Our Day Two hike near Laughlin NV was billed as “A Two Cave hike in the Oatman Area of AZ with rough rocky terrain and some scrambling.” Monday’s destination of Bighorn Cave, a large dry cave with stratified deposits, is in the Mount Nutt Wilderness in the Black Mountains of northwestern AZ, somewhere off Silver Creek Road. It all sounded very intriguing. Again, I put my faith in our hike leaders to locate the cave, since I had absolutely no idea where I was going.
I really do wish we had gotten there.
Driving up a sandy wash, we hadn’t even reached the trailhead when one of the vehicles bogged down and had to be freed from the wet streambed. While other great minds worked out how this extraction would be accomplished, I naturally wandered about looking at rocky outcrops.
These were not the garden-variety metamorphic rocks of the previous day’s hike. In fact, they did not appear to be metamorphic at all. Fine grained, with small clasts of what appeared to be plagioclase and possibly hornblende or pyroxene (I have definite personal issues with field identification of these two minerals) and weathered biotite, I concluded that the rocks I was looking at had to be a lithic tuff (compacted volcanic ash consisting mostly of crystalline rock fragments). I suspected this was all due to Tertiary (65-2.6 million years ago) volcanism and would have to do some research when I got home to be sure.
Meanwhile, I took no close-up pictures of the lovely tuff since I thought we would be coming back to the cars the same way. Silly me! Not only did a vehicle become mired in wet sand but it also turned out that, after walking a couple miles, we (we?) had chosen the incorrect wash to the cave. At this point, despite much animated discussion, no one was quite sure how to get there from there.
So we sat on a knoll burning daylight, grazed on our lunches, and waited while several folks from the vehicle-extraction committee walked the 45 minutes back to where we had parked and drove our cars to meet the rest of us along Silver Creek Road. By this time, it was too late to do much of anything except continue the few miles drive to the old mining town of Oatman, drink beer, and amuse ourselves by watching the feral burros lounge around the dusty streets.
However, several of us were not so thirsty that we didn’t first stop for an hour or so to investigate some nearby mine tailings. We found some nice calcite crystals but no undiscovered mother lode of gold – not even minuscule flecks.
Harris (1998) relates that the crystalline basement rocks in the Bullhead City-Black Mountains area consist of “…Precambrian schists (1.7 billion years) and intruded anorogenic (a time between mountain-building events) megacrystic (big crystals) or porphyritic granites (1.4 billion years)…typical of Precambrian granites throughout the southwest…It commonly displays a gneissic texture and is generally weathered and crumbly… The granite is composed of K-feldspar, plagioclase, quartz, biotite, and hornblende.” This is definitely what I was seeing on Sunday’s hike in the Dead Mountains just across the Colorado River. Interestingly, the dates are sort of but not quite comparable to those of the Beaver Dam Mountains in southwest Utah.
I was nowhere near a geological map but was generally spot-on about the general volcanics in Silver Creek. Harris (1998) goes on to say “A thick sequence of mid-Tertiary (30 to 15 million years) igneous rocks covers and is faulted against Precambrian basement … Volcanic rocks include tuffs, flows, and breccias of latite, andesite, and rhyolite.” Most of the mineralization in the Oatman district is hosted by the Oatman Andesite, and to a lesser extent by the Gold Road Dacite. In Silver Creek I figured it was a tuff but couldn’t be more specific than that.
Much has been written about Basin and Range volcanism. Harris (1998) sums it up nicely: “Two episodes of extensional tectonics are responsible for the present [Arizona] topography… A second major episode of faulting, the Basin and Range disturbance, affected western Arizona starting about 12-15 Million years ago … This faulting produced much of the present topography in the Basin and Range Province. Voluminous volcanism accompanied this later faulting.”
Right on! I’m maintaining two-for-two in my on-the-fly field identification skills.