THE PALEOCONTTOURITE AND ITS TECTONIC SIGNIFICANCE IN QINZI30U BASIN, GL'ANGXi, CHINA

Qinzhou Basin is located in Guangxi Province, South China, extending in NE-SW direction,200km long and 50-70km widc.Its southwest part passes through the Tonkin Gulf then ended in Vitnam.It is bounded on the east and west by two deep faults. This basin had had a continuous history from Silurianor earlier J to Permian during which the Silurian-Middle Devonian ilyschoid formation and Upper Devonian-Lower Permian siliceous-muddy formation formed.The basin is a particuliar tectonic unit in South China because of no uncomformity between the Silurian and Devonian Systcm.Its teesonic feature is very different from adjacent rrgions’in tithcr geological history or rock formation. The Silurian-Middle Devonian flyschoid formation here, about 9500m thick, is composed of sandstone, siltstone and mudstone of which make up rhythmic succcsions with thickness of about SOcm per unit.Conglomcrate can only be seen on sonic part of the bottom of the Silurian.Many kinds of fossil such as graptolite, tentaculitid and trilobite contained. most of them arc planktonic (few nectonic) and cosmopolitan taza.Inferably, scawater circulation and animal exchange;neat between this basin and outer oceans may have taken place during that period Onthc studies of field occurcncc, sedimentary structure and texture, and grmn一sift anallyscs ace., it is proposed the origin and tec:onic setting of the formation.It has a that h:ghcr malurl;y of borher;,incral and texture than typical turbidite. The average sandstone/m udstunc value(in thicknca) is about 2.Framcwork minerals are chiefly quartzcs.Thcre arc graded bcddings, lead srueture and cther charactcristics of turbidite on one hand, and little crossbcddings,flow marks,imbricated texture of mineral grains or faecle pellets,and other diagnostics of traction flow dcposis on the olhcr hand. Sedimentary structure and grain-size analyses show a transitional feature betwecn turbidcte and flvial (nr tractne cur-rent) deposit, only possibly the latter rcprescnecd a s mariac river一一a contour currcnt.lt Is important that there arc a kind of very L'1in (0.5 - cm) mudstunc。fairly sorted sandstone) lntcrclatcd into much thicker sandstone(or mudstonc).Such rocks arc very much liketos the muddy contourite(or sandstone) described by Stow&Lovc&Lovell (1979) Especially，the presence of pirnodal directions c.g.downslopc and alongslopc at about 90` is a key indreater in determintion of conearite. Furthermore, the sandstone here are chichy graywacke, most of tnem are quarly一nch，type one.and very similar to those of passive cononantal margin type sandstone in prochercal The featre of the contouritc can also be contrasted with those in the word. Summing up the above mcntiomd, it is concluded that the Silurion-Middle Devosion fiyschoid formation of the Qinzhoa Basin is a low dcnsy turbidce which obvioncy were rewerked or vuperimposed by paleocontcur current.Thcy formed on a continenyisl rise rechting the tectone setuag of the tectonie setuag of the ,Atlantic type of cast continental margin.