Application of ground-based LIDAR to constrain topographic strike-variability and facies proportions of progradational San Andres Formation clinoforms, Last Chance Canyon, NM




Scott, Samuel Zephyr

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Progradational mixed-carbonate/siliciclastic San Andres Formation clinoforms at Last Chance Canyon, New Mexico were quantitatively mapped and modeled using techniques developed for use with LIDAR (Light Dectection and Ranging). Fundamental questions addressed by this thesis are: (1) What is the nature of topographic strikevariability within a progradational mixed-siliciclastic setting, (2) How is topgraphic strike variability related to a sequence stratigraphic framework?, and (3) Can construction of a LIDAR-based digital outcrop model provide insight into the three dimensional relationship between clinoform morphology and resultant clinothem facies distributions within high-frequency cycles (HFCs)? Chapter one relates facies distributions within HFCs to depositional processes predominantly utilizing measured section and mapping data. Results indicate highstand Guadalupian 9 (G9) high-frequency sequence (HFS) clinoforms built basinward as a sinuous front composed of promontories and reentrants. Low relief lenticular bioherms composed of sub-wave base open-marine fauna colonized upper-slope settings in response to reduced sediment supply subsequent to HFC maximum flood. Resumed production of the carbonate factory near the previous slope break filled topographic lows between intervening autochthonous open marine bioherms during progradation of regressive hemicycles. Differential along-strike topography on the upper-slope was largely filled at HFC termini during the early highstand of the G9 HFS, while during the late-highstand strike exposures exhibit nested topographic depressions that persist through multiple HFCs. It is suggested that persistent topographic lows were generated and accentuated by siliciclastic bypass of the slope in response to increasing slope declivity, and a narrowing of the area of the carbonate factory. Focus of bypassing siliciclastic sands through persistent topographic lows reduced siliciclastic influx on adjacent topographic highs and provided a stable substrate for nucleation of a late highstand build-up. Chapter 2 documents construction of a LIDAR based Digital Outcrop Model (DOM), and demonstrates topographic strike-variability existed throughout the G9 HFS. Quantitative mapping of clinoform surfaces demonstrates the nature of siliciclastic bypass is sensitive to an equilibrium threshold in slope angle of approximately 15 degrees. When clinoform declivity exceeds this threshold, the slope is entrenched through bypass of shelf-derived siliciclastics. A reduction in HFC boundary slope angle below 15 degrees in youngest clinoforms is suggested to reflect a period of forced regression. Finally results of facies modeling for HFC 2 developed using experimental variograms derived from catergorical facies values along measured sections and a deterministic indicator kriging algorithm are presented. Results honor measured section date and further indicate clinoforms during the G9 highstand prograded as a sinuous clinoform margin characterized by promontories and reentrants


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