WILLIAM R. SEAGER Department of Earth Sciences, Box 3AB, New Mexico State University, Las Cruces, New Mexico 88003 Resurgent Volcano-Tectonic Depression of Oligocene Age, South-Central New Mexico ABSTRACT as other well-known resurgent cauldron cycles. Locally, deposition in the depression continued The Goodsight-Cedar Hills volcano-tectonic without interruption into early late Tertiary depression in south-central New Mexico time, when extensional faulting occurred. formed concurrently with and following depo- The depression is elongated parallel to the sition of about 550 km3 of Oligocene volcanic late Tertiary Rio Grande Rift and may be a and volcaniclastic rocks. The depression is an precursor of Basin and Range structure in the asymmetric basin about 80 km long and 38 km area. The late Tertiary fault pattern in the wide. It is filled to a maximum depth of about area indicates that earlier volcano-tectonic 550 m by early rhyolitic ash-flow tuff, medial structures had important effects on the pattern epiclastic strata derived partly from marginal of late Tertiary Basin and Range structures. raised rims (Bell Top Formation), and late Locally, late Tertiary faults were inherited basaltic andesite (Uvas Basalt). Much of the from and duplicate the position of earlier sub- 295 km3 of effusive rock was erupted from sidence faults or the synclinal moat. Elsewhere, vents located near the center of the depression the north-trending regional fault pattern was and from a major subsidence fracture zone considerably modified by structural and litho- along the eastern margin. Subsidence of the logic inhomogeneities of volcano-tectonic ori- depression floor was noncatastrophic and ap- gin, particularly the Sierra de las Uvas dome proximately kept pace with basin filling, except and its buried intrusive masses. along the eastern margin. Following eruption and broad, regional sub- INTRODUCTION sidence of the Uvas Basalt, the central floor of Eruption of about 295 km3 of Oligocene lava the depression was arched upward to form the and tephra from vents in the Sierra de las Sierra de las Uvas dome and adjacent synclinal Uvas-Cedar Hills area of Dona Ana County, moat. The fault pattern of the dome and its New Mexico, created the Goodsight-Cedar association with known vents and the thickest Hills volcano-tectonic depression (Fig. 1). The part of the volcanic pile suggest that it formed depression extends over an area of about 2,900 by vertical movement of magma from an km2 and is filled to a maximum depth of about underlying chamber. Its development, follow- 550 m by rhyolitic ash-flow tuff, andesite, ing subsidence of the Uvas Basalt, suggests that basaltic andesite, and associated epiclastic strata it is essentially resurgent in origin. Although no derived largely from raised rims. A prominent postdoming volcanism is known, indirect evi- domal uplift, the Sierra de las Uvas dome, dence indicates that intrusion of silicic magma formed near the center of the depression fol- probably caused resurgence. lowing eruption and subsequent subsidence of It is clear that the Goodsight-Cedar Hills the youngest basaltic andesite. The dome seems depression is not a resurgent cauldron of the similar to the central dome of well-known re- classic Valles or Toba types. Rather, it appears surgent cauldrons, but, as discussed in this to be a structure transitional in character be- paper, it differs in some important fundamental tween a cauldron and a fault trough. Subsid- ways. ence apparently followed volcanism. Resur- Late Tertiary fault blocks are superimposed gence followed eruption of basaltic andesite on the volcano-tectonic structures. Their ero- rather than the usual ash-flow sequence, and sion by the Rio Grande and its tributaries pro- the 10- to 12-m.y. history of the Goodsight- vides excellent exposures of the volcanic fill of Cedar Hills depression is 5 to 6 times as long the depression and of various structural and Geological Society of America Bulletin, v. 84, p. 3611-3626, 10 figs., November 1973 3611 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/11/3611/3428609/i0016-7606-84-11-3611.pdf by guest on 01 October 2021 3612 W. R. SEAGER Figure 1. Index map of Goodsight-Cedar Hills volcano-tectonic depressi on. vent features related to its eastern margin and genesis of the Goodsight-Cedar Hills depres- central dome. sion differs from them. Finally, some possibly The main objectives in this paper are to de- significant relations between the extensional scribe the volcano-tectonic depression and its late Tertiary fault pattern and earlier volcano- central dome. Comparison with other resurgent tectonic structures will be discussed. volcanic structures will emphasize how the Until recent years, little was known of the Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/11/3611/3428609/i0016-7606-84-11-3611.pdf by guest on 01 October 2021 07*00' i 10w Figure 2. Generalired gcologic nup of Goodught Mountaim-Cedar llill. Sierra de b» Urai arta, .outh-cemral New Meako. SEACER, FIGURE 2 Geological Society oí America Bulleun, v. M, no. 11 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/11/3611/3428609/i0016-7606-84-11-3611.pdf by guest on 01 October 2021 OLIGOCENE VOLCANO-TECTONIC DEPRESSION, NEW MEXICO 3613 geology of the area although Darton (1928) volcanic fill in the Goodsight-Cedar Hills de- and Dunham (1935) both reported observa- pression. tions in the Sierra de las Uvas and Caballo South of Interstate 10, Quaternary basalt Mountain area. Kelley and Silver (1952) flows and cinder cones of the West Potrillo named the middle Tertiary Palm Park and Mountains overlap older volcanic fill of the Thurman Formations from the Caballo Moun- Goodsight-Cedar Hills depression. The Quat- tains, and Kottlowski (1953) identified the Bell ernary volcanic rocks cover a rectangular area Top Formation and Uvas Basalt from the of about 800 km2 that is in line with, and Sierra de las Uvas. The Thurman, Bell Top, possibly above, a southward extension of the and Uvas rock units comprise the volcanic fill- older volcano-tectonic depression. ing of the volcano-tectonic depression. More recent work in the area involved detailed map- VOLCANO-TECTONIC DEPRESSION ping at a scale of 1:24,000 by Seager and FILL others (1971), Seager and Hawley (1973), and As in many other parts of southwestern demons and Seager (1973). Hawley (1970) New Mexico and adjacent Arizona, the Ter- summarized Cenozoic stratigraphy of the area tiary volcanic section in the Goodsight-Cedar in a guidebook to the Rio Grande Valley be- Hills depression consists of a tripartite sub- tween Hatch and Las Cruces, New Mexico, and division of andesite, rhyolite, and basaltic Seager and Clemons (1973) described the andesite (Seager and Clemons, 1973). Basal stratigraphy of the volcanic rocks. andesitic to latitic rocks are referred to the REGIONAL SETTING Palm Park and Rubio Peak Formations (Kelley and Silver, 1952; Elston, 1957), the medial The Goodsight-Cedar Hills volcano-tectonic rhyolitic section to the Bell Top-lower Thur- depression and Sierra de las Uvas dome are man Formations (Kottlowski, 1953; Kelley located near the southern end of the late and Silver, 1952), and upper basaltic andesite Tertiary Rio Grande Rift zone of Kelley and to the Uvas Basalt (Kottlowski, 1953). The Silver (1952), Kelley (1953), and Chapin volcanic sequence is more complex than the (1971; Fig. 1). Figure 2 shows a generalized threefold subdivision suggests because of the geologic map of the region. The Caballo, Sierra mutual intertonguing of rhyolite with basaltic de las Uvas, Cedar Hills, and other fault blocks, andesite and andesite. The volcanic rocks range together with adjoining grabens, are Miocene in age from Eocene to Oligocene and generally to Holocene intrarift structures that are super- overlie gently folded Paleozoic or Mesozoic imposed across the older, mainly Oligocene, strata, although in many places they are sepa- volcano-tectonic features. The position of some rated from them by fanglomerates or gypsifer- late Tertiary faults, however, appears to have ous red beds of the Love Ranch Formation been inherited from older subsidence fractures. (Kottlowski and others, 1956). Locally, the An interval of 2 to 4 m.y. lay between active volcanic rocks grade upward into fanglomerate volcanism in the depression and regional Basin of the Santa Fe Group, but more commonly and Range faulting in northern Doña Ana the volcanic rocks are overlain unconformably County. However, sedimentation and subsid- by the Santa Fe, especially along late Tertiary ence in the depression continued until rifting fault-block margins and along the eastern raised began, as indicated by local conformable rela- rim of the volcano-tectonic depression. The tions between Miocene Santa Fe Group Tertiary rock units are summarized in Table 1. fanglomerates, derived from rising fault blocks, and underlying volcaniclastic strata. The Eocene Palm Park and Rubio Peak Formations form the floor and eastern and Figure 1 also shows the position of the Good- western raised rim of the Goodsight-Cedar sight-Cedar Hills depression with respect to Hills depression (Figs. 3 and 4). The depression the better known and larger Mogollon-Datil is filled by the Bell Top-lower Thurman For- volcanic province farther west (Elston, 1957, mations and Uvas Basalt, which collectively 1968; Elston and others, 1968, 1970). Similar have a volume of about 550 cu km (Fig. 5). rock sequences, rock types, and ages character- ize the two provinces but the Goodsight-Cedar Hills field appears to be separate both geo- Bell Top-Lower Thurman Formations graphically and geologically from the Mogol- The Bell Top-lower Thurman rock units lon-Datil volcanic province. This is indicated comprise about two-thirds of the volcano- primarily by the provincial distribution of tectonic depression fill.