Showing papers in "Journal of Sedimentary Research in 1979"

Clay Diagenesis in Wilcox Sandstones of Southwest Texas: Implications of Smectite Diagenesis on Sandstone Cementation

Abstract: Sandstones and shales of the Wilcox Group (lower Eocene) in southwest Texas were examined by X-ray powder diffraction, electron microprobe, and petrographically to interpret their diagenetic history. Samples analyzed are from depths of 975 to 4650 m, representing a temperature range of 55°C to 210°C. No consistent trend of depositional environments is recognized with increasing depth, and mineralogic changes observed are interpreted as diagenetic. Major mineral distribution patterns are (1) disappearance of discrete smectite at temperatures >70°C, (2) gradation of mixed-layer illite/smectite to less expandable (more illitic) illite/smectite over the entire temperature range, (3) disappearance of kaolinite from 150-200°C accompanied by an increase in chlorite, and (4) replacement of calcite cement at about 117 120°C by ankerite. Calculations based on data of Hower and others (1976) indicate that the stability of smectite layers may be a function of composition. Smectites with high ratios of octahedral (Fe + Mg)/Al appear to resist conversion to illite until temperatures high enough to produce ordering are attained. A diagenetic model is proposed which involves the breakdown of detrital K-feldspar and of some smectite layers in illite/smectite to convert other smectite layers to illite. Silica and calcium released by the illitization of smectite is transferred from shales to sandstones to produce quartz overgrowths and calcite cements at temperatures as low as 60°C. Iron and magnesium released by the illitization reaction are transferred from shales to sandstones at temperatures >100°C and react with kaolinite to produce high-alumina chlorite and/or with calcite to produce ankerite.

Petrology and Provenance of Neogene Sand from Nicobar and Bengal Fans, DSDP Sites 211 and 218

Abstract: The Bengal-Nicobar submarine fan complex is part of a linked sedimentary chain consisting of molasse, deltaic, and flysch deposits resulting from the sequential closing of a remnant ocean basin. Ultimate sources for turbidite sand from this fan complex are the uplifted gneissic, sedimentary, and metasedimentary terranes of the Himalayas. Detailed point-counts of lithic grains, as well as standard QFL percentages, of 22 Neogene sand samples from DSDP sites 211 and 218 reveal very uniform compositions. Typical QFL percentages are: 55-30-15. Plagioclase/total feldspar is typically near 0.7. Lithic types are dominated by quartz-mica tectonite. quartz-mica aggregate, polycrystalline mica, and other sedimentary and metasedimentary varieties. Andesitic volcanic lithic grains are absent. The indicated provenance ("tectonic highlands") for Bengal-Nicobar sands contrasts markedly with that of sand and sandstone derived from magmatic arcs and rifted continental margins. Lithic populations of magmatic arc sand and sandstone are dominated by volcanic grains. whereas lithic populations of rifted continental margin sand and sandston are dominated by polycrystalline quartz and sedimentary grains. Two triangular plots of lithic grains distinguish the provenance of sandstone derived from major tectonic settings. Detailed point-counts of lithic grains from thick sedimentary accumulations of unknown tectonic setting are a powerful paleogeographic tool when used in the manner outlined here.

Limestone Response to Stress: Pressure Solution and Dolomitization

Abstract: Limestones that have been subjected to overburden or tectonic stress can be expected to have changed by some combination of deformation (plastic or brittle), neomorphism (as micrite conversion to microspar or aggrading pseudospar), and solution. Specific limestone units (beds, structures, or grains) may be preferentially resistant or responsive to change and with or without impurities (especially magnesium ions and clay particles) that will influence the character of change. Three styles of pressure solution response are recognized in Paleozoic limestones studied from Arizona, western Maryland, and southeast Kansas. Sutured-seam solution (common stylolites and grain-contact sutures) occurs in limestones that have structural resistance to stress and very little clay or platy silt content. Non-sutured seam solution (microstylolites, microstylolite swarms, and clay seams) occurs in limestones that have significant amounts of clay or platy silt. These limestones may be structurally resistant or responsive. Stress is accommodated both by solution and by lateral movement along microstylolite surfaces. Dolomite rhomb growth may or may not occur along microstylolite surfaces. Non-seam solution (pervasive solution thinning of a unit) occurs in clean limestones that have little or no structural resistance to stress. Dolomite growth is always observed to be associated with non-seam solution, but pervasive pressure solution without associated dolomitization should also be possible. Where clean, structurally resistant limestone units interface with responsive limestones or silty, clayey limestones, solution surfaces are not sutured. Nodular limestones are a characteristic product of non-sutured seam solution in silty, clayey limestones. Form and scale of fitted nodules may or may not be influenced by primary structure. The three forms of limestone pressure solution produce some types of finely crystalline stratigraphic dolomite, provide local sources of magnesium for dolomitization and calcite for porosity-filling cement, and severely distort primary sedimentary structures or create secondary structures.

Origin, sedimentary features, and significance of low-angle eolian "sand sheet" deposits, Great Sand Dunes National Monument and vicinity, Colorado

Abstract: Low-angle eolian deposits of a sand sheet constitute an areally extensive (710 km 2 ), transitional facies between high-angle eolian dunes and non-eolian deposits near Great Sand Dunes National Monument, Colorado. The bulk of the low-angle eolian deposits of the sand sheet lie in a 15-km-wide belt between the main dune mass at Great Sand Dunes and the Rio Grande River 45 km to the southeast, or "upwind." These low-angle eolian deposits have a number of distinctive sedimentary features which are useful in interpreting ancient rocks, including: (1) coarse-grained, high-index ripples (commonly horizontal); (2) gently dipping, curved or irregular surfaces of erosion (several meters in length) within the deposits; (3) abundant bioturbation traces, formed mainly by insects and plant roots; (4) patches or zones (extending up to 1 m below the ground surface) of bioturbated sand resulting from destruction of laminae by grass root growth; (5) convex upward laminations; (6) small scale cut-and-fill structures, perhaps due to scour around plant roots and subsequent infill; (7) gently dipping, poorly laminated layers resulting from adjacent grainfall deposition; (8) sets of normal- and inverse-graded laminae 1- to 4-mm-thick; (9) discontinuous thin 1- to 4-mm-thick layers of coarse sand intercalated with fine sand; (10) intercalation of non-eolian materials, including silty pond sediment and gravelly stream deposits; (11) occasional intercalation of high-angle eolian deposits. Low-angle eolian deposits of the sand sheet originate mainly by gentle deceleration of wind, in the lee of small topographic features, which produces a different style of deposition compared to the more extreme flow separation at the brink of a dune slipface. Low to moderate velocities, typical of those observed during this study, generally remove fine sand from exposed areas, and deposit the sand in nearby topographically lower or sheltered places, resulting in more texturally homogeneous sets of laminae (type "a") overlying more texturally heterogeneous deposits (type "b"). Recognition of low-angle eolian deposits may assist in identification of margins of ancient dune fields. From an economic point of view such information may be of value in predicting either (a) stratigraphic continuity of high-angle eolian reservoir rock, or (b) proximity of high-angle eolian rock.

Discrepancy between thin-section and X-ray estimates of dolomite in limestone

Abstract: Three independent errors affect X-ray diffraction estimates of dolomite in pre-Cenozoic carbonates. When using calcite/dolomite [104] peak ratios, each 1% of excess calcium in the dolomite lattice causes approximately a 2% overestimate of the amount of dolomite. Use of the [113] peak ratios avoids the stoichiometry problem; however. if quartz is present, the interference between the [102] quartz peak (d = 2.282 A) and the [113] calcite peak (d = 2.285 A) can cause a serious underestimate of the dolomite proportion. In addition, error will occur if the crystallite size in the powdered standards used to prepare X-ray calibration curves differs significantly from the crystallite size in the sample unknowns. All these errors can be avoided or corrected; however, point count r visual estimation of stained thin sections is a simple, reliable technique and is preferable for routine analysis of visibly crystalline, lithified, sediment.

Depositional Processes, Sedimentary Structures, and Predicted Vertical Sequences in Barred Nearshore Systems, Southern Oregon Coast

Abstract: Many of the straighter, less rocky parts of the southern Oregon coast are characterized by nearshore bars that extend obliquely out from shore and migrate alongshore. A typical oblique bar is attached to the foreshore at its upcurrent end, nearly parallel to shore through most of its length, and bowed seaward into a rip-channel-mouth bar at its downcurrent end. The main part of an oblique bar is separated from the foreshore by a longshore trough that curves seaward into a rip channel. The net wave-induced currents flow obliquely shoreward over the bar, parallel to shore through the longshore trough, and seaward through the rip channel and over the rip-channel-mouth bar. Medium-scale crossbedding formed by the migration of megaripples in the direction of net water flow is the dominant nternal structure in most of the nearshore. Small-scale crossbedding formed by wave-ripple migration is dominant in the inner offshore, and planar bedding is dominant on the foreshore. Most or all of the bar deposits would be destroyed if the coast prograded slowly but continuously. The vertical sequence produced during progradation should be characterized, in ascending order, by inner offshore deposits, possibly deposits of the lower seaward slope of the bar, a subhorizontal erosion surface corresponding to the deepest part of the rip channel, rip channel deposits, longshore trough deposits, deposits of the foreshore-trough transition, and foreshore deposits, Subhorizontal erosion surfaces probably occur in deposits formed by the slow progradation of most kinds of barred nearshore systems, but they should not occur in deposits formed by the progradation of a non-barred nearshore system. The nature of the deposits produced by the progradation of a barred nearshore system should vary with orientation of the bars relative to the shoreline, number of bars, wave energy, and available grain sizes.

Calcified filaments in Quaternary calcretes; organo-mineral interactions in the subaerial vadose environment

Abstract: A petrographic study of Quaternary (Pleistocene to Recent) calcretes from coastal regions of the western Mediterranean reveals that calcified filaments are abundant and characteristic structures in these terrestrial deposits. From their sizes, shapes and arrangements, these structures are interpreted as calcified organic filaments of soil fungi, algae, actinomycetes and root hairs of vascular land plants. The morphology of calcified filaments depends on: The origin and composition of the organic filament The state of preservation of the organic filament at the beginning and during its calcification Whether the mechanisms of calcification are essentially physicochemical, biochemical or a combination of both physicochemical, and biochemical processes. With the exception of two samples, the mineralogy of the calcified filaments is low magnesian calcite. The fabric of this calcite consists of micronsized needles (with long and crystallographic c-axes oriented perpendicular to the organic substrate), rhombs, or plates. The resulting calcified product, formed by mineral encrustation, impregnation or piece-meal replacement, may be a tube or solid rod depending on whether the organic mould of the filament has been left empty or filled with a micritic cement. The recognition of calcified organic filaments in calcretes indicates the former existence of organic activity and, moreover, indicates that these deposits functioned as biological soils.

The Role of Waves and Tidal Currents in the Development of Tidal-inlet Sedimentary Structures and Sand Body Geometry: Examples from North Carolina, South Carolina, and Georgia

Abstract: Morphologic variability in tidal inlets along the southeastern coast of the United States has been considered with respect to the distribution of large-scale sand bodies, intertidal and subtidal bedforms and internal sedimentary structures. Data indicate that the morphologic variability in these inlets can be largely explained as a response to waves and tides. Other factors (tidal prism, inlet cross-sectional area and shape, the nature of the back-barrier bay, the degree of flood or ebb dominance, fresh water input, relative changes in sea level and sediment supply) exhibit lesser controls and their effects are less easily quantified. Three types of inlets are identified: tide-dominated, wave-dominated and transitional. 1) Tide-dominated inlets are characterized by a deep, ebb-dominant main channel flanked by long, linear channel-margin bars. Flood-tidal deltas are poorly developed or non-existent. Sand bodies landward of the inlet throat are confined to tidal point bars further landward in the marsh creek system. 2) Wave-dominated inlets are characterized by large, lobate flood-tidal deltas building into wide, open lagoons. The ebb-tidal delta is small and extends only a short distance from the beach. Tidal channels are generally shallow (less then 6 m) and often bifurcate landward and seaward of the throat. 3) In transitional inlets, major sand bodies are typically concentrated in the inlet throat. These inlets v ry widely in morphology and sand body geometry. Logically, this variability should be expressed in the rock record. In a vertical section through a tide-dominated inlet channel, a coarse base, overlain by bidirectional trough cross stratification from the deep channel and ebb-oriented, planar and trough cross stratification from the shallower channel should be expected. Swash-generated, horizontal plane laminations or slightly inclined accretion beds formed along the channel margins are less likely to be preserved. In contrast, a wave-dominated inlet sequence would contain primarily landward-oriented, planar and cross stratification from the shallower channel bottom, overlain by dominantly horizontal or slightly inclined plane laminations from the shallow channel sides. Transitional inlets would produce a variety of sequences, the xact nature of which would reflect the relative importance of waves and tides.

A new approach to portable vibracoring underwater and on land

Abstract: An effective and inexpensive new method of vibracoring has been developed for acquisition of continuous cores up to 13 m long in unconsolidated sediments. This system can be used on land and in water depths up to 18 m. The principal components, a 7 hp. concrete vibrator, drill pipe and liner or thin-walled aluminum irrigation tubing, are stock items. The entire system weighs 150 kg and costs about $2,000. With this system approximately 200 cores averaging 4-6 m in length have been recovered in sediments ranging from clay to coarse sand. Percentage of recovery is usually 90 to 100% and distortion of delicate sedimentary structure is generally minor.

Buoyancy in Debris Flows

Abstract: Coarse granular solids in debris flows are supported within a clay-water matrix by strength and buoyancy. The magnitude of buoyancy is determined in part by the density contrast between coarse grains and matrix, but also by the pore-pressure increase associated with transfer of the weight of coarse grains as they are supported by matrix strength. Buoyancy therefore increases with concentration of coarse grains and can greatly increase the competence of the flow. Mobility of debris flows probably is also enhanced by increased buoyancy, due to reduction of internal friction. Therefore, large buoyancy forces, due to heavy loading of the matrix by coarse solids, may be significant in producing the high competence and mobility exhibited by many natural flows.

Fresh-Water Cementation of a 1,000-Year-Old Oolite

Abstract: Calcite cementation of aragonite ooid sand is producing oolite on Joulters Cays, Bahamas. During the last 1,000 years, calcite cement has formed at an average rate of between 27 and 55 cm3/m3/yr and is derived from dissolution of ooid aragonite in fresh water. The dissolution-reprecipitation of carbonate minerals in the aquifer results in ground waters of unusually high Sr content. Sea water and mixtures of fresh and sea water appear to inhibit cementation. A pronounced cement fabric change occurs across the water table and has produced an obvious petrographic record of fresh-water diagenesis. Above the water table, cement is typically near grain contact positions, where water is held by capillarity; below the water table, cement is more randomly distributed arou d grains. At the water table a transition zone, 1 meter thick, marks the boundary between cement textures. No porosity reduction is associated with cementation; calcite cement precipitation is apparently compensated by an equal or greater amount of aragonite dissolution in the interval undergoing cementation. Permeability is more variable above the water table than below it, reflecting early channelling of flow patterns in the vadose zone. Effective permeability below the water table is one to two orders of magnitude higher than above the water table because of entrained gas in the vadose zone. This permeability difference promotes preservation of unstable minerals above the water table and continued diagenetic alteration below the water table.

Rapid and Widespread Generation of Recent Oolitic Hardgrounds on a High Energy Bahamian Platform, Eleuthera Bank, Bahamas

Abstract: Contemporary intergranular submarine cementation occurs in close association with strongly agitated oolitic sands in the Schooner Cays tidal bar belt, resulting in widespread generation of surficial oolitic hardgrounds. Hardground formation takes place from near the shelf margin bankward up to 15-17 kilometers, within the zone of active oolitic sand, in water depths ranging from 1-II meters. Two forms of lithified oolitic sediment (hardgrounds) are recognized: in situ surficial crusts and unattached clasts (erosional crust fragments). Isopachous fibrous aragonite constitutes the major physicochemical cement; other cement forms of aragonite and high magnesium calcite are widespread but subordinate. Previously undocumented, probable chasmolithic/endolithic algae also effectively bind and cement surficial unconsolidated oolitic sediment by filament calcification. Characteristic decrease in cementation downward from crust surfaces substantiates their formation at or near the sediment-water interface. Patterns of hardground formation apparently result from the interaction of tidal bar surface topography, local hydrography and stabilization by algal scum mats. Radiocarbon dating, field relationships and crust petrography document the rapid and contemporaneous formation of oolitic hardgrounds. Initial crust formation can occur within a few months or less. Hand probing of tidal bars and radiocarbon dating of erosional oolitic clasts suggest that similar patterns of hardground formation occurred during earlier growth stages of the tidal bars.

Quartz-grain surface features of Mesozoic-Cenozoic sands from the Labrador and western Greenland continental margins

Abstract: The quartz-grain surface features of 39 Cretaceous and Paleocene sand samples from offshore Labrador and onshore western Greenland were examined by scanning electron microscope. The results of this study and those of other workers reveal the existence of 30 distinct surface features. The Labrador-Greenland sand grains show an abundance of pre-transportational surface features, imparted during liberation of the grains from crystalline source rocks. Impact features are relatively scarce, and features indicative of deltaic, aeolian and glacial environments are absent altogether. Among chemically produced surface features, those of precipitational origin far outweigh those due to silica dissolution. It is concluded that the Labrador-Greenland sands were derived, under acidic weathering conditions, from local crystalline parents and deposited in both marine and non-marine environments. Quartz overgrowths are present in all stages of development. A distinctive, high-relief surface texture ("gullying"), thought to originate by marine decapitation of pre-transportational pedologic overgrowths, is described and illustrated. Authigenic minerals adhering to quartz-grain surfaces include kaolinite, illite, dolomite, potassium feldspar, and pyrite.

The Silicified Evaporite Syndrome--Two Aspects of Silicification History of Former Evaporite Nodules from Southern Kentucky and Northern Tennessee

Abstract: Silicified evaporite nodules from Mississippian rocks of south-central Kentucky and adjoining Tennessee typify in many respects silicified evaporite nodules which have been reported in rocks of widely differing ages and diagenetic histories. The silicified evaporite syndrome is an assemblage of characteristics shared by evaporite-replacement quartz nodules and includes both mineralogical and textural aspects. The sequence of textures in most nodules begins with interlocking quartzine spherules near the outer nodule edge and ends with a mosaic of equant megaquartz in the nodule center. Proportions of the different textures vary considerably between nodules. Anhydrite inclusions are abundant in the centers of megaquartz crystals. Megaquartz in the nodules is characterized by strongly un ulose, radial extinction. Euhedral terminations often have a cubic appearance. Many nodules contain crusts of zebraic chalcedony on the euhedral megaquartz bordering internal cavities. Oxygen isotope analyses are useful in understanding the silicification history. Microcrystalline quartz and fibrous quartz types have O18 values which are heavier than those of megaquartz. The microcrystalline quartz nodules, associated with the silicified evaporites, formed earliest in fluids related to sea water. Length-slow fibrous quartz types formed later in water of intermediate composition. Megaquartz developed at somewhat higher temperatures and in fluids derived from meteoric water. Zebraic chalcedony in the nodules is also of meteoric origin. All silicification of these samples has probably occurred at temperatures of less than 40° C.

Inverse Grading and Hydraulic Equivalence in Grain-Flow Deposits

Abstract: Inversely graded grain-flow deposits are characterized by a hydraulic equivalence that differs from that based on settling velocities or entrainment. Dispersive equivalence, derived from the dispersive pressure hypothesis on how inverse grading develops, was found to agree reasonably well with observed relationships between grain sizes and densities in grain-flow deposits. Furthermore, observed relationships in deposits formed in subaerial and subaqueous environments were found to be independent of fluid density as is required by dispersive equivalence. The results suggest that dispersive pressure controls the development of the inverse grading common to beach foreshore laminations, slip-face foreset strata, the basal parts of some coarse-grained turbidites, and other diverse deposits

Lichen Stromatolites: Criterion for Subaerial Exposure and a Mechanism for the Formation of Laminar Calcretes (Caliche)

Abstract: Endolithic and epilithic lichens are pioneer colonizers of exposed calcrete (caliche) hardpans and fresh rock surfaces throughout the coastal regions of the western Mediterranean. Petrographic studies reveal that saxicolous (rock substrate) lichens cause characteristic textural and fabric changes of the colonized substrate. As a result of biophysical disintegration, biochemical decomposition and biosynthesis of mineral components, protosoils are formed. Induration of these superficial, biologically weathered rinds, followed by further lichen colonization, leads to the formation of lichen stromatolites. The effect of saxicolous lichens on exposed, indurated calcrete surfaces has been documented petrographically in terms of textural, fabric and compositional changes with respect to the underlying unaffected part of the calcrete profile. Such documentation provides criteria for the recognition of subaerially exposed substrates and is, thus, of potential importance in helping to delineate former land surfaces in ancient successions. Laminar calcretes contain organic-rich and organic-poor millimetre laminae, microborings, algal filaments, fungal hyphae, calcite spherulites, spherical structures and decimicron-sized calcite grains arranged in surface parallel layers. Hitherto, these features have been reported by numerous workers but their origin has not been considered. Comparative studies with modern lichens suggest that laminar calcretes examined here are lichen stromatolites, that is, that they owe their origin to successive cycles of organic (lichen) telogenesis, incipient pedogenesis and diagenesis. Recognition of lichen stromatolites has far reaching implications in terms of palaeoenvironmental reconstructions. Lichen stromatolites demarcate subaerial discontinuity surfaces, thus indicating terrestrial conditions and duplicating palaeotopography.

Authigenic iron sulfides as paleosalinity indicators

Abstract: A reasonably good correlation has been found between overlying water salinities and the pyrite-to-"FeS" (acid-volatile iron monosulfides) ratio in modern anoxic sediments below the zone of sulfate reduction. Values of FeS2/FeS greater than 10.0 are characteristic of marine sediments, whereas ratios from brackish estuarine or fresh water lake sediments are generally less than one. The low ratios are believed to result from an incomplete conversion of FeS to pyrite. Since the reaction is dependent upon available interstitial sulfate, it cannot proceed to completion in low sulfate-containing fresh or brackish water sediments. It is suggested that the presence of appreciable authigenic iron monosulfides in ancient sediments may be used as evidence that they were deposited in br ckish or fresh water environments.

Magnetic susceptibility and particle-size distribution in Recent sediments of the Loch Lomond drainage basin, Scotland

Abstract: Bedrock, till, soil, stream and beach samples have been analysed from the Loch Lomond drainage basin together with thirty sediment cores, either 6-m or 1-m long, from the lake itself. Magnetic analyses of susceptibility (chi ) and isothermal remanence (IRM) have been made on dried bulk samples and various particle-size fractions. Down-core variations in susceptibility show a series of maxima and minima which correlate with horizons of freer-and coarser-particles respectively. Magnetic susceptibility can thus be used to establish a lithostratigraphy for the lake sediments. This stratigraphy can be linked to a time scale based on palaeomagnetic, isotopic and pollen analyses. The source of the magnetic minerals in the lake could be either primary iron oxides in the bedrock or till, or alternatively secondary iron oxides developed in the soils. The possibilities of either as the major source are discussed in terms of three proposed models: a source model, an erosional/transport model and a depositional model. Variable erosion of primary magnetite explains most of the down-core, between-core and within-drainage basin changes in mineral magnetic properties.

The Snuggedy Swamp of South Carolina; a back-barrier estuarine coal-forming environment

Abstract: The Snuggedy Swamp is a back-barrier estuarine depositional environment. Extensive peat deposits as much as 15 feet (450 cm) thick are underlain by kaolinite-rich clay similar to the underclay associated with coal seams. These peat and underclay deposits are underlain by silt and clay which coarsen upward and were deposited in lagoons; the peat deposits are surrounded by eroding sand areas that are the remains of Pleistocene barrier islands. At present the salt marsh is encroaching on the swamp, destroying the fresh water vegetation that forms the peat and covering the peat with clays and salts. The thickest peat deposits in this swamp are orientated parallel to the surface trends of the barriers or preexisting tidal inlets. Peat islands are hypothesized as the method by which peat was deposited in this swamp. Initially this region consisted of extensive salt marsh and probably some open water areas. Then fresh water vegetation began to colonize those areas of the salt marsh that were farthest removed from the areas of tidal creek submergence, and eventually fresh water vegetation covered the surface of the marsh. These coalescing fresh water peat islands resulted in an undulatory contact between the peat and the underclay. Two types of splay deposits are interbedded with the peat in this area, crevasse splays and fire splays. Both are composed mainly of clay, but fire splays are dintinguished by a dark fusinite (charcoal) zone at their base.

Temporal and Spatial Variations in Shoreline Changes and their Implications, Examples from the Texas Gulf Coast

Abstract: Shoreline changes from sandy oceanic coasts worldwide show temporal and spatial variations that are similar to, and exemplified by, composite data from the Texas coast for four time periods extending from 1850 to 1975. The quantitative data, which exhibit remarkably similar distributions for the first two and latter two periods, show that both total length of eroding coast and rates of erosion have substantially increased since 1955-60. As a result, some shoreline segments that prograded following sea-level stillstand are presently eroding. Spatial variations in shoreline changes occur on regional and local scales. Regional tendencies toward accretion or erosion closely correspond with physiographic provinces, which in Texas are defined by transgressive and regressive beaches and barriers. Within each province, rates of accretion and erosion display periodicities that are irregular to quasi-sinusoidal in form and are attributed to shoreline rhythms resulting from differential rates of sediment transport. Natural processes, such as decreases in sediment supply and continued relative sea-level rise are largely responsible for long-term shoreline retreat. Short-term (historical) shoreline changes reflect the long-term trends. but they also reflect secular sea-level variations and human activities. The latter, in the form of coastal engineering modifications, are clearly responsible for the highest short-term rates of accretion and erosion. On the Texas coast nearly half of the total beach sand supplied by updrift erosion, presently a major sediment source, has been trapped by jetties at harbor entrances. This impoundment of sand at impermeable barriers together with reduced sediment influx from damming of rivers suggest that anthropogenic augmentation of natural shoreline erosion will likely increase from local to regional effects.

The Ooids of Baffin Bay, Texas

Abstract: The radial coatings of Holocene ooids from Baffin Bay, Texas, are Mg-calcite, whereas the tangential and micritic coatings are aragonite. Centrifugally deposited ooid coatings are similar in many respects to centripetally deposited marine cements. The reason(s) for the mineralogical and textural variation in these apparently inorganic carbonate precipitates are not certainly known, but the rate of nucleation and precipitation, degree of agitation and presence of certain types of organic compounds appear to be the most likely explanations.

On how some rays (Elasmobranchia) excavate feeding depressions by jetting water

Abstract: The feeding holes formed by bottom-feeding rays are not necessarily dug by a "wing-flapping" mechanism. The New Zealand eagle ray, Myliobatis tenuicaudatus, excavates hydraulically much of its vertical-sided feeding depression by the action of water jetted downwards through the mouth and/or gill clefts. We infer that other rays may use a similar mechanism, as evidenced by the deep, vertical-sided holes that they also dig. Although ray feeding holes are very numerous in present day intertidal and sub-tidal sand flats around northern New Zealand, no examples of fossil feeding excavations have been reported from the widespread Quaternary sediments of similar environments nearby.

Late Pleistocene Fan-Deltas Along the Dead Sea Rift

Abstract: Late Pleistocene lacustrine fan-deltas which developed in an arid, disequilibrated submergent environment along the western fault scarp of the Dead Sea Rift are characterized by: 1) alluvial fan deposits of crudely stratified conglomerate beds and horizontal and cross-bedded sandstones. The conglomerates and sandstones are cone-shaped and comprise the entire sequence in the fan head area; 2) fan-front thick and thin interlayers of ripple cross-laminated sandstones and mudstones which constitute a sedimentary belt a few kilometers wide in front of the fan; and 3) fan-influenced detrital laminated chalks. Both the interlayered and the laminated units appear as widespread sheet bodies whose basal contacts are gradational and upper contacts, sharp and disconformable. The rapid rise of lak level resulted in the buildup of upward-fining sequences. At the end of the Pleistocene, the lake level dropped; it began to rise again during the Holocene and, contemporaneously, new fan deltas began building up.

Lenticular gypsum; occurrences in nature, and experimental determinations of effects of soluble green plant material on its formation

Abstract: Lens-shaped gypsum crystals are common in both modern and ancient evaporite deposits, but laboratory experiments demonstrate that the presence of certain types of dissolved organic material is the major factor promoting growth of this particular habit rather than warm saline water conditions. A very widespread type of soluble organic material results from green plant decomposition. Experiments utilizing diffusion-controlled growth of gypsum within sediments of various types, and also utilizing evaporation-controlled solution growth indicate that this organic material promotes the growth of lenticular gypsum, but only under alkaline conditions. In acid conditions, both in the presence or absence of added plant material, only elongate prismatic gypsum develops. The type of sediment with n which gypsum grows has only minor effects on its morphology unless the sediment is contaminated with soluble organics. These observations help to explain why lenticular gypsum forms in some instances, and prismatic gypsum in others, within natural sedimentary environments.

Recognition and Sedimentologic Description of Recent Debris Flow Deposits from the Ross and Weddell Seas, Antarctica

Abstract: The Antarctic continental margin is inferred to be an active site of debris flow deposition and an ideal place to study these deposits. Although similar to the poorly sorted glacial-marine sediments from which they are derived, debris flow deposits can be distinguished, using textural criteria, from sediments deposited by floating ice. Antarctic debris flow units are poorly sorted, massive, texturally homogeneous throughout their thickness, and have sharp upper and lower boundaries. A crudely developed pebble fabric may be present in some units and all contain displaced shelf faunas or are unfossiliferous. Deposits from floating ice are poorly sorted, stratified, bimodal, and texturally heterogeneous within a single unit. They have gradational contacts and a crudely developed pebble f bric. These textural characteristics are useful in distinguishing debris flow deposits from floating ice sediments in the ancient record. No reliable textural criteria were found for differentiating between ancient debris flow deposits and tillites deposited by grounded ice.

Deep Sea Carbonates: Acoustic, Physical, and Stratigraphic Properties

Abstract: An examination of the relationship between acoustic, stratigraphic and physical properties of deep-sea carbonates from the eastern equatorial Pacific revealed that variations in acoustic impedance are almost entirely controlled by changes in saturated bulk density (or its inverse, porosity). Velocity changes are very small. Saturated bulk density changes are highly correlated with variations in percent calcium carbonate. High carbonate samples are dominated by high density platy carbonate material while low carbonate material is dominated by low density spiny siliceous microfossils. Stratigraphic control was achieved through: (1) comparison of the downcore variations in percent carbonate with the carbonate curves of well-dated cores (e.g., RC11-209), (2) stratigraphy of Pulleniatin coiling directions, (3) radiolarian zonation, (4) the extinction of P. lacunosa, and (5) last appearance of discoasters. Sedimentation rates in the area studied are found as 6.17 meters per million years for the last 700,000 years and 4.15 meters per million years before that time. The oxygen isotope record provided the final evidence linking the acoustic properties to the paleo-oceanographic record. Variations in saturated bulk density (or its inverse, porosity) that account for the changes in acoustic impedance are ultimately caused by climatic changes. Warm periods were times of enhanced dissolution which caused low percent carbonate and low saturated bulk density (and therefore low impedance) values. The opposite is true for cold periods. These climatic cycles can be matc ed with glacials and interglacials for the past one million years, but have also occurred before the advent of Pleistocene glaciation. Thus the acoustic record of equatorial carbonates contains a strong paleo-oceanographic signal.

Clastic Sills and Dikes Associated with Deep-Water Sandstones, Tourelle Formation, Ordovician, Quebec

Abstract: The Arenigian Tourelle Formation is dominated by thick, coarse, graded sandstone layers. These poorly-sorted sands were deposited on submarine fans which formed in response to uplift during initiation of the Taconic orogeny. The base of the formation consists of interbedded red and green shales, and rippled dolomitic siltstones, and is injected in spectacular fashion by fine to coarse sandstone dikes and sills. Sills are the dominant type of injection and may exceed 3 metres in thickness. Depth of burial during injection was on the order of 20 to 60 metres, with sand mobilization being the result of liquefaction and perhaps the thixotropic behavior of a primary clay matrix. The source of injected sand in the Cap Ste-Anne section, which was studied in detail, is inferred to have been a sand-filled submarine channel to the southwest of the present outcrop area. Sand was injected laterally along bedding planes and upward from the base to the top of the sequence. Liquefaction was facilitated by sediment loading and compaction under a clay seal, and was triggered by (1) slumps, which are present in the section, and/or (2) earthquakes associated with contemporary orogenesis. Clastic injection did not occur at higher levels in the Tourelle Formation for want of an impermeable clay seal capable of preventing escape of excess pore fluids during compaction.

An Explanation of the Logarithmic Spiral Plan Shape of Headland-Bay Beaches

Abstract: This paper presents an explanation for the logarithmic spiral plan form commonly observed in headland-bay beaches. The analysis focuses on the equilibrium state of such beaches. Basic ideas on longshore currents and wave refraction and diffraction are cast into simple analytical expressions and combined with an empirical relation between wave energy, grain size and beach slope to construct synthetic beach forms. The plan shapes predicted by theory are closely fitted by the logarithmic spiral and compare well with natural beaches. The model shows the interplay of the various factors responsible for the observed beach shape and provides a simple framework for the interpretation of a common feature of many shorelines.

Marine Aragonite Sea-Floor Growths and Cements in Permian Phylloid Algal Mounds, Sacramento Mountains, New Mexico

Abstract: A heretofore undocumented example of aragonite botryoid formation on the sea floor is described from shallow-water Wolfcampian (Permian) phylloid algal mounds in the Sacramento Mountains of New Mexico. The cores of these mounds consisted of botryoidal aragonite masses that stood in low relief on the sea floor. The upward growth and coalescence of these masses created a solid, anastomosing framework of inter-botyroid and internal cavities that were simultaneously filled with marine sediment and aragonite fan cements. The resulting inorganic boundstone became the substrate for later phylloid algal mound development. Subsequent algal colonization and contemporaneous aragonite precipitation also resulted in the development of a mixed inorganic-organic facies wherein algal blades are encas d in marine aragonite. Former aragonite constitutes from 50-85% of the total mound rock. The core facies of these inorganic "reefs" are texturally and, most probably, genetically similar to the many Paleozoic mudmounds throughout the world known variously as Waulsortian mounds. Stromatactis reefs, and zebra limestones. The former marine aragonite is represented by divergent-radial mosaics of coarsely crystalline calcite pseudospar that commonly are replaced by ferroan "baroque" dolomite. This pseudospar occurs as ray-crystals and composite crystals with included relicts of the precursor fibrous aragonite. This species of calcite is distinct from the radiaxial-fibrous mosaics that commonly replace acicular carbonate fabrics. Mineralogic stabilization of the aragonite to calcite via paramorphic inversion is believed to have occurred in the subaerial environment, with recrystallization to pseudospar occurring in the subsurface. Unresolved problems include the mechanics of fabric-retentive stabilization diagenesis of this aragonite, and the apparent anomaly of aragonite abundance in contrast to Holocene reefs, in which Mg-calcite is the dominant cement mineralogy.

Features and Origin of Italian Jurassic Radiolarites Deposited on Continental Crust

Abstract: Jurassic radiolarites that were deposited on continental crust and which are overlain and underlain by pelagic limestones were examined in four basins. Radiolarites studied range from bedded chert, where chert occurs in even, continuous beds separated by conscpicuous rhythmic shale interbeds, to nodular-lumpy chert, where chert occurs as isolated nodules in limestone at one extreme to lumpy beds of irregular thickness at the other extreme. Bedded chert formations are composed of microcrystalline quartz, clay minerals, and minor amounts of either hematite or organic matter pigment; nodular cherts contain in addition considerable micrite (altered coccolith ooze). Chert nodules, lenses, and beds all formed by the diagenetic reorganization of silica almost entirely of biogenic origin, chiefly from Radiolaria. Nodular-lumpy chert beds and even some bedded chert formed by partial to complete replacement of limestone. Lumpy beds formed because compaction, dissolution of calcite, and replacement of calcite were not uniform in either time or place within a bed. Many even-bedded chert layers were originally silts or sands composed almost entirely of current-deposited Radiolaria. The rhythmic bedding of radiolarite is the product of 1) episodes of rapid current-deposition (e.g., turbidity currents) of radiolarian sediment alternating with slow deposition of hemipelagic mud and 2) episodic growth of Radiolaria. Bioturbation blurs the evidence of these two different origins, but the following features indicate that many chert-precursor beds were current-deposited: sharp chert-shale bedding contacts, flute casts, graded beds, clay clasts, laminations and rapidly buried animal tracks and trails. Water depths during radiolarite deposition are uncertain. Regional stratigraphic data, evidence of redeposited radiolarian sand, and acceptance of the Bosellini-Winterer model of carbonate dissolution surfaces during Late Jurassic time leads EFM to conclude that well-bedded radiolarite was deposited at depths close to the CCD (2500 m) and that lumpy-bedded chert was deposited between the ACD and the CCD (1500 to 2000 m). Features indicating the presence of evaporites (evaporite pseudomorphs, breccias formed by crystal growth, quartzine, lutecite), paleosoil fabrics, stalactite-stalagmite fracture fillings, and disbelief in the validity of the CCD in Late Jurassic time in the Tethys leads RLF to conclude that the Lombardy radiolarites were deposited in environments that were, in part, shallow intertidal mudflats where local precipitation and solution of evaporite minerals and occasional subaerial exposure took place.