  We report ALMA Cycle 2 observations of 230 GHz (1.3 mm) dust continuum
emission, and $^{12}$CO, $^{13}$CO, and C$^{18}$O J = 2-1 line emission, from
the Upper Scorpius transitional disk [PZ99] J160421.7-213028, with an angular
resolution of ~0".25 (35 AU). Armed with these data and existing H-band
scattered light observations, we measure the size and depth of the disk's
central cavity, and the sharpness of its outer edge, in three components:
sub-$\mu$m-sized "small" dust traced by scattered light, millimeter-sized "big"
dust traced by the millimeter continuum, and gas traced by line emission. Both
dust populations feature a cavity of radius $\sim$70 AU that is depleted by
factors of at least 1000 relative to the dust density just outside. The
millimeter continuum data are well explained by a cavity with a sharp edge.
Scattered light observations can be fitted with a cavity in small dust that has
either a sharp edge at 60 AU, or an edge that transitions smoothly over an
annular width of 10 AU near 60 AU. In gas, the data are consistent with a
cavity that is smaller, about 15 AU in radius, and whose surface density at 15
AU is $10^{3\pm1}$ times smaller than the surface density at 70 AU; the gas
density grades smoothly between these two radii. The CO isotopologue
observations rule out a sharp drop in gas surface density at 30 AU or a
double-drop model as found by previous modeling. Future observations are needed
to assess the nature of these gas and dust cavities, e.g., whether they are
opened by multiple as-yet-unseen planets or photoevaporation.
