Straddling Rio Iguazú that forms the border between southeastern Brazil and northwestern Argentina is a semicircular waterfall of astounding proportions and incomparable beauty. Composed of Early Cretaceous basaltic magma that blanketed the Paraná Basin's vast and thick Paleozoic sediments, Iguazú Falls is a very recent addition on the landscape in geological terms, having formed in the Pleistocene.
And yet, its geomorphology, which is simple in construction, is the interaction and culmination of a complex succession of large-scale tectonic and regional geologic events that spanned more than a billion years of Earth history, and involved the assembly and break-up of three supercontinents. This is its evolutionary story from the bottom up in space and time.
|The Falls of Iguazú from the Upper Circuit|
Iguazú Falls is the second most popular tourist attraction in South America after Machu Picchu, drawing more than one million visitors annually. Here's a fantastic video of the Iguazú Falls taken from a drone.
This is Part I of a two-part post in which I summarize the evolution of the Paraná Basin from its Rodinian roots in the late Precambrian through its transition from a West Gondwanan depocenter in the Paleozoic to a pre-rift Pangaean large igneous province in the Cretaceous.
Part II, forthcoming, discusses various theories on the continental rifting process that separated the once-unified Paraná-Etendeka Volcanic Province of Western Gondwana and the hypothesized association between large igneous provinces and mass extinction events. It ends with a discussion of the acquisition of Iguazú Fall's geomorphology with a glimpse into the surrounding rainforest's rich and colorful biodiverse flora and fauna.
Pertinent definitions are italicized, important names are emphasized in bold and references are provided. Photographs were taken on a recent visit to Iguazú Falls in February 2017.
FAR MORE THAN JUST A BEAUTIFUL WATERFALL
• What geologic events culminated in the formation of Iguazú Falls across Río Iguazú in the Pleistocene? How did the Paraná Basin acquire its Proterozoic foundation, Paleozoic sedimentary supersequences, Mesozoic volcanics and Cenozoic epeirogeny? How did each acquisition influence those that followed?
• What promoted the emanation of continental flood basalts of the Serra Geral Formation across the basin? Why are they largely basaltic? What is the relationship of the Paraná Volcanic Province in South America to its trans-Atlantic counterpart, the Etendeka Province in Africa? Did its emplacement cause a mass extinction similar in global-scale to that suggested of other Large Igneous Provinces ? If so, how? If not, why?
• What triggered rifting between the South American and Africa plates during the break-up of West Gondwana? What accounts for uplift and segmentation of the Paraná Basin? Was the timing related to the onset of surface volcanism? What does it suggest about magma melting? Was it mantle plume-related or was it a plume-less, lithospheric process? What's the Tristan-Gough hotspot plume? Where is it now? Is it really there?
• When did the Paraná fluvial system and its Iguazú tributary become organized? What is the relationship of the development of the river basin to the dismemberment of Gondwana? How does its tectonic framework control drainage patterns? Why did Río Iguazú choose a westerly course over three plateaus instead of emptying eastward into the Atlantic Ocean?
• Why does Rio Iguazú's channel dramatically change course below the falls and convert from shallow, wide and serpentine to narrow, deep and linear? Are there lithologic and/or structural contributing factors to the channel's evolution and the fall's geomorphology? How and when were they acquired? Where does all the iron in the region come from?
• By what process of fluvial incision did the falls develop? How have knickpoint development, headward migration and channel-bed degradation been affected by the region's erosion-resistant bedrock? What is the fall's rate of regression, and where and when did it initiate? How does the genetic pattern and stratigraphy of Iguazú Falls compare to other great waterfalls?
|Iguazú Falls and Isla San Martín Facing East from the Upper Circuit on the Argentine Side|
"WETTING" ONES GEOLOGICAL APPETITE
In 1944, "Poor Niagara!" was First Lady Eleanor Roosevelt's purported response upon seeing the spectacular waterfalls for the first time. And yet, it's not the world's largest. Based on combined width and height, Victory Falls on the Zambezi River in southern Africa has that distinction. Iguazú is not the tallest. With an uninterrupted free fall, Angel Falls on a tributary of the Orinoco in Venezuela claims that title. Iguazú doesn't even possess the greatest rate of flow, ranked sixth below Boyoma Falls on the Lualaba River in the Congo.
But, it is the widest, four times Niagara and has the highest flow rate, although variable. Most times, it plummets as much as 269 feet over some 275 individual cascades down a three-tiered, nearly two mile-wide, J-shaped escarpment. And at flood stage, the falls becomes a single mesmerizing wall of iron-stained, sediment-laden water.
Indeed, Iguazú is arguably the planet's largest waterfall system. And, it's all on intimate display via a well-engineered system of metallic catwalks and balconies from both countries, a powerful Zodiac boat that plies the rapids below the falls or a helicopter for a thrilling bird's eye view.
|Where there's whitewater, there's always geology and adventure.|
Inflatable Zodiac boats make their way up the turbulent waters of Rio Iguazú directly below the falls.
With roaring falls, iridescent rainbows, drenching mist, alligators in the river, noisy parrots and toucans, hawks and vultures flying overhead, curious monkeys howling in the jungle and exotic butterflies fluttering everywhere, it's a spectacular sensory display of nature that you can't get enough of. It's no surprise that over 1.5 million visitors pay homage to the falls annually.
WHERE IS IGUAZÚ FALLS?
Cataratas do Iguaçu in Portuguese or Cataratas del Iguazú in Spanish straddles Rio Iguazú on the border of the northwestern corner of the Argentine Province of Misiones and southwestern corner of the Brazilian state of Paraná in central-southeast South America. Two-thirds of the falls are on the Argentinian side and are within sister national parks of both countries, which were declared World Heritage Sites by UNESCO in 1984.
Chosen by a global poll of 100 million votes in 2011, a confirmation of its enormous popularity, Iguazú Falls was elected to the list of man-made New7Wonders of the World (correct spelling) and is regarded as a distinctive Geomorphological Site by the Brazilian Commission of Geologic and Paleobiological Sites.
Copy the following co-ordinates into an on-line mapping program such as Google Earth and go to the Falls: 25°41'36.37"S, 54°26'16.33"W
The regional climate is humid subtropical with hot summers year-round (14 to 21 ºC). The falls is enveloped by a dense, intensely green, highly biodiverse rainforest, fed by abundant rainfall (1,275 to 2,250 mm/yr) that varies with season and is regionally drained by the large and complex system of the Rio Paraná and locally by Rio Iguazú. We'll take a closer look at the flora and fauna in post Part II.
|Salto Bernabé Mendez, Adan y Eva and Bosetti|
Iguazú Falls is composed of some 275 separately named waterfalls that meld into one great wall of thunderous water when flow is exceedingly high.
THE WATERS OF IGUAZÚ FALLS
Iguazú River is the lifeblood of the eponymous falls and important tributary of Rio Paraná, which is second in length to the Amazon in South America and sixth largest in the world. With a drainage basin of some 78,800 sq km, Rio Iguazú rises near the Atlantic Ocean within the Serra do Mar range. But, rather than heading a short distance east to the sea, Rio Iguazú River chose a meandering westerly course over sedimentary and volcanic rocks of the uplifted, fault-segmented, cuesta escarpment-punctuated, three-plateaued,
Paleozoic-Mesozoic Paraná Basin.
Through rainforests and farmlands, it continues over many minor falls and rapids that are neo-tectonically re-activated NW-SE lineaments that date back to the origins and evolution of the basin - the subject of this post. Eventually, near the western side of the basin, the river reaches Iguazú Falls where its fury is dramatically unleashed as it plunges off the plateau.
Although subtropical, unlike many other South American rivers where annual temperature variations are relatively limited, the volume, color and content of Iguazú's waters vary considerably with season. During the summer rains from October to March (the reverse of the North Hemisphere), the river swells within its channel and may become sediment-saturated with silt and clay from basaltic red soils, iron-stained due to oxidation of mafic minerals - largely hte minerals pyroxene and feldspar. During massive discharge, the river can completely obliterate the falls within a single massive wall of murky-brown water.
|Wide, Calm and Turbid, a Swollen Rio Iguazú Approaches the Falls in the Wet Season|
In the dry, winter season from April to July, Rio Iguazú transports a low quantity of suspended particulates, running clear or slightly greenish and unhurried within a moderately wide, basalt-floored, shallow channel across the gently downwest-sloping Paraná Plateau. To the joy of the millions that have visited it, most times the falls are richly endowed and spill off the Paraná plateau at about 1,500 cu ft/s over nearly 300 individual waterfalls carved into three tiers of basalt. It's a jaw-dropping spectacle for all the senses that you can't seem to get enough of!
During an extreme drought in May and June of 1978, the falls actually dried up completely for 28 days due to low flow. In contrast, 2014 rains in the Argentine and Brazilian regions of Misiones and Paraná reached historic levels that resulted in a flow rate of 46,300 cubic meters per second at the falls - 33 times the usual flow rate. The previous record of 36,000 was reached in 1992. Both times, officials were forced to close the catwalks around the falls for safety.
|Iguazú Falls During Extreme Drought and Overflow of Biblical Proportions|
Modified from airpano.com
After spilling off the basalt plateau of the uplifted Paraná Basin, about 25 km below the falls Rio Iguazú becomes a left-hand tributary at the confluence with Rio Paraná, which is achieved at the Triple Frontier between Brazil, Argentina and Paraguay, after Rio Iguazú has traveled west some 1,320 km from its source. Right-hand tributaries are far shorter owing to the tilt and funneling action of the Paraná Basin.
|The Paraná River and Wide Floodplain below the Confluence with the Iguazú River|
Where does all that water go? Further downriver, Rio Paraná is joined by Rios Paraguay and then Rio Uruguay before emptying into the Atlantic Ocean at Rio de la Plata between Buenos Aires, Argentina and Montevideo, Uruguay.
From the shoreline of Buenos Aires, Rio de la Plata looks more like a placid lake than a river, if one allows its classification as such considering its abbreviated length of 290 km. Geographically, the estuary (where tides enter the mouth of the river) is either a gulf of the Atlantic or the world's widest river (256 km). Historically, the "River of Silver" was named for the assumed abundance of the mineral in the region, which is plentiful but brought downriver from only far inland from Bolivia, barely accessible by the Paraná tributary of Paraguay and best reached via the Amazon.
IGUAZÚ SUPERIOR AND INFERIOR
Immediately above the falls, Rio Iguazú is referred to as Iguazú Superior as it flows over a number of small steps carved into the basaltic bedrock and skirts a few small islands set precariously in the channel. In the midst of initiating another meander, the channel dramatically widens to 1,500 m and shallows and begins a sweeping clockwise rotation, almost doubling back on itself.
Beyond the small island which is a haven for parrots and toucans, the Throat of the Devil sends a plume of mist skyward that can be seen from space and easily mistaken for a low cloud.
As the river churns and skirts the various islands and islets above the falls, it roils and aerates to the joy of a myriad of tropical birds and butterflies that find it a shady sanctuary for retreat and a meal.
On the verge of a precipitous drop, the river begins to violently swirl and froth as it quickens its pace, perhaps sensing what's to come. Studies of fluvial dynamics and waterfall evolution imply that flow rate and parameters such as knickpoint migration (the sharp change in channel slope) are related to the amplitude of the base level drop and other factors. More on fluvial dynamics and waterfall geomorphology in post Part II.
Frothy, brilliant white and churning every which way, the river plunges over over three tiers of vesicular basalt that follow a huge 2.7 km arc from Argentina to Brazil, while sending a cloud of spray skyward that's visible from space. At the brink, a large portion of Iguazú Superior converges into an enormous mist-shrouded, thunderous funnel that's 230 feet high. Called Garganta del Diablo in Spanish or Throat of the Devil, it curiously aligns with the strike of the river channel and gorge downstream from the falls, best seen from the air or map view.
|Staring into the Throat of the Devil|
Almost shouting to be heard, the spray is welcomed in the 90 degree heat.
Below the falls, Iguazú Inferior nearly completes a hair-pin reversal of direction as its channel peculiarly becomes linear within a narrow (~80-90 m) and deep canyon (~70 m) that is steeper along the north, right bank and more gradual on the south. Waters spilling off the Paraná plateau also formed the seven spectacular falls of Guairá upstream on the Parana River within a narrow gorge. A large regional attraction, in 1984, with great local disapproval, the falls were controversially submerged within the Itaipu Falls and Hydroelectric Dam, the largest in the world.
What structural aspects of the bedrock caused the river to nearly double back on itself and contribute to the river's distinctive channel morphology? What accounts for the step-like tiers of bedrock and the formation of the Devil's Throat? What is the regional and large-scale geologic explanation for the waterfall's evolution? How did the assembly and break-up of each of three supercontinent's contribute to the falls' geomorphology?
THE LEGENDARY ORIGIN OF IGUAZÚ FALLS
Native Guarani legend tells us that Iguazú Falls originated when members of the ancient Cainguengue tribe sacrificed a young girl during their annual ritual to appease the serpent god Mboi, son of Tupa, who lived in the river. Several tribes came to witness the event, which is how the young warrior Tarobá met the current offering Naipi, the beautiful daughter of Cacique Igobi. Tarobá pleaded that she be spared, but his requests were denied. To escape, the lovers fled downstream by canoe on the River Iguazú.
|Artist's Depiction of Naipi and her warrior lover Tarobá|
Image from Iguazú Falls Tours, artist unknown.
Enraged, Mboi sliced the river in two to prevent their union. The depression that formed created the falls and swallowed the young lovers in the deluge. As punishment, they were transformed into the landscape, Naipi turning to stone bathed by the waters of the river and Tarobá into a palm tree along its banks. Their fate was separation for an eternity, ever forcing them to gaze at one another from afar. It is only when the sun desires to shine that their loving hearts join with a rainbow that signifies their reunion.
Of course, geologists entertain a less mythological perspective. The evolution of Iguazú Falls is not merely the immediate consequence of erosion of the underlying strata that dictates its geomorphology but the culmination of large-scale, global events that produced the region's distinctive volcanic plateau.
That said, let's travel back in time a billion years to the acquisition of the region's oldest assumed basement foundation on a hypothesized supercontinent long gone. Where reconstructions, relationships and timing have been the subject of ongoing debate, I've tried to reflect the views of the consensus.
THE GEOLOGIC ORIGIN OF THE FALLS
We begin in the Middle to Late Proterozoic with supercontinent of Rodinia and continue with its continental successors, Gondwana and Pangaea. The transition proceeds according to the Supercontinental Cycle that hypothesizes how all or most of the world's landmasses cyclically assemble, dissociate and reassemble every 600 to 800 Ma. It includes the acquisition of new crust and the closure of intervening ocean basins. The process is speculated to influence biogeochemical cycles, which enhances biological productivity, biodiversity and alters the course of evolution.
Each supercontinent in the succession is geomorphologically and compositionally unique, yet each retains within its core elements of the parent continent that preserves a long-term record of the Earth's history that was acquired from it. In a sense, it mimics the
genetic evolution of life as ancient building blocks are tectonically passed on to continental progeny in addition to newly acquired crust. Indeed, tectonics and evolution are related on many levels, the former providing the impetus for the latter. Driven by plate tectonics, the cycle is a fascinating concept - mimicking life and being responsible for its evolution!
First conceptualized to have existed in 1970, long-lived, pole-spanning, crescent-shaped and massive, Rodinia (a.k.a. Paleopangaea) is thought to have achieved final assembly through worldwide Grenvillian (an elongate mountain range spanning North America from Mexico to Labrador to Scandinavia) and related orogenic (mountain and continent-building events) during the Middle and Late Proterozoic (~1.3 Ga to 0.9 Ga).
The process of plate tectonics is thought to have been shaping the planet for well over a billion years, possibly as much as three of its 4.6 billion-year history. Rodinia wasn't likely the first continent, although its predecessors were likely much smaller. Few doubt its existence, and no universal agreement exists regarding timing of assembly, its longevity, details of fragmentation, and the number and configuration of its constituent cratons (an interlocking Archean and Middle Proterozoic maze of basement-forming, rigid and stable crustal blocks).
Relevant to our Iguazú discussion, the Paranapanema cratonic block (red ellipse) was an inherited remnant of a preceding continent that was incorporated within central Rodinia between ~1000 and 850 Ma. It likely was associated with neighboring cratons of Amazonia (which is definitely Rodinian in origin by consensus) and Río de la Plata, Kalahari and Congo-São Francisco (which are likely "Non-Rodinian" that may have assembled during Gondwana's earliest collisional events).
In the Paleozoic within Gondwana, the successor supercontinent to Rodinia, the Paranapanema block would provide a stable foundation beneath a thick sequence of sedimentary rocks of the Paraná Basin, the location of Iguazú Falls.
After 150 million years of gradual accretionary cratonic assembly, Rodinia began to progressively break apart according to the hypothesized Supercontinental Cycle. It was a protracted (100-plus Ma) and diachronous (age varying from place to place) process. Rifting first occurred at its western margin (present co-ordinates) possibly as early as ~750 Ma and then southeast about the same time with complete break-up after ~600 Ma.
Mechanically and geothermally unstable and attributed by most to the presence of a mantle plume (or even the absence of one, stuff for post Part II), Rodinia rifted apart and spawned a flotsam and jetsam of landmasses both large and small separated by newly opened seas as they tectonically drifted across the globe.
Surrounded by the Panthalassa Ocean (a.k.a. proto-Pacific), the two largest were equatorically-situated Laurentia (North America's cratonic core) and australly-situated Gondwana (a South Pole-sprawling, massive parent to the continents of the Southern Hemisphere). The two mega-continents and sundry smaller micro-continents were separated by a widening Iapetus Ocean, named after the mythical Greek titan who fathered Atlas. The eponymous Atlantic Ocean would become the Iapetus successor, but first worldwide ocean closures and supercontinental re-assemblages would have to occur.
THE BIRTH OF GONDWANA, NEXT IN SUCCESSION
Gondwana is frequently referred to as a megacontinent or superterrane, since it not only formed in a shorter interval but didn't include all global landmasses. Regardless, it was the largest continental unit at the time and remained that way for over 200 Ma, spanning all southern paleolatitudes from the South Pole to over 20°N for most of the Paleozoic. It formed from the unification of over ten Precambrian cratons and covered almost 100 million sq km with remnants constituting 64% of all present-day land areas including the present-day continents of South America, Africa, most of Antarctica and Australia, Madagascar and India.
Another cycle contradiction, even in the final stages of Rodinia disassembly Gondwana had already begun to assemble in the latest Late Proterozoic and earliest Cambrian. It was largely together by ~600 Ma, although oceans (that would eventually close) still existed between Australia-East Antarctica, India and eastern Africa. Gondwana finally amalgamated by ~540-530 Ma.
Like its parent, Gondwana assembled from a collage of cratonic nuclei largely acquired from Rodinia (which were relics of older landmasses) and from newly-acquired crust as intervening oceanic domains closed. During the process, Rodinia's Paranapanema craton was a passive tectonic passenger that participated with numerous other cratons in Gondwana's assembly. In this manner, the future foundation of the Paraná Basin of Iguazú Falls transferred from Rodinia to Gondwana and will do so twice more!
EAST MEETS WEST
As with Rodinia, although the precise configuration and mechanisms of assembly are the subject of great ongoing debate, paleomagnetism and geochronology confirm that East Gondwana (yellow Australia, India, Madagascar and Antarctica, blue) and West
Gondwana (largely South America, Africa and Arabia, blue) unified through a succession of collisions and ocean closures via the consolidating Pan-African and Brasiliano orogenies.
Reminiscent of Rodinia's jigsaw-puzzle, building block construction, West Gondwana was an interlocking maze of cratonic blocks, shields (exposed, eroded Precambrian cratons) and mobile belts (ill-defined mountain-building, continent-unifying orogenies).
The cratonic mass contains a complex framework of faults, lineaments (linear surface features) and discontinuities (crustal structural changes that reflect bedding, faults, etc.) that influenced sedimentation patterns due to differential subsidence and uplift of the blocks. Many of the faults persisted within the crust and later tectonically reactivated.
Over time, cratonic relationships have remained fairly constant (note the yet-to-form Paraná Basin, arrow) during the evolution of West Gondwana, Pangaea and present-day South America within the tectonically stable, Precambrian-cellared South America platform of the eponymous tectonic plate.
WEST GONDWANA'S RESPONSE TO MARGINAL TECTONICS
Evolution of the Paraná Basin, the location of Iguazú Falls, was markedly influenced by the geodynamics of the southwestern region of Gondwana when it was subjected to a nearly continuous succession of orogenies of subducting oceanic lithosphere. In fact, a defining tectonic feature of Gondwana was the establishment of a peripheral subduction system that has arguably existed ever since.
Yet, Gondwana's stable continental interior and cratons remained relatively undistorted and undisturbed, even during the break-up and dispersal of Rodinia. In spite of this, their morphology was influenced by extensional regimes that resulted in their conversion to large sag basins. Also referred to as cratonic, intracratonic, interior and intercontinental basins, they are characterized by rapid subsidence and mainly siliciclastic deposition (non-carbonate, sandstone-based eroded rocks) with support conferred by a stable and rigid foundation - the Paranparema in the case of the Paraná Basin!
Cratonic basins are long-lived, circular or oval and saucer-shaped in cross-section with extents on the order of a few hundred thousand to a few million square kilometers. Marginal West Gondwanan collisions produced internal continental extension that induced flexural downwarping of the basins that created accommodation space for massive but gradual sedimentation.
Contingent on tectonics and climate, the Paraná Basin filled with ~3–6 km of mostly shallow marine and terrestrially-derived, layer-cake sediments that include estuarine and lacustrine, marginal and epeiric seas (shallow-shelf marine) with shales, limestones, aeolian sandstones and even glacially derived diamictites.
CRATONIC BASINS ARE FOUND GLOBALLY
Sag basins are some of the largest sedimentary basins on Earth. They cover over 10% of its continental surfaces and are abundant on the four continents that border the Atlantic domain (below). Inboard of passive margins, they often bear epeiric connections to the sea via failed rifts or even failed arms of triple junctions.
With all that is known, many aspects of origins and dynamics remain enigmatic such as their mantle associations. A large number of mechanisms have been invoked to their formation such as thermal contraction following heating, extension related to magmatic upwelling, deep crustal phase changes, reactivation of pre-existing sags, emplacement of basaltic underplates and the subduction of 'cold' oceanic slabs.
Attention is directed to the Paraná Basin (pink PAR) in south-central South America.
|Global Distribution of Typical Cratonic Basins Surrounding the Atlantic Ocean|
Basins are color-coding according to the timing of initiation.
Modified from Philip A. Allen et al
The Paraná Basin (encircled) initiated within the core of Gondwana subsequent to the break-up of Rodinia, as have many other cratonic basins worldwide, and continued forming within its supercontinental successor, Gondwana. Northern Canada's Hudson Bay is a familiar example of a large cratonic basin in North America encircled by rocks of the Canadian shield. The Anglo-Paris Basin is another in western Europe, delivered to the continent subsequent to Pangaea's break-up (read my blog post here about it).
THE PARANÁ BASIN - A BACIA DO PARANÁ
The Paraná and neighboring sag basins began to form shortly after Gondwana consolidation about 500 to 470 Ma. Elliptical in shape and named after the river system that flows through its central axis, the Paraná strikes NNE-SSW and occupies a wide area of the central and eastern portion of South America. About 65% lies in the Brazilian state of Paraná with the remainder in Argentina, Paraguay and Uruguay (bottom left).
The ~1.6 mil sq km and ~1,500 km wide depression classically represents the morphology of cratonic basins worldwide. Rather than viewed as a single entity, it consists of three superimposed basins (bottom right) that formed during the Silurian-Devonian, Permian and Jurassic-Cretaceous, although it was intermittently separate or linked with the Chaco-Paraná Basin to the west across the Asuncion Arch. The Paraná Basin's protracted geologic history has greatly assisted in understanding the origin and evolution of both Gondwana and Pangaea!
POLYCYCLIC SUPERSEQUENCES OF THE PARANÁ Basin
Its sedimentary origin began in the Late Ordovician (~450 Ma) when Gondwana was an insular supercontinent, continued when Gondwana participated in the formation of Pangaea in the late Paleozoic and ceased at the end of the Cretaceous (66 Ma) with widespread magmatism (the latter typical of many cratonic basins). Early Cretaceous magmatism in the Paraná was a precursor to break-up of West Gondwana (and greater Pangaea) in the Mesozoic!
Its record consists of a thick package (~7.5 km) of six unconformity-bounded, lenticular-shaped, lithostratigraphic units (a.k.a. supersequences, megasequences and Sloss sequences after the geo-pioneer). Deposited in intervals of roughly tens of millions of years and classified by mechanism of subsidence, the sequences are second-order (formed during tectono-eustasy versus first-order during global tectonic cycles). The concept is a long-standing paradigm of stratigraphic geology.
From bottom to top, the basalmost supersequences are Rio Ivaí (Ordovician-Silurian) and Paraná (Devonian) that correspond to early and middle Paleozoic marine transgressive-regressive cycles. The remainder are continental sedimentary packages acquired during and following Pangaea amalgamation: Gondwana I (Carboniferous-Early Triassic), Gondwana II (Middle-Late Triassic), Gondwana III (Late Jurassic-Early Cretaceous) and Bauru (Late Cretaceous) Gondwana III's uppermost igneous extrusive sequence, the Serra Geral Formation, is directly responsible for the geomorphology of Iguazú Falls.
Late Ordovician to Silurian...
During the early Paleozoic while in the Southern Hemisphere, Gondwana was surrounded by a number of lesser continents within the global Panthalassa Ocean. The nascent Paraná Basin (encircled), in communication with the Rheic Ocean that opened with the rifting of the Avalonia magmatic arc (that accreted with Laurentia's eastern margin), formed an epicratonic embayment when it received the transgressive supersequence of the Late Ordovician to Early Silurian (~440 to 370 Ma) Rio Ivaí Group. Deposited unconformably on the assumed Paranapanema basement, the shallow gulf includes sandstones, mudstones and glacial deposits (white star indicates the South Pole). Glaciation resulted in the deposition of diamictites and shales of the Iapó Formation.The following schematic maps are found in Torsvik and Cocks (see references).
In the Early Devonian, the Paraná Basin was at the inbound end of an epeiric sea (shallow continental shelf-flooding) recorded by transgressive shales of the Silurian Villa Maria Formation and post-glacial transgressions of the Devonian Paraná Group, the basin's second supersequence.
As Gondwana drifted from higher latitudes towards Laurentia between the Carboniferous and Early Permian, the Paraná Basin again received glacial deposits during the longest ice age of the Phanerozoic of ~90 million years. As the event waxed and waned, it affected eustatic sea level change that in turn influenced deposition in coastal basins globally. Foreland basins in communication with the sea were affected such as the Paradox Basin of the Ancestral Rockies with 30 transgressive-regressive cycles (read about it here).
Carboniferous to Early Triassic...
The 'second' Parana Basin began with a collisional cycle when an extensive mountain belt formed southwest of the basin. The event flexed internal portions of Gondwana that overloaded the continental lithosphere and contributed to basin subsidence. The marine Gondwana I Supersequence during the Carboniferous to Permian is the basin's largest and most complex sedimentary package.
It represents the invasion and exit of the Panthalassa Sea as the Paraná Basin finally closed, entrapped within continental West Gondwana. The basin records dramatic paleoenvironmental changes through time from glacial epochs in the Pennsylvanian (the Itararé Group and Aquidauana Formation), a marine transgressive section (Guatá Group) with sandstones and coals (Rio Bonito, Palermo, and Irati Formations), redbeds (Rio do Rasto) and the arid Triassic period of central Gondwanan Pangaea.
Basin-fill records radical paleoenvironmental changes through time that Gondwana was experiencing, everything from Pennsylvanian glacial to a marine transgression and then arid Triassic sands. Owing to the proximity of intracratonic basins within South America and Africa, nearly equivocal, syn-depositional supersequences are found across the Atlantic within Namibian-Angolan basins.
IMPORTANT GEOFACTS WORTHY OF MENTION
The Paraná Basin possesses famous and important paleontological representatives. Waning glaciation in the Middle Permian allowed Gondwana I shales to preserve fossils of Glossopteris (extinct order of seed fern) and Late Permian Mesosaurus (extinct freshwater crocodylian). Prior to the concept of plate tectonics, the enigma of their transoceanic locations were thought related to land bridges that spanned stable continents.
But their pronounced fit and distribution of related glacial deposits when plotted on a global geometric reconstruction led early 19th century geoscientists - such as Alfred Wegener in 1915 - to the concept of continental drift (a simple rift-to-drift theory) and the idea that the southern continents once formed a Pangaean supercontinent from a once-unified, dispersed Gondwana. Of course, more recent dating, paleomagnetic evidence and a better understanding of mantle dynamics led to the theory of plate tectonics in the 1960s.
Middle to Late Triassic...
Once formed, Gondwana remained independent for ~200 myr. Insulation ended in the Permian after completing a transequatorial tectonic journey across a closing succession of Iapetus and Rheic Oceans. It was then that the South American portion of West Gondwana obliquely collided with the eastern margin of southern Laurentia and African portion collided with northern Laurentia. The event formed Pangaea, the next supercontinent in the succession.
The collision marked a major depositional change for South America's sag basins, the Paraná in particular. Previously, they acquired marine, nonmarine and glaciogenic lithologies when Gondwana was insular. Subsequent to amalgamation with Pangaea, continental sedimentation (subaerial, lacustrine and fluvial deposits) prevailed within the basins entrapped within unified Pangaea. Thus began Middle to Late Triassic Gondwana II supersequences of fluvial and lacustrine red beds locally.
Late Jurassic to Early Cretaceous desertification and volcanic activity...
Two major events affected the Gondwana III supersequence that formed within the 'third' superimposed basin: desertification of interior Pangaea and the formation of a massive igneous province that heralded the dawn of global tectonic change.
Reflective of the extremely arid paleoclimate within the central supercontinent and West Gondwana, the region preserves 2 million sq km of cross-bedded eolian sandstones of the Botucatu Formation. Today, it holds the Guarani Aquifer, one of the world's largest beneath the surface of Argentina, Brazil, Paraguay and Uruguay.
Gondwana II's second member, the Serra Geral Formation, closed the the sedimentary depositional history of the Paraná Basin. It consists of extensive and voluminous basalts that flooded the continental landscape. The volcanics may have caused some degree of basin subsidence due to overloading and/or cooling of its deep intrusive plumbing. The extrusion of Serra Geral basalts was a sign of impending large-scale, global tectonic reorganization, and regionally, was responsible for the geomorphology of Iguazú Falls.
In the north-central corner of the Paraná Basin, a downwarped retro-arc basin formed in the Early Jurassic subsequent to Andean orogenics along South America's western margin. It accommodated the deposition of the region's final sedimentary unit, the Upper Cretaceous Bauru Supersequence. It consists of alluvial, fluvial and eolian lithologies and contains important plant, reptile and dinosaur bones, and eggs and teeth of gigantic titanosaurian sauropods in particular. The discoveries led to the assertion that Southern Hemispheric dinosaur biogeography was largely controlled by the progressive break-up of Gondwana.
SERRA GERAL FLOOD BASALTS OF THE PARANÁ VOLCANIC PROVINCE
Also called Arapey (flows) and Cuaró (sills) in Uruguay and the Alto Paraná Formation in Paraguay, the Serra Geral Formation, its most cited name, is derived from the eastern Serra Geral escarpment in Brazil in the southern portion of the Serra do Mar coastal range. Lying above the flat-lying Atlantic Coastal Plain, the eroded and dissected cliffs mark the easternmost extent of the massive lava field of the Paraná Basin.
With an area of ~917,000 sq km, volume of ~450,000 cu km extrusives at the surface and an estimated 112,000 intrusives within the subsurface as sills and dikes that propagated the ascent of magma and delivered lava to the surface, the Serra Geral lava field forms the Paraná Volcanic/Magmatic Province of the Paraná Basin of Brazil, Uruguay, Argentina and Paraguay. In its entirety, it also includes continental flood basalts that emplaced in the Etendeka region of southwest Africa in Namibia and Angola in the Early Cretaceous that were disproportionately divided by continental rifting.
Serra Geral basalts are a package of some 32 or more flows that emplaced between ~140 and 129 Ma and peaked ~133-130 Ma. The eruption period is relatively brief but poorly constrained and spans variably ~2.4 to 10 Myr.
Typical of extensional tectonic regimes, Serra Geral rocks are predominantly basalts, high volume, brief eruptions of low viscosity-fluid mafic magma (low-silica, dark-colored ferro-magnesian minerals) but also include some intermediate and felsic rocks (high-silica, light-colored, rhyolitic-granitic rocks). It's a bimodal igneous rock distribution that is asymmetrically distributed throughout the volcanic province and has implications for its evolution (more on that in post Part II).
The igneous lava rock distribution is as follows:
• ~90% tholeiitic lavas (a basalt sub-type with reduced olivines and higher quartz-mafic saturation typical of oceanic spreading centers)
• ~70% tholeiitic andesites (intermediate)
• ~3% rhyolites (light-colored, iron and quartz-rich felsic rocks).
In addition, a silicic geochemical subclass or suite of magma types with high- and low-Ti (titanium) exists (with additional incompatible elements) within various intermediate igneous latites (<5% quartz) and quartz latites. The distribution of igneous rocks within the Parana Basin has marked provinciality to the extent that volcanological genetic, magma melting and emplacement mechanisms are implied (again, Part II).
Earliest flows intercalated with uppermost arid Botucatu eolian sediments as pre-existing, re-activated NE and NW tectonic lineaments subdivided the flows, which further delineated the basin. Notable are Ponta Grossa arch, a major NW-SE-trending domal feature and site of the province's most important dike swarm. The N–S Asuncion Arch on the west separates the Paraná Basin from the Chaco-Paraná Basin. It's a western extension of the Paraná in Argentina with a contrasting evolutionary history that includes Andean foreland orogenics.
LARGE IGNEOUS PROVINCES
Our planet's geologic history is interspersed with the rapid extrusion of massive volumes of mainly flood basalts - upwards of 100,000 cu km and often exceeding 1,000,000 - that emplaced over a relatively brief time interval across the landscape of pre-rift continents. Unrelated to seafloor spreading at mid-ocean ridges and at subduction zones that occur at plate margins, these infrequent intraplate Large Igneous Provinces (LIPs) or Continental Flood Basalts (CFBs) are linked to regional uplift, continental rifting and break-up, and global environmental catastrophes and mass extinction events.
Consisting of Serra Geral basalts, the Paraná Volcanic Province is one such continental LIP that preceded rifting apart of the West Gondwanan component of Pangaea. Its emplacement resulted in the opening of the South Atlantic Ocean and dispersal of the continents that border the Atlantic realm - South America and Africa. Postulated genetic connections between the emplacement of LIPs, mantle plumes, hotspot activity and continental rifting have resulted in the emergence of several contrasting genetic models.
THE PARANÁ-ETENDEKA VOLCANIC PROVINCE
Before continental rifting and South Atlantic seafloor spreading separated West Gondwana and greater Pangaea broke apart, the future continents of South America and Africa were juxtaposed. Serra Geral basalts extruded over the cratonic basins of both continents in a once-unified LIP before the opening of the South Atlantic Ocean.
The massive lava field formed the combined Paraná (~1.2 mil sq km and up to ~1.7 km thick) in southeastern South America and Etendeka Volcanic Provinces in southwestern Africa (~78,000 sq km and ~1 km thick). Having formed coevally over a relatively short duration, the two provinces possess a close commonality of temporal, geochemical, petrological, stratigraphic and tectono-genetic attributes, although some differences do exist.
The bulk (~95%) of the formerly-unified, ocean-separated volcanic province is presently located within the Paraná Basin in Brasil and Argentina. In Africa, Etendeka Group Tsuhasis Basalts emplaced within the Huab Basin (~80,000 sq km, 900 m thick) of northwestern Namibia and the Kwanza Basin of southwest Angola within the Novo Redondo and Lucira Formations. In both provinces, massive dike swarms are exposed in areas of deeper erosion.
The Paraná-Etendeka Volcanic Province is the largest preserved LIP on the planet in terms of size and volume and is increasingly one of the most studied. Greatly eroded and likely once larger as implied by the location of central conduits and an extensive centrifugal array of dike swarms and ring-complexes that fed the volcanic fury, it currently ranks as the world's second largest LIP of the Phanerozoic and is surpassed only by the Siberian Traps (Swedish for eroded steps of basalt) in Russia's Tunguska sedimentary basin.
|Although also bimodal in composition, the percentage of silicic volcanics in the Etendeka (~50%) is proportionally higher than the Paraná (only ~3%), possibly related to asymmetry of the LIP.|
Three LIP events are linked to the opening of the Atlantic Ocean along its entire length, listed chronologically:
• the Central Atlantic Magmatic Province (CAMP) - between North America and Northwest Africa starting at ~195 Ma.
• the Paraná-Etendeka LIP - between South America and Africa starting at ~120 Ma.
• the North Atlantic Igneous Province (NAIP) - between Europe and Greenland at ~55 Ma.
The association of LIPs, continental break-up and the opening of the Atlantic suggests a definitive rift association (Yup! Part II). Obviously, the story of the Paraná Basin and Volcanic Province is far from over. Many important questions remain unanswered, and Iguazú Falls has yet to form.
• What was the trigger for magma generation, volcanism and continental rifting?
• Paraná-Etendeka magmatism is closely associated in space and time with continental rifting. Was the long-lived Tristan da Cunha-Gough mantle plume involved or was it a plume-less process related to plate tectonics?
• Did the plume provide passive heat for lithospheric melting or did it play a more active role by contributing material as well? Does the plume really exist? In fact, what's down there?
• To what extent was the sub-continental lithospheric mantle and depleted asthenospheric mantle involved? What do Serra Geral chemistries suggest? What is the correlation between the bimodal association of the Paraná-Etendeka's basaltic and silicic rocks? How does basin provinciality based on geochemistry play into volcanological genesis theories?
• Once the Paraná Basin acquired its Paleozoic sedimentary supersequences and Cretaceous igneous cover, what happened during the Cenozoic in regards to uplift, deformation and plateau segmentation?
• Recognizing the hypothesized temporal association between LIP eruptions and mass extinction events, how does the Paraná-Etendeka Volcanic Province compare to others of the Phanerozoic?
• What about Iguazú Falls? How did a billion years of geologic events affect its geomorphology? Does it behave like other falls on resistant bedrock globally?
Please visit my forthcoming post Part II for a continuation of this discussion.
Odysseys' Patagonia Tour Director Gabriel Blacher...
Virtually indispensable, his knowledge, expertise, attention to detail and adept handling of every conceivable situation (including the weather) was highly appreciated by all. Gabe's thoughtfulness, willingness to accommodate to everyone's needs, endless wit, amiable personality and sexy tango lessons on the bus will long be remembered.
Smithsonian Journeys Expert Wayne Ranney...
Wayne is a passionate geologist, experienced educator, river and trail guide, and well-published, multi-honored author that has acquired a wealth of knowledge on his travels to all seven continents and 85 countries. With a keen interest in archaeology, anthropology, history, foreign cultures, languages and everything related to our planet, his engaging and informative presentations always packed the house with the greatest of anticipation. His thorough explanations and insightful interpretations of the landscape and its evolution always puts things into a new and clearer perspective. No trip anywhere is complete without Wayne! Look for him here.
The Intrepid "Iguazú Crew"...
Forged by the bonds of world-class geology, travel adventure and central air conditioning, it was great fun exploring the falls and surrounding rainforest together! And thanks again to "Arizona" John for thoughtfully providing everyone with solar eclipse sunglasses. Fortunately, my vision has almost returned to normal.
|John, Pat, Ed, local guide Eduardo, Dee, Sandy and Sharon|
Lastly, I am extremely grateful to Edgardo M. Latrubesse, PhD of the University of Texas at Austin and Professor Eduardo Salamuni, PhD of the Federal University of Parana State in Brazil. Each contributed extremely helpful information on the evolution of the Paraná Basin and geomorphology of Iguazú Falls. Dr. Salamuni's personal communications (2017) were of great value in formulating many of the ideas found in these three posts.
EXTREMELY INFORMATIVE RESOURCES
• An Outline of the Geology and Petroleum Systems of the Paleozoic Interior Basins of South America by Edison José Milani and Pedro Victor Zalán, Episodes 2, 2014.
• A New Scheme for the Opening of the South Atlantic Ocean and the Dissection of an Aptian Salt Basin by Trond H. Torsvik et al, Geophys. J. Int 177, 2009.
• A Review of Wilson Cycle Plate Margins: A Role for Mantle Plumes in Continental Break-up Along Sutures? by Susanne J.H. Buitera and Trond H. Torsvik, Gondwana Research 26, 2014.
• Assembly, configuration, and break-up history of Rodinia: A Synthesis by Z.X. Li et al, Precambrian Research 160, 2008.
• Beyond Power: Bedrock River Incision Process and Form by Gregory S. Hancock and Robert S. Anderson, Geophysical Monograph 107, 1998.
• Climatic Events During the Late Pleistocene and Holocene in the Upper Parana River: Correlation with NE Argentina and South-Central Brazil by Jose C. Stevaux, Quaternary International 72, 2000.
• Contemporaneous Assembly of Western Gondwana and Final Rodinia Break-up: Implications for the Supercontinent Cycle by Sebastián Oriolo et al, Geoscience Frontiers, 2007.
• Continental Rift Evolution: From Rift Initiation to Incipient Break-up in the Main Ethiopian Rift, East Africa by Giacomo Corti, Earth-Science Reviews 96, 2009.
• Cratonic Basins by Philip A. Allen et al, Tectonics of Sedimentary Basins: Recent Advances, First Edition, Chapter 30, 2012.
• Cratonic Basins and the Long-term Subsidence History of Continental Interiors by John Joseph Armitage and Philip A. Allen, Journal of the Geological Society, 2010.
• Deep Crustal Structure of the Paraná Basin from Receiver Functions and Rayleigh-wave Dispersion: Evidence for a Fragmented Cratonic Root by J. Julià et al, Journal of Geophysical research, 2008.
• Foz do Iguaçú: Geomorphological Context of the Iguaçú Falls by Marga Eliz Pontelli and Julio Cesar Paisani, Landscapes and Landforms of Brazil, Chapter 31, 2015.
• Geophysical Definition of Paranapanema Proterozoic Block and Its Importance for the Rodinia to Gondwana Evolutionary Theories by M. Mantovani et al, Abstract 8053, EGS-AGU-EUG Joint Assembly, Nice, France, 2003.
• Gondwana Collision by T.S. Abu-Alam, Miner Petrol, 2013.
• Gondwana from Top to Base in Space and Time by Trond H. Torsvik and L. Robin M. Cocks, Gondwana Research 24, 2013.
• Gondwanaland from 650–500 Ma Assembly through 320 Ma Merger in Pangaea to 185–100 Ma Breakup: Supercontinental Tectonics via Stratigraphy and Radiometric Dating by J.J. Veevers, Earth-Science Reviews 68, 2004.
• Iguazu Falls: A History of Differential Fluvial Incision by José C. Stevaux and Edgardo M. Latrubesse, Geomorphological Landscapes of the World, Chapter 11, 2010.
• Landscapes and Landforms of Brazil by Bianca Carvalho Vieira et al, Springer Science, 2015.
• Large Igneous Provinces by Richard E. Ernst, Cambridge University Press, 2014.
• Magnificent Canyons Sculpted in the Aparados da Serra Scarps of the Volcanic Plateau of the Paraná Basin by Wilson Wildner et al, Geological and Paleotological Sites of Brazil on-line, 2006.
• New Insights on the Occurrence of Peperites and Sedimentary Deposits within the Silicic Volcanic Sequences of the Paraná Magmatic Province, Brazil by A. C. F. Luchetti
• Orogenias Paleozoicas No Dominio Sul-Ocidental do Gondwana e Os Ciclos de Subsidencia da Basin do Parana by Edison J. Milani and Victor A. Ramos, Revista Brasileira de Geociências 28, 1998.
• Paleomagnetic Poles and Paleosecular Variation of Basalts from Paraná Magmatic Province, Brazil: Geomagnetic and Geodynamic Implications by Luis M. Alva-Valdivia et al, Physics of the Earth and Planetary Interiors 138, 2003.
• Parana Magmatic Province-Tristan da Cunha Plume System: Fixed Versus Mobile Plume, Petrogenetic Considerations and Alternative Heat Sources by M. Ernesto et al, Journal of Volcanology and Geothermal Research 188, 2002.
• Planation Surfaces on the Paraná Basaltic Plateau, South America by Daniela Kröhling et al, Gondwana Landscapes in Southern South America, 2014.
• Review of the Areal Extent and the Volume of the Serra Geral Formation, Paraná Basin, South America by Heinrich Theodor Frank et al, Pesquisas em Geociências 36, 2009.
• Role of Subaerial Volcanic Rocks and Mantle Plumes in Creation of South Atlantic Margins: Implications for Salt Tectonics and Source Rocks by Martin P.A. Jackson et al, Marine and Petroleum Geology 17, 2000.
• Slab Pull, Mantle Convection, and Pangaean Assembly and Dispersal by W.J. Collins, Earth and Planetary Science Letters 205, 2003.
• South Atlantic Opening: A Plume-Induced Breakup? by T. Fromm et al, Geology 43, 2015.
• Synchrony Between the Central Atlantic Magmatic Province and the Triassic–Jurassic Mass-extinction event? by Jessica H. Whiteside et al, Palaeogeography, Palaeoclimatology, Palaeoecology 244, 2007.
• Tectonics and Sedimentation of the Paraná Basin by Pedro Victor Zalán, Atlas do III Simposio Sul-Brasileiro de Geologica 1, 1987.
• The Controversy over Plumes: Who Is Actually Right? V. N. Puchkov, Geotectonics 43, 2009.
• The Cretaceous Opening of the South Atlantic Ocean by Roi Granot and Jérôme Dyment, Earth and Planetary Science Letters 414, 2015.
• The Faroe-Shetland Basin: A Regional Perspective from the Paleogene to the Present Day and its Relationship to the Opening of the North Atlantic Ocean by David Ellis and Martyn S. Stoker, Geological Societyy, London, Special Publications 397, 2014.
• The Formation of Pangaea by G.M. Stampfli et al, Tectonophysics 593, 2013.
• The Origin and Evolution of the South American Platform by Fernando Flávio Marques de Almeida et al, Earth-Science Reviews 50, 2000.
• The Paraná Basin, Brazil in Interior Cratonic Basins by P.V. Zalan et al, Memoir Vol. 51, 1991.
• The Paranapanema Lithospheric Block: Its Importance for Proterozoic (Rodinia, Gondwana) Supercontinent Theories by M.S.M. Mantovani and B.B. de Brito Neves, Gondwana Research 8, 2005.
• The Cretaceous Alkaline Dyke Swarm in the Central Segment of the Asuncion Rift, Eastern Paraguay: Its Regional Distribution, Mechanism of Emplacement, and Tectonic Significance by Victor F. Velazquez et al, Journal of Geological Research 2011, 2011.
• The Fossilised Desert: Recent Developments in Our Understanding of the Lower Cretaceous Deposits in the Huab Basin, NW Namibia by Dougal A. Jerram et al, Communs geol. Surv. Namibia, 12, 2000.
• The Persistence of Waterfalls in Fractured Rock by Michael P. Lamb and William E. Dietrich, GSA Bulletin July/August 2009.
• The “Plate” Model for the Genesis of Melting Anomalies by G.R. Foulger, Department of Earth Sciences, Durham University, Durham, U.K., Mantleplumes.org.
• The South American Retroarc Foreland System: The Development of the Bauru Basin in the Back-bulge Province by Mirian Costa Menegazzo et al, Marine and Petroleum Geology 73, 2016.
• Thermotectonic and Fault Dynamic Analysis of Precambrian Basement and Tectonic Constraints with the Parana Basin by L.F.B. Ribeiroa et al, Radiation Measurements 39, 2005.
• Titaniferous Magnetite and Barite from the San Gregorio de Polanco Dike Swarm, Paraná Magmatic Province, Uruguay by Rossana Muzio, Earth Sciences Research Journal 17, 2013.
• Volcanological Aspects of the Northwest Region of Paraná Continental Flood Basalts (Brazil) by F. Braz Machado et al, Solid Earth 6, 2015.
• Zircon U-Pb Geochronology from the Paraná Bimodal Volcanic Province by Viter Magalhães
Pinto et al, Chemical Geology 281, 2010.