ABSTRACT

The most important nearshore environmental complexes on the south Texas coast are the inner shelf, barrier island complex, lagoon-bay complex, deltaic plain and aeolian sand plain. Each of these is a complicated facies tract subject to the effect of many climatic and hydrologic variables.

Important factors influencing the development and aerial distribution of nearshore depositional environments of this, or any other coast, are tectonic setting, hydrologic setting, geological history, climate, and modifying effects of catastrophic events (e.g. tropical cyclones, tsunamis, etc.). The tectonic setting of the south Texas coast is a depositional coastline that has been subsiding since before the start of the Tertiary. In regard to hydrologic setting, depositional coasts can be subdivided into three types -- tide-dominated coasts, mixed-energy coasts, and wave-dominated coasts (classification modified after Price, 1955). The south Texas coast belongs to the wave-dominated class, in that it is a region where wave energy dominates over tidal current energy in determining coastal morphology and sediment distribution patterns. The late Quaternary history of this area was covered thoroughly by LeBlanc and Hodgson (1959) and others, and, hence, will not be emphasized in this paper. The other two factors, climate and modifying effects of catastrophic events, will receive greatest emphasis.

The general climate of the south Texas coast is semi-arid with an average rainfall of 25 inches per year. Maximum periods of rainfall are concentrated during the late spring and September. The September peak correlates with tropical storm activity. The average temperature of the region ranges from 56-62°F in January to 82-86°F in July. Extreme temperatures recorded in the area during the past 40 years were 112°F and 10°F. The most important wind regimes that affect this region are the prevailing southeasterly winds, which blow strongest in the spring (20-30 mph.), and the sporadic, extremely strong northerly winds (occasionally up to 50 mph.), which accompany the movement of cold fronts onto the coast during winter months. Astronomical tides are slight in the open Gulf (mean range 1.5 feet at Port Aransas) and neligible in the enclosed bays and lagoons. Major fluctuations of sea level and strand positions in these bays, exclusive of the effects of hurricanes, are primarily responses to tides generated by the southeasterly and northerly wind regimes. Winds are also important in controlling shoreline configuration, sediment dispersal by wave action, accentuation of tidal currents, generation of longshore currents, and aeolian sediment transport.

Climate also strongly affects other variables on the south Texas coast, such as:

1. Salinity. -- Salinity in coastal regions is dependent upon several factors, for example amount of rainfall and runoff, rate of evaporation, and rate of exchange between bays and the open ocean. Salinity is extremely variable along the south Texas coast, ranging from mesohaline (10-20 ) in some of the northern bays (e.g., Mission, Corpus Christi, Copano) to metahaline (40-80 ) in the southern lagoon-bay complex of Laguna Madre and Baffin Bay. Salinities up to 102 in Baffin Bay were reported by Collier and Hedgpeth (1950). Laguna Madre and Baffin Bay are located in the most and part of the Texas coast, receive negligible runoff from streams, and have limited circulation with the open Gulf.

2. Amount of terrigenous material available. -- The bays and lagoons along the Texas coast are presently being filled with sediments that are transported by streams, carried by currents entering the bays through tidal passes, eroded from older deposits on the bay margins, washed across the barrier islands by storms, and blown in from back-island dune fields. The volume of alluvial sediments depends to a large extent upon the amount of runoff entering an area. Alluvial clays, silts, and sands are accumulating in the bays at a relatively rapid rate in the more humid northern part of the south Texas coast, whereas in the arid areas to the south, authigenic materials, such as carbonate mud, oolites, and shell accumulations, fill the vacuum left by a paucity of alluvial sediments.

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3. Vegetative cover. -- This factor is important in the preservation of geomorphic features (e.g. beach ridges) and in prohibiting the development of aeolian dunes. Extensive dune development occurs on Padre Island and the adjacent mainland, which is the most arid and least vegetated sector of the Texas coast.

Padre Island, Laguna Madre, and Baffin Bay are excellent examples of barrier island and lagoon-bay complexes in a semi-arid climate. These complexes have previously been studied by Fisk (1959), Rusnak (1960), Price (1958) and others. The definition of diagnostic features of deposits from the various subdivisions of these environmental complexes is essential for the interpretation of similar ancient sediments. These features are outlined below:

1. Inner shelf. -- The nearshore shelf sediments off the south Texas coast in water deeper than 40-50 feet are composed primarily of poorly sorted mixtures of fine sand, silt, and clay, with silt predominating. These sediments are thoroughly churned by bioturbination and most cores are homogenous; however, distinct burrow zones are not uncommon. The burrows are filled with fine sand and/or silt, and contain large percentages of skeletal material. The most important skeletal constituents are foraminiferal tests and the spines and plates of infaunal irregular echinoids. Mollusc shells occur in lesser amounts.

2. Barrier island complex. -- This complex is made up of several major subdivisions, which include barrier shoreface, beach zone, foredune ridge, barrier flat and back-island dune fields. Tidal channel and tidal delta deposits are associated with passes that cut through barrier islands. Such passes are common features of the Texas Coast north of Port Aransas and their deposits are important facies types. South of Port Aransas, however, the barrier is at present unbroken by natural passes. Corpus Christi Pass, located at the southern tip of Mustang Island, was a major natural pass forty years ago, but has not been open in recent years except for brief intervals after hurricanes. Other natural passes undoubtedly have been active along Padre Island (e.g. near the mouth of Baffin Bay). Surficial deposits have partially obscured evidence of these former passes.

Sediments of the barrier shoreface, which extends to a depth of about 40 feet, are usually extremely well sorted very fine sand. The sorting in this environment is even more pronounced than in beach and dune sediments. The surf zone (0-12 ft.) is composed of a series of submerged breakpoint bars, usually three in number.

The forebeach is composed of relatively well-sorted, polished terrigenous fine sand containing minor amounts of coarse to fine shell hash in most areas. A 20 mile stretch of central Padre Island, commonly referred to as "Big Shell" and "Little Shell", contains abundant highly polished shell fragments (up to 90+%. Low angle (usually 3-8°) cross-bedding dips seaward.

Separating the forebeach from the foredune ridge on Padre Island is a broad "hurricane beach", which averages 200 feet in width, and is composed of horizontal laminations of poorly sorted coarse to fine shell and fine sand. This "hurricane beach" is everywhere covered by a coarse shell pavement that is formed by deflation.

The foredune ridge of Padre Island contains very well sorted terrigenous sand with a mean grain size ranging from 2.8-3.0 in the north to 2.3-2.5 in the south. Placers of fine shell hash are present but are not volumetrically important. Cross-bedding in the foredune ridge on Mustang Island has an average dip of 24°, and 29% of the dip readings are greater than 30°, with a mean azimuth standard deviation of 75° (McBride and Hayes, 1963). Other diagnostic features are bimodal cross-bedding (dipping north and west) that results from the presence of pyramidal wind-shadow dunes, and steep seaward dipping beds associated with hurricane wave-cut cliffs.

The vegetated barrier flat behind the foredune ridge has more poorly sorted sandy sediments that are, in some places, riddled by distinct large diameter (0.5-1.0 in.) burrows. Most of these burrows are probably due to the activities of the "ghost crab", Ocypodes albicans, and/or rodents.

The lagoonal side of much of Padre Island is occupied by active dune fields. The dunes, which attain heights of 30-50 feet, are usually long, east-west oriented sief dunes. Barchan dunes are also common. The sediments of these dunes are well sorted fine sand with minor amounts of fine shell hash that is derived from hurricane washover sediments.

3. Lagoon-bay complex. -- Bay strands can be divided into high-energy and low-energy beaches. The distribution and orientation of beaches are related to the prevailing wind direction, the fetch, and local bottom configuration. High-energy beaches are

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generally located on the western or mainland side of the bays and lagoons, where they receive the full effect of the prevailing southeasterly winds. Sediments of high-energy beaches along Laguna Madre are usually fairly well sorted fine sand, but may contain large percentages of polished shell fragments, foraminiferal tests and oolites, as well as mats of dead "shoal grass" (Diplanthara wrightii).

Low-energy beaches are of two types, marshes and wind-tidal flats. Extensive marshes are not common along the semi-arid south Texas bay margins. Wind-tidal flats are broad flats covered at irregular intervals by lagoon or bay waters under the influence of wind-generated tides. These flats are devoid of higher vegetation, although blue-green and red algae bloom intermittently when the flats are submerged. Sediments of barrier bordering wind-tidal flats are typically composed of laminated blue-green algal mats and clay seams that are deposited during normal high wind-generated tides. These are interlayered with pure terrigenous sand that is blown from back-island dune fields while the flats are exposed. Mainland bordering wind-tidal flats also contain algal mats and clay laminae, but aeolian sand laminae are missing.

Submerged flats along the margins of bays and lagoons are often densely vegetated by marine "grasses", such as "shoal grass" Diplanthara wrightii, "turtle grass" (Thalassia testudinum) and "widgeon grass" (Ruppia maritima). The sediments of these submerged flats are extremely variable, ranging from moderately sorted fine to very fine sand and shell to poorly sorted muddy sand. The sedimentary structures also vary, however they are in most instances mottles produced by burrowing organisms. Distinct laminations of "shoal grass", blue-green algal mats and shells also occur.

The bay center facies is characterized by very fine grained sediments (clay and silty-clay) with shell contents that vary from very low in Baffin Bay to high in Corpus Christi Bay. The type of sedimentary structure depends upon the amount of bioturbination, which in turn depends upon salinity controlled population densities of infaunal organisms. These structures range through a spectrum of well laminated clay and silt in the metahaline Baffin Bay to irregular distinct laminae, distinct mottling and indistinct mottling in various parts of Corpus Christi Bay. Cores from this facies, which appear mottled or homogenous when wet, generally show some distinct laminae upon drying.

4. Aeolian sand plain. -- The unique aeolian sand plain of south Texas, which covers most of Kennedy and Brooks counties, as well as portions of neighboring counties, is composed primarily of well sorted terrigenous fine sand. However, marl deposits, as well as clay dune sediments, are associated with the many playa lakes that dot the sand plain.

The importance of catastrophic events as modifying agents cannot be overestimated. This fact was impressed upon us by observations of the effects of hurricane Carla of September, 1961. Three factors associated with hurricanes -- storm surge (elevation of water level), waves, and currents -- strongly modify nearshore environments.

Hurricanes cross the Texas coast at the rate of approximately 0.67 per year (based on weather records 1900-1963). Hurricane surge tides have frequently exceeded 10 feet, with a maximum of 22.0 feet occurring in Matagorda Bay near Port Lavaca during hurricane Carla. Twenty foot waves occur in the Gulf during hurricanes at the approximate frequency of once in 5 years.

Some major effects of hurricane Carla are cited below:

1. Inner shelf. -- During the advance of the storm, sediments and associated organisms from the nearshore shelf were washed onto the barrier beaches. After the storm passed inland, seaward flowing density currents deposited a fresh sand layer (0.5-1.0 in. thick) over the sandy mud bottom in depths of 45-60 feet, and a graded bed (up to 4 in. thick), composed of very fine sand, silt, and clay, over the homogenous mud bottom further out on the shelf. This graded bed can be traced to depths of at least 120 feet.

2. Barrier island complex. -- The beach zone was drastically altered by the deposition of a hurricane beach ridge. The foredune ridge of Padre Island was eroded back an average of 100 feet and breached by hurricane channels at many localities. In some parts of the island, where the foredune ridge was planed off, a trough-shaped hurricane washover runway developed behind the hurricane beach ridge. Currents running parallel with the axis of the barrier island deposited a unique assemblage of sediments in the washover runway. This hurricane Carla sedimentation unit consists of (from bottom to top): (1) sand megaripples containing clay blebs and some shell hash, with festoon crossbeds having dips as great as 30°, (2) micro-crossbedded

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and laminated sand deposited by ebbing currents, and (3) pure clay layer deposited by ponded storm surge waters.

3. Lagoon-bay complex. -- Washover fans consisting of sand and shell eroded from foredunes, beaches and inner shelf were constructed on the barrier bordering wind-tidal flats at several localities. These washover fans are associated with channels cut by the hurricane surge. A layer of pure mud, up to four inches thick, was deposited by Carla on the mainland bordering wind-tidal flats. The intervening lagoon acted as an effective trap for the landward-moving coarse sediments.