GeoLog

Stratigraphy, Sedimentology and Palaeontology

Imaggeo on Mondays: Angular unconformity

Imaggeo on Mondays: Angular unconformity

It is not unusual to observe abrupt contacts between two, seemingly, contiguous rock layers, such as the one featured in today’s featured image. This type of contact is called an unconformity and marks two very distinct times periods, where the rocks formed under very different conditions.

Telheiro Beach is located at the western tip of the Algarve; Portugal’s southernmost mainland region and the most touristic too.

The area, famous for its famous rocky beaches and great seefood, shows a spectacular Variscan unconformity between the highly-folded greywackes and shales of the Brejeira Formation (Moscovian-Carboniferous) and the horizontally placed red sandstones and mudstones of the Group Grés de Silves (of Late Triassic age: 237 and 201.3 million years old). There is a hiatus of about 100 million years between the two formations.

The Variscan period ranges from 370 million to 290 million year ago and is named after the formation of a mountain belt which extends across western Europe, as a result of the collision between Africa and the North American–North European continents.

The imposing sea cliffs produce a privileged place to observe the end of the Variscan Cycle and the beginning of the Alpine Cycle.

It is possible to visit the outcrop on foot, from the top of the cliffs to the beach, although the path is of high degree of difficulty. When going down to the beach one can begin to visualise the typical lithologies of the Grés de Silves. Toward its top you can see red to green Mudstones (dominant) intercalated with rare dolomites and immediately above the unconformity plane it is possible to observe the red sandstone with cross stratification. The highly-folded turbidites (a type of sediment gravity flow responsible for distributing vast amounts of clastic sediment into the deep ocean) of the Brejeira Formation are located below the unconformity.

The folds feature chevron geometries (where the rocks have well behaved layers, with straight limbs and sharp hinges, so that they look like sharp Vs). The folding is the result of the final deformation phase of the Variscan compression.

The beds of sedimentary rocks show sedimentary structures attributed to sedimentation in a turbidic environment (turbititic currents), namely the Bouma sequence and sole marks like flute, groove and load casts.

                                                                                                     By André Cortesão, Environmental Engineer and Geoscientist collaborator of the University of Coimbra Geosciences Centre

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at http://imaggeo.egu.eu/upload/

Imaggeo on Mondays: A spectacular view of moss-covered rocks

Imaggeo on Mondays: A spectacular view of moss-covered rocks

Geology has shaped the rugged landscape of the Isle of Skye – the largest island of Scotland’s Inner Hebrides archipelago. From the very old Precambrian rocks (approximately 2.8 billion years old) in the south of the island, through to the mighty glaciers which covered much of Scotland as recently as 14,700 years ago, the modestly-sized island provides a snap-shot through Earth’s dynamic history.

A far cry from its modern cold, foggy and drizzly weather, back in the Jurassic age (250 million years ago, or so), the island was part of hot and dry desert. Over time, the sea encroached the low-lying plain, depositing sands and muds, and later sandstones, as well as thin limestones and shales across the island. The best examples of these rocks are found on the western side of the island, on the Strathaird Peninsula, but they can also be found on northern and eastern coastal stretches too.

Fast-forward to the Tertiary period (approximately 60 million years ago) and the landscape changed dramatically. The calm tropical waters had made way for explosive eruptions, which vented lavas from crack’s in the Earth’s crust. The lavas blanketed large areas of the north of the island, covering the sediments deposited back in the Jurassic.

Long after the surface explosive activity ended, the cracks in the Earth’s crust continued to serves as pathways for molten magma to move below the surface. In the norther part of the island, the lava travelled sideways, pushing its way between the layers of Jurassic sedimentary rocks. The black lavas, layered between the lighter coloured limestones and sandstones (as pictured above), are in stark contrast with the present-day moss-covered cliffs.

The most spectacular examples of this layering of volcanic units atop sedimentary rocks can be seen not far from where this photograph was taken, at Kilt Rocks, in south Staffin. Visitors to the area can also enjoy Mealt waterfall, where water from Mealt Loch (the Scottish word for lake) tumbles spectacularly into the Sound of Raasay.

Marius Ulm, who captured today’s featured image, is a civil/coastal engineer meaning a totally different aspect of the geology captured his attention:

“From a coastal engineering point of view, what is interesting is the missing moss-cover at the cliff’s toe. There is a line which marks the transition where the rocks stop being covered by moss also indicates how high water regularly rises due to tides. It tells us the tidal range (difference between low and high water) reaches up to 5 m in this area.”

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at http://imaggeo.egu.eu/upload/.

 

Imaggeo on Mondays: Of ancient winds and sands

Imaggeo on Mondays: Of ancient winds and sands

Snippets of our planet’s ancient past are frozen in rocks around the world. By studying the information locked in formations across the globe, geoscientist unpick the history of Earth. Though the layers in today’s featured image may seem abstract to the untrained eye, Elizaveta Kovaleva (a researcher at the University of the Free State in South Africa) describes how they reveal the secrets of ancient winds and past deserts.

In summer 2016 we toured the Western US in a minivan. We visited many of the gems of Utah, Arizona, and New Mexico, such as Monument Valley, Antelope Canyon, Grand Canyon, The Arches, Bryce Canyon, White Sands Monument… But the most precious and memorable for me was Zion National Park in Utah. This canyon is a unique and special place. First, because you access it from the bottom, unlike most of the other canyons, which you observe from cliff tops, such as the Grand Canyon. Thus, as you drive along the road, leading into Zion National Park, you look upward into the magnificent cliffs and rock temples. Small hiking trails lead up to waterfalls, arches and breathtaking views.

The cliffs of Zion National Park are built of Navajo Sandstone and display aeolian deposits, which have been shaped by winds, on a massive scale. They are the remnants of an ancient fossil-bearing sand desert, one of the greatest and largest wind-shaped environments that has ever existed on Earth.

In the Early Jurassic, up to 200 million years ago, the Navajo desert covered most of the Colorado Plateau (which today includes the states of Utah, Colorado, New Mexico and Arizona). Fossils, found in these sand deposits, include ancient trees, dinosaur footprints and rare dinosaur bones.

In Zion National Park, the thickness of sand deposits reaches 762 m. Beautiful cross-beds are cross-sections through fossilized towering sand dunes. They indicate the direction of the ancient winds, which were mainly responsible for moving and accumulating the sand in the Navajo desert. On the top, the Navajo sandstone is abruptly truncated by a regional unconformity, which indicates the erosion of the overlying sediments, and is covered by Middle Jurassic sediments. In remains unknown how much of the Navajo sandstone was eroded from the top of the formation during this weathering episode. It might be that the thickness and height of the Navajo sand dunes used to be even more impressive than it is now.

The cliffs of Zion National Park. Pictured is Checkerboard Mesa (South-Eastern entrance to the Zion National Park. Credit: Credit: Elizaveta Kovaleva.

By Elizaveta Kovaleva, post-doctoral researcher at University of the Free State, in South Africa

Movement of ancient sand is one of the winners of the 2017 Imaggeo Photo Contest.

References

Ron Blakey and Wayne Ranney, Ancient Landscapes of the Colorado Plateau, Grand Canyon Association, 2008, p.156.

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at http://imaggeo.egu.eu/upload/.

 

Imaggeo on Mondays: Ice forming on Chesapeake Bay

Imaggeo on Mondays: Ice forming on Chesapeake Bay

Sandwiched between the U.S states of Mayland, Delaware, Pennsylvania, New York State, the District of Columbia and Virginia, lies Chesapeake Bay, the largest estuary in North America. It is of huge ecological importance: “the bay, its rivers, wetlands and forests provide homes, food and protection for countless animals and plants”, says the Chesapeake Bay Program. Up to 150 major rivers and streams feed into the bay’s watershed.

Geologically speaking, Chesapeake Bay isn’t very old. As recently as 18,000 years ago the bay was covered by dry land. Global sea levels were up to 200m lower than they are at present and the last of the great ice sheets to cover America was at its peak. The rivers which flowed along the east of the continent had to cut valleys in what is now the bay bottom, to reach the continental shelf, and drain out to sea.

Fast forward 8,000 years and rising global temperatures caused the ice sheets to melt rapidly. Global sea levels started to rise, flooding the continental shelf and coastal areas, which now make up the modern-day estuary.

The process was helped along by a remarkable, much older geological feature. During the late Eocene, 35 million years ago, the Atlantic margin of the U.S was struck by a 3.5 km bolide (an asteroid or meteorite). The impact crater is located about 200 km south of Washington D.C., buried below 300 -500 m of sediments in Chesapeake Bay. Though the crater didn’t form the estuary, it did create a long-lasting depression in the area which helped determine the location of the bay.

Landsat satellite picture of Chesapeake Bay (centre) and Delaware Bay (upper right) – and Atlantic coast of the central-eastern United States. Credit: Landsat/NASA. Distributed by Wikimedia Commons.

Further reading

The Chesapeake Bay Bolide Impact: A New View of Coastal Plain Evolution: USGS Fact Sheet 049-98.

Chesapeake Bay Program

Landsat Images Offer Clearer Picture of Changes in Chesapeake Watershed (Nasa Landsat Science)

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at http://imaggeo.egu.eu/upload/.