INTRODUCTION

The Paleozoic sediments of the Panhandle of Texas and northeastern New Mexico rest on a very complex sequence of igneous and metamorphic rocks. As a whole, the age of this complex is considered to be Precambrian. The southern part of the area is underlain by diabase and related igneous rocks. This area has been named the Swisher gabbroic terrane bv Flawn.(FOOTNOTE 3) The oldest sediment overlying the diabase is Upper Cambrian (Croixan). This relationship is present in the Maguire et al. Ritchie No. 1, Donley County, Texas. In a majority of the wells the oldest sediment known to overlie the diabase is Mississippian (Kinderhookian). The youngest sediment known to rest on the diabase in Texas is Pennsylvanian (Missourian). This relationship was found in the Humble Oil and Refining Compan 's Hyslop No. 1, Deaf Smith County, Texas.

An examination of the section below the top of the diabase or gabbro shows the presence of sediments which may or may not be metamorphosed.

Two wells recently drilled by the Humble Oil and Refining Company in the Plainview basin have added new information to the age concept of the basement. This information has necessitated a re-evaluation of the whole area. In some areas it is apparent that the presence of igneous rocks does not mean that the sedimentary section has been penetrated.

The following wells located on Figure 1 serve to outline the Swisher gabbroic terrane. The constriction between Swisher and Floyd counties may separate the terrane into two parts.

DESCRIPTION OF CORES AND CUTTINGS

BRISCO COUNTY, TEXAS

Humble Oil & Refining Company No. 1 Howard Ranch (Fig. 2). Elev. 3,228 ft. Loc. 1,880 ft. from N. and W. lines of Sec. 155, Blk. G & M, GC&SF Sur.
Age of sediment on igneous rock: Mississippian, Meramecian.

Cuttings
9,050-9,000 Diabase. Some hematite staining on contact of diabase and crystalline limestone.
9,060-9110 Diabase.

Core
9,110.5-9,122 Diabase, limestone, crinoidal; some patches of basaltic rock but no contact metasomatism. Magnetite skarn. Fig. 3.

Cuttings
9,122-9,200 Diabase.
9,200-9,265 Diabase. Some metasomatism.
9,265-9,300 Recrystallized limestone. Cherty, dolomitic limestone and serpentine containing epidote. Fossiliferous limestone containing algal balls and (or) fecal pellets and an agglutinated foraminifir test, all in calcite matrix.
9,300-9,325 Garnet-epidote-calcite skarn. Chips of fossiliferous limestone which consists of algal balls and (or) fecal pellets in fine-grained calcite matrix. Sponge spicules.
9,325-9,345 Basalt. Zoned, badly altered plagioclase. Pyroxene almost completely altered to chlorite.
9,345-9,360 Garnet-calcite-idocrase skarn. Fossiliferous limestone which consists of

FOOTNOTE 3. Flawn, P. T., 1956, "Basement Rocks of Texas and Southeast New Mexico," Univ. Texas Pub. 5605, pp. 43-46.

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Fig. 1. Map of Swisher gabbroic terrane.

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Fig. 1. Continued. See caption on page 1776.

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algal balls and (or) fecal pellets and pelecypod shell in sparry calcite matrix. Chips of fine-grained limestone.
9,360-9,365 Garnet-calcite-idocrase-wollastonite-diopside skarn. Chips of coarsely crystalline limestone marble.
9,365-9,613 Diabase. Sericitized plagioclase laths and chloritized pyroxene, less altered. Biotite, magnetite, apatite, and hornblende (?).

Core
9,613 Diabase. Not altered.

1. All of the igneous rocks encountered in the well are similar in composition and vary only in texture. The plagioclase is zoned, but its composition is predominantly around AN₇₀ (labradorite); the pyroxene is aegirite-augite; and minor magnetite and apatite appear in all samples. The texture is ophitic.

2. The degree of metamorphism and metasomatism of the carbonate rocks increases downward in the well. The uppermost intrusion has not altered the wall carbonate at all. Lower in the well, marbles and serpentinized limestones appear with the magnetite-quartz metasomatite. In the interval of 9,345-9,365 feet, directly above the lowermost diabase, garnet-idocrase-wollastonite-calcite-diopside skarns are found.

3. In most of the samples of cuttings below the uppermost diabase, there are chips of unrecrystallized fine-grained limestone and of sparry fossiliferous limestone with algal balls, fecal pellets, bryozoans, foraminifers, and pelecypods. There are no diagnostic fossils, but the assemblage is post-Cambrian. Paleozoic strata are therefore present below the top of the igneous intrusion (Pl. 1.)

DEAF SMITH COUNTY, TEXAS

Humble Oil & Refining Company No. 1 Reinauer (Fig. 2). Elev. 4,376 ft. Loc. 1,980 ft. from S. and E. lines of Sec. 7, T. 2 N., R. 1 E.
Age of sediment on igneous rock: Pennsylvanian, Missourian.

Cuttings and Cores
7,580-8,072 Quartz latite porphyry and much agglomerate.

Core
8,064 Potassium-argon date 650,000,000 years minimum. Indicated age, young Precambrian.

ROOSEVELT COUNTY, NEW MEXICO

Mid-Continent No. 1 Strickland. Elev. 4,280 ft. Loc. 1,980 ft. from S. and W. lines of Sec. 9, T. 4 S., R. 35 E.
Age of sediment on igneous rock: Permian, Wolfcampian.

Cores
7,508-7,513 Altered diabase.

Cuttings
7,608 Total depth. Altered albitized diabase.

Southern Union No. 1 Lucas. Elev. 4,290 ft. Loc. 660 ft. from S. and 1,980 ft. from W. lines of Sec. 5, T. 2 N., R. 30 E.
Age of sediment on igneous rock: Pennsylvanian, Derryan.

Cuttings
7,140-7,155 Altered diabase.

ARMSTRONG COUNTY, TEXAS

Hassie Hunt Trust No. 1 Helms. Elev. 3,174 ft. Loc. 330 ft. from E. and 1,200 ft. from N. lines of Sec. 2, Blk. 2, H&GN Sur.
Age of sediment on igneous rock: Mississippian, Kinderhookian.

Cuttings
6,070-6,180 Mixture of metasediments and sediments.
6,180-6,280 Calcite antigorite rock, probably dedolomitized dolomite.
6,300-6.570 Trachyte porphyry and rhyolite tuff.

It is possible that much of this material is coarse conglomerate composed of metasediment and volcanic detritus. Samples indicate the presence of sediments associated with sodic igneous rocks.

Hunt No. 4 Ritchie. Elev. 2,691 ft. Loc. 660 ft. from S. and E. lines of Sec. 122, but in Sec. 124, Blk. G-6, Adair & Goodnight Sur.
Age of sediment on igneous rock: Mississippian, Kinderhookian.

Cuttings
6,810-7,070 Diabase, more sodic in last 20 ft.

BAILEY COUNTY, TEXAS

El Paso Natural Gas No. 1 West Texas Mort. & Loan. Elev. 3,945 ft. Loc. 694 ft. from S. and 660 ft. from W. lines of Sec. 55, Blk. A, MB&B Sur.
Age of sediment on igneous rock: Mississippian, Kinderhookian?

Cuttings
8,710-8,720 Altered diabase.
8,720-8,790 Diabase.
8,790-8,800 Serpentinized dolomite.
8,800-8,950 Diabase.
8,960-9,050 Olivine diabase.
9,050-9,090 Calcareous sandy siltstone.
9,090-9,100 Olivine diabase.

Core
9,100-9,104 Diabase.
The sediments included in the diabase may or may not be Precambrian.

Lion Oil Company No. 1 Birdwell. Elev. 3,892 ft. Loc. 660 ft. from N. and E. lines of Sec. 78, Blk. A, MB&B Sur.
Age of sediment on igneous rock: Mississippian, Kinderhookian?

Cuttings
8,750-8,810 Some weakly metamorphosed dolomite and altered diabase.
8,810-8,950 Altered diabase.

Core
8,951-8,953 Olivine diabase.

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Fig. I. Continued.

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Fig. 2. Left, Humble Oil and Refining Company's Howard Ranch No. 1, Briscoe County. Right, Humble's Reinauer No. 1, Deaf Smith County.

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BRISCOE COUNTY TEXAS

Gulf Oil Corporation No. 1 Rodgers. Elev. 3,151 ft. Loc. 440 ft. from N. and 965 ft. from W. lines of Sec. 58, Blk. B-3, BS&F Sur.
Age of sediment on igneous rock: Mississippian, Kinderhookian.

Core
9,275 Garnet serpentinite. Core shows fine bedding or laminations.

Hunt No. 1 Ritchie. Elev. 2,492 ft. Loc. 1,393 ft. from N. and 223 ft. from E. lines of Sec. 68, Blk. A-1900, C. C. Cleveland Sur.
Age of sediment on igneous rock: Mississippian, Kinderhookian.

Cuttings
7,510-7,590 Diabase.
7,590-7,610 Dolomite and dolomitic quartzite. Thermal metamorphism.
7,610-7,650 Dolomite, silicified silty limestone and calcareous sandy siltstone. Brecciated sediment.
7,650-7,710 Sandstone, calcareous sandstone, sandy siltstone. Thermal and metasomatic metamorphism.
7,710-7,720 Ophicalcite (antigorite marble), serpentinized diopside rock. Strong thermal metamorphism followed by hydrothermal alteration.
7,720-7,740 Argillaceous sandstone.
7,740-7,760 Argillaceous sandstone. Thermal metamorphism.
7,760-7,770 Dolomite, ophicalcite (antigorite marble), steatite (soapstone). Pencatite (brucite marble). Thermal metamorphism of very siliceous dolomite.
7,770-7,790 Sandy siltstone. Thermal metamorphism.
7,790-7,890 Augite diorite. Hydrothermal metamorphism.
The diabase in this well appears to be of same age as that in Humble Oil & Refining Company's No. 1 Howard Ranch.

Hunt No. 2 Ritchie. Elev. 2,597 ft. Loc. 1,125 ft. from S. and 825 ft. from W. lines of Sec. 34, Blk. G-5, EL&RR Sur.
Age of sediment on igneous rock: Mississippian, Kinderhookian.

Cuttings
7,130-7,560 Olivine diabase.
7,630-7,640 Olivine diabase, some magmatic alteration.
7,640-7,660 Silty shale.
7,660-7,700 Dolomite, weak thermal alteration.
7,700-7,720 Hyalodacite, probably keratophyre.
7,720-7,750 Dacite or keratophyre tuff.

Hunt No. 10 Ritchie. Elev. 2,411 ft. Loc. 660 ft. from

Click to view image in GIF format. Fig. 3. [Grey Scale] Limestone fragments floating in skarn. No alteration at contact. Depth of core 9,120 feet, no magnification. Scale in centimeters (Humble Oil & Refining Company's Howard Ranch No. 1, Briscoe County, Texas).

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Click to view image in GIF format. PL. 1. [Grey Scale] Photomicrographs. All×70. A. Bryozoa. B. Algae. C. Foram. D. Foram. E. Algae. (Humble Oil & Refining Company's Howard Ranch No. 1, Briscoe County, Texas). Depth of core 9,116 feet.

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S. and W.lines of SE. corner of Sec. 70, but in Sec. 74, Blk. G-6, Adair and Goodnight Sur.
Age of sediment on igneous rock: Mississippian, Kinderhookian.

Cuttings
7,920-8,165 Olivine diabase cut by lamprophyre dike.

Midstates No. 1 Hickok and Reynolds. Elev. 2,355 ft. Loc. 660 ft. from N. and W. lines of Sec. 39, S. F. 2725, Craig Sur.
Age of sediment on igneous rock: Mississippian, Kinderhookian?

Cuttings
8,120-8,150 Diabase.

Standard of Texas No. 1 Owen. Elev. 3,295 ft. Loc. 2,571 ft. from S. and 735 ft. from W. lines of Sec. 142, Blk. M-10, D&SE Sur.
Age of sediment on igneous rock: Mississippian, Kinderhookian.

Cuttings
8,380-8,390 Altered diabase.

CASTRO COUNTY, TEXAS

Anderson-Prichard No. 1 Fowler-McDaniel. Elev. 3,667 ft. Loc. 660 ft. from S. and W. lines of Sec 12, Blk. 9-T, T&NO Sur.
Age of sediment on igneous rock: Mississippian, Kinderhookian.

Cuttings
9,650 Olivine diabase. Some alteration but no weathering on contact.

Sun Oil Company No. 1 Haberer. Elev. 3,865 ft. Loc 660 ft. from N. and E. lines of Sec. 8, Blk. 2, W. E. Halsell Sur.
Age of sediment on igneous rock: Mississippian, Kinderhookian.

Cuttings
8,860 Olivine diabase.
8,930 Magnesite.
8,950 Diabase. Extensive hydration.
8,980 Biotite diabase.
9,030 Contact biotite diabase. Extensive hydration in part deuteric.

Core
9,124-9,125 Sandy siltstone. Mild contact alteration.
This well drilled through diabase intrusion.

Sun Oil Company No. 1 Herring. Elev. 3,801 ft. Loc. 660 ft. from S. and 1,980 ft. from E. lines of Sec. 46, Blk. T-4, T. A. Thompson Sur.
Age of sediment on igneous rock: Mississippian, Meramecian.

Cuttings
9,370-9,790 Mostly arkose and sandstone showing vague metamorphism. Andesite or rhyolite weathered.

Core
9,804-9,810 Dark gray bedded pyroclastic and salmon-colored arkose. Some well rounded quartz grains.

Cuttings
9,790-10,065 Diabase, some alteration.
10,065-10,114 Hornfels and siltstone.

Core
10,114-10,128 Magnesite in serpentinite. Also antigorite cut by veinlets of chrysotile. Evidence of dynamic metamorphism. Rock probably was peridotite which has been hydrated.
10,128-10,133 Altered ferromagnesian-rich rock.

Cuttings
10,135-10,500 Olivine diabase.

Core
10,447-10,459 Olivine diabase. Little alteration.

Cuttings
10,460-10,500 Olivine diabase.

DEAF SMITH COUNTY, TEXAS

Honolulu Oil Company No. 1 Ponder. Elev. 3,991 ft. Loc. 8,700 ft. W. and 300 ft. S. of NW. corner of Sec. 73, Blk. K-8, TTRR Sur.
Age of sediment on igneous rock: Mississippian, Kinderhookian?

Cuttings
9,580- 9,610 Rhyolitic agglomerate.
9,610- 9,640 Magnesite and soapstone.
9,640- 9,670 Magnesite with maroon and green soapstone.
9,670- 9,700 Weathered rhyolite.
9,810- 9,830 Weathered rhyolite and dark green soapstone.
9,830- 9,850 Weathered rhyolite.
9,850- 9,980 Dense rhyolite.
9,980-10,191 Crystalline porphyritic rhyolite.

Humble Oil & Refining Company No. 1 Hyslop. Elev. 4,448 ft. Loc. 1,980 ft. from S. and E. lines of Sec. 18, T. 3 N., R. 1 E., League 418, State Capital Lands Sur.
Age of sediment on igneous rock: Pennsylvanian, Missourian.

Cuttings
7,710-7,730 Porphyritic rhyolite.

Core
7,730 Baked dark maroon shale.

Cuttings
7,750-7,800 Diabase, slight alteration.

Core
7,803-7,805 Porphyritic rhyolite.

LaMance Drlg. Co. Ltd. No. 1 Western Realty. Elev. 3,888 ft. Loc. 660 ft. from N. and W. lines of Labor 12, Gregg County School Lands Sur.
Age of sediment on igneous rock: Mississippian, Meramecian.

Cuttings
9,590-9,610 Olivine diabase. No weathering on contact.

DONLEY COUNTY, TEXAS

Maguire, Russell & Sunray Mid-Cont. No. 1 Ritchie. Elev. 2,938 ft. Loc. 4,580 ft. W. along N. line and 2,019 ft. S. of NE. corner of Sec. 6, Bik. C-5, TTRR Sur. T. D. 6,797 ft.
Age of sediment on igneous rock: Upper Cambrian (Croixan).

Cuttings
6,710-6,740 Weathered diabase.
6,740-6,798 Unaltered diabase. Some pink feldspar veins.

Placid Oil Company No. 1 Kelly. Elev. 2,520 ft. Loc. 660 ft. from S. and W. lines of Sec. 47, Blk. G-7, Adair and Goodnight Sur.
Age of sediment on igneous rock: Mississippian, Kinderhookian.

Cuttings
6,960-7,050 Coarse diabase, unaltered.

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7,053-7,068 Diabase. Alteration late-magmatic in part--weathering in part. Indurated feldspathic sandstone. Metasandstone, alteration by contact metasomatism. Serpentine. Massive antigorite with some calcite and minor quartz. This assemblage of rock types represents intrusive contact between diabase and indurated sediments. Sediments appear to have been altered by heat and hydrothermal agents due to diabase.

FLOYD COUNTY, TEXAS

Houston Oil Company No. 1 Lackey. Elev. 3,174 ft. Loc. 930 ft. from E. and 660 ft. from S. lines of Sec. 11, Blk. D-2, GC&SF Sur.
Age of sediment on igneous rock: Mississippian, Kinderhookian.

Cuttings
10,390-10,395 Diabase, unaltered.

HALE COUNTY, TEXAS

Amerada No. 1 Kurfees. Elev. 3,308 ft. Loc. 660 ft. from N. and W. lines of Sec. 6, Blk. N, H&OB Sur.
Age of sediment on igneous rock: Mississippian, Kinderhookian.

Core
10,245-10,250 Olivene diabase, altered.

Honolulu and Sinclair No. 1 Clements. Elev. 3,349 ft. Loc. 1,980 ft. from S. and 660 ft. from W. lines of Sec. 19, Blk. D-7, EL&RR Sur.
Age of sediment on igneous rock: Mississippian, Kinderhookian.

Cuttings
10,060-10,070 Olivine diabase.
10,140-10,150 Olivine diabase. Some deuteric alteration.

Standard of Texas No. 1 Keliehor. Elev. 3,280 ft. Loc. 1,980 ft. from N. and 10,000 ft. from W. lines of League 3, Lot 97, Callahan County School Lands Sur.
Age of sediment on igneous rock: Mississippian, Kinderhookian.

Cuttings
10,080-10,090 Quartz-diorite and granophyre.
10,140-10,170 Hornblende biotite quartz-diorite. Malchite (diorite lamprophyre). Quartz epidote rock.
10,220-10,300 Malchite (diorite lamprophyre). Dike in quartz-diorite.
10,300-10,400 Hornblende biotite quartz-diorite with aplite dike.
10,400-10,500 Hornblende biotite granodiorite.
10,500-10,600 Hornblende biotite quartz-diorite with aplite dike.
10,610-10,780 Hornblende biotite granodiorite.

PARMER COUNTY, TEXAS

Gulf Oil Corp. No. 1-A Keliehor. Elev. 3,989 ft. Loc. 1,980 ft. from N. and 660 ft. from W. lines of Sec. 5, League 2, Gregg County School Lands Sur.
Age of sediment on igneous rock: Mississippian, Kinderhookian.

Cuttings
9,560-9,570 Diabase, unaltered.

Core
9,578-9,584 Olivine diabase.
9,627-9,628 Olivine diabase, unaltered.

SWISHER COUNTY, TEXAS

Humble Oil & Refining Company No. 1 Nanny. Elev. 3,602 ft. Loc. 660 ft. from E. and 1,980 ft. from N. lines of Sec. 9, Blk. OD, S. B. Dinwiddie Sur.
Age of sediment on igneous rock: Mississippian, Kinderhookian.

Cuttings
9,590-9,600 Albite microdiorite, some alteration.
9,630-9,640 Diorite, altered.
9,658 Diabase, altered. Low-grade metamorphism.

Standard of Texas No. 1 Johnson. Elev. 3,386 ft. Loc. 660 ft. from S. and E. lines of S. ½ of Sec. 116, Blk. M-10, BS&F Sur.
Age of sediment on igneous rock: Mississippian, Kinderhookian.

Cuttings
9,150-9,200 Olivine diabase, some serpentine probably late magmatic; otherwise, rock is fresh.

Core
9,207-9,214 Olivine diabase cut by veins of antigorite.

CONCLUSIONS

From the evidence at hand it is apparent that the age of a part of the Swisher gabbroic terrane is younger than Precambrian. Furthermore, some of the rhyolites in the area may also be younger than Precambrian. The evidence further suggests that some, if not all, of the coarse diabase occurs as a very shallow sheet-like intrusion, some of which has risen into Paleozoic sediments, a conclusion suggested in part by Flawn (1956, p. 46).

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