Sedimentation and formation of Late Cretaceous and Paleogene ironstones in ancient epicontinental West Siberian Sea

Abstract

This study focuses on reconstructing the paleoenvironmental conditions responsible for the formation of Upper Cretaceous and Paleogene ooidal ironstones in Western Siberia, using detailed analyses of the Bakchar deposit. Two clastic and four chemical lithofacies are identified in the deposit. Morphometric parameters calculated based on an analysis of the authigenic and detrital fractions of ironstones and host rocks are used to determine the degree of undisturbed (autochthonous) or disturbed/re-transported (parautochthonous) chemical sediments. Spatial changes in sedimentation from the coastline are expressed by replacing chemical lithofacies with clastic lithofacies or by transitioning medium- and fine-grained sandstone lithofacies into sandy-clay deposits. Chemical lithofacies consist predominantly of iron-rich ooids, peloids and/or glauconite pellets with different types of cement. They are not laterally equivalent to one another, but each represents a distinct lithological type of ironstones formed at different times in the basin. Textural features of ooidal ironstones combined with assessment of the authigenic and detrital fractions make it possible to distinguish depositional hiatuses and intervals of autochthonous ooid deposition (autochthonous ooid interval). The Bakchar succession contains seven main autochthonous ooid intervals in the following times: Middle Santonian, Late Santonian, Middle Campanian, Campanian-Maastrichtian boundary, Middle Maastrichtian, Late Maastrichtian, and Paleocene-Eocene boundary. Autochthonous ooid intervals are usually transited up section by intraclast-rich ironstone layers with depositional hiatuses. Potential sources of detrital fraction were identified based on the detrital assemblages and the detrital zircon age. The weathering of Permian-Triassic intrusive rocks of the Tom-Kolyvan and the Salair folded systems is assumed to be a source of detrital minerals for the Santonian layers. Meanwhile, for the Maastrichtian and Paleocene successions, an additional part of the material was inputted from the intrusive Paleozoic formations of the Kuznetsk Alatau. Fingerprints of post-sedimentation (burial diagenetic) influences on ironstones are the presence of veinlet siderite cement, associations of siderite cement with pyrrhotite or wurtzite, and injections of ooids into each other. The lithofacies and structural characteristics of the ooid fraction (autochthonous ooid intervals), the mineral assemblages and the chemical composition of the ironstones testify to the repeated supply of ore-forming hydrothermal solutions to the basin of the Bakchar deposit. Fluid mobilisation events coincide with ironstone layers with an autochthonous ooid fraction, which are often overlain by layers with depositional hiatuses, expressed in an increase in the proportion of parautochthonous items of sedimentary rocks.