Paleoenvironmental reconstruction of the coastal Monte Léon and Santa Cruz formations (Early Miocene) at Rincón del Buque, Southern Patagonia: A revisited locality

Abstract

a b s t r a c tSedimentological, ichnological and paleontological analyses of the Early Miocene uppermost Monte Le on Formation and the lower part of the Santa Cruz Formation were carried out in Rinc on del Buque (RDB), a fossiliferous locality north of Río Coyle in Santa Cruz Province, Patagonia, Argentina. This locality is of special importance because it contains the basal contact between the Monte L eon (MLF) and the Santa Cruz (SCF) formations and because it preserves a rich fossil assemblage of marine invertebrates and marine trace fossils, and terrestrial vertebrates and plants, which has not been extensively studied. Ã 90 m-thick section of the MLF and the SCF that crops out at RDB was selected for this study. Eleven facies associations (FA) are described, which are, from base to top: subtidaleintertidal deposits with Crassotrea orbignyi and bioturbation of the Skolithos-Cruziana ichnofacies (FA1); tidal creek deposits with terrestrial fossil mammals and Ophiomorpha isp. burrows (FA2); tidal flat deposits with Glossifungites ichnofacies (FA3); deposits of tidal channels (FA4) and tidal sand flats (FA5) both with and impoverish Skolithos ichnofacies associated; marsh deposits (FA6); tidal point bar deposits recording a depauperate mixture of both the Skolithos and Cruziana ichnofacies (FA7); fluvial channel deposits (FA8); fluvial point bar deposits (FA9); floodplain deposits (FA10); and pyroclastic and volcaniclastic deposits of the floodplain where terrestrial fossil mammal remains occur (FA11).The transition of the MLFeSCF at RDB reflects a changing depositional environment from the outer part of an estuary (FA1) through the central (FA2e6) to inner part of a tide-dominated estuary (FA7). Finally a fluvial system occurs with single channels of relatively low energy and low sinuosity enclosed by a broad, low-energy floodplain dominated by partially edaphized ash-fall, sheet-flood, and overbank deposits (FA8e11). Pyroclastic and volcaniclastic materials throughout the succession must have been deposited as ash-fall distal facies in a fluvial setting and also were carried by fluvial streams and redeposited in both estuarine and fluvial settings. These materials preserve most of the analyzed terrestrial fossil mammals that characterize the Santacrucian age of the RDB's succession. Episodic sedimentation under volcanic influence, high sedimentation rates and a relatively warm and seasonal climate are inferred for the MLF and SCF section.Lateral continuity of the marker horizons at RDB serve for correlation with other coastal localities such as the lower part of the coastal SCF south of Río Coyle (~17.6e17.4 Ma) belonging to the Estancia La Costa Member of the SCF.

U-Pb geochronology of the Santa Cruz Formation (early Miocene) at the Río Bote and Río Santa Cruz (southernmost Patagonia, Argentina): Implications for the correlation of fossil vertebrate localities

“…dated the lowermost part of the SCF in Monte León at 17.71 Ma,which suggests no hiatus between deposition of the MLF and SCF. Sedimentological evidence shows a continuous transition between the sedimentary units (Feruglio, 1938;Raigemborn et al, 2015 (Fig. 1c).…”

“…Based on lithological differences, he divided the SCF into two members: a lower member (later called the Estancia La Costa Member, Tauber, 1997) dominated by fine-grained pyroclastic and epiclastic beds; and an upper member (later called the Estancia La Angelina Member, Tauber, 1997), which is dominated by epiclastic mudstones and sandstones. Recently, Matheos and Raigemborn (2012) and Raigemborn et al (2015) performed detailed sedimentologic analysis for the lower part of the SCF and its transition to the underlying MLF at the coastal localities south and north of Río Coyle, respectively. They interpreted the lower SCF beds as a low-energy, floodplaindominated fluvial system affected by pyroclastic input with poorly developed paleosols.…”

Stratigraphy and Depositional Environments of the Santa Cruz Formation (Early-Middle Miocene) Along the Río Santa Cruz, Southern Patagonia, Argentina

“…Several good exposures occur in the southern of the Province of Santa Cruz (Fig. 1.2), including the Río Chalía (= Sheuhen) (Vizcaíno et al, 2018) and the coastal zone between the Monte León National Park and the Río Gallegos estuary (Bown and Fleagle, 1993;Tauber, 1999;Matheos and Raigemborn, 2012;Raigemborn et al, 2015Raigemborn et al, , 2018aZapata, 2018). The southernmost counterpart for the SCF is the Cullen Formation, located ~300 km to the southeast in the northern part of the Tierra del Fuego Island (Olivero et al, 2015;Bargo et al, 2018).…”

Estratigrafía de Sr y evolución sedimentaria de los depósitos marinos del Mioceno temprano en el antepaís del norte de la Cuenca Austral (o Magallanes), Argentina.

“…Because the limit between both units is not clearly defined in the field, in this study we place it arbitrarily at the uppermost oyster bank present in the early Miocene marine-estuarine succession. This criterion was found useful to define the equivalent transitional passage in other regions of southern Patagonia (Cuitiño and Scasso, 2010;Cuitiño et al, 2012;Raigemborn et al, 2015). This criterion for the position of the limit implies that the lowermost part of the Santa Cruz Formation may be composed of oyster-free estuarine deposits.…”