Christine E. Turner-Peterson

Alteration of Magnetite and Ilmenite in Upper Jurassic Morrison Formation, San Juan Basin, New Mexico: Relationship to Facies and Primary Uranium Mineralization: ABSTRACT

Petrographic studies of outcrop and drill core samples in the Westwater Canyon Member of the Morrison Formation along the southern and western margins of the San Juan basin reveal a close spatial relationship among altered (iron-leached) detrital magnetite and ilmenite (FeTi oxides), depositional facies in the overlying Brushy Basin Member, and distribution of primary uranium deposits. Iron leaching of FeTi oxides resulted from passage of solutions containing soluble organic material; concentrations of this organic material are the sites of the primary uranium orebodies. Along the southern and western parts of the basin, FeTi oxides typically have been leached in the upper Westwater Member, but are unaltered in the lower Westwater; however, locally, leaching occurred thro ghout the Westwater. This zone of leaching systematically thins northward to zero, where unleached FeTi oxides occur throughout the Westwater. Regional patterns of alteration of FeTi oxides correspond to regional facies distribution in the overlying Brushy Basin Member. Extensive FeTi oxide leaching characterizes the Westwater beneath the smectite-rich mud-flat facies of the Brushy Basin, whereas negligible leaching characterizes the Westwater beneath the zeolite-rich playa facies of the Brushy Basin. This correspondence between facies and alteration patterns suggests that solutions responsible for solubilization of organic material, which in turn leached FeTi oxides in the Westwater, originated from the mud-flat facies of the Brushy Basin. Organic material that precipitated from these solutions concentrated uranium to form primary uranium orebodies; therefore, distribution of the Brushy Basin mud-flat facies may define, and restrict, distribution of primary orebodies in the Westwater. End_of_Article - Last_Page 937------------

Pore Throats to Plate Margins: An Integrated Approach to Basin Analysis: ABSTRACT

The basin analysis approach to modeling sedimentary basins affords the opportunity to view ore deposits or accumulations of fossil fuels in the context of the evolution of the entire basin. Integration of data from diverse specialties is now widely practiced, and the multidisciplinary approach to the study of basins has greatly enhanced our ability to understand and to predict the occurrence and distribution of economically important commodities. A significant outgrowth of the basin analysis technique is a more rigorous testing of scientific paradigms. Feedback from diverse specialties provides numerous constraints so that no conclusion can be drawn about one aspect of a basins history without affecting the interpretation of other aspects. Thus, when a conclusion from one line of evidence is at variance with a conclusion drawn from several other lines of evidence, it is necessary to challenge the assumptions that led to the different conclusions. Challenging such assumptions usually involves examining cherished theories or paradigms. Our general reluctance to discard prevailing theories reflects our heavy reliance on useful rules of thumb; without them we could not begin to interpret the geologic past. This reluctance to reli quish useful theories is more easily overcome when several lines of evidence point us toward new concepts that have exciting implications of their own. Basin analysis, by its very nature, pushes us toward new perspectives and thus serves to promote new discoveries in geoscience. A case study in the San Juan basin of New Mexico serves as an example of the basin analysis approach to a geologic problem and serves to illustrate that sometimes answers to questions that were never posed are the most significant (and surprising) outcome of the basin-analysis approach. The original goal of the San Juan basin study was to develop a genetic model for sandstone-type uranium deposits in the Jurassic Morrison Formation. Tectonic, geophysical, sedimentologic, petrographic, End_Page 1523------------------------------ hydrologic, and geochemical (organic and inorganic) studies were drawn together so that mineralization could be evaluated as one diagenetic event in the context of the entire depositional, structural, and diagenetic framework. The result was not only a model for uranium mineralization but also the development of several new concepts, many of them unrelated to the original problem. Spin-offs from the original study include significant advances in fields as diverse as hydrology, coal geology, saline, alkaline-lake geochemistry, and clay mineralogy. It was possible to document, for instance, that authigenic illite formed at near-surface conditions in the absence of the elevated temperatures commonly thought to be required. This finding limits the use of illite as a geothermometer and mak s it of considerable interest in petroleum geology. The integrative nature of the basin analysis approach allows a synoptic rather than a myopic view of specific geology problems. The approach can be applied to any geologic problem and will continue to lead us away from current paradigms as multiple lines of evidence force us to question our most cherished beliefs. Most certainly we will continue to learn surprising answers to questions we forgot to ask. End_of_Article - Last_Page 1524------------

Paleogeography of Northern Arizona During Deposition of Permian Toroweap Formation: ABSTRACT

Stratigraphic and facies analyses of the Toroweap Formation have yielded reliable indicators of the paleogeographic conditions that existed in northern Arizona during the Permian. Four depositional environments have been recognized that are based on definitive facies. The four environments and their respective facies laterally from west to east are: (I) open marine; skeletal packstone and wackestone, pelletal wackestone; (II) restricted marine; aphanitic lime mudstone, dolomite mudstone, sandy dolomite; (III) sabkha; gypsum, horizontal and gnarly bedded sandstone; and (IV) eolian dune; cross-bedded sandstone. A marine transgression encroached upon coastal and continental sabkhas eventually drowning a large eolian dune field in its eastward advance across northern Arizona depositing the Toroweap Formation. Eventually the sea slowly withdrew westward and thick prograding End_Page 769------------------------------ sabkha and dune facies were deposited over marine carbonate rocks. Desert conditions prevailed at the eastern edge of the sea as it transgressed eastward and regressed westward across northern Arizona. Eolian dunes with south-dipping cross-beds were formed by trade winds blowing southward toward the paleoequator south of Arizona during the Permian. Extensive coastal and continental sabkhas formed between the restricted mud flats and the dune fields. Westward, the restricted marine deposits of dolomite characterized by bivalves and gastropods gave way to shallow open-marine deposits of brachiopods, byrozoans, corals, and crinoids. The westward regression ended with a rapid transgression that deposited the Kaibab Limestone across the sabkhas and dune fields of the Toroweap and Coconino ormations. End_of_Article - Last_Page 770------------