San Andres Carbonates in the Texas Panhandle:

RI0121. San Andres Carbonates in the Texas Panhandle: Sedimentation and Diagenesis Associated with Magnesium-Calcium-Chloride Brines, by Amos Bein and L. S. Land. 48 p., 27 figs., 9 tables, 2 appendices, 1982.


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ABSTRACT
The San Andres evaporitic sequence in the Palo Duro Basin comprises several thick carbonate units in its lower part and many thin units in its upperpart. To the south, across the Northern Shelf of the Midland Basin, evaporites pinch out and carbonates predominate. Six lithofacies were differentiated in the Palo Duro and Northern Shelf carbonates: dolomudstone, pellet-oolite packstone-grainstone, filamentous (Girvanella-like) grainstone, sponge spicule packstone, wispy-laminated crinoid packstone, and skeletal packstone-grainstone. Facies distribution was controlled by water-body salinity, which increased from south to north. Within the Palo Duro Basin, the carbonates in the upper part of the sequence differ from those in the lower part in that the former lack skeletal lithofacies and have higher manganese, iron, and terrestrial organic matter content. Bromide (Br) content in halite in the lower part of the sequence is consistently high, whereas halite in the upper part is mostly depleted in bromide. Strontium (Sr) in dolomite, calcite and anhydrite, δ¹⁸O, δ^I3C values, and early diagenetic oxidizing conditions deduced from high pristane/phytane ratios are about the same throughout the entire San Andres Formation in the Palo Duro Basin. Depleted δ^I3 values in dolomites associated with low pristine/phytane ratios in the Northern Shelf formed under more reducing conditions in which organically derived carbon in the carbonates increased because of sulfate-reducing bacterial activity. Sodium/chloride and potassium/chloride ratios attributed to liquid inclusions in almost all carbonates are characteristic of marine brines evaporated beyond the level of halite saturation. Sodium content in the dolomite lattice is generally low and increases from north to south at the same stratigraphic levels.

Varied sedimentologic and geochemical properties of the rocks throughout the area reflect different primary depositional regimes. Properties that do not vary are attributed to diagenetic modification of the rocks in contact with brines having similar compositions. The lower part of the formation was deposited in a broad shelf basin or lagoon sufficiently deep to maintain long periods of steady-state circulation. During these periods neither halite dissolution nor potash-magnesia mineral precipitation occurred. The upper part of the formation was deposited in smaller water bodies sensitive to inflow fluctuations. Increased proportion of meteoric water in the depositional environment during this period is evidenced by high content of manganese, iron, and terrestrial organic matter, and the meteoric water was a source of dissolved carbonate for the deposition of many of the thin carbonate units.

Diagenesis of the San Andres carbonates occurred in contact with saline magnesium-calcium-chloride brines, which evolved from seawater by anhydrite and halite precipitation. Skeletal mold formation and subsequent anhydrite cementation, dolomitization, and high-strontium calcite cementation associated with celestite precipitation are all cogenetic processes controlled by this brine-rock interaction. The δ¹⁸O composition of dolomite, calcite, and chert indicates apparent equilibrium relations with the same solution. Possible low temperatures of 40° to 450° C (105° to 110° F) imply δ¹⁸O of such a solution to be about 2 to 3‰.The somewhat light δ¹⁸O composition of the proposed halite-saturated brine may have resulted from the reversal in the positive correlation between δ¹⁸O and increased evaporation in highly saline brines. San Andres carbonates in the Palo Duro Basin that were diagenetically altered in a halite-saturated magnesium-calcium chloride brine were plugged by precipitating salt and remain unchanged and isolated in a closed sedimentary basin. The Northern Shelf carbonates were modified by similar brines intermittently undersaturated with respect to halite because of mixing with seawater. As a result, some original porosity remained, and pressure solution occurred in the more deeply buried and more skeletal-rich sequence.

Keywords: anhydrite, carbonate lithofacies, geochemistry, lithofacies, Lamb County, Swisher County, Randall County, Panhandle, Texas, dolomudstones, Texas, Palo Duro Basin, San Andres Formation, Northern Shelf

CONTENTS

ABSTRACT

INTRODUCTION

GEOLOGIC SETTING

METHODS OF STUDY

Technique

Interpretation

CARBONATE LITHOFACIES

Dolomudstones

Pellet-oolite packstones and grainstones

Sponge spicule packstones

Filamentous (Girvanella-like) grainstones and boundstones

Wispy-laminated crinoid packstones

Skeletal packstones and grainstones

ANHYDRITE-DOLOMITE ASSOCIATION AND SILICIFICATION

Dolomite-anhydrite intergrowth

Anhydrite nodules

Replacement anhydrite

Blocky anhydrite cement

Silicification of anhydrite

GEOCHEMICAL DATA

Organic matter associated with carbonates

Trace elements in associated evaporites

Bromide in halite

Strontium in anhydrite

Sodium and chloride in carbonates

Potassium in carbonates

Strontium in carbonates

Iron and manganese in carbonates

Other trace elements in carbonates

Stoichiometry and order of dolomite crystals

Stable isotopes in carbonates and cherts

DISCUSSION

CONCLUSIONS

ACKNOWLEDGMENTS

REFERENCES

APPENDICES

Figures

1. Location and isopach map of the San Andres Formation in the study area

2. North-south cross section through the study area

3. Lithofacies distribution of SR2 rock units

4. Lithofacies distribution of LSR3 rock units

5. Lithofacies distribution of LSR4 rock units

6. Lithofacies distribution of YSR56 rock units

7. Photomicrographs of dolomudstones

8. Photomicrographs of oolitic and pelletoidal rocks

9. Photomicrographs of oolites and sponge spicule packstones

10. Photomicrographs of Girvanella-like grainstones and boundstones

11. Photomicrographs of skeletal packstones

12. Photomicrographs of replacement anhydrite fabrics

13. Gas chromatogram of saturated hydrocarbons in Swisher County core

14. Gas chromatogram of saturated hydrocarbons in S4 and L4 rock units

15. Relation between pristane/ phytane ratio and total organic carbon (TOC) in Swisher County and Lamb County samples

16. Relation between sodium/ chloride and chloride in YSR56 samples

17. Relation between sodium/ chloride and chloride in LSR4 samples

18. Relation between sodium/ chloride and chloride in SR23 samples

19. Relation between sodium/chloride and chloride mean values of each unit

20. Relation between potassium and aluminum in all San Andres rocks

21. Frequency histogram of strontium content in dolomite samples

22. Relation between aluminum and manganese in rock samples

23. Relation between aluminum and iron in rock samples

24. Relation between manganese and iron intercept value for zero aluminum

25. Hexagonal unit cell parameters a_o and c_o calculated from X-ray diffraction

26. Relation between δ¹⁸O and δ¹³C  in dolomites and calcites

27. Relation between δ¹⁸O and manganese in stratigraphically related samples

Tables

1. Rock unit symbols used in this study

2. Pristane/phytane ratio and total organic carbon in San Andres carbonates

3. Bromide content in halite and strontium and manganese content in anhydrite nodules and massive anhydrite beds
4. Carbonate constituents, trace elements, and stable isotopes in San Andres carbonates

5. Chemical composition of water used to leach ground dolomite samples

6.  δ¹⁸O values in quartz separated from partially silicified anhydrite nodules and sponge spicule packstones

7. δ¹⁸O equilibrium relation between water and carbonates as a function of temperature

8. δ¹⁸O equilibrium relation between water and chert as a function of temperature

9. δ¹⁸O and δ¹³C values in the Seven Rivers Formation, Guadalupe Mountains



Citation
Bein, Amos, and Land, L. S., 1982, San Andres Carbonates in the Texas Panhandle: Sedimentation and Diagenesis Associated with Magnesium-Calcium-Chloride Brines: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 121, 48 p.