Petrological and stratigraphical investigations of newly discovered phosphatic microfossils from the Sirban Formation, Hazara Basin, North Pakistan: Unveiling the late Sinian to middle Cambrian ecosystem

The late Sinian to Middle Cambrian Sirban Formation in Pakistan's Hazara Basin, located within the lesser Himalayas, provides critical insights into the depositional and diagenetic history of the Proto-Tethys Ocean. Originating from the breakup of Rodinia in the late Precambrian, this ocean persisted until the Devonian, influenced by the rifting of the Indian Plate from Gondwana. The Sirban Formation, comprises four lithofacies: fossiliferous limestone, dolomitic limestone, fossiliferous chert and stromatolitic brecciated and banded chert, which collectively reflect a spectrum of depositional environments ranging from high-energy shoreline to stable marine shelf. Petrographic studies reveal biosparite (bioclast packstone to grainstone), dolostone to oomicrite (ooidal packstone to grainstone), microcrystalline chert/mudstone and hematitebearing cryptocrystalline to microcrystalline chert microfacies indicative of varying conditions, including oxygen-restricted and biologically active settings, which suggest high productivity in Proto-Tethyan Ocean. Additionally, diverse porosity types in the dolomitic limestone, indicate a potential hydrocarbon reservoir. A rich fossil assemblage occurrence e.g., trilobites (Ptychoparia striata, Olenoides) and taxa like Dickinsonia and Hallucigenia, aligns the Sirban Formation with global Cambrian chronologies, providing biostratigraphic and ecological links across the Proto-Tethys. Notably, the presence of Dickinsonia and Kimberella marks the paraconformity in the area for the first time, indicating a significant stratigraphic transition. This study enhances stratigraphic correlations, particularly with the Burgess Shales Formation, and proposes a comprehensive depositional model for the Proto-Tethys marine shelf. By elucidating the interplay of tectonic, ecological and sedimentological controls, this research deepens our understanding of Late Sinian to Middle Cambrian marine environments, establishes critical linkage between regional stratigraphy and global tectonic-biological co-evolution.