REGIONAL PETROLOGIC VARIATION, JURASSIC AND CRETACEOUS SANDSTONE AND SHALE, NEUQUEN BASIN, WEST-CENTRAL ARGENTINA

The Neuquen Basin is one of a series of back-arc basins that developed along the eastern margin of the Andean Orogenic Belt during the Mesozoic. The basin overlies Paleozoic plutonic rocks and a wide Permo-Triassic magmatic arc. The basin contains more than 6 km of Lower Jurassic to Lower Cretaceous rocks, deposited in fluvial through submarine-fan environments.;Illite layers in mixed-layer illite-smectite in Jurassic Los Molles and Vaca Muerta shales increase from 30-40% on the basin margins to more than 90% in deeply buried parts of the basin interior as a result of burial diagenesis. Lower Cretaceous Agrio shales contain only chlorite and nonexpandable illite because of a change in provenance from that of Jurassic units.;Sandstones are feldspathic litharenites and lithic arkoses. Diagenesis of Jurassic sandstones (Cuyo Group and Lotena and Tordillo Formations) is complex due to large variations textural and mineralogical maturity. Volumetrically important cements include carbonate and clay minerals, anhydrite, and locally, zeolites. Lower Cretaceous sandstones (Mulichinco, Agrio, and Centenario Formations) are compositionally more mature than Jurassic sandstones and have a diagenetic sequence similar to that of Tertiary sandstones of the Gulf of Mexico Basin.;Detrital modes of Jurassic sandstones plot near the volcanoplutonic-sand-suite mixing line on Dickinson's (1985) tectonic discriminate diagrams. These compositions represent material derived from the underlying Permo-Triassic magmatic arc and from the Andean magmatic arc. Early Cretaceous sandstones plot on a mixing line between transitional-arc and transitional-continental terranes. These sandstones represent a mixture of detritus from the Permo-Triassic magmatic arc and from cratonic basement.;Maximum burial depths increase northwards from the southern margin of the basin and westwards from the eastern margin. Vitrinite reflectance (Ro) increases from 0.3-0.6% in shallowly buried areas near the basin margins and on structural highs to nearly 3.0% in deeply buried areas in the western part of the basin. Maximum burial temperatures never exceeded 250;In most areas of the basin plagioclase retains no memory of volcanic source areas because of albitization and dissolution of potassium-rich plagioclase. Proportions of potassium feldspar also have been substantially reduced through dissolution.