| Summary: | This study examined the processes responsible for the formation of sedimentary intrusions, also known as clastic dikes, observed in the AND-2A sedimentary rock core from Southern McMurdo Sound, Antarctica. Logging of natural fractures in AND-2A provided the means to determine the relationship these structures have to larger regional structures, the Victoria Land Basin and the Transantarctic Mountains. Sedimentary facies analysis revealed variations in the proximity of grounded ice sheets to the depositional site of AND-2A strata in the western Ross Sea region. Formation of clastic intrusions in such a setting can be related to deformation of substrate beneath grounded ice, vertical loading of the crust by ice sheets, as well as stresses associated with tectonic activity. This study examined photos of various scales, made petrographic observation of thin section samples, plotted clastic dike attitude data and compiled sedimentary facies interpretations to determine the processes and drivers involved with clastic dike formation in AND-2A strata. Observation of clastic dike attributes indicates that many processes were involved with clastic injection in AND-2A. The formation of clastic dikes in AND-2A was driven by high fluid pressures in cohesive silty sediments, mostly diamictite lithologies. Water velocities supporting clastic injection in AND-2A varied during injection, and typically flushed finer grains away, leaving an average grain size of fine to medium sand within the dike margin. Various modes of grading within clastic dike margins suggest that both laminar and turbulent flow regimes operated at the time of injection. Lamination of intrusive material in AND-2A clastic dikes indicates variations in the velocity and viscosity of sand charged fluids, and alignment of grains parallel to clastic dike margins suggests the upward injection of sand material in AND-2A. Impermeable diamictites present in AND-2A allowed for the maintenance of high fluid pressures that are required for clastic injection. Stereoplots and histograms of clastic dike orientation data suggest that a normal fault stress regime with significant differential stress operated at the time of their formation, and that clastic dikes formed after host sediments were largely compacted. Analysis of ice proximity to host strata at clastic dike sample depths showed that AND-2A clastic dikes form in ice-proximal to ice-distal depositional settings, and that while loading related to over-riding ice sheets likely contributed to the accumulation of overpressures, AND-2A clastic dikes can be attributed to tectonic activity in the VLB. NSF OPP 0342484 Shell Exploration and Production Company No embargo
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