An RMAG Adventure: Rafting the Upper San Juan River Canyon in Utah

On the Rocks Field Trip, June 14-16, 2022

By Denise M. Stone

OVERVIEW

Is there a river in the world that has more geology packed into a 27-mile course than the Upper San Juan River in UT? I doubt it. RMAG and Fort Lewis on the Water (FLOW) teamed up this summer to offer a 3 day-2 night raft trip on the Upper San Juan River Canyon. Floating through world class exposures of the Paradox Basin in southeastern Utah, participants saw spectacular geology, delicate river eco-systems, petroglyphs, and abundant wildlife. There was continuously something to see in every direction.

Passing through the entrenched meanders of the San Juan we saw the stratigraphic section from Jurassic through Pennsylvanian in age. We saw a monocline, anticlines, synclines, faults, fractures, mega-fossils, sedimentary structures, bioherms, red beds and even a diatreme! Strata are both clastic and carbonate, bedding is exposed at all different angles. All this was viewed from the comfort of rafts, piloted by energetic FLOW river guides. The weather was great, although a bit hot, but a dip in the river fixed that right away.

Professor of Geology, Dr. Gary Gianniny of Fort Lewis College in Durango, Colorado led the trip. He emphasized to everyone that our primary goal was to arrive at the end of the trip safe, healthy, and happy after having seen some amazing geology. That goal was met. Attending were 10 RMAG members, two geology students, six FLOW river guides, and Gary. That made 19 attendees, in a flotilla of 5 rafts and 4 inflatable kayaks. The kayaks were great for those wanting to paddle, get closer to the outcrops and navigate water shallower than the rafts could manage. Armed with hat, sunscreen, water shoes and wearing personal flotation devices, we set out to explore the river.

Location Map of the Four Corners area of southeastern Utah showing the San Juan River flowing from east to west. The RMAG group launched at Bluff and took out at Mexican Hat, floating 27 River Miles (RM).

This map is from the San Juan River Guide by Lisa Kearsley.

We launched at Bluff, and took out at Mexican Hat, both in Utah. Over the rafting route we experienced a few Class 2 rapids, but most of the time the water was flat. Class 2 (out of 6 Classes) refers to moderate to medium quick water with clear and open passages between rocks and ledges. The higher the class number the greater the difficulty. Discharge rates during our trip were 500-600 cubic feet per second (cfs), considerably lower than the 10,000 to 140,000 cfs that has been recorded during floods and monsoon rains. The source of the San Juan River is in southern Colorado and northern New Mexico. Its flow is controlled by the Navajo Dam at Navajo Lake State Park, New Mexico. At its termination, the San Juan River flows into Lake Powell joining the Colorado River through the Grand Canyon.

Annotated Google Earth Image showing the 27 River Mile (RM) journey of the RMAG rafters from Bluff (RM 0) to Mexican Hat, both in Utah. Structural features that stand out aerially are the Comb Ridge Monocline and the Lime Ridge and Raplee Anticlines. Comb Ridge Monocline divides the Paradox Basin (east/right) from the Monument Uplift (left/west) Structure and uplift of the Colorado Plateau have influenced the course of the highly down cut San Juan River

G. Gianniny & S. Hawthorne, River Miles in yellow after Whitis and Martin, 2009

NAVAJO SANDSTONE (LOWER JURASSIC) – DAY 1

Cliff exposures of eolian Navajo Sandstone (Jurassic) lined the channel for the first six river miles (RM) of the journey. From our launch point at Sand Island in Bluff to the strike of the Comb Ridge Monocline we could see large-scale cross-bedded ancient red dunes. Stop 1 for the day was at Butler Wash, RM 5.5, there Fort Lewis College geology major Chandler La Duke spoke to the group about his current study of eolian facies in the Navajo. He said the town of Bluff gets its water from the Navajo aquifer. Understanding the permeability is important to the water supply and production. Recently, there has been concern about arsenic contamination in the local water which may also have a facies component. Eolian sandstone permeability is a function of bedding and the nature of bed boundaries. Laterally extensive sub-horizontal surfaces in the Navajo, or “Stokes Surfaces” are permeability boundaries caused by wind scour, some continue for miles. Impressive transverse and barchan dune cross-bed sets 30 to 100 feet thick could be seen up close by kayak.

At RM 5.5 we left the rafts for a side hike up to River House Ruin, an ancestral Puebloan cliff dwelling with room-blocks, pictographs, and a kiva. This site also exposed the lowest horizontal permeability boundary in the Navajo Sandstone which aided in forming the alcove in which the ancestral Puebloan was built. The ruin was about 25-30 feet above the river level.

Regional Stratigraphic column for the Paradox Basin with red arrow showing the age range of strata exposed along the route of the RMAG rafters

Modified from Stevenson and Wray, 2009

VEGETATION, ARCHEOLOGY AND WILDLIFE

The San Juan River is a ribbon of green carved through an otherwise dry tan-to-red desert landscape. The delicate green ecosystem that exists on the riverbanks is thick with vegetation and contrasts against the red sandstone. Trees and grasses dominate, some are native, cottonwood and willow are abundant. Also abundant are tamarisk and Russian olive which are invasive, over the last 45 years they have gradually increased their presence along the channel stabilizing the sandy banks. Fragile cryptobiotic soils are also present, and we carefully avoided trampling them.

Lizards, birds, bighorn sheep, feral donkeys, wild turkeys, and night bats kept an eye on our group as we floated down the river. Our experienced river guides reported occasionally seeing snakes and scorpions, but we didn’t see any on our trip.

A smooth eroding cliff face of the Navajo Sandstone (Jurassic) along the Upper San Juan River Canyon. The top of the white area is the high-water level mark. The white is due to minerals deposited as the water level dropped. Angular lines through the white are cross beds formed by dunes. Vertical black stripes are deposits of manganese oxide, also known as desert varnish.

Rafters in San Juan River Canyon are taking a closer look at the Pennsylvanian Horn Point Limestone at water-level which is compacted and “draped” over subsurface phylloid algal mounds in the Lower Ismay Sequence of the Paradox Formation (Pennsylvanian).

FORT LEWIS ON THE WATER (FLOW) HISTORY

FLOW was founded in 2020 as an experiential educational program for all students at Fort Lewis College wanting to learn and experience the outdoors. Each year it enriches learning for a diverse spectrum of students in many majors including Geology, Biology and Adventure Education, as well as Philosophy and English! Each fall, all geology majors raft the river to study the better-than-text book geomorphology, stratigraphy, and structural in the canyons. The FLOW program was initiated when river permits (launches) and rafting equipment were purchased from a Grand Canyon outfitter that went out of business. FLOW has 34 river trips scheduled this summer alone. They also run trips for scientific societies (like RMAG!), environmental and outdoor groups. River trips are offered from early May to late October depending on the weather. In addition to the Upper San Juan River trip, FLOW takes rafters on the Lower San Juan, normally 5 days, through the famous “Goosenecks.” They also take groups for 3-day trips on the Chama River in New Mexico.

Rafters stop to take a closer look at a secondary black chert layer on a bedding plane exposed in the Lower Ismay Sequence of the Paradox Formation (Pennsylvanian). Trapped in the chert are impressive fossils including crinoid stem impressions, rugose corals, and sponges.

An outcrop of the Lower Ismay Sequence of the Paradox Formation exposed along the San Juan River bank contains a fan of white colonial rugose corals. Best seen when wet, the river water came in handy.

An exposure of the chert-replaced sclerosponge Chaetetes along the banks of the San Juan River in the Lower Ismay Sequence of the Paradox Formation.

After seeing lots of Navajo Sandstone, in the afternoon of day one, the geology began to change. At RM 8, Mule Ear came into view, which is a tooth-like promontory of the Wingate Sandstone with stronger cementation. In less than a mile, the Wingate beds had become almost vertical and next to the Mule Ear there was an anomalously dark body of rock on the horizon; the 25-million-year-old Mule Ear Diatreme. These features signaled our approach to the eastern side of Comb Ridge Monocline; a north-south trending regional feature. Nearly 80 miles long, this Laramide flexure was produced by the reactivation of a deep-seated Pre-Cambrian fault zone.

We spent the first of two nights at nearby Big Stick Camp at the base of the dip slope of Lower Cutler Formation (Permian) red beds of Comb Ridge. Big Stick is on a sandy floodplain deposit with large, shady, and billowing Cottonwood trees. It offered lots of space for camping. Before dinner, a spirited game of “cobble-ology” took place. Fist-sized cobbles from along the riverbank were collected and described. Natural groupings were made by their age and origin which prompted great discussion. The river guides remarked that geologists just love rocks, they even talk about them in their downtime.

A group of 6 River Guides from Fort Lewis on the Water (FLOW) piloted our rafts, cooked meals and organized our campsites during the 3-day float trip. They were all enthusiastic about geology, helpful to our group and safety minded.

Dinner was perfectly seasoned chicken curry over rice, delicious after the long day. We gathered around a circle of camp chairs to eat with a propane fueled “fire pit” flickering in the center. Seasonal fire restrictions were in place along the river due to dry conditions. Following dinner an energetic game of “rock bocce” ensued, two teams of three geologists joined in. They claimed the rules were fluid and there was plenty of laughing and cheering when someone scored. The full moon came up from behind the cliff across the river at 11pm. It lit up everything.

Trip leader Dr. Gary Gianniny, Professor of geology at Fort Lewis College, points to a fault in an outcrop of Navajo Sandstone (Jurassic) on the Upper San Juan River in southeastern Utah.

Fort Lewis College Professor of Geology, Dr. Gary Gianniny (ever the well-prepared field geologist) pulled out a tape measure to show the length of a 5-foot-long crinoid stem exposed in an outcrop of the Lower Ismay Sequence of the Paradox Formation (Pennsylvanian).

DAY 2

We were awakened at 5:30 am by the loud braying of feral donkeys. When that finally stopped, there was a “mashup” of birdsongs, each bird singing their unique song at the same time. Clear and uninterrupted there were no other sounds; no car engines, no background noise, just birds singing. It was a glorious message that you were not at home, you were waking up on the river. I was downwind from the kitchen and around 6:30 am I smelled fresh coffee.

After breakfast, a “fire line” of all campers loaded our gear back on the rafts. Since this would be the day with the most white water, we had a brief discussion on river safety and then got going. The flotilla followed the channel’s sharp turn to the west and entered the mouth of San Juan River Canyon. The walls were now tall and more enclosing, the amount of visible sky had shrunk. At the mouth of the canyon, we stopped for a brief look at the exposed section. There were several things to note: the red beds were disappearing, carbonates, increasing downsection had been introduced, and the depositional environment was changing from terrestrial to marine. We had left the red beds of the Permian Cutler and stopped to see low stand strata in the Pennsylvanian Honaker Trail Formation which had channel fill carbonate clast conglomerates. Fist to golf ball size clasts were impressively imbricated. Down the river a bit further we saw rhizoliths or fossilized plant roots, tidal grainstones and shallow water carbonates. There was herringbone cross-stratification indicating tidal influence. The cross strata were both siliceous and calcareous. into older and older rocks. Below the Honaker Trail we entered the Lower Ismay Sequence of the Paradox Formation, also Pennsylvanian. Its claim to fame is phylloid algal mounds. These are on display on both sides of the canyon, the rafts stop and tie up so we can have a closer look. The onlapping intermound beds contained beautiful sponges (Chaetetes), colonial rugose corals, and the tabulate coral Syringopora. Thirty miles east in the subsurface near Aneth, phylloid mounds and oomoldic grainstones form excellent reservoirs hosting the largest oil field in Utah.

The flotilla of rafts and kayaks approach the Raplee Anticline around a tight meander of the San Juan River Canyon, southeastern Utah.

Secondary black chert preserves the impression of a crinoid stem in the Lower Ismay Sequence of the Paradox Formation.

Secondary black chert was abundant as concretions and in thin layers along avenues of permeability. The chert is interpreted as a diagenetic product originating from the devitrification of siliceous sponge spicules. Trapped in the chert were impressive fossils including crinoid stem impressions and corals.

The RMAG and FLOW flotilla of rafts and kayaks heading toward Raplee Anticline in the distance. The town of Mexican Hat, and the Mexican Hat Syncline got their names from Mexican Hat rock in the upper right.

RMAG rafters and kayakers passing below huge cliffs of eroding Navajo Sandstone (Jurassic) along the San Juan River in southeastern Utah.

Fist to golf ball size clasts are impressively imbricated in the conglomerates of the Honaker Trail Formation (Pennsylvanian). The imbrication suggests current flow towards the basin center to the northeast.

Fort Lewis College Professor of Geology Dr Gary Gianniny sits in an inflatable kayak on the San Juan River in southeastern Utah. Behind him, on the horizon to the left is the Mule Ear Diatreme. Cutler Formation (Permian) redbeds in the foreground of the diatreme, are almost vertical and fold gently, decreasing in dip on the horizon to the right. This is the structural expression of the Comb Ridge Monocline.

STRUCTURE

Structural geology is responsible for the most dramatic largescale geological features along the river. We passed through the Lime Ridge Anticline, the Raplee Anticline, and the Mexican Hat Syncline. With no vegetation to obscure the rocks, the folded sedimentary layers red and tan of the Paradox, Honaker Trail and overly Permian formations were clearly visible. For the non-geologist, the dipping layers are easy to see and traceable at the surface.

As the flotilla followed tightening river meanders through dipping beds of the Raplee Anticline, it became easy to get disoriented. The Google Earth image in our guidebook helped us stay clear on our location by both river mile and strata exposed.

A rhizolith, or fossilized plant root coating in the Pennsylvanian Honaker Trail Formation on the banks of the San Juan River. A white halo almost 2 feet in length, shows the vertical path of the now-lithified root trace.

Fist to golf ball size clasts are impressively imbricated in the conglomerates of the Honaker Trail Formation (Pennsylvanian). Some clasts have dark gray caliche rims from alteration on the late Pennsylvanian landscape. The elongate black clasts are transported rhizolith fragments.

Tidal influenced shallow water carbonates with bidirectional current ripples and possible herringbone crossstratification in the Honaker Trail Formation (Pennsylvanian).

In conclusion, the Upper San Juan offers spectacular geology. It is only possible to see it up close and safely by raft. You cannot get to this extraordinary landscape any other way and better yet, the rocks come right to you. The scenery in every direction is constantly changing. Rafting provides an appreciation for the river, the geo history and the peace and tranquility of flowing water.

Next summer, RMAG On the Rocks (OTR) fieldtrip committee would like to launch a follow-up float trip to Lower Canyon of the San Juan River, from Mexican Hat to Clay Hills Crossing, Utah. This would continue our exploration of the Pennsylvanian strata through the Goosenecks and beyond. RMAG’s OTR Committee is working with FLOW and Dr. Gianniny on the idea.

Thanks to the FLOW program for teaming up with RMAG and to Dr. Gary Gianniny, an exceptional trip leader. He has been studying the area and leading raft trips on the San Juan for 32 years. His knowledge and enthusiasm for the area geology is evident. Thanks also goes to David Schoderbek for serving as Trip Coordinator for RMAG, he made the arrangements necessary for this successful float trip to happen. arrangements necessary for this successful float trip to happen.

River guides from Fort Lewis on the Water (FLOW) prepare dinner for the group on the first night of the trip. We set up camp at Big Stick point bar under a grove of giant cottonwood trees. Dinner was a warm and tasty curried chicken over rice.

A set of impressive vertical fractures in the Cutler Formation (Permian) redbeds along the Upper San Juan River Canyon, just before the Mexican Hat, Utah take-out point and the end of the trip.

To the right, the rafts approach a sandy point bar full of cottonwood trees, it was the campsite for the first of two overnights on the river. To the left on the skyline is Mule Ear, a tooth-like promontory of almost vertical beds of red Wingate Sandstone (Triassic). The lower orange sandstone ridge in left foreground is formed by steeply dipping beds of the Permian De Chelly Sandstone. The 25-millionyear-old Mule Ear diatreme is the dark rock in the distance behind the rafts.