Water-Level Controls on Halite Sedimentation: Permian Cyclic Evaporites of the Palo Duro Basin

RI0214. Water-Level Controls on Halite Sedimentation: Permian Cyclic Evaporites of the Palo Duro Basin, by S. D. Hovorka. 51 p., 29 figs., 1 table, 2 black-and-white plates, 1994. Print. To purchase this publication as a downloadable PDF, please order RI0214D.

ABSTRACT
Extensively cored cyclic evaporites of the San Andres Formation (Guadalupian) of the Palo Duro Basin, Texas Panhandle, provided fundamental information for interpreting evaporite depositional processes. These evaporites preserve exceptionally sensitive indicators of water-level fluctuation in their depositional environment: (1) sedimentary fabrics that formed during evaporite deposition; (2) sedimentary features, surfaces, and insoluble residues that formed during evaporite dissolution; and (3) diagenetic overprints that formed during evaporite brine evolution. Lateral and vertical facies relationships among evaporite facies defined on the basis of sedimentary fabrics were examined to document the evolution of the evaporite basin and the sedimentological response to water-level fluctuations.

San Andres evaporite sediments that were deposited in a variety of brine pool environments preserve evidence of water depth and brine pool character. Chevron structures defined by fluid inclusions trapped during crystal growth in halite indicate that the halite formed in very shallow, agitated and oxygenated water. Laminated, darker halite was interpreted to form in slightly deeper or more density-stratified brine pools by a combination of cumulate (surface-formed rafts) sedimentation and bottom growth. Facies relationships and fabrics indicate that bottom-grown fabrics in gypsum can form in slightly deeper water than bottom-grown fabrics in halite because density stratification in gypsum-saturated water is less.

This study documents using evaporite-dissolution features as paleoenvironmental indicators. Evaporite dissolution occurred (1) when water level rose and the evaporite basin became flooded by undersaturated marine water and (2) during emergence when water level fell and evaporites became exposed to undersaturated meteoric waters. The distinctive sedimentary fabrics and facies relationships created by both of these processes define the history of water-level fluctuation in the basin. High-frequency water-level fluctuation produced interbedded meter-thick zones of anhydritic halite (brine pool) and muddy halite (frequently exposed). Siliciclastic silt and clay concentration and structures that formed by karstic dissolution are recognized as evidence of exposure. Lower order cyclicity (parasequence scale) formed upward-shallowing cycles 2 to 30 m thick within the San Andres Formation. Insoluble residues that formed during the transgression that initiated cycles have a distinctive wavy "upside-down-accreted" fabric that may aid in identifying residues in other settings. Residue thickness was found to be inversely proportional to the amount of preserved halite in the underlying cycle. The relative timing of dissolution is documented by the facies and geometry of overlying sediments and by diagenetic phases incorporated in the residue.

Diagenetic overprints obscure primary textures in San Andres evaporites, but because the diagenesis is dominantly synsedimentary, it also provides information about the depositional environment. When water level fell, paleowater tables within the porous evaporites formed a vadose zone of halite dissolution and a phreatic zone of halite precipitation. The amount of brine pool fabric preserved is therefore a measure of water-level stability, in which the most altered fabrics are found to correspond to the highest frequency of fluctuation in updip or ephemeral environments.

The water-depth criteria developed by this study show that San Andres halite has preserved much depositional information and that cycle patterns in halite can be analyzed using the same techniques as are commonly used on carbonates. The techniques used in this study may be of use in other evaporite basins where water depth and basin evolution are unknown.

Keywords: cyclicity, depositional environment, evaporites, karst, salt

CONTENTS
Abstract

Introduction

     Geologic Setting and Basin Evolution

     Methods

     Cycle Patterns in the Palo Duro Basin

Description of Halite Rocks

     Sedimentary Structures, Fabrics, and Interbeds in Halite Rocks

         Cumulate Halite

         Bottom-Nucleated Halite and Chevron Structures

               Experimental Brine Pans

         Dissolution Surfaces

         Microkarst Pits and Pipes

         Mudstone Interbeds in Halite

         Chaotic Mudstone-Halite

           Diagenetic Alteration of Halite

     Classification of Halite

Depositional Facies Geometry in Halite Rocks

       Definition of Facies in Halite Rocks

           Anhydritic Halite Facies

           Muddy Halite Facies

       Detailed Halite Facies Cross Sections

           Case Studies of Facies Relationships in Halite

                     Laterally Traceable Units--San Andres Unit 4, Zones 21 and 23

                   Geometry of Muddy Halite Facies--San Andres Unit 4, Zone 19

           Geometry of Anhydrite Beds -

                   Thin Anhydrite Beds in Halite--San Andres Unit 4

                   Thin Anhydrite Beds in Halite--San Andres Unit 5

             Interfingering Anhydrite-Halite Relationships

             Dissolution during Transgression

                   Petrography of Base-of- cycle Dark. Anhydritic Mudstones

                   Small-Scale Example of Halite-Residue-Anhydrite Geometry, San Andres Unit 4,
                             Zones 24 and 25

                   Comparing Residue Thickness and Composition with Preserved Halite

                   Dissolution at the Top of San Andres Unit 4 and Formation of Residue at the Base of Unit 5

       Origins of Cyclicity in the Guadalupian Series of the Palo Duro Basin

Discussion--A Depositional Model of Thick Sequences of Bedded Halite Rocks in the San Andres Formation

     Brine Pool Geometry

         Lateral Extent

         Depth

       Brine Pool Instability

Mudstone Deposition and Exposure

         Microkarst

Causes of Restriction of Circulation

Summary and Conclusions

Acknowledgments

References

Figures

1. Geologic setting of the Permian Basin

2. Cross section A-A' of the Permian fill of the Palo Duro Basin

3. Stratigraphic nomenclature of the Permian strata of the Palo Duro Basin

4. Generalized Palo Duro Basin cycle composed of dark mudstone, carbonate, anhydrite, and halite

5. Summary of halite fabrics in the San Andres Formation, which compares fabrics seen in the San Andres with fabrics produced by modern or experimental halite precipitation

6. Photographs of color-banded cumulate halite

7. Photographs of chevron fabrics, defined by growth zones of halite rich in fluid inclusions alternating with clear halite

8. Photographs of truncation surfaces

9. Photographs of microkarst pits
10. Photographs of mudstone interbeds

11. Photographs of displacive halite in mudstone and chaotic mudstone-halite

12. Distribution of facies in San Andres unit 4, north-central Palo Duro Basin

13. Northwest-east cross section showing distribution of facies in five genetic cycles in San Andres unit 5

14. Northwest-southeast cross section showing distribution of facies in five genetic cycles in San Andres unit 5

15. Northwest-southeast cross section showing facies relationships and detailed correlation of thick and laterally persistent anhydritic halite

16. West-east cross section showing correlation of San Andres unit 4, zone 21, in Swisher County, demonstrating typical lateral variability of anhydritic halite

17. West-east cross section of San Andres unit 4, zone 19, showing typical variation in thickness and character of mudstone and muddy halite

18. Net-mudstone maps of San Andres unit 4: zone 1, zone 4, zone 6, zone 12, zone 22, and zone 24

19. South-north cross section of San Andres unit 4 anhydrite bed, zone 14, showing northward thinning into equivalent anhydritic chevron (Class A) and color-banded (Class B) halite

20. Map view of thickness of anhydrite beds in unit 4: zone 14, zone 16, and zone 26

21. West-east cross section of anhydrite bed, zone 15, San Andres unit 5, cycle 5c

22. Areal distribution of San Andres unit 5 anhydrite beds: cycle 5c, zone 15, cycle 5d, zone 17, and cycle 5d, zone 19

23. Photographs of dark, anhydritic mudstones at the base of San Andres cycles, strongly deformed and originating as insoluble materials derived from dissolution of halite rock

24. West-east cross section of zones 24 through 26 of San Andres unit 4 showing discontinuity of insoluble residue at the base of San Andres unit 4, zone 26

25. West-east cross section of zones 14 through 17 of San Andres unit 4 showing insoluble residue at the base of San Andres unit 4, zone 16

26. Distribution and thickness of anhydrite facies at the base of San Andres unit 5 and relationship to insoluble residue at the base of San Andres unit 5 and halite thinning at the top of unit 4

27. West-east cross section of facies and insoluble residue at the base of San Andres unit 5 and facies at the top of unit 4

28. Fischer plot of cycle thicknesses measured in DOE-Gruy Federal No. 1 Grabbe core
29. San Andres depositional environments, emphasizing scale and geometry of facies tracts relative to scale of study area, where brine pool is flooded, and gypsum and halite are deposited over wide areas; and where brine pool has shrunk, and evaporite-influenced mud flats have prograded over parts of the study area

Table

1. Textural classification of halite having genetic significance

Plates

1. Detailed cross section, San Andres unit 4, Palo Duro Basin

2. Detailed cross section, San Andres unit 5, Palo Duro Basin



Citation
Hovorka, S. D., 1994, Water-Level Controls on Halite Sedimentation: Permian Cyclic Evaporites of the Palo Duro Basin: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 214, 51 p.