2 research outputs found
Influence of Mechanical Layering and Natural Fractures on Undercutting and Rapid Headward Erosion (Recession) at Canyon Lake Spillway, Texas, U.S.A
This study investigates the role of mechanical layering and fractures on flood-related erosional undercutting and resulting rapid spillway recession. In the summer of 2002, 86 cm of rain fell in an 8-day period across the Guadalupe River drainage basin in central Texas, causing Canyon Lake reservoir to completely fill and overtop the emergency spillway for the first time. The resulting flood incised a gorge into the mechanically layered Glen Rose Formation and caused headward erosion (recession) at the downstream edge of the emergency spillway. Comparison of pre- and post-flood imagery and assessment of flood records indicates that maximum recession localized at the northern end of the emergency spillway where 28 m recession occurred. This recession occurred at an estimated rate of up to 10 m/day during the first ~3 days of the flood, which is among the highest rates of recorded bedrock recession. Analysis of historical photographs, field observations and measurement of erosional undercutting, along with measurements of fracture orientation, fracture spacing, and mechanical rebound are used to understand rock mass characteristics that influenced erosional undercutting and rapid recession of the spillway. Evidence of significant undercutting was observed where incompetent argillaceous wackestone (marl) underlies competent limestone. These results reveal that the greatest amount and rate of recession of the spillway was associated with undercutting and toppling collapse of fracture-bounded limestone blocks. Block size may be a factor in continuation of the process, in that large blocks may accumulate at the base of the scarp and inhibit continued erosional undercutting, whereas in other areas smaller eroded blocks can be carried away by the floodwaters and undercutting may continue, facilitating recession. The combination of mechanical contrast between layers and natural fractures in competent layers together contributed to exceptionally high rates of headward erosion. Observed rock mass erodibility behavior was in the range of medium to high erodibility in limestone with widely spaced fractures that would normally be expected to have very low erodibility. Bulk rock mass erodibility in this situation was similar to the most erodibile layer, specifically, the marl at base of spillway pour-off cliff
Mechanisms for Regional Dolomitization of the Triassic Yangtze Platform, Guanling Area, in the Nanpanjiang Basin of South China
Lower to Middle Triassic strata of the Yangtze Platform contain a dolomitized interior, undolomitized boundstone margin, and partially dolomitized slope facies. Anhedral to subhedral dolomite replaces micrite, carbonate grains, and intergranular sparry calcite. Coarse euhedral dolomite cement lines depositional cavities. Vugs and early fractures contain saddle dolomite, while later fractures are filled with calcite. Stylolites crosscut most fractures and all dolomite phases.
Burial history predicts a maximum temperature of 180°C at 5 km depth at the base of the platform and a maximum temperature of 96°C at 2.2 km depth at the top of the platform, using regional stratal thickness in conjunction with a thermal gradient of 30°C/km.
Stable isotope measurements within dolomite show δ18O values ranging from -7.7‰ to 0.8‰ (VPDB) and δ13C values ranging from 0.77‰ to 4.0‰ (VPDB). Vein calcite isotope values range from δ18O -18.4‰ to -5.2‰ and δ13C -6.1 to 3.4‰. 87Sr/86Sr ratios from 11 of 14 dolomite samples showed values ranging from 0.707677 to 0.708601, consistent with Triassic seawater, while 2 samples showed elevated 87Sr/86Sr values, and 1 sample showed depleted 87Sr/86Sr values.
Average homogenization temperatures (Th) and freezing point depressions (Tmice) from 35 euhedral dolomite crystals in the platform interior, slope, and basin indicate entrapment of saline brines (9.5 to 15 wt. % NaCl) over temperatures of 113-150°C.
87Sr/86Sr ratios of dolomite are consistent with modified seawater including radiogenic contribution of hydrothermal fluids. The range in δ18O within dolomite is consistent with enrichment by evaporative concentration of seawater, but also includes negative values consistent with high temperature fluids.
The spatial distribution of dolomite indicates that early marine cementation of the margin prevented permeation of dolomitizing fluids. Hypersaline reflux dolomitization remains a possibility for platform interior, but the partial dolomitization of the slope and basin pelagic carbonates and geothermometry are more consistent with late hydrothermal dolomitization. The late diagenetic stage, presence of saddle dolomite, and geothermometry from over 90 to 185°C point to burial dolomitization