广西桂林庙头上泥盆统融县组中的灰岩脉和角砾灰岩体及其成因

广西桂林庙头上泥盆统融县组台地边缘相具鸟眼或溶孔砂屑生屑藻粘结灰岩中发育了大量灰岩脉，灰岩脉呈岩墙状近直立地切过围岩，露头上可见连续垂直延伸长度达十余米，脉内仍是上泥盆统融县组灰岩组分，仅比围岩稍晚或同时代的：角砾状退白云石化藻粘结灰岩、角砾状藻粘结灰岩（快速堆积，角砾多数来自围岩）、细鲕粒颗粒灰岩、含钙球砂屑粒泥灰岩、藻砂屑腹足泥粒灰岩（正常沉积或液化变形软沉积流，均高于围岩层位）及大量栉壳状方解石脉（张性环境）等，灰岩脉内可见软沉积物变形痕迹及液化脉（地震液化），晚期灰岩脉可穿插早期灰岩脉；在其西侧同层位共生有一套楔状（地裂缝）、不规则状角砾灰岩体，角砾棱角状，大小不等，成分是灰色中厚层角砾状藻粘结灰岩、含钙球砂屑粒泥灰岩等。笔者等注意到灰岩脉均出露在北东向正断层的下盘（东侧），而角砾灰岩体则均出露在该断层的上盘（西侧），事实上，灰岩脉及角砾灰岩体分布走向与北东向断层走向一致，角砾灰岩体中的角砾成分显示其来自较灰岩脉围岩稍高层位，与灰岩脉内充填岩性相近，因此，是该断层控制了当时的沉积，此断层是晚泥盆世台地边缘同沉积正断层；灰岩脉是与断层伴生的张性裂隙被围岩角砾或稍晚时段的沉积物充填；角砾灰岩体是同沉积正断层形成的断层崖崩落角砾岩，指示一个消失殆尽的晚泥盆世碳酸盐岩同沉积正断层陡崖，因而，桂林台地，至少西段台地边缘，是与右江各孤立台地边缘一样——在地质图上应标示为同沉积正断层边界。其构造意义是：庙头地区的灰岩脉、角砾灰岩体及同沉积断层是桂林台地对晚泥盆世构造伸展作用的响应，从而说明广西晚古生代的板块拉张，不是从二叠纪才开始的，而是至少从晚泥盆世就开始了；晚古生代至早三叠世持续的孤立台地与深水盆地相间的古地理格局，是在晚泥盆世就奠定了基础。

Limestone Dykes and Breccia Limestone Bodies in the Upper Devonian Rongxian Formation in Miaotou Area, Guilin, Guangxi

The Rongxian Fm. bird-eye-or hollow-bearing calcarenaceous bioclast algal bindstones build up the carbonate shelf margin of the Upper Devonian, in which limestone dykes occur around Miaotou area, Guilin, Guangxi. Limestone dykes cut perpendicularly through the surrounding rocks and could be observed centimeters to tens centimeters wide and more than ten meters long from an outcrop. Limestones inside the dykes are younger than the surrounding rocks, even though both of them belong to the Rongxian Fm. The dykes are composed of breccia retro-dolomitized algal bindstones, breccia bindstones (both are rapid deposition, breccia are from surrounding rocks), fine ooid grainstones, calcisphere-bearing calcarenaceous wackestones, algal-calcarenaceous gastropod pckstones (normal sediments, or liquefied sediments flows, all younger than the surroundings), and amounts of pectinate calcites dykes (tensional background). Soft sediments deformation and liquefied vines are identified inside limestone dykes (seismic). Later dykes cut early ones. At the same outcrop, amounts of breccia limestone wedges or cumulates are observed lying in the west. The breccia are sharp-edged gray thick-bedded algal bindstones and calcisphere-bearing calcarenaceous wackestones, some of which are shatter breccia. Writers notice that limestone dykes outcropped only in the footwall of the northeastern strike normal fault (eastern wall), however breccia limestone bodies outcropped only in the overwall of the normal fault (western wall). In fact, the strike of the limestone dykes and the breccia bodies roughly along the strike of the normal fault. The breccia in the breccia limestone bodies indicated they were from higher position than the sounding rocks, roughly the same as the fillings inside the limestone dykes. Therefore, the northeastern strike normal fault controlled the deposition of the Late Devonian. In other words, the fault was synsedimentary normal fault occurring during Late Devonian carbonate shelf margin. The limestone dykes are tensional fissures caused by the faulting and filled with surrounding rocks breccia or later normal marine sediments. The breccia limestone bodies are scarp fall breccia, suggest a nearly disappeared Late Devonian synsedimentary normal fault scarp. Therefore the northwestern boundary of the Guiling carbonate shelf margin, the same as which of those in the Youjiang basin, should be drawn as a (paleo-)fault boundary in geological maps. The limestone dykes, the breccia limestone bodies and the synsedimentary normal fault are the response of the Guilin carbonate shelf margin to the structural stretch during Late Devonian. Thus the tensional stretch of the Late Palaeozoic Guangxi marine started at least at the Late Devonian, rather than the reported Middle Permian. The palaeogeographic pattern had been founded since Late Devonian.