Depositional cyclicity of lower Cambrian strata in the NW Himalayas: Regional sequence stratigraphy of the Indian passive margin

Abstract

Lower Cambrian (538-512 Ma) strata have been deposited in the northern Hazara, Kashmir, Salt Range and Zanskar-Spiti basins, which were undergoing subsidence during this period. However, the southern part of the Hazara Basin remained uplifted (Muzaffarabad-Hazara paleo-high, MHP) and acted as a sediment source. Unlike the previous work, the present paper provides an integrated sequence stratigraphic framework of the Lower Cambrian in this region, based on outcrop data, petrography, and well, seismic and gravity data, thereby enabling a correlation among the basins. In the NW Himalayas (Pakistan), the Lower Cambrian strata outcrop in the Salt Range, northern Hazara and Kashmir basins were penetrated during drilling in the Salt Basin (Potwar, Punjab Platform and Bikaner-Naguar basins). Lower Cambrian stratal terminations and subaerial/composite unconformities formed towards the Sargodha paleo-high and the MHP. The Jehlum and Abbottabad groups are well preserved in outcrops of clastic and carbonate sequences. The top of the Khewra/Sangargali sandstone marks an erosional surface and separates two sequences – a progradational and a retrogradational/progradational sequence. The progradational sequence began with sandstone deposition (Khewra and Tanaki Boulder Bed/Sangar Gali formation), just after Khewrite volcanism (538 Ma), with fining upward cycles during a falling stage system tract. Sea level dropped further, as marked by an intraformational subaerial unconformity, and the fluvio-deltaic facies of a lowstand system tract was deposited; this was again capped by a subaerial unconformity. Once subsidence began, a transgressive systems tract developed in a marginal marine setting. Further transgression occurred and a widespread tidalflat was established. These subtidal facies marked the maximum flooding surface in Jutana and Abbottabad formations and a highstand system tract was developed. During the late stages of the highstand system tract the clastic Baghanwala and Hazira formations were deposited and subaerial unconformities were formed at the top and bottom of these formations. The Kashmir Basin remained uplifted during the late stages of the highstand systems tract and no deposition occurred until the Danian. The maximum flooding surfaces of the subtidal facies are correlatable on both shoulders of the MHP. The deposition ceased in the Middle Cambrian with the intrusion of the Mansehra type granites.