Geology
The sedimentary beds that form the basin are of Paleozoic, Mesozoic and Paleogene age. In the eastern part of the basin, the sedimentary section is capped by flows of basalt of Miocene age. The basin is highly asymmetrical, the beds dipping more steeply on the west side than the east.
The sedimentary rocks of the basin are extensively intruded by igneous plugs, dikes and sills of Eocene to Oligocene age. Two large granitic intrusives near the axis of the basin form East Spanish Peak and West Spanish Peak. Dikes of felsic to intermediate composition radiate outward from East and West Spanish Peaks, and on the north side of the peaks have the appearance of large stone walls. Dikes of mafic and ultramafic composition trend east-northeast to west-southwest across the basin. Basaltic sills tend to intrude along the coal beds.
The site of the Raton Basin was a coastal plain at the end of Cretaceous and beginning of Paleogene time, and has a well-preserved sequence of rocks spanning the Cretaceous–Paleogene boundary. For this reason, the Raton Basin has been studied for evidence of the iridium anomaly thought to be evidence for a large meteor impact at the end of the Cretaceous that is in turn thought to have caused the Cretaceous–Paleogene extinction event. The boundary is represented in the basin by a 1-cm thick tonstein clay layer which has been found to contain anomalously high concentrations of iridium. The boundary clay layer is accessible to the public at Trinidad Lake State Park, among other places in the basin.
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