Glacial Attribute and Sedimentary Evolution of the Cryogenian Yuermeinak Formation in Aksu Area, Northwestern Tarim

Glaciers were a central element of the Cryogenian period in the Neoproterozoic era. Through their movement, glaciers shaped both depositional and erosional landforms. The latter serve as critical evidence for understanding glacier kinematics and represent the most definitive indicator of thermal regimes. These landforms hold significant implications for determining sedimentary environments and reconstructing paleogeography. Cryogenian subglacial erosional forms are preserved on several cratons globally, with China’s only example located in the Aksu area, NW Tarim Craton. However, numerous well-preserved erosional forms here remain unsystematically described and studied, hindering research on Cryogenian glacial dynamics, depositional environments, and paleogeographic reconstruction of the Tarim Craton. Detailed measurements and characterization were performed on eight sites of Cryogenian erosional forms in Aksu, identifying striations, various p-forms, and roche moutonnées, which indicate meltwater involvement at the ice-bedrock interface and reflect glacial kinematics with a northeast-to-southwest moving direction. Three lithofacies associations have been identified in the Yuermeinak Formation: (1) Massive diamictite, which can be divided into two types. The first type is widely distributed at the bottom of the succession in the study area. The most direct feature is that it directly and unconformably overlies the glacial pavement or basement rock, with a small thickness (several meters or thinner). The surfaces of gravels are characterized by striations and flatiron-like shapes, and the gravels show a weak directional arrangement. The second type is located at the top of the Yuermeinak Formation and is found in the northern part of the study area. The gravels here have no obvious orientation. Directly overlying it is the post-glacial marine cap dolomite, which can even be seen wedging into the underlying diamictite. (2) Thick-bedded pebbly sandstone intercalated with thin-bedded sandstone rhythms, which develop in the middle part of the succession in the study area. The gravels in the pebbly sandstone have obvious imbricate structures and are mainly composed of faceted and striated clasts. The sandstone develops trough cross-bedding and graded bedding, and generally shows an upward-fining sequence. (3) Purplish-red or greyish-green laminated silty mudstone with very few or no dropstones. This lithofacies association develops in the upper-middle part of the succession, transitioning upward gradually into pebbly sandstone or develops beneath the diamictite, or just directly underlying the cap dolomite. Based on the above facies markers, the following facies can be identified: (1) the subglacial lodgement facies represented by the first type diamictite and the underlying glacial pavement; (2) the periglacial outwash (fluvial-alluvial) fan represented by the pebbly sandstone intercalated with thin-bedded sandstone rhythms and laminated silty mudstone; (3) the subaqueous debris flow in the proximal glaciomarine environment indicated by the second type diamictite. Therefore, the vertical succession and evolutionary pattern of sedimentary characteristics of the Yuermeinak Formation from north to south collectively indicate a sedimentary environment evolutionary sequence of continental glacier: subglacial→periglacial→glaciomarine→normal shallow marine. This study is co-funded by the National Natural Science Foundation of China (42302123, 42472150, 42072135), Fundamental Research Fund of CAGS (JKYQN202421).