Hyperpycnal Shelfal Lobes - Some Examples of the Lotena and Lajas Formations, Neuquén Basin, Argentina*
Mariano Arcuri1 and Carlos Zavala1,2
Search and Discovery Article #50066 (2008)
Posted April 12, 2008
*Adapted from extended abstract prepared for AAPG Hedberg Conference, “Sediment Transfer from Shelf to Deepwater – Revisiting the Delivery Mechanisms,” March 3-7, 2008 – Ushuaia-Patagonia, Argentina
1Geología de Cuencas Sedimentarias (G.C.S.). Haiti 123. Bahía Blanca (8000), Argentina ( [email protected] )
2CONICET-UNS, Ushuaia, Argentina
Very thick massive sandstone bodies constitute new exploratory targets mainly in offshore mini-basins. Although these bodies are relatively well known from subsurface studies, there are few detailed field descriptions. This contribution focuses on the description and analysis of field examples of very thick massive sandstone bodies from Middle to Upper Jurassic Lajas and Lotena formations in the Neuquén basin, Argentina. The examples provided here are composed of nearly homogeneous fine- to medium-grained sandstone beds, up to 45 meters thick, and correspond to hyperpycnal shelfal lobes. These packages overlie a sharp or slightly erosive base, and they commonly lack internal bioturbation, mud deposition layers, or other evidences of pauses in the sedimentation. The existence of very thick massive sandstones was usually related to an “in mass” deposition induced by a gravitational collapse associated with surge flows having high suspended load. However, more recent studies have proposed an origin related to the progressive aggradation from long-lived and quasi-steady turbulent flows. Massive deposits could be related to the absence of a sharp surface between the flow and the deposit.
The Neuquén basin is a back-arc basin located in the west-central Argentina. Its sedimentary infill took place during Jurassic and Cretaceous with a mainly clastic succession, up to 7000 meters thick. The Lajas Formation (Early to Middle Jurassic) represents the first prograding unit after the initial marine flooding of the basin (Los Molles Formation). It is composed of conglomerates and sandstones that accumulated in different deltaic and shallow-marine environments. The Lotena Group (Middle to Late Jurassic) sharply overlies the Lajas Formation in the study area. This group constitutes a clastic to evaporitic unit, widespread in its development in the basin during middle Callovian to late Oxfordian times. It is internally composed of conglomerates, sandstones, mudstones, and limestones that accumulated in fluvial (Dellapé et al., 1979) to deep marine (Gulisano et al., 1984) environments.
The examples analyzed here correspond mainly to Lajas Formation in the Lonopué area and Lotena Formation at the Sierra de la Vaca Muerta (Figures 1 and 2). Some characteristics of the study examples are:
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