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Table of Contents

  1. The Gueydan, a new middle Tertiary formation from the southwestern coastal plain of Texas
    1. Introduction

    2. Discovery of the Gueydan formation

    3. Acknowledgments

    4. Location and delimitation of the area mapped

    5. Previous work

    6. Name

    7. Physiography

    8. General Statement

    9. Relief

    10. Physiographic Divisions

    11. Fayette Hills Belt (Wellborn Plain)

    12. Frio Plain

    13. Gueydan Hills Belt

    14. Gueydan Plain

    15. Oakville Hills Belt

    16. Reynosa Cuesta Plain

    17. Drainage

    18. Probable capture or diversion of the Nueces River

    19. Stratigraphy

    20. Other Formations Outcropping in the Vicinity of the Gueydan

    21. Underlying Formations (Jackson Group)

    22. Fayette Formation

    23. Frio Formation

    24. Overlying Formations

    25. Upper Miocene—Lower Pliocene Series Oakville Formation

    26. Pleistocene Series Reynosa Formation

    27. The Gueydan Formation

    28. Fant Member (Lower Gueydan)

    29. Section on Charlie York Creek, 4 Miles West of Three Rivers, between Three Rivers-Tilden Road and Frio River, Live Oak County

    30. Hill-side Section West of Simmons-Wentz Road, 2½ Miles North West of Simmons, Live Oak County

    31. Section of North HIll of "Chalk Bluffs" on Ray Ranch, 6 Miles South of Wentz, McMullen County. (See Plate III, Fig. 1.)

    32. Section on Bluff Located in Section 853, Webb County, 18 Miles North of Mirando City

    33. Soledad Member (Middle Gueydan)

    34. Section 2 Miles North of Government Wells, Duval Co., in Bluffs on Both Sides of Cotulla Road

    35. Section at South End of Soledad Hills, Northwest Duval County

    36. Section ¼ Mile East of the McMullen County Line, 4 Miles Southwest of Simmons, Live Oak County

    37. Section on Weedy Creek, Live Oak County, 2 Miles above Fant City Bridge

    38. Section on Branch of Rock Creek, ⅛ Mile North of Nopal-Nixon Road, 8 Miles South of Smiley, Gonzales County

    39. Chusa Member (Upper Gueydan)

    40. Section at Northeast End of Loma Alto Mesa

    41. Section on North Bank of Nueces River, 3 Miles above Mouth of Frio River, Live Oak County

    42. Section on West Bank of White Creek, 1 Mile above the Mouth, Live Oak County

    43. Description of Samples from H. Coquat and Associates, Hicks N0. 1 Well Located 1 Mile West of Simmons, Live Oak County

    44. Description of Samples from the Texas and Southern Petroleum Company's Wilson No. 1 Well, Located on Cherry Survey, about 4 Miles Northwest of Rockland, Texas

    45. Petrology of the Gueydan Rocks

    46. Classification of Pyroclastic Rocks

    47. Chemical Composition

    48. Petrography of the Gueydan Tuffs

    49. Composition

    50. Primary Constituents

    51. Secondary Minerals

    52. Petrographic Descriptions of Principal Types of Gueydan Tuffs

    53. Fant member

    54. Soledad Member

    55. Chusa Member

    56. Petrography of Lava Pebbles and Boulders

    57. Petrographic Descriptions of Lava Boulders from Fant Member

    58. Petrographic Descriptions of Boulders and Pebbles from Soledad Member

    59. Relative Abundance of Types of Pebbles in Soledad Conglomerate

    60. Petrographic Descriptions of Boulders from the Tuff of the Chusa Member

    61. Petrography of Non-Pyroclastic Rocks of the Gueydan-Catahoula

    62. Age of the Gueydan

    63. Mineral Veins in the Gueydan (Los Picachos Hills)

    64. Siliceous Knobs

    65. Intrusions in the Gueydan

    66. Origin of the Gueydan

    67. Structural Geology

    68. Faults

    69. Description of Principal Faults

    70. Folds

    71. Summary and Conclusions

  2. Illustrations
    1. Figure 1. Idealized Sketch Section of Reynosa Escarpment, ½ mile N. 20° E. of Mirando City, Webb County, a, Reynosa; b, Oakville; c, Gueydan.

    2. Figure 2a. Diagrammatic sketch showing approximate plan of vein outcrops at Los Picachos Hills.

    3. Figure 2b. Diagrammatic sketch of south hill of Los Picachos along line A-A' in Fig. 2a.

    4. Plate II Figure 1. Fissure in Oakville quartzitic sandstone on east side of San Caja Mesa showing cut in Rosalier Hills in alignment with it.

    5. Figure 2. Reynosa Escarpment at Mirando City Oil Field, Webb County, Texas, 40 miles east of Laredo.

    6. Plate III Figure 1. A Gueydan tuff cuesta scarp, looking northeast along strike. "Chalk Bluffs" on Ray Ranch in McMullen County, Texas. Note automobile at base of scarp.

    7. Figure 2. Exposure of Fant trachyandesite tuff in bed of creek, and massive-bedded, greenish-white, sand-tuff in banks of Lang Creek, 2½ miles west of Simmons, Live Oak County.

    8. Plate IV Figure 1. Unconformity in Gueydan near road bridge, 1½ miles west of Simmons. Reworked, light green, sandy, tuffaceous clay above and even-bedded sand-tuff below unconformity.

    9. Figure 2. Sun-cracks in Gueydan tuff, near north bank of Nueces River, 4½ miles southwest of Simmons, McMullen County.

    10. Plate V Figure 1. Sun-cracks in trachyte tuff in bed of Charlie York Creek, 4 miles west of Three Rivers.

    11. Figure 2. Exposure of trachyte tuff ½ mile south of Fant City, showing rugged character of outcrops.

    12. Plate VI Figure 1. Outcrop of Gueydan tuff, 4 miles west of Three Rivers, Live Oak County.

    13. Figure 2. Fragment from slab of tuff showing ropy flow structure. "Chalk Bluffs" on Ray Ranch, eastern McMullen County.

    14. Plate VII

    15. Figure 1. Andesite boulders in Gueydan tuff. Southwest corner of McMullen County. Figure 2. Soledad conglomerate containing a number of lava pebbles on La Mesa, at Mirando Valley Oil Field, northeastern Zapata County.

    16. Figure 1. Loess-like outcrop of Chusa tuffaceous clay at junction of White Creek and a tributary, 4 miles above mouth of White Creek, Live Oak County.

    17. Figure 2. View of south side of main Picachos Hill showing outcrop of thick chalcedony veins. Note dip of veins to left (west). Duval County.

    18. Plate IX Figure 1. Thin section of massive, very lumpy, vesicular, mudflow tuff from "Chalk Bluffs" on Ray Ranch, McMullen County. X 25. Large, white elongated spot is hole in section.

    19. Figure 2. Thin section of silicified, slightly vesicular tuff from ½ mile northeast of Wentz, McMullen County. Contains tridymite in cavities. X 25.

    20. Plate X Figure 1. Thin section of fairly hard, dense-textured, vesicular, white tuff containing tridymite crystals in cavities, from Charlie York Creek, 4 miles west of Three Rivers. X 25.

    21. Figure 2. Thin section of apatite-bearing trachyte or trachyandesite from a 1-foot boulder in the Fant member, 2 miles west of Chapote Ranch, McMullen County. Shows large resorbed oligoclase phenocrysts containing glass inclusions. Dark spots are limonitized pyroxene and magnetite. The two light-colored hexagonal crystals on each side of largest pyroxene crystal are yellow apatite. X 25.

    22. Plate XI Figure 1. Fine-grained Soledad volcanic conglomerate from south end of Soledad Hills, Duval County. Clear areas are chalcedony surrounded by a band of opal. Phenocryst with high relief is augite, black phenoeryst is altered augite. Other phenocrysts are albite-oligoclase and oligoclase. Plane polarized light. X 25.

    23. Figure 2. Fine-grained volcanic conglomerate from south end of Soledad Hills, Duval County. Dark areas are opal. One pebble is trachyte and the other is andesite. Crossed nicols. X 25,

Gueydan Formation

175

a group of rolling hills projects westward for a mile or more beyond the Reynosa escarpment. Such rounded hills (not mesas) are also present west of the Schott- A viators field, four miles south of Mirando City, but are rarely pres ent at other points along the Reynosa scarp. The topography and the scanty geologic evidence obtainable from exposures thus suggest small anticlines with north-south axes at the Ojuelos and Schott- A viators fields in southeastern Webb County.

Gentle opposing tilts of the siliceous sandstone cappings of the mesas which partially surround the Mirando Valley oil field in northeastern Zapata County indicate the presence there of a small dome or nearly equidimensional anticline.

The large reentrant in the Reynosa scarp near Randado, in northwestern Jim Hogg County, with an extensive east ward projection of older rocks into the reentrant, suggests the presence of an anticline in this region. An oil field has been discovered in northwestern Jim Hogg County since the writers last visit, which furnishes confirmatory evidence of such a structure.

Faulting may have played a minor role in the causation of petroleum and natural gas accumulation in the Webb- Zapata and Jim Hogg county fields but the surface evidence points to small anticlines or, as Sellards 84 suggests, anti clinal noses as the important agencies in the accumulation of petroleum and natural gas.

The widening of the Gueydan outcrop and the broad reentrant of Gueydan in the vicinity of Moglia, in east central Webb County, suggests the existence of a north-south structural terrace or of gentle anticlinal and synclinal fold ing in this region. Important additional evidence of fold ing here is afforded by the predominance of north dips of 1-5° at the northern margin of this reentrant of Gueydan. The Carolina-Texas gas wells west of Moglia may owe their existence to one of these inferred structural features. Some doubt is cast on the validity of folding in this region by the

84 Sellards, E. H., "Notes on the Oil and Gas Fields of Webb and Zapata Counties," Univ. Texas Bull. No. 2230, pp. 5-29, 1922.