DEPOSITIONAL HISTORY OF THE CAMBRIAN KARSHA AND KURGIAKH FORMATIONS, ZANSKAR REGION, INDIAN HIMALAYAS

Sedimentological and stratigraphic analysis was conducted on the Middle to Upper Cambrian strata of the Karsha and Kurgiakh formations of the Zanskar Valley in the northern Indian Himalayas. The lower Mauling Member of the Karsha Formation is composed of approximately 1100 m of shale and sandstone deposits with minor carbonate beds. This member contains evidence for storm influenced sedimentation in shallow marine environments. Lithofacies range from outer shelf shale to amalgamated HCS shoreface to minor trough cross-bedded flluvial deposits. The Zanskar deposits are less cyclical, more shale rich and thus more distal than the roughly time-equivalent deltaic strata of the Kunzum La Formation exposed futher east in the Spiti Valley. Thick carbonate-rich deposits of the upper two members of the Karsha Formation are overlain by an ~250 m thick coarsening-upward succession of shale and sandstone of the Kurgiakh Formation. This transition has been interpreted to represent a shift from passive margin deposition to flysch deposition in an active foreland basin, based on the purported presence of turbidites within the Kurgiakh Formation. However, our detailed measured sections indicate that the upper 50 m of the formation contains amalgamated sandstone intervals and shoaling cycles up to several meters thick that include abundant hummocky cross-stratification and other storm-generated structures. These indicate deposition in a shelf setting above storm wave base. The stratigraphic shift to shale at the base of the Kurgiakh Formation represents deepening to outer shelf depths. This was followed by the shift to inner shelf and shoreface environments. These interpretations remove the underlying basis for the interpretation of a major tectonic event in this region.