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Table of Contents

  1. Geology of the Marathon region, Texas
    1. Geology of the Marathon Region, Texas.

    2. Contents.

    3. Illustrations

    4. Geology of the Marathon Region, Texas

    5. Abstract

    6. Introduction.

    7. Location.

    8. Previous work

    9. Field work

    10. Acknowledgments.

    11. Geography.

    12. Physical Features of Trans-Pecos Texas

    13. Basin and Range province.-

    14. Mexican Highlands province.-

    15. Great Plains province.

    16. Climate.

    17. Vegetation

    18. Erosional Agencies.

    19. Agencies observed in the Marathon region.-

    20. Marathon Region.

    21. General Features.

    22. Escarpments Bordering the Marathon Basin

    23. Escarpments and Plateaus on the East and South Sides.

    24. Escarpments on the north side

    25. Escarpments on the west side.

    26. Relation of escarpments bordering the Marathon Basin to the later tectonic movements –

    27. Ridges of the Marathon Basin.

    28. Novaculite ridges.

    29. Dimple limestone ridges.—

    30. Even summit levels on the ridges.

    31. Lowlands of the Marathon Basin.

    32. Rock floors, their nature and origin.-

    33. Comparison of rock floors previously described with those in the Marathon Basin-

    34. Rock floors of the Marathon Basin.

    35. Erosion and deposition on the rock floors

    36. Streams of the Marathon Basin.

    37. Stratigraphy.

    38. General Outline.

    39. PRE-CAMBRIAN ROCKS

    40. Pre-Cambrian rocks north and south of Marathon.

    41. Fragments of crystalline rocks in the Paleozoic sedi-

    42. Depth of the pre-Cambrian floor in the Marathon Basin.

    43. Cambrian System

    44. DAGGER FLAT SANDSTONE

    45. Local Features.

    46. Dagger Flat area-

    47. Threemile Hill-

    48. Woods Hollow Tank.-

    49. Marathon anticlinorium.-

    50. Microscopic Character.

    51. Fossils and Age.

    52. Stratigraphic Relations.

    53. Problem of the Brewster Formation.

    54. Ordovician System

    55. Historical Summary.

    56. General Features.

    57. Marathon Limestone.

    58. General Features

    59. Local Features.

    60. Marathon anticlinorium.

    61. Dagger Flat anticlinorium,

    62. Fossils and Age.

    63. Stratigraphic Relations.

    64. Alsate Shale

    65. General Features.

    66. Local Features.

    67. Marathon anticlinorium,

    68. Dagger Flat anticlinorium.-

    69. Relation of the Marathon and Dagger Flat areas.—

    70. Microscopic Character

    71. Fossils and Age.

    72. Stratigraphic Relations.

    73. Fort Pena Formation.

    74. General Features.

    75. Local Features

    76. Marathon anticlinorium.-

    77. Dagger Flat anticlinorium.-

    78. Jones ranch area.

    79. Microscopic Character

    80. Fossils and Age

    81. Stratigraphic Relations.

    82. Woods Hollow Shale.

    83. General Features.

    84. Local Features.

    85. Woods Hollow Mountains.—

    86. Simpson Springs and East Bourland Mountains.—

    87. Other localities

    88. Microscopic Character.

    89. Fossils and Age.

    90. Stratigraphic Relations.

    91. Maravillas Chert.

    92. General Features.

    93. Local Features.

    94. Marathon anticlinorium.

    95. Monument Spring and Rock House Gap.-

    96. Dagger Flat anticlinorium-

    97. Southeastern Exposures.

    98. Microscopic and Chemical Character.

    99. Fossils and Age.

    100. Stratigraphic Relations.

    101. General Problems of Ordovician Stratigraphy.

    102. Correlations and Regional Relations.

    103. General correlations.

    104. Correlations in trans-Pecos Texas.-

    105. Correlations with central Texas.

    106. Correlations with Oklahoma and Arkansas.-

    107. Conditions of Deposition.

    108. Faunal fades of the Marathon Ordovician.-

    109. Source of the sediments.

    110. Depth of water during deposition.-

    111. Origin of the chert beds.-

    112. Origin of the boulder beds-

    113. DEVONIAN (?) SYSTEM

    114. Local Features.

    115. Northwestern exposures (Jades 1).-

    116. Marathon anticlinorium (fades 2).

    117. Dagger Flat anticlinoriwn (facies 8 and 4)-

    118. Microscopic and Chemical Character.

    119. Banded chert.-

    120. Sandstone

    121. Fossils and Age.

    122. Stratigraphic Relations.

    123. GENERAL PROBLEMS OF DEVONIAN (?) STRATIGRAPHY

    124. Correlations in trans-Pecos Texas.-

    125. Correlations with Oklahoma and Arkansas.-

    126. Origin of the Capallos Novaculite.

    127. Definition of novaculite.-

    128. Theories of origin.-

    129. Significant features in the novaculite.-

    130. Significant features in the banded cherts.-

    131. Conclusions.-

    132. Carboniferous System.

    133. Pennsylvanian Series.

    134. Tesnus Formation.

    135. Local Features

    136. Tesnus and Haymond area.-

    137. Southeastern part of Marathon Basin.

    138. Pena Colorada synclinorium.-

    139. Dugout Creek area,

    140. Microscopic Character.

    141. Fossils and Age.

    142. Stratigraphic Relations.

    143. Dimple Limestone.

    144. General Features.

    145. Local Features.

    146. Eastern part of Marathon Basin

    147. Western part of Marathon Basin

    148. Fossils and Age.

    149. Stratigraphic Relations.

    150. RAYMOND FORMATION

    151. General Features.

    152. Haymond Area.

    153. General relations.

    154. Lower members of formation.-

    155. Boulder-bed member.

    156. Upper member of formation.-

    157. Other Areas of Haymond Formation.

    158. Gap Tank area.-

    159. Exposures west of Marathon.-

    160. Microscopic Character.

    161. Arkosic sandstones.

    162. Thin-bedded sandstone and shale.-

    163. Boulder-bearing mudstone.-

    164. Fossils and Age.

    165. Indigenous fossils.-

    166. Fossils of the exotic blocks.-

    167. Stratigraphic Relations.

    168. Gaptank Formation.

    169. Historical Summary.

    170. General Features.

    171. Bed 6.

    172. Bed 10.

    173. Bed 15

    174. Bed 19

    175. Beds beneath the Uddenites zone.

    176. Uddenites Zone.

    177. Correlation of the Gaptank Strata at Gap Tank and Wolf Camp. Correlation of the Gaptank Strata at Gap Tank and Wolf Camp.

    178. Faunas of the Area West of Marathon.

    179. Basal beds.

    180. The ammonoid fauna.

    181. Northeastern part of Payne Hills.

    182. Milepost 580 area.

    183. Relation of the Strata West of Marathon to those of the Type Section. TYPE SECTION

    184. Stratigraphic Relations.

    185. PENNSYLVANIAN STRATIGRAPHY

    186. Stratigraphic Sequence.

    187. Correlations and Regional Relations

    188. General relations.

    189. Correlations in trans-Pecos Texas.-

    190. Correlations with central Texas, southern Oklahoma, and Arkansas-

    191. Pennsylvanian-Permian boundary-

    192. conditions of Deposition of the Pennsylvanian Strata

    193. Younger formations.

    194. PROBLEMS OF THE HAYMOND FORMATION

    195. General features.

    196. Origin of the arkoses.

    197. Origin of the boulder-bed member.

    198. Permian Series.

    199. Historical Summary.

    200. General Features.

    201. WOLFCAMP FORMATION

    202. Local Features.

    203. Wolf Camp area.-

    204. Exposures in Monument Spring quadrangle.-

    205. Fossils and Age.

    206. Fossils of the Type Section.

    207. Gray limestone member.

    208. Bed 8-

    209. Bed 9

    210. Bed 12-

    211. Bed 11

    212. Age of the Wolfcamp Formation in the Type Section.

    213. Fossils and Age of the Wolfcamp Formation in the Monument Spring Quadrangle.

    214. Stratigraphic Relations.

    215. LEONARD FORMATION

    216. General Features.

    217. Exposure in the Monument Spring Quadrangle.

    218. Fossils and Age.

    219. General Features.

    220. Long-ranging fossils in the formation.-

    221. Characteristic lower Permian fossils of the formation.-

    222. A possible lower Leonard faunule.

    223. Faunal jades of the Leonard formation.

    224. Correlation of the Leonard formation.

    225. Stratigraphic Relations.

    226. WORD FORMATION

    227. Exposures in the Monument Spring Quadrangle.

    228. Fossils and Age.

    229. General character of the fauna.

    230. Faunas of different parts of the formation.

    231. Stratigraphic Relations.

    232. Capitan Limestone.

    233. Origin of the Name.

    234. General Features.

    235. Exposures in the Monument Spring Quadrangle.

    236. Fossils and Age.

    237. Fossils of the Altuda shaly member.

    238. Fossils of the Vidrio massive member.

    239. Fossils of the Gilliam thin-bedded member.

    240. Correlation of the Capitan limestone.

    241. Stratigraphic Relations.

    242. Tessey Limestone.

    243. GENERAL PROBLEMS OF PERMIAN STRATIGRAPHY

    244. Sedimentation of the Permian Series in the Glass Mountains.

    245. Source of the Clastic Sediments in the Permian.

    246. Triassic (?) System.

    247. Bissett Conglomerate.

    248. General Features.

    249. Fossils and Age.

    250. Stratigraphic Relations.

    251. Post-Bissett and Pre-Cretaceous Time Interval.

    252. Cretaceous System.

    253. General Features.

    254. Lower Cretaceous (Comanche) Series.

    255. Trinity Group.

    256. Glen Rose Formation.

    257. General Features.

    258. Local Features.

    259. East side of Marathon Basin

    260. South side of Marathon Basin.

    261. West side of Marathon Basin

    262. MAXON SANDSTONE

    263. Local Features.

    264. Age of Formation.

    265. Fredericksburg Group.

    266. Walnut and Comanche Peak Formations.

    267. General Features.

    268. Local Features.

    269. Edwards Limestone.

    270. Washita Group.

    271. Georgetown Limestone.

    272. Del Rio Shale and Buda Limestone.

    273. UPPER CRETACEOUS (GULF) SERIES)

    274. Possible Higher Beds.

    275. Quaternary System.

    276. General Features.

    277. Gravel Deposits.

    278. Alluvium.

    279. Igneous Rocks.

    280. Volcanic Rocks.

    281. Intrusive Rocks.

    282. STRUCTURAL GEOLOGY

    283. Marathon Basin.

    284. General Features.

    285. Marathon Anticlinorium (Including Dugout Creek Area). CREEK AREA)

    286. General features .

    287. Exposures of the Duqout Creek overthrust-

    288. Structure of the overridden rocks.-

    289. Faults of the overriding block.-

    290. Folds of the overriding block.

    291. Pena Colorada Synclinorium.

    292. General features —

    293. Synclinal area south of Monument Spring-

    294. Simpson Springs and East Bourland Mountains.

    295. Woods Hollow Mountains.-

    296. Dagger Flat Anticlinorium.

    297. General features.

    298. Structural features of the older rocks.-

    299. Folding in the younger rocks.

    300. Folded overthrusts in the younger rocks-

    301. Tear faults in the younger rocks.-

    302. Ridges southeast of the anticlinorium.-

    303. Synclinorial area between Tesnus and Gap Tank.

    304. General features.

    305. Folds.

    306. Faults.

    307. Southeastern Part of Marathon Basin.

    308. General features.-

    309. Hells Half Acre fault.

    310. Structural features south of Hells Half Acre fault.—

    311. General Structureal Problems in the Marathon Basin.

    312. Theoretical problems of the overthrust faults.

    313. Theoretical problems of the steep thrust faults.-

    314. Theoretical problems of the tear faults.

    315. Theoretical problems of the folds.-

    316. Age of the Deformation.

    317. Regional Relations of the STructural Features at Marathon.

    318. Structural Features of the Glass Mountains.

    319. Post-Cretaceous Structural Features.

    320. Eastern margin of the Marathon dome.-

    321. Western margin of the Marathon dome.-

    322. Age of the Marathon dome.-

    323. Regional relations of the post-Cretaceous structural features.

    324. Economic Geology.

    325. Ore Deposits.

    326. Novaculite.

    327. Building Stone.

    328. Water Supply.

    329. Oil and Gas.

    330. Index.

    331. U.S. Geological Survey.

  2. Illustrations
    1. Untitled

    2. Index map of trans-Pecos Texas showing location of Marathon region( The shaded area is that covered by the detailed geologic map (pi. 24). Figure 1

    3. A. VIEW LOOKING SOUTHWEST FROM SUMMIT OF HORSE MOUNTAIN. Shows novaculite ridges.

    4. B. HOUSETOP MOUNTAIN FROM SOUTH. Shows angular unconformity between Lower Cretaceous (Ke, Edwards limestone; Kcm, Comanche Peak limestone, Walnut Clay, and Maxon sandstone; Kgr, Glen Rose formation) and Pennsylvanian (Ct, Tesnus formation).

    5. Figure 2.—Views of the Marathon Basin from the escarpments on the east and west. (See also pi. 1, A, a view of the basin from the top of Horse Mountain.) A, View from summit of Housetop Mountain south and southwest across Marathon Basin. On the left and in the background are the eastern and southern bordering escarpments of Cretaceous limestone. TH, Tres Hermanas Mountain; SC, Siena del Carmen; CM, Chisos Mountains; TP, Twin Peaks; HM, Horse Mountain; SP, Santiago Peak. (Compare pi. 1, B, a view of Housetop Mountain from the plains to the south.) B, View from summit of Del Norte Mountains 2 miles north of Del Norte Gap, looking northwest across Marathon Basin. The Glass Mountains form the sky line on the left. RR, Roberts Ranch; PH, Payne Hills; TM, Iron Mountain; LM, Leonard Mountain; M, Marathon; Rh, Rock House Gap; SS, Sunshine Springs.

    6. Untitled

    7. Figure 3.—Sections of the Cretaceous escarpments on the east (A) and south (B) sides of the Marathon Basin. The southern scarp has been called the Maravillas scarp.

    8. Figure 4.—Map showing deposits of gravel on the uplands east of the Marathon region. From data furnished by N. H. Darton.

    9. Untitled

    10. Figure s.—Block diagrams of the escarpments on the west side of the Marathon Basin. The upper diagrams (A and C) show the form of the restored structural surface, and the lower diagrams (B and D) show diagrammatically the present surface form. A and B are in the Del Norte Mountains. Here erosion has carved out the weak beds on the upthrown side of the fault so that the resistant beds on the downthrown side stand up in an escarpment (an obsequent fault-line scarp). Compare sections M-M' and N-N', plate 21. C and D are in the Santiago Mountains. Here the resistant beds on the west have been raised to their present position by a normal fault. Erosion of the raised beds in places makes an escarpment near the fault line (as toward back of block), but in others has cut back into steeply folded beds to the west (as toward front of block). Compare sections Q-Q'-Q", R-R', and S-S', plate 21.

    11. Figure 6.—Block diagrams of three anticlinal mountains the Marathon Basin, arranged in sequence to show forms displayed during the course of their erosion. Op, Fort Pefia formation (resistant); Ow, Woods Hollow shale (nonresistant); Omv, Mara villas chert; and Dc, Caballos novaculite (resistant); Ct, Tesnus formation (nonresistant). A, Horse Mountain, which is covered by novaculite; B, East Bourland Mountain, where the novaculite cover is removed and where there is a small axial anticlinal valley cut in the Woods Hollow shale; C, mountain between Woods Hollow and Little Woods Hollow, with an extensive axial anticlinal valley and a low axial ridge on the Fort Pefia formation in the center.

    12. Figure 7.—Block diagram of the alluvial fan of Antelope Creek, at the base of the Del Norte Mountains, northwest of Altuda. (See pi. 23.) The building of the alluvial fan has ponded the drainage coming from the back side of the block, producing a patch of swampy land north of Strobel.

    13. Untitled

    14. Untitled

    15. Figure 9.—Maps showing possible stream history in south half of the Marathon Basin. Ais a hypothetical map at a stage considerably before the present, when the land surface stood several hundred feet above its present position. A consequent drainage radiates from the original center of the Marathon dome and has been superimposed on the hard and soft beds of the Paleozoic rocks which underlie the original cover of the dome. B, map at the present time, showing modification of the original drainage by the cutting of subsequent streams along belts of weak rock in the Paleozoic area.

    16. D. UPPER CHERTS, LIMESTONES, AND SHALES OF FORT PENA FORMATION. In bed of Alsate Creek 3 miles west-southwest of old Fort Pena Colorada.

    17. B. MONUMENT SPRING DOLOMITE MEMBER OF MARATHON LIMESTONE. North of old Fort Pena Colorada. Shows characteristic weathering of member.

    18. C. WOODS HOLLOW SHALE. Near old Louis Granger place, 6 miles southeast of Marathon. Shows alternation of shales and thin flaggy sandstones and limestones.

    19. A. UPPER MEMBER OF MARATHON LIMESTONE. Exposed in bed of Alsate Creek 3 miles west-southwest of old Fort Pena Colorada. Shows alternation of flaggy limestones and shales.

    20. D. DETAIL OF LOWER NOVACULITE MEMBER OF CABALLOS NOVACULITE. At gap south of old Fort Pena Colorada.

    21. C. MARAVILLAS CHERT AT ROCK HOUSE GAP. Shows alternation of limestone and black bedded chert.

    22. B. BASAL CONGLOMERATE OF MARAVILLAS CHERT AT ROCK HOUSE GAP. Hammer rests on a 5-foot boulder of calcareous sandstone.

    23. A. VERTICAL LAYERS OF CABALLOS NOVACULITE. Near old Louis Granger place, 6 miles southeast of Marathon. Photograph by C. L. Baker.

    24. A. GAP IN NOVACULITE RIDGE SOUTH OF OLD FORT PENA COLORADA. White ledges are novaculite, and darker slopes to right are Maravillas chert, Looking Bouthwest.

    25. D. VIEW SOUTHWEST FROM HACKBERRY TANK. C. NOVACULITE RIDGE SOUTHEAST OF HACKBERRY TANK. About 3 miles southwest of summit of Horse Mountain, looking southwest. Shows characteristic weathering of upper chert layers of Caballos novaculite. Shows novaculite ridges in southern part of Marathon Basin. In the foreground is white novaculite gravel, washed down from the ridges.

    26. li. MARAVILLAS CHERT NORTHEAST OF GAP SHOWN IN A. Woods Hollow shale crops out in creek bed at base of cliff, and Caballos novaculite forms ledges at top.

    27. Untitled

    28. E. AXIAL ANTICLINAL VALLEY IN WOODS HOLLOW SHALE AT SOUTHWEST END OF SIMPSON SPRINGS MOUNTAIN. Cambrian and early Ordovician fossils were collected in boulders embedded in Woods Hollow shale in this valley. Looking northeast. Dc, Caballos novaculite; Omv, Maravillas chert; Ow, Woods Hollow shale. Photographs A, B, C, and D by C. L. Baker.

    29. A. EAST BOURLAND MOUNTAIN FROM SOUTHEAST, FROM NORTHEAST END OF SIMPSON SPRINGS MOUNTAIN Shows flanking hogbacks of Caballos novaculite. Maravillas chert forms a high ridge along the axis.

    30. B. EAST BOURLAND MOUNTAIN FROM NORTHEAST. View along axis showing axial anticlinal valley carved from Woods Hollow shale, flanked by ridges of Maravillas chert and Caballos novaculite. From a field sketch

    31. C. WEST BOURLAND MOUNTAIN, LOOKING NORTHEAST. Shows folding of Dimple limestone and of Tesnus and Haymond formations. Ow, Woods Hollow shale; Omv, Maravillas chert; Dc, Caballos novaculite (1, lower chert member and lower novaculite member; 4, upper chert member); Ct, Tesnus formation; Cd, Dimple limestone; Ch, Haymond formation.

    32. A. NOVACULITE RIDGES IN LIGHTNING HILLS, LOOKING NORTHWEST FROM ELEVATION POINT 4,450. Lightning overthrust crops out in valley in foreground. Convoluted ridges of folded and faulted novaculite in middle distance.

    33. li. NOVACULITE RIDGES IN LIGHTNING HILLS, LOOKING SOUTHWEST FROM ELEVATION POINT 4,462. Lowlands in distance carved from Ordovician strata on crest of Dagger Flat anticlinorium. Ow, Woods Hollow shale; Omv, Maravillas chert; Dc, Caballoa aovaculite; Ct, Tesnus formation.

    34. C. FOLDING IN NOVACULITE ON EAST SIDE OF MARAVILLAS CREEK AT MARAVILLAS GAP.

    35. Figure 10.—Section through Threemile Hill showing relation of Dagger Flat sandstone (-Cd) to Woods Hollow shale (Ow), Maravillas chert (Omv), and Caballos novaculite (De). Small wedges of Marathon limestone, not shown in the section, are present along the plane of the overthrust along the northwest slope of hill. For general relations, see section I-I'-II", plate 21.

    36. Figure 11.—Sketches showing structural features in the Marathon limestone. A, Sponge bed and other features near Alsate Creek; B, animal burrow near Monument Spring; C, mud cracks in calcareous shale near top of formation, Alsate Creek,

    37. Figure 12.—Section in the bed of Alsate Creek, 3 miles west-southwest of old Fort Pena Colorada, showing sequence and structure of part of the Ordovician rocks. Q, Alluvial deposits. From observations by Josiah Bridge and C. L. Dake.

    38. Figure 13.—Sketches showing structural features in the Maravillas chert. A, Conglomerate interbedded with lenses of primary chert, near base, Monument Spring; B, beds and ellipsoidal masses of chert near top, Monument Spring. Chert is indicated by shading.

    39. Figure 14.—Section across novaculite ridge at picnic grounds south of old Fort Pena Colorada, showing sequence and structure of the Ordovician and 'Devonian (?) rocks. Ow, Woods Hollow shale. Omv, Maravillas chert. Dc, Caballos novaculite, divided into la, lower chert member; lb, lower novaculite member; 2, middle chert member; 3, upper novaculite member; 4, upper chert member. Og, gravel deposits. Compare plate 5, A and B.

    40. Figuee 15.—Thin sections of pre-Carboniferous cherts and novaculites; based on camera lucida drawings in plain light, magnified 21 diameters. A, Granular laminated chert, Maravillas formation, Buttrill ranch; note scattered quartz grains. B, Spotted chert, Maravillas formation; 3 miles southwest of Fort Pefia Colorada. C, Finely granular chert, Fort Pefia formation, near Fort Pena Colorada. Note sponge spicules, calcite crystals (cross-ruled), and quartz grains (unshaded). D, Porcelainlike novaculite, lower novaeulite member of Caballos formation; gap south of Fort Pefia Colorada; note scattered quartz grains (unshaded). E, Brown chert, Caballos formation, Payne Hills, in northwestern part of Marathon Basin. Note the numerous spherical tests, probably of Radiolaria. F, Banded green and white chert, middle chert member of Caballos formation, gap south of Fort Pefia Colorada. Note bands of clay particles and tests of Radiolaria. F', Test of radiolarian from slide, considerably enlarged.

    41. Figure 16.—Hypothetical block diagram showing probable geography of the Marathon region in Ordovieian time. Length of section, several hundred miles, a, Interbedded limestone, shale, and sandstone facies as found in the Marathon region, which probably passes southeastward into a more clastic facies, 6, and northwestward intoa more calcareous facies, c, such as is found near Van Horn and El Paso. For later appearance of the region (in Carboniferous time), see plate 20.

    42. Untitled

    43. Untitled

    44. CORRELATED STBATIGRAPHIC SECTIONS OF PEJNNSYLVANIAN ROCKS OF THE MARATHON BASIN.

    45. Figure 18.—Sections of lower part of Tesnus formation along Rough Creek, in southeastern part of Marathon Basin (Dove Mountain quadrangle; see pi. 23 for location). A, General section. (See sec. D-D'-D", pi. 21, for general relations.) Dc, Caballos novaculite; Cts, basal shale member of Tesnus formation; Ct, upper part of Tesnus formation with white quartzite member (Ctq). B, Detailed section on Rough Creek, a mile above its mouth, showing apparent overlap of basal shale member of Tesnusformation onto Caballos novaculite. Novaculite unshaded; thin-bedded chert and siliceous shale, probably a part of Caballos formation, shaded. Cts, dark clay shale of basal member of Tesnus formation.

    46. Figure 19.—Section across middle part of Devils Backbone showing structure and relations of white quartzite beds (Ctq) in Tesnus formation. The remainder cf the formation in the section is sandstone and shale (Ct). In the northern fault block the upper quartzite appears to lie a short distance stratigraphically below the Dimple limestone (Cd). (For general relations of Devils Backbone, see sec. E-E'-E", pi. 21.)

    47. Figure 20.—Sections between East and West Bourland Mountains showing structural features in Tesnus formation along line of stratigraphic section described in the text. A, General section between the two mountains. (For general relations, see sec. H-H'-H", pi. 21.) Omv, Maravillas chert; Dc, Caballos novaculite; Ct, Tesnus formation; Cd, Dimple limestone; Qal, alluvium. B, Detailed section on bank of Pefla Colorada Creek near middle of A,' showing complex folding of middle of Tesnus formation; note sandy concretionary masses in the shales.

    48. Figure 21.—Structure section across southern part of Payne Hills, 5 miles northwest of Roberts ranch, where a stratigraphic section was measured. Note small thickness of Tesnus formation, a characteristic feature of this part of the region. Ow, Woods Hollow shale; Omv, Maravillas chert; Dc, Caballos novaculite; Ct, Tesnus formation; Cd, Dimple limestone. See middle part of section D-D', plate 16, for location and relations of this section.

    49. D. FOLDING AND THRUSTING OF THIN-BEDDED SHALES AND SANDY SHALES OF UPPER PART OF HAYMOND FORMATION. In cut on Dugout Creek 1J.4 miles southeast of Dugout Mountain. Photograph by C. R. Longwell.

    50. B. SANDSTONES AND SHALES OF LOWER PART OF HAYMOND FORMATION. In cut of San Francisco Creek 3 miles south of Haymond station. Photograph by C. L. Baker.

    51. C. SHALES AND SANDY SHALES OF UPPER PART OF HAYMOND FORMATION. In cut on Dugout Creek 2J4 miles south of Dugout Mountain.

    52. A. MASSIVE SANDSTONE OF UPPER PART OF TESNUS FORMATION. On San Francisco Creek north of Devils Backbone.

    53. li. WATER-WORN COBBLES OF DIMPLE AND GAPTANK LIMESTONES IN FIRST CONGLOMERATE MEMBER OF GAPTANK FORMATION. A mile and a half south of Gap Tank.

    54. A. BLOCK OF NOVACULITE IN BOULDER-BED MEMBER OF HAYMOND FORMATION. Five miles southeast of Gap Tank. Photograph by C. L. Baker.

    55. D. ROUNDED BOULDER OF BRECCIATED CHERT FROM CABALLOS FORMATION IN BOULDER-BED MEMBER. A mile and a half north of summit of Housetop Mountain.

    56. C. EXPOSURES OF BOULDER-BED MEMBER. A mile west of Housetop Mountain (locality of sec. D-D', fig. 23). Knobs on hillside in middle distance are boulders of novaculite and of Pennsylvania!! limestone. Letters refer to boulders: N, Caballos novaculite; T, Teenus formation; P, Pennsylvanian limestone. Diameter of each boulder given in feet. Ch, Sandstones and shales of I [aymond formal ion underlying boulderbed member.

    57. F. BOULDER OF PENNSYLVANIAN LIMESTONE. Projecting from the mudstone matrix of the boulder-bed member 1 mile west of summit of Housetop Mountain.

    58. E. BOULDER OF NOVACULITE IN BOULDER-BED MEMBER. About IY2 miles north of summit of housetop Mountain. The boulder has byeii shattered by later deformation and by weathering.

    59. Untitled

    60. Untitled

    61. Untitled

    62. Figure 25.—Thin section of the mudstone matrix of the boulder bed, from a specimen collected on the dump of a well near the north end of the exposure. Based on camera lucida drawing in plain light. Note angular fragments of quartz (unshaded), limestone (stippled), and feldspar (ruled).

    63. Figure 26.—Plan of part of outcrop of boulder bed between sections C-C and D-D' of figure 23, showing how, where the large limestone boulders are very numerous, they might be interpreted as parts of a single stratum of limestone, subsequently broken by faulting. P, Limestone of Pennsylvanian age; N, novaculite. Figures are longer dimensions of boulders in feet.

    64. Figure 28.—Stratigraphic diagram of Permian rocks in the Glass Mountains. The diagram extends from the southwest to the northeast end of the mountains. Based on field work by P. B. and R. E. King.

    65. Figure 29.—Map of trans-Pecos Texas showing structural features of Permian time. Below is a stratigraphic diagram extending across the area, showing the relation of the Permian strata to the Delaware Basin.

    66. CORRELATED STRATIGRAPHIC SECTIONS OF COMANCHE SERIES ON THE EAST AND WEST SIDES OF THE MARATHON RASIN. Ra, Railiolights; Po, Porocystis; Or, Orbilolii a: He, Requienia; Ex, Exogyra; G, Gryphaea.

    67. , Buda limestone; Kdr, Del Rio shale; Kg, Georgetown limestone; Ke, Edwards limestone; Kcm, Comanche Peak limestone, Walnut clay, and Maxon sandstone: Kgr, Glen Rose formation; Ch, Haymond formation. D. NORTH END OF HOUSETOP MOUNTAIN. Limestones, marls, and sandstones of Comanche series are exposed in peak. Strata of Haymond formation exposed in foreground.

    68. B. DEL NORTE GAP FROM SUMMIT OF DEL NORTE MOUNTAINS, TO THE NORTH. In the distance vertical strata of Glen Rose formation (Kgr) are thrust over flaggy limestones of Eagle Ford formation.

    69. A. BLACK PEAK, IN THE DEL NORTE MOUNTAINS, FROM NORTHEAST. The peak is a mass of limestone of the Gaptank formation (Cg) thrust against the limestones (Kg, Ke) of the Comanche series, which crop out in the background.

    70. C. BLACK PEAK FROM SOUTH. Shows exposure of Black Peak thrust. Upthrown block consists of limestoaes of Gaptank formation (Cg) dipping to the east. On downthrown block, limestones of Edwards formation (Ke) are dragged to a vertical position near the fault.

    71. A. LIMESTONES OF GLEN ROSE FORMATION (Kgr) INTRUDED RY IGNEOUS ROCK (Ti). On south margin of Marathon Basin. The sharp peak, which is composed of igneous rock, is elevation point 4,748. Looking southwest. Ct, Tesnus formation

    72. B. LIMESTONES OF GLEN ROSE FORMATION (Kgr) OF THE MARAVILLAS SCARP, DIPPING SOUTH OFF MARATHON UPLIFT. Sandstones of Tesnus formation (Ct) crop out on lower part of canyon walls. View looking southward from point at which view shown in A was taken.

    73. Figure 30.—Map of trans-Pecos Texas showing progressive northeastward overlap of Comanche series and shore lines at successive stages during the epoch. A, Shore line in late Jurassic and early Cretaceous time; B, shore line at end of Trinity time (stippled area shows extent of marginal sandstone facies of Trinity group); C, shore line at end of Fredericksburg time. Based chiefly on observations of W. S. Adkins, C. L. Baker, and P. B. King.

    74. AERIAL PHOTOGRAPH OF NORTHEASTERN PART OF DAGGER FLAT ANTICLINORIUM. Center of area shown is about 2 1/2 miles southwest of Lightning ranch. Shows isoclinal northeastward-pitching folds of Marathon (Om), Alsate (Oa), Fort Pefia (Op), and Woods Hollow (Ow) formations and some outcrops of Maravillas chert (Omv). Scale, about 1,500 feet to the inch. Photograph by Edgar Tobin Aerial Surveys.

    75. AERIAL PHOTOGRAPH OF AREA OF CARBONIFEROUS ROCKS SOUTH OF HAYMOND STATION. Center of area about 3 miles northeast of summit of Horse Mountain. Shows basal shale member (Cts) and upper sandstone member (Ct) of Tesnus formation, Dimple limestone (Cd), and Haymond formation (Ch) folded into a broad syncline. Note that folds are more open here than in area shown on plate 17. Scale, about 1,500 feet to the inch. Photograph by Edgar Tobin Aerial Surveys.

    76. Figure 31.—Restoration of structural features in the younger rocks of the Dagger Flat anticlinorium. Figures B and D are pieced together from observations at different places along pitching folds, such as those shown in figures B and C, plate 19. No single section may have contained all the features represented on these restorations, but a complexity of structure of equal degree is probable. Figure A represents the probable structure of section B, and figure C the probable structure of figure D, after the first folding and overthrusting in the region, and before the final folding. A and B are in the southwestern part of the Dagger Flat anticlinorium, between Threemile Hill (on right) and the Garden Springs district (on left). For sections in Garden Springs district, see figure B, plate 19. C and D are in the northeastern part of the Dagger Flat anticlinorium, in the Warwick and Lightning Hills. For sections in this district, see figure C, plate 19. Ct, Tesnus formation; Dc, Caballos novaculite; Omv, Maravillas chert; Ow, Woods Hollow shale; Om, Marathon limestone; Cd, Dagger Flat sandstone.

    77. BLOCK DIAGRAMS SHOWING FOLDED OVERTHRUSTS OF DAGGER FLAT ANTICLINORIUM. A, Enlarged view of locality C, figure C. B, Region of Ridge and Garden Springs. C, Region of Warwick and Lightning Hills.

    78. Figure 32.—Sections of Caballos novaculite in the Warwick Hills, showing change in thickness of members and its relation to the Warwick overthrust. For location of sections, see block diagram C, plate 19. Note the difference in thickness of the two novaculite members in the overriding and overridden block of the overthrust.

    79. Figure 33.—Diagram showing estimates of crustal shortening in different parts of the Marathon Basin. The curved lines show the attitude of the beds from which the estimates were made.