Detrital Zircon Analysis of the Taza-Guercif Basin and the Adjacent Rif and Middle Atlas Mountains of Morocco and X-Ray Fluorescence Chemofacies Analysis of the Maness Shale of East Texas

Jonathan Richard Pratt, University of South Carolina - Columbia

Abstract

The research in this compilation encompasses three studies leveraging quantitative chemical analyses to interpret the sedimentary record to reconstruct geologic history. The first study surveys the detrital zircon U-Pb and zircon fission-track geochronology of the sedimentary Cretaceous Ketama and Tisiren units of the Rif Mountains, and the Bou Rached sandstones of the Middle Atlas Mountains. All analyzed samples contain a population of Mesoproterozoic crystallization ages formerly unknown to northwest Africa. Possible sources for these ages include the Avalonian terranes now present on the northeastern seaboard of North America, and/or zircons derived from the Amazonian craton and preserved in the Pan-African Volta basin in central Africa. The detrital zircon spectra of both the Rif and Middle Atlas samples are dominated by U-Pb ages associated with the Pan-African orogeny (700-560 Ma) and West African craton (2.2-1.8 Ga). In addition to these ages, the Middle Atlas samples contain a 400-460 Ma U-Pb population that most likely derived from the Sehoul block, a small Avalonian allochthon, or from Avalonia itself, recording cross-continental sediment transport during the Variscan orogeny.

In the second chapter of this work, the combined detrital zircon U-Pb and zircon fission-track geochronology of the Taza-Guercif basin is analyzed to assess tectonic cause for the closure of the Rifean Corridor. The Rifean Corridor was the last major Miocene marine connection between the Atlantic Ocean and Mediterranean Sea that when tectonically severed led to the Messinian Crisis. A total of 499 U-Pb ages and 98 fission-track ages are presented from the basin stratigraphy. These ages are compared with the detrital zircon signature for the Rif and Middle Atlas mountains, which were actively deforming during corridor closure, as a proxy for tectonism. A key population of Triassic-centered fission-track ages within the basin sediments indicates that the majority of detrital sediment present in the basin derived from the Middle Atlas Mountains. The lack of any significant provenance shifts towards a Rif Mountain source during the emergence of the basin and the closure of the Rifean Corridor suggests that uplift of the Middle Atlas Mountains was the primary cause for the emergence of the Taza-Guercif basin and was an important contributor to the Messinian Salinity Crisis.

The final chapter characterizes the depositional and post-depositional history of the Maness Shale of the East Texas basin. This stratigraphic section is significant to the petroleum industry as it is interpreted as age equivalent to the nearby prolific hydrocarbon producing Eagle Ford Shale of South Texas. To evaluate the complexity of and subtlety of this mudrock dominated section, X-ray fluorescence analysis was applied at 5 cm resolution across four separate slabbed cores in the Maness Shale. Altogether 5371 samples were analyzed for 29 elemental abundances, generating 155,759 data points. Hierarchical cluster analysis was applied to divide samples into clusters that display similar covariance of elemental abundances. The cluster, interpreted as chemofacies, guide the evaluation of the Maness Shale. The cores vary in the amount of sandy hyperpycnite deposits from an incipient fluvial-deltaic system located to the north. The deposition of hyperpycnites often coincides with oxygenation events recorded in iron-rich sediments distinguished by the hierarchical cluster analysis. Oxygenated conditions were cyclically re-established on a probable precessional time-scale and coincide within increases in siliciclastic deposition. The analysis shows that the basin never approached the sediment starvation or anoxic conditions necessary to produce the same organic preservation present in the Eagle Ford Shale.