The tertiary history of the Chiricahua volcanic field of southeastern Arizona is essentially that of rhyolitic ash-flow deposition and concomitant block faulting in the period from 29 to 25 m.y., as determined by K-Ar analysis. The Rhyolite Canyon ash-flow sheet is the youngest of three sheets, each more than 1000 feet thick. Its distribution is limited mainly to the Chiricahua and northern Pedregosa Mountains with a lesser amount of deposits in the neighboring Swisshelm and Peloncillo Mountains. It is estimated that the original areal extent was of the order of 700 square miles and that the volume of deposits was around 100 cubic miles. The source area of the Rhyolite Canyon sheet is identified as a 13-mile diameter caldera, named the Turkey Creek caldera. This is the first major caldera of the Valles type described in the Mexican Highland and Sonoran Desert sections of the Basin and Range. It is unique because of its denudation. Erosion to 5000-foot depth locally has exposed thick sections of moat deposits and a fine grained monzonite pluton associated with central doming. Rhyolite Canyon tuff in the caldera, some 3000 feet thick, is domed and intruded by the monzonite. More than 1500 feet of tuff breccia, tuffaceous sediments, and rhyolite flows are exposed in the moat, along with 3000 feet of monzonite forming annular segments a couple miles wide abutting or overlying rocks forming the caldera wall. The most monzonite is similar to that in the dome and was emplaced amidst the period of deposition in the caldera. Petrographic and trace element analyses indicate a cogenetic relation between the Rhyolite Canyon sequence and the moat rhyolites. The K-Ar age of the Rhyolite Canyon tuff is very close to that of the monzonite. The ash-flow sheet immediately underlying the Rhyolite Canyon sheet is also very close in age as indicated by K-Ar analyses. Block faulting and tilting took place between the two sheets and also following the deposition of the Rhyolite Canyon sheet. There is evidence that the present basin-range structure was not established until after the Rhyolite Canyon sheet had been emplaced.

PhD (Geology) / Department of Mining and Environmental Geology / Cretaceous-Tertiary Periods marked the break-up of Gondwana, a large landmass composed of most of the present-day southern continents. In understanding the events of the supercontinental break-up, paleoenvironmental studies need to be carried out. In such studies, kaolinites could be used as paleoenvironmental proxies due to their small particle sizes and large surface area. It is in this context that this research sought to reconstruct the paleoenvironments in which selected Cretaceous-Tertiary kaolin deposits in the Douala Sub-Basin in Cameroon formed. To achieve this objective, mineralogical and geochemical characterisations were carried out using x-ray diffractometry, scanning electron microscopy, Fourier transform infrared spectrometry, thermal analyses and x-ray fluorescence spectroscopy. Trace elements and stable isotopes were analysed using mass spectrometries. Ages of zircons in the kaolins were determined using laser ablation magnetic sector-field inductively coupled plasma mass spectrometry (LA-SF-ICP-MS) U-Pb geochronology. Diagnostic evaluation for industrial applications of the kaolins were carried out using particle size distribution, texture, moisture content, pH, and electric conductivity. Six kaolin deposits from Cretaceous-tertiary Formations of the Douala Sub-Basin were studied; namely, Bomkoul (Tertiary), Dibamba (Tertiary), Ediki (Cretaceous), Logbaba (Cretaceous), Missole (Tertiary) and Yatchika (Cretaceous). The nature and occurrences of these kaolin deposits in Cameroon were determined through thorough mineralogical and geochemical characterisations of bulk (< 2 mm size fraction), silt (2-63 μm size fraction) and clay samples (< 2 μm size fraction). By quantifying the mineral phases present, the morphology and the functional groups in the kaolins are presented as the mineralogical characteristics of kaolins of each study site; whereas, the major oxides geochemistry and the micro-elemental composition constitute the geochemical characteristics of these kaolins. The minerals’ geneses were also determined and the prevailing paleoenvironmental and paleoclimatic conditions in which they were formed were reconstructed using trace elements and stable isotopes of oxygen and hydrogen in kaolinite. The maximum age of the kaolins were determined using U-Pb LA-SFICP-MS dating of zircons in the kaolin deposits. Diagnostic evaluation of the kaolins was carried out, and involved the determination of physical characteristics (particle size, texture, colour and moisture content) and physico-chemical characteristics (pH and electrical conductivity). Results showed that kaolinite and quartz (as major phases), smectite and/or illite (as minor phases), anatase and rutile (as minor or trace phases), goethite and hematite (as trace viii phases) were the mineral phases present in bulk and silt samples. Whereas, in the < 2 μm fractions, the mineral phases are made up of kaolinite and smectite (as major phases), smectite and/or illite (as minor phases), anatase and rutile (as minor or trace phases), goethite and hematite (as trace phases). The kaolins are mostly made up of thin platy or pseudo-hexagonal particles or flakes, books or stacks of kaolinite. The Dibamba, Logbaba and Missole II kaolins have well-ordered structures. Exothermic peak temperatures were generally between 943-988oC. The most abundant major oxides are silica and alumina, followed by iron oxide and titania; though Logbaba and Missole II had higher titania than iron oxide. 85% of the kaolins, portrayed extreme silicate weathering (chemical index of alteration > 80%) and are compositionally mature (index of compositional variability > 0.78). The geochemical composition of the kaolins showed that source rocks of these kaolins vary between rhyolite/granite and rhyolite/granite + basalt. The geochemistry also suggested that the kaolins formed in a marine environment (except Logbaba samples). Trace elements results revealed that Cretaceous-Tertiary kaolins in the Douala Sub-Basin are mainly enriched in rare earth elements compared to the upper continental crust, and have negative Eu anomaly. Large ion lithophiles (mainly Rb and U) were highly enriched in samples, high field strength elements (Y and Nb) were enriched in studied samples of all fractions; and transition trace elements generally had concentrations quite similar to upper continental crust values. Stable isotopes showed that the kaolins were formed in a supergene environment; and temperatures of kaolinitisation (assuming equilibrium with the global meteoric water line) were 26.58oC ± 9.65oC for Cretaceous kaolins and 29.40oC ± 7.22oC for Tertiary kaolins. Assuming equilibrium with the local (Douala) meteoric water line, the temperatures of kaolinitisation were 24.64oC ± 9.48oC for Cretaceous and 27.42oC ± 7.08oC for Tertiary kaolins. Four main zircon populations were identified from radiogenic dating: the 1st between 550 and 650 Ma, the 2nd between 950 and 1050 Ma, the 3rd around 1600 Ma and the 4th between 2800-3200 Ma. These four zircon populations belong to the Proterozoic (Neo-, Meso- and Paleoproterozoic) and the Archean. The maximum depositional ages of the kaolins, reflected by the youngest weighted averages of zircon populations varied between 588 ± 2 Ma and 612 ± 2 Ma, all belonging to the Ediacaran Period (Neoproterozoic). The diagnostic evaluation of the kaolins revealed that the kaolins are very sandy, with 50% of the samples having a sandy loamy clay or sandy loam texture. The colour of the samples varied considerably from white to darker colours (dark grey); with 15% of the kaolins being light reddish brown. The moisture content was generally very low (< 2 wt %) in all size fractions, except in Yatchika samples (moisture content > 2 wt %). The kaolins are generally acidic, with ix a pH(KCl) varying between 3.06 and 3.81, except in Missole I samples, which had a pH (KCl) < 2. The electrical conductivity (EC) generally varied between 20 to ~ 50 μS/cm, except Dibamba and MSL II 01 samples which had EC values in the interval 50 μS/cm < EC < 80 μS/cm; and Missole I samples having an EC > 7500 μS/cm. In conclusion, no great distinction was found between Cretaceous and Tertiary kaolins of the Douala Sub-Basin based on their mineralogy and geochemistry. The best kaolins in terms of these characteristics, and in comparison with the Georgia Kaolins (known for their high kaolinite quality), were the Dibamba (Tertiary), Logbaba (Cretaceous) and Missole II (Tertiary) kaolins. Based on their compositional maturity and mineralogical characteristics, these three kaolins are considered to be second cycle sediments; unlike Bomkoul, Yatchika and Ediki kaolins, which are believed to be first cycle sediments. Based on the trace elements and stable isotopes composition, Cretaceous and Tertiary kaolins of the Douala Sub-Basin were derived from felsic rocks. However, Cretaceous kaolins were formed in a cooler anoxic reducing environment; whereas the Tertiary kaolins were formed in a warmer oxidising environment, with higher precipitation. Ages of zircons in Cretaceous-Tertiary kaolins suggested that the zircon formed during two main tectonic events: the Eburnean orogeny, during which older zircons crystallised and the Pan-African orogeny, during which younger zircons crystallised. The maximum depositional ages of the kaolins varied between 588 ± 2 Ma and 612 ± 2 Ma. The main identified sources of these zircons are the Archean Ntem Complex, the Paleoproterozoic Nyong Group and the Neoproterozoic Yaounde Group. The diagnostic evaluation indicated that the particle size greatly influences the mineralogy and geochemistry of the kaolins because the finer particles (< 2 μm) have higher amounts of kaolinite and Al2O3. The moisture content of the kaolins makes them suitable as paint fillers and in soap production. Paper coating, paper filler, ceramics, pharmaceutics and cosmetics are potential applications for the kaolins, though particle size reduction and beneficiation will give them a higher quality. However, because these kaolin deposits are not big and extensive, they cannot be recommended for large scale industrial applications; but they can be used for bricks, pottery and stoneware manufacturing.