Description
Clay minerals have been used as geothermometers to determine source rock maturation and basin analysis in the oil and gas industry, as well as to understand heat generation along faults during rupture. In both of these applications the proportion of discrete illite in mixed-layer illite/smectite is correlated to heat. However, in our study we will evaluate if the presence of smectite can be used to limit the temperature regime of emplacement of an andesitic dome, located in Carlsbad, CA. Until recently, Mount Calavera was thought to be the solidified neck of an ancient volcano. Notably, recent research by Mohammad El-Najjar suggests that the feature is instead extrusive in nature, and is most likely a dome with andesitic composition. Understanding the temperatures at which the dome was emplaced may help us further understand its genesis. On the northern, eastern, and southern sides of Mount Calavera,the Miocene andesitic dome is in contact with sediments of the middle Eocene Santiago Formation. A bake zone, the focus of this study, is exposed along the NW portion of the western flank of the dome. Seven samples were collected, four from the baked sediments, two from the unbaked sediments, and one from the andesite. All seven samples were analyzed for clay mineralogy by x-ray diffraction. In addition, thin sections of each sample were evaluated for changing textures and mineralogy across the baked zone. The < 4 μm fraction of samples CA 1, CA 2, CA 3, CA 4 (baked Santiago Formation), CA 5, and CA 6 (unbaked Santiago Formation), include smectite, illite, and lesser amounts of quartz. The < 4 μm fraction of CA 7 (andesite) was dominated by smectite with lesser amounts of quartz and feldspar. We view the strong presence of smectite in all the sediment samples as an indication that temperatures during emplacement were < 200 °C, the point at which smectite generally collapses to an illite structure. The determination of a low heat signature corroborates the idea that Mount Calavera is a surficial dome-lik feature, rather than a volcanic plug.
