Despite our trip reaching its formal conclusion, we snuck in one last (amazing) geologic experience: Visiting Great Sand Dunes National Park. A crook in the line of Sangre de Christo Mountains has allowed strong westerly winds blowing pleistocene Lake Alamosa sediments to form a vast accumulation of sediments. Small rivers around the margins of the dunefield help keep the sand tightly confined, growing to over 200 meters.

We spent our final morning playing in this dramatic interaction of Precambrian / Laramide uplifted Sangre de Christo mtns, eroded San Juan material, paleolakes and modern winds. We then spent the afternoon picking sand out of everywhere during our egress to the airport. A great trip conclusion!

Great Sands and the Sangres in the distance

Last (full) day of field tripping! After familiarizing ourselves yesterday with the stratigraphic relations of units of the La Garita caldera, we worked our way into the Creede caldera, one of the most well preserved calderas of the central San Juans. Named for the small mining town on its north end, the Creede Caldera occupies the center of the immense (75 x 35 km) La Garita nested caldera system. Creede is a beautifully well preserved example of caldera collapse and resurgent doming. This site provided some of the most spectacular views of caldera collapse structures we’ve seen since the Valles Caldera. Not to mention the turning Aspens and constantly changing weather to improve the views!

Reaching the capstone moment of our trip, the UO crew admires the very impressive, and aspen-accented, resurgent dome (Snowshoe Mtn) of Creede Caldera

Thus, our trip is formally concluded; however the rocks seen, lessons learned, and friendships and collaborations formed will live on throughout our entire careers.

Thank you to Mike Dungan, Paul Wallace, and the UO Staples fund for putting on such a wonderful trip! And a La Garita-sized (ie., huuuge) THANK YOU to our USGS colleagues who shared their time and expertise with us!

Closing in on the home stretch of this field trip, things just keep getting bigger and better! We ventured into one of the world’s largest, and best studied “nested caldera” system – the Central San Juan Caldera Complex.

This complex erupted a phenomenal amount of material, 8000 km^3 of magma, in a shockingly short period of 2 million years (trust us, that is short geologically speaking!). For comparison, the cataclysmic 1980 eruption of Mt. St. Helens in Washington, which produced ashfall all the way into Montana, led to lahars for decades, partially dammed the Columbia River, and killed over 50 people, produced 1 km^3 of magma. In this light, the San Juan volcanic field is immense on almost unimaginable proportions. Our official goals for our final two days of the trip are:

[Investigating the] Central  San  Juan  caldera complex,  which  counts  the  twin  distinctions  of:  (1)  being  one  of  the  volumetrically  largest  multicyclic caldera  complexes  of  its  kind  (>8000  km^3 of  tuff  in  eight  units),  and  (2)  being  as  well  mapped  and studied as any Cenozoic caldera system in the world…. We  will  view  many  of  the  highlights  of  this  system  for  the purpose of completing an overview of caldera‐related volcanism. Among these are the Fish Canyon Tuff (at  least  5000  km^3),  the  highly  unusual  Pagosa  Peak  Dacite  (~200  km^3),  the  immense  scale  of  the  La Garita caldera (90 x 45 km),  geologic relations along the margins of  calderas that are nested within the La Garita, the resurgent domes of the La Garita, Creede, and Bachelor calderas, and more.

Basal mudstones, lacustrian deposits (or maybe surges?? [probably not]), draping plinian pumice deposit, and base of the massive Fish Canyon Tuff. This site has, and continues to, cause much controversy

Saying farewells to the Land of Enchanment, we crossed over from New Mexico into Colorado (although the Rio Grande rift knows no political borders). Venturing out of the Rio Grande basalts and paleo- Lake Alamosa, we set our sights to the massive calderas of the southern San Juan Volcanic Field (SJVF), which occurred in huge bursts between 25-30 Ma. We first visit tuffs sourced from the fascinating Platoro Caldera, in the southeast SJVF:

The Platoro caldera complex was the source of eight, largely dacitic Members of the Treasure Mountain Tuff. These units range in volume up to ~500‐1000 km3 … This  system  is  an  instructive  example  of  the  difficulty  of linking outflow  tuff  sheets, which present  relatively  straightforward  stratigraphic successions,  with  the much  less  well‐preserved  corresponding  units  in  their  caldera  sources.

Did we mention we get to see beautiful rivers and aspen-infused hills of Colorado high-country too?

Densely welded tuff of the Treasure Mtn group. Glassy black areas are fiamme: squashed pumice	Sam Johnstone, USGS Mendenhall Fellow, gets up close an personal with some Treasure Mountain vitrophyre
Mike Dungan protecting some lovely fiamme-bearing ignimbrite	Visiting an overlying, much more poorly welded member of the Treasure Mountain Tuffs
Paleontologist stoked about multiple types of pumice in the ignimbrite!	Paleontologist very happy to see hydrothermally improved rocks in the Platoro Caldera
Incredibly Densely welded intracaldera tuff of the Platoro caldera	Pre-caldera Conejos lava flow in foreground, with overlying thick package of course lava-derived sediments

We continued our venture onto, and into, the Taos Plateau Volcanics around the Rio Grande and Red River Gorges, exploring the surprisingly diverse range of volcanic rocks (Servilleta Basalts to andesite and dacites), earlier sub-surface paleotopography, and how exciting incipient landslide blocks can be.

Christine Chan (USGS) sizes up the scene

Part of the Red Gorge package of dacites, basalts, and andesites

Baked paleosol underlying andesite flow

Next we visited La Junta – the confluence between the Rio Grande and Red Rivers revealed an amazing section of ~4 million year old Servilleta basalts with features as fresh as Kilauean lava flows on Hawaii

Beautiful dragged vesicle pipe in Servilleta basalt

Discussing flow features in Servilleta basalt

La Junta – A great place for contemplating fluvial and landslide geomorphology (Sam Johnstone [USGS] and Paul Richardson [UO]

View up-gorge from the incorrectly named “buried volcano” location

Leap of faith over incipient fractures that will eventually lead to another slump block in the Rio Grande Gorge

Dacite lobes interspersed with Servilleta basalts

Mike Dungan (UO) and Ren Thompson (USGS) working through the geologic map. Photo by Prof. Marli Miller

Evening at the Sagebrush, in Taos

Today’s mission:

Examin[e]  diverse  structural,  sedimentary,  and  volcanic manifestations of the early Miocene to  late Pleistocene evolution of the San Luis Basin. The Rio Grande gorge, which is deeply incised into lavas of the Taos Plateau volcanic field and intercalated sediments of the Santa Fe Group, provides a window into the late Miocene to Pleistocene portion of this history. One aspect of the gorge’s history that has recently come to light is the integration of the lower and upper Rio Grande drainages during rapid incision related to late Pleistocene flooding from Lake Alamosa.

The striking Taos Plateua sharply incised by the Rio Grande Gorge

Ren Thompson (USGS) provides a masterful overview of the Taos Plateau volcanic and structural history.

Marveling at an amazing piece of the world

The Rio Grande carved into hundreds of meters of Servilleta Basalts, which were deposited 3-5 Ma. The Rio is the lifeblood of New Mexico, before flowing through Texas, and eventually trickling into the Gulf of Mexico

UO group getting introduced to the Rio Grande Gorge and toreva slump blocks. Photo by Prof. Marli Miller

Ren Thompson (USGS) showcases the 3D gravimetric investigation of the San Luis Basin, which reveals surprises about subsurface structures and just how much (or how little!) the Rio Grande Rift has subsided

Mission success!