Density Turbidity Currents Research Articles

Pyrite-bearing non-marine ostracods from the Lower Cretaceous Maracangalha Formation, Recôncavo Basin, NE Brazil: Depositional controls and paleoecological implications

Ostracods are aquatic microcrustaceans which use calcium dissolved in water to synthesize their carapaces, along with a low magnesium content. Depending on the conditions of the depositional environment and diagenetic processes, ostracods can be preserved in the fossil record, with or without alteration of the original chemical composition. In this study, samples from shales, siltstones and silty sandstones of the Maracangalha Formation (Recôncavo Basin) along the Gameleira and Manguinhos (Ilha de Itaparica) and Praia da Falha (Ilha dos Frades) outcrops, Baía de Todos os Santos, were analyzed. Results show that the pattern of ostracod carapaces preservation was controlled by distinct sedimentological processes. In the Manguinhos and Praia da Falha outcrops, ostracod specimens show total or partial replacement of the carapace by pyrite, with the more expressive pyritization observed in shale from Manguinhos. In the Gameleira outcrop, on the other hand, no pyritized carapaces were observed. Results of the Energy Dispersive Spectrometry (EDS) from Manguinhos and Praia da Falha outcrops indicate the presence of Fe and S pointing to pyrite mineralization. The Scanning Electron Microscopy (SEM) images revealed the euhedral and framboidal habit of pyrites present in the carapaces. The low recovery of specimens from Gameleira and Praia da Falha, as well as the abundance of specimens in Manguinhos, seem to have depositional controls. At Gameleira and at Praia da Falha, facies associated with low-to-high density turbidity currents may have hampered preservation, thus low recovery of ostracods. Despite the occurrence of fragmented carapaces, the anoxic conditions of the Manguinhos shale samples allowed a better preservation of specimens. The Manguinhos facies are interpreted as relatively deep lacustrine deposits. The presence of framboidal pyrite suggests early conditions of diagenesis at low burial depths. Pyrite mineralization may have occurred through a hydrogen sulfide environment, rich in organic matter, with dissolution of the carbonate carapace through the acidic pH of the medium and relative rapid post-mortem burial. The sedimentary facies described, along with the taphonomic results of ostracod preservation, corroborate a lacustrine environment with occasional sediment gravity flows to the Maracangalha Formation.

“Bottomsets” of the lava−fed delta of James Ross Island Volcanic Group, Ulu Peninsula, James Ross Island, Antarctica

Abstract Sedimentological study of the three geographically separated outcrops of bottom− sets of a single lava−fed delta (Pliocene) in the James Ross Island (Antarctica) allows recognition of six lithofacies.Deposits of traction currents, deposits of volcaniclastic debris flows and products of such flows transformations (both low− and high−density turbidity currents) and glacigenic deposits (subaqueous debris flows and traction/turbidity currents) were all recognised. Existence of submarine proglacial environment formed prior to formation of volcani− clastic deposits partly covering the subaqueous slopes of volcano is supposed. The principal role of mass flow processes was recognised and explained by relative steep slopes of the lava−fed delta. The distribution of lithofacies significantly differs in the individual outcrops. These variations in sedimentary succession and also in thickness of volcaniclastic deposits of “bottomsets” of the single lava fed delta suggest principal role of local conditions and paleogeography for development and preservation of this part of delta depositional system. Moreover proximal and distal setting can be followed and direct vs. more distant relation to over−riding lava−fed delta supposed. The sedimentary succession terminated by foresets of hyaloclastite breccia.

On the dense water spreading off the Ross Sea shelf (Southern Ocean)

In this study, current meter and hydrological data obtained during the X Italian Expedition in the Ross Sea (CLIMA Project) are analyzed. Our data show a nice agreement with previous data referring to the water masses present in this area and their dynamics. Here, they are used to further analyze the mixing and deepening processes of Deep Ice Shelf Water (DISW) over the northern shelf break of the Ross Sea. In more detail, our work is focused on the elementary mechanisms that are the most efficient in removing dense water from the shelf: either classical mixing effects or density currents that interact with some topographic irregularity in order to drop to deeper levels, or also the variability of the Antarctic Circumpolar Current (ACC) which, in its meandering, can push the dense water off the shelf, thus interrupting its geostrophic flow. We also discuss in detail the (partial) evidence of dramatic interactions of the dense water with bottom particulate, of geological or biological origin, thus generating impulsive or quasi-steady density-turbidity currents. This complex interaction allows one to consider bottom particular and dense water as a unique self-interacting system. In synthesis, this is a first tentative analysis of the effect of bottom particulate on the dense water dynamics in the Ross Sea.

A new model for marine density-turbidity currents with criteria for ignition

Abstract We discuss the hydrodynamic stability properties of a one-dimensional quasi-steady marine current, driven by a density excess caused by low temperature or high salinity, and flowing over a regular slope, taking bottom-erosion phenomena into consideration. The term density-turbidity current is used here for a thermohaline density current, with that density increased by entrained sediment. Thermohaline currents are of fundamental importance with regard to the Earth's climate, and the same must apply to density-turbidity currents. To simplify this complex problem, we schematize the flow as a thin turbulent quasisteady current, with gravitational and frictional forces in approximate equilibrium; the effects of small-scale perturbations, and of interaction with the bottom sediment, are then schematised by assuming a heuristic model of sediment evolution. Indeed, as in recent work by Caserta et al. (1990), we postulate that density variation due to bottom erosion or deposition is a function only of the...

Análisis paleoambiental de los depósitos detalud de la Formación Kotick Point (Cretácico Inferior) en el área de Punta Stoneley, Isla James Ross, Antártica (64° S)

ABSTRACT. Paleoenvironmental analysis of the slope deposits on the Kotick Point Formation (Lower Cretaceous) at 5toneley Point, James Ross Island, Antarctica (64°5). The Kotick Point Formation (Aptian•Albian), exposed at Stoneley Point, James Ross Island, Antarctica, is interpreted as having been deposited on a fault controlled slope. Eight lithofacies have been defined, forming three distinct depositional elements: slope foot wedge, open slope and sand lobes. Lithofacies A (breccias and conglomerates with projected clasts) and B (graded or massive sandstones) make up the slope foot wedge where sandy debris flows, fluidized f10ws and high•density turbidity currents were the dominant processes. A large Jurassic glide block is associated to this coarse grained wedge. Lithofacies e (medium•grained conglomerates to coarse sandstones) represents the development of a unique channelized turbidite system. Open slope is represented by lithofacies D (very fine sandstones with lag deposit); E (highly bioturbated medium to fine•grained sandstones); F (parallel stratified tabular sandstones) and G (non lithified mudstones). The latter is dominant, suggesting suspension sedimentation, whereas the former three represent deposition from esporadical turbidity currents and probable bottom currents reworking. Sand lobes are characterized by progradational cycles of lithofacies H (finegrained sandstones to mudstones).lntrabasinal tectonism and sea level changes played a major role in slope sedimentation. High sea level stand and tectonic quiescence are inferred for the deposition of hundreds of meters of mudstone packets. Formation of the slope foot wedge and the channelized turbidite system may have been related to fault reactivation.

Self-preserving solutions for turbidity currents

Subaquatic density currents and turbidity currents flowing down an incline are analyzed. By noting that at least depositing turbidity currents on a constant slope depend on the source conditions, it is proposed to consider them as self-preserving or as piecewise selfpreserving in analogy to other shear flows. The requirements for self-preservation are examined and the limiting conditions for realizable flows are derived.