Ana Luiza de Oliveira Borges

Processes and products of turbidity currents entering soft muddy substrates

New laboratory experiments reveal that cohesionless turbidity currents are able to enter cohesive soft muddy substrates without losing their shape. These intrabed currents are driven by bed shear stress exceeding bed cohesive strength, and by flow density exceeding bed den - sity. The flows produce unique turbidites with internal mud layers, mixed cohesive-nonco - hesive sediment layers, and flame and load structures. A depositional model for intrabed (I) turbidites is proposed, comprising, from base to top: I1�sand-bearing mud, with a scoured base, dispersed mud, and mud clasts; I2�muddy sand from the intrabed portion of the tur - bidity current; I3�sandy mud with a speckled appearance; and I4�mud-poor sand from the suprabed portion of the flow. Complete I1�I4 turbidites are inferred to dominate loca - tions in nature where the currents mix with the bed and deep erosional scours form, filled with deformed or chaotic sand-mud mixtures. Further downflow, base-missing I2�I4 and I4 sequences signify gradual deceleration, loss of erosivity, and termination of intrabed flow

Sedimentologia e estratigrafia de um depósito deltaico gerado por modelagem física empregando amostragem tipo testemunho

In natural ambient, the analysis of deposit cores is a method widely used in sedimentological and stratigraphic studies. It is proposed in this paper, a new method to sample sedimentary deposits generated by physical modeling in small scale. This method was adapted in order to extract additional details and information in terms of geological point of view of the deposit generated. At the lab, the sampling process, consisted in a vertical hole on the deposit and removal sedimentary material, and then, filling the hole with resin. The resin penetrates horizontally a few inches in the sedimentary layers along all vertical cores and, after impregnation, allows the removal of a well preserved core. A total of 33 cores was collected and described , producing vertical profiles of facies which are then interpreted and correlated by using the models and concepts of sedimentology and sequence stratigraphy. Similar geometries and sedimentary features related to natural deltas were observed . It was also possible evaluate the deltaic facies associations and established a longitudinal section of the deposit in order to understand its stratigraphic evolution.

Prediction of the bedforms generated by density currents based on fluvial phase diagrams

Density currents, whose movement takes place by the density difference between the flow and the ambient fluid around it, can interact with the substract generating bedforms similar to the fluvial environments. However, there are no specific bedform phase diagrams capable to predict this type of phenomenon. This study aims to compare the prediction of fluvial bedforms phase diagram with those generated by experimental saline currents. Bedforms were generated in two-dimensional tilting plexiglass flume submerged in a larger tank filled with water with three different mobile beds and varied values of discharge and salt concentration. It was observed three types of bedform (lower plane bed, ripples and dunes), which, with the concomitant calculation of hydrodynamic parameters (mean velocity, energy and mobility) allowed the use of the phase diagram. It was observed that the fluvial phase diagrams did not present good predictions for bedforms generated by density currents. This fact is associated to the hydrodynamics differences (velocity and concentration profiles) and the limitation of the dimensional parameters in the extrapolation of results. Therefore, it is indicated the need to draw up a proper phase diagram to density currents.