As I write this I am hoping I will have enough new material to offer from our second visit to the Alamo Breccia that I did not say in the original Alamo Breccia article of November 2002.
But, I guess I will start by saying that when we were there a few weeks ago I thought it had only been 8 years. Then I looked at the magazine back issues and found to my surprise that it had been ten years ago. I looked over the original article and said to myself that is pretty good, I clearly had more time in my life to do research and to spend on writing. But, I will give it a shot anyway again this month.
The Alamo Breccia is a prominent layer of rock that can be traced visually for a long distance undulating from ridge to ridge and mountain to mountain over a wide area of Nevada. Mostly a gray color it is seen usually as two close together thick bands of rocks with some thinner layers added. Up close investigation shows these apparently separate layers to actually be from the same event.
At the Hancock Summit site the exposures are easy to find visually. However, to get up onto the top of one of them is a good hike up steep terrain. You will be making your own path most of the way since there is not a real trail to follow. So pick a rate of climb that you are comfortable with and then make your way from boulder to boulder up to the top of the ridge. A portion of the rocks projects forward as a separate prominence. The remainder of the mountain rises behind you. From there you can stand and look out over the valley. I am really uncomfortable up on top of the ridge. I managed to get up there but did not stay long. I retreated to a spot just a little lower where I could collect specimens from the side of the exposure rather then off the very top.
On our way up we had determined to find the stromatolites. We had missed the best deposit the first trip. So after photographing the single petroglyph that I showed in last month’s article we made sure to find the stromatolites. We knew they were near the bottom of the mountain. We began hiking up a little more westward this trip and found the stromatolites pretty easy. We also immediately saw that there had been a lot of core sampling done since we were there last. In several places numerous cores had been removed. Paul suggested that there may have been so many done in order to provide each person in a large group a core of their own. That makes quite a bit of sense. There were more cores taken then needed for dating or other analysis, dozens.
From the top of the ridge you get a great view out toward Area 51 and part way around the valley that leads ultimately after a few more mountains to Rachel, Nevada. As I said at the beginning the layer of Alamo Breccia can be visually traced from ridge to ridge and across the intervening valleys and washes. I had been noticing what I was sure was the continuation of the layers on the other side of the road from where we parked. After eating lunch we headed across the road. The Alamo Breccia there was tilted much flatter showing more surface of each layer. It was much like playing cards fanned out rather than the cliff-like exposure of the entire thickness on the other ridge. But, like the exposure of the morning hike if you when farther around the hill it too became a cliff as can be seen in the following picture.
On the previous trip years ago we had hiked over to the next mountain in the other direction, about, I would say half a mile away southward from the high difficult ridge. There the rocks were slightly different again. In this part of Nevada it is not hard to find the Alamo Breccia. For example, when we returned to Rachel we drove off the road and parked to take some pictures of the abandoned mine that is outside town. As we were standing there I noticed that the rocks at our feet were mostly Alamo Breccia that had washed down out of the mountains near Rachel. The Tempeute Mountain deposit near there is 4×4 accessible with a serious hike at the end. Yet even miles away at the abandoned mine the ground is covered with mainly pieces of breccia.
Your normal piece of Alamo Breccia from the Hancock Summit area is a gray stone with a distinct glassy feel and clink when struck. It is not rock that one would want to work with for very long without gloves. It is sharp often and wears out the skin fast when trying to pull it from the ground or even carry it. We put the pieces into back packs to get it to the 4×4 and then stored it in plastic tubs I had brought. We did not bring back really that much; a dozen pieces I guess from each of the two locations.
Alamo Breccia is easy to work from a lapidary standpoint. It is not particularly hard and it polishes up nice. But, like most impact breccias it is prone to having cracks and to falling apart. Pieces will often crumble a little in the saw and portions may dislodge on the diamond lapping disks during smoothing. This has been the case with the interesting white crystal filled material from the site near the road. The crystal is rather fibrous and chunks cleave off very easy around the edges. It is stable in the middle of slices and as can be seen in the accompanying photo it is quite nice looking material.
The Alamo Breccia rock unit is made up of several layers. At Hancock Summit there is a thick layer of huge mega blocks and giant boulders. In places the blocks are so large that it looks like a regular layer of gray rock rather then one made of blocks at all. Another layer consists of large clasts from say half an inch up to several inches with very little tiny broken rock in it. A third version is made up of material much more pulverized. It can contain little or no large clasts. There are many pieces which are a blending of these last two. Sometimes tiny breccia material will be found filling the gaps and cracks between the larger cobbles and boulders of broken rock. The shells and coral present at the impact site were incorporated into the rock as well.
The Alamo Event as it is sometimes referred to was a very significant impact. The minimum area involved is on the order of 100,000 square kilometers. The rock layers are found to one extent or another at 25 different mountain ranges. Estimates for the cubic volume of rock transported by the event are all over the place. Estimates range from hundreds of cubic kilometers to a staggering 1000 cubic kilometers, to the unimaginable 2500 cubic kilometers. My geology training is pretty old at this point, though I have continued to use it over the years. Some impact products known today were not known when I was a student. So my descriptions of the layers as they are seen are my impression only and should be taken with a grain of salt. But, it is a difficult problem for professionals too as I read the written material. Several writers have made comments about not having access to places in the Area 51 portion of the impact region. Places that hold information necessary to aid the investigation of the Alamo Impact.
Clearly the feel and sound of the rocks leads the visitor to the site to regard many of them as heat generated. However, the way that the beds themselves appear suggests water transport played a part. There is also the problem of differentiating what is immediate fall back material and what is later infill from subsequent land slide and tsunami activity. On top of these issues, you need to know in what zone the deposit you are looking at resides. In the case of the Hancock Summit sites where there are usually three or four outcrops marked on older study maps, you are outside the crater in one of the transitional zones. It is sometimes referred to as the Ring Realm. So any impressions of water reworking of material are certainly justified. The impact was into an ancient sea and the water was displaced , tsunami waves were formed, and then the water moved back into the sea after the event. Clearly some areas were stirred up considerably.
There has been modeling by investigators that incorporates a series of tsunami waves being made along with collapse and landslide wave action as well. Presumably while all this was happening layers of broken rock ejecta and carbonate accretionary lapilli were being laid down. These however, are now incomplete layers disrupted and partially erased by the continuing water and settling processes. But, pockets of mostly small clast size breccia and scattered samples of the carbonate accretionary lapilli layer can be found if you hunt around a little. Some of the layers made up of large boulder size masses have fill material of finer grain size. I am guessing that this is reworked ejecta material that has settled and washed into the fracture spaces of the layer made up of boulders and megablocks.
The most interesting material for me as a collector who is going to cut it up and polish it comes from the layer at or near the top made of mixed smaller size particles. This is the material which to me is so visually attractive when cut. Solid chunks of dolomite are in this writer’s opinion near the top of the list of boring rocks when cut. In the case of the ones involved in the Alamo Event they are plain dark gray to nearly black featureless stone. But, mix them up with a more than a dozen different limestones and other rocks, throw in a nice helping of fossil shells and you have a great impact breccia. At Hancock Summit this portion of the Alamo Breccia section is not very thick so you have to do some hunting around up on top to find the nice specimens. This layer of material is called in the literature the A Unit and varies from site to site greatly in thickness. I would estimate that up on the ridge it is only 1-2 meters thick. Though hunting around at the side of the cliff and the base of the ridge would probably yield many good specimen amongst the tallus that has fallen down. Across the road at the new spot we went to this trip the smaller grain breccia is spotty, less abundant but easier to get to. That is where we found the vein of breccia mixed with white crystalline rock. I hesitate to say that this was one of the well documented dikes associated with the Alamo Breccia at many locations. To me with the opportunity we had to briefly study the thing, it looked like a vein of rock that formed and filled a crack at a later time. I have not researched the dikes in the Alamo Breccia to see if they were immediate formations of injected material or later filled cracks running perpendicular to the bedding. The vein was an interesting feature however it came to be.
I could probably be happy revisiting the Alamo Breccia sites every few years. Maybe come up with some alternating schedule. Meteor Crater one year, Alamo Breccia another with a couple other favorite areas for a couple years in between. Every time I go I find new material that I have only read about. As they say a picture is worth a lot of words so here is a small album of a few more pictures of the Alamo Breccia and the Hancock Summit location.
Until next month, enjoy your meteorites and have a great up coming new year. Jim