First articulated stalked crinoids from the Mesozoic of South America: two new species from the Lower Cretaceous of the Neuquén Basin, west-central Argentina

Two new isocrinids are described from the Lower Cretaceous Agrio Formation of the Neuquén Basin, west-central Argentina. Isocrinus (Chladocrinus) covuncoensis new species is based on several beautifully preserved specimens from Valanginian beds of the Pilmatué Member. It is characterized by a small size, multiramose crown with six arm divisions, 240 arm tips, mostly isotomous branching, seven (or rarely eight) secundibrachials, smooth and stout column, short noditaxis, and pentalobate columnals. The species occurs in a 30 m thick interval of cross-bedded sandstones and mixed clastic-carbonate sediments that represent the migration of large, tidally influenced, subaqueous dunes developed in the offshore. Sudden burial of crinoids that dwelled on the dune toes and interdunes, possibly by the acceleration of the lee face migration, provided the exceptional preservation of specimens and thus this finding can be considered as a local crinoid Konservat Lagerstätte. Isocrinus (Chladocrinus) pehuenchensis new species is described from a single articulated specimen preserved in a silty calcareous concretion collected from a late Hauterivian concretion level of the Agua de la Mula Member. It is characterized by isotomous branching, eight or nine secundibrachials (IIBr), slender column ornamented with medial ridge of fine tubercles, interradius acuminated with fine tubercles on its tip, short noditaxis, and pentastellate columnals. It is associated with low-energy fall-out deposits in the offshore. The excellent state of preservation was due to an early cementation process by carbonate that enhanced lithification around the specimen.UUID: http://zoobank.org/4763ae70-9d0a-4015-b1c9-6a7ceeedfb1a

GEOACOUSTIC PATTERNS OF THE GUAÍBA RIVER BOTTOM AND SUBBOTTOM AND THEIR RELATIONSHIP WITH SEDIMENTARY AND HYDRODYNAMIC PROCESSES

ABSTRACT. This work aimed at studying the bottom and sub-bottom geoacoustics of the Guaíba River, whose echo patterns are related to hydrodynamic patterns and sedimentary processes. A total of 324 km of seismic transects were surveyed using the Stratabox high-frequency (10 kHz) profiler with concomitant imaging using a side-scan sonar in the northern and central areas of the Guaíba system. The records were interpreted with the aid of complementary data (bathymetry and bottom samples), and maps containing the spatial arrangement of echo-characters were developed. The echo-characters were classified into seven different types according to the presence of sediments controlled by the hydrodynamics of the system or modified by human action. In general, the echoes related to the deposition of fine sediments occurred in deeper areas, the navigation channel or surrounding areas. The echoes in the shallower areas presented strong acoustic reflectivity at sites with lower deposition of fine sediments and predominance of sandy sediments. The occurrence and morphology of asymmetric subaqueous dunes indicated a predominant north-to-south flow and a river behavior.Keywords: shallow geophysics, sedimentology, hydrodynamics, geoprocessing.RESUMO. Este trabalho tem por objetivo o estudo geoacústico de fundo e subfundo do rio Guaíba, cujos padrões de ecos são relacionados aos padrões hidrodinâmicos e aos processos sedimentares atuantes. Foram levantados 324 km de perfis sísmicos em extensão, com o perfilador de alta frequência (10 kHz) Stratabox e concomitante imageamento com Sonar de Varredura Lateral nas áreas Norte e Central do Guaíba. Os registros foram interpretados com o auxílio de dados complementares (batimetria e amostras de fundo) e foram confeccionados mapas com a disposição espacial dos eco-caráteres. Os eco-caráteres foram classificados em sete diferentes tipos, de acordo com a presença de sedimentos controlados pela hidrodinâmica do sistema ou modificados pela ação humana. De uma forma geral, os ecos relacionados à deposição de sedimentos finos ocorrem nas áreas mais profundas, no canal de navegação ou adjacências. Os ecos nas áreas mais rasas apresentam forte refletividade acústica, em locais com menor deposição de sedimentos finos e predomínio de sedimentos arenosos. A ocorrência e morfologia de dunas subaquosas assimétricas indicam um fluxo preponderante no sentido Norte – Sul e um comportamento fluvial do sistema aquático.Palavras-chave: geofísica rasa, sedimentologia, hidrodinâmica, geoprocessamento.

Relief and internal structure of the subaqueous sand dunes in the Vistula River mouth

This paper presents the results of bathymetric and seismoacoustic measurements carried out in the canal of the Vistula River mouth (pol. Przekop Wisły). The surveys were conducted with the use of a multibeam echosounder and a parametric sub-bottom profiler. It made it possible to obtain a high resolution bottom morphology measurement, and to identify and characterize an internal structure of subaqueous bedforms. Presence of multiple series of small to large dunes, with their length ranging from 5 up to 55 m and height from 0.1 up to 1.5 m, was established. They were composed mainly of poorly graded medium-grained sands. The nature of the sediment graining was relatively uniform. Significant diversity in terms of the formation sizes was observed, and itwas only loosely dependent on their depth and location within individual relief units. Collateral medium and small formations were found everywhere among the large formations. The bedforms consisted of well-developed steep cross-coset stratification structure. It has been ascertained that the cross-coset thickness of this formation was greater (or equal) than its height. The scope of this work is to identify the relief and structure of subaqueous dunes for the purpose of the assessment of flow properties and bottom conditions in both recent and palaeoenvironments. The obtained results are crucial for determining the state of the Przekop Wisły canal, indicating potential limitations to unconstrained flow of spate water and ice drifting as well as navigation conditions.