The Sacta Limestone Member (early Wenlock): Cool-water, temperate carbonate deposition at the distal foreland of Gondwana's active margin, Bolivia

Abstract

The Sacta Limestone Member is located towards the base of the Silurian Kirusillas Formation in central Bolivia, and forms part of the sedimentary record of the Peru–Bolivia Palaeozoic Basin. The early Wenlock (Sheinwoodian) age of this thin carbonate unit has been established by conodont and foraminifer biostratigraphy. During the Silurian, a thick siliciclastic wedge was deposited in the foredeep of the basin, related to tectonism along the active margin of Gondwana. Strong subsidence and downslope resedimentation (slides, slumps, debris flows, turbidites) dominated towards the west of the basin during the Llandovery and early Wenlock. In contrast, a much thinner record was deposited at the distal margin of the foreland basin, pinching out towards the northeast and onlapping the stable Precambrian basement. The Sacta Member represents cool-water, temperate carbonate deposition at mid-to-high latitude on the distal stable margin of this foreland basin. Its sedimentation took place towards the end of a phase of important tectonic instability, and the carbonate bed was itself also partially resedimented. Late Ashgill?–Llandovery glaciation of nearby reliefs along the active margin of Gondwana was followed by brief and local cool-water, temperate carbonate deposition in the early Wenlock, and subsequently by the development of a storm- and wave-dominated shallow clastic shelf during the rest of the Silurian. The Sacta Member records carbonate deposition in western Gondwana during a brief and singular event with important palaeogeographic and palaeoclimatic implications.