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Authors: Alessandro Cantelli, Wonsuck Kim, John Martin, James Mullin, Chris Paola, Nikki Strong
The XES facility is a large experimental basin (13 m x 6.5 m), developed and built with funds from NSF and the University of Minnesota , that permits the formation of stratigraphy through the use of a flexible subsiding floor. The goal is to reproduce the real-world (i.e. spatially variable) kinematics of subsidence, as determined by geophysical modeling and backstripping of real basins.
The floor is a honeycomb of 432 independent subsidence cells (Fig. 1) through which a gravel "basement" is slowly removed to provide accommodation space for deposition. At the beginning of an experiment, the basin is filled with dry, well sorted commercial gravel. The top of the gravel is covered with a thin rubber membrane. The experimental deposit is formed on top of this membrane. Subsidence is induced by withdrawing gravel from the bottoms of the hexagonal cells. Each hexagon forms the top of a cone that tapers into a standard elbow pipe (Fig. 2). The gravel in the cone rests at the angle of repose in this elbow. Subsidence is induced by firing a pulse of high-pressure water into the gravel in the elbow. A small volume of gravel is knocked out of the elbow and falls into an exhaust line, where it is transported out of the system and stored for later reuse. Each subsidence cell has its own sealed pressure tube that drives the pulses via a computer-controlled solenoid valve. We have refined and calibrated the pulsing so that each pulse produces about 0.12 mm of subsidence: the "earthquake slip" in the experiments. This is about equal to the resolution with which the basement elevation can be read (described below), and also to the typical grain size of sediment in the experiments. Hence the subsidence is effectively smooth and continuous in time. The subsidence is also spatially continuous. The cells are separated only at floor level, so the gravel can flow laterally to accommodate differential subsidence with no breaks at the cell boundaries. Firing a single cell, for instance, produces a smooth bowl-shaped subsidence pattern that extends over the six adjoining cells. Extensive testing has shown that the underlying honeycomb structure is not imprinted on the subsidence at the surface until the rubber membrane (the top of the basement) has been lowered to within about 0.2 m of the honeycomb. This leaves about 1.3 m of usable accommodation space in the basin. As long as the gravel basement is loaded, lateral slopes of up to 60 can be produced between adjoining cells
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