Facies analysis and palaeoecology of the Jurassic Spiti Shale Formation in the Spiti area, Northern India

The Spiti Shale Formation is a widely distributed stratigraphic unit of the passive northern margin of the Indian craton, deposited between the Callovian and earliest Cretaceous. The siliciclastic strata are dominated by dark-grey to black argillaceous silt. As the formation has undergone intense tectonic stress involving folding and faulting, it is very difficult to document a complete section. In the type area, the Spiti Valley, six sections have been measured that document parts of the three informal members of the formation, the Lower, Middle, and Upper members. Despite its uniform appearance, eight facies/biofacies types could be distinguished, ranging from the anoxic shale facies, with ammonites and belemnites as the only faunal elements, to the dysoxic Malayomaorica and Bositra biofacies, and the oxic to anoxic offshore shelf facies, which are characterized by low-diversity macrobenthos associations. Other facies are the condensed glauconitic-phosphoritic mudrock facies and the Fe-oolitic siltstone facies (both characterized by sediment starvation), the aerated argillaceous silt–sandstone facies, and the tide-influenced nearshore shelf facies. The benthic macrofauna represents four bivalve-dominated associations all characterized by a very low to low species diversity. They are the Bositra buchii, the Australobuchia spitiensis, the Palaeonucula cuneiformis‒Pruvostiella hermanni‒Indogrammatodon egertonianus, and the Malayomaorica sp.‒Australobuchia spitiensis‒Retroceramus haasti association. Sediments and macrobenthic associations indicate that the Spiti Shale Formation represents outer to inner shelf environments, which for much of the time were subjected to upwelling and anoxic to dysoxic conditions. Distinct shallowing at the top characterizes the transition to the overlying Lower Cretaceous Giumal Formation.