In the Neuquén Basin of Argentina, ’beef' (bedding-parallel veins of fibrous calcite) is widespread within Late Jurassic black mudstones. A typical vein consists of two grey inner zones and two white outer zones. The inner zones contain inclusions of wall rock and hydrocarbons. Calcite fibres are perpendicular to the margins. In the outer zones, the angle between fibre and margin varies from about 45° at the vein tips to 90° in the centre. Imprints of fossils are offset, proving that the fibres have grown antitaxially. We infer that the veins opened in two phases. During Phase 1, the opening was vertical, against gravity. During Phase 2, the veins resisted tectonic shortening, so that shear stresses acted at the margins. The senses of shear account for the fibre angles. At outcrop, igneous intrusive rocks have cut and metamorphosed the veins. From burial curves, maturity calculations, growth strata, and ages of igneous intrusions, we estimate that the inner zones of the beef formed in the oil window, during the Aptian to Albian, and that the outer zones formed in the gas window, during the Cenomanian to Campanian. We infer that the beef is evidence for fluid overpressure during hydrocarbon generation.