The stratigraphic and paleoenvironmental significance of the regressive Monte Observación Member, Early Miocene of the Austral Basin, Patagonia


  • Ana Parras CONICET-Universidad Nacional de La Pampa, Instituto de Ciencias de La Tierra y Ambientales de La Pampa (INCITAP). Facultad de Ciencias Exactas y Naturales, Uruguay 151, 6300 Santa Rosa, La Pampa, Argentina.
  • José I. Cuitiño Instituto Patagónico de Geología y Paleontología (IPGP), CCT CONICET - CENPAT. Boulevard A. Brown 2915, 9120 Puerto Madryn, Chubut, Argentina.

Palabras clave:

Monte León Formation; Sedimentology; Fossil Concentrations; Neogene; Southern South America; Argentina


The stratigraphy of the Oligocene-Miocene southern Patagonia marine deposits has been extensively discussed. However, the Monte Observación Member (MOM) was vaguely defined and many aspects related to its boundaries, distribution, lithology and paleoenvironments of deposition remain uncertain. In this paper we present results obtained from the field survey of several early Miocene outcrops exposed along the Atlantic coast of southern Santa Cruz Province (Austral-Magallanes Basin). They correspond to the transitional interval between the marine shelf Monte León Formation and the terrestrial Santa Cruz Formation. Sedimentological analysis allowed to define six Facies Associations (FAs) for the studied beds: FA-1 corresponds to inner shelf to lower shoreface deposits; FA-2 corresponds to prodelta to distal delta-front deposits; FA-3 corresponds to proximal delta-front deposits; FA-4 corresponds to terminal distributary channels and meandering channels in the delta plain; FA-5 corresponds to inter-distributary bays and tidal flats; and finally FA-6 corresponds to the upper delta plain deposits. In turn, analysis of fossil concentrations allowed defining three main genetic types: biogenic, sedimentologic, and mixed biogenic-sedimentologic concentrations. Biogenic concentrations are represented by beds and lenses of the marginal marine oyster Crassostrea orbignyi (Ihering), which is a significant fossil invertebrate typical of the transitional deposits and is the last invertebrate showing marine influence in all the studied sections and beyond. Sedimentologic and mixed biogenic-sedimentologic concentrations exhibit different attributes and evidence the action of different processes. They include from mostly monospecific within-habitat reworked concentrations, with little post-morten disturbance to polyspecific concentrations bearing the imprint of various taphonomic processes. The distribution of FAs suggest a regressive system that prograded over shelf deposits in the form of deltaic depositional systems with tidal influence. This analysis allowed to define better the MOM, which shows: 1) null to very low bioturbation degrees; 2) a reduced autochthonous invertebrate faunal content; 3) a well-stratified pattern given by the alternating lithologies, and 4) abundant decimetric intercalations of gray to greenish sandstone beds. The fast accumulation of the MOM, which would be part of the regressive phase of the Monte León Formation, could be the consequence of a peak of siliciclastic and volcaniclastic input into the Austral Basin, produced as a result of rapid exhumation of the Southern Patagonian Andes during the early Miocene.


Ameghino, F., 1894. Enumeration synoptique des espèces de mammifères fossiles des formations éocènes de Patagonie. Boletín de la Academia Nacional de Ciencias de Córdoba 13:259-445.

Ameghino, F., 1898. Segundo Censo de la República Argentina (1895), Tomo I Territorio. Capítulo 1, Tercera Parte, Sinópsis Geológico-Paleontológica, 112-255.

Ameghino, F., 1900-1902. L’age des formations sédimentaires de Patagonie. Anales de la Sociedad Científica Argentina vol. 50 (1900): 109-130, 145-165, 209-229; vol. 51 (1901): 20-39, 65- 91; vol. 52 (1901): 189-197, 244-250; vol. 54 (1902): 161-180, 220-249, 283-342.

Ameghino, F., 1906. Les Formations sedimentaires du Crétacé Supérieur et du Tertiaire de Patagonie. Anales del Museo Nacional de Buenos Aires 15 (serie 3, Tomo VIII), Buenos Aires, 568 pp.

Aramendía, I., J.I. Cuitiño, M. Ghiglione and P.J. Bouza, in press. Tectonostratigraphic significance of the Neogene sedimentary record of the northwestern Austral Basin, Argentinean Patagonia. Latin American Journal of Sedimentology and Basin Analysis.

Bahr, L.M. and W.P. Lanier, 1981. The Ecology of Intertidal Oyster Reefs of the South Atlantic Coast: a Community Profile. United States Fish and Wildlife Service, Office of Biological Services, Washington, D.C. FWS/OBS-81/15, 105 pp.

Barreda, V. and S. Palamarczuk, 2000. Palinomorfos continentales y marinos de la Formación Monte León en su área tipo, provincia de Santa Cruz, Argentina. Ameghiniana 37(1): 3-12.

Behrensmeyer, A.K., S.M. Kidwell and R.A. Gastaldo, 2000. Taphonomy and paleobiology. Paleobiology 26(4):103-147.

Bertels, A., 1970. Sobre el “Piso Patagoniano” y la representación de la época del Oligoceno en Patagonia austral, República Argentina. Revista de la Asociación Geológica Argentina 25(4):495-501.

Bertels, A., 1980. Estratigrafía y foraminíferos (Protozoa) bentónicos de la Formación Monte León (Oligoceno) en su área tipo, provincia de Santa Cruz, República Argentina. II Congreso Argentino de Paleontología y Bioestratigrafía y I Congreso Latinoamericano de Paleontología Actas 2:213-273, Buenos Aires (1978).

Bhattacharya, J.P., 2010. Deltas. In N.P. James and R.W. Dalrymple (Eds.), Facies Models 4, GeoText 6, Geological Association of Canada: 233-264.

Bown, T. M. and J. G. Fleagle, 1993. Systematics, biostratigraphy, and dental evolution of the Palaeothentidae, later Oligocene to early-middle Miocene (Deseadan-Santacrucian) caenolestoid marsupials of South America. The Paleontological Society, Memoir 29:1-76.

Buatois, L.A., M.K. Gingras, J. MacEachern, M.G. Mángano, J.P. Zonneveld, S.G. Pemberton, R.G. Netto and A. Martin, 2005. Colonization of brackish-water systems through time: evidence from the trace-fossil record. Palaios 20(4):321-347.

Choi, K.S., R.W. Dalrymple, S.S. Chun and S.P. Kim, 2004. Sedimentology of modern, inclined heterolithic stratification (IHS) in the macrotidal Han River delta, Korea. Journal of Sedimentary Research 74:677-689.

Comité Argentino de Estratigrafía (CAE), 1992. Código Argentino de Estratigrafía. Asociación Geológica Argentina, Serie “B” Didáctica y Complementaria 20, Buenos Aires, 64 pp.

Crawford, R.S., S. Casadío, R.M. Feldmann, M. Griffin, A. Parras and C.E. Schweitzer, 2008. Mass mortality of fossil decapods within the Monte León Formation (early Miocene), southern Argentina: victims of Andean volcanism. Annals of Carnegie Museum 77(2):259-287.

Cuitiño, J.I., M.M. Pimentel, R. Ventura Santos and R.A. Scasso, 2012. High resolution isotopic ages for the “Patagoniense” transgression in southwest Patagonia: stratigraphic implications. Journal of South American Earth Sciences 38:110-122.

Cuitiño, J.I, R. Ventura Santos and R.A. Scasso, 2013. Insights into the distribution of shallow-marine to estuarine early Miocene oysters from southwestern Patagonia: sedimentologic and stable isotope constraints. Palaios 28(9):583-598.

Cuitiño, J.I., J.C. Fernicola, M.J. Kohn, R. Trayler, M. Naipauer, M.S. Bargo, R.F. Kay and S.F. Vizcaíno, 2016. 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. Journal of South American Earth Sciences 70:198- 210.

Dalrymple, R.W. and K. Choi, 2007. Morphologic and facies trends through the fluvial–marine transition in tide dominated depositional systems: A schematic framework for environmental and sequence-stratigraphic interpretation. Earth-Science Reviews 81:135-174.

Dalrymple, R.W., E.K. Baker, P.T. Harris and M.G. Hughes, 2003. Sedimentology and stratigraphy of a tide-dominated, foreland-basin delta (Fly River, Papua New Guinea). In F.H. Sidi, D. Nummedal, P. Imbert, H. Darman and H.W. Posamentier (Eds.), Tropical Deltas of Southeast Asia— Sedimentology, Stratigraphy, and Petroleum Geology. SEPM, Special Publication 76:147-173.

Darwin, Ch., 1839. Narrative of the surveying voyages of His Majesty’s Ships Adventure and Beagle, between the years 1826 and 1836, describing their examination of the southern shores of South America, and the Beagle’s circumnavigation of the globe. Colburn, H. Ed., London, vol. 3, 615 pp.

Darwin, Ch., 1846. Geological observations on South America. Being the third part of the geology of the voyage of the Beagle, under the command of Capt. Fitzroy, R. N. during the years 1832 to 1836. Smith Elder and co Ed., London, 279 pp.

Di Paola, E.C. and H.G. Marchese,1973. Litoestratigrafía de la Formación Patagonia en el área tipo (Bajo de San Julián-desembocadura del río Santa Cruz). Provincia de Santa Cruz. República Argentina. V Congreso Geológico Argentino Actas: 207-222, Córdoba (1972).

Fernicola, J.C., J.I. Cuitiño, S.G. Vizcaíno, M.S. Bargo and R.F. Kay, 2014. Fossil localities of the Santa Cruz Formation (Early Miocene, Patagonia, Argentina) prospected by Carlos Ameghino in 1887 revisited and the location of the Notohippidian. Journal of South American Earth Sciences 52:94-107.

Feruglio, E., 1938. Relaciones estratigráficas entre el Patagoniano y el Santacruciano en la Patagonia Austral. Revista del Museo de La Plata, Tomo 1, Geología 4, 129-159.

Feruglio, E., 1949. Descripción Geológica de La Patagonia. Ministerio de Industria y Comercio de La Nación, Dirección General de Yacimientos Petrolíferos Fiscales, vol. 2, Buenos Aires, 349 pp.

Fleagle, J.G., T.M. Bown, C. Swisher and G. Buckley, 1995. Age of the Pinturas and Santa Cruz Formations. VI Congreso Argentino de Paleontología y Bioestratigrafía Actas: 129-135, Trelew.

Fosdick, J.C., M. Grove, J.K. Hourigan and M. Calderón, 2013. Retroarc deformation and exhumation near the end of the Andes, southern Patagonia. Earth and Planetary Science Letters 361:504–517.

Fürsich, F.T. and W. Oschmann, 1993. Shell beds as tools in basin analysis: the Jurassic of Kachchh, western India. Journal of the Geological Society of London 150:169-185.

Ghiglione, M.C., J. Quinteros, D. Yagupsky, P. Bonillo-Martínez, J. Hlebszevtich, V.A. Ramos, G. Vergani, D. Figueroa, S. Quesada and T. Zapata, 2010. Structure and tectonic history of the foreland basins of southernmost South America. Journal of South American Earth Sciences 29:262-277.

Ghiglione, M.C., M. Naipauer, Ch. Sue, V. Barberón, V. Valencia, B. Aguirre-Urreta and V.A. Ramos, 2015. U-Pb zircon ages from the northern Austral basin and their correlation with the Early Cretaceous exhumation and volcanism of Patagonia. Cretaceous Research 55:116-128.

Ghiglione, M.C., V.A. Ramos, J.I. Cuitiño and V. Barberón, 2016. Growth of the Southern Patagonian Andes (46–53°S) and Their Relation to Subduction Processes. In Folguera et al. (Eds.), Growth of the Southern Andes. Springer Earth System Sciences, Springer International Publishing, Switzerland: 201-240.

Griffin, M. and A. Parras, 2012. Oysters from the base of the Santa Cruz Formation (late Early Miocene) of Patagonia. In S.F. Vizcaíno, R.F. Key and M.S. Bargo (Eds.), Early Miocene Paleobiology in Patagonia: high-latitude paleocommunities of the Santa Cruz Formation. Cambridge University Press, Cambridge: 83-90.

Hatcher, J.B., 1897. On the geology of southern Patagonia. American Journal of Science, Series 4 Vol. 4:327-354.

Hatcher, J.B., 1900. Sedimentary rocks of southern Patagonia. American Journal of Science, Series 4 Vol. 9:85-108.

Hatcher, J.B., 1903. Narrative of the expeditions Geography of Southern Patagonia. In W. B. Scott (Ed.), Reports of the Princeton University Expeditions to Patagonia, 1896-1899. J. Pierpoint Morgan Publication Foundation, Princeton, vol. 1, 314 p.

Hervé, F., R.J. Pankhurst, C.M. Fanning, M. Calderón, and G.M. Yaxley, 2007. The South Patagonian batholith: 150 my of granite magmatism on a plate margin. Lithos 97(3-4): 373-394.

Ihering von, H., 1897. Os Molluscos dos terrenos terciarios da Patagonia. Revista do Museu Paulista, vol. 2: 217-382, pls. 3-9.

Kidwell, S.M. and S.M. Holland, 1991. Field description of coarse bioclastic fabrics. Palaios 6:426-434.

Kidwell, S.M., F.T. Fürsich and T. Aigner, 1986. Conceptual framework for the analysis and classification of fossil concentrations. Palaios 1:228-238.

MacEachern, J.A., K.L Bann, G. Pemberton and M.K. Gingras, 2007. The ichnofacies paradigm: high-resolution paleoenvironmental interpretation of the rock record. In J.A. MacEachern, K.L. Bann, M.K. Gingras and S.G. Pemberton (Eds.), Applied Ichnology. SEPM Short Course Notes 52:27-64.

Martini, I. and F. Sandrelli, 2015. Facies analysis of a Pliocene river dominated deltaic succession (Siena Basin, Italy): Implications for the formation and infilling of terminal distributary channels. Sedimentology 62(1):234-265.

Matheos, S.D. and M.S. Raigemborn, 2012. Sedimentology and paleoenvironments of the Santa Cruz Formation. In S.F. Vizcaíno, R.F. Kay and M.S. Bargo (Eds.), Early Miocene Paleobiology in Patagonia: high-latitude paleocommunities of the Santa Cruz Formation. Cambridge University Press, Cambridge: 59-82.

Náñez, C., M.E. Quattrocchio and L. Ruiz, 2009. Palinología y micropaleontología de las Formaciones San Julián y Monte León (Oligoceno - Mioceno temprano) en el subsuelo de cabo Curioso, provincia de Santa Cruz, Argentina. Ameghiniana 46(4):669-693.

Olariu, C. and J.P. Bhattacharya, 2006. Terminal distributary channels and delta front architecture of river-dominated delta systems. Journal of sedimentary research 76(2):212-233.

Parras, A. and M. Griffin, 2009. Darwin’s Great Patagonian Formation at the mouth of the Santa Cruz River: a reappraisal. In B. Aguirre-Urreta, M. Griffin and V.A. Ramos (Eds.), Darwin in Argentina. Revista de la Asociación Geológica Argentina 64(1):70-82.

Parras, A., G.R. Dix and M. Griffin, 2012. Sr-Isotope chronostratigraphy of Paleogene-Neogene marine deposits: Austral Basin, southern Patagonia (Argentina). Journal of South American Earth Sciences 37:122-135.

Perkins, M.E., J.G. Fleagle, M.T. Heizler, B. Nash, T.M. Bown, A.A. Tauber and M.T. Dozo, 2012. Tephrochronology of the Miocene Santa Cruz and Pinturas Formations, Argentina. In S.F. Vizcaíno, R.F. Kay and M.S. Bargo (Eds.), Early Miocene Paleobiology in Patagonia: high-latitude paleocommunities of the Santa Cruz Formation. Cambridge University Press, Cambridge: 23-40.

Raigemborn, M.S., S.D. Matheos, V. Krapovickas, S.F. Vizcaíno, M.S. Bargo, R.F. Kay, J.C. Fernicola and L. Zapata, 2015. Paleoenvironmental reconstruction of the coastal Monte León and Santa Cruz formations (Early Miocene) at Rincón del Buque, Southern Patagonia: A revisited locality. Journal of South American Earth Sciences 60:31-55.

Raigemborn, M.S., V. Krapovickas, E. Beilinson, L.E.G. Peral, A.F. Zucol, L. Zapata and A.N. Sial, 2018. Multiproxy studies of Early Miocene pedogenic calcretes in the Santa Cruz Formation of southern Patagonia, Argentina indicate the existence of a temperate warm vegetation adapted to a fluctuating water table. Palaeogeography, Palaeoclimatology, Palaeoecology 500:1-23.

Raigemborn, M.S., V. Krapovickas, A.F. Zucol, L. Zapata, E. Beilinson, N. Toledo, J. Perry, S. Lizzoli, L. Martegani and E. Passeggi, in press. Paleosols and related soil-biota of the early Miocene Santa Cruz Formation (Austral-Magallanes Basin, Argentina): a multidisciplinary approach to reconstructing ancient terrestrial landscapes. Latin American Journal of Sedimentology and Basin Analysis.

Riggi, J.C., 1978. La importancia de los sedimentos piroclásticos y de la sílice biogenética en la estratigrafía de la Formación Patagonia. Revista de la Asociación Geológica Argentina 33(2):158-171.

Riggi, J.C., 1979. Nuevo esquema estratigráfico de la Formación Patagonia. Revista de la Asociación Geológica Argentina 34(1):1-11.

Rossi, V.M. and R.J. Steel, 2016. The role of tidal, wave and river currents in the evolution of mixed energy deltas: Example from the Lajas Formation (Argentina). Sedimentology 63(4):824-864.

Russo, A. and M.A. Flores, 1972. Patagonia Austral Extraandina. In A.F. Leanza (Ed.), Geología Regional Argentina. Academia Nacional de Ciencias de Córdoba: 707-725.

Russo, A., M.A. Flores and H. Di Benedetto, 1980. Patagonia Austral Extraandina. In J.C. Turner (Ed.), Geología Regional Argentina. Academia Nacional de Ciencias de Córdoba: 1431- 1462.

Sacomani, L.E., J.L. Panza, H. Pezzuchi, C.Parisi and G. Pichersky, 2012. Hojas Geológicas 5169-I y 5169-II, Puerto Coig y Puerto Santa Cruz, provincia de Santa Cruz. Boletín del Instituto de Geología y Recursos Minerales, Servicio Geológico Minero Argentino 392:1-133.

Scasso, R.A. and J.I. Cuitiño, 2017. Sequential development of tidal ravinement surfaces in macro to hypertidal estuaries with high volcaniclastic input: the Miocene Puerto Madryn Formation (Patagonia, Argentina). Geo-Marine Letters 37:427- 440.

Thomson, S.N., F. Hervé and B. Stöckhert, 2001. Mesozoic- Cenozoic denudation history of the Patagonian Andes (southern Chile) and its correlation to different subduction processes. Tectonics 20(5):693-711.

Vizcaíno, S.F., R.F. Kay and M.S. Bargo, 2012. Early Miocene Paleobiology in Patagonia: High-latitude Paleocommunities of the Santa Cruz Formation. Cambridge University Press, Cambridge, 378 pp.

Walker, R.G., 1992. Facies, facies models and modern stratigraphic concepts. In R.G. Walker and N.P. James (Eds.), Facies Models, response to sea-level change. Geological Association of Canada: 1-14.

Wells, H.W., 1961. The fauna of oyster beds, with special reference to the salinity factor. Ecological Monographs 31(3):239-266.




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Parras , A. ., & Cuitiño, J. I. . (2021). The stratigraphic and paleoenvironmental significance of the regressive Monte Observación Member, Early Miocene of the Austral Basin, Patagonia. Latin American Journal of Sedimentology and Basin Analysis, 25(2), 93-115. Recuperado a partir de



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