Glauconitic Condensed Sections(CS): The Key to Correlation Of Sequence Stratigraphy in the Craton Basin──Characteristics and Implication of the Cambrian Condensed Section of North China

Condensed sections (CS) are related to the maximum flooding stage (lag dePOsits) of a relative sea-level Change. They are present in each second and third order sequences in the Middle-Upper Cambrian strata.I. Main features of condensed sections(CS)1. Rock types and sub factes of condensed sections.CSs are composed of seven rock types which are distributed in three factes and five sub factes:Rock types:(1) Glauconitic siltstone-sandstone; (2) Glauconitic oolitic limestone 1 (3) Glauconitic Binclastic limestone; (4)Glauconitic micritic limestone; (5)Glauconitic storm calcirudite;(6)Glauconitic hardground; (7)Glauconitic clay.Three factes: inner shelf, middle shelf and outer shelf Five subfacies: (1) Quartz sandstone-siltstone and Glauconity; (2) Glauconitic oolitic limestome; (3) Glauconitic intraclastic muds; (4) Glauconitic muds and hardgrounds; (5) Deep water muds +phosphorus and manganese.2. Mineralogy and geochemistry of condensed sections.Glauconite, dolomite, pyrite, limonite and clays are the main mineral components of CSs.Chemical analyses of CSs have shown concentrations of some trace elements, such as Mn, lr,P,K and Ti, one of the most important features of condensed sections is the abundance of bingenic material.The potassium content and texture of glauconite permit the assessment of the evolutionary stage and duration of a condensed section. From this, it is apparent that the length of time for the development of CSs is in the range of 10⁴～10⁶ years. This is similar to the duration of the maximum flooding stage of second or third order sea level changes.Ⅱ. Conclusion1. A new concept of condensed section is given in this paper:A condensed section is a low--rate marine sedimentation unit which formed at the time maximum flooding close to the inflection-point from TST to HST during the long-term (2nd to 4th order cycle) global sea-level changes.2. Condensed sections are not only the products of maximum flooding and early highstand, but also the ties of TST and HST. They are the key stratigraphic units of sequence division and stratal correlation. Sea-level changes of the different orders result in the formation of condensed sediments. Studies of the maturity of glauconite can be used to distinguish different orders of sea-level changes.