Magnetic Fabric Characteristics and Sedimentary Environment of Subaerial Tidal Sand Body in Subei Area

Key words: 
• mag netic fabric /
• tidal sand body /
• postglacial period

Abstract: There exists a sub-recent buried tidal sandbody system among standplain muddy deposit areas,with a fanshape stretching towards east from Dongtai and similar to that of the radial tidal sand ridge pattern in the southern Yellow Sea.The area of subaerial tidal sandbody system is about 3 000 km\+2.Tidal sedimentary structures,such as muddy streak,flaser structure and bi-direction cross-bedding,show tidal genesis of the sandbody system.The tidal sand strata are underlain by littoral unit or terrigenous hard clay and overlain by tidal flat deposits.\;Magnetic fabrics are measured and analyzed at two representative drilling cores,Tongshang and Xinhai,which have penetrated tidal flat strata,tidal sand body and hard clay.Two drilling cores ard sampled successively.The samples are measured and magnetic fabric parameters K、P、F、L and q are calculated,then successive curves of these magnetic fabric parameters are acquired.These curves show that K、P、F、L and q of the tidal sand body demonstrate an apparent difference from those of underlain paleosol and overlain strata,and lower and upper boundaries of the tidal sand body can be defined by the curves.There are abrupt changes of K、P and F at the boundary between tidal sand strata and the underlying paleosol,with sharply increasing of these parameters in the tidal sand strate.Thus.it can be inferrd that environmental energy in the tidal sand strata is abruptly increased.Under the strong and stable tidal current,the deposition is coincided with hydrodynamics and most of q is less than 0.5 in the tidal sand strata.However the hard clay(paleosol) formed in low energy environment of floodplain contains a lot of sedimentary units with q higher than 0.7,which corresponds to a quick deposition The tidal sand body unit transits gradunally upward to tidal flat deposits,but the boundary between them is still cleraly discernable by the magnetic fabric curves.Smaller K and P reflect hydrodynamic decline from the tidal sand body unit to tidal flat deposts.\;Abrupt variation of K and P within the tidal sand strata may reflect superimposition of sand ridge and tidal current trough depositional units in vertical profile,which shows micro-hydrodynamic changes on the tidal ridge and in the trough.Horizontally,that values of P、K in core Xinhai are higher than those in core Tongshang shows variations of hydrodynamics matching the convergent-divergent tidal current field in the study area.Contents of heavy minerals also show the similar tendency of tidal current changes.Besides,variations of K and content of varied magnetic minerals among heavy mineral also indicate multi-provenance of the study area,with“the Yellow River source” prominent at north and “the Changjiang Rover source” at south.