Distinguishing muddy turbidites and wave-influenced muddy turbidites from hemipelagic deposits using grain fabric : an experimental approch
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Since mudstones comprise the majority of deepwater deposits, an ability to distinguish between the various depositional styles associated with these rocks is scientifically and economically important. Of particular significance is distinguishing between hemipelagic deposits that typically represent a relative shutdown of the depositional system and muddy turbidites representing the distal fringe of active depocenters. Distinguishing hemipelagic deposits from muddy turbidites is also important to the study of unconventional hydrocarbon reservoirs because systematic spatial change in deposit properties that include laminae thickness, grain size and grain sorting are only expected in the case of muddy turbidites. Despite their importance to marine and submarine sedimentology and stratigraphy, there are few measurements that directly compare the grain fabrics of hemipelagites and muddy turbidites. We have now collected such a data set using experimentally created muddy turbidites, muddy wave-influenced turbidites, and hemipelagic deposits that start with the identical original distribution of mud-size quartz grains. We are able to unambiguously compare the grain fabric generated by each depositional style in three dimensions. Using high-resolution, X-ray computed tomography, we are able to measure and quantify the trend and plunge for the long axes of individual silt grains composing each style of deposit. All three deposit types preserve approximately the same distribution of dip angles for the long axes of grains, making this an unreliable metric for distinguishing between the three studied styles of mud deposition. Fortunately, a significant difference was found in the compass direction for the long axes of grains in the three deposit types, as well as primary porosity. Grains in turbidites and wave-influenced turbidites have a preferred long-axis trend that is oriented parallel to the transport direction and a secondary orientation that is aligned transverse to the flow direction. This grain alignment is best developed in those grains that are most oblate- and blade-shaped, as measured using the Corey Shape Factor and Flatness factor, respectively. The settling-only deposits connected with hemipelagic sedimentation record no preferred orientation for the long axes of grains.