Quaternary Evolution of Playa Lakes on the Southern High Plains--A Case Study from the Amarillo Area, Texas

RI0236. Quaternary Evolution of Playa Lakes on the Southern High Plains--A Case Study from the Amarillo Area, Texas, by S. D. Hovorka. 52 p., 40 figs., 3 tables, 1995. Print.

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About This Publication
This report documents the stratigraphy beneath playa lakes and associated lake-margin, playa-basin, and upland settings. Genetic facies interpretation of the observed lithologies was undertaken to better predict the nature of interplaya stratigraphic variation and to extract information about the long-term geologic and paleoclimatic evolution of the area.

ABSTRACT
Playa lakes are abundant small ephemeral lakes (generally ~0.5 km in area) that occur in shallow depressions (generally <11 m deep) on the surface of the Southern High Plains. This study, based on analysis of excavations and 63 hollow-stem auger cores taken from 10 lake basins in the study area around the Pantex Plant northeast of Amarillo, Texas, resolves longstanding controversies regarding origin, evolution, and recharge behavior of playa lakes.

Origin of playa lakes has been debated for decades because the lakes are abundant and the processes that form them obscure. All of the lake basins studied have had a long history, originating as topographic lows before or during the early phases of Blackwater Draw deposition. The origin of some topographic lows can be related to dissolution of underlying Permian salt beds. Other basins appear to have been formed by other geomorphic processes. Stacked depositional cycles identified in lake sediments record repeated phases of (1) initial highstand, (2) ephemeral lake conditions, and (3) lake shrinkage and prolonged subaerial exposure. Although sedimentary structures show that the lakes were ephemeral during all phases, duration and frequency of flooding varied, thus changing relative amounts of sediment accumulation, deflation, and soil formation.

Playa lake basins in the study area contain 5 to 18 m of Quaternary lake sediments, including gray clays, oxidized red-brown clays, heterogeneous lacustrine delta deposits, fine lacustrine-eolian sand and silt beds and laminae, and admixed sand and clay. Lake sediments interfinger with calcic soils and red-brown loam of the Blackwater Draw Formation near the edges of the playas. As many as 12 calcic soil horizons are identified in the Blackwater Draw Formation in the upland. A sand unit underlies both playas and uplands at depths of 10 to 20 m.

Although the ephemerally ponded playa lakes are floored by clay soils, ground-water and unsaturated zone investigations show that playa lakes serve as sites of focused recharge. Description of a spectrum of playa basins of various sizes and recharge behaviors documents the long-term maintenance of the seasonally ephemeral lakes and their responses to past climatic changes. All of the lakes are underlain by thick but laterally and vertically heterogeneous clay sections. However, evidence of preferential pathways controlling flow is abundant beneath playas, including shrink-swell cracks and roots in clay soils and gleying, illuviated clay, leached carbonate, or mineralized fractures in older sediments. Sand interbeds within the lacustrine deposits may also influence flow rates. Vertical fractures served as conduits for both oxidizing and reducing fluids in high-permeability, well-sorted sands, as well as in low-permeability sediments.

The observed lateral and vertical heterogeneity within, and the range of variation among, playas can be used to constrain assumptions that will be made about subsurface stratigraphy of the playas at the Pantex Plant. Preferential pathways similar to those observed in all the studied playas can also be inferred to influence recharge behavior at the plant.


Keywords: Ogallala recharge, playa lakes, Quaternary climate, Texas

CONTENTS

Abstract

Introduction

            Geologic Setting

            Purpose of Study

            Methods

Geomorphic Subdivisions of Playa Basins

Sediments within Playa Basins

            Upland and Basin Slope Accretionary Eolian Facies

                        GPR Character

                        Trench Description

                        Genetic Facies Interpretation

            Lacustrine Clays

                        Youngest Clays

                        Older, Gray Clays

                        Older, Reddish Clays

                        GPR Character

                        Trench Description

                        Genetic Facies Interpretation

            Lacustrine-Eolian Sand Beds

                        Genetic Facies Interpretation

            Lacustrine Delta Deposits

                        Genetic Facies Interpretation

            Lower Fine to Medium Sand

                        Description

                        Correlation Problems

Geometry of Playa Basin Sediments

            South Washington Street Playa

            Coulter Street Playa

            TDCJ Playa Basin

                        Dating

            Wink Playa

            Vance Playa

            Sevenmile Basin

            Bradshaw Playa

            Finley Playa

            SCS (Murrey) Playa

Origin, Evolution, and Recharge of Playas

            Facies Relationships in Playa Basins

            Inferred Distribution of Permeability in Playa Basins

            Playa Basin Evolution Model

            Cyclic Deposition of Playa Sediments

            Origin of Playa Basins

            Limits of Study

Conclusions

Acknowledgments

References

Figures

1. Location map of the Southern High Plains and geologic setting of the study area

2. Quaternary stratigraphy of the Southern High Plains

3. Location map of investigated playas and others in the area of the Pantex Plant

4. Detailed location map of TDCJ playa, along with the GPR survey line, trenches, and boreholes

5. Photos of typical appearance of Wink playa lake, north shoreline of Vance playa lake, floor of TDCJ playa, and eastern annulus of TDCJ playa

6. Soil map of TDCJ playa

7. Photos of playa shoreline features

8. Generalized playa stratigraphic section showing major facies

9. Photos of features typical of upland and basin slope eolian accretionary facies

10. Lithologic log of hollow-stem auger core from TDCJ playa and Borehole No. 1 and offset No. 1b

11. Segment of the GPR survey, in the upland east of TDCJ playa

12. Segment of the GPR survey, on the east slope of TDCJ playa basin

13. Segment of the trench in the slope setting on the east side of TDCJ playa basin, showing typical stratigraphy near station

14. Photos of representative sedimentary structures in surface lake clay

15. Photos of characteristics of lacustrine sediments in trenches

16. Photos of fine-grained lacustrine facies

17. Photos of fine-grained oxidized and gray lacustrine facies

18. Segment of the GPR survey

19. Lithologic log of hollow-stem auger core from TDCJ Borehole No. 3

20. Segment of trench 2 in the annulus setting at TDCJ playa. showing typical stratigraphy from the high-water mark down into the playa

21. Photos of features typical of the lacustrine-eolian sand facies

22. Photos of delta deposits

23. Photos of the lower fine to medium sand

24. Drawing from a photomosaic of east side of flood-control structure at South Washington Street playa, Amarillo

25. Photomosaic of west side of flood-control excavation at South Washington Street playa, Amarillo

26. Cross section showing geometry of sediments exposed on north side of flood-control excavation near Coulter Street, south Amarillo

27. North-south cross section A-A' of TDCJ playa

28. West-east cross section B-B' of the southern annulus of TDCJ playa

29. North-south cross section C-C' of TDCJ playa, showing relationships along the major drainage

30. Wink playa basin topographic map, showing well locations

31. North-south cross section D-D' of Wink playa basin

32. Vance playa basin topographic map, showing well locations

33. North-south cross section E-E' of Vance playa basin

34. Sevenmile Basin topographic map showing locations

35. West-east cross section F-F' of Sevenmile Basin

36. West-east cross section G-G' of Bradshaw playa

37. Finley playa basin topography and soils map

38. West-east cross section H-H' across Finley playa basin

39. Schematic cross section of facies relationships at the playa margin beneath the annulus, showing interrelationships among sediments deposited at various lake levels and emphasizing distribution of potential preferential pathways in playa sediments

40. Conceptual model of the evolution of playa basins, showing alternating episodes of lake expansion and lake shrinkage

Tables

1. Characteristics of playa basins examined during this study

2. Hollow-stem auger cores collected for playa studies

3. Radiocarbon analysis of soil samples from TDCJ playa

Citation
Hovorka, S. D., 1995, Quaternary Evolution of Playa Lakes on the Southern High Plains--A Case Study from the Amarillo Area, Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 236, 52 p.