B. Charlotte Schreiber

Spring Peas from New York State: Nucleation and Growth of Fresh Water Hollow Ooliths and Pisoliths

ABSTRACT Delicate hollow spheroids (1 to 2 mm) composed of radially fibrous calcite occur in, and on the surface of, calcareous travertine being deposited from water of the Orenda Spring, Saratoga Springs, New York State. Field observations suggest that spheroids form by the precipitation of calcite on the surface of water droplets that contain newly formed bubbles of CO2. Although many of the spheroids subsequently disintegrate, some grow by the addition of laminae consisting of radial bundles of fibrous calcite, commonly separated by thin (? seasonal) laminae of fine-grained calcite and detritus. These spheroids are either incorporated into laminar travertine sheets or grow by successive increments into fresh-water ooliths and pisoliths with both a concentric and a radial structur .

Evidence for Early Aragonite in Paleo-Lacustrine Sediments

ABSTRACT Distinct pseudomorphs of displacive pseudohexagonal aragonite crystals are found in organic-rich lake deposits of the rift basins of the east coast of North America (Triassic-Jurassic). These crystals developed while the sediments were relatively uncompacted and soft, and hence are displacive, readily mistaken for molds and casts of pre-existing halite and/or thinolite (ikaite). A second form of early aragonite is found in more evaporative sediments of the Newark Basin (Passaic Formation), developed as pseudomorphic replacement of precursor, more soluble minerals such as glauberite and/or gypsum that formed during deposition. In all instances the aragonite is now replaced by calcite and dolomite, but still preserves much of the original and unusual crystal form and associated inter al structure. This aragonite is probably formed due to release of CO2 by bacterial breakdown of organic matter in bottom sediments. Elevated Mg/Ca in the pore waters may also have promoted preferential formation of aragonite over calcite.

Experimental Simulation of Plagioclase Diagenesis at P-T Conditions of 3.5 km Burial Depth

Dissolution of plagioclase under the physical conditions at shallow to intermediate burial depths is a prime candidate for secondary porosity generation in feldspathic siliciclastic sediments. The diagenetic behavior of granular aggregates of plagioclase feldspar and quartz has been investigated by experimentation performed in a Bridgeman-type pressure vessel. The experiments, each of two weeks duration, simulated pressure-temperature conditions approximating 3.5 km burial depth. By using a double-acting pore-fluid reservoir, solutions of various chemistries were cycled through samples composed of oligoclase or labradorite feldspar and quartz (90:10 wt% respectively).

Sedimentation and Diagenesis of Upper Smackover Grainstone, Jay Field Area, West Florida: ABSTRACT

The examination of core and logs from a well 3 mi (5 km) southwest of the Jay field has given considerable insight into the upper Smackover facies distribution, diagenesis, and the application of recent models for the sedimentation and diagenesis of this Jurassic reservoir. A 63-ft (19 m) thick unit of oolitic and oolite-oncolite grainstones is recognized in the upper Smackover. High-angle inclined bedding, visible on both core and dipmeter, with a consistent 15 to 20° northeast dip, demonstrates the presence of oolite bars. These bars formed a barrier which affected subsequent deposition and diagenesis in the Jay field area. A complicated diagenetic history of marine and vadose cementation, and pervasive and selective dolomitization have left a unique imprint on the porosity and permeability of these rocks. Dipmeter results and petrographic analysis of the grainstones indicate that cementation and diagenesis have not been uniform. Within the large-scale cross-strata, permeable beds are interstratified with tightly cemented or compacted, impermeable beds. Horizontal flow should be greatest along the strike of the inclined units, because the flow would remain within the permeable planes of the inclined strata. Thus, dipmeter correlation permits an interpretation of the direction of bedding permeability anisotropy produced by the inclination of the pore system. The characteristics of sedimentation and facies distribution in the Jay field area have previously been compared with a modern analog from Joulters Cay in the Bahamas. The Trucial Coast of the Persian Gulf in the Abu Dhabi region may be a better model. The style of deposition and distribution of carbonate and evaporite sediments, and diagenetic characteristics in the grainstone barriers and lagoons closely fit the sedimentation and diagenetic pattern in the Jay Field area. End_of_Article - Last_Page 950------------

Evaporitive Limestone: Its Generation and Diagenesis: ABSTRACT

The Calcare-di-base is the only significant carbonate facies of the upper Miocene evaporites of Sicily. It was deposited within an exceedingly saline but marine water body adjacent to older, exposed carbonate terranes which surround the depositional basin. This somewhat dolomitic limestone is commonly cavernous or brecciated at outcrop and contains numerous zones of evident halite dissolution. It also interfingers with thin gypsum stringers in many areas and overlies diatomites of variable thickness. Based on petrographic and isotopic studies, it appears that this limestone was produced by diagenetic processes from an origininal aragonitic mud containing displacive halite hoppers and massive halite zones. Regional variation in carbon-13 (^dgrC13 from 0 to 49 pp PDB) and oxygen-18 (^dgrO18 from +6 to -5 ppt PDB) can be tied to variations in the mineral content of the original sediment and to later diagenetic waters and their organic content. The very negative carbon-13 values and the presence of native sulfur are indicative of the calcitization of gypsum, the by-product of the life processes of sulfate-reducing bacteria. These bacteria utilize part of the available and commonly copious organic matter associated with both the carbonate and with the diatomite. Inversion of the original aragonite to calcite has resulted in expulsion of strontium from the carbonate crystal lattice and the formation of celestite (SrSO4) which now fills the voids left by the dissolution of halite, and the associate pore spaces. End_of_Article - Last_Page 988------------