[No abstract submitted] / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

Ph.D., Faculty of Science, University of the Witwatersrand, 2011 / The Wahweap Formation is an ~400 m thick clastic sedimentary succession of fluvial and estuarine channel sandstones and floodbasin mudrocks that was deposited in western North America during the Late Cretaceous. It preserves important mammal, dinosaur, crocodile, turtle and invertebrate fossils that have been the subject of recent palaeontological investigations. The Wahweap Formation can be divided into lower, middle, upper, and capping sandstone members based on sand:mud ratios and degree of sandstone amalgamation. Facies analysis reveals the presence of ten facies associations grouped into channel and floodbasin deposits. Facies associations (FAs) from channels include: (1) single-story and (2) multistory lenticular sandstone bodies, (3) major tabular sandstone bodies, (4) gravel bedforms, (5) low-angle heterolithic cross-strata, and (10) lenticular mudrock, whereas floodbasin facies associations include: (6) minor tabular sandstone bodies, (7) lenticular interlaminated sandstone and mudrock, (8) inclined interbedded sandstone and mudrock, and (9) laterally extensive mudrock. The lower and middle members are dominated by floodbasin facies associations. The lower member consists dominantly of FA 8, interpreted as proximal floodbasin deposits including levees and pond margins, and is capped by a persistent horizon of FA 3, interpreted as amalgamated channel deposits. FAs 4 and 6 are also present in the lower member. The middle member consists dominantly of FA 9, interpreted as distal floodbasin deposits including swamp, oxbow-lake and waterlogged-soil horizons. FAs 1, 2, 5, 6, 7, 8, and 10 are present in the middle member as well, which together are interpreted as evidence of suspended-load channels. The upper member is sandstone-dominated and consists of FAs 1, 2, 3, 5, 7, and 8. FAs 5 and 7, which occur at the base of the upper member, are interpreted as tidally influenced channels and suggest a marine incursion during deposition of the upper member. The capping sandstone is characterized by FAs 3, 4, and 6, and is interpreted to represent a major change in depositional environment, from meandering river systems in the lower three members to a low-accommodation, braided river system. Combined results of facies and palaeosol analyses suggest that the overall climatic conditions in which the Wahweap Formation was deposited were generally wet but seasonally arid, and that iv conditions became increasingly moist from the time of lower member deposition up to the time of middle member deposition. Improved age constraints were obtained for the Wahweap Formation by radiometric dating of two devitrified ash beds (bentonites). This allowed for deposition to be bracketed between approximately 81 Ma and 76 Ma. This age bracket has two important implications: firstly, it shows that the Wahweap Formation is synchronous with fossiliferous deposits of the Judithian North American Land Mammal Age, despite subtle differences in faunal content. Secondly, it shows that the middle and upper members were deposited during the putatively eustatic Claggett transgression (T8 of Kauffman 1977) in the adjacent Western Interior Seaway. This is consistent with facies analysis which shows a marked increase in tidally-influenced sedimentary structures and trace fossils at the top of the middle and base of the upper members. Following recent alluvial sequence stratigraphic models, the middle member is interpreted as the isolated fluvial facies tract, while the upper member represents the tidally influenced and highstand facies tracts. Maximum transgression occurred during deposition of the lowest part of the upper member, synchronous with the Claggett highstand in other parts of the Western Interior Basin. The sequence boundary is placed at the base of the overlying capping sandstone member, diagnosed by a major shift in petrography and paleocurrent direction, as well as up to 4 m of fluvial incision into the underlying upper member. The capping sandstone member is interpreted as the amalgamated fluvial facies tract of an overlying sequence. Analysis of the western-most exposures of the Wahweap Formation on the Markagunt and Paunsaugunt plateaus shows facies variations in the proximal and distal parts of the central Western Interior Basin. The inconsistent thickness and variations in fluvial architecture, as well as the presence of unconformities and generally poor exposure in the west, hinder correlation attempts and also prevent the subdivision of the Wahweap Formation into members. Only the capping sandstone, which can be positively identified west of the Paunsaugunt fault, has a consistent thickness and fluvial architecture across the west-east extent of the Wahweap Formation. The capping sandstone also bears remarkable lithological similarity to the Tarantula Mesa Formation which is exposed to the east in the Henry Mountains Syncline, and it is suggested that these two units be equated under the name “Tarantula Mesa Formation”, which has precedence.

An array of stratigraphic, chemical, isotopic, and mineralogical evidence indicates that an impact terminated the Cretaceous Period. The 180-km-diameter Chicxulub crater, which now lies buried on the Yucatan peninsula of Mexico, was probably formed by the impact. The impactor was probably a long-period comet. Shock devolatization of the thick carbonate/evaporite sequence impacted at Chicxulub probably led to a severe and long-lasting greenhouse warming and a prompt pulse of sulfuric acid rain. The fallout of crater ejecta formed two layers: a lower layer which varies in thickness following a power-law relation based on distance from the Chicxulub crater and an upper, globally-distributed, uniformly ∼3-mm-thick layer. The upper layer probably represents the fallout of condensates and entrained solid and liquid particles which were distributed globally by the impact fireball. The lower layer consists of brecciated rock and impact melt near the crater and largely altered tektites far from the crater. The clasts of this layer were probably ballistically transported. The Raton, New Mexico K/T boundary section preserves the fireball and ejecta layers in a coal-free nonmarine environment. Siderophile, chalcophile, and lithophile trace element anomalies occur similar to those found at marine K/T boundary localities. Soot occurs peaking in the 3-mm-thick fireball layer and the immediately overlying 3 mm of sediment, implying prompt burning of the Cretaceous forests. The Brazos River, Texas continental-shelf K/T sections preserve coarse boundary sediments which were probably produced by impact waves. Siderophile and chalcophile trace-element anomalies occur suggesting that the fireball layer and possibly part of the ejecta layer are interbedded with the coarse boundary sediments. The Beloc, Haiti deep-sea K/T sections preserve a thick ejecta sequence including altered and unaltered tektites and shocked minerals capped by the fireball layer. The thick K/T ejecta preserved at this and other nearby K/T localities require a source crater of Chicxulub's size and location. The composition of the tektites and shocked grains require an impact into recently extracted continental crust with a carbonate/evaporite component as found at the Chicxulub crater.

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The rudist bivalves were one of the many and varied groups of organisms to be extinguished at the end of the Cretaceous Period. They were a group of bivalves which evolved during Late Jurassic times to dominate the carbonate shelves on the margins of the Tethys Ocean during the Cretaceous Period. Through Late Cretaceous times their diversity climbed to a peak and then entered a period of rapid decline, resulting eventually in the complete extinction of the group. Theories as to the cause of that extinction should clearly be based upon a knowledge of the detailed pattern of the decline. Most important is the question of the timing of the extinction: How long did it take from the peak of diversity to the elimination of the entire group? Is there one main extinction event - or are there several - or is the pattern a gradual decline? In answering these questions this work adopts a new approach to dating end Cretaceous strata by using strontium isotope stratigraphy. The method works by measuring the 87 Sr/86Sr of palaeo-seawater preserved in marine carbonate, such as the thick low-Mg calcite layer of rudist shells. The 87Sr/86Sr of seawater was changing fairly rapidly through time in the latest Cretaceous. The pattern of change has been established in detail using samples from Boreal sequences of which the ages are known with respect to the belemnite stratigraphy. Using this as a standard graph, Tethyan rudist samples have been dated by a comparison of the Sr isotope ratio. The use of Sr isotope stratigraphy has enabled a time axis to be established, against which the ranges of rudist bivalves and of their facies have been ploued. From these data it can be clearly seen that the ranges of many of the established biostratigraphic markers are in error and that the stage boundaries, as defined by the belemnite and planktonic foraminiferal stratigraphies, are offseL When the pattern of rudist diversity is plotted against this time scale it can be seen that, at a specific level, the rudists were at their most diverse at the Campanian/Maastrichtian boundary. Their decline lasted until I almost the end of the Maastrichtian after which the only survivors are Tertiary forms. The decline is related to the disappearance of rudist facies with the end Cretaceous regression, though that graph is slightly displaced from that of rudist diversity. This shows that the rudist extinction is not merely due to non-exposure of terminal Maastrichtian strata.

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Historical research reveals a repeating pattern of uplift, erosion, and deposition in the region of the Llano Uplift, central Texas. This report examines the topography of the pre-Cretaceous landscape. The data consist of points, in three dimensions, that are located on the erosional surface, as determined by three methods. Category I data lie upon the contact between Cretaceous strata and underlying Paleozoic sediments or Precambrian basement; Category II data are defined in the subsurface from well logs; and Category III data are topographic high points where the Cretaceous has eroded away, but the underlying unit has not eroded (an exhumed surface). Digital mapping procedures were used to create triangulated irregular networks, three dimensional scenery, and topographic profiles. The digitally reconstructed surface is compound, consisting of higher, older erosional surfaces, incised into by rejuvenated stream activity to create lower, younger surfaces. This valley/divide topography, which is regional in extent, could not have been visualized without modern GIS technology. / text

Cretaceous bauxite deposits from Hall and Veneer mines, Wilkinson County, Georgia are composed of kaolinite, gibbsite, goethite, anatase, nordstrandite and bohemite. Quartz and micas are absent in the samples. The presence of boehmite and goethite are evidence of intense weathering forming the bauxite deposits. The extremely high values of the Chemical Index of Alteration (CIA) which is over 99, and the low values of the alkali metals and alkali earth metals, support an intense weathering origin for the bauxite deposit. There is evidence of deposition in the mines based on the presence of pisoids in the bauxite samples and the composition of the parent materials, which vary markedly by the non-uniform TiO2/Al2O3 values which represent the accumulation of transported materials from contrasting source areas. Kaolin minerals were first produced by the hydrolytic weathering of aluminous sediments and then gibbsite was formed as early kaolin was desilicated.