Lithofacies, architecture, and depositional environments of the Santa Cruz Formation in Chilean Patagonia Raúl Ugalde 1 *, Enrique Bostelmann 2 , Karina E. Buldrini 2,3 , and José Luis Oyarzún 4 1: PEDECIBA Geociencias, Facultad de Ciencias, Universidad de la República, Iguá 4225, Montevideo, Uruguay. 2: Red Paleontológica U-Chile, Laboratorio de Ontogenia y Filogenia, Departamento de Biología, Facultad de Ciencias, Universidad de Chile, Santiago, Chile. 3: Laboratorio de Zoología de Vertebrados, Facultad de Ciencias, Universidad de Chile, Santiago, Chile. 4: Parque Geopaleontológico La Cumbre-Baguales, Ruta 9 km 284, Magallanes, Chile. * email: raugaldep@gmail.com Abstract. Early Miocene fluvio-lacustrine deposits are widespread throughout Patagonia. In Chile, remarkable but isolated outcrops of the Santa Cruz Formation include the highest plains on the Meseta Cosmelli in Aysén, and the Sierra Baguales in Magallanes. At both areas, stratigraphic sections were measure and fossil content recorded. The Pampa Guadal profile at Meseta Cosmelli includes the formation’s lower packages, which consists of fine-grained mudstones and sandstones, interbedded with subordinate conglomeratic horizons. Abundant and well-preserved fossil mammals are present in the upper half of the section. The strata characterize a fluvial environment, with thick floodplains and restricted gravel to coarse sand channels, recording a change from an anastomosing to a meandering river system. The Morro Bayo profile at Sierra Baguales begins with sedimentological aggregates of the estuarine Patagonian oyster Crassostrea orbignyi, followed by mudstone and fine sandstone levels, interbedded with subordinate coarse-grained sandstone. Clastic sedimentary structures, abundant fossil mammals, and floodplain facies characterize the top of the succession, indicating a transition from a low energy estuarine system to a meandering river, with wedged bars. Both localities represent a variable energy fluvial environment with high sedimentary input, similar to those described for the westernmost exposures of the Santa Cruz Formation in Argentinean Patagonia. Key word: Santa Cruz, Fluvial, Lithofacies, Early Miocene, Aysén, Magallanes, Chilean Patagonia. 1 Introduction The Santa Cruz Formation is an extensive early Miocene siliciclastic fluvial unit of the Magallanes-Austral basin, initially described by Florentino Ameghino based on field observations gathered by his brother Carlos during successive expeditions in the homonymous Argentinean province (Ameghino, 1906). Its abundant fossil content, mostly mammals, had made it a well-known study unit since the mid-19th century (Vizcaíno et al., 2012). In Chile, the formation has been recognized in two major areas: the highlands of the Meseta Cosmelli immediately south of Lake General Carrera, Aysén, and the northern valleys of Sierra Baguales, in Última Esperanza province, Magallanes (Figure 1). At both localities initial investigations were performed during the middle to late 90’s (Flint et al., 1994; Flynn et al., 2002), followed by later work in the last decade (De la Cruz and Suárez, 2006; Bostelmann and Buldrini, 2012; Bostelmann et al., 2013). In spite of its relevance, most of these studies did not address in further detail the lithostratigraphic and depositional attributes of the units. This study contributes new lithofacies analysis and depositional architecture of two relevant outcrops at each locality: the Pampa Guadal in Meseta Cosmelli, and the Morro Bayo section at Sierra Baguales. Figure 1. Locations of the study areas in the Chilean Patagonia. 1: Pampa Guadal, Meseta Cosmelli, General Carrera province, Aysén. 2: Morro Bayo, Sierra Baguales, Última Esperanza province, Magallanes. 1.1 Stratigraphic context At Pampa Guadal, the Santa Cruz Formation overlies the Guadal Formation, an epiclastic shallow marine unit deposited during the early Miocene “Patagonian” transgression (Bostelmann et al., this congress). Estimated thickness of the outcrops measured from the San Martín River is >700 m, composing the nucleus of the Las Dunas syncline (De la Cruz and Suárez, 2006). In Sierra Baguales, the Santa Cruz Formation transitionally overlies the Estancia 25 de Mayo Formation (Cuitiño and Scasso, 2010), which also represents the local expression of the “Patagonian” Sea in southwestern Santa Cruz province. The upper contact of the Santa Cruz Formation is