Clay minerals in structurally complex settings influence fault zone behavior and characteristics such as permeability and frictional properties. This work aims to understand the role of fault zones on clay authigenesis in arkosic, high-porosity sandstones of the Cretaceous Rio do Peixe basin, northeast Brazil. We integrated field, petrographic and scanning electron microscopy (SEM) observations with X-ray diffraction data (bulk and clay-size fractions). Fault zones in the field are characterized by low-porosity deformation bands, typical secondary structures developed in high-porosity sandstones. Laboratory results indicate that in the host rock far from faults, smectite, illite and subordinately kaolinite, are present within the pores of the Rio do Peixe sandstones. Such clay minerals formed after sediment deposition, most likely during shallow diagenetic processes (feldspar dissolution) associated with meteoric water circulation. Surprisingly, within fault zones the same clay minerals are absent or are present in amounts which are significantly lower than those in the undeformed sandstone. This occurs because fault activity obliterates porosity and reduces permeability by cataclasis, thus: (1) destroying the space in which clay minerals can form; and (2) providing a generally impermeable tight fabric in which external meteoric fluid flow is inhibited. We conclude that the development of fault zones in high-porosity arkosic sandstones, contrary to other low-porosity lithologies, inhibits clay mineral authigenesis.

The aim of this research was to compare the efficacy and tolerability of iomeprol, 150 mg iodine/mL, a new nonionic contrast medium, and iopamidol, 150 mg iodine/mg in intraarterial (IA) peripheral digital subtraction angiography (DSA) in 100 patients; a group of 40 patients were also submitted to a complete coagulation screening to check the influence of contrast media on blood clotting. The study was a comparative, double-blind clinical trial. The compound was assigned to each patient according to a randomization list. Small size (4-5 French) catheters were used in all patients to minimize arterial trauma and bedding time and to assess the quality of x-ray pictures in this condition. Vital signs, EKG tracings and laboratory parameters were monitored before and after the angiographic procedure; the coagulation screening included: thrombin time, prothrombin time, partial thromboplastin time, euglobulin lysis time, plasma thromboplastin antecedent, and plasminogen activator inhibitor (PAI). Both contrast media did not produce any adverse reaction or clinically significant alteration of studied parameters; in the 40-patient group subjected to massive coagulative screening, no important alteration after contrast media administration was reported. The score for contrastographic efficacy was very good with both media with a prevalence of better results in the iomeprol group.