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.
|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.
|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."
|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.
|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.
|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.
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.
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.
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.
|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.
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.
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!
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.
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.
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!
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.
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).
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.
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.
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.
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.
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.
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.
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.
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.
RAPID REFLOODING OR PROGRESSIVE REFILLING FORMS A NEW SEA
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).
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.
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
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.
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.
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.
First generation (E-W) folds affected the foredeep's late Miocene claystones and mudstones that is reflected in the axial trace and slumping of overlying Messinian evaporites prior to the deposition of the unconformable early Pliocene Trubi Formation. Second generation (N-S to NNE-SSW) folds that folded the first generation deformed the Trubi and its Plio-Pleistocene overburden. On a grander scale, convergence created an E-W syncline that deflected about a N-S, north-plunging anticline.
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.
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.
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.
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.
|Calcareous Nanofossil Amaurolithus delicatus and Planktonic Foraminifer Globorotalia miotumida|
Both biostratigraphic marker species for the
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.
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.
|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.
|Realmonte's Official Behavioral Regulations at Scala dei Turchi|
But a danger does lurk, if one gets too close to the cliff face.
|Scala dei Turchi by Floriana Quaini|
Please visit the artist here.
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!
|Hal and Marti Bracing Against the Wind on the Staircase's Precariously Smooth, Inclined Steps|
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