Iron concretion formation and diagenesis in the Upper Cretaceous Woodbine Formation, north-central Texas

Iron concretions are abundant throughout the Upper Cretaceous Woodbine Formation of north-central Texas. Analysis of these concretions from six outcrops in the Dallas/Fort Worth area of Texas indicates that these features formed diagenetically in an organic-rich environment. The overall mineralogy of the concretions was determined by studies using both translucent and reflected light microscopy. Sphaerosiderite and pyrite provided the original source of iron in the concretions that were formed by bacteria. All of the concretions have subsequently undergone diagenesis, but the textures of original minerals have been preserved. These original iron minerals were diagenetically altered by groundwater to form two different iron minerals, hematite and goethite, which along with quartz, are the predominant minerals present in the concretions today.