Héctor A. Leanza

Las principales discordancias del Mesozoico de la Cuenca Neuquina según observaciones de superficie

The main unconformities of the Mesozoic of the Neuquen Basin based on surface observations. A general summary of the main unconformities which produced major order break-ups during the evolution of the Mesozoic sedimentary infill of the Neuquen Basin is offered in this paper. Based on its hierarchy, they are categorized in three groups, as follows: 1) key unconformities widely spread along the whole basin; 2) «Intracuyan» unconformities related to the Dorsal de Huincul, and 3) «Intraprecuyan» unconformities restricted to isolated depocentres. Having into account tectonic, eustatic, paleoclimatic and/or paleontological aspects of the sedimentites situated below- and above of each unconformity, precisions about their ages are given. The number of the recognized key unconformities sums 10, as follows: Huarpican (Intratriassic), Rioatuelican (Intraliassic), Lotenican (Intracallovian), Araucanican (ca. Oxfordian/Kimmeridgian boundary), Huncalican (Intravalanginian), Coihuequican (Intrahauterivian), Pampatrilican (Intrabarremian), Pichineuquenican (Intraaptian), Patagonidican (ca. Albian/Cenomanian boundary) and Huantraiquican (Intracampanian). In relationship with the area realated with the Dorsal de Huincul, three «intracuyan» unconformities were identified, as follows: the Intratoarcian, only recognized at subsurface, the Seguelican (Intrabajocian 1) and the Lohanmahuidican (Intrabajocian 2). However, the influence of these unconformities diminish towards northern areas of the basin, as the distance increase with regard to the cited morphostructural element. The «intraprecuyan» unconformities recorded in the Cordillera del Viento and Chachil depocentres are of local character and less hierarchy. They are the result of short interruptions in the volcano-sedimentary infill of these halfgrabens. Given its geographical restriction to the cited depocentres, no formal names were ascribed to these unconformities. Once defined the key unconformities, a summary in three tables of genetically related formational units bounded by them, using the classical lithostratigraphic categorization in Groups, the latter ones basically coincident with the concept of tectosedimentary units (TSU), is presented. The obtained results lead to suggest some new ideas with regard to the evolution of the Neuquen Basin and to keep attention on still open nomenclatural problems.

Igneous sill and finger emplacement mechanism in shale-dominated formations: a field study at Cuesta del Chihuido, Neuquén Basin, Argentina

Seismic reflection data and field observations have revealed the presence of voluminous igneous sill complexes emplaced into organic-rich shale formations in sedimentary basins worldwide. Damage and structures associated with sills have major implications for fluid flow through basins. Constraining the distribution of these structures requires a good understanding of the sill emplacement mechanism. However, most mechanical models of sill emplacement assume elastic host behaviour, whereas shale is expected to deform inelastically. This contradiction calls for new field observations to better constrain sill emplacement mechanisms. In this paper, we report on detailed field observations of spectacularly exposed fingers and a sill emplaced in shale at Cuesta del Chihuido, in the Neuquén Basin, Argentina. Exceptional outcrop conditions allow detailed descriptions of both (1) the entire cross-section of the intrusions, and (2) the deformation structures accommodating intrusion propagation in the host rock. All intrusions exhibit irregular, blunt or rectangular tips. The structures accommodating the tip propagation are systematically compressional, including reverse faults, folding and imbricate thrust system. Our observations suggest that the studied intrusions have propagated by pushing the host rock ahead, as a viscous indenter. Our observations suggest that the viscous indenter model is probably a dominant mechanism of sill emplacement in shale.

Control of strike-slip fault on dyke emplacement and morphology

Strike-slip faults are commonly assumed to influence magma transport and emplacement in the Earths crust. However, direct observations of magma conduits within strike-slip faults are lacking. Here we provide some of the first detailed field observations of dykes emplaced within strike-slip faults in the Neuquén Basin, Argentina. We show how fault planes within strike-slip fault zones affect the emplacement of dyke offshoots, resulting in complex dyke morphology. Our study also emphasizes the importance of pre-existing strike-slip fault array on the development of dyke swarms, showing that orientations of dyke swarms may not systematically relate to the principal tectonic stress axes.

Structure and evolution of volcanic plumbing systems in fold-and-thrust belts: A case study of the Cerro Negro de Tricao Malal, Neuquén Province, Argentina

Magma ascent and emplacement in compressional tectonic settings remain poorly understood. Geophysical studies show that volcanic plumbing systems in compressional environments are vertically partitioned into a deep level subject to regional compression and a shallow level subject to local extension. Such vertical partitioning has also been documented for the plumbing systems of mud volcanoes, implying common, yet unresolved, underlying processes. In order to better constrain the mechanisms governing this depth partitioning of emplacement mechanisms, we studied the structure and evolution of the Cerro Negro intrusive complex emplaced in the Chos Malal fold-and-thrust belt in the foothills of the Neuquen Andes, Argentina. The Cerro Negro intrusive complex consists of sills and N-S–striking dikes that crosscut the sills. The most prominent structures in the study area are N-S–trending folds, and both E- and W-vergent thrusts. We provide new U-Pb ages of 11.63 ± 0.20 Ma and 11.58 ± 0.18 Ma for sills and 11.55 ± 0.06 Ma for a dike, which show that the Cerro Negro intrusive complex was emplaced in a short period of time. Our ages and field observations demonstrate that the emplacement of the Cerro Negro intrusive complex was coeval with the tectonic development of the Chos Malal fold-and-thrust belt. This implies that the dikes were emplaced perpendicular to the main shortening direction. The systematic locations of the dikes at the anticlinal hinges suggest that their emplacement was controlled by local, shallow stresses related to outer-arc stretching at the anticlinal hinge. We conclude that folding-related outer-arc stretching is one mechanism responsible for the vertical partitioning of igneous plumbing systems in compressional tectonic settings.

Facies de patch reefs en la Formación Picún Leufú (límite Jurásico/Cretácico) en la región de Zapala, Cuenca Neuquina

Patch reefs facies in the Picun Leufu Formation (Jurassic/Cretaceous boundary) in the Zapala region, Neuquen Basin, Argentina. In the locality of Cerrito Caracoles, situated in the nearby of Zapala city, in the western central region of the Neuquen province, Argentina, the basal part of the Picun Leufu Formation (Jurassic/Cretaceous boundary) crops out. The facies and microfacies association analysis points out to an inner shelf margin environment, with development of well preserved patch reefs. The description of these bodies (coral and algae bafflestone, bryozoan bafflestone and algal boundstone), and of a low relief mound reef (skeletal algal packstone) are presented in this paper. The associated facies are represented by low energy deposits (mudstone), bar complexes (sandy skeletal packstone and skeletal packstone) and channels (skeletal intraclast floatstone and intraclast skeletal ooidal floatstone).

Storage and Transport of Magma in the Layered Crust—Formation of Sills and Related Flat-Lying Intrusions

Abstract Even though dykes are the main upward magma pathways through the Earth’s crust, the last two decades of research showed that significant parts of volcano plumbing systems consist of flat-lying igneous intrusions, namely sills. Sills form mainly in the layered parts of the crust, principally in volcanic deposits and sedimentary basins. Sills exhibit various shapes, strata-concordant, transgressive sheets, and saucer-shaped. Lateral magma flow through sill complexes and networks can reach several hundred kilometres. Sills represent intermediate feeder structures for volcanic eruptions, and therefore better understanding of sill emplacement and evolution is essential for assessing volcanic hazards. Sills emplaced in sedimentary basins also deeply affect petroleum systems and are essential components in exploring hydrocarbons. Finally, the massive and fast emplacement of sills resulting from LIPs in sedimentary basins triggered catastrophic climate changes and mass extinctions during Earths history.

Finding of Neogene synorogenic sedimentites around Chos Malal, Neuquén Basin, Argentina

Finding of Neogene synorogenic sedimentites around Chos Malal, Neuquen Basin, Argentina. The Chos Malal Formation nov. is proposed to group a 140,50 m thick neogene succession of epiclastic sedimentites including sandstones, silstones and mudstones with tuffaceous input, which were previously ascribed to the Early Cretaceous Rayoso Formation. They were accumulated during the formation of a syncline elongated in a NNW-SSE direction, being deposited in angular unconformity on the Rincon Member of the Rayoso Formation, and covered in the same way by the Middle Miocene-Early Pliocene Rincon Bayo Formation. According to its stratigraphic position and correlations with near geographically situated units, they are ascribed to the Middle Miocene.