ENVIRONMENTS AND FORMING- PROCESSES OF IRON BEARING SEDIMENTS IN THE SILURIAN-EARL y DEVONIAN SHELF OF PAT AGONIA, ARGENTINA.

Abstract

Marine strata of the Upper Silurian - Lower Devonian Sierra Grande Formation in northeastern Patagonia (Argentina) comprise silty to very coarse quartzites, siltstones, quartzglomerates and ferrilites.Based on sediment - body geometries, mechanical and biogenical structures and textures, eight sedimentary facies were recognized and grouped into five facies associations.The studied deposits were formed in a shallow open marine shelf dominated by wave and storm processes. Most (sandstone and conglomerate) facies were deposited between the beach and the upper - shore -face. The offshore deposits are represented by heterolithic facies formed under alternating fair weather and storm conditions. Paleocurrent measurements of directional structures (wave-ripples and cross beds) suggest that the local position of the shoreline was northeast-southwest. They also show that subtidal shad bars were mainly induced by longshore currents.Iron concentration was favoured by open and shallow marine environments, very slow sedimentation rate, warm paleoclimatic conditions in the source areas and a general sea level rise during the deposition of the Sierra Grande Formation.