FLUID FLOW AND DEFORMATION DURING EVAPORITE-DETACHED FOLDING OF CUPIDO FORMATION, NORTHEASTERN MEXICO DERIVED FROM STABLE ISOTOPE AND TRACE ELEMENT PATTERNS

The results of an integrated study of the structural relations, petrography, and geochemistry of rocks and veins from the Early Cretaceous Cupido Formation, Cortinas Canyon, Sierra Madre Oriental, Mexico, provide insight into the history of fluid flow during burial diagenesis and fold-related deformation.  The evolution of the carbonate sequence is divided into three main stages, each characterized by distinct: (i) temperature/pressure distribution, (ii) fluid flow patterns (iii) geochemical signature.

Stage I was characterized by marine and shallow burial diagenesis associated with early dolomitization (slow and steady subsidence).  The isotopic values of rocks vary in a narrow range with average values of d¹⁸O_PDB=-4.1 ± 1.3‰ and d¹³C_PDB =+2.6 ± 0.6‰, consistent with slightly modified Cretaceous marine values.  There are no veins that preserve isotopic values similar to host rocks from this stage.

Stage II was characterized by deep burial diagenesis, compressional tectonics, fracturing of carbonate strata and vigorous hydrothermal fluid flow.  Burial diagenesis was associated with micritization, late dolomitization, and carbonate recrystallization.  Host rocks display lighter isotopic composition than carbonates of the earlier stage, with values of d¹⁸O_PDB=-7.1±1.1‰ and d¹³C_PDB =+1.2 ± 1.1‰.  The geochemical signatures of vein fillings discriminate between mineral precipitation from: (i) locally derived fluids, in which the isotopic signature of veins is similar to the host rocks (Δ _{host rock-vein} < 2‰), and (ii) externally derived fluids in which the isotopic composition of veins is different from the host rocks (Δ _{host rock-vein}=2-12‰).  A complex pattern of interaction from advective, diffusive, and/or pervasive fluid flow in the preexisting carbonates results in an intricate isotopic distribution pattern in which large gradients of d¹⁸O exist on a sub-millimeter scale in both rock and vein material.  Geochemical evidence supports the idea of large scale fluid flow associated with intense deformation during folding.

Stage III was characterized by post-deformation carbonate precipitation either as secondary vein fillings or low temperature tufa associated with meteoric-phreatic environment.  The isotopic values of carbonates from this stage are d¹⁸O_PDB=-7.9 ± 0.5‰, d¹³C_PDB =-6.8 ± 0.9‰.