Bulletin 63—Geologic studies of Union County, New Mexico

By B. Baldwin and W. R. Muehlberger, 1959, reprinted 1981, 171 pp, 5 tables, 28 figs., 17 plates, 1 appendix, 1 index.

This bulletin is divided into two parts: I. Geology of Union County, and II. Volcanic rocks of Des Moines quadrangle. Union County, 3,817 mi² in area, is in the northeast corner of NM. Mesozoic formations recognized at the surface include the Baldy Hill Formation, Travesser Formation, Sloan Canyon Formation, and Sheep Pen Sandstone, all of the Triassic Dockum Group; the Jurassic Exeter Sandstone and Morrison formation; and the Cretaceous Purgatoire Formation, Dakota Formation, Graneros Shale, Greenhorn Limestone, Calile Shale, and Niobrara Formation. These units aggregate nearly 3,000 ft.

Undifferentiated upland deposits consist largely of the Pliocene Ogallala Formation and include caliche and algal limestone. Basaltic rocks cover a fifth of the county and rest on upland deposits. The flows are subdivided into the Raton, Clayton, and Capulin units of Collins. However, the type Clayton may in fact be of younger Raton age. Capulin flows are Recent, whereas Raton flows are possibly of Pliocene age. About 80 volcanic centers, including cinder and shield cones, occur in aligned sets.     

Locally, Exeter Sandstone truncates open folds of the Dockum Group. Mesozoic formations dip eastward from the Sierra Grande arch, with local interruptions by a monocline, an anticline, and other warpings. Possible warping in late Ogallala time may have triggered volcanic activity. Mineral resources include scoria, gravel, and possibly clay. There has been intermittent production of CO² from wells and of copper from several pre-Exeter Sandstone plugs. The report contains a colored geologic map (125,000 scale), 44 columnar sections, and 115 lithologic logs of water wells. A summary of subsurface stratigraphy appears in the report, together with structure contour and isopach maps.

Late Cenozoic volcanic rocks cover about 725 mi² of Union County. On geomorphic criteria, the basalts have been divided by earlier workers into three major groups (from oldest to youngest): the Raton, Clayton, and Capulin basalts. The Red Mountain dacites, pre-Clayton post-Raton local silicic differentiates found in several vents in Colfax County, have as their only possible representative in Union County, Sierra Grande, the largest volcano in this region.

The Des Moines 15-min quadrangle and vicinity were mapped in detail. The stratigraphic sequence of basalts from 11 Clayton vents and 5 Capulin vents was determined. Petrographic, petrologic, paleomagnetic, and geomorphologic methods were used to correlate the volcanic rocks over the remainder of the county, because stratigraphic relationships could not be determined across the covered areas. The volcanic sequence in Des Moines quadrangle began with the outpouring of the extensive Raton basalt sheets that now cap the high mesas. No representative of the younger sequence of Raton basalts recognized to the west of Union County is present.

A time interval long enough to lower stream gradients several hundred feet along the Dry Cimarron River intervened prior to the initial eruption of the Folsom sequence of Clayton basalt. Emery Peak, East Emery Peak, Big Hill, and East Big Hill volcanoes erupted nearly simultaneously and thoroughly blocked the drainage of the Dry Cimarron River 3 mi east of Folsom, forming a lake. Into this lake, in succession, flowed basalt from Augite Vents and Purvine mesa, tuff from Mud Hill and Great Wall, and basalt from Bellisle Mountain. Sheets of lava from Robinson Mountain and finally from Jose Butte, both lying just west of Union County, completed the filling of the lake basin.