Characteristics of Tempestite of Lower Ordovician Tongzi Formation, in the Longshan Area, Northwestern Hunan and Its Geological Significance

Key words: 
• tempestite /
• succession /
• Tongzi Formation /
• Longshan area /
• northwest Hunan province

Abstract: This is the first time to report storm deposit in the Tongzi Formation, Lower Ordovician in the northwest Hunan Province. Meanwhile, the author demonstrates the distribution and manifestation of the sedimentary deposits systematically. Based on careful field observation and laboratory analysis, indications of the tidal flat environment such as hemispheric stromatolites, Herringbone cross bedding are widely present in the succession. The sedimentary structures of the storm deposit such as erosion -tear structure, graded bedding, rhythmic bedding, deformed bedding and the sand grain-wave bedding have been identified, excluding hummocky cross-stratification. Based on the analysis, the storm deposit in this Formation is the tidal flat storm deposit. And, four types of storm deposits have been recognized. Meanwhile, big slump gravity flow deposits are identified in Tongzi Formation, which must be the different material records of the same hurricane in different environments. Study of the storm deposit indicates that the tempestites carry important poleoclimate inFormation and frequent occurrences of storms responded to climate changes from late Cambrian to lower Ordovician. The discovery of tempestite, the storm turbidite and slump breccias provide new lines of evidence for the Western Hubei platform depression in this period and suitable reference for fine stratigraphic division and correlation of ore-containing block Formation as well as more reliable reference data for the analysis of the north draft of the South China Plate.