秦岭刘岭群风暴沉积的基本特征

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

摘要:

风暴沉积(storm deposits)是由风暴浪引起的一种密度流沉积,主要发生在具开阔海面的浅海陆棚区。风暴岩(tempestite)一词是由凯林(G.Kelling,1973)提出,用以描述受风暴扰动后再沉积的浅海盆地的递变浊积岩。库马尔和桑德(N.Kumar和J.E.Sander,1978)以及艾格内尔(T.Aigner,1979)将其含义扩充为风暴沉积,泛指非正常天气风暴作用所形成的一系列沉积。

Abstract:

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.