“The purity of the outlines, the softness of everything, saggy of colors,
the harmonious unity of the sky with the sea and the sea with the land ...
who saw them a only once, he owns them for life.”
German writer and statesman Johann Wolfgang von Goethe on Sicily in 1787
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| Scala dei Turchi Large and small-scale rhythmic bedding of the Staircase has weathered the Trubi Formation's whitish marly limestone and calcaeous marls into a series of repetitive notches. Gray-white-beige-white-colored cycles of deposition are enhanced by differential erosion making them cropout in parallel sinuous bands. Here's your chance to do some magnificent coastal geology under the intense Sicilian sun. |
Taking a long-term geological view, the Staircase's genesis began with the fragmentation of a supercontinent followed by the re-amalgamation of its daughter continents and their an inevitable disassembly. The story includes the Tethyan forerunners of the Mediterranean Sea that opened and closed throughout the Phanerozoic and culminated with the convergence of Africa and Eurasia. In order for the Staircase to form, global tectonic and glacial events working in concert first had to desiccate the sea in the late Miocene, reflood it in the early Pliocene and uplift it in the Pleistocene. This is its story.
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| Facing West Towards the Staircase Looking downbeach, a side view of Scala dei Turchi revels a northward dip of the strata and eroded repetitive lithological cycles that stand out in both color and relief. |
ABOUT THIS POST
Taking a break from our road trip through Sicily, our party of four headed for the beach to relax and catch some glorious Sicilian sun. It turned out to be a most enlightening geological excursion. Landscapes, landforms and the compendium of rock types that comprise them don't simply form by accident or randomly out of disorder. They are the culmination of a succession and interaction of geologic and tectonic processes and events that occurred both regionally and even globally. Such is the case with Scala dei Turchi.
There are a plethora of models, interpretations and reconstructions that explain the paleo-geography and paleo-tectonics that occurred during the Phanerozoic for the evolution of Pangaea and the Mediterranean. Whenever possible, I've tried to convey a consensus of opinion and degree of simplicity. Relevant items are italicized and defined, and important place names and events are in boldface at first mention. All directions refer to modern global coordinates.
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| The Intersection of Sky, Sea and Staircase Scala dei Turchi is extremely photogenic but challenging with the extremes of contrast at mid-day. |
WHERE ARE WE?
Scorched by the sun and warmed by African currents that course through the Strait of Sicily, Scala dei Turchi in Italian or Staircase of the Turks, as it's popularly known, lies midway along Sicily's south Mediterranean coast. It's a rather small but spectacular, ~2.5 km-long, serpentine series of stepped-cliffs officially called Punta di Maiata (point of in Italian) in the small municipality of Realmonte. To reach it, park along the cliff-top road Contrada Scavuzzo (SP68), and find the sandy footpath that switchbacks down to the sea. Plug these coordinates into your GPS: 37°17'26.16"N, 13°28'21.24"E
The Trubi Formation is dramatically exposed at the Staircase but crops out with progressively younger strata as far as Eraclea Minoa to the west some 25 km. Trubi calcareous marls and marly limestones are also submerged some distance out to sea to the south and buried inland within a tectonically transported seafloor basin (more on that later). In Sicilian dialect, Trubi (or trubbu) means "whitish rocks."
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| Google Earth View of Scala dei Turchi The Trubi Formation is dramatically exposed at Scala dei Turchi, laterally along the coast, submerged a distance out to sea and outcrops inland within the Caltanissetta basin. |
From a distance it resembles the White Cliffs of Dover, England's most recognizable landmark along the southeast coast. Brilliant white in the sun, both are marine in origin composed of calcium carbonate, but their genesis and stratigraphy differ. More homogeneous morphologically, the Cliffs was deposited in a passive continental margin setting, while the Staircase, as we shall see, is far more complex, deposited in the foredeep of an active collisional and ongoing tectonic regime.
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| Impressive and Dramatic, the White Cliffs of Dover Stand Watch Over the English Channel |
The Cliffs are a 100 million year-old, Cretaceous-age, fairly easily-pulverized, silica-speckled chalk, a powdery form of limestone. Formed under relatively deep marine conditions, it consists of coccoliths (settled seafloor remains of single-celled, shelled marine algae). In contrast, the Staircase, also abyssal, was deposited in the Early Pliocene some 65 million years later and constructed of hard, fine to very fine-grained, detrital limestone (transported, settled and lithified skeletal fragments).
Also referred to as a calcarenite (a sandstone-equivalent), the Staircase's limestone is combined with marl, a siliciclastic (weathered silicate rock) seafloor sediment formed from mud and clay. The marly-limey stew imparts a creamy, faint beige hue to the Staircase compared to the White Cliff's brilliant white. Upon close investigation, rhythmicity (repetition in the bedding) indicates an astronomical process that was operational during deposition, in part, the subject of this post.
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| The Undeniable Allure of Scala dei Turchi from Above "How do I get down there from here?" |
If Italy is in the shape of a boot, then Sicily is a three-sided soccer ball being kicked at the narrow Strait of Messina. The island is actively tectonically developing along the roughly E-W boundary of the converging African and Eurasian plates, the Calabrian arc in particular.
It's a rather unique subduction zone, where a segment of the elongate continent-continent collisional interface makes a swooping bend or arcuate front. Although it's one of the shortest slab segments in the world (<150 km), its geodynamics are very complex and only partially understood, hence actively debated. The same can be said for the tectonic puzzle of the Central and Greater Mediterranean.
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| Relief Map of Sicily in the Central Mediterranean The Staircase (arrow) lies along the southeast coast. The island is mostly hilly and mountainous in a belt across Calabria and North Sicily. It's bordered by the Tyrrhenian sub-Sea to the north and Ionian to the east. Sicily's strategic location has made it a melting pot for the various ethnic groups and civilizations that sought its shores and natural resources. Go there: 37°17′24″N, 13°28′22″E. |
The island's contemporary formation began some 80 million years ago in the Late Cretaceous, when north-migrating African continental crust took a subductive deep-mantle dive and is still doing so. For now, let's just say Sicily formed along the boundary that links the African Maghrebides range and the Southern Apennine chain of mainland Italy across the Calabrian accretionary wedge, which is heaped up African seafloor onto the edge of the overriding Eurasian plate. It makes Sicily a mountainous place of exceptional beauty and accounts for volcanism on and around the island.
Sicily's triangular geographical shape is a consequence of tectonic evolution and accounts for its Greek name Thrinakia, "island of the three capes", and Roman name Trinacrium. It's derived from tinacria, a religious symbol used in the 8th century meaning "star with three points." It's also a triskelion, a triple-spiral motif used by various European civilizations for some five to six thousand years. Geology and history. Ever inseparable.
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| Pottery found in nearby Palma di Montechiaro, Sicily Probably produced near Gela at the end of the Seventh Century BC, the clay vessel displays the pattern of the triquetra in Latin (triskeles in Greek). While produced under the influence of Rhodinian models, the design reflects the personality of the local master craftsman. From Museo Archeologico Regionale di Agrigento |
The highly recognizable motif is on the Sicilian flag and on pottery, tee-shirts and mugs on-sale everywhere. Three stalks of wheat for fertility surround the head a winged Medusa (a Gorgon with snakes for hair that is a mythical Greek creature in literature with the power to turn viewers to stone) that is attached to three bent, running legs. They're suggestive of rotation and to some, symbolize Sicily's three sides, shores and capes.
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| The Flag of Sicily As for the flag's red and yellow colors, they signify Sicily's founding cities of Palermo and Corleone that united against French occupation in 1282. By the way, Homer alluded to Thrinakie in the Odyssey because of Sicily's shape, and Dante Alighieri in Paradise of the Divine Comedy described Sicily as "the beautiful Trinacria." It's all related to geology and tectonics. It's what makes Sicily so unique and so appealing on every level. |
A "MIDDLE OF THE EARTH" MARGINAL SEA
The Mediterranean is a marginal sea versus an epeiric or epicontinental one that is relatively shallow within a continental plate that floods during periods of high global sea level. The modern-day Hudson Bay and the long-gone Cretaceous-age Western Interior Seaway of central North America come to mind.
Marginal seas are land-locked or nearly so. The Mediterranean is open to circulation from the Central Atlantic through the narrow and shallow Strait of Gibraltar between Spain (of the Eurasian plate) and Ceuta-Morocco (on the African plate). The tectonic history of the inter-oceanic causeway is controversial. Whether it was restricted or completely closed, it has affected the sea's saline evolution, basinal depositional history and, as a consequence, Scala dei Turchi.
Thought to have been a desiccated hypersaline marginal sea or even a vast canyon-incised evaporite-floored desert as recent as six million years ago, the modern sea has a saline content (38 ppt) that's higher than that of the world's open oceans (34 to 36 ppt). Rather than being a holdover from its hypersaline past, it's due to a high evaporative rate with the only source of recharge from the strait, precipitation and fluvial sources, which replenishes the sea every 250 millions years or so.
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| A tiny beach-goer at the Staircase is not sure what to make of all those hats. |
Another distinction, marginal seas are typically deeper than epeiric ones and contain tectonically-derived submarine ridges such as the Sicily Sill (actually two) that played a role in the sea's evolution between Sicily and Tunisia-Sardinia. As such, the Mediterranean has an average depth of 1,500 km (4,900 ft) but plunges to 5,267 m (17,280 ft) in the Ionian Sea (a region of intense tectonic debate).
Lastly, the marginal Mediterranean Sea resides between two converging plates versus epeiric seas that are typically on a single continental plate. Eventually, they will squash the Mediterranean Sea out of existence, that is consume it tectonically. Appropriately, the Romans called it Mare Mediterraneum, which literally means "sea in the middle of the Earth" in reference to Europa and Africa. They were astute geographically in spite of their lack of knowledge tectonically.
A UNESCO PARADISE AND GEOLOGICAL DISNEYLAND
Slightly smaller than Massachusetts at 9,927 sq km, Sicily is the largest and most densely populated Mediterranean island. In addition to friendly Sicilians, spectacular landscapes, glorious cuisine and sumptuous wine, and magnificent architecture and art, it boasts an astounding seven UNESCO World Heritage sites based on historical, cultural, artistic and natural significance.
The two natural ones are volcanological and related to the convergence of Africa and Eurasia: Mount Etna and the Aeolian Islands. The latter are in the Tyrrhenian Sea off Sicily's north coast. It's a seven island, half-dozen seamount (submarine volcano) island arc (ribbon-like chain of oceanic volcanoes formed from the subduction of an oceanic plate). Most famous are Vulcano and Stromboli that lend their names to types of eruptions that are short and violent and more explosive, respectively.
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| Mount Etna from the Greek Amphitheater atop Taormina Shrouded in a seemingly persistent, dense mist, a long whitish plume of gaseous vapor or brownish plume of ash continuously wafts to the east from stratovolcano Etna's multi-cratered summit. With a classic steep-sided, conical shape and basal diameter of 40 km, it's the largest and highest volcano in Europe over 3,000 meters. A geological post on Etna is forthcoming. |
Currently 3,329 meter-high and rising, Mount Etna has been making the news regarding the potential catastrophic collapse of the east flank into the Ionian Sea. The event would destroy towns along the heavily populated coast and trigger a massive tsunami. Etna is the largest volcano in Europe and one of the world's most continually active. Its origin is more enigmatic than that of the Aeolian Islands (to be discussed in a forthcoming post).
ANCIENT LEGEND OR HISTORICAL FACT?
Scala dei Turchi is named after feared Saracen marauders or "Turks", a generic term in Sicilian dialect for Islamic peoples from nearby North Africa. Purportedly, having moored their ships offshore in the early Middle Ages (284 to 1000), they repeatedly ascended the steps and ravaged and looted coastal villages.ANCIENT LEGEND OR HISTORICAL FACT?
Although some question the story's historical accuracy, Turkish invasions along Sicily's coast are well documented and highly conceivable at the Staircase given the island's location at the "Crossroads of the Mediterranean" and proximity to Africa.
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| The Uplifted, Tilted and Differentially Eroded Steps of the Staircase The steps formed from differential erosion as the Staircase was tectonically uplifted and tilted from the sea floor. The cliff face was acted upon by sea level that changed under the influence of glacio- and tectono-eustasy. The prominent terrace or platform formed during a wave-stand that existed during transgression (rising) or recession (falling) sea level. Sun worshipers and tourists vastly outnumber geologists. |
In fact, due to its location, Sicily has experienced 13 dominations beginning with three prehistoric tribes (Sicani, Sicels and Elymians) that continued with almost three thousand years of occupations and conquests by Phoenicians from Carthage, Greeks, Romans, Germanic Vandals, Gothic Ostrogoths, Byzantines, medieval Arab Saracens, French Normans, Anglo-French Angevins, Spanish Aragonese, French Bourbons and modern Italians. As a result, Sicily is a melting pot of ethnic, cultural and culinary diversity, which is evident in its architecture, art, music, cuisine, dialect and people.
UNESCO LISTING APPLIED FOR
Realmonte and its Staircase are a stone's throw from the ancient Greek archaeological site of Valle dei Templi (Valley of the Temples) in Agrigento, UNESCO listed in 1977. Known as Akragas almost 3,000 years ago, it was one of many independent Greek city-sates in southern Italy collectively referred to as Magna Graecia during classical times.
Realmonte applied for listing of the Staircase along with the Roman site of Villa Aurea in spite of the fact that the beach is privately owned, although open to the public. Regardless of inclusion, the Staircase is extremely popular, highly visited and listed in enumerable sightseeing guidebooks and geological trip guides. It makes a perfect day at the beach with some great geology for extra measure. Go there!
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| The Temple of Concordia It's one of seven Doric temples in the Valley of the Temples in the ancient Greek city of Akragas in modern Agrigento, which by car is only a few kilometers from the Staircase on the coast. The "valley" is a misnomer and is actually a plateau. It's an outstanding example of Magna Graecia, a group of independent ancient Greek cities on the southern Italian coast. The site rests on the Pliocene-Pleistocene Agrigento section of the Monte Narbone formation that overlies Miocene-age marls of the Trubi formation. |
SUPERCONTINENT UNDER CONSTRUCTION
Our global geologic story begins in Late Proterozoic time when supercontinent Rodinia amalgamated (~1.0 Ga) from all-known landmasses and diachronously fragmented apart (~0.75 Ga). By the early Paleozoic, the event spawned the megacontinents of Laurentia, located equatorially, and massive Gondwana, sprawling across the Southern Hemisphere and South Pole. In cyclical supercontinental succession, which is thought to have occurred every 300 to 500 million years, they re-united to form Pangaea by the late Paleozoic.
Its unification progressed in increments, first with the accretion of several peri-Gondawnan superterranes to Laurentia, then Laurussia (Laurentia, Greenland and Europe) and finally to Laurasia (Laurussia and northern Asia). Pangaea was completed with the arrival of the massive Gondwana continent. Today, peri-Gondwanan remnants are scattered across the continents of the circum-Atlantic domain subsequent to Pangaea's fragmentation and opening of the Atlantic Ocean. The same can be said of the many Tethyan seas, all Mediterranean forerunners that opened and closed concomitant with Pangaea's geologic history.
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| East Hemispheric View of the Early Paleozoic World Beginning in latest Neoproterozoic-Cambrian time (by ~490 Ma), the elongate Avalonia-Cadomia-Serindia superterrane was the first of a compendium of peri-Gondwanan terranes to detach from the northern margin of African or South American Gondwana. The event opened the Proto-Tethys Ocean (Eastern Rheic Ocean). By the Late Silurian (by ~440 Ma), it converged on Laurentia and Baltica, as the Iapetus Ocean closed and the Rheic opened. At ~420 Ma, the Hun superterrane had separated from Gondwana. In this manner, Pangaea formed by terrane accretion at the expense of intervening seas that opened and closed. Modified from Stampfli, 2002. |
The first terrane to separate from Gondwana in latest Neoproterozoic-Cambrian time was
Avalonia-Cadomia-Serindia (above). The ribbon-like volcanic island arc initiated a pattern of rifting from the northern margin of Gondwana that would dominate the geodynamic evolution of every Tethyan sea in the Phanerozoic by drifting trans-equatorially across a body of water that closed in its path as a new one opened in its wake. The superterrane and those that followed variably attached to the eastern margin of Laurentia and southern margin of Eurasia (formative Europe and Asia, the northern part of Pangaea).
MAGICALLY APPEARING AND DISAPPEARING TETHYAN SEAS
Pangaea's construction occurred through most of the Paleozoic. It was a ~300 million year, multi-phasic, protracted affair that added crust to the supercontinent's growing mass as mountain belts were built and ocean basins opened and closed. The process is represented in the Wilson Cycle of Canadian geophysicist and geologist J. Tuzo Wilson in 1966. It's one of the great unifying theories in geology. The Staircase, the island of Sicily and the Mediterranean Sea are products of that incredible process!
As a newly-formed terrane (distinctive crustal block) rifts from a continental margin and begins to drift, a new ocean basin gradually opens and widens. The ocean, caught in the tectonic path of the terrane, progressively closes via subduction (crustal descent) of oceanic lithosphere. Convergence of the terrane with another results in the ocean's demise and leaves remnants preserved within the suture. Again and again, the cycle repeats, opening and closing ocean basins with the formation and accretion of each new terrane.
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| The Wilson Cycle of Opening and Closing of an Idealized Ocean Basin Each cycle (A) begins with a craton, an old, stable continent with low relief. Rifting (B) fragments it and opens a new ocean basin, as new continental plates drift apart (C). Closing (D) begins when a subduction zone forms within the basin that faces either direction. It's consumed (E) as it subducts beneath an adjacent oceanic basin (forming a volcanic island arc) or beneath the buoyant lithosphere of a continent (forming a cordillera volcanic mountain belt). Basin consumption (F) ends the cycle with continental collision and uplift. All that's left is erosive peneplanation and repetition of the cycle (A). Modified from L.S. Fichter, JMU. |
THE SEAS OF THE TETHYAN DOMAIN
As the Avalonia superterrane rifted from Gondwana, the Iapetus Ocean (between Rodinia and the approaching terrane) closed while the Rheic and Proto-Tethys Oceans (or "Eastern Rheic) opened between the drifting arc and trailing Gondwana. Named after Tethys, the mythological Greek goddess of the sea and daughter of Uranus and Gaia, it was the first body of water in the Tethyan lineage that spanned a half billion years!
In punctuated succession, rifting of the Hun superterrane (parts of southern Europe and Asia devastated by Attila) in the Devonian opened the Paleo-Tethys between it and northern Gondwana. It was followed by the Permo-Triassic-age Cimmerian superterrane (parts of Turkey, Iran, Afghanistan, Tibet and SE Asia) that opened the Neo-Tethys. A new Tethyan sea formed as an old one closed in concert with the inception and accretion of each peri-Gondwanan superterrane during the Paleozoic!
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| East Hemispheric View of the Middle Paleozoic World By ~400 Ma, the Hun superterrane had rifted from Gondwana's northern margin, as the Rheic Ocean closed and the Paleo-Tethys to the south opened. As with previous Gondwana-derived terranes, the Hun converged on Laurussia (Laurentia plus accreted Scandinavian Baltica). By ~300 Ma, the Rheic was consumed, and Gondwana had collided with Laurasia (Laurussia and acquired Eurasia that included Siberia and North China) to complete Pangaea with the Paleo-Tethys open to the east. At ~260 Ma, the Cimmeria terrane detached from Gondwana and opened the Neo-Tethys to the south. Modified from Stampfli, 2002. |
The collision of massive Gondwana in the south and Laurasia (Laurentia + Europe and northern Asia) in the north completed the formation of Pangaea. Consisting of most of the world's landmasses in the late Paleozoic and earliest Mesozoic, it sprawled nearly pole to pole and was C-shaped to the east. The Panthalassic Ocean (Proto- or Paleo-Pacific Ocean) bathed the entire globe as the Paleo-Tethys Ocean, with which it communicated, swirled within Pangaea's crescentic embrace.
Of genetic interest: The debate rages on over Pangaea's shape, why and when it broke apart, from where and whether a sub-lithospheric superplume was involved, whether it involved plume-less, shallow lithospheric processes or if peripheral tensional stresses acting on pre-existing suture zones tore it asunder. Similarly, hypotheses flourish as to which ocean's demise, Paleo-Tethys or Panthalassic, triggered Pangaea fragmentation.
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| East Hemispheric View of Pangaea in the Late Early Permian (~280 Ma) The late Paleozoic was a time of major plate reconfiguration that culminate with the formation of Pangaea. During the Permian, the opening of the Neo-Tethys was coeval with a major dextral rotation of Laurasia relative to Gondwana in which Africa becomes situated south of Europe or Asia and South America is placed south of Europe or North America depending on the model used. Eventually, an E-W trending trans-Pangaean seaway connected the Paleo-Tethys to the global Panthalassic Oceans. Modified from Stampfli, 2002. |
THE BIRTH OF NEW PLATES AND A NEW OCEAN
Long-lived Pangaea began to disassemble in the Late Triassic after some 100 million years of unification. Diachronous seafloor spreading from the Mid-Atlantic Ridge began to open the Atlantic Ocean, spawning the diverging continents of the Atlantic domain - North and South America in the West Hemisphere and Eurasia and Africa in the East.
Drifting of the Cimmeria terrane in the Permian closed the Paleo-Tethys as the Neo-Tethys Ocean (proto-Mediterranean or just Tethys) opened. Beginning in the Late Triassic, as the Atlantic Ocean began to open, the Neo-Tethys began to widen scissor-like to the east, as the Paleo-Tethys descended beneath Asia (below).
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| East Hemispheric View of the Mesozoic World At ~220 Ma, Pangaea was initiating break-up after some 160 million years of existence. The Paleo-Tethys is fully consumed via convergence of Cimmeria with Eurasia concomitant with expansion of the Neo-Tethys. In the Late Triassic to Early Jurassic, the Alpine Tethys (aka Penninic) has opened connecting the Central Atlantic with the opening western Neo-Tethys. In the Cretaceous, Central and South Atlantic began to open between South American and African Gondwana. Neo-Tethyan rifting began to tear India from the eastern margin of Africa. Northward drift and counter-clockwise rotation was beginning to drive the African plate toward the Eurasian plate, further narrowing the Neo-Tethys. Modified from Stampfli, 2002. |
By the Late Cretaceous with Gondwana independent from Laurussia, the Central Atlantic and Neo-Tethys were confluent. With the Paleo-Tethys fully-consumed, the Neo-Tethys nearly fully-formed, the Alpine-Himalayan mountain chain uplifting across southern Eurasia, and Pangaea almost fully-disassembled, Africa was finally separated from South America.
Smaller and tectonically controversial, the Alpine Tethys Ocean (Western Tethys) opened E-W along the Central Atlantic-Neo-Tethys equatorial axis. The Alpine and Neo-Tethys were the third and fourth Tethyan seas in the progression and, in a sense, precursors of the modern Mediterranean!
AFRICA TAKES ON EURASIA
Once independent from Gondwana, the African plate began to rotate counter-clockwise and head toward the Eurasian plate. As convergence progressed, the Atlantic continued to open as the Neo-Tethys became entrapped Wilson-style. The event has dominated the evolution of the ocean ever since, although its west and east histories differ markedly.
Convergence of the two plates formed an E-W elongate Africa-Eurasia plate boundary at the interface. The complex morphology of the Mediterranean region is reflected in the number of deep back-arc sub-basins, arcuate fault-and-thrust belts, extensional and transtensional boundaries and a compendium of independent micro-plates that originated since the Late Cretaceous. Let's briefly focus on the evolution of the Western Tethys - the youngest part of the nascent Mediterranean Sea - as it pertains to Sicily and the Staircase.
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| Kinematic Tectonic Map of Africa and Eurasia and the Western Alpine-Himalayan Belt The map illustrates the level of complexity of tectonic boundaries in the developing Mediterranean Sea. The East Tethys records long-term and complex convergence between the Eurasian, Indo-Australian, and Pacific plates since Pangaea breakup. The West Tethys along the Africa-Eurasia boundary, germane to this post, is no less complex with subduction processes, arcuate compressive fold-and-thrust belts and deep back-arc extensional basins. From Wikimedia Commons and Woudloper. |
SICILY TAKES SHAPE - A GEODYNAMIC PUZZLE
No less complex than the Central Mediterranean in which it lies, Sicily developed along a small component of the African-Eurasian convergent boundary. It's a segment of the African Maghrebides mountain range and mainland Italy's Southern Apennines across the Calabrian accretionary wedge (accreted clastic sediments from the overriding Eurasian plate). Subduction, thrusting and back-arc extension that continues to the present gave rise to the curved Calabrian arc of Sicily.
The arc's three "collisional" components were derived from Eurasian and African plates and paleo-Tethyan elements include the Trapani-Peloritani mountain chain (across northern Sicily), the Hyblean plateau (a tableland of carbonate rocks in the southeast) and the highly complex Appennino-Maghrebian chain. The foreland basin system that developed and advanced with the front is critical to the formation of Scala dei Turchi.
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| Geologic Setting of Sicily and the Calabrian Arc In the Central Mediterranean, the Calabrian arc is a turning point along the roughly E-W Africa-Eurasia collisional plate boundary. To the east, slab rollback in the late Miocene resulted in sinking of Mesozoic-age Ionian oceanic crust, proposed to be the oldest worldwide, while to the north, the Tyrrhenian Sea is a back-arc basin. The Gela Nappe is the outermost and youngest thrust sheet that transported above the African-Pelagian Foreland along a south-facing arcuate front. The nappe represents the structural element of the Gela-Catania fordeep that originated from collapse of the northern margin of the Gela-Catania foreland following its emplacement. The foredeep is E-W elongated and filled with gravity-flow deposits from the Miocene onward. The basin is filled with Licata Formation clays followed by Tripoli pre-Messinian evaporites, "Calcare di Base" Messinian evaporites and gypsum and truncated by Trubi deep marine calcareous marls and marly limestones. Modified from Ghisetti |
ORIGIN OF A FOREDEEP
As the subducting African descended beneath the Eurasian plate, crustal thickening of the orogenic wedge induced by the Apennine-Maghrebian fold-and-thrust belt downwarped African continental lithosphere into an elongate and wide, multi-component trough or foreland basin to the south of Sicily. Nearest the front on the north, a foredeep (deep depozone of the foreland system) received late Miocene to Pleistocene continent-derived sands and marine-derived muds and limestones.
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| Schematic Orogenic Belt and Foreland Basin System Forelands typically consist of four depozones: thrusted wedge-top sediment zone; foredeep (a region of deep sediment parallel to the front of the thrust belt; forebulge (zone of flexural uplift); and a broad and shallow back-bulge (depozone of sediment often carbonates). Modified from DeCelles and Giles |
The sediments include a number of diverse lithologies and sub-members. Three at the Staircase are germane to this post that lie on pre-evaporitic Messinian deposits: the Gessoso-Solfifero Group of Messinian evaporites and Trubi Formation marly limes and limey clays and overlying Monte Narbone Formation marly clays. They formed during a period of major geological and marine biological change in the nascent Mediterranean Sea. We must digress to discuss the micro-plate partly responsible for these litho-entities.
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| Regional Geologic Map of Sicily, the Gela-Catania Foredeep and its Main Depocenters For orientation, identify the outlines of Sicily and Calabria. The foredeep's inner boundary is the arched front of the Gela Nappe along Sicily's South Coast. It's the most external and youngest thrust sheet of the Apennine fold-and-thrust belt, and its outer boundary is the rest of the Gela-Catania foreland. The nappe (recumbent fold) is advancing above the foredeep, which is split by tectonic-highs into three sub-basins: the Pina (A), Gela (B) and Catania (C) basins. In the early Pliocene, continental sedimentation prevailed that gave way to transgressive marine conditions of Trubi marls and clays and eventually deep-water deposits. |
GEO-GYMNASTICS OF A MOBILE MICROPLATE
As mentioned, a number of micro-plates formed in the Neo-Tethys as Africa rotated into and converged upon Eurasia in the late Cretaceous. In particular, the motion of the Iberian micro-plate (future Spain, Portugal, Corsica and Sardinia) played a key role in the evolution of the Mediterranean, the world's oceans and Scala dei Turchi locally.
Only partially understood, following the opening of the Atlantic, Iberia initially moved as part of the African and then Eurasian plate. It detached from Eurasia (at France) and, moving independently, variably rotated, left-lateral strike-slipped (fault parallel motion) and converged into a more recognizable position between colliding Africa and Eurasia.
In the process, it narrowed the oceanic gap between the two plates across the Betic and Rif gateways of the proto-Strait of Gibraltar- the only marine communication between the Atlantic and Mediterranean.
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| Progressive Closure of the Central Atlantic-Neo-Tethys Marine Corridor Iberia rifted from the Grand Banks of Newfoundland prior to the initiation of Atlantic seafloor spreading. Following opening of the Atlantic, Iberia rifted from France with the onset of spreading at the Bay of Biscay. Somewhere from the Middle Jurassic through Early Cretaceous (~130 ma) to the earliest Oligocene (~33 Ma), the independent Iberian micro-plate, at the expense of the widening eastern Central Atlantic Ocean, rotated into position between converging southwestern Europe and northwestern Africa. Modified from Rosenbaum. |
TRANSFORMATION OF A SMALL OCEAN INTO A VAST HYPERSALINE LAKE
By the end of the Miocene, Iberia had either entirely closed or more likely vastly diminished the inter-oceanic Central Atlantic-Western Tethys marine corridor between the converging plates. The restricted circulation is thought to have triggered the Messinian Salinity Crisis in the Neo-Tethyan Sea from ~5.98 to ~5.33 Ma.
Named after the northeast Sicilian city of Messina where an evaporative deposit is of the same age, the event was of immense geological, environmental and ecological proportions and provided the lithological environment for the Staircase's deposition. What process or mechanism may have closed the marine gateway?
Once thought to have been preposterous and still highly debated over several aspects that are controversial, the hydrologic event may been a consequence of Antarctica-induced, glacio-eustatic global sea level drop, although some deem a ~60 m drop to have been insufficient and not timed to the event itself. Others ascribe to Gibraltar arc uplift or tectonic slab-tear in the gateway that horizontally or vertically diminished or closed it. Secondarily, uplift may have created a rainshadow in the already-arid equatorial, paleo-climate that further exacerbated the crisis. Regardless of the hydrologic crisis trigger, it reflects a common theme - the interaction of tectonics, climate and sea level.
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| A Modern Analogue of Badwater Basin in Death Valley National Park in California The Mediterranean seafloor is thought to have been converted to a number of hypersaline lakes or a patchy brine-rich desert analogous to the salt pan floor of Death Valley. A similar endorheic condition (internally draining) may have prevailed with fluvio-lacustrine systems intermittently fed by African and Eurasian watersheds to the north and south. |
AFFECTS OF HYDROLOGIC CONSTRICTION
The ensuing evaporative drawdown resulted in brine saturation that converted some 2.2 million sq km of Mediterranean seafloor into a vast Dead Sea-style, hypersaline lake below Atlantic sea level with possibly a segregated mosaic of subbasins separated by structural highs or more dramatically, a Death Valley-style, brine-rich desert. Confirmation comes from deeply-buried Messinian-age evaporates on the seafloor that have been recovered by the deep-sea research drilling vessel Glomar Explorer in the 1970s.
The extreme desiccation radically affected water chemistries globally with a reduction in salinity (~6%) and depth (a few meters). Within the Mediterranean basin, drawdown created a larger than Grand Canyon, fluvially-incised system of Miocene-age paleo-canyons on the Mediterranean seafloor. Free from the immense water-load, the seafloor isostatically rebounded in a manner similar to that of the landscape when a glacier melts.
In concert with the seafloor, river beds of the paleo-Rhone, Nile and others rebound and eroded below Atlantic sea level as they progressively became desiccated and filled with precipitated salts. The concept of a V-shaped gorge with a cascade of waterfalls in Egypt from the Sudan to the delta cut by a Messinian Nile filled with evaporites buried beneath Cairo conjures up an incredible image as does the Mediterranean converted to a briny desert wasteland or system of interconnecting salt lakes below sea level.
A CRISIS OF DESSICATION AND ENVIRONMENTAL DETERIORATION
The ensuing evaporative drawdown resulted in brine saturation that converted some 2.2 million sq km of Mediterranean seafloor into a vast Dead Sea-style, hypersaline lake below Atlantic sea level with possibly a segregated mosaic of subbasins separated by structural highs or more dramatically, a Death Valley-style, brine-rich desert. Confirmation comes from deeply-buried Messinian-age evaporates on the seafloor that have been recovered by the deep-sea research drilling vessel Glomar Explorer in the 1970s.
The extreme desiccation radically affected water chemistries globally with a reduction in salinity (~6%) and depth (a few meters). Within the Mediterranean basin, drawdown created a larger than Grand Canyon, fluvially-incised system of Miocene-age paleo-canyons on the Mediterranean seafloor. Free from the immense water-load, the seafloor isostatically rebounded in a manner similar to that of the landscape when a glacier melts.
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| Artist Conception of a Desiccated Paleo-Mediterranean Sea During the Messinian Salinity Crisis With the inter-oceanic gateway between the Central Atlantic and West Tethys constricted or completely closed and intensified by Antarctica-induced glacio-eustasy, the sea basin was converted to a rebounded seafloor punctuated by hypersaline subbasins and incised by fluvially-generated paleo-canyons. Mammalian remains (camel, hippo, rodent and canine) at Venta del Moro (Spain) indicate the presence of an ephemeral land bridge (inset) in the paleo-Strait of Gibraltar between Africa and Iberia just before 6.2 Ma. From Wikimedia and Paubahi |
In concert with the seafloor, river beds of the paleo-Rhone, Nile and others rebound and eroded below Atlantic sea level as they progressively became desiccated and filled with precipitated salts. The concept of a V-shaped gorge with a cascade of waterfalls in Egypt from the Sudan to the delta cut by a Messinian Nile filled with evaporites buried beneath Cairo conjures up an incredible image as does the Mediterranean converted to a briny desert wasteland or system of interconnecting salt lakes below sea level.
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| Artist's Conception of a Hypersaline Paleo-Dead Sea Analogous to the hypothesized Messinian Grand Canyon of the Mediterranean, paleo-shorelines record a desiccating body of water much as the paleo-Mediterranean is thought to have experienced during the Messinian Salinity Crisis. From NPS and Lisa Lynch |
The dessication event and evaporite beds are thought to have formed in three stages over a geologically short ~500 Ka beginning with precipitation of gypsums, halites and K-Mg salts within shallow subbasins on the Tethys seafloor. Halite starts to precipitate when the remaining solution is reduced to 10% of the original seawater volume, which implies a dramatic sea level drop in the second stage. The crisis peaked with large-scale fluctuations that severely diminished the size of the Mediterranean, transforming it into a vast hypersaline lake referred to as Lago Mare.
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| Seafloor Map of Messinian Evaporates in the Mediterranean Region A, A trilogy of deeper tripartite cycles of deposits of a halite unit sandwiched between two gypsum units, undifferentiated where they are indistinguishable. B, Inset showing the main evaporative depocenters (dotted areas). Modified from Manzi. |
TONS OF BURIED SANDWICHED SALT
Although the precise cause of the Salinity Crisis is actively debated, there appears to have been a complex interplay between tectonic, glacio-eustatic control and evaporative drawdown. A million cubic kilometers of seafloor evaporates with a 1,500 meter-thickness that accumulated in a geologically brief period of 700 ka reflected a a three to ten-fold increase in normality lie on the Mediterranean seafloor. Curiously, seafloor salt is also found subaerially, albeit buried, in Central Sicily. How did it get there?
External to the advancing orogenic front and part of the Apenninic-Maghrebian foredeep, the Caltanissetta basin (CB on map above) was thrust upward during plate convergence. The depression is a wedge-top basin (thrust-top or piggyback) transported between two thrust stacks. The basin corresponds to the main depozone of the foreland system. It provides a nearly complete record of the evaporitic crisis and the Trubi carbonatic cycle that followed. It serves as confirmation that Sicily was once a shallow Neo-Tethyan subbasin that uplifted during plate convergence.
SICILIAN SALT DIRECTLY FROM THE SEA
Incidentally, long before mining of Sicilian salt on land, it was obtained directly from the sea along 30 km of the West Coast between Trapani on the north and Marsala on the south. Beginning with the Phoenicians 2,700 years ago, it was used as a method of trade, currency and means of preserving and flavoring food.
The "White Gold" was extracted from seawater by progressively concentrating it in a series of interconnecting shallow salt pans (a concentration basin) via solar evaporation. It was an effective but slow process facilitated by the Mediterranean's high salt content, Sicily's shallow coast perfect for salt pans, a near-constant and intense summer sun, and scorching and constant African winds that powered windmills to pump sea water from basin to basin and grind extracted salt into a usable form.
Although the precise cause of the Salinity Crisis is actively debated, there appears to have been a complex interplay between tectonic, glacio-eustatic control and evaporative drawdown. A million cubic kilometers of seafloor evaporates with a 1,500 meter-thickness that accumulated in a geologically brief period of 700 ka reflected a a three to ten-fold increase in normality lie on the Mediterranean seafloor. Curiously, seafloor salt is also found subaerially, albeit buried, in Central Sicily. How did it get there?
External to the advancing orogenic front and part of the Apenninic-Maghrebian foredeep, the Caltanissetta basin (CB on map above) was thrust upward during plate convergence. The depression is a wedge-top basin (thrust-top or piggyback) transported between two thrust stacks. The basin corresponds to the main depozone of the foreland system. It provides a nearly complete record of the evaporitic crisis and the Trubi carbonatic cycle that followed. It serves as confirmation that Sicily was once a shallow Neo-Tethyan subbasin that uplifted during plate convergence.
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| Cross-section of Deformed Saline Lens in the Realmonte Mine Less than a kilometer north of the Staircase in the Caltanissetta basin, the mine's halite deposits record the evaporative crisis. Astronomical forcing produced the cyclicity, while tectonic plate collision drove the basin on-land and synclinal deformation that produced the artistic display that was uncovered during mining. |
SICILIAN SALT DIRECTLY FROM THE SEA
Incidentally, long before mining of Sicilian salt on land, it was obtained directly from the sea along 30 km of the West Coast between Trapani on the north and Marsala on the south. Beginning with the Phoenicians 2,700 years ago, it was used as a method of trade, currency and means of preserving and flavoring food.
The "White Gold" was extracted from seawater by progressively concentrating it in a series of interconnecting shallow salt pans (a concentration basin) via solar evaporation. It was an effective but slow process facilitated by the Mediterranean's high salt content, Sicily's shallow coast perfect for salt pans, a near-constant and intense summer sun, and scorching and constant African winds that powered windmills to pump sea water from basin to basin and grind extracted salt into a usable form.
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| From Seawater to Salt From 750 meters atop the ancient Greek city of Eryx and Sicilian hilltop town of Erice, a maze of interconnecting salt pans fills the harbor of Trapani on Sicily's West Coast. The tiny conical dots are no-longer-used windmills. Their cogwheels and gears pumped salt from basin to basin as the Sicilian sun gradually converted Mediterranean seawater into 'white gold.' Please watch for my upcoming post on the Geology of Sicilian Sea Salt. |
Incidentally, the waters of the Mediterranean are getting saltier and warmer at least for the last 40 or 50 years. It appears to be heating up at ~0.015 to 0.04°C per decade. The evaporative basin, calculated for net gain and loss from evaporation versus and precipitation, river runoff and gateway inflow and outflow, loses 50 to 100 cm/yr of freshwater. It was first thought related to damming of the Nile during the 1960s. Models suggest that evaporation is increasing in response to the warming climate and the changing water balance.
RAPID REFLOODING OR PROGRESSIVE REFILLING FORMS A NEW SEA
RAPID REFLOODING OR PROGRESSIVE REFILLING FORMS A NEW SEA
Beginning ~5.5 Ma, the crisis intermittently rejuvenated when climate change initiated fluvial run-off from the African and European mainland. It ultimately ended ~5.33 Ma in the Early Pliocene, when the gateway permanently re-opened as the present-day Strait of Gibraltar. Whether cataclysmically or in pulses, flowing directly or cascading over waterfalls, Atlantic waters re-entered the western and then eastern Tethys across the Sicily Sill in the Strait of Sicily between Sicily and Tunisia.
In what may have been the largest flood in the geological record, the outburst restored marine conditions and oceanic exchange in a phenomenal Zanclean Mega-Flood in the Zanclean-age of the Early Pliocene in a hypothesized period of months to two years. Breach causative theories include tectonic uplift in the gateway (due to lithospheric slab tear and rollback beneath the Gibraltar Arc), subsidence (collapse of a pull-apart graben from extension) or regressive fluvial erosion (headward river-incision induced by base-level drop in Tethyan sea level).
In what may have been the largest flood in the geological record, the outburst restored marine conditions and oceanic exchange in a phenomenal Zanclean Mega-Flood in the Zanclean-age of the Early Pliocene in a hypothesized period of months to two years. Breach causative theories include tectonic uplift in the gateway (due to lithospheric slab tear and rollback beneath the Gibraltar Arc), subsidence (collapse of a pull-apart graben from extension) or regressive fluvial erosion (headward river-incision induced by base-level drop in Tethyan sea level).
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| Artistic Interpretation of Zanclean Flooding of the West Tethys from the Central Atlantic Viewed from the southwest across the breached proto-Gibraltar Strait, Atlantic waters spilled over two thresholds, the Camarinal and Spartel Sills, in a mega-outburst flood between Africa and the Iberian Peninsula of Europe. Notice the Sicily Sill barrier between the West and East Tethys. Image from Roger Pibernat under supervision of Daniel Garcia-Castellanos and Wikimedia Commons |
THE STRAIT OF GIBRALTAR
The deluge created the Mediterranean at a theorized depth of ~10m/day and flow rate of 1,000 times the Amazon River and decreased global sea level ~10 meters and salinity of the world's oceans, however, the effect that the crisis had on global climate has yet to be fully explosed.
To this day with the Iberian micro-plate affixed to the Eurasian plate and open at the Strait of Gibraltar, the Mediterranean is more saline than the Atlantic - 38 or more ppt (parts per thousand) versus 34 to 36, enhanced by a high evaporative rate that exceeds precipitation and fluvial recharge.
To this day with the Iberian micro-plate affixed to the Eurasian plate and open at the Strait of Gibraltar, the Mediterranean is more saline than the Atlantic - 38 or more ppt (parts per thousand) versus 34 to 36, enhanced by a high evaporative rate that exceeds precipitation and fluvial recharge.
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| Western View Digital Elevation Model of the Modern Strait of Gibraltar Viewed from the west, the strait is 58 km long and narrows to 13 km in width between Point Marroquí, Spain, and Point Cires, Morocco. C, Ceuta; G, Gibraltar; TN, Tanger; TR, Tarifa. From Loget, 2006. |
Should the oceanic gateway re-close as plate convergence progresses, which is a likely occurrence, it could re-isolate the Mediterranean from Atlantic inflow and re-trigger desiccation and a rise in salinity in less than a theorized 1,000 years. Regardless, the Mediterranean basin will meet its demise in a Wilson-style, collisional oceanic closing, when Africa and Eurasia join as a single mega-continent.
IT'S ALL IN THE RHYTHM, ASTRONOMICAL THAT IS
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| Three Dominant Cycles of the Earth's Orbit Attributable to Serbian geophysicist and astronomer Milutin Milanković in the 1920s, eccentricity (deviation from orbital circularity) varies primarily due to the gravitational pull of Jupiter and Saturn, obliquity (axis tilt) that is largely a seasonal affect and precession (axis wobble) related to solar orbit cumulatively affect climate by placing parts of the planet closer or further from the sun. The periods vary from 100,000 and 400,000, 41,000 down to 23,000 and 26,000 years, respectively. It's also influenced by paleography and paleocurrents. Image from K. Cantner, AGI. |
Celestial orientations of the Earth and Moon about the Sun induce climatic oscillations that gravitationally affect stratigraphic deposition within the Neo-Tethys sedimentary basin. These parameters are omnipresent but are expressed on the landscape under certain conditions. In the nascent Mediterranean basin, it's due to the high sedimentation rate, the overall shallowness of the marginal basin and its nearly land-locked condition.
TRUBI RHYTHMICITY
As African-Eurasian convergence progressed and the Sicilian-Maghrebian fold-and-thrust belt developed, subsidence within the Gela-Catania foredeep provided accommodation space for the Trubi Formation in the Pliocene in concert with coeval Mediterranean reflooding and astronomical sediment forcing.
Astronomically induced variations in incoming solar radiation manifest as cooler-dry and warmer-wet phases of the climate that modify the composition of the sediment (cyclically bedded couplets called rhythmites) were deposited on the seafloor. Intensified precipitation and fluvial discharge during warmer-wet periods, when precession is at a minimum, promotes the formation of carbonate-poor layers, while carbonate-rich marls, during arid phases, form when precession is maximal. As a result, biological productivity varied in response to changes in astronomical parameters.
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| Side-by-Side Stratigraphy Cross-sections of the Caltanissetta Basin and Foredeep The construction of an astronomical timescale allowed the depositional and paleo-environmental processes that led to the crisis to be understood. It allowed correlation to other sequences in the Mediterranean and the Atlantic Ocean. It showed that the transition to evaporitic conditions occurred at 5.96 ± 0.02 Ma, synchronously between the western and eastern Mediterranean. Modified from Roveri et al |
Evaporative drawdown is not a uni-directional water level excursion, but precession-modulation of solar insolation assures a rise and fall of water levels at the precession- periodicity. The amplitude of the excursions is a function of changes in insolation from one cycle to the next and the evolving architecture of the foreland basin.
SCALA DEI TURCHI SEES THE LIGHT OF DAY
As plate convergence progressed in the Pleistocene, the foreland system experienced extensional and shear stress. The foredeep's sedimentary package of Miocene evaporitic sequences, overlying Pliocene chalky marls and overburden of mass gravity flows testify to slope instability as it was uplifted from the sea, tilted north ~30° and mildly deformed.
Extension flexure is recorded in a complex geometry that includes systematic sets of widely and evenly-spaced joints in a bedding plane-parallel, perpendicular and oblique orientation that are conjugate (formed together) and both linear and curvilinear. Joints and fractures not only provide an indication of the direction, cause, sequence and timing of force propagation but provide a plumbing system for the movement of ground water that enhances bedrock erosion.
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| Repetitive Deformational Joints in Lithological Trubi Sequences In addition to the effect of deformation, precession-related rhythmic sedimentary cycles of the Trubi are exposed with a grey-white-beige-white alternation. The influence of obliquity is recognized by marked alternations in the distinctness of beige layers in successive basic cycles. Eccentricity related cycles are visible in the weathering-profile of the cliff-exposure. |
EXHUMATION AND EROSION
Global glaciogenic sea level fluctuations during the ice ages of the Pleistocene variably exposed and submerged portions of the South Coast. Combined affects of erosion from wind, wave, tide and salts (haloclasty) differentially carved the Trubi's tilted beds into steps and broader wave-cut marine terraces during stillstands as the cliff face was exhumed and retreated to the north partly driven by stream incision.
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| On the Path Down to the Beach A ~20 meter-wide platform that formed during a marine stillstand provides a natural walkway. The Staircase's subaerial presence is testimony to vertical crustal movements that affected Sicily's southern coastal sector. Over time, sea level change has been in competition with coastal uplift during the Pleistocene and subsequent Holocene glacial-interglacial sea level fluctuations. Again, Capo Rossello looms in the distance. |
As mentioned, the marly-calcareous biogenic sequences of the Staircase unconformably overly hundreds of meters-thick of Messinian evaporitic sediments. The Trubi marks the end of the Messinian Salinity Crisis, first with uppermost continental runoff and ending with the return of open-marine (pelagic) conditions as tectonic convergence progressed within the developing foredeep and the Mediterranean reflooded.
Deep-marine conditions eventually shallowed-upwards to calcarenites (limestone with over 50% transported sand). Finally, the Trubi is para-unconformably overlain (juxtaposed sediments have remained parallel) with brownish-red, laminated marly clays (sapropels) of the Monte Narbone Formation (below) that has been astronomically calibrated as well.
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| The Upper Trubi-Basal Monte Narbone Formation Contact atop the Staircase Resting para-unconformably on the Trubi marls, evidenced by the irregular contact, the brownish-red Monte Narbone Formation consists of gray clays and white calcarenites that grade upwards into a coarse calcareous sandstone. The hemi-pelagic sediments of terrigenous origin formed on the continental shelf and versus Trubi biogenic material derived from the foredeep. Slumps, joints and faults are within the Trubi. |
OFFICIAL STRATOTYPE BOUNDARY
The Salinity Crisis laid a foundation of evaporites for deposition of the Trubi Formation. It marked the end of hydrologic desiccation and a return to normal marine conditions. Only 1.92 Ma in duration, it was a major geologic event to the extent that the Messinian age, the last time frame of the Miocene epoch ending in 5.33 Ma, was named after it and the town of Messina for its evaporite deposits. The Zanclean age of the earliest Pliocene records when reflooding occurred, named after Messina's Greek name Zanclea.
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| The Late Tertiary (Neogene) Messinian Miocene-Zanclean Pliocene Boundary Evaporite deposits are sandwiched in between deep marine sediments of Tortonian and Zanclean-age. Tuning has resulted in an age of 7.25 Ma for the base of the Messinian and of 5.96 Ma for the onset of evaporite formation during the Messinian Crisis. The officially accepted Miocene-Pliocene boundary is coincident with the base of the Trubi marls and Zanclean age, as defined in the Mediterranean at Eraclea Minoa (see below). Modified Wikipedia time scale. |
CHRONOSTRATIGRAPHY
The South Coastal exposure has been crucial in developing a timescale for the Miocene-Pliocene and the Pliocene-Pleistocene boundaries both locally and throughout the Mediterranean. Its age and many sequences were constrained with a combination of argon-argon radiometric dating, astronomical tuning and the use of biostratigraphic index fossils. The latter include planktonic (surface-floating) foraminifera (single-celled protists that settled on the seafloor) and nannofossils (unicellular algae that produce calcitic platelets that fell to the seafloor).
Neo-Tethyan micro-fauna greatly diminished in abundance during the restrictive marine conditions and became barren in response to the conditions of extreme hypersalinity, stagnation, sediment starvation. Subsequently, they dramatically diversified during the evolutionary outburst that followed in response to reflooding. In addition, chronostratigraphic correlations were also facilitated by magnetostratigraphy that registers reversals in the Earth's polarity calibrated to a geomagnetic timescale.
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| Calcareous Nanofossil Amaurolithus delicatus and Planktonic Foraminifer Globorotalia miotumida |
ERACLEA MINOA
Due to a far better preserved paleomagnetic signal, the official base of the Trubi is formally defined at the Eraclea Minoa, some 25 km west of the Staircase on the South Coast. It's defined by the International Commission on Stratigraphy (here) as the GSSP (Global Stratotype Section and Point) for the Messinian-Zanclean boundary.
The abrupt lithological transition at the exposure records the end of the hydrologic crisis at the top of the evaporites with catastrophic reflooding of the Mediterranean Sea and the resoration of marine conditions and a through-flowing connection between the Atlantic and Mediterranean. The lower part of the overlying Monte Narbone clays is also contained in the Eraclea Trubi section.
Eraclea Minoa is also of extraordinary aesthetic, archaeological and cultural value. The top of the cliff exposure is dominated by Greek era ruins from the sixth century BC. In fact, gypsum for construction was obtained from the Messinian sedimentary interval covering the transition from the gypsum to the Pliocene marine deposits.
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| The Lower Trubi Stepped Sea Cliffs at Eraclea Minoa Almost identical to Scala dei Turchi, Eraclea Minoa consists of lower Trubi marly limestones organized in quadripartite lithological cycles. Rhythmically bedded foraminiferal pelagic ooze and astronomically calibrated rhythmites constrain the base of the Trubi at Eraclea (the basal Rossello Composite Section), while Scala dei Turchi includes the middle Trubi (middle Rossello). The display is widely exposed in the nappes of central Sicily, in Calabria and cores from different parts of the Mediterranean seafloor. |
A CLOSE LOOK AT THE TRUBI
In addition to micro-fauna, macro-fauna of the foredeep includes bivalves, gastropods, ostracods, fish, corals and burrowing worms. They provide valuable evidence for the bathymetric conditions of the foredeep and a return of open marine conditions following re-flooding. Indirect evidence includes various ichnofacies (trace fossils of suspected organism) that provide paleo-environmental marine conditions such as water depth, salinity, turbidity and energy.
A recognizable example is Zoophycos, found in the Trubi's marly calcareous pelagic marine pelgic ooze. It's one of nine recognized marine archetypes identified on continental shelves and slopes such as found within the foredeep. It is thought to have been created by polychaetes (marine worms) while feeding, burrowing or helical swirling.
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| Zoophycos Ichnofacies in the Trubi Formation Ichnofacies in shallower water of the photic zone (uppermost sunlit region) are generally vertical (more secure in nearshore, high energy turbulent waters), whereas those in deeper are more horizontal and patterned, related to food abundance. |
Another is the presence of occasional cylindrical, branched trunk-like, knobby-surfaced structures that are firmly attached to the Trubi's surface. They may be a coral or fossilized invertebrate burrows within the sediment such as Thalassinoides, a branched trace fossil ichnogenus made by a number of marine organisms. Its resistance to erosion may be related to the diagenetic iron content. In addition, elongate cream-colored structures on and within the Trubi surface are reminiscent of infilled invertebrate burrows.
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| Invertebrate Fossil or Ichofacies? |
ON A FINAL NOTE
By official decree of the Commune of Realmonte, at Scala dei Turchi there's no collecting, removing blocks of limestone (assumedly for collecting), sunbathing on the steps, "covering one's body with mud derived from the white marl", "expelling physical needs" (use your imagination), boating or playing games (perhaps injurious physical ones). Fortunately, observational geology is not on the list.
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| Realmonte's Official Behavioral Regulations at Scala dei Turchi |
But a danger does lurk, if one gets too close to the cliff face.
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| Climber Beware Before complete lithification, soft sediment deformation affects the most ductile marls of the Trubi Formation. Structures such as clastic breccias, diapyr-like injections and thixotropic wedges of fluidized water-escape columns are discernible here and there. |
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| Scala dei Turchi by Floriana Quaini Please visit the artist here. |
SPECIAL THANKS
Much appreciation is extended to Dr. William B.F. Ryan, Doherty Senior Scholar and Professor and Earth Institute Affiliate at Lamont-Doherty Earth Observatory for his communications and forwarded articles regarding the Messinian Salinity Crisis. In addition, immense gratitude is expressed to my wife Diane and dearest travel companions cousin Hal and his wife Marti for generously and patiently acquiescing to my geological proclivities on this glorious beach day and countless others on our road trip through Sicily. Great company. Sicilian sun. Good food and wine. Magnificent geology. What a combination!
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| Cousin Hal and I Quaternary tectonic uplift, a consequence of ongoing African-Eurasian plate convergence, in concert with glaciogenic sea level fluctuations (global rise and fall during glaciation-deglaciation phases), affected Sicily's southern coastal sector. The morphological consequences to Scala dei Turchi on its astronomically-forced rhythmic-beds include differential erosion with remarkable horizontal continuity and larger wave-cut terraces formed during a stillstand. Out to sea, submerged Trubi marls on strike, demonstrate the same erosional sequelae. The north coast of Africa at Tunisia is only about 160 miles due west, although if you measure from Pantellaria, a small volcanic satellite island of Sicily in the Strait of Sicily, it's only 41 miles away! |
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| Hal and Marti Bracing Against the Wind on the Staircase's Precariously Smooth, Inclined Steps |
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| Unknowingly, Hal's Pose Demonstrates Deformation, Strike and Dip of the Staircase The highly polished surface and broad wavecut terraces of the lower exposure of the paleo-cliff is in contrast to the upper strata. The Staircase's subaerial disposition testifies to uplift and deformation generated by the advancing thrust of the Apenninic-Maghrebian front seen in the repetitive jointing and wrench-features in the foreground. A few large, displaced calcareous blocks are scattered on the terrace that formed at highstand. |
REFERENCES ON GLOBAL AND REGIONAL TECTONICS AND THE TRUBI FORMATION
• Atlas of Paleogeographic Maps (Mollweide Projection) by C.R. Scotese, PALEOMAP Atlas for ArcGIS, PALEOMAP Project, Evanston, IL, 2014.
• Diagenesis and Remanence Acquisition in the Lower Pliocene Trubi Marls at Punta di Maiata (Sountern Sicily): Paleomagnetic and Rock Magnetic Observations by J. Dinarès-Turell et al, Special Publication 151 of the Geological Society of London, 1999.
• Foreland Basin Systems by Peter G. DeCelles and Katherine A. Giles, Basin Research 8, 1996.
• Geodynamics of Collision and Collapse at the Africa–Arabia–Eurasia Subduction Zone – an Introduction by Douwe J.J. Van Hinsbergen et al, Geological Society of London, Special Publications 311, 2009.
• High-Frequency Cyclicity In the Mediterranean Messinian Evaporites: Evidence For Solar-Lunar Climate Forcing by Vinicio Manzi et al, Journal of Sedimentary Research, April 2013.• Imprint of Foreland Structure on the Deformation of a Thrust Sheet: The Plio‐Pleistocene Gela Nappe (Southern Sicily, Italy) by Francesca C. Ghisetti et al, Tectonics 28, 2009.
• Modeling the Magnitude and Timing of Evaporative Drawdown During the Messinian Salinity Crisis by William B. F. Ryan, Stratigraphy 5, 2008.
• On the Origin of the Strait of Gibraltar by Nicolas Loget and Jean Van Den Driessche, Sedimentary Geology 188-189, 2006.
• Paleozoic Evolution of Pre-Variscan Terranes: From Gondwana to the Variscan Collision by Gérard M. Stampfli, GSA Special Paper 364, 2002.
• Plio-Pleistocene Sedimentary Facies and Their Evolution in Centre-south-eastern Sicily: a Working Hypothesis by A. Di Grande and V. Giandinoto, EGU Stephan Mueller Special Publication Series, 1, 211–221, 2002.
• Reconstruction of the Tectonic Evolution of the Western Mediterranean since the Oligocene by G. Rosenbaum et al, Journal of the Virtual Explorer, June 2014.
• Relative Motions of Africa, Iberia and Europe during Alpine Orogeny by Gideon Rosenbaum et al, Tectonophysics 359, 2002.
• Structural Styles and Regional Tectonic Setting of the "Gela Nappe" and Frontal Part of the Maghrebian Thrust Belt in Sicily by W. Henry Lickor et al, Tectonics 18, 1999.
• Tectonic Evolution of Western Tethys from Jurassic to Present Day: Coupling Geological and Geophysical Data with Seismic Tomography Models by Maral Hosseinpour et al, International Geology Review 58, No.13, 2016
• Tectonic History of the Western Tethys Since the Late Triassic by Antonio Schettino and Eugenio Turco, GSA Bulletin 123, 2011.
• Tethyan Ocean by G.M. Stampfli, Geological Society London Special Publication, 2000.
• The African Plate: A History of Oceanic Crust Accretion and Subduction since the Jurassic by Carmen Gaina et al, Tectonophysics 2013.
• The Base of the Zanclean Stage and of the Pliocene Series by John A. Van Couvering et al, Epsiosdes 23, 2000.
• The Calabrian Arc: Three Dimensional Modelling of the Subduction Interface by Francesco E. Maesano et al, Nature, August 2017.
• The Evolution of the Tethys Region throughout the Phanerozoic: A Brief Tectonic Reconstruction by Fabrizio Berra and Lucia Angiolini, Petroleum systems of the Tethyan region: AAPG Memoir 106, 2014.
• The Formation of Pangaea by G.M. Stampfli, Tectonophysics 593, 2013.
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