湖北大洪山区域地质问题

Gcneral Geological Features The Ta-hungshan area is a rolling country with several rather high andnarrow mountain ranges stretching east-west on the north and north-west onthe south. All the ranges meet together eastward to make up a few mountainpeaks about 700800 meters bigh. The area is bordered on both sides by theHan river on the west and theYun river on the east. Both the rivers with theirflooded planes are winding their courses approximately in the south-southeastwarddirection and joined together on the south to form a swamp area. So it makesthe whole area a mountain belt of north-northwestern trend. The subdivisions of rock formations may be summarized as follows in des-cending order. Quarterary: Alluvial clay and sands. Loess. Red loam. Often on the terraces about 8Ometers high. Tertiary: Tunghu sandstone. Red sandstone and gravels. 150m. Triassic: Chinglung limestone. Thin-bedded limestone and laminated limestone divided into thick strata. Thickness unknown. Permian: Lungtan series. Sandstone and black shale with coal seams, about 40m. in thickness. Kuhfeng beds. It is chiefly composed of chert beds aud siliceous shalesin the south sections of the Kingshan city, but sandy and calca-reous shales yielding fossils of ammonites and dwarf Brachiopodsin the Huchiachi area. About 40m.Chihsia limestone. Dark gray limestone. About 40m.Carboniferous:Lower Yangsing limestone. Pinkish and light gray limestone intercalatedwith dark gray limestone beds. 60m.Devonian:Yungtaikuan quartzite. Coarse quartzose sandstone and conglomerate.Few in the Ta-hungshan proper, but very well developed on thesouthem neighbouring lands.Silurian:Sintan shale. Mainly alternating beds of greenish-yellow shale and san-dstone, with several black shale beds about 50 meters at the base,yielding abundant fossils of Graptolites, such as Monograptus,Rastrites, Climacograptus etc.; submetamorphosed and often withquartz veins are also characteristic. It is wide spread all over theTa-hungshan area with wonderful thickness, 3000m. at least.Ordovician:Pagoda limestone. Greenish-yellow, slabby, argillaceous limestone, oftenwell bedded, abundant in fossils of Cephalopods, yielding Orthoce-ras, Vaginoceras etc. 100m.Siaokisze limestone. Thin-bedded limetone intercalated with shales,yielding Cameroceras. 400m.Cambrian:Wuchiachi limestone. Thin and thick bedded limestones with chertnodules, often phosphorous. 621m. Lumensze limestone. Thin-beded and shaly limestone and shale, withGirvanella beds at the top. Fossiles of Trilobites are abundant;Redlichia meitanensusis and Kootenia asiatica are the leadingtypes. It is also partly phosphorous. 300m. in thickness.Shipai shale. Red shale and thin-bedded argillaceous limestone, witha conglomerate bed at the base. Fossils are abundant in the redshale in the middlepart of the formation, yielding Redlichia chi-nensis, Palaeolenus sp. and many other species of Redlichia.340m. in thickness.Sinian:Toying limestone. Siliceous limestone with bands of chert, the basalpart not exposed at the Ta-hungshan area.The structure of the whole Ta-hungshan area is a synclinorium. It cohsistsof a few narrow anticlines and rather broad synclines. Faults both along thestrikes and across the strikes of the folding are predominant. The chief foldunits may be divided from north to south as follows.1. The Changshan anticline. It is chiefly composed of the lower Cambrianstrata, only with the Sinian rock in the axial part. All the rocks are metamorph-osed into schists in the eastern part. The axis of the anticline holds the east-west directed strike for the most part, but tums to the north-northwesterndirection near the Han river, where a fault might be prerent. Near the axialpart the rocks are overturned and thrust to the north.2. The Lianglaishan syncline. This is a hilly country of widely spreadingSilurian rocks. All the rocks are metamorphosed into schist on the east. Itsstrike is almost east-west.3. The Ta-poshan anticline. It is chiefly composed of the Ordovicianlimestone. Its strike is east-west, but turns toward northwest on the west.Many faults exist on both the-flanks. 4. The Pantongkong-Chowchiachi syncline. It consists of a large area ofwide-spreading Silurian shales. In the midway from Pantongkong to Chowchia-chi, there is an elongated dome about 15km. long and 3km. wide, which isbuilt up of middle Ordovician rocks, especially the Pagoda limestone. 5. The Wenhsiakou anticline. It is chiefly composed of Ordovician rocks,and is overturned and thrust to the south, Its strike is northwest. 6. The Sunchiachiao syncline. This is the southern most wide synclinecomposed of Silurian rocks. Its strike is west-northwest. On the south, it iscontinuous with the southem structure near Kingshan, but abuts against thefaults near Tungchiao on the west, where the Silurian rocks meet the Devoniandetrital deposits. Discussion The stritigraphical succession shows that the lower palaeozoic rocks playthe leading role in making up the structure statue of the Ta-hungshan area.Owing to tremendous thickness and complex structure, the marine deposits fromCambrian to Silurian are chiefly composed of detrital materials. The cyclic dep-osition of the thin-bedded limestones, shales and sandstones is also characteristic.It shows that it might be a geosyhclinal depositional district in the time fromCambrian to Silurian period. Since the Devonian time the condition becomesotherwise. No more deposits had been preserved except some foothill detritalsof later Devonian and early Tertiary ages. As the lower palaeozoic rocks areinvolved in the complex structure and again metamorphosed in some districts,it would be no doubt to suppose that the caledonian orogeny is the controllingcycle in this area. It would be helpful in discussing the problems that we make some inspec-tions about the neighbouring lands for comparison with them and try to findout what are the structure relations between them. On the south, we take a section not far from the Kingshan city （fig. 6）. In this section, the succesion of the strata is somewhat different from the Ta-hungshan proper. Marine calcareous rocks from Carboniferous to Triassic are welldebeloped. If we compare this section with that of the Nanking Depression, notonly the rock faces and subdivisions, but also the attitudes of the strata are verysimillar. It is better to attribute the area south of Kingshan city to NankingDepression rather than to the Ta-hungshan structure unit. Both of them aretransitional in structure, but they have quite different tectonic history. Now let us turn to the south-western neighbouring land to the Huchia-chi area-whichis a small town on the west bank of the Han river. Here a secti-on had been taken from the west hill of the town. In this section it is veryclear to see that the geological history here is quite different. The Archean gra-nite is exposed, upon which overlies unconformably the Sinian siliceous limestone.The younger strata are made chiefly of two sets of marine deposits. One is fromSinian to Silurian in age. In this set siliceous, argillaceous and dolomitic limestoneand detrital deposits are important. The other is of Permian to Triassic ages,the set is chiefly composed of carbonate rocks of epicontinental origin. Thereis no deposition in the long interval of Devonian and Carboniferous times exceptthe weathering crust of Fowling type iron ore deposit. From stratigraphicalsuccession, faunal assemblages （in which the Ordovician fauna is the most typi-cal） and the attitude of the rocks, this area is a part of the so-called Yangtzeplatform. From the strike discordance, there might be a fault along the Hanriver to divide the two area into two independent structure units. On the north-west near the Siangyang city, the successions are the sameas the Ta-hungshan area and the structures are continuous. The Changshananticline could be traced westward across the Han river to Pienshan, a smallhill on the south of Siangyang. Some hills of east-west trend near Siangyangmay be traced along the strike westward to the Tsinling geocyncline apparently, On the north-east and east of the Ta-hungshan area, there is no hills here but the flooded planes of the Yun river. The sporadically exposd rocks in whichdistributed shales and schists of the Ta-hungshan type may be lower palaeozoicin age. The strike of these rocks stretches eastward to the west flank of theHwaiyang shield. Now, let us make some temporal conclutions in regard to the Ta-hungshanarea. The Ta-hungshan area was a south-southeastward extending depressionalarea in the lower Palaeozoic time. It was bordered by the Hwaiyang shield onthe east and northeast, and by the Yangtze platform on the west and southwest.It is a prolongation of the southem part of the Tsinling geosyncline and prolongsto the Nanking depression. From the stratigraphical point of view it might beparageosyncline instead of a orthogeosyncline. At the close of the caledonianstage, by the compressive pressures from the resistant massifs on west and east,the depression area folded into synclinorium and uplifted. In the time of Devo-nian, it was rather high and afforded more detrital materials to the warped Nan-king depressin. But after that time the whole area became a low land andafforded detritals no more. At the close of this paper I would like to take a chance to say somethingabout the metamorphosed rocks of the Ta-hungshan area. the paper conceringthe general geological features of the Ta-hungshan area devoted by Dr. C. C.Yu and Mr. W. P. Shu, 1929（1）, the metamorphosed schists which widely spr-ead in the north-east part of this area are assigned to the Proterozoic ageand described as the Wutai series. From our obserbations in 1955（3）, it is un-true. First, the wide-spreading lower palaeozoic rocks are gradually meta-morphsed into schists eastward, any faults or other features could not be seenbetween them. The writer himself had a chance to observe that the Palaeolenusand Redlic