Texas Bureau of Economic Geology Padre Island National Seashore: A Guide to the Geology, Natural Environments, and History of a Texas Barrier Island

COVER

Cover: View of the surf zone from sand dunes that have been partially stabilized by driftwood and sea oats (Uniola paniculata), Padre Island National Seashore. Photo by William A. White.

PREFACE

INTRODUCTION

PURPOSE AND SCOPE OF THE GUIDE
   The guidebook
   The Geology and Natural Environments Map
GENERAL SETTING
   Location
   Physiography and general geology
      Padre Island
      Laguna Madre
   Climate
      Precipitation and evaporation
      Temperature
      Winds
   Hurricanes and tropical storms
ESTABLISHMENT AND MANAGEMENT OF PADRE ISLAND NATIONAL SEASHORE
   Creation of a national seashore
   U.S. National Park Service and seashore management
   Visitor facilities
   Access to island and lagoon environments
      Island access
         Roads
         Beach travel
      Access to lagoon environments
         Gulf Intracoastal Waterway
         Mansfield Channel

HOW TO USE THE GEOLOGY AND NATURAL ENVIRONMENTS MAP

MAP CONSTRUCTION
MAP ORIENTATION
COORDINATE SYSTEMS
   Latitude and longitude
   Universal Transverse Mercator System
   Location grid system
MAP SCALE
MAP LEGEND
MAP SYMBOLS AND LABELS

ORIGIN AND GEOLOGIC HISTORY OF PADRE ISLAND

HISTORY OF ISLAND DEVELOPMENT
PRESENT SHORELINE CONDITIONS
FUTURE OF PADRE ISLAND

THE DYNAMIC BARRIER ISLAND

THE DYNAMIC GULF SHORELINE
   Waves and longshore currents
   Evidence of converging longshore currents
      Effects of jetties on longshore drift
      Accumulation of shells in zone of convergence
   Effects of waves on the upper shoreface and beach
      Offshore submerged sandbars (wave breakpoint bars)
      Swash bars
      Berms
      Beach steepness
      Beach cusps
   The beach — a source of sand for dunes
WINDS AND DUNES ON PADRE ISLAND — A DYNAMIC SYSTEM
   Winds as agents of sediment transport
   Characteristics of dunes and associated deflation areas
   Influence of southeasterly and northeasterly winds on sand migration
   Net direction of sand transport
   Rate of dune migration
   Internal structure of dunes
TIDES AND THEIR EFFECTS ON LAGUNA MADRE
HURRICANES AND THEIR EFFECTS ON PADRE ISLAND
HUMAN INTERACTION WITH THE DYNAMIC BARRIER ISLAND
   Linking gulf and lagoon waters
   Effects of people on the beach and fore-island area
   Creation of experimental dunes
   Human reaction to windblown sand

ENVIRONMENTS OF PADRE ISLAND AND LAGUNA MADRE

BARRIER SYSTEM
   Beach (B1)
   Sandflat and/or coppice dune field (B2)
   Fore-island dune ridge (B3)
   Vegetated barrier flat (B4, B5)
   Stabilized blowout dunes (B6)
   Washover channel (B7)
   Wind-deflation flat, storm runway, and washover fan (B8)
   Ephemeral fresh- to brackish-water ponds and marshes (B9)
   Back-island dune field and fore-island blowout dunes (B10)
   Back-island sandflat (B11)
LAGOON SYSTEM
   Wind-tidal flat (L1, L2, L3)
   Lagoon sand and shell berms (L4)
   Lagoon-margin sand (L5)
   Grassflat (L6)
   Lagoon-center sand (L7)
   Serpulid reefs (L8)
MAN-MADE AND MAN-MODIFIED UNITS
   Spoil (M1, M2, M3)
   Land modified by human activity (M4)

HISTORY OF HUMAN ACTIVITY ON PADRE ISLAND

KARANKAWA INDIANS
SPANIARDS
   Exploration
   The shipwrecks of 1554
   Colonization
   Padre Ballí
AMERICANS
   Smuggler Kinney
   A boundary dispute and the Mexican War
   John Singer and his buried treasure
   The Civil War
   The King Ranch
   The meat packeries
   Patrick Dunn — the "Duke" of Padre
   The wreck of the Nicaragua
   Colonel Sam's dreams of an island resort
   Movement toward an island park
SUMMARY OF NATURAL RESOURCE USE

FIELD TRIP THROUGH THE ENVIRONMENTS OF NORTH PADRE ISLAND

ROAD LOG

ACKNOWLEDGMENTS

REFERENCES

HISTORICAL REFERENCES

GLOSSARY

INDEX (omitted from the online edition)

1. Index and location maps for Padre Island and the surrounding area

2. Generalized cross section of north Padre Island environments, from the Gulf shoreline to Laguna Madre

3. Annual average precipitation along Texas coast and monthly precipitation data for Corpus Christi and Port Isabel, Texas

4. Annual average temperatures along Texas coast and average monthly temperatures for Corpus Christi and Brownsville, Texas

5. Annual frequency of surface wind directions as determined over the period 1951 to 1960

6. Index map of the northern part of Padre Island National Seashore

7. Observation deck and walkway at Malaquite Beach visitor facilities

8. Paved campground just north of Malaquite Beach visitor facilities

9. Four-wheel-drive warning sign

10. Beach marker showing the approximate mileage from the south beach access road near Malaquite Beach

11. Intracoastal Waterway, approximately 12 feet deep, paralleled by a row of spoil heaps

12. Petroleum company service channel dredged from the Intracoastal Waterway to provide access to a drilling site

13. Mansfield Channel

14. Three theories of barrier island origin

15. Sketches representing several stages in the history of the southern Texas coast

16. Sea-level changes during the past 18,000 years, as interpreted by various authors

17. Spit accretion

18. Cross section of central Padre Island near beach mileage marker 30

19. Net shoreline changes along Padre Island National Seashore based on time periods of various lengths between 1862 and 1975

20. Waves approaching shoreline at an oblique angle, producing longshore currents

21. Direction of littoral, or longshore, drift along a straight shoreline

22. Effect of the concave Texas Gulf shoreline on directions of littoral drift

23. Geologically effective winds (predominant winds) on Padre Island

24. Jetties at Mansfield Channel

25. Little Shell Beach sediment, consisting primarily of shells of the small clam Donax

26. Big Shell Beach sediment, consisting primarily of abraded shells of large clams

27. Characteristics of a wave that define wave steepness

28. Waves breaking along offshore submerged sandbars (wave breakpoint bars)

29. Swash bar on a shell beach on central Padre Island

30. High, partially eroded berm of a steep, central Padre shell beach

31. Observation trenches dug along the gulfward edge of berms on Big Shell Beach

32. Beach cusps

33. Beach cusps near Malaquite Beach

34. Closely spaced cusps on a steep, central Padre shell beach

35. Examples of salt-spray-tolerant plants that help trap and hold sand to form stabilized dunes along the Gulf shoreline

36. Well-vegetated portion of central Padre Island in the vicinity of grids H-11, J-11, and K-11, plate I

37. Sparsely vegetated segment of Padre Island immediately north of Mansfield Channel

38. Generalized diagram depicting the sun as the energy source behind the wind

39. General characteristics of dunes and common dune types, shown in relation to wind direction

40. Active back-island dune on Padre Island

41. Deflation trough or flat formed along the trailing edge (windward side) of an active dune

42. Diagrammatic cross section of deflation troughs and interlying stabilized ridges compared with a graph of rainfall

43. Generalized sketch of an active dune and related deflation troughs

44. Aerial photograph of active dunes shaped by southeasterly winds

45. Oblique, low-level aerial photograph of active dunes shaped by northeasterly winds

46. Active dunes in a state of transition as they respond to changing wind directions

47. Lagoonward migration of sand in response to onshore (southeasterly) winds

48. Aerial photograph showing the direction of elongation of active dune fields in relation to their net direction of migration

49. Movement of active dunes with respect to major wind directions

50. Comparison of the distribution of island environments as they appeared in (a) 1956, (b) 1967, and (c) 1975

51. Active longitudinal dune on the lagoon side of Padre Island

52. Ripple marks showing sorting or segregation of sediments by the wind

53. Ripple marks in which flakes of shell material have been sorted or segregated from finer grained quartz sand by the wind

54. Trench dug in hurricane washover channel on Padre Island

55. Alternating layers of sediment, composed predominantly of quartz sand and shell fragments, in a fore-island dune on Padre Island

56. Trench dug behind a small dune (vegetated with sea oats) located near the beach on Padre Island

57. Close-up view of trench shown in figure 56

58. Dune crossbedding, Padre Island

59. Truncated crossbedding exposed on nearly horizontal surfaces windward of an active back-island dune on Padre Island

60. Sequence of events that can produce crossbeds dipping in different directions

61. Aerial photograph of washover channels along central Padre Island that were activated by storm surge accompanying Hurricane Beulah

62. Sketch of washover channels and washover fans, shown in relation to other natural environments on central Padre Island

63. Natural shell "pavement" lagoonward of the fore-island dunes on Padre Island

64. Seawall on south Padre Island, south of the National Seashore

65. Northern end of pedestrian beach, along which vehicular traffic is not allowed

66. Trails worn in fore-island dunes by pedestrians

67. Experimental fore-island dune ridge

68. Bulldozer scraping windblown sand off the paved parking lot at Malaquite Beach

69. Tongues of sand blown by onshore winds onto a road near Malaquite Beach

70. Road rerouted because of migrating dunes

71. Natural gas wellhead threatened by burial under windblown sand

72. Methods used to stabilize loose sand and prevent it from being blown by the wind

73. Shell beach

74. Generalized profile from upper shoreface to fore-island dunes

75. Beach profiles recorded June 17-18, 1975

76. Malaquite Beach, typical of north Padre beaches, having a flat profile and broad, shallow cusps

77. Coppice dune field lying between the beach and the fore-island dune ridge

78. Broad coppice dune fields in the southern part of the National Seashore near Mansfield Channel

79. Fore-island dune ridge of central Padre Island, as viewed from the surf zone

80. Fore-island dune ridge at one of its widest points

81. Heavily vegetated barrier flat supporting various grasses and small shrubs

82. Sparsely vegetated barrier flat

83. Novillo, the northernmost line camp constructed by Pat Dunn for cattle ranching on the island during the late 1800's and early 1900's

84. Park Road 22, which cuts through stabilized blowout dunes just north of the Bird Island Basin road intersection

85. Hurricane washover channel at the 35-mile beach marker

86. Wind-deflation flat at the trailing edge of a back-island dune field

87. Washover fan spreading onto a deflation flat

88. Ponds occupying long wind-deflation troughs, which parallel back-island dune fields

89. One of the marshy ponds that have developed in deflation flats left by fore-island blowout dunes

90. Blowout complex that originated at the dune ridge, where stabilizing vegetation had been destroyed and loose sand was free to blow out

91. Back-island dune field including several large, shifting dunes

92. Trench dug on a wind-tidal flat, showing the alternating layers of sand (light layers) and clay (dark layers)

93. Wind-tidal flat covered by extensive algal mats

94. Gypsum rosettes

95. South Bird Island, a natural island in Laguna Madre

96. North Bird Island

97. Lagoon-margin sand

98. Large sand waves and ripples on lagoon-margin sand exposed during very low tide

99. Small cliff at the edge of the barrier flat, eroded by high wind and storm tides

100. Lobes of lagoon-margin sand oriented southward, in the direction of prevailing lagoonal currents

101. Subaqueous grassflat in Laguna Madre

102. Lagoon grassflat exposed during low tide

103. Dead shoalgrass that has been washed onto a wind-tidal flat, dried in mats, and bleached white by the sun

104. Rows of bleached shoalgrass marking varying tide levels on the wind-tidal flats and reworked spoil

105. Middle Ground, an area of wind-tidal flats and shallow grassflats

106. Muddy sand on the center bottom of Laguna Madre, just south of the Land-Cut Area

107. Dredged spoil along the Intracoastal Waterway

108. Petroleum company facilities on the northern part of the Seashore

109. Historical sites on Padre Island

110. South Texas

111. Route for north Padre field trip

112. Padre Island National Seashore entrance on Park Road 22 (Observation Point 1)

113. Beach at north beach access road (Observation Point 2)

114. Effects of beach traffic on vegetation near north beach access road

115. Trailhead of the Grasslands Nature Trail (Observation Point 3)

116. Barren, active blowout dune near the Grasslands Nature Trail

117. Shells of the oyster Crassostrea, dredged from nearby bays and placed on barren dunes on the east side of Park Road 22 to prevent loose sand from blowing across the highway (Observation Point 4)

118. Marshy ponds occupying wind-deflation troughs (Observation Point 5)

119. Exposed roots of live oaks

120. Large, elongate dune in a back-island dune field (Observation Point 6)

121. Grassflats, environments of very high biological productivity, lying in calm water just beyond the lagoon-margin sand (Observation Point 7)

1. Characteristics of Hurricanes Carla, Beulah, and Celia, as measured at Texas stations

I. Geology and natural environments of Padre Island National Seashore, Texas (omitted from the online edition)