Petrographic And Chemical Microheterogeneity In Dolomite, Bahariya Oasis, Egypt

Abstract

Dolomite samples from the lower member of the El-Hafhuf Formation (Turonian-Santonian) possess noticeable petrographic and chemical heterogeneity among their fabric components. Petrographically, the study samples can be subdivided into three types of dolomite: 1) main dolomite intraclasts embedded in 2) dolomite matrix and bounded by 3) laminated dolomite. Dolomite intraclasts are the more stoichiometric having 8I8O values that range from (-0.5 to -3.2%o PDB) and enriched 8I3C signatures (+2.0 to +6.0%o PDB). They possess nonplanar crystal boundaries and polymodal crystal size distributions. The dolomite matrix shows the lowest Sr2+ (av. 80 ppm) and Na+(av. 320 ppm) values and the highest Mn2+ (av. 1150 ppm) content. They exhibit narrow5I8O and 8I3C ranges (-3.5 to -4.7 and 0.0 to -1.8%o PDB, respectively). Crystals comprising this matrix have dominantly euhedral planar forms possessing polymodal distribution. Laminated dolomites exhibit the highest Sr2+ (av. 230 ppm) and Na+ (av. 470 ppm) values and depleted SI8O (-5.1 to -6.4%o PDB) and 5I3C (-2.5 to -5.\%o PDB) signatures. Their crystal boundaries are nonplanar while crystal size distribution is polymodal. Such petrographic and chemical heterogeneity among the investigated dolomite types within the studied samples are related primarily to the variable origins of these dolomites and their diagenetic stabilization pathways.