A Sedimentologic Description of a Microtidal, Flood-Tidal Delta, San Luis Pass, Texas

ALAN M. ISRAEL AND FRANK G. ETHRIDGE
Department of Earth Resources
Colorado State University
Fort Collins, Colorado 80523

AND

ERNEST L. ESTES
Department of Marine Sciences
Texas A & M University at Galveston
Galveston, Texas 77553

ABSTRACT: San Luis Pass is a microtidal tidal inlet located at the southwest end of Galveston Island, Texas. Continuous cores taken with a portable vibracoring rig, and surface grab samples provide data for constructing vertical sequences and three-dimensional sedimentologic descriptions of the flood-tidal delta complex. These descriptions are based on the type and vertical and lateral distribution of lithofacies, sedimentary structures, textures, and trace fossils.

A vertical sequence through the flood-tidal-delta complex consists, from base to top, of: highly bioturbated bay muds and associated oyster reefs; highly bioturbated clayey sand/sandy clay of the tidal delta margin; burrowed sand and shelly sand of the tidal delta; and rooted or burrowed mud of the marsh or mud flat. Washover shell hash deposits may occur at random intervals throughout this sequence. A vertical sequence in the vicinity of San Luis Pass consists of a basal tidal inlet deposit of graded layers of sand and shell overlain by burrowed to shelly sand of the barrier spit. This description of a microtidal, flood-tidal delta differs significantly from descriptions presented for mesotidal, flood-tidal-delta systems in the 1) general lack of large-scale, high-angle sedimentary structures; 2) presence of intense bioturbation; 3) presence of washover deposits; and 4) general coarsening-upward nature of the vertical sequence.

INTRODUCTION

Stratigraphic models have been developed for most of the major depositional systems of the Texas Gulf Coast (Bernard et al. 1970; Fisher et al. 1972; and Morton and McGowen 1980). Two notable exceptions are the ebb and flood-tidal deltas associated with tidal inlets. This gap in our knowledge of microtidal coastal depositional environments is partially filled by the presentation in this paper of vertical sequence and three-dimensional sedimentologic descriptions of a microtidal (tidal range <2.0m) flood-tidal delta. These descriptions permit recognition of deposits in the rock record that were formed by similar processes and provides contrasts with descriptions of mesotidal (tidal range, 2.0 to 4.0 m) flood-tidal deltas (Hubbard and Barwis 1979).

LOCATION

San Luis Pass is a microtidal inlet which connects the Gulf of Mexico with Bastrop, Christmas, and West bays and is located at the southwest end of West Bay in Galveston and Brazoria counties, Texas (Fig. 1). The inlet separates Galveston Island from Follets and San Luis islands.

The study area, which includes San Luis Pass and its associated flood-tidal delta, is approximately 80 km south of Houston, Texas and 32 km southwest of the city of Galveston, Texas. The flood-tidal delta includes Mud, Bird, and Moody's islands, associated channels, and various submerged shoals and channels east of these islands and west of Galveston Island (Fig. 1). The study area is bounded on the west by Christmas and Bastrop bays, on the north and east by West Bay and Galveston Island, and on the south by the Gulf Mexico. San Luis Pass was chosen for this study because it is the only major inlet on the Texas Coast which is 1) not stabilized by jetties, 2) accessible by road, and 3) close to support facilities at Texas A&M University at Galveston.

PREVIOUS RESEARCH AND METHODOLOGY

PREVIOUS RESEARCH

Three previous studies which have particular relevance to this study are those by Barwis and Hayes (1978), Hub- bard et al. (1979), and Hodge (1982). Based on available data from modern and inferred ancient flood-tidal deposits, Barwis and Hayes (1978) reviewed the general characteristics of flood-tidal-delta deposits. The study by Hubbard et al. (1979) involved a detailed investigation of waves and tidal currents in the origin of tidal-inlet sedimentary structures and sand-body geometry along the east coast of the United States. They predicted, based on detail studies of three-foot-deep surface trenches, that well-preserved sedimentary structures are not common in wave-dominated flood-tidal-delta deposits and that the tabular geometry of the flood-tidal-delta and its associated washover deposits was a characteristic feature that could be used to identify ancient tidal-inlet-tidal-de1ta deposits. Our work with continuous cores at San Luis Pass generally SUPPORTS THE PREDICTIONS MADE IN BOTH STUDIES. Hodge (1982), reported that flood-tidal deposits such as those at the mouth of the dammed Santee River in South Carolina might make good petroleum reservoirs given sufficient burial.

The great majority of studies of tidal inlets and their associated tidal deltas have been conducted along the mesotidal portions of the east coast of the United States. The studies have been devoted, in large part, to developing a better understanding of processes, bedforms, and morphology. Hypothetical vertical sequences for flood-tidal delta deposits have been developed based on studies of surface bedforms, shallow trenches, and box cores and 0 2. 50 MI.iS on the application of Walther's Law (Kumar and Sanders 1974; Hayes 1975; Hubbard and Barwis 1977; and Hayes1979). Most previous studies of tidal inlets and tidal deltas along the Texas Coast have concentrated on their origin, recent geologic history, processes, and sand resources (Fisher et al. 1972; Morton 1977; Williams et al.1979). Little research has been conducted on the vertical and lateral sequence of structures and textures and on the development of idealized sequences for recognition of ancient microtidal flood-tidal-delta deposits. Exceptions include Bernard et al. (1970), who concluded, on the basis of information obtained from a core in the spit at the southwest end of Galveston Island, that the tidal-inlet sequence at San Luis Pass consisted of graded layers of shell and sand and that these deposits were probably cross-bedded. Munson (1975) studied the distribution and geometry of Holocene environments at Harbor Island, Texas but did not present a vertical sequence of sedimentary structures for flood-tidal deltas. Morton and McGowen (1980) state that the typical sedimentary structures found in flood-tidal-delta deposits along the Texas Coast include trough cross-beds, graded beds, and shell layers. The few descriptions of microtidal flood-tidal-delta deposits along the Texas Coast (Piety 1972; Fisher et al.1972; Morton and McGowen 1980) are based on studies of surface bedforms, washdown holes, and eight continuous cores (in the Packery channel area-northern Padre/southern Mustang Island). More recently Boothroyd et al. (1985) and Berelson and Heron (1985) discuss the geology of micro-tidal coastal lagoons--flood-tidal deltas along the Rhode Island and North Carolina coasts, respectively. Cores penetrating flood-tidal delta/lagoonal deposits along the Rhode Island coast reveal a general coarsening-upward sequence (Boothroyd et al. 1985) similar to that observed in the San Luis Pass flood-tidal delta. However, no detailed sedimentologic characteristics and no stratigraphic sequence are presented for microtidal flood-tidal delta deposits in this general review paper which emphasizes sedimentary processes and morphology. Cores penetrating Holocene sediments beneath a lagoonal marsh behind the Shackleford Banks barrier-island system, North Carolina, reveal two upward-fining sequences consisting of a lower, coarse-grained sand zone, a middle, laminated zone, and an upper, muddy zone. These deposits are inferred to represent two relic flood-tidal-delta deposits (Berelson and Heron 1985). Data presented in Boothroyd et al. (1985) and this study describe coarsening-upward sequences in Rhode Island and Texas flood-tidal delta deposits.

METHODOLOGY

Field work for this study was carried out over a period of two field seasons with the primary goal being the collection of core and surface sediment samples. Forty-five continuous cores up to 6.4 m in length and 7.6 cm in diameter were obtained (Fig. 2) using a vibracoring apparatus modified from one described by Lensky et al. (1979). In addition to the vibracores, 21 short cores up to 0.6 m in length and 0.8 cm in diameter and 17 longer cores up to 1.8 m long and 0.8 cm in diameter were obtained by forcing hollow plastic tubes into the unconsolidated sediment. Two cores, located seaward of the tidal inlet (COE #25 and COE #28), were obtained from the U.S. Army Corps of Engineers (Williams et al. 1979). One hundred eighty-two grab samples were also collected - from the surface of the tidal-inlet-tidal-delta deposits. Sieve analyses were performed on these grab samples and on an additional two hundred thirty-three samples obtained from the continuous cores in order to assess lateral and vertical textural variations.

All cores were transported to the laboratory and split lengthwise. One half of each core was photographed, described, and archived, and the other half was sampled in textural and compositional analyses

MICROTlDAL VERSUS MESOTlDAL
FLOOD-TlDAL DELTAS

Hayes (1979) has proposed plan-view and vertical-sequence models for mesotidal flood-tidal deltas based on studies of barrier islands along the Massachusetts and South Carolina coastlines. Figure 3 is a plan-view model for a mesotidal flood-tidal delta illustrating the five geomorphic units that are common to this depositional system. Time velocity asymmetry of tidal currents creates a vertical and horizontal segregation of ebb and flood-tidal currents resulting in five distinct geomorphic units including 1) a seaward-facing slope on the sand body over which the main force of the flood current is directed, resulting in flood-oriented sand waves; 2) flood channels which are dominated by flood-tidal currents and. whi.ch bifurcate off the flood ramp; 3) a topographically high nm or margin on the landward side (termed the ebb shield); 4) ebb spits on the seaward side formed by the ebb-tidal currents; and 5) lobate bodies of sediment (termed spillover lobes) formed by unidirectional currents.

Similar geomorphologic units have been observed on the principal flood-tidal delta and on a small deposit north of the spit at the southwest end of Galveston Island (Fig 4). This smaller deposit probably resulted from wind-driven currents that breached the spit during the passage of hurricane Alicia in August 1983. Units such as the flood ramp (1), ebb shield (3), and ebb spits (4) are clearly visible on both deposits. Although the various geomorphic units are easily seen from the air, they are difficult to pinpoint on the surface and impossible to recognize in core. They are, therefore, not discussed further. Instead, the characteristics and vertical and lateral relationships of various lithofacies and despositional environments recognized on the surface and in core are discussed, and an idea vertical sequence and block diagram for a mesotidal flood-tidal delta is presented.

LITHOFACIES

Six lithofacies are recognized in cores from the San Louis Pass flood-tidal delta and tidal inlet. These lithofacies include 1) highly oxidized red clay; 2) oxidized red sand clay; 3) medium-dark grey to dark grey clay; 4) sand to shelly sand; 5) sandy clay to clayey sand; and 6) shell to shell-hash gravel.

HIGHLY OXIDIZED RED CLAY

Lithofacies 1, which underlies most of the study area, consists of stiff, reddish brown, highly calcareous clay with sparse organic debris. In cores on the bayward side of Bird Island the lithofacies was encountered at depths ranging from 2.1 to 6.1 m below sea level (fig. 5A, top half). Clay of this lithofacies is generally structureless, contrains zones of moderate to weak bioturbation, and has little or no sand. Burrows are mainly sand filled. The lithofacies generally has a gradational contact with lithofacies 2 or 3. Morton and McGowen (1980) considered this type of lithofacies to be the deposit of an earlier lobe of the Brazos River delta.

OXIDIZED RED, SANDY CLAY

Lithofacies 2 is a structureless, reddish brown, oxidized, sandy clay (Fig. SA, bottom half) containing wood fragments, rare clay, or sand interbeds and shell fragments. Burrows are usually sand filled or, rarely, clay filled or clay lined. This unit was probably also deposited as part of an earlier lobe of the Brazos River delta (Morton and McGowen 1980) and tends to have sharp upper contacts with lithofacies 3, 4, or 5. The lower contact is generally gradational with lithofacies 1.

Medium Dark Gray to Dark Gray Clay

Lithofacies 3 is a structureless or extensively bioturbated and mottled medium-dark gray to dark gray (Fig. 5B). Distinct burrow structures are rare but, when present, are generally sand filled. Lower contacts lithofacies 1 or 2 and upper contacts with lithofacies 5 range from sharp to gradational.

Sand to Shelly Sand

Lithofacies 4 is variable and consists of very finegrained, subrounded to rounded quartzose sand with varyable amounts of whole shells or shell fragments (Fig. 5C). The degree of bioturbation ranges from weak to strong. Distinct burrow structures are commonly clay lined (Fig. 5D), filled with sand, silt, or clay, and may be stained by iron oxide cement. Organic debris and wood fragments are scattered throughout the unit. Primary sedimentary structures include swach laminae (Fig. 5E), ripples, small-scale trough cross-stratification (Fig. 5F), and rare small-scale, high-angle, planar cross-stratification. Upper contacts with lithofacies5 are usually sharp or rarely gradational.

Sandy Clay to Clayey Sand

Lithofacies 5 is also quite variable and consists of dark gray to olive-gray sandy clay to clayey sand, which may be interlaminated with clay (Fig. 5G). burrows are both horizontal and vertical and are either sand (fig. 5H) or clay filled and may be clay lined. In some instances the degree of bioturbation is so intense that the lithofacies is totally mottled (Fig. 5I). Iron oxide stained root traces (Fig 5J), plant fragments, root fragments, and heavy mineral concentrations may be present locally. Rare primary sedimentary structures may include horizontal to low-angle cross-laminae. Upper contacts with lithofacies 4 are sharp and erosive, whereas lower contacts with lithofacies 2,3 or 4 vary from gradational to sharp.

Shell or Shell Hash Gravel

Lithofacies 6 is composed of both intact and fragmented oyster shells in a very fine-grained or sandy clay matrix (Fig. 5K) and fragmented shell-hash deposits (Fig. 5L, top third). The lithofacies is generally structureless but locally contains horizontal beds of bidirectional cross-beds. Upper and lower contacts with lithofacies 3,4 or 5, within which the unit is incorporated, are sharp and lower contacts are decidedly erosive. In a few areas this unit overlies lithofacies 1.

DEPOSITIONAL ENVIRONMENTS

Twelve depositional environments were recognized and differentiated on the basis of lithofacies type, type and degree of bioturbation, sedimentary structures, geographic and stratigraphic position, and data provided in Fisher et al. (1972), McGowen et al. (1977), and Morton and McGowen (1980). These environments include bay, oyster reef, mud flat, marsh or grass flat, distal tidal delta, tidal delta, sand flat, abandoned inlet, tidal inlet, wash over, back barrier, and beach. Only six of these (marsh or grass flat, tidal delta, distal tidal delta, washover, oyster reef, and bay) are incorporated in the idealized vertical sequence that will be presented here. Other environments are associated with the adjacent tidal-inlet and barrier island systems and are shown in the cross-sections and the three-dimensional block diagram that are presented in the following sections. Characteristics of the deposits of each environment are summarized in Table 1 and are briefly reviewed below.

Modern bays of the Texas coast are shallow, low-energy, open-water environments, located on the landward side of barrier islands (Fisher et al. 1972). Bay deposits at San Luis Pass are composed of structure less or mottled, homogeneous muds with variable amounts of shell fragments and burrows. Closely associated with and surrounded by bay muds are the oyster-reef deposits. These deposits are composed of intact pelecypod shells of Crassostrea virginica in a matrix of very fine-grained sand or sandy clay/clayey sand. Modern reefs in the vicinity of San Luis Pass tidal delta vary in plan view from circular to elongate with the larger ones tending to be more elongate. Reefs are typically oriented perpendicular to the dominant current direction which is advantageous to the entrapment of nutrients and the removal of waste materials (Fischer et al. 1972). Washover deposits are composed of poorly sorted, fine- to very fine-grained sand or shelly sand and locally of shell-hash deposits. These deposits are found on the bay side of barrier islands and are chiefly responsible for irregular backbarrier shorelines. Washover deposits within the flood-tidal-delta complex at San Luis Pass are associated with bay, distal delta, and delta deposits.

Distal-tidal-delta deposits are composed of highly bioturbated, sandy clays or clayey sands. They gradationally overly bay deposits and are overlain conformably by tidal-delta-deposits. Modern distal-tidal-delta deposits at San Luis Pass are located landward of tidal-delta deposits and seaward of bay deposits. Tidal-delta deposits are composed of mottled sand to shelly sand and may be extensively burrowed or contain low-angle swash laminae or horizontal laminae.

Deposits of the marsh or grass flat environments are composed of rooted-mottled, sandy clay and clayey sand. Root traces are invariably iron oxide stained, and the deposits may contain plant fragments, sand-filled burrows, and scattered shell fragments. Abundant root structures differentiate deposites of this environment from those of other environments with similar textures. The march and grass flat environment occupy the upper-subtidal to supratidal areas on the flood-tidal delta. Mud-flat deposits consist of sparsely rooted to homogeneous or mottled sandy clay or clayey sand. The common occurrence of sand-filled burrows and the low density of root structures allow differentiation of this environment from the marsh or grass flat. Deposits of this environment are found close association with the marsh or grass flat deposits overlying tidal-delta or distal-tidal-delta deposits. Sand flat deposits consist of well-sorted, very finely grained sand. Clay matrix, organic clays, root structures, and shell fragments may be present locally. Deposits of the sand flat generally overlay distal-tidal-delta or mud-flat deposits.

Abandoned inlet deposits consist of structureless clay or clayey sand with rare, low-angle swash laminae and scattered clayey and bioturbated zones. Overall, there is an upward decrease in sand and shell content above an erosional contact which marks the base of abandoned inlet deposits. Tidal-channel deposits consist of normally graded to cross-bedded layers of sand and shells. Sands are poorly sorted, and shell content generally decreases upwards. Deposits of this environment generally have a sharp erosional lower contact with bay deposits and grade upwards into abandoned inlet or deltaic deposits.

Back-barrier deposits are composed of interbedded, burrowed, and laminated sand or sandy clay/clayey sand with lenticular shell-hash deposits. Beach deposits consist of well sorted, very finely grained or finely grained sand which is locally shelly. Burrows are rare and sedimentary structures include low-angle swash laminae and rare forest beds.

CROSS-SECTIONS

Figure 6 shows the location of cross sections L-L', E-E', H-H', and I-I' illustrated in Figures 7-10 and discussed in this section. Each cross-section displays the lateral and vertical distribution of lithofacies and environments within the flood-tidal delta and associated tidal-inlet and barrier-island systems and is the upper portion of the pretidal-inlet/tidal-delta deposits represented by lithofacies 1 and 2.

Cross section L-L' (Fig. 7) traverses the more distal portion of the flood-tidal-delta complex. Between vibracore VC-36 and VC-24 the section transects Mud Island, an emergent portion of the flood-tidal-delta systems, which has been stabilized by marsh or grass flat deposits. The typical flood-tidal-delta succession, as shown by this section, begins with bioturbated, homogeneous clay (lithofacies 3) of the bay, which grades upward into bioturbated, shelly, clayey sand and sandy clay (lithofacies 5) of the distal delta; bioturbated shelly sand and sand of the delta; and rooted, clayey sand and sandy clay of the marsh or grass flat. This vertical succession of environments and the thinning and intertonguing of tidal-delta and distal-tidal-delta deposits to the northeast results from the bayward progradation of the flood-tidal system and exemplifies the coarsening-upward sequence which typifies the microtidal, flood-tidal-delta system (Fig. 7 Lenticular oyster-reef deposits (lithofacies 6) are found encased in bay mud in YC-36 and YC-24

Cross-section E-E' (Fig. 8) transects the flood-tidal delta immediately landward of San Luis Pass tidal inlet. Sediments in the area of the flood-tidal delta are generally more coarsely grained, contain more shells and shell hash, and exhibit less biotubation than in the distal portion of the flood tidal delta. A core obtained by Shell Oil company (#3687; Bernard et al. 1970) at the southwest end of Galveston Island penetrates 3.1 m of graded shelly sand and shell hash beds of lithofacies 4 and 6 below a depth of 3.9 m. This sequence, not encountered in the shallower vibracores of this study, represents active tidal-inlet and spit-accretation deposits associated with San Luis Pass. Deposits northeast of Cold Pass in the upper portion of the cross-section consist of sandy clay and clayey sand of the distal delta overlain by shelly sand and sand of the flood-tidal delta. These are in turn overlain and interfinger with beach and washover deposits of Galveston Island. On the southwest side of San Luis Pass delta, distal delta and bay deposits show a complex interfingering relationship that is not easily explained by a uniform progradation of a flood-tidal delta complex. These deposits are underlain by oxidized clays and sandy clays of lithofacies 1 and 2 and overlain by rooted marsh or grass flat clays of Moody and Mud Islands. Cross-section G-G' (Fig. 9) parallels the northeast of Mud Island and extends from the mainland to San Luis Island. Northwest of Cold Pass the section reveals a single coarsening-upward flood-tidal-delta sequence beginning with bay clay and progressing up through distal delta and delta deposits and capped by fine-grained rooted clay of the marsh or grass flat. In the vicinity of and southeast of Cold Pass, a similar but earlier flood-tidal-delta complex is revealed in VC-9, VC-38, and VC-31. This earlier complex is overlain by beach, back-barrier and mudflat deposits of San Luis Island and may be related to pre-1853 depositional patterns when San Luis Island was separated from Follets Island by Cold Pass, which had an outlet to the Gulf (Lee 1966).

Cross-section I-I' parallels section G-G' along the northeast side of San Luis Pass (Fig. 10). Bayward of Galveston Island this section reveals a single, prograding flood-tidal-delta complex overlying oxidized red clays of lithofacies 1. Storm shell-hash deposits are interbedded with distal delta and delta deposits in this area. Cores penetration Galveston Island reveal a complex interrelationship among beach, washover, and back barrier deposits.

IDEALIZED SEQUENCE AND BLOCK DIAGRAM

Using grain-sized, sedimentary-structure, and trace-fossil data obtained from continuous cores of San Louis Pass flood-tidal delta, an idealized vertical sequence (Fig. 11) and a hypothetical three-dimensional block diagram, (fig 12) for microtidal flood-tidal deltas have been constructed. The sequence and block diagram, illustrate the following characteristics:

  1. The flood-tidal-delta sand body is asymmetric perpendicular to depositional strike, has a jagged scalloped landward edge (Barwis and Hayes 1978), and may be elongated parallel to strike assuming inlet migration.
  2. Sands of the flood-tidal delta thin and interfinger landwards with sandy clay and clayey sand of the distal delta and with clays of the bay. Seward, these deposits overlie tidal-inlet sands and shell-hash deposits and interfinger with and are overlain by barrier and back barrier deposits.
  3. Flood-tidal-delta deposits conformably overlie bay deposits and may unconformably overlie nonmarine deposits. At San Luis Pass these non-marine deposits comprise delta-plain clays and sandy clays of a former Brazos River delta lobe. Along portions of the Atlantic coast they are freshwater peats (Morton and Donaldson 1973; Boothroyd et al. 1985).
  4. Flood-tidal delta deposits are in part conformably overlain by rooted sandy clay and clayey sand of emergent marsh or grass flats (Fisher 1962; Fisher et al. 1972).
  5. Lenticular, coarse-grained, shell-rich layers, including oyster reef and washover deposits, can occur throughout the flood-tidal-delta deposits
  6. A vertical profile within the main body of the flood-tidal delta reveals a coarsening-upward sequence from bay clay to distal delta sandy clay and clayey sand to tidal-delta sand and shelly sand. This sequence is commonly interrupted by coarse-grained shell hash (washover deposits) or shell beds (oyster reefs) and is capped by finegrained, rooted clayey sand of the marsh or grass flat
  7. Flood-tidal delta deposits are described as relatively clean sands by Barwis and Hubbard (1976) and Barwis and Hayes (1978). Flood-tidal-delta deposits at San Luis Pass are generally not clean. These deposits consist mostly of clayey sands and sandy clays which are extensively bioturbated and display a general lack of primary sedimentary structures and a low primary porosity (Israel1983). Rarely, horizontal laminations and low- to high angle cross-stratification may be preserved in the delta deposits.
  8. The thickness of the flood-tidal-delta deposits will, vary depending upon complex interrelations among such factors as sediment supply and rates of subsidence or sea level change, lagoon and inlet depths, tidal range and prism, inlet stability, and so on. The San Luis Pass flood- tidal delta has an average thickness of 4.6 m. Microtidal flood-tidal delta deposits along the Rhode Island coast average about 3 m in thickness (Boothroyd et al. 1985).

CONCLUSIONS

Flood-tidal deltas along the microtidal Texas coast constitute significantly large geomorphic features and are major components of the barrier/lagoon depositional system.

The flood-tidal delta produced by San Luis Pass and associated environments consist of six lithofacies ranging from highly oxidized red clay and sandy clay of a former lobe of the Brazos River delta to dark gray clay, sandy clay, clayey san, sand, shelly sand , and shell and shell hash gravel that comprise the tidal delta and associated modern depositional environments. Specific depositional environments, recognized on the basis of lithofacies type, sedimentary structures, trace fossils and stratigraphic position include bay, oyster reef, distal delta, marsh or grass flat, mud flat, abandoned inlet, tidal channel, washover sand flat, back barrier and beach.

Cross-sections constructed from lithologic logs of cores reveal lateral relations of lithofacies and environments and provide data for the construction of an idealized vertical profile and a three-dimensional block diagram for microtidal flood-tidal-delta systems. The vertical profile exhibits a coarsening-upward trend from bioturbated bay muds to bioturbated distal-tidal-delta sandy clay and clayey sand and tidal-delta sand and is capped by rooted, sandy clay and clayey sand of the emergent marsh or grass-flat. This profile is, as might be expected, different from published profiles of mesotidal flood-tidal deltas. Microtidal flood-tidal-delta deposits in general comprise an assemblage of strike-oriented, highly bioturbated sandy clay clayey sand, and sand which generally lack primary depositional sedimentary structures, attain an average thickness of 4.6 m, thin and interfinger with bay clay in a landward direction, and interfinger with back-barrier and tidal-channel sand and shelly sand in a seaward direction

The high degree of bioturbation, interstitial and inter laminated clay, and generally low primary porosity, of the San Luis Pass microtidal flood-tidal-delta deposits suggest that these deposits would make poor-quality hydrocarbon reservoirs inferred ancient micotidal flood-tidal-delta deposits form important components of hydrocarbon reservoirs in the Frio Formation of the Texas Gulf Coast (Galloway and Cheng 1985).

ACKNOWLEDGMENTS

This report represents a part of a master's thesis by Alan M. Israel completed in the Department of Earth Resouces at Colorodo State University. The research was supported by the division of Earth Resouces, National Science foundation, under Grant EAR-80-04127 to Frank G. Ethridge.

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G. Ethridge. HA~, M. 0., AND KANA, T. W., 1979, Terrigenous clastic depositional Dave Cart and Brad Cross collected most of the short enVIronments: some examples: a field course s~onsored by the AA~G: dId. thfitfiIdSS.fTech. Rept. 11-CRD, Dept. of Geology, Umv. of South Carolina, an ong core urmg e rs e season. teve aVlg 0 315 p.

Colorado State University and Robert S. Lemmons, Ste- HODGE, J. A., 1982, Three-dimensional study of a modem flood-tidal phen W. Simpson, Michael A. Kom, George C. Creigh- delta in South Carolina (abs.): Am. Assoc. Petroleum Geologists Bull., ton, David Rose, and Jim ~wing of Texas A&M Uni- v. 66, p. 582-583. versity at Galveston provided knowled ge mId HUBBARD, D.. K., AND BARWIS, J. H., 1979, DIS~ussIon of ~dal inlet ...' usc e, an sand deposits: Examples from the South Carolina Coast, In Hayes, companIonshIp dunng the second field season. Sue Oney M. 0., and Kana, T. W., eds, Terrigenous Clastic Depositional En- helped with logistics. Margaret Hall assisted with plant vironments, Some pxi!.mples: a Field Course Sponsored by the AAPG: identification and Charles Coleman helped with the core Tech. Rept. No. 11-CRD,-Dept. of Geology, Univ. of South Carolina, preparation and storage at the Fort Crockett Sedimen- p. 11-128 to 11-142. tat. Lab tJffAhbkMFIII.HUBBARD, D. K., OERTEL, G., AND NUMMEDAL, D., 1979, The role of Ion ora ory. e uc n ac, ~tt u e~, Me a~Ie waves and tidal currents in the development of tidal-inlet sedimentary Keenen, Herb Saperstone, and Dan HIppe assIsted WIth structures and sand body geometry: examples from North Carolina, various aspects of the drafting. Rufus LeBlanc provided South Carolina and Georgia: Jour. Sed. Petrology, v. 49, p. 1073- core photographs obtained by Shell Oil Company on the 1092. west end of Galveston Island. S. Jeffrey Will. f th ISRAEL, ~. M., 1983, A se~mentologic study of a Holocene mi~rotidal USAnnCfE.CI.Iam~ 0 e flood tIdal delta-San Lws Pass, Texas [unpub1. master's thesIs]: Col- ..Y orps 0 ngIneers, oasta EngIneenng Re- orado State Univ., 185 p. search Center, and G. L. Shideler of the U.S. Geological KUMAR, N., AND SANDERS, J. E., 1974, Inlet sequence: a verticalsucces- Survey, Corpus Christi provided access to cores obtained s~on ,;> f sedime~tary structures and textures by lateral migration of offshore from San Luis Pass, Texas by the U.S. Army tIdal m1ets: Sedimentology, v. 21, p. 491-532. CfE.LEE, H. T., 1966, Passes of the Texas coast: Internal report, Texas Parks OrpS 0 ngIneers. and Wildlife Dept., unpublished. The authors are especially grateful to Robert A. Mor- LENSKY, D. E., LoGAN, B. W., BROWN, R. G., AND HINE, A. C., 1979, ton, W. E. Galloway, and J. H. Barwis for much-needed A new approach to portable vibracoring under water and on land: constructive criticism of earlier versions of this paper Jour. Sed. Petrology, v. 49, p. 654-657. .MORTON, R. A., 1977, Nearshore changes at jettied inlets, Texas Coast: Coastal Sediments '77, Symposium, Am. Soc. Civil Engineers, p. 267- ~ REFERENCES 286. ., MORTON, R. A., AND DoNALDSoN, A. C., 1973, Sediment distribution BARWIS, ]; H., AND HAYES, M. 0., 1978, Regional patterns of modem and evolution of tidal deltas along a tide-dominated shoreline, Wa- barrier island and tidal inlet deposits as applied to paleoenviron- chapreague, Virginia: Sedimentary Geo1., v. 10, p. 285-299. mental studies, in Ferm, J. C., and Home, J. C., ed., Carboniferous MORTON, R. A., AND McGOWEN, J. H., 1980, Modem depositional Depositional Environments in the Appalachian Region: Dept. Ge- environments of the Texas coast: Bur. Econ. Geol., the Univ. of Texas ology, Univ. of South Carolina, Am. Assoc. Petroleum Geologists at Austin, 167 p. Field Course Guidebook, p. 472-498. MUNSON, M. G., 1975, Tidal delta facies relationships, Harbor Island, BARWIS, J. H., AND HUBBARD, D. K., 1976, The relationship of flood- Texas [unpub1. master's thesis]: Univ. of Texas at Austin, 126 p. tidal delta morphology to the configuration and hydraulics of tidal PiETY, W. D., 1972, Surface sediment facies and physiography of a inlet-bay systems (abs.): Geol. Soc. America, Abstr. w. Progr., v. 8, recent tidal delta, Brown Cedar Cut, central Texas [unpubl. master's p. 128-129; thesis]: Univ. of Houston, 208 p. BERELSON, W. M., AND HERON, S. D., 1985, Correlations between Ho- WILLIAMS, S. J., PRINs, D. A., AND MEISBURGER, E. P., 1979, Sediment locene flood tidal delta and barrier island inlet fill sequences: Bock distribution, sand resources, and geologic character of the inner con- Sound-Shackleford Banks, North Carolina: Sedimentology, v. 32, p. tinental shelf off Galveston County, Texas: U.S. Army Corps of En- 215-222. gineers Coastal Engineering Research Center, Fort Belvoir, VA., Misc. BERNARD, H. A., MAJOR, C. F., PARROTT, B. S., AND LEBLANC, R.. J., Rept. No. 79-4, 71 p.



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