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One of the remarkable characteristics of the Neuquén Basin is that a group of Cenozoic shallow microgabbroic sills exhibit gas and oil shows, and some have produced hydrocarbons for up to 20 years. Their capacity to act as reservoirs is caused by the increase in the effective porosity owing to the simultaneous development of petrological and fracture processes during cooling that generate systematic variations inside the intrusive bodies.

In this work we describe in detail an unusual intracolumnar joint system formed by two joint sets, referred to here as concentric and radial joint sets. This atypical joint system is developed within the small stacked polyhedrons into which certain zones of classic prismatic columns are transformed during the cooling processes. These small polyhedrons are laterally bounded by the vertical planes that limit the columns, and at the top and bottom by transversal joint planes oriented almost perpendicular to the column axis.

As a result of detailed macroscopic analysis, the genesis of the concentric joint sets have been interpreted as resulting from either the development of natural convective flows or from complex processes of cooling influenced by the joint-bounded planes of the columns. When the radial joint set is present, it is clearly related to the development of thermal stresses between the outer and inner parts of the small polyhedrons.

In geology textbooks and previous published papers the genesis of what is informally called ‘onion skin’ joint sets in basic intrusive and extrusive igneous rocks has been attributed to processes related to either weathering or deuteric alteration. However, based on detailed macroscopic observations, our work suggests that, in most cases, any type of alteration process only accentuates the planes previously generated during the cooling process.

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