Natural condition Characteristics of geological structure in Long An province
Long An is located in the South Eastern edge of Da Lat zone. This is independent tectonic - metallogenetic zone, with foundation of Precambrian continental crust, subsided in early – middle Jura and experienced epicontinental regime in late Mesozoi.
The East Sea tide in Soai Rap estuary has amplitude of high from 3,5 to 3,9 m. The process of intruding salt water is deeper and longer into the inland. The reason is the strong operation of tide, northeast wind, less water from upstream and especially many exploitations of surface water in dry season. In the past, Vam Co Tay river had been intruded by salt water in Tuyen Nhon of about 5 km, since 1993, it has reached to Vinh Hung. The salinization has started from January to June with 2 - 4 gam/liters. The salinity of Vam Co Dong river has gradually reduced by the influence of Dau Tieng lake.
Besides, the earth tends to be hotter, facilitating sea level raising, developing the salinization deeply to inland.
The salinization has changed the ecological system of region which has been stable for many years, resulting in the influence to production and daily life of people. To reduce this process, should not exploit too much in dry season and the investment in irrigation systems should consider the impact of salt water intrusion on the general ecological system.
At the end of Mesozoi and in Kainozoi, Da Lat zone was activated forcefully. In Neogen – Quaternary, this region involving Mekong basin was subsided forcefully and filled up by terrigenous sediments . In Long An province, it mainly developed formations of sediments of Neogen Quaternary:
Sediments of conglomerate, sandstone, clay siltstone of Can Tho formation (N21ct) have large area of allocation.
Gravelstone, sandstone, siltstone, claystone of Nam Can formation (N22nc) have large area of allocation.
Formations of stream deposition including pebble, gravel, grit, sand, clay, kaolin of Cu Chi formation (aQ23cc) are allocated mainly in Loc Giang, Duc Lap Thuong, My Hanh Bac, Hoa Khanh Dong, An Ninh Tay communes under Duc Hoa district.
Formations of sea and rivers depositions including silt, clay with few gypsum of Moc Hoa formation (amQ21mh) are allocated mainly in areas of Duc Hue, Moc Hoa, Vinh Hung, Tan Hung districts.
Formations of stream deposition including pebble, gravel, grit, sand, clay kaolin of Hau Giang formation (mQ21hg) are allocated mainly in areas of Tan Thanh, Moc Hoa, Tan Hung districts.
Formations of sediments of Middle-Upper Holocen (aQ22-3) including stream deposition (sand, silt, clay), river - marsh (silt, clay, remains of plants and peat), are shelf deposits, alluvial flat and river bed deposit, allocating most of areas of districts in Long An province.
According to result of geological survey at the ratio of 1:200.000, it is recorded that there are 2 main fault systems in the North Western - South Eastern direction and North Eastern - South Western direction. These fault systems were found according to geophysical materials and indirect geological signs.