Matthew Leung

The Cenomanian-Campanian Mancos Shale located in the San Juan Basin, New Mexico is an important unconventional play. Compared to its correlative sandstones, such as the Gallup and Tocito, less is known about its stratigraphy and processes of deposition. The Mancos shale was deposited in the Cretaceous Western Interior Seaway (CWIS) and can be divided into two segments, known as the Lower Mancos and Upper Mancos Shale; with the Basal Niobrara unconformity separating the two. The duration of the deposition of each division was approximately 7.5 million years. This study primarily investigates the depositional processes and high-frequency cyclicity of the Upper Cretaceous Mancos Shale by examining fifteen shale rock cores. Close sedimentological examination of the shale rock cores revealed a variety of sedimentary structures, including wave, combined flow, current, and tidal cross-laminations that indicate a mixture of shallow marine processes. The most ubiquitous structures found were wave ripple and combined flow cross laminations indicating strong storm-influence. Numerous upward coarsening parasequences were observed, bounded by flooding surfaces. Parasequences range from 2 – 5 meters thick. A 103 meter core, which includes approximately a fifth of the Upper Mancos Shale, revealed approximately 26 parasequences. These results indicate that high-frequency cyclicity had an important eustatic control on distal sediments; and storm-wave processes were the dominant mode of deposition for the sediment in the Upper Mancos shale.