Three deep holes(Well TX1

XH1 and BT5)encountered clastic sedimentary rocks underlying the Cambrian carbonates in the Bachu uplift of the Tarim Basin.After detailed study of these clastic rocks through petrographic observations

clastic composition statistics and geo-chemical analyses

their provenances and the tectonic setting for their formation were discussed in depth.The clastic compositions of conglomerates and sandstones together with the geochemical characteristics of mudstones indicate that the provenances of the clastic sedimentary rocks underlying the Cambrian carbonates in the three wells include sedimentary rocks

igneous rocks

metamorphic rocks

among which the intermediate-acidic volcanic rocks and intrusive rocks are predominant.The angular-to-sub-rounded appearance of the clastic particles in the conglomerates and sandstones

the presence of erodible compositions in gravels(such as carbonates

schists)and the lower Th/U ratio of mudstones collectively show that these clastic rocks are near-source deposits with low maturity.By comparing the clastic composition

geochemical characteristics

and age structure of detrital zircons

we could determine that the detrital components of these clastic rocks are mainly derived from the underlying basement.The detrital models of conglomerates and sandstones

geochemical characteristics of mudstones and in-situ trace element characteristics of detrital zircons suggest that the source area

that is the underlying basement

was located in an arc-related tectonic setting on the active continental margin in the Neoproterozoic(peak: 790-730 Ma).The newly obtained evidences of clastic rocks are well consistent with the previous studies on the age and tectonic setting of the andesites and dacites at the bottom of Well Tong-1 and the granodiorites and diorites at the bottom of Well Tashen-1 in the Katake uplift

demonstrating that the central uplift zone(Bachu uplift-Katake uplift-Guchengxu uplift)across the central Tarim Basin is a Neoproterozoic continental marginal arc

while the basement of the Tarim Basin is the result of amalgamation caused by Neoproterozoic orogeny.