Ancient Lacustrine Hyperpycnites: A Depositional Model from a Case Study in the Rayoso Formation (Cretaceous) of West-Central Argentina

Abstract

Hyperpycnal flows originate when sediment-laden fluvial discharges enter standing, lower-density water. Because of their excess density, the flows plunge near the river mouth and continue to travel basinward as a quasi-steady and fully turbulent underflow. The related deposits are hyperpycnites, and constitute a particular type of turbidite with poorly known facies and facies tracts. Although hyperpycnal flows seem to be quite common in present times, their occurrence in fossil strata is poorly documented. This paper addresses the characteristics and depositional processes of shallow lacustrine sandy hyperpycnites, on the basis of the field analysis of well-exposed Lower Cretaceous strata (Rayoso Formation) in the Neuquén Basin of west-central Argentina. The Rayoso Formation is composed of clastic (and minor evaporitic) red beds up to 1200 m thick, deposited in a shallow perennial lake of variable salinity affected by long-lived hyperpycnal flows. Main clastic facies are composed of fine-grained sandstones with climbing ripples and plane beds. Other common sandstone facies include massive beds and low-angle cross stratification. Most sandstone facies are related to traction plus fallout processes, and often show a vertical fluctuation between sedimentary facies originated under different traction-plus-fallout conditions within single beds. These fluctuations are interpreted to be evidence of deposition from flow fluctuations in sustained hyperpycnal flows. Most beds internally show the existence of three depositional phases, acceleration (AP), erosion plus bypass (EP), and deceleration (DP), which record the complete evolution of a single long-lived hyperpycnal flow at a fixed point. Additionally, the depositional evolution of a single long-lived hyperpycnal flow with distance records initially the progressive basinward migration of the AP and EP phases, and finally an overall deposition under the DP phase both in proximal and distal areas. This evolution provides an adequate explanation for the basinward extension of channelized features, and for the occurrence of fine-grained sandstones with climbing ripples both in proximal and distal positions within the same hyperpycnal system. Consequently, facies analysis derived from application of the Bouma sequence is not valid for deposits of quasi-steady hyperpycnal flows. 42 C. ZAVALA ET AL.

Origin and depositional model of deep-water lacustrine sandstone deposits in the 7th and 6th members of the Yanchang Formation (Late Triassic), Binchang area, Ordos Basin, China

“…Massive sandstones such as these could have been deposited by grain flow, sandy debris flow or hyperpycnal flow (Bagnold 1956;Zavala 2006;Li et al 2011a;Gao et al 2012). Hampton (1975) concluded, based on experimentation, that a clay content of less than 2% is required for the development of grain flow.…”

Sedimentología e Icnología de deltas fluvio-dominados afectados por descargas hiperpícnicas de la Formación Lajas (Jurásico Medio), Cuenca Neuquina, Argentina.

“…2). Internamente estos depósitos difieren de los arreglos anteriormente mencionados, ya que muestran un pasaje transicional y recurrente entre diferentes estructuras sedimentarias con importantes variaciones texturales y múltiples superficies de reactivación interna (Zavala et al, 2006;Ponce y Carmona, 2011).…”

“…Consecuentemente el desarrollo de modelos depositacionales integrando datos sedimentológicos e icnológicos en estos sistemas resulta clave, ya que permite tener un detallado control de la distribución que presentan las facies sedimentarias, así como una caracterización de sus principales reservorios. Durante las descargas fluviales, importantes volúmenes de sedimento son transferidos a cuencas marinas y lacustres a través de plumas hipopícnicas, homopícnicas o hiperpícnicas (Mulder y Syvitski, 1995; Syvitsky y Morehead, 1999;Zavala et al, 2006). El desarrollo de uno u otro tipo de proceso de sedimentación en la cuenca depende de una serie de factores; uno de los más significativos es la concentración del flujo debido al volumen de sedimentos transportados durante la descarga fluvial.…”

“…Por el contrario, durante descargas fluviales generadas por eventos de crecida (descargas extraordinarias), la alta concentración de sedimento transportado por el flujo puede superar el valor de Ccs, produciendo corrientes de densidad hiperpícnicas (Mulder y Syvitski, 1995). Las hiperpicnitas se caracterizan por presentar depósitos de espesor variable integrados por un pasaje transicional y recurrente entre diferentes estructuras sedimentarias (Zavala et al, 2006Ponce et al, 2008;Ponce, 2009;Ponce y Carmona, 2011). En ambientes marinos, las descargas fluvioderivadas incorporan importantes volúmenes de sedimentos embebidos en agua dulce a la cuenca, produciendo notables variaciones en la salinidad, oxigenación, temperatura y turbidez de la columna de agua (Wetzel, 2008).…”

Stratigraphy of the Neuquén Group, Upper Cretaceous of the Neuquén Basin (Argentina): new proposal for lithostratigraphic ordaining

“…Rayoso. Como consecuencia de ello, esta discontinuidad fue establecida de forma definitiva como la correspondiente a la base del Grupo Neuquén (Discordancia Patagonídica), criterio que fue seguido en todos los trabajos posteriores realizados en el área (Ramos, 1981;Gazzera & Spalletti, 1990;Ponce et al, 2002;Leanza, 2003;Zavala et al, 2006). Recientemente, Tunik et al (2010) efectuaron análisis en zircones provenientes de depósitos asignables las "Areniscas Violáceas", obteniendo un valor de edad para los zircones más jóvenes de 98,6 ± 2,5 Ma.…”