Biogenic and physical sedimentary structures in latest Cambrian-earliest Ordovician mudrock facies (Famatina Range, Northwestern Argentina)

Abstract

Lamination is a common feature in shales and other mud-dominated rocks. When combined with other geological evidence, the diverse lamination types can provide important information about sedimentary processes and depositional environments. Petrographic and Scanning Electron Microscope studies performed in the latest Cambrian- earliest Ordovician lower Member of the Volcancito Formation (Famatina Range) have revealed three distinct types of lamination in mudrocks. Thick horizontal lamination (> 0.5 mm thick) is characterized by dark clay laminae alternating with calcite and quartz-rich light colored silt laminae. Silty laminae show micro-scours at the base, and unidirectional micro-ripple cross-lamination. This type of lamination is dominant in the lower part of the succession, and it is interpreted as formed by bottom flowing currents, probably induced by storms. Wavy lamination shows clear undulation surfaces at megascopic scale. In thin section, wavy lamination is composed by alternating black organic-rich laminae less than 0.2 mm thick, and lighter colored carbonate-rich clay and fine silt laminae (0.1-0.2 mm thick). This second type of lamination is observed at the lower and middle parts of the sequence and it is interpreted as produced by benthic microbial mat development (probably cyanobacteria). Thin horizontal lamination (< 0.5 mm thick) is characterized by dark organic and clay-rich laminae alternating with lighter colored quartz-rich silty laminae. This lamination may be associated with suspension settling from dilute suspended fine-grained sediment flows. Thin horizontal lamination is predominant at the upper part of the succession. Changes in lamination and sedimentary processes observed from base to top of the studied succession record a major deepening event in the basin during the latest Cambrian - earliest Ordovician, which finally triggered the deposition of a thick lower Tremadoc black shale package. This event was also responsible of a significant faunal change through the sequence. The representing fauna of the Parabolina frequens argentina Biozo-ne changes upwards from predominant benthic trilobites (e.g. Parabolinella, Angelina hyeronimi (KAYSER), Shumardia erquensis KOBAYASHI, Onyclopyge, etc.) to pelagic trilobites (exclusively Jujuyaspis), which occur associated with planktonic graptolites (Rhabdinopora). Changes in facies pattern and faunal components record a relative sea level rise, which resulted in a major transgression at the base of the Ordovician.