Sedimentology and depositional setting of the Sarmiento Formation (Eocene-Miocene) in Central Patagonia

BELLOSI, E.S.

La Plata

Congreso; XV Congreso Geológico Argentino; 2005

Asociación Geológica Argentina

ABSTRACT The Sarmiento Formation (Middle Eocene – Lower Miocene) is a pyroclastic continental succession, mostly known because of its fossil mammal assemblages. At the type locality Gran Barranca (Spalletti and Mazzoni, 1979), located in south-central Chubut province, this unit is now divided into five members, bounded by unconformities and subordinate discontinuity surfaces (Bellosi and Genise, 2004). The Gran Barranca Member (Middle Eocene) is a 76 m thick unit of poorly stratified, thick to very-thick, even and parallel-bedded fine-grained tuffaceous mudstones (chonites) and bentonitic claystones. Silica and gypsum concretions are common in the lower portion of this member. Paleosols are not abundant and weakly to moderately developed. Sediment supply was dominantly eolian and the prevailing sedimentation was subaerial on extensive plains. Bentonites and some pedofeatures of poorly-drained conditions, suggest the existence of ponded areas, probably ephemeral. The thin Rosado Member (late Middle Eocene) is a massive pedogenic calcrete, bearing abundant and diverse land snails and a low diversity and peculiar ichnofauna. It records the dryest period during Sarmiento time (semi-arid to arid). The Puesto Almendra Member is an 80 m thick, well-stratified unit comprising lenticular and tabular-bedded tuffaceous sediments. It can be divided into two sections; the Lower section (latest Middle to Late Eocene) includes cross-bedded tuffaceous sandstones, chonites, bentonitic claystones and moderately developed paleosols. The Upper Section (Early Oligocene to Early Miocene ?) was deposited after an episode of very deep fluvial erosion and is characterized by lenticular channel-fill bodies of tuffaceous sandstones and intraformational conglomerates, tuffaceous muds, bentonites, lenticular basalt flows, and stacked paleosols. A mixed sedimentation prevailed during the Puesto Almendra time. Fluvial sedimentation was restricted to small and sinuous channels, which reworked pyroclastic and soil materials. The intercalated eolian episodes resulted in volcanic ash accumulations on plains. Both fluvial and eolian deposits were modified by pedogenesis. The lava flows filled fluvial channels and the subsequent intra-formational erosion caused local inversion of topography. The Vera Member (latest Eocene – Early Oligocene) is a 110 m thick, very poorly-stratified succession of massive chonites of relatively fresh volcanic ash. This unit was not originally recognized in the type profile (Profile MMZ), and is intercalated between the lower and upper sections of the Puesto Almendra Member. It records the continuous accumulation of distal fine-grained volcaniclastic material by eolian processes (loessic sedimentation), interrupted by occasional and very short pedogenic events, which originated paleosols bearing calcareous nodules and small root traces. The Colhue Huapi Member (Early Miocene) is a 64 m thick succession of tuffaceous sediments that represents the infill of an incised paleovalley in the ancient coastal plain of central Patagonia. The deep erosion at its base was related to a significant sea level fall. To the east, this event also produced intense erosion in the contemporaneous Lower Miocene marine Chenque Formation. The Colhue-Huapi Member comprises a succession of fining-upward fluvial cycles punctuated by air-fall tuffs. The lower section includes intraformational conglomerates and cross-bedded pyroclastic sandstones. The middle section has better developed paleosols and is very rich in fossil mammals. The upper section includes thick and massive beds of chonites and tuffaceous paleosols with carbonate nodules. Accumulation initially occurred in fluvial channels and on pedogenized floodplains. The last stage would have been loessic, suggesting drier conditions. Gran Barranca (Spalletti and Mazzoni, 1979), located in south-central Chubut province, this unit is now divided into five members, bounded by unconformities and subordinate discontinuity surfaces (Bellosi and Genise, 2004). The Gran Barranca Member (Middle Eocene) is a 76 m thick unit of poorly stratified, thick to very-thick, even and parallel-bedded fine-grained tuffaceous mudstones (chonites) and bentonitic claystones. Silica and gypsum concretions are common in the lower portion of this member. Paleosols are not abundant and weakly to moderately developed. Sediment supply was dominantly eolian and the prevailing sedimentation was subaerial on extensive plains. Bentonites and some pedofeatures of poorly-drained conditions, suggest the existence of ponded areas, probably ephemeral. The thin Rosado Member (late Middle Eocene) is a massive pedogenic calcrete, bearing abundant and diverse land snails and a low diversity and peculiar ichnofauna. It records the dryest period during Sarmiento time (semi-arid to arid). The Puesto Almendra Member is an 80 m thick, well-stratified unit comprising lenticular and tabular-bedded tuffaceous sediments. It can be divided into two sections; the Lower section (latest Middle to Late Eocene) includes cross-bedded tuffaceous sandstones, chonites, bentonitic claystones and moderately developed paleosols. The Upper Section (Early Oligocene to Early Miocene ?) was deposited after an episode of very deep fluvial erosion and is characterized by lenticular channel-fill bodies of tuffaceous sandstones and intraformational conglomerates, tuffaceous muds, bentonites, lenticular basalt flows, and stacked paleosols. A mixed sedimentation prevailed during the Puesto Almendra time. Fluvial sedimentation was restricted to small and sinuous channels, which reworked pyroclastic and soil materials. The intercalated eolian episodes resulted in volcanic ash accumulations on plains. Both fluvial and eolian deposits were modified by pedogenesis. The lava flows filled fluvial channels and the subsequent intra-formational erosion caused local inversion of topography. The Vera Member (latest Eocene – Early Oligocene) is a 110 m thick, very poorly-stratified succession of massive chonites of relatively fresh volcanic ash. This unit was not originally recognized in the type profile (Profile MMZ), and is intercalated between the lower and upper sections of the Puesto Almendra Member. It records the continuous accumulation of distal fine-grained volcaniclastic material by eolian processes (loessic sedimentation), interrupted by occasional and very short pedogenic events, which originated paleosols bearing calcareous nodules and small root traces. The Colhue Huapi Member (Early Miocene) is a 64 m thick succession of tuffaceous sediments that represents the infill of an incised paleovalley in the ancient coastal plain of central Patagonia. The deep erosion at its base was related to a significant sea level fall. To the east, this event also produced intense erosion in the contemporaneous Lower Miocene marine Chenque Formation. The Colhue-Huapi Member comprises a succession of fining-upward fluvial cycles punctuated by air-fall tuffs. The lower section includes intraformational conglomerates and cross-bedded pyroclastic sandstones. The middle section has better developed paleosols and is very rich in fossil mammals. The upper section includes thick and massive beds of chonites and tuffaceous paleosols with carbonate nodules. Accumulation initially occurred in fluvial channels and on pedogenized floodplains. The last stage would have been loessic, suggesting drier conditions.