Li County, Gansu - Ma Areas Along The Carboniferous Lithostratigraphic Units And Basin Evolution

Lixian-Mayanhe fissure-sink basin lies in the west of Qingling.lt close to the junction areas of Qilian and north Qingling .It's stratum classifiction belong to Central Qinling.At the present,the Carbonifefous system stratums have not classified clearly and it's position is not clear. Because this area stratum exposed not completely and great change in traverse. There are few appraisal significance fossil in this basin.As to above-mentioned circumstances ,at the base of 1:250000 regional geological mapping of Tianshui .This article main stuyed basin Carboniferous period statigraphic composition , depositional environment and basin evolution. Through a com- prehensive study of the reginal geology of this area ,the following major results have been made:(1) Because of tectonic processes,at the east of the basin only exposed early Carboniferous stratum.By the synthetical study of lithofacies characteristic and fossil assemblage ,these indicate following depositional environments:deep-water basin deposit; basin's dissection water channel deposit; clinothem deposit of carbonate platform's leading edge ; carbonate platform deposition; tidal flat-lagoon deposition. But at east of basin only exposed later Carboniferous stratum.It's characteristics indicate following depositional environments:deep-water basin's edge deposit; shelf edge deposit ;shelf channel deposit;deltaic deposit.(2) Through out-door studying, the basin's north and south edge is Shujiaba and Xihanshui of Devonian period.Because of tectonic processes ,they contacting by the fracture in this area.(3)By studying event deposit .Concluding that Lixian-Mayanhe basin experienced about three developing stages at Carboniferous period: I, The first fissure - sink and spread period. II, The second fissure - sink and spread period. III, Extruding and going up period. At the last of Carboniferous period ,great changes have taken place in the basin's deposit patten. At this period ,the basin change from undercompensation to ultra-compensation. In the basin, cumulusing a large terrigenous gravity flow deposition .It manifestation that quartzose microconglomerate, granulite, sandstone and mudstone which contains lots of Plant fossils.