2013 Geology Posts That Never Quite Made It

Ancient West African Crust in Boston; Enigmatic Beach Sands of Florida; Living Fossils in Backbay; Cretaceous Oysters in New Jersey; Alpine Bogs in New Hampshire and a Precambrian River in Newton Center, Massachusetts

Every blogger knows the challenge. What shall I blog about next? What photos should I use? By the time the end of the year rolls around, there are always a few posts that never quite made it. And so, with this final post of the year, here they are from here and there. Please visit the same for 2012 here.

January
Flying High Above Boston’s West African Harbor Islands

Looking frigid and uninviting in mid-winter, Boston’s Harbor Islands are best explored during the summer months. The harbor is sprinkled with 38 of them, most designated as National Recreation Areas. Many have fascinating histories such as Georges Island (apostrophes are not used) with Civil War-era Fort Warren used as a Confederate prison and its resident ghost, the Lady in Black. Little Brewster is home to Boston Light, the oldest continually used lighthouse in the U.S. from 1716. Worlds End has plantings and roads by legendary 19th century landscape architect Frederick Law Olmsted, the designer of Central Park in NYC and Boston's Emerald Necklace park system. There’s even an abandoned, off-limits Nike missile silo on Long Island.

As for the region’s geology, Boston Harbor is a glaciated structural basin that has been inundated and modified by post-glacial sea level rise in the last 15,000 years. It contains dozens of exposed and submerged Pleistocene-age drumlins and other glacial features modified by coastal processes. The bedrock crops out at numerous locations and consists of the Late Proterozoic Boston Bay Group, rocks of the Avalonia terrane that accreted to Laurentia during the Middle Paleozoic.

The group consists of fine-grained clastics of the Cambridge Formation (“Argillite”) and coarse-grained clastics of the Roxbury Conglomerate better known as “puddingstone”, the Commonwealth’s state rock. The terrane of Avalonia rifted from its peri-Gondwanan, Southern Hemisphere-berth off the northern edge of the West Africa craton (although some advocate a northern South America provinence). It then drifted some 6,000 miles during the Ordovician across the Iapetus sea to its present location in Boston Harbor, accreting (attaching) in the process to a large portion of the Appalachian orogen along Laurentia’s northeast coast. 

February
On an Appalachian-Derived Beach at Fort Lauderdale

This Fort Lauderdale beach scene is far more welcoming meteorologically this time of year. It depicts a commonplace entity with the warming climate – beach erosion and restoration. Sediment (mostly sand) is typically lost through longshore drift (movement of material by waves that approach at an angle to the shore but recede directly away from it) and from changing ocean currents and storms. A wider beach reduces damage to coastal structures by dissipating energy across the surf zone. It also protects upland structures and infrastructure from storm surges, tsunamis (not on this passive marginal coast) and unusually high tides.

Of course, Floridians will need to deal with the issue that everyone must confront, rising sea levels from melting glacial ice. It won’t be the first time it has risen. Fluctuating glacial periods of the Pleistocene triggered vacillating high seas that periodically flooded coastal plains. Before that, during the Cretaceous, North America’s central continental and coastal lowlands were completely submerged by global high seas of the Tejas transgression.

By the way, Lauderdale’s beaches are composed of brownish, quartz sand not whitish, calcium carbonate, which is not what one would expect considering Florida’s carbonate-platform heritage. Silicon dioxide-rich sand was transported downbeach from the eroding Appalachians Georgia-way by longshore currents during the Cenozoic. Next time you stroll along the beach further south, check out a handful of sand. It gets whiter as its carbonate content increases with distance from its granitic source up north.

April
Living Cretaceous Fossils in Bloom in Boston’s Backbay

The annual explosion of pink and white magnolias in bloom is one of Boston’s first rites of spring. The city's floriferous trees have more to offer than large flowers, showy colors and fragrant scents. There's a tale of evolution to be told here.

You see, beetles pollinate magnolias, not bees as one might expect. Bees were not around in the mid-Cretaceous (about 100 million years ago), when magnolias were evolving. That pollinator relationship has changed little over the millennia since the co-evolution (mutual evolutionary influence) of insects and angiosperms (flowering plants). Magnolia flowers don't produce nectar, the sugary secretion that encourages insect visitation (and hence pollination). They do produce large quantities of pollen that's high in protein, which beetles use for food, and in the process, cross-fertilize (transfer) pollen from the male anther of one flower to the female stigma of another. The high proportion of beetle-pollinated systems within the Magnolia family has perpetuated the long-standing theory that modern flowers were derived largely from beetle-pollinated proto-angiosperms. Indeed, many paleobotanists have devoted their attention to plants such as magnolias in their attempts to unravel the events of angiosperm evolution. 

Magnolia's ancestral floral characteristics include: its large blossom with its tepal structure (magnolia's petals and usually green sepals in higher plants all look alike); its central, cone-like receptacle of spirally-arranged, male stamens at the base and similarly-arranged spiral, female carpels; its radial symmetry; its actinomorphism (floral parts similar in size and shape); and its leathery beetle-durable petals. 

One of many botanical classification systems, Cronquist's interpretation assigns magnolias to the most archaic positions of all living angiosperms, the subclass Magnoliids, along with water lilies and buttercups. The concept that magnolias are amongst the most basal angiosperms has been refuted by higher-level phylogenetic analyses, yet they remain one of the most important lineages in the early radiation of angiosperms. Appearing long before the radiation of flowering plants, Charles Darwin called their abrupt appearance in the fossil record “an abominable mystery.” What's more, the magnolia qualifies as a "living" fossil, having changed little since it first appeared.

By the way, magnolias acquired their name from the 17th century French botanist and physician Pierre Magnol. Now back to enjoying spring in Boston!  

June
Luxuriating in the Grenville-Age High Peaks of the Adirondacks

This High Dynamic Range photo of glacial Heart Lake was taken from the summit of lowly Mount Jo in the High Peaks region of the Adirondack Mountains in uppermost New York State. The tall peak to the right is Algonquin. Colden is the rock slide-scarred summit in the center, and to the left, Mount Marcy is the highest in the state, each separated by Precambrian faults re-activated during the Paleozoic.

We see almost two billion years of geological scenery in the making, beginning with the meta-anorthosite bedrock that emplaced during the Grenville orogeny. The protracted, multi-phasic tectonic event culminated with the formation of the Late Proterozoic supercontinent of Rodinia and a transglobal Grenville Mountain spine. Rodinia’s subsequent fragmentation in the latest Proterozoic formed two megacontinental siblings: smaller equatorial-positioned Laurentia and larger australly-located Gondwana. The two incrementally re-assembled throughout the Paleozoic into the supercontinent of Pangaea along with its Appalachian Mountain spine.

In the Late Cretaceous, the peneplaned Grenville’s, now internal to Laurentia, began to dome upward triggered by the region's proximity to the Great Meteor Hotspot that tracked southeastward from Canada beneath the drifting North American plate. The hotspot crossed the Mid-Atlantic Ridge, after tracking beneath the North American plate generating seamounts in its path, and is currently off the coast of Africa beneath the African plate.

Having been glacially sculptured during the ice ages of the Pleistocene, the Adirondack’s ascent of “new mountains from old rocks” (namely Grenville basement crust domed into a mountain range) possibly continues to this day. What’s more, we geologically recognize that the Adirondack’s (located cratonward) are distinctly non-Appalachian in origin (paralleling the coast)!

July
A Summer’s Wade in the Late Cretaceous Marl of Big Brook

This lazy stream, a “piddly little dribble” in the words of the New York Paleontological Society's field guide, courses through one of the oldest and prolific collecting sites for marine fossils on the East Coast. Collectors, both amateur and professional, have been extricating both vertebrate and invertebrate faunal remains out of the clear-flowing waters of Big Brook in Monmouth County of coastal Central New Jersey for well over a hundred years.

The diverse, age-spanning list includes Cretaceous bullet-shaped belemnite guards (a squid-like mollusc), brachiopod, oyster and clam shells, steinkerns (shell casts), hadrosaur (washed down from the mainland), shark and mosasaur teeth, alligator scutes, Pleistocene sloth and mammoth remains, Holocene Lenape arrowheads and even Colonial nick-knacks such as smoking pipes and pottery.

As the brook wends its way to the sea through farmlands, forests and the gentrified estates of rural New Jersey, it flows through a Late Cretaceous continental shelf setting and dissects its way down through Pleistocene and Holocene alluvial surface-overburden along the way. Although the banks are off limits for active fossil exploration, the brook does most of the work for fossil hunters as the bounty virtually collapses in from the upland Navesink Formation and glauconitic Mount Laurel Formation of the streambed. All that’s needed to sift through the streambed is a wire-mesh screen, a garden trowel, a pair of waders and a little patience.

Simply park your car, stroll a short distance through the woods, step into the stream, and travel back in time 66 to 70 million years near the end of the Age of Dinosaurs! 

August
Monster Mushrooms in Chestnut Hill, Massachusetts



This astounding three-foot beauty appears like clockwork every August near the base of a massive oak in my Boston suburb of Chestnut Hill. The rather drab, cream-colored mushroom is intricately branched with overlapping caps, yet surprisingly emanates from a single stalk. Its mycelial network remains dormant beneath the soil until summer rains and heat cause the fungal “roots” to germinate into a gargantuan “plant” above the soil. It gives the impression of growing from the ground, but it actually has colonized the buried roots of the tree, making it parasitic.

Once considered to be plants, with which they share many traits, fungi actually belong to their own kingdom of classification. As for the mushroom (the fruitbody), it’s relationship to the parent fungus is as the apple (the fruit) to the tree. This Bondarzewia berkeleyi is a bracket fungus, so called because many within the family grow shelf-like from the sides of trees. Its reproductive spores are manufactured within tiny tubes on the underside of the fruitbody rather than within the more accustomed gills we're used to seeing. For this reason, species within this group are called polypores. If cut when fresh, the pores exude latex. It’s not considered edible because of its leathery and woody texture, not that you're tempted.

September
My Lofty Visit to an Alpine Bog in New Hampshire

Artificially located above the treeline due to ravaging fires in the early 19th century and below the climatic treeline of higher mountains in the region, this exquisite alpine bog hides on a corner of the summit of Mount Monadnock at the foot of the White Mountains of New Hampshire.

The tops of mountains, where the climate is cold, windy and rainfall is scant, are amongst the harshest biomes on our planet. Only a select few plants and animals can exist in these severe conditions. Depressions in the bedrock collect rain and retain what little soil exists on the summit, keeping it permanently saturated. The lifeforms encountered here are similar to those found in the arctic tundra further north. Well-adapted to the bog’s poorly-drained, nutrient-poor and acidic peat soil are Sphagnum mosses, which form a carpet on which the bog’s dwarf shrubs and herbs grow. Look for Deerhair bog sedge, sheep laurel and tufted cotton-grass interspersed with patches of Labrador tea, leatherleaf, cranberry and round-leafed sundew to name a few. 

Mount Monadnock’s rocky core at higher elevations is composed of highly metamorphosed schists and quartzites of the Devonian-age Littleton Formation, which extends well north into the White Mountains of New Hampshire. The mountain represents an overturned syncline derived from compressional forces exerted during the Acadian orogeny, the second of three tectonic collisions that created the northern Appalachians and contributed to the crustal growth of Laurentia (proto-North America).

As our planet experiences progressively warmer climatic conditions, alpine flora and fauna will be challenged as they attempt to progress to a higher elevation to survive. They can only climb so high before being eradicated from their biome. If changing climatic conditions regionally prevail globally, the lifeforms will become extinct. This occurrence, species extinction, has been going on naturally since life appeared on our planet, but we understandably become concerned when its thought to be anthropogenic (man’s fault).

Henry David Thoreau spent some time on Monadnock in the mid-1800's, writing in his journal about the regional botany and geology. There's supposedly a bog up here named for him. This might be it!

October
High Atop Laccolithic Katahdin in the Remote North Woods of Maine



Congratulations are in order! You’re approaching the flat Tableland of mile-high Mount Katahdin in the wilderness of northern Maine from its west flank. Notice the botanical succession you've witnessed with elevation: deciduous hardwoods in autumnal splendor that blanket the lowlands; evergreens foresting the mountain's slopes; and alpine tundral sedge in the foreground.

The bedrock of Katahdin is a Devonian-age laccolith that has achieved its lofty status through intrusive buoyancy, surface erosion and post-glacial isostatic rebound. Katahdin (Mainers and climbers in the know drop the “Mount” from the name) formed during the Acadian orogeny, the second of three tectonic collisional phases that built the Appalachian Mountain chain and contributed to the crustal growth of Laurentia, the Paleozoic continent of North America.

Once Pangaea fully assembled following the third orogeny, the Appalachians graced the supercontinent with a Himalayan-esque mountainous backbone. The pluton of Katahdin, along with the other regional peaks, emplaced within a sea of Late Silurian rock during the Acadian collision in what is thought to have been a retro-arc setting.

Getting here was no easy task, especially if you just trekked 2,180 miles along the Appalachian Trail from Springer Mountain in Georgia to this point at the trail’s terminus. But you're not quite finished. To reach the Tableland you still have to complete the “A.T.’s” final assault via the Hunt Trail’s Spur on a near-vertical, 

quad-burning, heart-pounding, lichen-encrusted, truck-sized boulder-strewn ascent of pink Katahdin granite. Once on the Tableland's plateau, you must strive for Katahdin’s penultimate summit of Baxter Peak, one of five that rim its three cavernous glacial cirques on its east flank.

"Press on. You’re almost there. The view is spectacular!”

November
The Remnants of Historic Fort Bowie within the Apache Pass Fault Zone

Apache Pass is a natural opening and low point at the juncture of the Dos Cabezas and Chiricahua Mountains in southern Arizona. Since prehistoric times, it’s been of importance to humans as a major travel route connecting the San Simon and Sulphur Springs Valleys.

Part of the Basin and Range physiographic province of southeastern Arizona, the surrounding mountains rise abruptly like islands of rock in an arid desert from relatively flat, sediment-filled basins that formed during an extensional tectonic regime about 20 million years ago. Even older is the Apache Pass fault zone, initiated over a billion years ago as strike-slip and more recently reactivated as normal faults during Basin and Range extension. Precambrian rocks on the southwestern side of the fault (on this side of the fort) have been moved upward relative to the Paleozoic and Mesozoic strata on the northeastern side (the hills just beyond the fort). Thus, the fort rests on Permian Horquilla Limestone of the Naco Group, while, amongst other rocks, the hills are Late Jurassic to Cretaceous Glance Conglomerates of the Bisbee Group. Erosion of the fault zone's shattered rocks formed the saddle of Apache Pass.

The Apache people, who arrived in America with their Navajo cousins sometime after 1000 AD, hunted and camped in the area, and drank from Apache Spring that emanates within the fractured and faulted rocks within the fault zone. With the arrival of the Anglos in the mid-1800’s, Puerto del Dado, the Spanish name for the “Pass of Chance”, became the site of Fort Bowie (actually the second) by 1868 to insure the safe movement of the Butterfield Overland Mail, a stagecoach and mail service that connected Memphis and St. Louis with San Francisco. Prior to this, the arduous route was by ship across the Gulf of Mexico to the Isthmus of Panama, and on to California via the Pacific Ocean. For years, the Apache Wars led by Cochise and later Geronimo of the Chiricahua Apache waged upon the U.S. military. It all ended in 1886 with Geronimo's surrender and expatriation to Florida, leaving the foundations of the fort to decompose into the landscape.

The region’s complex geologic history contributed to the strategic importance of the pass and delivered dependable water into the fracture zone. It's another reminder of the importance of geology and geographic setting in shaping the course of civilization and human history.

December

A Six Hundred Million Year Old West African Riverbed in Newton, Massachusetts

Oblivious to most passersby alongside Beacon Street, a major thoroughfare out of Boston, is a cross-section of an ancient streambed embedded within a cliff wall. The stream bed appears as a semi-circular channel outlined perfectly by fallen leaves. The transected bed and its banks consist of fine-grained, thinly-bedded, fissile (easily split along its planes) siltstone (mud rock) that displays a large infill of conglomerate rock over its entirety. The siltstone preserves the contours of an ancient landscape that was buried by subsequent deposition.

Upon close inspection, laminations within the streambed display whorls of sediment indicative of stream turbidity currents and slump features indicative of settling. The manmade wall at the top is composed of stacked conglomerate boulders.

The flat-lying rocks of the entire assemblage, being sedimentary, were deposited horizontally under the action of gravity. Subsequent to their deposition, compaction, cementation and lithification (conversion to solid rock), the assemblage and the rocks in the region were tilted by tectonic forces, which accounts for the angulation seen in the photo. These rocks belong to the Roxbury Conglomerate, a 2,000 foot thick formation of coarse arkosic sandstone with small to medium-size, rounded clasts (rounded fragments of stones). In 1830, the American poet Oliver Wendell Holmes likened the Roxbury to puddingstone, its common name, since it reminded him of raisins in English bread pudding.

The puddingstone's sandy matrix and rocky inclusions indicate they were deposited in a high-energy depositional and/or transport system such as a cascading mountain stream or a massive submarine flow. The Roxbury is exposed almost everywhere in the neighboring towns to the west and southwest of Boston. The channel's siltstone is a facies change, a clastless sediment within the Roxbury Formation. Along with the Cambridge Argillite (or Slate), the Roxbury Conglomerate comprises the sedimentary strata of the Boston Bay Group. As mentioned in the first vignette at the top of this post, the group was deposited on the microcontinent of Avalonia in an extensional regime, such as a faulted rift basin in Late Proterozoic-time between 595 and 540 million years ago.

Avalonia originated as an elongate volcanic island chain along the edge of the megacontinent of Gondwana, possibly of West Africa cratonic provenance in the southern hemisphere. Avalonia’s deeper basement is volcanic in origin, and, in the vicinity of the Boston Basin, they include the Brighton, Dedham, Mattapan, Lynn and Westwood granites, which underlie the rocks of the Boston Bay Group. During the Acadian orogeny, Avalonia welded to the continent of Laurentia about 370 million years ago. Can't get enough of the Roxbury Conglomerate? Check out my previous post here.

The "unnoticed" streambed is an example of my masthead statement at the top of my blog. "Geology is all around us, scarcely thought of as we go about our lives." Perhaps I should add, "but not by all of us!"

Happy New Year from Franklin the Border Collie (and Jack)!

High Dynamic Range digital photograph

The Adirondack Mountains of New York State: Part II – What do we know about their geological evolution?

 Yours truly atop Wright Peak in the High Peaks region of the Adirondacks

HUMAN HABITATION
The rugged and insular geomorphology of the Adirondack Mountains is attributed to their complex tectonic and glacial history. The mountains' geological past promoted a similarly colorful and varied history of human habitation. The word Adirondack is thought to be derived from a derisive Iroquois term toward the Algonquin tribe meaning “bark-eaters.” The phonetic spelling sounded similar to atiru’ taks. On old English maps the region was called “Deer Hunting Country” with “Adirondack” coming into usage around 1837.

Pleistocene deglaciation about 16,000 years ago opened the door to Native American hunting and fishing parties. During the eighteenth century, the Adirondack’s periphery saw the French and English struggle for control of North America. In the nineteenth century, the mountains enticed loggers and iron-miners, guides and hikers, dreamers and artists, and philosophers and poets. In the twentieth century, they witnessed titanium and magnetite-miners, climbers and naturalists, sportsmen and outdoorsmen, forest fires and logging-denudation followed by preservationists, environmentalists and tourists. 

Once blighted by logging and industry, the region has undergone a renaissance of woods and waters.” * Today, in the twenty-first century, the Adirondacks lives on as “a remarkable mix of wilderness and small towns in the midst of one of the most heavily developed regions in the world.” **

* Adirondack Park – Forever Wild by Verilyn Klinkenborg, National Geographic

** The Great Experiment in Conservation – Voices from the Adirondack Park by William F. Porter et al, 2009



BUILDING THE FOUNDATION OF A SUPERCONTINENT

“We now understand this ancient (Adirondack) terrain as a product of global tectonic processes that gave rise to the continents and ocean basins” of our planet. * In order to better understand how these processes formed the Adirondacks, we must look to some of the continent’s oldest rocks.

* The Great Experiment in Conservation:  Geology of the Adirondack Mountains by McLelland and Selleck

The ancient nucleus of the North American craton is the Canadian Shield (red) that formed during the Archaean and Early Proterozoic. It’s a two and a half to four billion year old, stable, igneous and metamorphic mosaic of accreted terranes and micro-plates that were progressively fused together by the process of plate tectonics. Shaped like a warrior’s shield, it was the first part of North America to remain permanently above sea level. One more massive terrane was needed to attach to the shield in order to finalize the supercontinent of Rodinia.

Today, the once-mountainous shield is a vast, gently-undulating, heavily-eroded and extensively-glaciated physiographic region of over three million square miles. From north to south, it extends from the islands of the Arctic Archipelago to the upper Midwestern states of Minnesota, Wisconsin and Michigan. From east to west, it extends from Greenland and Labrador of the Canadian Maritimes to the Canadian Northwest Territories. The Shield also exists in the subsurface beneath the Western Cordillera in the west and the Appalachians in the east.

Geologic bedrock map of North America with the Canadian Shield (red) embracing Hudson Bay.
The pointer is directed at Grenville bedrock (orange) and specifically the Adirondack outlier.
Notice the orange inliers in the Hudson Highlands, Reading Prong and within the Appalachians.
 (Modified from USGS)



ACCRETION OF THE GRENVILLE PROVINCE
During the Middle Proterozoic from ~1,300 to ~1,050 Ma, the Grenville Province (orange bedrock above) accreted to the Canadian Shield along its southeast boundary (contemporary coordinates). This was accomplished in a complex, long-lived, global-scale, tectonic collisional event called the Grenville Orogeny (after an exposure in a Canadian town in Quebec). The collision not only formed the Grenville orogen, an immense mountain belt, but it served to complete the final assembly of the supercontinent of Rodinia by bringing together most of the landmasses on the planet.

The ~3,000 kilometer-long and 600 kilometer-wide, supercontinent-spanning orogen was of Himalayan proportions that in North America extended from Labrador in eastern Canada to Mexico. Globally, the orogen reached as far as Australia, Antarctica and beyond in the west (contemporary coordinates), and in the east, Greenland, Scandinavia (Norway and Sweden), South America (Amazon) and Africa (Kalahari). This axis-sideways view of Middle Proterozoic Earth depicts the global extent of the orogen across Rodinia. The mountains in the region of the future Adirondacks (red ellipse) are Grenvillian NOT Adirondack, but the Grenville Province on which they would rise (orange blob at the arrow above) was in place!
 

(Modified from Scotese.com)



This cartoonish representation (~700 Ma) shows the extent of the Grenville orogen (reddish-brown) running through Rodinia’s building blocks. After Rodinia’s final assembly, it would fragment (rift) apart. Smaller cratonic blocks would be sent tectonically adrift along with the Grenville rocks they acquired. After the craton of Amazonia fragmented from Rodinia, the region of the future Adirondack’s (white dot) would assume a coastal locale. Geologists are studying the Grenvillian rocks on ancient continents far-adrift in an attempt to piece together the collisional events that formed Rodinia, and the details and timing of its fragmentation.

(Modified after Callan Bentley, 1991)

VESTIGES OF RODINIA

The fate of all orogens is their eventual reduction to a low-lying peneplain. Thus, the mountain belt’s long and complex history of igneous intrusion, metamorphism and deformation is represented today by ongoing degradation (erosion) and exhumation (exposure). In North America, the Grenville Province’s presence in the subsurface of the Appalachians (diagonal lines) is extensive, having been overprinted subsequently by the Appalachian Orogeny (although recently the southern and central Appalachian basement crust appears to be exotic). Surficially, it extends into southeastern Canada (yellow) and outliers of the Adirondack Mountains (green AD). It surfaces again in the Hudson Highlands, the Manhattan Prong of New York and inliers of the Appalachians (black blobs), and down south in Texas and Mexico. Globally, vestiges of Rodinia are present in the cratons of rifted landmasses that once formed the supercontinent.

Allochthonous (yellow and green) Grenville rocks thrust upon autochthonous (indigenous) rocks,
making much of the Grenville Province “reworked” older continental crust.
The Grenville Front separates the Grenville Province from the Canadian Shield.  
 (Modified after Rivers et al, 1989)

 

DEMYSTIFYING THE GRENVILLE OROGENY

Lay descriptions of the orogeny depict it as a singular, protracted mountain-building event. In reality, it consisted of a multitude of events spanning perhaps 300 million years and is best viewed as a collection of collisional and magmatic phases separated from each other by 50 to 80 million years. The scenario is somewhat analogous to the more recent long-lived Appalachian Orogeny that includes Taconic, Acadian and Alleghenian phases or episodes.

Although dates and details vary considerably and are controversial, the phases of the collective Grenville event are: the Elzevirian orogeny (1350 to 1220 Ma), the Shawinigan orogeny (1180 to 1170 Ma), magmatism of the enigmatic AMCG (anorthosite-mangerite-charnockite-granite) suite (1160 to 1150 Ma), the Ottawan orogeny (1090 to 1050 Ma) and the Rigolet orogeny (1010 to 980 Ma). The Grenville timeline might look something like this.

A-F coincides with panels below
(Timeline by Doctor Jack)

DEMYSTIFYING THE PHASES OF THE GRENVILLE

To gain a sense of how the Adirondack’s bedrock was derived, here’s a VERY abbreviated synopsis of the Grenville’s phases assimilated from numerous sources most notably from McLelland et al.* Importantly, the proposed terrane of Adirondis (red letters) is thought to have formed the basement of portions of Quebec to New Jersey (MC, VT, NY, NJ) and includes the Adirondack region!

The Canadian Shield (light gray) experienced rifting (gray arrows), opening and closing (black arrows) of the Central Metasedimentary Belt (CMB) of the Grenville Province in the Middle Proterozoic. This allochthonous belt was thrust to its location in the ensuing arc-collision. Adirondis is thought to have rifted from the North American craton and then reattached (A-D). The Elzevirian (B) and Shawinigan (D) orogenies and the enigmatic, mantle-derived AMCG suite magmatism (E) provided additional metamorphism, deformation, and further contributed to the formation of the Adirondacks. Note that the AMCG suites formed anorogenically due to lithospheric delamination and tectonic transportation in large thrust slices and nappes, and were emplaced in two intervals (1160-1130  and 1080-1040 Ma). 

The Phases of the Grenville Orogeny
 (A) Adirondis rifting; (B) Elzevirian east-directed subduction zone;
(C) Back-arc basin closure and Adirondis accretion; (D) Shawinigan CMB thrusting;
(E) AMCG suite intrusions; (F) Ottawan thrusting of Grenville rocks over the shield’s foreland.
MA, Marcy Anorthosite of the High Peaks region.  
(Modified from McLelland et al, 2010)

The Grenville Orogeny ended with deformation and metamorphism during the Ottawan phase (F) which is considered the main orogen-wide, continent-continent collision and the culminating event in the evolution of the Grenville Province. Convergence is thought to have occurred when one or more continental blocks (likely including the South American craton of Amazonia although collisions with Baltica and the Kalahari have been implicated) collided with Adirondis and the previously accreted Grenville terranes. The orogeny is comparable to the convergence of India with Asia that created the Himalayan Mountains and the Tibetan Plateau in terms of magnitude, crustal thickness, metamorphic fabric and tectonic design.

* Review of the Proterozoic Evolution of the Grenville Province, its Adirondack Outlier, and the Mesoproterozoic Inliers of the Appalachians  by McLelland, Selleck and Bickford, GSA, Memoir 206, 2010.

THREE GEOLOGIC SUBDIVISIONS OF THE ADIRONDACKS
The final outcome of the multi-phasic orogeny was the Grenville Province that includes a southern extension or outlier in northern New York, the locale of the future Adirondack Mountains. The tectonic and magmatic history of the Adirondacks is extremely complex. The timing of deformation, the identification of sutures, and the clarification of phases responsible for structural features remain unclear due to overprinting, metamorphic obscuring of boundaries and bedrock inaccessibility.

Today, the Adirondacks are divided into three terranes based on metamorphic grade, rock type and structure. Their rocks are metamorphic almost without exception, having been subjected to high temperatures and pressures at depths of 19-25 miles (30-40 km).

The three recognized subdivisions are:
 

1.) The Central Highlands (red HL) is a mountainous terrain underlain by erosion-resistant igneous rocks that were metamorphosed under granulite facies conditions (high temperature and pressure during the Shawinigan and Ottawan orogenies). Its meta-plutonic rocks include orthogneisses, meta-anorthosite, a voluminous AMCG suite and olivine meta-gabbro. The High Peaks region is located within the center of the Highlands with the Marcy Massif as its centerpiece. The red ellipse denotes the region of our geologic ascent in post Part III.

The three subdivisions of the Adirondacks in northern New York State
(Modified from Huemann et al, 2006)

2.) The Northwest Lowlands (red LL), a smaller, topographically-subdued region. Its varied rocks include metamorphosed sedimentary rocks of shallow-marine origin (notably marble, quartzite and gneiss) that are folded, faulted, and then intruded by metamorphosed volcanic rocks. These supracrustal rocks were metamorphosed to amphibolite facies (intermediate temperatures and pressures) during the Shawinigan orogeny. The Lowlands are contiguous with the main Grenville Province in Canada via the Frontenac Arch which extends across the St. Lawrence River in the region of the Thousand Islands. It is a terrane that is lithologically similar to the Lowlands, and many consider the Lowlands to be part of it.

3.) The Carthage-Colton Mylonite Shear Zone (red CCZ) is a kilometers-wide, major northeast-trending, ~45º northwest-dipping fault and terrane boundary that separates the two above domains. Its shear zone is a major Ottawan Orogeny extensional feature. The Lowlands were thrust over the Highlands along a  suture zone coincident with the present Carthage-Colton Zone.


WHAT GOES UP MUST COME DOWN

With the orogen and mountain-building complete, and the removal of convergent tectonic driving-forces, compression changed to extension. The constructive phase of mountain building was succeeded by a late-stage, destructive phase as erosion and sediment transport overwhelmed the orogen. The orogen’s over-thickened crust gave way under its own weight spreading laterally. Syn- (at the time of) through post-orogenic collapse is a fundamental process in the tectonic evolution of mountain belts.

Tectonically in brief, the over-thickened lithosphere of the orogen is removed either by delamination or convection which allows asthenosphere to well upward. The buoyant asthenosphere undergoes compression melting forming ponded gabbroic magmas that further fractionate, and exerting upward (POP UP) and outward (Fb), extensional vectors. In this manner, it is thought that the plagioclase-rich anorthosite (black squares) and the enigmatic AMCG suite (MCG) typical of the anorthositic massifs of the High Peaks may have developed. Obviously over-simplified, but we can see how orogenic collapse contributes to the formation of the Adirondack’s magmas. The genesis of the magmas is referred to as “anorogenic” emplacement (versus orogenic emplacement). 

Overthickened collisional orogen undergoing lithospheric delamination, consequent orogen rebound
and collapse along low-angle, normal faults during late phases of orogenesis.
(From McLelland, 2010)

In addition, many of the NE-striking faults found throughout the region may have originated as normal faults during this period of Late Proterozoic extension. These faults and additional from the Paleozoic were re-activated at various times and are responsible for much of the Adirondack’s contemporary landscape!

Cartoon of orogen collapse after asthenospheric upwelling has produced orogen rise,
lateral spreading and extensional faulting.
(Modified from Selverstone, 2005)

By ~1,020 Ma, the orogen's broad, elevated topography began to gravitationally collapse (the destructive phase). The Rigolet Orogeny (1,010 to 980 Ma) was an independent, final phase involving renewed orogen-wide contraction and additional collapse. Over 30 km of rock was stripped away as the majestic Grenville range was reduced to a peneplain of low relief, exposing the deep core of the mountain belt at the surface. The Adirondack Mountains still had not yet formed, but their basement rocks, the very core of the Grenville orogen, were now in place!

BREAKING UP IS HARD TO DO
Rifting typically follows the final consolidation of a supercontinent and ultimately results in its demise. Its continental crust is both thick and brittle, and becomes a trap for the buildup of heat. Tectonic movements generate stresses greater than the crust can sustain causing the supercontinent to rift apart, often along inherently-weak convergent boundaries. Following Rodinia’s breakup, fragmented cratonic blocks as newly-formed continents were sent tectonically adrift throughout the globe taking along their share of the Grenville.

Traditional Rodinia models argue that breakup on Rodinia’s west coast commenced with the opening of the Panthalassic Ocean (Paleo-Pacific) at 800 to 700 Ma between the conjugates of Australia and East Antarctica, while on the east coast, the Iapetus Ocean (Paleo-Atlantic) opened by 600 to 535 Ma. With the cessation of ongoing tectonic activity both coasts were converted from an active rift-margin into a passive rifted-margin.

(Modified from Dalziel, 1997 and Torsvik et al, 1996)

This Mollweide Projection (note the equator for orientation) shows the postulated position of Rodinia (~750 Ma) shortly after breakup with South American terrane of Amazonia beginning to disengage. The newly-formed continents of Laurentia (~550 Ma) and Western Gondwana are separated by the nascent southern Iapetus Ocean. Black shaded areas are Grenville mobile belts. Red arrow points to the region of the future Adirondack Mountains.

 (Modified from Cocks and Torsvik, 2005)

RIFTING TO DRIFTING > ACTIVE TO PASSIVE > SUBSIDENCE AND SEDIMENTATION

As the developing rift widened into the expanding Iapetus Ocean on the east (south using Cambrian coordinates), Laurentia’s passive margin was characterized by subsidence and sedimentation. Low-lying coastal regions including the region of the future Adirondacks were flooded by rising global seas (possibly caused by the many shallow ocean-basins following Rodinia’s fragmentation, rapid seafloor rift-spreading and/or thermal subsidence of passive margins). As mentioned, many of the NE-striking faults found in the region of the Adirondacks and throughout the state may have originated as normal faults during this rifting-period of Late Proterozoic extension.

Middle Cambrian (500 Ma) Laurentia with flooded coastal and cratonic regions
inlcuding the region of the future Adirondack Mountains.
(From Ron Blakey, Colorado Plateau Geosystems, Inc. and courtesy of Wayne Ranney)

As the rising Cambrian Sauk seas flooded the landscape, a thick wedge-shaped blanket of siliciclastic sand and mud covered the surface of the Grenville basement followed by an overlying carbonate system in deeper waters. The sandstone-shale-limestone assemblage transgressed with the rising seas advancing landward and drowning most of Laurentia’s craton. For the record (and everyone that thrives on names and details), the entire sedimentary package is referred to as a Sauk (the first global high-water of the Phanerozoic of which there are six) Supersequence (a conformable, time-orderly succession of strata) of Sloss (the proposing sedimentary geologist).


ADIRONDACK REGION IN THE EARLY PALEOZOIC

Thus, in the region of the future Adirondacks, the eroded Middle Proterozoic Grenville basement rocks were overlain by Late Cambrian to Early Ordovician Potsdam Sandstone (yellow) followed by an overlying limestone-dolostone sequence of the Theresa Formation and the Beekmantown Group (light gray). The contact between the two rock layers represents a billion-year-plus gap in time called an unconformity. It formed due to a prolonged interruption in deposition and/or protracted erosion, likely both. The amount of missing time (and strata) is so massive that it has achieved capital letter status in the geological literature called the Great Unconformity. And, it’s global in its extent, found wherever a Paleozoic sequence overlies a Precambrian basement.

(Modified from the Geology of New York, 2000)

The Potsdam Sandstone is the geological and temporal equivalent of the Tapeats Sandstone, the basalmost strata of the classic-textbook, time-transgressive Tonto Group within the Grand Canyon. The Great Unconformity between Middle Proterozoic Vishnu Schist and the overlying Middle Cambrian Tapeats formed on Laurentia’s west coast. It is the same time-gap that we see on the periphery of the Adirondacks!


ADIRONDACK REGION IN THE MIDDLE TO LATE PALEOZOIC AND MESOZOIC

From the Devonian through the Mesozoic, the Adirondack region remains poorly constrained. With the arrival of the Taconic Orogeny in the Middle Ordovician, loading and subsidence due to Taconic Allochthon overthrusting resulted in the creation of additional normal faults within the Grenville basement and the reactivation of pre-existing Grenville ones, as well as burial of much of the eastern Adirondacks. Like the Taconic, the subsequent Acadian Orogeny during the Middle to Late Devonian further subsided and buried portions of the Adirondack region.

The final event of the Appalachian orogenic cycle in late Pennsylvanian to Permian time brought the Alleghenian phase to the northeast, this time with the eastern Adirondacks experiencing slow uplift and exhumation. Mesozoic continental rifting of Pangaea likely prolonged regional exhumation. Still, no mountains existed in the region of the Adirondacks, but the geological stage was set with a Grenville basement covered by a Sauk sequence, exposed and fault-scarred!

The following map displays known faults and lineaments within the State of New York. The strike pattern is the cumulative result of Grenville and Appalachian orogenesis, Rodinian and Pangaean rifting. The scars within the basement structure will serve to dictate the presentation of landforms in the Holocene.

(Modified from Fakundiny et al, 2002)

ADIRONDACK REGION IN THE EARLY CRETACEOUS

As the North American plate tectonically drifted northwest, it passed over the stationary Great Meteor hotspot (also called the New England hotspot). A hotspot is a hypothetical region of mantle-derived, voluminous volcanism in the form of a thermal plume that upwells to the surface. The plate’s passage produced a somewhat linear track or age progression of igneous intrusions of various compositions on the surface.

The hotspot track can be traced by a line of kimberlite dikes in the Laurentian Uplands of Quebec to Mont Royal in Montreal, the Monteregian Hills magmatic complex east of Montreal, into northern New York and New England with intrusions of hypabyssal dikes, and off the coast of Massachusetts with the New England Seamounts (e.g. Corner, Nashville, Gosnold and Bear). The seamounts are a line of extinct, submarine volcanoes that extend over 1,000 km along the track. At about 80 million years, the Mid-Atlantic oceanic spreading center migrated to the west over the hotspot. The track of the hotspot continues on the African Plate at the Great Meteor Seamounts off the coast of West Africa from which the hotspot gets its name.

Generalized map of the Great Meteor hotspot track
(Modified from Duncan, 1984)

This topographic map demonstrates the Great Meteor’s surficial features. Trace the track from the Monteregian Hills (M) through New England (NEM) including the Adirondacks (red arrow) and past the Great Stone Dome (GSD), an intrusion into passive margin sediments domed by pressure-release melting. The track follows the submarine New England Seamounts across the Dynamic Gap and to the Cormer Seamounts (offset due to seafloor spreading). It then crosses the mid-Atlantic ridge to the African plate and continues as the Great Meteor Seamounts off the African coast.

(Modified from Smith and Sandwell, 1997)

THE ADIRONDACKS GET THE LIFT THEY NEEDED

The hotspot is thought to have induced regional heating between ~125 and 100 Ma in the vicinity of the Adirondack Highlands, as the North American plate on which it rides migrated over it. The scenario is analogous to the Hawaiian Island chain and Yellowstone magmatism. Mantle lithosphere under the hotspot is suspected to have delaminated thereby producing dynamic uplift as the buoyant asthenosphere welled up to replace the mantle lithosphere.

The result is ~1 km of domal uplift of the Grenville basement of rocks giving rise to the Adirondack Mountains forming “new mountains from old rocks.” In addition to re-activated normal faults in the Adirondacks during the orogenies of the Paleozoic, it is plausible that thermal doming may have contributed to additional re-activation in the region.

(Modified from Geology of New York)

 

 

THE GREAT UNCONFORMITY OF THE ADIRONDACKS
The thermal doming of the Adirondacks unroofed the Early Paleozoic Sauk sequence that once covered the region and re-exposed the Middle Proterozoic Grenville basement. On the periphery of the dome where uplift is minimal, the sedimentary cover and the intervening time gap of the Great Unconformity can be found.

(Modified from Geology of New York)

ADIRONDACK GRAVES

How do we know that the region of the Adirondacks was once covered by sandstones and limestones, if the sediments were unroofed and now missing from the dome? Because the transgressive sequence surrounds the periphery of the range and from down-dropped grabens that contain Cambrian and Ordovician rocks in the southern Adirondacks. These geological “graves” that formed in the extensional Grenville regime protected the landscape from erosion while uplifted horst-blocks were eroded during regional uplift. We are reminded of the preservation of the Grand Canyon Supergroup within erosion-protected, down-dropped grabens.




(Modified from Artemis at MIT)

ENIGMATIC UPLIFT *
Q.  Why did doming occur in the Adirondack region and not elsewhere along the hotspot track? Why is there not a train of Adirondack-like mountains along the track?
A.  The lack of an uplifted-track may be due to a failure of the plume to penetrate the Canadian Shield or a strengthening of the plume as it tracked eastward. The answer likely lies in the structure of the lithosphere and mantle under the Adirondacks relating to dynamic support.

An alternative interpretation of the hotspot model relates to the inferred hotspot as it encountered a progressively thinning lithosphere due to the motion of the overriding plate. Notice the path of the earthquake epicenters (black line) along the hotspot track in Quebec and New England. Earthquakes can be used as an indirect measure of magmatism and to measure its track out to sea. The track crosses two large orogenic belts that cut across the region, that of the Grenville and Appalachian orogenies. The heavy lines are failed rift arms (characterized by normal faults and mafic dikes) emplaced subsequent to the rifting of Rodinia and the opening of the Iapetus Ocean. A comparison of the track with pre-existing crustal structures suggests that a reactivation of structural features may have occurred. The emplacement of buoyant asthenosphere may account for the systemic evolution on the surface of kimberlite dikes to more voluminous crustal magmatism and Adirondack doming.

Earthquake epicenters align with the Great Meteor hotspot track (dashed line),
while Grenville and Appalachian orogenic belts transect the region.
Adirondack region at red arrow.
(Modified from Shutian and Eaton, 2007)

Q.  Why are there seamounts in the Atlantic basin along the track?
A.  Seamounts occur along hotspot tracks in oceanic lithosphere which is thinner than continental crust. Hotspots readily melt material at the base of the crust generating submarine magmatism.

Q.  If cooling is occurring in the Adirondack region with the passage of the hotspot, could uplift still be taking place other than from glacial isostatic rebound?
A.  If uplift is indeed present, it would be related to dynamic support within the lower crust and mantle.

Q.  Why are there no extrusive volcanics in the Adirondacks as in hotspot-related Yellowstone and the Hawaiian Islands?
A.  The possibility exists that magmatism may have occurred in places within the mountains and has since eroded away. Perhaps the intrusive stocks in Canada are erosive remnants that fed long-extinct volcanoes. Projecting the track to the west in Canada where it appears devoid of surficial volcanic activity, intrusives may not have reached the surface. Unconfirmed seismic reflectors in the middle and lower crust under the eastern Adirondacks do imply the presence of a mafic intrusion of the same age at depth. Again, we must look to the mantle for an answer.

* Personal communication, name withheld

ICING ON THE CAKE

With incipient accumulations in the Middle Pliocene and in earnest by the Pleistocene, the two-mile thick North American Laurentide continental ice sheet covered hundreds of thousands of square miles throughout the majority of Canada and northern United States a multitude of times. Better known as the Ice Ages, the furthest southern extent of the continental glaciations surpassed New York City and Chicago with a mid-continent terminus of approximately 38º latitude. The ice sheet created much of the surface geology of southern Canada and northern United States by gradually bulldozing its way through the landscape.




The northeast extent of the Laurentide Ice Sheet during the Late Wisconsinan Stage.
Blue, 14,000-18,000 ky; Turquoise, 10,000-14,000 ky; Dark blue, 6,000-10,000 ky.
Red line is the end moraine. Red arrow points to the Adirondack region.
(Modified from Geographie Physique et Quaternaire from erudite.com)

After some two million years of glaciation, about 10,000 years ago the ice had fully retreated from the Northeast including the Adirondacks. With the coming of interglacial warming trends alpine glaciers continued the work of scouring the upper reaches of the Adirondack’s now-elevated landscape and are responsible for the distinctive, sculpted and scoured appearance of the region today. The eroded, domal architecture of the Adirondacks has dictated the configuration of its landforms and the path of drainage that its waterforms have chosen to take. Once radial in design, the Adirondack’s lakes, rivers and streams have begun to adapt a trellis pattern as they eroded into resistant Grenville bedrock and followed the NE-trending faults in the landscape. This NASA satellite photo of the Adirondack Mountains shows the ranges, valleys and waterways that orient with the strike of the prevailing bedrock structures within the Adirondack Mountains. 




(From earthobservatory.nasa.gov)

Some workers have proposed that the Adirondacks are still experiencing uplift at a rate of ~1 to 3 mm/yr due to prolonged thermal doming; however, this hypothesis remains controversial. Other hypotheses explain contemporary uplift, if truly active, by an isostatic response to crustal thickening relating to Great Meteor Mesozoic magmatism or post-glacial isostatic rebound.


THE ADIRONDACKS OF TODAY

We’ve witnessed the emplacement of the Adirondack’s crystalline basement via Middle Proterozoic Grenville orogenesis well over a billion years ago. After Late Proterozoic mountain belt collapse and erosion, exhumation brought the deep roots of the orogen to the Earth’s surface. Latest Proterozoic rifting fragmented Rodinia, and Early Paleozoic high seas flooded the region with the Sauk sequence of deposits. Multi-phasic Appalachian orogenesis further exhumed and scored the region with faults and fracture zones. Late Cretaceous passage near the Great Meteor hotspot uplifted the Grenville foundation into the Adirondack range followed by Pleistocene glaciation that sculpted the region. Voila!

The Adirondack’s complex geological history explains their enigmatic intraplate locale at a considerable distance from the Appalachian passive margin of the continent. We now understand how the Adirondack Mountains appear to be part of the Appalachian chain but are uniquely independent geographically, tectonically and temporally. And finally, having derived their structure from ancient Precambrian rocks, we see they are truly “new mountains from old rocks.”

Please visit my upcoming post on the Adirondacks entitled Part III "Climbing the Geology."