UTIG Seminar Series: GRA Presentations
||December 4, 2015 at 10:30 am
||December 4, 2015 at 11:30 am|
| ||Location:||PRC, 10100 Burnet Road, Bldg 196, Rm 1.603, Austin, TX 78758|
| ||Contact:||Nick Hayman, email@example.com, 512-471-7721|
| ||URL:||Event Link|
"Point-bar scaling and application to the Lower Miocene of the Gulf of Mexico Basin" (Jie Xu)
Fluvial systems are major sediment conveyers from source terranes to basinal sinks. Fluvial channel geometry results from interaction between fluid flow and sediment particles, and scales with water discharge and sediment flux. Point bar deposits formed in bends of meandering rivers provide a good proxy to estimate the depths of paleochannels (Ethridge and Schumm, 1978; Blum et al., 2013). Modern observations show that bankfull channel depth (or point bar thickness) well correlates with bankfull discharge and drainage area (Blum et al., 2013). However, this scaling relationship has not been rigorously tested in ancient rock records, especially for large passive margin basins that have diverse climate, tectonic, topographic, lithological and geomorphic regimes.
In this study, we try to test whether this scaling relationship applies to the large drainage systems in the Gulf of Mexico in early Miocene time. Point bar thickness data were collected from subsurface well logs that are within major paleo-fluvial axes. Point bar data from each major fluvial axis display a wide range of thickness and a combination of different river types, including point bar thickness from trunk stream, local small streams, and valley fills. The Paleo-Mississippi and Red rivers in eastern Texas-Louisiana have the thickest point bar deposits (23 m and 20 m), while Paleo Houston-Brazos and Guadalupe rivers in central Texas deposited the thinnest point bars in early Miocene (12 m and 14 m). Paleo-Rio Grande River has intermediate values, 16m. The results show a clear scaling relationship between the point bar thickness and paleo-drainage area, length of channel, and sediment supply rate. This study indicates that such point bar scaling relationship can be used to ancient sedimentary rock to constrain paleodrainage system size. In addition, the sediments deposited in point bars can be applied to predict the fan run out length and size where the seismic image quality does not allow one to detect such features beneath a salt canopy (e.g. lower Miocene subsalt reservoir in GOM deep-water basin).
"Revisiting the 1899 Earthquakes of Yakutat Bay, Alaska" (Maureen Walton)
North of Yakutat Bay in southeastern Alaska, the subducting Yakutat Block intersects with the Fairweather transform fault system. A series of large earthquakes occurred in the region in September of 1899, including a Mw 8.2 event on 10 September that resulted in >14 m of coseismic uplift and a 6 m tsunami in Yakutat Bay. Despite recurrence risk of the 1899 or similar events in the region, the fault(s) that ruptured in 1899 remain unidentified. Previous efforts to map active Yakutat Bay faults carried out by Plafker and Thatcher (2008) used post-1899 bedrock uplift measurements to infer the location of potentially important structures, including the Esker Creek and Bancas Point thrusts. As measurement error was not assessed in their study, we revisit the uplift measurements by quantifying uncertainty; effects of glacial isostatic adjustment (GIA), in particular, are significant. We also combine new seismic reflection data with existing topography, bathymetry, GPS, and satellite photo data to update earlier fault maps. Our reevaluation of uplift measurements suggests that primary slip and uplift during the 10 September earthquake was limited to northwest of Yakutat Bay. Additionally, a high-resolution seismic reflection survey we conducted in Yakutat Bay during August 2012 constrains faulting to on- or near-shore based on the absence of bay-crossing faults. Collectively, our results imply that predominantly strike-slip and transpressive horsetail-type faults are southeast of Yakutat Bay, with compressional structures related to Yakutat Block subduction/collision to the northwest. We interpret the 10 September 1899 event to be the result of complex rupture somewhere within the Yakutat subduction/collision system. Based on our updated map of coseismic uplift and fault structure, we favor a rupture model where primary slip occurred along the Esker Creek system locally with possible induced coseismic slip along the neighboring Boundary transpressive fault system.