Marine depositional signatures of the Aptian Oceanic Anoxic Events in the Eastern Tethys, Lower Indus Basin, Pakistan

Abstract

The depositional signatures of the Cretaceous OAE1a and OAE1b intervals in the Parh Formation, Lower Indus Basin, Pakistan, Eastern Tethys, have been studied to better constrain the Aptian–Albian ocean–climate system. Based on the microfacies attributes, an outer ramp to deep basin is suggested for the deposition of Parh Formation. The quasi-periodic occurrences of radiolarian-rich microfacies and the cyclicity between the outer ramp and deep basin settings throughout the section suggest eutrophication in this part of the Eastern Tethys influenced by Milankovitch cycles. The clastic influx during OAE1a and OAE1b intervals indicate the role of continental runoff in the eutrophication of the basin. The overall negative covariance between radiolarian-rich microfacies and clastic input suggests that upwelling was a major driver of nutrient enrichment rather than continental runoff. The organic enrichment along with pyrite framboids and lack of burrows suggest that dysoxic–anoxic bottom water conditions were common throughout the Aptian and not just in the OAE1a and OAE1b intervals. The poor ventilation of the sea floor was a result of restricted circulation and may have triggered sustained anoxia during Aptian in this part of the Eastern Tethys. Bottom-water currents recharged coarser clastics to the deeper part of the basin during OAE1a and OAE1b intervals. However, the effects of deep ventilated oxic water during the events were reduced by persistent ocean anoxia. Furthermore, the dominance of carbonate sediments throughout the Aptian–lower Albian succession suggests relatively alkaline oceans in this part of Eastern Tethys. A pronounced sea-level rise, as suggested elsewhere was not observed during OAE1a and OAE1b intervals in this part of the Tethys.

KEY POINTS

1. The Aptian–Albian hemipelagic sequence of the Parh Formation, Lower Indus Basin, Pakistan, Eastern Tethys has been deposited in an outer ramp to deep basin environment.

2. The quasi-periodic occurrences of radiolarian-rich microfacies and cyclic alternation of depositional environments suggest eutrophication of the basin influenced by Milankovitch cycles.

3. Organic enrichment, pyrite framboids and lack of burrows suggest dysoxic–anoxic bottom-water conditions throughout the Aptian to early Albian.

Keywords:

• microfacies
• eutrophication
• anoxic
• Tethys
• Milankovitch cycle
• upwelling
• productivity

Acknowledgements

The authors are thankful to the Higher Education Commission of Pakistan and School of GeoSciences, University of Edinburgh, UK, for support.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This project was funded by Strategic Pilot Science and Technology Projects of the Chinese Academy of Sciences, Award XDB26000000.