Archive for category: Blog

It was a nice and sunny Saturday when we started our journey on I-10 going west of Houston. We crossed Pleistocene and thin Pliocene rocks until we were near the Little Low Miocene Hills of Columbus, a very small strip. Finally, we arrived at our first stop where lower Cretaceous rocks onlap onto basement rocks. We noticed that Oligocene rocks were missing.

Day 1:

Walnut Formation Road Cut (Stop 1)

Location N 030.29947 W 097.82767

Elevation: 297 m

At this stop, we saw Glenrose basement rock overlain by Walnut Formation (110 Ma). The lithology of the Walnut Formation is a lime mud marl (limestone), and the beds seen are horizontal and fissile. The beds contain fossils and some of the fossils found include the famous Devil’s toenail (exogyra), oyster, snails and warm burrows. The worm burrows are indicative of a depositional environment with ubiquitous bioturbation. Since it was quiet enough for worms to burrow and mud to be deposited, we speculated that the depositional environment was lagoonal. If this is the case, it could be an indicator of paleo sea level.
There is an enigma surrounding the Walnut formation. If it was formed close to sea level, how is it that the elevation is now 297 m today? The question of when and how the Llano Uplift (part of the Edwards Plateau) occurred is still a mystery in geology. Some researchers have pinned it to Oligocene time. As far as how it occurred, several theories exist: (1) magmatic underplating from ultramafic intrusions along the Balcones Escarpment; (2) a flexural response from forebulge to the coastal basin; and (3) Jurassic salt moving to the surface, causing Cretaceous rocks to slide and creating offset.

Transition from Glen Rose to Walnut Formation moving upwards. Glen Rose has harder, regular bedding, where Walnut has muddy Cretaceous limestone.

Close-up of the Walnut Formation. Contains visible shelled organisms and exhibits diffuse bedding.

Ellenberger Group, Honeycut Formation (Stop 2)

Location: N 030.553777 W 098.26020

Elevation: 246 m

The Honeycut Formation is Ordovician in age. About 100 m of limestone has been eroded here in comparison to the Walnut Formation, and this is reflective of the erosional window of the Llano Uplift. The formation is highly laminated and contains bacterial mats (stromatolites which grow on tidal flats). There might also be some stylolites although it is not certain. Barnett Shale, which is Mississippian-Devonian in age, can also be seen at this outcrop, but it is very thin.
In West Texas, the Ellenberger limestone is a major carbonates reservoir where it was exposed, karsted, faulted, and later sealed. The faults here are related to the Ouachita orogeny (which is younger than 300 Ma) and analogous to the Alleghanian orogeny of North America. Although the Honeycut marble is the downthrown side of the fault while the granite is upthrown side, the granite appears to be eroding faster than the marble. The result of this is a reverse topography instead of what we would expect from faulting.

Stromatolite in the Honeycut Formation.

Backbone Ridge (Stop 5)

Location: N 030.64519 W 098.41574

Elevation: 289 m

The Cambrian rocks are beneath the Town Mountain Granite, so we should have been in a graben (central downdrop block), but instead we were standing at a local high point. This is another instance of inverted topography due to erosion of the granite which has relatively low resistance. At this stop, we were standing on normal fault with two more faults running perpendicular in the Cambrian rocks before us. The Lion Mountain green sandstone across the road was formed by worm droppings in an anoxic environment, creating a mineral called glauconite. The sandstone is pelletal in texture and contains white lenses that we identified as trilobite hash. There are little muscovites on the surface of the trilobite deposits suggesting past metamorphism. Underlying the Lion Mountain is the upper Hickory Sandstone containing iron oolites, indicating a ferriferous depositional environment. Above the dark green sandstone we observed a color change to whitish-green and a compositional change to coarser grains containing more quartz. This is indicative of the depositional environment moving inland up the strata with the upper rock likely deposited in a shoreface to backshore setting. The whitish-green then transitions upward into a pink and green rock. The presence of herringbone bedding within this upper section signifies a tidal phase, perhaps an offshore to lagoonal environment. Overall the Lion Mountain Sandstone is less resistant than the overlying strata. Within the whole body we found an oxidized strike slip fault with mellow plunge in the slickenlines.

Predominately strike-slip fault (with some thrusting) in Lion Mountain Sandstone. Directionality indicated by slickenlines on red Fe-stained parts of glauconite on right side of fault.

Students looking at the Lion Mountain Sandstone.

Valley Spring Gneiss and Granite, Inks Lake State Park (Trip 6 PC Guide Stop 3)

Location: N 030.74855 W 098.35748

Elevation: 278 m

Here we saw heavily deformed quartzofeldspathic gneiss among other schists and gneiss. Up to four generations of folds and five associated foliations were able to be identified, all of which except the last were penetrative and seen through the whole area. Granitic dikes and intrusions were present and responsible for local metamorphism.

Granitic dike cutting through the Valley Spring Gneiss.

Distinct F3 fold in the Valley Spring Gneiss.

Valley Spring Gneiss (Trip 6 PC Guide Stop 2)

Location: N 030.75404 W 098.67634

Elevation: 281 m

For our last stop on Day 1, we examined a synform in the Valley Spring Gneiss by the Llano River. We measured the attitudes for several mineral foliations, plotted them on a stereonet, and attempted to match the data to our observations. At another spot by the river, we observed a Valley Spring pluton dated approximately 1250 Ma with a dynamothermal overprint dated 119 Ma. We noted that there was not much foliation in the country rock. The texture of the rock suggests that it was not highly metamorphosed.

Old Valley Spring pluton by the Llano River.

Day 2:

Highway 71 Outcrop (Mosher Day 1 Stop 4)

Location: N 030. 63010 W 098.52912

Elevation: 296 m

At this stop we observed the graphitic Packsaddle Schist. Lightened zones indicate the graphite was volatilized during intrusion, oxidizing the rock along the boundary and bleaching it. Overall the schist is blue in color with white folded quartz veins. It is fairly resistive and contains S1 and S2 folds. Little knots of quartz represent the old fold axis of the formation, and the initial folds appeared to be very ductile. We measured the trends and plunges of various foliations. The folds are typically isoclinal with some shearing, and the plunges of the fold axes range from steep to shallow. Within the schist we observed some vestige of the sedimentary protolith, a dark shale stacked atop a light sand. We noted sheaf folds and contemporaneous multi-directional folding, unique kinematic indicators for shear. The timing of the main assembly of the schist dates to 1140 Ma, predating the intrusion of the Town Mountain Granite. These schists are roughly the same grade as the gneiss we saw the previous day; both were metamorphosed at the amphibolite facies.

Some trends and plunges of fold axes:

1. N50ºW, 43º
2. N20ºW, 34º
3. N40ºW, 15º
4. N55ºW, 31º
5. N15ºW, 39º
6. N60ºW, 12º

Packsaddle Schist containing graphitic layers (dark banding) and folded quartz veins (white).

Cordierite (Added Stop)

Location: N 030.53540 W 098.44107

Elevation: 282 m

The next roadcut we stopped at was notable because of the presence of cement-like cordierite nodules which statically overprint the rock’s foliation. Cordierite is characteristic of high temperature, low pressure contact metamorphism associated with later stage plutonism. It contains no clear crystal structure and is only visible in thin section. Since the cordierite overprints foliation but exhibits none itself, we know that this stage of plutonism postdates folding. The Packsaddle rock here is more schist-like than what we saw at the last stop; the protolith is less obvious.

Packsaddle Schist with cordierite indicated by the weathering pattern. The cordierite is more resistant than the schist.

White Creek Ford (Mosher Day 1 Optional Stop 3)

Location: N 030.45692 W 098.482931

Elevation: 255 m

At this stop, we had an opportunity to see a rock specific to Texas, llanite. Llanite typically has blue quartz phenocrysts and K-feldspar. We saw some of these diagnostic characterics in the rocks surrounding the llanite dike that cut through the area. This dike is dated at 1098 Ma and indicative of late-stage rhyolitic igneous activity. Due to the lack of metamorphic overprint, the area is estimated to be at least 20 million years younger than the Town Mountain Granite not far from there. Further, the surrounding rocks had an aphanitic texture due to being injected into the mix at a shallower depth. They had to formed near the surface to avoid crystal growth and develop this texture.

An interesting feature we observed in this area included veins that were folded. First, in order for the veins to be formed, the rocks had to exhibit brittle behavior by cracking, and then later on exhibit ductile behavior for the veins to then be folded. This brought up the transition between brittle and ductile behavior which can occur due to changes in temperature, overburden pressure, or strain rate. The boudinage of the veins also indicated flow after formation. We also saw some crenulation folding that indicates a later extensional event due to its lack of offset from the main formation.

Classic “boudinage” feature.

Sandy Creek Shear Zone (Mosher Day 1 Stop 3a)

Location: N 030.54169 W 098.56100

Elevation: 292 m

The Sandy Creek Shear Zone is home to unique rocks that were crushed at high pressure and low temperature. We inferred these conditions from the crushed rock grains that lack evidence of recrystallization. At this stop we could also see another rock very specific to Texas, the Town Mountain Granite. This felsic granite appears to have orthoclase, hornblende, K-feldspar, quartz, and mica. The granite owes its pink color to abundant K-spar. In some places, the rock looks very “beat up” in that it has discrete fractures and seems to have undergone violent deformation at temperatures much lower than the solidus of the rock. Very little of the protolith was left untouched. The granite has been sheared but not recrystallized due to its location at shallow depths in the crust.

This formation brought up the brittle vs. ductile deformation question again when we observed that the sheared granite was brecciated to a fine-grained texture. Does the cataclastic but cohesive nature of the rock mean that there was some sort of cataclastic flow? This formation was a prime example of the blurred and ill-defined boundary between brittle and ductile deformation and could perhaps represent some sort of boundary condition between these two categories. The fact that there are mylonites half a kilometer down the road from this formation and its relative location indicates that the Sandy Creek Shear Zone is a point of contact between the Coal Creek and Packsaddle domains.

Brecciated sheared granite exhibiting cataclastic flow.

Augen Gneiss (Mosher Day 1 Stop 3b)

(Approximate)

Location: N 030.54169 W 098.56100

Elevation: 292 m

The gneiss here is similar in lithology to the rock at the Sandy Creek Shear Zone. Unlike the previous stop, this rock has been recrystallized after flowing. High-temperature ductile flow and partial melting occurred. This outcrop is a mylonite, meaning that grain size was reduced by plastic deformation during recrystallization. Minerals more resistant to grain size reduction, such as orthoclase feldspar, were rolled during deformation, resulting in clear mineral lineations and classic augen gneiss eye-like feldspar features. These rocks were clearly hotter than at the previous site, yet sit topographically higher, indicating that they likely did not undergo contemporaneous deformation.

Augen gneiss outcrop with mineral lineations lying on a foliation plane.

Red Mountain Ranch (Mosher Day 1 Stop 3c)

Location: N 030.51822 W 098.55237

Elevation: 312 m

Here the cataclastic flow and mylonite from the previous two stops (4 and 5 respectively) are interwoven. This leads us to believe that the two different flow features at their respective sites must represent the early and late stage of the same shear zone. Additionally, this locality is near the transition into the Packsaddle Schist, sitting at the base of the Coal Creek domain.

Interwoven cataclastic flow and mylonite.

Serpentinite Quarry (Mosher Day 1 Stop 2)

Location: N 030.47915 W 098. 63175

Elevation: 332 m

The serpentinite seen at the quarry here is a 6 km long section of the Coal Creek Domain. Derived from ultramafic, low crustal rocks, the serpentinite records a complex history of multiple instances of metamorphism, serpentinization, and deformation. It is thought that the serpentinite was deposited originally as a olivine-rich harzburgite and was subsequently serpentinized during tectonic emplacement. Following emplacement, the unit underwent regional deformation and metamorphism during which it was de-serpentinized. Finally, as granites were emplaced in the area, associated metamorphism anthophyllite grew and, as temperatures fell, serpentinization happened again due to hot water from the granite.

Serpentinite quarry at the final stop.

Sept 30^th: Dr. Helge Gonnermann’s Volcanic, Magmatic, and Hydrothermal Processes class set off dark and early Wednesday morning from Houston to the Cascades of California, Oregon, and Washington. We ventured across this varied volcanic terrain interpreting and learning about local and regional geologic events. The Cascades are an active continental volcanic arc resulting from the subduction of the Farallon plate beneath North America. Our first day consisted mostly of driving from Portland to Mt. Shasta and setting up at our lodging, a sweet house called Falconhurst (I still can’t get over the Bronte-ness of that name).

Oct 1^st: Thursday, we drove through Mt. Shasta’s expansive debris flow deposits; these volcanic remains used to be mapped as glacial due to their poor sorting and extensive coverage. This is the largest subaerial debris flow on earth with an expanse of 450 km². The huge hills we saw at every roadside stop were bits of ancestral Mt. Shasta’s flanks, deposited during the landslide 300,000-360,000 years ago.

Mt. Shasta and hummocks, pieces of its ancestral edifice deposited during a landslide.

Helge lectures on Mt. Shasta and debris flows

After dinner, several trip members were feeling peaky, so I made some medicinal grade ginger honey lemon maple tea, which was said to have “quite a kick”, and to be “reviving” (kind euphemisms for how spicy it was). The official recipe is as follows, but may be adjusted to taste.

Water                      1500 mL
Peeled ginger        150 g
Lemons                   2
Honey                    45 mL
Maple syrup         45 mL

Crush ginger and boil for 20 minutes. Squeeze lemons and strain into a jug. Add honey and syrup to your liking. (Personal notes: more ginger and longer boiling time results in a stronger, spicier brew.)

Nightly discussion featured vibrant political debate. Dr. Thomas Giachetti, who was Helge’s post-doc at Rice and is now a professor at the University of Oregon, joined us for dinner.

Oct 2^nd: I awoke Friday morning to the dulcet tones of the smoke alarm wailing as the breakfast bacon burned. If you can’t handle thermodynamics, stay out of the kitchen! As we embarked, Sriparna noticed that every person in our car was from a different country (India, China, Iran, France, Brazil, USA), which I think is unique almost anywhere.

We hiked up Mount Lassen at Lassen Volcanic National Park. Lassen Peak is a dacitic dome, with pyroclastic dacite deposits containing phenocrysts and mafic enclaves, and dacitic pumice away from the peak. From the top, you can see Mount Shasta and the U shaped valleys carved out by glaciers in the last ice age.

View from Lassen Peak with Mt. Shasta in the background. Photo credit Thomas Giachetti.

Andesitic enclave within dacitic matrix at Lassen Peak

We continued on to an a’a flow! I could hear the stratification of bubbles within the solidified flow as we trod over it, from the deep clunk of fragments of the dense inner core to the high-pitched glassy crunch of the sharp and vesicular upper crust. The flow had angular shards, round bubbly blobs, and solid dark fragments with visible feldspar phenocrysts.

At 8:15 pm we pulled the cars onto a side road on the way back to Mt. Shasta in the pitch dark to see the stars. The Milky Way was a full, bright arc directly above us, all the way across the sky. Satellites and shooting stars burned tiny white trails through the dark velvety firmament. I made a wish. It seemed impossible to take pictures of the sky, and we all decided it was best to experience the moment without needing to take it back home to show anyone else.

Oct 3^rd: Our first stop Saturday was a basaltic pahoehoe and a’a lava flow, featuring a network of lava tubes. Flow direction was evident in the ropy pahoehoe sections by concentric surface flow features. Across the road was a scoria deposit; its clasts had smaller vesicles on the outside, smoothed from air fall, while their insides had larger bubbles where bubbles had enough time to coalesce.

Scoria airfall clast. Note the small vesicles on the outside (from airfall smoothing) and the larger ones inside (due to bubble consolidation).

We ate lunch at one of the highest points in the Medicine Lake volcanic area, with a great view of Little Glass Mountain (which is not so little) and Mt. Shasta’s head in the clouds. LGM is an effusive (non-explosive volcanic) obsidian rhyolite flow from 1000-1100 years ago. It is pancake-shaped with steep sides and a small dome in the middle near the vent.

Little Glass Mountain . Mt. Shasta in the clouds in the background. Photo credit: Thomas Giachetti.

Our next stop was Big Glass Mountain, another obsidian dome featuring glossy black volcanic glass banded with reticulite (a frothy, lace-like obsidian glass) in complex folds and beautiful flows. The banded textures are thought to result from magma exsolving dissolved gases; during eruption the shearing motion and pressure causes concentration of the gas in some layers but not others, or, alternately, shearing causes volatile exsolution and concentration. It was impossible to take enough pictures. We found beautiful folds that looked almost like fabric, and red layers intermingled with black obsidian. We discussed whether the reddish colors are iron oxidation, sericite from plagioclase microlite alteration, or some other alteration product.

Exsolution bubbles and textures at Big Glass Mountain, CA. Photo credit: Thomas Giachetti.

My carmates were exceedingly kind about my eclectic music taste, with only minimal teasing about how ridiculously miscellaneous it is.

Oct 4^th: Sunday morning we drove up to Portland via the magnificent volcanic Crater Lake and Mt. Mazama. The ash flow tuffs at the bottom of Mt. Mazama’s flanks are from the climactic eruption; their composition changes stratigraphically, indicating compositional changes of the magma chamber from which they erupted. This climactic event produced 40 times the eruptive volume of Mt. St. Helens. We estimated that the lake in the caldera could fit about 45 Rice campuses on its surface.

The layered flows on the cusp of the crater tell stories of the cataclysmic eruption. A red, weathered dacite lava flow on bottom is overlain by an orangey layered welded tuff flow, with a white pumice air fall deposit on top. Volcanic glass is not stable at atmosphere temperature and pressure, so it has begun to devitrify, crystallizing into spherulites over time.

Layered flow deposits on the rim of the Mazama caldera. Photo credit: Thomas Giachetti.

Volcanic glass devitrification (spherulites). Photo credit: Thomas Giachetti.

Today in the car, while sharing music, we learned that the word ‘zindagi’ means life in both Hindi (जिंदगी) and Persian (زندگی). Persia and India have previously shared a border and continue to share culture, which persists in the language. Further north we began to see the Columbia River Flood Basalts, thick and hilly due to erosion, and golden from the grain fields covering each surface.

Columbia River Flood Basalts, eroded and with a splash of alpenglow.

Oct 5^th: Monday we had breakfast on the road; Helge told us stories of growing up on a farm in Germany with his favorite dairy calf, Sputnik.

We had uncommonly clear weather for our visit to Mount St. Helens. Small rock falls in the crater released bright dust clouds visible from kilometers away. Dr. Mike Poland, geophysical researcher for the USGS and expert on remote deformation measurement, was our guide. Mt. St. Helens is a member of the Cascades, famous for its catastrophic 1980 eruption. A cryptodome of pressurized magma began to build, pushing up the rock above it. When the rock above became gravitationally unstable, it fell, depressurizing the magma and causing the eruption and pyroclastic surge. All was quiet until 2004-2008 when a spine of hot crystalline rock, called the “whale back,” emerged within the crater.

As Dr. Poland discussed the things we can and cannot explain about eruptions, I’m struck again by a recurring thought of how young geology is as a science. People have been observing natural phenomena for millennia, but really understanding what they mean is an art. Seeing eruptive phenomena in person clarifies processes that are otherwise obfuscated by time or preservation bias. One can piece together that hummocks downhill of Mt. Shasta are pieces of the edifice, but until you see Mt. St. Helens erupt and move material in the same way, you can only guess. Geologists have time working both for and against us; eons are preserved in the rock record, but we only have our own perceptions, experiences, and conceptual limitations to guide us.

Mt. St. Helens. Slight dust visible at the top of MSH from gravitationally unstable rockfall.

Our group in front of Mt. St. Helens with Dr. Mike Poland, who was kind enough to guide us. Photo credit: Thomas Giachetti.

I get the sense that losing volcanologists to volcanoes still stings. The way Mike and Helge reminisced about Dr. David Johnston, a geologist who died in the 1980 eruption, and Gerry Martin, the radio operator on the ridge behind him, there was a persistence of loss, and a solemnity underlying the oft-retold stories. Volcanology is scientifically fascinating, which draws us to it, but its dangers must be taken seriously. Sometimes curiosity comes at a price.

Thanks so much for a fantastic day, Mike!

Monday night we ate at Beaches, a beach-themed (not Bette Midler themed?!) restaurant on the water in Portland. Our waitress was sweet and bubbly. At the end of our meal, she gave us saltwater taffy, and handed us packages of stick-on mustaches, telling us to get crazy and have fun. We took a group photo wearing them, and I think we all looked very distinguished.

Distinguished and also possessed. You CAN have it all! Photo credit: Julin Zhang

Oct 6^th: Tuesday morning we left the house at 4 am to get to the airport. When we all arrived home and unpacked, the TSA had searched each of our bags. I guess there’s something suspicious about carrying home pounds and pounds of rocks as souvenirs? This trip greatly expanded my understanding of volcanic processes, and was a wonderful experience overall.