How much rock has been removed
Nick, you asserted that " a hundred metres or more of rock have been destroyed." to give current surface spherule concentration.
Squyres etal Nature p1608, give an estimate of several metres, 2 orders of magnitude less! Their words "Because spherules are sparsely distributed in the soil, concentrated primarily at the surface, the total inventory of spherules in the soil may be the equivalent of a region-wide layer of spherules no more than a centimeter thick. The observed concentration of spherules in the underlying rock is about 1 to 2% by volume (28), so only about 1 m of rock needs to have been eroded away to produce all of the spherules observed in the soil. "
How did you reach your figure?