Northwest Gulf Research Articles

Cercaria anadarae sp. n. Parasitizing a Bivalve Mollusc, Anadara brasiliana (Lamarck), from the Northwest Gulf of Mexico

Cercaria anadarae sp. n., a cytophorous cercaria of a digenetic trematode of the superfamily Hemiuroidea, was found developing in the nephridia of 39 to 356 specimens of Anadara brasiliana (Lamarck), the incongruous ark clam, collected from the beachfront in the vicinity of Galveston, Texas. The germinal sacs containing cercariae were found to leave the host clam before liberation of cercariae when infected clams were isolated in bowls of seawater. This is the first record of a hemiuroid larva from a bivalve host, all previous hemiuroids having been reported from gastropod molluscs. The Hemiuroidea (Trematoda: Digenea) are the only flukes known to produce cystophorous cercariae, and in all of the approximately 40 known species, the cercariae develop in rediae in gastropod and scaphopod molluscs (Dollfus, 1950). In most known hemiurid life cycles the cercariae are planktonic and are ingested by copepods, and the adults are found in the digestive tract of carnivorous fishes (Schell, 1970). During a survey of the helminth parasites of marine molluscs of the Galveston Bay area, Texas (Wardle, 1974), a cystophorous cercaria was found in Anadara brasiliana (Lamarck) which inhabits the sandy substrates in and beyond the shallow surf zone along the Gulf of Mexico side of Galveston Island, Texas. During the winter months, combinations of strong winds and low tides agitate the substrate and the clams are washed from their shallow sand burrows and deposited on the strand line of the beach where they were hand-collected. Two species of larval flukes were found in this clam, one a bucephalid from the gonadal area and the other a member of the Hemiuroidea which was unusual in that the host mollusc is a bivalve rather than a scaphopod or gastropod which is the usual host. The cercaria is named after the generic name of the host clam. Since many more adult hemiuroid flukes than larvae have been reported from the northwest Gulf of Mexico, and since the host and its infection are quite common, it is likely that the adult will prove to be a previously described species from one of the common coastal fishes. Measurements are in micrometers unless otherwise noted. Cercaria anadarae sp. n. (Figs. 1, 2) Diagnosis: Cystophorous cercaria (Fig. 1), body 72 to 110, elongate, pyriform, tail cyst spherical, 54 to 65 in diameter, bearing 2 latitudinal rows of 8 to 9 long curved setae each. Center of cysts bearing sac housing 9-segmented protrusile delivery tube 50 long when extended by coverslip pressure. Normally, the tube is withdrawn into the cyst. Mouth and oral sucker indistinct. Pharynx and intestine apparently lacking, body densely packed with parenchyma and with a network of larger nucleated cells connected by fibrils forming a meshwork pattern. Excretory bladder inverted pyriform, size variable in live specimens 12 to 20 long by 15 wide, not visible in preserved specimens. Excretory pore posterior and terminal. Common collecting tubules entering bladder on anterior margin. Four flame cells observed on each side of body. Further details of excretory system not observed. Body cuticula thick and aspinose. Cercariae developing in germinal sacs (Fig. 2) located in nephridia of host clam. Germinal sacs pale orange in color, very mobile, elongate, 1.3 to 4.8 mm in length by 0.30 mm wide with conical anterior end bearing birthpore, and containing hundreds of cercariae. Germinal sac appearing externally segmented due to anterior muscular annulations approximately 100 apart. Host: Anadara brasiliana (Lamarck), incongruous ark clam. Locality: Galveston Beach, Texas. Prevalence: Thirty-nine of 358 clams (10.8%).

Comparative Growth of Hard Shell Ciams (Mercenaria mercenaria LINNÉ AND Mercenaria campechiensis Gmelin) and Their F1 Cross in Temperate, Subtropical, and Tropical Natural Waters and in a Tropical Artificial Upwelling Mariculture System1

ABSTRACTThe growth of hard shell clams (Mercenaria mercenaria Linné and Mercenaria campechiensis Gmelin) and their F1 cross in a controlled experimental environment was compared to growth in uncontrolled natural environments. The clams were spawned by Paul Chanley of Shelter Island Oyster Company, Inc., Greenport, New York.In the controlled environment of the artificial upwelling mariculture system on St. Croix, U.S. Virgin Islands, pollutants and predators were absent and fouling was minimal. Salinity was 34.75 to 34.95 ppt and water temperature varied between 22 and 30 C during the experiment. Natural environments in temperate, subtropical, and tropical waters were selected as sites for comparative studies: Southold on Southold Bay, New York; Wachapreague on Bradfords Bay, Virginia; Alligator Harbor on the northwest Gulf Coast of Florida; Kupeke Pond in Pukoo, Molokai, Hawaii; and Salt River Inlet, St. Croix.Clams from each population‐M. mercenaria, M. campechiensis and their F1 cross‐were planted at Southold, New York, and at both St. Croix sites. Only M. campechiensis and F1 clams were sent to Virginia and Florida, and F1 clams and M. mercenaria were sent to Hawaii. Increase in length, “wet” weight, and survival were measured at the different locations from April 1973 through May‐June 1974.M. campechiensis and F1 clams reached market size (greater than 25.5 mm thick) in 6.5 to 13 months in the St. Croix artificial upwelling system. These clams did not reach market size in Southold, New York; Wachapreague, Virginia; or Salt River Inlet, St. Croix. The F1, clams in Molokai, Hawaii, and in Alligator Harbor, Florida, were close to market size when the experiment was terminated after 13 months. Survival and growth of M. mercenaria was poor in the artificial upwelling controlled environment, in Salt River Inlet, St. Croix, and in Molokai, Hawaii.

Submarine Current Measurements, Northwest Gulf of Mexico: ABSTRACT

In February 1966, Exxon Corporation undertook a geophysical and geologic survey of the continental slope of the Gulf of Mexico from Texas to Florida. They were joined in May by Standard Oil Company of California and in August by Gulf and Mobil Oil Companies. As part of this survey the Caldrill 1 was to drill 42 coreholes at 36 sites. Submarine current measurements were to be recorded while the vessel was on station. Because of technical difficulties, submarine-current records were obtained at only 6 northwestern sites. Vertical current profiles at 5 of the 6 stations show two prevailing directions which may represent distinct water masses. The upper current flows generally east or northeast, increasing from about 0.2 knots in the west to about 0.4 knots in the east. Below this system is a current that flows west or northwest at similar rates. The interface between these water masses deepens End_Page 1834------------------------------ westward, from about 400 to about 600 m. The deeper mass may be related to the Yucatan Current. At 5 sites the meter was maintained for up to 9 hours a short distance off the sea floor to determine the sustaining nature of the currents. Records obtained at 3 locations are consistent in both velocity and direction with the short-term measurements. These few scattered measurements would not seem to warrant broad conclusions regarding the movement and structure of the water masses over the upper continental slope. However, the data clearly document certain water movements in the spring of 1966. Most of these data are internally consistent, and the velocities are sufficient at times to transport fine to medium sand. End_of_Article - Last_Page 1835------------

Sediments and history of the Postglacial transgression in the Persian Gulf and northwest Gulf of Oman

Large parts of the Persian Gulf receive little recent sedimentation (see Fig. 2). In these areas the bottom samples contain considerable quantities of parautochthonous relict sediments with radiocarbon ages of 7,000–13,000 years. The relict grains have a shallow water origin. Studies of their locally very differentiated sedimentary facies (see Fig. 5) as well as the morphology help to decipher the palaeogeographic history during the Late Pleistocene-Early Holocene transgression. The deepest traces of sub-fossil, water-line sedimentation (oöliths, reef material) are today found at the shelf break in water depths of 105–125 m (see Fig. 10). At the time of this deposition the Persian Gulf was essentially a dry, flat river valley of an ancient Shatt al Arab. Between 100 and 65 m water depth “polymict coquinas”, in the shallower part covered by thick unlithified aragonite mud, reflect a rapid transgressive migration of intertidal environments. Behind the Central Swell a temporary lagoon was formed which filled with aragonite mud and some terrigenous deltaic sediments. Transgression standstill periods are indicated at 64–61 and 53–40 m by coarse, frosted quartz and oöid concentrations embedded in lithified aragonite mud. These, together with a fossil ridge and channel system, are interpreted as drowned relicts of strand dunes. Their water depths are analogous to similar occurrences on other shelf regions (see Fig. 8). A possible third standstill lies at about 30 m. The Late Pleistocene climate, as evidenced by the absence of Zagros river sediments, was probably more arid than the present day Persian Gulf climate.

Geology of Upper Continental Slope in Alaminos Canyon Region: ABSTRACT

The surficial sediments of an area of the upper continental End_Page 1895------------------------------ slope in the Alaminos Canyon area, Gulf of Mexico, indicate sand input from the ancestral Colorado-Brazos and Mississippi River systems. The clay minerals in the area were derived from indeterminate sources and were incorporated in coarse samples through resuspension of former sediment. Vermiculite, as well as tubular hallaysite, were identified in clay samples. The first mineral is unreported in the northwest Gulf, and the latter is only known from the Mississippi delta in the northwest Gulf area. The hummocky nature of the bathymetry in the area resulted from salt diapirism and scouring by tractive and/or density flow. Sand-size sediment was transported to the area from river systems by longshore drift during the Holocene transgression or through channels still identifiable on the present continental shelf. The lineation of one of these features, the Outer Colorado-Brazos Channel, is probably due to salt tectonics and not the result of a barrier spit as previously reported. End_of_Article - Last_Page 1896------------

Bathymetry of Recent Marine Ostracoda in Northwest Gulf of Mexico: ABSTRACT

Holocene bottom samples from the northwest Gulf of Mexico, examined for their Ostracoda faunas, yielded a total of 171 species. A large majority of them have never been described. The number of species per sample varied from 2 to 40; the richest and most diversified faunas occur in middle to outer shelf environments. Numerous species exhibit rather restricted depth ranges, and distinctly different faunal assemblages characterize the various bathymetric environments from shallow freshwater lakes and bayous down to the abyssal depths of the Sigsbee Deep. A bathymetric chart illustrates that at the generic level there exists a marked diversity of faunal composition at different depths. A pronounced relation has been noted between the size of 2 common species of Echinocythereis and the depth at which they were collected. A similar connection was observed in 12 species of the genus Krithe. The results of this study will be of considerable use in evaluating the paleoecologic significance of the rich, ostracod assemblages commonly found in the clastic Tertiary and Upper Cretaceous deposits of the Gulf Coast area. End_of_Article - Last_Page 1904------------

The Distribution of Modern Benthonic Foraminifera in the Northwest Gulf of St. Lawrence

The Distribution of Modern Benthonic Foraminifera in the Northwest Gulf of St. Lawrence

Geology of West Flower Garden Bank, Northwest Gulf of Mexico: ABSTRACT

The Flower Garden banks, the most prominent of a series of topographic highs in the northwest Gulf of Mexico, have been noted and studied for years. Most of these physiographic expressions have been related to salt uplift. Previous investigations of the Flower Garden banks, located 130 mi south-southeast of Galveston, Texas, suggest that biohermal development has contributed significantly to the overall topographic expression. The results of this study demonstrate that biohermal development during the present interglacial period occurred in a deep water environment in the northwest Gulf of Mexico; therefore, it is possible that buried fossil reefs formed within structural and depositional environments similar to those existing today on the outer continental shelf. If there are such fossil reefs, they offer a previously unrecognized exploratory objective on known structural features. End_of_Article - Last_Page 2039------------

PETROLEUM EXPLORATION PREVIEW FOR 1967

Exploration for petroleum in Australia paused in 1966 and this pause is likely to continue in 1967.The number of wells drilled and seismic activity will both be slightly less in 1967 than in 1966, but the work will generally be directed to more specific targets. The cost of exploration will be somewhat higher than in 1966.During the year off-shore drilling should increase and it is expected that five mobile rigs will be drilling in the offshore areas by the end of the year. The immediate structural targets available for these rigs are in the Gippsland, Bass and Otway Basins (Victoria-Tasmania), the North-West Shelf and Timor Sea-Bonaparte Gulf, and in the Gulf of Papua.Onshore exploration will be concentrated in the western Australian basins, the Surat Basin, the central Great Artesian Basin, and the Gidgealpa region of the southwestern Great Artesian Basin.The success or otherwise of the off-shore drilling will determine the rate of exploration over the next few years. If important discoveries are made off-shore, these may and probably would result in more intensive exploration of the same stratigraphic intervals onshore.The exploration patterns of Australia and several other countries before and after first commercial discovery are compared. This indicates that Australian discovery came early, as compared with other countries, where production has developed since the war, but that post-discovery effort in Australia has been very much less. The main obvious differences appear to be that in Australia the average size of the exploration concession is very much larger and the number of operators (having regard to the areas concerned) is much smaller, than in the other successful countries.There has been a gradual movement towards reducing the size of operating areas in Australia either by obligatory relinquihment or by farmout, but if discoveries are to be made at a satisfactory rate more operators are needed in every basin.The economic environment of Australia vis-a-vis Middle East oil and oil markets is probably the main basic reason for the peculiar exploratory pattern, which has impelled the Government to provide financial incentives to encourage exploration and development.

Physical Properties and Stability of Continental-Slope Deposits, Northwest Gulf of Mexico: ABSTRACT

ABSTRACT Sediments from the continental slope and shelf edge in the northwest Gulf of Mexico were tested to determine their strength, stability, and consolidation characteristics. Consolidation tests for Sabine River delta samples and abyssal plain samples were used to aid in the interpre tation of the shelf and slope samples. To approximate in situ conditions, a direct shear-test machine was used to measure shear strength, and an Anteus Back-Pressure Consolidometer was used to determine consolidation. The topography of this area has been attributed to local slumping of unconsolidated and unstable marine sediments. Although the Sabine River delta samples were underconsolidated, the continental shelf and slope samples were all overconsolidated. The overconsolidation of these samples and the high values of shear strength are assumed to be the result of low deposition rates and incipient cementation of the mineral grains. Analysis of the void ratio versus log-pressure consolidation curves indicated that the sediments were sensitive, and therefore, relatively undisturbed by the sampling process. The shear strength ranged from 0.05 TSF for the upper meter of sediment to over 0.5 TSF for a simulated depth of 20 meters. The shear strength tests indicate that the slope sediments should be stable to great thicknesses on even higher slopes than exist in this area.

Patterns of Foraminiferal Distribution, Northwest Gulf of Mexico: ABSTRACT

Within the past 30 years, several investigators have collected material and reported on Recent Foraminifera in the northwest Gulf of Mexico. The investigations include lagoon, delta, shelf, slope, and abyssal environments. Each report, however, includes only a limited area of investigation. This report is an attempt to evaluate and integrate the existing data in the form of geographic-distribution patterns. The species reported in approximately 1,200 samples form the basis of this study; environmental data come from sedimentologic and oceanographic investigations. Marginal marine environments: Faunas reported primarily from Texas bays and lagoons change geographically in composition. On the southwest, the Laguna Madre and lower Corpus Christi and lower Aransas bays are dominated primarily by miliolids and secondarily by species of Elphidium and Streblus. The miliolids are replaced by the Elphidium-Streblus fauna in Aransas, lower San Antonio, Matagorda, and Galveston Bays and in the pass area of Sabine Lake. These areas contain the so-called Lower Bay fauna. The landward parts of these same water bodies contain an Upper Bay fauna, usually dominated by the arenaceous Ammotium salsum; where streams enter the bays, Eponidella usually is characteristic. Sparse data from Louisiana bays indicate that the Upper Bay fauna predominates throughout the coastal area. The southwest-t -northeast changes occur in response to changing climatic zones, increasing stream discharge, and sediment type. Marine environments: The continental shelf extending from the Gulf beach to about 70 fm. contains a variety of geographically bounded faunas. Despite the emphasis on depth facies, many shelf genera show a pronounced change along strike. For example, the inner shelf from the beach to about 20 fm. generally is dominated by species of Elphidium and Streblus. Offshore from south Texas, a band of Elphidium is dominant nearshore and is succeeded by a band of Streblus in somewhat deeper water; the bands are reversed offshore from southeast Texas and Louisiana. The middle shelf, 20-50 fm. deep, is characterized by a laterally diverse system of dominant taxa. Hanzawaia, Virgulina, and Proteonina occur in elongate and commonly interfingering bands. These dominant forms, however, are replaced in the vicinity of the Mississippi delta by Nonionella, Buliminella, and Epistominella. The outer shelf contains dominant abundances of Cibicides, Uvigerina, and Bigenerina; species of Bolivina occur mainly on the outer shelf off southeastern Louisiana. The continental slope, in contrast to the shelf, is characterized by more uniform lateral distributions of genera. The upper slope from 70 to approximately 500 fm. is strongly dominated by species of Bolivina. This genus is replaced by Bulimina from 500 to 900 fm. and can be considered as indicative of the middle-slope environment. The lower-slope and abyssal environments deeper than 900 fm. are characterized by Pseudoparrella, Eponides, and Glomospira. The number of species per sample increases from a low average of about 10-12 in the bays and nearshore zone to a maximum of more than 55 at or near the shelf-slope break. The faunal diversity decreases again on the slope reaching a minimum of 10-15 species in abyssal samples. Planktonics increase uniformly in abundance from the beach into the Sigsbee Deep. Their increase is related directly to decreasing rates of dilution by sediments and other organisms. End_of_Article - Last_Page 2322------------

Structural framework of the continental terraces northwest Gulf of Mexico : Moore, D. G., and J. R. Curray, 1963. J. Geophys., Res., 68 (6): 1725–1747.

Structural framework of the continental terraces northwest Gulf of Mexico : Moore, D. G., and J. R. Curray, 1963. J. Geophys., Res., 68 (6): 1725–1747.

Structural framework of the continental terrace, northwest Gulf of Mexico

Intricate detail of stratigraphy and structure, to depths of about 5000 feet (probably Miocene), is shown in Rayflex Electro-Sonic Profiler records collected on the continental shelf and the upper continental slope of the northwest Gulf of Mexico. The Rayflex Electro-Sonic Profiler is a continuous-recording, acoustic reflection surveying system which utilizes a high-intensity spark source, rather than chemical explosives. The recorded sections of the continental terrace of this area show essentially horizontally stratified deposits underlying the continental shelf (probably shelf, littoral, deltaic, lagoonal, and continental facies) changing smoothly over the shelf break into seaward-dipping continental slope deposits. The upper part of this terrace has thus formed during the late Tertiary and Quaternary by both upbuilding and outbuilding. This basic framework has been modified locally by folding and faulting, which are largely associated with salt or shale intrusion structures. The irregular topography of the upper continental slope, previously attributed to slumping, gravity tectonics, and turbidity current erosion, is shown to be controlled by these intrusive structures modified by varying amounts of contemporaneous deposition of sediment. No evidence of extensive slope erosion is shown by records of this survey.

Recent Sediments, Northwest Gulf of Mexico: ERRATUM

The following error appeared in column 2, page 232, in the paper by Joseph R. Curray, Sediments and History of Holocene Transgression, Continental Shelf, Northwest Gulf of in Recent Sediments, Northwest Gulf of Mexico, ed. F. P. Shepard et al., published by The American Association of Petroleum Geologists, 1960. The equation was printed: [EQUATION] where µ = horizontal component of orbital velocity at the bottom H = height of wave at the depth of water considered T = wave period in seconds = depth of water L = wave length at the depth of water considered. The equation should read: [EQUATION] The symbol sinh represents the hyperbolic sine of the expression to follow rather than the sine of h times the expression to follow. End_of_Article - Last_Page 560------------

Bathymetry and Sediments of Bay of Campeche, Gulf of Mexico: ABSTRACT

During February and March, 1957, the Department of Oceanography and Meteorology, Agricultural and Mechanical College of Texas, conducted a cruise to the Bay of Campeche. Twenty-nine cores and one grab sample were collected at 30 stations. Approximately 1,5000 nautical miles of sounding tract was accomplished. The sediment and bathymetric data indicate that the area can be subdivided into three broadly defined sedimentary and geomorphic provinces: (1) Western Bay, separated from the Southern Bay by a deep-water re-entrant of the Gulf of Mexico centered along 95° W. Longitude; (2) Southern Bay, separated from the Eastern Bay by Campeche Canyon; and (3) Eastern Bay. The concave upper part and step-like lower part of the continental slope in the Western Bay suggest that the slope is structurally controlled and little modified by sedimentation. The convex upper part and highly irregular lower part of the continental slope in the Southern Bay suggest that the topography of the slope is the result of the behavior of unstable sediment under stress, as Gealy (1955) suggested for the topography of the Northwest Gulf of Mexico. It was not possible to shed new light on the origin of the continental slope off Campeche Bank in the Eastern Bay. The sediments from the Western and Southern Bay were found to be composed mainly of clastic material which grades sharply seaward from sands near shore to silts and clays on the continental slope. The CaCO3 content of the surface sediments increases seaward because of a relative increase in the quantity of Foraminifera and decrease in the quantity of clastic material. The clay mineral type changes seaward from dominantly kaolinite-illite on the continental shelf to dominantly montmorillonite on the continental slope. Calcareous sediments dominate the Eastern Bay Campeche Bank. Conclusive evidence of sediment displacement was found in cores from Campeche Canyon, and evidence suggesting sediment displacement was found in cores from the deep-water re-entrant between the Wester and Southern Bay. End_of_Article - Last_Page 1781------------

Mid-Tertiary Rock Outcrop on Continental Shelf, Northwest Gulf of Mexico: GEOLOGICAL NOTES

Mid-Tertiary Rock Outcrop on Continental Shelf, Northwest Gulf of Mexico: GEOLOGICAL NOTES

Recent Marine Sediments and Environments of Northwest Gulf of Mexico

This paper presents the results of a megascopic study of eighty-five sediment cores, ranging in length from 3 to 10 feet and averaging 7 feet, taken by the Woods Hole Oceanographic Institution in the northwest Gulf of Mexico. The study has led to the following conclusions. 1. Recent sediments in this region are accumulating in five major marine environments, each with its characteristic sedimentary facies. These are: (a) the continental shelf, extending seaward to a depth of about 600 feet, with a coarse-textured facies consisting of sand, silty clay, and shell clay; (b) the rises, a group of isolated topographic highs at the outer edge of the shelf and upper part of the slope, with a coarse-textured facies consisting of sandy and silty glauconitic foraminiferal ooze, calcareous biostromal deposits, and other deposits similar to those of the shelf facies; (c) the upper slope, from depths of 600 to 3,000 feet in the central and western parts of the region but extending to 8,000 feet in the eastern part, with a green-gray to brown-gray homogeneous clay fa ies; (d) the lower slope-western Sigsbee Deep, from depths of 3,000 to 12,000 feet in the central and western parts of the region, with a light yellow-brown mottled clay facies; and (e) the eastern Sigsbee Deep, from depths of 8,000 to more than 11,000 feet in the eastern part of the region, with a foraminiferal ooze facies. 2. Most of the continental slope and Sigsbee Deep cores show a three-fold zonation. The intermediate zone is reddish brown and/or dark olive-gray clay in slope cores and red or red-streaked clay in Sigsbee Deep cores. Because changes from warm- to cold-water faunas generally occur at the stratigraphic End_Page 813------------------------------ level of this zone, the base of the reddish brown, red, or red-streaked clay is taken to be the contact between the Recent and the last glacial stage of the Pleistocene. There is no evidence of the Recent-Pleistocene contact in shelf cores or, with one possible exception, in rise cores. 3. The sedimentary facies distribution pattern is the result of two factors: (a) the great volumes of sediment carried into the Gulf by a system of several major and many minor rivers with a large combined drainage area and (b) the transport of this sediment by the counter-clockwise prevailing current. Because of factor (a), most of the northwest Gulf is an area of clastic deposition. Because of factor (b), the non-clastic ooze facies, an extension of the large eastern Gulf calcareous area, terminates southwest, rather than directly south, of the Mississippi Delta. 4. The thickness of the Recent, as shown by the isopach pattern, appears to be controlled by the same two factors. Though variable, it decreases on the average from slope to deep and from west to east within the deep. Averages for the upper slope, lower slope, western deep, and eastern deep are 56, 42, 37, and 20 inches, respectively. The corresponding average sedimentation rates are 360, 480, 540, and 1,000 years per inch, on the assumption that the Recent epoch began 20,000 years ago. 5. On the slope and in the deep, the good to excellent core-to-core correlation and the absence of graded bedding indicate that turbidity current activity has been non-existent, or at most of minor importance, during the time represented by the cored strata. 6. Coarse-textured deposits similar to those of the shelf are found in the Pleistocene sections of Sigsbee Deep cores. Evidence for turbidity current origin, such as graded bedding, is lacking. It is postulated either that they accumulated in a shelf environment and were downfaulted to their present depth during late Pleistocene or early Recent time, or that they are products of the coarser loads contributed to the Gulf by the rejuvenated streams of the last glacial stage.

Fauna and Bathymetry of Banks on Continental Shelf, Northwest Gulf of Mexico

Considerable new information has been obtained on the bathymetry and fauna of several calcareous banks on the continental shelf of the northwest Gulf of Mexico. A compilation of depths to tops and bases of 130 banks on the shelf revealed primary modes of depth occurrences at 9, 31, and 45 fathoms. These modes correspond with well developed terraces at the same depths on the banks studied in detail. Several of the banks were characterized by semi-ring depressions, steep central pinnacles, and flat terraces at several depths. The fauna collected on these banks at depths of between 24 and 31 fathoms are characteristic of intertidal depths in the West Indies and Caribbean. Many of the mollusks from these banks exhibited sub-specific differences from the species which have been collected from their normal range in the West Indies. Despite the existence of dead reef coral, no living forms were collected. Living lithothamnioids were common on all banks. Evidence suggests that the tops of these banks were in shallow intertidal waters at a time when over-all water temperatures were warmer than at present, and when populations of these faunas were continuous to the West Indies. As sea-level and hydrographic conditions changed, the original fauna became isolated and the corals which appear to form the primary organ c capping ceased their growth, leaving horizontal surfaces. The presence of large crystals of gypsum and inclusions of anhydrite from the flanks of one bank may have a bearing on the original topographic high at this place.

TOPOGRAPHY OF THE CONTINENTAL SLOPE IN NORTHWEST GULF OF MEXICO

Research Article| February 01, 1955 TOPOGRAPHY OF THE CONTINENTAL SLOPE IN NORTHWEST GULF OF MEXICO BETTY LEE GEALY BETTY LEE GEALY 221 OHIO ST., CORPUS CHRISTI, Texas Search for other works by this author on: GSW Google Scholar Author and Article Information BETTY LEE GEALY 221 OHIO ST., CORPUS CHRISTI, Texas Publisher: Geological Society of America Received: 02 Jul 1953 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Copyright © 1955, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment. GSA Bulletin (1955) 66 (2): 203–228. https://doi.org/10.1130/0016-7606(1955)66[203:TOTCSI]2.0.CO;2 Article history Received: 02 Jul 1953 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation BETTY LEE GEALY; TOPOGRAPHY OF THE CONTINENTAL SLOPE IN NORTHWEST GULF OF MEXICO. GSA Bulletin 1955;; 66 (2): 203–228. doi: https://doi.org/10.1130/0016-7606(1955)66[203:TOTCSI]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Topographic charts of the continental slope in the northwest Gulf of Mexico were constructed largely from soundings from unpublished hydrographic surveys of the U. S. Coast and Geodetic Survey. The configuration of the slope and the nature of its sediments suggest a genetic interpretation for its varied topography and high relief.Twenty-one profiles show that the break in slope at the edge of the continental shelf is marked only by a gradual steepening. A hummocky zone on the upper part of the slope shows features resembling landslide scars and deposits, sediment-flow scars, and surface expressions of faults. These features indicate that the upper part of the slope is underlain by a mass of relatively unstable sediments, probably deposited during the Pleistocene when sea level was near the present 75-fathom curve.Steep slopes and a lack of hummocks on the lower part of the slope indicate that most of the underlying sediments are relatively stable. Many steep-sided troughs cut this portion of the slope. Their position and configuration suggest a history which may involve (1) subaerial erosion, (2) submergence and tilting, (3) masking of the upper part of the slope by Pleistocene sediments, and (4) extensive modification by the failure of sediment under the stress of gravity.The Sigsbee Scarp, at the base of the slope, is at least 340 miles long and as high as 4800 feet. It may represent one of a series of large faults surrounding the Gulf of Mexico and downthrown toward the Sigsbee Deep.The Sigsbee Deep is a basin of gentle slopes and subdued relief extending from the base of the Sigsbee Scarp, at 1700 fathoms, to a maximum depth of about 2200 fathoms.Although salt domes may underlie a number of features of low relief on and near the edge of the continental shelf, there is no evidence which indicates they have been an important geomorphic factor on the continental slope.The topography and sediments suggest that in the northwest Gulf of Mexico there is a large mass of sediment under the stress of gravity and failing by landslides, faults, creep, sediment flows, and intermediate types of failure. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Dynamic Geology of the Modern Coastal Region, Northwest Gulf of Mexico

Dynamic Geology of the Modern Coastal Region, Northwest Gulf of Mexico