The Chronology of the Geological Record: N.J. Snelling (Editor). The Geological Society Memoir No. 10. Published for The Geological Society by Blackwell Scientific Publications, Oxford, England, 1985, vii + 343 pp., £55.00, Hardback

Abstract

Dolomites from the Ordovician Majiagou Formation are considered as major hydrocarbon reservoir rocks in the Yangshuiwu area, Jizhong Depression. However, the origin of the dolomites remains unclear. This study utilizes a composite analysis of seawater-normalized rare earth elements (REEs), petrography and isotopes (C, O and Sr) to trace dolomitizing fluids and processes. Three types of dolomite are identified: planar-s to nonplanar microcrystalline (type D1) dolomite (<3 μm), planar-s to relatively planar-e micrite-silty crystalline (type D2) dolomite (3–50 μm), and planar-s to nonplanar silty-fine crystalline (type D3) dolomite (50–250 μm). Their dolomitizing fluids are seawater or seawater-derived because their seawater-normalized REE partition patterns are similar to micrites, and their δ13C values mainly fall within the Ordovician seawater range while δ18O values exhibit varying degrees of negative excursion characteristics. Type D1 is characterized by a tight texture with dark to dull red cathodoluminescence (CL), gypsum crystals, low order degrees (mean 0.58), and high 87Sr/86Sr ratios (0.708938–0.713809). In combination with Z values, Ce and Eu anomalies, it suggests an evaporitic near-surface origin (i.e., supratidal flat) with high-Mg/Ca brines as dolomitizing fluids. Type D2 is the product of seepage-reflux dolomitization, indicated by petrography, high 87Sr/86Sr ratios (0.709015–0.710551), and slightly lower positive Ce anomalies and higher negative Eu anomalies than type D1. Frequent sea-level fluctuations control the formation of types D1 and D2 and their high 87Sr/86Sr ratios are perhaps related to the presence or dissolution of gypsum-salt. Type D3 has the lowest positive Ce anomalies among the three types and slightly negative Eu anomalies. Together with its large crystals with a dirty core and clear rim and high order degrees (mean 0.70), this points to a formation by burial dolomitization mediated by a mixture of Permian-Carboniferous seawater with residual seawater stored in the strata which causes high 87Sr/86Sr ratios (0.709067–0.709527).