Phosphogenesis of Kakul Garlaniyan phosphorites during the early Cambrian in northeastern Pakistan

Abstract

The largest phosphogenesis event in Earth’s history occurred during the Precambrian-Cambrian transition, characterized by the formation of extensive global phosphorite deposits. One of the most notable examples is the Kakul Garlaniyan phosphorite, located at the topmost part of the Sirban member of the Abbottabad Formation in the Hazara region of northeastern Pakistan, dating to the Early Cambrian (∼541 Ma, Terreneuvian). Despite its significance, limited studies have addressed the origin and formation processes of the phosphorite deposits in this area. To enhance our understanding, we conducted petrographic and geochemical analyses on samples from three intercalated phosphorite layers within the Sirban Member. The phosphorite comprises both primary and reworked grains, with quartz supplied by terrestrial input. The grains are derived from a marine origin, enriched by upwelling of nutrient-rich bottom seawater. Additionally, the radiogenic ⁸⁷Sr/⁸⁶Sr isotope ratios (0.7113–0.7129) indicate terrestrial flux to the inner shelf environment through the chemical weathering of continental rocks. Rare earth and yttrium (REY) patterns exhibit a seawater-like, post-Archean Australian Shale (PAAS)-normalized REY distribution, with significant negative Ce/Ce* anomalies (0.40–0.54) and super-chondritic Y/Ho ratios (∼50–60), indicating a hydrogenous origin and supporting oxic conditions. The (La/Yb)_N and (La/Sm)_N ratios in Kakul Garlaniyan phosphorite layer suggest REY adsorption during early diagenesis, consistent with modern seawater conditions. Based on these findings, we propose a phosphogenesis depositional model: phosphorite formed via upwelling and reworking rather than hydrothermal activity under mainly oxic (with rarely anoxic conditions) and abiotic conditions. Winnowing and transgression events transformed pre-existing phosphate mud into reworked grains, occurring in the absence of microbial mediation. These findings provide new insights into the processes governing phosphorite formation during the Early Cambrian period in the Abbottabad region.