Microfacies shift in the Late Paleocene–Early Eocene Patala Formation in the Upper Indus Basin (Pakistan): Implications for development of the Ceno-Tethys Ocean

Abstract

Field observations, petrography, and laboratory interpretations of the Patala Formation were utilized in an integrated petrologic study aimed at deciphering the genesis mechanism and controlling factors of the limestone/shale alternation during the mature development of the Ceno-Tethys Ocean in the Upper Indus Basin of Pakistan. To achieve this goal, the Patala Formation is categorized into four integrated microfacies (MM-I to MM-IV). Multiple cycles of transgression/regression are noted through fluctuations in eustatic sea level, controlled by climate and regional tectonism. Consequently, a deepening, upward retrogradation sequence was deposited on a carbonate platform of the Ceno-Tethys Ocean. The sediments of the Patala Formation exhibit a facies shift from a shallow-marine to a relatively deep-marine platform. It is indicated by the transition in fossils from shallow-marine green algae, trending towards a relatively deeper-shelf comprising benthic foraminifera such as Discocyclina, Ranikothalia, red coralline algae, echinoids (Echinothrix), and Alveolina, species. The deeper species possess robust shells adapted by organisms to maximize oxygen and light at greater depths. Additionally, mineralogical features, including gypsum (deposited in a lagoon within the platform) and pyrite (found in a deep slope toward a deep shelf), confirm deepening upward interpretations. Thermophilic dinoflagellates, thermal dissolution fragments, and hardground surfaces indicate a Paleocene-Eocene Thermal Maximum (PETM; in the lower part, ca. 56 Ma) of the Patala Formation, assigning a relative chronological age from the late Paleocene to early Eocene. These results highlight the joint influence of climate change, sea level rise, and continuous tectonic activity (associated with the Indo-Eurasian collision) on the pattern of sediment filling, and facies shift. Furthermore, the study contributes to understanding sedimentary environments, paleoenvironmental reconstruction, and paleotectonic evolution of the basin. This ideal case study provides a breakthrough advancement in globally comprehending the complex relationships between periodic autocyclic processes and aperiodic allocyclic events.