Lithofacies Characters and Significance of the Submarine Fan of the Liufengguan Group in Qinling

Abstract

Abstract:Field investigation and laboratory research on flysch of the Liufengguan Group in Qinling indicate the following: (1) Sandstone of the Liufengguan Group is categorized as feldspathic lithic graywacke with a minor amount of lithic graywacke in the QFR triangular diagram. Grain size ≤0.3 mm. Bedding plane structures such as groove casts and suspected flute casts can be found at the bottom of the sandstone. It is inferred that currents may have come from the southeast during deposition. Bedding structures such as ripple marks, graded bedding, parallel bedding, small‐scale cross bedding, climbing bedding, suspected convolute bedding, microlamination and sliding structures have also been observed, which are of indicative significance. It is thought that the Liufengguan Group has the sedimentary characteristics of bedding, bedding plane structures and lithological assemblages of deep‐sea low‐density turbidity current deposits. The vertical succession of the Bouma sequence in the inner fan subfacies zone is generally incomplete: the assemblage of Ta and Tabc is commonly seen; the succession of the middle fan subfacies zone is relatively complete; and divisions Te and Tb are common in the outer fan subfacies zone. (2) The flysh of the Liufengguan Group is a sequence of deep‐sea argillaceous‐arenaceous submarine fan deposits, in which the authors recognize the inner, middle and outer fan subfacies and also nine types of lithofacies: normal graded sandstone (A1), medium‐ to thick‐bedded, fine‐grained sandstone (A2), medium‐ to thick‐bedded and massive siltstone (A3), thin‐bedded, fine‐grained sandstone and mudstone (B1), irregular interbeds of thin‐bedded, fine‐grained sandstone and siltstone (B2), thin‐bedded, fine‐grained sandstone (C1), very thin‐bedded, fine‐grained sandstone (D1), olistostromes (E1) and deep‐sea mudstone (F). The inner fan consists of four microfacies: natural levee (A1), water channel (A2, A3) and olistostromc (E1); in the middle fan there also occur four microfacies, i.e., branch channel (B1), branch channel (B2), interdistributary bay (D1) and olistostrome. The outer fan is made up of the branch channel (C1) and sheet sand (D1) microfacies, which alternate vertically with sediments of deep‐sea plain subfacies (F). There occur fining‐ and thin ning‐upward channel deposits in the outer‐fan subfacies zone of the submarine fan of the Liufengguan Group observed in this study. The quartz content of the graywacke of the deposits is all higher than 40% and may reach as high as 60%. Therefore, on the basis of the aforementioned features, this flysh should be formed in a passive continental‐margin tectonic environment.