Late Holocene history of the Central coast from Galveston Island to Pass Cavallo

BRUCE H. WILKINSON Department of Geology and Mineralogy, University of Michigan, Ann Arbor, Michigan 48109 ROBERT A. BASSE Department of Geology, Stanford University, Stanford, California 94305

ABSTRACT tidal passes. Unlike Follets Island and Matagorda Peninsula, Gal- veston and Matagorda Islands are progradational barriers which Mid-Holocene fades underlying Matagorda Peninsula and Gal- have been built gulfward over the past several thousand years. veston Island record a complex history of transgressive and regres- The climate of this region is humid subtropical. Surface winds sive fluvial, deltaic, and estuarine sedimentation along the central are primarily onshore from the south-southeast. Net longshore Texas coast. Data from 80 borings and jet-down holes reveal a dis- drift along this coastal segment is to the southwest. similar sequence of depositional events for northeastern and The drainage pattern of the central Texas coast is dominated by southwestern Matagorda Peninsula. Along northeastern Mat- two major rivers and several lesser streams. The largest, the Brazos agorda Peninsula and the area now occupied by the Colorado- River, has occupied and abandoned several meander belts within its Brazos delta plain, all major river valleys were flooded during the alluvial plain. It abandoned the Oyster Creek ~ 1,000 yr ago (Ber- Holocene transgression; gulfward, a barrier system developed and nard and others, 1970). The has undergone a simi- was driven landward by rising sea level. As the rate of sea-level rise lar westward shift. Prior to a few hundred years ago, the Colorado decreased, the Colorado and Brazos Rivers completely filled their River flowed within a broad alluvial valley now occupied by Caney common estuary and prograded across ancestral Matagorda and Creek (LeBlanc and Hodgson, 1959). This flow was captured by a West Bays. Deltaic progradation terminated after the landward- headward-eroding stream (McGowen and others, 1976b) and migrating barrier system overrode the delta front; at that time, sed- began discharging into the north end of . The delta iment was delivered directly to the . Shortly thereaf- of the modern Colorado River developed in a relatively short ter, beach and shoreface progradation began along barrier island period of time, having prograded across Matagorda Bay between systems lateral to the now rapidly eroding deltaic headland. This 1929 and 1935 (Wadsworth, 1966; Kanes, 1970). Unlike other riv- was accompanied by renewed deposition of estuarine mud in ers of the central Texas coast, the Colorado and Brazos Rivers have Matagorda and West Bays. drainage basins that extend northwest across the entire state, cross- Fades underlying southwestern Matagorda Peninsula indicate ing extensive exposures of Permo-Triassic red beds. As a result, Pass Cavallo was initially established over the axis of the Lavaca mud carried by these rivers has a distinctive reddish color. For most River valley. Southwestward migration of the pass accompanied by of the past 10,000 yr, the ancestral (Oyster Creek) landward migration of the barrier peninsula has resulted in a well- and the ancestral Colorado River (Caney Creek) occupied a com- preserved fades tract of bay, distal and proximal flood tidal delta, mon alluvial valley and discharged into a common estuary. and spit-related sediments now underlying this coastal segment. With the exception of the alluvial plain sediments of the Modern physiographic features, as well as rates of shoreline ero- Colorado-Brazos system, the central Texas coast consists of late sion and recession, are directly related to these middle and late Pleistocene deltaic sediments (Bernard and others, 1970; McGo- Holocene depositional events. wen and others, 1976a). These deposits were dissected, oxidized, and calichified during the Wisconsin lowstand of the Gulf of INTRODUCTION Mexico. The subsequent Holocene transgressive sequence that composes and underlies Matagorda Peninsula rests upon this The central Texas coast consists of several major physiographic weathered surface. Pleistocene sediments extend from the mainland provinces. The central part of the area consists of a fluvial-deltaic shoreline out under the modern gulf, where they are buried under a complex of sediments deposited by the Colorado and Brazos Rivers thin blanket of recent sediment. during the latter stages of the Holocene transgression. The Data on the thickness and distribution of facies underlying bar- shoreline along this area projects gulfward as a bulge (Fig. 1) and riers of the central Texas coast from Bernard and others (1970), experiences some of the highest erosion rates observed along the Wilkinson (1975), and the present study, permit a reconstruction of Texas coast. Two barrier peninsulas extend laterally from this the important depositional and erosional events that have given rise headland: Follets Island to the northeast and Matagorda Peninsula to the present physiography of this coastal segment. to the southwest. Both of these peninsulas are also actively eroding For the purpose of this discussion, the area has been divided into (McGowen and Brewton, 1975; Morton and Piper, 1975) and have two parts: the first extends from northwestern Matagorda Penin- been for the past several thousand years. Two barrier islands, Gal- sula to Galveston Island, and the second extends from southwest- veston and , are separated from the peninsulas by ern Matagorda Peninsula across Pass Cavallo to Matagorda Island.

Geological Society of America Bulletin, v. 89, p. 1592-1600, 9 figs., October 1978, Doc. no. 81013.

1592

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Deltaic Sand and Mud WH?;--- Fluvial-deltaic Sand and Mud 3 Marsh Mud

Strandplain Sand Barrier Island Sand Deltaic Sand Holes and Line of Section

Figure 1. Simplified geologic map of central Texas coast showing location of borings and jet-down holes and line of section. Modified from Brown and Fisher (1974), McGowen and others (1976a, 1976b), and Bernard and others (1970).

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NORTHEASTERN MATAGORDA PENINSULA rectly on the oxidized Pleistocene surface, is a gray, sparsely fos- siliferous estuarine mud. The contact with the underlying stiff Pleis- Matagorda Peninsula is a narrow, shell-rich barrier separating tocene mud is always sharp and well defined. Where not dissected the Gulf of Mexico from Matagorda Bay. It is an erosional barrier by valleys cut into the weathered surface, this contact gradually which has continued to recede landward over the past several rises from depths of 12 m northeast of Maverick Bayou to depths of thousand years. This retreat results from bayward transport of 4 m southwest of Brown Cedar Cut. This rise in elevation reflects barrier sediment during storms. The numerous hurricane surge the configuration of the Pleistocene surface prior to "Wisconsin low- channels that cross the peninsula normal to the gulf shoreline attest ering of sea level. The gray mud unit is largely confined to the major to the frequency of this catastrophic process. valley underlying the modern course of the Colorado River and to Fifty jet-down holes along the margin of the bay and thirty smaller valleys to the southwest. But, the gray estuarine mud did logs of borings, provided by the Corps of Engineers through fades not bury interdrainage highs during its deposition. The lack of even now underlying the barrier sands, provide a wealth of data on thin blankets of this sediment on these highs suggests they may Holocene depositional events in this area. have been subaerially exposed during deposition in lateral flooded valleys. Fades Distribution The gray mud fades is overlain by a homogeneous reddish- brown prodelta mud of the ancestral Colorado and Brazos Rivers. Northeastern Matagorda Peninsula from Greens Bayou to It is notably fine-grained, contains little sand, and is only sparsely Brown Cedar Cut is underlain by a laterally continuous sequence of fossiliferous. Where present, the fauna is typically estuarine. The Holocene fades (Fig. 2). The late Pleistocene surface in this area has contact with the lower gray mud is relatively sharp and distinct. been dissected by two major river valleys. The modern Colorado Like the Pleistocene-Holocene contact, the surface separating these River overlies a narrow valley that is in excess of 30 m deep. The two units rises from the southwest to the northeast. Under Greens maximum depth of this valley is unknown; a hole located south- Bayou, the contact is encountered at 18 m; under Brown Cedar west of the valley axis penetrated 30 m of Holocene estuarine mud. Cut, the contact is at 9 m. This fades forms a continuous blanket An unknown thickness of transgressive fluvial and deltaic sediment under most of Matagorda Peninsula and thickens to the southwest that underlies this mud was not encountered. where it interfingers with a flood tidal delta sequence. The other major river valley was incised by the ancestral It in turn is overlain by an upper gray estuarine mud lithologi- Colorado-Brazos fluvial system. This valley system is poorly cally similar to the lower gray mud unit. The contact with the un- defined. The northeastern hole in the line of section penetrated 24 derlying reddish-brown fades is less distinct; however, the color m of reddish-brown prodelta mud without encountering the Pleis- change occurs in less than one metre. The upper gray mud thins and tocene surface. The total thickness of Holocene sediment lying occurs at shallower depths from the southwest to the northeast. within this valley and the location of the valley axis is unknown. Under Greens Bayou, the unit is first encountered at a depth of 8 m Four Holocene facies overlie Pleistocene sediment along the and is 3.5 m thick; under Brown Cedar Cut, it is first encountered northeastern portion of the peninsula. The lowest, which lies di- at a depth of 2 m and is only 0.6 m thick. This fades is moderately

Wisconsin

Lavaca, etc. Valley-

Wisconsin Brazos Valley f=

Figure 2. Strike section along Matagorda Peninsula from Pass Cavallo to Brown Cedar Cut. Radiocarbon age determinations are: (1) 930 ± 60; (2) 4,350 ± 70; (3) 4,350 ± 80; (4) 5,300 ± 90; and (5) 6,670 ± 100 radiocarbon yr. See Figure 1 for location of the section.

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shelly and contains a normal estuarine fauna dominated by Cras- was somewhere gulfward of the line of borings. (2) Reddish-brown sostrea. The southwestward thickening of the upper gray mud prodelta mud delivered by the ancestral Colorado-Brazos fluvial fades and the underlying red mud fades appears to be a function of system was deposited in the study area between 5,000 and 4,000 yr the gradually increasing depth to the firm Pleistocene surface seen ago. Upon termination of deltaic sedimentation, estuarine condi- to the southwest. The thickness of mud units in this area appears to tions returned. (3) Reddish-brown Colorado-Brazos deltaic mud be primarily a function of paleo-water depth rather than proximity underlies southwestern Galveston Island and most of Matagorda to their source. ' Peninsula. Since most of Galveston Island is progradational and The upper gray estuarine mud is overlain by the shelly sand that prograded during constant sea level, the deltaic sediment probably makes up most of subaerial Matagorda Peninsula. It is a laterally predates stabilization of sea level. (4) The surfidal expression of continuous fades, which, like the underlying argillaceous units, abandoned distributaries on the Colorado-Brazos delta plain show thins to the northeast. In places, relatively thin discontinuous beds no lateral reworking (McGowen and others, 1976b), indicating of sandy mud, resulting from deposition in abandoned hurricane progradation into an environment that was protected from the at- surge channels, veneer the barrier sand. tack of Gulf waves. (5) The Holocene Colorado-Brazos deltaic plain extends out into the Gulf of Mexico as a broad bulge in the AGE Texas coastline. While this plain consists of Holocene sediment and must have been formed by progradation, erosion rates along most Five radiocarbon age determinations of shell fragments derived of this area are among the highest found along the Gulf coast (Mor- from these Holocene deposits agree well with published ages of ton and Piper, 1975). other Texas Holocene sediments. Two ages from the lower gray es- With these facts in mind, the following sequence of events is indi- tuarine mud are 6,670 ± 100 and 5,300 ± 90 radiocarbon yr. Two cated for this area. During Wisconsin glacial advances, sea level ages from the top of the reddish-brown prodelta mud are 4350 ± was lowered as much as 140 m (Fisk, 1944). During this time, the 70 and 4,350 ± 80 radiocarbon yr. A single determination from the ancestral Colorado and Brazos Rivers cut deep valleys into the ex- top of the upper gray estuarine mud is 930 ± 60 radiocarbon yr posed Pleistocene surface (see Fig. 3). In addition, these Pleistocene (Fig. 2). These ages reflect continuous estuarine and deltaic muds were highly oxidized and calichified. sedimentation in this area for at least the past 7,000 yr. About 900 yr ago, Matagorda Peninsula first migrated over the area occupied by the line of section. Additional temporal constraints on the history of sedimentation in this area can be inferred from data published by Bernard and others (1970) on the Brazos alluvial plain and Galveston Island. Galveston, which lies northeast of the Colorado-Brazos deltaic plain, is a barrier island that has prograded gulfward throughout much of its history. This progradation is reflected as prominent ridges and swales on the island's surface. Radiocarbon data for Galveston exhibits a high variation due to numerous shells re- worked from older sediment exposed on the continental shelf. Ber- nard and others (1970) fitted a series of isochrons to the barrier, indicating maximum ages for stages of progradation. Their data indicate that island progradation was initiated no more than 5,300 yr ago, although, perhaps, much more recently. Furthermore, since this progradation is reflected in a series of low ridges and swales, the entire progradational history of Galveston Island postdates stillstand of the Gulf of Mexico. The chronology of progradation of Galveston Island can be re- lated to Matagorda Peninsula since the reddish-brown deltaic mud that underlies Matagorda Peninsula also underlies the southwest- ern end of Galveston Island. These reddish deltaic sediments pre- date the stabilization of Galveston Island and predate the migration of Matagorda Peninsula to its present position. Age determinations of the deltaic mud beneath Galveston Island fall between 4,790 ±190 and 3,940 ±210 radiocarbon yr (Bernard and others, 1970), similar to ages from the same unit under Matagorda Peninsula.

HISTORY OF DEVELOPMENT Several temporal and spatial limitations must be honored when reconstructing the late Holocene history for this portion of the cen- Figure 3. Area of the modern Colorado-Brazos alluvial plain tral Texas coast. These may be summarized as follows: (1) The old- about 20,000 yr B.P. Sea level is at —75 m. The ancestral Colorado est Holocene sediment beneath Matagorda Peninsula is estuarine and Brazos Rivers have deeply eroded exposed Pleistocene units. gray mud deposited 5,000 to 7,000 yr ago in a protected lagoonal The "X" in this and following figures is a fixed reference point (the environment. No facies reflecting marine conditions are found be- present location of Brown Cedar Cut). Vertical relief greatly exag- neath or landward of the estuarine mud, suggesting that a barrier gerated.

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About 20,000 yr ago, sea level began to rise in response to wan- orado and Brazos Rivers began to prograde into the lagoon. By ing glaciation. The area now occupied by Matagorda Peninsula was 4,500 yr ago, with sea level at —3 m, reddish-brown deltaic mud flooded at least 7,000 yr ago as waters moved into and up valleys carried by the ancestral Brazos River was being deposited in the cut during glacial maxima. The oldest Holocene sediment pene- area now occupied by Matagorda Peninsula, Follets Island, and trated in these valleys is estuarine mud rather than facies indicating Galveston Island (Fig. 5). The barrier system continued to migrate normal marine salinity. Frazier (1974) reported beach sand on the landward by storm overwash in response to rising sea level. The Texas shelf at depths between 14 and 18 m which was deposited at rate of landward barrier migration decreased along the front of the this time. By at least 7,000 yr ago, a barrier system had formed on prograding deltaic system as the barrier was pushed onto the the Texas shelf, and it protected this area from the influence of sedimentary platform. Laterally, ancestral Follets Island, Galveston marine waters. Island, and Matagorda Peninsula continued to migrate landward. Figure 4 is a reconstruction of this segment of the Texas coast Dominant southeasterly winds pushed turbid river waters to the about 6,000 yr ago. At this time, sea level was at —6 m and rising. north and west, depositing an even blanket of reddish mud on the The valleys cut by the ancestral Colorado and Brazos Rivers and lagoon floor. the drainage valley presently occupied by the Colorado River were Deposition of reddish-brown sediment continued in this area flooded. Deltas associated with the Colorado and Brazos systems until about 3,900 yr ago; at that time, the gulfward-prograding had retrograded far to the north in response to rapidly rising base delta and the landward-migrating barrier met. Matagorda Penin- level, and, at this time, they occupied separate estuaries. A barrier sula overrode the front of the Colorado-Brazos deltaic complex, system protected the area from the attack of Gulf storms, but it and sediment carried by this system was discharged directly into the migrated landward in response to rising sea level. Matagorda Bay Gulf of Mexico. Conditions in the lagoon reverted to estuarine be- and were one continuous system in which the lower gray cause deltaic sediment and fresh river waters were now delivered to estuarine mud was being deposited. the Gulf. Deposition of the upper gray estuarine mud began at this Approximately 5,000 yr ago, the rate of sea-level rise began to time. decrease significantly (Curray, 1960; Nelson and Bray, 1970; Figure 6 depicts this area —3,000 yr ago. By this time, the Gulf of Frazier, 1974), and rates of sedimentation exceeded rates of inun- Mexico was very close to its present level. A portion of the dation. Even though sea level was still rising, the ancestral Col- landward-migrating barrier stabilized northeast of the deltaic

Figure 4. The central Texas coast —6,000 yr ago. Sea level is at Figure 5. The central Texas coast about 4,500 yr. B.P. Com- —6 m and rising. Deltas of ancestral Colorado and Brazos Rivers bined deltas of the ancestral Colorado and Brazos Rivers have filled have retrograded inland in response to rising base level. A barrier their estuaries and have begun prograding across Matagorda Bay. system has developed offshore, forming an estuary. The lower gray The barrier system continues to migrate landward while reddish- mud is being deposited in ancestral Matagorda Bay. brown prodeltaic muds blanket the estuary floor.

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complex as the nucleus of Galveston Island. Rapid erosion began Matagorda Bay. The growth of the modern Colorado River delta along the front of the deltaic complex, which was exposed to the across Matagorda Bay took place between 1929 and 1935, follow- attack of Gulf waves. To the southwest of the deltaic complex, ing the removal of a log jam on the lower reach of the river (Fig. 7). Matagorda Peninsula continued to migrate landward in response to The amount of erosion that has taken place along the front of the frequent washover during storms. In the area now occupied by delta plain over the past 4,000 yr is difficult to determine accurately Matagorda Peninsula, deposition of the upper gray mud continued. with the available data. Caney Creek exhibits a broad meander belt During the period from 3000 to 1000 yr B.P., conditions along extending nearly to the modern beach. LeBlanc and Hodgson this area were relatively stable. Galveston Island experienced a (1959) stated that this channel probably extended "quite some dis- period of progradation, especially along its northeastern end, tance" gulfward while it was active. Morton and Piper (1975), ex- which gave rise to numerous ridges and swales on the island's sub- amining historic shoreline changes along this segment (1852-1974), aerial surface. The front of the exposed Colorado-Brazos deltaic found that the entire area was in a state of erosion except for the complex experienced rapid rates of erosion. This recession was area around the mouth of the Brazos River which underwent probably a result of several processes, including subsidence from short-term periods of accretion and erosion. Their data show, how- compaction of underlying mud, refraction of wave energy onto the ever, that the 70-km coastal segment along the front of the projecting deltaic headland, and possibly a decrease in the volume Colorado-Brazos delta plain has eroded at an average rate of 1.6 of sediment supplied to the Gulf shore through climatic changes m/yr for the past 122 yr. If this rate is at all representative of past and decreased discharge. Matagorda Peninsula continued to mi- erosion rates, then —6.5 km of recession has taken place along this grate landward during this time interval. Barrier sands were not part of the Texas coast in the past 4,000 yr. deposited in the area of the line of section prior to about 900 yr ago. SOUTHWEST MATAGORDA PENINSULA Approximately 1,000 yr ago, the Brazos River shifted its course to the west. The Oyster Creek course was largely abandoned at this The southwestern part of Matagorda Peninsula is underlain by time. Several hundred years ago, the lower part of the ancestral only one major river valley cut into underlying Pleistocene sedi- Colorado River, Caney Creek, was captured by a headward-eroded ment. This valley was formed by the drainage from the confluence stream. After its capture, the Colorado began discharging into of the principal systems draining into northwestern Matagorda Bay: the Lavaca-, Carancahua Creek, and Tres

Figure 6. The Colorado-Brazos area —3,000 yr ago. Fluvial dis- charge is directly into the Gulf of Mexico while gray estuarine mud Figure 7. The central Texas coast as it appears today. Rapid is being deposited in Matagorda and West Bays. Rapid erosion oc- erosion continues between Brown Cedar Cut and southwestern curs along the bulge in the shoreline which, in this figure, is greatly Galveston Island. Matagorda Peninsula continues to migrate land- exaggerated. ward.

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Placios Creek. The fact that only one valley was penetrated under migrate by erosion along their southwestern margin and deposition Matagorda Peninsula between Pass Cavallo and the modern Col- along their northeastern margin until they attain equilibrium. This orado River indicates that these three systems had joined to form migration is primarily in response to strong winds periodically one major channel by the time they reached the present position of blowing directly out of the north during winter months. A south- Matagorda Peninsula. The depth of this valley is unknown. The erly position for tidal passes most effectively drains water pushed deepest penetration (26.5 m) bottomed in gray estuarine mud. Un- south during the passage of these cold fronts. Pass Cavallo is ac- derlying transgressive fluvial-deltaic sediments were not encoun- tively migrating toward the southwest. Erosion rates along the tered. Projection of the three major tributary drainage gradients southwestern margin of Pass Cavallo are presently about 5 m/yr. southeastward suggest that the valley axis is —45 m deep along the Evidently this migration will continue until Pass Cavallo is located line of section. directly south of Matagorda Bay. The Holocene sequence underlying the peninsula between The northeastern extent of proximal tidal delta sediment along Greens Bayou and Matagorda Island (Fig. 2) is dominated by two Matagorda Peninsula indicates the approximate position of the fades not encountered to the northeast. These are assoaated with southwestern margin of Matagorda Peninsula when Pass Cavallo the deposition of sediment along the margins and landward of Pass first developed in this barrier chain. It is interesting to note that this Cavallo. The two fades are: distal flood-tidal delta muddy sand position is almost directly over the axis of the old val- and sandy mud deposited in Matagorda Bay, and shelly fine sand ley cut into underlying Pleistocene sediment. Morton and deposited on the proximal flood-tidal delta and on the southwest- Donaldson (1973) observed a similar relationship between tidal ern end of Matagorda Peninsula. Both of these fades were depo- passes and Pleistocene valleys on the Virginia coast. There, as well sited as Matagorda Peninsula migrated landward and Pass Cavallo as along ancestral Matagorda Peninsula, tidal passes developed migrated to the southwest. The synchronous operation of both of over older river valleys while barrier island segments developed on these processes has resulted in a complex association of facies. The interdrainage divides. Although no data are available on the mor- development of the relationship between tidal delta, estuarine, and phology of this peninsula prior to several thousand years ago, it is barrier island sediment is depicted in Figure 8. Here, a system of conceivable that this tidal pass was located over the Lavaca valley barrier islands and a tidal pass is migrating landward and during much of the Holocene transgression. Migration of this pass downdrift. In so doing, tidal delta sediments on the eastern margin to the southwest may be a process that has become important only of the lobe are progressively covered by bay mud as the pass mi- since the level of the Gulf of Mexico stabilized. grates to the southwest. Barrier sand is driven landward and is de- posited on bay mud at the same time. As a result, in the northeast- CONCLUSIONS ern hole (1) barrier sand overlies bay mud (Phillips Bayou in Fig. 2); in the central hole (2) barrier sand overlies bay mud which in turn Figure 9 summarizes the sequence of depositional events re- overlies tidal delta muddy sand (Greens Bayou in Fig. 2); and in the corded in fades underlying the barrier islands and peninsulas of the southwestern hole (3) shelly sand makes up most of the sequence central Texas coast. As sea level rose past —30 m during the (Ship Channel in Fig. 2). The entire tidal delta sequence thickens Holocene transgression, valleys that were cut into the underlying and becomes coarser grained to the southwest due to the increased Pleistocene deltaic sediment were gradually flooded by brackish es- depth of scour and higher energy conditions of the axis of the tidal tuarine water. Transgressive fluvial and deltaic fades, which fill the channel. ancestral Colorado, Brazos, and Lavaca drainages, have not been Price (1947) has shown that the equilibrium location for tidal encountered due to their great depth along the modern shoreline. passes on the Texas coast is at the southernmost position possible These transgressive deposits are probably similar to those reported with respect to the bays that they connect to the Gulf of Mexico. by Wilkinson and Byme (1977) underlying , located Passes that initially developed north of this equilibrium point will higher on the Texas coastal plain.

SW NE

Bay Mud

Figure 8. The migration of a tidal pass landward and downdrift. The vertical sequence is (1) barrier sand over bay mud; ^—er (2) barrier sand over bay mud over tidal Tz delta muddy sand; (3) shelly sand deposited ... I v. on the northeastern margin of the pass.

Drift

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o „[TRANSGRESSIVE] ~a u o FLUVIAL-DELTAIC SAND] k_ a CD O a> « O O Q£ O Rate of Sedimentation Exceeds O Rate of Flooding-Progradation Begins * C [GRAY ESTUARINE MUD] O Figure 9. Sequence of late Holocene o Delta Fills Bay-head Estuary-Begins depositional events on the central Texas "D Progradation Across Matagorda Bay D coast. A general transgressive-regressive se- a: a> O) quence was terminated when Matagorda CÛ [RED PRODELTAIC MUD] Û O Peninsula overrode the Colorado-Brazos Landward-migrating Matagorda Peninsula delta about 4,000 yr ago. Sea level curve CQO£ Overrides Delta Front-Progradation Ceases modified from Curray (1960), Nelson and < Bray (1970), and Frazer (1974). a> _c

O) c 0) [GRAY ESTUARINE MUD] o c

o _c to

Landward Margin of Matagorda Peninsula Overrides Position of Section Line 3 o [BARRIER ISLAND SAND] •Q .

I —r- I I VO o »O o IO o CM CM ro

SEA LEVEL (m)

The oldest sediment encountered along this area is estuarine mud Deltaic discharge into the Matagorda-West Bay system was at a depth of 30 m under southwestern Matagorda Peninsula. At abruptly terminated about 4,000 yr ago as landward-migrating an- this time, approximately 10,000 yr ago, estuarine conditions cestral Matagorda Peninsula and Follets Island overrode dis- existed in the deeper river valleys while a barrier island system had tributaries of the Colorado-Brazos system. From this time on, sev- developed on what is now the Texas shelf and had migrated land- eral new processes became important in the development of the ward in response to rising sea level. Sometime between 6,000 and central Texas coast. 10,000 yr ago, as rates of sea-level rise declined, deltaic sedimenta- Gray mud was once again deposited in Matagorda Bay and West tion rates of the ancestral Colorado and Brazos Rivers exceeded Bay, and it continues to be deposited at present. The direct dis- rates of flooding by estuarine waters, and the system began pro- charge of the Colorado-Brazos system into the Gulf of Mexico also grading gulfward, filling its estuary. The exact time that prograda- provided an abundant supply of sand-sized material to littoral cur- tion began cannot be determined from the available data. While rents. At this time, barrier segments downdrifting from the deltaic this change is shown at 6,000 yr on Figure 9, the presence of red headland began prograding gulfward. This progradation is dis- prodelta mud at depths of 24 m in the Brazos River valley indicates played as ridges and swales on Matagorda Island, St. Joseph Island, this transition may have taken place as early as 8,000 to 9,000 yr and Mustang Island to the southwest. In addition, with bending of ago. By 5,000 yr ago, this delta had prograded southward beyond the Matagorda Peninsula—Follets Island system around the its valley walls. Discharge of sediment was into the northern end of Colorado-Brazos deltaic complex and with migration of the barrier the Matagorda-West Bay system. Currents generated by southeast- over the delta front, Gulf waves began to rapidly erode the deltaic erly winds carried much of this sediment west and north, depositing complex. a blanket of reddish clay over older gray estuarine mud. Red deltaic Over the past 4,000 yr, these processes have continued. Galves- mud has been encountered under Lavaca Bay, 60 km west of the ton Island has prograded gulfward more than 3 km at its northeast- modern Brazos River mouth. ern end and lesser amounts closer to the deltaic headland. The

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headland itself has undergone very rapid erosion except in the im- Fisk, H. N., 1944, Geological investigation of the alluvial valley of the Mis- mediate area of the Brazos River mouth. A small wave-dominated sissippi River: Vicksburg, Mississippi River Commission, 78 p. Frazier, D. E., 1974, Depositional episodes — Their relationship to the delta has formed there. Matagorda Peninsula has continued to mi- Quaternary stratigraphic framework in the northwestern portion of grate landward, primarily during times of hurricane surge with the Gulf basin: University of Texas Bureau of Economic Geology landward transport of sediment as washover lobes. This landward Geological Circular 74-1, 28 p. migration has been accompanied by a shift of Pass Cavallo from its Kanes, W. H., 1970, Fades and development of the Colorado River delta in initial position over the Lavaca River valley southwest to its present Texas, in Morgan, J. P., and Shaver, R. H., eds., Deltaic sedimenta- tion, modern and ancient: Society of Economic Paleontologists and location. Mineralogists Special Publication No. 15, p. 78—106. The River also flows across a broad Holocene deltaic LeBlanc, R. J., Sr., and Hodgson, W. D., 1959, Origin and development of plain in south Texas. Like the Colorado-Brazos system, this is an the Texas shoreline: Gulf Coast Association of Geological Societies Transactions, v. 9, p. 197-220. area of shoreline erosion, and it projects gulfward, forming an even McGowen, J. H., and Brewton, J. L., 1975, Historical changes and related more prominent bulge in the Texas-Mexico coast. Although little coastal processes, Gulf and mainland shorelines, Matagorda Bay area, data are available on the thickness, distribution, and age of fades Texas: University of Texas Bureau of Economic Geology, 72 p. which comprise this system, a late Holocene history similar to that McGowen, J. H., Proctor, C. V., Jr., Brown, L. F., Jr., Evans, T. J., Fisher, of the central Texas coast seems likely. W. L., and Groat, C. G., 1976a, Environmental geologic atlas of the Texas coastal zone — Port Lavaca area: University of Texas Bureau of Economic Geology, 107 p. ACKNOWLEDGMENTS McGowen, J. H., Brown, L. F., Jr., Evans, T. J., Fisher, W. L., and Groat, C. G., 1976b, Environmental geologic atlas of the Texas coastal We would especially like to acknowledge Joseph H. McGowen zone — Bay City—Freeport area: University of Texas Bureau of Economic Geology, 98 p. and A. J. Scott, from the University of Texas, without their encour- Morton, R. A., and Donaldson, A. C., 1973, Sediment distribution and agement and interest in studies of the Texas coast over the past evolution of tidal deltas along a tide — dominated shoreline. Washap- years this project would have never been initiated. Thanks are due reague, Virginia: Sedimentary Geology, v. 10, p. 285-299. to the Galveston Office of the Corps of Engineers for logs of many Morton, R. A., and Piper, M. J., 1975, Shoreline changes in the vidnity of borings through southwestern Matagorda Peninsula. Rufus Le- the Brazos River delta ( to Brown Cedar Cut) — An analysis of historical changes of the Texas Gulf shoreline: University Blanc of Shell Oil Company, Henry Chafetz of the University of of Texas Bureau of Economic Geology Geological Circular 75-4,47 p. Houston, and Joseph McGowen read the manuscript and made Nelson, H. F., and Bray, E. E., 1970, Stratigraphy and history of the helpful suggestions. James Struhsaker aided with all aspects of the Holocene sediments in the Sabine-High Island area, Gulf of Mexico, field work. This study was supported by a Faculty Research Grant in Morgan, J. P., and Shaver, R. H., eds., Deltaic sedimentation, mod- from the Horace H. Rackham School of Graduate Studies at the ern and ancient: Society of Economic Paleontologists and Mineralogists Speaal Publication No. 15, p. 48-77. University of Michigan. Price, W. A., 1947, Equilibrium of form and forces in tidal basins of coasts of Texas and Louisiana: American Association of Petroleum REFERENCES CITED Geologists Bulletin, v. 31, p. 1619-1663. Wadsworth, A. H., Jr., 1966, Historical delineation of the Colorado River, Bernard, H. A., Major, C. F., Jr., Parrott, B. S., and LeBlanc, R. J., Sr., Texas, in Deltas in their geological framework: Houston, Texas, 1970, Recent sediments of southeast Texas, in A field guide to the Houston Geological Soaety, p. 99-105. Brazos alluvial and deltaic plains and the Galveston Barrier Island Wilkinson, B. H., 1975, Matagorda Island Texas: The evolution of a Gulf complex: University of Texas Bureau of Economic Geology coast barrier complex: Geological Society of America Bulletin, v. 86, Guidebook 11, p. 1-16. p. 959-967. Brown, L. F., Jr., and Fisher, W. L., 1974, Environmental geologic atlas of Wilkinson, B. H., and Byrne, J. R., 1977, Lavaca Bay — Transgressive del- the Texas coastal zone, in Wermund, E. G., ed., Approaches to en- taic sedimentation in a central Texas estuary: American Association vironmental geology: University of Texas Bureau of Economic Geol- of Petroleum Geologists Bulletin, v. 61, p. 527-545. ogy Report of Investigations 81, p. 25-51. Curray, J. R., 1960, Sediments and history of Holocene transgression, con- tinental shelf, northwest Gulf of Mexico, in Shepard, F. P., Phleger, F. B., and van Andel, Tj. H., eds., Recent sediments, northwest Gulf of MANUSCRIPT RECEIVED BY THE SOCIETY FEBRUARY 18, 1977 Mexico: Tulsa, Oklahoma, America Association of Petroleum REVISED MANUSCRIPT RECEIVED JUNE 6, 1977 Geologists, p. 221-266. MANUSCRIPT ACCEPTED OCTOBER 12, 1977

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