Final W/ Base Comprel/Splitlong
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U.S. DEPARTMENT OF THE INTERIOR GEOLOGIC INVESTIGATIONS SERIES U.S. GEOLOGICAL SURVEY I–2631 105°37'30" 35' 32'30" 30' 105°27'30" 32°07'30" 32°07'30" 11 Mountain and on Wind Mountain; two additional anticlines lacking exposed igneous rocks in their cores Psa Psa Cornudas are present north of Flat Top and southwest of Alamo Mountain. Between the anticlines are a series of Py Psa Psa Psa Mountain Ph 3 8 subparallel to obliquely trending synclines or structural sags. The most pronounced syncline can be 7 ° Qc traced along the west side of Wind Mountain where sedimentary rocks dip as much as 60 away from Py Psa 13 12 the peak. The complex, locally merging and commonly curvilinear patterns defined by the traces of the Black Qc 4 Ph Ti ၤ Qc Qaf Qaf axial planes of folds in the Cornudas Mountains bears a strong similarity to folds associated with intru- North p Mountain Qaf Qaf 5 Qa Psa 6 Qaf sive rocks so well described by Hunt (1953) in his study of the Henry Mountains, Utah. pၤ Qaf Psa 13 Psa Ti To the north, Cornudas Mountain is within an anticline that can be traced for 12 mi (20 km) com- 3 Qfy 000 Ku Psa 4 13 pletely across the map area; the Hueco Formation is well exposed in that part of the fold that is cen- Py Py 7 Qa Ph 11 tered on the intrusive rocks. The anticlinal axis trends northwest on the east and folded Hueco lime- Ti Psa Qa 14 5 4 Qaf Unconformity Ph Qa 13 12 Qaf stones appear to extend beneath the east side of Cornudas Mountain. On the west, the fold deforms East Psa Qaf 9 Hueco limestones that overlie the intrusion; the fold axis trends west in this area but is displaced about 1 Wind pၤ 9 mi (1.6 km) to the north where it merges with a north-trending structural sag or syncline before reach- pၤ Qc Qc 7 plift Mtn. ing the western map boundary. Spatial relations between the Cornudas Mountain intrusive and the brian lrocks u of 6 13 Psa Psa 4 adjacent folded rocks suggest that an elongate intrusive body underlies the entire anticlinal fold and that 5000 5 3 5 2 Ph Precam Qaf Psa Psa the exposed Cornudas intrusive is an irregular-shaped, sill-like body that plunges gently west, yet is bulg- Pederna Flat Top Qc Psa 17 6 Py 7 Psa ingly discordant on the east (fig. 2); Hunt (1953) might have termed this apophysis of the underlying 3000 Ti 7 500 3 Ph Ph 0 Qc intrusive a sphenolith. 5000 ၤ Py 7 p Qaf Psa Qa Wind Mountain is the core of a dome in which dips of enclosing sedimentary rocks of the Hueco ၧၤu ၤ Psa Qc 9 p 3000 14 Formation are locally more than 60°. A structural sag appears to separate Deer Mountain on the west SEA ၤ 3000 LEVEL p ၤ from the main part of the dome; however, the Deer Mountain intrusive also intruded the Hueco and is p Qaf 6 Alamo 7 3 4 structurally in the core of a west-trending anticlinal flexure that is a part of the Wind Mountain dome 10 Mountain Ku Psa SEA 6 Qaf 15 (fig. 2). The similarity in composition of the Wind and Deer Mountains intrusives suggests that they are Psa LEVEL 4 8 6 SEA 0 14 petrologically related as well. The Wind Mountain dome cannot be traced east of the peak. The Wind LEVEL Ku 5000 FEET Psa Psa Mountain intrusive has been interpreted as a laccolithic intrusion (McLemore and others, 1996); howev- Ti Ph 1 South Psa Ph Qaf 23 6 13 er, geologic and geophysical relations mapped during this and a previous study (Nutt and others, 1997) Ti 3000 Qaf argin 2 Qa 16 7 suggest that an alternative interpretation is justified. There is no evidence that sedimentary rocks floor ၧၤu Qaf Qfy 4 Qfy the intrusion; all sedimentary host rocks are strongly deformed adjacent to the intrusion and dip steeply Py ၧၤ 3 Qaf u pၤ 5 4 25 16 Qaf away from the peak. Foliation within the Wind Mountain intrusive defines a circular pattern that is SCALE IN MILES SEA Psa 6 LEVEL Qfy 4 mimicked by the enclosing sedimentary rocks. The aeromagnetic signature at Wind Mountain is the 5000 Psa 11 est-facing,of Pedernal faulted uplift m 10 10 8 strongest, most prominent magnetic anomaly in the Cornudas Mountains (Nutt and others, 1997) even W Psa 17 6 EXPLANATION 4 5 though topographically similar peaks are present; all other peaks are clearly floored by sedimentary 3000 Qc 14 rocks and, based on the magnetic signature, lack the lithologic mass of Wind Mountain. The Wind Ti Tertiary intrusive rock Ph Permian Hueco Fm. 12 22 Qaf 10 Qaf 7 Mountain intrusive appears to be a steep-sided plug that extends downward to its intersection with a Ku Cretaceous sedimentary rock ၧၤu Pennsylvanian to Cambrian rock Qa Psa parent intrusive body that underlies the combined Wind-Deer Mountains anticlinal flexure. Hunt (1953) 4 Qaf SEA 12 19 described intrusive forms similar to Wind Mountain as bysmaliths. Psa Permian San Andres Fm. pၤ Precambrian rock 13 LEVEL 15 Qa An anticlinal, convex-northward upwarp is present north of Flat Top. The fold plunges west and 48 Tqs Qa Py Permian Yeso Fm. 12 Qfy 21 terminates near Alamo Mountain; on the east it appears to merge with and become a part of the Wind- West Qa 14 Qfo 21 23 Qfy Psa Deer Mountains anticline. A second anticlinal flexure is mapped southwest of Alamo Mountain; the fold 10 Qaf Figure 2. Schematic fence diagram showing stratigraphic, structural, and intrusive relationships discussed in text. Qfy Qfo Qaf can be traced southeast at least 7 mi (11 km) into Texas. Based on the direct correlation of anticlinal 16 Ti Qaf 4 flexures and doming with known intrusive rocks, we suggest that these two additional upwarps are also Qaf 3 cored by igneous rocks. Qaf 9 14 Ph Synclines mapped in the Cornudas Mountains appear to be related to igneous intrusions as well. Psa Qa 10 Tqs 14 They occur as partial ring structures around Wind Mountain, as structural sags above and between bur- 9 ied intrusive bodies, and as paired anticline-syncline accommodation folds formed in response to dilation Ph 6 Qfy of intruded sedimentary rocks. 4 18 106°00' 105°00' 7 15 17 Qc Peaks in the map area other than Cornudas Mountain and Wind Mountain do not appear to be 3 Qfy Psa associated with strongly deformed sedimentary rocks. Alamo Mountain, San Antonio Mountain, Chatt- –992 32°30' 235 3500 Qfy Qaf 10 7 8 field Mountain, Black Mountain, and Flat Top are similar to one another in that they are concordant 0 Qa 8 21 18 –2500 8 Tqs and clearly sills or laccoliths, and, unlike Wind and Cornudas Mountains, they were injected into sedi- 24 Qaf 17 –1500 Qc T 4 mentary host rocks at or above the Permian-Cretaceous unconformity (fig. 2). San Antonio Mountain, –500 IF –1000 500 2500 33 3000 Qa Psa perhaps the only true laccolith of the Cornudas Mountains and only partly exposed in the south-central –2000 L Psa 9 2000 6 part of the map area, is floored by basal Cretaceous rocks (Kues and Lucas, 1993). Chattfield Moun- 850 1000 Qao –3000 Qfy 3 500 UP 1500 Qaf tain, at least on the north where exposed in the map area, also is underlain by flat-lying Cretaceous stra- –3500 1 10 Qaf 10 Ph 2000 5' 10 Qfy 5' ta. Black Mountain, a multiple sill complex east of Wind Mountain, was emplaced in part directly above 19 Ph 9 2500 1000 Ph 7 1664 12 8 8 the unconformity as well as higher in the Cretaceous section. The Flat Top sill is underlain by the San 3000 Qc 1560 Qc 11 Py Andres Formation; no Cretaceous rocks were observed on this peak. The stratigraphic position of the L Qaf 9 16 1935 10 unconformity east of Flat Top (north of Wind Mountain) as well as on the west (beneath Alamo Moun- A Psa 5 10 Qc Qaf 9 tain) relative to the statigraphy at Flat Top suggests that the Flat Top sill was intruded along or very near 1214 N Psa Qfo Qa R Qfy the Permian-Cretaceous unconformity. E Qfy 481 D Qc Qfy The Alamo intrusion is perhaps the most complex of the stratigraphically higher intrusions (fig. 2). E 11 7 P 13 12 The intrusive body is discordant on the north where it probably intruded gently west-dipping limestones Qaf Qa of the San Andres. Along the south and west base of the peak, Cretaceous rocks are exposed. Flow 8 foliation within the intrusive is complex. On the north flank, foliation dips steeply north; midway Qaf 5 between the north flank and the crest of the peak, foliation is gently inclined to the north. Directly 1232 Ph 2280 12 6 Qa beneath the crest of the peak, the foliation steepens whereas at the crest the attitude of the foliation Qa Qa Psa 21 9 Psa again becomes nearly flat lying. The contemporary morphology of the peak, as viewed from the north- –2933 NEW MEXICO 12 32°00' Qa Py Qfy 3 west, reflects the abrupt changes of foliation attitude. Hornfelsed Cretaceous rocks rest on the intrusion TEXAS Qao 14 Psa on its north side (shown as Km) suggesting that the present exposed upper surface of the igneous rocks 0 20 MILES 12 10 8 12 13 probably represents the exhumed upper contact of the intrusion.