Matagorda Island, Texas: the Evolution of a Gulf Coast Barrier Complex

Matagorda Island, Texas: The Evolution of a Gulf Coast Barrier Complex

BRUCE H. WILKINSON Department of Geology and Mineralogy, The University of Michigan, Ann Arbor, Michigan 48104

ABSTRACT Two sources of sand contributed to this 6.4 km wide. It is bound on the northeast barrier complex. Prior to stillstand, erosion by Pass Cavallo and on the southwest by Matagorda Island is a wide, sand-rich, of Pleistocene strandplain sand and middle Cedar Bayou. It is separated from the main- barrier-island complex on the central Texas Holocene fluvial-deltaic sand which was land by Espiritu Santo Bay, San Antonio coast. This barrier initially formed as an exposed on the shelf supplied most of the Bay, and Mesquite Bay. intermittently emergent sand shoal which sediment to the early barrier. Following migrated landward during the late stillstand, with progradation, shelf sands GEOMETRY AND LITHOLOGY Holocene transgression and then became were too deeply submerged to be eroded by OF THE ISLAND COMPLEX stabilized as the Gulf of Mexico reached Gulf waves. Sand, discharged into the Gulf stillstand. The subaerial portion of the is- by the Colorado and Brazos Rivers and Sediments which underlie and make up land complex rests on a blanket of middle transported southwestward by longshore Matagorda Island have been divided into Holocene bay-estuarine mud which was currents, was deposited on the beach and several basic units.. Figure 2, a strike sec- deposited behind the landward-migrating shoreface of Gulfward-building Matagorda tion through the island from Cedar Bayou sand body and then was overridden by it. Island. Key words: marine geology, to Pass Cavallo, and Figure 3, a dip section Following stillstand, Matagorda Island sedimentation, barrier islands, Texas coast. across the south end of the island, illustrate prograded Gulfward approximately 1.6 the general relationships among these units. km. During this progradation, two large INTRODUCTION The division of Holocene sediments into tidal passes, which connected the Gulf of middle and late is informal, designating Mexico with San Antonio and Mesquite Matagorda Island is located in the central only stratigraphic position, and is not Bays, were closed. The island was further portion of the Gulf coast, and is one of a meant to imply the time at which they were modified by migration of Cedar Bayou sev- series of barrier islands and peninsulas deposited during the Holocene epoch. eral miles to the west across the island's which lie off the mainland coast of Texas Pleistocene sedimentary units consist of a southern end. (Fig. 1). The island is 54 km long and 1.6 to thick, mud-rich, deltaic sequence with a

Geological Society of America Bulletin, v. 86, p. 959-967, 11 figs., July 1975, Doc. no. 50712.

959

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and others, 1975) indicates that this relatively thick sequence represents interdistributary mud, a component fades of a mud-rich lobate Pleistocene delta, which prograded across the area during an earlier interglacial. Interdistributary mud grades upward into .1 to .2 m of very tough, hard, bright orange to tan mud. This soil represents a period of lower sea level, subaerial exposure of mud-rich deltaic sediments, with subsequent weathering and oxidation. The oxidized interval is overlain by 1 to 2 m of tan to yellow sand, the Ingleside sand, which is part of a broad strandplain sand- belt running along the mainland coast from Pass Cavallo to south of Corpus Christi. The Ingleside sand under Matagorda Island is easily recognized by the strong contrast in color between the yellowish orange of these sediments and the dull gray of overlying PLEISTOCENE MIDDLE HOLOCENE LATE HOLOCENE middle and late Holocene sand. | | Interdistributary mud Fluvial - deltaic sand fcffi] Barrier island sand

r Soil zone Bay-estuarine mud Bay-estuarine mud Middle Holocene Units Ingleside sand Lower shoreface mud

Figure 2. Northeast-southwest strike section through Matagorda Island from Pass Cavallo to Middle Holocene sediments consist of a Cedar Bayou. sequence of medium to dark gray sand, as much as 10 m thick. The sand is highly micaceous, contains little or no shell, and is thin soil zone on its surface, overlain by a Pleistocene Units moderately organic. Although the sequence thin sand veneer. Holocene units are a consists mostly of clean, well-sorted, very transgressive, sandy, fluvial-deltaic se- Interdistributary mud and associated dis- fine sand, intervals of thinly interbedded quence beneath a body of bay-estuarine tributary sand are the northernmost and sand and muddy sand are not uncommon. mud, which is in turn overlain by sand of oldest sediments exposed in this area (Fig. A net sand isopach of this interval shows a the barrier-island nucleus and bayward- 4). Mapping of the coastal zone by the pronounced dip orientation to the sand extending lobes of flood tidal deltas. Bureau of Economic Geology (McGowen body (Fig. 5), which is in contrast to the predominantly strike-oriented sand and mud bodies which overlie this interval. Lithological characteristics, thickness trends, and stratigraphic relationships of these middle Holocene sediments indicate that they represent a fluvial-deltaic sequence which filled valleys incised into the Pleistocene surface during the Wisconsin lowstand of the Gulf of Mexico, and were deposited by rivers which were aggrading and back-filling valleys during the Holocene transgression. Pleistocene units that occur at about the same depth below sea level as the middle Holocene fluvial- deltaic sand are representative of drainage divides which were subsequently covered. Middle Holocene sand and muddy sand are overlain by a sequence of medium greenish-gray mud up to 8 m thick. The mud is soft to moderately firm and contains little shell and plant debris. The shell is representative of a grassflat to bay fauna. This PLEISTOCENE MIDDLE HOLOCENE LATE HOLOCENE mud was deposited in bay and estuarine en- I I Interdistributary mud Fluvial-deltaic sand lllllll Barrier island sand vironments during later stages of the Holocene transgression. Figure 6 is a net \ / \ Soil zone [HZ-^ Bay-estuarine mud mud isopach of the area. A comparison be- I | Ingleside sand tween Figure 5 and Figure 6 reveals that the Figure 3. Northwest-southeast dip section through the northern end of Matagorda Island. thickest mud sequences underlying

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Matagorda Island correspond with Pleis- tocene valleys. The thicker mud deposits «JpsffiB were probably deposited in valleys incom- „S pletely filled with middle Holocene fluvial-deltaic sediments. When the Gulf of Mexico reached its present level, middle Holocene mud underlay much of the area now occupied by Matagorda Island. Most of the late Holocene sand of the island complex rests upon this mud unit.

Late Holocene Units

The island complex has been divided into several units (Figs. 7 and 8). The fore-island area of Matagorda Island consists of a series of ridges and swales which run paral- lel to the present Gulf shoreline. Ridge and swale topography is exceedingly straight ^ tfMMimm and well defined, with individual ridges and swales traceable for more than 16 km. Sed- iments consist of clean, very fine, subangular, well-sorted sand. The sand contains an abundant Gulf fauna. The fore-island sand unit was deposited on the middle and upper shoreface, and subaerially, during the Gulfward progradation of Matagorda Is- PLEISTOCENE HOLOCENE land. ifsflj Deltaic sands and muddy sands Fluvial-deltaic valley-fill sands and muddy sands The back-island sand unit is characterized by prominent northwest-oriented Deltaic muds and sandy muds Bay-estuarine muds and sandy muds

hurricane surge channels. This is in contrast Strandplqin sands Barrier- island and peninsula sands and muddy sands to the northeast orientation of ridges and swales which typify fore-island sand. The Figure 4. Simplified geologic map of the central Texas coast (modified from McGowen and others, boundary between the fore-island area and 1975). the back-island area is sharp and easily recognized along most of Matagorda Island. These sediments consist of highly burrowed and root-mottled very fine sand with scat- tered shell debris. Unlike fore-island sand, back-island sand contains a mixed fauna consisting of normal Gulf molluscan genera as well as bay-margin grassflat and the deeper bay genera. The back-island area was frequently overrun by storm tides soon after it became emergent. The southern segment of Matagorda Is- land has been modified by the migration of Cedar Bayou to its present position, across several kilometers of the island nucleus. During this migration, ridge and swale topography was obliterated. The part of the island through which this pass has migrated is characterized by a series of arcuate active and inactive vegetated dunes and interdune lows, concave toward the northeast. Lithologically, these sediments are unique; the sand is clean, very fine grained, well sorted, subangular to subrounded, and contains abundant plant material. Plant debris consists of seaweed, twigs, bark, and rounded wood chips that range from several millimeters to several centimeters in length. In general, concentration of plant debris, especially wood fragments, tends to increase downward to the base of this se-

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Contour interval 10 feet ( 3.1 meters)

0 5 Miles 1 i'i i'I 'i I'I I 0 8 Kilometers

Figure 6. Net middle Holocene bay-estuarine mud underlying Matagorda Island. Figure 7. Geologic map of late Holocene sands which make up Matagorda Island.

Figure 8. Net late Holocene barrier-island sand subaerially exposed on Matagorda Island.

quence. Rounded wood fragments are drowned river valleys, which underlie the tion of the area. These passes were both rarely found in other sand bodies of island, from the open Gulf was in existence large and active at the time progradation Matagorda Island. when sea level was at least 13 m below its was initiated. Both are bounded by strongly Flood deltas of active and inactive tide present level. The oldest island sand in the recurved ridges and swales along the chan- passes make up a significant volume of the area, the back-island sand, contains a . nel (Fig. 9B). sand of Matagorda Island barrier complex. mixed Gulf-bay fauna. Any mechanism for Island progradation continued until Three flood delta complexes are recogniz- the initial emplacement of early island Matagorda Island had attained a total able in the area. All are lobate sand bodies sands must incorporate a source for these width of from 1.6 to 6.4 km. This growth that extend behind the barrier nucleus (Fig. brackish-water genera. These sands exhibit was accomplished by seaward progradation 7). Two of the flood deltas are roughly a well-preserved series of washover or surge of about 1.6 km along the entire length of symmetrical, are presently inactive, and are channels, indicating significant sediment the island and by bayward growth of flood in various stages of destruction due to sub- transport from the Gulf. deltas associated with the tidal passes. Dur- sidence and erosion by waves. The active For these reasons, it is thought that ing this time, both remaining passes were flood delta at Pass Cavallo, however, is Matagorda Island initially formed in this sealed off from the Gulf and became inac- unique, due to its strong asymmetry to the area as a sand shoal, migrating landward in tive. It is doubtful if Matagorda Island ever southwest. response to rising sea level, moving over a extended farther Gulfward. At this time, Lithologic units which make up these fea- previously deposited blanket of bay mud. the island seems to have reached its max- tures consist of fairly clean, well-sorted, Migration of this sand body ceased as the imum size (Fig. 9C). fine-grained sands that interfinger with and Gulf of Mexico reached stillstand with the At the present time, not much is happen- overlap late Holocene bay mud. The sand shoal coming to rest on a sequence of ing on Matagorda Island, at least from a amount of mud in these sand lobes in- bay-estuarine muds. The mixed fauna geological point of view. The Gulf shoreline creases toward the surface, which in many within this early sand body reflects rework- appears to be largely stabilized, except for places consists of a veneer of dark organic ing of bay-estuarine sediments which were the northern segment near Pass Cavallo, mud. In general, the fauna of the clean tidal constantly exposed and eroded in the which has been prograding Gulfward. Most delta sand is dominated by Gulf shell with shoreface as the sand body moved to its of the bay shoreline of the island is under- lesser numbers of bay genera. Bay species present position. going erosion at rates of less than .3 m per become more dominant and Gulf species The boundary between fore-island and year on an average. As a result of this ero- become more scarce as the mud content of back-island sand represents a time prior to sion, the margin environments of marshes, the section increases vertically. which the island was frequently overridden ponds, and bays are expanding southward by storms because of ever-rising sea level at the expense of higher ground. The west- HISTORY OF DEVELOPMENT and after which the island prograded under ern margin of Pass Cavallo is eroding conditions of stable sea level. As stillstand toward the southwest at the extremely high The oldest Holocene sediments encoun- was reached, Matagorda Island consisted of rate of 3 to 5 m per year in most places. tered in this area were deposited during the numerous lobate shoal areas oriented Vertical accretion occurs on the Pass Holocene transgression and consist of northwest. These intermittent shoals were Cavallo flood delta during storm surge; at fluvial-deltaic sand that accumulated in val- cut by numerous small and large tidal this time, sediment also moves through leys cut into the Pleistocene surface during passes and were frequently awash during tidal channels into Espiritu Santo Bay. lowstand of the Gulf. Rivers were storms (Fig. 9A). back-filling valleys as sea level rose to its Shepard (1960) reported that several BARRIER STABILIZATION present position. As the Gulf continued to cores taken through the bay-estuarine muds rise, these deltas were flooded and estuaries of San Antonio Bay contain faunas which During the Holocene transgression, sea developed. In much of the area presently indicate higher salinities than those re- level rose from about —125 m to its present overlain by Matagorda Island, this flooding corded in the modern bay. He further level (Fisk, 1944). As sea level reached and change in depositional environment states: about —8 m, the first barrier sands were occurred when sea level was about 13 m deposited in the area that is now below its present position. Mud deposition The thick columns of oyster reef (encountered in Matagorda Island. This deposition was in continued until many of the valleys were borings in San Antonio Bay) . . . are particularly the form of a sand shoal which was migrat- filled and most of the drainage divides were interesting. As the middle portion of the reefs ing landward in response to rising sea level. blanketed with mud. The oldest part of contain Ostrea equestris, a relatively high salinity As sea level rose to its present level, this Matagorda Island rests on these bay- oyster, it would appear that the salinity was high sand shoal migrated to the position now during the time when they were deposited. estuarine deposits. seen as back-island sands on Matagorda Whether this was due to larger openings in the No boundary between pre-stillstand and barrier or due to greater aridity causing less Island. post-stillstand sedimentation is discernible inflow of fresh water from streams is difficult to The final resting place of the migrating on lithologic characteristics in the bays be- say. sand shoal evidently depended on two fac- hind Matagorda Island. Shepard (1953, tors — the slope of the Pleistocene surface 1960) indicated that waters first flooded the The many passes and surge channels in the in this area and the configuration of the San Antonio Bay River Valley as sea level back-island sands of Matagorda Island in- shoreface at stillstand. The horizontal dis- reached —25 m. Deposition of bay sedi- dicate that the first of these two possibilities tance between the Gulfward exposure of ments has continued from that time to the is probably the case. the Ingleside sand and the Gulfward extent present, at a rate approximately .4 m per Soon after stillstand, Matagorda Island of the back-island area (the width of the century (Shepard, 1960). shifted into a phase of progradation. The lagoon at stillstand) is amazingly constant, As the Gulf transgressed over sharp boundary between back-island and ranging between 8.8 and 10.3 km. In other valley-filling deltas in the area, conditions fore-island sands suggests that initial pro- words, when the sand shoal ceased migrat- changed from fresh to brackish water. gradation was abrupt and fairly rapid. By ing landward at stillstand, the Gulf There is no evidence that normal marine the time the island had prograded 250 m shoreline lay about 9 km from the mainland conditions existed in this area prior to the seaward, all but two tidal passes in the area shoreline. deposition of late Holocene sand which were closed. One of these was located on The top of the Pleistocene, as constructed makes up Matagorda Island. This indicates the south-central portion of the island. The from holes through the island and from that some form of barrier separating the other was located in the north-central por- mainland exposures, has been projected

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seaward to the shelf surface in a profile through Matagorda Island. This profile represents the surface of the mainland margin at lowstand and during the early Holocene transgression (Fig. 10A). If the modern shoreface profile (Fig. 10B) is superimposed on this mainland profile, assuming a constant sea level, the distance between the intersection of the shoreface profile with sea level at stillstand and the mainland shoreline is 8 km (Fig. 10C). This distance agrees well with that actually measured in this area. From this relationship, the following are suggested: 1. The position of Matagorda Island with respect to the mainland (the width of the lagoon excluding flood deltas and shell spits) is a function of the slope on the Pleis- tocene basement and the equilibrium profile of the shoreface. 2. The profile of the shoreface along the earliest formed part of Matagorda Island (the back-island sand) was similar to the present equilibrium profile. 3. Superposition of the equilibrium profile and the profile of the Pleistocene basement, with sea level constant, dictates the location of initial barrier development following stillstand. 4. During the late Holocene transgression, there was no great amount of erosion of Pleistocene sediments on the shelf surface. 5. At present, in water depths in excess of 16 m, Pleistocene sediments are probably exposed on the shelf in many places or are covered only by a thin veneer of Holocene sand and mud. The exception to this would be areas which were deeply dissected during periods of lower sea level.

SOURCE OF SAND

There has been much discussion in the literature about the origin of barrier islands since deBeaumont suggested that barrier islands form from upward building of offshore bars (1845). Most of the previously published ideas concerning barrier-island formation are rather simplistic, deal primarily with the origin of the original re- lief on the coastal surface, and have little bearing on the problem at hand. The question of sand source must be discussed considering the mode of deposition, fauna, and underlying units of this barrier complex. First, it is necessary to consider the two basic components of the barrier complex, namely back-island sands and fore-island sands, and the volumes of sediment involved, as well as a variety of possible source areas and their availability both prior to stillstand and since stillstand of the Gulf of Mexico. In general, sand can be made available for barrier building from Figure 9. Reconstructions of Matagorda Island during the late Holocene. A. Matagorda Island about 4,000 yr ago, soon after stabilization of the back-island sand shoal. B. Matagorda Island about two general areas. One area is offshore 3,000 yr ago, during the initial stages of progradation, exhibiting three active tidal passes. C. from the building barrier; the sand for that Matagorda Island about 2,000 yr ago, following progradation, but prior to the migration of Cedar area is provided by wave erosion and land- Bayou. ward transport of sediment (Johnson,

1919). The other source area of sand is lat- sea-level rise — only that sufficient sand early Gulf beaches exists only if eral to the area in question, with sand in- was available for supplying this portion of fluvial-deltaic sedimentation by these rivers troduced by longshore drift (Gilbert, 1885). the barrier complex. was equal to or exceeded the amount of Sand is made available to longshore cur- The possibility that sand was transported valley flooding taking place during the rents either through erosion of up-current westward by longshore drift from the area marine transgression. I know of no studies coastal areas or by rivers discharging their of eroding coastline during the Holocene conducted to date which would shed light bedload alongshore, somewhere up-drift. transgression is most difficult to evaluate. on this problem. Joseph H. McGowen (per- Before considering possible source areas At the present time, there are a few areas sonal commun.) has noted bay muds over- of sand, it is perhaps best to consider the along the Texas coast where Pleistocene lain by fluvial-deltaic sediments in an exca- island itself and volumes of sand involved. sediments are being extensively eroded vation in the Colorado-Brazos deltaic plain As discussed earlier, the nucleus of along the shoreline. One of these is an area near Surfside, Texas. The presence of such Matagorda Island is made up of two between Sabine Pass and Rollover Pass on muds would indicate that at stillstand, this physiographically and genetically distinct the eastern Texas coast; another is along valley was at least not completely filled. If sand bodies: back-island sands and fore- parts of Matagorda Peninsula. The extent an estuary did exist in the valleys of these island sands. A series of cross sections to which Pleistocene sediments were eroded rivers during the Holocene transgression, through Matagorda Island were drawn to during transgression along areas east of then a lateral source by river discharge is scale. Assuming that the shoreface at still- Matagorda Island is unknown. Nelson highly unlikely. stand was similar to the present shoreface and Bray (1970) and Frazier (1974) re- seen on the island, the amount of sand de- ported drowned Holocene sand barriers PROGRADATIONAL SOURCES posited prior to stillstand (back-island lying on the shelf near High Island and on sand) was calculated and compared to that the central Texas shelf. The presence of It is highly doubtful that sediments deposited during island progradation, since these sand bodies and the evidence suggest- offshore served as a source of sand once stillstand (fore-island sand). ing a migrating sand shoal off what is now Matagorda Island began to prograde Gulf- These cross-sectional areas were aver- Matagorda Island would indicate that, at ward. Figure 11 depicts a hypothetical case aged over an island length of 54 km. The least in places along the Texas coast, an in which island progradation is accom- results are as follows: total volume of sand incipient barrier system had developed on plished by erosion of offshore sands. It has making up Matagorda Island = 2.3 X 109 the shelf during the later stages of the been drawn to scale, the shoreface is that m3; total volume of sand deposited prior to Holocene transgression. seen on Matagorda Island today, and the stillstand = 1.2 X 109 m3; total volume of Holes through Matagorda Island show slope in the shelf is about .8 m per kilom- sand deposited since stillstand = 1.1 x 109 that the largest river valleys in the area had eter, approximately that of the modern m3. These figures point out several interest- been completely filled by the time the early shelf in the area. A number of problems ing facts. First, more than 2.3 sq km of sand barrier system transgressed in the area. Nel- arise if this model is applied to Matagorda was deposited in this area during Holocene son and Bray (1970) also indicate that Island. It requires that waves initially erode time. Secondly, of this volume, approxi- Sabine Pass had been filled with sediment at sand in the shoreface and later, with pro- mately one-half of the sand that makes up the time the early barrier formed in that gradation, deposit sand in the same area. Matagorda Island was in place prior to area. If the other major drainages cut into This is not a plausible situation if we can progradation. In other words, one-half of the Pleistocene surface had been filled, and assume that the amount of energy imparted the sand had been deposited in this area if an incipient barrier system had de- to the Gulf shoreface in this area has not before Matagorda Island became a well- veloped, then there is no reason why sand changed significantly in the past few thou- developed barrier. could not be delivered into this area by lat- sand years. Also, this model requires an eral erosion of Pleistocene sands and trans- initial slope that is less than that of the PRE-PROGRADATIONAL SOURCES ported by longshore drift. However, possi- shoreface being established. Holes through bility of transport is all that can be claimed. the island show that the slope on the Pleis- An offshore source for back-island sands It cannot be demonstrated at this time tocene surface was sufficiently steep, that seems highly likely in light of available data. whether this mechanism was operative or these sands were probably never subjected The presence of sand on the shelf and a whether significant quantities of sand to reworking once progradation was reasonable mechanism for transport land- reached this area during the Holocene initiated in the area. ward were both at hand. Ingleside strand- transgression. At the present time, and presumably dur- plain sands blanketing older Pleistocene Were the rivers draining the Texas ing much of the time since stillstand, sediments extended from several kilometers coastal plain during the Holocene trans- roughly 128 km of coastline, east of to the north and west of the present main- gression, contributing sand to early barrier Matagorda Island along Matagorda Penin- land coast, at least to the present Gulf systems along the Texas coast? With re- sula, is undergoing and has undergone ero- shoreline of Matagorda Island, and proba- spect to all Texas rivers except the Col- sion. Rough calculations indicate that only bly considerably farther Gulfward. Waves orado, Brazos, and the Rio Grande, about one-half as much sand could be de- were cutting a profile of equilibrium into fluvial-deltaic sedimentation was not able rived from this area as would be necessary the underlying Pleistocene and early to keep up with flooding by marine waters. to make up the progradational portion of Holocene sands as the sand shoal moved Even after thousands of years since still- Matagorda Island, to say nothing of St. across this area. The amount of downcut- sand, these rivers are depositing their bed- Joseph, Mustang, and parts of Padre Island ting into the shelf is not known, for lack of loads high on the coastal plain at the heads to the southwest. While erosion of shoreline data on the nature of modern shelf sedi- of their respective estuaries. There is little areas updrift is probably a valid mechanism ments in this area. Calculations show that doubt that these river valleys were drowned for delivering sand to the prograding in order to derive 1.2 X 109 m3 of sand (the during much of the Holocene transgression, Matagorda Island, it seems that this source volume that makes up the back-island por- precluding sediment contribution to early is only of minor importance, due to the rel- tion of Matagorda Island) from the Gulf Gulf beaches. atively small volumes of sediment available. shelf during transgression, it would be The Colorado and Brazos Rivers, on the Delivery of sand to a prograding necessary to remove only 1.2 m of the shelf other hand, carry significantly higher vol- Matagorda Island by river discharge into as sea level rose the last 13 m. This is not to umes of bedload than did other Texas the Gulf of Mexico east of the area is highly imply that 1.2 m is the proper figure for streams. The possibility that either or both probable with respect to the Colorado and scour, or that 13 m is the exact figure for of these rivers were able to deliver sand to Brazos Rivers. Other streams draining the

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PLEISTOCENE BASEMENT

B • SEA LEVEL -

-Modern shoreface

Figure 11. Hypothetical sections showing the progradation of a barrier complex by the erosion of offshore sediments. Regular pattern represents Figure 10. Sections showing the relationship between a profile on the sand deposited above the original shelf surface. Stippled pattern represents Pleistocene surface, the modern equilibrium profile of the Gulf shore- areas below the shelf surface where both erosion and deposition must take face, and the stable location of the back-island sand shoal at stillstand. place.

Texas coast are currently dumping their supply sufficient quantities of sand to sup- bly have been made available from this bedloads at the head of their estuaries. ply sediment to a prograding Matagorda Is- source. The quantity is insignificant when These rivers with a smaller bedload are con- land as well as to St. Joseph Island, Mus- compared to the volume of sand (2.3 x 109 tributing no sand at present to modern tang Island, and northern Padre Island to m3) deposited to form Matagorda Island. beaches, and surely have not since stillstand. the southwest. In conclusion, it would seem that prior to The Colorado and Brazos have filled their One additional sand source must be con- stillstand, while back-island sands were ac- estuaries and have been supplying sand to sidered in the case of Matagorda Island. As cumulating as a landward migrating sand longshore currents for some period of time. previously mentioned, the entire western shoal, an offshore sand source was the most The amount of sediment delivered by these side of Pass Cavallo is eroding toward the important. Following stillstand, when drainages depends on the rapidity at which west at a very rapid rate. One result of this Matagorda Island was prograding Gulf- sediments filled these river valleys during erosion is that sediment is currently being ward, sediment carried to the Gulf by the the Gulf transgression. Calculations by removed from the subaerially exposed In- Colorado and Brazos Rivers was the most McGowen (1973) indicate that these two gleside sands along Pass Cavallo and is important source of sand. During this time, rivers were carrying sufficient bedload to fill being carried south toward Matagorda Is- offshore sands were too deeply submerged their valleys during, or soon after, the latter land. Since the thickness and width of the to serve as an important sediment source. stages of the Holocene transgression and Ingleside sands in the area are known, a have been discharging sand and mud di- possible maximum volume of sand that ACKNOWLEDGMENTS rectly into the Gulf of Mexico for at least could have been removed since stillstand several thousand years. Further, several was calculated. The results show that no This paper is based on a dissertation 9 3 thousand years of such discharge would more than . 1 X 10 m of sand could possi- completed at The University of Texas under

the supervision of A. J. Scott. Portions of ern portion of the Gulf basin: Texas Univ. Econ. Paleontologists and Mineralogists this work were supported by a Penrose Be- Bur. Econ. Geology, 28 p. Spec. Pub. no. 15, p. 48-77. quest Research Grant from The Geological Gilbert, G. K., 1885, The topographic features of Shepard, F. P., 1953, Sedimentation rates in Society of America, Inc., and by a research lake shores: U.S. Geol. Survey, 5th Ann. Texas estuaries and lagoons: Am. Assoc. Rept., p. 69-123. Grant-in-Aid from the Sigma Xi Society. Petroleum Geologists Bull., v. 37, p. Johnson, D. W., 1919, Shore processes and 1919-1934. J. H. McGowen, L. F. Brown, W. L. Fisher, shoreline development: New York, John 1960, Gulf coast barriers, in Shepard, F. P., and R. J. LeBlanc read and improved the Wiley 8c Sons, Inc. Phleger, F. B., and van Andel, Tj. H., eds., text. McGowen, J. H., 1973, Modern Texas rivers; ' Recent sediments, northwest Gulf of Mex- Their contribution to shoreline stability: ico: Am. Assoc. Petroleum Geologists Pub., Texas Acad. Sci., March. p. 197-220. REFERENCES CITED McGowen, J. H., Proctor, C. V., Brown, L. F., Jr., Evans, T. J., Fisher, W. L., and Groat, deBeaumont, E., 1845, Leçons de geologie pra- C. G., 1975, Environmental geologic atlas tique: Paris, p. 223-252. of the Texas coastal zone, Port Lavaca area: Fisk, H. N., 1944, Geological investigation of the Texas Univ. Bur. Econ. Geology (in press). alluvial valley of the Mississippi River: Nelson, H. F., and Bray, E. E., 1970, Stratig- Vicksburg, Mississippi River Comm., 78 p. raphy and history of the Holocene sedi- MANUSCRIPT RECEIVED BY THE SOCIETY MAY 6, Frazier, D. E., 1974, Depositional episodes: ments in the Sabine-High Island area, Gulf 1974 Their relationship to the Quaternary of Mexico, in Morgan, J. P., ed., Deltaic REVISED MANUSCRIPT RECEIVED NOVEMBER 11, stratigraphie framework in the northwest- sedimentation, modern and ancient: Soc. 1974

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