GEOLOGIC MAP OF THE THREE RIVERS 7.5-MINUTE QUADRANGLE, OTERO COUNTY, NEW MEXICO BY DANIEL J. KONING New Mexico Bureau of Geology and Mineral Resources, New Mexico Tech, 801 Leroy Place, Socorro, NM 87801-4796;
[email protected] May, 2009 Revised November, 2012 INTRODUCTION Below are descriptions for geologic map units depicted in the Three Rivers 7.5-minute quadrangle, New Mexico. The topography in the eastern part of this quadrangle generally consists of gentle slopes, with isolated hills and mesas. Quaternary alluvium and slopewash occupy most of the surface area here, but sufficient outcrops of bedrock units exist to map Permian, Triassic, and Cretaceous formations, as well as the structures that deform them. The north- to northwest-striking Alamogordo fault passes near the center of the quadrangle. Relatively recent movement along this fault has created fault scarps in the Quaternary alluvium. To the west of the fault are west-sloping alluvial fan deposits. Alluvial fan sediment consists primarily of interbedded clayey-silty sand and sand-gravel beds, with common gravel near the mountain-front and clay becoming abundant in the west. The Phillips Hills extend into the northwestern corner of the map, and are underlain primarily by limestone, dolomite, and gypsum. DESCRIPTION OF MAP UNITS Most of the map units on the Three Rivers quadrangle are Quaternary or Pliocene in age. Locally, sediment of the Santa Fe Group was observed (map unit Tsf), probably Miocene in age. A variety of intrusive sills, laccoliths, and dikes intrude older Mesozoic and Permian strata. The Quaternary units on this quadrangle are grouped into the following six categories: Modern alluvium, artifical fill and excavations, eolian and slopewash deposits, terraces and terrace deposits, alluvial fan deposits west of the Alamogordo fault, and alluvial slope deposits east of the Alamogordo fault.