Abstract:On the basis of detailed analysis of sedimentary facies and strata geometry, the present study reinterprets the Laiyang Formation (Lower Cretaceous, Lingshandao, Qingdao), previously considered as a depositional system of deep-water, as a plate subduction depositional system. In the sandstone beds of the LF, planar, low-angle and undulating laminations with weakly asymmetric hummocky and swaley bedforms, combined-flow ripples, accretionary hummocky and swaley cross-stratification-like (HCS-like and SCS-like), and anisotropic HCS-like and SCS-like suggest the action of oscillatory currents combined with unidirectional currents in forming the deposits. Most storm sediments and tempestite presently recognized lie in south Beilaishi profile, but surprisingly, equally as many may have also occurred in north Beilaishi profile and that have most distinct and perfect features of storm sediments and tempestite.
Noteworthy features of the hummocky cross-stratification include the presence of depositional and eroded storms and tsunamis have differences, but also commonalities such as onshore flooding and infrastructure damage domes in addition to scoured depressions, the absence of significant bedform migration, and the presence of a small proportion of dip angles greater than the angle of repose (>55 °) in addition to the large proportion of low (<15°) dip angles. The lower, stratified, fining-upward part of the cycle (up to the top of the shale bed) is interpreted as having accumulated under conditions of initially great but gradually decreasing current velocity and deposition rate. The currents probably had a strong oscillatory component, and the depositional event is inferred to have been a storm. The part of the planar- and ripple-bedded sandstone above the shale bed was probably deposited during relatively fair weather after the storm but before re-establishment of a normal benthic fauna. The bioturbated sandstone is interpreted to have been deposited during fair weather or during minor storms separated by long intervals of fair weather. Recentlly, a set of sandstone hummocky cross-stratification is coming to be recognized in sedimentary deposits on Lingshan Island. In this paper we described an excellent example of lacustrine sandstone characterized by cycles of storm and nonstorm layers and by hummocky and swaleyy cross-stratification in the storm layers.
The present paper focuses on the part of the sequence that is characterized by storm-nonstorm cycles and associated hummocky cross-stratification. We give some new descriptive details of hummocky cross-stratification and offer a more detailed interpretation of the storm processes than has previously been attempted. The HCS beds range a few decades of centimenters to 220 cm thick and contain well-preserved hummocks, which are generally asymmetric with steeper ends to the east. The heights of HSC and SCS are in many cases up to more than 1 meter.
Key words：Storm; storm sediments; tempestite; hummocky structure; swaley structure; (anti)dual structure; lacustrine and Lingshan island