TROPICAL POOLS AND COLD-WATER SEEPS: FORMATION AND DIAGENESIS OF A SHORELINE-TO-LAGOON SYSTEM IN THE CRETACEOUS

The Lower Cretaceous Bluff Dale Sand underlies the Glen Rose Formation, and is the uppermost member of the Twin Mountain Formation in Bluff Dale, Texas. The Bluff Dale Sand is exposed at the base of the hillsides in the surrounding area. The sand has previously been interpreted as equivalent to the Hensel Sand member of the Twin mountain formation, which is found exposed 165 miles to the south as part of the Kerr basin, with no intermediary exposures. The Kerr Basin is one of the most underexplored basins in Texas, and information about regional geology is limited. To the north, the Bluff Dale sand is part of the flanks of the Fort Worth Basin. These two basins are separated by the Llano uplift, and lacking evidence indicating their equivalence, the Bluff Dale Sand is interpreted as a separate member. In the field, this outcrop presents numerous coarsening upward units, varying from muds to fine quartz sands. Included in this outcrop are two distinctly resistant units of barite cemented sands, as well as fragments of bivalves, vertebrate parts, calcite veins, and iron concretions. Sedimentary structures, such as ripples, laminated beds, and associated fossils support the interpretation of this unit as a transitional shoreline-to-lagoon environment. Barite cement formation is interpreted as having occurred as diagenetic cementation, likely due to the combination of basin subsidence, sea level rise, and probable faulting in the area.