Base Level Control on the Sequence Patterns of Ancient Low-Stand Meandering Valley of Middle Jurassic Shaximiao Formation in the Western Sichuan Depression

Abstract

The middle Jurassic Shaximiao formation encompasses large-scale low-stand meandering valley (LMV) successions in the western Sichuan foreland basin. Ancient LMVs were first discovered in the area and further studied by sequence stratigraphy analysis and seismic sedimentology methods. The present study aimed to investigate the sedimentary characteristics and sequence architecture of LMVs deposited in tectonically active basin by field survey, seismic section, seismic amplitude image, core description, and comprehensive application of drilling data. Using an integrated approach of depositional facies analysis of outcrop profile, seismic facies, and electrofacies, we described three facies associations, including channel, overbank, and delta facies, in the LMVs. Abandoned channel and point bar constituted the channel facies, which are mainly found in LMV infills. An in-depth examination of available data revealed the following sedimentary characteristics of the LMVs of ancient Shaximiao formation. (1) According to the core data, mudstone intervals are brown and brown-red, suggesting a shallow water sedimentary environment. (2) The types of sedimentary structure present on the well cores include large-scale tabular (trough) cross-bedding, inclined bedding, parallel bedding, and erosion surfaces. (3) Well-logging curves are mainly cylinder bell; the seismic reflection configuration is characterized by strong amplitude, low frequency, and poor continuity. Considering the transgressive-regressive curve, accommodation, system tracts, and valley shape, we grouped the sequence architecture of LMVs into three types, namely, A1, A2, and A3. On the basis of sequence architecture and sedimentary characteristics, we constructed a typical vertical section and a sedimentary model of LMVs in different stages; this model was used as a reference to interpret the sedimentary evolution of ancient LMVs developed on the foreland basin margin. Key words: base level fluctuated; sequence patterns; low-stand meandering valley; western Sichuan foreland basin; Shaximiao formation

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016