Depth-related Changes Research Articles

Burial Diagenesis and Geochemical Alterations and Their Relationship to High Fluid Pressures, Frio and Vicksburg Shales: ABSTRACT

Depth-related mineralogic changes in Frio and Vicksburg shales from Ann-Mag field, Brooks County, Texas, were studied to assess the nature and extent of burial diagenesis and geochemical alterations, and to determine their relationship to development of high fluid pressures. Mixed-layer illite-smectite and illite dominate the mineralogy of the shales. Burial diagenesis has transformed randomly interstratified illite/smectite to regularly interstratified illite/smectite and to illite, over a depth range of 6000-10,400 ft (1828-3170 m). The quantity of expandable smectite layers in the fine clay fraction decreases from 70% in the shallowest sample to 22% in the deepest sample, whereas illite increases from 27 to 61%. Alteration of feldspar in the fine silt fraction probably provided the potassium required in this reaction. An abrupt increase of smectite to illite alteration occurs at a depth of approximately 8900 ft (2713 m), corresponding to a temperature of 224/sup 0/F (107/sup 0/C). Fluid pressures estimated from conductivity data and measured pressures show that, at this depth, a significant increase in fluid pressure takes place. Associated with this increase is a decrease in shale density. These relationships suggest that high fluid pressures in this field are caused by clay transformation reactions that expel interlayer water into themore » pore spaces. Upward movement of pore fluids in thick shales is also suggested by extractable cation determinations.« less

Depth-Related Changes in Sagitta Morphology in Six Macrourid Fishes of the Pacific and Atlantic Oceans

On suggere que la taille du sagitta est controlee par les facteurs de l'environnement et n'a que peu de signification taxonomique

Diagenesis in Upper Miocene Sandstones, Louisiana Gulf Coast: ABSTRACT

Study of diagenesis in upper Miocene sandstones of coastal Louisiana documents depth-related cementation and geochemical changes of primary detrital mineralogy. Samples were collected from depths of 8,000-20,000 ft (2,600-6,500 m) in an area roughly corresponding to the upper Miocene depocenter in the Terrebonne trough of southeast Louisiana. Sandstones are primarily subarkoses and sublitharenites with minor amounts of feldspathic litharenites and lithic arkoses. Plagioclase feldspar (oligoclase/andesine composition) composes approximately 60% of the detrital feldspar. Dominant rock fragments are siltstone or mudstone, silicified volcanic rock, and chert. Authigenic minerals and cements occur in the following order: dolomite, chlorite grain coats, albite overgrowths on plagioclase and K-feldspar, quartz, calcite, kaolinite, and ankerite. Calcite composition remains nearly constant with depth, but ankerite composition differs both with depth and within individual samples. In general, the mineralogy and order of cements resemble that of the lower Tertiary sandstones of the Texas Gulf Coast; however, in the upper Miocene, the volume of each cement is much less and the depth of first occurrence is greater. Feldspars have reacted substantially with pore fluids. With increasing depth, feldspar becomes more sodic because of albitization and dissolution of calcic plagioclase. At approximately 20,000 ft, 75% of the plagioclase is nearly pure albite. Alteration of K-feldspar is not common above 17,000 ft; below 17,000 ft occurrences are rare because of dissolution. End_of_Article - Last_Page 257------------

Patterns of diagenesis in the Sherwood Sandstone Group (Triassic), United Kingdom

AbstractThe Triassic Sherwood Sandstone Group comprises a complex of continental red beds deposited by a major fluvial system flowing dominantly down a northerly inclined palaeoslope. Sedimentation took place in several distinct, tectonically active basins with varying maximum burial depths, ranging from shallow (<1 km) to deep (>3 km). Despite proximal to distal variations in stream type, a distinct suite of early diagenetic events can be recognized throughout all the depositional basins, which is related to the depositional environment. These events are best preserved in those basins with shallow burial histories, and show many similarities to the processes recorded from modern red beds of the Sonoran Desert, Baja California, although a more advanced grade of diagenesis has been reached in the Sherwood Sandstone. On the margins of the Irish Sea Basin in Cumbria, where burial was shallow, these early diagenetic textures are well preserved. The detrital grains underwent changes aimed at reaching equilibrium with the near-surface chemical environment. Unstable silicates were dissolved or replaced and the released ions were capable of precipitating authigenic phases, typically mixed-layer illite-smectite, K-feldspar, non-ferroan carbonates and hematite. Lateral variations in the early diagnetic assemblages reflect chemical and spatiotemporal changes ofinterbasin depositional and diagenetic environments. Deeply buried equivalents in the Irish Sea reached a maximum burial depth in excess of 3 km towards the end of the Mesozoic. Superimposed on the early diagenetic fabric are a series of depth-related changes. In the absence of early cements, compaction reduced porosity to low levels. Mixed-layer illite-smectites were converted to highly crystalline illites with low Fe and Mg contents. Early framework-preserving non-ferroan carbonates were extensively dissolved, generating widespread secondary porosity. Late pore-filling carbonate cements comprise ferroan dolomites and ankerites with compositions up to Ca(Ca0·02Mg0·43Fe0·53Mn0·02). Following late Mesozoic burial, inversion of the Triassic basins resulted in the re-exposure of basin margin sequences and shallow burial basins. Modern groundwaters are typically very dilute, with essentially neutral pH, and are capable of dissolving carbonate, sulphate and halite cements. In the Wessex Basin, however, the Triassic sandstones of the South Devon aquifer have been extensively modified by the percolation of post-inversion acidic groundwaters. These low-pH waters have resulted in thein situbreakdown of feldspar to produce abundant authigenic kandite, widespread dissolution of carbonate cements and removal of early-formed iron oxide cements. In contrast, the concealed, deeply buried eastern margin of the Wessex Basin exhibits a diagenetic evolution resultant of deep burial and has no development of authigenic kandite.

Field experiments for in situ measurement of photosynthetic efficiency and quantum yield

A method for the in situ determination of kc¯ , the average in vitro spectral specific attenuation coefficient of chlorophyll is described. This method takes into account changes in the spectral distribution of underwater light throughout the water column using a commonly available cosine collector. The kc¯ values, together with irradiance and primary productivity data gathered during the first GAP workshop at Lake Constance are used to calculate photosynthetic efficiencies Ez and quantum yields φ at different depths. The depth related changes found in φ and kc¯ are discussed, as is the question of constancy of φ max at low irradiance levels.

Optical parameters of euphausiid eyes as a function of habitat depth

The relationships between habitat depth, eye diameter relative to body length, and the dimensions of rhabdoms and crystalline cones have been examined for 13 species of three oceanic euphausiid genera with habitats ranging from near-surface waters to the deep-sea. Rate of eye growth decreases with depth. Longer rhabdoms may increase the visual sensitivity to point and extended light sources by an eye of a particular size with depth. Larger interommatidial angles suggest that visual acuity decreases at depth. Depth-related changes in euphausiid eyes are considered with respect to the probable roles of vision and bioluminescence in the deep-sea. Unusual features of the eyes of several species are described.

Depth related changes in the [formula omitted] ratio of skeletal carbonate deposited by the Caribbean reef-frame building coral Montastrea annularis: further implications of a model for stable isotope fractionation by scleractinian corals

Systematic variations in the isotopic composition of skeletal carbonate deposited by the Caribbean reef-frame building coral Montastrea annularis are correlated with water depth, location of the corallites within the corallum, and polyp packing density, as is demonstrated by isotope ratio measurements for 426 samples collected at 4.6 m depth intervals between 0 and 27.4 m at St. Croix, U.S. Virgin Islands. These data support a model, based on a study of Indo-Pacific scleractinians, proposed earlier for stable isotope fractionation by corals. Of particular interest is the fact that, within this species, ecotypic differentiation into shallow-water and deep-water subpopulations, with a boundary close to 20m, is reflected by changes in skeletal ° 13C. Stable isotope geochemical studies of both modern and fossil coral-derived carbonate may contribute to the solution of several problems having geologic and paleontologic significance.

Influence of X-Rays on Limb Regeneration in Urodele Amphibians

Influence of X-Rays on Limb Regeneration in Urodele Amphibians