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Geology and Tertiary Igneous Activity of the Hen Egg Mountain and Christmas Mountains Quadrangles...Trans-Pecos Texas


Geology and Tertiary Igneous Activity of the Hen Egg Mountain and Christmas Mountains Quadrangles, Big Bend Region, Trans-Pecos Texas, by C. D. Henry, J. G. Price, and D. E. Miser. 105 p., 35 figs., 6 tables, 1 appendix, 1989. Print Version.

For a downloadable, digital version: RI0183D.

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RI0183. Geology and Tertiary Igneous Activity of the Hen Egg Mountain and Christmas Mountains Quadrangles, Big Bend Region, Trans-Pecos Texas, by C. D. Henry, J. G. Price, and D. E. Miser. 105 p., 35 figs., 6 tables, 1 appendix, 1989.

To purchase this publication as a downloadable PDF, please order RI0183D.


Cretaceous sedimentary rocks and Tertiary intrusive, extrusive, and volcaniclastic rocks dominate the geology of the Hen Egg Mountain and Christmas Mountains quadrangles in the Big Bend region of Texas. The Cretaceous rocks consist of limestones, shales, mudstones, and sandstones that are host rocks for the Tertiary intrusions.

This report focuses on the Tertiary intrusive and extrusive rocks. The intrusions were emplaced in several distinct intervals between 47 and 20 Ma ago (Eocene to Miocene) but are dominantly 44 to 40 Ma old. All intrusions are small (less than a few kilometers in diameter) and were shallowly emplaced as laccoliths, sills, and dikes. The Eocene intrusions fall into two petrographically and chemically distinct suites: (1) a silica-undersaturated series from nepheline-normative hawaiite to syenite and (2) a silica-oversaturated series from trachyandesite to high-silica, peralkaline rhyolite. The silica-undersaturated rocks dominantly form sills or dikes and, more rarely, laccoliths having little structural relief. The silicic rocks form laccoliths having considerable structural relief, including trap-door laccoliths, and, less commonly, sills or dikes. The undersaturated rocks occur primarily in the central part of the Hen Egg Mountain quadrangle; the oversaturated intrusions occur throughout both quadrangles.

A single nepheline-normative hawaiite sill was emplaced 28 Ma ago (late Oligocene) in the southeastern corner of the Christmas Mountains quadrangle. Abundant north-northwest-trending dikes of strongly nepheline-normative hawaiites and basalts were emplaced at 24 to 20 Ma (early Miocene), contemporaneous with initial Basin and Range faulting. These latter rocks commonly host mantle xenoliths.

Silicic volcanism accompanying late Eocene intrusion occurred in two areas. A caldera complex in the Christmas Mountains represents an unusual eruptive laccolith, a caldera type not previously recognized. These calderas developed on the Christmas Mountains dome, a large (8 x 5 km), elliptical laccolithic uplift that resulted from emplacement of the caldera magma body. The complex produced an extensive series of rhyolitic to quartz trachytic airfall and ash-flow tuffs, silicic lavas and domes, and coarse debris-avalanche deposits; these rocks are divided into five stratigraphic sequences. At least four caldera collapses accompanied eruption, and the fill in the two youngest calderas dips steeply inward, indicating post-eruption subsidence. More minor volcanism consisted of massive air-fall or ash-flow tuff associated with high-silica, peralkaline rhyolite domes in the northwest corner of the Hen Egg Mountain quadrangle.

The major structural features of the quadrangles are domes produced by intrusive emplacement. Laramide folding either did not occur or was a minor influence. Three north-northwest-trending Basin and Range fault zones transect the quadrangles. Displacement is mostly down to the west and less than 1 km.

The only significant mineral production in the area was of fluorite formed at the contacts between alkalic, silicic intrusions and Cretaceous carbonate host rocks. The presence of numerous prospects, the unusual trace-element enrichment in several of the fluorite deposits, and the overall geologic setting and types of igneous rocks suggest significant potential for deposits of rare metals.

alkalic rocks, ash-flow tuffs, calderas, K-Ar, laccoliths, ore deposits, Tertiary, Trans-Pecos Texas





Previous Work

Regional Geologic Setting


Precambrian and Paleozoic Rocks

Cretaceous Rocks

Comanchean Series

Del Carmen Limestone

Sue Peaks Formation

Santa Elena Limestone

Del Rio Clay

Buda Limestone

Gulfian Series

Boquillas Formation

Pen Formation

Boquillas and Pen Formations undifferentiated

Aguja Formation

Javelina Formation

Tertiary Volcanic and Sedimentary Rocks

Chisos Formation

Alamo Creek Basalt Member

Undifferentiated, dominantly reworked tuffs

Devil's Graveyard Formation

Basal conglomerate

Basalt lava

Tuffaceous sediments

Undifferentiated Devil's Graveyard Formation

Tuff at Hen Egg Mountain

Quaternary Deposits


Terrace deposits

Alluvial-fan deposits

Colluvium and landslide blocks

Intrusive Rocks


Middle Eocene

Upper Middle Eocene.

Silica-undersaturated series

Hawaiitic sills and dikes

Christmas Mountains gabbro

Silica-oversaturated series


High-silica peralkaline rhyolite

Low-silica peralkaline rhyolite

Flow-banded rhyolite

Quartz trachyte

Sparsely porphyritic, peralkaline quartz trachyte

Abundantly porphyritic, peralkaline quartz trachyte

Metaluminous quartz trachyte

Altered quartz trachyte


Dikes and small sills

Upper Oligocene

Lower Miocene

Igneous Petrology and Geochemistry

Silica Saturation and Chemical Classification of Igneous Rocks

Silica-undersaturated Rocks

Silica-oversaturated Rocks

Rocks of Contrasting Tectonic Settings.

Alkalic Rocks

Future Studies

The Christmas Mountains Caldera Complex

Stratigraphy of Caldera-Related Rocks

Sequence I: initial tuffs and lavas

Initial tuffs

Lower quartz trachyte lavas

Debris flows and tuffs

Ash-flow tuff at Little Christmas Mountain.

Upper quartz trachyte lavas

Undifferentiated sequence I

Related rocks in Big Bend National Park

Sequence II: debris-flow deposits, rheomorphic tuff, and lava dome

Lower debris-flow deposits

Rheomorphic tuff

Middle breccia

Quartz trachyte porphyry

Upper debris flows

Sequence III: massive debris-avalanche deposits

Sequence IV: caldera-fill tuff and breccia.

Western caldera

First tuff

Limestone collapse breccia

First breccia

Second tuff

Second breccia

Third breccia

Third tuff

Fourth breccia

Eastern caldera

First tuff

First breccia

Second tuff

Second breccia

Third tuff

Sequence V: quartz trachyte porphyry intrusions

Structure of the Christmas Mountains Dome and Calderas

The Christmas Mountains dome

First and second calderas

Main western caldera

Main eastern caldera

Late subsidence

Evolution of the Christmas Mountains Caldera Complex

Economic Geology

Fluorspar, Beryllium,and Molybdenum



Other Mineral Commodities

Water Resources


Laramide Structures

Structures Related to Igneous Activity

Metaluminous quartz trachyte laccolith

Paisano Peak.

Adobe Walls dome

Wildhorse Mountain intrusion

Paynes Waterhole anticline

"Pop-up" intrusions

Hen Egg Mountain intrusions

East-northeast-trending faults

Basin and Range Faults

Terlingua Creek graben.

Wildhorse - Hen Egg fault zone

Christmas Mountains fault zone

Implications of Dike Trends for Paleostress Orientations

Geologic History



Appendix: Chemical Analyses of Igneous Rocks, Christmas Mountains and Hen Egg Mountain Quadrangles



1. Location of the Hen Egg Mountain and Christmas Mountains quadrangles and other selected features of Trans-Pecos Texas and northern Mexico

2. Pre-mid-Tertiary tectonic elements of Trans-Pecos Texas and adjacent Mexico

3. Approximate distribution of Tertiary volcanic rocks of Trans-Pecos Texas and adjacent regions of Mexico, New Mexico, and Arizona

4. Published geologic maps that depict areas in and adjacent to the Hen Egg Mountain and Christmas Mountains quadrangles

5. Stratigraphy and thickness of Cretaceous rocks in the Christmas Mountains area

6. Photographs of Cretaceous rocks

7. Stratigraphy of volcanic and volcaniclastic rocks in and adjacent to the Hen Egg Mountain and Christmas Mountains quadrangles

8. Photographs of Tertiary rocks

9. Typical formation of Quaternary colluvium

10. Distribution of major mapped intrusive rock types

11. Photomicrographs of intrusive rocks

12. Flow-banded rhyolite in sill 1 km northwest of Adobe Walls dome, Hen Egg Mountain quadrangle

13. View to south of dikes of lower Miocene hawaiite in southwest corner of Hen Egg Mountain quadrangle

14. Igneous rocks in the Hen Egg Mountain and Christmas Mountains quadrangles in the chemical classification of alkalic rocks devised by Barker (1979)

15. Histogram of silica content of all samples listed in the appendix

16. Silicic igneous rocks of the Hen Egg Mountain and Christmas Mountains quadrangles

17. Plot of alkalinity, molar (Na + K)/A1, versus differentiation index

18. AFM diagram for nepheline-normative rocks 19. Plots of MgO and TiO, versus differentiation index

20. Alkali-silica and lime--s ilica plots of rocks from the Hen Egg Mountain and Christmas Mountains quadrangles

21. Index map of calderas in the Christmas Mountains caldera complex

22. Stratigraphy of initial tuffs and lavas west and south of the Christmas Mountains

23. General distribution of volcanic rocks of sequence I in the northwestern part of the Tule Mountain quadrangle.

24. Photographs of rocks of sequences I, II, and III

25. Stratigraphy of sequence II

26. Stratigraphy and thicknesses of caldera fill in the main western and eastern calderas

27. Photographs of caldera-fill rocks of sequence IV

28. Photographs of quartz trachyte intrusions of sequence V

29. Photographs illustrating structure of Christmas Mountains calderas

30. Relative enrichment of various trace elements in fluorspar associated with igneous rocks of Trans-Pecos Texas and adjacent New Mexico

31. Photographs of intrusive structures

32. Photographs of Basin and Range fault structures

33. Generalized distribution and orientation of dikes in the Christmas Mountains area

34. Histogram of K-Ar ages of igneous rocks of the Christmas Mountains area

35. Diagram showing evolution of the Christmas Mountains area



1. Magmatic-tectonic phases in Trans-Pecos Texas

2. K-Ar isotopic data,Christmas Mountains rocks

3. General characteristics of intrusive rock types

4. General stratigraphy of the Christmas Mountains caldera complex

5. Chemical analyses of Christmas Mountains area fluorspar

6. Chemical analyses of samples from mercury prospects on the south side of the Christmas Mountains


Plate (in inside back pocket of book)

Geologic map of the Hen Egg Mountain and Christmas Mountains quadrangles

Henry, C. D., Price, J. G., and Miser, D. E., 1989, Geology and Tertiary Igneous Activity of the Hen Egg Mountain and Christmas Mountains Quadrangles, Big Bend Region, Trans-Pecos Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 183, 105 p.

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