Updated nomenclature: The Burro Mesa Formation consists of the lower Wasp Springs Tuff Member (fomerly the Wasp Spring Flow-Breccia) and the upper Burro Mesa Rhyolite. The formation was erupted between 29.4 and 29.0 Ma from several lava dome volcanoes. The Burro Mesa Formation is no longer considered to be corrlative with the rhyolite found on top of Emory Peak, Casa Grande Peak, and Lost Mine Mountain, which is now known as the Emory Peak Rhyolite Member of the South Rim Formation. The Burro Mesa Formation consists of the Wasp Springs Tuff Member underneath the Burro Mesa Rhyolite Member. The Burro Mesa Rhyolite Member consists of an upper and lower unit of potassium and sodium (alkali) rich lava and ash-flow tuff, separated by a mugearite lava flow (a basalt rich in sodium and potassium -?). The Burro Mesa Formation apparently originated through eruptions of several lava dome volcanoes. According to the www.maroon.com/bigbend/ website the conglomerate here is composed of quartz and rhyolite gravel and cobbles in a matrix of tuffaceous sand. One website says the pouroff is 150 feet, which may be the total drop, and another says it is 40 feet to the top of the vertical drop. However, it is clear the drop is well over 40 feet and more like 80 feet but definitely not 150 feet. Stratigraphy of Burro Mesa Pouroff Burro Mesa Formation Burro Mesa Rhyolite Member (29 Ma, upper Burro Mesa; 28 Ma, Cerro Castellan) - phyric (>1% phenocrysts) and aphyric (<1% phenocrysts) comendite lava and ash-flow tuff (comendite = sodic rhyolite) Wasp Spring Tuff Member (29 Ma) Unidentified conglomerate of Chisos Group Chisos rocks with Bee Mtn Basalt Nomenclature update for South Rim Formation (younger to older): Burro Mesa Rhyolite --> Emory Peak Rhyolite Member (rhyolitic welded ash-flow tuffs, 32 Ma; apparently the E to NE trending dikes between Casa Grande and Lost Mine Peak are feeders for this rhyolite) Wasp Spring Flow-Breccia/Lost Mine Rhyolite --> Boot Rock Member (quartz trachyte to rhyolite lavas, ignimbrites, and maar surge deposits, between 32 and 33 Ma) Brown Rhyolite --> Pine Canyon Rhyolite Member (caldera forming ignimbrite, 32-33 Ma) All members are alkali-rich, with the Emory Peak Rhyolite being the least alkaline. Updated nomenclature: The Burro Mesa Formation consists of the lower Wasp Springs Tuff Member (fomerly the Wasp Spring Flow-Breccia) and the upper Burro Mesa Rhyolite. The formation was erupted between 29.4 and 29.0 Ma from several lava dome volcanoes. The Burro Mesa Formation is no longer considered to be corrlative with the rhyolite found on top of Emory Peak, Casa Grande Peak, and Lost Mine Mountain, which is now known as the Emory Peak Rhyolite Member of the South Rim Formation. The Burro Mesa Formation consists of the Wasp Springs Tuff Member underneath the Burro Mesa Rhyolite Member. The Burro Mesa Rhyolite Member consists of an upper and lower unit of potassium and sodium (alkali) rich lava and ash-flow tuff, separated by a mugearite lava flow (a basalt rich in sodium and potassium -?). The Burro Mesa Formation apparently originated through eruptions of several lava dome volcanoes. According to the www.maroon.com/bigbend/ website the conglomerate here is composed of quartz and rhyolite gravel and cobbles in a matrix of tuffaceous sand. One website says the pouroff is 150 feet, which may be the total drop, and another says it is 40 feet to the top of the vertical drop. However, it is clear the drop is well over 40 feet and more like 80 feet but definitely not 150 feet. 1431 - in first pouroff of trail. Looks like novaculite. Get flint smell when struck with rock hammer. 1432 - ls over novaculite. Since Tesnus is stratigraphically above novaculite, looks like could be thrust fault here. 1433 - ss over clay in side of wash. Could be Tesnus. 1434 - novaculite boulders in wash. 1435 - looking at inside of fold. Axis runs from upper left to lower right at an angle of about 30 deg with the horizontal. Beds above hammer dip toward camera, originally wrapped around axis and continued as beds dipping away from camera (below hammer). Fold exposed in small pouroff. 1436 - same pouroff. Fault runs upper left to lower right just above hammer. Note beds striking differently above and below hammer. Note fault breccia. Looks like novaculite on both sides of fault. 1437 - resistant novaculite forms pouroffs. 1438 - fault in novaculite 1440-1442 - fault in novaculite. Hard to correlate beds across fault, but looks like reverse fault. Fault breccia seen in closeups. 1443 - novaculite over Tesnus, exposed in wash. Thrust fault with fault breccia 1397 - syncline in Chisos rocks, N end of Sierra Quemada, SW of Ward Mtn. Resistant ledge high on foreground mtn may be Bee Mtn Basalt. Note dike extending from foreground left into mtn. Note syncline. Map says gully on right must be a fault associated with Burro Mesa Fault Group, down on right. Looks like dark stratum surrounded by ashy-looking beds on right behind bush may be correlated with similar-looking bed above in on left. Stratigraphy of Burro Mesa Pouroff Burro Mesa Formation Burro Mesa Rhyolite Member (29 Ma) - phyric and aphyric comendite lava and ash-flow tuff (comendite = sodic rhyolite) Wasp Springs Tuff Member (29 Ma) Unidentified conglomerate of Chisos Group Chisos rocks with Bee Mtn Basalt 1399 - BMR over WST over unnamed cg. Note weathered top of WST 1402 - Igneous intrusion/volcanic plug 1362 - road to Study Butte, about 5 mi W of junction with Maxwell Scenic Dr. N end of Burro Mesa. Chisos Group, topped by Tule Mtn Trachyandesite 1363 - Tule Mtn from 118 (to Study Butte). Topped with TMT over Mule Ear Tuff, which may be barely exposed beneath the TMT. Prob outcrops of Ash Spring Basalt at base of mtn to left 1364 - Study Butte. Trachyte sill intruded into Pen Fm. Sill a source of mercury (cinnabar). Mine used to be on left side of butte. 1366 - S Lajitas Mesa. Topped by TMT over MET. Basalt below that, apparently some BMB and some ACB (Alamo Creek Basalt). Ash (tuff) above road is Chisos. 1367 - S side of S Lajitas Mesa 1369 - landslide. Ash above and below what is probably a flow of the BMB. 1370 - Hoodoos in ash 1371 - Ash cliffs across the Rio in MX 1372 - Cgl on basalt 1373 - Ditto 1374 - Appears to be trachyandesite 1376 - Trachyandesite (?) on cgl. Note old erosional surface. 1377 - Cave in cgl beneath lava flow 1378 - Spring along dry creek bed 1379 - Upstream from spring. Shelf of solidified stream deposits on hard-rock bank. 1380 - Layer of former soil between cgl and lava flow. Paleosol apparently a layer of weakness. 1381 - Boulder of cgl 1382 - Vesicular basalt 1383 - Red ss containing cobbles 1384 - Looking S down canyon 1385 - Layers of gravelly/cobbley ss. Note disconformities. 1386,7 - Spring at head of canyon 1388 - View toward spring area? 1389 - Layers of lava (trachyandesite?) 1390,1 - spring water flowing back underground 1392 - View to W down Rio from Big Hill on S side of Santana Mesa. Hill made of tuff. 1394 - Mule Ears in distance 1395 - Chisos from Terlingua Chisos Group: Tule Mtn Trachyandesite 32 Ma Mule Ears Spring Tuff 33 Bee Mtn Basalt 34-30 (interbedded with MEST and TMT) Ash Spring Basalt 34.5 Alamo Creek Basalt 47-34.5 New stratigraphic names in Chisos Mtns: South Rim Formation now includes the Pine Canyon Rhyolite Member, the Boot Rock Member, and the Emory Peak Rhyolite Member. Burro Mesa Rhyolite --> Emory Peak Rhyolite, quartz-bearing rhyolitic ash-flow tuffs, 32 Ma. A series of E and NE trending dikes of chemically identical rock cut through Chisos Fm and younger rocks. Wasp Spring Flow Breccia and Lost Mine Rhyolite --> Boot Rock Member, quartz trachyte to rhyolite lavas, ignimbrites, and maar surge deposits, ~32 Ma Brown Rhyolite --> Pine Canyon Rhyolite, ignimbrite that formed Pine Canyon Caldera, 33-32 Ma 1338 - Pine Canyon 1339 - tuff and volcanics of Chisos Group. Crown Mtn on right 1340 - Juniper Canyon. South Rim on right, Crown Mtn on left, Tortuga Mtn (?) in distance 1342 - Juniper Canyon and South Rim 1344 - S Rim and Toll Mtn separated by Boot Canyon with linear igneous intrusion 1346 - feet on Wasp Spring, back on dike, Pine Canyon in background with Crown Mtn in center, Lost Mine Pk on left. Drop off due to fault 1347 - Lost Mine Pk 1348 - Contact between WSFB (left) and dike (right), fault-formed cliff beyond backpack 1349 - Juniper Canyon. Elephant Tusk in distance, Tortuga Mtn between ET and camera, Crown Mtn on left 1350 - WSFB with strata containing a variety of clast sizes, poor resolution 1351 - Green Gulch. NE corner of Pullium Bluff left of center in distance 1352 - debris from rock falls off Vernon Bailey Peak 1353 - conglomerate-like alluvium exposed by Oak Creek 1354 - looking N toward Vernon Bailey Pk. Linear structure prob due to jointing not dike 1316 - alluvium in Nine Point Draw on way to Dog Canyon 1317 - horizontal Santa Elena strata on left and right (foreground) with beds dipping up into drag fold background right due to thrust faulting 1318 - coming out of Dog Canyon. Erosion in alluvial material where draw changes direction 1319 - toward end of Dog Canyon. Santa Elena smoothed by abrasion in draw 1320 - looking wsw toward entrance to Dog Canyon. Santa Elena nearly horizontal in distance but beginning to curve upward on right nearest camera 1321 - a bit farther E in canyon than 1320, looking at S side of canyon. Santa Elena curves to vertical then overturned. 1322 - same position as 1321, looking at N side of canyon. Santa Elena clearly turns over in fold 1323 - essentially same position as 1320. SE strata go from nearly horizontal to nearly vertical 1324 - nearly horizontal SE at entrance to canyon 1325 - essentially same view as 1317 1326 - similar to above except farther out of canyon to W 1327 - fluted edge on top of canyon due to dissolution of limestone by water flowing over canyon rim 1328 - deeply eroded floor of draw. Would be deep pool during high water flow. Wet on bottom due to recent wet weather 1329 - scalloped pattern on limestone due to dissolution. Large ripples in ss lying over ls 1330 - mountain strata, top to bottom, according to Maxwell: tan Glen Rose on top. Thinly exposed line of Buda below it. Since Glen Rose is older, contact must be thrust. Boquillas below Buda, then Buda again, forming ledge. MacCloud doesn't show upper Buda outcrop. If Buda appears twice must be thrust fault contact between Buda and Boquillas, since Buda is older. Del Rio below Buda covered with alluvium, according to MacCloud. Hill to left is S side of canyon and topped by Santa Elena 1332 - Hill to S of Dog Canyon formed of drag-folded Santa Elena 1333 - like 1330 but farther away. Buda outcrop dips toward W and is better exposed in middle ground on right, showing thickness of unit more fully 1334 - hogbacks on W side of Dagger Mtn anticline. N extent of Dagger Mtn at left 1335 - Rosillas Mtns from Nine-Point Draw. Highest point is Rosillas Peak at 5445` 1336 - cuesta in middle distance. Santiago Mtns in background.