WHAT’S AN ANGULAR UNCONFORMITY?
If the successive, horizontal deposition of sedimentary rock layers is interrupted, say by erosion of a layer or a failure of deposition, the gap in time between the strata of different ages is called an unconformity. Unconformities are extremely common in the rock record and generally indicate a regional or even global geological event.
Angular unconformities occur where an older, underlying package of sediments has been uplifted, tilted and truncated by erosion, followed by a younger package that was deposited horizontally on the erosion surface. This gap in the rock record generally occurs from a regional tectonic event which changes the altitude and attitude of the bedding before sedimentation resumes. Compare the diagram below with the photo above.
WHERE ARE WE?
We’re on the Colorado Plateau in east-central Utah within the Paradox basin of late Paleozoic time. Paleogeographic reconstructions place us between 5º and 15º north of the paleo-equator during the Triassic, the time of deposition of the Chinle Formation. The town of Moab and Canyonlands National Park are off to the southwest, while Arches is just to the north.
The unconformity is east of town within Moab Canyon along the Colorado River across from Scenic Byway 128. Running from the northeast to the southwest, the Colorado transects a succession of NW-SE-trending, salt-generated, anticlinal valleys (first Onion-Fisher-Sinbad, then Salt-Cache, Castle-Paradox Valley) before entering Moab Canyon (the location of our unconformity and others), and then emerges from the canyon into another salt-intruded anticline at Moab-Spanish Valley.
|The Colorado River flows NE to SW through a succession of salt-intruded valleys.|
The Chinle unconformity in the photo is exposed at river level within Moab Canyon.
It is displayed at numerous locations throughout the basin.
Once again, what processes are responsible for the formation of the unconformity? Hint: The region’s many anticlines, synclines and the unconformity share a common genesis.
THE PENNSYLVANIAN AND PERMIAN PERIODS OF THE LATE PALEOZOIC
The Pennsylvanian and Permian Periods herald the close of the late Paleozoic, a time of expansion for marine invertebrates, gigantism amongst arthropods, the diversification of terrestrial stem tetrapods, and the advent of the amniote egg. Pennsylvanian coal forests in eastern North America’s more northerly paleo-latitudes attest to swampy, humid conditions, while western paleo-equatorial North America was largely arid. At the South Pole, extensive glaciation repeatedly waxed and waned causing global sea level to successively rise and fall. The wide range of climatic extremes was related to the development of a supercontinent, when things came together tectonically.
|Pangaea before the initiation of break up in the Early Permian (280 Myr)|
Note the orogen within the Laurussian-Gondwanan collision zone
and the South Polar continental ice sheet.
Ron Blakey and Colorado Plateau Geosystems, Inc.
Near the end of the Mississippian Period, the majority of our planet’s landmasses began to assemble into a supercontinent called Pangaea (Greek for “all lands”). It spanned the poles and was surrounded by a vast global sea called the Panthalassic (Greek for “all oceans”). Pangaea was largely the unification of the megacontinents of equatorial-situated Laurussia (North America and Eurasia) and australly-situated Gondwana (most of the modern South Hemisphere continents), and lasted for over 100 million years.
GLOBAL AND REGIONAL OROGENESIS
When continents tectonically collide, there’s nowhere to go but up. Orogeny (literally “mountain creation”) occurs when landmasses converge. The competition for space within the Laurussian-Gondwanan collision zone created a Himalayan-esque, trans-global mountain chain. Today, the eroded remnants are distributed amongst Pangaea’s globally-rifted siblings, and in North America, form the Appalachians.
The unification of Laurussia and Gondwana brought Africa into contact with North America’s eastern margin (using contemporary coordinates) along the Appalachian-Caledonian-Herycnian suture, which extends through Greenland into western and northern Europe. Along the collision zone to the southeast, South America accreted at the Ouachita-Marathon-Sonoran suture, building mountains from Arkansas and Texas into Mexico.
Curiously, the South American collision is thought to have created a second mountain system further to the west of the suture within Laurussia’s interior called the Ancestral Rocky Mountains (circled on the map below).
|The red dot depicts the location of the future Chinle unconformity. |
Late Pennsylvanian paleomap (300 Myr ago)
Modified from Ron Blakey and Colorado Plateau Geosystems, Inc.
Traditionally, the uplift of the Ancestral Rocky Mountains has been ascribed to a continent-continent collision of the conjoined masses of Laurussia and Gondwana. But not all tectonic aficionados agree with the intraplate geometry of a South American collision from the southeast having raised a range that trends NW-SE and so far-afield from the effects of the Ouachita-Marathon convergent margin. They also find fault (pun intended) with the extensionally-derived, “pull apart” structure of the marine basins that also formed as a part of the Ancestral Rockies. Opponents advocate for a volcanic arc-collision occurring somewhere from the southwest, likely within Mexico, which fits better with the Ancestral’s orientation and the compressionally-derived, foreland structure of its basins.
|The arrow indicates the traditional collision vector from the southeast.|
Modified from Wood (1987) and Houch (1998)
A third hypothesis (and there’s undoubtedly more) evokes pre-existing weaknesses within the craton that, when compressed, uplifted the range along deep Proterozoic basement lineaments, a Precambrian "inherited" defect, if you will. In "Canyonlands Country" by geologist Donald Baars, he says "These deep-seated Precambrian faults set the geological stage, and will come back to haunt us throughout geologic time."
Rodinia was the supercontinent that preceded Pangaea by half a billion years, give or take. When Antarctica separated from Rodinia’s southwest paleo-shore in the Late Proterozoic-Early Cambrian, the rifting event sent extensional shockwaves through the craton. Notice the orientation of the normal faults within Rodinia's interior (below). The NW-SE trend of the Ancestral’s ranges and basins reflects these deep-seated, basement-penetrating structures.
These zones of structural weakness were predisposed to future re-activation during Pennsylvanian-Permian compressional tectonics and even Cretaceous-Tertiary age Laramide contractional structures (such as monocline orientation). Tectonic inheritance of structural features in continental cycles, especially with intraplate orogenesis, is a recurring theme in the science of plate tectonics. We’ll see inheritance resurface later (literally) in our discussion of the Chinle unconformity.
Incidentally, the Late Proterozoic rifts that formed throughout Rodinia when it fractured apart likely induced "inversion" tectonics (extensional faults rejuvenating contractionally) in cratonic platforms of its rifted siblings worldwide.
ANCESTORS OF THE ROCKIES
The Ancestral Rocky Mountains, named after the modern Rockies that would eventually reside in roughly the same locale, rose from the sea in western equatorial Pangaea beginning in the Late Mississippian, reached their greatest intensity in the Middle Pennsylvanian, and ended their ascent in the Early Permian. An enigma to this day, they rose amagmatically (without volcanism) in an intra-cratonic and intra-plate setting far from any known plate boundary (1,500 km).
They consisted of a collection of crystalline, Precambrian basement-cored, NW-SE-trending ranges (referred to as highlands and uplifts) and paired fault-bounded depressions (referred to as basins and troughs) from Chihuahua, Mexico, through Oklahoma, Texas, Colorado, Utah and up to British Columbia, Canada. Initially, the many basins were in communication with the marine waters of the Panthalassic Ocean.
THE UNCOMPAHGRE UPLIFT AND THE PARADOX BASIN
On the southwest flank of the Ancestral range, the Uncompahgre (UH) highlands (alternately called an uplift) was bordered on the east by the Central Colorado basin (CCB) and on the west by the elongate Paradox basin (PaB). Tectonically associated with the highland’s rise, the Paradox basin rapidly subsided and assumed an asymmetric profile 200 miles in breadth and as much as 33,000 square miles (about the size of Maine). As the entire range ascended, erosion worked to bring it down, shedding deposits into the waters of the intervening basins in large debris fans. The Paradox basin's relationship with the sea became intermittent but with astounding regularity.
|Map of the Paradox Basin, the extent of which is delineated by salt of the Paradox Formation.|
The red ellipse encloses the region of our unconformity.
Modified from Nuccio and Condon, 1996.
Closest to the rising front, 16,000 feet of the Uncompahgre’s arkosic, Precambrian sediments were shed into the Paradox basin as it rapidly subsided (northeast in diagram). Moving away from the highlands, the high seas poured into the deepest portion of the basin from the north and south. When the cyclically-oscillating global seas dropped low enough, the basin’s shallow shelf (labelled southwest) prevented the entry of sea water.
Cut off from the sea, the basin became a hypersaline lake as water evaporated within the restricted basin in the hot, arid Pennsylvanian climate of western Pangaea. Salt brines precipitated from the briny solution and settled to the deepest depression of the basin where they accumulated. The depositional scenario reversed when sea level cyclically rose again, reentered the basin and diluted the briny concentrate. And so on.
The Paradox Formation was conformably succeeded by the alternating terrestrial eolian and fluvial, and marine shales and limestones of the Honaker Trail formation, the uppermost unit of the Hermosa Group within the Paradox basin. Like the Paradox Formation, the Honaker Trail Formation continued to record cyclic sea level fluctuations but contained no evaporites.
ABSAROKA HIGH SEAS
The rising Pennsylvanian and Permian seas that flooded the Paradox also inundated other neighboring basins and low-lying regions both regionally and worldwide. Called the Absaroka transgression, it was not a smooth event but progressed with sea levels that constantly rose and fell, withdrawing and advancing onto land and communicating basins.
For the record, the Absaroka wasn’t the first marine highstand to flood the planet. It was actually the fourth of six complete transgressive-regressive cycles during the Phanerozoic. Why global changes in sea level occur, called eustasy, is a complex process partially involving tectonoeustasy (with the shallowing of ocean basins in rift zones) and glacioeustasy (as climate triggers glaciation and deglaciation).
PENNSYLVANIAN POLAR ICE
Pangaea lasted about 100 million years from the Late Mississippian period until the Late Triassic, when it ultimately fragmented apart. Like previous supercontinents, its enormous landmass profoundly influenced the Earth’s geosphere, atmosphere, hydrosphere and biosphere. With progressive cooling, Pangaea was thought to possess extensive continental glaciers at the South Pole that locked up a substantial portion of the planet’s water, enough to lower the level of the global seas. Conversely, deglaciation flooded the seas and basins with which the seas communicated. We are witnessing this process today in reverse as deglaciation adds water to the planet’s hydrologic budget and triggers a rise in sea level.
Thus, the basins of the Ancestral Rockies received marine waters that cyclically fluctuated with the waxing and waning of glacial ice, estimated to range from 100 to 230 meters of sea level change. Spanning 60 million years, the late Paleozoic ice age was the most severe glaciation in the Phanerozoic, far exceeding the more familiar ice ages of the Pleistocene in the northern latitudes.
Why South Polar glaciation was triggered during the late Paleozoic has a great deal to do with the formation of Pangaea. Stretching from pole to pole, ocean and atmospheric circulation was drastically altered. Mountain ranges were uplifted that altered wind patterns and precipitation. Climate determinants, however, were not only terrestrial but extra-terrestrial.
Cyclic sedimentation in Pennsylvanian rocks is not unique to the Paradox basin but has been recognized in basins around the world. After all, the Absaroka transgression was a global event that affected all low-lying regions in communication with the sea. The consensus is that the sea level changes were caused by regular climate fluctuations that triggered the alternating accumulation and melting of glacial ice in Pangaea’s South Polar region. While the waxing and waning of Pennsylvanian polar ice is the source of the cyclic changes in sea level, the cause of the fluctuations of the climate is thought to be extra-terrestrial or astronomical.
Our planet derives its energy from the sun, but the amount of energy we receive is not always the same. The late Paleozoic sun was less bright than it is today, 3% less than modern values. But solar luminosity (the amount of energy that reaches us) is also related to sunspots and the Earth’s orbit. The Earth gyrates and wobbles in its solar orbit such that the amount of sun reaching our planet varies. Milutin Milankovitch, a Serbian geophysicist in the 1920’s and 30’s, hypothesized that climatic fluctuations are related to the position of the Earth as it travels about the sun.
Orbital factors such as precession (axis wobble), obliquity (axis tilt) and eccentricity (roundness) effect the amount of light reaching the Earth’s surface (solar insolation), and hence affect the planet’s climate. Each of these motions possesses a time period, the sum of which affects climate by driving the hot and cold cycles that produce glaciation. Orbital variations clearly had a substantial impact on Pangaean ice volume. Within the cyclothems of the Paradox basin, the repetitive successions (cyclicity) of Pennsylvanian marine and non-marine sediments are considered to be the stratigraphic signature of orbitally-controlled ice volume fluctuations during the late Paleozoic.
Why are the effects of the Milankovitch cycles “suddenly” seen in the late Paleozoic? The cycles have likely been occurring over a vast period of geologic time, but conditions were optimal for recording the changes with Pangaea sprawling across the South Pole, a climate perfect for glaciation and deglaciation, and shallow marine conditions within the basins of the Ancestral Rocky Mountains. Small periodic changes in sea level profoundly affected evaporite sedimentation and cyclization within the Ancestral’s basins.
The entire process of mountain-uplift, basin-subsidence, oscillating sea level and cyclic salt deposition continued throughout the Middle Pennsylvanian and into the Early Permian. During the Permian, highland uplift and basin subsidence continued but at a declining rate as deposits of the Cutler Group (strat column above) derived from the Uncompahgre uplift blanketed the cyclic deposits of the Hermosa Group. Eventually, the Paradox basin was overtopped as the Panthalassic shoreline made a final wavering westward retreat.
Although greatly eroded in the Triassic, the remnants of the Ancestral Rockies (assisted by the Mogollon highlands to the south and the distant Southern Appalachians to the east) covered the Paradox basin in its entirety with the Lower Triassic Moenkopi Formation’s deep red mix of tidal flat and coastal plain sandstones, mudstones and shales. The Triassic closed with sandstones, siltstones, conglomerates, mudstones and limestones of the Upper Triassic Chinle Formation deposited within an alluvial and lacustrine environment. Like the Moenkopi, the Chinle was derived regionally from the same sources especially the much-reduced Uncompahgre highlands.
The angular unconformity within the Chinle Formation is located several miles west of the uplifted front of the Uncompahgre highlands in the shadow of its eroding flanks and within the confines of the deepest portion of the infilled Paradox basin. And let the truth be told, the beds of the underlying Moenkopi Formation and the even-deeper Cutler Formation also possess similar unconformities from the same regional geological scenario, which has yet to be discussed.
As for the once precipitous Ancestral Rockies, it wasn’t until the Jurassic that eolian sediments finally buried the once great range. Deposition and burial continued with the epeirogenic inland seas of the Cretaceous and Early Tertiary, further entombing the detritus of the Ancestral Rockies, the only remaining record of their existence.
THE BIG PICTURE BEGINS TO TAKE SHAPE
In summary, a complex relationship likely exists between Rodinia’s fragmentation, tectonic inheritance and Ancestral Rocky Mountain orogenesis; and between the Pangaean climate, astronomical solar forcing, cyclical South Polar glaciation, Absaroka glacioeustasy and cyclical evaporite sedimentation.
But there’s more to the story, and I’ve run out of space. We still must explain the genesis of the intra-formational, angular unconformity within the Chinle Formation, and if you haven't guessed by now, it has to do with salt.
Stay tuned for Part II.
VERY INFORMATIVE RESOURCES
"Ancient Landscapes of the Colorado Plateau" by Ron Blakey and Wayne Ranney, 2008.
"A Traveler’s Guide to the Geology of the Colorado Plateau" by Donald L. Baars, 2002.
"Geological Evolution of the Colorado Plateau of Eastern Utah and Western Colorado" by Robert Fillmore, 2011.