The Relationship between Sequence Stratigraphy and Groundwater of Quaternary Sediments in Relation to Global Sea-level Change in the Downstream Red River Delta Area

The study of the distribution law and origin of groundwater in Quaternary sediments downstream of the Red River Delta is approached in the direction of research on sequence stratigraphy and the distribution law of lithofacies according to space and time. However, the distribution law of lithofacies and groundwater depends on global sea-level changes and tectonic activities as the two most important reasons. Quaternary sediments of the Red River delta downstream have 5 sediment cycles corresponding to 5 sequences: (1) sequence 1 with Early Pleistocene age (Sq₁–\({\text{Q}}_{1}^{1}\)); (2) sequence 2 with early Middle Pleistocene age (Sq₂–\({\text{Q}}_{1}^{{2{\text{a}}}}\)); (3) sequence 3 with late middle Pleistocene age (Sq₃–\({\text{Q}}_{1}^{{2{\text{b}}}}\)); (4) sequence 4 with early Pleistocene age (Sq₄–\({\text{Q}}_{1}^{{3{\text{a}}}}\)); (5) sequence 5 with late Pleistocene–Holocene age (Sq₅–\({\text{Q}}_{1}^{{3{\text{b}}}}\)–Q₂). Each sequence is composed of three sedimentary systems tracts. The lowstand systems tract (LST) is characterized by a lowstand alluvial muddy sand facies complex that acts as a primary freshwater aquifer (S_marLST). The transgressive systems tract (TST) is characterized by a coastal swamp sand mud facies (M_samtTST) and a maximum transgression bay-lagoon mud facies (MmtTST) acting as an aquifuge. The highstand systems tract (HST) is characterized by sand bar facies (SamhHST) and delta plain sandy mud facies (M_samhHST). Thus, vertically Pleistocene sediments have 5 aquifers, in which each aquifer is constituted by the superposition of two successive facies complex: (1) upper part: lowstand alluvial gravelly sand facies complex (LST) and (2) lower part: highstand dune sand bar facies complex (SarLST/SamhHST). The spatial distribution of freshwater aquifers in 3 provinces Northwest Thai Binh, Ninh Binh—southeast Nam Dinh, and northwest Nam Dinh demonstrate the absence of shallow marine-bay mud facies.