New Mexico Geological Society Downloaded from: http://nmgs.nmt.edu/publications/guidebooks/31 Quaternary faulting in Salt Basin Graben, West Texas Lisa K. Goetz, 1980, pp. 83-92 in: Trans Pecos Region (West Texas), Dickerson, P. W.; Hoffer, J. M.; Callender, J. F.; [eds.], New Mexico Geological Society 31st Annual Fall Field Conference Guidebook, 308 p. This is one of many related papers that were included in the 1980 NMGS Fall Field Conference Guidebook. Annual NMGS Fall Field Conference Guidebooks Every fall since 1950, the New Mexico Geological Society (NMGS) has held an annual Fall Field Conference that explores some region of New Mexico (or surrounding states). Always well attended, these conferences provide a guidebook to participants. Besides detailed road logs, the guidebooks contain many well written, edited, and peer-reviewed geoscience papers. 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GOETZ Conoco, Inc. 9301 Indian School Rd., NE Albuquerque, New Mexico 87112 INTRODUCTION der, south to the latitude of Van Horn, there are at least five major The Salt Basin graben is the easternmost limit of modern Basin structural breaks. and Range tectonism in Texas and is the easternmost structure The northernmost structure line shown in Figure 2, a map of Salt related to the Rio Grande rift in the United States. Quaternary Basin, was dashed in by King (1948) and bears southeast from the faulting in Salt Basin appears to be controlled by pre-existing struc- faulted limestone south of Dell City to the south side of Bitterwell tural zones of weakness. Over one hundred Quaternary fault Mountain. The next apparent break in the basement blocks is pos- scarps and photolineaments were mapped in Salt Basin (Goetz, tulated by the author to bear east-west between the Babb Flexure 1977; fig. 1). Three types of fault scarps were recognized: lake- and the south side of Bitterwell Mountain. This structural zone has shore scarps, mid-fan breaks, and bedrock boundary faults. The not been mapped before and will be discussed later. The Babb faults, particularly the mid-fan breaks, frequently act as semi- Flexure may continue beneath the Salt Basin and intersect the pervious barriers separating fresh water on the mountain side from Apache Mountain trend, thus forming the northern side of the saline water on the playa side. Scott Canyon fan embayment. A tectonic origin is suggested by the orientation of the scarps The Victorio Flexure strikes in a more easterly direction than and the proximity of the Mayfield fault scarps to the estimated epi- does the Babb Flexure. Movement on the flexure apparently centers of the 1931 Valentine earthquakes (intensity VIII on the downdropped the basement block to the north along an east-west modified Mercalli scale). It is suggested that these fault scarps have trend beginning in Victoria (sic) Canyon, crossing the north side of been produced and maintained by intermittent activity rather than the Baylor Mountains and intersecting the Apache Mountains on by a single event. A Holocene age for some of the scarps is the south side of the Scott Canyon fan embayment. Both the Babb strongly suggested by a comparison of the erosion rate there with and Victorio flexures are Early Paleozoic, north-dipping faulted that on the 1968 Borrego Mountain, California, scarps (Clark and monoclines. others, 1972) and by interpolation using the caliche development The southern limit of the Wildhorse sub-basin, the southernmost scale developed by Giles and others (1970) for southern New Mex- basement block in the study area, is formed by the Hillside Fault ico. and the Wylie Mountain Fault. These two faults and the faults that The frequency of small earth tremors felt by the residents of the separate the grabens of Lobo Valley and Michigan Draw from the basin and recorded by temporary seismograph stations indicate Salt Flat graben and have downdropped the sub-basin of Salt Flat that tectonic adjustments are presently occurring (Dorman and and the toe of the Wylie Mountains (Hood Scalapino, 1951, others, 1976; see also Dumas, this volume). Baker, 1952 and Hay-Roe, 1958). These faults lie in the northwest- I suggest that the basement blocks of the graben floor are being trending zone of structural transition known as the Texas Linea- actively downdropped to the west-southwest along the eastern ment (Wiley and Muehlberger, 1970; see Muehlberger, this hinge of the half-graben. Evidence for this includes the preferential volume). This regional anomaly forms a 100- to 150-m-wide zone alignment of Quaternary and Holocene fault scarps along the extending from the Transverse Ranges of California along the western side of the graben. The westward shifting of the playa southern edge of the Colorado Plateau to Trans-Pecos Texas. lakes and the preferred orientation of giant desiccation polygons Locally it separates the stable Diablo Plateau from the mobile Chi- also were probably induced by this movement (see second paper huahua Trough. by Goetz, this volume). The approximate depth of the alluvium and the general tilt of the The basement blocks are broken by four transverse structural lin- bedrock blocks beneath Salt Basin are indicated in figure 3, con- eaments crossing the Salt Basin graben mapped by King (1948, structed from the resistivity and well log data assembled by White 1965) and Wiley (1970). A fifth zone trending east-west between and others (1977). With the exception of the block or zone adja- the Babb flexure and Bitterwell Mountain has been proposed by cent to Bitterwell Mountain, all the bedrock blocks appear to be the author (Goetz, 1977). downdropped to the west and have increasing displacements to the south. STRUCTURAL SETTING The graben as a whole is asymmetric, with the eastern horst ele- Salt Basin graben is a block-faulted half-graben composed of four vated higher than the western horst in the Salt Basin section of the segments (fig. 2). Each segment is offset to the west of its adjacent graben. The reverse is true for the Salt Flat section. The difference southern segment along faults that parallel Late Paleozoic fault in elevations is approximately 1400 m at the north end of Salt Basin trends. The two southernmost segments form Salt Flat. The south- and decreases to approximately 180 m at the southern end. The ernmost section bifurcates to the south at the latitude of Van relief between El Capitan Mountain in the Guadalupe Mountains Horn. The northern Salt Flat section also bifurcates at its southern (2624 m) and the adjacent basin floor (1080 m) is the most extreme end. The basin arm west of the Baylor Mountains is much less sig- found in the state of Texas. nificant than the basin arm that widens to become Wildhorse Flat. The steep slope and high elevation on the eastern side of the The two northern sections of the area form the Salt Basin and are graben at its north end is similar to the slope and asymmetry of the essentially rectangular and parallel in outline. Rio Grande rift. This characteristic shape is used by Chapin and The graben floor appears to be formed by a series of southwest- Seager (1975) as the basis for including this basin as part of the rift west-dipping basement blocks. From the New Mexico-Texas bor- (fig. 3). 84 GOETZ 32°00 30 31 00 state I 250.000 E-1V F-1 E 1=3 - CONTOUR INTERVAL 200 FEET WITH SUPPLEMENTARY CONTOURS AT 100 FOOT INTERVALS TRANSVERSE MERCATOR PROJECTION Magnetic Declination 11.NE Figure 1. Map showing the locations of fault scarps found in Quaternary alluvium in Salt Basin, West Texas. Base map from U.S.G.S. sheet NH 13-2, Van Horn, Texas, 1954; limited revision, 1964. Roads updated by Goetz, 1977. Heavy solid lines are Quaternary fault scarps in alluvium; dashed heavy lines are anomalous breaks in alluvial slopes (possible Quaternary fault scarps); dotted lines are photolineaments. QUATERNARY FAULTING IN SALT BASIN GRABEN 85 105 32 New Mexico _ . Texas u IQ B i tterwelf • Brea 31 0 20 40 60 80 --1 1 KILOMETERS Figure 2. Structural map of the area about and including Salt Basin, adapted from King, 1965. The Rio Grande rift or depression is a series of structurally (Cordell and Kottlowski, 1975), which closely resemble the faults aligned basins which lacked a master integrated drainage system similarly defined in Salt Basin graben (Wiley, 1970; Decker and until sometime in the late Pliocene (Strain, 1958).