The basement of the Chaco-Pampean Plain: Ediacaran igneous complexes and a Cambrian epicontinental sea

We present the first evidence for the composition of basement beneath the Chaco-Pampean plain of southern South America, based on partly unpublished borehole reports and our analysis of recovered samples. At 1864 m the easternmost borehole reached unmetamorphosed granodiorite with a new U–Pb zircon age of 556 ±5 Ma. This is similar in age, shallow emplacement depth and geochemistry to plutonic-volcanic complexes nearby in Paraguay: together they represent the basement beneath this part of the Chaco-Pampean plain. Above the Ediacaran granodiorite is a thick succession dominated by monotonous quartz-rich sandstones with a maximum age of 550 Ma, also found in other drill holes to the west. We propose the name Quebrachal Group for these Cambrian subsurface formations of the Chaco-Pampean plain and endorse their equivalence to the Mesón Group of the Cordillera Oriental of Argentina and Bolivia. The combined Quebrachal–Mesón megabasin would have occupied > 375 000 km 2 during a mid-to-late Cambrian interval ( ca. 510–490 Ma). Sedimentation occurred in a vast epicontinental sea, for which we propose the name Vadum Maris (shallow sea); it is considered a geological marker of the stable stage of the southwestern region of the Gondwana megacontinent.