This publication — covering the Hesperus Quadrangle in La Plata and Montezuma counties — includes a cross-section, map unit correlation, shaded-relief map with geology overlay, booklet of extended descriptions of map units, geologic setting, geochemistry of igneous rocks, mineral and energy resources, and references. 33 pages. 1 color plate (1:24,000). Digital PDF download. OF-00-04D
From the Author’s Notes:
The Hesperus quadrangle straddles a major regional structural boundary that separates the Four Corners platform from the San Juan dome in southwestern Colorado. The quadrangle is in the eastern part of the Four Corners platform, a broad, northeast-trending structural bench that extends from northwestern New Mexico into southwestern Colorado and is underlain by relatively flat-lying sedimentary rocks. The platform structurally separates several surrounding uplifts and basins in southwestern Colorado and northwestern New Mexico. Within the southwestern part of Hesperus quadrangle, rocks in the platform generally dip about 5 degrees southwest, south, or southeast.
The San Juan dome is a roughly circular, broad, domal uplift that forms the northeastern side of the Four Corners platform. The Laramide-age (early Tertiary-Upper Cretaceous) San Juan dome extends across much of the mountainous part of southwestern Colorado and is in large part concealed by the middle Tertiary San Juan volcanic field. A smaller, more abrupt dome, the laccolithic La Plata dome, is superimposed upon the southwestern end of the larger San Juan dome.
The La Plata domal uplift resulted from intrusion of 75 to 60 Ma plutonic rocks. As a result of this period of intrusion, the late Paleozoic and Mesozoic sedimentary section has been inflated 6,000 to 8,000 ft. The plutonic rocks consist largely of sill-shaped stocks and sheets emplaced at various stratigraphic horizons and numerous sills and dikes that cut surrounding country rock. These plutonic rocks are dominantly dioritic to syenitic in composition, and include late-phase lamprophyre dikes. The mountainous northern end of the Hesperus quadrangle is carved out of the southern part of the La Plata dome. Along the southern margin of this dome the sedimentary country rocks are generally only slightly metamorphosed to unmetamorphosed.
The southeastern flank of the Four Corners platform is bordered by the northwestern margin of the San Juan Basin, another large, Laramide structure. This major structural boundary coincides with the Hogback monocline, a sharply southeastward dipping structure that lies only three to six miles beyond the southeastern corner of the Hesperus quadrangle. In the southwestern part of the Hesperus quadrangle, rocks in the Four Corners platform generally dip about five degrees southwest, south, or southeast. Although some minor faults and folds may be present in this area, none were mapped at the surface during this investigation. A few minor rolls or folds have been encountered in the underground workings of the King Coal mine, but no faults have been intersected.