Sedimentology, geochemistry and palaeoenvironments of Jurassic oolitic ironstones from the eastern Tethys (Indus Basin, northwestern Himalayas, Pakistan)

Oolitic ironstones in Lower (Toarcian) and Middle (Bajocian) Jurassic strata of the Indus Basin, Pakistan, were deposited in the eastern part of Tethys and are similar to those from India. Both units are represented by shallow-marine iron ooidal-siliciclastic wacke-packstone carbonates with sedimentary structures including ripple marks, cross lamination and trace fossils. Both the Toarcian and Bajocian iron ooids are mainly composed of goethite, the Toarcian ones have a higher Fe content than those in the Bajocian but the older unit has fewer silicate grains and lower contents of Si, Al, K and Sr. Thin to thick carbonate laminae occur between the Fe-rich laminae in the Bajocian iron ooids. The iron ooids in the Toarcian unit are related to phases of relative sea-level fall and represent deposition in a nearshore lagoonal setting whereas the Bajocian unit was deposited in a relatively broad spectrum of environments. The Fe itself was sourced from weathering of ferruginous soils and higher contents of K, Na, Ti and Ti/Al indicate a continental source. The cortical growth of the iron ooids in the Toarcian unit mainly took place under oxic conditions whereas conditions were more oxic to suboxic and anoxic in the Bajocian unit. The occurrence of spastoliths indicates the former presence of berthierine and the alternation of Fe-rich and carbonate-rich laminae within the Bajocian iron ooids suggests frequent burial and reworking of ooids in the near subsurface sub-oxic to anoxic environment. The subsequent exposure to oxic conditions on the seafloor likely altered the berthierine to goethite, which is stable in oxidizing conditions.