Resumen: This work shows the results of a detailed structural analysis of the deformation belts of Lavinia Planitia. Ridge belts and graben and groove belts can be observed at the studied area, while wrinkle ridges and large individual grooves predominate in the smooth plains. Transcurrent
components of displacement are commonly observed, and transpression and transtension zones are the rule rather than the exception at most of the studied belts. Along strike azimuth changes of deformation belts are accommodated by internal variations in the predominance of contractional, transcurrent or extensional structures. The material of the surrounding plains embays most of these deformation belts. The kinematic analysis of this complex network of tectonic structures suggests a broadly synchronous activity of contractional, transcurrent and extensional structures. The maximum horizontal shortening axis determined in this work describes a steady,
semicircular pattern centered at Alpha Regio. This deformation continued, although with subdued activity, after embayment of the deformation belts by the material of the plains. Future study of the tectonic evolution of the lowland plains should take into account the importance of the coeval history of neighboring uplands and lowlands.
Resumen: A new type of trace fossil from the Lower Cretaceous of Patagonia that may record the oldest evidence of insect
agriculture is represented by biconvex, sub-spherical, carbonate balls composed of a dense mass of rhizoliths.
The best preserved balls show an external wall, a tunnel entrance, and vertical, large rhizoliths crossing from
top to base, from which a
mass of smaller rhizoliths arise. In some cases there is a pelletal surface texture in
internal layers of the wall. Some specimens show meniscate tubes attributed to organisms different than the
constructors of the chambers, probably attracted by the original organic matter. Micromorphology, analyzed
by thin sections and SEM, shows strong evidence of high fungal activity associated to the original roots.
Four different hypotheses are analyzed to explain the origin of the balls. The less probable ones are either that
the balls were produced only by a self induced and localised overgrowth of secondary and tertiary rootlets from
a group of neighbouring primary roots, or that they were crayfishes' feeding chambers. Rhizolith arrangement,
as well as other characters, essentially rule out both hypotheses. The hypotheses involving social insects,
termites and ants, are more risky, considering the lack of support from the body fossil record, the biogeography
of these extant insects, and particularly the lack of definitive evidence of discrete fungus or bacterial gardens
in the balls. However, the trace fossils described herein partially resemble the nests and behaviour of the
African termite Sphaerothermes sphaerothorax, particularly because of the presence of roots inside a thick
lined chamber excavated around them to cultivate bacterial combs. The ant origin of these balls is at least
a hypothesis to explore considering the size, shape, wall, entrance tunnel, presence of traces of associated
organisms, and particularly roots. The roots and wall relate these balls to hypothesised scenarios for the origin
of fungus agriculture in ants. One hypothesis suggests that primitive Attini developed fungus agriculture from
the habit of feeding on fungi associated with roots (mycorrhizae) and still other proposes that adventitious
fungi on nest walls were responsible for the arise of fungiculture. The balls would supply physical evidence that
by the lower Cretaceous some social insects excavated chambers around uncut roots, which in turn provided
optimal conditions for fungi development. These rhizolith balls possibly constitute the oldest physical evidence
for the origin of fungiculture in social insects, and represent a critical contribution of ichnology to this hot topic
intensively discussed on the basis of a scattered body fossil record and molecular phylogeny
Resumen: Se han estudiado seis sondeos, realizados en la cuenca neógena del Tajo, con el fin de conocer los procesos genéticos de los materiales arcillosos de la cuenca. Estos sondeos cortan facies correspondientes a distintos ambientes deposicionales. La evolución observada puede relacionarse con dos etapas distintas de sedimentación.
La primera corresponde a la sedimentación
de la Unidad de Yesos; la asociación de minerales de arcilla en esta unidad está constituida principalmente por illita, menores contenidos de esmectita, interestratificados I/Sm, Cl/Sm, Glauconita/Sm, palygorskita y clorita. El origen de esta asociación es en parte detrítica y en parte neoformada; los aportes detríticos corresponden a illita y esmectita, mientras que los minerales neoformados son principalmente illita e interestratificados, clorita y palygorskita. Esta etapa se caracteriza por la existencia de procesos de illitización. En la segunda etapa de sedimentación (Unidad de Arcillas Verdes), se observa un incremento de los aportes detríticos y una evolución hacia condiciones de menor salinidad. El ambiente de formación de esta unidad se encuentra enriquecido en magnesio y los minerales de la arcilla más característicos de las facies que componen dicha unidad son las esmectitas magnésicas (saponita y estevensita) y, en menor proporción, los interestratificados
Cl/Sm. Esta asociación mineralógica se origina por la reacción entre las partículas detríticas arcillosas y los cationes del medio.
[ABSTRACT]
The Tagus Basin is an intramontanous basin that had an important sedimentary activity during the Neogene. Six drill boreholes have been studied; these boreholes pierce
through facies corresponding to different depositional environments. Such sedimentary environments correspond to different sedimentation stages. During each stage the basin
shows areas with different geochemical gradients which rule the neoformation and transformation processes of the detrital particles that reach it.
A first stage corresponds to the sedimentation of the Gypsum Unit. Clay minerals during this stage are mainly illites, with minor contents of smectite, I/Sm, Cl/Sm, Glauconite/Sm, palygorskite and chlorite. The origin of such association is partly detrital and partly neoformational. Detrital contribution is composed by illite and smectite. Neoformed minerals are mainly illite and mixed-layer I/Sm, chlorite and palygorskite. This
stage is characterised by illitization processes.
The second sedimentation stage corresponds to the Green Clays Unit which shows an increase in detrital supply, the environment is enriched in magnesium and silica. The
most characteristic clay minerals of these facies are magnesium smectites (saponites and stevensites) and, to a lesser extent, mixed-layer Cl/Sm. The mineralogical assemblage of this unit must have been originated by reaction between detrital clay particles and cations in the sedimentary environment.
Palabras clave: Cuenca del Tajo, Saponita, Interestratificados, Sepiolita, Illitización, The Basin Tagus, Saponite, Sepiolhe, Illitization.
