No description available.

No description available.

A sequence of Older Precambrian volcanic and sedimentary rocks more than 15,000 feet thick occurs in the Blackjack Mountains and White Ledges, 20 miles north of Globe, Arizona. This sequence consists of (from older to younger) the Redmond formation (acidic volcanic rocks) and the Hess Canyon group (clastic sedimentary rocks). The Hess Canyon group is subdivided into: the White Ledges formation (interbedded quartzites and argillaceous rocks); the Yankee Joe formation (argillaceous strata with interbedded graywackes and arkoses); and the Blackjack formation (argillaceous quartzites). These rocks have been intruded by the Ruin Granite (a porphyritic quartz monzonite) and subsequently eroded to approximately the present level of exposure prior to the deposition of the Younger Precambrian Apache Group. The unconformity between the Older and Younger Precambrian strata is well exposed at Butte Creek north of Haystack Butte. Diabase has intruded the Blackjack formation, the Ruin Granite and the Apache Group. No Paleozoic or Mesozoic rocks are known to occur within the surveyed area. Sediments and volcanic and sedimentary rocks of Tertiary and Quaternary age partly conceal the older rocks. The Hess Canyon group can be correlated with the Deadman Quartzite, Maverick Shale, and Mazatzal Quartzite of the Mazatzal Mountains (Wilson 1939a) and also the Houden Formation of the Diamond Butte Quadrangle (Gastil 1958). Whole rock Rb-Sr dating indicates an age of 1,510 ± m.y. for the Redmond formation. Isotopic dating of the Ruin Granite near the Blackjack Mountains and of the granitic rocks intruded the Mazatzal Quartzite of Four Peaks in the southern Mazatzal Mountains indicates that the Mazatzal Orogeny (the Mazatzal Revolution of Wilson, 1939a) occurred 1,425 to 1,380 m.y. ago in central Arizona. This orogeny followed the deposition of the Mazatzal Quartzite and the Hess Canyon group, terminating older Precambrian time in Arizona and was followed by the deposition of the Younger Precambrian Apache Group. Isotopic dating of volcanic metamorphic and plutonic rocks in the Pinal and Tortilla Mountains and near Roosevelt Dam on the Salt River indicate that portions of the Pinal Schist in the type locality are greater than 1,730 m.y. old and that these rocks have experienced a complex series of events in Older Precambrian times. The Madera Diorite of Ransome (1903) consists of rocks 1,730 ± 30 m.y. old as well as rocks about 1,500 m.y. old. The Older Precambrian igneous rocks in this part of Arizona appear to have developed from material similar in Rb to Sr ratio to average shallow continental crust. These rocks formed during the interval 1,730 to 1,370 m.y. ago. The continental crust in this region probably originated no earlier than about 1,800 m.y. ago. Igneous rocks younger than 1,370 m.y. have not been derived soley from average shallow crustal material.

published_or_final_version / abstract / Earth Sciences / Doctoral / Doctor of Philosophy

published_or_final_version / Earth Sciences / Doctoral / Doctor of Philosophy

Four lithostratigraphic alluvial formations and eight chronostratigraphic "accumulation phases" occur along the middle Duck River Valley. Each accumulation consists of about 2 m of sandy and gravelly bottom stratum facies overlain by 2.5-4 m of clayey and silty top stratum facies. An additional 1-2 m of clayey and silty terrace veneers blanket all but the youngest accumulation. Based on numerous excavations into this fill, and 14 radiocarbon dates, a history of floodplain sedimentation can be traced: 1) Yellowish brown clay loam was deposited during the late Pleistocene, above a bedrock thalweg 5 in higher than the present level. Severe bedrock and floodplain erosion then occurred. 2) During the early Holocene, aggradation of dark yellowish brown clay loam occurred, over a bedrock valley floor already as deep as that at present. Scattered within this unit are early Archaic (ca. 9000 yr B.P.) chert artifacts. 3) Following a brief interval of stability, brown silty clay loam accreted, but by 7200 ¹⁴C yr B.P. the floodplain surface was again stable and soil formation dominated over deposition. Abundant mid-Archaic chert artifacts as young as 6400 ¹⁴C yr B.P. in age were left behind by their makers on this fossil floodplain surface, and pollen analytical studies document an effectively drier climate in the region during this time. 4) By 6400 ¹⁴C yr B.P., renewed overbank accretion was underway and pollen analyses indicate an increasingly humid climate. Aggradation continued up to 4200¹⁴C yr B.P., by which time the older surface, artifacts, and soil were buried by veneers of dark brown silty clay loam, itself containing late Archaic artifacts. 5) Two periods of floodplain stability and soil formation, separated by aggradation, occurred during the past 4000 years. The last period of stability ended in the early 1800's; the introduction of row crop agriculture into the basin at this time probably caused the historic episode of renewed accretion which is still underway. In response to altered hydrologic regimes, the Duck River forms new floodplains by suspended load deposition on older floodplain surfaces, on vegetated channel banks, and on the higher portions of vegetated in-channel bars. In contrast, lateral accretion of point bar sands and gravels is not an important floodplain forming process along this river.

M.Sc. / The configuration of the supercontinent Rodinia, at the end of the Mesoproterozoic to the beginning of the Neoproterozoic (1100-750 Ma), and its subsequent break up into cratonic fragments that would later result in the formation of Gondwana (Early Palaeozoic), is still not completely understood. This is largely due to ambiguity surrounding relationships between cratons, craton evolution and timing of significant tectonic or sedimentary events. Particular to this study is the evolution and palaeogeographic history of the Kalahari Craton and a comprehensive provenance analysis of Neoproterozoic to early Palaeozoic clastic sedimentary rocks from the Kango Inlier (Saldania Belt, South Africa). This includes the Cango Caves and Kansa Groups as well as the Schoemanspoort and the adjacent Peninsula Formation (Table Mountain Group, Cape Supergroup). A well established lithostratigraphy, in addition to recent establishment of age constraints by UPb zircon dating and microfossil evidence, allowed for strategic sampling with the objective of gaining insight to the crustal evolution of SW Gondwana. In this study, a progression from immature, moderately altered rocks in the Cango Caves Group (Upper Neoproterozoic) to mature, strongly altered rocks in the Lower Palaeozoic Kansa Group and overlying formations is observed. Thus, rapid sedimentation of the former is anticipated, while the subsequent formations developed at a passive/rifted margin culminating in the laterally extensive deposition of the Peninsula Formation. Ongoing extensional movement is evident due to chronologically deeper-water facies and the progressive influence of a less fractionated component in the Cango Caves Group, particularly in the Huis Rivier Formation. The association of these rocks with an active margin is not certain since index trace element concentrations are too high for typical arc terranes. Thus, the mixing of a younger (570-600 Ma) magmatic source (close to an active margin) with mafic and felsic rocks of the older Mesoproterozoic Natal- Namaqua Mobile Belt (NMB) is the most likely possibility. A maximum, pre-Cape Granite age of 571 Ma can be assigned to the Huis Rivier Formation (Cango Caves Group) by detrital zircon dating, and thus correlation with the Malmesbury Group can be made. Ediacaran age zircons might be related to the active continental margin (Trans Antarctic Orogen) surrounding southern Gondwana, but this is still hypothetical. The post-Cape Granite Kansa Group and overlying Schoemanspoort Formation were most likely deposited as basin infill subsequent to folding and transtensional tectonics affecting the underlying Cango Caves Group. The Kansa Group may be comparable with the Klipheuwel Formation (southwest South Africa) in terms of its stratigraphic position beneath the Table Mountain Group. Deposition of the Table Mountain Group is much younger than previously believed in light of Ordovician zircon ages (471, 485, 499 Ma) obtained from the underlying Kansa Group. However, the provenance of these thus far unheard of ages for magmatic events in South Africa is a matter of contention. The proximal Ordovician Ross-Delamerian Orogenic event in Antarctica is the most likely source. Peninsula Formation deposition represents a cover sequence i.e. the culmination of small isolated basins (e.g. the Kansa Group and lower Table Mountain Group) into a larger, laterally extensive basin where reworking played a dominant role. This basin is likely to be a rift-related. However, it is not clear which crustal entity rifted away from vi South Africa and if, during the Ordovician an, active continental margin further to the south - bridging the South American Famatina Orogen with the Ross-Delamerian arc in Antarctica - existed. The Natal-Namaqua Mobile Belt appears to be the predominant source throughout the succession as indicated by Nd-isotope data and zircon populations. This implies that simple crustal recycling of Natal-Namaqua basement (or rocks with similar Nd-isotope characteristics) led to the genesis of the magmatic material younger than 1 Ga, observed in this study.

M.Sc. / Please refer to full text to view abstract

A dissertation submitted to the Faculty of Science of the University of the Witwatersrand for the degree of Master of Science / Detailed high precision geochronological studies have been performed on the 2054 Ma old Bushveld Complex, in an attempt to unravel its tectonic and thermal evolution in the period immediately following intrusion and crystallisation. The geochronological techniques used have been specifically chosen to sample specific temperature episodes in the cooling of the Complex, rather than to necessarily provide an accurate emplacement age, The Bushveld Complex is seen in this study as part of the Bushveld Magmatic Province, rather than as an isolated intrusion, The geochronological data are therefore interpreted in the context of the current understanding of the Proterozoic tectonic and thermal history of the Kaapvaal Craton. The development of clean chemical methods and accurate geochronological methods are essential to this type of study. The reduction of laboratory blanks, especially for lead and the development of laboratory techniques for the analysis of small samples therefore played an important part in this study. It has been possible to lower analytical blanks, especially lead blanks to levels where the analysis of small samples is possible. In addition, the zircon evaporation technique was attempted. Phlogopite micas from the Critical Zone of the Bushveld Complex give a wlde range of Rb-Sr model ages, some almost 100Ma older than the preferred age. This indicates a period of hydrothermal alteration of considerable duration at the same time as the intrusion. The slightly young Rb-Sr age recorded for all the mica and whole rock data collected for this study indicates the alteration of the micas which is evident from petrographic and electron microprobe studies. U-Pb and Pb-Pb zircon ages are also Significantly younger than the preferred age, indicating a degree of alteration. This is also seen in the discordance of the zircons seen in the U-Pb data. / AC2017

Thesis (M.Sc.)--Memorial University of Newfoundland, 2006. / Bibliography: leaves 296-319. Also available online.