High-temperature dolomite in the Lower Cretaceous Cupido Formation, Bustamante Canyon, northeast Mexico: petrologic, geochemical and microthermometric constraints

Abstract

The Lower Cretaceous Cupido Formation, a carbonate system developed in northeastern Mexico, like many ancient carbonate platforms contains numerous dolomite bodies. These diagenetic features are particularly well exposed at Bustamante Canyon (Nuevo Leon State) where the Cupido Formation crops out from base to top along 6 km. Dolomitization affected practically all facies and crosscuts bedding planes; dolomite bodies are irregular in outer and margin platform facies and tabular/subhorizontal in inner platform facies. Most dolomite is replacive and also occurs as cement in small amounts. Crystal shape of replacement dolomite varies from nonplanar, planar-s to planar-e, whereas the dolomite cement consists mostly of saddle dolomite. Dolomite is non ferroan and shows dull red luminescence, its δ¹⁸OPDB varies from -4.2 to -6.4‰ and its δ¹³CPDB from 1.8 to 3.4‰. ⁸⁷Sr/⁸⁶Sr ratios of replacement dolomite vary from 0.70754 to 0.70770. Homogenization temperatures in dolomite from fluid inclusion analysis range from 190 ºC to 200 ºC and are interpreted as the minimum temperatures for the dolomite formation. Petrographic data, geometries and distribution of dolomite bodies, microthermometric results from fluid inclusions and geochemical information suggest that the dolomitization occurred under deep-burial diagenetic conditions. Similar homogenization temperatures were determined in dolomite and post- dolomite calcite cement of the Cupido Formation from southern locations including Potrero Chico and Potrero Minas Viejas. The high temperatures recorded in the Cupido Formation dolomites are the result of a regional thermal anomaly developed probably around salt structures. ⁸⁷Sr/⁸⁶Sr values, oxygen stable isotopes, and trace element composition of dolomite suggest that the dolomitizing fluid was perhaps a hot mixture of formation water (modified sea water) of the Cupido Formation and brines derived from the updip La Virgen Formation, a carbonate-evaporite succession equivalent in age to the Cupido Formation. Dolomite distribution was apparently not controlled by major tectonic features (e.g., faults or fractures); the dolomitizing fluid seems to have followed subhorizontal or lateral flowing circulating patterns controlled by the former porosity and permeability of the calcareous facies.