Stratigraphic Division between Xujiahe Formation and Zhenzhuchong Member in the NE Part of Central Sichuan Basin

Key words: 
• NE part of central Sichuan Basin /
• stratigraphic division between Triassic and Jurassic /
• LUNA principle /
• to judge the reached stratigraphic level with cuttings

Abstract: Studies of the corresponding seismic data revealed the division plane is a sequence boundary (SB) (T3x truncated by the boundary and along which J1z onlaps). So it can be dertermined by LUNA principle. LUNA regards sequence boundary unconformity as can be recognized by strata geometry of abrupt change of accomodation generation rate which is marked by the paleochannel scale and the superimposition of channel sandstones, and base of the almagmated sandstone body is just the situ of the sequence boundary. In welllog, SB is presented by break point of Gr and Rt curvesthe reverse rhythm of the underlain sequence turns into positive rhythm of the overlying sequence upwards at the point. This plane coincides well with the base of basal conglomerate. Measuring the transboundary short columns along the west edge of Huayingshan and coresection studies have revealed that apparent differences of paleontologypaleoecology and sedimentology exist in between T3x and J1z successions, which reflect the inreversibility of geological evolution. Paleontology: mostly fossil plants, plants are plentiful and wellpreserved in T3x6 black shale, and plant stems are considerably common in coarse sandstone of the same member. Fossils are rare in J1z, however, allochthonous small carbonized plant debris or occasionally coal "pebbles" are seen. Sedimentary and diagenetic feafures: most aspects are quite different beween T3x6 and J1z, except that siderite concretions occur in the interval from below to above the boundary by several meters. Lithologythe lowermiddle part of T3x6 is represented by thick graywhite median (coarse)fine sandstone with single bed thickness of several to decades meters, the upper part by interbeds of light gray, yellowish gray fine sandstone to siltstone and black shales; however J1z is characterized by intercalations of light gray, grayishgreen thin siltstone and red, stained mudstome at its lower part, the mudstone are grading into gray colour and sandstone are getting thicker at its upper part. Composition——the guartz content in lower J1z is higher than the sandstones of T3x6, but its chert and metamorphic rock debris are apparently lower than the later; Texture maturity——J1z siltstone is much worsesorted than the T3xv6 sandstone, the thin turbidites in J1z are not seen in T3x successions. Sedimentary sturcturesbiclaylayers, herringbone structure, abrupt facies change, lenticular and flaser bedding and thin interbedds are common in T3x but not seen in J1z. Siliceous cobblethe J1z basal cobble is a reliable boundary marker which is easily differentiated from the basal ones of T3x6; the storminduced conglomerate and "oxhorn coal" occur only in T3x; rootlets are common seen in J1zv and occassionaly presented in T3x. Sedimenfary facies: the abovemention differences between T3x and J1z are thought to be derived from original sedimentary environments, the authors attribute T3x to shallow sea and estuarine origin and J1z to lacustrine delta system. The two successions are also different from each other in welllogging responses and can be easily discerned. This paper proved that to judge the reached stratigraphic level during welldrilling swiftly on the basis of cutting information can be achieved.