MAIN CHARACTERISTICS OF THE STORM DEPOSITS IN THE LIULING GROUP IN THE QINLING MOUNTAIN AREA

A succession of tempestites with typical sedimentary sequences of the storm deposits is developed in the first lithologic member at the bottom of the Qingshiya Formation of the Liuling Group in the Qinling Mountain area.
The tempestites in the study area are characterized by abrupt sole structures,graded beds, parallel lamination, ripple cross-lamination, hummocky cross-stratification and diagnostic trace fossils. The sole structures are interpreted to be attributed to ablation of the substrates by storm currents and subsequent filling of scoured pits by lag deposits. The graded beds composed of the shell beds with normal grading are laterally discontinuous, orientedly arranged and have infiltration fabrics. Parallel lamination and wave ripple cross-lamination are common, whereas superposed wave ripple bedding, hummocky cross-stratification and convolute bedding are rare. There are sharp confacts between parallel lamination and underlying shell beds, in which vertical or oblique burrows and escape structures are abundant.
The tempestite sequences in the Qingshiya Formation of the Liuling Group may be divided, from base to top, into. A) sharp basal contact; B) graded beds; C) parallel lamination division; D) ripple cross-lamination, division; E) plates. Six types of the tempestite sections have been distinguished in the Qingshiya Formation of the Liuling Group. The presence of shell lag deposits and argillaceous beds deposited under fair-weathers and the absence of the upper lamination units in Type Ⅰ suggest typical proximal tempestites, whereas the common upper lamination units and rare basal scour and infill structures from Types Ⅱ to Ⅵ indicate distal tempestites. The existence of Type Ⅰ in the lower part of the tempestites in the study area and Types Ⅱ to Ⅵ in the upper part of it shows a sedimentary environment from shallow-water shelf to deep-water shelf. It is inferred from the turbidity current deposits in the northern overlying Tongyusi Formation that the Liuling Group in the study area may be a transgressive sequence.