| Geology: Verde Valley! |
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89A SH89A WEST SEDONA. This is after you leave the city limits going down the hill. The road cut on the north side is the only place in Sedona where you can see some of the Redwall Limestone from about 320m years ago. Way below Midgley Bridge is another exposure. CR CATHEDRAL ROCK. This formation is the most well known around Sedona. Most of it is Schnebly Hill sandstone. In the middle-height, is the Ft Apache limestone from being under the Pegregosa Sea. Below that is the Bell Rock sandstone member (beach) while above the Ft Apache is the Sycamore Pass member (away from the beach). The shape occurs from a subsequent uplift with erosion in the cracks. GB GRAYBACK MOUNTAIN. Also known as Thunder Mountain and Capitol Butte. The old-timers here don't appreciate all the more recent names! It's a nice mountain just to gaze at from a restaurant in West Sedona. On the far western edge is Lizard Rock, and indeed it looks like a lizard! The mountain is mainly permian Coconino sandstone overtop the Schnebly Hill red sandstone. On top are hints of the Kaibab limestone. There's a trail all along its base. If you cash a check at the Wells Fargo drive-thru, and use the RIGHT drive-thru, you'll see a red rock arch in the making straight ahead (can't see from the left drive-thru!). There are quite a few arches in this area, which start out as erosion BEHIND a rock facing, with the center-part eventually dropping out. HM HOUSE MOUNTAIN. This is an interesting mountain. As you drive south on SH89A toward Cottonwood, it's on your left or to the southeast. It's flat-ish with what looks like a little house on top. Some old-timers say it was named for the House Ranch on its eastern side; others say it was Maggie Dumas that named it for the little 'house' on top. Only recently has its origin been traced to a shield volcano by Wayne Ranney. If you look at it from the Village of Oak Creek side, you'll notice the north side is missing! That's because the ancestral Mogollon Rim was likely located next to where the volcano erupted. 'Shield' refes to the shape of a shield! And the liquidy lava comes from DEEP in the earth (compared to the San Francisco Peaks). Estimating the date of the House Mountain ash, the Mogollon rim is eroding at about 12" every 625 years, per Wayne. (compare that to Oak Creek downward erosion of 12" every 3000 years). Imagine the newly available land for building!! If you go further south on SH179, you'll come to the Dry Beaver Creek and ANOTHER geologic story of the ancient Mogollon highlands. It's complicated ... buy Wayne's 'Sedona Through Time' book! MM MINGUS MOUNTAIN. Goodness, what a history! Start back in precambrian times, only 2 BILLION years ago (about half the age of the earth). That's under Mingus Mountain about the same height as Cottonwood. For comparison, it's the same as the BOTTOM of the Grand Canyon. And geographically, we're near the South Pole! Deep under today's Antarctic(!), hot magma welled up forming the rhyolite (similar to granite) that you can view on SH89A south and above of Jerome. Eventually a 2000 ft dome built up under the ocean with volcanic vents spewing dilute liquid copper, gold and silver, cooling on the surface of the sea. That is what the Jerome miners were digging up at the open pit. Eventually this dome collapsed, with more volcanic eruptions into the caldera. The tuff from the eruptions created Cleopatra Hill under Jerome and the Little Daisy Mine inside Cleopatra Hill. Remember we're still under the ocean! With the colliding microcontinents, the whole area was then forced UP thousands of feet. At this point, a full quarter of the earth's history in this area disappeared ... 'The Great Unconformity'. We pick up the story over a billion years later! About 525m years ago, you begin the sedentary layer building, most of which is also viewable at the Grand Canyon. First was the 'Tapeats' sandstone, in which can be found mollusks, brachiopods and trilobites. Outcrops of the Tapeats can be found ABOVE Jerome, and all the way north to Montana. Geographically, you're now near the equator (you moved a LOT!). For a close up view of the Tapeats, walk around Jerome ... it was used for building material. Next comes the Martin limestone at about 380m years ago. We're BACK deep below the ocean AGAIN, and the limestone is from marine deposited brachiopods and sponges. Watch out! It was just BEFORE this, that another uncomformity occurred plus two mass extinction events (beginning of the fish age). Next at 340m years ago is the Redwall limestone and it's up above the Martin limestone. Sedona gets most of its water below this layer, and geographically we're still around the equator and UNDER the ocean. At least its a warm ocean. Since we're on Mingus Mountain, at this point, everything you see over in Sedona is happening ABOVE us (permian layers). But obviously you can't see them since they're eroded away. It's only when you get to the time of House Mountain (see above), that you also get volcanic eruptions on Mingus ... known as the Hickey formation. That would be 15-6m years ago, with almost 300m years missing of permian, triassic, jurassic, cretaceous, and most of the 'cenes' time. WOW. It's about this time that the basin/range period began, with mountains tipping over in southern Arizona. Mingus had multiple volcanic eruptions, with the results on TOP of Mingus Mountain (a little counter intuitive!). I don't know about you, but I'm exhausted just thinking about this ... the Verde down-drop hasn't even occurred yet (see below)! SC SYCAMORE CANYON. This is a wilderness area (thank goodness), that is southeast of Sedona, but very similar in geology. It's the namesake of the top Schnebly Hill formation member known as Sycamore Pass sandstone. Because the sandstone has less clay silt than the Bell Rock member, it supports much more vertical angles. If you get the chance, take the Verde Canyon Railroad excursion. You may meet Mike Ward (he's often a guide on the train). The train goes up into the canyon past the Sycamore area, where you can see many of the late volcanic flows (4.8mya), eroded by the Verde River. SH SCHNEBLY HILL ROAD. Named for early settlers Carl and Sedona Schnebly, who had a small homestead where Los Abrigados is. The road goes all the way to I-17 but you need a high-clearance vehicle after the first 2 miles. It's both a nice drive and hike, displaying most of the permian layers ... Hermit shale, Schnebly Hill sandstone, Coconino sandstone, kaibab limestone and basalt on top. It's the old Munds wagon trail and you can see a whole series of carvings: Camel Rock, Merry-Go-Round and the Cowpies. A good overlook allows you to see the Wilson Mountain bench and faultline. VV VERDE VALLEY. The story begins about 8m years ago, when the Jerome side pulled apart from the Sedona side along the Verde fault, creating an almost 6000 feet downdrop in the Verde Valley area. A lake began to fill this in, when about 5m years ago, volcanic flows (likely from Sycamore Canyon) flowed into the valley (see above). Again, a sizable lake formed, with substantial limestone deposited. This is why, as you come down SH89A from Sedona, the dirt slowly becomes white ... you're driving into the bottom of an old lake! The red rocks, that you know and love, are thousands of feet BELOW you! Now, if you noticed a cement plant over in Clarksdale (next to a closed electric plant), it's mining the Redwall limestone ... much earlier and not downdropped. It's this Redwall cement that was used to build Glen Canyon Dam. So, if you drive up to Page, imagine the Redwall (340mya) plugging up the Glen Canyon sandstone group(about 140mya)! Even more interesting is that the Redwall is deep below Cottonwood, being mined near Clarksdale, and is near the top of Mingus Mountain. Goodness, it gets around. Party-boy!! |
| Copyright ©, 2006, dmbarnhart |
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