Origins and Geochemistry of Dolomites and Their Dissolution in the Middle Triassic Leikoupo Formation, Western Sichuan Basin, China

Abstract

Triassic dolomites occur pervasively in the Western Sichuan Basin. Although these strata have been deeply buried and affected by multiple phases of dolomitization and dissolution, some intervals in the upper part of the Leikoupo Formation have developed high porosity. Based on their petrographic and geochemical features, three major types of dolomite fabrics are recognized, namely, dolomicrite, fabric-retentive dolomite, and fabric-destructive dolomite. Geochemical evidence indicates that the dolomicrite formed following the Sabkha model in a low-temperature hypersaline environment, as these rocks exhibit abnormally high Sr and Na contents, lower Fe and Mn contents, δ 18 O values generally ranging from −1.70 to −1.67 (with an average value of −1.69 ), and higher Mg/Ca ratios. The fabric-retentive dolomite formed following the seepage-reflux model in a shallow burial environment, and these rocks exhibit the highest 87 Sr/ 86 Sr ratios, δ 18 O values generally ranging from −6.10 to −2.50 (with an average value of −3.98 ), and a wide range of Fe and Mn contents, indicating that they may have been altered by meteoric water. The fabric-destructive dolomite formed following the burial model at elevated temperatures; these rocks exhibit the lowest Sr and Na contents, δ 18 O values generally ranging from −7.01 to −6.62 (with an average value of −6.79 ), relatively higher Mg/Ca values, and lower 87 Sr/ 86 Sr ratios. The early Sabkha, seepage-reflux dolomitization and penecontemporaneous periodic meteoric freshwater selective dissolution processes formed multi-period, overlapping moldic pores, algal framework pores, and intragranular dissolution pores. The superposition of organic acid dissolution during the burial period is the main controlling factor of the formation of deeply buried, high-quality dolomite reservoirs in the Leikoupo Formation.

Crystals structures of carbonate phases with Mg in Triassic rocks, mineral formation and transitions

“…The proto-dolomite crystal structure was prepared on the basis of the ordered dolomite crystal structure, include the reduced number of magnesium ions determined in proto-dolomite. Ordered dolomite is characterized by stoichiometric value of Mg for this carbonate phase (MgO—21.86%, Mg—13.18%) 18 – 20 , 22 , 26 , 48 , 51 , 52 . Mg content in ordered dolomite of Triassic limestones ranges from 13.20 to 13.67%.…”

Identification of subsidence hazard zone by integrating engineering geological mapping and electrical resistivity tomography in Gunung Kidul karst area, Indonesia

“…In general, subsidence is triggered by the presence of spaces/cavities below the surface either due to human activities (subsurface mining) or natural processes of rock dissolution (Font-Capo et al, 2015). The degree of dissolution will be significantly influenced by the type and characterization of the rock (Zhang et al, 2018). The dissolution rate will be more intensive if the rock is fractured or jointed (Sun et al, 2018).…”

“…The term subsurface cavity denotes all subsurface structures, including caves, cavities, and sinkholes (Cui et al, 2015). The most naturally occurring cavities in nature are developed by dissolution carbonate rocks and evaporation (Zhang et al, 2018). Karst's topography is created by the dissolution process of meteoric water percolation into the soil (Sun et al, 2018).…”

The Origin of Quartz Cement in the Upper Triassic Second Member of the Xujiahe Formation Sandstones, Western Sichuan Basin, China

“…The Indosinian movement at the end of the Late Triassic resulted in the collision of the Yangtze plate with the North China plate. The Longmenshan island chain along the western edge of the Sichuan Basin gradually evolved into a strong thrust nappe orogenic belt and entered the stage of foreland basin evolution, and the coal-bearing clastic rocks of the Xujiahe Formation were deposited [17,18,[22][23][24].…”