Abstract: As a kind of sedimentary rock,primary,contemporaneous and penecontemporaneous dolostones with mud-or silt-sized crystalline structures are generally related to restricted-evaporative sedimentary setting.However,those formed by replacement or hydrothermal alteration during burial process could not be used as direct indicators of sedimentary facies,at least not until the relict texture of the dolostone is recognized and the original limestone depositional setting is restored.This is critical to sedimentary-and reservoir-related research on dolostone.The Middle-Lower Ordovician dolostone from the Central Uplift Zone,Tarim Basin,may serve as a good example.It is commonly regarded as signature of restric-ted-evaporative platform facies.However,an investigation of the dolostone in the study area proved otherwise.We identified the lithology of samples taken from the area through systematic core/thin section observation and established criteria for relict texture identification by comparing the dolomitized and the un-dolomitized parts of the same samples.The results show that dolostones in the area are characterized by three relict textures,among which the relict ghost texture dominates.The other two relict textures,i.e., the relict granular/biogenetic texture and the relict grain shape texture,are uncommon.We therefore suggest that the Ordovician dolostones originated mainly from a set of grained shoal limestone,and the sedimentary facies during the Early-Middle Ordovician was an open platform rather than a restricted platform.The predominant microfacies are intraplatform high-energy arenstritic shoal.

Key words: dolostone, relict texture, open platform facies, Ordovician, Tazhong area, Tarim Basin