The shell-comb and grape-like dolomite of the Dengying Formation in the Sichuan Basin has an unusual textures.The petrographic data

carbon

oxygen and strontium isotope analysis on matrix and grape-like dolomites microdrill-samples have been used to investigate the origin of shell comb and grape-like dolomite.The grape-like dolomite is characterized by length-fast fascicular-optic fibrous aragonite cement

and bladed radial slow high-Mg calcites as well as granular calcites.The micritic dolomite in opposite direction growth

which consists of different size of either spherical and dumbbell forms or rhombic shape

has formed in pattern rhythm layers of banding or laminated along the terrace of C-axis of crystal.The zonation

which consists of microcrystalline

fine crystalline dolomite

has generally a more distinctive negative values of δ

13

C(PDB) and δ

18

O(PDB) than that of the black and white fibrous dolomite of grape-like dolomite.The black or dull fibrous dolomite has a positive δ

13

C(PDB) and δ

18

O(PDB)

compared with that of white fibrous dolomite.It may mainly indicate a marine origin for shell comb and grape-like dolomite.At least three different fluid environment oscillation to generate the banding of shell comb grape-like dolomite:① One of them is characteristic of lower negative of δ

13

C(PDB)and δ

18

O(PDB) and much higher 87Sr/86Sr compared to that of matrix dolomite or seawater.The wide medium bright orange luminescent bands with interval of dull bright zones is supposed to form in seawater overprinted with fresh water

indicate an periodic environment changes between weak oxidation and weak reduction;② The second has a sharp variation of sequences

with slow decrease of δ

13

C(PDB) and increase of δ

18

O(PDB) from its centre to rim of zonation.Normal seawater value of

87

Sr/

86

Sr

medium bright orange luminescent in the centre to dull or non-luminescent in outer zones indicates it generated in a seawater with continuous recharge of fresh water

and weak reduction in the center shifted to weak oxidation in the outer of zone;③ The third one does not have very clear zonation

and negative

18

O(PDB) in the rim with a slow increase of δ

13

C(PDB) and increasing and later decreasing of δ

18

O(PDB).It also shows a cathodoluminescence feature similar to that of matrix dolomite

which has been interpreted as a weak reduction and later oxidation environment.The environment first developed in condense seawater and was ventilated by fresh water in later diagenesis periods.In conclusion

the shell comb and grape-like dolomite has been developed in an unusual Neoproterzoic seawater characterized by very high Mg/Ca.It is coeval with the precipitation of algal laminated stromatolite and thrombolite

in forms of the microbial mat

mound

biofilms and reef.The environment model can most likely account for rapid growth of isopachous

fibrous aragonite and bladed high-Mg calcites cement

and growth of later granular calcites induced or promoted by bacterial microbial modulation

and para-contemporaneous dolomitization in condense seawater.The fluid is oversaturated seawater at the beginning

and later superimposed by a cyclic subaerial exposure of fresh water.