New outcrops of Sierra Grande Formation (Silurian-Devonian) were recognized during mapping of metamorphic basement at Arroyo Tembrao area, North Patagonian Massif.

These rocks had been previously mapped as belonging to the Mina Gonzalito Complex, a Proterozoic metamorphic unit that includes gneisses, schists and crystalline calcareous mudstones.

The outcrops constitute a 140 m thick sequence. Three groups with transitional contacts were recognized.

The basal group, 20 m thick, is coarse grain and corresponds to arkoses. Quartz grains are subrounded, equigranular with penetrative contacts. No matrix was recognized. Up to 5% of opaque minerals with ovoidal forms remembering flattened oolites are present.

The middle group (40 m) presents banks 30 to 60 cm thick, with festoon cross-stratification, ripples and antidunes. Channelled forms and point bar migration structures were observed. Paleoflow measuring indicates a NW. (N 310º-340º) provenance.

Toward the superior part of this group, two levels of pure quarzites are recognized.

The upper group (80 m) is conformed by arenites with a coarse, diffuse stratification in banks of 1 m.

The Arroyo Tembrao Stock (Lower Permian) intrusion generated a contact metamorphism evidenced by muscovite and sillimanite (fibrolite) crystallization, conforming a spotted texture.

Textural and compositional characteristics, and also facies arrangement and structures allow us to infer that the basal and middle groups were deposited in a fluvial environment. To the top an increasing of marine influence is observed.

Rocks of similar characteristic were mapped in the Ao. Nahuel Niyeu area, 80 km north-westward. They constitute 500 m of conglomerates, quarzites and arkosic quarzites.

The basal section presents the characteristics of a fluvial environment that passes to a marine environment.

Traditionally it was considered that these rocks were deposited in a marine environment but the new evidence allow us to indicate that some sections would be deposited in a continental environment.