The Great Unconformity at Baker's Bridge: Part III - Regional Geological and Global Bio-Evolutionary Significance

In my first post on the Great Unconformity, Part I (here), I discussed its contiguous stratigraphy within the Grand Canyon. In my second post, Part II (here), I offered a more global interpretation of the contact. In this post, I discuss the time gap at Baker's Bridge in southwest Colorado and the Great Unconformity's hypothesized significance to biological evolution.

WELCOME TO BAKER'S BRIDGE
If you follow the Animas River some 14 miles upstream from Durango, Colorado, you’ll arrive at Baker’s Bridge in the southern foothills of the San Juan Mountains. The short bridge traverses the amazingly green-hued waters of small but strikingly scenic Animas Gorge, formed by erosion-resistant, varnish-stained walls of pale brownish-red Bakers Bridge granite. The region's beauty is a compendium of almost two billion years of geological evolution.

Animas Gorge from atop Baker's Bridge looking north

On this hot summer’s day throngs of locals were sunning, swimming and jumping from the bridge in what has been described as a right of passage in these parts. Chances are you won't notice (I didn’t) that this locale was used in the filming of the escape scene in the 1969 movie "Butch Cassidy and the Sundance Kid." The two leaders of the Hole-in-the-Wall Gang are cornered on a cliff by the approaching posse and make a dramatic, expletive-echoing leap to freedom from a ledge of Bakers Bridge granite into the Animas Gorge.

Camera magic made the cliff appear higher than it really is, as Robert Redford and Paul Newman leaped into the Animas, purportedly photographed somewhere in California. Hollywood has never been concerned with geological correctness, as every geologist knows. Here’s the jump scene from the movie. Check out the Bakers Bridge granite.

Robert Redford and Paul Newman making a leap from Bakers Bridge granite

THE GREAT UNCONFORMITY AT BAKER'S BRIDGE
We're not here for a photo op or to take the plunge. A stone's throw from the bridge, our draw is geological. Where Precambrian and Paleozoic rocks are in contact, there exists a temporal discontinuity or gap in time of enormous proportions. It's on the order of 1.2 to 1.3 billion years - 25% of the earth's history missing from the geological record. It's called the Great Unconformity, distinguished with capital letters by geologists in honor of its immensity. It's at Baker's Bridge, yet it exists globally, providing you know where to look.

The Great Unconformity at Baker's Bridge just west of the gorge

The Great Unconformity at Baker's Bridge spans the contact between underlying medium to coarse-grained igneous rock of late Early to early Middle Proterozoic (~1700 Ma) Bakers Bridge granite. Above the contact are marine sandstones of the Upper Cambrian(?) Ignacio Formation, the Tapeats sandstone equivalent found in the Grand Canyon and on the Colorado Plateau. The question mark denotes uncertainty on the part of geologists concerning the Ignacio's age at the time of deposition. More on that later.

SO WHO WAS BAKER?
Charles Baker arrived in the region with one thing on his mind - mineral wealth, but he wasn't the first to seek his fortune in the San Juans. El Dorado-seeking Spanish explored for gold deeply into the mountains during the eighteenth century, evidenced by their abandoned openings and discarded prospecting tools. In 1860 and 1861, Charles and his mining party established camp on the Animas River's east side. They called it Animas City (not to be confused with the later-named suburb of Durango) and built the first Baker's Bridge of logs across the narrow gorge.

Charles Baker's bridge of logs across the Animas River (c. 1898 photograph)
From The San Juan Highway by Frederic B. Wildfang

Unfortunately, the prospectors found little placer gold. Deterred by hostile Utes, extreme winter hardship and the Civil War looming, the group disbanded. Charles went back east to join the Confederate forces and achieved the rank of captain. Charles returned after the war only to be killed by Utes while preparing to lead a party into the Grand Canyon. So the story goes.

The 1870's witnessed a rush for gold in the San Juan Mountains upstream in Baker’s Park, where rich lodes were discovered. Through the 1890's, silver reigned as the predominant metal. Legend has it that Charles' cache of gold is buried somewhere in the hills around Baker's Bridge. The following memorial at the bridge recalls his trials and tribulations. The contemporary concrete bridge was built in the 1930's.


Memorial to Captain Charles H. Baker framed in Bakers Bridge granite
Erected by the State Historical Society of Colorado

THE GRANDEST GREAT UNCONFORMITY
Discussions of the Great Unconformity invariably begin or end with mention of the spectacular display within the Inner Gorge of the Grand Canyon in northern Arizona. The contact exists between 1.7 billion year-old Early Proterozoic Vishnu Schist and overlying 525 million year-old Cambrian Tapeats Sandstone - 1.2 billion years of missing time. In Annals of the Former World, Pulitzer Prize-winning author John McPhee states, "More time is absent than is represented. If a gap of five hundred million years were the right five hundred million years, it could erase the Grand Canyon."

The Great Unconformity within the Grand Canyon's Inner Gorge
The time gap within the contact is 1.2 billion years, more time than it took to form all of the canyon's layers.

John Wesley Powell, Civil War general, geologist, explorer and head of the U.S. Geological Survey, documented the Great Unconformity during his first trip through the Grand Canyon in 1869, but at the time couldn't have fully appreciated its enormity and age. We know a great deal more about the events associated with it, yet since its recognition the Great Unconformity has remained something of an enigma. Scientists enjoy torturing themselves with questions about how it formed and what happened during the long interval geologically and biologically.

UNCONFORMITIES DEFINED
The passage of time is recorded in the rock record with deposition that might seem to occur without interruption, layer upon layer in a continuous sequence. But our dynamic planet forms new layers of rock at its surface with fits and starts, while older ones are inexorably worn away, and later redeposited upon. As a result, a hiatus or interruption within the rock record is the norm and is measured by missing time. Time isn't really missing; the anticipated rock layers are.

Gaps in the rock record are called "unconformities" and represent rock layers that either never formed or were eroded away. The "interruption" in the depositional sequence brings strata into contact of different ages. Many are long-term gaps - tens of millions to hundreds of millions of years. The vastness of the Great Unconformity qualifies it as a unique geological entity, but as we shall see, not just because of its size. 




Artist’s sketch of an unconformity near Edinburgh discovered by James Hutton in 1787
The landscape erodes into a seafloor limestone that buries an older “puddingstone” conglomerate that had eroded from a mountain long gone. Below the unconformity lies a buried erosion surface of a “once mountain proclaimed below” by its vertical, folded roots. Hutton spotted the exposure in a river cutbank in Scotland.
From Geowords.com by Hugh Rance

Knowing how the gap in the rock record formed may provide important clues about crustal activity or movement such as uplift, erosion and subsidence. Thus, the "time gap" can be of tremendous geological value...even biological value!

THOSE THAT CAME BEFORE AND LOOKED INTO THE ABYSS OF TIME
Horizontal strata resting atop the eroded edges of inclined strata was recognized in the early 1700’s as an indication that a significant period of erosion and non-deposition had occurred before the younger formation came to bury the older formation. In 1788, James Hutton of Scotland, the "Father of Modern Geology," looked into the abyss of time at the angular contact at Siccar Point along the northeast coast of Scotland, arguably one of the most important geological sites in the world. He described it as “a beautiful picture of a junction washed bare by the sea” and envisioned the process as an endless succession of deposition “with no vestige of a beginning, no prospect of an end,” a famous phrase in geology.

The contact gave evidence to Hutton that deep burial of an erosion surface had occurred after prolonged erosion. His vision was limited to the realization that uplift had raised the land and was unable to see the mountains that had once existed. It wasn’t until 1805 that Prof. Robert Jameson of Edinburgh University called the surface separating two discordant formations an “unconformity.” Since then, American geologists expanded upon the entity, adding new definitions and interpretations.




In the inset, “the uppermost sedimentary formation oversteps the erosionally truncated, upended, strata of the overlying schistus formation; itself older than apophyses of an inferred underlying body of granite.”
The buried erosion surface is analogous to the unconformity examined by Hutton.
Modified from Geowords.com by Hugh Rance

THREE FLAVORS OF UNCONFORMITIES
Geologists categorize an unconformity based on the strata that embrace the contact above and below. Disconformities are found between horizontal sedimentary layers. Angular unconformities are between underlying metamorphosed, tilted and uplifted strata and overlying horizontal strata. Nonconformities are between younger, overlying sedimentary rock and older, underlying igneous or metamorphic rocks. At Baker's Bridge, igneous and metamorphic rocks below the Great Unconformity and sedimentary rocks above identify the contact as "nonconformable."

The three types of temporal stratigraphic gaps

SO WHERE IN COLORADO IS BAKER'S BRIDGE?
Baker's Bridge resides in southwest Colorado in the southern foothills of the San Juan Mountains, whose area embraces about 12,000 square miles, the combined size of Massachusetts and Connecticut. The San Juans are a high and rugged range of the Southern Rocky Mountain province that geologist and author Donald L. Baars refers to as the "American Alps."

Replete with alpine crags, glacially carved valleys and deep canyons, many of the peaks in the central San Juan's exceed 13,000 feet, and a few exceed 14,000. Composed of erosion-resistant granitic and quartzitic rocks, the range falls within the Colorado Mineral Belt, an area with abundant ore deposits, notably gold, that played an integral role in the region's settlement history. The San Juan Mountains contain numerous sub-ranges such as the Needles, the Grenadiers and the La Platas.

Lofty and angular Needle Mountains under threatening summer skies
Along the southwestern edge of the San Juan Volcanic Field, the Needles preserve evidence of the oldest mountain-building event in the San Juans. The Irving Formation and Twilight Gneiss are remnants of Precambrian-accreted, oceanic plate-derived, magmatic-arc mountains between 1.8 and 1.75 Ga. The crustal block was annexed to Laurentia in a regional tectonic event called the Yavapai orogeny.

The San Juan Mountains possess an extremely complex history, beyond the purview of this post to elucidate. Simply stated, they are the erosional remnant of a large composite volcanic field that covered much of the southern Rocky Mountains in middle Tertiary time, about 30-35 million years ago, in Colorado and adjacent parts of New Mexico. Notice Baker's Bridge (red dot) in the foothills.

Geologic map showing the principal rock units of the San Juan Mountains in southwestern ColoradoLocate Durango and Baker's Bridge (red dot) for reference. The San Juan Volcanic Field's more recent Late Cretaceous to Tertiary volcanic and intrusive rocks rest on earlier Paleozoic and Mesozoic assemblages that in turn lie on a foundation of Early to Middle Proterozoic rocks.
Modified from the Geologic Map of the Hermosa Quadrangle, Colorado Geological Survey, 2003

THE SAN JUAN MOUNTAIN'S RELATIONSHIP TO THE COLORADO PLATEAU
Beginning in the latest Jurassic, subduction of the oceanic Farallon plate beneath the North American plate was responsible for east-directed compression that added crust to western North America. With ongoing subduction, compression wrinkled the Rockies skyward and uplifted the Colorado Plateau en masse, a physiographic province with minimal deformation and a handful of small volcanic intrusions in the region of the Four Corners.

Triggered by a change in the geometrics and speed of Farallon descent into the mantle during the Tertiary, extension followed compression. Still intact internally, the Colorado Plateau's surrounding regions were extensively faulted, intruded by pluton-forming magma and covered by volcanic deposits. The outcome on the landscape was the formation of the Basin and Range province and the generation of voluminous lava on three sides of the Colorado Plateau, one of which was within the San Juan Volcanic Field on the east.

Overstated in its simplicity, this schematic of the changing geometry of Farallon plate subduction beneath the North American plate offers one interpretation of the development of the western landscape from compression to extension.
Modified from unknown source

Today, the eastern boundary of the Colorado Plateau (wide black line) makes a curious skirt to the west to exclude the uplifted igneous and metamorphic rocks of the San Juan Volcanic Field (red). In Plateau Magazine (Volume 6, 2007), geologist and author Wayne Ranney states, "It is interesting to note that these volcanic deposits were erupted upon and rest on a Plateau-like surface; prior to 30 million years ago, the area of the present-day San Juan Mountains could have been considered a part of the Colorado Plateau.

Distribution of volcanic and igneous centers (shaded gray) around the margins of the Colorado Plateau
The San Juan Volcanic Field is shaded in red, outside the Plateau's physiographic boundary.
Modified from Geology of the Colorado Plateau from Foos and after Hunt, 1956

If it wasn't for late Cretaceous plutonic and late Tertiary volcanic activity that typifies the region, the San Juans would likely be included within the Plateau's eastern boundary. If you know the Colorado Plateau's Precambrian basement and its Paleozoic through Mesozoic stratigraphy, the rock units at Baker's Bridge will be fairly familiar. Later in this post, we'll briefly tap into that knowledge base for our discussion of the Great Unconformity at Baker's Bridge, in particular the strata that formed above the unconformity. First, let's visit the river that flows through Baker's Bridge.

"THE RIVER OF LOST SOULS"
Perhaps prophetic, Spanish explorers in the mid-1700's named the river El Rio de las Animas Perdidas. Its 126-mile descent headwaters at an elevation of 11,120 feet in the mining ghost town of Animas Forks, over forty canyon-cut, river-miles upstream from Baker's Bridge in the heart of the San Juans. Fed by a host of tributaries, the Animas watershed drains much of the southwestern San Juans.

The Silverton Caldera and the Animas River
Twelve miles downstream from its source at Animas Forks in the heart of the San Juans, the Animas courses through the historic mining boomtown of Silverton at an elevation of 9,318 feet. Silverton resides within "Baker's Park", who entered the region in 1860 and 1861 in search of gold. Silverton is synonymous with the Sunnyside Mine, one of Colorado's largest. We're within the Silverton caldera, one of 20 or so in the region that formed when stratovolcanoes collapsed into their violently-evacuated magma chambers in the mid-Tertiary (35 to 30 million years ago). Radial and concentric faulting served as plumbing for upward migration of hot, acidic, mineral-laden waters. Hydrothermal solutions were charged with gold, silver, lead, zinc and copper that leached from surrounding rocks and percolated upward. Reduced temperature and pressure encountered as the ore-fluids neared the surface within the host rocks precipitated their bounty within the faults as veins.

Suspended in time
Obscure mine addits (entrances) that lead to underground veins are springled all over these hills, located by the tailings (waste rock) that emanate downslope from them. Today, the mines are silent with the exclusion of tourists that visit them. As the price of metals fluctuate in the market place, Silverton's dormant mines could reopen ending another cycle of "boom and bust." Stretching between the Mayflower Mill (built in 1929) and its mines (one of forty), an aerial tram's bucket still dangles against the backdrop of 13,000-foot Galena Mountain, one of several that rim the Silverton caldera. Of all the innovations introduced to the mining industry, it was one of the most important. With severe winters and the majority of the mines above timberline, trams allowed year-round work by delivering ore down to the mills, and transporting miners to and from the mines. 

Engineer Mountain
From the San Juan Skyway (U.S. 550), Engineer Mountain is just a few feet short of being a "13er." It's roughly halfway between Baker's Bridge and Silverton, eight miles west of the Animas River. Recalling the San Juan's excluded relationship from the Colorado Plateau, Engineer displays a section of late Paleozoic, plateau-typical rocks. Cyclic marine sediments of the Pennsylvanian Hermosa Group form Engineer's forested lower slopes, while terrestrial Pennsylvanian and Permian Cutler redbeds form higher slopes, a reminder of the Ancestral Rocky Mountains (the Uncompahgre uplift specifically) that once towered over the region to the north. The light-colored, cliff-forming, columnar-jointed, intrusive sill of quartz trachyte that rimrocks the summit is likely of Late Cretaceous or early Tertiary origin. While intrusive igneous bodies were travelling through the earth's crust in the San Juan's high ranges, sills forced their way into sedimentary strata such as at Engineer. Between the Cutler and the sill is a region-wide contact known as the "Telluride erosion surface" formed during the Eocene. It bevelled the San Juan dome, a Laramide-age, basement-cored uplift, by eroding its flank. Talus on lower flanks is composed of sill material, especially the large, downslope-migrating rock glacier on the cirqued north slope out of view. Glacial thickness in this region during the Pleistocene is estimated between 2,500 and 3,000 feet but spared Engineer's summit making it a nunatak, an Inuit word. Glacier's that originated in the San Juans bulldozed through Baker's Bridge and down the Animas Valley to Durango.

LAS ANIMAS CAÑON
As the Animas courses south towards Baker's Bridge, its gradient varies from moderate to steep, slicing through narrow, forested canyons carved into the San Juan's erosion-resistant granites and quartzites. The Baker's Bridge granite is exposed for a few miles above the bridge within Animas Canyon.

Sadly, the upper reaches of the Animas are contaminated by toxic heavy metals of lead, cadmium, copper, manganese, zinc and iron that discharge from the countless mines and tailings (waste) piles in Baker's Park, legacies of the gold and silver that lured the hordes of prospectors and miners beginning in the last quarter of the nineteenth century. The degradation in water quality has adversely impacted all forms of aquatic wildlife, although further downstream, particularly below Baker's Bridge, its quality is greatly improved through dilution from inflow of high quality tributaries.

Animas Canyon
Railroad transportation opened the San Juans by reaching its remote, land-locked mining towns and getting ore downriver. In this William Henry Jackson photo made into a 1906 postcard, the Durango & Silverton Narrow Gauge Railroad, founded by the Denver & Rio Grande Railway in 1881, follows the Animas Canyon upriver to Silverton. In continuous operation for over 130 years, the train is a popular tourist attraction, National Historic Landmark and feat of nineteenth century engineering. It's also a great way to see the remote geology within the canyon. Notice that the Needle Mountains upstream were artfully hand-colored into the photo. 

A CHANGE IN GEOMORPHOLOGY BELOW BAKER'S BRIDGE
At an elevation of 6,761 feet, Baker's Bridge (red arrow below) lies at the juncture of a profound change in the topography owing to the presence (or absence) of the locale's Precambrian crystalline basement. Above the bridge into the mountains, the Animas Canyon confines a swift Animas River within its narrow, resistant walls.

At Baker's Bridge, where our photo of the Great Unconformity is well displayed, the Precambrian basement is in contact with the overlying Paleozoic strata. Just below Baker's Bridge where the Animas Gorge ends, the eponymous granite dives into the subsurface buried under alluvium and Paleozoic rocks.

Animas Gorge and the upper Animas Valley
Looking down the upper Animas Valley toward Durango from atop Bakers Bridge granite, the Animas Gorge ends abruptly where granite enters the subsurface. Immediately beyond the gorge, the channel dramatically widens, its gradient lessens and its current diminishes forcing the river to drop its sediment load. These features are evident on the elevated topo-map below. The hills that frame Animas Valley are Paleozoic and Mesozoic rock assemblages coincident with those on the Colorado Plateau.

Looking north, Baker's Bridge (red arrow) lies at the head of wide Animas Valley and floodplain and at the bottom of narrow Animas Canyon. The Great Unconformity is exposed at Baker's Bridge and for a few miles upriver. The change in terrain is at Baker's Bridge is synonymous with its granite entering the subsurface.

Within and around Animas Valley, yellow and tan colors designate Quaternary Pleistocene and Holocene glacial, alluvial and colluvial deposits, and turquoises and shades of purple designate Paleozoic bedrock. Immediately above Baker's Bridge, the bedrock (taupe) is Paleoproterozoic Baker's Bridge Granite and Irving Formation. See references below for the on-line location of this map.

Shaded relief map of the Hermosa quadrangle with geology and topography overlay
Modified from the Geologic Map of the Hermosa Quadrangle, La Plata County, Colorado, 2003

THE ANIMAS RIVER VALLEY BETWEEN BAKER'S BRIDGE AND DURANGO
During the Pleistocene, the San Juan Mountains experienced 15 or more glacial advances that blanketed the region with a 1,900 square mile ice-field. The high cirques became ice-free at least 15,000 years ago with the peaks of some remaining above ice. The forty mile-long Animas glacier, one of the longest in the Southern Rockies, scoured out Animas Valley, evidenced by lateral moraines over 1,000 feet above the valley floor.

Today, the U-shaped valley floor is flat, having been filled with Pleistocene glacial outwash and moraines, and Holocene alluvium (stream deposits) and colluvium (slope deposits). The valley is continually being modified by mass wasting with landslides, mudflows, debris flows and creep, and periodic Animas flooding from voluminous winter snowmelt and summer monsoons.

The valley's walls are formed from 16,000 feet of limestone, shale and sandstone sedimentary beds of Mesozoic and Paleozoic-age. At the mouth of the Animas Valley (map below) near Durango, we ascend the Mesozoic column beginning with the Late Triassic Chinle Formation at the base of Animas City Mountain (photo below) and progress through the Middle Jurassic San Rafael Group beginning with the Entrada Sandstone. The slope-forming Late Jurassic Morrison Formation follows with a resistant cap of the Late Cretaceous Dakota Sandstone.

MOUTH OF THE LOWER ANIMAS VALLEY
We’re standing on a glacial moraine facing the mouth of the lower Animas Valley looking north across the Animas River and floodplain. Baker’s Bridge is 12 miles upvalley in the foothills of the San Juans. The valley's U-shape was scoured by the forty mile-long, Pleistocene Animas glacier that originated within the San Juan Mountains. Withdrawing late during the Pinedale glaciation, its sediments accumulated in Glacial Lake Durango, a proglacial lake that formed in a deeply scoured basin cut into bedrock. The valley's flat floor is filled with lake sediments, glacial till, and Holocene alluvium and colluvium.

Within this expanse of the lower Animas Valley, the Ariver has become a very low energy system of oxbows and vegetation-rich, abandoned cutoffs on its flood plain. Durango, to the south behind us, is built on late Cretaceous sedimentary rock, principally the Mancos Shale, with a veneer of Animas River gravels. Higher portions of Durango are built on glacial outwash, terraces and moraines generated from the recent 18,-25,000 year old Wisconsin glacial event and others earlier such as the Pinedale.

Mouth of the Animas Valley, Animas City Mountain and the Animas' floodplain and oxbow lake
Animas City Mountain, across the valley, is capped with Late Cretaceous Dakota Sandstone with white Entrada Sandstone mid-slope and Upper Triassic Chinle Formation (called Dolores locally) at its base. The formations dip southwest about 7º towards the San Juan structural basin, observable in the subtle tilt of Animas City Mountain's strata. As one travels upvalley and up the dip-slope, one encounters increasingly earlier assemblages that reveal Mesozoic and then Paleozoic strata. After the Chinle, earlier deposited Lower Permian Cutler Formation is revealed, and so on. By the time Baker's Bridge is reached we're stratigraphically into the Devonian Period with majestic cliffs in the Permian Hermosa Formation surrounding the valley.

PALEOZOIC HERMOSA CLIFFS FRAME BAKER'S BRIDGE
At Baker’s Bridge, down-section in the upper Animas Valley, an early to middle Paleozoic assemblage is revealed, Cambrian(?) through Pennsylvanian. This photo was taken from atop an elongate ridge of Leadville Limestone (a Redwall equivalent) and a patchy, paleosol-veneer of reddish Molas Formation. Underlying it is Devonian Ouray Limestone that forms the cliff above the Great Unconformity at Baker's Bridge nearby.

Looking to the northwest, Middle Pennsylvanian Hermosa Group cliffs (the type-section) dramatically rise with forested, lower slopes in the Paradox Formation. Recall that although we are positioned outside the physiographic boundary of the Colorado Plateau, the stratigraphy is Paleozoic-Mesozoic syn-depositional and concordant. The entire display is beautifully exposed throughout the Animas Valley.

Hermosa cliffs above Baker's Bridge

HOW DID THE CONTIGUOUS STRATIGRAPHY OF THE GREAT UNCONFORMITY FORM AT BAKER'S BRIDGE?
The Great Unconformity is the expression of geological events that occurred globally; that is, conditions existed worldwide to promote the development of this massive gap in time. At Baker's Bridge, the stratigraphy embracing the unconformity is an outcome of regional tectonic controls, in many respects, a microcosm of the global event.

Acquisition of Proterozoic crust...
The Southwest's oldest rocks, its crystalline basement, formed in a flotsam and jetsam of tectonic collisions of juvenile volcanic arcs and marine basins during the Early to Middle Proterozoic. In succession, first the Mojave, then the Yavapai (1.8-1.7 Ga), and finally the Mazatzal (1.7-1.65 Ga) provinces collided with pre-2.5 billion year old rocks of the Archean Wyoming Province of the Canadian Shield, a portion of the craton or ancient nucleus of the nascent North American continent to the north.

Using Southeast Asia as a modern analogue, the Southwest may have appeared something like this during the Early to Middle Proterozoic. The region of Baker's Bridge (encircled) received crust largely from Yavapai tectonic derivatives.

Rodinia's Archean core hosts tectonic convergence and crustal growth during the Middle Proterozoic
This scenario illustrates an early stage of crustal growth at the southwest margin of the supercontinent of Rodinia. For reference, note the outline of the states. The Mojave province previously accreted to the Wyoming province (the continent's Archean cratonic) followed by Yavapai and Mazatzal oceanic magmatic arcs. Southwest Colorado at Baker's Bridge received largely Yavapai crust.
Blakey, R. C., 2012, Paleogeography and paleotectonics of Southwestern North America:
Colorado Plateau Geosystems, DVD Flagstaff, Arizona.

Once accreted, the Yavapai basement in the region of Baker's Bridge and the future San Juan Mountains were twice metamorphosed during the Middle Proterozoic. The first epsiode was part of a mountain-building event called the Boulder Creek orogeny (1.72 to 1.667 Gma) in which Twilight Gneiss metamorphosed from andesite and the Irving Formation from basalt. By 1.5 Gma, the eroded Boulder Creek Mountains were covered by marine sediments of the Uncompahgre Formation.

The emplacement of the Baker's Bridge granite...
During the second, milder metamorphic episode of the Silver Plume orogeny (~1.5 Gma), magma intruded the Twilight and Irving metamorphic rocks with felsic plutons of both Baker' Bridge and Tenmile granites (part of the statewide Routt Plutonic Suite), and metamorphosed the Uncompahgre sediments into quartzites, slate, phyllites and schist. These Precambrian rocks can be found in the various sub-ranges of the San Juan's.

Rifting to drifting to exposure and weathering...
In the late Middle Proterozoic (1.4 to 1.0 Ga), the supercontinent of Rodinia finally assembled with the Grenville orogeny that united the majority of the world's landmasses and built a transglobal Grenville mountain chain. Following Rodinia's break-up in the latest Proterozoic, its crust distributed globally with the drifting apart of its continental siblings. Once exposed, the basement rock experienced extensive weathering over a prolonged period. The eroded Precambrian crust, both Archean and Proterozoic, is the foundation on which the Great Unconformity formed.

The remnants of Rodinia's Archean and Proterozoic crustal core are distributed throughout the globe after the continents reassembled at the end of the Paleozoic into the supercontinent of Pangaea, and then redistributed upon its fragmentation.


World map with terranes of Precambrian Archean and Proterozoic crust
Dark areas are Archean crust: unshaded with numbers. Proterozoic crust: shaded lines in areas either under ice or preventing direct access. Phanerozoic orogenic belts: dot pattern. Proterozoic terranes are divided into three categories: 1 (confirmed), 2 (incomplete analysis) and 3 (exposed but unconfirmed).
Modified from Proterozoic Crustal Evolution by K.C. Condie, 1992.

Great floods of the Phanerozoic...
On the short term, the level of the sea rises and falls, whether lunar orbitally-induced or regionally weather-related. Sea level also possesses a long-term oscillation that typically lasts hundreds of millions of years, related to celestial parameters (that trigger glaciation cycles) and planetary tectonic events (that change the holding capacity of ocean basins).

Six times in the Phanerozoic, the level of the sea substantially rose and fell, flooding low-lying regions of the continents globally. With each landward advance (transgression) and withdrawal (regression), the seas deposited continental-scale, unconformity-bounded, sedimentary sequences. Centered on the Cambrian, the earliest was the Sauk sequence from the latest Proterozoic through the early Ordovician.

 

The Sauk transgression-regression...
At its peak, the Sauk flooded the low-lying, weathered Precambrian margins of the drifted paleo-continents and their cratonic interiors from the latest Proterozoic and into the beginning of the Paleozoic for 50 million years or so. In North America, the continent was blanketed largely with a sequence of well-sorted sandstones and clastic carbonates (excluding topographic highs such as the NE-SW-trending Transcontinental Arch that extended into Arizona and parts of the raised Canadian Shield well to the north).

Middle to Late Cambrian paleomap of the North American Southwest
Rising seas of the Sauk transgression bathed North America's west coast largely during the Cambrian. As water invaded the land, the shoreline eventually reached the region of Baker's Bridge. The Ignacio Formation was deposited on the Baker's Bridge granite forming the Great Unconformity at Baker's Bridge, while elsehwere in the southwest, the equivalent Tapeats Sandstone formed the Great Unconformity on uquivalent igneous and metamorphic rock.
Blakey, R. C., 2012, Paleogeography and paleotectonics of Southwestern North America:
Colorado Plateau Geosystems, DVD Flagstaff, Arizona.

The birth of the Great Unconformity...
The siliciclastic, near-shore Ignacio Formation (the Grand Canyon's Tapeats equivalent) was the first sedimentary rock deposited in the region of Baker's Bridge and the future San Juan Mountains. Voila! With covering of the Precambrian basement by the Cambrian transgressive-regressive sequence, the Great Unconformity had formed.

The Great Unconformity, so well exposed in the Grand Canyon and on grand display at Baker's Bridge, can be traced across Laurentia (here) and found globally on Rodinia's tectonically dispersed landmasses - including Gondwana (largely the Southern Hemisheric continents), Baltica, Avalonia and Siberia. That makes the Great Unconformity "the most widely recognized and distinctive stratigraphic surface in the rock record" (Peters and Gaines, 2012). Geologists can't resist touching it and pausing for reflection!




Sauk sequences in North America that overlie the Great Unconformity
Distribution and age of the oldest Phanerozoic rocks in North America. Early Cambrian sediments (light gray) on the margins of the paleo-continents and later Cambrian sediments (medium and dark gray).
Peters and Gaines, Nature, 2012.

WHAT HAPPENED DURING THE GREAT UNCONFORMITY?
The Late Proterozoic and the time of the Great Unconformity was a turning point in the development of "the modern earth system" (Shields-Zhou, 2011). Its half-billion or so years were enough time to allow for a total re-invention of earth's geosphere, atmosphere, hydrosphere and biosphere.

Geologically, it accommodated a complete reorganization of the Earth’s tectonic plates, the uplift and erosion of vast mountain ranges to sea level, a rifting apart of the supercontinent of Rodinia into smaller continental fragments and their drifting throughout the globe. Biologically, it was enough time to evolve completely new and diverse lifeforms, and accommodate their radiation.

The two are thought to be related in that the geological processes that resulted in the formation of the Great Unconformity provided the impetus for the burst of biological diversity of the Cambrian Explosion of multi-cellular animal life. There are many competing theories that are highly controversial and lack a consensus of opinion. That said, let's attempt to assimilate them into one apologetically simplistic scenario. It's a rather unlikely tale of a fragmenting supercontinent, a dimly lit planet that became entombed in glacial ice, a hothouse heat wave, an oceanic geochemical infusion and an explosion of multi-cellular animal life within the sea.

LATE PROTEROZOIC WEATHER REPORT
During the Great Unconformity, the breakup of Rodinia (900-750 Ma) occurred largely during the Tonian Period (Greek meaning “to stretch”) early in the Late Proterozoic. In the Cryogenian (Greek for "cold" and "birth") Period (850-635 Ma) in the mid-Late Proterozoic, our planet experienced two intense and widespread glaciations that were notably equatorial in locale - as opposed to more familiar high latitude Phanerozoic glaciations such as those of the Pleistocene.

These were the Sturtian (715-680 Ma) and Marinoan (680-635 Ma) glaciations. A third Cryogenian glaciation, the Gaskier or Varanger, was less extreme, likely short-lived and not global in extent.

A COSMIC BALL OF ICE
The ice ages prevailed with such intensity that the surface oceans in the tropics froze. The event has been anointed with the colorful and description of “Snowball Earth” (Kirschvink coined the term in 1992, and Hoffman presented the hypothesis in 1998). It’s a bold and imaginative theory that remains speculative and controversial, yet very attractive since it successfully explains the geological findings regarding the glaciations (e.g. equatorial glacial tillites and diamictites, banded iron formations, post-glacial marine “cap” carbonates, carbon isotopic anomalies, etc.).

These events – Rodinia's assembly, break-up and the ensuing glaciation - preceded the recognition of multicellular animal life during the Ediacaran Period (named for the type-section at the Ediacara Hills of Australia), the final time period of the Proterozoic.

TRIGGERS OF THE DEEP FREEZE
What are the causes of extreme climate deterioration, that is, the prolonged cooling that led to equatorial snowball glaciations? Proposed explanations include extraterrestrial triggers such as galactic cosmic rays driving cloudiness, the dimmer young sun (solar luminosity 83-94% of present-day values) and high planetary obliquity (> 54º of axial tilt) that resulted in a colder low-latitudinal climate than at the poles. Terrestrial triggers include methane degassing from anoxic oceans (boosting the hydrologic cycle) and tectonic influences.

The latter trigger - tectonics - relates epochs of glaciation to a reduction in the partial pressure of atmospheric CO2 caused by supercontinental break-up. It’s an attractive model. Here’s how it works.

FRAGMENTATION AND ATMOSPHERIC pCO2…
Rodinia fragmented apart a good 70 million years before the first snowball event. Although Rodinia may have stretched from pole to pole, its paleo-orientation and that of its rifted constituents was likely, largely equatorial, at least initially. Its fragmentation generated intense magmatic activity within the Laurentian magmatic province. It also opened many seaways as the severed continental blocks drifted apart, which in turn increased precipitation and temperature along newly rifted margins, largely in a low-latitudinal locale.

Equatorial cluster of Rodinia's fragmented landmasses during the global glaciations that occurred during the Cryogenian Period. Our planet witnessed the continents reassemble at the end of the Paleozoic and again redisperse. Geological relics of the debris left behind when the ice melted are exposed on the contemporary land surface. Namibia (red dot) is a notable example.
From Snowball Earth by Paul F. Hoffman and Daniel P. Schrag, Scientific American, 1999

The continental runoff increased weathering, particularly of silicates on freshly generated basaltic surfaces, and hence atmospheric consumption (q.v. long-term carbon cycle). The “drawdown” (reduction) of CO2, a greenhouse gas, resulted in long-term climate cooling, trending toward an icehouse. As ice accumulated equatorially, a “runaway ice-albedo” 
(reflective ice reduced solar absorption leading to more cooling; Budyko, 1968) drove the earth into a snowball glaciation. Thus, supercontinental break-up is thought to have had a profoundly cooling effect at low latitudes during the Late Proterozoic and to have been the main trigger in reducing atmospheric CO2.

WHAT AWAKENED THE EARTH FROM ITS CRYOGENIC SLUMBER?
The Snowball Earth hypothesis also postulates that millions of years of glaciation ended when sufficient volumes of volcanically-derived CO2 emissions accumulated within the atmosphere. Degassing overcame the effect of the runaway albedo climate, which collapsed weathering and allowed the planet to transition from an icehouse to a greenhouse world that melted the ice and liberated the planet from its snowball state.

A geochemical infusion of weathering products...
During the Great Unconformity, continental exposure and chemical weathering of silicate materials effected seawater chemistry and global bio-geochemical cycling in the atmosphere and the oceans. The weathered-runoff was delivered to rivers and oceans in massive quantities conducive to the evolution of new forms of life, in particular, Ediacara-type fauna that flourished as a prelude to more diverse forms of the Phanerozoic world.

The final stages of the Great Unconformity are thought to have acted as a “geological trigger” by infusing the oceans with continental weathering products including carbonates, calcium, potassium, sodium, magnesium and iron (Peters and Gaines, 2012). That had profound implications for ocean chemistry at the time that complex life was proliferating and initiated a biochemical response seen in the Cambrian Explosion.

EDIACARA-TYPE FAUNA
Lifeforms that existed before and after the Precambrian-Cambrian boundary differed immensely. During the Ediacaran Period lifeforms were plant-like, suspension-feeding metazoans (multi-cellular) that lacked the morphological capacity (form and structure) for locomotion and were non-biomineralized (without hard calcified tissues like shells and bones).

They are generally viewed as the oldest unequivocal animals and passive inhabitants of their ecosystem, tethered to a cyanobacterial microbial mat on the ocean floor. By and large, the Ediacara fauna became extinct by the end of the Ediacaran Period, although proponents of Burgess-type ancestral relationships believe some lifeforms persisted into the Cambrian.

Ediacara-type fauna
Although appearing plant-like morphologically, the Late Proterozoic Ediacara-type fauna were the first multicellular marine animals. Benthic (substrate-loving) and mostly afixed to a cyanobacterial microbial mat on the ocean floor, they lacked the capacity for locomotion, vertical bioturbination and predation.
Wikipedia.org

BURGESS SHALE-TYPE FAUNA
Following the Ediacaran-Cambrian boundary, new lifeforms were distinguished by the emergence and rapid diversification of more complex multicellular animals, by their acquisition of biomineralized skeletons (phosphate and carbonate salts of calcium), and by their innovative body plans with movable, muscular body parts.

With new bodies came new lifestyles - possessing the ability to bioturbinate the substrate (rework the sediments by burrowing), move vertically through the water column and exploit new habitats. Predation had begun across the threshold of the gap in time, and along with it, the ability of prey to protect themselves and escape from capture. Things have never been the same. This period is referred to as the Cambrian Explosion of life because of the seemingly abrupt timing of the biological event. Evolutionary biologists single the Cambrian Explosion as the event that generated all the phyla that have persisted to the present.

Burgess Shale-type fauna
The Cambrian Explosion typifies a fundamental change in marine ecosystems. Ediacaran two-dimensional mat scraping was replaced by three-dimensional infaunal burrowing (within the substrate). The Middle Cambrian Burgess Shale-type fauna was empowered by the development of muscular, biomineralized body parts and innovative body plans, which not only enabled surface grazing but movement throughout the water column.
Artist John Sibbick of Time Magazine

WE SEE THE GREAT UNCONFORMITY IN A NEW LIGHT
Alas, we have come to perceive the Great Unconformity as more than just a gap in time but as “a unique physical, environmental boundary condition” (Peters and Gaines, Nature, 2012). Reflecting on the dramatic transition from Ediacaran to Burgess-style lifeforms across the Great Unconformity, Robert R. Gaines (personal communication, 2012) believes the "geological circumstances surrounding its formation led to the Cambrian Explosion."

Analyses of seawater chemistries “provide support evidence for changes in tectonic activity and enhanced (and extensive Late Proterozoic) continental weathering during the formation of the Great Unconformity” (Peters and Gaines). The chemical infusion that was delivered to the sea – with minerals such as calcium that rose precipitously – has been proposed as a mechanism for the origin of biomineralization of animals during the Cambrian Explosion that evolved in Cryogenian to Ediacaran time.



Summary of major tectonic, geochemical and sedimentary patterns over the past 900 Myr
The wavy dark gray blob (global mean isotopic Sr) is a measure of the progressive exposure of weathering granitic continental crust required to form the Great Unconformity. The wavy light gray blob (global mean εNd) is a measure of young arcs versus old continental crust. The vertical green bars identify continent-scale sedimentary sequences particularly the Sauk transgression (the first of six global high seas during the Phanerozoic) that blanketed the Great Unconformity in the Cambrian. The vertical blue bars indicate major global glaciations during the fragmentation of Rodinia and preceding the radiation of the Ediacara-type fauna. The timing of Rodinia fragmentation in relation to the Great Unconformity is designated across the top. Following the fragmentation of Rodinia, the Great Unconformity is defined by a shift from widespread continental weathering (red bar) to widespread sedimentation (yellow bar).
From Peters and Gaines, Nature, 2012.

LET'S RETURN TO BAKER'S BRIDGE FOR A FINAL LOOK AT THE CONTACT
Now that we have an enlightened geological and biological perspective of the events that occured during and surrounding the Great Unconformity, let's take one last look at the contact at Baker's Bridge. The Ignacio Formation, the stratum anticipated to cap the Great Unconformity, appears absent at Baker's Bridge.

Close-up of the Great Unconformity at Baker's Bridge
The regional paleotopography - its ancient landscape - is indicated above the Bakers Bridge granite, where the Ignacio Formation is absent and the Elbert Formation is overlies the Precambrian basement. Elsewhere regionally in the San Juans, the Ignacio can be found overlying the contact.

Geologists have found it difficult to distinguish between the sandstones of the McCracken from the underlying Ignacio. Petrographic analyses at Baker's Bridge has found that the sandstone overlying the Great Unconformity is more similar to the Devonian McCracken Sandstone Member of the Elbert Formation rather than the Cambrian Ignacio Formation, which contacts the granite elsewhere regionally. The following paleo-map depicts the widespread Devonian through the Mississippi-age Kaskaskia transgression (on the sequence map above) that deposited Redwall limestones in northern Arizona, and Elbert and Leadville Formations in the region of Baker's Bridge in Colorado.

Late Devonian paleomap of the North American Southwest
The Kaskaskia transgression, the third eustatic event to reach the Southwest in the Phanerozoic, deposited the Elbert and Leadville Formations. Where the Ignacio was absent, the Elbert capped the Great Unconformity.
Blakey, R. C., 2012, Paleogeography and paleotectonics of Southwestern North America:
Colorado Plateau Geosystems, DVD Flagstaff, Arizona.

The McCracken-Ignacio age-uncertainty has persisted in the literature for over a hundred years, much to the surprise and even disappointment of some Plateau geologists that expect the Tapeats or its regional equivalent overlying the Great Unconformity. The problem has been with the precise dating of the Ignacio, which has been difficult since it's regionally depauperate (greatly diminished or devoid of ecosystem fossils). In the strata above the Ignacio, trace burrows, and brachiopod and fish remains are plentiful in the Elbert and overyling Leadville, which does allow reliable dating.

I asked the question "What stratum overlies Baker's Bridge granite at Baker's Bridge?" to Dr. David Gonzales, Professor and Chair in the Department of Geosciences at Fort Lewis College in Durango). He responded, "Unfortunately, there is not a definitive answer (personal communication, 2013). He continued, "The Ignacio Formation at Baker's Bridge is somewhat unique on a regional level. In a number of locations, Devonian limestone lies directly on Proterozoic rocks. So, clearly, depending on what area you are in, there is both Cambrian(?) and Devonian rock units on the Proterozoic. Unless more fossils are found, I am not sure the issue will be easily resolved."

A thesis interpretation (Maurer, 2012) of the strata-conundrum attributes Ignacio's absence at Baker's Bridge to its deposition mostly as estuarine within an incised valley sequence. Thus, it was not deposited at Baker's Bridge but located elsewhere regionally. With the return of the sea, the landward advance of the Kaskaskia transgression left McCracken marine sediments upon Bakers Bridge granite at Baker's Bridge.

The Ignacio question at Baker's Bridge is surprising to geologists that anticipate its presence. But technically - recalling our definition of the Great Unconformity - the overlying strata can very greatly both regionally and even globally contingent on the circumstances of deposition. 



This exposed surface of the shallow-marine Mississippian Leadville Formation displays 350 million year old, crinoid stem-ossicles, bivalve shell fragments, rugose horn corals and much later Pleistocene glacial polish and striations. It is a unique occurrence to find the Leadville Limestone glaciated compared to its non-glaciated Redwall-equivalent in the Grand Canyon.

PLEISTOCENE GLACIAL AND HOLOCENE POST-GLACIAL PROCESSES LEAVE THEIR MARKS
Back at the bridge, glacial and fluvial erosion have stripped the sedimentary cover from the granite, and allowed the Animas River to carve a channel through the Animas Gorge. Notice the smooth glacial polish and multiple, parallel glacial striations produced by the Animas Glacier moving downvalley some 18,000 years ago. Once exposed, repetitive freeze-thaw cycles have also taken their toll on the granite as it began to exfoliate at the surface into gently curved slabs.

VERY INFORMATIVE PRINTED RESOURCES
Ancient Landscapes of the Colorado Plateau by Ron Blakey and Wayne Ranney,
Plateau: The Land and People of the Colorado Plateau, Vol. 6, Nos. 1 and 2, 2009.
Snowball Earth by Paul F. Hoffman and Daniel P. Schrag, Scientific American, 1999.
The American Alps by Donald L. Baars, 1992.
The Garden of Ediacara by Mark A.S. McMenamin, 1998.
The Geology of the American Southwest by W. Scott Baldridge, 2004.
The Roadside Geology of Colorado by Halka Chronic and Felicie Williams, 2002.
The Western San Juan Mountains by Rob Blair, 1996.
Wonderful Life by Stephen Jay Gould, 1989.

VERY INFORMATIVE ON-LINE RESOURCES
Geologic Bedrock Map of the Hermosa Quadrangle, Colorado Geological Survey, 2003.
Reinterpretation of the Ignacio and Elbert Formations by Joshua T. Maurer, 2012.
Petrologic Evolution of the San Juan Volcanic Field by Peter W. Lipman et al, 1978.
Formation of the Great Unconformity as a Trigger for the Cambrian Explosion by Shanon E. Peters and Robert R. Gaines, Nature, 2012.