Abstract

Geological processes producing barrier island and other shoreline sand bodies throughout the world during the Recent standing sea level stage have produced similar sand bodies during standing sea level stages of the geologic past. It seems logical that lenticular barrier island sands probably form reservoir rocks containing large quantities of hydrocarbons in both structural and stratigraphic traps in many sedimentary basins. Several examples have been described in the literature. Many accumulations of the stratigraphic type remain to be found and there is a real need for criteria for their recognition and for the prediction of their location. Therefore, a knowledge of the geologic processes, setting (framework), history, lithologic character, genesis and sequence, directional features, shape, and trend of a barrier island sand mass, such as that of Galveston Island, should be of value to a geologist in the exploration for and exploitation of reservoirs of this type.

Prevailing winds, waves and currents along the southeast Texas coast are from the southeast and northeast quadrants. Wave heights average two and four feet in ten and fifty foot water depths respectively off Galveston and Bolivar beaches. Ten to twenty foot waves may occur during hurricanes. The combined effect of the winds, surface currents, and waves refracting shoreward produce the prevailing westward longshore currents.

Tides are semi-diurnal and diurnal, and range in height from less than 1 foot to 2.5 feet. Currents in Bolivar Roads vary from less than 1 knot to a maximum of 3.3 knots during flood and 4.3 knots during ebb tides. Similar currents occur in San Luis Pass. The direction, force, and duration of the wind has a considerable effect on the tides and currents. Fifteen foot tides may be expected during severe hurricanes and very low tides may accompany strong northerlies of long duration.

Surface water temperature averages slightly less than 90° F and ranges between 80 and 100° F during the late summer. During the winter the average is slightly less than 60° F and the range is between 35 and 80° F. Salinity varies with the run-off and averages approximately thirty parts per one thousand during the late summer and early fall and twenty-two parts per one thousand during the winter and spring.

Most of the sand deposited in this area is derived from an easterly source along the shore; and it appears that much of the finer grained sediment is derived from the Trinity and San Jacinto rivers.

Galveston Island, Bolivar Peninsula (a land tied island) and associated tidal deltas together comprise a sand barrier feature separating the Gulf of Mexico from Galveston Bay, East Bay and West Bay. This composite sand mass extends parallel to the shoreline from Caplen on the east to San Luis Pass on the west, a distance of fifty miles. This barrier feature is the easternmost part of a line of barriers which continue (with few interruptions) south along the Texas and Mexican Gulf Coast for approximately six hundred miles. The width of Galveston and Bolivar at sea level averages one and one-half miles, and their base extends two miles seaward and a few hundred feet lagoonward from the islands' shore. At Bolivar Roads the long axis of Galveston Island is offset seaward and lies en echelon with that of Bolivar Peninsula. This offset is related to the westerly longshore drift.

The history of the Recent is subdivided into standing and rising sea level stages. The subdivision is based upon the published results of Fisk (1944, 1947), McFarland (1955), LeBlanc and Bernard (1954), and LeBlanc and Hodgson (1959) and is supported by more than two hundred and fifty radiocarbon dates of shells and wood collected along the Texas-Louisiana coastal and offshore area, including those from borings through the Galveston barrier. The standing sea level stage dates from the present to 5,000-6,000 years ago. The latter part of the rising sea level stage has been dated relatively accurate back to 13,000 years, when sea level was about one hundred and eighty feet below its present level. However, the exact details of the positions of sea level during this stage are not entirely clear.

Galveston Bay resulted largely from the drowning of the Trinity and San Jacinto valleys which were entrenched during the previous falling and low sea level stages. The lower parts of the valleys were filled with alluvial and deltaic sediments during the low and early rising stages. Because of the relatively small volume of sediment contributed to the area, the valleys were drowned during the latter part of the rising stage. Shore erosion and deposition during the past 5,000-6,000 years have rounded the shoreline of the major parts of the bays. Thus, the shape of Galveston Bay is controlled by the position of the trenches, subsequent shoreline erosion and deposition, and also by the nature and distribution of the sediments of the previous Pleistocene standing sea level stage.

Only within the past one thousand years has the Trinity River adjusted its gradient to the present base level and begun to advance its delta across the upper part of Galveston Bay. Given sufficient time and a constant base level, the Trinity and San Jacinto deltas should fill the bays and advance seaward across the barrier. This sequence of events should repeat those which resulted in the development of the more mature Pleistocene Beaumont coastal plain and associated sediments.

The depositional morphology of Galveston Island and Bolivar Peninsula consists principally of numerous narrow, parallel beach ridges which vary in elevation from a few inches to twelve feet, and intervening swales. These features trend almost parallel to the present shoreline, but are recurved along the backside and western ends of the islands. The greater widths or backshore bulges of Bolivar Peninsula are caused by closely spaced and well developed recurved beach ridges. "Tidal guts" on the lagoonward side of the island are swales between the recurved spit-like beach ridges. These trend at an approximate angle of 30° with the long axis of Bolivar and 70° from that of Galveston. The "guts" are kept open by floods during heavy rainfalls, tides, and storm washovers. A few sand dunes of the hummocky type occur on the beach ridges and are more common on the westward end of Galveston. Most of the sand dunes form on the storm berm but are destroyed during severe storms. Subaqueous tidal deltas, referred to as bars by coastal pilots, occur on both the seaward and bay sides of Bolivar and San Luis passes. Flood and ebb channels, modified subaqueous barchan dunes and giant ripples are the more common features of these deltas.

The slope of the beach averages less than 5°. Prominent beach cusps are uncommon. Seaward from the beach, the bottom slopes approximately thirteen feet per mile to the thirty foot contour, which is about two miles offshore. Between the thirty and fifty-two contours the slope decreases to about four feet per mile, and seaward it is less than one foot per mile. Normally, four breaker bars (longshore bars or balls) may be observed in the shallow water immediately off the Gulf beach. Their position, height, and number vary with the wave height, tides, and possibly with the longshore currents. It is believed that these breaker bars are destroyed during severe storms.

The bays behind the barriers are less than ten feet deep. Maximum depths of approximately twenty-five feet were measured in the tidal passes before the jetties were constructed.

During the standing sea level stage Galveston Island began as a small bar on the southwestern side of the mouth of the estuary about four miles offshore in five to eight feet of water. The island emerged and grew seaward by beach accretion and southwestward by spit accretion in the direction of the prevailing longshore drift. The areal extent of the island was approximately one-half its present size about three thousand years ago. This history is supported by the offlap relationship of the beach ridges and swales, radiocarbon dates, and other data. The lagoonward growth of the island by the accretion of tidal delta, washover fan, and aeolian deposits is of secondary importance. The depositional features of Bolivar Peninsula record a similar history of development, but the peninsula became land-tied upon the filling of Rollover Pass near Caplen.

The surface of Galveston Island consists of very well sorted, fine to very fine sands. Shells and Formanifera are common in the recently deposited sand near the shore, but leaching processes have removed most of these from the older sands occurring above the water table. Sand containing shells extends seaward along the Gulf bottom to the break in offshore slope near the thirty foot contour. Seaward of this contour the bottom consists of silt and clay with shells. However, sand, silt, and shells occur on subdued topographic highs which are probably not related to the development of Galveston Island. The distribution of the sediments and the decrease in grain size offshore are closely related to the offshore profile of equilibrium. Sand fringes the back side of the island and is the dominant sediment in the tidal channels and deltas. Silt and clay containing shells make up most of the sediments in the open parts of the bays and lagoons.

Sedimentary features are very useful in distinguishing the Recent facies of the Galveston barrier and environs. Most of the bay and lagoonal deposits are bored and churned by organisms. Some are massive. Bedded and laminated sand and clay occur in the lagoonal deposits near the tidal passes where currents are stronger and more frequent. Laminations which are essentially horizontal, and very low angle cross-bedding, are the most common depositional features of the backshore, beach crest, and foreshore sands. The initial attitude of the laminated foreshore sands conform with the slope of the beach. These deposits are commonly bored and churned by organisms. Aeolian sands forming the uppermost barrier island deposits are variably cross-bedded (festooning may be common), horizontally laminated, contorted, and bored. The bottom deposits from the beach gulfward to the break in offshore slope consist of laminated, bored, churned and occasionally cross-bedded sand with shells. The initial attitude of the bedding appears to conform with the bottom slope. Cross-bedding is probably common in the tidal delta deposits. The deposits seaward of the break in offshore slope are interbedded and bored sand and silt-clay. A few thin graded beds of very fine sand and coarse silt have been observed. In places where bottom currents are stronger and more frequent, bored and churned features are less common.

The unconformity between the Recent and the Pleistocene sediments below Galveston Island is easily recognized. Its depth below sea level varies between six and sixty feet and is greater than sixty feet at the eastern end of the island. The contact is below one hundred feet in the Trinity trench between Galveston and Bolivar Peninsula. During the lower sea level position, the Pelistocene sediments were exposed, weathered, oxidized, and leached. Because the uppermost Pleistocene mottled red and brown sediments lost much of their moisture through desiccation, they are stiffer than the soft, grey Recent deposits. At the contact the Pleistocene in most places is bored and in many places contains calcareous nodules.

Sections across Galveston Island reveal an offlap sequence of Recent deposits which is caused by the seaward growth of the island during the standing sea level stage. The barrier island sand body overlies interbedded deposits. The vertical sequence of sedimentary features from top to bottom repeats the offshore sequence of sedimentary features from shore to sea. Laminated, bored, and churned sand containing shells overlies interbedded and bored sand and silt-clay layers with shells. The decrease in median and maximum grain size with depth in the barrier is, as expected, similar to the offshore decrease in grain size.

Arenaceous Foraminifera characterize the marsh and near marsh sediments behind the barrier. Calcareous 'Rotalia' and Elphidium are the most common Foraminifera in the open bay and lagoonal sediments. 'Rotalia' and Elphidium plus a few open Gulf Foraminifera characterize the sediments accreting to the Gulf side of the barrier. Crassostrea, Rangia, Mercenaria (Venus), and Thais are the more common larger invertebrates found in the bay sediments. Littorian is commonly found in the brackish and salt water marshes. Anadara, Busycon, Dinocardium, Donax, Noetia, Oliva, and Polincies are the more common larger invertebrates found in the Gulf shore and tidal pass deposits. The sequence of foraminiferal and larger invertebrate assemblages with depth in borings through Galveston Island is similar to the offshore sequence of assemblages.

The grain orientation in the beach deposits results from the to and fro action of the waves and is approximately at right angles to the Gulf shore (Nanz 1955). Thus, the subsurface trend of beach deposits should be predictable from oriented cores.

Barrier island sands range in thickness from approximately fifteen to fifty feet. Along the South Texas coast aeolian deposits increase the thickness of the barrier island sand mass by as much as forty feet. Individual barrier island deposits are relatively narrow, from less than one mile to over eight miles wide. They are relatively long, from a few miles to over fifty, and are parallel to the regional depositional strike. A chain of Recent barrier islands connected by tidal delta deposits and interrupted at only a few places by river deltas, extends a distance of over six hundred miles along the Texas and Mexican coasts. Adjacent chains of barrier island deposits, the Recent and the late Pleistocene (Smith Point-Live Oak) occur in the same "stratigraphic" position. In places they are adjacent to each other, and in others are separated by lagoonal and/or marine deposits. Thus, the width of composite barrier island sand may be as much as or more than fifteen miles.