Academic literature on the topic 'Pisolites (Petrology)'

AbstractPisolites in the bauxitic laterites of Darling Range, Western Australia may contain up to 50% AI2O3, although no major Al-containing phases can be identified using standard X-ray diffraction techniques. These pisolites have been investigated using a range of techniques, and it has been demonstrated that Al is present in the form of x-alumina, a poorly crystalline form of A12O3. The x-alumina exhibits an indurating morphology and appears to have formed from a hydrated precursor.

The Permian reef complexes of West Texas and New Mexico are among the classic carbonate sequences in the world and have significantly influenced geologic thinking for over half a century. Study of the reefs can be subdivided into 6 broad periods. The first period involved early exploration of the region, establishment of regional stratigraphic relationships and attempts at dating stratigraphic units. The Guadalupian Fauna typifies this early period. The second period, during the 1920-30's, was a time of early petroleum exploration in the region, following on discovery of the Kendrick Field in Winkler County, Texas, and resulted in attempts to explain the complicated subsurface stratigraphy. Development of a marginal reef model and research on facies relationships between the basin and shelf resulted in refinement of stratigraphic nomenclature.The third period, here termed the King period, was a time of more intense study of the outcrops and their subsurface extensions. It was a time when facies became more clearly differentiated and when the great diversity and abundance of fossils in the region became appreciated. This period ended when World War II curtailed research in the region. The fourth period began after the war, with heightened interest in reefs and paleoecology. It was a time when carbonate petrology and paleoecology rose as major fields of interest. It was also a time of mega-paleontology. Tens of tons of fossiliferous limestones were processed at the U.S. National Museum and the American Museum of Natural History and collections of literally millions of fossils were assembled. The earlier publication of Geology of the Southern Guadalupe Mountains, Texas and the later publication of The Permian Reef Complex of the Guadalupe Mountains Region, Texas and New Mexico characterize the period.The fifth period is marked by the return of industry investigators to study the reefs and associated rocks, perhaps spurred as much by Dunham's "Vadose pisolites in the Capitan reef" as by any single paper. The period was one of concern about origins of the distinctive pisolites of the complex, nature of the massive Capitan Limestone, diagenesis of carbonates and by concern for understanding the economically significant rocks of the backreef sequence. The sixth period, termed the Wisconsin phase, continued research along lines of the fifth period but was a time when faculty and students of the University of Wisconsin, and their associates, re-examined all facies of the Guadalupe Mountain reef complexes as a major effort, while industry became less broadly involved. Those efforts, and those now initiated by faculty and students of the University of Nebraska and Rice University, bring us essentially to date, but much still remains to be discovered and understood about the reef complexes.

The Beck Spring Dolomite is the medial unit of the Middle to Late Proterozoic Pahrump Group, the oldest sequence of sedimentary rocks in eastern California. Stratigraphic sections of the Beck Spring Dolomite examined in the eastern Mojave Desert and Death Valley regions consist of four members. These are, in ascending order, a lower cherty member, a lower laminated member, an oolitic–pisolitic member, and an upper cherty member. More than 80% of the Beck Spring Dolomite is algal-laminated dolomite with a possible Middle to Late Riphean stromatolite assemblage characterized by cf. Conophyton, eroded, irregular columnar forms similar to Kussiella or Baicalia, and several types of stratiform Stratifera. Petrographic, X-ray diffraction, and atomic absorption spectroscopic analyses indicate that the formation is composed of well-ordered replacement dolomite with less than 25% acid-insoluble residue. Concentrations of Fe and Mn are two to six times higher in the algal-laminated members than in the oolitic–pisolitic member, whereas the concentrations of Ca, Mg, Ba, Sr, Na, and K show no systematic variations. Stratigraphic relationships, primary and secondary sedimentary structures, petrology, and stromatolite assemblages suggest deposition during Middle to Late Proterozoic time on a platform that most likely included offshore shoals, restricted lagoons, and broad tidal flats with ponds, channels, and levees.