is a very fine-grained tuffaceous limolite with incipient laminatedstructure, that topped the upper and most prolific fossiliferous level(diatomitie). Under microscope, it is mostly formed by volcanic ash; abundantdiatoms and sponge spicules were also recognized. Zircon crystals were dated inboth samples in the LA.TE.ANDES (Argentina; U-Pb LA-ICP-MS) and the Boise StateUniversity Isotope Geology Laboratory (USA; U-Pb TIMS) respectively, giving asimilar early Oligocene age (~33.5 Ma).Depositionof these strata occurred coevally to the early to mid stages of the El Maiténvolcanic belt, situated at about 40 km to the west of the new fossiliferouslocality, which activity could be the source of the pyroclastic materials. Thestudied section constitutes the first record of early Oligocene plants innorthern Patagonia and co-occurring with the first Cenozoic glaciation phase(Oi-1); rising the importance of these new northern Patagonian records in orderto achieve a compressive evolution of Patagonian continental biotas through theEocene-Oligocene transition.