José I. Cuitiño

Fossil pollen records indicate that Patagonian desertification was not solely a consequence of Andean uplift

The Patagonian steppe-a massive rain-shadow on the lee side of the southern Andes-is assumed to have evolved ~15-12 Myr as a consequence of the southern Andean uplift. However, fossil evidence supporting this assumption is limited. Here we quantitatively estimate climatic conditions and plant richness for the interval ~10-6 Myr based on the study and bioclimatic analysis of terrestrially derived spore-pollen assemblages preserved in well-constrained Patagonian marine deposits. Our analyses indicate a mesothermal climate, with mean temperatures of the coldest quarter between 11.4 °C and 16.9 °C (presently ~3.5 °C) and annual precipitation rarely below 661 mm (presently ~200 mm). Rarefied richness reveals a significantly more diverse flora during the late Miocene than today at the same latitude but comparable with that approximately 2,000 km further northeast at mid-latitudes on the Brazilian coast. We infer that the Patagonian desertification was not solely a consequence of the Andean uplift as previously insinuated.

Insights into the distribution of shallow-marine to estuarine early Miocene oysters from southwestern Patagonia: Sedimentologic and stable isotope constraints

ABSTRACT The lower Miocene Estancia 25 de Mayo Formation consists of shallow-marine to estuarine deposits that bear numerous oyster shell beds distributed throughout the succession. Facies analysis reveals that oysters grew in the nearshore paleoenvironments of both the lower Quién Sabe and the upper Bandurrias members. Two oyster species were identified: Crassostrea(?) hatcheri, distributed in the lower two thirds of the column, and Crassostrea orbignyi, distributed in the upper third of the column within the transition from marine to the fluvial deposits of the overlying Santa Cruz Formation. Petrographic, cathodoluminiscence, and carbon and oxygen stable isotope analyses of individual growth increments were performed on carbonate from all the oyster beds, together with consecutive time-series analyses for one specimen of each species. The isotopic composition of the shell seems to be associated with the microstructure of the growth increments. Selective diagenetic alteration affected chalky growth increments; whereas translucent (foliated and prismatic) growth increments are well preserved and can be used to infer paleoenvironmental conditions. Isotopic data indicate that C.(?) hatcheri lived in normal marine waters with a range of paleotemperatures from 10.6 to 20.5 °C. Monospecific beds of this oyster are related to opportunistic and quick colonization of the sea bottom. More negative &dgr;13C and &dgr;18O values in C. orbignyi shells likely reflect decreased paleosalinities, and the resultant environmental stress controlled the development of these low-diversity oyster accumulations. The separate stratigraphic distribution of each species was therefore determined by the paleoenvironmental conditions.

Growth of the Southern Patagonian Andes (46–53°S) and Their Relation to Subduction Processes

The Cretaceous-Cenozoic evolution of the Southern Patagonian Andes is one of the most prominent examples of coupling between subduction processes and climatic, magmatic, deformational, and sedimentary events. Three orogenic and magmatic cycles can be particularly related to processes in the subduction zone (1) Late Cretaceous closure of the Rocas Verdes marginal basin, (2) Paleogene collision of the Farallon-Aluk seismic ocean ridge, and (3) Miocene subduction of the Chile seismic ridge beneath South America. Andean orogenic growth started during Late Cretaceous times, in a tectonic scenario that included the Rocas Verdes back-arc oceanic basin, widening from 49°S toward the south. The Andean segment south of 49°S experienced a strong Cenomanian–Santonian deformational event during the closure of the back-arc basin and progressive subduction of its ocean floor. The final closure produced the Coniacian–Santonian exhumation of the Sarmiento Ophiolitic Complex and propagation of the orogenic front toward the foreland. The second cycle, during Paleogene deformation, coincided with an Eocene volcanic arc gap, and seems to be related to the Fallaron-Phoenix seismic ridge collision. The resulting slab window produced OIB volcanic plateaux represented by the Chile Chico and Posadas Basalts, erupted in the foothills and retroarc. The third cycle of accelerated Andean uplift started during the Oligocene, as a consequence of orthogonal and fast subduction of young lithosphere, while the Chile seismic ridge between Antarctica and Nazca was approaching the trench. Kinematic plate reconstructions show that at approximately 14–18 Ma the Chile oceanic ridge entered the South America trench and migrated northward from 53°S to its present-day position at 46°S. The early Miocene ridge collision and resulting slab window produced an extensive OIB magmatism between 10 and 3 Ma in extra-Andean Patagonia. The space-time unraveling of tectonic uplift is well known from geochronometers, and shows a migration from the basement domain to the external fold and thrust belt accompanied by lower Miocene synorogenic sedimentation. Orogenic growth led to middle Miocene rain shadow in the foothills, followed by the late Miocene—Pliocene desertification of Patagonia.

Miocene Marine Transgressions: Paleoenvironments and Paleobiodiversity

Two major marine transgressions covered part of Patagonia during the Miocene and both are recorded in the Peninsula Valdes region. The older (early Miocene) is represented by the volumetrically scarce outcrops of the Gaiman Formation, composed by shelf mudstones and fine sandstones. The late Miocene transgression is represented by the Puerto Madryn Formation, widely distributed in Peninsula Valdes and composed of mudstones, sandstones and shell beds, being the focus of this work. Sediments of this unit were deposited in inner shelf, nearshore, tidal channel and tidal flat environments. Fossil content is abundant and diverse, including palynomorphs, foraminifers, marine invertebrates (dominated by molluscs), cetaceans, pinnipeds, marine fishes and birds, as well as continental mammals, birds, and fishes. Isotopic and biostratigraphical data suggest a late Miocene age for the Puerto Madryn Formation, although some middle Miocene biostratigraphical indicators are present. Paleoenvironmental information suggests oceanic and continental temperatures warmer than present day, evidenced by the Caribbean molluscan association and the continental vertebrate and palinological associations, respectively. Instead, cetaceans, dinoflagellates, and some marine fishes, suggest colder oceanic temperatures. Precipitations were also higher than present, evidenced by the presence of freshwater mammals, birds, fishes, and plants. This work highlighted some gaps in the geological and paleontological knowledge including geochronology, stratigraphic control of paleontological studies and the knowledge of poorly known fossil groups, which should be the focus of future investigations.

The Late Oligocene–Early Miocene Marine Transgression of Patagonia

The most important Cenozoic marine transgression in Patagonia occurred during the late Oligocene–early Miocene when marine waters of Pacific and Atlantic origin flooded most of southern South America including the present Patagonian Andes between ~41° and 47° S. The age, correlation, and tectonic setting of the different marine formations deposited during this period are debated. However, recent studies based principally on U–Pb geochronology and Sr isotope stratigraphy, indicate that all of these units had accumulated during the late Oligocene–early Miocene. The marine transgression flooded a vast part of southern South America and, according to paleontological data, probably allowed for the first time in the history of this area a transient connection between the Pacific and Atlantic oceans. Marine deposition started in the late Oligocene–earliest Miocene (~26–23 Ma) and was probably caused by a regional event of extension related to major plate reorganization in the Southeast Pacific. Progressive extension and crustal thinning allowed a generalized marine flooding of Patagonia that reached its maximum extension at ~20 Ma. It was followed by a phase of compressive tectonics that started around 19–16 Ma and led to the growth of the Patagonian Andes. The youngest (~19–15 Ma) marine deposits that accumulated in the eastern Andean Cordillera and the extra-Andean regions are coeval with fluvial synorogenic deposits and probably had accumulated under a compressive regime.

The early Miocene balaenid Morenocetus parvus from Patagonia (Argentina) and the evolution of right whales

Balaenidae (right and bowhead whales) are a key group in understanding baleen whale evolution, because they are the oldest surviving lineage of crown Mysticeti, with a fossil record that dates back ∼20 million years. However, this record is mostly Pliocene and younger, with most of the Miocene history of the clade remaining practically unknown. The earliest recognized balaenid is the early Miocene Morenocetus parvus Cabrera, 1926 from Argentina. M. parvus was originally briefly described from two incomplete crania, a mandible and some cervical vertebrae collected from the lower Miocene Gaiman Formation of Patagonia. Since then it has not been revised, thus remaining a frequently cited yet enigmatic fossil cetacean with great potential for shedding light on the early history of crown Mysticeti. Here we provide a detailed morphological description of this taxon and revisit its phylogenetic position. The phylogenetic analysis recovered the middle Miocene Peripolocetus as the earliest diverging balaenid, and Morenocetus as the sister taxon of all other balaenids. The analysis of cranial and periotic morphology of Morenocetus suggest that some of the specialized morphological traits of modern balaenids were acquired by the early Miocene and have remained essentially unchanged up to the present. Throughout balaenid evolution, morphological changes in skull arching and ventral displacement of the orbits appear to be coupled and functionally linked to mitigating a reduction of the field of vision. The body length of Morenocetus and other extinct balaenids was estimated and the evolution of body size in Balaenidae was reconstructed. Optimization of body length on our phylogeny of Balaenidae suggests that the primitive condition was a relatively small body length represented by Morenocetus, and that gigantism has been acquired independently at least twice (in Balaena mysticetus and Eubalaena spp.), with the earliest occurrence of this trait in the late Miocene–early Pliocene as represented by Eubalaena shinshuensis.

Sequential development of tidal ravinement surfaces in macro- to hypertidal estuaries with high volcaniclastic input: the Miocene Puerto Madryn Formation (Patagonia, Argentina)

The late Miocene beds of the Puerto Madryn Formation (Provincia del Chubut, Argentina) are formed by shallow marine and estuarine sediments. The latter include several tidal-channel infills well exposed on the cliffy coast of the Peninsula Valdés. The Bahía Punta Fósil and Cerro Olazábal paleochannels are end members of these tidal channels and show a fining-upward infilling starting with intraformational channel lag conglomerates above deeply erosional surfaces interpreted as fluvial ravinement surfaces (the erosion surface formed in the purely fluvial or the fluvially dominated part of the estuary, where erosion is driven by fluvial processes). These are overlain and eventually truncated (and suppressed) by the tidal ravinement surface (TRS), in turn covered with high-energy, bioclastic conglomerates mostly formed in the “tidally dominated/fluvially influenced” part of an estuary. Above, large straight or arcuate point bars with alternatively sandy/muddy seasonal beds and varying trace and body fossil contents were deposited from the freshwater fluvially dominated to saline-water tidally dominated part of the estuary. The upper channel infill is formed by cross-bedded sands with mud drapes and seaward-directed paleocurrents, together with barren, volcaniclastic sandy to muddy heterolithic seasonal rhythmites, both deposited in the fluvially dominated part of the estuary. Volcanic ash driven by the rivers after large explosive volcanic eruptions on land resulted in sedimentation rates as high as 0.9 m per year, preserving (through burial) the morphology of tidal channels and TRSs. The channel deposits were formed in a tide-dominated, macrotidal to hypertidal open estuary with well-developed TRSs resulting from strong tidal currents deeply scouring into the transgressive filling of the channels and eventually cutting the fluvial ravinement surface. The TRSs extended upstream to the inner part of the estuary during long periods of low sedimentation rates, extended channel migration and sediment bypass, interrupted by transient, high volcaniclastic input. The tidal channels of the Puerto Madryn Formation constitute a unique example of estuary sedimentation with pulsed sediment supply in a macrotidal to hypertidal estuary.

Climatic, Tectonic, Eustatic, and Volcanic Controls on the Stratigraphic Record of Península Valdés

The Peninsula Valdes region is situated in an intraplate position of the South American Plate, in the Patagonian foreland close to the Argentine Continental shelf. This region has a complex geotectonic evolution that started more than 400 Ma and involves the conformation of Northern Patagonia as a part of Gondwana during the Paleozoic, the opening of the Atlantic Ocean during the Mesozoic and the configuration of the Andean margin during the Cenozoic. At different scales, the interplay between climate, tectonic, sea-level, and volcanic processes, set the sedimentary routing system that had governed the final geologic records of the Peninsula Valdes region and control the transfer of terrigenous sediments from source to sink. The stratigraphic record of the region was not only influenced by local factors. Processes developed far away from Peninsula Valdes, both in the Southern Andes or in the continental shelf had influenced the late Cenozoic record of this region.

Selected contributions from the 9th International Conference on Tidal Sedimentology, November 2015, Puerto Madryn, Patagonia, Argentina: an introduction

This special issue of Geo-Marine Letters presents selected contributions from the 9th International Conference on Tidal Sedimentology held on 17–19 November 2015 in Puerto Madryn, Chubut Province, Patagonia, Argentina. The guest editors are the conference organizers Roberto A. Scasso and José I. Cuitiño. Gerardo M. Perillo was the head of the Scientific Committee. The conferences on tidal sedimentology have been traditionally held every 4 years. However, only 3 years separated the last conference held in Caen (France, 2012) from this conference. Increasing numbers of contributions and the growing interest in tidal sedimentation have been the reasons for shortening the inter-conference period. The 2015 conference served as a discussion forum focusing on advances in modern and ancient tidal sedimentation at different locations worldwide. The papers presented in this Special Issue provide a selective view of the latest research results, the main topics dealing with tidal hydrodynamics and sediment transport, tidal coastal morphodynamics, modern and ancient tidal sedimentation, geotechnical processes in tidal environments, and tidal basins, facies and reservoirs.