EVIDENCES OF SYNSEDIMENTARY ACID VOLCANISM IN THE EVAPORITIC SEQUENCE OF THE JURASSIC MINAS VIEJAS FM. IN GALEANA, NE MEXICO

The thick Mesozoic stratigraphic column of the Sierra Madre Oriental (SMO) in the State of Nuevo León (NE Mexico) is mainly composed by calcareous and shaly sediments, strongly deformed during the Laramide Orogeny. However, in the vicinity of Galeana the Upper Triassic to Middle Jurassic Huizachal Group (Alamar, La Boca and La Joya Fm.) composed by red beds and the evaporitic Upper Jurassic Minas Viejas Fm. crop out, representing the basis of the SMO sequence.

During the last decade volcanic beds intercalated within those evaporites have been described. This unit is now known as Primavera Member. In the former descriptions this volcanism was related to a carbonatitic event active during the opening of the Gulf of Mexico. This interpretation was made considering its high content of modal calcite, dolomite and iron oxides, possibly from primary origin. However C and O isotopic determinations exclude any relationship with magmatic carbonates.

The volcaniclastic layers show a rhyodacitic to dacitic composition with a continental arc affinity. Epiclastic levels are also present. They display a deep alteration that could be explained by a deposition in a very aggressive chemical environment, contemporaneous to the sulphatic sedimentation.

The explosive, volcanism, probably phreatoplinian, could belong to the last episodes of the Jurassic Nazas arc. That means that the activity of this major volcanic arc reached up to Late Jurassic times.