Abstract Distributions of living Foraminifera were analyzed from 76 stations in the Gulf of California. Approximately 220 living species were identified. Standing crops of living Foraminifera at depths greater than 1,000 fm average 26.5 specimens/10 ml of wet surface sediment. Standing crops are larger in shallow water. Areas of unusually large living populations at the head of the Gulf may be due to river influence, hypersalinity, or the effect of the silty substrate. Large living populations at three locations on the border of the Gulf may be due to high organic production caused by upwelling. The following boundaries between depth biofacies are indicated by ranges of 66 common species- 15-20 fm, 30-35 fm, 40-50 fm, 70 fm, 90 fm, 200 fm, 400-500 fm, 600-800 fm, 1,000 fm, 1,300 fm, and 1,500 fm. Ecological explanations for these 12 depth assemblages must await further understanding of the physical and chemical environments of the area. Relative rates of deposition based on living-total ratios of Foraminifera suggest a very slow overall present rate of deposition in the Gulf, with small areas of faster deposition around the borders. The deposition rates south of the entrance to the Gulf, in depths greater than 1,000 fm, appear to be much faster than those within the Gulf.

Abstract All but one of the papers that follow represent a symposium summarizing the results of work carried on in Project 51 of the American Petroleum Institute. This had as its objective during the period 1951-1958 the study of modern sediments along the northwest margin of the Gulf of Mexico. The paper by Grover E. Murray, on the other hand, was not part of the project but was contributed to us to give the geological setting for the area of investigation. This investigation of sediments came as a result of many years of deliberation by committees of the American Petroleum Institute and The American Association of Petroleum Geologists, which finally led in 1951 to the creation of A.P.I. Project 51. Many petroleum geologists contributed toward the creation of this largest of the AP.I. geologic projects, and others have given generously of their time in advisory capacities. Much of the background which led to the project was prepared by Shepard W. Lowman, who first conceived the idea and headed a research committee in which preliminary plans were discussed. A. Rodger Denison, Clarence L. Moody, Marcus A. Hanna, Hugh A. Bernard, and R. Dana Russell have served as chairmen of the project advisory committees and all were of great assistance. The complete membership of the committees is given in an appendix at the end of the book. A F. Frederickson has been of considerable help to us in reviewing for the A.P.I. committee all the papers in this volume. The project was given

Abstract The Gulf Coastal province of the United States is a segment of the Mesozoic-Cenozoic coastal geosyncline of eastern North America which can be traced continuously from Newfoundland to Guatemala. The geosyncline is roughly lens-shaped in cross section; approximately equal parts exist (1) submerged beneath the waters of the Atlantic Ocean and Gulf of Mexico, and (2) partly emerged adjacent to their shores. The Gulf Coastal portion of the geosyncline has an area of more than 150,000 square miles and contains about 50,000 feet of predominantly arenaceous-argillaceous, marginal to shallow-marine strata, although calcareous materials predominate in the Florida Peninsula and in the Cretaceous in Texas. The geosynclinal mass overlies Precambrian-Paleozoic rocks of variable facies, structure, and degree of metamorphism; • their top surface possesses an over-all slope toward the Gulf of Mexico. Strata of the Gulf geosyncline also possess a general gulfward slope, at least as far basinward as the edge of the continental slope, and thus constitute a great sedimentary structural arc from Florida to Mexico. Deformation from primary compressional movements is presently unknown. Instead, deformation appears to be principally a result of vertical movements due mainly to isostatic adjustments, density differences between a thick bed, or thick beds of salt and superjacent strata, and igneous emplacements. Larger structural anomalies, such as the Sabine and Monroe uplifts and the East Texas embayment, constitute incipient growing “welts and furrows” in the developing geosynclinal province. Axes of the larger anomalies trend primarily northwest and northeast but secondary north and east orientations also exist. Some evidence suggests that certain of these larger anomalies are, in fact, related to and probably positioned and controlled by Paleozoic structures and trends. Major systems of normal, strike faulting, or of pronounced downflexing, believed to be associated with (1) down warping of the margins of the Gulf of Mexico basin in the earlier stages of its development, and (2) subsidence related to rapid sedimentation, break the general continuity of the strata constituting the geosynclinal prism. The faults are of variable orientations; their dip ranges from 35° to 70°. Faults of younger, gulfward systems ordinarily dip more steeply than those of older, landward systems. Pre-Cretaceous, Coahuilan, Comanchean, and early Gulfian rocks are predominantly red-bed elastics in the eastern and up-dip portions of the Gulf province of the United States; westward and down dip marine equivalents of these beds predominate. Extensive marine deposits constitute the middle and late Gulfian, arenaceous facies predominating in the east and argillaceous-calcareous materials being prevalent westward and down dip. Tertiary deltaic sediments center in Mississippi, Louisiana, and Texas; marine facies are prevalent eastward and down dip. Fluviatile and deltaic Quaternary deposits occur as a surficial mantle over much of the region; down dip they are replaced by marine equivalents. Stratigraphic studies indicate that major sedimentary units are arranged in belts subparallel to the modern northern shore line of the Gulf of Mexico. They have the general shape of flattened link sausages, the thicker lenses being centers or loci of deposition (depocenters) for a particular depositional epoch. Depocenters of the same age constitute a regional axis of deposition (depoaxis). Sedimentary materials have accumulated repeatedly in depocenters to thicknesses of 2,500 feet, or more, after which the depositional locale has shifted. Depocenters are not generally known to occur vertically above immediately older ones. Depoaxes have generally shifted gulfward from the Jurassic to the modern day; major reversals of this gulfward progression occurred in the Cretaceous and Tertiary and resulted in the formation of widespread cyclic depositional sequences (cyclothems). During Jurassic and Cretaceous time the major source of sediments for the region was apparently eastern and central United States. In the Cenozoic appreciable quantities of material appear to have come from western United States.

Abstract Principal sediment sources in the northern Gulf of Mexico are the Mississippi and Rio Grande basins, which supply subarkosic sands with highly unstable heavy mineral suites derived from mixed sedimentary, volcanic, plutonic, and metamorphic rocks. Components belonging to the last three groups predominate in the heavy mineral assemblages. Much of the detritus has been transported over very long distances. Technically, the source areas include both cratons and orogens. The distributive provinces are homogeneous, and sands, silts, and clays spread over the entire width of the shelf. The drainage basins of the rivers of western Louisiana, Texas, and the area east of the Mississippi represent less abundant sediment sources. With the exception of the metamorphic and sedimentary suite of the Colorado River, sediments are orthoquartzitic with stable heavy mineral suites, and are derived from the Cretaceous and Tertiary margins of the Gulf Coast basin. Sands from these sources are distributed mainly in the nearshore zone, whereas the clays are carried by Gulf residual currents and deposited on the middle and outer shelf together with finegrained Mississippi and Rio Grande material. The distribution patterns of the two major textural groups, sand and silt-clay, are virtually independent, and in many instances the sand, silt, and clay modes of the same locality have different sources. The sequence of nearshore environments produced by the Holocene transgression has resulted in the deposition of a complex pattern of sediments from a variety of sources. Modification of the composition of the sand fraction by agents operating in the depositional basin appears to be of little significance except for the removal of pyroxenes by weathering from Mississippi and Rio Grande deposits exposed on the continental shelf during the Pleistocene. A review of Gulf sediments in the light of theories on tectonic control of sediment properties leads to the conclusion that the tectofacies of the source and depositional areas has little influence on sediment composition in this area. Sediment texture is controlled almost exclusively by depositional environment, which is only to a small extent dependent upon tectonics. Consequently, in the Gulf of Mexico both textural and compositional properties of the sediments are to a large degree independent of tectofacies.

Abstract The study of more than a thousand samples from the Mississippi Delta margins and from shallow borings made into the delta has provided some criteria for recognizing ancient delta facies. Almost all Mississippi Delta sediments are high in wood fibers, mica, and ferruginous aggregates. It is suggested that this rapidly advancing delta includes the traditional top-set, fore-set, and bottom-set beds, although the depositional slope of the fore-set beds is almost everywhere less than 1°. The top-set beds are divided into subaerial and shallow marine. The subaerial include channel deposits, a mixture of sandy and clayey sediments with rare Foraminifera introduced by the salt wedge which penetrates the river mouth at low water; levee deposits consisting of laminated sands and silty clays; and marsh deposits which include peat beds and an abundance of rootlets along with layers of silty sand. The shallow marine includes delta-front platform deposits which are commonly laminated and show rapid lateral and vertical changes from silty sand to silty clay, and interdistributary bay deposits which contain almost as much sand as the delta-front platform deposits and differ only in having somewhat thicker layers of clayey sediments and less lamination. The fore-sets beds show a marked change from the platform deposits in having very little lamination and consisting almost entirely of poorly stratified silty clays. The fore-set beds resemble the top-set in having a scarcity of organic remains. The bottom-set beds are also silty clays, but they have a considerably greater percentage of Foraminifera, echinoids, and shell fragments than found in the other environments, evidently owing to slower deposition and higher productivity at a moderate distance beyond the river mouth. Mottling patterns due to reworking by organisms appear in the bottom-set beds and are found to an even greater degree in the more sandy deposits of Breton Sound. Reworked delta deposits from which the fines have been removed are found in Breton Island and on the sea floor to the east, whereas a sandy old shelf deposit occurs farther east and to the south. These old shelf deposits are being overlapped by the bottom-set beds of the modern delta.

Abstract Study of the Mississippi Delta and several others shows that there are order and pattern in delta building. The characteristic stratigraphic sequences form during a delta cycle which consists of constructional and destructional phases. Marine deltas are seaward-thickening embankments of sediments deposited during the constructional phase and modified by the destructional phase. Sediments of the embankments are mostly land-derived elastics deposited in orderly sequence on the sea floor about distributary mouths. They make up the top-set, fore-set, and bottom-set beds of classical delta literature. Because of the way deltas grow, sediments change vertically in completed deltas in the same way they change seaward during construction; definite similarities between different deltaic sequences can be seen when they are compared. Changing sediment properties are produced by seaward-changing depositional environments. Sedimentary environments of modern deltas, and ancient ones as well, are complicated when studied in detail, but the general relationships now are well known. The environments are defined by (1) sediment sources, (2) processes and their intensities, and (3) rates of deposition. Source areas determine the raw materials, and these differ from delta to delta. Similar marine and fiuviatile processes, similar distributions of process intensity, and relatively high rates of deposition are the environmental properties that cause deltaic sequences to be similar. Relatively rapid deposition is the fundamental characteristic of deltas. Deltas rarely build indefinitely in one direction; rather, the river shifts for a shorter route to the sea when it becomes over-elongated. The recently built delta is abandoned and modified by compaction coupled with marine wave and current action. This is the destructional phase, the time of winnowing of fine sediment and concentration of coarse material into thin veneers, beach ridges, and barrier islands. Large alluvial plains at river mouths are built up in a step-by-step manner. Lqcal delta construction is followed by partial destruction, and later, by another constructional phase. A large alluvial plain consists of several imbricating deltas, each lying partly on the toes of earlier deltas and partly on the surface that existed prior to any delta building. The stratigraphic components of younger deltas become seaward extensions of their older counterparts. When the beds are buried beneath still younger ones, their full history can be understood only by recognizing their deltaic origin and by knowing how deltas are built.

Abstract At 138 stations in the eastern Mississippi Delta area measurements were made of phytoplankton production (C 14 O 2 uptake), chlorinity, temperature, suspended solids, Secchi disc depth, inorganic phosphate, soluble silica, and soluble Kjeldahl nitrogen. In addition, the phytoplankton in the water were identified and enumerated. These stations were generally located inshore of those taken by G. A. Riley (1937). The following results were obtained. 1. Surface phytoplankton production off the delta is equal to or greater than that of highly productive tropical or subtropical pelagic or neritic areas. 2. Surface production is quite variable; variation can be as much as seven-fold from one day to the next, at a given location. 3. Although seaward traverses made on single days showed that production generally increased at seaward locations, there were no over-all statistical differences between river, plume, and gulf areas at any given season. 4. During the period of high river discharge (May), surface production at the most seaward locations (gulf) was significantly greater than during the fall months. Phosphate and Kjeldahl nitrogen were also significantly greater in May than during the fall months in this area. 5. Integrated production in the water column was three to six times that occurring at the surface. Measured in situ water column production agreed within 20 per cent with that calculated by Ryther’s (1956) method in three out of six determinations. In the other determinations, the measured value was three to six times greater than the calculated value. 6. Rough calculations of the rate of sedimentation of organic carbon off the delta were compared with phytoplankton production. This comparison showed that it is unlikely that production contributes much organic carbon to pro-delta slope sediments. Production would have to proceed at the maximum rate throughout the year without losses of phytoplankton carbon to equal the rate at which carbon is deposited in these sediments. In the bottom-set beds farther seaward we cannot estimate from the present data the relative contribution of organic matter by phytoplankton to these sediments. 7. Theoretical calculations indicate that nitrogen is a more likely limiting nutrient in these waters than phosphate. Silicate may limit phytoplankton only in highly saline water, where no silicate could be detected. 8. Two hundred species of phytoplankton were identified from the delta, of which 86 per cent were diatoms. Two associations of primary species were recognized. One of these, consisting of species of Cyclotella, Melosira , and Navicula , was found in the river plume, and Blind Bay. The other consisting of species of Nitzschia, Thalassionema, Thalassiothrix, Skeletonema, Asterionella , and Chaetoceros , was found in the gulf, plume, and Breton Sound. During any given season the relative concentrations of phytoplankton do not differ significantly from area to area. Significantly fewer phytoplankton were found in the river and plume at times of low water, but no significant difference in phytoplankton concentration in the gulf was found between May and the fall months.

Abstract Approximately 1,000 samples have been obtained from the bays of the centra! Texas coast. Their size parameters, constituents, and organisms have been studied in relation to salinity, depth of water, and relations to entering rivers and to inlets. All of these bays are cut off from free connection with the gulf by barrier islands, although widely-spaced inlets allow some exchange with gulf waters. Among the bay facies are (1) bays near stream mouths which have alternating silty clays and sands, commonly laminated and containing abundant plant fibers and aggregates, along with abundant ostracods in some localities; (2) shallow bays not related to stream mouths, which have an abundance of marine plants, a large number of gastropods, and higher sand content than most other bay environments; (3) deep central bays which have unstratified silty clay sediments commonly with a high content of benthonic Foraminifera in the coarse fraction; (4) central bays with oyster reefs which have been built up above the surrounding bay surfaces, and (5) bays near inlets or narrows where the currents have introduced considerable sand, and the sediment is a bimodal sandy clay with relatively small silt content. Borings into the bays and into the deltas recently built over their northern ends show that as much as 80 feet of bay sediment has accumulated during the past 9,000 years, since the ocean first came up into the valleys eroded during glacial stages, in what is now the bay area. The samples from the borings along most of San Antonio Bay show a remarkable constancy of sediment types with the only significant changes being from central bay facies to oyster-reef facies. The faunas, however, indicate salinity changes. No lamination and little stratification were found in the borings except near or under the delta at the head of the bay.

Abstract Laguna Madre is a linear coastal lagoon, developed on the pre-Holocene 3 3 Throughout the paper the age of transgressive de posits following the last glaciation is referred to as “Holocene,” in keeping with its generally accepted usage. erosion surface by the buildup of a barrier island, resulting from the transgression brought about during the postglacial rise in sea level. The Holocene deposits average 10-20 feet in thickness, although local depressions in the pre-Holocene surface may contain a considerably thicker section. The bulk of the Holocene deposits consists of subgraywacke and subarkosic sand. Local concentrations, however, include limestones, gravels, oolites, and relatively pure clays. Source of the clay is unknown, but heavy and light mineral analyses of the sand fraction indicate locally reworked earlier deposits. The northern Laguna sediments are derived from the pre-Holocene sediments off the Nueces delta province and the southern Laguna sediments are derived from the Holocene and pre-Holocene deposits of the Rio Grande distributary province. The immediate source of the lagoonal fill is the barrier island sand, which is believed to be derived from the earlier deposits of the nearshore gulf. During and after sea-level rise, the near-shore gulf floor was actively eroded by wave action to establish an equilibrium profile. The eroded material was sorted and the sand fraction was transported landward. Wind action piled the new beach sands into coastal dunes on the barrier. Where the earlier deposits were relatively un-consolidated, as offshore northern Padre Island, thick dune deposits could form because of large supply. Where the earlier deposits were more consolidated, and consequently less easily eroded, as in the vicinity of the Rio Grande delta, the dunes are lower and the barrier narrower. Subsequent to the formation of the lagoon about 5,000 years ago, partial filling was accomplished by washover storm waves, wind transport, and tidal activity.