عنوان

Computer simulation of landform evolution, Western Desert, Egypt and paleoclimate implications

TLpq304246919

انگلیسی

Computer simulation of landform evolution, Western Desert, Egypt and paleoclimate implications

[Thesis]

W. Luo

R. E. Arvidson

Washington University in St. Louis

1995

275

Ph.D.

Washington University in St. Louis

1995

Field and remote sensing observations demonstrate that depressions of the Western Desert of Egypt (specifically, Kharga, Farafra, and Kurkur regions) are mainly occupied by shales that are impermeable, but easily erodible by rainfall and runoff, whereas the surrounding plateaus are composed of limestones that are permeable and more resistant to fluvial erosion under semiarid to arid conditions. Scallop-shaped escarpment edges and stubby-looking channels cut into the plateau units are suggestive of slumping of limestones by groundwater sapping at the limestone-shale interfaces, removal of slump blocks by weathering and fluvial erosion, and consequent scarp retreat. Computer simulations were conducted using a simple two layer model to simulate the first order geomorphic features with a permeable, resistant layer over an impermeable, friable unit. Erosion, deposition, slumping were modeled in a parameterized way based on cellular automata techniques. Groundwater flow was modeled based on finite difference techniques with Dupuit assumption, i.e., ignoring vertical component of groundwater flow. Groundwater sapping rate and spring-derived tufa deposition rate were assumed to be proportional to groundwater discharge. The surface and subsurface processes were coupled together as a system that allows interactions between the processes. The model simulates landform evolution under different climatic conditions and records the resultant landform, deposits and age information for comparison with realities. Simulations using geologically reasonable parameters demonstrate that relatively rapid erosion of the shales by surface runoff, groundwater sapping and slumping of the limestones, and detailed control by hydraulic conductivity inhomogeneities associated with structures explain the depressions, escarpments, and plateau terrains. Episodic wet pulses, keyed by usd\delta\spusdO deep sea record, reproduce actual U/Th tufa ages, and provide further evidence that northeastern African wet periods occurred during interglacial maxima, probably because of enhanced monsoonal activity. Finally, the usd\delta\spusdO-forced model replicates the dramatic decrease in fluvial and sapping activity over the past million years, as northeastern Africa moved toward hyperarid conditions.

Earth sciences

Geology

groundwater sapping

Hydrology

R. E. Arvidson

W. Luo

p

[Thesis]

276903

a

Y