I am speculating here but have seen similar things with the packed vegetation in the floor of very old mines; packrats make nests of sticks, twigs, rope etc and over time these are abandoned and with the result that water from rains washes in and carries in dirt that eventually buries the old nest. What would "seal the deal" would be if you were to find some rat droppings in the very bottom, practically UNDER the twigs.
That is quite an interesting find amigo, I had heard there was a mercury mine in the Superstitions but never found it. If you suspect that pack of vegetation might be just a "plug" as sometimes forms in old mines, rather than risk life and limb by stepping on it (and maybe falling through) you could make up a steel probe out of rebar; that is get a length of half inch (thicker would be better but then is just too danged heavy to pack) rebar, cut two pieces out of it, one being around six feet or so (shorter is okay) and another about two feet long then weld the short one on the end of the six footer. The other end of your "T" then you put on a grinder and sharpen it to a somewhat rounded point, not TOO sharp either or it can "stick" in the ground too much. If you have the patience it also helps to grind the whole length of the part you shove into the ground (the long leg of the T) so that it is fairly smooth otherwise the rough pattern on the rebar tends to grab the dirt and can be tough to pull it out. To use it, just push it in with your hands, wiggle it some every few inches and push some more etc DON'T hammer on it or you will be leaving it there forever. If that turns out to be a "plug" you will quickly find out as it will suddenly get much easier or almost fall as you are pushing so be careful doing this.
Mercury has been bringing some pretty fancy prices recently, it might be worth working that mine if it were legally feasible.
One more thing, the gold mine of Apache Jack was mentioned; I am fairly convinced this was quite a different mine from the one of Jacob Waltz, based on his description of the ore itself; he said it was black quartz with spots of gold showing "...like stars in the sky" which is unlike the known specimens of Waltz's ore. Being black in color is also indicative of a different type of gold lode deposit, as this color most commonly occurs (in Arizona) in what are called "Epithermal" veins, that is they are formed at very shallow or near-surface deposits, while Waltz's ore has the hallmarks of a vein formed at great depth, called a "Hypothermal" vein. I may be mistaken of course, having never found either Waltz's mine nor the one Apache Jack spoke of, just judging by the ore description compared to what we see from old Jacob.
Thank you for sharing this Jim, really cool photos and story, my compliments!
Roy ~ Oroblanco