Depositional and stratigraphic evolution of a Permian megalake system: Implications for seiche-influenced models

Abstract

The Permian succession of the Paraná Basin records the progressive disconnection from the Panthalassic Ocean that bathed the southwestern Gondwana Supercontinent from the Ordovician to the early Permian. The development of the Gondwanides Belt in the southwestern part of the continent acted as an orographic barrier, restricting marine connections and trapping marine waters in a megalake. In this study, we present a detailed analysis of the transition from marine to continental environments in the Serra Alta and Corumbataí formations, documenting high-resolution stratigraphic sequences with significant hydrological and salinity changes. Our data illustrate how sedimentation and accommodation dynamics shaped the evolution of this continental-scale lake system, which was greatly influenced by meteorological phenomena, including storms and seiches. The megalake experienced transitions between overfilled, brackish to freshwater balanced-fill and saline underfilled stages, associated with distinct fourth-order transgressive and regressive stratigraphic sequences. These changes in water balance and salinity fostered the development of a unique, endemic bivalve-dominated fauna derived from marine ancestors, highlighting the basin's response to changing environmental conditions. Notably, this study identifies tectonic events and climate shifts as primary allogenic forces controlling deposition. Meanwhile, local sediment dynamics and episodic events such as storms and seiches originated key autogenic changes in the resulting stacking patterns. Such meteorological phenomena generated an intriguing heterolithic pattern in the fine-grained lake deposits, which would otherwise be confused with astronomical tides. Our results provide insights into the understanding of sedimentological processes in large lacustrine systems, with implications for paleoclimatic and paleoenvironmental reconstructions in megalakes from the geological record.