Browsing by Subject "Turbidity current"
Now showing 1 - 2 of 2
- Results Per Page
- Sort Options
Item Distinguishing muddy turbidites and wave-influenced muddy turbidites from hemipelagic deposits using grain fabric : an experimental approch(2015-12) Brown, David Michael, M.S. in Geological Sciences; Mohrig, David; Buttles, James L; Kim, WonsuckSince 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.Item Dynamics of dilative slope failure(2013-12) You, Yao; Flemings, Peter Barry, 1960-; Mohrig, DavidSubmarine slope failure releases sediments; it is an important mechanism that changes the Earth surface morphology and builds sedimentary records. I study the mechanics of submarine slope failure in sediment that dilates under shear (dilative slope failure). Dilation drops pore pressure and increases the strength of the deposit during slope failure. Dilation should be common in the clean sand and silty sand deposits on the continental shelf, making it an important mechanism in transferring sand and silt into deep sea. Flume experiments show there are two types of dilative slope failure: pure breaching and dual-mode slope failure. Pure breaching is a style of retrogressive subaqueous slope failure characterized by a relatively slow (mm/s) and steady retreat of a near vertical failure front. The retreating rate, or the erosion rate, of breaching is proportional to the coefficient of consolidation of the deposit due to an equilibrium between pore pressure drop from erosion and pore pressure dissipation. The equilibrium creates a steady state pore pressure that is less than hydrostatic and is able to keep the deposit stable during pure breaching. Dual-mode slope failure is a combination of breaching and episodic sliding; during sliding a triangular wedge of sediment falls and causes the failure front to step back at a speed much faster than that from the breaching period. The pore pressure fluctuates periodically in dual-mode slope failure. Pore pressure rises during breaching period, weakens the deposit and leads to sliding when the deposit is unstable. Sliding drops the pore pressure, stabilizes the deposit and resumes breaching. The frequency of sliding is proportional to the coefficient of consolidation of the deposit because dissipation of pore pressure causes sliding. Numerical model results show that more dilation or higher friction angle in the deposit leads to pure breaching while less dilation or lower friction angle leads to dual-mode slope failure. As a consequence, pure breaching is limited to thinner deposits and deposits have higher relative density.