Ironstones in the Mesozoic passive margin sequence of the Tethys Himalaya (Zanskar, Northern India): sedimentology and metamorphism

Abstract

Summary The sedimentary succession of the Zanskar continental terrace comprises Triassic (Quartzite Series), Jurassic (Ferruginous Oolite) and Cretaceous (Giumal Sandstone) shelf siliciclastic units, all of which contain either chamositic ironstones or glauconitic greensands associated with reworked phosphorites. Ironstones are found above major unconformities at the top of shoaling arenaceous sequences, and generally mark the rapid transition from shallow-marine sands to highstand offshore pelites or pelagic foraminiferal limestones. Petrographical and sedimentological features of condensed intervals indicate deposition by transgressive fronts. During sea-level rise, high-energy waves mixed ferruginous and phosphatic grains formed in the course of earlier starved stages at low sedimentation rates, with detritus from shoreline and paralic sources reworked during stepwise coastal retreat. The timing of ironstone deposition apparently coincides with breakup stages affecting the Indian continental margin. Ironstones are also important metamorphic markers in the Tethyan sedimentary zone, which has undergone intense fold-thrust deformation at very low to low metamorphic grade during the Tertiary Himalayan orogeny. In the Triassic iron oolites of central Zanskar, the occurrence of stilpnomelane suggests upper anchimetamorphic conditions ( c. 300° C). In the Cretaceous greensands, however, K-rich glauconite is only peripherally replaced by incipient stilpnomelane growth, pointing to metamorphic conditions comparable to lower prehnite-pumpellyite facies ( c. 260° C).